Investigations into high temperature separation of antimony from metal oxide varistors Toni Gutknecht a, * , Christer Forsgren b , Britt-Marie Steenari a a Industrial Materials Recycling, Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Kemiv€ agen 4, 412 96 Gothenburg, Sweden b Stena Metall AB, Fiskhamnsgatan 8D, Box 4088, 400 40 Gothenburg, Sweden article info Article history: Received 7 February 2017 Received in revised form 12 May 2017 Accepted 5 June 2017 Available online 12 June 2017 Keywords: Antimony Characterisation Recycling Reduction Separation abstract Very little research has been done into methods for recycling antimony from end of life products. This study investigates separation of antimony from metal oxide varistors (MOVs), where antimony is present between 3 and 5 wt%. In MOVs, antimony is found in spinel (Zn 2.33 Sb 0.67 O 4 , Zn 7 Sb 2 O 12 ) and pyrochlore (Zn 2 Bi 3 Sb 3 O 14 ) compounds. Due to the low concentration of antimony in MOVs, a leaching pretreatment using dilute sulfuric acid was performed where antimony remained in an insoluble solid. The leaching pretreatment resulted in a more than fivefold increase in the antimony concentration. In order to separate antimony from the MOV leaching residue, it was subjected to a simple thermal treatment and carbothermal reduction at temperatures of 500e1300  C. Results showed that 80% of antimony can be separated from the leach residue by heat treatment at 1100  C, while 85% of antimony can be separated from the leach residue at 1100  C using carbothermal reduction. Once separated, antimony can be recovered from the gaseous phase due to its volatile nature. Results show that antimony recycling from MOVs is a promising source of secondary antimony. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Antimony is a strategic and critical metal due to its supply, availability and increased use in consumer goods (CRM-EU, 2010). Antimony is listed as a critical metal for the European Union (CRM- EU, 2010) as well as a strategic metal for the United States Department of Defense (NDS, 2009). The European Union is entirely dependent on imports of antimony, though total EU consumption in 2007 (792 tonnes) only accounted for 0.5% of global production (CRM-EU, 2010). It is predicted that, apart from antimony, a na- tional security emergency in the US would entail a severe shortage of just three other metals (NDS, 2009). In 2013 the US imported 25,000 metric tonnes of antimony ores and concentrates, oxides and metal for consumption (Guberman, 2014). In total, 85% of the antimony used in the US was imported, predominantly from the People’s Republic of China (74%), Mexico (10%) and India (7%). In 2008 a total of 187,000 tonnes of antimony was produced, 91% of which was produced in China. It is predicted that there will be a large deficit of antimony by 2020 (CRM-EU, 2014), and antimony will reach its peak production by 2018 (Sverdrup et al., 2017). The amount of extractable antimony remaining is 1.8 million tonnes (Carlin, 2013), while the amount extracted up to 2012 was 6.7 million tonnes (Sverdrup et al., 2017), which means that only 21% of antimony remains for extraction. However, the recoverable amount of antimony is 7 million tonnes, not including undiscovered extractible antimony (Sverdrup et al., 2017). Antimony is typically present in small amounts in industrial and commercial products, thus making recycling difficult (Graedel and Reck, 2014). Conventional recycling routes are typically not appli- cable for rare metals such as antimony if no prior pretreatment step(s) are performed (Rombach and Friedrich, 2014). Pretreatment is performed in order to increase the antimony concentration or to allow for proper separation of metals in further hydro- or pyro- metallurgical recycling steps. Antimony recycling in the USA is currently limited to recycling of lead-acid batteries where the secondary antimony is put back into the construction of new lead- acid batteries. In the Netherlands about 375 tonnes of antimony is recycled per year while annual consumption is approximately 1300 tonnes (van Velzen et al., 1998). Over 50% of antimony used is as a flame retardant (Kirk-Othmer, 1992) in, for instance, plastics, curtains, etc. Many items containing antimony end up as municipal solid waste (MSW) having an * Corresponding author. Contents lists available at ScienceDirect Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro http://dx.doi.org/10.1016/j.jclepro.2017.06.033 0959-6526/© 2017 Elsevier Ltd. All rights reserved. Journal of Cleaner Production 162 (2017) 474e483