A method for resource recovery of silicon-containing alkaline wastewater based on silicon concentration differences

Abstract

This invention relates to the field of alkaline wastewater treatment and resource recovery technology in the semiconductor or photovoltaic industry, specifically to a resource recovery method for silicon-containing alkaline wastewater based on silicon concentration differences. Addressing the problems of low silica recovery rate, low product added value, high energy consumption, and environmental pollution in existing technologies, this invention constructs a "two products, two pathways" technical system to achieve targeted recovery of high-purity silica gel from low-silicon alkaline wastewater and high-purity silica from high-silicon alkaline wastewater, combining efficient resource utilization with environmental friendliness.

Description

Technical Field

[0001] This invention relates to the field of alkaline wastewater treatment and resource recovery technology in the semiconductor or photovoltaic industry, specifically to a resource recovery method for silicon-containing alkaline wastewater based on silicon concentration differences. Background Technology

[0002] The silicon-containing alkaline wastewater generated by the semiconductor and photovoltaic industries contains high concentrations of silicon dioxide (SiO2) and strongly alkaline substances (such as NaOH and Na2SiO3). If discharged directly without effective treatment, the strongly alkaline substances in the wastewater will significantly alter the pH value of the water body, inhibit the survival of aquatic organisms, and the infiltration of alkaline wastewater can easily lead to soil salinization and threaten drinking water safety through groundwater migration. Excessive silicon dioxide deposition may block waterways, affecting the recycling of water resources. Furthermore, the value of silicon dioxide as an important industrial raw material (such as silica gel and precipitated silica) has not been fully explored.

[0003] Currently, silicon-containing wastewater treatment technologies include: (a) Neutralization precipitation: pH is adjusted by adding acid to separate silicon dioxide into a precipitate. However, this method is difficult to control product purity, often resulting in residual metal ions, and the precipitate requires further treatment. (b) Evaporation crystallization: Sodium silicate crystals are recovered through heating and concentration, but this method is energy-intensive and produces a single product, failing to meet diverse market demands. (c) Membrane separation technology: While it can retain silicon dioxide, membrane fouling is a serious problem, and operation and maintenance costs are high. Meanwhile, the above technologies generally have the following defects: (1) Low added value of products: Traditional acid precipitation products are mostly heterogeneous silica precipitates with low purity and many impurities. They fail to achieve high-value conversion of silica and only remain at the primary recovery stage; (2) Insufficient resource utilization: The single precipitation recovery rate is low, often accompanied by a large loss of silica, which cannot meet the resource utilization needs of low-concentration silica wastewater. Moreover, there is a lack of targeted design for wastewater concentration differences, making it difficult to meet the needs of different working conditions; (3) Weak control capability: Due to the lack of multi-stage conditioning or intermediate dissolution processes, the precipitate particle size and structure are difficult to control, and the product performance fluctuates greatly; (4) High environmental load: In order to ensure precipitation efficiency, a large amount of strong acid needs to be added, resulting in a large amount of by-product waste liquid and a heavy treatment burden, and the overall environmental protection is insufficient. Summary of the Invention

[0004] To address the aforementioned problems in existing technologies, this invention proposes a resource recovery method for silicon-containing waste alkaline water based on differences in silicon concentration. This method constructs a technical system of "two products and two pathways" to achieve targeted recovery of high-purity silica gel from low-silicon waste alkaline water and high-purity silica from high-silicon waste alkaline water, thus combining efficient resource utilization with environmental friendliness.

[0005] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0006] A method for resource recovery of silicon-containing waste alkaline water based on silicon concentration differences includes the following steps: dividing the silicon-containing waste alkaline water into low-silicon waste alkaline water and high-silicon waste alkaline water according to the silicon concentration difference, and recovering silica gel and precipitated silica from the low-silicon waste alkaline water and high-silicon waste alkaline water respectively;

[0007] The process for recovering silica gel from low-silica waste alkaline water is as follows:

[0008] S11. Add sulfuric acid solution to the low-silica waste alkaline water reaction tank until the pH value of the low-silica waste alkaline water drops to 7.5~8, and then react for a period of time to enrich silica into gel. Centrifuge to separate and obtain filter residue.

