Effectiveness of secondary copper electrolytic refining slime decopperization

The relevance of replacing the slime–H₂SO₄–H₂O system used for processing slimes from secondary copper electrolytic refining (SCER) with a slime–NH₃·H₂O–(NH₄)₂SO₄–H₂O system has been substantiated. Comprehensive studies of the characteristics of SCER slime samples were conducted. It was found that about 90 % of the copper is distributed between the Cu₂O phase and other phases, with a total copper content of 55.12 %. A new phase, Cu₄(OH)₆SO₄, corresponding to the mineral brochantite, was discovered, with a content in the slime of 6.40 %. Silver, with a concentration of 2.43 % in the slime, is present in metallic form at 69.1 %, with the remainder in the form of AgCl. The contents of associated components PbSO₄, BaSO₄, and SnO₂ are 13.52 %, 9.33 %, and 4.73 %, respectively. To substantiate the feasibility of low-temperature hydrometallurgical opening of the slime components and the conditions necessary for its implementation, determined by the specific qualitative and quantitative compositions of the slime, a thermodynamic analysis of the slime–NH₃·H₂O–(NH₄)₂SO₄–H₂O system was performed. This analysis allowed for the discovery and mathematical description of the dependencies of copper leaching indicators on the composition of the ammonia-ammonium mixture (ammonia buffer). A nomogram for the theoretical calculation of the minimum excess NH₃·H₂O/NH₄⁺ over the stoichiometrically necessary amount required for the complete formation of the copper ammine complex was constructed according to the equilibrium ammonia-ammonium solution's pH and copper concentration. Thermodynamic calculations determined the optimal composition and consumption of ammonia-ammonium solutions, as well as the characteristics of the leach pulp, such as the concentration of [Cu(NH₃)₄]²⁺ and the redox potential. Technological studies demonstrated the possibility of effective and selective extraction of copper from SCER slimes at a rate of no less than 99 % in the slime–NH₃·H₂O–(NH₄)₂SO₄–H₂O system, which was confirmed experimentally. Studies of the kinetics of copper leaching from slime in the slime–NH₃·H₂O–(NH₄)₂SO₄–H₂O system were conducted. The activation energy of the ammonia-ammonium copper leaching process from SCER slime (E_a = 5±0.25 kJ/mol) was determined within the temperature range from 15 to 45 °C at a total buffer system concentration [NH₃·H₂O] + [NH₄⁺] of 1 and 2 mol/L, as well as the order of reaction at a temperature of 24±1 °C, which is 0.24±0.02 and 0.91±0.05 for [NH₃·H₂O] + [NH₄⁺] concentrations above 1.5 mol/L and below 1.5 mol/L, respectively. A change in the kinetic mode of leaching with the limitation of the reaction rate by adsorption of reagents on the surface of solid particles to diffusion was detected when the total buffer system concentration [NH₃·H₂O] + [NH₄⁺] was reduced below 1.5 mol/L. The equation for the formal kinetics of the investigated process in the slime–NH₃·H₂O–(NH₄)₂SO₄–H₂O system was determined.

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