Method for efficiently recovering and recycling salt in production of titanium dioxide by sulfuric acid method

Abstract

The invention discloses a method for efficiently recovering and recycling salt in sulfuric acid method titanium dioxide production, which comprises the following steps: S1, pretreating salt-containing wastewater generated in sulfuric acid method titanium dioxide production to remove solid particles in the wastewater; s2, salt in the wastewater is concentrated, and the ion proportion and pH in the concentrated solution are adjusted; s3, concentrating and crystallizing to obtain aluminum potassium sulfate dodecahydrate solid; s4, dehydrating the aluminum potassium sulfate dodecahydrate solid to obtain aluminum potassium sulfate hemihydrate; and S5, grafting the aluminum potassium sulfate hemihydrate with a water-soluble high-molecular compound containing an active functional group to obtain the flocculating agent. According to the method, the salt recovery rate and purity can be remarkably improved, the energy consumption is reduced, and efficient cyclic utilization of the recovered salt is realized.

Description

A method for efficient salt recovery and recycling in the sulfuric acid process for titanium dioxide production. Technical Field

[0001] This invention belongs to the field of titanium dioxide preparation technology, specifically relating to a method for efficient recovery and recycling of salt in the sulfuric acid process for titanium dioxide production. Background Technology

[0002] The sulfuric acid process is one of the main processes in titanium dioxide production. Its core steps include acid hydrolysis of ilmenite, hydrolysis, salt treatment, and calcination. Among these, the salt treatment step is crucial for improving the whiteness, hiding power, and weather resistance of the titanium dioxide product. By adding salt additives such as potassium hydroxide and aluminum sulfate to the titanium dioxide hydrolysis product (metitanic acid), excessive particle growth of titanium dioxide can be inhibited and crystal morphology optimized during subsequent calcination. Simultaneously, lattice defects are eliminated, ensuring the product meets optical performance standards. However, after salt treatment, a pre-kiln pressure filtration process is needed to separate the metatitanic acid filter cake from the saline phase. The resulting filtrate is a waste liquid containing high concentrations of potassium, aluminum, and sulfate ions—the salts in this waste liquid mainly originate from potassium ions that were not fully adsorbed during the salt treatment process. + Al 3+ and SO4 2- Currently, these waste liquids are usually treated by neutralization followed by direct discharge or simple evaporation and crystallization, which not only wastes resources such as potassium and aluminum, but also increases wastewater treatment costs and environmental pressure.

[0003] Existing technologies have made some attempts to recover salt from titanium dioxide production, such as recovering potassium aluminum sulfate through reverse osmosis concentration and freeze crystallization. However, these methods have the following drawbacks: 1. Low salt recovery efficiency, typically below 80%; 2. Low purity of the recovered salt, with impurities exceeding 5%, making it difficult to utilize directly; 3. High energy consumption, especially in the freeze crystallization stage; 4. Lack of a systematic recycling scheme, preventing the recovered salt from being efficiently reused in the production process; 5. Inadequate treatment of byproducts during the recovery process, easily causing secondary pollution.

[0004] Therefore, developing a high-efficiency, low-consumption, and high-purity process for salt recovery and recycling in the sulfuric acid process for titanium dioxide production has significant economic value and environmental implications. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide an improved method for efficient recovery and recycling of salt in the production of titanium dioxide using the sulfuric acid process. This process can significantly improve the salt recovery rate and purity, reduce energy consumption, and achieve efficient recycling of the recovered salt.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: The first aspect of this application provides a method for efficient recovery and recycling of salt in the production of titanium dioxide using the sulfuric acid process, comprising the following steps: S1. Pre-treating the saline wastewater generated during the production of titanium dioxide using the sulfuric acid process to remove solid particles from the wastewater; S2. Concentrating the salt in the wastewater and adjusting the ion ratio and pH of the concentrated solution to achieve a K+ concentration of... + Al 3+ SO4 2- The ion molar concentration ratio is between 98% and 102% of the ion molar ratio in the KAl(SO4)2 compound, and the solution pH is 1 to 1.5; S3. Concentrate and crystallize to obtain solid potassium aluminum sulfate dodecahydrate; S4. Dehydrate the solid potassium aluminum sulfate dodecahydrate to obtain potassium aluminum sulfate hemihydrate; S5. Graft the potassium aluminum sulfate hemihydrate with a water-soluble polymer compound containing active functional groups to obtain a flocculant.

[0007] Preferably, the pretreatment in step S1 employs multi-stage ceramic membrane filtration.

[0008] Preferably, the concentration in step S2 is achieved by reverse osmosis concentration, and the mass percentage concentration of salt in the concentrated solution is 10-12%.

[0009] Preferably, step S3 further includes: S31. MVR gradient concentration; in the first stage, concentration is carried out at 65~75℃ and 0.03~0.07MPa to a solid content of 12~18%, and in the second stage, concentration is carried out at 75~85℃ and 0.06~0.1MPa to a solid content of 22~28%; S32. Two-stage flash evaporation is used; in the first stage flash evaporation, potassium aluminum sulfate dodecahydrate seed crystals are added at an amount of 0.1~0.3% of the solution mass; in the first stage flash evaporation, the temperature is reduced to 40~50℃ at a cooling rate of 5~8℃ / min. S31. Temperature is reduced to 25-35℃ and pressure is reduced to 0.015-0.025 MPa; S2 flash evaporation is carried out at a cooling rate of 1-2℃ / min to reduce the temperature to 25-35℃ and the pressure to 0.008-0.012 MPa; S33. Freeze crystallization is carried out by step cooling; in the first stage, the temperature is reduced to 15-25℃ at a cooling rate of 4-6℃ / h, and then stirred for 1-3h; in the second stage, the temperature is reduced to 3-7℃ at a cooling rate of 1-3℃ / h, and then stirred for 3-5h; S34. Solid-liquid separation is performed to obtain the potassium aluminum sulfate dodecahydrate solid.

[0010] Preferably, when the concentrated crystallization in step S3 yields solid potassium aluminum sulfate dodecahydrate, a concentrated mother liquor is also obtained. The concentrated mother liquor is purified by a modified diatomaceous earth adsorption column, and the solid content of the purified concentrated mother liquor is detected. When the solid content is ≥10%, it is sent to step S3 to be mixed with the solution to be concentrated and crystallized. When the solid content is <10%, it is mixed with the concentrated solution in step S2.

[0011] Preferably, when the modified diatomaceous earth adsorption column outlet solution Fe 3+ When the concentration is >50ppm, a sulfuric acid solution with a mass percentage concentration of 10-30% is used for elution. After elution, the solution is rinsed with water until the pH of the outlet solution is 2-3. The eluent is then transported to the sulfuric acid titanium dioxide hydrolysis stage and mixed with the hydrolyzed titanium dioxide material. The amount of eluent transported is 1-2% of the mass of the hydrolyzed titanium dioxide material.

[0012] Preferably, the dehydration method in step S4 is a microwave method, wherein the microwave frequency is 2400~2500MHz, the power is 800~1000W, and the time is 6~10min.

[0013] Preferably, the water-soluble polymeric compound containing active functional groups is selected from at least one of polyacrylamide, acrylamide-sodium acrylate copolymer, chitosan, or polydiallyldimethylammonium chloride.

[0014] Preferably, step S5 further includes: adjusting the solid content of the hemihydrated potassium aluminum sulfate to 20-30% by adding water, then adding a water-soluble polymer solution containing active functional groups, and reacting at 60-70°C for 20-40 minutes; the mass percentage concentration of the water-soluble polymer solution containing active functional groups is 4-6‰, and the amount of the water-soluble polymer solution containing active functional groups added is 0.6-1.0% of the mass of the hemihydrated potassium aluminum sulfate, based on the water-soluble polymer solution containing active functional groups therein.

[0015] Preferably, the solution after the reaction is spray-dried to obtain the flocculant, wherein the average particle size of the flocculant is 100~200μm.

