Method for removing silicon and aluminum in brine for chlor-alkali production

Abstract

The present invention belongs to the technical field of brine treatment, and particularly relates to a method for removing silicon and aluminum in brine for chlor-alkali production. The method is as follows: The desulfurized wastewater from power plants and low-silicon water are introduced into a salt sludge washing tank together, and then pumped into a filter press for pressure filtration. The filtrate from the pressure filtration enters a mixing tank, where alkaline substances are added for reaction, and then enters an inclined plate settler for sedimentation. The supernatant is filtered to produce intermediate water for reuse as low-silicon water, and the concentrated water enters a chemical silicon removal unit; the concentrated water is introduced into a concentrated water reaction tank, and a silicon removal coagulant is added for silicon removal, and then pumped into a filter press for pressure filtration. The filtrate from the pressure filtration enters an aluminum removal unit; the filtrate from the pressure filtration is introduced into a buffer tank, and hydrochloric acid is added to adjust the pH value, and then it is filtered through a Kail membrane to obtain refined brine with silicon and aluminum removed. According to the existing state of silicon in brine, the present invention combines physical silicon removal and chemical silicon removal, and at the same time, by controlling the pH value in the brine, deep removal of silicon and aluminum in brine for chlor-alkali production is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of brine treatment, and particularly relates to a method for removing silicon and aluminum in brine for chlor-alkali production. Background Technique

[0002] The technological process of chlor-alkali production is to feed the secondary refined brine into the anode chamber of the ion-exchange membrane electrolyzer. At the same time, dilute alkali solution diluted with pure water is injected into the cathode chamber, and high-voltage direct current is passed through to promote the electrolysis reaction of the materials in the electrolyzer. Chlorine gas and dilute brine are generated on the anode side, and caustic soda with a concentration of ≥ 32% and hydrogen gas are generated on the cathode side.

[0003] The anode chamber and the cathode chamber of the electrolyzer are separated by an ion-exchange membrane. The refined brine enters the anode chamber, and pure water is added to the cathode chamber. The ion-exchange membrane is a polymer membrane containing charged groups. It is composed of a polymer base membrane composed of hydrocarbon chains and ion functional groups fixed on the base membrane, and has the ability to carry current, so it has the characteristic of selectively permeating the ions of the separated substances.

[0004] SiO2 itself has no influence on the membrane. It will only affect the performance of the ion-exchange membrane and increase the cell voltage when it exists simultaneously with aluminum, calcium, magnesium, strontium, etc. SiO2 dissolves in brine in an ionic state under alkaline conditions, and cannot be removed by the HVM membrane filter and resin tower; non-ionic silicic acid is formed in acidic brine and enters the membrane under the influence of the movement of Na+. After silicon dioxide enters the membrane, silicate is formed in the membrane or it acts as a soluble cation and is attracted by the negative charge of the membrane and the cathode electric field. After entering the ion-exchange membrane, as the pH value rises, it will form anionic silicate. This ion cannot pass through the membrane due to the influence of the cathode electric field and accumulates on the membrane after being washed by water flow. As an anion, it can combine with any cation to form a precipitate. The reaction position, crystal size and precipitation amount will determine the performance impact. When exceeding the specified range, silicate can form precipitates with calcium ions, aluminum ions, and even sodium ions, causing damage to the cathode surface of the membrane and reducing the current efficiency. Therefore, it is necessary to deeply optimize and reduce the silicon and aluminum content in brine.

