Calcium carbonate as an important filler is widely used in the processing and production of PVC pipes, but the PVC produced by many manufacturers has various degrees of yellow, brittle and other problems. The alkalinity of calcium carbonate has a great impact on the brittleness and yellowing of PVC pipes, and the alkalinity is often a factor that is easily ignored by many manufacturers.

The alkalinity of industrial precipitated calcium carbonate is actually the free base caused by some reason in the production process. Free base refers to the substance in the production of calcium carbonate, the product is not converted into calcium carbonate but exists in the form of calcium hydroxide. The alkalinity is too high, it will react with other plasticizers of plastics, causing plastics to be brittle and yellow. Therefore, free alkali is an important technical index in calcium carbonate products, which must be strictly controlled in production.

The main reason for the high alkalinity of calcium carbonate is the formation of basic calcium carbonate.

Lime overburning

In the process of lime calcination, due to the different size of the block, poor control, it is easy to make the lime overburning phenomenon, overburnt lime digestion water, low water temperature digestion is incomplete, resulting in lime particles. The calcium carbonate produced during carbonization takes fine particles as crystal nuclei and is deposited on them to form calcium carbonate coated with calcium oxide particles. We know that calcium oxide crystallizes as a cube and calcium carbonate crystallizes as a rhemihedron, and the intercrystalline Angle of these two crystals is different, and different expansion coefficients occur after heating up, resulting in grain breakage, which makes calcium oxide free and obvious alkaline.

hyperalkaline

Basic calcium carbonate, especially in cold days, due to the low temperature of calcium hydroxide solubility, in the carbonization process, there are calcium hydroxide solids and water-soluble calcium hydroxide ions in the lime milk, and then carbonization reaction with carbon dioxide, which occurs in an alkaline solution, so the basic calcium carbonate is produced. This basic calcium carbonate changes with the temperature of the carbonizing liquid and the amount of carbon dioxide injected, transforming into three different structures of calcium carbonate (calcite type, spirit stone type, aragonite type). When carbonized to the end point, PH=8 ~ 10, more alkaline, can not destroy the basic calcium carbonate. These basic calcium carbonate has not had time to change, enter the next process, when entering the rotary dryer, the temperature rises, the basic calcium carbonate decomposes calcium hydroxide and calcium carbonate, so that the alkalinity is high.

High alkalinity, cold weather is more prominent than hot days, the key is hot days high temperature, high water temperature, lime digestion is better, while the temperature of the carbonization tower is also relatively high, and calcium hydroxide solubility is low, not easy to produce plate basic calcium carbonate, easy to convert into calcium carbonate. Thus, the alkalinity of hot days is lower than that of cold days.

Therefore, when processing PVC pipe fittings, we should not only pay attention to the particle size, whiteness, moisture, settling volume, mineral elements, etc., but also pay attention to the alkalinity of calcium carbonate.