[0009] S12. Transport the filter residue to the secondary reaction tank, add sodium hydroxide solution until the filter residue is completely dissolved, and obtain a transparent sodium silicate-containing solution;

[0010] S13. After the sodium silicate solution is heated to a certain temperature in a heater, sulfuric acid solution is added until the pH value of the sodium silicate solution drops to 8-8.5 to carry out the aging reaction. After the reaction is completed, the product is obtained by centrifugation.

[0011] The process for recovering silica from high-silica waste alkaline water is as follows:

[0012] S21. In the high-silicon waste alkaline water reaction tank, add precipitated silica seeds at a mass of 0.4% to 0.6% of the high-silicon waste alkaline water, then add sulfuric acid solution until the pH value of the high-silicon waste alkaline water drops to 7.5 to 8. React for a period of time to allow the silica to fully precipitate, and then separate the precipitate by centrifugation.

[0013] S22. The precipitate is washed, dried and pulverized to obtain the silica product.

[0014] Preferably, the silicate content in the low-silica waste alkaline water is less than 5 g / L; the silicate content in the high-silica waste alkaline water is greater than 10 g / L.

[0015] Preferably, the mass fraction of sulfuric acid solution in S11 is 25% to 35%; the sulfuric acid solution is added while stirring, and the stirring speed is controlled at 400-600 rpm, with the addition rate of sulfuric acid solution being 0.4 to 0.6 pH / min.

[0016] Preferably, after adjusting the pH in S11, the reaction is carried out at 300-400 rpm for 60-120 min, and then aged at 100-200 rpm for 60-120 min to fully enrich the silica into a gel.

[0017] Preferably, the concentration of sodium hydroxide solution in S12 is 1.5-2 mol / L, and the concentration of the transparent sodium silicate solution is 10%-15%.

[0018] Preferably, the temperature in S13 is heated to 80℃~90℃; the mass fraction of the sulfuric acid solution is 25%~35%; the sulfuric acid solution is added while stirring, and the stirring speed is controlled at 400-600 rpm, with the addition rate of the sulfuric acid solution being 0.4~0.6 pH / min.

[0019] Preferably, after adjusting the pH in S13, the aging reaction is carried out for 60–120 min.

[0020] Preferably, the mass fraction of sulfuric acid solution in S21 is 8% to 12%; the sulfuric acid solution is added while stirring, and the stirring speed is controlled at 400 to 600 rpm, with the addition rate of sulfuric acid solution being 0.4 to 0.6 pH / min.

[0021] Preferably, after adjusting the pH in S21, the reaction is carried out at 400-500 rpm for 60-120 min, and then aged at 100-200 rpm for 100-140 min to allow silica to fully precipitate.

[0022] Preferably, in step S22, the precipitate is washed 4 to 6 times with deionized water to remove impurity ions, and then spray-dried at a temperature of 100 to 110°C for 7 to 9 hours. The dried product is then pulverized using an air jet mill.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] (1) This invention proposes a silicon product recycling method with high resource utilization and differentiated product output capabilities by constructing a technical system of "two products and two paths" based on alkaline wastewater with different silicon contents generated by the semiconductor or photovoltaic industry. Low-silicon alkaline wastewater is enriched and concentrated to prepare high-purity silica gel (SiO2≥95%), while high-silicon alkaline wastewater is directly prepared into silica (SiO2≥96%~99%) through precise control, breaking through the limitations of the traditional single-product recycling mode. This scheme not only realizes the full high-value utilization of silicon resources, but also significantly reduces acid and alkali consumption and energy consumption through closed-loop process design, combining environmental friendliness and economic benefits, and providing an innovative solution for the green and sustainable development of the semiconductor / photovoltaic industry.

[0025] (2) The three-step silica gel recovery method of "acid precipitation → alkali dissolution → acid precipitation" proposed in this invention has significant advantages:

[0026] Innovative Path Structure: This invention is not a simple superposition of the traditional acid precipitation method, but rather a reaction chain based on the difference in solubility of silicon under different pH conditions. It constructs a stepwise enrichment + secondary phase transformation and reconstruction reaction chain, so that silicon dioxide is first converted into soluble sodium silicate under controlled conditions, and then reconstructed into high-purity silica gel. The recovery path has a clear division of labor and functional design, which belongs to the process chain hierarchical reconstruction.

