Pachuca leach

Fig, 2.3. Pachuca leaching tanks at Western Reefs Uranium Plant, South Africa (Arden, T.V. Ref. 1). 

The leaching is 50—60°C without external heating. The cone-bottom tanks are equipped with a pipe from just above the solution level to near the bottom through which air is blown forming bubbles which lower the density of the slurry. These leach tanks, caHed Pachucas, are fairly efficient and are stiH popular although many plants employ mechanical agitators. Most plants use 3—5 tanks in series with acid and calcine being fed to the first and, in some cases, downstream from the first tank as weH. 

The pulp-water systems encountered in the agitation of paper stock represent a highly specialized case of solid-liquid agitation, discussed in a few papers (CIO, K5, 04), which will not be reviewed here. Lamont (L2) discusses the operation of pachuca tanks, used for ore-leaching operations, in which agitation results from air introduced at the tank bottom. 

The selective leaching of the nickel and cobalt from the gangue material is carried out in Pachuca-type leaching vessels with 98% sulfuric acid at temperatures between 450 and 500°F and with equivalent pressures between 400 and 600 psi. A typical leach liquor contains 6 gm/liter Ni, 0.6 gm/liter Co, 0.6 gm/liter Fe, 2.3 gm/liter Al, and 67 gm/liter SO4. Each batch is leached for about 90 min to extract 95% of the metal values. Preheating and heating of the ore slurry is by direct steam absorption in the reaction vessel. The barren residue is separated from the leach liquor by countercurrent decantation to a density of 55-60% solids. The solids are washed with very little dilution and about 99.5% of the soluble value are recovered. 

Immersion leach low solute leach rate. Product of residence time x concentration of leachant for acid leach = constant for a given particles diameter. Tends to have a separate leacher followed by system to separate and wash sohds. Pachuca plus CCD, settler particle diameter <70 pm sohds 30 to 60% w/w agitator 0.07 to 0.2 kW/m for CCD 1.5 to 2 kg liquid solvent carryover/kg inert solid. Autoclaves plus CCD particle diameter <70 pm solids 30 to 60% w/w 0.7 to 1.3 kW/m. ... 

Finely divided solids can be suspended in leaching solvents by agitation, and for batch operation a variety of agitated vessels are used (see Chaps. 6 and 11). The simplest is the Pachuca tank (Fig. 13.5), which is employed extensively... 

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