Study On The Influence Of Calcium Sulfate Crystallization Under Microwave Radiation And The Special Effect

In this thesis, the characteristics of microwave heating and the applications of microwave in chemical engineering productions are introduced. After discussing the current research on the influence of microwave on the crystallization process, we present the importance and necessity to study such influence and then introduce the experimental study method on microwave non-thermal effect and the results of the study on microwave special effects.The main contents and innovation points of this thesis are as follows:First, based on the reaction of using vitriol and potassium chloride to get calcium sulphate, we studied the influence of different fields, especially microwave, on the crystallization of CaSO₄. Through special designed device, the same temperature rising has been achieved in microwave heating and traditional water bath. However, the structure and size of the calcium sulfate under microwave heating are quite different. Using XRD, we tested the materials scattered in the microwave oven due to explosion and found that there was a lot of calcium sulfate hemihydrate in the deposit under microwave heating. We measured the temperature rise at eight points at the beaker surface and found hot spots at the interface between the solution and the beaker wall. This is further proved by the picture obtained from the thermometer. This shows that even if the same temperature rising at the central point has been achieved in microwave heating and traditional water bath by means of stirring, we cannot guarantee the same temperature rising everywhere. The hot spot leads to the existence of calcium sulfate hemihydrate, which belongs to microwave special effect, rather that non-thermal effect.Second, based on the reaction to get calcium sulphate, we started and ended at the same temperature and then studied through experiment the difference of weight of calcium sulfate obtained through microwave heating and traditional water bath. We measured the temperature rise at the central point, and then calculate the weight of calcium sulphate theoretically under both conditions. We compared the calculated results with those from experiment and found good agreement between them, as the difference is within our expectation. This shows that it is not correct to prove the existence of non-thermal effect by comparing the productions under different conditions, even though the starting and ending temperatures are the same.Third, concentration has an obvious influence on the starting time of explosions. With the increase of the solution concentration, the starting time of explosion will decrease, i.e., explosion is easier to occur. Under the same concentration, the starting time of explosion will increase when stirring is engaged. If the power density of the microwave oven increases, explosion is easier to occur. The conductivity of solution has little to do with the starting time of explosion, which is related with the average density of the solution system.