Method for removing SiO2 and Al in primary brine by adopting magnesium compound method

Abstract

The invention discloses a method for removing SiO2 and Al in primary saline water by adopting a magnesium compound method, magnesium compounds are put into the primary saline water, the temperature of the primary saline water is increased, the primary saline water is hydrolyzed under an alkaline condition to form complex molecules of MgO and Mg (OH) 2, the surface activity of MgO and Mg (OH) 2 is high, and the silicon removal process can be accelerated by increasing the temperature of water, so that the silicon removal effect is improved; therefore, silicic acid compounds and aluminum-containing substances existing in various forms in the primary saline water with the water temperature increased can be adsorbed to the surfaces of complex molecules of MgO and Mg (OH) 2 and precipitated, and then the primary saline water stays in the clarifier and is filtered, so that insoluble substances are filtered and precipitated; siO2 and Al on silicic acid compounds and aluminum hydroxide are also removed from the primary brine along with insoluble substances, so that the primary brine is effectively clarified. According to the method, SiO2 and Al in the saline water are effectively removed, so that the content of SiO2 and Al is reduced.

Description

Technical Field

[0001] The present invention relates to a method for removing SiO 2 and Al from primary brine, in particular to a method for removing SiO 2 and Al from primary brine by using a magnesium compound method. Background Art

[0002] In current ion-exchange membrane caustic soda plants, all primary brine uses refined salt. The calcium and magnesium contents in the refined salt of most manufacturers are relatively low. SiO 2 and Al brought by the refined salt and makeup water in the primary brine system, while the SiO 2 and Al in the makeup water used exceed the standard. Therefore, SiO 2 and Al in the brine system cannot be removed together with calcium carbonate and magnesium hydroxide during the refining process when all using refined salt as raw materials, and will accumulate and increase in the system. The control indexes of Al and SiO 2 in primary brine are respectively less than 100 PPb and less than 5000 PPb. However, the Al content in primary brine reaches up to 1179 PPb at its highest, and the SiO2 content reaches 9344 PPb. The serious exceeding of SiO 2 and Al in the brine causes great harm to the operation of the ion-exchange membrane electrolyzer, resulting in an increase in the electrolyzer voltage, a decrease in the current efficiency, and a significant increase in the AC power consumption of caustic soda. Summary of the Invention

[0003] The present invention provides a method for removing SiO 2 and Al from primary brine by using a magnesium compound method, which can effectively remove SiO 2 and Al from the brine, reducing the contents of SiO 2 and Al.

[0004] The present invention adopts the following technical solution: A method for removing SiO 2 and Al from primary brine by using a magnesium compound method, characterized in that it first puts the magnesium compound into the primary brine and raises the water temperature of the primary brine, so as to hydrolyze under alkaline conditions to form complex molecules of MgO and Mg(OH) 2 . Due to the large surface activity of MgO and Mg(OH) 2 , and the acceleration of the silicon removal process and the improvement of the silicon removal effect by raising the water temperature, silicate compounds and aluminum-containing substances in various forms in the primary brine after raising the water temperature can be adsorbed onto the surface of the complex molecules of MgO and Mg(OH) 2 and precipitate, thus forming complex insoluble substances. Then, it passes through a clarifier and stays for filtration, so that after filtering and precipitating the insoluble substances, the SiO 2Ca and Al are also removed from the primary brine along with the insoluble substances, and the primary brine is effectively clarified.

[0005] Furthermore, the pH value of the alkaline condition is 10.1 - 10.3, and Mg in the magnesium compounds 2+ forms complex molecules of MgO and Mg(OH)₂ within this pH range, and MgO and Mg(OH)₂ play an adsorption role.

[0006] Furthermore, the silicate compounds in the primary brine coexist in the forms of silicon dioxide and silicate; the aluminum-containing substances coexist in the forms of aluminum hydroxide and aluminate.

[0007] Furthermore, the water temperature of the primary brine is raised to 40 °C, so that the residual silicon in the effluent of the primary brine at 40 °C can reach below 1 mg / L.

