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Tools for Mining: Techniques and Processes for Small Scale Mining (GTZ, 1993, 538 p.)
	Technical Chapter 15: Gold Benefication
		15.1 Amalgam press
		15.2 Amalgamating table
		15.3 Amalgamating barrel
		15.4 Rocker, cradle
		15.5 Mechanized, compact gold processor
		15.6 Hydraulic gold trap
		15.7 Retort for, amalgam
		15.8 Centrifugal separator
		15.9 Gold pan, batea
		15.10 Jigs with jig bed, russel jig
		15.11 Gold leaching
		15.12 Gold separation by smelting
		15.13 Gold-coal-agglomeration

Tools for Mining: Techniques and Processes for Small Scale Mining (GTZ, 1993, 538 p.)

Technical Chapter 15: Gold Benefication

15.1 Amalgam press

Gold Mining
Beneficiation, Gold Processing

germ.:

Amalgampressen

span.:

prensa de amalgama

Manufacturer:

ASEA, Zutta

TECHNICAL DATA:

Dimensions:

starting at approx. 0.5 × 0.5 × 1 m

Weight:

approx. 40 kg

Extent of Mechanization:

not mechanized

Form of Driving Energy:

manual

Mode of Operation:

intermittent

Throughput/Capacity:

very high, several 100 kg of Amalgam-Hg-mixture per day

Technical Efficiency:

very high degree of separation in comparison to amalgam extrusion performed in cloths without the assistance of a press; residual amalgam contains about 50 - 65 % HgOperating Materials:

Type:

possibly hot water

ECONOMIC DATA:

Investment Costs:

when locally produced approx. 100 DM

Operating Costs:

low

CONDITIONS OF APPLICATION:

Operating Expenditures:

low |————|————| high

Maintenance Expenditures:

low |————|————| high

Effects on Personnel:

high health hazard due to handling of toxic operating materials

Recovery:

very high, < 0.2 % Au remains in the Hg, which is recycled back into the process with the Hg

Replaces other Techniques:

simple extruding without auxiliary apparatuses

Regional Distribution:

very seldom used, to date unknown in developing countries

Operating Experience:

very good |————|————| bad

Environmental Impact:

low |————————| very high

Suitability for Local Production:

very good |————|————| bad

Under What Conditions:

metal manufacturing

Lifespan:

very long |————|————| very short

Bibliography, Source: Schnabel, Schennen, Villefosse

OPERATING PRINCIPLE:

The Amalgam Press takes advantage of differences in viscosity between pure metallic mercury and the gold amalgam alloy (Au3Hg and Au2Hg, viscous masses). The mixture is pressed through leather (chamois or deer) or dense cloth (such as linen) whereby the amalgam remains completely in the press while pure mercury, with an insignificant quantity of gold (< 0.2 %), is forced through the filter cloth/leather and collected.

AREAS OF APPLICATION:

Separation of amalgam from amalgam-mercury mixtures from amalgamating processes in order to reduce feed quantities entering the distillation retort.

REMARKS:

In small-scale mining in Latin America, the amalgam and Hg are often separated without the use of an Amalgam Press. This involves manually removing the mercury from the thick, paste-like amalgam by squeezing it out the side with the fingers. The amalgam is then wrapped in a damp cloth and wrung, whereby the Hg beads are forced out through the cloth and collected In a batea (pan). The high toxic effects of mercury, combined with the low recovery values obtained, advise against the use of this method.

In Colombian gold mining, the mixture is warmed up in hot water prior to pressing it in order to improve the separation of the amalgam from the mercury. The rise in temperature leads to a reduction in the viscosity of the compound and consequently to a better separation of the individual components as it is forced through cloth or squeezed in a press.

SUITABILITY FOR SMALL-SCALE MINING:

A simply-constructed amalgam press can be produced locally at low cost. Its use reduces health and ecological risks associated with the handling of mercury. For this reason, the small-scale mining industry should implement the use of amalgam presses in combination with distillation wherever amalgamation is being performed.

Fig.: Amalgam Press. Source: Rittinger.

15.2 Amalgamating table

Gold Mining
Beneficiation, Gold processing

engl.:

copper plates

germ.:

Amalgamierherd, Amalgamiertisch

span.:

place de amalgamacion, place electroplateada, mesa de amalgamacion, planca de cobre

TECHNICAL DATA:

Dimensions:

4 × 2 × 1 m WDH and smaller; 2.5° - 12° inclination angle

Weight:

several 100 kg as free-standing apparatus with stand, otherwise constructed on the ground

Extent of Mechanization:

not mechanized

Form of Driving Energy:

only processing water

Mode of Operation:

semi-continuous

Throughput/Capacity:

3 t/d per m², table surface area

Operating Materials:

Type:

water

Hg see comments below

Quantity:

slurry density 20 % solids

>= 50 g/m²

ECONOMIC DATA:

Investment Costs:

when of imported construction with stand, high costs due to copper or muntz metal plates, approx. 10.000 DM; when locally produced with copper plates in masonry sluices built on the ground, less than 1000 DM

Operating Costs:

relatively low, almost exclusively costs of reagents

Related Costs:

subsequent hydraulic heavy-material trap for recovery of discharged amalgam and Hg; sun shade above plates to reduce mercury evaporation.

CONDITIONS OF APPLICATION:

Operating Expenditures:

low |————|————| high

Maintenance Expenditures:

low |————|————| high

Effects on Personnel:

high health risk from handling toxic material

Location Requirements:

water necessary

Grain Size of Feed:

50 µm - 1 mm; good classification necessary to avoid mechanical wear (abrasion) of the amalgam

Special Feed Requirements:

Au must exist as liberated free gold; gold may not be encrusted, for example by limonite; a slightly basic pH-value of the slurry improves the amalgamation

Recovery:

60 - 80 % of the free gold, in the fine grain-size range generally higher than with gravimetric sorting

Replaces otherEquipment:

sluices, tables, amalgamating barrels

Regional Distribution:

earlier widely distributed especially in the USA

Operating Experience:

very good |————|————| bad

Environmental Impact:

low |————————| very high

possible sources of environmental contamination are: discharge of large quantities of Hg due to mechanical abrasion of the amalgam and to discharge of foreign-metal amalgams, such as antimony and arsenic amalgam; evaporation of Hg through the water film into the air; discharge of reagents during preparation of the copper plates prior to the amalgamation, e.g., cyanide and silver nitrate

Suitability for Local Production:

very good |————|————| bad

Under What Conditions:

good copper plates must be available

Lifespan:

very long |————————| very short

Bibliography, Source: Schnabel, von Bernewitz, Schennen, Clennell, Treptow, Hentschel, Amalgamation ABC, Taggert. Hypolito, Escobar Alvarez and Echeverri Villa

OPERATING PRINCIPLE:

A slurry composed of the liberated, ground feed material flows along the flat, inclined copper plates. The angle of Inclination must be so chosen as to prevent sedimentation of the mineral particles (dependent on the specific density of the heaviest accompanying minerals). Gold flows at the bottom of the slurry flow and is amalgamated by the mercury. The table surfaces are cleaned of the gold amalgam several times daily and prepared anew for the next amalgamation processing.

AREAS OF APPLICATION:

The amalgamation of finely stamped or ground Au ores. The recovery of Hg following amalgamation in stamp mills or Chilean mills.

REMARKS:

- Besides copper plates, muntz metal plates (60 % Cu, 40 % Zn) are recommended.
- The pre-treatment of the plates is complex and time-consuming:

- Polishing with fine sand
- Degreasing using a 1 % Na- or K-cyanide solution
- Polishing with fine sand
- Use of salmiac solution to remove the oxides of non-precious metals
- Coating with mercury

- Silver amalgam is more effective than pure mercury in amalgamating gold. This is produced either by adding silver nitrate solution to the Hg or alloying with silver foil. Alternatively, the copper or muntz metal plates can be activated by plating them with a thin layer of silver. Pure copper tends to oxidize on the surface. Upon the application of Hg, the Hg surface is made inert by the partly soluble copper salts. Furthermore, these hydrated layers prevent the mercury from adhering to the surface of the plates. The application of silver on the surface prevents this.
- Acidic mine water is detrimental to the amalgamation. By grinding limestone in with the feed, this effect can be neutralized. Correspondingly, It is recommended that this be done only in deposits which have ores containing little or no sulfidic accompanying minerals.

The development of the amalgamation occurred in the 1st century AD. during the reign of Emperor Nero (54 - 68 AD.) in Bosnia.

The inclination of the table has a critical effect on Au recovery. Tables should be inclined just enough so that heavy-material grains do not settle out. The slurry should flow with small periodic waves over the table surface. Small steps improve the amalgamation. The plates comprising the table surface must, in any case, be completely smooth.

To avoid evaporation of mercury especially when the flow of slurry is turned off, the amalgamation table should always be protected from direct sunlight.

- Soluble lead minerals, arsenic (in arsenopyrite, arsenic sulfphides etc.), antimony, and bismuth react either with the mercury, forming amalgam or chemical coatings, or dissolve Hg or precious metal amalgam out of the compound, which lead to substantial losses of precious metals and mercury. Fresh pyrite and copper pyrite, to the contrary, do not affect amalgamation. Barite, talc, steatite and viscous hydrogenized magnesium and aluminum silicates also cause disruptions or losses during amalgamation.
- Oils, grease or lubricants are extremely deleterious and instantly lower the recovery achieved from the amalgamation.
- Prior to amalgamation on the amalgamation table, the feed must be thoroughly classified to ensure that no coarse grains flow over the table which could cause mechanical abrasive wear of the amalgam.

Contaminated, impure mercury is much less active than fresh mercury. While the latter forms ideal beads, almost perfectly round with a bright metallic glow, the contaminated mercury can be recognized by its dull surface, deformed shape of the beads, and the tendency of the beads to adhere somewhat and form a tail when rolled over a smooth inclined surface.

There are several methods of cleaning and reactivating contaminated mercury;

- by screening with a very fine-mesh screen (~ 200 mesh)
- by washing with wood ashes and water (whereby calcium carbonate contributes to the saponification of impurities)
- by washing the mercury with water containing tensides or with special plant-sap solutions, both of which are capable of saponifying grease and greasey substances and bringing them into solution
- by washing with reagents such as ammonia, ammonium chloride, cyanides, hydrochloric acid, nitric acid, etc.
- by distilling the mercury in the retort, which removes slightly volatile impurities
- by admixing sodium amalgam with the mercury, whereby the Na-amalgam is transformed into NaOH upon contact with water, which in turn dissolves surface components of the impure mercury. An effective solution concentration is 1 part sodium to 2000 parts mercury. (The production of Na-amalgam through electrolysis is performed as follows: A vessel is filled with mercury and brought into contact with a carbon cathode placed inside an insulated tube (e.g. glass or plastic). A sodium chloride bath (10 - 15 % solution), which is connected to a carbon anode, is poured over the mercury. Through the direct current of a car battery, Na+-ions are transferred onto the Hg surface and amalgamated as metallic sodium. After 10-15 minutes, sufficient concentrations are reached. The Na-amalgam obtained should be stored under air-tight conditions, for example under petroleum.

REPORTS OF OPERATING EXPERIENCES:

In gold mining in northern Chile, the copper plates are cleaned with urine before being coated with the mercury.

In gold mining in Colombia, amalgamating tables are widely distributed. The amalgamating tables are cleaned of amalgam every 6 hours and newly prepared for reuse. Cleaning of the copper plate, and then the mercury surface, is performed using either a strong detergent or the sap from sisal leaves (cabulla, fique). Sisal contains tenside-like substances which dissolve grease. At the same time, sisal sap helps prevent flotation of gold particles during the amalgamation process. This is accompilshed by placing a piece of leaf in the stamp mill and grinding it in with the ore feed' thereby releasing the sap.

SUITABILITY FOR SMALL-SCALE MINING:

For the processing of finely intergrown gold ores which do not have high sulfide contents, the amalgamation table represents a sorting method which is very effective, reasonably-priced and simple to operate. The use of amalgamation tables should, however, automaticallyinclude the use of devices for the recovery of amalgam and mercury (retorts and hydraulic heavy-material traps) for protection of both health and the environment.

Fig.: Amalgamating table directly behund a stamp mill. Source: Uslar.

Fig.: Amalgamating table. Source: Uslar.

