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Bio-oxidation for refractory gold.



Recent developments bring bio-oxidation closer to being a competitive method for increasing the recovery of precious metals from refractory ores. Davy McKee, which has been investigating bio-leaching for the past eight years, is now at the stage of planning a 3 mt/d pilot plant that is designed for rapid scale-up to commercial operation.

Bio-oxidation is not a panacea for all ore bodies, some hardly respond at all, but Davy McKee believes it will prove to be an economic alternative to pressure leaching for complex ores at relatively small throughputs. Capital costs are estimated to be higher than roasting with clean-up of off-gases but considerably less than those of pressure leaching.

Apart from grass-roots projects, bio-oxidation is seen by Davy to have a role in the expansion of plants currently using pressure leaching. Partial oxidation in a bio-leach plant ahead of pressure leaching should dramatically increase autoclave capacity, with the added bonus that oxygen-rich off-gases from the autoclave can be used to oxygenate leach.

The use of bacteria, for the natural degradation of sulphides as an alternative to roasting or pressure leaching in the treatment of refractory gold ores has been studied for many years. However, the successful application of this seemingly simple process has been hindered by lack of understanding of the reaction mechanisms involved and the variability of ore types.

The mode of bacterial attack can be broken down into three distinct actions: * direct contact with mineral surfaces where pyrite is broken down to ferrous iron and sulphide radical, which are acid-soluble. * indirect bacterial action in which solubilized ferrous iron is converted to ferric, which in turn breaks down arsenopyrite to soluble arsenic, sulphide radical, and ferrous iron. * indirect bacterial actions on the solubilized sulphide radical that convert it to sulphuric acid.

Whatever the dominant mode, the process has a high oxygen demand and the bacteria are sensitive to temperature, toxic build-up of heavy metal ions, and shear conditions in the tanks. In conventional plants using a string of bio-leach tanks, balancing these conflicting demands has proved to be the major difficulty. In addition, the long retention times necessary, typically three days or more, and the low pulp densities required to reduce shear have made the resultant large tankage economically prohibitive.

A breakthrough was the realization that 100% degradation of the sulphides was not necessary for high gold extraction because oxidation takes place preferentially along the mineral grain boundaries, liberating the gold particles for subsequent cyanidation long before all the sulphide is converted to sulphate. In many ores, 40-50% oxidation is sufficient for gold recoveries of over 90%. This, together with the realization that ferric-sulphate leaching in acid, solutions also plays a role in sulphide matrix degradation, led to the development of the Davy McKee Separator Generator Concept. This separator generator concept has been found to be particularly suitable for less refractory arsenopyritic ores.

In this design, bacteria are cultivated in a separate vessel where conditions of temperature, agitation, and concentration of metal species can be closely controlled to optimize biomass growth, while leaching takes place in subsequent tanks where close control of conditions is not necessary. Bioliquor from the generator is pumped to the leach vessels. Recycle liquor, containing dissolved metal species that might, if in excess, hinder bacterial activity, is pumped directly to the leach stages, bypassing the biomass tank.

Little consideration is needed to ensure that the subsequent leach stages are designed to encourage bacteria growth because most of the oxidation reactions are carried out by the ferrous-ferric couple oxidation potential with respect to arsenic, leaving the bacteria to play only a minor role. High-shear agitation at high-pulp density can therefore be employed in the leach stages reducing plant size and cost.

The recycle liquors, although high in arsenic, contain mainly ferrous iron, which is converted to ferric by bacteria in the presence of oxygen in a converter ahead of the leach. The resultant ferric sulphate then passes to the leach where arsenopyrite oxidation occurs.

Bio-oxidation of pyritic ores is much more difficult as it requires the presence of bacteria on the mineral surface. Conditions within the reactor are most important--temperature must be kept within 37 to 40 [degrees] C and low shear conditions should prevail to avoid damaging the biomass. Davy McKee had devoted considerable effort in reactor design to minimize shear conditions and ensure that oxygenation is never a limiting factor. Agitation and air sparging are kept separate.

The process is applicable to both run-of-mine ores and sulphide concentrates. Bioleaching of concentrates has the advantage of reducing the mass to be treated, but the quantity of sulphide to be oxidized remains almost the same, and since the reaction is exothermic, it may be necessary to cool the leaching tanks to avoid killing the bacteria.

Testwork for the evaluation of bioleaching consists of a preliminary determination of whether the ore is amenable or not. This will give indicative gold recovery data and a general understanding of the predominant methods of bacterial attack. From this, a conceptual bio-oxidation plant design can be developed.

The second stage will include a detailed examination of the bio-oxidation kinetics and an investigation of the requirements of the downstream processing plant, such as solid/liquid separation and the range of cyanide and alternative leaching processes. Critical parameters to be investigated include particle size, pH redox potential, slurry density, aeration requirements, and temperature. All these factors will be incorporated into the operation of a continuous laboratory-scale plant and will provide the design data for a pilot-scale, site-based plant.

A few bio-leach plants are already operating and it seems the technology will play an increasing, if more limited, role than first thought in the treatment of refractory ores.

References for both bio-oxidation and solvent extraction articles [1]Chile's Biggest SX/EW Plant Comes on Line. J.Edwards. AIME 89-175 [2]Stage III Expansion of Nchanga Tailings Leach Plant. S.Mwenechanya, J.Mawer & S.Chowdhury. Copper 87 Vol. 3. [3]SX/EW For the Copper Industry. A.Lillo. Copper 87 Vol. 3. [4]Practical Aspects of Copper Solvent Extraction. J.Slerakoski & H.Hein. Copper 87 Vol. 3. [5]The Cobrex Process. A.Himsley & J.Bennett. Copper 87 Vol. 3. [6]Cuprex - New Chloride Based Hydrometallurgy to Recover Copper from Sulphide Ores. R.F.Dalton, R. Price, E.Hermana & B.Hoffman, Mining Engineering Jan 1988. [7]Mathematical Modelling of Metal Extraction with Minchem II Computer Program. J.W.Morrison & B. Townson. Extraction Metallurgy 89. [8]Influence of Ore Type and Leaching Mechanism on Design of Bioleaching Plants. P.C. Miller & M.T. Errington. Extraction Metallurgy 89.
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Author:Suttill, Keith R.
Publication:E&MJ - Engineering & Mining Journal
Date:Sep 1, 1989
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