Study On The Removal Of Calcium And Magnesium Impurity From Sodium Chromate Alkali Solution

Sodium dichromate is an important chemical raw materials,when manufacturing sodium dichromate by electrolyzing sodium chromate alkali solution,the polyvalent metal ions,especially calcium and magnesium ions,would penetrate the ion exchange membrane to form scal which block the pores of the membrane,it would result in shorting the service life of the membrane.Therefore,it is essential to refine sodium chromate alkali solution before electrosynthesis of sodium dichromate.In this study,after comparing several different methods of impurity removal,ion exchange method was chosen to investigate the basic problems in the process of removing calcium and magnesium ions by using chelating resin,which provided a detailed reference for further research.The effects of process parameters such as contact time,pH,temperature and resin dosage on process equilibrium were studied.The obtained optimum conditions were T=50?,pH=8.5,t=6 h and resin dosage=1.0 g/100 mL.The desorption experiment showed that when the temperature was 35?,the concentration of HCl was 1.0 mol/L HCl and the reaction time was 3h,the desorption of calcium and magnesium ions were completed.After recycling used 8 times,the adsorption capacity of the resin remained above 90%,indicating a good performance for reusing.The behavior of the resin adsorption process was studied.The Langmuir,Freundlich,Temkin and Dubinin–Radushkevich(DR)models were used to fit single solute sorption data.The Langmuir model was the best,indicating the major adsorption was single molecular layer adsorption.The maximum ion exchange capacity was found to be 34.32 mg/g for calcium and 22.26 mg/g for magnesium.The Sheindorf-Rebhun-Sheintuch(SRS)model,the extended Freundlich model(EFM),the modified extended Langmuir model(MELM)and the IAST-Freundlich model were used to fit bi-solute sorption data.The SRS model fitted the data better than the others,the inhibition effect of calcium on magnesium was greater than that of magnesium.According to the MELM model,the sorption of one metal ion was suppressed by the presence of competing metal ion.The thermodynamic of the adsorption process was investigated.The positive?H~0 value indicated endothermic sorption processes.All the sorption processes are spontaneous in nature according to the negative values of?G~0.The kinetic data were tested using Lagergren-first-order,pseudo-second-order,Elovich and Particle diffusion kinetic models.Kinetic data correlated well with the pseudo-second-order kinetic model,indicating that the chemical adsorption was the rate-limiting step.The activation energy of ion-exchange(Ea)was determined as 49.46 kJ/mol for calcium and39.84 kJ/mol for magnesium according to the Arrhenius equation.Effects of flow rate,initial ion concentration and column height on the adsorption breakthrough curve had also been investigated.The breakthrough time increased with column height but decreased as initial ion concentration and flow rate increased.Specifically the bed depth service time(BDST)and Thomas models were applied to the experimental data to predict the breakthrough curves and determine the column parameters,the results showed that the exchange capacity increased with initial ion concentration and column height but decreased as flow rate increased.In the mixed solution containing sodium chromate,the sorption of calcium and magnesium was suppressed by the presence of sodium chromate,the breakthrough curves were shortened too.