The presence of solid insolutes in the primary brine led to excessive levels of calcium and magnesium in the secondary brine.
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This post was last edited by Valve Mingming on 2015-6-26 at 10:23. Analysis request: Our company’s design capacity for caustic soda is 250,000 tons, but the current production volume is 270,000 tons. Two days before the major maintenance on the 4th of this month, the calcium and magnesium levels in the secondary brine suddenly exceeded the limits again; the calcium and magnesium levels in the primary brine were normal. The current level was reduced as a corrective measure, and the resin tower was switched over in advance. During the major maintenance, tank V-409 was opened once, and tank d-150 was opened twice; a large amount of calcium sludge was found at the bottom, which is essentially solid suspended matter or solid precipitates, with particles that are very fine. We analyzed the possibility that there was a leak in the Kaier membrane filter, but the tests showed no abnormalities; the calcium and magnesium levels were 0.8 PPM ； It was also analyzed whether too little sodium carbonate had been added, resulting in excess alkalinity. The laboratory kept the level of excess alkalinity in the samples within the range of 0.4–0.5. It is suspected that insufficient reaction time could be the cause. The two reaction tanks in the brine reaction vessel V-401AB are designed to hold 240 cubic meters of liquid; we maintain a volume of 280–300 cubic meters for the crude brine. Samples are taken from the first reaction tank for analysis, and the excess alkalinity level there is kept between 0.3–0.5. I was wondering if anyone knows how calcium carbonate is formed? Is it a problem with overloaded reaction time? Is it the KAI membrane of the equipment that’s leaking? Is it still due to an inappropriate sampling port location? Why control it at 0.3-0.5? Should the control level for alkali also be set at a higher limit when the load increases? How is the membrane capacity for saline solutions designed? Experts, please provide an explanation. I need some advice! ~~~~~~~~~~~~~~~~Crude salt is sun-dried salt, with 0.2% calcium and 0.01% magnesium
It’s easy to determine when there is a leak in the Kay membrane; it’s likely due to an insufficient reaction time with saltwater. Check whether there is a short circuit in the reaction tank or if salt sludge has accumulated. Additionally, do not overlook whether there are any changes in the quality of the raw salt, as well as any changes in other raw materials and auxiliary materials. Saltwater production requires stability; all adjustments, changes in raw and auxiliary materials, as well as the method of adding refining agents must be made in a gradual manner, avoiding any sharp fluctuations.
In other words, no matter which step it is, the operation must be performed steadily.
If the actual flow rate during production exceeds the designed flow rate, it becomes difficult to ensure the quality of the output water. The control parameter for the excess alkali amount remains unchanged; if the flow rate of the saline increases, it is necessary to increase the flow rate of sodium carbonate in a timely manner to maintain the value within the range of 0.3–0.5. Sediment was found at the bottom of the tank used for filtering saltwater; our unit also encounters this during maintenance. As long as operations are carried out carefully, it will not affect the quality of the saltwater. Therefore, I suspect that the low liquid level in your brine filtration tank has caused the sediment layer to be washed away by the brine, resulting in excessive levels of concentrated brine. The membrane capacity of a saltwater system is determined by the actual flow rate of saltwater being produced, which in turn determines how many membrane units are needed and what the flux will be. Under normal conditions, the flux of the Kae membrane should be slightly higher than the amount required for production. The poster can consider installing membrane bags at the spare openings on the KAI membrane’s ceiling panels to increase water production.
Our company has also encountered similar situations to yours. Our precision filtration brine storage tanks are two tanks with a capacity of 1300 m3 each, and the inlet and outlet are not on the same level. When the carbonate concentration at the outlet of the KAI membrane filter exceeds 0.5 g/l, it becomes necessary to take turbidity into consideration. The formation of calcium carbonate in the filtered saltwater tank is normal; it’s because there is an excess of carbonate and hydroxide ions in your highly purified saltwater. Over time, calcium carbonate and magnesium hydroxide will certainly precipitate at the bottom of the tank. We usually clean it every two to three months: we remove the accumulated debris, and then use gluten from flour to stick out the uneven sediment. Salt, acid, and sodium sulfite need to be added to tank #3 after the Kai membrane filter. This is done not only to remove free chlorine from the liquid but also to balance the pH level at around 9. It also serves to convert calcium carbonate and magnesium hydroxide in their combined form into ionic forms. You can find out the specific reasons by checking the pH values at which magnesium hydroxide and calcium carbonate precipitate, which makes it easier for the resin tower to remove them. The conventional analytical methods used to detect calcium and magnesium levels in brine after passage through Kay membrane filters generally cannot detect these substances; ICP analysis is required, but the levels detected in this way are quite high. The resin tower filters calcium and magnesium in ionic form; if the concentration of carbonate ions is too high, a thin layer of scale forms on the resin, which affects the filtration pressure difference. If the levels of calcium and magnesium in ionic form are too high, it is easy for the resin column to be damaged; in such cases, the column needs to be replaced and rapidly regenerated at least twice. In general, load increase has little to do with the levels of excess reduction (including carbonate and hydroxide), as as long as these values remain within the specified range, there is not much impact. The most crucial point is that, as mentioned above, there should not be large fluctuations in the raw materials and auxiliary materials used; the pre-treater should be completely clear, and the pressure difference across the Kai membrane filter needs to be controlled properly. If the Kai membrane requires frequent backwashing, then salt or acid cleaning should be considered. Making use of the spare port is also a solution. Regarding the design capacity of Kay membrane filters, the manufacturer should determine the number of membrane tubes to be used based on your production capacity.
I would like to ask whether, regardless of whether the indicators are normal and the alkalinity level is within the normal range, it has nothing to do with the reaction time. Also, after adding acid to the saline solution, we keep the pH at around 10. In other words, the formation of calcium carbonate is inevitable in concentrated brine, and it’s not possible to remove all of the calcium carbonate precipitate? I need an answer, thank you.
Under normal circumstances, each device designed (such as the front reaction tank, preprocessor, rear reaction tank, Kaier membrane filter, etc.) is equipped with a reaction reserve of about 10%; even if it operates under overload conditions, there should still be sufficient reaction time, as long as the reduction in capacity remains within the normal range. It is impossible to remove completely the portion of calcium carbonate and magnesium hydroxide that dissolves in water; salts and acids must be added to dissolve them into ionic forms so that they can be removed using resin columns.
We keep the pH of the brine at 10 – isn’t that a bit high? I want to keep it at 9.0, but since the optimal adsorption capacity of Bayer resins from Germany is 10, there is a conflict – what level is appropriate for the brine concentration in each step?
This post was last edited by Yongbu※Yanqi on June 29, 2015, at 21:40. Well, you should consult the manufacturers of Bayer resin regarding your problem. If the pH level is maintained at 10, the adsorption capacity of the resin will be optimal. There must be certain requirements concerning turbidity as well. Alternatively, you could install a larger storage tank for refined brine to serve as a buffer. (Personal opinion)
11 or 11.5 can be considered; it’s fine if the pH of the primary brine is on the higher side, as long as it’s not too high, so as to avoid the need to add more acid to the secondary brine
The brine tanks are covered in mud; it’s definitely a problem with the filtration system – the amount of impurities exceeds what the system can handle. If the pre-treatment process is working properly, then it’s necessary to check the issues related to the calcium removal system. 1. Reaction time: the longer, the better; at least half an hour. 2. Amount of sodium carbonate added: when calcium levels are high, more sodium carbonate needs to be added. In our case, the maximum amount of carbonate used is 0.8. 3. Quality of the membrane installation: it’s important to check whether there are any leaks in the membrane components.