15.3 Amalgamating barrel

Gold Mining
Beneficiation, Gold Processing

germ.:

Amalgamiertrommel

span.:

tromel de amalgamacion, chancho, amalgamadora, barril de amalgamacion

Manufacturer:

Svalcor

TECHNICAL DATA:

Dimensions:

Berdan pan: 1.0 × 0.6 × 0.6 m HOOD, inclination angle approx. 15°, 20 - 30 min^-1

Weight:

from approx. 50 kg up to several 1000 kg

Extent of Mechanization:

not mechanized/semi-mechanized

Form of Driving Energy:

electric motor or internal combustion engine, hydromechanical, manual, pedal drive

Mode of Operation:

intermittent

Throughput/Capacity:

depends on size

Operating Materials:

Type:

mercury

possibly steel balls

water

reagents to activate surface of Hg, e.g. NaOH, sodium amalgam, ammonium chloride, cyanide, nitric acid, tensides

Quantity:

approx. 1 - 3 kg/kg Au

or stone pebbles

ECONOMIC DATA:

Investment Costs:

dependent on extent locally produced and type of drive-system, starting at approx. 100 US$

Operating Costs:

cost of reagents and energy

Related Costs:

amalgamating press, distillation retort, pre-concentrating equipment

CONDITIONS OF APPLICATION:

Operating Expenditures:

low |————|————| high

depends on power source

Maintenance Expenditures:

low |————|————| high

Personnel Requirements:

training in handling of Hg is necessary

Location Requirements:

very little water needed

Grain Size of Feed:

20 · 50 ym to 2 mm for Au-fraction

Special Feed Requirements:

free, liberated gold without crustations (for example by fine Fe-oxides); possibly treatment with reagents, no platy floured gold, low content of antinomy minerals, possibly prior flotation, but then problematic surface conditions (hydrocarbons)

Recovery:

very high under good working conditions; in some cases > 95 %

Grade of Concentrate:

amalgam contains up to 50 % Au, but lower concentrations of gold in the amalgam are practical, with subsequent concentration in an amalgam press

Replaces other Equipment:

manual amalgamation in batea (pan), amalgamation in combination with other processes

Regional Distribution:

worldwide in gold mining

Operating Experience:

very good |————|————| bad

compared to amalgamation in sluices, Chilean mills, stamp mills, etc., Hg-emissions can be kept at very low levels; in particular the addition of reagents and reduction of slurry velocity prevent the occurrence of floured mercury.

Environmental Impact:

low |————|————| very high

Suitability for Local Production:

very good |————|————| bad

Under What Conditions:

metal manufacturing involving pipe sections, sheet metal, bearings, etc.

Lifespan:

very long |————|————| very short

Bibliography, Source: Hentschel, Born, Bernewitz,

OPERATING PRINCIPLE:

The amalgamating barrel operates according to the same principle as the ball mill. The mineral feed consists of rich pre-concentrates. During rotation of the barrel, this feed material is extensively mixed with water, mercury (in a quantity about three times the amount of gold expected to be recovered), grinding bodies and the above-mentioned reagents. During the process, Au-particles come into contact with the mercury and amalgamate. Under influence of the grinding bodies, the gold is worked into the mercury; this occurs so effectively that even the finest gold-fraction, which otherwise could not be recovered In the amalgam due to the surface tension of the mercury, penetrate into the mercury. Upon completion of the rotation process, vibrations and pounding impact forces enhance the density differentiation, whereby the amalgam-Hg mixture collects at the deepest portion of the barrel and is removed following flushing or scimming off of the tailings.

AREAS OF APPLICATION:

For amalgamation of pre-concentrates in a closed reactor.

SPECIAL AREAS OF APPLICATION:

For the simultaneous grinding and amalgamation of non-liberated pre-concentrates in a non-continuous operation, which requtres, without exception, a higher filling-gradient (more grinding bodies) in the ball mill.

REMARKS:

Instead of a ball mill, small-scale mining operations often use cement mixers for barrel amalgamation.

The feed material for barrel amalgamation should always be thoroughly washed prior to amalgamating In order to wash out soluble minerals which could cause inactivation of the Hg-surface.

During a purely amalgamating process, the rotation speed of the amalgamating barrel lies at about 50 % of the optimal rotation speed for a mill of comparable size. At this low speed, the formation of floured mercury is avoided. In simultaneous grinding and amalgamation, an intermediate rotational speed must be applied as a compromise between losses in efficiency due to sliding of the grinding bodies and unfavorable conditions for the amalgamation.

The American "berdan pan" is a slowly-running one-ball mill equipped with a circular ecliptic track, whereby the mill shell rotates and the ball remains in the deepest portion of the track where it floats on the mercury.

The advantage of amalgamating barrels is their contribution to environmental protection by eliminating or reducing losses In metallic mercury (as floured mercury) during amalgamation. During processing, the surface characteristics of the Hg in the closed barrel can be maximized according to the feed material through the addition of reagents.

SUITABILITY FOR SMALL-SCALE MINING:

Amalgamating barrels can be highly recommended for their effectiveness in preventing the release of Hg during amalgamation. They combine the advantages of easy low-cost construction, diverse mechanization possibilities, and ability to control the surface activity of mercury through the addition of reagents with the utilization of the good separating characteristics of amalgamation. Pre-concentrating of the feed material is a prerequisite.

Fig.: Berdan pan, single-ball mill for amalgamation. Source: Bernewitz.

Fig.: Amalgamating barrel as rod mill. Source: Bernewitz

Fig.: Amalgamation in agitators, by Agricola. Source: Agricola.

15.4 Rocker, cradle

Gold Mining
Beneficiation, Gold Processing

germ.:

Rocker, Wiege, Wiegensieb

span.:

cuna, criba cuna, chinchola, cuna californiana, cuna siberiana, lavador, concentrador

Manufacturer:

Keene

TECHNICAL DATA:

Dimensions:

up to several m in length

Weight:

20 - 50 kg

Extent of Mechanization:

not mechanized

Form of Driving Energy:

manual

Mode of Operation:

semi-continuous

Throughput/Capacity:

500 - 1000 kg/MS including refining with batea (panning)

Technical Efficiency:

approx. 6 - 10 t feed with 2 persons in 10 h when solely pre-concentrating

Operating Materials:

Type:

water

Quantity:

400 - 3000 1/10 h

ECONOMIC DATA:

Investment Costs:

starting at approx. 200 DM if locally produced

Operating Costs:

mainly labor costs

CONDITIONS OF APPLICATION:

Operating Expenditures:

low |————|————| high

Maintenance Expenditures:

low |————|————| high

Special Feed Requirements:

lowest possible content of sticky, tenacious clays or partially consolidated sediments that cannot be crushed with the rocker during processing and therefore prevent liberation of the gold

Recovery:

somewhat low recovery in fine fractions, therefore suitable for high quantities of feed (high throughput) containing relatively coarse-grained gold (> 100 ym).

Replaces other Equipment:

sluices, batea (panning)

Regional Distribution:

Chile, Colombia

Operating Experience:

very good |————|————| bad

Environmental Impact:

low |————|————| very high

Suitability for Local Production:

very good |————|————| bad

Under What Conditions:

rockers can be manufactured in simple wood manufacturers using good screen material

Lifespan:

very long |————|————| very short

Bibliography, Source: Rittinger, Buch der Erfindungen 1890 ("Book of Inventions 1890", in German), Silva

OPERATING PRINCIPLE:

The rocker consists primarily of a classifying device and a trough washer. The classifying device has the form of a deep screen box for receiving the feed. Located underneath it is an Inclined wooden riffled trough with transverse slats. The inclination of the trough varies depending on the grain size of the feed material. For feed with high clay content, the angle of inclination is less than for material of coarse grain size. This entire unit is mounted on rockers, so that the whole upper portion can be rocked back and forth by means of a lever handle. The input and discharge of processing water for flushing is performed manually, requiring a total of four people for the mining and processing of gold using one rocker: one for mining the raw ore, one for transporting and feeding the ore into the rocker, one to do the rocking, and one for flushing with water.

AREAS OF APPLICATION:

For processing of loose or slightly-consolidated gold-containing sediments.

REMARKS:

The rocker represents the classic processing tool of North American gold miners and was used up until the present century.

In gold mining in North America, the pre-concentrates obtained from processing with the rocker (heavy-mineral sands with gold) were subsequently dried and sorted by wind-classifying.

The proper adjustment of the sluice's inclination greatly affects the recovery achieved with the rocker. The discussion presented for sluices is applicable here as well.

Rockers are particularly well suited for arid regions due to their low water requirements.

SUITABILITY FOR SMALL-SCALE MINING:

The advantages of a rocker (light-weight construction, portable non-powered technique, highly suitabile for local production) make it appropriate as a mobile beneficiation apparatus for processing sedimentary gold ores during prospecting and mining. Especially for loose materials which do not require crushing, the rocker offers the advantage of a combined classification and sorting.

Fig.: Elevation and plan views of a rocker. Source: Bernewitz.

Fig.: Design plans a rocker. Source: Silva

15.5 Mechanized, compact gold processor

Gold Mining
Beneficiation, Gold Processing

engl.:

gold saver, washplants

germ.:

Mechanisierte kompakte Goldaufbereitungen, z.B. Gold Saver, Prospektor

span.:

beneficios de oro mecanizados en forma compacta, por ejemplo gold saver, prospector

Manufacturers:

Denver, Goldfield, Svalcor, Buena Fortuna, Fima, Metal Callao E.P.S., Met. Lacha

TECHNICAL DATA:

Dimensions:

1 17x74x153 cm HWD

Weight:

270 kg

Extent of Mechanization:

fully mechanized

Driving Power:

3 PS

Form of Driving Energy:

internal combustion engine or electric motor

Alternative Forms:

hydromechanical ?

Mode of Operation:

seml-continuous

Throughput/Capacity:

2-3 m³/h

Operating Materials:

Type:

water

fuel

lubricants

Quantity:

20 - 351/min

approx. 2 - 5l/h

small quantities

ECONOMIC DATA:

Investment Costs:

approx. 4000 us$ if locally produced; approx. 25.000 DM fob factory

CONDITIONS OF APPLICATION:

Operating Expenditures:

low |————————| high

3 people: 1 to feed, 1 to remove tailings, 1 to operate

Maintenance Expenditures:

low |————|————| high

Grain Size of Feed:

< 100 mm

Special Feed Requirements:

relatively coarse gold grains

Recovery:

can be substantially increased with subsequent sluice or amalgamating table

Replaces other Equipment:

sluices, Long Tom

Regional Distribution:

Colombia

Operating Experience:

very good |————|————| bad

Environmental Impact:

low |————|————| very high

depending on type of feed material, high sludge loading can occur

Suitability for Local Production:

very good |————|————| bad

according to Degowski, a Gold Saver has already been reproduced locally in Pasto, Colombia

Under What Conditions:

good metal and welding workshops

Lifespan:

very long |————|————| very short

Bibliography, Source: Degowski

OPERATING PRINCIPLE:

Washplants for gold processing, for example those offered by Denver or Goldfield, consist of a classification drum with a coarse screen for separating and autogenous crushing of consolidated chunks of host rock. Fine material is subsequently processed in a vibrating sluice at a frequency of 200 - 220 min-1. The over-flow can then be further processed, for example, in a sluice or amalgamating table.

AREAS OF APPLICATION:

Extraction of gold from non or slightly-consolidated sediments containing coarse lumps < 100 mm in size.

REMARKS:

A Gold Saver of similar construction as described above was locally produced and applied in a GTZ-project (German Technical Assistance) in Colombia.

Gold Savers may find little acceptance compared to traditional methods. Especially in the extraction of gold from fluviatile alluvial deposits' mine operators consider the following aspects as problematic: the dependency on fuels, the danger of rising water levels in the river which necessites relocation of the Gold Saver to higher ground at the end of each work day, and the very high throughput which requires a corresponding increase in production capacity.

SUITABILITY FOR SMALL-SCALE MINING:

The advantage of the Gold Saver is its compact and mobile construction, serving as a complete processinq unit for loose and slightly-consolidated sediments. For stationary application, however, or when powered hydromechanically. the employment of individual components is a more cost-effective solution.

15.6 Hydraulic gold trap

Gold Mining
Beneficiation, Gold processing

germ.:

Hydraulische Schwergutfalle

span.:

trampas hidraulicas pare material pesado

Manufacturer:

Zutta Hermanos, ASEA-Perini

TECHNICAL DATA:

Dimensions:

up to 60 × 60 × 60 cm depending on throughput capacity

Weight:

from approx. 2 - 20 kg

Extent of Mechanization:

not mechanized

Form of Driving Energy:

only processing water, no drive system

Mode of Operation:

continuous

Throughput/Capacity:

several t/d

Operating Materials:

Type:

water for counter flow

Quantity:

10 - 50 I/min with hydrostatic pressure of 0.1 - 0.5 bar

ECONOMIC DATA:

Investment Costs:

very low, approx. 100 - 200 DM depending on size

Operating Costs:

very low

CONDITIONS OF APPLICATION:

Operating Expenditures:

low |————|————| high

Maintenance Expenditures:

low |————|————| high

Location Requirements:

trap is built into sluice systems, water necessary

Grain Size of Feed:

range of grain size should have an upper limit of 1 - 3 mm

Output::

only coarse gold grains are separated from the mass flow, for example to prevent their being subject to further grinding; no amalgamation occurs, contrary to the similarly designed Jackpot where amalgamation occurs at the deepest point

Replaces other Equipment:

sluices for coarse fractions

Operating Experience:

very good |————|————| bad

Environmental Impact:

low |————|————| very high

since no mercury is used

Suitability for Local Production:

very good |————|————| bad

Under What Conditions:

metal workshop

Lifespan:

very long |————|————| very short

Bibliography, Source: v. Bernewitz, Schennen

OPERATING PRINCIPLE:

The hydraulic gold trap functions like a small artificial sedimentation basin which is placed in the material flow. In so doing, the flow is not interrupted, so that only heavy particles sink down. The sedimentation chamber is kept free of light material by means of a supplementary underwater flow. Therefore, the hydraulic gold trap is comparable to a single-celled settling box with clear water countercurrent.

