Copper solution - extraction from high alkali deposits
Determined to reduce levels of acid consumption, Alexander Mining plc, based in London, UK, has developed a new technique for extracting copper from high alkali deposits. Michael Forrest investigates.
The downward trend of exploration expenditure has turned a corner, after a precipitous fall in 2009 of over 40% from the peak of US$13.2bln. This is according to the advance notice of the annual exploration expenditure survey results, published by the Metals Economic Group, in Nova Scotia, Canada.
This is good news but the number of large finds has been in decline over the past decade, despite record amounts spent on exploration. The reason for this is not entirely clear, but the premise that the easy targets have been found must be high on the list.
We have already seen how mined grades of copper and gold have fallen, yet companies have remained profitable, compensated in part by rising prices for gold, which have reached new heights, and for copper, which has almost recovered to the peak levels of 2008.
Nevertheless, Dr Matt Sutcliffe, Executive Chairman of Alexander Mining plc, an AIM-listed company, based in London, UK, suggests that the combination of a lack of large-scale new finds and increasing demand for metals from India and China will put pressure on the supply chain. ‘One way to combat this double hit,’ he says, ‘will be to sweat mines harder, that is, we will have to get more metal out of existing mines and known deposits.’
Of the 15.8Mt of copper produced globally each year, about one quarter comes from oxide ores that are leached using sulphuric acid. The remainder is treated using various pyrometallurgical techniques and all that entails in terms of gas discharges. Environmental concerns have, therefore, led to an increasing proportion of production from oxide deposits over the past 20 years. There is, however, a sizable proportion of copper oxide deposits that are difficult to treat by acid heap leaching because of their high alkali content. It is this deposit type that Alexander Mining has been tackling.
‘It was more by necessity than design,’ states Sutcliffe. ‘We were working towards feasibility at our Leon copper project in Salta Province, northwest Argentina, and had defined a resource of 6.7Mt of ore grading 0.62% copper and 18g/t silver. The ore was stratabound, primarily in limestone, with 70% of the mineralisation malachite and azurite to a depth of 70m. Although relatively easy to mine, the problem lay in recovering the metals, as conventional leaching resulted in acid consumption of several hundred kilogrammes per tonne. [This is] totally unacceptable in terms of cost, leaching/processing efficiencies and neutralisation on disposal of tailings. Clearly something had to be done.’
Alexander Mining’s engineers and metallurgists believed ammonia leach might provide the answer.
Ammonia as a leaching agent is not new in metal recovery. In 1916, Kennecott Copper and Hecla Mining in the USA precipitated copper using the chemical, and, in the 1950’s, Sherritt Gordon developed a process to recover nickel and cobalt from complex sulphides in an autoclave. Two decades later, Anaconda Mining Inc developed its Arbiter process for the dissolution of copper sulphides with ammonia and oxygen, followed by Adelaide Chemical Co, which used an ammonia plant on its Gunpowder deposit ore in the 1980s.
However, none of these worked in a heap leach ambient temperature and pressure environment.
Sutcliffe says, ‘Our research at Leon led us to believe that an ammonia route might be the answer to high acid consumption, but a number of objectives had to be met to make it viable. First a ready and safe supply of ammonia would be required in this part of Argentina. Secondly, [there must be] a method of recycling [it] to prevent consumption of the reagent as occurs with acid leaching. And thirdly, the ability to sequentially reclaim metals from a polymetallic ore body.’
Alexander investigated ammonia use, and, based on favourable results, built a pilot plant to demonstrate the process, producing cathode copper with low reagent consumption of less than five kilogrammes per tonne of ammonia. Alternative acid technologies can consume over 100kg/t have been reported, with 30-50kg/t common.
As a result of the pilot’s success, the company recognised that its technology was patentable and applicable to other high acid consuming copper oxide deposits in many parts of the world. Subsequent test work showed that the process was also well suited to copper-cobalt and cobalt deposits. Alexander then registered the trademark of AmmLeach for its proprietary process.
