The impact of saltwater pH changes on production
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The pH of brine is generally kept between 8 and 10. I have a few questions regarding this: 1. Is the pH maintained within this range because it corresponds to the optimal adsorption capacity of the chelating resin? 2. If the brine contains no free chlorine, is it theoretically possible to keep the pH outside this range? What would be the consequences? (Acidic conditions could dissolve precipitates such as CaCO3, so that’s not an option; what about a pH higher than 10?) 3. The pH is adjusted by controlling the amount of alkali added. If the amount of sodium carbonate used exceeds 0.5 g/L, and sufficient HCl is not added after passing through the membrane, what effects will this have? Could excessive sodium carbonate trigger chemical reactions that are detrimental to the production process? I’m asking these questions because today the Sr content in the raw salt was too high, so more sodium carbonate was added. It occurred to me then what might happen if too much sodium carbonate or alkali is used, and what effects a pH outside the recommended range could have. I hope everyone can share their insights
1. What type of resin is being used? Resin manufacturers have specific requirements regarding the pH level of saltwater; this can be found in the technical specifications. 2. As with the first question, the resin has requirements regarding pH. So don’t deviate. 3. The pH is within the acceptable range, and the excess alkalinity has no impact.
So, according to you, the main reason for controlling pH is the selectivity of the chelating resin for pH? What are the main reactions that occur in the residual sodium carbonate and sodium hydroxide in saltwater during subsequent production processes? Has it already reacted with HCL during the mixing process with the recirculated brine, which is why there is basically no sodium carbonate or sodium hydroxide in the brine that actually enters the tank?
One of the main reasons for controlling pH is the selectivity of chelating resins toward pH, and the other is to ensure the reaction between calcium, magnesium, sodium hydroxide, and sodium carbonate. Acid must be added to the brine fed into the electrolyzer; carbonates react with acid to produce carbon dioxide, while sodium hydroxide reacts with acid to yield sodium chloride and water.
PH is probably not the direct factor in the reaction between calcium and magnesium ions with sodium carbonate and sodium hydroxide. First of all, sodium carbonate and sodium hydroxide naturally react with calcium and magnesium ions; it is actually the excess alkali that affects the pH, rather than the pH affecting the progress of the reaction, right? So I think that, if it’s not for chelating resins, the amount of excess alkali should theoretically be able to be controlled to exactly match the mass of calcium and magnesium ions
This post was last edited by Du Xiaoping on 2016-4-21 08:41. There are two concepts: excess alkalinity and pH value. The pH value indicates the acidity or alkalinity of a solution, while the excess value represents the concentration of that substance in the solution. pH is the prerequisite; under acidic conditions, one must ask how a reaction will take place. For example, if ammonia is present in brine and needs to be removed, sodium hypochlorite is used for this purpose. But how can ammonia be removed if the environment is not alkaline (i.e., when pH is less than 7)? Obviously, it cannot be removed. But in specific situations such as when magnesium is present in saltwater, you are right – the excess alkali level affects the pH, and the magnitude of this excess influences the speed of the reaction. What I mean is: many reactions are based on the pH level; only when the pH meets the required value can the reaction take place, and it is then that we can talk about excess amounts.
1. Magnesium hydroxide precipitate dissolves at pH 10.5, while calcium carbonate precipitate dissolves at pH 9.4. If the pH in the chelating resin column is controlled at 9, calcium and magnesium exist in ionic form, making them more easily adsorbed by the chelating resin column. Furthermore, the adsorption capacity of the chelating resin column is proportional to pH. The adsorption capacity increases rapidly between PH7 and PH9. 2. PH is a key factor in brine and throughout the entire chlor-alkali system. If the pH in the saline solution is too low, the reaction does not proceed completely; if the pH is too high, certain precipitates such as silicon will dissolve. Furthermore, too high a pH leads to rapid formation of magnesium hydroxide, which is detrimental to sedimentation. The reaction tank is generally kept at 10.5. 3. Excessive sodium carbonate can affect the acidity at the inlet and outlet of the electrolyzer; adding acid can lead to instability, and excessive addition of acid may damage the membrane. If too much sodium carbonate is used, it may also affect the saline feed to the cell.
If the pH is high, magnesium hydroxide is formed rapidly; with the help of sedimentation aids and clarifiers, it becomes easier to remove this magnesium hydroxide precipitate, thereby reducing the amount of magnesium ions that can be generated in the brine. Isn’t that better? Why is it said that too rapid formation of magnesium hydroxide is undesirable?
Furthermore, if calcium carbonate and magnesium hydroxide dissolve at pH > 9.5, then after passing through the Caionex membrane, brine is formed as primary brine. Is it possible to avoid adding HCl for neutralization and keep the pH of this primary brine high, thereby facilitating its adsorption in the chelating resin column?
Too much is as bad as too little: the previous system wasted caustic soda, while the subsequent system added salts and acid for neutralization. This leads to increased consumption
It dissolves at a value lower than this. If acid addition does not significantly improve the results when it is not used at all, then the process of adding acid after treating with brine once can be considered. Under normal circumstances, the pH of the reaction tank remains constant all the way to the resin tower. So theoretically, around PH10.5 might be more economical and reasonable.