Oxide leaching

Smectite is the first secondary mineral to form upon rock weathering in the semi-arid to sub-humid tropics. Smectite clay retains most of the ions, notably Ca2+ and Mg2+, released from weathering primary silicates. Iron, present as Fe2+ in primary minerals, is preserved in the smectite crystal lattice as Fe3+. The smectites become unstable as weathering proceeds and basic cations and silica are removed by leaching. Fe3+-compounds however remain in the soil, lending it a reddish color aluminum is retained in kaolinite and A1-oxides. Leached soil components accumulate at poorly drained, lower terrain positions where they precipitate and form new smectitic clays that remain stable as long as the pH is above neutral. Additional circumstances for the dominance of clays are ... 

Oxidative leaching with ammonium thiosulfate (Section 9.17.3.1) generates solutions containing a mixture of the mono- and di-thiosulfato complexes, [Au(S203)] and [Au(S203)2]3> Gold can... 

APOL [Alkaline pressure oxidation leaching] A process for extracting gold from refractory ores, developed by Davy McKee (Stockton, UK). 

Ledgemont A process for removing sulfur from coal by an oxidative leach with lime and ammonia. Developed by Hydrocarbon Research. 

Screening tests, in the study of iron oxides leaching by sucrose in sulphuric acid solution, using statistical methods. Hydrometallurgy 35 293-311... 

Based on dissolved ions only, the titanate waste showed an overall leach rate of x 10 5 g/cm day and a rate of 5 3 x 10 7 g/cm day for the fission waste oxides only. The results indicate that the leaching which is occurring is associated with the silicate phases in the ceramic, i.e., the Si02 formed from the silicon and the zeolite. The glass samples showed overall leach rates of 6-15 x 10 5 g/cm day and fission waste oxide leach rates of 1.8-2.7 x 10 g/cm day, where the higher rates in both cases were observed in the phosphate-containing glass. 

In the Nippon Mining Company process,163 the aqueous feed consists of a solution of cobalt, nickel and other metal sulfates obtained by the high-pressure oxidative leaching of a mixed metal sulfide ore. Following the removal of iron and copper by chemical precipitation methods, and the successive extraction of zinc and cobalt by organophosphorus acid reagents (Section 63.3.2.2) under weakly acidic conditions, the pH value of the solution is adjusted to between 9 and 9.5 with ammonia, and nickel is extracted into a 25% solution of LIX 64N in an alkane solvent. 

One problem with them is their sensitivity to the medium they are employed in, as regards metal oxide leaching. For example, in f-butanol, the catalysts are relatively stable, but in co-ordinating solvents such as acetonitrile, considerable leaching is detected. It is possible that if the leaching problem can be overcome, the MOx-Si02 xerogel catalysts could have a future on an industrial scale. 

EXAFS results confirm that the Cu atoms in reduced PtCu/NaHY and PdCu/NaHY are leached out of the PtCu and PdCu bimetal particles and migrate to small cages on heating to 500°C in an inert atmosphere (185,202). For PdCu in NaHY, additional evidence for oxidative leaching of Cu from the bimetal particles is obtained from an examination of the hydride decomposition. 

Buckley A. N., Wouterlood H. L, and Woods R. (1989) The surface composition of natural sphalerites under oxidative leaching conditions. Hydrometallurgy 22, 39-56. 

Hackl R. P., Dreisinger D. B., Peters E., and King J. A. (1995) Passivation of chalcopyrite during oxidative leaching in sulfate media. Hydrometallurgy 39, 25—48. 

Pratt A. R., Nesbitt H. W., and Muir I. J. (1994b) Generation of acids from mine waste oxidative leaching of pyrrhotite in dilute H2SO4 solutions at pH 3.0. Geochim. Cosmochim. Acta 5%, 5147-5159. 

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