The secret world of gold processing

Various methods have been employed to process gold, becoming more labour- and technology intensive, but also more successful in recovering, writes independent consultant Dr Nicolaas C. Steenkamp, Bowline Professional Services MD Breton Scott, and Leon Louw, editor, writer and specialist in African affairs and mining.   

While there has been a lot of talk in the mining industry about the mechanisation and modernisation of mines, most of these discussions focus on the methods to extract the rock material in which the ore bodies are hosted, rather than on the processing methods after it has been unearthed. Yet, the development of new and innovative ways to liberate gold from the rock material is important and represents the modernisation of an entire mining process. Not only do these processing methods need to be more efficient and effective, but it is imperative that they are also environmentally friendly. Effective processing of the ore plays a key role in the viability of a project or, at the end of the day, the profit margins.

Types of gold ore

There are two types of gold mineralisation: free gold and refractory gold. Native gold or free gold is exactly what the name implies: it is not bound in any other mineral that needs to be removed and can be concentrated by a combination of gravity methods, such as leaching (for example cyanidation) and/or direct smelting. This type of mineralisation is usually associated with heavy mineral placer deposits, such as alluvial gold deposits in streams and rivers or quartz veins. Therefore, most gold deposits are found through gold panning initially before locating the hard rock source.

Refractory gold, on the other hand, is bound with other minerals, usually sulphides and associated chalcophile elements. This type of gold requires more complex metallurgical recovery processes to liberate it. These ores are naturally resistant to recovery by standard cyanidation and carbon adsorption processes. These refractory ores usually require pre-treatment of some sort for cyanidation or other leaching methods to be effective.

Over time, various gold recovery methodologies have been developed. In ancient times, technology only allowed for visible free gold to be recovered by gravity concentration methods and then smelting. In the more recent past, a better understanding of the mineral and metallurgical properties of the gold-bearing ores has encouraged more sophisticated methods of recovery. Owing to the nature of technological advancement today, the greater demand for cost efficiencies, and higher recovery performance, more effective and creative technology advancements are being developed.

Historical processing methodologies

Historically, gravity concentration has been the most important way of extracting the native metal using pans or washing tables. Gravity concentration separates minerals based on differences in specific gravity. Various concentrator equipment has evolved over time to simulate and automate the panning methods used by the ‘old timers’, but now the recovery of even finer particles of free gold is possible. Once a gravity concentrate is produced, this can undergo direct smelting to produce gold metal but, as would be expected, the gold is of a lower purity.

Similarly, gold amalgamation processes were used. Amalgamation is a concentrating process in which metallic gold is mixed with mercury, either in an amalgamation drum or table. Here the precious metal bonds with the mercury to form the metal-laden mercury amalgam and the waste (barren) ore pulp is caused to travel different paths to effect separation. Refractory or sulphide precious metal ores are difficult to amalgamate using mercury, due to the complex iron-sulphur-gold and other metals present, which do not allow the gold to come into contact with the mercury. The mercury vapour generated is highly toxic and, therefore, special care has been taken, over time, to develop methods of retorting the mercury. Retorting means distilling the mercury from the amalgam and it is done in a cast iron retort or steel retort. A retort is a vessel having a cover that can be fastened on so tightly that no fumes of mercury escape, except through the condenser, which leads from the cover to a vessel containing water, where the fumes of mercury are condensed to a metallic state. These methods are still very common among artisanal gold miners across the world.

Modern metallurgical processes

With the advancement of technology after the Industrial Revolution, the means to evaluate mineral properties along with their metallurgical behaviour became available. Experimentation with various chemicals such as acids were conducted to find ways to leach out the gold metal and then recover it from a liquid solution. Highly corrosive acids such as hydrochloric and hydrofluoric acids were used with fair success. However, they are extremely dangerous to work with and large concentrations of the acid remain in the waste material (tailings) produced, which is difficult to treat and discard in an environmentally safe manner.

In 1887, John Stewart MacArthur developed the cyanide process for gold extraction. During the cyanidation process, a slurry of ground ore is mixed with a sodium cyanide solution in the presence of activated carbon. The carbon has a very high affinity for the aurocyanide complex and adsorbs the gold out of the solution, resulting in very high gold loadings on the carbon. At the end of the leach, the loaded carbon is removed from the slurry and the adsorbed gold is stripped out at high temperature and pressure with sodium hydroxide and cyanide solutions to form a high-value electrolyte solution. Gold bullion is then recovered from the electrolyte by electro-winning. There are many variations, such as carbon-in-leach (CIL), carbon-in-pulp (CIP), resin-in-leach, and heap leach, but the basic cyanidation principles apply.

