Research On Application Of Raman Spectroscopy In Industry Crystallization

As a solid-liquid separation technology,crystallization is more difficult and more complicated compared to other chemical unit operation.Supersaturation is the driving force of crystallization processes.It is important to get supersaturation information during crystallization process.Raman spectroscopy is considered as a powerful tool for on-line monitoring technology.It can monitor real-time changes in crystallization and provide real-time information of supersaturation.However,a few studies have been reported in the literature on measuring changes in solid-liquid suspension using Raman spectroscopy. Based on the theoretic analysis of predicting solubility using Pitzer model,this work selects two experimental systems to research on the application of Raman spectroscopy in industry crystallization.One is crystallization process of producing potassium chloride by dissolving carnallite,the other is anti-solvent crystallization of potassium dihydrogen phosphate.For the crystallization process of producing potassium chloride by dissolving carnallite, the model for predicting the solubility of potassium chloride was developed by calculating solubility of potassium chloride in ternary system of KCl-MgCl₂-H₂O using Pitzer model under 25℃.And using literature data,the solubility prediction model of potassium chloride in the above system with different temperatures were established.Solubility of potassium chloride is correlated to concentration of magnesium chloride in the solution with the second polynomial.Based on the effect of concentration of magnesium chloride on the solubility of potassium chloride,the experiments for studying the mechanism of KCl from dissolving carnallite were arranged.It is experimenting proved that Raman spectroscopy can not be used on the on-line monitoring carnallite dissolving process.It means that Raman spectra can not apply to single bond such as Cl^- or substance which is composed of single bond.In order to get the crystallization mechanism of producing potassium chloride by dissolving carnallite,the other experimental method was used.Experiments about crystallization mechanism of KCl by dissolving carnallite were carried out with altering amounts of carnallite in the solution under the conditions of constant temperature 20℃,at a fixed stirring rate and unchanged amounts of water.The results showed that carnallite dissolved in the water was a process that potassium chloride dissolved firstly and then KCl crystallization appeared when certain supersaturation of KCl was attained.Raman spectroscopy has successfully researched on the anti-solvent crystallization of potassium dihydrogen phosphate.As the ternary system of KDP-water-acetone has not been reported yet,so this paper select this system to on-line monitoring solvent induced crystallization.The solubilities of KDP in four temperatures were measured firstly.The relationship between solubility of KDP and temperature is:solubility of KDP increased with increasing temperature;at the same temperature,it decreased with increasing concentration of acetone.The solubility equations with different concentration of acetone were attained by empirical equation and solubility data.The function relationship between acetone and temperature in this system using simplified Pitzer model was established. Based on solubility data,the calibration model of Raman spectra was created using a series of KDP-water-acetone ternary or binary solutions,and it can calculate the concentration of KDP.The crystallization processes were conducted by adding acetone into an initially pure aqueous KDP solution saturated at 25℃using Raman spectroscopy.Based on supersaturation profiles attained in crystallization process,the anti-solvent crystallization of KDP was studied.It can be concluded that the feed rate of the anti-solvent acetone has remarkable effect on the KDP crystallization process.No remarkable effect of feed mode was found for the experimental conditions used in this study.The results showed that Pitzer model can be effectively applied to prediction of solubility. And it also can be successfully applied to the two systems of KCl-MgCl₂-H₂O and KDP-acetone-water.The supersaturation can be monitored in anti-solvent crystallization process by Raman spectroscopy which can on-line monitor solute concentration. Consequently,the research and control of crystallization process can be carried out. Furthermore it has certain guiding significance for development of crystallization.