Preparation Of Chromium Oxide From Sodium Chromate Electrolytes

Chromium salts are one of the main varieties of inorganic salts and play an important role in the national economy.The pollution problems of chromium salts production are mainly concentrated in the front end of the chromium salts industry.The electrochemical oxidation and dissolution technology using ferrochrome as the raw material has changed the traditional production mode of chromium salts.It has the characteristics of short process flow,mild reaction conditions,strong controllability,high resource utilization and zero discharge of waste residue containing Cr(VI).About 1 g/L of silica in sodium chromate electrolytes prepared by electrochemical oxidation and dissolution technology of ferrochrome affects the quality of subsequent chromium oxide products.In order to prepare high-purity chromium oxide cleanly from sodium chromate electrolytes,the neutralization method using chromic acid as the acidifier to remove silica from sodium chromate electrolytes was carried out;the p H value control range of sodium chromate electrolytes was determined by measuring the titration curve of sodium chromate electrolytes titrated with chromic acid;hydrothermal reduction of sodium chromate electrolytes after silica removal with carbon black and thermal decomposition of chromium hydroxide were studied experimentally.The optimum parameters of silica removal by the chromic acid neutralization method,hydrothermal reduction with carbon black and thermal decomposition of chromium hydroxide were obtained respectively.The experimental results showed that:(1)The titration curve of sodium chromate electrolytes titrated with chromic acid showed that,when the chromic acid neutralization method was used to remove soluble silica from sodium chromate electrolytes,the p H value of sodium chromate electrolytes should be controlled between 7.50 and 10.00.(2)The optimum experimental conditions for removing silica from sodium chromate electrolytes by the chromic acid neutralization method were as follows: p Hvalue of sodium chromate electrolytes of 8.00,reaction temperature of 30 ?,reaction time of 16 h and stirring speed of 300 rpm.Under the optimum experimental conditions,the concentration of silica in sodium chromate electrolytes was reduced from about 1 g/L to about 200 mg/L.The purity of chromium oxide prepared from sodium chromate electrolytes after silica removal was 99.43%,which met the purity requirement of pigment grade chromium oxide(? 99.00%).(3)The optimum experimental conditions for hydrothermal reduction of sodium chromate electrolytes after silica removal with carbon black were as follows: reaction temperature of 280 ?,reaction time of 6 h,sodium chromate concentration of less than 2.40 mol/L and ratio of actual amount of carbon black to theoretical amount of1.4.Under the optimum experimental conditions,the reduction rate of Cr(VI)was99.99%.(4)The optimum experimental conditions for thermal decomposition process of chromium hydroxide were as follows: thermal decomposition temperature of 800 ?,thermal decomposition time of 0.5 h and heating rate of 10 ?/min.Under the optimum experimental conditions,the crystallinity of the obtained chromium oxide was good;the morphology was approximately spherical,without obvious agglomeration;the particles distribution was uniform,and the average particle size was 78.11nm;the color was bright green.In-situ purification of sodium chromate electrolytes was realized by removing silica from sodium chromate electrolytes with chromic acid.The technology of preparing chromium oxide by electrochemical oxidation and dissolution of ferrochrome had the characteristics of short process and high purity of products,which had great development and application prospects.