Oxidative ammonia leaching of pure zinc sulfide in the presence of lead compounds
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Solution Concentration, 1M H2SO4 HzSO 4
a2so 4
HCI
HC1
Conditions bulk, sweep voltammetry 5 mV/s (stirred) carbon electroactive paste, sweep voltammetry, 5 mV/s (unstirred) carbon electroactive paste, sweep voltammetry, 100 mV/s (stirred) carbon electroactive paste, sweep voltammetry, 5 mV/s (stirred) carbon electroactive paste, sweep voltammetry, 100 mV/s (stirred) Mean value Standard deviation
V
E1/2, v s SHE 0.51
M.K. G H O S H , S. ANAND, and R.P. DAS
0.49
Pure zinc sulfide is known to be resistant to leaching under oxidative ammonia leaching conditions, tll However, zinc sulfide associated with sphalerite concentrate, complex nmltimetal sulfide ores, and concentrates is easily leachable, t2-7] In an earlier investigation, it was established that the addition of small amounts (10 m g / 5 g of ZnS) of metal ions such as Cu(I), Cu(II), Ag(I), and Pb(II) enhances the rate of zinc extraction from pure zinc sulfide in the order Cu(I) = Cu(II) > Ag(I) > Pb(II). tsl Though copper is known to have a catalytic effect during ammonia leaching of sphalerite, t9'1~ metallic copper, t111 and chalcopyrite, u2j no studies are reported on the effect of lead during ammonia leaching of zinc sulfide. Since galena is invariably associated with naturally occurring sulfidic zinc minerals, the present study was taken up to know the effects of various lead compounds during zinc sulfide leaching. Pure zinc sulfide used during leaching experiments was Reidel-de-Haan analytical grade reagent. Galena mineral containing 98 pct PbS and 2 pct acid insolubles was crushed, ground, and sieved to obtain 100 pct < 4 5 / x m fraction. Pb304 and Pb(NOa)2 were of AnalaR reagent grade. Lead sulfate was prepared by the addition of sulfuric acid to lead nitrate solution. Leaching experiments were carried out in a Parr 2 L stirred autoclave equipped with automatic temperature controller and having provisions for gas inlet, outlet, and sampling. Ten grams of pure zinc sulfide were placed in the reactor and one liter of 3.7 molar ammonia (as NH4OH) having a pH of 11.6 was added, followed by the addition of the desired amount of lead compound. In order to avoid any oxidation during heating, the pressure vessel was flushed with nitrogen by raising its pressure to 303 kPa. Such flushing was done three times. Samples collected on attaining the temperature refer to zero hour samples. It took about half an hour to achieve 373 K in the reactor. Oxygen was introduced to the system after attaining the desired temperature. Samples were taken out at regular intervals and immediately filtered. Zinc was estimated volumetrically by Ethylene diamine tetraacetic acid (EDTA) method t131 and lead in the leach solution by an atomic absorption spectrophotometer. Reacted lead was determined by heating the known amount of leach residue with 20 pct ammonium acetate t14j solution and estimating dissolved lead volumetrically by EDTA. f131 During an earlier study, tsl it was observed that by varying the oxygen partial pressure in the range of 202 to 405 kPa and leaching te
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