[0016] The second aspect of this application provides a method for applying a flocculant obtained by the above-described method for efficient salt recovery and recycling in the sulfuric acid process for titanium dioxide production. The method involves adding 0.05-0.1% by mass of a flocculant to titanium dioxide production wastewater with a pH of 2-3, reacting the reaction, adjusting the pH of the wastewater to 6-7, then adding another 0.05-0.1% by mass of a flocculant for another reaction, and finally performing solid-liquid separation to obtain a supernatant with suspended solids removed.

[0017] This application concerns the saline wastewater (rich in potassium) generated during the salt treatment process in the sulfuric acid process for titanium dioxide production. + Al 3+ SO4 2-With "efficient resource recovery - high-value product utilization - closed-loop utilization throughout the entire process" as its core technological concept, this technology integrates "pretreatment - concentration and regulation - crystallization separation - modification and value-added - recycling and energy saving" into a unified process. This overcomes the pain points of existing technologies, such as low recovery efficiency, insufficient purity, high energy consumption, and insufficient utilization, and achieves multiple goals, including efficient salt recovery, high-value product utilization, and energy conservation and environmental protection. Detailed Implementation

[0018] The saline wastewater generated during the sulfuric acid process for titanium dioxide production contains large amounts of sulfate, aluminum, and potassium ions. As is well known to those skilled in the art, potassium aluminum sulfate is a traditional water purification agent with good efficacy in wastewater treatment. Since titanium dioxide production generates a large amount of saline wastewater, if the sulfate, aluminum, and potassium ions in this wastewater are recovered and reused to obtain potassium aluminum sulfate, and then further used to purify the titanium dioxide production wastewater, efficient recovery and high-value utilization of the saline wastewater generated during the sulfuric acid process for titanium dioxide production can be achieved.

[0019] On the other hand, the processes of titanium dioxide acid hydrolysis, water washing, and pressure filtration generate a large amount of wastewater. This wastewater contains a large amount of suspended solids (such as titanium dioxide particles and acid hydrolysis impurities), heavy metal ions, and residual sulfuric acid. When treating this wastewater, flocculants must be used first to remove suspended solids and heavy metals.

[0020] The specific differences between the titanium dioxide production wastewater treated by the flocculant and the saline wastewater treated in this application are shown in Table 1.

[0021] Table 1

[0022] Potassium aluminum sulfate, obtained by recovering sulfate, aluminum, and potassium ions from saline wastewater, is generally obtained through concentration and crystallization. Direct concentration and crystallization typically yields potassium aluminum sulfate dodecahydrate, which, due to its high water content (approximately 45%) and dense crystal structure, has encapsulated surface active sites. This results in a small contact area with pollutants in titanium dioxide wastewater (acidic, high in suspended solids, and containing heavy metals). It can only form loose flocs through simple charge neutralization, exhibiting drawbacks such as weak adsorption bridging ability, slow floc settling speed, poor resistance to water quality fluctuations, high dosage requirements, and high sludge production. Furthermore, its removal efficiency for heavy metal ions in the wastewater is limited, failing to meet the demands for efficient purification of titanium dioxide wastewater. Therefore, further modification treatment is necessary before using the recovered potassium aluminum sulfate for flocculation of titanium dioxide production wastewater.

[0023] Based on the above, this application focuses on the saline wastewater (rich in K) generated during the salt treatment process in the sulfuric acid process for titanium dioxide production. + Al 3+ SO4 2-With "efficient resource recovery - high-value product utilization - closed-loop utilization throughout the entire process" as its core technological concept, this technology integrates a unified process of "pretreatment - concentration and control - crystallization separation - modification and value-added - recycling and energy saving". Furthermore, by optimizing the use of technologies such as ceramic membrane pretreatment, reverse osmosis concentration, MVR gradient concentration, step-by-step freeze crystallization, and dehydration and modification of potassium aluminum sulfate dodecahydrate, this technology overcomes the pain points of existing technologies, such as low recovery efficiency, insufficient purity, high energy consumption, and insufficient utilization. This achieves multiple goals of efficient salt recovery, high-value product utilization, and energy conservation and environmental protection.

[0024] This application provides a method for efficient salt recovery and recycling in the production of titanium dioxide using the sulfuric acid process, comprising the following steps: S1. Pre-treating the saline wastewater generated in the production of titanium dioxide using the sulfuric acid process to remove solid particles from the wastewater; the main function of this step is to fully remove suspended particulate matter, mainly titanium dioxide, from the wastewater to avoid affecting subsequent treatment. Methods such as sand filtration and ultrafiltration can be used.

[0025] This application preferably employs multi-stage ceramic membrane filtration, as ceramic membranes have stable chemical properties and good separation performance. A further preferred method is three-stage ceramic membrane filtration, with pore sizes of 1μm, 0.5μm, and 0.1μm for the three stages, and operating pressures of 0.2~0.25MPa, 0.25~0.3MPa, and 0.3~0.35MPa, respectively. This method removes particulate matter from the wastewater stage by stage, reducing the filtration pressure on each ceramic membrane while effectively removing solid particles from the wastewater.

[0026] More preferably, the feed liquid temperature for ceramic membrane filtration is adjusted to 40~45℃. Increasing the temperature reduces the viscosity of the feed liquid, optimizes mass transfer efficiency, and improves separation performance. The viscosity of the feed liquid decreases with increasing temperature, reducing adhesion and clogging on the membrane surface, increasing filtration rate and throughput per unit time, while also reducing the risk of membrane fouling and extending the lifespan of the ceramic membrane.

[0027] More preferably, a sampling port is set at the outlet of each ceramic membrane filtration unit to test the turbidity of the filtrate every 30 minutes to ensure that the turbidity is ≤1 NTU. The purpose is to control the filtration effect in stages, achieve step-by-step interception and purification, protect the membrane components, ensure the stable operation of subsequent processes, and fully remove solid particles from the wastewater.

[0028] The particles retained by the ceramic membrane are mainly composed of titanium dioxide residue. It is preferable to recycle them to the sulfuric acid process for titanium dioxide hydrolysis, mix them with the hydrolyzed materials, and reuse them in the titanium dioxide preparation process.

[0029] S2. Concentrate the salts in the wastewater and adjust the ion ratio and pH of the concentrated solution to ensure the K+ concentration is high. + Al 3+ SO42- The ion molar concentration ratio is between 98% and 102% of the ion molar ratio in the KAl(SO4)2 compound, and the solution pH is between 1 and 1.5.

[0030] Because the overall ion concentration in the wastewater is low, generally between 0.5% and 1%, to facilitate subsequent concentration and crystallization to precipitate potassium aluminum sulfate dodecahydrate solid, the ion concentration is first concentrated, and the ion concentration ratio in the concentrate is adjusted to fully utilize the K, Al, and SO4 ions. Theoretically, KAl(SO4)2 contains K... + Al 3+ SO4 2- The ion molar ratio is 1:1:2, therefore, when K in the solution... + Al 3+ SO4 2- When the ion molar concentration ratio is close to this ratio, K can be used. + Al 3+ SO4 2- All ions are fully utilized.

[0031] The preferred concentration method for this step is reverse osmosis concentration. After concentration, the salt concentration in the solution is 10-12% by mass. The higher the concentration, the greater the reverse osmosis pressure. This concentration facilitates the subsequent concentration and crystallization to precipitate potassium aluminum sulfate dodecahydrate solid, while also reducing the reverse osmosis pressure.

[0032] More preferably, a two-stage reverse osmosis system is used, with the first-stage reverse osmosis operating at a pressure of 1.5~2.0 MPa and the second-stage reverse osmosis operating at a pressure of 2.0~2.5 MPa.

[0033] The clear liquid produced by reverse osmosis (accounting for ≥70% of the total filtrate) can be used in the washing process of titanium dioxide production to replace part of the fresh water for washing.

[0034] Preferably, "adjusting the ion ratio and pH in the concentrated solution" further includes: conveying the reverse osmosis concentrate to a mixing tank, the mixing tank being equipped with a heating jacket to maintain the concentrate temperature at 30-35°C, and stirring with a high-shear stirrer; installing an ion chromatograph on the outlet pipe of the mixing tank to detect K in the concentrate every 10 seconds. + Al 3+ SO4 2- Concentration, calculate n(K) + ):n(Al 3+ ):n(SO4 2- When the ion molar ratio exceeds 1:1:2±2%, specifically (0.98~1.02:0.98~1.02:1.96~2.04), accurately add dilute K2SO4 or Al2(SO4)3 solution until the ion ratio meets the standard.

