| The chromic salts production in China accounts for～35% of the total yield in the world. Calcium-free oxidative roasting process is the technological tendency of the chromic salts production for environmental protection. Sodium vanadate is easily dissolved into the solution together with sodium chromate, which results in the increase of the concentration of V2O5 upto 1g/L. And then, the existence of vanadium in chromic product will change chromate color and deteriorate its using properties, such as electroplating, catalysts, etc. Therefore, it's necessary to remove the vanadium before conversion into sodium dichromate. Furthermore, removal of vanadium from chromate solution both meets the needs of chromate production and acts as vanadium resources.Precipitation n ion exchange,solvent extraction or oxidation-reduction are usually employed for extraction vanadium from solution. However, the removal of vanadium is preferably carried out by addition of calcium oxide to the sodium chromate solution. Now the main problems are:1) much NaOH is added to adjust pH from 7 to 12, leading to more sodium sulfate discharged in the evaporation operation; 2) much lime is added (the molar ratio of [CaO]/[V2O5]> 30, more than ten times the theoretical amount), and much residue was formed, leading to large losses of sodium chromate (>20%) and serious environmental problems.In order to overcome these above problems, the reaction behavior of vanadium in the process of chromic salt production was investigated based on the calculation of reaction Gibbs free energy and equilibrium solubility, and the formation principle of calcium vanadate and influences on vanadate removral rate, as well as the structure of vanadate, were studied by experiments. The method for recovering valuable metals from vanadate residue was also explored.The thermodynamic calculation indicated that VO3-, CO32-, SO42-, VO43- and CrO42- could sequentially be precipitated with addition of lime at 298-373K. Meanwhile, the mutual transformation occured between calcium salts, for example Ca3(VO4)2 and CaCrO4 could react with CO32-, and CaCrO4 could be converted to Ca3(VO4)2 in the existence of VO43-. In addition, the formation of CaCO3, CaCrO4, CaSO4·nH2O, as well as Ca3(VO4)2, Ca2V2O7 may account for the more lime in the removal of vanadium. The IR structural studies on pure vanadate solution indicated that some VO43- converted to V2O74- with the pH decreasing from about 13 to 9 and lime could be added to remove vanadium from the vanadate solution.The experiment results showed that improving the solution pH value, reaction temperature, the molar ratio of CaO/V2O5, V2O5 concentration or adding active lime favored the removal of vanadium, but increase of concentration of sodium chromate, SO4- or CO3- was disadvantageous to the vanadate removal. And results also showed that the vanadate removal rate could reach 85% under the following optimized conditions:the molar ratio of [CaO]/[V2O5] 18 (adding lime) or 9(adding active lime slurry), pH-10, T-80℃and t-30min. The vanadate residue was comprised of various calcium salts, including CaCO3, Ca(OH)2, Ca5(CrO4)3(OH), Ca0.5V3O5, CaV3O7 and Ca2V2O7 with excess lime added. Industrial results in removal of vanadium were in agreement with lab experimental results and the product quality and properties were conformed to the national standard.The results of decomposition of carbonate showed that increase of the temperature, lower pH value, strengthening mixing, lower partial pressure of CO2 in the gaseous phase or reducing the concentration of sodium chromate was advantageous to the carbonate decomposition in the solution. In the industrial experiment, the concentration of Na2CO3 dropped less than 4g/L under the conditions:T-85℃, pH 7.4～7.8, t～60min.Meanwhile, the technology of separation vanadate from chromate was also come up with by hydro-treating the industrial vanadate residue.
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