Comprehensive utilization of mother liquor containing sodium hydroxide, sodium carbonate, and sodium chloride (continued)

“However, the sodium chloride content in the filter cake is relatively high; it reaches 10% in the wet filter cake.” If the filter cake is washed with a small amount of water and then tested again, what will be the result? Alternatively, sodium chloride is formed as a result of co-crystallization; you can also redissolve it in hot water and then cool it to recrystallize baking soda. The mother liquor can simply be returned to the raw material solution. “If salt is not recycled, starting from soda ash, should one first carry out carbonation to remove sodium hydroxide and most of the sodium carbonate, then add salt until saturation, and finally perform ammonia absorption and carbonation? ” In my reply to the original thread “Comprehensive utilization of a mother liquor”, I once suggested that: “By simply adding carbon dioxide, most of the sodium hydroxide and sodium carbonate will crystallize out in the form of sodium bicarbonate, since the concentration of sodium chloride in the mother liquor is already quite high.” This approach is mainly intended to eliminate the step involving ammonia, thus saving effort. As for whether the obtained sodium bicarbonate is pure or not, first one must examine the phase diagram, and second, conduct experiments. ”

If salt is not to be recovered, the mother liquor can first be concentrated. Based on the phase diagram of the four-component system consisting of sodium hydroxide, sodium carbonate, sodium chloride, and water, one can determine the optimal relationship among the crystallization temperature, concentration, and pH value for sodium chloride crystallization. By conducting multiple experiments and making trial-and-error adjustments, sodium chloride can be filtered out first; only sodium hydroxide and sodium carbonate remain in the mother liquor. Subsequently, carbonation, crystallization, and filtration yield a wet filter cake of sodium bicarbonate.

The problem with the concentration method is that it is necessary to raise the alkali concentration to a very high level in order to crystallize most of the salt; moreover, the resulting salt also contains sodium carbonate. In this system, the contents of sodium chloride and sodium carbonate are closely related to that of sodium hydroxide; generally, the higher the concentration of sodium hydroxide, the higher the contents of sodium carbonate and salt. Since the concentration of sodium hydroxide is the lowest in this system, concentrating it would require a large amount of steam. My original intention was to economically extract one of the components from it. A mother liquor containing a single component can be easily handled using our own equipment.

However, the sodium chloride content in the filter cake is relatively high; it reaches 10% in the wet filter cake, which is then washed using a filter-washing machine. When washing, pay attention to the temperature and composition. I won’t specify the exact criteria; figure them out on your own. Feel free to make some improvements to the showerhead. This is very crucial. In a laboratory setting, the vacuum filtration method can be used. It’s similar to large vacuum drum filters. However, 10% is indeed a bit high. It’s not certain that it can be washed to meet the criteria for industrial applications. Moreover, if more additives are added, the total volume of the system will increase... Starting with soda ash, it is first carbonated to remove sodium hydroxide and most of the sodium carbonate; then salt is added until saturation is reached, followed by ammonia absorption and further carbonation. This is the ammonia-soda process. The system seems acceptable, but the cost is not. Additionally, it seems that there’s no separate sodium hydroxide in the soda ash process... Will this affect the equilibrium?

Regarding the issue of sodium hydroxide being present, if we start from the premise that the final product is soda ash, is it possible to first carry out carbonation to convert only the sodium hydroxide into sodium carbonate? Subsequently, ammonia can be absorbed followed by another round of carbonation. In this way, the salt content in the resulting sodium bicarbonate would not be high. The overall process would then be similar to the combined soda process. From a purely technical feasibility standpoint, is this approach viable?

The problem is that industrial production and the carbonization process are very complex. Can it be ensured that all sodium hydroxide is reacted without any other impurities being produced? I find it very difficult. You need to conduct experiments to prove this; without tests, I can’t be sure.