[0035] Potassium aluminum sulfate dodecahydrate is a typical double salt crystallizer. Its crystallization process is extremely sensitive to the ion ratio. If the ion ratio deviates from the theoretical value by more than 2%, it will lead to an excess of a single ion (such as K⁺ excess), which cannot participate in the crystallization reaction, resulting in resource waste and making it impossible to achieve the goal of "total salt recovery rate ≥95%".

[0036] If the ion ratio deviates from the theoretical value by more than 2%, impurities (such as potassium sulfate and aluminum sulfate single salts) are easily generated during the crystallization process, which leads to a decrease in product purity (the purity may be lower than 98%), and will affect the application effect when it is subsequently modified into a flocculant.

[0037] The concentrated solution with the ion ratio meeting the standard is sent to the pH adjustment tank, and the pH is adjusted to 1.0~1.5 using dilute sulfuric acid.

[0038] The core purpose of adjusting pH is to ensure the efficient precipitation and purity of potassium aluminum sulfate dodecahydrate in the subsequent concentration and crystallization process, specifically as follows: 1. Inhibit ion hydrolysis, Al 3+ It is highly susceptible to hydrolysis. This generates aluminum hydroxide colloids or precipitates, leading to Al 3+ Loss, unable to connect with K + SO4 2- They combine to form potassium aluminum sulfate crystals. Adjusting the pH to 1.0-1.5 (strongly acidic environment) allows them to pass through H+. + Increased concentration inhibits Al 3+ Hydrolysis ensures Al 3+ It exists stably in solution in a free state, ensuring that the ion ratio meets the requirements for crystallization.

[0039] 2. To promote crystal precipitation and avoid impurity eutectic, the solubility of potassium aluminum sulfate dodecahydrate is easier to control under strongly acidic conditions. If the pH is too high, impurities such as Al(OH)3 may precipitate in the solution, which are easily trapped within the potassium aluminum sulfate crystals during crystallization, leading to a decrease in product purity. If the pH is too low (<1.0), the solution is extremely acidic, increasing the solubility of potassium aluminum sulfate and reducing the crystallization yield. A pH range of 1.0–1.5 balances the "crystal precipitation efficiency" and the "impurity inhibition effect," ensuring a product purity ≥98%.

[0040] The permeation flux of a reverse osmosis membrane is positively correlated with the feed solution temperature. Within the range of 40-45°C, increasing the temperature enhances the rate at which water molecules permeate through the membrane while reducing salt deposition on the membrane surface, thus improving concentration efficiency. Therefore, it is preferable to adjust the reverse osmosis solution temperature to 40-45°C.

[0041] S3. Concentrate and crystallize to obtain solid potassium aluminum sulfate dodecahydrate; the concentrated solution after adjusting the ion ratio and pH can be further concentrated and crystallized to precipitate solid potassium aluminum sulfate. When potassium aluminum sulfate precipitates in water, it precipitates as a dodecahydrate.

[0042] Preferably, step S3 further includes: S31. Employing MVR gradient concentration; in the first stage, concentration is carried out at 65-75℃ and 0.03-0.07MPa to a solid content of 12-18%, and in the second stage, concentration is carried out at 75-85℃ and 0.06-0.1MPa to a solid content of 22-28%; S32. Employing two-stage flash evaporation; in the first stage, potassium aluminum sulfate dodecahydrate seed crystals are added at a rate of 0.1-0.3% of the solution mass; the addition of potassium aluminum sulfate dodecahydrate seed crystals facilitates the crystallization and precipitation of potassium aluminum sulfate dodecahydrate; the first stage flash evaporation is carried out at 5-8℃ / m S31. The cooling rate is reduced to 40~50℃ and the pressure is reduced to 0.015~0.025Mpa; the second-stage flash evaporation uses a cooling rate of 1~2℃ / min to reduce the temperature to 25~35℃ and the pressure to 0.008~0.012Mpa; S32. Freeze crystallization is carried out by step cooling; the first stage uses a cooling rate of 4~6℃ / h to reduce the temperature to 15~25℃, and then stirs for 1~3h; the second stage uses a cooling rate of 1~3℃ / h to reduce the temperature to 3~7℃, and then stirs for 3~5h; S33. Solid-liquid separation is performed to obtain solid potassium aluminum sulfate dodecahydrate.

[0043] MVR gradient concentration uses a two-stage temperature and pressure gradient control to gradually increase the solid content of the feed liquid to 22-28% with low consumption, avoiding local over-concentration or boiling over, and providing a stable feed for subsequent crystallization. Two-stage flash evaporation uses a negative pressure environment combined with seed crystal addition and gentle cooling to directionally induce the formation of potassium aluminum sulfate dodecahydrate crystal nuclei, while reducing the precipitation of impurity crystals. Stepped cooling and freezing crystallization gradually reduces the temperature to 3-7°C with a rapid initial cooling rate followed by slow cooling, combined with segmented stirring, to promote the regular growth of crystals around the crystal nuclei into large particles, improving solid-liquid separation efficiency and product purity. The three processes work together to achieve efficient salt enrichment, directional crystallization, and low-consumption separation, ensuring the effectiveness of subsequent product recovery.

[0044] More preferably, solid-liquid separation is performed by centrifugation at a speed of 3000-3500 rpm and a separation factor of 1500-2000 to obtain crude aluminum sulfate dodecahydrate and concentrated mother liquor.

[0045] Before undergoing step S4 modification, the crude solid potassium aluminum sulfate dodecahydrate is stored under sealed conditions, maintaining a relative humidity of 30-40% and a temperature of 25-30℃. The core purpose of these storage conditions is to prevent the crude solid potassium aluminum sulfate dodecahydrate from efflorescence, moisture absorption, or agglomeration, ensuring the stability and purity of the crystal water, avoiding impurities, and simultaneously ensuring a uniform moisture content to facilitate subsequent dehydration and modification grafting processes, thus guaranteeing the stability of the modified product's performance.

[0046] The concentrated mother liquor is purified by a modified diatomaceous earth adsorption column. The solid content of the purified concentrated mother liquor is tested. When the solid content is ≥10%, it is sent to step S3 and mixed with the solution to be concentrated and crystallized for direct concentration and crystallization. When the solid content is <10%, it is sent to step S2 and mixed with the concentrated solution for further reverse osmosis concentration to increase the salt content.

[0047] After treatment with modified diatomaceous earth, Fe in the mother liquor can be effectively removed. 3+ Ti 4+ Heavy metal ions were removed. The mother liquor after impurity removal was then processed according to different conditions.

[0048] More preferably, when the Fe at the outlet solution of the modified diatomaceous earth adsorption column... 3+ When the concentration is >50ppm, start the adsorption column regeneration program and use a sulfuric acid solution with a mass percentage concentration of 10~30% for elution to fully remove the metal ions adsorbed by the adsorption column. After elution, rinse with water until the pH of the outlet solution is 2~3.

[0049] The eluent contains a certain amount of metal ions and low concentration of sulfuric acid. The eluent is transported to the sulfuric acid titanium dioxide hydrolysis stage and mixed with the hydrolyzed titanium dioxide material. The amount of eluent transported is 1 to 2% of the mass of the hydrolyzed titanium dioxide material.

[0050] MVR evaporation and concentration generates high-temperature secondary steam, which can be used for energy recovery, further improving resource recycling efficiency. Preferably, the MVR secondary steam is introduced into a heat pump unit through an insulated pipe, compressed, and then fed into a chilled water heater to heat the chilled water from 5°C to 20°C. The heated chilled water is then fed into a ceramic membrane feed tank preheater to preheat the feed liquid from 25°C to 40-45°C.

[0051] As mentioned earlier, if potassium aluminum sulfate dodecahydrate is used directly as a flocculant to treat titanium dioxide production wastewater, the flocculation effect is poor. Furthermore, if it is modified by grafting water-soluble polymers containing active functional groups onto it to improve the flocculation effect, the grafting rate will also be low because these polymers are rich in water of crystallization, and the water exists in a free state in the interstitial spaces of the crystal lattice. This water will encapsulate the active sites (hydroxyl groups, ionic bonds) of potassium aluminum sulfate.

