Hebei Messi Biology Co., Ltd. stated that magnesium hydroxide is a weak base. It has three main uses. One is used as a polymer filler and acts as a flame retardant. As a new type of flame retardant, magnesium hydroxide has the advantages of being non-toxic, non-volatile, not producing corrosive gases, and having a long-lasting effect. Secondly, it is used in environmental protection to treat acidic waste gas, waste liquid treatment, heavy metals and organic matter. In waste liquid treatment, exhaust desulfurization and other environmental protection measures, it is safe to use and pollution-free. It is a preferred green and safe neutralizer; thirdly, it is used in fine high-tech products such as oil additives, electronic components or coatings, and electronic ceramics. ; Therefore, magnesium hydroxide is regarded as one of the green products that is beneficial to environmental protection and ecological development, and has received widespread attention at home and abroad.
There are physical and chemical methods for producing magnesium hydroxide. Chemical methods mainly include calcium hydroxide method, ammonia method and sodium hydroxide method. Each method has its own advantages and disadvantages. The purity of magnesium hydroxide is not high. The magnesium hydroxide obtained by the ammonia method has high purity but the magnesium conversion rate is low. The product obtained by the sodium hydroxide method at room temperature has high purity but the filtration performance of the slurry is poor or even impossible to filter. This research mainly starts from improving filtration performance and looking for better methods and process conditions.
Combined with the law of nucleation and growth of magnesium hydroxide crystal grains, the test conditions were determined. Magnesium chloride and sodium hydroxide were prepared into a solution of a certain concentration. The magnesium chloride solution was first heated to a certain temperature, and then a certain amount of human substance was added to the magnesium chloride solution. Measure the slurry of magnesium hydroxide seed crystals, stir, wait until the specified temperature, add the prepared sodium hydroxide solution to the system at a certain speed, take out the slurry after a certain reaction time, and use a funnel and filter paper of the same specification to vacuum filter , record the filtering time as an inspection indicator.
Effect of crystal seed coefficient on slurry filtration performance
Initial tests show that without adding any additives, due to the rapid alkaline precipitation reaction of sodium hydroxide, a large number of magnesium hydroxide crystallites are generated in a short period of time, and the generated fine grains grow into large-sized grains. The process is extremely slow. The magnesium hydroxide produced by the sodium hydroxide method is in the form of colloids, and there is no washing or filtration at all. After adding additives such as flocculants and dispersants, the phenomenon of colloid aggregation has not been substantially improved. The addition of flocculant only promotes the agglomeration of crystal grains through the attraction of opposite charges, and does not actually form large-sized crystal grains. The filter washing performance was greatly improved after adding seed crystals.
Tests have proven that the method of adding seed crystals can effectively improve the agglomeration phenomenon of magnesium hydroxide. The slurry can be filtered and washed, and the filtration performance becomes better as the amount of seed crystals increases. This is because the addition of seed crystals provides crystal nuclei for the newly generated magnesium hydroxide and reduces the nucleation barrier of the seed crystals. It promotes the crystallization of new crystal grains on the surface of the seed crystal and shortens the crystal growth time. At the same time, the movement and erosion of a large number of crystal seeds under the action of stirring slows down the agglomeration of colloids to a certain extent and plays a role in dispersion. It can be seen from the experimental data that, taking into account the influence of crystal seed consumption and reaction volume, R=1 was selected as the seed coefficient for later experiments.
Effect of crystallization temperature on crystallization properties
In order to examine the change of crystallization performance with temperature after adding seed crystals, a single factor test on the effect of reaction temperature on filtration performance was conducted to find a suitable reaction temperature. Tests show that as the temperature increases, the filtration and washability of magnesium hydroxide slurry becomes better, because higher temperatures promote the crystal growth rate, and the generation of large-sized crystal grains improves the filtration performance of the crystal. This is consistent with the magnesium hydroxide slurry. The mechanism of hydrothermal modification 4.8] is consistent. The filtration time at 90°C is the shortest, which is 22 minutes. This temperature is selected as the reaction temperature.
Effect of reaction time on filtration performance
In order to examine the change of crystallization properties with time after adding seed crystals, a single factor test on the influence of crystallization time on crystallization properties was conducted to find a suitable crystallization time. The test results show that the reaction time has no significant effect on the filtration performance at a temperature of 90°C, and the filtration performance does not change much with time. The reaction time is selected as 1 hour.
Orthogonal test for preparing magnesium hydroxide by sodium hydroxide method
Under these conditions, the magnesium conversion rate is >98% and the filtration time is <24min. In order to obtain the best test conditions, an orthogonal test was conducted based on the single factor test results. To further investigate the best test conditions, the order of factors derived from the orthogonal test results is: temperature-concentration-time-R. The influence of crystallization temperature The maximum value should be 90°C. The concentration factor also has a great influence. Taking Mg2+=40g, the reaction time and species coefficient have little impact on the junction performance. Combined with the single factor test results, 60min and 1 were taken respectively, and the optimal process conditions were obtained. It is: the seed coefficient is 1, the reaction time is 1h, the reaction temperature is 90°C, and the magnesium ion concentration is 40g/L.
Hebei Messi Biology Co., Ltd. stated that the use of seed crystal method can significantly improve the filtration performance of magnesium hydroxide slurry. The optimal process conditions for preparing magnesium hydroxide by the sodium hydroxide method are: the seed coefficient is 1, the reaction time is 1 hour, the reaction temperature is 90°C, and the magnesium ion concentration is 40g/L. This study has certain reference value for the development of a process for producing magnesium hydroxide using the sodium hydroxide method.
