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MEI Online: Hydrometallurgy: Latest News: November 23rd 2010


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:: New Base Metal Hydrometallurgy Processes

MetaLeach Limited is a wholly owned subsidiary of Alexander Mining plc, and was formed in 2007 to enable the commercialisation of its proprietary hydrometallurgical mineral processing technologies. These technologies have the potential to revolutionise the extraction processes for many base metals deposits by reducing costs, and hence enhancing operating margins, at the mine site.

MetaLeach® owns the intellectual property to two ambient temperature, ambient pressure, hydrometallurgical technologies, namely AmmLeach® (patents pending) and HyperLeach® (patents pending). These technologies are environmentally friendly, cost effective processes for the extraction of base metals from amenable ore deposits and concentrates allowing the production of high value products at the mine site (i.e. metal powder or sheets). These technologies were created as a result of the Company’s work at its Leon copper project in Argentina and subsequent research and development undertaken by the Company and its consultant Dr. Nicholas Welham.

The AmmLeach® process (patents applied for) is a new process developed for the extraction of base metals, especially copper, zinc, nickel and cobalt from ore deposits and concentrates. The process utilises ammonia-based chemistry to selectively extract metals from ores under ambient conditions of temperature and pressure. The target ores will typically be high acid consuming, although AmmLeach® is also a viable alternative to acid leach processes as it is far more selective and offers a considerable number of technical and economic benefits.

The technology consists of the same three major stages as acid processes i.e. leaching, solvent extraction (SX) and electrowinning (EW). The leaching occurs in two steps, an ore-specific pre-treatment which converts the metals into a soluble form and the main leaching step, which uses recycled raffinate from the solvent extraction stage. Solvent extraction is used to separate and concentrate the metals, whilst also changing from ammoniacal media to acid sulphate media from which metals can be directly electrowon using industry standard unit operations. One of the key benefits of the AmmLeach® process is that, unlike some new technologies, it requires no special purpose built equipment. The AmmLeach® process can directly replace acid leaching in an existing operation.

AmmLeach® technology is suitable for both low grade heap leaching and higher grade tank leaching; the choice is dictated by the grade and deposit economics. Polymetallic deposits can be readily handled using standard solvent extraction and solution purification techniques. The difference from acid leaching is that the leaching is conducted in moderately alkaline solution with ammonia present to selectively leach base metals. The use of alkaline conditions allows the use of AmmLeach® on high-carbonate ores where acid consumption would be prohibitive.

The AmmLeach® process has an extremely high selectivity for the target metal over iron and manganese, which are insoluble under AmmLeach® conditions. Calcium solubility is also significantly suppressed by the presence of carbonate and extremely low sulphate levels in the leaching solutions. These features ensure that there are no potential problems due to jarosite or gypsum precipitation reducing permeability in the heap or scaling problems in the solvent extraction plant. Additionally, silica is also insoluble in the AmmLeach® process, removing problems associated with formation of unfilterable precipitates within an acid leach plant during pH adjustment and 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.

The following metals are particular targets for the AmmLeach® process:

  • copper and copper/cobalt in carbonate and weathered oxide deposits;
  • zinc (and cadmium) in mixed oxide deposits;
  • nickel and cobalt in lateritic deposits;
  • gold, silver and copper in leached porphyries;
  • pre-treatment of gold ores with high cyanide-soluble-copper levels;
  • polymetallic base metal deposits, especially uranium; and
  • leaching of base metals from roaster concentrates.

Of these, the copper process has already been demonstrated at pilot plant scale for heap leaching, and at bench scale for agitated leaching. The cobalt (or copper and cobalt oxide ore) process has been bench scale tested successfully for both heap and agitated leaching.

Further development of the zinc process has led to a new solvent extraction process for zinc from ammoniacal solutions, for which patents are pending. This patent application is for the recovery of zinc from ores which do not require pre-treatment before ammoniacal leaching. A patent covering a process allowing selective leaching of zinc from sideritic zinc ores has also been applied for.

The nickel process is still under development but early trials on a wide variety of ores show promise.

Because of the tailored pre-treatment step, almost any ore type is amenable to the AmmLeach® process. Thus far, it has been demonstrated on predominantly oxide ores but some sulphides have also been shown to leach after appropriate pre-treatment. This advance allows the treatment of mixed oxide-sulphide ores which are often present in the transition from weathered to un-weathered ore. As a project proceeds, the AmmLeach® process can be modified to cope with the changing mineralogy from oxide to sulphide without substantial capital expenditure.




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