Method for preparing high-purity sodium chloride from chlor-alkali wastewater

Abstract

The application discloses a method for preparing high-purity sodium chloride from chlor-alkali wastewater, wherein a reducing agent is added to the chlor-alkali wastewater, and the mixture is stirred at 10-20 rps for 20-30 min to obtain a reduction treatment solution; free residual chlorine is removed; then, a pH regulator is used to adjust the pH value of the reduction treatment solution; and high-purity sodium chloride is obtained through evaporation crystallization. In the application, formic acid is used as the reducing agent, and the formic acid reduction method is used to remove free residual chlorine, and the removal efficiency can reach 83%-99%; meanwhile, the formic acid can effectively neutralize a large amount of hydroxyl ions in the wastewater, so that the pH value is reduced, which is beneficial to the evaporation of sodium chloride in the crystallization process. The application further processes the solution from which the free residual chlorine is removed, and high-purity sodium chloride is prepared. The application simultaneously realizes the treatment and purification of the chlor-alkali wastewater and the recycling of resources, and has the advantages of simple operation, high efficiency and the like.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, specifically relating to a method for preparing high-purity sodium chloride from chlor-alkali wastewater. Background Technology

[0002] The chlor-alkali industry is the largest sector in modern chemical industry, a relatively mature industry that mainly produces caustic soda, chlorine, and hydrogen through the electrolysis of brine. Chlor-alkali products are diverse and highly interconnected, with thousands of downstream products, possessing high economic value. It is widely used in various vital sectors of the national economy, including agriculture, petrochemicals, light industry, textiles, building materials, power, metallurgy, and national defense, playing a pivotal role in my country's economic development. However, chlor-alkali industrial wastewater is characterized by large volume, significant variations in water quality, high salinity, high chloride ion content, and complex composition, making it one of the most challenging aspects of industrial wastewater treatment.

[0003] The chlor-alkali industry generates a large amount of waste gas, containing significant amounts of tail chlorine. Currently, the common method is alkaline absorption, which involves reacting sodium hydroxide with chlorine to produce sodium hypochlorite, which is then recycled to absorb the tail chlorine, thus treating accidental chlorine gas and eliminating chlorine pollution. This process inevitably produces wastewater containing sodium hypochlorite. Sodium hypochlorite solution has strong oxidizing properties. Due to the high asymmetry of the valence electron configuration of the anion and the large ionic potential (Z / r) of the central chlorine atom, hypochlorite is unstable and has a strong ability to gain electrons to transform into more stable Cl2 molecules or Cl... - The ability, that is, manifested as ClO - It possesses strong oxidizing power. This wastewater not only produces toxic and corrosive fumes upon high-temperature decomposition, but also releases free chlorine that can cause poisoning, burns, and sensitization. Therefore, how to harmlessly treat chlor-alkali wastewater containing sodium hypochlorite has always been a hot topic in the chlor-alkali industry.

[0004] The method proposed by Huang Ziliang, Xia Jianhui, et al. in their article "Research on the Treatment of Sodium Hypochlorite-Containing Wastewater with Sodium Sulfite" involves reducing sodium hypochlorite to sodium chloride and sodium sulfate with sodium sulfite, and discharging the wastewater into the sewage network after passing the test. However, this method requires precise control of the amount of reducing agent added. If the amount of reducing agent is too small, the free chlorine in the wastewater will not meet the standard; if the amount of reducing agent is too large, the COD in the wastewater will exceed the standard. This method has disadvantages such as high operational precision and small wastewater treatment volume. Yang Song, Hu Dan, et al. in their article "Research on Chlor-Alkali Wastewater Treatment Process" proposed a combined treatment process of ultraviolet photocatalysis and hydrogen peroxide method; however, this method has disadvantages such as low photocatalytic efficiency and long treatment time. CN109879505B discloses a chlor-alkali wastewater treatment process, which includes the following steps: primary sedimentation, primary pH adjustment, primary treatment, secondary pH adjustment, secondary treatment, secondary sedimentation, and primary catalytic treatment. In the secondary treatment, a composite reducing agent composed of ferrous sulfate, sodium sulfite, and sodium thiosulfate in equal mass ratios is added, which achieves the purification of chlor-alkali wastewater. CN109912075A discloses a method for recycling and reusing chlor-alkali wastewater. First, the chlor-alkali wastewater is deeply oxidized using an oxidant, then coagulated and precipitated using a flocculant, then passed through a multi-media filter and an activated carbon filter in sequence, and finally a resin softening process is used to reduce the hardness so that it meets the recycling and reuse index, thus achieving zero discharge and recycling of chlor-alkali wastewater.

