The AmmLeach® process is a proprietary ammonia based process for the selective extraction of base metals from amenable ore deposits and concentrates. The technology consists of the same three major stages as conventional sulphuric 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 (the portion of an original liquid that remains after other components have been dissolved by a solvent) from the SX stage. SX 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.
The AmmLeach® process has many advantages compared to conventional acid leaching. The primary difference from acid leaching is that the leaching is conducted in a moderately alkaline solution, which allows the use of AmmLeach® on high carbonate ores where acid consumption would be prohibitive. The major diffferential in reagent consumption for moderate to high (50 to >100kg acid per tonne ore) acid consuming ores compared to using AmmLeach® (<5kg ammonia per tonne ore), and the fact that, unlike acid leaching, undesirable metals and other impurities are either insoluble or significantly suppressed, leads to major capital and operating cost savings.
One of the key advantages of the AmmLeach® process is that, unlike some new technologies, it requires no special purpose built equipment. The AmmLeachВ® 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.
As mentioned, one of the major benefits is that 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 SX plant. 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, does not react with aluminosilicates and ferrosilicates, whose products can cause drainage and permeability problems in heaps.
Compared with previous ammoniacal processes, almost any ore mineralogy can be treated as the pre-treatment step is specific to each ore body and the whole AmmLeach® process is tailored to individual ore bodies. 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 unweathered 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.
Polymetallic ores can also be processed by AmmLeach® with separation achieved using solvent extraction to separate metals and produce multiple revenue streams. The minimisation of ammonia transfer allows these metals to be recovered directly from their strip solutions by precipitation, crystallisation or electrowinning.
Decommissioning of the heap is extremely simple as no neutralisation is necessary 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 vegetate, with the residual ammonia acting as a fertiliser.
The alkaline residue allows immediate application of 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. Work is currently underway to incorporate precious metal recovery within the AmmLeach® process. Preliminary work on the leaching of cyanide consuming metals prior to precious metal leaching with cyanide looks highly promising.