Calcium carbonate as an important filler is widely used in PVC pipe processing and production, but many manufacturers of PVC have varying degrees of yellowing, brittle and other issues. The alkalinity of calcium carbonate has a great influence on the brittleness and yellowing of PVC pipes, and the alkalinity is often a factor that many manufacturers are easy to ignore.
The alkalinity of industrial precipitated calcium carbonate is actually the free alkali caused by some reason in the production process. Free alkali refers to the production of calcium carbonate, the product is not converted into calcium carbonate but in the form of calcium hydroxide.
If the alkalinity is too high, it will react with other plasticizers in plastics, causing brittle and yellowing of plastics. Therefore, 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 basic calcium carbonate.
In the process of lime calcination, due to the different sizes of lumps and poor mastery, it is easy to cause the phenomenon of over-burning lime. The over-burning lime uses more water to digest, and the water temperature is low and the digestion is incomplete, resulting in lime particles. The calcium carbonate produced at the time of carbonization is deposited on fine particles as crystal nuclei to form calcium carbonate-coated calcium oxide particles. We know that calcium oxide crystals are cubic and calcium carbonate crystals are orthorhombic hexahedron. The intercrystalline angles of these two crystals are different, and different expansion coefficients occur after heating, resulting in broken grains, freeing calcium oxide and showing alkaline.
Basic calcium carbonate, especially in cold days, due to the low temperature calcium hydroxide solubility, in the carbonization process, there are calcium hydroxide solid and water-soluble calcium hydroxide ions in lime milk, and then carbonization reaction with carbon dioxide occurs in alkaline solution, thus producing basic calcium carbonate. This basic calcium carbonate changes with the temperature of the carbonization solution and the amount of carbon dioxide introduced, and changes into three different structures of calcium carbonate (calcite type, Lingshi type, aragonite type). When the carbonization to the end, PH = 8~10, alkaline, can not destroy the basic calcium carbonate. These basic calcium carbonate has not had time to change and enter the next process. When entering the rotary dryer, the temperature rises and the basic calcium carbonate decomposes calcium hydroxide and calcium carbonate, making the alkalinity high.
Alkalinity is high, cold weather is more prominent than hot days, the key is hot days high temperature, water temperature is also high, lime digestion is better, at the same time the temperature of the carbonization tower is relatively high, and calcium hydroxide solubility is low, not easy to produce plate-like basic calcium carbonate, easy to convert into calcium carbonate. It can be seen that the alkalinity of hot weather is lower than that of cold weather.
Therefore, when processing PVC pipe fittings, we should not only pay attention to the particle size, whiteness, moisture, sedimentation volume, mineral elements, etc. of calcium carbonate, but also pay attention to the alkalinity of calcium carbonate.