[0027] Product quality has been significantly improved: the silica gel product obtained by secondary acid precipitation has a purity of ≥95%, with uniform particle size distribution, white color, and significantly lower impurity ions than traditional processes, meeting the performance requirements of high-end silica gel and being significantly superior to the primary silica precipitates obtained by existing acid precipitation methods.

[0028] Adaptability to low-concentration wastewater: This solution is specifically designed for silicon-containing wastewater with SiO2 concentrations below 5 g / L. By combining concentration and segmented control methods, it makes it possible to effectively recover low-silicon wastewater that is difficult to recover using traditional methods. The technology has a wider range of applications and higher resource utilization.

[0029] Balancing resource utilization and environmental friendliness: This invention effectively reduces acid consumption by more than 40% by adjusting the acid-base addition acceleration rate (0.5pH / min), precise pH control (8.0~8.5), and temperature regulation (85℃). The clear liquid generated in the entire process can be recycled, achieving zero wastewater discharge, which is significantly better than conventional methods with high acid consumption and the risk of secondary pollution.

[0030] (3) Based on the characteristics of high-concentration silicon-containing wastewater (>10g / L), this invention constructs an integrated process chain of "acid precipitation + precise pH control + multi-stage post-treatment", which has the following significant advantages compared with the traditional acid precipitation method:

[0031] Precise pH control improves precipitation efficiency and selectivity: This solution employs a synergistic regulation strategy of slow sulfuric acid addition, real-time pH monitoring, and constant-speed stirring to control the pH value to gradually decrease within the range of 7.5 to 8. This effectively avoids colloidal instability and co-precipitation of impurities caused by localized acidification, resulting in more complete silica precipitation and a more uniform structure, significantly improving silicon recovery rate and product stability.

[0032] An aging process is introduced to improve the morphology and crystallinity of the precipitate: a 60-minute stirring aging process is set after precipitation to promote particle growth and structural homogenization, which helps with subsequent washing and drying, improves the product's flowability and dispersibility, and provides a guarantee for the production of high-performance silica.

[0033] Systematic post-processing steps enhance product purity and application value: Through a standardized processing procedure of "five deionized water washes + 105℃ constant temperature drying + airflow pulverization (particle size ≤20μm)," the resulting silica product has a stable SiO2 content of 96% to 99%, which meets the national standard HG / T3061-2020 and is far superior to traditional coarse silica products, making it suitable for high-end rubber, electronic packaging and other fields.

[0034] The process is green and low-consumption, and has the potential for industrial promotion: except for the precipitation stage which requires temperature control and stirring, the rest of the process is basically carried out at normal temperature and pressure; by accurately adding sulfuric acid and recycling the filtrate, waste acid discharge and reagent waste are reduced, which has good environmental friendliness and economic operation. Attached Figure Description

[0035] Figure 1 The process flow diagram of silicon-containing waste alkaline water resource recovery based on silicon concentration difference provided by the present invention;

[0036] Figure 2 A diagram illustrating the engineering application of silicon-containing waste alkaline water resource utilization based on silicon concentration differences, provided by this invention.

[0037] Figure 3 This is a photograph of the silicone product recovered in this invention.

[0038] Figure 4 This is a photograph of the silica product from Example 1 of this invention.

[0039] Figure 5 This is a diagram of the pilot-scale equipment for Embodiment 2 of the present invention;

[0040] Figure 6 This is a photograph of the silica product from Example 2 of this invention.

[0041] Figure 7 The XRD patterns are of the silica products from Embodiments 1 and 2 of this invention. Detailed Implementation

[0042] The present invention will be further described below with reference to embodiments. Example 1

[0043] This invention provides a method for the resource recovery of silicon-containing waste alkaline water in the semiconductor / photovoltaic industry. Based on the difference in silicon concentration, it recovers silica gel and precipitated silica from low-silicon and high-silicon waste alkaline water, respectively. Through two pathways, the targeted recovery of silica gel and precipitated silica products is achieved. The process flow diagram is shown below. Figure 1 and Figure 2 As shown.

