Gold cyanidation

Gold cyanidation (also known as the cyanide process or the MacArthur–Forrest process) is a hydrometallurgical technique for extracting gold from low-grade ore through conversion to a water-soluble coordination complex. It is the most commonly used leaching process for gold extraction. Cyanidation is also widely used in silver extraction, usually after froth flotation. Production of reagents for mineral processing to recover gold represents 70% of cyanide consumption globally. While other metals, such as copper, zinc, and silver, are also recovered using cyanide, gold remains the primary driver of this technology. The highly toxic nature of cyanide has led to controversy regarding its use in gold mining, with it being banned in some parts of the world. However, when used with appropriate safety measures, cyanide can be safely employed in gold extraction processes. One critical factor in its safe use is maintaining an alkaline pH level above 10.5, which is typically controlled using lime in industrial-scale operations. Lime plays an essential role in gold processing, ensuring that the pH remains at the correct level to mitigate risks associated with cyanide use.

History In 1783, Carl Wilhelm Scheele discovered that gold dissolved in aqueous solutions of cyanide. Through the work of Bagration (1844), Elsner (1846), and Faraday (1847), it was determined that each gold atom required two cyanide ions, i.e. the stoichiometry of the soluble compound.

Industrial process

The expansion of gold mining in the Rand of South Africa began to slow down in the 1880s, as the new deposits found tended to contain pyritic ore. The gold could not be extracted from this compound with any of the then available chemical processes or technologies. In 1887, John Stewart MacArthur, working in collaboration with brothers Robert and William Forrest for the Tennant Company in Glasgow, Scotland, developed the MacArthur–Forrest process for the extraction of gold from gold ores. Several patents were issued in the same year. By suspending the crushed ore in a cyanide solution, a separation of up to 96 percent pure gold was achieved. The process was first used on the Rand in 1890 and, despite operational imperfections, led to a boom of investment as larger gold mines were opened up. By 1891, Nebraska pharmacist Gilbert S. Peyton had refined the process at his Mercur Mine in Utah, "the first mining plant in the United States to make a commercial success of the cyanide process on gold ores." In 1896, Bodländer confirmed that oxygen was necessary for the process, something that had been doubted by MacArthur, and discovered that hydrogen peroxide was formed as an intermediate. Around 1900, the American metallurgist Charles Washington Merrill (1869–1956) and his engineer Thomas Bennett Crowe improved the treatment of the cyanide leachate, by using vacuum and zinc dust. Their process is the Merrill–Crowe process.

Chemical reactions

The chemical reaction for the dissolution of gold, the "Elsner equation", follows:

4 Au + 8 NaCN + O2 + 2 H2O → 4 Na[Au(CN)2] + 4 NaOH Potassium cyanide and calcium cyanide are sometimes used in place of sodium cyanide. Gold is one of the few metals that dissolves in the presence of cyanide ions and oxygen. The soluble gold species is dicyanoaurate. from which it can be recovered by adsorption onto activated carbon.

Application The ore is comminuted using grinding machinery. Depending on the ore, it is sometimes further concentrated by froth flotation or by centrifugal (gravity) concentration. Water is added to produce a slurry or pulp. The basic ore slurry can be combined with a solution of sodium cyanide or potassium cyanide; many operations use calcium cyanide, which is more cost effective. To prevent the creation of toxic hydrogen cyanide during processing, slaked lime (calcium hydroxide) or soda (sodium hydroxide) is added to the extracting solution to ensure that the acidity during cyanidation is maintained over pH 10.5 - strongly basic. Lead nitrate can improve gold leaching speed and quantity recovered, particularly in processing partially oxidized ores.

Effect of dissolved oxygen Oxygen is one of the reagents consumed during cyanidation, accepting the electrons from the gold, and a deficiency in dissolved oxygen slows leaching rate. Air or pure oxygen gas can be purged through the pulp to maximize the dissolved oxygen concentration. Intimate oxygen-pulp contactors are used to increase the partial pressure of the oxygen in contact with the solution, thus raising the dissolved oxygen concentration much higher than the saturation level at atmospheric pressure. Oxygen can also be added by dosing the pulp with hydrogen peroxide solution.

Pre-aeration and ore washing In some ores, particularly those that are partially sulfidized, aeration (prior to the introduction of cyanide) of the ore in water at high pH can render elements such as iron and sulfur less reactive to cyanide, therefore making the gold cyanidation process more efficient. Specifically, the oxidation of iron to iron (III) oxide and subsequent precipitation as iron hydroxide minimizes loss of cyanide from the formation of ferrous cyanide complexes. The oxidation of sulfur compounds to sulfate ions avoids the consumption of cyanide to thiocyanate (SCN−) byproduct.

Recovery of gold from cyanide solutions In order of decreasing economic efficiency, the common processes for recovery of the solubilized gold from solution are (certain processes may be precluded from use by technical factors):

Carbon in pulp Electrowinning Merrill–Crowe process

Cyanide remediation processes The cyanide remaining in tails streams from gold plants is potentially hazardous. Therefore, some operations process the cyanide-containing waste streams in a detoxification step. This step lowers the concentrations of these cyanide compounds. The INCO-licensed process and the Caro's acid process oxidise the cyanide to cyanate, which is not as toxic as the cyanide ion, and which can then react to form carbonates and ammonia:

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