Does the excess alkalinity in saltwater affect the levels of Al and Si?

Considering the recovery of CNOOC discussed in another post, high pH does indeed dissolve silicon precipitates, resulting in excessive SI ions in the brine; however, the effect on aluminum remains unclear

It will definitely have an impact, especially with caustic soda. The aluminum hydroxide precipitate that forms is amphoteric; at pH 9, this precipitate can be easily filtered using diatomaceous earth membranes or ceramic membranes. However, once the pH reaches 9.5, the precipitate begins to dissolve. Silicon can only be removed partially through reaction with aluminum salts; there are quite a few methods for removing silicon, such as magnesium salts, aluminum salts, electrocoagulation, etc....:$

It might still be the effect of PH; si is rather complex. Data shows that Si is harmless on its own, but when present together with elements such as calcium, strontium, and aluminum, it causes a decrease in current efficiency. And both Si and these ions exist in the form of complexes, which is quite complex. When PH is greater than 11, substances of this form dissolve. Therefore, Si and Al plasmas are dissolved simultaneously.

Excess alkali content (NaOH) can affect the Si/Al ratio by increasing it; however, the requirements vary for different sections of the brine system. The optimal pH for the chemical brine is between 8 and 9, as both too high and too low pH levels can increase the Si/Al ratio. The reaction tank must have an adequate amount of excess alkali to maintain control, but if sediment accumulates in the reaction tank, it will also lead to an increase in the Si/Al ratio. Which paragraph does the original poster’s statement “The content of sodium hydroxide (our company keeps it at 0.01–0.1 g/l)” refer to? In the pre-treater (clarification tank) stage, pH affects precipitation; according to data from Asahi Kasei, a pH range of 10–10.5 yields the best sedimentation results, which is beneficial for the removal of Si/Al copolymers.

Well, I forgot that aluminum hydroxide is an amphoteric compound; that is to say, in high pH conditions, aluminum hydroxide precipitates and then dissolves again, resulting in an increase in aluminum ions. Can it be understood in this way?

During the salt dissolution stage, the value is kept between 0.01 and 0.1; in fact, there is no detailed control over the pH at each individual step – generally, the same excess alkali level is applied during salt dissolution, membrane treatment, and the use of secondary brine. I’m not able to understand your claim that a pH value greater than 10 during the clarification stage is beneficial for precipitation; I recall that an excessively high pH actually causes magnesium hydroxide and aluminum hydroxide to precipitate and dissolve again, which is not conducive to precipitation. I hope you can clarify this for me.

That is why there is a process for acid neutralization. The quality of the CaCO3 and Mg(OH)2 precipitates depends largely on the amount of excess alkali and the pH value; generally, the amount of caustic soda used is 0.1–0.3 g/L, while that of soda ash is 0.5–0.7 g/L, with adjustments made based on the mass of the brine； When PH is greater than 10.5, the newly formed Mg(OH)2 precipitate does not dissolve; for CaCO3, this value is around 9.4. The resulting precipitate is filtered through a microfiltration membrane to produce qualified primary brine. Chelating resins can only absorb calcium and magnesium ions, but not calcium and magnesium particles. The forum mentioned phosphates once; it is understood that their effect is not good. Phosphates act as shielding agents, affecting ICP testing, and have a significant impact on atomic absorption and ICP-OES testing, while their impact on ICP-MS is relatively smaller. The above are my personal opinions; I hope they are helpful to you:$

During the processing, the concentration of excess alkali is calculated to be within an appropriate range, so as to prevent excessive dissolution of silicon and aluminum. Compounds of silicon and aluminum require a strong alkaline environment to dissolve, and by keeping the excess alkali at less than 0.1 g/L, the alkali concentration is only 0.0025 M, corresponding to a pH of 11.4; thus, the dissolution of silicon and aluminum is limited. Moreover, most of the silicon and aluminum impurities should have been removed during the initial purification and refinement steps.