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Several processes have been developed commercially to oxidise refractory mineral sulphides, such as bacterial and pressure leaching. An alternative technology for the oxidation of refractory sulphides is the Albion Process. The process was developed in 1993 and has been patented worldwide. The Albion Process technology was developed by MIM Holdings (now Xstrata Plc) to treat concentrates produced from refractory base and precious metals ores.
The Albion Process is a combination of ultrafine grinding and oxidative leaching at atmospheric pressure. The Albion Process incorporates the revolutionary Isamill to produce an activated, finely ground concentrate at relatively low specific energy inputs. This finely ground concentrate is then leached at atmospheric pressure in conventional agitated tanks. The capital costs of an Albion Process plant can be substantially lower than a comparable bacterial or pressure leach plant, due to the simplicity of the process flowsheet. 
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The key to the Albion Process is the ultrafine grinding stage. The process of ultrafine grinding results in a high degree of strain being introduced into the mineral lattice. As a result, the number of grain boundary fractures and lattice defects in the minerals increases by several orders of magnitude, relative to unground minerals. The increase in the number of defects within the mineral lattice "activates" the mineral, facilitating leaching. The rate of leaching is also enhanced, due to the dramatic increase in the mineral surface area. Passivation of the mineral surface by sulphur based leaching products is also minimised by ultrafine grinding. Typically, precipitates that form on the surface of a leaching mineral will slowly passivate the mineral, by preventing the access of chemicals to the mineral surface. Passivation is normally complete once this precipitated layer is 2 - 3 microns thick. Ultrafine grinding of a mineral to a particle size of 80 % passing 8 - 12 microns will eliminate passivation, as the leached mineral will disintegrate prior to the precipitate layer becoming thick enough to passivate the mineral. The Isamill, marketed exclusively by Xstrata Technology, is the most advanced ultrafine grinding technology available today, with 18 mills installed worldwide. The oxidative leaching stage is carried out in agitated tanks operating at atmospheric pressure. Oxygen is introduced to the leach slurry to assist the oxidation. Leaching is autothermal, not requiring any external heat. Temperature is controlled by the rate of addition of oxygen, and by the leach slurry density. The general reaction in an Albion Process leach is: MeS + 1/2 O2 + H2SO4 = MeSO4 + S0 + H2O In the Albion Process, sulphides are oxidised preferentially to elemental sulphur, resulting in significant savings in oxygen costs relative to pressure leaching. Albion Process flowsheets have been developed for a range of base and precious metals, and Xstrata Technology have extensive testing capabilities, including pilot plants capable of producing up to 120 kg per day of metal at their Hydrometallurgy Research Laboratories in Brisbane, Australia. Xstrata continues to actively develop the technology for its copper and zinc projects. top
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