[0005] In the chlor-alkali industry, the main sources of silicon dioxide and aluminum ions are raw salts. Due to different salt production processes, the contents of silicon dioxide and aluminum ions in sea salt and mineral salt are different. The silicon dioxide and aluminum ions brought in from the salt dissolve into the brine system through salt dissolution. Both silicon dioxide and aluminum are amphoteric substances and can form ionic states and dissolve in the solution under acidic and alkaline conditions. Silicon dioxide and aluminum ions enter the ion-exchange membrane electrolyzer with the brine. Due to the selective permeability of the ion-exchange membrane of the electrolyzer, silicon dioxide and aluminum are blocked on the anode side of the ion-exchange membrane, blocking the through-pores of the ion-exchange membrane, thus causing problems such as a decrease in the current efficiency of the electrolyzer and an increase in the cell voltage. Therefore, the indexes of silicon dioxide and aluminum ions in brine are the key factors affecting the control indexes of ion-exchange membrane electrolysis power consumption.

[0006] Removing silicon and aluminum from brine is a technical problem faced by the chlor-alkali industry and an issue that many chlor-alkali manufacturers urgently need to solve. The main method for the chlor-alkali industry to reduce silicon and aluminum is through the discharge of salt mud. However, currently, due to the requirement of "zero discharge" for environmental protection, the direct discharge of salt mud is no longer allowed. It is necessary to wash the salt mud with high silicon and aluminum content to reduce the chloride ions in the salt mud and achieve the comprehensive utilization of salt mud. However, during the washing of salt mud, the silicon and aluminum impurities that should have been carried out by the salt mud re-enter the brine system again, resulting in the total content of silicon dioxide and aluminum ions in the brine being about 10,000 ppb, far exceeding the range of the brine control index of 1750 ppb (total silicon and aluminum), increasing the difficulty of controlling the brine index. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a method for removing silicon and aluminum from brine in chlor-alkali production. According to the existing state of silicon in the brine, a combination of physical silicon removal and chemical silicon removal methods is adopted. At the same time, by controlling the pH value of the brine, the content of silicon dioxide in the final brine is reduced to 1300 - 1800 ppb, and the content of aluminum ions is reduced to 20 - 30 ppb, realizing the deep removal of silicon and aluminum in chlor-alkali production brine.

[0008] The method for removing silicon and aluminum from brine in chlor-alkali production according to the present invention includes the following steps

[0009] (1) Physical silicon removal: The desulfurized wastewater from the power plant and low-silicon water are introduced into the salt mud washing tank together for salt mud washing, and then pumped into the secondary filter press for filtration. The filtered water enters the mixing tank, reacts after adding alkaline substances, and then enters the inclined plate settler for sedimentation. The sedimented sludge returns to the salt mud washing tank, and the supernatant of the sedimentation passes through a multi-media filter, a cation bed, an ultrafiltration membrane group, and a reverse osmosis membrane group for filtration in sequence. The produced intermediate water is recycled as low-silicon water, and the concentrated water enters the chemical silicon removal unit;

[0010] (2) Chemical silicon removal: The concentrated water produced by the reverse osmosis membrane group is introduced into the concentrated water reaction tank, and after adding a silicon removal coagulant for silicon removal, it is pumped into the primary filter press for filtration. The filtered water enters the aluminum removal unit;

[0011] (3) Aluminum removal: The filtered water in step (2) is introduced into a buffer tank, and at a temperature of 18 - 20 °C, hydrochloric acid is added to adjust the pH value to 8 - 9.5, and then it is filtered through a Kail membrane to obtain brine with silicon and aluminum removed.

[0012] In step (1) of the present invention, the chloride ion content in the desulfurized wastewater from the power plant is 20 - 30 g / l, the silicon dioxide content is 9000 - 11000 ppb, and the aluminum ion content is 400 - 600 ppb.

[0013] In step (1) of the present invention, the low-silica water is industrial water or the intermediate water produced by the reverse osmosis membrane group or a mixture of both; the volume ratio of the low-silica water to the power plant desulfurization wastewater is (1 - 2):1.

[0014] In step (1) of the present invention, the chloride ion content in the pressure-filtered water is ≤15 g / l, and the chloride ion content in the salt sludge filter cake is ≤1.0 g / l.

[0015] In step (1) of the present invention, the thickness of the filter cake of the secondary pressure filter is 3 - 5 cm.

