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Wednesday, 16 March 2011 21:06

Gold Smelting and Refining

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Adapted from the 3rd edition, Encyclopaedia of Occupational Health and Safety.

Gold mining is carried out on a small scale by individual prospectors (e.g., in China and Brazil) and on a large scale in underground mines (e.g., in South Africa) and in open pit mining (e.g., in the United States).

The simplest method of gold mining is panning, which involves filling a circular dish with gold-bearing sand or gravel, holding it under a stream of water and swirling it. The lighter sand and gravel are gradually washed off, leaving the gold particles near the centre of the pan. More advanced hydraulic gold mining consists of directing a powerful stream of water against the gold-bearing gravel or sand. This crumbles the material and washes it away through special sluices in which the gold settles, while the lighter gravel is floated off. For river mining, elevator dredges are used, consisting of flat-bottomed boats which use a chain of small buckets to scoop up material from the river bottom and empty it into a screening container (trommel). The material is rotated in the trommel as water is directed on it. The gold-bearing sand sinks through perforations in the trommel and drops onto shaking tables for further concentration.

There are two main methods for the extraction of gold from ore. These are the processes of amalgamation and cyanidation. The process of amalgamation is based on the ability of gold to alloy with metallic mercury to form amalgams of varying consistencies, from solid to liquid. The gold can be fairly easily removed from the amalgam by distilling off the mercury. In internal amalgamation, the gold is separated inside the crushing apparatus at the same time as the ore is crushed. The amalgam removed from the apparatus is washed free of any admixtures by water in special bowls. Then the remaining mercury is pressed out of the amalgam. In external amalgamation, the gold is separated outside the crushing apparatus, in amalgamators or sluices (an inclined table covered with copper sheets). Before the amalgam is removed, fresh mercury is added. The purified and washed amalgam is then pressed. In both processes the mercury is removed from the amalgam by distillation. The amalgamation process is rare today, except in small scale mining, because of environmental concerns.

Extraction of gold by means of cyanidation is based on the ability of gold to form a stable water-soluble double salt KAu(CN)2 when combined with potassium cyanide in association with oxygen. The pulp resulting from the crushing of gold ore consists of larger crystalline particles, known as sands, and smaller amorphous particles, known as silt. The sand, being heavier, is deposited at the bottom of the apparatus and allows solutions (including silt) to pass through. The gold extraction process consists of feeding finely ground ore into a leaching tub and filtering a solution of potassium or sodium cyanide through it. The silt is separated from the gold cyanide solutions by adding thickeners and by vacuum filtration. Heap leaching, in which the cyanide solution is poured over a levelled heap of coarsely crushed ore, is becoming more popular, especially with low grade ores and mine tailings. In both instances, the gold is recovered from the gold cyanide solution by adding aluminium or zinc dust. In a separate operation, concentrated acid is added in a digest reactor to dissolve the zinc or aluminium, leaving behind the solid gold.

Under the influence of carbonic acid, water and air, as well as the acids present in the ore, the cyanide solutions decompose and give off hydrogen cyanide gas. In order to prevent this, alkali is added (lime or caustic soda). Hydrogen cyanide is also produced when the acid is added to dissolve the aluminium or zinc.

Another cyanidation technique involves the use of activated charcoal to remove the gold. Thickeners are added to the gold cyanide solution before slurrying with activated charcoal in order to keep the charcoal in suspension. The gold-containing charcoal is removed by screening, and the gold extracted using concentrated alkaline cyanide in alcoholic solution. The gold is then recovered by electrolysis. The charcoal can be reactivated by roasting, and the cyanide can be recovered and reused.

Both amalgamation and cyanidation produce metal that contains a considerable quantity of impurities, the pure gold content rarely exceeding 900 per mil fineness, unless it is further electrolytically refined in order to produce a degree of fineness of up to 999.8 per mil and more.

Gold is also recovered as a by-product from the smelting of copper, lead and other metals (see the article “Copper, lead and zinc smelting and refining” in this chapter).

