Hydrometallurgy and Metal Finishing

The chemistry of hydrogen peroxide and Caro s acid can be used for many applications in metal extraction and separation from ores or waste (hydrometallurgy), and in the surface treatment of metals and alloys. Compared to alternative treatments, direct chemical costs are sometimes higher, but the difference is often outweighed by advantages in simplicity of operation (cost savings on equipment) and in lower overall effluent production. The properties of the two peroxygens which are relevant to these applications are  

Hydrogen peroxide can act as an oxidant or reductant towards metals in various oxidation states. In some cases (e.g. Fe, Cu, Se, Te) no net reductive behaviour is exhibited at any pH. Where either net reduction or net oxidation is observed (e.g. V, Cr, Mn, Ce), reductive behaviour is usually shown at acidic pH and oxidative behaviour at alkaline pH. This can be useful in precipitating or separating metals from solution, since higher oxidation states are often either less soluble or will form anionic species (i.e. opposite charge to other components). Where it is required to oxidise these metals at acidic pH, Caro s acid is a useful alternative to H2O2, from which it can be generated on site (see section 9.3.3.1). 

Two types of indirect oxidation using H2O2 are used. The first involves reoxidation of an intermediate, or relay , metal which in turn acts as an oxidant in the recovery process - examples include Fe(III) oxidation of U(IV) and of thiourea in gold production. The second involves oxidation to assist acid dissolution of metals, chiefly in finishing of stainless steel, brass or copper. 

Several metals form peroxo-complexes in their highest oxidation states, when treated with hydrogen peroxide (e.g. V, Mo, W, see section 9.3.4.4) and these are much more water-soluble than the corresponding oxides. This therefore provides a more efficient means of extracting these metals selectively into solution in, for example, scrap recycling (e.g. WC), and also potentially a method of purifying crude metal oxides (e.g. Zr02). 

Each of these application types will now be described in a little more detail. 

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This chapter will focus on three important industries where peroxygens have had a significant impact, particularly with respect to environmental issues chemical purification, pulp and paper bleaching and hydrometallurgy and metal finishing. 

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