Precipitation of magnesium ions from aqueous solutions by reagent method [in Ukrainian]

Current trends in negative climate and environmental changes force us to pay more attention to environmental protection and the future of humanity. The hydrosphere, which is perhaps the most sensitive to discharges of various pollutants, suffers especially from human anthropogenic activity. Untreated or insufficiently treated wastewater, when entering surface water bodies and underground horizons, significantly reduces the quality of water in them and prevents their direct consumption and use. At the same time, along with a mass of various pollutants, relatively non-toxic calcium and magnesium compounds are discharged into the components of the hydrosphere, which significantly increase the hardness of water and its mineralization. Increased levels of hardness have a negative impact on both individual human organs and the technological equipment of industrial enterprises. The classic technology for reducing the content of hardness salts today is considered to be the reagent soda-lime or soda-sodium technology. The main disadvantages of these technologies are insufficient efficiency at normal ambient temperatures and the need to use a significant amount of reagents. The most widespread today is the ion-exchange technology of water softening, but its use is accompanied by the discharge into the hydrosphere of significant volumes of regeneration solutions of sodium chloride with impurities of calcium and magnesium compounds. This leads to the formation of an anthropogenic cycle of these compounds. Therefore, there has always been a need to develop and implement environmentally safe water softening processes. One of such promising processes for reducing the content of hardness salts is electrochemical technology today. It allows you to regulate the pH without the use of chemical reagents and transfer calcium and magnesium ions to a solid state. Our studies have shown that even when maintaining the hydrogen indicator in the cathode compartment at a level of 11.5-12.5, magnesium ions are not always quickly hydrolyzed and can be removed from the technological process. The reason for this situation is the long incubation period of magnesium hydroxide formation and the dependence of the properties of the formed solid particles on the conditions of their deposition. Since for diluted solutions at pH ≤ 11.3 the solid phase is not visually fixed at all, the range of studies was limited to the range of 11.5-12.5. The settling of the suspensions obtained during precipitation showed the absence of a significant effect of pH on the settling rate and physicochemical properties of the precipitate. No significant effect of the pH of precipitation was recorded on the parameters of separation of the solid phase by filtration. The effect of temperature was more diverse. At temperatures of 10 °C at the time of decantation of the solutions, the solid phase is not visually fixed at all. During settling, turbidity of the solution and the formation of highly dispersed particles begin to be observed at first, which practically do not settle during the hour of settling. At the initial temperature of the model solutions of 21.3 and 41.7 °C, after treatment with sodium hydroxide and settling, the formation of amorphous structures is observed, which begin to aggregate and float to the surface of the mother liquor. The solution lightens simultaneously throughout the volume. When settling at a solution temperature of 32.1 °C, the formation of developed aggregates of flakes is observed throughout the volume, which immediately begin to settle to the bottom of the vessel. Thus, the most acceptable temperature for the precipitation of magnesium hydroxide by the reagent method using sodium hydroxide can be considered a solution temperature of 30 °C. Since the results of both settling and filtering were unsatisfactory, it was proposed to use solid phase recirculation. It was shown that the recirculation of the precipitate even in an amount of 260 mg/dm3 allows to accelerate the formation of the solid phase and clarification of the mother liquor. At the same time, the total apparent volume of the solid phase after settling for 1 h does not change significantly, despite a significant increase in its total mass. An increase in the efficiency of magnesium ion removal is also observed. In general, by selecting the pH level and precipitation temperature, it is possible to ensure the necessary softening efficiency.