[0052] Therefore, this application first dehydrates potassium aluminum sulfate dodecahydrate to convert it into potassium aluminum sulfate hemihydrate, removing a large amount of crystal water to form a loose and porous structure, exposing sufficient active sites. Then, it is grafted with a water-soluble polymer compound containing active functional groups to construct an "inorganic core-organic chain" composite system. This system retains the charge neutralization advantage of inorganic components and promotes floc aggregation and growth through the adsorption bridging effect of organic long chains, significantly improving flocculation efficiency and sedimentation speed. At the same time, it enhances acid resistance and heavy metal adsorption capacity, ultimately achieving the goal of reducing the dosage by 20%~25% and increasing the suspended solids removal rate to over 98%, perfectly meeting the actual needs of titanium dioxide wastewater treatment and achieving high-value utilization of the recovered product. The specific steps are as follows: S4. Modify the solid potassium aluminum sulfate dodecahydrate and dehydrate the potassium aluminum sulfate dodecahydrate to obtain potassium aluminum sulfate hemihydrate.

[0053] As mentioned earlier, potassium aluminum sulfate dodecahydrate contains 12 molecules of water of crystallization, resulting in a high water content in its crystal structure. This water exists in a free state within the interstitial lattice, encapsulating the active sites (hydroxyl groups, ionic bonds) of potassium aluminum sulfate. This makes it difficult for modifiers such as PAM to approach and react, leading to an extremely low grafting rate (typically <30%). In contrast, hemihydrate potassium aluminum sulfate retains only 0.5 molecules of water of crystallization, with most of the interstitial lattice spaces open, exposing a large number of polar hydroxyl groups (-OH) and ionized sulfate ions (SO42-). 2- These groups can form hydrogen bonds or ionic bonds with the amide groups (-CONH2) on the PAM molecular chain, providing sufficient active sites for the grafting reaction and increasing the grafting rate to over 80% (as in the examples, the grafting rate is ≥80%). However, if anhydrous potassium aluminum sulfate is used, the anhydrous form has a dense crystal structure, resulting in low reactivity during subsequent grafting modification and making it difficult to form a stable bond with polyacrylamide (PAM).

[0054] Therefore, the hemihydrate removes most of the free water of crystallization while retaining some polar hydroxyl groups (-OH), and its lattice structure has moderate porosity, providing active sites for the grafting reaction.

[0055] Dehydration can be achieved using methods such as microwave drying, hot air drying, vacuum drying, and infrared radiation drying. Preferably, microwave drying is used, with a microwave frequency of 2400~2500MHz, a power of 800~1000W, and a time of 6~10min.

[0056] Microwaves at around 2450MHz are used to selectively heat potassium aluminum sulfate dodecahydrate. The energy is preferentially absorbed by the water of crystallization and converted into internal energy through high-frequency polarization vibration. Within a power of 800~1000W and a processing time of 6~10min, the hydrogen bonding between the water of crystallization and the crystal framework is efficiently destroyed, achieving precise dehydration and conversion into hemihydrate potassium aluminum sulfate. At the same time, endogenous heat transfer is used to avoid the overall high-temperature decomposition of the crystal, promoting the orderly reconstruction of the crystal lattice to form a porous structure with a high specific surface area. This provides sufficient active sites for subsequent polymer grafting, achieving a balance between low consumption, high efficiency and product stability.

[0057] S5. Grafting a water-soluble polymer compound containing active functional groups onto potassium aluminum sulfate hemihydrate yields a flocculant.

[0058] Preferably, step S5 further includes: adjusting the solid content of the hemihydrated potassium aluminum sulfate to 20-30% by adding water, then adding a water-soluble polymer compound solution containing active functional groups, and reacting at 60-70°C for 20-40 minutes to achieve grafting of the water-soluble polymer compound containing active functional groups, with a grafting rate ≥80%.

[0059] The mass percentage concentration of the water-soluble polymer containing active functional groups in the solution is 4-6‰, and the amount of the water-soluble polymer containing active functional groups added is 0.6-1.0% of the mass of potassium aluminum sulfate hemihydrate, based on the amount of the water-soluble polymer containing active functional groups in the solution.

[0060] As will be understood by those skilled in the art, active functional groups generally refer to active groups such as hydroxyl, carboxyl, and amino groups.

[0061] Preferably, the water-soluble polymeric compound containing active functional groups may be selected from at least one of polyacrylamide, acrylamide-sodium acrylate copolymer, chitosan, or polydiallyldimethylammonium chloride.

[0062] The following describes the mechanism of action of grafted polyacrylamide (PAM) as an example: The core function of grafted polyacrylamide (PAM) is to enhance the adsorption and flocculation efficiency of flocculants for suspended solids in titanium dioxide wastewater. The mechanism is as follows: the active groups on the PAM molecular chain form chemical bonds with the hydroxyl groups on the surface of potassium aluminum sulfate hemihydrate, giving the inorganic flocculant (potassium aluminum sulfate hemihydrate) the long-chain bridging properties of organic polymers—retaining the properties of Al... 3+ The aluminum hydroxide colloid generated by hydrolysis adsorbs suspended solids and crosslinks and aggregates the dispersed micro flocs into large particles through the PAM long chain, accelerating solid-liquid separation. Ultimately, the suspended solids removal rate is increased from the conventional 90% to over 98%, while reducing the amount of flocculant added by 20% to 25%.

[0063] Preferably, the solution after reaction is spray-dried to obtain the finished flocculant, and the average particle size of the flocculant is 100~200μm.

[0064] Another aspect of this application provides a method for applying a flocculant, specifically as follows: A flocculant with a mass fraction of 0.05-0.1% is added to titanium dioxide production wastewater with a pH of 2-3, and after reaction, the pH of the wastewater is adjusted to 6-7. Then, another flocculant with a mass fraction of 0.05-0.1% is added, and the reaction is repeated. Finally, solid-liquid separation is performed to obtain a supernatant with suspended solids removed. The pH adjuster can be sodium hydroxide, carbide slag (whose main component is calcium hydroxide), etc.

[0065] The flocculant is added in stages with intermediate pH adjustments. The core function is to adapt the flocculant's activity to the acidic environment of titanium dioxide wastewater: the first part of the flocculant is added when the initial pH is 2-3, allowing its active groups to initially adsorb tiny suspended solids in the wastewater, forming small flocs; the second part of the flocculant is added after adjusting the pH to 6-7 (the optimal pH range for flocculant operation), which enhances the long-chain bridging of polymers such as PAM and Al. 3+ The synergistic adsorption effect of hydrolyzed colloids promotes the aggregation of small flocs into large particles, improves solid-liquid separation efficiency, and ultimately achieves efficient removal of suspended solids.

[0066] Preferably, a PLC control system is used to connect sensors and actuators at each stage, achieving fully automated control of the entire process, reducing labor costs by 30-40%, and improving process stability.

[0067] Compared with existing technologies, this invention has the following advantages: 1. High salt recovery rate, with a total recovery rate of over 95%, far exceeding the below 80% of existing technologies; 2. High purity of recovered potassium aluminum sulfate, reaching over 98%, which can be directly used as a flocculant after modification; 3. A modified potassium aluminum sulfate composite flocculant has been developed, reducing the dosage by 20-25% compared to conventional flocculants, and increasing the suspended solids removal rate from 90% to over 98%; 4. The use of MVR gradient concentration and energy cascade utilization technology reduces overall process energy consumption by 25-30%; 5. Highly efficient recycling of reverse osmosis supernatant is achieved, with a recycling rate ≥80%, significantly reducing the consumption of fresh water.

[0068] The present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the content described.

