Technical introduction of industrial water softening equipment

How does the industry soften water?

There is a wrong consumption concept and awareness in our current understanding of household water treatment: as long as the “drinking” part meets the standard and other aspects of water use almost does not matter. In fact, in addition to drinking, domestic drinking water also includes eating, bathing, washing, and flushing toilets. In fact, one-third of the various substances in the water are absorbed into the human body through the skin through bathing, etc. Good water can improve the washing power of water, reduce the amount of washing powder, and reduce water pollution. Good water can also reduce the smell of toilet flushing and improve the indoor environment. Therefore, in addition to the “drinking” part, people’s water for bathing, washing, washing, etc. should also be clean, hygienic and free from pollution. Brown et al. studied the skin’s absorption of volatile organic compounds in water. According to adult drinking water 2 liters / day, infant drinking water 1 liter / day, the bathing time of both is 15 minutes / day, the skin absorption of common volatile organic compounds in drinking water and The oral intake ratio is 63/37 and 40/60 for adults and infants. Andelaman reported on indoor respiratory intake caused by trichloroethylene in drinking water. Based on the water consumption of 2 liters/person·day and the water consumption of 40-95 liters/person·day for bathing, the respiratory intake of trichloroethylene during the shower is several times the oral intake of drinking water.

Therefore, the harm to human health caused by harmful substances in water is not simply caused by drinking. According to foreign reports, the proportion of harmful substances in water absorbed by the human body is roughly: 1/3 is absorbed by the oral cavity; 1/3 is absorbed by the skin during washing and bathing; 1/3 is absorbed by the respiratory tract along with water vapor during bathing.

There are many places where water is used in industry, and different treatment methods are used according to the quality of the water used to achieve the required standards. The industrially common method of softening water is the ion exchange method.

Ion exchange water treatment refers to the use of ion exchangers to make the exchangeable ions in the exchanger and the aqueous solution produce reversible exchange in accordance with the amount of other substances, resulting in improved water quality but the structure of the exchanger is not substantial (chemical) Changing water treatment methods. In this water treatment method, only cations participate in the exchange reaction, called cation exchange water treatment; only anions participate in the exchange reaction, called anion exchange water treatment; both cations and anions participate in the exchange reaction, called cation and anion exchange Water treatment. Because the quality of raw water varies greatly, and the requirements for effluent water quality are diverse, there are many types of ion exchange and a combination of water treatment methods. These water treatment methods are used to soften the raw water, remove alkali and remove salt. When the ion participating in the exchange reaction in the ion exchanger is sodium ion Na+, this method is called sodium (Na) type ion exchange method, this exchanger is called sodium (Na) type cation exchanger, similarly, there is hydrogen (H ) Type ion exchange method and hydrogen (H) type cation exchanger, etc.

Sodium ion exchange method is the most common water treatment method for industrial boiler feed water. When the raw water passes through the sodium ion exchanger, the cations such as Ca2+ and Mg2+ in the water are exchanged with the Na+ in the exchanger, which reduces the hardness of the water and softens the water. Therefore, this method is also called sodium ion exchange softening method.

(1) Exchange process

carbonate hardness (temporary hard) softening process:
Ca(HCO3)2 + 2NaR——CaR2 + 2NaHCO3
Mg(HCO3)2 + 2NaR——MgR2 + 2NaHCO3
Non-carbonate hardness (permanent hard) softening process:
CaSO4 + 2NaR——CaR2 + Na2SO4
CaCl2 + 2NaR——CaR2 + 2NaCl
MgSO4 + 2NaR——MgR2 + Na2SO4
MgCl2 + 2NaR——MgR2 + 2NaCl
can also be expressed by an ionic formula combining the above reaction formulas:
Ca2+ + 2NaR——CaR2 + 2Na+
Mg2+ + 2NaR——MgR2 + 2Na+

(2) Regeneration process

In the sodium ion exchange process, when the soft water has hardness and the residual hardness exceeds the water quality standard, it is considered that the sodium ion exchanger has failed. In order to restore its exchange capacity, the exchanger needs to be regenerated (or reduced). The regeneration process is a process in which a sodium chloride (NaCl) solution containing a large amount of sodium ions passes through a failed exchanger layer to restore its exchange capacity. At this time, the sodium ions are absorbed by the ion exchanger, and the calcium and magnesium ions in the exchanger are replaced into the solution. The regeneration process of sodium ion exchanger can be expressed by the following reaction formula:
CaR2 + 2NaCl——2NaR + CaCl2
MgR2 + 2NaCl——2NaR + MgCl2

Common salt (NaCl) solution is often used as a regenerant in production. Because table salt is relatively easy to obtain, and the products (CaCl2, MgCl2) formed during the regeneration process are soluble salts, which are easily discharged with the regeneration solution. Most of the salt for regeneration uses industrial salt, and the impurity content should not be too much. The salt solution needs to be clarified and filtered before use. It is generally believed that the hardness of a 10% salt solution should not exceed 40mmol/L, and the suspended solids should not exceed 2%. When the ion exchanger is regenerated, a clarified 8-10% salt solution is generally used. The total regeneration contact time varies with the degree of crosslinking of the ion exchange resin. For a strong acid styrene cation exchange resin with a general crosslinking degree of about 7%, the total contact time between the regeneration agent and the resin should be at least 45 minutes.

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