Kinetics study of selenium removal from copper sulfate–sulfuric acid solution Mohammad Mokmeli ⁎, Berend Wassink, David Dreisinger University of British Columbia, Department of Materials Engineering, 6350 Stores Road, Vancouver, BC V6T 1Z4, Canada abstract article info Article history: Received 8 November 2012 Received in revised form 15 March 2013 Accepted 29 June 2013 Available online 13 July 2013 Keywords: Biselenate Kinetics Cuprous Copper sulfate Selenium The chemistry and kinetics of the removal of selenium from copper sulfate–sulfuric acid solution by cuprous ion reduction and precipitation were studied. Continuous measurement of [Cu + ] was carried out by potenti- ometric monitoring. The stoichiometry of the overall reaction was found to be: HSeO 4 − + 9.96 Cu + aq + 7H + = Cu 1.98 Se + 4H 2 O + 7.98Cu +2 at 95.1 °C. The reaction mechanism appears to be based on an initial one electron transfer to form intermediate Se(V). The kinetics fit the rate law: −d Se VI ½  dt ¼ k1 Cu þ ½  2 Se VI ½  k −1 k 2 Cu þ2 ½ þ Cu þ ½  at constant acidity. The values of k 1 and k −1 /k 2 were 0.0055 M −1 s −1 and 1.5 × 10 −4 at [H 2 SO 4 ] = 100 g/L, [Cu +2 ] = 50 g/L at 95.1 °C. Rate constants are almost independent of cupric concentration at constant ionic strength but decrease slightly with increasing cupric concentration and increasing ionic strength. Activation ener- gy of 85.5 kJ/mol for k 1 was calculated by varying the temperature from 86.1 to 98.6 °C. Cuprous selenide precip- itates react with cupric ion and generate cuprous ion and non-stoichiometric Cu x Se precipitate (1.88 b x b 1.94). Increasing solution acidity significantly increases the biselenate reduction reaction rate. An empirical correlation of k 1 = 0.0058 [H 2 SO 4 ] 1.25 was found between the sulfuric acid concentrations in the range of 10 to 200 g/L and k 1 . The biselenate reduction reaction was also investigated in the non-complexing perchlorate medium to study the effects of acidity and ionic strength without the complication of weak dissociation of HSO 4 − and ion pairing effects. Cuprous is oxidized slowly by perchlorate species. The rate of cuprous oxidation was first order with respect to cu- prous concentration ([Cu + ] = [Cu + 0 ]×e −kt ). This rate equation was used to modify the rate of biselenate reduc- tion reaction in perchlorate medium. An empirical correlation of k 1 = 0.0159 [H + ] 0.9 was found between the H + concentrations in the range of 10 to 150 g/L HClO 4 and I = 4.35 m at 95.1 °C. Biselenate removal rate increases significantly by increasing the ionic strength of the solution. k 1 values vary from 0.0075 to 0.0412 M −1 s −1 in so- lutions containing 0 to 3 M NaClO 4 at 95.1 °C and [HClO 4 ] = 100 g/L, [Cu(ClO 4 ) 2 ] = 50 g/L. Linear correlation be- tween log k 1 and solution nominal ionic strength was obtained as log k 1 = −9.135 + 10.800 × [I 1/2 /(1 + I 1/2 )] in perchlorate medium at 95.1 °C. Despite the results achieved in the perchlorate medium, biselenate removal rate decreases in sulfate bearing solutions by addition of sulfate based salts. This almost certainly has to do with decreasing free acid levels due to formation of bisulfate anion. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The presence of selenium in the base metal sulfide minerals and pyrometallurgical products results in contamination of leach solu- tions and electrolytes in hydrometallurgical treatment of sulfide ores and residues (Zigaro and Cooper, 1974). Contamination of cop- per metal by selenium affects the physical properties of electroplated copper such as annealability and the recrystalization temperature (Cooper, 1985). For instance, wire-bars produced from cathodes with selenium content between 6 and 10 ppm contained cracks (Charles and Hannaert, 1970). Hence, selenium concentrations must be kept to less than 1 ppm in grade 1 Electrolytic Copper Cathode according to ASTM B115-00, 2004 specification (Se + Te + Bi less than 3 ppm) and accordingly must be maintained much lower than 1 mg/L in electrolyte. Cuprous may be used to remove impurities at higher oxidation states such as selenate and selenite from solutions containing cupric ion. Metals such as Fe, Ni, Co and Zn also have the possibility of reduc- ing selenate/selenite to selenide (Mondal et al., 2004). However, in cupric-bearing solutions, couples with electrochemical potentials below that of the cupric–copper couple will precipitate the copper as well. Distinct advantages of cuprous generated by reaction be- tween cupric salts and copper metal are as follows. It is an environmentally clean process (compared to the SO 2 /HSO 4 − oxidation half reaction), it adds no impurities to copper EW electro- lytes (the only soluble reactant and final product is cupric ion), the re- duction reaction effectively occurs at low pH (refined solution may transfer to copper electrowinning tank house without further acidity adjustment) and finally, the reduction reaction occurs below the Hydrometallurgy 139 (2013) 13–25 ⁎ Corresponding author. Tel.: +1 604 822 1357; fax: +1 604 822 3619. E-mail address: mokmeli@alumni.ubc.ca (M. Mokmeli). 0304-386X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hydromet.2013.06.016 Contents lists available at ScienceDirect Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet