Caustic leaching

This dissociated zircon is amenable to hot aqueous caustic leaching to remove the siHca in the form of soluble sodium siHcate. The remaining skeletal stmcture of zirconia is readily washed to remove residual caustic. Purity of this zirconia is direcdy related to the purity of the starting zircon since only siHca, phosphate, and trace alkaHes and alkaline earth are removed during the leach. This zirconia, and the untreated dissociated zircon, are both proposed for use in ceramic color glazes (36) (see Colorants for ceramics). 

More recendy, the molten caustic leaching (MCL) process developed by TRW, Inc. has received attention (28,31,32). This process is illustrated in Eigure 6. A coal is fed to a rotary kiln to convert both the mineral matter and the sulfur into water- or acid-soluble compounds. The coal cake discharged from the kiln is washed first with water and then with dilute sulfuric acid solution countercurrendy. The efduent is treated with lime to precipitate out calcium sulfate, iron hydroxide, and sodium—iron hydroxy sulfate. The MCL process can typically produce ultraclean coal having 0.4 to 0.7% sulfur, 0.1 to 0.65% ash, and 25.5 to 14.8 MJ/kg (6100—3500 kcal/kg) from a high sulfur, ie, 4 wt % sulfur and ca 11 wt % ash, coal. The moisture content of the product coal varies from 10 to 50%. 

J. L. Anastasi and co-workers, "Molten-Caustic-Leaching (Gravimelt Process) Integrated Test Circuit Operation Results," Report to the Gravimelt Process Advisory Board, Summer 1989. 

Also, the tellurium metal can be prepared by thermal reduction of dioxide. However, prior to reduction crude dioxide is refined by successive caustic leaching and neutralization steps mentioned above. 

Fig. 12. Morphology of Raney-nickel-coated cathodes for hydrogen evolution from caustic electrolytes (a) surface of Ni-Zn precursor coatings, (b) surface of Raney-nickel coating prepared by caustic leaching of the Zn content of the precursor, (c) cut through a Raney-nickel coating.

The principal advantage of skeletal catalysts is that they can be stored in the form of the active metal and therefore require no prereduction prior to use as do conventional catalysts which are in the form of the oxide of the active metal supported on a carrier. These catalysts can also be prepared on demand by a simple caustic leaching procedure. They have very high activity since the BET surface area (typically up to 100 m2g-1 for skeletal nickel and 30 m2 g l for skeletal copper) is essentially the metal surface area. Skeletal catalysts are low in initial cost per unit mass of metal and therefore provide the lowest ultimate cost per unit mass of active catalyst. The high metal content provides good resistance to catalytic poisoning. 

A model for the nano-structural evolution of Raney-type nickel catalysts (widely used in hydrogenation reactions) from the constituent intermetallic phases present in nickel-aluminium precursor alloys is presented here. Nano-porous nickel catalysts are prepared via a caustic leaching process where the NiAl alloy powder (typically 50-50 at.%) is immersed in concentrated NaOH solution in order to leach away the aluminium present to leave a highly-porous nickel catalyst (often referred to as spongy nickel). 

The mined zinc ores retrieved from the mines are too low in zinc content for direct reduction to refined metal thus, they are first concentrated. Production of concentrates requires crushing and grinding followed by gravity or magnetic methods of separation or flotation. These processes may be combined, depending on the complexity of the ore. A caustic-leach process is used to decrease the extent of metal loss during the concentration process. In this process, the metal is leached by caustic soda, the resulting electrolyte is purified with zinc dust and lime, and the zinc is electrodeposited. The crude zinc may be dissolved in sulfuric acid and purified by electrodeposition. 

FIGURE 26 Molten-Caustic-Leaching (Gravimelt Process) flow schematic. 

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