Gold ore, cyanidation

A proposed method which avoids cyanide consists of treating gold ore with gaseous chlorine at elevated (<250° C) temperatures to volatilise gold as chloride Au2Clg [12446-79-6] or AuMCl, (M = Fe [12523-43-2] A1 [73334-09-5], or Ga [73334-08-4]) and recovering it by condensation (23). 

Gold ores can be concentrated by froth flotation, the resulting concentrate being roasted at 600-800°C to oxidize off sulphur and arsenic as their oxides. The product is extracted with cyanide under oxidizing conditions (using either peroxide or air itself) before displacement with powdered zinc. More reactive metals (silver etc.) can be removed by chlorination of molten gold. 

This hematite is not soluble in the cyanide solution. The oxidative pretreatment of gold ores thus reduces the cyanide consumption. Some impurity elements inhibit leaching reactions, examples include elements, carbon, sulfur and arsenic in gold ores are such impurities, but these can be removed by heating in air. 

The precipitation of gold occurred on the surface of the reductant charcoal. The charcoal was subsequently burnt and the gold recovered. This process was used successfully for some time until it was withdrawn in favor of using cyanide solution because of the ineffectiveness of chlorine water to dissolve silver, which more often than not co-occurs with gold ores. 

Gold ores grouped as refractory are those in which the gold does not lend itself easily to dissolution in cyanide solutions. There are at least three different types of refractory ores that are encountered ... 

Gonen, N. Leaching of finely disseminated gold ore with cyanide and thiourea solutions. Hydrometallurgy 2003,69,169-176. 

Prior to gold extraction by cyanidation, refractory gold ores are either roasted or pressure oxidized to liberate the gold contained as submicroscopic particles or in solid solution in arsenopyrite and arsenic-rich pyrite. Gold extraction from such ores require roasting or pressure oxidation or bacterial oxidation prior to cyanidation to destroy the sulfide structure. 

Hydrometallurgical methods are normally employed for recovery of gold from oxidized deposits (heap leach), low-grade sulphide ores (cyanidation, CIP, CIL) and refractory gold ores (autoclave, biological decomposition followed by cyanidation). 

The U.S. Bureau of Mines-Reno Research Center is conducting studies on the use of thiosulfate as an alternative to cyanide for refractory gold ores as a means of pollution prevention and on the fate of cyanide in solution after land application. 

In 1803 WiUiam Hyde Wollaston (1776—1828), an Enghsh chemist who also discovered rhodium, isolated palladium at the time he analyzed the platinum and gold ores sent to him from Brazil. Dissolving the platinum in aqua reg)a acid, Wollaston then treated the residue with mercuric cyanide to produce the compound of palladious cyanide that was reduced by burning it to extract metallic palladium. 

Aquatic Contamination by Gold Ore Extractants. The separate use of mercury and cyanide has led to the contamination of freshwater systems. 

Gold ores can also be treated with potassium cyanide (KCN) or some other kind of cyanide. The gold combines with the cyanide to form a new compound, gold cyanate. The gold cyanate is then treated with an active metal, such as zinc. The active metal replaces gold in the compound, leaving pure gold. 

Metallurgical Practice.— Leaching of sands by percolation was very widely used in the United States up to a few years ago in the cyanide treatment of gold ores, and is still largely used in the Transvaal. The reason for its decline was the successful development of processes for the treatment of slimes at lower costs which resulted in the plants treating all their material as slime instead of separate treatments as before for sands and slimes. The question of removing the leached material from the tanks is handled in a number of different ways. 

Roasted gold ore, magnetite, hematite, maghemite, cyanide, passivation, galvanic corrosion, XPS, SEM-EDS... 

Figure 4 - The influence of agitation speed on the galvanic potential (a) and current (b) between gold (Au) and roasted gold ore (RGO) electrodes, pH 10.5,25 °C, cyanide cone. 0.01 M, 25 °C, saturated atmospheric oxygen, Au electrode surface area 0.25 cm2, RGO area 4.9 cm2...

Figure 6 - Galvanic couple potential and ZRA vs. time at 100 rpm agitation, pH 10.5, 0.01 M cyanide cone., saturated atmospheric oxygen, 25 °C, (a) RGO (roasted gold ore electrode 4.9 cm2) and Mag (magnetite disc electrode 4.9 cm2) (b) RGO (roasted gold ore electrode 4.9 cm2) and Hem (hematite disc electrode 4.9 cm2)...

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