Introduction to the sulfate method except

Thread Content

Currently, there are mainly 6 mature technical methods for the separation and removal of sulfate ions, namely the barium chloride method, calcium chloride method, freezing method, barium carbonate method, ion exchange method, and membrane separation method. 1. Barium chloride method: The barium chloride method involves reacting with the substances present in brine to form a precipitate. Since the solubility product of this compound is very low, this method is effective for removal purposes. Before the year 2000, most chlor-alkali plants in China used this method to remove sulfate ions. However, when using this method, care must be taken to avoid excess, as excess will react with the NaOH in the cell to form a precipitate that can clog the cell’s diaphragm. In particular, the heavy metal ion barium will deposit on the surface of the metal anode, forming non-conductive compounds that reduce the activity of the anode coating and increase the voltage. Barium ions also have a severe impact on ion membranes. Although chemical removal is effective and has a high reaction rate, it is highly toxic; it requires strict storage conditions, and improper handling can lead to excessive levels of Ba, which in turn damages the ion exchange membrane ； Its biggest drawback is the high operating cost; taking a 100 kt/a ion-exchange membrane caustic soda plant as an example, the annual processing cost amounts to over 11 million yuan. This method can produce barium sulfate as a by-product. The amount of barium chloride used increases accordingly, resulting in high operating costs. Moreover, this substance is highly toxic, and it is difficult to recycle the by-products as well as the packaging bags for barium chloride, which poses significant challenges to production and on-site management. 2. Calcium chloride method: This method involves using calcium chloride to react and form a precipitate. Due to its high solubility product, especially in saline solutions where the solubility increases by three to four times, this method is not as effective at removing impurities as other methods. However, if the amount of brine used is not large, the mass concentration of impurities in the saltwater after treatment can be reduced to below 7 g/L; under normal circumstances, it cannot be reduced to below 5 g/L. The removal process using this method is similar to that of the calcium chloride method for removing sulfate ions; it requires less investment. Moreover, since calcium chloride is relatively inexpensive, this method has a certain degree of competitiveness. Its drawback is that due to the high solubility product of calcium sulfate, the precipitate formed is slightly soluble, and as a result of the salt effect, its solubility in saturated salt water is 2–3 times higher than in aqueous solutions. The efficiency of removing sulfate ions is not high, and it also increases the amount of calcium ions in the saltwater, leading to an increase in the amount of salt sludge which is difficult to handle. This method does not meet the emission reduction requirements set forth by relevant regulations, and its effectiveness is inferior to that of the barium chloride method. To address the issues related to crystallization and back-dissolution, xx Company designed a homogeneous flow reactor that combines reaction and pre-clarification into one unit, effectively controlling the particle size of the crystals. It also achieves a more ideal level of clarity. The pre-cleared denitrified brine from the homogeneous reactor enters the HVM membrane filter; the mass fraction of SS in the filtered denitrified brine is less than 1, thereby achieving complete separation between the crystals and the brine. This process equipment has been used by numerous chlor-alkali and soda ash manufacturers in China. It is understood that a patent has been applied for this homogeneous flow reactor. A homogeneous flow reactor is a specialized device for nitrate removal using the membrane calcium method. The economic advantage of calcium-based nitrate removal over barium-based nitrate removal lies in the fact that it becomes more cost-effective as the concentration of salts (brines) increases. Due to the low price of , the initial investment in using HVM membranes is 100,000 yuan less compared to the barium method for caustic soda production. If 24 kg of is required per ton of caustic soda to be processed, the investment cost for HVM membranes can be recovered in less than half a year. In addition, Su Hengxi studied multi-component inorganic salt composite systems, adding to remove sulfate ions, and investigated factors such as the amount of , reaction temperature, and reaction time; laboratory data show that these methods can meet the requirements set by enterprises for sulfate ion removal. Wen Zhen and others studied the process of using waste salt sludge to remove halide ions from brine. It essentially utilizes the calcium ions in the salt sludge, and its nature remains that of the calcium chloride method. 3. Barium carbonate method: The barium carbonate method utilizes the difference in solubility products of barium carbonate and barium sulfate to achieve the separation of sulfate ions. XX Chemical Co., Ltd. and others have developed a method for removing sulfate ions from brine using barium carbonate. This method involves adding an appropriate amount of barium carbonate to fresh brine from an ion-exchange membrane caustic soda plant, or to recycled brine from an asbestos diaphragm caustic soda plant, at a temperature of 65–80°C; the brine concentration is between 150–250 g/L. After that, the barium carbonate is mixed thoroughly with the brine under stirring to form a suspension of barium carbonate ； A barium carbonate suspension is added from the top into a reaction tank containing brine with sulfate ions and calcium ions, so that the sulfate ions in the brine react with barium carbonate; the reaction time is 20–40 minutes, and a stirring device is installed in the reaction tank ； The brine from the reaction tank is pumped by a brine pump into the central guide tube of the clarification tank; there, the brine undergoes separation. The clear liquid resulting from this reaction overflows from the upper overflow weir of the clarification tank into the brine tank, where it is then fed into a desalination