[0069] Example 11. Filtrate Collection and Pretreatment a. The filtrate discharged from the filter press during the pre-kiln filtration process in the sulfuric acid process for titanium dioxide production (containing K) is collected and pretreated. + Al 3+ SO4 2-The plasma is transported to a dedicated storage tank via 316L stainless steel corrosion-resistant pipes. The tank is equipped with a level sensor (measuring range 0~5m, accuracy ±0.01m) and a stirring device (paddle type, speed 70rpm); b. A 316L stainless steel corrosion-resistant centrifugal pump (flow rate 8m³ / h) is used. 3 The filtrate in the storage tank is transported to the ceramic membrane feed tank (1.5m³ / h, 25m head) via a pump. 3 a) A 100μm filter screen is installed at the inlet of the feed tank for preliminary removal of large-particle suspended solids; b) The filtrate in the feed tank is filtered through a three-stage series ceramic membrane filtration system. The pore sizes of the three ceramic membranes are 1μm, 0.5μm, and 0.1μm, and the operating pressures are 0.22MPa, 0.28MPa, and 0.32MPa, respectively. A sampling port is set at the outlet of each filtration unit, and the turbidity of the filtrate is tested every 30 minutes to ensure that the turbidity is ≤1NTU; c) The titanium dioxide residue (moisture content ≤80%) retained by the three-stage filtration is collected and returned to the sulfuric acid titanium dioxide hydrolysis stage, mixed with the hydrolyzed material for reuse, with a titanium dioxide recovery rate ≥95%.

[0070] 2. Salt Concentration and Ion Adjustment a. The clarified liquid after ceramic membrane filtration is fed into a two-stage series reverse osmosis membrane concentration system. The first-stage reverse osmosis operates at a pressure of 1.8 MPa, and the second-stage reverse osmosis operates at a pressure of 2.2 MPa, increasing the salt concentration of the clarified liquid from 0.8% to 11%; b. The clarified liquid produced by reverse osmosis (accounting for 75% of the total filtrate) is transferred to the storage tank of the titanium dioxide washing process; c. The reverse osmosis concentrate is transferred to the mixing tank (volume 2.5 m³). 3 The mixing tank is equipped with a heating jacket to maintain the concentrated solution temperature at 32℃, and is stirred by a high-shear stirrer (anchor blade, 180 rpm); d. An ion chromatograph (detection range 0~1000 mg / L, accuracy ≤0.1 mg / L) is installed on the outlet pipe of the mixing tank to detect K in the concentrated solution every 10 seconds. + Al 3+ SO4 2- Concentration, calculate n(K) + ):n(Al 3+ ):n(SO4 2- e. When the ion ratio exceeds the range of 1:1:2±2%, accurately add dilute K2SO4 or Al2(SO4)3 solution using a twin-screw atomizing feeder (atomizing particle size 80μm). During addition, increase the stirrer speed to 250rpm, and recheck the ratio every 30s until the standard is met; f. Send the concentrated solution with the ion ratio meeting the standard into the pH adjustment tank (volume 1.5m³). 3 ), add 20% by mass of dilute sulfuric acid using a metering pump (flow rate 0.3 L / min, accuracy ±1%) to stabilize the pH at 1.2.

[0071] 3. Concentration, Crystallization, and Solid-Liquid Separation: a. The pH-adjusted concentrate is fed into an MVR gradient concentration system. The first stage concentrates the solution to 15% solids content at 70℃ and 0.05MPa, and the second stage concentrates it to 25% solids content at 80℃ and 0.08MPa; b. The MVR concentrate is then fed into a primary flash tank (0.8m³). 3 The pressure was reduced to 0.02 MPa, and the temperature was lowered to 45°C at a rate of 6°C / min. Seed crystals were added; the seed crystals were potassium aluminum sulfate dodecahydrate from the previous batch (purity ≥99.5%, particle size 80 μm), and the addition amount was 0.2% of the solution mass. c. The solution after the first-stage flash evaporation was transferred to the second-stage flash tank (volume 0.8 m³). 3 The pressure was reduced to 0.01 MPa, and the temperature was lowered to 30°C, with a cooling rate controlled at 1.5°C / min; d. The solution after secondary flash evaporation was transferred to a cryogenic crystallization tank (volume 2.5 m³). 3 The temperature was lowered in a stepped manner: in the first stage, the temperature was lowered from 30℃ to 20℃ at a rate of 5℃ / h, and stirred for 2 hours; in the second stage, the temperature was lowered from 20℃ to 5℃ at a rate of 2℃ / h, and stirred for 4 hours. The crystallized mixture was then fed into a horizontal screw discharge centrifuge (drum diameter 650mm) for separation. The centrifuge drum speed was 3200rpm, the separation factor was 1800, and the crude aluminum sulfate dodecahydrate solid and filtrate were obtained. The moisture content of the crude solid was ≤10%, and the solid content of the filtrate was ≤1%.

[0072] 4. Product Processing and Recycling a. The crude solid product (purity ≥98%) separated by the centrifuge is sent to a closed silo (volume 1.5m³). 3 a. A dehumidification device (dehumidification capacity 8 kg / h) is installed in the silo to maintain a relative humidity of 35% and a temperature of 28℃; b. The centrifuge filtrate is transported to a modified diatomaceous earth adsorption column (500 mm in diameter, 2000 mm in height) to remove Fe. 3+ Ti 4+ To ensure the presence of heavy metal ions, control the filtrate flow rate to 1.5 m / h, and guarantee a contact time ≥ 30 min. The outlet Fe... 3+ Ti 4+ Concentration ≤ 30ppm; c. The adsorbed filtrate enters the online refractometer detection unit (range 0~30%, accuracy ±0.1%). When the solid content is ≥10%, it is sent to the feed end of the MVR concentration system; when the solid content is <10%, it is sent to the concentrate section of the mixing tank; d. When the Fe at the adsorption column outlet... 3+When the concentration is >50ppm, start the regeneration program, automatically switch to the standby adsorption column, and introduce 20% mass fraction dilute sulfuric acid (45℃) into the saturated adsorption column. The elution flow rate is 0.8m / h, and the elution time is 2h. After elution, rinse with 32℃ water until the outlet pH=2.5, and blow dry with 0.15MPa compressed air for later use. e. Transport the eluent after adsorption column regeneration to the sulfuric acid process titanium dioxide hydrolysis stage and mix it with the hydrolyzed titanium dioxide material. The eluent delivery volume is 1.5% of the titanium dioxide slurry mass.

[0073] 5. Product Modification and Application a. The solid crude product is fed into a microwave modification furnace (frequency 2450MHz, power 900W), equipped with a stainless steel conveyor belt (speed 0.8m / min), and remains in the furnace for 8 minutes to convert it into hemihydrate potassium aluminum sulfate (moisture content reduced to 4%); b. The microwave-modified product is then fed into a modification reactor (volume 1.5m³). 3 Add deionized water to adjust the solid content to 25%, then add polyacrylamide (molecular weight 9 million) at a mass fraction of 0.8% of potassium aluminum sulfate hemihydrate. PAM is pre-dissolved into an aqueous solution with a mass percentage concentration of 5‰ (dissolution time ≥2h). Stir at 65℃ for 30min to achieve PAM grafting with a grafting rate ≥80%. c. The grafted product is spray-dried (inlet air 190℃, outlet air 85℃) to form a powder with an average particle size of 150μm, which is used as a customized flocculant for titanium dioxide wastewater treatment.

[0074] d. Wastewater from titanium dioxide production [pH 2.5, suspended solids 800 mg / L, main suspended solids include core component titanium dioxide particles, incompletely hydrolyzed titanium hydroxide and other titanium compound derivatives, and Fe(OH)3 and Al formed by acid hydrolysis of iron and aluminum impurities associated with the raw materials]. Particles of sparingly soluble metal compounds such as (OH)3, fine salt impurities such as CaSO4 and CaSiO3 generated by the reaction of sulfuric acid with impurities such as calcium, magnesium, and silicon, as well as fiber debris generated by the wear of filter cloth in the filter press and small solid debris from the titanium dioxide calcination process, are introduced into the equalization tank. The agitator (90 rpm) is used to maintain uniformity. In the first stage, 0.075% of the wastewater mass of customized flocculant is added, and the stirring speed is 150 rpm. Small flocs are formed after 10 minutes. In the second stage, a 30% sodium hydroxide solution is added to adjust the pH to 6.5, and 0.075% of the wastewater mass of flocculant is added. The stirring speed is reduced to 50 rpm. Large flocs are formed after 5 minutes. After flocculation, the wastewater enters the sedimentation tank (retention for 30 minutes), and the suspended solids in the supernatant are ≤20 mg / L.