The concentrate can be withdrawn during processing by opening the outlet valve.

AREAS OF APPLICATION:

Separation of coarse gold and heavy-mineral sand fractions from beneficiation processing circuits, e.g. prior to further grinding, sorting, amalgamating, leaching, etc.

SPECIAL AREAS OF APPLICATION:

Also applied for separating amalgam and mercury after the amalgamating process, for example following a stamp mill to collect the amalgam.

Also appropriate for collecting jig-bed particles discharged from a Jig.

REMARKS:

Suitable only in deposits which contain coarse fractions of gold; not practical for use in deposits containing exclusively fine. grained gold fractions.

In addition to the above-described construction with cross-current flow, some hydraulic gold traps have a slurry-feed input through a centralized pipe under the slurry surface. This forces a reversal in the direction of slurry flow, allowing the gold to settle out. This system therefore functions analogous to the continuous (rake) thickener, or Dorr-type thickener.

SUITABILITY FOR SMALL-SCALE MINING:

Hydraulic gold traps are a simple, efficient, and inexpensive alternative for preliminary separation of coarse gold grates or nuggets. Hydraulic gold traps also play an important role as amalgam or mercury traps succeeding any type of amalgamating equipment.

Fig.: Simple hydraulic trap. Source: Schennen.

Fig.: Cross-section through a hydraulic trap. Source: Taggert.

Fig.: A hydraulic gold trap. Source: Bernewitz

Fig.: Basic operating principle of a simple hydraulic trap for mercury and amalgam. Source: Escobar Alvarez.

15.7 Retort for, amalgam

Gold Mining
Beneficiation, Gold Processing

engl.:

still, distillation retort

germ.:

Destillierkolben

span.:

retorta de destilacion, retorta de amalgama, matraz de destilacion

Manufacturers:

Keene, Svalcor, Talleres J.G., Taller "Centro del Muchacho Trabajador", ASEA, Zutta

TECHNICAL DATA:

Dimensions:

approx. 10 × 10 × 60 - 80 cm, crucible approx. 5 cm in diameter, 5 cm deep

Weight:

approx. 10 kg

Extent of Mechanization:

not mechanized

Form of Driving Energy:

thermal from biological fuels

Mode of Operation:

intermittent

Throughput/Capacity:

depending on design, between 0.5 and 70 or more kg capacity, duration of distillation approx. 15 - 25 min

Operating Materials:

Type:

water Heat by burning wood, coal, gasoline, diesel, gas, etc

Quantity:

small quantities For cooling

ECONOMIC DATA:

Investment Costs:

Retort: 100 to 500 DM if locally produced, serial production should be targeted to lower the cost of production, however amortization possible through recovery of Hg.

Operating Costs:

relatively high due to heating; however in open-circuit distillation, costs are also incurred for fuel

Related Costs:

blow torch approx. 30 DM

CONDITIONS OF APPLICATION:

Operating Expenditures:

low |————|————| high

Maintenance Expenditures:

low |————|————| high

Location Requirements:

even the distillation of amalgam in retorts should take place in sufficiently ventilated environments, preferably in open air

Special Feed Requirements:

the amalgam should be cleaned (see 15.2)

Recovery:

heating of the gold amalgam to a temperature above the boiling point of mercury (350° C) separates the amalgam into gold (residue) and mercury (vapor). Recovery is nearly 100 %.

Replaces other Techniques:

must replace separation processes openly exposed to the atmosphere!!

Regional Distribution:

widespread in beneficiation laboratories in Latin America, seldom found in production plants

Operating Experience:

very good |————|————| bad

Environmental Impact:

low |————|————| very high

minimal Hg-vapor pollution due to opening of the crucible or loose cover/seal

Suitability for Local Production:

very good |————|————| bad

Under What Conditions:

simple metal manufacturer, produced from pipe sections (Rossi-Retort)

Lifespan:

very long |————|————| very short

Bibliography, Source: v. Bernewitz, Stout, Albes, Villefosse, Rittinger, Ullmann, Company Information

OPERATING PRINCIPLE:

Through the inparting of heat to the crucible, the gold-mercury-alloy is dispersed into its components at around 600°C and mercury is vaporized. In the condenser tube, the Hg-vapor condenses as droplets (counter-current cooling with water) and flows into a catchment vessel containing water (which prevents further evaporation).

AREAS OF APPLICATION:

For the separation of amalgam into mercury and the valuable metal (gold or
silver)

REMARKS:

Due to the short utilization period of distillation processes, the distillation retorts should, when possible, be cooperatively purchased and used in order to more widely distribute the investment costs.

Retort constructions which are built from steel-pipe sections are comparatively inexpensive and lend themselves to mass production without major cost or effort.

In all types of retorts, extra care must be taken that fittings, valves or closures are air-tight.

In cases where amalgamating with open Hg-circuits cannot be eliminated, then efforts should be taken to encourage the gold buyer to purchase the amalgam instead of the processed gold. The amalgam could then be distilled in the presence of the mine operator, the gold weighed, and payment made.

Before distillation, the amalgam is wrapped in paper. The ashes of the burned paper form a non-adhesive intermediate layer between gold and the bottom of the retort. It would be better to dust the lining of the retort with a thin layer of graphite, limestone, gypsum or talc prior to inserting the amalgam so that the gold does not stick to the bottom of the retort following distillation. In no case should greasy or fatty substances be used; these evaporate with the mercury and Inactivate its surface.

If a retort is not tightly sealed, the leak can be sealed prior to distillation with a mass made of fine, moist clay mixed with ash applied at the fitting between the cover and the bottom. The clay may not contain any grains, however.

It could often be observed that the acceptance of distillation devices in small-scale mining in developing countries is relatively low. Even when retorts are already present, amalgamation is still performed in open Hg-circuits, first because this proceeds faster, and secondly because the color of the gold produced after evaporating the mercury in a ceramic crucible is lighter, and therefore brings a higher price from the buyers. These disadvantages can possibly be counteracted by lining the Inner surface of the retort with a highly corrosion-resistant material. On the other hand, impurities in the amalgam are partly responsible for the discoloration. In small-scale mining in Ecuador, for instance, the gold concentrate is washed with the juice from sugar-cane oranges or lemons in order to complex silver and thereby increase the fine-gold content. Such impurities impart a black coating during air-tight distillation. A good washing of the concentrate or amalgam can remove these substances, which cause later discoloration, from the feed prior to distillation.

Finally, there are other amalgams, for instance amalgams of arsenic, antimony or copper, which cause impurities In the gold as well as in the Hg. Depending on the boiling temperature and the vapor pressure of the metal, these evaporate With the mercury during the distillation process, or remain as a crust on the gold. These amalgam impurities can be washed out before distillation. First, the amalgam is degreased using a milk of quick lime, then freed of iron chips, and finally washed with diluted hydrochloric acid.

In order to increase acceptance of disillation retorts in small-scale mining regions, the process can be demonstrated to the miners using glass retorts made of heatproof glass. These demonstrations would make the process understandable to the miners and remove their fears of allowing their gold or amalgam to be processed in a closed reactor. This type of retort could also be produced in developing countries by technically-qualified glass-blowers.

In constructing retorts, It is important that the condensation surface for the mercury is kept as small as possible. This minimizes the loss of mercury through cohesion of the finest beads on the inner wall of the retort. Therefore the cooling pipes should be of the least possible diameter, and must be made of iron or steel pipe sections since brass amalgamates with mercury, The inside surface of the pipe must be smooth, and seams should always face upwards to avoid losses due to dripping. Nevertheless, about 2 - 5 g mercury always remain in the device which can only be recovered by washing. For this reason it is advantageous to always collect larger quantities of amalgam for distilling together in one operation.

During operation, the distillation retort must always be heated so that the entire crucible and its lid, as well as the rising portion of the vapor tube, become hot enough to prevent a condensation of mercury in this zone. Otherwise, this liquid mercury runs down again Into the deepest part of the retort and has to be revaporized.

Instead of amalgamating in a distillation retort, the distilling process is unfortunately often conducted in an open-circuit process involving open flat clay or ceramic bowls which are heated by means of blow torches, allowing the mercury to evaporate out of the amalgam. Highly toxic Hg vapors develop.

In Ecuador, a fresh banana leaf is placed over the bowl or crucible in order to recover part of the mercury escaping as vapor from the amalgam. Hg condenses on the surface of the leaf and collects at the edges. Colombian miners use orange peels or cabbage leaves for this purpose.

Besides distillation in retorts, there are also chemical methods for separating mercury from the amalgam. Among these is the analytical method of dissolving Hg out of the amalgam with (diluted) nitric acid. The transformation reaction occurs as follows:

HgAu + 2 HNO₃ ® Hg(NO₃) + Au + H₂

After the precipitated spongy gold-residue has been separated from the dissolved nitrate, the mercury can be recovered through ion exchange with copper or other non-precious metals, with the copper nitrate being discarded. In using chemical separation methods, the danger exists with silver-containing gold ores that Ag gradually concentrates in the mercury, which consequently requires a periodic cleaning by distillation. The chemical reaction for mercury separation is:

Hg(NO3)₂ + Cu ® Cu(NO₃)₂ + Hg

Mercury losses through distillation in retorts are very minimal (< 0,1 %).

WARNING!!!

The separation of the amalgam in the atmosphere is endangering to life and toxic to the environment (Hg-vapors).

In all distillation retorts, care must be taken that, upon completion of heating, no water is sucked in which can penetrate inside the crucible during cooling. This can lead to an explosion of the still-hot crucible due to sudden evaporation. This danger can be prevented through the use of water sacks or similar devices, or by maintaining a minimal distance between the suspended cooling pipe and the catchment bucket.

SUITABILITY FOR SMALL-SCALE MINING:

For economic and especially ecological and health reasons, every amalgamation plant should, without exception, amalgamate in closed mercury circuits, i.e. employ distillation retorts.

Fig.: Design drawing of distilation retort. Source: Bernewitz.

Fig.: Retort for amalgam distillation, designed by Proiekt-Consult,made in Colombia.

Fig.: Retort for distilation of amalgam. Source: Ulsar.

Fig.: Types of retort constructions for distillation. Source: Stout.

Fig.: Simplest retort made of standard pipe joints, threaded couplings and pipe sections. Source: Apropriate Technology

Figure 1. The Hypolito retort or RHYP (above)
Figure 2. The condensation tube (1), tampion (2), elbow (3), and double nipple (4), wich together make up the retort (below).

Fig.: Design sketch of a distillation retort made of pipe sections. Source: Bernewitz.

15.8 Centrifugal separator

Gold Mining
Beneficiation, Gold Beneficiation

engl.:

Knelson concentrator, Knudson bowl

germ.:

Zentrifugalscheider

span.:

concentrador centrifugo, Knelson concentrador, Knudson bowl

Manufacturers:

Knelson International Sales Inc., Mineral Deposits, Steve and Duke's Manufacturing Co. or Lee-Mar Industries Ltd., Goldfield, Falcon Concentraters, FUNDEMIN, VARDAX, Met Lacha, INCOMEC

TECHNICAL DATA:

Knelson 7.5"

Knelson 12"

Knudson

Dimensions LWH

33" × 22" × 26"

31" × 31" × 34"

~ 860 × 760 × 585 mm

Weight

117 kg

154 kg

172 kg (without motor)

Driving Capacity

3/4 PS

1 PS

7.5 PS

Throughput

650 kg/in

5 m³/h

3 - 4 t/h

Quantity of Concentrate

1.5 kg

5 kg

Slurry

17 gal/min

30 gal/min

Backwater Quantity

20 gal/min

35 gal/min

-

Backwater Pressure

2 - 3 bar

aprox. 4 bar

-

Washing Water

Maximum Grain �

< 4 mm

< 4 mm

< 4 mm

Minimum Grain �

> 30 µm

> 30 µm

50 - 70 µm

RPM

~ 400

100 - 400

100 - 105

Centrifugal acceleration

60 g

Separation Cut-off

Mode of Operation

semicont.

semicont.

discont.

