Treatment with chemical reagents for maintaining the flowability of the coal charge

Abstract

The main factors affecting coal freezing are considered. It is shown that the treatment of coal concentrates with chemical additives reliably prevents them from freezing in winter during transportation. Coal can be easily unloaded from railway wagons without high energy costs for heating in special garages. Calcium and magnesium chlorides (bischofite), as well as acetates of metals of group IA and IIA, have been investigated as anti-freezing agents. The method is based on the fact that the inter-cube moisture is replaced by a solution with a low eutectic freezing point, and the resulting ice is characterized by a defective structure, flaky structure and, as a result, low mechanical strength. In order to find new reagents, sodium, potassium, calcium and magnesium acetates, as well as a mixture of calcium and magnesium acetates, were investigated. Attention is focused on their physicochemical characteristics, methods of production and introduction into coal concentrate, as well as their impact on the freezing process. Our research shows that acetates match – and in some cases surpass – metal chlorides in their ability to prevent the freezing of coal. They are also minimally corrosive, have negligible environmental impact, and break down with the release of heat, which may ensure more uniform heating over the thickness of the batch. Metal acetates are safe for most surfaces, including concrete, metal and wood. They are biodegradable by bacteria, non-toxic and do not cause any noticeable corrosion of metals. Studies show that acetates are as good as, and in some cases better than, metal chlorides in preventing coal from freezing. It has been found that the freezing point depends on the way the salt was produced in the coal layer. In particular, if the salt production reaction produces gas, the freezing point is reduced by 1.2 °C. The results show that the best way to reduce freezing point is  by using mixed calcium and magnesium acetate, which can be obtained from natural dolomite, due to its synergistic effect. Thus, metal acetates are the best alternative to metal chlorides in preventing the freezing of coal on rail transportation in winter.