The key inventive steps in AmmLeach are protected in the extensive international patenting programme that Alexander has implemented. In essence, AmmLeach is a two-stage process that begins with a proprietary ore-specific pre-treatment stage, which mobilises the copper within the ore and redeposits it onto the surface. This enables easy access for heap or tank leaching with ammonium carbonate solution. The company claims that no special purpose-built equipment is required – proven standard SW-EX equipment can be used.
Body of work
Compared with previous ammoniacal processes, almost any ore mineralogy can be treated over the course of a mine’s life, as the pre-treatment step is specific to each orebody or mineralogically distinct part of an orebody. The entire AmmLeach process, is tailored to individual ore bodies and consequently is said to incur substantially lower ammonia losses than earlier processes. In theory, all the ammonia can be recovered, however, in practice small losses do occur. Furthermore, the technique is not limited by high carbonate content like other acid technologies that dissolve all metal-bearing and gangue minerals.
AmmLeach has extremely high selectivity for the target metal over iron, aluminium and manganese, which are insoluble under these conditions, and the carbonate and the low sulphate levels in the leaching solutions significantly suppress calcium and magnesium solubility. These features inhibit jarosite or gypsum precipitation, which ordinarily reduces permeability in the heap, and prevent scaling problems in the solvent extraction plant.
Additionally, silica is also insoluble in the process, removing any problems associated with unfilterable precipitates forming within the acid leach plant during pH adjustment, and eliminating the need to handle high viscosity solutions. Ammonia, unlike acid, doesn’t react with aluminosilicates and ferrosilicates, whose products can cause drainage and permeability problems in heaps.
End of the line
Heap decommissioning is said to require no neutralisation and the potential for acid mine drainage is virtually eliminated. After final leaching, the heap is simply washed to recover ammonia and then left to re-vegetate, with the residual ammonia acting as fertiliser. The alkaline residue enables cyanide leaching of gold and silver in ores where there is an economic precious metal content after removal of high cyanide consuming metals such as copper.
Alexander has now refocused the business and formed MetaLeach Ltd to commercialise its proprietary hydrometallurgical technology. ‘We have the invaluable assistance of the research facility at the University of Ballarat in Victoria, Australia, where our consultant Dr. Nicholas Welham and his team carried out R&D, and are instrumental in our ongoing technical work,’ explains Sutcliffe.
‘Although we have proved the technology in Argentina, investors and potential users were keen to find out the economics. To that end we contracted David Lunt of Stirling Process Engineering, in Perth, Australia, to compare AmmLeach to conventional Acid SX-EW on carbonate-hosted oxide ores from the Democratic Republic of the Congo Copperbelt, which typically run upwards of two per cent copper and 0.2% cobalt’.
Lunt’s independent report shows that for a heap leach 50,000t/y, (contained) copper mine capital and operating cost savings are 30% and 42% respectively, with similar recoveries of around 80%. The largest operating savings are in reagent costs per annum, nearly US$14m for ammonia compared with over US$41m for sulphuric acid.
The capital cost was US$223m and US$317m for AmmLeach and acid leach, respectively. The capital cost differential of US$94m is, according to the report, mainly due to the fact that the AmmLeach option does not require an acid plant and the cost of the ammonia plant to generate the reagent from urea is significantly lower. Also the cobalt circuit, associated with AmmLeach, incurs less investment than the acid route.
Sutcliffe declares, ‘We have now completed successful trials on zinc oxide ores that promise equal savings in processing costs. Promising results are also being obtained from molybdenum/copper ores and nickel laterites. So far, we have AmmLeach-tested ores from dozens of mines and deposits around the world. Our test work is continuing, and we are developing a dual-approach business model that will license the technology for a royalty/fee structure or minority project interest, and will identify and secure direct equity interests in amenable base metals deposits. The latter includes our consultancy arrangement with Canadian company RPT Resources’.