[0044] Pathway 1: Low-Silica Waste Alkaline Water → Silica Gel: Suitable for wastewater with silicate content below 5g / L. This path relies on a "step-by-step concentration + precision precipitation" process chain. Through the enrichment, dissolution, and reprecipitation of silica in low-silica waste alkaline water, high-purity silica gel products are prepared. The process flow is as follows:

[0045] S11. Measure 200 ml of low-silica waste alkaline water (silicate content 4880 mg / L, pH 11.5) and pour it into a beaker, placing it on a magnetic stirrer. Slowly add 30% sulfuric acid solution dropwise, controlling the stirring speed at 500 rpm, with the sulfuric acid solution added at a rate of 0.5 pH / min, adjusting the pH to 7.5-8. As sulfuric acid is added, the solution gradually changes from clear to a milky white gel. After adjusting the pH, maintain stirring at 350 rpm for 60 minutes to allow silica to fully form a gel precipitate. Then reduce the stirring speed to 150 rpm and continue aging for 60 minutes to promote gel stabilization. After the reaction is complete, centrifuge to obtain a silica-containing filter residue and filtrate. The filter residue is then processed further.

[0046] S12. Transfer the filter residue obtained in S11 to a new beaker, add 14.5 ml of 2 mol / L sodium hydroxide solution until completely dissolved, and control the stirring speed at 350 rpm. The gel gradually dissolves, forming a transparent sodium silicate-containing solution. Stir at room temperature for 30 minutes to enrich the sodium silicate concentration to 10%–15%. The silicate content is measured to be 10.71% by spectrophotometry, and the concentration is completed after confirmation.

[0047] S13. Transfer the concentrated sodium silicate solution from S12 to a water bath beaker, heat to 85°C and maintain a constant temperature. Slowly add 30% sulfuric acid solution, controlling the stirring speed at 500 rpm, adjusting the pH to 8-8.5 at an acceleration rate of 0.5 pH / min. With the addition of sulfuric acid, the solution rapidly gels; continue aging for 60 minutes to promote gel stability. After the reaction is complete, centrifuge to obtain the silica gel product, such as... Figure 3 As shown.

[0048] This process, conducted on a laboratory beaker scale, achieves a silicon recovery rate of ≥98% and a product purity of ≥95%, while reducing energy consumption by 80% compared to traditional evaporation and concentration methods. Heating is only required after enrichment and concentration; the remaining steps are performed at room temperature, reducing acid consumption by approximately 40%, allowing for filtrate reuse, and resulting in near-zero emissions. Compared to traditional chemical precipitation methods, this process combines high-efficiency resource utilization with environmental advantages, making it suitable for treating silicon-containing alkaline wastewater in the semiconductor / photovoltaic industry.

[0049] Pathway 2: High-Silica Waste Alkaline Water → Silica: Suitable for wastewater with silica content higher than 10g / L. This path relies on a process chain of "acid precipitation + precise pH adjustment + post-treatment". High-purity silica is prepared by controlling the precipitation of silica in high-silica waste alkaline water. The process flow is as follows:

[0050] S21. Measure 200 ml of high-silica waste alkaline water (silicate content 10868.6 mg / L, pH 12.34) and pour it into a beaker. Place the beaker on a magnetic stirrer. Add precipitated silica seed crystals (purity ≥99%) at 0.4%–0.6% of the high-silica waste alkaline water mass. Mix at a stirring speed of 450–550 rpm for 10–15 min to form a uniform suspension. Then slowly add 10% sulfuric acid solution dropwise, controlling the stirring speed at 500 rpm. Monitor the pH value in real time. Stop adding acid when it drops to 7.5–8, with an addition rate of 0.5 pH / min. As sulfuric acid is added, precipitate gradually forms. After adjusting the pH, maintain stirring at 350 rpm for 60 minutes to allow the silica to fully precipitate. Then reduce the stirring speed to 150 rpm and continue aging for 60 minutes. After the reaction is complete, centrifuge to obtain the silica-containing precipitate.