[0008] Furthermore, the residence time in the clarifier: when the water temperature of the primary brine is 30 °C, the residence time should be greater than or equal to 1 h; when the water temperature of the primary brine is 40 °C, the residence time should be less than or equal to 1 h; when the water temperature of the primary brine is 120 °C, the residence time should be 20 - 30 min.

[0009] Furthermore, the magnesium compound is magnesium chloride or magnesium oxide.

[0010] The present invention has the following beneficial effects: After adopting the above technical solutions, the present invention mainly utilizes the principles of adsorption and coagulation. Generally, magnesium chloride or magnesium oxide is used as the magnesium compound. Under alkaline conditions, magnesium chloride or magnesium oxide hydrolyzes to form complex molecules of MgO and Mg(OH) 2 , which have relatively large surface activity and can adsorb silicate compounds, aluminum hydroxide precipitates or colloids existing in different forms in water onto the particle surface, forming complex insoluble compounds. After precipitation and filtration, the purpose of removing SiO 2 and Al in the primary brine can be achieved, thereby reducing the contents of SiO 2 and Al in the primary brine. The Al content in the primary brine is less than 0.02 ppm, and the SiO₂ content is less than 2.6 ppm. Moreover, this method is simple to operate, has low cost, and operates stably and reliably during operation. By adopting this removal method Embodiment

[0011] The preferred embodiments of the present invention are described in detail below, so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making the protection scope of the present invention more clearly defined.

[0012] The present invention provides a method for removing SiO in primary brine using magnesium compounds 2The method of [substance name] and Al is characterized in that it first puts magnesium compounds into primary brine and raises the water temperature of the primary brine. The magnesium compounds are magnesium chloride or magnesium oxide. In this embodiment, the magnesium compound is magnesium chloride. Thus, hydrolysis occurs under alkaline conditions to form MgO and Mg(OH) 2 complex molecules. Due to the large surface activity of MgO and Mg(OH) 2 raising the water temperature can accelerate the silicon removal process and improve the silicon removal effect. Therefore, silicate compounds and aluminum-containing substances existing in various forms in the primary brine after raising the water temperature can adsorb onto the surface of the complex molecules of MgO and Mg(OH) 2 and precipitate to form complex insoluble substances. Then, it passes through a clarifier and stays for filtration. After the insoluble substances are filtered and precipitated, the SiO 2 on the silicate compounds and aluminum hydroxide and Al are also removed from the primary brine along with the insoluble substances, and the primary brine is effectively clarified. The pH value of the alkaline condition is 10.1 - 10.3. In this embodiment, the pH value of the alkaline condition is 10.1. Mg in the magnesium compound 2+ forms complex molecules of MgO and Mg(OH) 2 within the pH value range. MgO and Mg(OH) 2 play an adsorption role. In this embodiment, the silicate compounds in the primary brine coexist in the forms of silicon dioxide and silicate; the aluminum-containing substances coexist in the forms of aluminum hydroxide and aluminate. In this embodiment, the water temperature of the primary brine is raised to 40 °C, so that the residual silicon in the effluent of the primary brine at 40 °C can reach below 1 mg / L. The residence time in the clarifier of this embodiment: when the water temperature of the primary brine is 30 °C, the residence time should be greater than or equal to 1 h; when the water temperature of the primary brine is 40 °C, the residence time should be less than or equal to 1 h; when the water temperature of the primary brine is 120 °C, the residence time should be 20 - 30 min.