[0016] In step (1) of the present invention, in the pressure-filtered water of the secondary pressure filter, the contents of silicon dioxide and aluminum seriously exceed the indexes. The reason is analyzed as follows: adding low-chloride water to re-wash the filter cake (salt sludge) after the first pressure filtration reduces the chloride ions in the salt sludge, but at the same time dissolves the silicon and aluminum ions carried in the salt sludge, and the silicon and aluminum ions enter the salt sludge washing water. However, through a large number of experiments, it is found that by adjusting the pH value, raising the temperature, increasing or decreasing the amount of washing water, etc., no obvious effect is achieved, and the thickness of the filter cake will affect the silicon and aluminum impurities in the filtrate.

[0017] In step (1) of the present invention, the alkaline substance added to the mixing tank is a mixture of sodium hydroxide and sodium carbonate with a mass ratio of 1:(0.5 - 2); the mass-volume ratio of the alkaline substance to the pressure-filtered water is (1 - 5) g:1 L.

[0018] In step (2) of the present invention, the silicon-removing coagulant added to the concentrated water reaction tank is a rare earth metal salt; preferably one or more of cerium sulfate and lanthanum carbonate. Using a rare earth metal salt as the silicon-removing coagulant, the floc formation speed is fast and the particles are large. It far exceeds ordinary flocculants in terms of silicon-removing ability, has the advantages of less flocculant dosage and high silicon-removing ability. At the same time, the flocculated particles are carried out of the chlor-alkali production system with the salt sludge, so the flocculant will not cause a destructive impact on the ion-exchange membrane of the electrolytic cell.

[0019] In step (2) of the present invention, the mass-volume ratio of the silicon-removing coagulant to the concentrated water is (10 - 30) mg:1 L.

[0020] In step (2) of the present invention, the thickness of the filter cake of the first pressure filter is 2 - 5 cm, and the formed filter cake can block the silicon in the water from entering the pressure-filtered water.

[0021] In step (2) of the present invention, in the pressure-filtered water of the first pressure filter, the silicon dioxide content is between 1300 - 1800 ppb, and the aluminum content is about 70 ppb.

[0022] In step (3) of the present invention, aluminum ions are greatly affected by the pH value, so it is necessary to precisely control the pH value of the brine between 8 - 9.5.

[0023] In step (3) of the present invention, in the refined brine from which silicon and aluminum are removed, the silica content is 1300 - 1800 ppb, and the aluminum ion content is 20 - 30 ppb.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] (1) According to the existence state of silicon in the brine, the present invention combines two methods of physical silicon removal and chemical silicon removal. At the same time, by controlling the pH value of the brine, the silica content in the final brine is reduced to 1300 - 1800 ppb, and the aluminum ion content is reduced to 20 - 30 ppb, realizing the deep removal of silicon and aluminum in the brine for chlor-alkali production;

[0026] (2) After being treated by the present invention, the brine has extremely low silicon and aluminum contents, reducing the deposition of impurities such as silicon and aluminum on the ion exchange membrane, improving the current efficiency of the electrolytic cell, reducing the cell voltage of the electrolytic cell, and prolonging the service life of the ion exchange membrane. Specific embodiments

[0027] The present invention will be further described below in conjunction with embodiments. The raw materials used in the embodiments are all commercially available conventional raw materials unless otherwise specified; the process methods used in the embodiments are all conventional methods in the art unless otherwise specified.

[0028] The silica content of the power plant desulfurization wastewater used in the embodiments is 10500 ppb, the aluminum ion content is 462 ppb, and the chloride ion content is 24 g / l. The hydrochloric acid used in the embodiments is hydrochloric acid with a concentration of 10%.