[0051] S22. The precipitate is washed repeatedly with deionized water 5 times to remove residual sodium, sulfate and other impurity ions; then it is spray-dried at 105℃ for 8 hours to obtain a loose powdery silica precursor; the dried product is then processed by an air jet mill (particle size ≤20μm) to finally obtain the silica product (SiO2 content 96%~99%, conforming to HG / T3061-2020 standard). See the product sample below. Figure 4 As shown. Example 2

[0052] The beaker experiment was further expanded to conduct pilot-scale engineering. The pilot-scale process is detailed below. Figure 2 See pilot-scale equipment Figure 5 The concentration of silica-containing alkaline wastewater in the raw water storage tank was tested. Low-concentration silica-containing wastewater with a silicate content of less than 5 g / L was classified as low-silica alkaline wastewater. This wastewater was then fed into a neutralization reactor, and a 30% sulfuric acid solution was added. The acid flow rate was controlled in real time using an online pH sensor. When the pH dropped to 7.3–7.7 (target range 7.5–8.0), acid addition was stopped, and the stirring rate was maintained at 480–520 rpm during this stage. After the pH reached the target, the stirring rate was adjusted to 300–400 rpm, and the reaction was continued for 60 minutes. Then, the mixture was aged for 60 minutes with stirring at 100–200 rpm to fully enrich the silica into a gel, which was then deposited in the gel aging tank. Solid-liquid separation was performed using a horizontal screw centrifuge at 3000–4000 rpm, and the filter residue and filtrate were collected. The filtrate was collected in a filtrate recovery tank and reused as cleaning water in the workshop. The filter residue is placed in a dissolving tank, and 2 mol / L sodium hydroxide solution is added until the filter residue is completely dissolved to obtain a transparent 10% to 15% sodium silicate solution (sodium silicate concentrate). This solution is then transferred to a constant temperature water bath reactor, where 30% sulfuric acid solution is added for secondary gelation. The product in the secondary gelation tank is the obtained silica gel product with a purity ≥95%.

[0053] High-concentration silicon-containing wastewater with a silicate content higher than 10 g / L is classified as high-silica alkaline wastewater. High-silica alkaline wastewater with a silicate content of 10,000–12,000 mg / L and a pH of 12.0–13.0 is fed into a precipitation reactor with a single batch processing capacity of 200–500 L. Precipitated silica seed crystals (purity ≥99%) are added at 0.4%–0.6% of the mass of the high-silica alkaline wastewater, and the mixture is stirred at a stirring rate of 450–550 rpm for 10–15 min to form a uniform suspension. A 10% dilute sulfuric acid solution was added dropwise to the reaction system at a rate of 0.4–0.6 pH / min, with the acid flow rate controlled in real time by an online pH sensor. Acid addition was stopped when the pH dropped to 7.3–7.7 (target range 7.5–8.0), maintaining a stirring speed of 480–520 rpm during this stage. After the pH target was reached, the stirring speed was adjusted to 300–400 rpm, and the reaction was continued for 60 min to generate silica precipitate. The stirring speed was then reduced to 100–200 rpm, and the mixture was aged at atmospheric pressure and 20–30℃ for 100–140 min to promote particle densification. Solid-liquid separation was performed using a horizontal screw centrifuge at 3000–4000 rpm, collecting the filter residue (precipitate) and filtrate. The filtrate was collected and reused in a filtrate recovery tank as cleaning water in the workshop. The precipitate was washed 4–6 times countercurrently with deionized water in a washing tank until the filtrate conductivity was ≤100 μS / cm. The wet filter cake is fed into a spray drying kiln and dried at an inlet temperature of 100–110℃ for 7–9 hours to obtain powder with a moisture content ≤5%. This powder is then processed using an air jet mill (particle size ≤20μm) to obtain finished silica (SiO2 content ≥96%). See the attached image for the actual product. Figure 6 As shown.

[0054] XRD analysis of the silica products obtained in Examples 1 and 2 is shown in the figure. Figure 7 (Products from Example 1 are labeled Sample1#, and products from Example 2 are labeled Sample2#), and their performance parameters are shown in Table 1:

[0055] Table 1. Product index parameters of silica obtained in Examples 1 and 2

[0056]

[0057] As can be seen from Table 1, all indicators of the silica products obtained in Examples 1 and 2 meet the relevant standards, thus proving that silica products were successfully prepared in Examples 1 and 2.

