The alkalinity of industrial precipitation of calcium carbonate is actually a free alkali caused by some reason in our production process. Free alkali refers to a substance that exists in the form of calcium hydroxide instead of converting the product into calcium carbonate in the production of calcium carbonate.
If the alkali is too high, it will react with other plasticisers of the plastic, causing the plastic to be brittle, yellow, etc. It can be seen that free alkali is an important technical index in calcium carbonate products and must be strictly controlled in production.
The main reason for the high alkalinity of calcium carbonate is the formation of alkaline calcium carbonate.
1. Lime overburning: In the process of lime calcination, due to the different size of the block size and poor mastery, it is easy to overburn the phenomenon of lime. Over-burning lime has a lot of water for digestion, low water temperature and incomplete digestion, resulting in lime particles. The calcium carbonate produced during carbonisation is deposited on it with fine particles as the nucleus, forming calcium carbonate wrapped with calcium oxide particles. We know that calcium oxide crystallisation is a cube and calcium carbonate crystalisation is a morphobic hexahedron. The two crystals have different inter-crystalline angles, and different expansion coefficients occur after heating, resulting in grain fragmentation, making calcium oxide free and alkaline.
2. High and prominent free alkali in cold weather: alkaline calcium carbonate, especially in cold days, due to the low temperature and large solubility of calcium hydroxide, in the process of carbonisation, there are calcium hydroxide solids and water-soluble calcium hydroxide ions in lime milk, and then carbonisation reaction with carbon dioxide occurs in alkaline solution, so alkali is produced. Type calcium carbonate. This alkaline calcium carbonate changes with the temperature of the carbonisation solution and the intake of carbon dioxide, and is transformed into three different structures of calcium carbonate (calcite type, spirit stone type, and stone type). When carbonisation reaches the end, PH = 8~10, alkaline, and cannot destroy the alkaline calcium carbonate. These alkaline calcium carbonate have not yet had time to change and enter the next process. When entering the rotary dryer, the temperature rises and the alkaline calcium carbonate decomposes calcium hydroxide and calcium carbonate, making the alkalinity high. The alkalinity is high, and the cold weather is more prominent than hot days. The key is that the temperature is high in hot days, the water temperature is also high, and the lime digestion is good. At the same time, the temperature of the carbonisation tower is also relatively high, while the solubility of calcium hydroxide is low, which is not easy to produce plate alkaline calcium carbonate, which is easy to turn into calcium carbonate. It can be seen that the alkalinity of hot weather is lower than that of cold weather.