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Precipitation softening method and method of water
What is the precipitation softening method of water? What are the methods?
The process of transforming calcium and magnesium ions in natural water into insoluble compounds and precipitating them to reduce water hardness is called precipitation softening of water.
There are two methods of precipitation softening: one is thermal softening method; One is chemical softening.
(1) Thermal softening method
Thermal softening method is to heat water to 100 ℃ (or above) and boil it. At this time, the "temporary hardness" in the water will be deposited from the water to form precipitation and removed.
The thermal method can only basically remove the "temporary hardness" in the water (it is impossible to completely remove it, because the generated CaC03 and № (OH) 2 still have a certain solubility), and can not remove the non carbonate hardness in the water. Therefore, the water treated in this way can not meet the requirements of the boiler for water quality. Moreover, this method is heated with steam, which consumes a lot of energy and has poor economy. It is generally rarely used.
(2) Chemical softening method
Chemical softening method is also called reagent softening method. It is to add chemicals to water to precipitate calcium and magnesium ions into insoluble compounds. Because the commonly used drugs in chemical softening method are lime, soda ash, caustic soda, trisodium phosphate, etc; According to the requirements of raw water quality and different water quality after treatment, one agent or several agents can be used at the same time, such as lime method, lime soda method, etc.
Chemical softening method is suitable for water treatment of feed water of low-pressure (no more than 1.5MPa) anhydrous cold wall boiler, circulating cooling water and industrial washing water in industrial production process; "Dealkalination" treatment of some water with high bicarbonate hardness.
The operator of chemical softening method has high labor intensity and poor working conditions, and the equipment will be blocked or worn due to the low purity of treatment reagent.
If the hardness of water is caused by calcium bicarbonate or magnesium bicarbonate, this hardness is called temporary hardness. After the water is boiled, the calcium bicarbonate contained in the water will be decomposed into calcium carbonate insoluble in water, and the magnesium bicarbonate contained in the water will form magnesium carbonate precipitation insoluble in water. When these precipitates are precipitated, the hardness of water can be reduced, so that the water with higher hardness can be softened. If the hardness of water is caused by sulfate or chloride of calcium and magnesium, this hardness is called permanent hardness. Permanent hardness cannot be softened by heating. Most natural water has both temporary hardness and permanent hardness. Therefore, generally speaking, the hardness of water refers to the sum of the above two hardness. Mineral water is drinking water rich in minerals beneficial to human body, while distilled water is pure water made by evaporation and condensation.
Among them, cooling water is the most important water consumption link, accounting for about 70% ~ 80% of the water consumption of the whole plant [2]. Therefore, the reuse of urban domestic sewage for industrial cooling water system or boiler make-up water and industrial water has become the research focus in recent years. Hardness control is very important in the treatment process of municipal sewage reuse for power plant make-up water. Among them, lime softening method is the most commonly used method. However, due to the insufficient source of high-purity lime, the use of inferior lime will lead to poor operation reliability, poor operation conditions, difficult automation, and a large amount of sludge after treatment, which is easy to block the pipeline. Since 1972, the Netherlands first developed fluidized bed crystallization technology. For decades, the United States, Japan, Belgium and other countries have continuously studied this technology. At present, it has been successfully applied to water softening, phosphorus removal and heavy metal treatment and recovery [4]. The method does not need to add any coagulation or flocculant. The crystallized product can be recycled and easily separated from water. In recent years, China has also started research in this area, but it is still in its infancy [5]. The key to water treatment by crystallization is the control of solution supersaturation. Therefore, the main purpose of this study is to combine precipitation and crystallization, determine the crystallization growth operating conditions of calcium carbonate, and carry out laboratory scale water softening crystallization by controlling supersaturation.