[0029] Example 1

[0030] In this example, the influence of the filter cake thickness of the secondary filter press on the silicon removal effect in the physical silicon removal step was studied. The specific operation was as follows:

[0031] The power plant desulfurization wastewater with a silica content of 10500 ppb and an aluminum ion content of 462 ppb and low-chlorine water (volume ratio 1:1.5) were introduced into the salt sludge washing tank for salt sludge washing together, and then pumped into the secondary filter press for filtration. The filter cake thickness was adjusted, and the changes in the silica and aluminum contents in the filtrate of the secondary filter press were observed. The results are shown in Table 1.

[0032] Table 1

[0033]

[0034] As can be seen from Table 1, within the range where the filter cake thickness of the secondary filter press is greater than 3.0 cm, the aluminum ion and silica contents are significantly reduced, and with the increase of the filter cake thickness, there is no obvious change in the reduction of silicon and aluminum.

[0035] Example 2

[0036] In this example, the influence of the silicon removal coagulant on the silicon removal effect in the chemical silicon removal step was studied. The specific operation was as follows:

[0037] The primary brine was used as the experimental liquid (silicon dioxide content was 4528 ppb, and aluminum ion content was 169 ppb), and different silicon removal coagulants were added for silicon removal. The changes in the silicon dioxide and aluminum contents after silicon removal were observed. The results are shown in Table 2.

[0038] Table 2

[0039]

[0040] As can be seen from Table 2, for removing silicon in the brine, the organic polymer basically has no effect; sodium hydroxide and rare earth metal salts have better effects. The addition of calcium hydroxide solution increased the calcium ion in the brine from 10 ppm to 89 ppm, and at the same time, the brine solution became viscous after adding calcium hydroxide, so it was not considered. The addition of rare earth metal salts utilizes its own adsorption property, without increasing the increase of other ions in the brine, and at the same time, the brine quality remains pure and transparent.

[0041] Example 3

[0042] In this example, the influence of the filter cake thickness of the primary filter press on the silicon removal effect in the chemical silicon removal step was studied. The specific operation was as follows:

[0043] The silicon-removed brine with a silicon dioxide content of 2385 ppb and an aluminum ion content of 109 ppb was pumped into the primary filter press for filtration. The filter cake thickness was adjusted, and the changes in the silicon dioxide and aluminum contents in the filtrate of the primary filter press were observed. The results are shown in Table 3.

[0044] Table 3

[0045]

[0046] As can be seen from Table 3, when the filter cake thickness of the primary filter press is greater than 2.0, the aluminum ion and silicon dioxide contents decrease significantly, and with the increase of the filter cake thickness, the decrease of silicon and aluminum does not change significantly.

[0047] Example 4

[0048] In this example, the influence of the system temperature on the aluminum removal effect in the aluminum removal step was studied. The specific operation was as follows:

[0049] The filtrate from the filter press with an aluminum content of 72 ppb and a silicon dioxide content of 1796 ppb was introduced into the buffer tank. At a temperature of 19 °C, hydrochloric acid was added to adjust the pH value, and then it was filtered through a Kail membrane. The changes in the silicon dioxide and aluminum contents in the filtrate were observed. The results are shown in Table 4.

[0050] Table 4

[0051]

[0052] As can be seen from Table 4, in the range of pH value from 8 to 9.5, the value of aluminum ions is the lowest. When the pH value > 10, the aluminum ions increase significantly. When the pH value < 8, the aluminum ions also show an increasing trend; the influence of silicon dioxide on the change of pH value is relatively small.

[0053] Example 5

[0054] In this example, the influence of the system temperature on the aluminum removal effect in the aluminum removal step was studied. The specific operation was as follows:

[0055] The pressure - filtered effluent with an aluminum content of 72 ppb and a silicon dioxide content of 1796 ppb was introduced into the buffer tank. Under the condition of pH = 9, the system temperature was adjusted, and then it was filtered through a Kail membrane. The changes in the silicon dioxide and aluminum contents in the filtered effluent were observed. The results are shown in Table 5.

