The Agglomeration of Fine Material for Bacterial Heap Leaching

Résumé du livre

In almost all instances, the mineralogy of an ore determines its amenability to treatment and the ease with which the metals it contains can be extracted. This paper examines the favourable and unfavourable mineralogical parameters that determine the bacterial- assisted leachability of sulphide ores in general and of those containing nickel in particular. Such an examination also requires a better understanding of the physical and chemical changes that occur in minerals before and during bacterial leaching. Deposits of nickel sulphides are invariably magmatic in origin, and have a relatively simple and consistent sulphide and silicate (gangue) mineral assemblage. Any variation in this condition can be ascribed to post-magmatic alteration, which inevitably takes place. These processes, which have an important effect in shaping the final sulphide and silicate mineralogy of these deposits, are examined in detail and are ascribed to hypogene, metamorphic, or super-gene alteration. Except under oxidizing conditions, their effect on the sulphide minerals is usually minimal, but fibrous and talcose minerals are produced in the silicate gangue. These changes naturally affect the leachability low-grade ores. The role of the bacteria is primarily to oxidize the sulphide minerals in sulphuric acid solution, a process that is enhanced by their conversion of ferrous iron to ferric iron, the latter being a powerful oxidant in its own right. The bacteria are extremely sensitive to their environment, however, and the variables (temperature, pH, and the availability of oxygen and nutrients) need to be optimum and stable for an acceptably high rate of metal recovery to be achieved. Nevertheless, it should be stressed that bacterial leaching is used only as a pretreatment in the recovery of the base metals. To be treatable, the base-metal sulphides must be acid-soluble, which explains why the acid-insoluble platinum-group elements are not amenable to leaching. The leachability of a sulphide mineral depends on its texture and its galvanic interaction with other sulphides, both of which are examined in this paper. The gangue minerals are important only in that carbonate, chlorite, and fibrous species react with, or absorb, the sulphuric acid leaching solution, since this affects the pH value of the leaching solution and, consequently, the performance of the bacteria. In a heap-leaching process, the permeability of the heap to oxygen and to the leaching solution containing the bacteria is of prime importance and is discussed in detail. This paper confirms that an understanding of the mineralogy of the primary ore and gangue minerals and of their secondary post-magmatic counterparts is essential to the successful assessment of the suitability of an ore deposit to bacterial leaching. A knowledge of the mineralogy of those minerals allows factors like metal availability, reagent-consumption potential, mineral dissolution rates, ore permeability, and other vital parameters in the leaching process to be determined. Acid-bacterial leaching is attracting a considerable amount of research effort and industrial interest in many countries. Although time-consuming, it is cheap and versatile enough to be applied at any stage of a beneficiation operation, from the in situ leaching of material to the leaching of a sulphide concentrate.