Falcon B 12

Falcon B 6

Vardax 801

Dimensions LWH

36" × 60" × 73"

19" × 20" × 32"

72" × 18" × 24"

Weight

800 kg

100 kg

110 kg with classifier

Driving Capacity

7.5 PS

1.0 PS

2 PS

Throughput

~ 6 t/h

0.5 t/h

- 2 t/h

Quantity of Concentrate

- 4.5 kg

~ 1 kg

~ 80 kg

Slurry

7 - 12 gal/min

Backwater Quantity

-

-

Backwater Pressure

-

-

Washing Water

5 gal/wash

2 gal/wash

Maximum Grain �

< 1.5 mm

< 0.9 mm

< 6 mm

Minimum Grain �

> 30 µm

> 30 µm

RPM

Centrifugal acceleration

300 g

300 g

Separation Cut-off

4 g/cm³

Mode of Operation

discont.

discont.

discont.

Extent of Mechanization:

fully mechanized

Form of Driving Energy:

electrical, optional internal combustion engine, possibly convertable to hydromechanical drive with difficulty

Technical efficiency:

concentration up to more than 1: 8000; comparatively very high recovery with well-liberated free gold

ECONOMIC DATA:

Investment Costs:

Prices from original manufacturer fob factory:

Knelson 7.5":

6850 US$

Knelson 12":

12500 US$

Knudson:

4500 US$

Falcon B 12:

34000 US$

Falcon B 6:

7000 US$

Vardax 801:

2400 US$

Vardax Sec.:

14500 US$

Operating Costs:

cost of energy and minimal labor costs

Related Costs:

possibly costs for thickener, sedimentation basin or sludge pond

CONDITIONS OF APPLICATION:

Operating Expenditures:

low |————|————| high

Maintenance Expenditures:

low |————|————| high

Location Requirements:

water

Grain Size of Feed:

< 6 mm

Special Feed Requirements:

the feed may only contain a small proportion of clay minerals or partly-consolidated sediments, since these envelope the gold and, due to their consistence, prevent it from existing in a liberated state.

Output:

According to the manufacturers, approx. 95 % free Au up to grain size > 500 mesh (approx. 30 ym). Mineral Deposits lists 50 - 70 µm as the lower grain-size limit for Knudson centrifuge. BGR in Burundi with Knelson concentrator, 95.5 % recovery, 0.63 - 0.063 mm grain size.

Replaces other Equipment:

sluices

Regional Distribution:

rare

Operating Experience:

very good |————|————| bad

Environmental Impact:

low |————|————| very high

Suitability for Local Production:

verygood |————|————| bad

Lifespan:

very long |————|————| very short

Bibliography, Source: Beyer, Hersteller, BGR

OPERATING PRINCIPLE:

Knelson:

Centrifuge with radial acceleration up to 60 g. Slurry is fed through a pipe at the deepest point of the centrifuge and rotated in circular grooves within the drum. Counter-current water flow is forced into the circular grooves from the outside which loosens the material, similar to a fluidized bed. Light material flows over the wall into the next higher circular groove.

Knudson:

The similarly-built Knudson centrifuge operates without a counter-current flow, which greatly simplies the construction. Instead of the water, a vertical blade within the rotating drum assists in spinning the material In the circular grooves, leading to a purification of the concentrate material.

Falcon:

The Falcon centrifuge is comprised of a vertically rotating cylinder with a partially cone-shaped inner surface. A central slurry feed-intake pipe directs the suspended solids onto the rotating feed-intake disk. Under the influence of centrifugal acceleration, the material migrates outward onto the upward-widening conically-formed centrifuge wall. This effectively results in a radial density-sorting of the slurry in which the heaviest particles remain adhered to the smooth wall. The light material flows over the top of the upper cylindrical portion of the centrifuge and is carried out. A ring-formed zone of concentrate with a wedge-shaped cross-section develops. After cessation of feed input, this concentrate is washed off with supplementary rinsing water, whereby the solid materials between the feed-intake disk and the wall of the centrifuge are flushed through the hollow axis into a receptacle for collecting the concentrate.

AREAS OF APPLICATION:

Sorting of feed material containing high proportions of fine gold, specifically alluvial gold.

SPECIAL AREAS OF APPLICATION:

In addition to its application in gold deposits, the profitable winning of by-products with the Knelson centrifuge is also possible in some cases, such as obtaining gold from gravel pits or heavy industrial minerals from kaolinite deposits. The very high throughput capacity of the Knelson centrifuge concentrator allows it to be integrated directly into the processing circuit.

Knudson centrifuges have also been used in some instances for amalgamating.

REMARKS:

In Brazil, Knelson centrifuges have already been manufactured locally. A problem encountered with local production is the centrifuge bearings; worn bearings must be taken up or replaced if necessary.

It is extremely important that the feed material to be centrifuged is completely liberated or suspended prior to processing in the centrifuge. Clay-like sediments or gold occurring in laterites require a partially expensive pre-processing before sorting by centrifuging. Mineral components in the feed which have very heavy specific densities, particularly arsenic gravel, are also recovered with the concentrate. If the proportion of these minerals is very high, the separation accuracy of the centrifugal sorting is impaired.

The product of the centrifuge is a pre-concentrate which then requires subsequent cleaning (purifying) either by amalgamating, leaching or similar processing.

The counter-current water flow of the Knelson centrifuge must consist of clear water, otherwise the fine perforations in the centrifuge shell could become clogged, and consequently the centrifuged material in the circular grooves could not be loosened. Furthermore, it is absolutely necessary that the water pressure be kept constant, since even negligible increases in pressure can result in fine-grained concentrate, especially particles of high specific surface area, being carried out in the overflow. Likewise, the flow of feed should not be interrupted. These difficulties with process regulation do not arise in simply-designed centrifuges.

On account of their simple construction, Knudson concentrators and possibly Falcon centrifuges are quite suitable for local manufacturing.

In comparind fluid-bed centrifuges (Knelson) to simple centrifuges, Knelson has the advantage of being able to recover significantly smaller grains of the precious metals and achieving a higher degree of concentration, as a result of the fluidized-bed structure. Whereas the Knelson concentrator can operate continuously for many weeks and, as a result, yields high concentrations of gold, the construction of simple centrifuges allows these to operate semi-continuously with only brief pauses and to produce comparatively larger amounts of pre-concentrate (for example, turbulence can be generated in the Knudson centrifuge only very incompletely and the sorting barrel rotates at lower rpm).

SUITABILITY FOR SMALL SCALE MINING:

The Knelson centrifuge is a very suitable apparatus for winning even the finest fractions of gold from alluvial deposits. Despite relatively high investment costs and the necessity to import the equipment, the investment is amortized relatively quickly through the income from the high recovery of the gold fines. Suitable deposit characteristics are a prerequisite for successful application.

Simple centrifugal separators are, especially when they can be manufactured locally, the most suitable for small-scale mining due to their sturdier, simpler construction.

Fig.: Schematic cross-section diagram through the centrifuge of a Knelson concentrator. Source: Knelson.

15.9 Gold pan, batea

Gold Mining
Beneficiation, Gold Beneficiation

germ.:

Sichertrog, Waschpfanne, Schiffchen, Niersch, Saxe

span.:

batea, chua, challa, prune, zuruca (v.), sarten lavador, batea en forma de bote

Southeast Asia:

dulong, dulang

Manufacturers:

Krantz, Keene

TECHNICAL DATA:

Dimensions:

20 50 cm 0, 5 - 25 cm depth / 15 × 15 × 150 cm HWD / ca 45 - 50 cm diameter, 10 cm depth, 35°- 40° inclination (USA)

Weight:

0,5 - 5 kg

Extent of Mechanization:

not mechanized

Form of Driving Energy:

manual

Mode of Operation:

intermittent

Throughput/Capacity:

cat 1 - 5 kg/mini daily performance 100 pans at 20 lbs = 1 t/d

Operating Materials:

Type:

water

Quantity:

small (can be used in non-flowing water)

ECONOMIC DATA:

Investment Costs:

approx. 10 to 20 DM

Operating Costs:

labor costs only

Related Costs:

none

CONDITIONS OF APPLICATION:

Operating Expenditures:

low |————|————| high

Maintenance Expenditures:

low |————|————| high

Personnel Requirements:

lots of experience is essential for accurate sorting with high recovery

Grain Size of Feed:

< approx. 30 mm

Special Feed Requirements:

free Au as valuable mineral or valuable mineral with very high density

Recovery:

high, also in the fine grain-size range (lower grain-size limit 20 µm), flakes (flour gold) down to 50,um are recoverable in the gold pan

Regional Distribution:

worldwide

Operating Experience:

very good |————|————| bad

Environmental Impact:

low |————|————| very high

Suitability for Local Production:

very good |————|————| bad

Under What Conditions:

simple wood manufacturer or sheet-metal workshop for trays made of galvanized sheet metal

Lifespan:

very long |————|————| very short

Bibliography, Source: Treptow, Schnabel, Agricola, Calvor, Ramdohr, Lepper, Clennell, Silva, Treptow Collection/Freiberg

OPERATING PRINCIPLE:

Through shaking of the pan, gold settles at the deepest point on the pan's bottom. During washing, the tray is moved in such a way that the middle and deepest part, containing the heavy gold particles, remains almost stationary and the lighter tailings, carried along by the flowing water and subject to the radial acceleration forces near the pan's rim, are discharged. This procedure is repeated until only the gold or the black gold-containing sands remain. The final step performed by the gold penner is to slightly tip the pan and lightly knock on the back of the rim in the direction of inclination. This resembles the bumping-table effect whereby the gold accumulates at the highest point of concentrate fan.

AREAS OF APPLICATION:

- for analysis in almost all beneficiation processes
- for sorting of pre-concentrates, for instance, from sluices
- for sorting of gold-containing alluvial deposits
- for semi-quantitative analysis of contents exceeding around 10 g/t

REMARKS:

Gold pans are manufactured from various materials, such as:

- metal
- wood
- half-shell or rind of pumpkin, squash or melon
- PVC
- animal horn (poruna), historically from Argentina and Mexico (bull horn); still being used in arid mining regions in Chile
- rubber (car tires)

The best have proven to be those made of black PVC:

Advantages:

- cracks do not develop

Disadvantage:

- the surface repells water

- good gold visibility

- durable and long-lasting

- light weight

Chromite or ilmenite sands are recommended as contrast medium (added to the raw
material)

Gold tends to undergo flotation. 1 or 2 drops of detergent added to the water, or often also sap from plants (e.f. sisal, spanish: fique) can prevent flotation.

Panning was already described by the Swede Peter Mansson who died in 1536.

The number of individual particles per ounce of gold depends upon the grain size:

small nuggets

10 - 20 mesh:

2200/oz.

big flakes

20 - 40 mesh:

12000/oz. gold

fine flakes

< 40 mesh:

40000/oz.

The minimum particle size visible with the naked eye in a black pan is around 20 ym

The lower limit for manual removal of gold particles is 1 - 2 mm, smaller particles require amalgamation or leaching.

Gold pans are often clearly different in their design depending upon the type of feed to be processed: gold pans used In alluvial deposits are generally significantly flatter (shallower) than those pans used in vein ore mining.

For hand-sorting of fine pre-concentrates, small spray bottles with thin elongated nozzles for sucking up the grain are preferred.

In gold mining in Ecuador, for example, amalgamation is also performed in gold pans. The gold is worked in with a stone for about an hour, after which the mercury, divided into fine beads, is recombined by knocking on the rim of the gold pan. The finest beads, or floured mercury, cannot be refused, due in part to the high surface tension of the mercury or encrustations of fine oxidic mineral dusts, and is carried off and released into the environment during washing of the amalgam. For this reason, this procedure must be considered extremely dangerous and should not be used.

In large-scale facilities, the surface tension of the mercury is relieved by adding cyanide or nitric acid, or less frequently sodium amalgam, caustic soda or ammonium chloride. This is not possible when processing in a gold pan.

SUITABILITY FOR SMALL-SCALE MINING:

Gold pans are used in small-scale mining because of their high degree of separation in all areas of application (prospecting, exploration, analysis during processing, and beneficiation); their use is indispensible. In beneficiation they are primarily employed for cleaning of pre-concentrates. They are characterized by very low throughput quantities and investment costs.

Fig.: Different types of gold pans, above left: "Freiberger" pan, above right: "Salzburger" pan, below left: North American from, below right: Latin American form. Source: Treptow (above) and Schnabel (below).

Fig.: Gold pan designs form the Rhine gold mining region (Germany). Source: Lepper.

