Investigation of a thermosetting method for the recovery of regenerating solutions of Na-cationite filters

In this paper, we analyzed a method for the thermochemical recovery of spent regeneration solutions of Na-cation exchange filters containing chlorides, calcium and magnesium ions, with the introduction of one precipitant (soda ash) and heating in an autoclave at a pressure of 0.2-0.41 MPa, as well as proposed on its basis the technology of recycling solutions of sodium chloride. Spent regeneration solutions after the process are still discharged into wastewater or directly into natural reservoirs. The concentration of chlorides in such effluents significantly exceeds the MAC for wastewater sent to water bodies (MAC = 350 mg/dm3), which affects the properties of water in natural water bodies. It should also be noted that the MAC of wastewater discharged after regeneration and washing of cation exchangers is also higher in terms of magnesium content (MAC Mg = 50 mg/dm3), and in regeneration solutions the concentration reaches 200 mg-eq/l. Known reagent methods for the recovery of sodium chloride solutions (alkaline, lime-sulfate, phosphate, oxalate, etc.) allow the precipitation of hardness salts only when they are used in combination, which leads to a multi-stage purification process, and also increases the number of pieces of equipment used. According to technical and economic indicators, these methods are not implemented in production. The method developed and proposed in the article can eliminate and prevent the discharge of spent sodium chloride solutions into water bodies while obtaining products suitable for sale, which will ensure the recycling of Na-cationite filter solutions and the low-waste technology for their purification. When developing the method, special attention was paid to indicators of residual hardness and filtering properties of the resulting suspension. Guided by the results obtained on the content of calcium and magnesium ions and the removal of the solid phase, the influence of the content of sodium carbonate and sodium chloride on the solubility of calcium and magnesium carbonates in aqueous systems is also considered: NaCl - CaCO3 - H2O, NaCl - CaCO3 - Na2CO3 - H2O and NaCl - (MgOH)2 CO3 - H2O и NaCl - (MgOH)2 CO3 - Na2CO3 - H2O in the temperature range 120÷140 0C.