[0058] This process achieves efficient resource utilization of silicon-containing wastewater of different concentrations through tiered concentration and precise control, combining economic efficiency, environmental friendliness, and stability, and is suitable for large-scale wastewater treatment in the semiconductor / photovoltaic industry.

Claims

1. A method for resource recovery of silicon-containing alkaline wastewater based on differences in silicon concentration, characterized in that... The process includes the following steps: dividing silicon-containing waste alkaline water into low-silicon waste alkaline water and high-silicon waste alkaline water according to differences in silicon concentration; recovering silica gel and precipitated silica from the low-silicon waste alkaline water and high-silicon waste alkaline water respectively; wherein the silicate content in the low-silicon waste alkaline water is less than 5 g / L; and the silicate content in the high-silicon waste alkaline water is greater than 10 g / L. The process for recovering silica gel from low-silica waste alkaline water is as follows: S11. Add a sulfuric acid solution with a mass fraction of 25% to 35% to the low-silica waste alkaline water reaction tank until the pH value of the low-silica waste alkaline water drops to 7.5 to 8. Then react for a period of time to enrich the silica into a gel and centrifuge to obtain the filter residue. S12. Transport the filter residue to the secondary reaction tank, add sodium hydroxide solution until the filter residue is completely dissolved, and obtain a transparent sodium silicate solution with a concentration of 10% to 15%. S13. The sodium silicate solution is transferred to a heater and heated to 80℃~90℃. Then, a sulfuric acid solution with a mass fraction of 25%~35% is added until the pH value of the sodium silicate solution drops to 8~8.

5. An aging reaction is carried out. After the reaction is completed, the product is obtained by centrifugation. The process for recovering silica from high-silica waste alkaline water is as follows: S21. In the high-silica waste alkaline water reaction tank, add precipitated silica seeds at a mass of 0.4% to 0.6% of the high-silica waste alkaline water, and then add sulfuric acid solution with a mass fraction of 8% to 12% until the pH value of the high-silica waste alkaline water drops to 7.5 to 8. Then react under stirring at 400 to 500 rpm for 60 to 120 minutes, and then age under stirring at 100 to 200 rpm for 100 to 140 minutes to fully precipitate the silica. Centrifuge to separate the precipitate. S22. The precipitate is washed, dried and pulverized to obtain the silica product.

2. The method for resource recovery of silicon-containing alkaline wastewater based on silicon concentration differences according to claim 1, characterized in that: While adding sulfuric acid solution to S11, stir at a speed of 400-600 rpm. The addition rate of sulfuric acid solution is 0.4-0.6 pH / min.

3. The method for resource recovery of silicon-containing alkaline wastewater based on silicon concentration differences according to claim 1, characterized in that: After adjusting the pH in S11, react for 60-120 minutes with stirring at 300-400 rpm, and then age for 60-120 minutes with stirring at 100-200 rpm to fully enrich silica into a gel.

4. The method for resource recovery of silicon-containing alkaline wastewater based on silicon concentration differences according to claim 1, characterized in that: The concentration of sodium hydroxide solution in S12 is 1.5–2 mol / L.

5. The method for resource recovery of silicon-containing alkaline wastewater based on silicon concentration differences according to claim 1, characterized in that: While adding sulfuric acid solution to S13, stir at a speed of 400-600 rpm. The addition rate of sulfuric acid solution is 0.4-0.6 pH / min.

6. The method for resource recovery of silicon-containing alkaline wastewater based on silicon concentration differences according to claim 1, characterized in that: After adjusting the pH in S13, the aging reaction is carried out for 60–120 minutes.

7. The method for resource recovery of silicon-containing alkaline wastewater based on silicon concentration differences according to claim 1, characterized in that: While adding sulfuric acid solution to S21, stir at a speed of 400-600 rpm. The addition rate of sulfuric acid solution is 0.4-0.6 pH / min.

8. The method for resource recovery of silicon-containing alkaline wastewater based on silicon concentration differences according to claim 1, characterized in that: In S22, the precipitate is washed 4 to 6 times with deionized water to remove impurity ions, and then spray-dried at a temperature of 100 to 110°C for 7 to 9 hours. The dried product is then pulverized using an air jet mill.

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