15.10 Jigs with jig bed, russel jig

Gold Mining
Beneficiation, Gold Beneficiation

germ.:

Setzmaschinen mit Setzbett

span.:

jig con came de boles de promo

Manufacturers:

Mineral Deposits, Denver, IHC Sliedrecht, Goldfield

TECHNICAL DATA:

Dimensions:

from 1 × 0,75 × 1 m up to 3,60 × 3 × 3 m LWH with 0,5 × 0,2 m (2 jig beds) up to several m jig bed sizes

Weight:

from 50 kg

Extent of Mechanization:

fully mechanized

Throughput/Capacity:

4 - 65 t/h

Form of Driving Energy:

electric

Power/Performance:

0,5 to several kW, 50 - 300 thrusts/minute, approx. 25 mm lift

Alternative Forms:

with internal combustion engine, possibly even manual operation for small machines

Operating Materials:

Type:

lead balls grease

Quantity:

8 - 10 DM/kg

ECONOMIC DATA:

Investment Costs:

from approx. 3000 DM when locally produced

Operating Costs:

mainly costs of energy and labor

CONDITIONS OF APPLICATION:

Operating Expenditures:

low |————|————| high

Maintenance Expenditures:

low |————|————| high

Location Requirements:

water must be available

Grain Size of Feed:

50 ym - 2.5 mm gold grain-size fraction is recovered; nuggets larger than 2,5 mm are concentrated on the screen bottom underneath the jig bed.

Special Feed Requirements:

gold must exist as liberated free gold

Replaces other Equipment:

troughs, sluices, other Jigs, Gold Saver

Regional Distribution:

available on the world market, widespread in gold mining in Australia, Indonesia

Operating Experience:

very good |————|————| bad

Environmental Impact:

low |————|————| very high

Suitability for Local Production:

very good |————————| bad

Under What Conditions:

possibly as manual jig or jig with pedal-drive, can be entirely produced by local metal manufacturing shops. For motorized jigs, imported drive units are mostly used.

Lifespan:

very long |————|————| very short

Bibliography, Source: Manufacturer's information, Silva, Schubert, Horway

OPERATING PRINCIPLE:

The Russel Jig with Jig bed for processing gold is a two-staged diaphragm Jig with an internal double-action diaphragm membrane. In comparison to conventional Jigs, this type of construction substantially reduces the drive-power required. The pulsating water flows through a coarse screen mesh of, for example, 1.6 mm wire thickness and 2.5 mm screen openings. A Jig bed, consisting of lead balls (SPb approx. 11.3 g/cm³ ), rests on the bottom of this screen; the lead ball diameter of 4.1 mm has been dimensioned to correspond to the screen openings. Thereby, the Jig bed, which settles onto the screen bottom in the densest volumetric arrangement, is capable of completely closing the screen openings, resulting in higher separation precision and lower quantities of concentrate. The jig's pulse frequencies can be regulated between 50 and 300 min-1. Similarly, the pulse width and the quantity of supplementary water added can also be regulated. The addition of water occurs in the Jig bed above the screen. In this respect, jig types which operate with a constant water quantity (with suction) are differentiated from those in which water is added only during the respective suction-stroke of the diaphragm (without suction). The later is accomplished by regulating with a rotary piston valve.

AREAS OF APPLICATION:

Jigs of the above-described design are used to produce concentrates from alluvial gold or platinum ores. For larger quantities of feed material, the jig is used initially to achieve pre-concentrates, which are then refed into the jig for secondary processing to yield concentrates. Smaller feed quantities are jigged only once and then subsequently processed in an amalgam trap for secondary cleaning or to produce final concentrates.

SPECIAL AREAS OF APPLICATION:

Jigs with jig beds, or Russel jigs, are standard components of mobile pilot-scale beneficiation plants and small mobile production units.

REMARKS:

The simple construction and low specific-energy requirement of the diaphragm jig with Jig bed (Russel Jig) appear to support local production of a manually or pedal-driven Jig of this type. Jig bed size in the order of 2 × 40 × 20 cm should still permit manual operation.

Modern Russel jigs are of modular construction, consisting of several units. These are fashioned from circular segments which are assembled into a round unit. This has the advantage that, with a relatively simpler central feed input, large quantities of feed material can be processed and sorted. The geometry of the jig bed causes the cross-flow to become relatively smaller toward the rim, which increases the processing duration and therefore separation precision.

By modifying the bed material (balls of lighter specific-density), these fine-grain Jigs can also be used for winning well-classified tin or tungsten ores.

The thickness of the jig bed is determined by the granulation of the feed material: for coarse feed, the bed should be 7 to 12 times thicker than the upper grain-size of the concentrate, and for finer feed (< 2 mm) about 20 times the maximum grain-size. The diameter of the bed grains should be 3 to 4 times that of the upper grain-size in the concentrate.

If a Jig is operated with suction, fine fractions are quickly and accurately drawn through the processing, while coarse fractions migrate only very slowly through the jig bed and screen mesh. In jigs run without suction, the effect is reversed.

If several consecutively-arranged jig beds are used, the grain sizes of the jig-bed particles increase In the direction of feed input.

The length of duration of feed material in the jig can be varied by changing the ratio of cross-current flow: quantity of supplementary water.

The use of a heavy-material trap, such as a riffled sluice, installed in the light-material discharge outlet in the Jig bed is highly recommended for the recovery of jig-bed material which has been undesirably flushed out.

The large diaphragm dilations can be achieved by using a rubber car hose as the membrane element The complicated insertion of the diaphragm connecting-rod through the jig's settling-box wall can also be accomplished with the help of a locally-available standard part, namely the bellow which covers the gear-shift-lever slot in a car, which can be used to seal the opening around the rod.

As bed material for locally-manufactured jigs, lead buckshot (for hunting purposes) can be used, which is freely sold on the market in developing countries.

The concentrate or underflow valves in the jig bed should always be slightly open during operation and the concentrate continuously discharged in order to avoid sedimentation and clogging of drain outlets.

SUITABILITY FOR SMALL-SCALE MINING:

Jigs with Jig beds for producing gold pre-concentrates at high factors of concentration with comparatively high throughput are very appropriate for small-scale mining; they require, however, a motorized drive system.

15.11 Gold leaching

Gold Mining
Beneficiation, Gold Beneficiation

engl:

cyanide leaching, (agitation leaching, vat leaching, heap leaching)

germ.:

Goldlaugung, cyanidische Laugung (Ruhrlaugung, Behalterlaugung, Haufenlaugung)

span.:

lixivacion de oro, lixiviacion, lixiviacion con cianuro (lixiviacion por agitacion, lixiviacion en tanques, lixiviacion en piles)

Manufacturers:

HBS-Equipment (cell for electrolytic + absorptive separation), Denver

TECHNICAL DATA:

Dimensions:

leaching tanks several m³ in volume

Weight:

brick masonry basins

Extent of Mechanization:

fully mechanized

Mode of Operation:

semi-continuous, continuous

Power:

varies according to type of leaching procedure chosen, from 100 W for small percolation leaching plants (pump drive) up to several kW for larger agitation-leaching plants

Form of Driving Energy:

electric drive for pumps and filter

Technical Efficiency:

three various categories of efficiency are differentiated: leaching efficiency, adsorption efficiency (= f (activated carbon quality, etc.)), stripping efficiency (degree of stripping does not influence processing results if activated carbon is reused in the circuit because gold is not lost in the process) the sum of the different efficiencies leads to recovery values which lie at approx. 90 - 95 % for agitation leaching, approx. 80 - 90 % for vat leaching and ca 50 - 80 % for heap leaching.

Operating Materials:

Type:

sodium cyanide (NaCN) or calcium cyanide/black cyanide (Ca(CN)₂)

Quantity:

Concentration: 1 - 5 kg/t (ave. 1,5), Consumption: up to 8 kg/t, compressed air, CaO, Zn and PbNO₃ or activated carbon

ECONOMIC DATA:

Investment Costs:

for small percolation-leaching plants only minimal costs for the masonry construction of leaching basins, precipitating basins and leach collection containers, totalling approx. 1000 DM depending upon cost of materials. For industrial-scale plants, (e.g. CIP - carbon-in pulp process) costs are extremely high at 1.000.000 DM minimum.

Operating Costs:

high costs due to consumption of reagents, approx. 50 % of operating costs are for cyanide; also energy-intensive technique, especially the Merrill-Crowe-Procedure

Related Costs:

costs for sludge ponds, especially in leaching facilities where finely-ground ores are processed

CONDITIONS OF APPLICATION:

heap leaching

Operating Expenditures:

low |————|————| high

CIP

Maintenance Expenditures:

low—|high

depends on type of leaching

Personnel Requirements:

of extreme importance is a precise control of the process, especially with regard to the homogeneity of concentrations, feeds and slurry characteristics

Location Requirements:

very high space requirements for heap leaching

Grain Size of Feed:

< 0.1 mm for agitation leaching; <10 mm for vat leaching of 2 - 4 day duration; < 50 mm for heap leaching of 3 - 6 week duration

Special Feed Requirements:

minerals of arsenic, antimony, manganese and especially soluble oxidic copper, which are extremely deleterious and cause high cyanide consumption, high gold losses in organic CH-bonding, graphite, etc., require the use of CIL (carbon-in-leach process). Pyrrhotine (magnetic pyrite) is also a detrimental mineral: it binds cyanide ions and consumes oxygen during decomposition.

Replaces other Equipment:

all other methods of cleaning pre-concentrate in gold beneficiation, e.g. manual sorting, amalgamation, gravimetric processes, smelting

Regional Distribution:

worldwide more than 70 % of all gold is won by leaching. Cyanide leaching, however, is cost intensive, difficult to control and involves complicated technical equipment, restricting its application to large-scale mining operations.

Operating Experience:

very good |————|————| bad

Environmental Impact:

low |————|————| very high

The dangers associated with leaching operation lie in the possibility of cyanide being released as a result of leaks, haphazard handling, etc. Cyanide is highly toxic; only trained personnel should operate leaching operations to ensure safe use; large space requirements heap leaching

heap leaching

Suitability for Local Production:

very good |————|————| bad

CIP

Under What Conditions:

for small percolation leaching plants, the simple brickwork provides very good possibilities for local construction

Lifespan:

very long |————|————| very short

Bibliography, Source: Krone/Erzmetall (German publication), Ullmann, Rollwagen in Erzmetall 41/2, Beyer, Bugnosen, DE 3429458 Al, Seeton, Meza S.

OPERATING PRINCIPLE:

Agitation leaching

The cyanide gold leaching exploits the ability of gold to build soluble cyanide complexes. Specifically, the ore is subject to the following procedure: After grinding of the raw ore to < 0,1 mm, the resulting slurry is treated with CaO as lime milk to bring the pH-value to around 10 · 11,5 and then thickened to 40 - 50 % solids (by mass). Sodium cyanide is added to the slurry in the agitation tank at a concentration of 100 ppm NaCN; the solids are held in suspension either by means of a stirrer or by injected compressed air (In Pachucas). At a pH-value of between 10 and 1 1,5, at which point the dissociation balance shifts In favour of the cyanide ions, the following chemical reaction takes place:

2Au + 4NaCN + 1/2 O₂+ H₂O ® 2NaAu(CN)₂+ 2NaOH

The leaching duration lasts between 12 and 24 hours, producing solutions containing gold. concentrations of about 4 - 6 ppm.

Vat leaching

In vat leaching, pre-crushed or agglomerated ores in containers (vats) are flooded with a cyanide leaching solution of specified pH-value. The leaching process proceeds analagous to the above-described chemical process. At the end of the exposure time, a clear leachate solution is withdrawn through a filter tube.

Percolation leaching

Tailings derived from gravimetric beneficiation processes are often leached by percolation leaching. Large open tanks (of up to more than 100 m in volume) are equipped with a leachate outlet on the bottom, sometimes constructed as a double bottom with filter cloth or gravel. These tanks are filled with the ores to be leached, the leachate solution is alternately added and then allowed to seep down through the ore: the trickling speed should exceed 8 - 10 cm/h; rates below 2 cm/in are very disadvantageous, and indicate that the feed material requires prior desilting or desliming. After sprinkling of the leachate solution, the solution level sinks to the bottom, during which air and therefore oxygen for oxidation penetrate into the ore layer. This process is repeated daily for the duration of leaching, which ranges from a few days to more than a month.

The leachate can be added in varying concentrations, that is: first, highly concentrated, then less concentrated, and later as washing leachate.

Heap leaching

Depending upon the leachability, coarsely-crushed ore is placed onto a leakproof basin on the ground which has been sealed with clay, asphalt and/or a tarp covering. The ore is then doused with leachate solution. Upon completion of the leaching process, a clear gold-containing cyanide complex solution is drawn off.

