Corrosivity Of The Second Brine In Chlor-Alkali Industry And Its Solution

In chlor-alkali industry, the target of the ion-exchange membrane electrolysis is high current efficiency and low voltage. The experiences have indicated that the quality of the brine plays an important role in the process of electrolysis. China have imported the ion-exchange membrane caustic soda technology from abroad for nearly 20 years. The level of operational control has been improved greatly, and the effect of the quality of the brine on electrolysis has been realized. The increase of the sodium chlorate concentration will accerlate the corrosion rates of carbon steel equipments and the electrolyzerslack, which imperceptibly decreases the competition of our enterprises. Many enterprises in china are lack of comprehension in negative effect of sodium chlorate content on chlor-alkali industry. Therefore, how to reduce the sodium chlorate content is an important project. In this paper, we investigate the effect of sodium chlorate content on the corrosion rate, and then seek the method to control the concentration of sodium chlorate. On the basis of above results, we design the equipments for this method. According to JUHUA's ion-exchange membrane caustic soda equipments' serious corrosion, the low current efficiency and the experiences of other enterprises, A reasonable conclusion can be drew that a high chlorate content may lead to above problems, which is also proved by the analysis results of the quality of brine. Considering the applicability of method in practice, we made a study on the reducing of chlorate. Through the tests we have obtained the following conclusions:1)The higher the reaction temperature, the faster and the higher decomposition rate of chlorate; 2) The higher acidity, the faster and the higher decomposition rate of chlorate; 3) At 91℃, acidity of 0.8mol/L or more (non-mixing), the chlorate decomposition rate is the fastest, and the decomposition rates of up to 70%; 4) Under the conditions of stirring, the chlorate decomposition is faster by 3 times than that under the unstiring conditions.On the basis of above experimental data and the requirements of practical production, we made an industrial design from seven aspects to reduce the chlorate content: 1) determine the process; 2) select equipments and their materials; 3) optimize equipment shape and mixing method; 4) determine the total amount of chlorate; 5) analyze the decomposition ability of chlorate; 6) design the tank to decompose chlorate ion; 7) calculate the area of heat exchanger.The equipements for chlorate decomposition were put into operation in 2008, which significantly reduced the chlorate content. It is found that the chlorate ion concentration decreases from 40 g/L to 6 g/L (or less). The decrease in chlorate ion concentration in the brine can improve the electric efficiency, the concentration of salt without the phenomenon of crystallization, and reduce the side-reactions and corrosion rates of equipments, and keep electric cell voltage stable. The experiences of practical operation indicate that the concentration of sodium chloride can increase from 300 g/L to 310 g/L, and that the electric efficiency of ion-exchange membrane electrolyzer can increase by 1.0%. The ammount of ionic membrane caustic soda can increase by 3800 t/y, thus obtaining the profits nearly 2.3 million yuan. In addition, the cost for equipment repair can reduce by more than 80 million per year after this techincal reformation. Furthermore, the cost for anti-corrosion or equipment replacement can also save over 1 million yuan per year.The chlorate decomposition tank nearly two years of practice in our work proves that the theoretical analysis and the design of industrial is realistic in chlor-alkali enterprises. It not only solve corrosion problems of equipment in our factory, but also provided a valuable approach to domestic chlor-alkali counterparts.