| Dicyandiamide is an important fine chemical material,which is widely used in pharmaceutical,electronics,textile,printing and other industries.Due to not complete of the productive process of decalcification,resulting in the subsequent dicyandiamide crystallization mother liquor containing Ca(OH)2,CaCO3 and other calcium compounds,Content of Ca2+ is about 1500mg·kg-1.The presence of impurities seriously affected the purity and quality of dicyandiamide.This article is based on the original production process,and introduce the ion exchange method to decalcification in the crystallization section,The optimum conditions were obtained by studying the ion exchange-crystallization coupling process.And finally,the content of Ca2+ in the dicyandiamide products has meet the stander of pharmaceutical grade dicyandiamide products.First of all,studing the adsorption capacity of Ca2+ in dicyandiamide crystallization mother liquor by comparing four kinds of cation exchange resins D001,001×4,001×7,001×8,the resuls show that the adsorption capacity of calcium chloride was the highest in 001×7 cation exchange resin,unit volume adsorption capacity of 68.15mg,and consider the economy,this article selected 001x7 type resin at the same experimental conditions.The static and dynamic exchange experiments of Ca2+removal in dicyandiamide crystallization mother liquor were carried out.The results of static experiments show that:reaction time was 80 min,temperature is 45-50 ℃,resin dosage is 40mL(porosity 0.42,the same below).when the stirring rate is 250r·min-1,200mL dicyandiamide crystallization mother liquor was processed.200mL dicyandiamide crystallization mother liquor was processed,Ca2+ removal rate reached to 98.12%,and the content of calcium ion in dicyandiamide crystal was 27.17mg·kg-1.The dynamic experimental apparatus was a glass column(a height of 600mm and an inner diameter of 40mm).100mL of resin was used to treat 500mL of dicyandiamide mother liquor.The mother liquor flow rate is 5mL·min-1,the resin bed height ratio is 5.3:1,the exchange temperature of the resin bed in the exchange column is 45-50℃.Under all this condition,decalcification rate has reached to 97.32%,making the calcium content of the mother liquor reached to 3.71mg·kg-1.At the same time while the through function of the process is obtained,and the dynamic process of the behavior of the simulation is used to reveal the characteristics of dynamic ion exchange.Based on the existing fractal porous media theory and,combining with dynamic ion exchange process,the calculation equation of the fractal dimension of the resin in the ion exchange process is obtained by the experimental data(correlation coefficient of 0.992).Using the slope of the equation,the fractal surface spectral dimension of the exchange process is ds=2.21,the mathematical model was obtained.By analyzing the relationship between the ideal surface and the fractal surface dynamics equation,the ideal surface is linear,fractal surface is a power function.At last,through the previous single factor experiment results and optimization of crystallization process by response surface methodology,the best process conditions was obtained.The results showed that cooling rate is 0.2℃·min-1.Stirring rate is 400r·min-1,seed grain size is 80 mesh,raising time is 40min.At the optimum condition,Ca2+ removal rate reached to 98.73%,the calcium content of the dicyandiamide crystal product is 18.75mg·kg-l.All these have achieve the stander of the content of Ca2+ at the pharmaceutical grade dicyandiamide products.All the results of this study provide the basis for industrial production of high quality dicyandiamide. |
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