Depending upon the nature of the gold-cyanide complex solution, various methods can be used for further processing:

For clear solutions:

- In the Merrill-Crowe Process the solution is separated from unsoluble components in a cloth-lined vacuum filter or a suspended filtering candle and is completely deaerated with a vacuum cylinder (otherwise oxidation occurs in subsequent steps which leads to high gold losses). The oxygen concentration is consequently reduced to about 0.5 mg/l. Following the addition of zinc dust and lead nitrate solution (from local elements), the cementation of the gold takes place according to the following reaction:
2 NaAu(CN)₂ + 2Zn ® 2 Au + Na₂[Zn(CN)₄].
Gold and excess zinc are filtered through a filter press (the cyanide solution is re-circulated), and the solid materials are then treated with diluted sulfuric acid in order to wash out the excess zinc. The gold slime is then calcined at approx. 800° C and subsequently melted at 1200° C with borax and silicate fluxing agents.
- A significantly simplified variation, however associated with higher gold losses, is the zinc precipitation method. This is performed using a calotte of nested screens containing loosely-layered fine zinc shavings with large specific surface area The leachate solution is then applied from underneath, flowing upward through the screens. The gold separates out onto the zinc shavings and becomes visible as a black discoloration. When all the shavings are loaded with gold, these are melted and the gold collected (possibly performed by buyers or service organizations).
Recent patent literature also describes the cementation of gold from slightly-turbid cyanide leachate in reaction vessels filled with zinc granules and shaken by means of a vibrator which supposedly leads to faster cementing, higher recovery, lower zinc consumption, and a greatly simplified processing procedure.
- A third possibility is the CIC (Carbon in Column) Process in which clear gold-cyanide- complex solution flows from the bottom upwards through a cylinder filled with activated carbon, whereby the gold adsorbs onto the carbon. Activated carbon which is completely saturated with gold can contain as much as 20 - 30 kg gold/l, marketed either as ashed gold or as gold concentrate.

For slurries:

- In the Carbon-in-Pulp Process, in which granulated activated carbon is added to the slurry, the precious-metal cyanide complex is adsorbed onto the activated carbon. This is then mechanically separated (by screening) and washed out with a strong alkaline sodium-cyanide leaching solution, possibly under conditions of increased pressure and temperature; subsequently the gold is recovered electrolytically by collecting it together with silver and copper on steel wool electrodes. Upon completion of the process, the activated carbon must be regenerated (a costly endeavor). Alternatively, the gold-saturated activated carbon can be incinerated to ash.
- Gold-containing ores that also contain organic substances (which as semi-activated medium can potentially absorb gold from the leachate), are processed using the CIL (Carbon-in-Leach-Process), in which the leachate already contains the activated carbon when it is added to the ore. The more active carbon absorbs, which can then be mechanically separated as described above for the CIP Process. Slurries with a small proportion of suspended material can be filtered in gravel-bed filters; this is the cheapest method, involving the least equipment, for purifying solutions.
For purifying slurries, the CCD method (counter current decantation) is also applied, a process in which several thickeners are charged in counter-current system (opposite direction); the thickened sludge from one is again fed back into the previous thickener, and the overflow is directed into the next thickener.

REMARKS:

The process was developed in South Africa in 1889.

In agitation leaching operations, the leaching can partly begin already in the mill by performing wet grinding in a cyanide leach. This has the advantages in that absolutely fresh (uncontaminated) mineral surfaces come into contact with the solution.

In the ClP-Leaching, it is often the extremely high cost of the activated carbon which renders the procedure uneconomical in developing countries. At the same time, there are good possibilities in many areas in developing countries where activated carbon could be locally produced. Raw materials such as coconut husks are particularly well suited for such purposes. Coconut shell carbon is known particularly for its hardness and fine porosity. However, the raw material, namely the coconut shells, which are usually used as heating fuel, are relatively expensive. Moreover, the quality standards are very high: the shells must be clean and very fresh. For these reasons, imported activated carbon has been employed so far in small-scale mining in developing countries.

High costs occur due to consumption of reagents, particularly the consumption of cyanide through oxidation, release of HCN and reactions with accompanying substances in the ores. In ClP-leaching, abrasion of the loaded activated carbon is problematic (gold losses).

The agitation leaching processes ground products or finely-crushed ores, the vat leaching processes pre-crushed ores, the heap leaching processes coarsely crushed crude ore. Heap leaching is less expensive, but with respect to the comparatively low recovery (approx. 50 %) is less recommended (the process is better suited for low grade ores). Ores which can be leached in the stockpile in a coarsely-crushed form are rare. Gold particles which, for example, are bound within quartz cannot be leached without grinding to liberate them.

Tailings from gold leaching operations must be stockpiled or collected in a sludge pond. Excessive CN-contents decompose over time under the influence of ultra-violet radiation.

In general, agitation leaching and vat leaching, and also heap leaching for easily-leachable ores, appear to be the most suited for small-scale mining; these methods require substantially less equipment for the leaching, adsorption and winning of gold from clear slurries than other methods.

High temperatures tend to cause decomposition of cyanide leaching solutions, whereas low temperatures drastically reduce the speed of reaction. The economical optimum lies at a leaching temperature of about 20 C, which in colder climates is attained through artificial heating. In all cases, leaching tanks and vessels should be covered since UV-radiation leads to decomposition of the leaching solution.

To avoid environmental hazards, leaching tanks and vessels should be covered on top with wire mesh to prevent humans, animals and especially birds from gaining direct access to the toxic solution. Pachuca tanks for compressed-air agitation leaching of the slurry have a height which corresponds to at least three times the diameter of the tank.

Leaching tanks for percolation leaching should not be too deep. Very deep tanks inhibit the penetration of air during the sinking of the leachate level, thereby insufficiently supplying the ore with the necessary oxygen for leaching.

Leached ores can be removed from vat leaching plants and tanks by means of a bottom gate or, even simpler, by flushing with large quantities of water.

Cyanide leaching allows the processing of a very wide spectrum of gold ores, for example, ores with fine-grained free gold (down to sub-microscopic gold occurences such as in vulcanises or carbonates), gold from soluble sulfides, and gold attached to the surface of sulfides. Refractory ores, for example, with gold-containing pyrites are not leachable without further processing (e.g. roasting).

Ores which exhibit varying intergrowth relationships can be selectively comminuted in order to attain liberation without overgrinding. Ores with coarse gold intergrowths in quartz or with finer gold intergrowths in sulfides and their interspaces are frequently encountered. In this case, the sulfides from the primary grinding circuit can be gravimetrically separated (for example in a jig) and selectively finely ground in a second grinding circuit.

Gold-containing cyanide leachates should always be immediately further treated, otherwise there is danger that colioids (mostly aluminum, iron or magnesium hydrates) settle out of the clear solution which hinder the precipitation of gold onto the zinc shavings or zinc dust. Furthermore, the Ca-rich leachates can precipitate calcium carbonate by absorbing CO₂ from the air.

Zinc wool for precipitation should have a thickness of approx. 0.02 mm. It then exposes between 10 and 20 m² of specific surface area pro kg and has a volume of approx. 10 lifers. Optimal quantities are approx. 30 lifers of zinc wool for every m³ leachate/24 hours. Solutions which have passed through the zinc wool precipitation are dropped into the supply tank from greater heights so as to enrich it with oxygen as it falls through the air.

High, cylindrical leach tanks, such as pachucas, can be built out of cement rings stacked on top of each other. For leaching in an acidic medium (for example, with thiourea), these cement rings can simply be lined with synthetic resin.

Leaching is especially suitable for ores containing fine-grained gold particles of high specific surface area With feed of coarser grain-size fractions the leaching speed drops. Therefore, these coarser fractions are usually separated in a prior gravity beneficiation, leaving only the tailings and the fine fractions to be leached.

Leaching speed may be increased by leaching under pressure; this method, however, is characterized by enormously high investment costs and is therefore not appropriate for small-scale mining.

EXPERIENCES IN LEACHING IN SMALL-SCALE MINING:

In Brazilian gold mining at its smallest scale, a technique could be observed in which raw ores are mixed with cement via shovelling and thus agglomerated. A leaching process which produces pure solutions is conducted in small vessels (for example, diesel barrels). The adsorption of the gold-cyanide-complex takes place on locally produced Babacu-nut-carbons, which are subsequently incinerated to ashes.

In small-scale gold ore mining in Colombia and Ecuador, percolation leaching plants are in operation in which the tailings from amalgamation plants and gravimetric beneficiation processes are leached. Here the slurry-flow falls in a sedimentation basin, which has the effect of desilting the sands. The sedimented material is then sufficiently permeable to produce a pure solution during leaching. The brick leaching tanks have a capacity of 20 - 100 t and are situated above the precipitation basin in which the gold is precipitated onto the zinc shavings. Subsequently, the solution drains into the leachate supply tank. A small pump, driven by a gasoline-engine, pumps the leachate into the leaching tank once daily. The investment costs for this type of facility are minimal and, depending upon the cost of building materials and wages, total less than 5000 DM.

Electrolytic separation of gold from the cyanide leachate is performed in Philippine mining using locally-produced cells constructed from batteries, where the anodes are made of stainless-steel screen mesh and the cathodes of steel wool The gold-containing leachate continuously flows through these cells in which a 12 V, 60 A electrical current has been applied The gold is later shaken out of the steel wool and collected, and the steel wool reused.

SUITABILITY FOR SMALL-SCALE MINING:

Cyanide leaching yields high recovery particularly in the beneficiation of ores containing fine gold fractions. However, its dependence on large quantities of reagents, some toxic, and the difficulties in controlling the processes are problematic for small-scale mining application. In medium-scale plants where specialized knowledge is readily available, gold leaching is an economic alternative which can even serve, among others, as a substitute for the environmentally detrimental amalgamation processing.

Fig.: A zinc precipitation plant made of wood. Source: Armstrong.

15.12 Gold separation by smelting

Gold Mining
Beneficiation, Gold Beneficiation

germ.:

Schmeiztrennung von Gold

span.:

separacion del oro por fundicion

TECHNICAL DATA:

Dimensions:

oven approx. 1 × 1 × 1 m

Throughput/Capacity:

thermal heating up to about 1200° C

Form of Driving Energy:

gasoline, oil, coal or wood burner; or electric oven

Mode of Operation:

intermittent

Technical Efficiency:

very high recovery

Operating Materials:

Type:

various fluxing agents, coating agent and precious-metal collector (the latter only when fire assay is applied (Dokimasie))

ECONOMIC DATA:

Investment Costs:

refractory (fireclay) crucible, graphite crucible, oxide-ceramic crucible, crucible tongs, agitator, mortar, iron crucible, and heating facility (crucible oven) to 2.000° C, totalling about 5000 DM when of Latin American production

Operating Costs:

cost of energy, labor costs, cost of reagents, cost of crucibles (20 to 35 (max) melts/crucible)

Related Costs:

possibly cost of presses for the manufacture of crucibles and cupels.

CONDITIONS OF APPLICATION:

Operating Expenditures:

low |————|————| high

Maintenance Expenditures:

low |————|————| high

Personnel Requirements:

extensive experience is necessary, especially for the quantitative separation by fire assay

Recovery:

through the use of precious-metal collectors, recovery can be quasi quantitative (100 %)

Replaces other Equipment:

as a beneficiation technique, replaces other methods for winning pure gold from pre-concentrates, e.g. hand picking. As an analytical technique, fire assay is the simplest, fastest and above all the most accurate method of gold analysis.

Regional Distribution:

in gold analysis worldwide, as a beneficiation technique very rare

Operating Experience:

very good |————|————| bad

Environmental Impact:

low |————|————| very high

during use of this process, gases of volatile and possibly toxic components are emitted into the atmosphere; also, depending upon energy source, detrimental exhaust gases.

Suitability for Local Production:

very good |————|————| bad

Under What Conditions:

oven cannot be locally produced by non-specialized manufacturing plants; crucibles and cupels can be locally manufactured

Lifespan:

very long |————————| very short

short service life especially of crucibles and cupels

Bibliography, Source: Chemikerausschu� der GDMB

OPERATING PRINCIPLE:

To separate gold from the heavy minerals, the enriched pre-concentrate is placed in a crucible with borax and baking powder or ammonium chloride (NH₄CI) and heated to a temperature of 1200° C. This causes the oxidic minerals such as limonite, ilmenite, etc. to melt. In the solid-liquid system which is created, liquid gold concentrates at the bottom of the crucible underneath the slag. The melting temperature of gold is 1063° C.

In the "fire assay" method, the material to be separated, normally a sample for determing precious metal content, is melted together with the excess lead and flux. Through the addition of oxidation or reduction agents, as well as slag-forming agents, the melt separates into the precious-metal containing lead regulus and a slag extensively free of precious metals. The lead regulus is then separated and further processed by the so-called cupellation process. The regulus is heated in a cupel a crucible made of bone ashes - whereby the lead is oxidized in the liquid melt and is either volatized or absorbed by the cupel material (especially the magnesium component therein). What remains is an almost perfectly round preclous-metal grain which allows precise quantitative conclusions to be drawn, either by weighing or by size-comparison with a linear scale, concerning the Initial content of the feed material.

