What are the processes for preparing pure water, and what are the characteristics of each?

Distillation method

According to the distillation vessel can be divided into glass, quartz distiller, metal materials include copper, stainless steel and platinum distiller, etc. According to the number of distillations, it can be divided into primary, secondary and multiple distillation methods. In addition, in order to remove some special impurities, some special measures need to be taken. For example, adding some potassium permanganate in advance can remove easy oxides; adding a little phosphoric acid can remove trivalent iron; adding a little non-volatile acid can prepare ammonia-free water and so on. Distilled water can meet the water requirements of ordinary analytical laboratories. Because it is difficult to exclude the dissolution of carbon dioxide. Therefore, the resistivity of water is very low, not reaching the MΩ level. Cannot meet the needs of many new technologies.

Ion exchange method

There are two main preparation methods:

  • Multiple bed type, that is, connect and produce deionized water according to the way of cation bed-cation bed-cation bed-cation bed-mixed bed; this method is mostly used in the early days to facilitate resin regeneration.
  • Mixed bed type (2-5 stages in series), the effect of mixed bed deionization is good. But regeneration is not convenient.
    Ion exchange method can obtain more than ten MΩ deionized water. But organic matter cannot be removed, and TOC and COD values ​​are often higher than raw water. This is because the resin is not good, or the pretreatment of the resin is not complete, the oligomers, monomers, additives, etc. contained in the resin are not removed, or the resin is unstable and continuously releases decomposition products. All of this will be expressed in the form of TOC or COD indicators. For example, when the COD value of tap water is 2mg/L, the COD value of deionized water obtained after deionization treatment is often between 5-10mg/L. Of course, good results will be obtained when a good resin is used, otherwise it will not be possible to prepare ultrapure water.

The electrodialysis method

Produced in 1950, is often used as the pretreatment step of the ion exchange method due to its low energy consumption. Under the action of an external DC electric field, it uses an anion and cation exchange membrane to selectively allow anion and cation to pass through, so that part of the ions can migrate through the ion exchange membrane to another part of the water, so that part of the water is purified and the other part of the water is concentrated. This is the principle of electrodialysis. Electrodialysis is one of the commonly used desalination techniques. The purity of the produced water can meet the needs of industrial water. For example, using raw water with a resistivity of 1.6KΩ·cm (25°C) can obtain 1.03MΩ·cm (25°C) produced water. In other words, when the total hardness of the raw water is 77mg/L, the total hardness of the produced water is ∽10mg/L.

Reverse osmosis

Reverse osmosis is currently the most widely used desalination technology. Although reverse osmosis membranes existed in 1977, their large-scale production and extensive use in desalination have been a matter of recent years. The reverse osmosis membrane can remove inorganic salts, organic matter (molecular weight>500), bacteria, heat sources, viruses, suspended matter (particle size>0.1μm), etc. The resistivity of produced water can be increased by nearly 10 times compared with the resistivity of raw water.

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