Hazards and Their Prevention

Gold ore occurring in great depths is extracted by underground mining. This necessitates measures to prevent the formation and spread of dust in mine workings. The separation of gold from arsenical ores gives rise to arsenic exposure of mine workers and to pollution of air and soil with arsenic-containing dust.

In the mercury extraction of gold, workers may be exposed to high airborne mercury concentrations when mercury is placed in or removed from the sluices, when the amalgam is purified or pressed and when the mercury is distilled off; mercury poisoning has been reported amongst amalgamation and distilling workers. The risk of mercury exposure in amalgamation has become a serious problem in several countries in the Far East and South America.

In amalgamation processes the mercury must be placed on the sluices and the amalgam removed in such a manner as to ensure that the mercury does not come in contact with the skin of the hands (by using shovels with long handles, protective clothing impervious to mercury and so on). The processing of the amalgam and the removal or pressing of mercury must also be as fully mechanized as possible, with no possibility of the hands being touched by mercury; the processing of amalgam and the distilling off of mercury must be carried out in separate isolated premises in which the walls, ceilings, floors, apparatus and work surfaces are covered with material which will not absorb mercury or its vapours; all surfaces must be regularly cleaned so as to remove all mercury deposits. All premises intended for operations involving the use of mercury must be equipped with general and local exhaust ventilation. These ventilation systems must be particularly efficient in premises where mercury is distilled off. Stocks of mercury must be kept in hermetically sealed metal containers under a special exhaust hood; workers must be provided with the PPE necessary for work with mercury; and the air must be monitored systematically in premises used for amalgamation and distilling. There should also be medical monitoring.

Contamination of the air by hydrogen cyanide in cyanidation plants is dependent on air temperature, ventilation, the volume of material being processed, the concentration of the cyanide solutions in use, the quality of the reagents and the number of open installations. Medical examination of workers in gold-extracting factories has revealed symptoms of chronic hydrogen cyanide poisoning, in addition to a high frequency of allergic dermatitis, eczema and pyoderma (an acute inflammatory skin disease with pus formation).

Proper organization of the preparation of cyanide solutions is particularly important. If the opening of drums containing cyanide salts and the feeding of these salts into dissolving tubs is not mechanized, there can be substantial contamination by cyanide dust and hydrogen cyanide gas. Cyanide solutions should be fed in through closed systems by automatic proportioning pumps. In gold cyanidation plants, the correct degree of alkalinity must be maintained in all cyanidation apparatus; in addition, cyanidation apparatus must be hermetically sealed and equipped with LEV backed up by adequate general ventilation and leak monitoring. All cyanidation apparatus and the walls, floors, open areas and stairs of the premises must be covered with non-porous materials and regularly cleaned with weak alkaline solutions.

The use of acids to break down zinc in the processing of gold slime may give off hydrogen cyanide and arsine. These operations must therefore be performed in specially equipped and separated premises, with the use of local exhaust hoods.

Smoking should be prohibited and workers should be provided with separate facilities for eating and drinking. First-aid equipment should be available and should contain material for immediately removing any cyanide solution that comes in contact with workers’ bodies and antidotes for cyanide poisoning. Workers must be supplied with personal protective clothing impervious to cyanide compounds.

Environmental Effects

There is evidence of exposure to metallic mercury vapour and methylation of mercury in nature, particularly where the gold is processed. In one study of water, settlements and fish from gold mining areas of Brazil, the mercury concentrations in edible parts of locally consumed fish surpassed by almost 6 times the Brazilian advisory level for human consumption (Palheta and Taylor 1995). In a contaminated area of Venezuela, gold prospectors have been using mercury to separate gold from auriferous sand and rock powders for many years. The high level of mercury in the surface soil and rubber sediments of the contaminated area constitutes a serious occupational and public health risk.

Cyanide contamination of wastewater is also a great concern. Cyanide solutions should be treated before being released or should be recovered and reused. Emissions of hydrogen cyanide gas, for example, in the digest reactor, are treated with a scrubber before being exhausted out the stack.



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