[0075] 6. Energy Recovery and Automation Control a. MVR secondary steam (85℃, 0.08MPa) is introduced into the heat pump unit (heating power 80kW) through insulated pipes and compressed to 95℃, 0.12MPa; b. The compressed steam is then fed into the chilled water heater (heat exchange area 6.5m²). 2 a) Heat the chilled water from 5°C to 20°C; b) Pass the heated chilled water into the ceramic membrane feed tank preheater (heat exchange area 4m²). 2 d. Preheat the feed liquid from 25℃ to 42℃; d. Use Siemens S7-1200 series PLC, equipped with a 10-inch Chinese HMI, to connect sensors and actuators at each stage to achieve full-process automated control.

[0076] Using the process in this embodiment, the total salt recovery rate reaches 96.5%, the purity of the recovered potassium aluminum sulfate reaches 98.7%, the overall process energy consumption is reduced by 28%, the filtrate circulation rate reaches 82%, the dosage of the customized flocculant is reduced by 23% compared with the conventional flocculant, and the suspended solids removal rate reaches 98.5%.

[0077] Comparative Example 11. Filtrate Collection and Pretreatment (Same as Example 1) The filtrate discharged from the filter press was transported to the storage tank through a 316L stainless steel pipeline, and then fed into the ceramic membrane feed tank (100μm filter screen) by a centrifugal pump. It was filtered through a three-stage series of ceramic membranes (pore size 1μm / 0.5μm / 0.1μm, pressure 0.22MPa / 0.28MPa / 0.32MPa) to ensure that the turbidity is ≤1NTU, and the titanium dioxide residue was retained for reuse.

[0078] 2. Salt Concentration and Ion Adjustment (Same as Example 1): The ceramic membrane supernatant is fed into a two-stage series reverse osmosis system (stage 1.8 MPa, stage 2.2 MPa) and concentrated to a salt concentration of 11%. The reverse osmosis supernatant is reused in the water washing process. The concentrated solution is fed into a mixing tank (heated and stirred at 32°C), and is analyzed by ion chromatography and replenished as needed. Prepare a dilute solution, ensuring an ion ratio of 1:1:2 ± 2%, and adjust the pH to 1.2.

[0079] 3. Concentration, Crystallization, and Solid-Liquid Separation (Same as Example 1): The concentrated liquid was concentrated using an MVR gradient (70℃ / 0.05MPa to 15% solid content, 80℃ / 0.08MPa to 25% solid content), followed by sequential primary flash evaporation (0.02MPa, 45℃, with 0.2% seed crystals added), secondary flash evaporation (0.01MPa, 30℃), and step-crystallization (30℃→20℃, stirring at 5℃ / h for 2h; 20℃→5℃, stirring at 2℃ / h for 4h). The solid was then separated using a horizontal screw centrifuge (3200rpm, separation factor 1800) to obtain crude potassium aluminum sulfate dodecahydrate (purity 98.5%) and filtrate.

[0080] 4. Product processing and recycling (same as in Example 1) The crude solid product is fed into a sealed silo (humidity 35%, temperature 28°C); the filtrate is treated by a modified diatomaceous earth adsorption column (flow rate 1.5 m / h) and then recycled to the batching tank or MVR system according to the solid content; the eluent from the adsorption column regeneration is reused in the sulfuric acid titanium dioxide hydrolysis stage.

[0081] 5. Product Modification and Application (Compared to Example 1: Microwave modification and dehydration omitted) Solid crude potassium aluminum sulfate dodecahydrate was directly taken from a sealed silo and fed into a modification reactor. Deionized water was added to adjust the solid content to 25%, followed by the addition of 0.8% polyacrylamide (molecular weight 9 million, pre-dissolved in a 5‰ aqueous solution). The mixture was stirred at 65°C for 30 minutes to achieve PAM grafting (grafting rate 45%). The grafted product was spray-dried (inlet air 190°C, outlet air 85°C) to produce a 150μm powder flocculant. Wastewater Treatment: Wastewater in the equalization tank (pH 2.5, suspended solids 800mg / L) was stirred evenly at 90rpm. In the first stage, 0.075% of the wastewater mass of flocculant was added, and the mixture was stirred at 150rpm for 10 minutes. In the second stage, 30% sodium hydroxide was added to adjust the pH to 6.5, and 0.075% of the wastewater mass of flocculant was added, and the mixture was stirred at 50rpm. Stir for 5 minutes, then transfer to a sedimentation tank and let stand for 30 minutes. Analyze the supernatant parameters.

[0082] 6. Energy recovery and automation control (same as Example 1): The secondary steam of MVR is compressed by a heat pump to heat the chilled water and preheat the feed liquid of ceramic membrane; the whole process is controlled by PLC.

[0083] The final test results showed that the salt recovery rate was 95.8%; the purity of potassium aluminum sulfate was 97.3%; the suspended solids removal rate was 82.3% (suspended solids in the supernatant were 141.6 mg / L). The dosage needs to be increased to 0.198% to achieve suspended solids ≤20 mg / L, and the floc settling rate was 1.2 cm / min.

[0084] Comparative Example 21. Filtrate collection and pretreatment (same as Example 1) is the same as in Example 1, with three-stage ceramic membrane filtration pretreatment completed, titanium dioxide residue retained for reuse, and the turbidity of the clear liquid ≤1 NTU.

[0085] 2. Salt concentration and ion adjustment (same as in Example 1): The reverse osmosis concentration, ion ratio control and pH adjustment steps are completely consistent with the example. The concentrated liquid index reaches 11% salt concentration, ion ratio 1:1:2±2% and pH 1.2.

[0086] 3. Concentration, crystallization and solid-liquid separation (same as in Example 1): After MVR concentration, two-stage flash evaporation, step-by-step freeze crystallization and centrifugation, crude solid potassium aluminum sulfate dodecahydrate was obtained.

[0087] 4. Product processing and recycling (same as in Example 1) The solid crude product is stored in a sealed container (humidity 35%, temperature 28°C), and the filtrate and adsorption column regeneration eluent are recycled according to Example 1.

[0088] 5. Product Modification and Application (Compared with Example 1, only microwave dehydration modification was performed, without PAM grafting) The solid crude product was fed into a microwave modification furnace (2450MHz, 900W, 8 min) to be converted into hemihydrated potassium aluminum sulfate (4% water content). Without PAM grafting, it was directly pulverized into 150μm powder as a flocculant. Wastewater treatment: The same staged addition process as in Example 1 was adopted. In the first stage, 0.075% of the wastewater mass of flocculant was added (stirred at 150 rpm for 10 min). In the second stage, after adjusting the pH to 6.5, 0.075% of the wastewater mass of flocculant was added (stirred at 50 rpm for 5 min). The product was then settled in a sedimentation tank for 30 min before testing.

[0089] 6. Energy recovery and automated control (same as in Example 1) is consistent with Example 1, realizing energy recovery and full-process automated control.

[0090] The final test results showed that the salt recovery rate was 96.3%; the purity of potassium aluminum sulfate was 98.6%; the suspended solids removal rate was 78.5% (suspended solids in the supernatant were 172 mg / L). The dosage needs to be increased to 0.205% to achieve suspended solids ≤20 mg / L, and the floc settling rate was 0.8 cm / min (the flocs are fine and easily dispersed).

[0091] Comparative Example 3 (Direct use of commercially available conventional potassium aluminum sulfate flocculant for titanium dioxide wastewater treatment) 1. Source of flocculant: Commercially available conventional potassium aluminum sulfate flocculant (98% purity, 150μm particle size) was purchased and used directly for wastewater treatment without going through the "salt recovery-modification" process described in this application.

[0092] 2. Wastewater treatment process (same as the application steps in Example 1): Take the same batch of titanium dioxide wastewater (pH 2.5, suspended solids 800 mg / L) and send it to the equalization tank, stirring evenly at 90 rpm; in the first stage, add 0.075% conventional potassium aluminum sulfate flocculant and stir at 150 rpm for 10 min; in the second stage, add 30% sodium hydroxide solution to adjust the pH to 6.5, add the remaining 0.075% conventional potassium aluminum sulfate flocculant, and stir at 50 rpm for 5 min; after flocculation, the wastewater enters the sedimentation tank and stays for 30 min, and the supernatant indicators are tested.