[0013] Damage mechanism of impurity silicon and aluminum: Silicon and aluminum exist in some salts, usually in the forms of clay and sediment, and may also exist in the water used to make brine. SiO 2 dissolves into the brine in an ionic state (silicate) in a strongly alkaline environment, and cannot be removed by using filters and resin towers; in an acidic environment, non-ionic silicic acid is formed, generating SiO of different degrees 2Colloidal particles enter the membrane under the influence of the movement of Na⁺. The pH value inside the membrane gradually increases from the anode side to the cathode side. When silicic acid moves towards the cathode side in the membrane, it gradually dissociates into anions in the form of silicate. The anions are repelled by the electric field force of the cathode, but are continuously pushed forward by the water transport force of the membrane to the water molecules. Therefore, they continuously accumulate in the ion-exchange membrane. As anions, they will combine with any available cations and form precipitates, clogging the voids of the ion-exchange membrane, causing the membrane to form pinholes and resulting in an increase in the cell voltage, directly affecting the performance, current efficiency, and service life of the membrane. (SiO₂ + 2NaOH = Na₂SiO₃ + H₂O), aluminum will exist in the form of clay components in the salt. If the filtration is not successful, these clays will enter the anode chamber with the brine. Since the anode chamber is in an acidic state, the clay will dissolve at this time, releasing aluminum ions. For other ions (such as silicate), it acts like a trap and forms complex precipitates with them, damaging the physical structure of the ion-exchange membrane and causing a decrease in current efficiency. In addition, aluminum will turn into anions in the form of aluminate with the increase of pH value. The anions are repelled by the cathode electric field force and leave the membrane, but are also pushed by the driving force of the ion-exchange membrane to transport water forward, thus accumulating in the membrane and forming precipitate compounds with other cations, damaging the ion-exchange membrane, reducing the current efficiency, and consuming the product sodium hydroxide.

[0014] Al 3+ + 3OH⁻ = Al(OH)₃ Al(OH)₃ + NaOH = NaAlO₂ + 2H₂O.

[0015] Influence of the raw water quality of the primary brine in this embodiment: When the hardness of the raw water is high, it is beneficial to the effect of magnesium agent desilication. The content of silicon compounds in the raw water has an impact on the specific consumption of the magnesium agent. The specific consumption of the magnesium agent decreases with the increase of the content of silicon compounds in the raw water and increases with the increase of the proportion of colloidal silicon in the water, generally in the range of 5 - 20.

[0016] Without being limited thereto, any changes or substitutions that can be thought of without creative work should be covered within the protection scope of this invention patent. Therefore, the protection scope of this invention patent should be subject to the protection scope defined by the claims.

Claims

1. A method for removing SiO 2 and Al from primary brine by using a magnesium compound method, characterized in that It first puts magnesium compounds into primary brine and raises the water temperature of the primary brine, so that hydrolysis occurs under alkaline conditions to form complex molecules of MgO and Mg(OH) 2 , and due to the large surface activity of MgO and Mg(OH) 2 , raising the water temperature can accelerate the silicon removal process and improve the silicon removal effect. Therefore, silicate compounds and aluminum-containing substances in various forms in the primary brine after the water temperature is raised can adsorb onto the surface of the complex molecules of MgO and Mg(OH) 2 and precipitate, thus forming complex insoluble substances. Then, it stays in a clarifier for filtration. After the insoluble substances are filtered and precipitated, SiO2 and Al on the silicate compounds and aluminum hydroxide are also removed from the primary brine along with the insoluble substances, and the primary brine is effectively clarified.

2. The method for removing SiO and Al in primary brine by using magnesium compound method according to claim 1, characterized in that 2 The pH value of the alkaline condition is 10.1 - 10.3, and Mg in the magnesium compound 2+ forms complex molecules of MgO and Mg(OH) within the pH value range 2 , and MgO and Mg(OH) 2 play an adsorption role. ​ 3. The method for removing SiO and Al from primary brine by using magnesium compound method according to claim 1, characterized in that 2 Silicate compounds in the primary brine coexist in the forms of silicon dioxide and silicate; aluminum-containing substances coexist in the forms of aluminum hydroxide and aluminate. ​ 4. The method for removing SiO 2 and Al in primary brine by using a magnesium compound method, characterized in that Raise the water temperature of the primary brine to 40 °C, so that the residual silicon in the effluent of the primary brine can reach below 1 mg / L at 40 °C.

5. The method for removing SiO and Al from primary brine by using a magnesium compound method according to claim 1, characterized in that 2 and Residence time in the clarifier: when the water temperature of the primary brine is 30 °C, the residence time should be greater than or equal to 1 h; when the water temperature of the primary brine is 40 °C, the residence time should be less than or equal to 1 h; when the water temperature of the primary brine is 120 °C, the residence time should be 20 - 30 min.

6. The method for removing SiO and Al from primary brine by using a magnesium compound method according to claim 1, characterized in that 2 The magnesium compound is magnesium chloride or magnesium oxide. ​