Study of the efficiency of sodium silicate water softening processes
Authors: Radovenchyk Iaroslav , Hordiienko Kateryna , Bakunovsky Oleg , Ivanova Veronika 
Published: July 2024

The current state of natural water resourses requires preliminary softening for almost all drinking waters and waters for energy purposes. Therefore, the search for effective softening technologies and reagents is especially relevant today. Researches in this field are constantly increasing every year due the intensive quality deterioration of the natural waters. The processes of calcium and magnesium ions removing from water are on central place in such researches. It is traditionally considered that magnesium ions are more difficult to remove from water than calcium ions because magnesium ions form less quantity of insoluble compounds. The total efficiency of water softening processes depends on the residual content of both cations. Therefore, researchers also pay a lot of attention to magnesium ions removing. Sodium hydroxide and sodium phosphate are considered as a perspective reagents in such processes. However, the use of first reagent is associated with a significant increase of pH index, and the use of second reagent requires a big quantity of the reagent and needs the alkaline medium for sufficient efficiency. Therefore, in this work, we investigated the effectiveness of sodium silicate in the processes of magnesium and calcium ions removing from the aqueous medium. A characteristic feature of the solid phase formation in reaction between magnesium cations and silicate anions is the significant reaction dependence on the pH index. In neutral environment, the formation of a solid phase in model solutions was not visually fixed. However, some decrease of water hardness after filtering through "blue tape" filter paper is still observed. And it is decrease with increasing of pH index. The most effective softening process can be obtained at pH levels > 10. It was not possible to obtain any positive results during study of settling and filtering processes of the formed solid phase. At the same time, the ratio between the components and the initial water hardness does not affect the clarification process practically. However, sodium silicate can be quite effective in stoichiometric or higher ratios. But a significant disadvantage of such method is a pH index increasing when the components are mixed. It requires futher adjusting the pH level in the treated water. And this is aditional stage of water treatment. It was not possible to obtain positive results when aluminum hydroxide was used as a settling and filtering intensifier. With the ratio K = [SiO32-, mg-eq]/[Mg2+, mg-eq] = 1 and different doses of aluminum ions, there is no significant intensification of sedimentation processes in the range of aluminum ions doses from 10 to 70 mg/dm3. And with aluminum doses less than 30 mg/dm3, there is even inhibition of the settling process. Like in the case of sodium silicate silicate treatment of magnesium containing solutions, the precipitation of calcium silicate strongly depends on the pH level. High efficiency of process is observed only in a highly alkaline environment. Under other conditions, the softening efficiency is significantly reduced. With an excess or lack of a precipitant, more compact particles are formed. The settling process for such particles occur more faster. At the same time, for a stoichiometric or near ratio between the components, particles of the branched structure are formed. Such particles settle twice slowly and occupy a larger imaginary volume. With a stoichiometric ratio of components, the speed is significantly slowed down, and when K = [SiO32-, mg-eq]/[Ca2+, mg-eq] ≥ 1,0 all filter pores are blocked during the first 10 min and liquid phase transportation stops. Reducing the initial water hardness to 20 mg-eq/dm3 does not significantly change the situation. Clarification of the suspension occurs partially. A significant amount of suspended particles remains in the mother solution. The flocculating ability of silicon compounds does not appeared under these conditions. When the content of calcium ions is reduced to 10,2 mg-eq/dm3, treatment with an equivalent amount of sodium silicate does not lead to a solid phase formation. Although after filtering through the "blue tape" filter the water hardness decreases. At the same time, the filtration properties of the solid phase formed during the softening process are the same as the filtration properties of carbonates, phosphates and hydroxides, so the reagent cannot be recommended for use in low- and medium-productivity systems, where the filtration is a main phase separation process.