All that glitters may not be gold, but recovery technologies for this sought-after metal present a sparkling diamond for oxygen, as Tony Wheatly finds out.
Gold may be the most noble of metals chemically speaking and its long-term price trend reflects continued demand, but the terrible cost of winning this coveted metal from ‘mother earth’, both in terms of human injury or death and environmental devastation, are mounting.
Gold is not only different from most metals because of its chemical and physical properties but the market supply and demand also exhibits unique properties. Despite continual research and periodic announcements to the contrary, gold is not an industrially valuable commodity.
Gold demand is very much harder to evaluate than production, because while production is concentrated in a relatively small number of mines, demand is distributed throughout the world and some of the difficulties are:
Many buyers of gold are deliberately secretive.
In particular the recycling of scrap does not lend itself to measurement because recycling can utilise scrap supply and meet a demand without going anywhere near a statistician.
Unallocated gold is difficult to measure because it is often notional.
The demand for gold for jewellery manufacture remains the largest market segment and has been stable at about 80% for many years with India, the US and China consuming at least 50%, while industrial demand has seldom exceeded 10% of global usage.
Retail investment demand for gold, to be held as a private reserve asset, displays the greatest fluctuation between 4% and 11% in response to downward movements in the market price.
Annual gold-mining output peaked at 2,573 metric tons in 2000, falling to 2,518 metric tons in 2005, then 2469 metric tons in 2006, and finally 2,444 metric tons in 2007 according to the GFMS consultancy – a London-based research group that serves gold mining companies and supplies the World Gold Council with its data.
A report issued by the South African Chamber of Mines states that traditional gold producing countries such as South Africa, Canada, Australia, the US, and Brazil are experiencing declines in production, while emerging producers such as China, Peru and Indonesia are becoming more dominant – with China replacing South Africa as the top producer.
The balance between the fabrication demand for gold and new mine supply has remained in deficit since 1988 and this has meant that about 835 Mt/y of above-surface gold holdings has been added to the supply equation to satisfy demand.
The report states that the continued reduction in the mining sector’s propensity to hedge forward, when combined with increased research into possible industrial uses for gold and improved marketing of gold jewellery, also supports positive market dynamics.
According to Gold World Staff, “Quarterly world gold production numbers have been at their lowest levels in years during 2008. Estimates suggest that the world will only produce 76.8 million troy ounces during 2008. This represents a 9% decline in world gold production since 2001 and provides another solid positive fundamental on which the gold bull market can stand.”
Gold recovery technologies
The most commonly used process for extracting gold from complex, low grade ore (0.6 – 1.5 g/Mt), is a metallurgical technique commonly known as the cyanide process that converts gold into water soluble aurocyanide metallic complex ions.
The process was commercialised through the pioneering work of Doctors Robert and William
Forrest in collaboration with John MacArthur in 1887. Though initially patented in both Britain and the US, the Mac-Arthur-Forrest process patents were disallowed after long and expensive litigation costing the inventors $1m.
Originally the cyanide process was applied in the context of conventional ore milling; that starts with crushing, grinding and then agitation leaching in steel tanks into a diluted solution of sodium cyanide and calcium hydroxide (slaked lime).
The gold containing ions are then separated from the pregnant solution by adsorption, onto beds of activated-carbon or anionic ion exchange resin and recovered by elution, or simply by incineration of the carrier at high temperature.
Over the past 20 years, heap leaching techniques have been developed into an efficient way to extract precious metals from small, shallow deposits, as well as an attractive way to treat large, low grade, disseminated deposits.
Heap leaching offers several advantages; including simplicity, lower capital and operating costs, shorter start-up times and less intensive environmental regulatory concerns.
One specific disadvantage of heap leaching is that there is limited flexibility of the leach time and where variability of ore being mined results in inconsistent leaching cycle times, this can result in a lower percentage of metal extraction.
Gold metal is recovered from the pregnant leach solution by electrolytic precipitation onto steel wool cathodes. The cathodes are then dissolved with hydrochloric acid, leaving a gold dore´ which is smelted into dore´ bullion containing up to 90% pure gold. An external refinery will later purify these into 999.9 parts-per-thousand gold bullion.
Oxygen is one of the re-agents consumed during dissolution of gold into the cyanide solution and insufficient available oxygen can delay this process. Air is often bubbled through the pulp and where economically viable, this air stream is enriched with pure oxygen, to increase its concentration.
Oxygen enrichment can also be applied by dosing the pulp with hydrogen peroxide. When processing sulphidised ores, iron and sulphur in the pulp will demand additional oxygen unless passivated by aeration and pH management.
Of the two basic gold ore types, gold is more easily recovered from low-grade oxide gold ores, by heap leaching with cyanide and obviously these were extracted first.
In the remaining low-grade sulphide ores, sulphide within the rock matrix encapsulates the gold particles requiring the development of new technology. Using bio-oxidation pre-treatment, bacteria break down the sulphides to expose the gold particles and permit the cyanide to reach and dissolve them.
The process is controversial, due to the highly toxic nature of cyanide. However, free cyanide breaks down rapidly when exposed to sunlight, although the less toxic compounds such as cyanates and thiocyanates may persist for some years.
The worst disasters tend not to kill many people, as humans can be warned not to drink or go near polluted water.
However, cyanide spills can have a devastating effect on rivers, killing everything for several miles downstream. Fish are the most obvious casualties, but in fact the whole food chain collapses, from phytoplankton to ospreys.
However, the pollution is soon washed out of river systems and as long as organisms can migrate from unpolluted areas upstream, affected areas can soon be repopulated.
The American state of Montana and several other countries have banned cyanide mining altogether.
Brussels introduced Directive 2006/21/EC on the management of waste from extractive industries. Article 13(6) requires that, ‘the concentration of weak acid dissociable cyanide in the pond is reduced to the lowest possible level using best available techniques’ and at most, all mines started after 1st May 2008 may not discharge waste containing over 10ppm WAD cyanide.
Mines built or permitted before this date are allowed no more than 50ppm initially, dropping to 25ppm in 2013 and 10ppm by 2018. Under Article 14, companies must also put in place financial guarantees to ensure clean-up after the mine has finished.
The industry has come up with a voluntary Cyanide Code that aims to reduce environmental impacts with third party audits of a company’s cyanide management. Over 90 mines worldwide now use an Inco SO2/air detoxification circuit to convert cyanide to the much less toxic cyanate before waste is discharged to a tailings pond.
Of even greater concern is the fact that amalgamation with mercury is the dominant gold extraction method used by 10 million small scale gold miners, in more than 50 countries.
It is estimated that this results in hundreds or possibly thousands of tons of mercury being released into soil, groundwater and the air annually.
Alternative extraction technologies
Environmental concerns over the use of cyanide in the hydrometallurgical process of leaching gold from gold ores has spawned the search for alternative lixiviants; thiosulfate appears to be one of the most promising alternatives.
After the dissolution, the gold in solution must be recovered from the solution phase. The ideal recovery process would have simple operational control, fast kinetics, high recovery, and low unit cost.
A researcher at Queen’s University has developed a novel process for the recovery of gold from thiosulfate leaching solution. The process is suitable to either pre-existing or green-field industrial thiosulfate leaching processes.
The process has significant advantages over existing precipitation techniques, including the use of a low-cost environmentally inert reagent; no introduction of harmful ions into the leaching solution; and easy retrofit of existing recovery facilities.