Hebei Messi Biology Co., Ltd. stated that in the process of extracting slurry magnesium hydroxide from concentrated seawater, the presence of calcium is an important factor affecting the purity and quality of the product. This paper first studied the pretreatment process of extracting magnesium from concentrated seawater, and investigated the reaction conditions of calcium removal by sodium carbonate. The separation method of calcium removal by sodium carbonate, the amount of sodium carbonate added, the stirring speed, the aging time and the stirring time on the calcium ion removal rate and the magnesium ion loss rate in concentrated seawater were studied, and the optimal operating conditions were determined. Under the optimal operating conditions, the calcium removal rate was controlled at more than 60%. The calcium content of the product in the process of extracting slurry magnesium hydroxide from concentrated seawater was effectively reduced, and the purity of the subsequent synthetic magnesium hydroxide product was fully guaranteed.
At the same time, it provides low impurity conditions for the further use of the mother liquor after extracting the magnesium hydroxide product. Secondly, a magnesium chloride solution with the same concentration of 3 times concentrated seawater obtained in the process of vacuum membrane distillation seawater desalination was used to prepare slurry magnesium hydroxide by sodium hydroxide precipitation. In order to improve the filtration performance of slurry magnesium hydroxide and increase its solid content, the effects of reaction conditions such as reaction temperature, alkali concentration, feeding speed, aging time, pH value on particle size distribution were studied. Finally, the conditions when the particle size of magnesium hydroxide obtained by the reaction of magnesium chloride and sodium hydroxide was the largest were determined. However, during the reaction, the filtration and washing of the slurry became very difficult due to the extremely fine magnesium hydroxide particles generated, and the solid content of the magnesium hydroxide slurry did not meet the use requirements. Therefore, when brine and sodium hydroxide were used as reactants, the crystallization mechanism and formation conditions of large-particle magnesium hydroxide prepared by the seed method were mainly studied.
The effects of the seed method on the solid content of the product, the effects of the number of seed crystals on the particle size distribution, the washing of the slurry, and the changes in the ultrafiltration membrane flux were investigated. Finally, slurry magnesium hydroxide was prepared by lime milk precipitation method using three times concentrated seawater as raw material. Mainly targeting the causes of calcium oxide impurities, combined with experiments, methods for removing or reducing calcium oxide impurities were proposed one by one. On this basis, the reaction conditions were optimized and the seed method was investigated, so that slurry magnesium hydroxide that met the requirements of HY standards was prepared.