[0005] The methods described above all generate a large amount of impurities, requiring multiple sedimentation and filtration processes for treatment. Furthermore, the treatment only yields purified wastewater, without clearly specifying whether the generated impurities can be recycled. Therefore, it is necessary to develop a new process that can efficiently treat chlor-alkali wastewater while simultaneously recycling the generated impurities. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for preparing high-purity sodium chloride from chlor-alkali wastewater. This method utilizes formic acid to reduce sodium hypochlorite, achieving efficient removal of free residual chlorine from the wastewater. Furthermore, the solution from which residual chlorine has been removed can be used to prepare high-purity sodium chloride crystals, thus realizing resource utilization.

[0007] To achieve the above objectives, the present invention provides a method for preparing high-purity sodium chloride from chlor-alkali wastewater, comprising the following steps:

[0008] (1) Dechlorination: Add a reducing agent to chlor-alkali wastewater and stir to obtain a reduced treatment solution;

[0009] (2) Crystallization: Add pH adjuster to the reduction solution obtained in step (1), and then crystallize by evaporation to obtain high-purity sodium chloride and evaporated condensate.

[0010] Preferably, the chlor-alkali wastewater in step (1) has a pH of 12-13, a free residual chlorine content of 13000-18000 mg / L, and a TDS content of 8-11 g / L.

[0011] Preferably, the reducing agent in step (1) is formic acid, and the amount of reducing agent used is 1.2-1.4% of the volume of chlor-alkali wastewater.

[0012] Preferably, the stirring speed is 10-20 rpm, the reaction temperature is 30-50℃, and the reaction time is 20-30 min.

[0013] Preferably, the amount of pH adjuster used in step (2) is 0.67% of the volume of the reduction treatment solution.

[0014] Preferably, the pH adjuster is hydrochloric acid with a concentration of 10-12 mol / L.

[0015] Preferably, the conditions for evaporation and crystallization in step (2) are a temperature of around 110°C and a reaction time of 2-3 hours.

[0016] Preferably, the sodium chloride used in step (2) for evaporation crystallization has a purity of 90%.

[0017] Preferably, the evaporation condensate in step (2) has a pH of 7.0-7.2, a free residual chlorine content of 0.12 mg / L, a TDS content of 217 mg / L, and a salinity of 0.0 ppt, and can be used as industrial cooling circulating water.

[0018] The beneficial effects of this invention are as follows:

[0019] 1. Formic acid was selected as a reducing agent to treat free residual chlorine in chlor-alkali wastewater, achieving purification of the wastewater and reducing the removal rate of free residual chlorine to 83%-99%, significantly improving the removal efficiency. Gas chromatography-mass spectrometry (GC-MS) was used to detect the gases generated during the reaction; qualitative analysis revealed that the gases were carbon dioxide and a small amount of volatile formic acid.

[0020] 2. High-purity sodium chloride was prepared by evaporation and crystallization of wastewater after the removal of free residual chlorine. It can be reused in the chemical industry, realizing the resource recovery of sodium and chlorine elements, improving economic efficiency, and avoiding the secondary impact on the environment caused by the direct discharge of wastewater with high salt content.

[0021] 3. Using formic acid as a reducing agent can effectively neutralize a large number of hydroxide ions in the wastewater, reducing the alkalinity of the chlor-alkali wastewater from strong alkalinity (pH value above 12) to weak alkalinity (pH value 7-8), saving the amount of pH adjuster used, and at the same time making it more conducive to the evaporation of sodium chloride in the evaporation crystallization process, thus improving the purity of sodium chloride. Attached Figure Description

[0022] Figure 1 The figure shows the effect of different reducing agents on the removal of free residual chlorine in chlor-alkali wastewater in Example 1. Figure A shows the effect of different reducing agents on the removal of free residual chlorine in chlor-alkali wastewater, and Figure B shows the effect of formic acid and hydroxylamine hydrochloride on the removal of free residual chlorine in chlor-alkali wastewater.

[0023] Figure 2 The graph shows the effect of the reducing agent dosage in Example 2 on the removal of free residual chlorine in chlor-alkali wastewater.

[0024] Figure 3 This is a graph showing the effect of reaction temperature on the removal of free residual chlorine in chlor-alkali wastewater in Example 3.

[0025] Figure 4 This is a graph showing the effect of reaction time on the removal of free residual chlorine in chlor-alkali wastewater in Example 4.

