Electrochemical enhanced oxidative decomposition of chromite ore in highly concentrated KOH solution Zhonghang Wang a,b , Hao Du a , Shaona Wang a , Shili Zheng a,⇑ , Yi Zhang a , Seyeon Hwang c , Nam Soo Kim c , Tae Eui Jeong d a National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China b University of Chinese Academy of Sciences, Beijing 100049, China c The University of Texas at El Paso, El Paso 79968, USA d Seokyeong University, Seoul 136-704, Republic of Korea article info Article history: Received 5 October 2013 Accepted 5 December 2013 Available online 22 December 2013 Keywords: Electrochemical Chromite ore KOH Sub-molten salt abstract A novel method which introduces an electrochemical field to enhance the oxidative decomposition of chromite in a KOH sub-molten salt medium was proposed and proven to be feasible and efficient. Under optimal reaction conditions (slot current density 750 A/m 2 , alkali concentration 60 wt.%, reaction tem- perature 150 °C, alkali-to-ore mass ratio 6:1, and particle size <200 mesh), the extraction rate of chro- mium reached 99%, after reacting for 480 min. In comparison with the current liquid-phase oxidation technologies, the reaction temperature in the new approach is 150–250 °C lower, and the alkali concen- tration of the reaction medium is lower by more than 20%, showing substantial advantages in terms of energy efficiency, equipment corrosion alleviation and prospects for industrial application. The reaction kinetics study shows that the extraction process under optimal reaction conditions is jointly governed by surface chemical reaction and solid product layer diffusion with the apparent activation energy calcu- lated to be 17.56 kJ/mol. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Chromate production from chromite ore is an important metal- lurgical process but is also a source of major pollution. In the tra- ditional chromate production process, chromite ore is calcined with sodium carbonate at 1200 °C in a rotary kiln with the addition of limestone and dolomite. Due to the poor mass transfer efficiency inherent with the roasting process, up to 75% of chromium was recovered in subsequent leaching steps, causing a substantial waste of resources. Furthermore, large amounts of chromite ore processing residue (COPR, 2.5–3.0 tons per ton of chromate prod- uct), chromite dust, and waste gases are discharged during the roasting stage, creating serious environmental pollution. Although there have been many improvements to the roasting method, such as roasting with less or no calcareous additives (Antony et al., 2006), which improve the chromium yield to nearly 90% and re- duce the hazardous residue to 0.8 ton per ton of chromate product, the problems associated with high temperature roasting technolo- gies including low overall resource utilization, high energy con- sumption, as well as the environmental pollution due to the formation of hexavalent COPR remain unresolved. To optimize the chromate production process, many alternate methods, such as acid leaching (Li et al., 2011; Vardar et al., 1994), carbon reduction at high temperature (Chakraborty et al., 2010), and liquid-phase oxidation (LPO) (Kashiwas et al., 1974a), have been proposed, among which the LPO technology is regarded as the most promising alternative. In early reports regarding LPO, chromite ore was treated with molten NaOH–NaNO 3 medium or highly concentrated KOH–KNO 3 aqueous solution under oxidative conditions using air, oxygen or nitrate as the oxidant (Kashiwas et al., 1974b; Sun et al., 2009; Zhang et al., 2010). The main differ- ences between the roasting methods are, the oxidation reactions in these LPO approaches are pseudo-homogeneous or homogeneous, and the mass transfer efficiencies are much higher. Fundamental studies suggest that in the molten medium or highly concentrated alkaline solutions (namely the sub-molten salt medium, SMS), chromite could be oxidized by oxygen, air, nitrate, or the reactive oxygen species (ROS), which could be obtained via the pyrolysis of hydroxyl or nitrate ions and the oxygen reduction reaction (ORR) process (Chong et al., 2008; Jin et al., 2010; Sun et al., 2009). While ROS could cause mineral lattice distortion or catalytic oxidation of metal suboxides in the ore, the above characteristic are what contribute to the high reaction efficiency of LPO processes in comparison with the traditional roasting method. For example, in the NaOH–NaNO 3 binary molten salt medium, the chromite 0892-6875/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2013.12.009 ⇑ Corresponding author. Tel./fax: +86 10 8254 4858. E-mail address: slzheng@home.ipe.ac.cn (S. Zheng). Minerals Engineering 57 (2014) 16–24 Contents lists available at ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng