Abstract:
In an earlier work (Das et al., 1995a) it was established that Ni(II) precipitates as NixMn(1-x)(NH3)(4)S2O6.2H(2)O (where x varies between 0.05 and 0.25) from the Ni(II)-NH3-SO2-MnO2 system. In the present work it is attempted to determine conditions for minimum precipitation of nickel by varying parameters such as time, reaction temperature (80-120 degrees C), storage temperature (10-30 degrees C) ammonia concentration (20 to 100 g/l) and ammonium sulphate concentration (20 to 200 g/l) The standard conditions maintained were: Ni 1 g/l, NH3 80 g/l, (NH4)(2)SO4 80 g/l, (NH4)(2)SO3 57.1 g/l, pulp density 3.165%, reaction temperature 120 degrees C and storage temperature 30 degrees C, while the above-mentioned parameters were varied one at a time to study its effect. Nickel precipitation decreases from 66 to 16% with the increase in ammonium sulphate concentration from nil to 80 g/l and remains the same when further increasing its concentration to 200 g/l. Precipitation of nickel decreases from 39 to 7% with the decrease of ammonia concentration from 100 to 40 g/l on storing for 68 h at 30 degrees C. Increase in initial nickel concentration from 1 to 3 g/l results in increase of nickel precipitation from 6.5 to 31.5%. The minimum nickel loss (0-6.5%) conditions for the slurry obtained at 120 degrees C were: NH3 less than or equal to 40 g/l, (NH4)(2)SO4 greater than or equal to 80 g/l, NI(II) 0.75-1 g/l, storage time between 0 and 68 h and storage temperature 30 degrees C. The Eh of the solution for minimum precipitation (0-6.5%) varied between -150 to -200 mV with corresponding pH variation between 9.9 and 10.5.