[0026] Figure 5 The graph shows the effect of stirring speed on the removal of free residual chlorine in chlor-alkali wastewater in Example 5.

[0027] Figure 6 The graph shows the effect of the number of times the reducing agent is added on the removal of free residual chlorine in chlor-alkali wastewater in Example 6.

[0028] Figure 7 This is a line graph showing the effect of the amount of reducing agent on the pH of chlor-alkali wastewater in Example 7.

[0029] Figure 8 XRD pattern of sodium chloride preparation from chlor-alkali wastewater in Example 8

[0030] Figure 9 The XRD patterns are of sodium chloride prepared at different pH values ​​in Example 9. Detailed Implementation

[0031] The technical solution of the present invention will be further explained and described below with reference to the accompanying drawings and specific embodiments. It is worth noting that the following embodiments are only preferred embodiments of the present invention and should not be construed as limiting the present invention. The scope of protection of the present invention should be determined by the contents of the claims. Modifications and substitutions made by those skilled in the art to the technical solution of the present invention without creative effort all fall within the scope of protection of the present invention.

[0032] Formic acid: purchased from Tianjin Beichen Fangzheng Reagent Factory, AR value is 88%.

[0033] Chlor-alkali wastewater: This wastewater, containing sodium hypochlorite, is produced by a chlor-alkali chemical plant after sodium hydroxide absorbs residual chlorine from the tail gas. Its physicochemical properties are shown in the table below:

[0034]

[0035] Example 1: Effect of Reducing Agent Type on Free Residual Chlorine Removal Efficiency

[0036] (1) Take chlor-alkali wastewater, the content of its free residual chlorine is 11360 mg / L;

[0037] (2) Different reducing agents were added to the chlor-alkali wastewater in step (1), and the free residual chlorine content in the wastewater was determined by stirring at 20 rpm for 20 min using an LH-C10F residual chlorine detector (detection range: 0-10 mg / L). The reducing agents included powdered reducing agents and liquid reducing agents. The powdered reducing agents included hydroxylamine hydrochloride, glucose, and humic acid, while the liquid reducing agents included formic acid, methanol, and ethanol. The amount of reducing agent added was 1.2% of the volume of the chlor-alkali wastewater.

[0038] (3) For the reducing agents that were screened and found to have the best effect on removing free residual chlorine, the dosage was increased to 25% of the volume of chlor-alkali wastewater and a large-dose experiment was conducted; the content of free residual chlorine in the chlor-alkali wastewater was 12360 mg / L.

[0039] The results are as follows Figure 1 As shown in Figure A, methanol, ethanol, and glucose as reducing agents showed poor removal efficiency for residual chlorine, only 8%-16%; humic acid had the best removal efficiency, but the resulting solution was dark brown with high turbidity, which was not conducive to subsequent evaporation and crystallization; while using hydroxylamine hydrochloride as a reducing agent reduced the content of free residual chlorine to 6560 mg / L, with a removal rate of 42%; and using formic acid as a reducing agent reduced the content of residual chlorine to 7240 mg / L, with a removal rate of 36%.

[0040] The results are as follows Figure 1 As shown in Figure B, the addition of a large dose of formic acid can reduce the content of free residual chlorine in chlor-alkali wastewater to 260 mg / L, while the content in hydroxylamine hydrochloride is only reduced to 5120 mg / L. This indicates that when the dosage of reducing agent is increased, formic acid can effectively reduce the content of free residual chlorine in wastewater, and its removal rate can reach 98%. Therefore, formic acid was selected as the reducing agent in subsequent experiments.

[0041] Example 2: Effect of reducing agent dosage on the removal efficiency of free residual chlorine

[0042] Take chlor-alkali wastewater and add 0.8%, 1.0%, 1.2%, 1.4%, and 1.6% formic acid solutions of different volumes of chlor-alkali wastewater, respectively. Stir and react at 30℃ and 20 rpm. After 20 minutes, detect the content of free residual chlorine in the wastewater.

[0043] The results are as follows Figure 2As shown, with the increase of formic acid dosage, the free residual chlorine content in chlor-alkali wastewater gradually decreased from 11840 mg / L to 145 mg / L. When the formic acid dosage exceeded 1.4%, the removal efficiency of residual chlorine remained essentially unchanged. This indicates that increasing the dosage of formic acid can significantly improve the removal efficiency of free residual chlorine. Specifically, when the formic acid dosage accounts for 1.2%-1.4% of the chlor-alkali wastewater volume, the removal rate can reach 83%-99%, indicating that the optimal dosage of formic acid is 1.2-1.4% of the chlor-alkali wastewater volume.