[0056] Table 5

[0057]

[0058] As can be seen from Table 5, in the temperature range of 18 - 20 °C, the contents of aluminum ions and silicon dioxide are both relatively low. As the temperature increases, the contents of aluminum ions and silicon dioxide both increase. When the temperature decreases, the changes in the contents of aluminum ions and silicon dioxide are not significant. Considering that the temperature of the pressure - filtered effluent is generally between 18 - 20 °C, this temperature range is selected as the optimal temperature range.

[0059] Example 6

[0060] A method for removing silicon and aluminum in brine for chlor - alkali production includes the following steps:

[0061] (1) Physical silicon removal: The desulfurized waste water from the power plant and low - silicon water (industrial water or the middle water produced by the reverse osmosis membrane group) are introduced into the salt sludge washing tank for salt sludge washing at a volume ratio of 1:1, and then pumped into a secondary filter press for pressure filtration, keeping the filter cake thickness of the secondary filter press at about 3 cm. The chloride ion content in the pressure - filtered effluent ≤ 15 g / l, and the chloride ion content in the salt sludge filter cake ≤ 1.0 g / l. The pressure - filtered effluent is introduced into the mixing tank, and a mixture of sodium hydroxide and sodium carbonate (mass ratio 1:2) is added for reaction. The concentration of this mixture in the pressure - filtered effluent is 1 g / L, and then it enters the inclined - plate settler for sedimentation. The sedimented sludge is returned to the salt sludge washing tank, and the supernatant of the sedimentation is filtered through a multi - media filter, a cation bed, an ultra - filtration membrane group, and a reverse osmosis membrane group in sequence. The produced middle water is recycled as low - silicon water, and the concentrated water enters the chemical silicon removal unit;

[0062] (2) Chemical desilication: The concentrated water produced by the reverse osmosis membrane module is introduced into the concentrated water reaction tank, and cerium sulfate and lanthanum carbonate (mass ratio 1:1) are added for desilication. The total concentration of cerium sulfate and lanthanum carbonate in the concentrated water is 30 mg / L. Then it is pumped into a primary filter press for filtration, keeping the filter cake thickness of the primary filter press at about 5 cm. The filtrate after filtration enters the de-aluminum unit;

[0063] (3) De-aluminum: The filtrate after filtration in step (2) is introduced into a buffer tank. At a temperature of 20 °C, hydrochloric acid is added to adjust the pH value to 9.5, and then it is filtered through a Kail membrane to obtain brine with silicon and aluminum removed. After testing, its silicon dioxide content is 1572 ppb and its aluminum ion content is 24 ppb.

[0064] Example 7

[0065] A method for removing silicon and aluminum in brine for chlor-alkali production, comprising the following steps:

[0066] (1) Physical desilication: The desulfurized wastewater from the power plant and low-silicon water (industrial water or intermediate water produced by the reverse osmosis membrane module) are introduced into the salt sludge washing tank together according to a volume ratio of 1:2 for salt sludge washing, and then pumped into a secondary filter press for filtration, keeping the filter cake thickness of the secondary filter press at about 5 cm. The chloride ion content in the filtrate after filtration ≤ 15 g / l, and the chloride ion content in the salt sludge filter cake ≤ 1.0 g / l. The filtrate after filtration is introduced into a mixing tank, and a mixture of sodium hydroxide and sodium carbonate (mass ratio 2:1) is added for reaction. The concentration of this mixture in the filtrate after filtration is 5 g / L, and then it enters an inclined plate settler for sedimentation. The sedimented sludge is returned to the salt sludge washing tank, and the supernatant after sedimentation is filtered through a multi-media filter, a cation bed, an ultrafiltration membrane module, and a reverse osmosis membrane module in sequence. The produced intermediate water is recycled as low-silicon water, and the concentrated water enters the chemical desilication unit;

[0067] (2) Chemical desilication: The concentrated water produced by the reverse osmosis membrane module is introduced into the concentrated water reaction tank, and hafnium sulfate and lanthanum carbonate (mass ratio 1:1) are added for desilication. The total concentration of hafnium sulfate and lanthanum carbonate in the concentrated water is 50 mg / L. Then it is pumped into a primary filter press for filtration, keeping the filter cake thickness of the primary filter press at about 2 cm. The filtrate after filtration enters the de-aluminum unit;

[0068] (3) De-aluminum: The filtrate after filtration in step (2) is introduced into a buffer tank. At a temperature of 18 °C, hydrochloric acid is added to adjust the pH value to 8, and then it is filtered through a Kail membrane to obtain brine with silicon and aluminum removed. After testing, its silicon dioxide content is 1465 ppb and its aluminum ion content is 25 ppb.