tank using a desalination pump for desalination ； The unreacted barium carbonate present at the bottom of the clarification tank is pumped back into the reaction tank using a sedimentation pump for repeated reaction. The main advantage of this invention is that, due to the repetitive reaction, the reaction rate of barium carbonate is high ； It is cheaper and safer than the barium chloride method for removing sulfate ions ； It can produce a certain amount of sodium carbonate, reducing the consumption of this refined substance (sodium carbonate) and thus saving on the costs associated with its purchase. Furthermore, the use of barium carbonate, which is cheaper than barium chloride, to remove sulfate ions results in lower costs for sulfate ions in the deionized water. Due to its lower cost, the significant difference in the mass fraction of barium between the two options – 70% in the former and 56% in the latter – as well as the production of soda ash as a by-product during the reaction process, which allows for reduced usage of crude brine in the purification process, this method has attracted attention within China’s chlor-alkali industry. However, the disadvantages of this method are also obvious: its solubility is low, and pipe blockages often occur in practical use. This technology is not yet mature, and the existing problems need to be further addressed in production. Currently, only one chlor-alkali enterprise in China uses this method to remove sulfate ions. 4 Freezing method: This method takes advantage of the fact that the solubility of sodium sulfate and sodium chloride changes with temperature in order to achieve separation. The process flow is as follows: A high-sodium sulfate saline solution with a mass concentration of 30 g/L is heat-exchanged with frozen saline, causing its temperature to drop from room temperature (25°C) to 8–10°C, thereby forming a crystal slurry. The crystals are then separated further using a centrifuge. The resulting slurry is heat-exchanged with another 30 g/L high-sodium sulfate saline solution before being sent to the salt purification unit; it can also be cooled further before being sent to that unit. The high-salt brine undergoes two stages of cooling. In the first stage, heat is exchanged with the separated slurry at a temperature of 8–10°C, reducing the temperature from 25°C to 15°C. In the second stage, heat exchange takes place with frozen brine, further lowering the temperature to 8–10°C. This method can produce crystals as a by-product, offers good removal efficiency, and can meet the requirement that the saltwater used for electrolysis contain less than 5 g/L. Its drawback is the high cost of investment, as it requires centrifuges, freezing stations, heat exchangers, belt conveyors, as well as associated storage tanks and pumps. Principle: High-magnesium saline is prepared, then frozen; in this way, it can be removed in the form of , making it suitable for the denitrification of saline solutions with a mass concentration of over 25 g/L. Advantages: It can produce mirabilite as a by-product. Disadvantages: High initial equipment investment, high energy consumption, and it is not economical when the mass concentration of the raw material is less than 25 g/L. 5 Ion exchange method: The ion exchange method involves using ion exchange resins to remove sulfates from brine, with the resins being backwashed regularly. The ion exchange resin used by XX Company in Country X to remove sulfates is Lewatit E304/88, whose functional group is polyamide. Test results show that when the mass concentration of sodium chloride is 100–150 g/L, after exchange with E304/88 resin, the mass concentration of sulfates in the brine is reduced to about 0.2 g/L. The exchange cycle is completed when the mass fraction of sulfates reaches about 50%; at this point, the exchange capacity of the resin is approximately 15 g/L. The resin is then backwashed using purified brine. The sulfurate that flows out can be frozen to produce mirabilite, or it can be discharged directly without being recovered. In recent years, the new type of resin for desorption developed by Japan’s Seiko Kagaku Kogyo Company features fast adsorption and desorption rates, good oxidation resistance, low susceptibility to salt damage, and easy operation, making it an effective method for removing contaminants. In 1997, the Takasago Industrial Division of the company began to successfully apply this resin for desalinating seawater. 6 Membrane separation: In recent years, with the application of ion-exchange membrane soda-making technology, saltwater with excessive levels has occurred frequently in chlor-alkali plants, and the costs associated with removing such impurities remain high. How to reduce costs has quickly become a topic of concern for experts at home and abroad. The SRS membrane separation technology recently developed by Canada’s K.C. Company is one of the more successful new technologies, with excellent results in industrial applications. This method was successfully applied in 1997 to the de-systematization device of electrolyzers at xxxx Chemical Industry Company in Country X. The key to this technology is the NF membrane, which can effectively separate monovalent anions from saline solutions. In simple terms, this NF membrane has selective permeability: it allows certain substances to pass through, while high-valent anions cannot; cations can pass through freely, thereby achieving the purpose of removal. NF membranes are relatively sensitive to saline suspensions (SS can cause scaling on the membranes), making them particularly suitable for the dechlorination of brackish water using ion exchange membranes. For example, in the brackish water from ion-exchange membrane cells, the mass concentrations of NaCl are 200 g/L and 10 g/L respectively; after treatment with an NF membrane, a concentrated solution is obtained in which the mass concentrations of NaCl are 200 g/L and 85 g/L respectively. The main equipment of this method includes a feed pump, a membrane filtration system, and automated instruments, etc. The process flow is shown in Figure 2. This method has advantages such as easy operation, low operating costs (electric energy consumption per 1 kg removed ≤ 1 kW•h), and rapid return on investment; it is currently one of the more advanced methods for removing sulfate ions internationally.