[0093] The final test results showed that the suspended solids removal rate was 85.1% (suspended solids in the supernatant were 119.2 mg / L). The dosage needs to be increased to 0.195% to achieve suspended solids ≤20 mg / L and floc settling rate of 1.5 cm / min.

[0094] Table 2 shows a comparison of the core metrics of the examples and comparative examples.

[0095] Table 2

[0096] A comparison of the entire process in the comparative examples and embodiments reveals that the combined process of "microwave dehydration modification + PAM grafting modification" in this application is the core of improving flocculation performance. Meanwhile, the "three-stage ceramic membrane filtration + precise ion control + MVR - flash evaporation - freeze crystallization" process ensures high salt recovery rate and purity. Omitting microwave dehydration modification leads to insufficient grafting rate, omitting PAM grafting modification results in the loss of long-chain bridging effect, and using conventional flocculants lacks structural optimization. All three factors lead to a significant decrease in suspended solids removal rate (≤85.1%) and a significant increase in dosage, fully demonstrating the significant advantages of the process in this application in terms of resource recovery efficiency, product purity, and application effect.

[0097] Example 21. Filtrate Collection and Pretreatment: In the sulfuric acid process for titanium dioxide production, the filtrate discharged from the filter press (containing potassium) from the pre-kiln filtration process is collected and pretreated. + Al 3+ SO4 2- The plasma is transported to a dedicated storage tank via a 316L stainless steel corrosion-resistant pipeline. The storage tank is equipped with a level sensor (measuring range 0~5m, accuracy ±0.01m) and a stirring device (paddle type, speed 70rpm). A 316L stainless steel corrosion-resistant centrifugal pump is used to transport the filtrate in the storage tank to a ceramic membrane feed tank. A 100μm filter screen is installed at the inlet of the feed tank. The filtrate in the feed tank is filtered through a three-stage series ceramic membrane filtration system. The pore sizes of the three ceramic membranes are 1μm, 0.5μm, and 0.1μm, and the operating pressures are 0.2MPa, 0.25MPa, and 0.35MPa, respectively. A sampling port is set at the outlet of each filtration unit, and the turbidity of the filtrate is tested every 30 minutes to ensure that the turbidity is ≤1NTU. The titanium dioxide residue (moisture content ≤82%) retained by the three-stage filtration is collected and returned to the titanium dioxide slurry preparation process for reuse, with a titanium dioxide recovery rate ≥94%.

[0098] 2. Salt Concentration and Ion Adjustment: The clarified liquid after ceramic membrane filtration is sent to a two-stage series reverse osmosis membrane concentration system. The first-stage reverse osmosis operates at a pressure of 1.5 MPa, and the second-stage reverse osmosis operates at a pressure of 2.5 MPa, increasing the salt concentration of the clarified liquid from 0.5% to 12%. The clarified liquid produced by reverse osmosis (accounting for 70% of the total filtrate) is sent to the water storage tank of the titanium dioxide washing process to replace fresh water. The reverse osmosis concentrate is sent to a mixing tank equipped with a heating jacket to maintain the concentrate temperature at 30°C, and is stirred by a high-shear stirrer. An ion chromatograph (detection range 0~1000 mg / L, accuracy ≤0.1 mg / L) is installed on the outlet pipe of the mixing tank to detect K in the concentrate every 10 seconds. + Al 3+ SO4 2- Concentration, calculate n(K) + ):n(Al 3+ ):n(SO4 2- The ratio is adjusted as follows: When the ion ratio exceeds the range of 1:1:2±2%, K2SO4 or Al2(SO4)3 dilute solution is precisely added through a twin-screw atomizing feeder. During the addition, the stirrer speed is increased to 240 rpm, and the ratio is re-checked every 30 seconds until the standard is met. The concentrated solution with the standard ion ratio is sent to the pH adjustment tank, and 20% mass fraction of dilute sulfuric acid is added through a metering pump to stabilize the pH at 1.0.

[0099] 3. Concentration, Crystallization, and Solid-Liquid Separation: The pH-adjusted concentrate is fed into an MVR gradient concentration system. In the first stage, concentration is achieved at 65℃ and 0.03MPa to a solid content of 15%, and in the second stage, concentration is achieved at 75℃ and 0.06MPa to a solid content of 22%. The MVR concentrate is then fed into a primary flash tank, where the pressure is reduced to 0.025MPa, and the temperature is lowered to 50℃ at a rate of 5℃ / min. Seed crystals (potassium aluminum sulfate dodecahydrate from the previous batch, purity ≥99.5%, particle size 70μm) are added at a rate of 0.1% of the solution mass. The solution after primary flash evaporation is then fed into a secondary flash tank, where the pressure is reduced to 0.012MPa, and the temperature is lowered to 35℃ at a controlled rate of 1.0℃ / min. The solution after secondary flash evaporation is then fed into a cryogenic crystallization tank, where a stepped cooling process is employed: in the first stage, the temperature is lowered from 35℃ to 25℃ at a rate of 4℃ / h, with stirring for 1 hour; in the second stage, the temperature is lowered at a rate of 1℃ / h... The temperature was reduced from 25°C to 7°C and stirred for 3 hours. The crystallized mixture was then fed into a horizontal screw discharge centrifuge for separation. The centrifuge drum speed was 3000 rpm and the separation factor was 1500. The crude aluminum sulfate dodecahydrate solid and the filtrate were obtained. The moisture content of the crude solid was ≤11% and the solid content of the filtrate was ≤1.2%.

[0100] 4. Product Processing and Recycling: The crude solid product (purity ≥98.2%) separated by the centrifuge is sent to a sealed silo. A dehumidification device is installed in the silo to maintain a relative humidity of 30% and a temperature of 25℃. The centrifuge filtrate is then fed to a modified diatomaceous earth adsorption column (450mm diameter, 1800mm height) to remove Fe. 3+ Ti 4+ To ensure the presence of heavy metal ions (control the filtrate flow rate to 1 m / h), and guarantee a contact time ≥ 30 min, the outlet Fe... 3+ Ti 4+ Concentration ≤35ppm; the filtrate after adsorption enters the online refractometer detection unit. When the solid content is ≥10%, it is sent to the feed end of the MVR concentration system; when the solid content is <10%, it is sent to the concentrate section of the mixing tank; when the Fe at the adsorption column outlet... 3+ When the concentration is >50ppm, start the regeneration program and automatically switch to the standby adsorption column. Pass 20% mass fraction dilute sulfuric acid (40℃) into the saturated adsorption column, with an elution flow rate of 0.7m / h and an elution time of 1.8h. After elution, rinse with 30℃ water until the outlet pH=2, and blow dry with 0.15MPa compressed air for later use. The eluent after adsorption column regeneration is then transported to the sulfuric acid titanium dioxide hydrolysis stage and mixed with the hydrolyzed titanium dioxide material. The eluent transport volume is 1% of the titanium dioxide slurry mass.

[0101] 5. Product Modification and Application: The crude solid product was fed into a microwave modification furnace (frequency 2450MHz, power 800W), equipped with a stainless steel conveyor belt (speed 0.6m / min). It remained in the furnace for 5 minutes, converting it into hemihydrated potassium aluminum sulfate (moisture content reduced to 5%). The microwave-modified product was then transferred to a modification reactor, where deionized water was added to adjust the solid content to 20%. Then, sodium polyacrylate (molecular weight 8 million) at 0.5% of the mass fraction of the hemihydrated potassium aluminum sulfate was added. The sodium polyacrylate was pre-dissolved in a 4‰ aqueous solution (dissolution time ≥2h). The mixture was stirred at 60℃ for 30 minutes to achieve sodium polyacrylate grafting, with a grafting rate ≥78%. The grafted product was then spray-dried (inlet air 180℃, outlet air 80℃) to obtain 100μm... Powder, used as a customized flocculant for titanium dioxide wastewater treatment: Titanium dioxide wastewater (pH 2.5, suspended solids 800 mg / L) enters the equalization tank, and the agitator (90 rpm) maintains uniformity; in the first stage, 0.080% customized flocculant is added, and the stirring speed is 150 rpm, after which small flocs are formed after 10 minutes; in the second stage, carbide slag is added to adjust the pH to 6.5, and 0.075% flocculant is added, and the stirring speed is reduced to 50 rpm, after which large flocs are formed after 5 minutes; after flocculation, the wastewater enters the sedimentation tank (retention for 30 minutes), and the suspended solids in the supernatant are ≤20 mg / L.