[0044] Example 3: Effect of Temperature on the Removal of Free Residual Chlorine

[0045] Chlor-alkali wastewater was taken, formic acid was added to it, and the mixture was stirred and reacted at different temperatures. After 20 minutes, the content of free residual chlorine in the wastewater was measured. The amount of formic acid used was 1.2% of the volume of chlor-alkali wastewater, and the reaction temperatures were 30℃, 40℃, 50℃, and 60℃.

[0046] The results are as follows Figure 3 As shown, the removal rate of free residual chlorine increased from 93% to 99% as the reaction temperature increased from 30℃ to 50℃. However, when the temperature continued to rise to 60℃, the removal rate of free residual chlorine decreased, indicating that the removal of free residual chlorine needs to be controlled within a certain temperature range to achieve good removal results, with the optimal reaction temperature being 50℃. However, the treatment effect was also good at a reaction temperature of 30℃, requiring less heating energy and reducing costs.

[0047] Example 4: Effect of reaction time on the removal efficiency of free residual chlorine

[0048] Take chlor-alkali wastewater, add 1.2% by volume of formic acid, stir and react at 30℃ and 20 rpm, and take samples every 10 minutes to detect the content of free residual chlorine.

[0049] The results are as follows Figure 4 As shown, the initial content of free residual chlorine in chlor-alkali wastewater was 14540 mg / L. After 20 min of reaction, it gradually decreased to 970 mg / L. However, as the reaction time was extended from 20 min to 30 min, the content of free residual chlorine increased slightly, but it was still significantly lower than the content at 10 min of reaction, indicating that the optimal reaction time was 20-30 min.

[0050] Example 5: Effect of stirring speed on the removal efficiency of free residual chlorine

[0051] Take chlor-alkali wastewater, add 1.2% by volume of formic acid, stir and react at 30℃, and detect the content of free residual chlorine in the wastewater after 20 minutes. The stirring speeds are 10 rpm, 20 rpm and 30 rpm.

[0052] The results are as follows Figure 5As shown, the removal efficiency of free residual chlorine did not change much (92%-93%) as the rotation speed increased from 10 rpm to 20 rpm; however, when the rotation speed increased to 30 rpm, the removal rate became 88%. This indicates that 10-20 rpm is the optimal reaction speed range.

[0053] Example 6: Effect of the number of formic acid additions on the removal efficiency of free residual chlorine

[0054] Chlor-alkali wastewater was collected, and formic acid was added to it at different times: once, twice, four times, and six times, ensuring that the total amount of formic acid was 1.2% of the volume of chlor-alkali wastewater. The reaction temperature was 30℃, and the stirring speed was 20 rpm. When the total reaction time after adding formic acid was 20 minutes, samples were taken to detect the content of free residual chlorine. The interval between the two formic acid additions was 10 minutes, and the formic acid added each time was 0.6%; the interval between the four formic acid additions was 5 minutes, and the formic acid added each time was 0.3%; the interval between the six formic acid additions was 3 minutes, and the formic acid added each time was 0.2%.

[0055] The results are as follows Figure 6 As shown, when formic acid is added in a single dose, the free residual chlorine content in the treated chlor-alkali wastewater is 970 mg / L, with a removal rate of 93%. This indicates that a single dose of formic acid can effectively treat chlor-alkali wastewater within 20 minutes, and the treatment effect on free residual chlorine deteriorates as the interval time increases. Therefore, the optimal method for adding formic acid is a single dose.

[0056] Example 7 Effect of reducing agent dosage on pH

[0057] Take chlor-alkali wastewater, measure the initial pH value, add formic acid in 6 portions, each time adding 0.2% of the volume of chlor-alkali wastewater, for a total of 1.2% formic acid. After each addition of formic acid, stir for 1 minute and then measure the pH value.

[0058] The results are as follows Figure 7 As shown, the chlor-alkali wastewater is strongly alkaline (12.65). With increasing formic acid dosage (0.2% → 1.2%), the pH of the treated chlor-alkali wastewater decreases (12.65 → 7.98). This indicates that adding 1.2% formic acid by volume can neutralize the chlor-alkali wastewater, and also suggests that adjusting the pH of the wastewater may enable the preparation of high-purity sodium chloride.

[0059] Example 8: Preparation of Sodium Chloride from Chlor-Alkali Wastewater

[0060] (1) Take chlor-alkali wastewater, add 1.2% formic acid to it, stir and react at 40℃ and 20rpm, and obtain a reduced treatment solution after 20min. The pH value is measured to be 7.62.

