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Leaching with Alkalies
Published in C. K. Gupta, T. K. Mukherjee, Hydrometallurgy in Extraction Processes, 2019
In practice, dissolution of nickel present as Fe-Ni alloy takes place in the first leaching stage at a redox potential of — 500 mV (as measured by silver electrode V/S silver chloride electrode) maintained by controlling the O2 input. In the subsequent stages, the redox potential is permitted to increase to about – 150 mV, this being sufficient to oxidize the sulfide form and convert the soluble ferrous ammonium carbonate complex [FeCO3·(NH4)2CO3] to ferric hydroxide. The sulfur compounds in the leach liquor are present mainly as sulfate, thiosulfate, and trithionate. The leach residue generated after thickening, washing, etc. goes for con- version to premium-grade iron ore. The pregnant liquor containing less than 1% Ni is subsequently treated for nickel carbonate precipitation and recovery of ammonia and CO2 for recycle.
Conventional Metal Recycling Techniques
Published in Hong Hocheng, Mital Chakankar, Umesh Jadhav, Biohydrometallurgical Recycling of Metals from Industrial Wastes, 2017
Hong Hocheng, Mital Chakankar, Umesh Jadhav
This thiosulfate is further completely oxidized to tetrathionate (Williamson and Rimstidt 1994, Schippers et al. 1996). Tetrathionate is then degraded to various sulfur compounds, that is, trithionate, pentathionate, elemental sulfur, and sulfite (Schippers et al. 1996, Druschel 2002, Schippers 2004, Druschel and Borda 2006). All these sulfur compounds are finally oxidized to sulfate in chemical and/or biological reactions.
Metal Recovery Processes
Published in C. K. Gupta, T. K. Mukherjee, Hydrometallurgy in Extraction Processes, 2017
The well-known Sherritt-Gordon process involving ammoniacal pressure leaching of a nickel-, cobalt-, and copper-bearing sulfide concentrate provides a fine example of the processing of the leach liquor by sulfide precipitation. In the process, copper separation is accomplished by its precipitation in a sulfidic form. A typical ammoniacal leach liquor contains 40 to 50 g/l Ni, 0.7 to 1 g/l Co, 5 to 10 g/l Cu, 120 to 180 g/l ammonium sulfate, 5 to 10 g/l S as thiosulfate and polythionate, and 85 to 100 g/l free ammonia. This type of solution cannot be subjected to direct H2 reduction, because in that case both nickel and copper metal would separate out simultaneously. It is, therefore, mandatory to make the solution copper-free first. It is also desirable to recover free ammonia for recycle. To fulfill these objectives, the solution is heated to 121°C in closed vessels at a total pressure of 0.93 MPa.84 As the temperature of the solution is increased, copper and nickel ammines dissociate to lower ammines and release ammonia, which is recovered. With the removal of ammonia, the unsaturated sulfur ions, thiosulfate (S2O32) and trithionate (S3O62−), undergoes combination reactions with cupric ions to precipitate copper sulfide. The reactions are shown here: () Cu2++S2O32−+H2O→CuS+2H++SO42− () Cu2++S3O62−+2H2O→CuS+4H++2SO42−
Review: Removal of Thiosalt/Sulfate from Mining Effluents by Adsorption and Ion Exchange
Published in Mineral Processing and Extractive Metallurgy Review, 2019
Bhagya M. K. Range, Kelly A. Hawboldt
Thiosalts occur as intermediates in the oxidation of sulfide ores, such as pyrite. Pyrite oxidation occurs via two major oxidants Ferric ion (Fe3+) and oxygen (O2) (Miranda-Trevino et al. 2013). Pyrite oxidation is a complex process and products will depend on the conditions of the solution. Thiosulfate (S2O32-) is considered to be the first thiosalt product of pyrite oxidation. Other sulfur species resulting from pyrite oxidation are trithionate (S3O62-), tetrathionate (S4O62-), and sulfate (SO42-) (Miranda-Trevino et al. 2013). Several studies have identified key factors in the generation of thiosalts in the mining process such as sulfur content in the ore, grinding and flotation pH, residence time in the mill, agitation rate, temperature, SO2 addition, dissolved oxygen in the grinding solution, air flow in flotation, and chemicals used in the system (Negeri et al. 1999; Miranda-Trevino et al. 2013).