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INTRODUCTION
As an important chemical raw material widely used in many fields, sodium dichromate is a precursor of many chromium compounds (Wang et al. 2015), but the traditional chromic salts industry is facing a serious challenge because of its substantial environmental pollution. Compared with sulfuric acid methods and carbonization methods, the electrochemical synthesis process of chromate production has attracted greater attention as a green technology and is still in the development stage (Ahmed et al. 2001). It not only solves environmental pollution problems in the production of chromate, but also produces sodium hydroxide, hydrogen and oxygen by-products which have high economic value. The compounds of silicon and aluminium are the main impurities in the sodium chromate solution. Silicate ions react with other ions in the sodium chromate solution to form precipitation as desilication reaction products (DSPs). In the electrolysis procedure, DSP can deposit as scale on the electrolytic tank, affecting the continuous operation of the electrolytic process and the quality of the products, decreasing electrolysis efficiency. Therefore, only the removal of silicates from sodium chromate solution before the electrolysis stage can ensure the smooth progress of the entire follow-up process.
Chemical treatment (primary precipitation) is considered as a suitable method valuable to wastewater treatment (Indah et al. 2016). The removal of silicates from solution is mostly achieved by introducing chemicals which can react with silicate ions to produce insoluble compounds. At present, high capacity desilication agents that have been reported most are calcium compounds, including CaO (Zhao et al. 2002), Ca(OH)2 (Gu et al. 2002), 3CaO·A12O3·6H2O (Yuan & Zhang 2009), CaSO4·2H2O (Ma et al. 2011), Friedel's salt (Ma et al. 2009), etc. However, the reaction between sodium chromate and calcium ion will form the carcinogen of calcium chromate (Reyes-Gutiérrez & Rodríguez-Castillo 2009). Therefore, adding calcium salt for desilication is not applied for sodium chromate solution. Iron salts also can be used to remove the silicate (Jolsterå et al. 2010), which is expensive. There are two other methods of desilication: increasing the alkalinity (Groen et al. 2007a, 2007b; Jin et al. 2009) and increasing the temperature of the solution (Ma et al. 2009). However, it is difficult to adjust the pH value for the high alkali concentration in raw liquor of sodium chromate, so both methods...