[0061] (2) Take half the volume of the reduction treatment solution and evaporate it at 110℃ to obtain crystals, which are recorded as sample 1;

[0062] (3) Add 12 mol / L concentrated hydrochloric acid to the other half of the reduction treatment solution to adjust the pH value to 5.77, and evaporate and crystallize at 110℃ to obtain crystals, which are recorded as sample 2.

[0063] The results are as follows Figure 8 As shown, using the pure NaCl (100% purity) (PDF#05-0628) officially released in the PDF-5+ standard diffraction database by the International Center for Diffraction Data (ICDD) as a reference, it was found that samples 1 and 2 partially match the characteristic diffraction peaks of pure NaCl, indicating that sodium chloride can be prepared from chlor-alkali wastewater. Compared with sample 1, the impurity peaks of sample 2 were significantly reduced, indicating that the purity of sodium chloride can be further improved by adjusting the pH of the treatment solution.

[0064] Example 9: Effect of pH on Sodium Chloride Purity

[0065] (1) Take chlor-alkali wastewater, add 1.2% formic acid to it, stir and react at 40℃ and 20rpm, and obtain a reduced treatment solution after 20min. The pH value is measured to be 7.06.

[0066] (2) The reduction solution was divided into four equal parts, and the pH was adjusted to 6.68, 6.58, 4.26 and 0.61 respectively with 12mol / L concentrated hydrochloric acid. Then, the solution was evaporated and crystallized at 110℃ to prepare sodium chloride.

[0067] The results are as follows Figure 9 As shown, using pure NaCl (100% purity) (PDF#05-0628) from the officially released PDF-5+ standard diffraction database of the International Center for Diffraction Data (ICDD) as a reference, it was found that sodium chloride contained many impurities when the pH value was as low as 4.26. When the pH was between 4.26 and 6.68, the impurity peaks of the obtained sodium chloride decreased significantly, and the purity of sodium chloride gradually increased as the pH value decreased. When the pH was in the range of 6.5 to 6.7, the obtained sodium chloride corresponded perfectly to the standard card, without any impurity peaks, indicating that its purity was quite high.

[0068] (3) The purity of the prepared sodium chloride was analyzed by ion chromatography: 10 mg of sodium chloride prepared in step (2) was accurately weighed as a sample (prepared at pH 6.5-6.7) and placed in a 250 mL volumetric flask. It was diluted with water to the mark and extracted in an ultrasonic oscillator for 5 min. After extraction, it was cooled to room temperature, and the supernatant was filtered through a 0.45 μm filter membrane for analysis. The above sample was analyzed by ion chromatography. The chloride ion content was determined from the peak area on the standard working curve. The calculated chloride content was 8.99 mg, so the purity of the sample was approximately 90%.

Claims

1. A method for preparing high-purity sodium chloride from chlor-alkali wastewater, characterized in that: Includes the following steps: (1) Dechlorination: Add a reducing agent to chlor-alkali wastewater and stir to obtain a reduced treatment solution; (2) Crystallization: Add pH adjuster to the reduced solution obtained in step (1), and then obtain high-purity sodium chloride and condensate by evaporation and crystallization; The reducing agent mentioned in step (1) is formic acid, and the amount of reducing agent used is 1.2-1.4% of the volume of chlor-alkali wastewater.

2. The method for preparing high-purity sodium chloride from chlor-alkali wastewater according to claim 1, characterized in that: The chlor-alkali wastewater described in step (1) has a pH of 12-13, a free residual chlorine content of 11000-18000 mg / L, and a TDS content of 8-11 g / L.

3. The method for preparing high-purity sodium chloride from chlor-alkali wastewater according to claim 1, characterized in that: The stirring speed is 10-20 rpm, the reaction temperature is 30-50℃, and the reaction time is 20-30 min.

4. The method for preparing high-purity sodium chloride from chlor-alkali wastewater according to claim 1, characterized in that: The amount of pH adjuster used in step (2) is 0.67% of the volume of the reduction treatment solution.

5. A method for preparing high-purity sodium chloride from chlor-alkali wastewater according to claim 4, characterized in that: The pH adjuster is hydrochloric acid with a concentration of 10-12 mol / L.

6. The method for preparing high-purity sodium chloride from chlor-alkali wastewater according to claim 1, characterized in that: The conditions for evaporation and crystallization in step (2) are a temperature controlled at 100-120℃ and a reaction time of 2-4 h.

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