Ore that has limited sulphide compositions or that has oxidised over time, leaches extremely well. Refractory ores, because of their sulphidic composition, do not leach well. Therefore, pre-treatment or pre-oxidation needs to take place. In the 20th century, techniques such as roasting, pressure oxidation, atmospheric leaching (Albion), and even biological oxidation (BIOX) have been developed. Roasting is used to oxidise both the sulphur and organic carbon at high temperatures using air and/or oxygen. BIOX involves the use of bacteria that promote oxidation reactions in an aqueous environment. Pressure oxidation is an aqueous process for sulphur removal, carried out in a continuous autoclave, operating at high pressures and somewhat elevated temperatures. The Albion process uses a combination of ultrafine grinding and atmospheric, auto-thermal, oxidative leaching techniques.

The importance of pre-oxidation

According to Juan D. van der Merwe, business development manager at Maelgwyn Mineral Services Africa, most contemporary gold mining projects have shown themselves to be more difficult to process than their historical counterparts. Projects have to deal with lock-up aspects such as gold that is not easily accessible to the reactants, the presence of significant portions of fast-reacting sulphide minerals such as pyrrhotite, and gold outright contained in pyrite or arsenopyrite. Alternatively, as is the case in many tailings reclaim operations, the presence of large amounts of unstable oxidation products, such as ferrous iron or meta-stable sulphide oxidation products, places a heavy demand on oxygen and cyanide consumption.

“To sustain gold leach efficiencies, these inhibiting aspects have directed focus to suitable pre-oxidation steps, followed by CIL, and it is in that phase of the process where our technology becomes applicable,” says Van der Merwe. Maelgwyn Mineral Services Africa is a wholly owned subsidiary of Maelgwyn Minerals Services based in Wales.

One of Maelgwyn’s prime products is the Aachen High Shear Reactor Technology, which is the enabling technology to achieve efficient high-level oxygen mass transfer along with shear exposure. Slurry is pumped from the process tank through the reactor, where the oxygen is introduced at high shear/high velocity in a specially engineered aerator system. After the oxygen has been transferred under pressure, shear and high velocity, the slurry is returned to the same process tank. If the flow through the reactor is a multiple of the slurry flow rate at plant level, a system of multiple pass rates is established, usually leading to increased benefits. “The purpose of deploying the Aachen technology is to assist with realising hidden potentials in reagent savings, gold recovery, or throughput increases,” says Van der Merwe.

Although cyanide is a highly toxic substance, it has maintained the position of preferred lixiviant of auriferous oxide-silicate-carbonate ores. It does however require the application of adequate water management and cyanide management plants, particularly descriptions of how cyanide-containing solutions and slurries are to be handled, stored, contained, and monitored, and in many cases, a plan will also include a description of treatment plants used to remove cyanide from solutions or slurries.   

Environment friendly cyanide  

According to a recent article written by Martin Sadongdong in the Philippine newspaper Manila Bulletin News, a Singapore-based firm introduced a new environment-friendly way of mining through a powder solution in lieu of toxic cyanide to extract gold and other precious metals.

Zack Ho Xuan Yi, managing director of a company called Perfect 9, said in an interview with Sadongdong that he has developed a new powder solution, called P9 Au-568, that can be used to replace cyanide. From tests, Yi said that they realised that the powder worked, along with the benefits of it being faster, a higher extraction rate, and being non-toxic to people handling it.

Aside from being environmentally friendly as it incorporates a non-toxic process, Yi claims that the powder solution can increase the gold extraction rate up to 90% and is four to eight times faster than the current extraction method being used. “It literally takes seconds to extract gold from an electronic chip,” said Yi. 

Future developments

Research continues on the processing of gold, with the focus on becoming more cost-effective and environmentally friendly. Thiosulfate and alpha-cyclodextrin, a substance commonly extracted from corn starch, respectively, are researched to process and recover gold. Thiosulfate and bromide are used to leach the gold, in a similar way to cyanide, whilst the alpha-cyclodextrin is used in a similar fashion to carbon to recover the gold.

Work is also being done using the amino acid glycine to extract gold. Glycine is manufactured from by-products of the natural gas industry. It forms a stable soluble complex with gold, which is soluble in water. Technical-grade glycine is cheaper than cyanide and available in roughly the same quantities. It is used in a similar manner to existing gold refining processes such as CIL and CIP, while it also eliminates toxic waste disposal problems. The opportunity exists to recover and reuse the glycine, lowering the net costs. Amino acids need to be heated to 40–50°C to dissolve gold, thus reducing the energy requirements for such a process. In sufficiently warm underground environments, the process could be applicable to solution mining, where solvents are injected into an ore body and recovered along with the gold from a central borehole. Future applications are aimed at in situ heap leach extraction projects — a process that is expected to make mining low-grade deposits in remote locations more viable. Being an alkaline compound, it is well suited to extracting gold in alkaline ores such as dolomite, unlike sulphuric acid. Another upshot of this method is that the ores treated in this way require no milling, reducing energy consumption and associated costs.

As civilisation progresses through the 21st century and global population numbers grow exponentially, the demand for water, energy, food, land, and other natural resources significantly increases. Therefore, no matter what gold recovery process is eventually adopted for a mining business, all stakeholders and the industry need to be sufficiently responsible to protect the environment, reduce energy consumption, manage costs responsibly, and forever find ways to recover, re-use, and recycle waste material produced.