Leaching of manganese nodule in ammoniacal medium using ferrous sulfate as the reductant
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rous ammine is stable in the range of 9.5 to 9.8, 36 and its solubility increases with increase in the total concentration of ammonia and ammonium salt. 37 The reducing nature of ferrous ammine is exploited to reduce the metal oxides present in the nodule. Various overall chemical reactions taking place would be: MnO2 + 2FeSO4 + 2H20 + 4NH3
>MnO + 2(NH4)2SO4 + Fe203
S. ANAND, S.C. DAS, R. P. DAS, and P.K. JENA Leaching of manganese nodule for recovering copper, nickel, and cobalt in ammoniacal medium has been extensively studied. During earlier attempts, the ground nodules were subjected to reduction at high temperature 1-~2prior to leaching, similar to processing of lateritic nickel ores. 13'14 Studies were extended to leach copper, nickel, and cobalt values in ammoniacal solution at high temperature without any prereduction. 15-18Subsequently it was observed that reductive leaching in ammonia as well as acid medium could be done under aqueous conditions to recover metal values from nodules. Some of the reductants used for such studies in ammoniacal medium include cuprous ion together with carbon monoxide, manganous ion, organic reductants like glucose, formaldehyde, and carbohydrates, 19-29 whereas in acidic medium sulfur dioxide and charcoal have been used. 3~ The use of ferrous ion as a reductant in acid solution for manganese dioxide 33'34'35 is quite well known and can also be used for the nodule materials. Though ferrous iron has not been used as a reductant in ammoniacal solution, its capacity to form a soluble ferrous ammine 36 at particular pH values does render it a good reducing agent. This property of the ferrous ammine has been exploited in the present work to effect aqueous reduction for extraction of copper, nickel, and cobalt values from Indian Ocean nodules. The Indian Ocean manganese nodule samples were provided by the National Institute of Oceanography (CSIR), Goa, India. These were crushed, ground, and sieved to obtain 100 pct < 250/zm. The chemical analysis of the major constituents of the manganese nodule was 0.34 pct Cu, 0.56 pct Ni, 0.12 pct Co, 10.5 pct Fe, 13.0 pct Mn, and 25 pct moisture. 5 g sample was leached in a 250 ml conical flask, fitted with a condenser and a thermometer. The required quantities of ammonia, ammonium salt, and water were added and heated on a heater cum stirrer. Ferrous sulfate was added to the leachant when the desired temperature was attained. Samples were taken at ten-minute intervals. The samples collected from time to time were immediately filtered into a graduated cylinder containing a known amount of hydrochloric acid to avoid any precipitation of iron and manganese on keeping the solution. After required dilution of the sample solutions, the copper, nickel, cobalt, and manganese values were determined by a PerkinElmer atomic absorption spectrophotometer. Ferrous iron dissolves in ammonia-ammonium salt solution forming a ferrous ammine complex which on further oxidation precipitates as ferric hydroxide. The pH potential diagram of the NH3-Fe2+-SO4Z--H20 s
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