Studies on the Recovery of Secondary Antimony Compounds from Waste
The global supply of antimony is dominated by one country which has led the European Union to classify antimony as a critical raw material. Other than the rare-earth elements antimony has the highest supply risk. Therefore, a feasibility study on the recycling of antimony and antimony compounds is of great importance. This research focuses on two types of antimony containing wastes: (1) metal oxide varistors (MOVs) and (2) municipal solid waste incineration (MSWI) fly ash to determine if it is possible to recover secondary antimony. Recovery of antimony is highlighted because it is a critical element and the known natural resources are rapidly becoming depleted.
The MOVs contain a significant amount of antimony but have minimal waste volumes. Characterization of the MOV determined the microstructure of the MOV contained the three major phases (1) the zinc rich phase dominated by ZnO grains, (2) the antimony rich phase containing Zn2.33Sb0.67O4, Zn7Sb2O12, and Zn2Bi3Sb3O14 compounds, and (3) the bismuth rich phase made up of Bi2O3. To increase the antimony concentration, the MOV was pulverized and leached in a pre-treatment step. The pretreatment step removed the bulk ZnO grains from the MOV resulting in a zinc-sulfate leachate and antimony rich insoluble residue having a fivefold increase in antimony concentration. Removal of the minor metals such as cobalt, manganese, and nickel from the zinc sulfate leachate was done by activated cementation using Cu/Sb as activators and zinc dust as the reducing agent. It was determined for the MOV leachate system that the optimized cementations conditions for copper concentration, antimony concentration, zinc dust addition, temperature and pH were found to be 0.8 g·L-1 Sb, 0.4 g·L-1 Cu, T = 40 °C, 0.2 g·L-1 Zn addition in a solution of pH 5.0. A two-step batch cementation process resulted in 98 % removal of cobalt and 100 % removal of nickel making the purified zinc sulfate leachate suitable for zinc electrowinning. The antimony rich leach residue was subjected to heat treatment as well as carbothermal reduction in nitrogen atmosphere. Findings suggest that thermolysis of the MOV leaching residue and separation of antimony was not possible below 1000 °C. However, carbothermal reduction of the MOV leaching residue showed it was possible to separate antimony at temperatures between 700 – 825 °C by the decomposition of Zn7Sb2O12. In this process antimony oxide, Sb4O6(g), was separated from the MOV residue leaving zinc oxide and other metals including bismuth, cobalt, manganese, and nickel. The volatilized Sb4O6 can be recovered through condensation from the gaseous state.
Two MSWI fly ash samples were used (1) untreated and (2) HALOSEP-treated fly ash to determine the amount of antimony which could be extracted from MSWI fly ash as a function of pH. The HALOgen SEParation (HALOSEP) process is used to remove water soluble salts such as NaCl, KCl, CaCl2, and MgCl2 and to alter the leaching properties of the fly ash with respect to metals such as antimony, lead, and zinc. Results showed that the maximum amount of antimony that could be extracted from the ashes was approximately 20 % using a pH 1 HCl solution with L/S of 20. Increasing the pH gave lower antimony yields. It was determined that very little of the antimony could be recovered or removed from the fly ash, which makes commercialization of such a process difficult. However, leaching did cause the concentration of antimony in the dry residue to increase due to dissolution of bulk minerals, which suggests a two-step leaching process could be preferable for antimony recovery.
Municipal Solid Waste
Metal Oxide Varistor