Leaching residues

The chemical-grade ore, containing about 30% chromium, is dried, cmshed, and ground in ball mills until at least 90% of its particles are less than 75 ]lni. It is then mixed with an excess of soda ash and, optionally, with lime and leached residue from a previous roasting operation. In American and European practice, a variety of kiln mixes have been used. Some older mixes contain up to 57 parts of lime per 100 parts of ore. However, in the 1990s manufacturers use no more than 10 parts of lime per 100 parts of the ore, and some use no lime at all (77). The roasting may be performed in one, two, or three stages, and there maybe as much as three parts of leached residue per part of ore. These adaptations are responses to the variations in kiln roast and the capabihties of the furnaces used. 

Production as practiced in Chile consists of leaching residues from their Na nitrate operation to produce a weak brine of K nitrate which is coned by solar evapn When the proper concn of K nitrate is achieved it is recovered by crystn (Ref 14)... 

Oxisols Highly weathered and leached, residual Fe and A1 oxides Tropical areas with a stable landscape... 

The scheme E involves dead roasting of the concentrate by which Fe203 forms and the nickel content is oxidized to nickel ferrite (NiFe204). This material is selectively reduced to produce an iron-nickel alloy which is then leached in an ammoniacal ammonium carbonate solution for nickel recovery, leaving hematite as a leach residue. 

The scheme F involves sulphating roasting. The conditions are so chosen that iron is converted to its oxidic form (Fe203) while nickel and copper are converted to nickel sulfate and copper sulfate respectively. The product is subsequently water leached to take the sulfates into solution, leaving the iron oxide in the leach residue. 

The process is selective for nickel and cobalt, with iron, the major component of the ore, remaining in the leach residue. Overall metal recovery is low - typically 75-80% recovery of nickel and only 40-50% recovery of cobalt. 

Iron in the feed concentrate is rejected either as unreacted pyrite mixed with elemental sulfur or as jarosites in the leach residue. The pyrite/sulfur mixtures said to be suitable for indefinite storage, but the well known environment effects caused by pyrite weathering are likely to make storage of this material a less than straightforward problem. Besides this, there are problems associated with the disposal of the leach residues from the pressure leach process. 

Falconbridge Also called the matte leach process. A process for extracting copper and nickel from matte (a sulfide ore that has been roasted to remove most of the sulfur). Most of the nickel is leached out with hydrochloric acid and recovered as nickel chloride crystals. The leach residue is roasted and leached with sulfuric acid to dissolve the copper. The process has been operated in Canada and Norway since 1970. 

Leachates from solid residues from the processing of primary minerals such as flotation and leach residues, and from landfill... 

It may be mentioned that starting with ash and soot from crude oil-fired stations, the resulting metals are the same, but the main leaching residue is carbon. This residue is initially burned and the ash is leached again to increase the total yield of vanadium. In the same operation, the concentration of iron is reduced by precipitation of jarosite. During leaching, the redox potential is controlled by SO2 addition to keep vanadium in its IV-valent state. 

Due to chemical passivation in old waste materials, subsequent leaching of the leach residue with sulfuric acid, with perhaps some oxidation, is necessary to achieve very high yields. If the pH is kept about 3 in this second treatment, leaching of iron is negligible. Also, to reduce iron in the leach solution, spent potassium hydroxide electrolyte from discarded batteries can be used for iron hydroxide precipitation. 

The procedure has several advantages iron will remain in the leach residue nickel and cadmium are almost completely leached and form metal-amine complexes in solution and, finally, the buffer capacity of the solution... 

If the low rade ore is a zinc mineral, then zinc concentrate obtained from the flotation process is calcined and leached with water to remove zinc. Silver and lead are left in leach residues. Residues are treated like lead concentrates and fed into lead smelters. Silver is recovered from this lead concentrate by various processes described above. 

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