Mandrika A. Preparation of monomeric orthosilicic acid as a modifier of aluminum-based coagulants

Рrovides a theoretical justification and experimental confirmation of the processes of stabilization of orthosilicic acid. Quantum-chemical modeling of orthosilicic acid clusters with inorganic acids was performed. It is shown that the highest stability of the H4SiO4 molecule is observed in the presence of HPO42–, anions, the sum of intermolecular hydrogen bonds of which is –186.27 kJ/mol. Calculations have shown that the stability of the H4SiO4 molecule increases in the series HSO4− < CH3SO3− < H2PO4− < SO42− < PO43− < HPO42- due to the formation of two hydrogen bonds with HO groups of orthosilicic acid. It was also shown for the first time that the most stable cluster of orthosilicicic acid with methanesulfonate anion is [H4SiO4 · 4CH3SO3–]. This fact indicates that methanesulfonate anion is able to stabilize the monomer of orthosilicic acid and reduce the possibility of its dimerization.To obtain solutions of orthosilicic acid with the maximum monomer content, we analyzed the graphs of the forms of orthosilicic acid depending on the pH of the solution, and found that in the pH range from 1 to 4, orthosilicic acid is in the undissociated form in solutions. To obtain solutions of orthosilicic acid with the maximum monomer content, the pH range from 1.5 to 3.5 was chosen. The yield and stability of orthosilicic acid solutions were investigated depending on the type of acid used for hydrolysis and the silicate module. It was shown that the yield of orthosilicic acid at a lower silicate module is higher, and the stability of solutions increases in the series of anions SO42− < Cl− < CH3SO3− < HPO42−. It has been suggested that this is due to less polymerization in solutions of liquid glass with a lower silicate module. It has been experimentally proven that the yield of the orthosilicic acid monomer depends on the influence of the concentration of background electrolytes and the pH value. The maximum relative content of orthosilicic acid was observed at a solution pH of 3.5 and at a minimum concentration of background electrolytes. Using quantum chemical calculations, the complexes of aluminum hydroxychloride with orthosilicic acid in its monomeric, dimeric, and trimeric forms were calculated. Two reaction mechanisms for the formation of AlCl2OH with orthosilicic acid in aqueous solution were proposed (for example, the monomeric form of orthosilicic acid). It has been shown that the Gibbs energy of interaction of aluminum hydroxychloride with the monomeric form of orthosilicic acid by mechanism I is 48.38 kJ/mol, which is slightly higher than the energy of interaction with the dimeric and trimeric forms –32.94 kJ/mol and –41.86 kJ/mol, respectively. A decrease in the concentration of the monomeric form of orthosilicic acid after its mixing with aluminum hydroxychloride was experimentally confirmed, which confirms the formation of a compound between them. Two methods of modifying aluminum hydroxychlorides were investigated: by acid hydrolysis of liquid glass using low-base aluminum hydroxychloride and by mixing a previously prepared solution of orthosilicic acid with a high monomer content (above 50 %) with aluminum hydroxychlorides of different basicity. The stability of the obtained modified coagulants was also studied for the first time. It has been experimentally confirmed that the combined acid hydrolysis of liquid glass and aluminum hydroxychloride leads to cloudy, unstable solutions, but mixing the finished solution of orthosilicic acid obtained using methanesulfonic acid with highly basic aluminum hydroxychlorides in the range of Al/Si ratios of 250–50 leads to stable, transparent solutions with a shelf life of 6 months and more.The interaction between aluminum hydroxychloride and orthosilicic acid and the preservation of this bond after re-dissolution was proved by drying the modified alumina coagulant and its subsequent dissolution to form transparent concentrated solutions. In the fifth Chapter, the effectiveness of the modified alumina-silica coagulant in the purification of natural water was studied and its 12 % higher degree of turbidity removal and 8 % lower concentration of residual aluminum at low temperatures were proved. The explanation was put forward that this is due to an increase in the average molecular weight of aluminum complexes in interaction with orthosilicic acid. The larger hydraulic size of the flocs of the modified coagulant was experimentally confirmed. It has been experimentally established that the modification of aluminum hydroxychlorides with solutions of orthosilicic acid with a monomer content above 50 % and in the ratio of Al/Si from 100 to 20 allows obtaining stable solutions of coagulants. In Chapter 6, on the basis of theoretical and experimental results, a technological scheme was developed and the costs of producing an aqueous solution of orthosilicic acid using methanesulfonic acid and modification of aluminum hydroxychlorides of different basicity were estimated.