[0102] The implementation results of the process in this embodiment show that the total salt recovery rate reaches 95.2%, the purity of the recovered potassium aluminum sulfate reaches 98.3%, the overall energy consumption of the process is reduced by 25%, the filtrate circulation rate reaches 80%, the dosage of the customized flocculant (sodium polyacrylate grafted) is reduced by 20% compared with the conventional flocculant, and the suspended solids removal rate reaches 98.0%.

[0103] Example 31. Filtrate Collection and Pretreatment: In the sulfuric acid process for titanium dioxide production, the filtrate discharged from the filter press (containing potassium) from the pre-kiln filtration process is collected and pretreated. + Al 3+ SO4 2- The plasma is transported to a dedicated storage tank via 316L stainless steel corrosion-resistant pipes. The storage tank is equipped with a level sensor (measuring range 0~5m, accuracy ±0.01m) and a stirring device (paddle type, speed 75rpm). A 316L stainless steel corrosion-resistant centrifugal pump is used to transport the filtrate in the storage tank to a ceramic membrane feed tank. A 100μm filter screen is installed at the inlet of the feed tank. The filtrate in the feed tank is filtered through a three-stage series ceramic membrane filtration system. The pore sizes of the three ceramic membranes are 1μm, 0.5μm, and 0.1μm, and the operating pressures are 0.23MPa, 0.27MPa, and 0.33MPa, respectively. A sampling port is set at the outlet of each filtration unit, and the turbidity of the filtrate is tested every 30 minutes to ensure that the turbidity is ≤0.8NTU. The titanium dioxide residue (moisture content ≤78%) retained by the three-stage filtration is collected and returned to the titanium dioxide slurry preparation process for reuse, with a titanium dioxide recovery rate ≥96%.

[0104] 2. Salt Concentration and Ion Adjustment: The clarified liquid after ceramic membrane filtration is sent to a two-stage series reverse osmosis membrane concentration system. The first-stage reverse osmosis operates at a pressure of 1.8 MPa, and the second-stage reverse osmosis operates at a pressure of 2.3 MPa, increasing the salt concentration of the clarified liquid from 0.9% to 11%. The clarified liquid produced by reverse osmosis (accounting for 78% of the total filtrate) is sent to the water storage tank of the titanium dioxide washing process to replace fresh water. The reverse osmosis concentrate is sent to a mixing tank equipped with a heating jacket to maintain the concentrate temperature at 34°C, and is stirred by a high-shear stirrer. An ion chromatograph (detection range 0~1000 mg / L, accuracy ≤0.1 mg / L) is installed on the outlet pipe of the mixing tank to detect K in the concentrate every 10 seconds. + Al 3+ SO4 2- Concentration, calculate n(K) + ):n(Al 3+ ):n(SO4 2-The ratio is adjusted as follows: When the ion ratio exceeds the range of 1:1:2±2%, K2SO4 or Al2(SO4)3 dilute solution is precisely added through a twin-screw atomizing feeder. During the addition, the stirrer speed is increased to 260 rpm, and the ratio is re-checked every 30 seconds until the standard is met. The concentrated solution with the standard ion ratio is sent to the pH adjustment tank, and 20% mass fraction of dilute sulfuric acid is added through a metering pump to stabilize the pH at 1.4.

[0105] 3. Concentration, Crystallization, and Solid-Liquid Separation: The pH-adjusted concentrate is fed into an MVR gradient concentration system. In the first stage, concentration is achieved at 75℃ and 0.07MPa to a solid content of 18%, and in the second stage, concentration is achieved at 85℃ and 0.1MPa to a solid content of 28%. The MVR concentrate is then fed into a primary flash tank, where the pressure is reduced to 0.015MPa, and the temperature is lowered to 40℃ at a rate of 8℃ / min. Seed crystals (potassium aluminum sulfate dodecahydrate from the previous batch, purity ≥99.5%, particle size 90μm) are added at a rate of 0.25% of the solution mass. The solution after primary flash evaporation is then fed into a secondary flash tank, where the pressure is reduced to 0.008MPa, and the temperature is lowered to 25℃ at a controlled rate of 1.8℃ / min. The solution after secondary flash evaporation is then fed into a cryogenic crystallization tank, where a stepped cooling process is employed: in the first stage, the temperature is lowered from 25℃ to 15℃ at a rate of 6℃ / h, with stirring for 3 hours; in the second stage, the temperature is lowered at a rate of 3℃ / h... The temperature was reduced from 15℃ to 3℃ and stirred for 5 hours. The crystallized mixture was then fed into a horizontal screw discharge centrifuge for separation. The centrifuge drum speed was 3400 rpm and the separation factor was 1900. The crude aluminum sulfate dodecahydrate solid and the filtrate were obtained. The moisture content of the crude solid was ≤9% and the solid content of the filtrate was ≤0.8%.

[0106] 4. Product Processing and Recycling: The crude solid product (purity ≥98.9%) separated by the centrifuge is sent to a sealed silo. A dehumidification device is installed in the silo to maintain a relative humidity of 38% and a temperature of 29℃. The centrifuge filtrate is then fed to a modified diatomaceous earth adsorption column (550mm diameter, 2200mm height) to remove Fe. 3+ Ti 4+ To ensure the presence of heavy metal ions, control the filtrate flow rate to 1.8 m / h, and guarantee a contact time ≥ 30 min, the outlet Fe... 3+ Ti 4+ Concentration ≤25ppm; the filtrate after adsorption enters the online refractometer detection unit. When the solid content is ≥10%, it is sent to the feed end of the MVR concentration system; when the solid content is <10%, it is sent to the concentrate section of the mixing tank; when the Fe at the adsorption column outlet... 3+When the concentration is >50ppm, start the regeneration program and automatically switch to the standby adsorption column. Pass 20% (w / w) dilute sulfuric acid (48℃) into the saturated adsorption column, with an elution flow rate of 0.9m / h and an elution time of 2.2h. After elution, rinse with 34℃ water until the outlet pH=2.8, and blow dry with 0.15MPa compressed air for later use. Transfer the eluent after adsorption column regeneration to the titanium dioxide slurry pretreatment tank, with a dosage of 1.8% of the titanium dioxide slurry mass.

[0107] Product Modification and Application: The crude solid product was fed into a microwave modification furnace (frequency 2500MHz, power 950W), with a stainless steel conveyor belt (speed 0.9m / min) inside. It remained in the furnace for 9 minutes, converting to hemihydrated potassium aluminum sulfate (moisture content reduced to 3.5%). The microwave-modified product was then transferred to a modification reactor, where deionized water was added to adjust the solid content to 28%. Then, 0.9% polyacrylamide (molecular weight 10 million) was added, and PAM was pre-dissolved into a 6‰ aqueous solution (dissolution time ≥2h). The mixture was stirred at 68℃ for 30 minutes to achieve PAM grafting, with a grafting rate ≥83%. The grafted product was then spray-dried (inlet air 195℃, outlet air 88℃) to produce 180μm powder, which was used as a customized flocculant for titanium dioxide wastewater treatment: Titanium dioxide wastewater (pH 2.5, suspended solids 800mg / L) entered an equalization tank, where a stirrer (speed 90rpm) maintained uniformity. The first stage dosage was 0.074%. A customized flocculant was used, and small flocs were formed after 10 minutes at a stirring speed of 150 rpm. In the second stage, 30% sodium hydroxide solution was added to adjust the pH to 6.5, and 0.074% flocculant was added. The stirring speed was reduced to 50 rpm, and large flocs were formed after 5 minutes. After flocculation, the wastewater entered the sedimentation tank (retention for 30 minutes), and the suspended solids in the supernatant were ