Laterite mineral processing

MOSSBAUER SPECTROSCOPY IN THE STUDY OF LATERITE MINERAL PROCESSING... 

Chapter 31). The variables control in the laterite mineral processing using Mossbauer spectroscopy is another example (Chapter 32). 

Mossbauer spectroscopy has been used as a characterization tool in a wide range of mineral processes including ilmenites, chalcopyrites, pyrrhotites, bauxites, as well as laterites to some extent [5-9]. The Mossbauer technique yields detailed information on the phases present, their composition, structure, and their relative amounts. In the case of laterite processing in particular, in addition to the above useful data, it gives critical information on the overall degree of metal reduction. 

In these studies, we can see the power of Mossbauer spectroscopy for shedding light on many important aspects of mineral processing in this example of laterite processing. 

The treatments used to recover nickel from its sulfide and lateritic ores differ considerably because of the differing physical characteristics of the two ore types. The sulfide ores, in which the nickel, iron, and copper occur in a physical mixture as distinct minerals, are amenable to initial concentration by mechanical methods, eg, flotation (qv) and magnetic separation (see SEPARATION,MAGNETIC). The lateritic ores are not susceptible to these physical processes of beneficiation, and chemical means must be used to extract the nickel. The nickel concentration processes that have been developed are not as effective for the lateritic ores as for the sulfide ores (see also Metallurgy, extractive Minerals recovery and processing). 

Herbillon, A. J. and Nahon, D. (1988). Laterites and laterization processes. In "Iron in Soils and Clay Minerals" (J. W. Stucki, B. A. Goodman, and U. Schertmann, eds), pp. 267-308. Kluwer Academic Publishers Dordrecht, The Netherlands, NATO ASI Series C Mathematical and Physical Sciences 217. 

Virnig, M. J. Mackenzie, J. M. W. Wolfe, G. A. Boley, B. D. Nickel laterite processing Recovery of nickel from ammoniacal leach liquors. Miner. Metall. Process. 2001, 18, 18-24. 

In nature aluminium oxide is mostly mined as the minerals bauxite and laterite, but these as extremely impure. Most bauxite is purified according to the Bayer process which removes the oxides of iron(III), silica and titanium. This takes place by autoclaving the bauxite with sodium hydroxide and sodium carbonate. The precipitated aluminium hydroxide is subsequently heated, or calcined. Calcination involves a heat treatment of a powder as a result of which the latter breaks down ... 

Solidification of minerals in carbonate rocks or in sea shells, or that of silica and alumina in lateritic or desert soils, is a very slow process. It takes years and centuries and even geological time to consolidate some minerals. Unfortunately, little is known on the exact chemical reactions and the resulting hardening process. As a result, the chemical hardening in nature cannot be translated into technological applications where accelerated hardening and solidification are desired. 

Some silicate minerals are also formed in a similar manner. The process is very slow, slower than even carbonate formation, because of the very low solubility of silicate minerals. In clay minerals, or in lateritic soils, silicates dissolve very slowly to form an intermediate product, silicic acid (H4Si04), which subsequently will react with other sparsely soluble compounds and form silicate bonding phases. Thus, a dissolution-precipitation process seems to be crucial to forming some silicate minerals. 

Aluminum is the second most abundant metal on earth s crust. It is a common metal in tropical soils called laterites (red soils). It is extracted from bauxite that is a rich laterite by Bayer process that involves dissolution and separation of the oxide in caustic soda solution between 150 and 250°C and 20 atm of pressure. Though abundant and inexpensive, alumina based CBPCs are difficult to form because even in an acid solution the solubility of alumina is very low. This solubility, however, can be enhanced by a mUd thermal treatment and suitable CBPCs can be formed. Alumina is available commercially as calcined alumina called corundum, or as its hydrated forms such as aluminum hydroxide (Al(OH)3), as bohmite, (A1203-3H20), gibbsite (AI2O3 H2O) or in impure forms as in kaolin clay. These mineral forms and their use in ceramic formation are discussed in Chapter 11. 

In spite of this limitation, the method is very useful, because it provides a means of forming a ceramic of one of the most common and inexpensive oxides. As discussed before, iron oxide is a component of lateritic soils and red mud, high-volume iron mine tailings, and machining swarfs. Thus, useful products of several mineral waste streams can be formed by the process described in this chapter. Development of ceramics using red mud and swarfs is discussed in Chapter 14. 

The division into laterite and ferricrete used in this chapter represents a useful process-based distinction, but the practicality of determining whether mineral components of a profile are allochthonous or autochthonous is problematic because many lateritic weathering profiles are subsequently modified by the introduction of allochthonous materials. Conversely, once formed, ferricretes can be subject to weathering processes in situ and evolve toward more lateritic-type profiles. Nevertheless, the distinction between dominantly autochthonous weathering profiles or allochthonous alteration profiles is an important one because it places constraints upon the processes operating during duricrust evolution, and also upon contemporaneous climatic and geomorphological conditions. 

The mineralogy of ferricrete alteration profiles can be complex and varied because of the incorporation of mechanically derived materials and the retained importance of host rock composition after the formation of secondary minerals. In general, ferricrete profiles do not display the progression of alteration minerals observed in laterites. Where ferricretes are formed by mechanical accumulation, they can lie disconformably above unaltered bedrock (Bowden, 1987, 1997). In these instances, the ferricrete mineral assemblage will be inherited, in part, from the derived materials, and in part from later cementation processes that involve remobilised iron and alumina deposited as neo-formed oxyhydroxides. In such examples, determining the sequence of mineralogical transformations becomes exceptionally difficult. 

The first point of discussion is the influence of the bedrock nature on the chemical composition of waters. We can see in Table II and in Fig. 6 that waters in the chloritic schist or argillaceous sand areas do not differ from those in the granitic area. This is not surprizing, in spite of the well-known control of rock mineral on the groundwater composition (M. Schoeller, 1962 Tardy, 1969) in fact, in the whole bioclimatic sequence considered, the lateritization processes have destructed all the primary minerals, except quartz, and the weathering minerals are always the same kaolinite, iron oxide and oxy-hydroxide, the stabilities of which in the surface are very great. 

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