AREAS OF APPLICATION:

Melting to produce pure precious metals from concentrates, employed as a separation technique In small-scale mining.

Fire assay, as the most important method of analysis for gold contents in mined ore, can no longer be ignored; In addition to free gold, other gold occurrences, such as gold contained in pyrite, can also be determined.

REMARKS:

This technique or method is not applicable for platinum group metals due to their significantly higher melting point (platinum 1769° C, rhodium 1966° C, palladium 1550° C, osmium 2700° C, iridium 2454° C).

As an analytical method, fire assay captures and reveals the entire gold content of a sample, including the diadochic contents in the pyrite lattice such as dispersed gold. The recoverable free-gold contents are determined by panning.

Cost of analysis with imported crucibles and cupels is about 4 - 6 US$ per sample, compared to 2 - US$ when locally. manufactured crucibles and cupels are used. Crucibles of clay can be locally formed and fired. It must be taken into consideration, however, that only clay which is completely free of gold should be used, since during melting the crucible material is also melted, and any gold contained in the crucible material could accordingly alter the analytical results. Cupels can also be locally manufactured from a mixture of bone ashes or magnesium and common, commercially-traded cement.

SUITABILITY FOR SMALL-SCALE MINING:

Gold separation by smelting is an inexpensive and very accurate method of achieving marketable products when the concentration of gold is adequate (determined by previous sample melting). Fire assay is the most Important analytical method for determing gold contents in ores.

Cupels made of Bone Ashes

Size

Upper diameter mm

Lead adsorption capacity gr

Height mm

Weight gr

1

22

4

11

4

2

24

7

13

7

4

30

13

14

13

5

33

18

16

18

6

35

24

18

24

7

40

30

19

30

8

50

60

25

60

9

60

100

27

100

10

88

300

33

300

Fig.: Cupels: sizes, lead-adsorption capacity and shape.
Source: Frick-Dausch

Table: The most important sample reagents. Source: Frick-Dausch

Name of Sampler

Composition

Tasks and Characteristics

Quartz

SiO₂

scorification, fluxing agent, acidic

Glass

x Na₂O y CaO z SiO₂

scorification, fluxing agent, acidic (weaker than quartz)

Borax

Na₂B₄O₇ 10H₂O

scorification, fluxing agent, acidic

Borax glass or molten borax

Na₂B₄O₇

scorification, fluxing agent, acidic

Phosphorus salt

Na(NH₄)HPO₄ 4H₂O

scorification, fluxing agent, seldom used

Soda

Na₂CO₃

scorification fluxing agent, desulfurization

Sodium bicarbonate

NaHCO₃

scorification fluxing agent, desulfurization

Potassium carbonate

(Potash)

K₂CO₃

scorification, fluxing agent, desulfurization

Lead (II) oxide

PbO

scorification, fluxing agent, desulfurization,

oxidizing, collector, basic

Tartaric

KHC₄H₄O₆

reducing agent and basic fluxing agent

Charcoal

reducing agent

Flour

reducing agent

Potassium Cyanide

KCN

reducing agent and neutral fluxing agent

Iron

Fe

reduing agent, desulfurization, basic scorification agent

Salpeter (salts of nitric acid)

KNO₃ (NaNO₃)

oxidizing agent, desulfurizination, basic fluxing agent

Assay lead

Pb

collector

White lead

2 PbCO₃ Pb(OH)₂

collector, also desulfurizing, oxidizing basic fluxing agent

Lead acetate

Pb(CH₃COO)2 3H₂O

collector, sometimes also desulfurizing basic fluxing agent

Sodium chloride

NaCI

coating agent

Fluorite

CaF₂

inert neutral fluxing agent

Greenland spar cryolite

Na₃AlF₆

dissolves Al₂O₃

Ammonium carbonate

(NH₄)2CO₃

desulfurizing, volatilizing

1 The earlier common names have been kept

Fig.: Phase diagram of gold-lead melting. Source: Frick-Dausch.

Fig.: Crucible tongs. Source: Frick-Dausch. a) strainght from; b) curved form

Fig.: Mold for fire assay. Source: Frick-Dausch.

Fig.: Standardized reference scale for the determination of gold content from fire assays. Source: Frick-Dausch.

15.13 Gold-coal-agglomeration

Gold Mining
Beneficiation, Gold Beneficiation

engl.:

coal-gold agglomeration, CGA

germ.:

Gold-coal-agglomeration

span.:

aglomeracion oro-carbon

TECHNICAL DATA:

Extent of Mechanization:

fully mechanized

Power:

for agitating and pumping, several kW

Form of Driving Energy:

electric for mixing, agitating

Throughput/Capacity:

not known, since it has not yet been applied in large-scale industrial operations, British Petroleum operates with a 1 t/h pilot plant (see House), 30 min contact time gave optimal results

Technical Efficiency:

loading of the agglomerate in large-scale plant operations to between 1000 and 5000 g/t is possible

Operating Materials:

Type:

oil, activated carbon

ECONOMIC DATA:

Investment Costs:

not known

Operating Costs:

high cost of reagents, conservative cost estimates show that operating costs for CGA are less than those for leaching

Related Costs:

costs of grinding and settling basin, thickener or sludge pond for treating tailings

CONDITIONS OF APPLICATION:

Grain Size of Feed:

0.2 - 200 ym gold particles can be agglomerated

Special Feed Requirements:

gold, electrum (Ag-bearing Au), gold telluride, also for ores with significantly less than 1 g Au/t

Recovery:

over 90 % in pilot plants

Replaces other Equipment:

this technique is used for obtaining meltable gold concentrates and should replace amalgamation, leaching, and other methods.

Regional Distribution:

so far not applied in large scale operations; used in pilot plants in Australia

Operating Experience:

information not yet available

Environmental Impact:

low |————|————| very high

high sludge loading; pollution through discharge and consumption of reagents, which are produced by highly energy-intensive and environmentally detrimental processes (for example the production of activited carbon). Positive effect if it proves successful as a substitute for amalgamation.

Bibliography, Source: F 530793, USA 4, 597, 791, House

OPERATING PRINCIPLE:

In a BP-gold-coal-agglomeration pilot project, the high-grade gold-containing fine-grained slurry is transposed by activated carbon into an oil suspension. The hydrophobia of the gold is being utilized in this process. The gold then agglomerates onto the oil-saturated activated carbon particles. Following agitation of the slurry-reagents-mixture, the gold-oil-activated carbon agglomerate is mechanically separated.

CGA Process Flowsheet:

SUITABILITY FOR SMALL-SCALE MINING:

Suitability for small-scale mining application is not yet assessible due to insufficient operational data.

germ.:	Amalgampressen
span.:	prensa de amalgama
Manufacturer:	ASEA, Zutta

TECHNICAL DATA:
Dimensions:	starting at approx. 0.5 × 0.5 × 1 m
Weight:	approx. 40 kg
Extent of Mechanization:	not mechanized
Form of Driving Energy:	manual
Mode of Operation:	intermittent
Throughput/Capacity:	very high, several 100 kg of Amalgam-Hg-mixture per day
Technical Efficiency:	very high degree of separation in comparison to amalgam extrusion performed in cloths without the assistance of a press; residual amalgam contains about 50 - 65 % HgOperating Materials:
Type:	possibly hot water
ECONOMIC DATA:
Investment Costs:	when locally produced approx. 100 DM
Operating Costs:	low

Operating Expenditures:	low \|————\|————\| high
Maintenance Expenditures:	low \|————\|————\| high
Effects on Personnel:	high health hazard due to handling of toxic operating materials
Recovery:	very high, < 0.2 % Au remains in the Hg, which is recycled back into the process with the Hg
Replaces other Techniques:	simple extruding without auxiliary apparatuses
Regional Distribution:	very seldom used, to date unknown in developing countries
Operating Experience:	very good \|————\|————\| bad
Environmental Impact:	low \|————————\| very high
Suitability for Local Production:	very good \|————\|————\| bad
Under What Conditions:	metal manufacturing
Lifespan:	very long \|————\|————\| very short

engl.:	copper plates
germ.:	Amalgamierherd, Amalgamiertisch
span.:	place de amalgamacion, place electroplateada, mesa de amalgamacion, planca de cobre

TECHNICAL DATA:
Dimensions:	4 × 2 × 1 m WDH and smaller; 2.5° - 12° inclination angle
Weight:	several 100 kg as free-standing apparatus with stand, otherwise constructed on the ground
Extent of Mechanization:	not mechanized
Form of Driving Energy:	only processing water
Mode of Operation:	semi-continuous
Throughput/Capacity:	3 t/d per m², table surface area
Operating Materials:
Type:	water	Hg see comments below
Quantity:	slurry density 20 % solids	>= 50 g/m²
ECONOMIC DATA:
Investment Costs:	when of imported construction with stand, high costs due to copper or muntz metal plates, approx. 10.000 DM; when locally produced with copper plates in masonry sluices built on the ground, less than 1000 DM
Operating Costs:	relatively low, almost exclusively costs of reagents
Related Costs:	subsequent hydraulic heavy-material trap for recovery of discharged amalgam and Hg; sun shade above plates to reduce mercury evaporation.

germ.:	Amalgamiertrommel
span.:	tromel de amalgamacion, chancho, amalgamadora, barril de amalgamacion
Manufacturer:	Svalcor

TECHNICAL DATA:
Dimensions:	Berdan pan: 1.0 × 0.6 × 0.6 m HOOD, inclination angle approx. 15°, 20 - 30 min^-1
Weight:	from approx. 50 kg up to several 1000 kg
Extent of Mechanization:	not mechanized/semi-mechanized
Form of Driving Energy:	electric motor or internal combustion engine, hydromechanical, manual, pedal drive
Mode of Operation:	intermittent
Throughput/Capacity:	depends on size
Operating Materials:
Type:	mercury	possibly steel balls	water	reagents to activate surface of Hg, e.g. NaOH, sodium amalgam, ammonium chloride, cyanide, nitric acid, tensides
Quantity:	approx. 1 - 3 kg/kg Au	or stone pebbles
ECONOMIC DATA:
Investment Costs:	dependent on extent locally produced and type of drive-system, starting at approx. 100 US$
Operating Costs:	cost of reagents and energy
Related Costs:	amalgamating press, distillation retort, pre-concentrating equipment

Operating Expenditures:	low \|————\|————\| high
	depends on power source
Maintenance Expenditures:	low \|————\|————\| high
Personnel Requirements:	training in handling of Hg is necessary
Location Requirements:	very little water needed
Grain Size of Feed:	20 · 50 ym to 2 mm for Au-fraction
Special Feed Requirements:	free, liberated gold without crustations (for example by fine Fe-oxides); possibly treatment with reagents, no platy floured gold, low content of antinomy minerals, possibly prior flotation, but then problematic surface conditions (hydrocarbons)
Recovery:	very high under good working conditions; in some cases > 95 %
Grade of Concentrate:	amalgam contains up to 50 % Au, but lower concentrations of gold in the amalgam are practical, with subsequent concentration in an amalgam press
Replaces other Equipment:	manual amalgamation in batea (pan), amalgamation in combination with other processes
Regional Distribution:	worldwide in gold mining
Operating Experience:	very good \|————\|————\| bad
	compared to amalgamation in sluices, Chilean mills, stamp mills, etc., Hg-emissions can be kept at very low levels; in particular the addition of reagents and reduction of slurry velocity prevent the occurrence of floured mercury.
Environmental Impact:	low \|————\|————\| very high
Suitability for Local Production:	very good \|————\|————\| bad
Under What Conditions:	metal manufacturing involving pipe sections, sheet metal, bearings, etc.
Lifespan:	very long \|————\|————\| very short

germ.:	Rocker, Wiege, Wiegensieb
span.:	cuna, criba cuna, chinchola, cuna californiana, cuna siberiana, lavador, concentrador
Manufacturer:	Keene

TECHNICAL DATA:
Dimensions:	up to several m in length
Weight:	20 - 50 kg
Extent of Mechanization:	not mechanized
Form of Driving Energy:	manual
Mode of Operation:	semi-continuous
Throughput/Capacity:	500 - 1000 kg/MS including refining with batea (panning)
Technical Efficiency:	approx. 6 - 10 t feed with 2 persons in 10 h when solely pre-concentrating
Operating Materials:
Type:	water
Quantity:	400 - 3000 1/10 h
ECONOMIC DATA:
Investment Costs:	starting at approx. 200 DM if locally produced
Operating Costs:	mainly labor costs

engl.:	gold saver, washplants
germ.:	Mechanisierte kompakte Goldaufbereitungen, z.B. Gold Saver, Prospektor
span.:	beneficios de oro mecanizados en forma compacta, por ejemplo gold saver, prospector
Manufacturers:	Denver, Goldfield, Svalcor, Buena Fortuna, Fima, Metal Callao E.P.S., Met. Lacha

