Issue regarding the amount of refined saltwater to add (seeking experts’ advice)

The amount of sodium hypochlorite added is related not only to organic substances but, most importantly, to total amines (inorganic ammonium and organic amines). Generally, the amount added is twice the theoretical amount, or the free chlorine content at the outlet of the pretreatment unit (in the case of air flotation processes) is kept below 10 ppm (traditionally, it is considered to be 1–3 ppm). Organic amines are easier to remove compared to inorganic ammonium, but the removal rate does not reach 100%; it is usually around 60–70%, while the removal rate for inorganic ammonium is 30–40%. The organic substances in raw salt are mainly long-chain unsaturated compounds such as humic acid. After adding sodium hypochlorite, it is intended to break these chains, either through air blowing or by removing them via the sludge from the pretreatment unit. However, all of these processes are invisible to the naked eye, and they cannot be detected using conventional analysis methods. The TOC value can provide some indication, but such instruments are generally not available in chlor-alkali plants; only large enterprises or research institutions have access to them (especially when measuring TOC in brine, as sodium chloride interferes with the readings). Nevertheless, the organic substances in raw salt have a significant impact on the clarification of brine (solid-liquid separation). Using microfiltration processes (such as Gore or HVM) also has a considerable effect on the flux of the tetrafluoro membranes (it is estimated that this value should be above 50 l/m2·h). Therefore, it is necessary to control the quality of raw salt. As for other factors, the amount of ferric chloride added should be around 50–150 ppm, and this depends mainly on the quality of the raw salt (such as magnesium and organic substances levels). Of course, more can be added, but this will change the color of the sludge. Over time, the membrane tubes and covers of filters will also turn red, as well as the pipes and resins used in the brine treatment process. The amount of sodium hydrosulfite added is related to the free chlorine content at the outlet of the pretreatment unit, and a certain safety factor must be taken into account, as many manufacturers need to adjust the pH level, requiring the use of high-purity hydrochloric acid. There’s no need to explain further regarding caustic soda and soda ash.

The amount of complete reaction is calculated based on the composition of the saline or brine, which facilitates precise control. But the most important factor is to determine it based on the excess levels of various additives in saline water; for example, the excessive amount of NaOH in analysis is 0.08–0.15 g/l, while the excess level of Na2CO3 is 0.3–0.5 g/l (in addition to calcium removal, SY3 removal also needs to be taken into account). The excess amount of NACLO is 10 PPM, and FECL3 should be ≤30 PPM. For NA2SO3, it is mainly determined by an ORP value of less than 50 MV. That’s all for now. That’s incorrect; please correct me.

1. The amount of ferric trichloride to be added should be adjusted according to the ratio of calcium to magnesium; this can be done by observing the appearance of the floating sludge and its shape； 2. For sodium hydroxide, the standard set by foreigners is 0.2–0.3 g/l, in order to maintain the balance of various ions ； 3. Sodium carbonate: The recommended range for foreigners is 0.5–0.7 g/l, to ensure that calcium can react rapidly ； 4. For sodium hypochlorite, it is adjusted based on ORP, and then corrected using sodium sulfite ；

Is there a certain proportional relationship between the added FeCl3 and the solids in the saline solution?

The amount of refining agent added is determined based on the impurity content in the raw salt, but ultimately it is the parameters of the refined salt water after treatment that are decisive; if those parameters do not meet the required standards, the amount of refining agent must be adjusted. After all, it comes down to the outcome of the processing.

1. Regarding the addition of NaClO and FeCl3, a friend on the second floor provided excellent comments. I won’t say more. 2. The addition of NA2CO3 follows the same principle as that of NAOH: first, all the calcium and magnesium ions in the brine must be involved in the reaction; thereafter, the time required for the reaction to complete must be taken into account, which necessitates a certain excess amount to facilitate a faster completion of the reaction. Theoretically, if the excess amount of NA2CO3 is above 0.3 g/L, the reaction can be completed in just half an hour; therefore, it is sufficient to measure the excess amount of NA2CO3 in the saline solution after adding NA2CO3 and initiating the reaction. In standard processes, the excess amount is controlled at 0.3–0.5 g/L; too much of it will instead increase consumption. 3. It should also be noted that the addition of the refining agent requires a certain temperature; only within an appropriate temperature range can the reaction rate remain at its highest, allowing for effective use of the equipment’s capacity. Otherwise, if the reaction time is insufficient, the reaction will not be complete, which can lead to excessive levels of calcium, magnesium ions, and SS in the brine. This in turn affects the operational lifespan of the membranes (whether they are Kay or Gore membranes) and increases the frequency of acid regeneration.

The reaction time between sodium hydroxide and magnesium is 0.5 hours (at 50–60 degrees), while the reaction time between sodium carbonate and calcium is about 1 hour. Of course, when an excess amount of reagent is used, the excess amount of NaOH is 0.05–0.2 g/L, and the excess amount of Na2CO3 is 0.4–0.6 g/L. The concentration of sodium hypochlorite is 5–15 PPM. As for the concentration of ferric trichloride, it is determined based on the magnesium content; when the magnesium content is 0.15%, the addition amount of ferric trichloride is 15 PPM. The amount of sodium hypochlorite added is determined based on a redox potential of less than 50 MV

Previously, the amount of ferric chloride added was adjusted based on the flow rate of the saline solution; now it is controlled by the amount of impurities introduced. After further study, it seems that it is now controlled by the amount of magnesium.