Discussion on canceling the pretreater for once-through brine

I didn’t understand the technique used by the original poster; I hope they can explain it further. There are a few points that I’m not quite clear about: 1. The purpose of adding calcium hydroxide is to introduce calcium ions; the original poster also said that increasing calcium ions helps to precipitate magnesium ions. Since the solubility of calcium hydroxide is much higher than that of magnesium hydroxide, does the original poster mean that with the presence of calcium hydroxide, magnesium ions and magnesium hydroxide can be completely removed? 2. The poster said that magnesium hydroxide should be precipitated in the pre-reaction tank, with calcium carbonate sludge added as a co-precipitant. I’m not sure how precipitation and reaction can occur simultaneously in that tank; I hope the poster can clarify this. 3. Regarding the calcium hydroxide issue, how much calcium hydroxide does the poster need to add? What will be the cost increase resulting from the increased consumption of sodium carbonate? Will the increased load on the organic membrane filter as a result affect the filter’s flux? If the filtration flux decreases, does that mean an increase in equipment investment? I hope the original poster will think deeply from three aspects: investment, operating costs, and technical feasibility. Friends are also welcome to join in the discussion.

The solubility of calcium hydroxide might not be sufficient; if lime water is added to saline water, there may not be enough calcium hydroxide. If lime milk is used, precipitation and mixing can result in calcium hydroxide being included in the sludge, leading to waste. Additionally, it is necessary to evaluate whether it is cost-effective to use calcium hydroxide over the long term. The ultimate reason for using a pre-treater in the primary purification of domestic brine is not the low calcium-to-magnesium ratio, but rather the extensive production methods used for raw salt. As the original poster mentioned, even if the calcium-to-magnesium ratio is increased to above 2:1, these precipitates will still be removed during the membrane filtration process, which will certainly place additional strain on the membranes, leading to more frequent backwashing and an increased workload for the workers. Therefore, I believe that to eliminate the need for a preprocessor, the solution lies in using refined salt. The chlor-alkali industry in China has reached a consensus on this, encouraging salt producers to carry out secondary processing in order to increase the added value of raw salt.

If calcium is added, the consumption of sodium carbonate also increases accordingly. In fact, if refined salt is used and the salt is dissolved in pure water, very little calcium and magnesium are introduced into the system, so a pretreater may indeed not be necessary

1. Since magnesium precipitates as flocculent small particles that do not settle easily, and the calcium-to-magnesium ratio is low, magnesium ions fail to settle readily and tend to stick to the membrane; therefore, the pretreater is designed to remove magnesium more effectively. 2. Since the calcium-to-magnesium ratio is low, there is less calcium precipitation and fewer large particles are formed; therefore, increasing the calcium-to-magnesium ratio aims to produce more calcium precipitation to aid sedimentation. The purpose of introducing calcium ions into the system is to increase the calcium-to-magnesium ratio. For imported Australian and Mexican salts, the calcium-to-magnesium ratio is above 10:1, so there is no issue with the lack of a pretreater in the brine processing process. So when Gore was promoting it back then, the foreigners simply told us to figure out the magnesium ion problem on our own; the pre-treatment process was also specifically designed to address the characteristics of salts in China, and it wasn’t a process that already existed from the beginning. Abroad, the process involves removing calcium first and then magnesium, without a pre-treater. 3. Refined salt: Even when refined salt is used, the problem remains that the calcium-to-magnesium ratio of the salt is low; the precipitates that form are fine and not easy to settle, and there is still a lack of large calcium particles that could aid in the precipitation process. Since the introduction of calcium ions is necessary, and using calcium hydroxide or calcium oxide presents cost issues, a salt sludge recycling system is employed to achieve this goal; however, its effectiveness may not be very good. It might be possible to use some salt sludge along with some calcium hydroxide. 4. At present, refined salt is mostly used in China, yet pre-treatment units are still not eliminated. I have inspected a few facilities where such pre-treatment units have been removed; the salt quality in Zhangshu is quite good. When problems arose with the pre-treatment units, the brine was routed through a bypass, but in the end those units were reinstated. This shows that for domestic salt, some problems still arise even after the preprocessor is removed, and the same is true for Sevi. 5. If the use of refined salt and pure water can ensure that no calcium or magnesium is introduced, in such an ideal situation, I think membrane filters would not be necessary at all. But such investment will definitely be high. 6. The reason for such discussions is mainly that preprocessors are not available abroad but are available in China; this is what has led to these discussions among people

My ability to express myself is not very good; I hope everyone can forgive me. If there are any issues, let’s discuss them together. What I want to talk about is the process related to refined salt. Since vacuum-refined salt has high quality, with very low levels of calcium and magnesium, adding two alkalis to it after salt purification and then sending it to the pre-treater still seems like an overkill. Using refined salt incurs additional costs, yet the brine processing process isn’t optimized as a result. So, since the calcium-to-magnesium ratio is low, is it possible to increase the calcium ion content through certain methods?