TECHNICAL DATA:
Dimensions:	1 17x74x153 cm HWD
Weight:	270 kg
Extent of Mechanization:	fully mechanized
Driving Power:	3 PS
Form of Driving Energy:	internal combustion engine or electric motor
Alternative Forms:	hydromechanical ?
Mode of Operation:	seml-continuous
Throughput/Capacity:	2-3 m³/h
Operating Materials:
Type:	water	fuel	lubricants
Quantity:	20 - 351/min	approx. 2 - 5l/h	small quantities
ECONOMIC DATA:
Investment Costs:	approx. 4000 us$ if locally produced; approx. 25.000 DM fob factory

Operating Expenditures:	low \|————————\| high
	3 people: 1 to feed, 1 to remove tailings, 1 to operate
Maintenance Expenditures:	low \|————\|————\| high
Grain Size of Feed:	< 100 mm
Special Feed Requirements:	relatively coarse gold grains
Recovery:	can be substantially increased with subsequent sluice or amalgamating table
Replaces other Equipment:	sluices, Long Tom
Regional Distribution:	Colombia
Operating Experience:	very good \|————\|————\| bad
Environmental Impact:	low \|————\|————\| very high
	depending on type of feed material, high sludge loading can occur
Suitability for Local Production:	very good \|————\|————\| bad
	according to Degowski, a Gold Saver has already been reproduced locally in Pasto, Colombia
Under What Conditions:	good metal and welding workshops
Lifespan:	very long \|————\|————\| very short

germ.:	Hydraulische Schwergutfalle
span.:	trampas hidraulicas pare material pesado
Manufacturer:	Zutta Hermanos, ASEA-Perini

TECHNICAL DATA:
Dimensions:	up to 60 × 60 × 60 cm depending on throughput capacity
Weight:	from approx. 2 - 20 kg
Extent of Mechanization:	not mechanized
Form of Driving Energy:	only processing water, no drive system
Mode of Operation:	continuous
Throughput/Capacity:	several t/d
Operating Materials:
Type:	water for counter flow
Quantity:	10 - 50 I/min with hydrostatic pressure of 0.1 - 0.5 bar
ECONOMIC DATA:
Investment Costs:	very low, approx. 100 - 200 DM depending on size
Operating Costs:	very low

engl.:	still, distillation retort
germ.:	Destillierkolben
span.:	retorta de destilacion, retorta de amalgama, matraz de destilacion
Manufacturers:	Keene, Svalcor, Talleres J.G., Taller "Centro del Muchacho Trabajador", ASEA, Zutta

TECHNICAL DATA:
Dimensions:	approx. 10 × 10 × 60 - 80 cm, crucible approx. 5 cm in diameter, 5 cm deep
Weight:	approx. 10 kg
Extent of Mechanization:	not mechanized
Form of Driving Energy:	thermal from biological fuels
Mode of Operation:	intermittent
Throughput/Capacity:	depending on design, between 0.5 and 70 or more kg capacity, duration of distillation approx. 15 - 25 min
Operating Materials:
Type:	water Heat by burning wood, coal, gasoline, diesel, gas, etc
Quantity:	small quantities For cooling
ECONOMIC DATA:
Investment Costs:	Retort: 100 to 500 DM if locally produced, serial production should be targeted to lower the cost of production, however amortization possible through recovery of Hg.
Operating Costs:	relatively high due to heating; however in open-circuit distillation, costs are also incurred for fuel
Related Costs:	blow torch approx. 30 DM

engl.:	Knelson concentrator, Knudson bowl
germ.:	Zentrifugalscheider
span.:	concentrador centrifugo, Knelson concentrador, Knudson bowl
Manufacturers:	Knelson International Sales Inc., Mineral Deposits, Steve and Duke's Manufacturing Co. or Lee-Mar Industries Ltd., Goldfield, Falcon Concentraters, FUNDEMIN, VARDAX, Met Lacha, INCOMEC

TECHNICAL DATA:
	Knelson 7.5"	Knelson 12"	Knudson
Dimensions LWH	33" × 22" × 26"	31" × 31" × 34"	~ 860 × 760 × 585 mm
Weight	117 kg	154 kg	172 kg (without motor)
Driving Capacity	3/4 PS	1 PS	7.5 PS
Throughput	650 kg/in	5 m³/h	3 - 4 t/h
Quantity of Concentrate	1.5 kg	5 kg
Slurry	17 gal/min	30 gal/min
Backwater Quantity	20 gal/min	35 gal/min	-
Backwater Pressure	2 - 3 bar	aprox. 4 bar	-
Washing Water
Maximum Grain �	< 4 mm	< 4 mm	< 4 mm
Minimum Grain �	> 30 µm	> 30 µm	50 - 70 µm
RPM	~ 400	100 - 400	100 - 105
Centrifugal acceleration	60 g
Separation Cut-off
Mode of Operation	semicont.	semicont.	discont.
	Falcon B 12	Falcon B 6	Vardax 801
Dimensions LWH	36" × 60" × 73"	19" × 20" × 32"	72" × 18" × 24"
Weight	800 kg	100 kg	110 kg with classifier
Driving Capacity	7.5 PS	1.0 PS	2 PS
Throughput	~ 6 t/h	0.5 t/h	- 2 t/h
Quantity of Concentrate	- 4.5 kg	~ 1 kg	~ 80 kg
Slurry		7 - 12 gal/min
Backwater Quantity	-	-
Backwater Pressure	-	-
Washing Water		5 gal/wash	2 gal/wash
Maximum Grain �	< 1.5 mm	< 0.9 mm	< 6 mm
Minimum Grain �	> 30 µm	> 30 µm
RPM

Centrifugal acceleration	300 g	300 g
Separation Cut-off	4 g/cm³
Mode of Operation	discont.	discont.	discont.
Extent of Mechanization:	fully mechanized
Form of Driving Energy:	electrical, optional internal combustion engine, possibly convertable to hydromechanical drive with difficulty
Technical efficiency:	concentration up to more than 1: 8000; comparatively very high recovery with well-liberated free gold

ECONOMIC DATA:
Investment Costs:	Prices from original manufacturer fob factory:
	Knelson 7.5":	6850 US$	Knelson 12":	12500 US$
	Knudson:	4500 US$	Falcon B 12:	34000 US$
	Falcon B 6:	7000 US$	Vardax 801:	2400 US$
	Vardax Sec.:	14500 US$
Operating Costs:	cost of energy and minimal labor costs
Related Costs:	possibly costs for thickener, sedimentation basin or sludge pond

germ.:	Sichertrog, Waschpfanne, Schiffchen, Niersch, Saxe
span.:	batea, chua, challa, prune, zuruca (v.), sarten lavador, batea en forma de bote
Southeast Asia:	dulong, dulang
Manufacturers:	Krantz, Keene

TECHNICAL DATA:
Dimensions:	20 50 cm 0, 5 - 25 cm depth / 15 × 15 × 150 cm HWD / ca 45 - 50 cm diameter, 10 cm depth, 35°- 40° inclination (USA)
Weight:	0,5 - 5 kg
Extent of Mechanization:	not mechanized
Form of Driving Energy:	manual
Mode of Operation:	intermittent
Throughput/Capacity:	cat 1 - 5 kg/mini daily performance 100 pans at 20 lbs = 1 t/d
Operating Materials:
Type:	water
Quantity:	small (can be used in non-flowing water)
ECONOMIC DATA:
Investment Costs:	approx. 10 to 20 DM
Operating Costs:	labor costs only
Related Costs:	none

Advantages:	- cracks do not develop	Disadvantage:	- the surface repells water
	- good gold visibility
	- durable and long-lasting
	- light weight

small nuggets	10 - 20 mesh:	2200/oz.
big flakes	20 - 40 mesh:	12000/oz. gold
fine flakes	< 40 mesh:	40000/oz.

germ.:	Setzmaschinen mit Setzbett
span.:	jig con came de boles de promo
Manufacturers:	Mineral Deposits, Denver, IHC Sliedrecht, Goldfield

TECHNICAL DATA:
Dimensions:	from 1 × 0,75 × 1 m up to 3,60 × 3 × 3 m LWH with 0,5 × 0,2 m (2 jig beds) up to several m jig bed sizes
Weight:	from 50 kg
Extent of Mechanization:	fully mechanized
Throughput/Capacity:	4 - 65 t/h
Form of Driving Energy:	electric
Power/Performance:	0,5 to several kW, 50 - 300 thrusts/minute, approx. 25 mm lift
Alternative Forms:	with internal combustion engine, possibly even manual operation for small machines
Operating Materials:
Type:	lead balls grease
Quantity:	8 - 10 DM/kg
ECONOMIC DATA:
Investment Costs:	from approx. 3000 DM when locally produced
Operating Costs:	mainly costs of energy and labor

engl:	cyanide leaching, (agitation leaching, vat leaching, heap leaching)
germ.:	Goldlaugung, cyanidische Laugung (Ruhrlaugung, Behalterlaugung, Haufenlaugung)
span.:	lixivacion de oro, lixiviacion, lixiviacion con cianuro (lixiviacion por agitacion, lixiviacion en tanques, lixiviacion en piles)
Manufacturers:	HBS-Equipment (cell for electrolytic + absorptive separation), Denver

TECHNICAL DATA:
Dimensions:	leaching tanks several m³ in volume
Weight:	brick masonry basins
Extent of Mechanization:	fully mechanized
Mode of Operation:	semi-continuous, continuous
Power:	varies according to type of leaching procedure chosen, from 100 W for small percolation leaching plants (pump drive) up to several kW for larger agitation-leaching plants
Form of Driving Energy:	electric drive for pumps and filter
Technical Efficiency:	three various categories of efficiency are differentiated: leaching efficiency, adsorption efficiency (= f (activated carbon quality, etc.)), stripping efficiency (degree of stripping does not influence processing results if activated carbon is reused in the circuit because gold is not lost in the process) the sum of the different efficiencies leads to recovery values which lie at approx. 90 - 95 % for agitation leaching, approx. 80 - 90 % for vat leaching and ca 50 - 80 % for heap leaching.
Operating Materials:
Type:	sodium cyanide (NaCN) or calcium cyanide/black cyanide (Ca(CN)₂)
Quantity:	Concentration: 1 - 5 kg/t (ave. 1,5), Consumption: up to 8 kg/t, compressed air, CaO, Zn and PbNO₃ or activated carbon
ECONOMIC DATA:
Investment Costs:	for small percolation-leaching plants only minimal costs for the masonry construction of leaching basins, precipitating basins and leach collection containers, totalling approx. 1000 DM depending upon cost of materials. For industrial-scale plants, (e.g. CIP - carbon-in pulp process) costs are extremely high at 1.000.000 DM minimum.
Operating Costs:	high costs due to consumption of reagents, approx. 50 % of operating costs are for cyanide; also energy-intensive technique, especially the Merrill-Crowe-Procedure
Related Costs:	costs for sludge ponds, especially in leaching facilities where finely-ground ores are processed

	heap leaching
Operating Expenditures:	low \|————\|————\| high
	CIP
Maintenance Expenditures:	low—\|high
	depends on type of leaching
Personnel Requirements:	of extreme importance is a precise control of the process, especially with regard to the homogeneity of concentrations, feeds and slurry characteristics
Location Requirements:	very high space requirements for heap leaching
Grain Size of Feed:	< 0.1 mm for agitation leaching; <10 mm for vat leaching of 2 - 4 day duration; < 50 mm for heap leaching of 3 - 6 week duration
Special Feed Requirements:	minerals of arsenic, antimony, manganese and especially soluble oxidic copper, which are extremely deleterious and cause high cyanide consumption, high gold losses in organic CH-bonding, graphite, etc., require the use of CIL (carbon-in-leach process). Pyrrhotine (magnetic pyrite) is also a detrimental mineral: it binds cyanide ions and consumes oxygen during decomposition.
Replaces other Equipment:	all other methods of cleaning pre-concentrate in gold beneficiation, e.g. manual sorting, amalgamation, gravimetric processes, smelting
Regional Distribution:	worldwide more than 70 % of all gold is won by leaching. Cyanide leaching, however, is cost intensive, difficult to control and involves complicated technical equipment, restricting its application to large-scale mining operations.
Operating Experience:	very good \|————\|————\| bad
Environmental Impact:	low \|————\|————\| very high
	The dangers associated with leaching operation lie in the possibility of cyanide being released as a result of leaks, haphazard handling, etc. Cyanide is highly toxic; only trained personnel should operate leaching operations to ensure safe use; large space requirements heap leaching
	heap leaching
Suitability for Local Production:	very good \|————\|————\| bad
	CIP
Under What Conditions:	for small percolation leaching plants, the simple brickwork provides very good possibilities for local construction
Lifespan:	very long \|————\|————\| very short

germ.:	Schmeiztrennung von Gold
span.:	separacion del oro por fundicion