Claims

1. A method for removing silicon and aluminum in brine for chlor-alkali production, characterized in that: It includes the following steps: (1) Physical desilication: Feed the desulfurized wastewater from the power plant and low-silicon water into the salt sludge washing tank for salt sludge washing, then pump it into the secondary filter press for filtration. The filtrate after filtration enters the mixing tank, reacts after adding alkaline substances, and then enters the inclined plate settler for sedimentation. The sedimented sludge returns to the salt sludge washing tank, and the supernatant after sedimentation is filtered successively through a multi-media filter, a cation bed, an ultrafiltration membrane module, and a reverse osmosis membrane module. The produced reclaimed water is reused as low-silicon water, and the concentrated water enters the chemical desilication unit; (2) Chemical desilication: Feed the concentrated water produced by the reverse osmosis membrane module into the concentrated water reaction tank, add a desilication coagulant for desilication, and then pump it into the primary filter press for filtration. The filtrate after filtration enters the de-aluminum unit; (3) De-aluminum: Feed the filtrate after filtration in step (2) into the buffer tank, adjust the pH value to 8 - 9.5 by adding hydrochloric acid at a temperature of 18 - 20 °C, and then filter it through a Kai membrane to obtain refined brine with silicon and aluminum removed; In step (1), the filter cake thickness of the secondary filter press is 3 - 5 cm; In step (2), the filter cake thickness of the primary filter press is 2 - 5 cm; In step (2), the desilication coagulant added to the concentrated water reaction tank is a rare earth metal salt; the rare earth metal salt is one or more of cerium sulfate and lanthanum carbonate.

2. The method for removing silicon and aluminum in brine for chlor-alkali production according to claim 1, characterized in that: In step (1), the chloride ion content in the desulfurized wastewater from the power plant is 20 - 30 g / l, the silicon dioxide content is 9000 - 11000 ppb, and the aluminum ion content is 400 - 600 ppb.

3. The method for removing silicon and aluminum in brine for chlor-alkali production according to claim 1, characterized in that: In step (1), the low-silicon water is industrial water or the reclaimed water produced by the reverse osmosis membrane module or a mixture of both; the volume ratio of the low-silicon water to the desulfurized wastewater from the power plant is (1 - 2):

1.

4. The method for removing silicon and aluminum in brine for chlor-alkali production according to claim 1, characterized in that: In step (1), the chloride ion content in the filtrate after filtration ≤ 15 g / l, and the chloride ion content in the salt sludge filter cake ≤ 1.0 g / l.

5. The method for removing silicon and aluminum in brine for chlor-alkali production according to claim 1, characterized in that: In step (1), the alkaline substances added to the mixing tank are a mixture of sodium hydroxide and sodium carbonate with a mass ratio of 1:(0.5 - 2); the mass-to-volume ratio of the alkaline substances to the filtrate after filtration is (1 - 5) g:1 L.

6. The method for removing silicon and aluminum in brine for chlor-alkali production according to claim 1, characterized in that: In step (2), the mass-to-volume ratio of the desilication coagulant to the concentrated water is (10 - 30) mg:1 L.

7. The method for removing silicon and aluminum in brine for chlor-alkali production according to claim 1, characterized in that: In step (3), in the brine with silicon and aluminum removed, the silicon dioxide content is 1300 - 1800 ppb, and the aluminum ion content is 20 - 30 ppb.

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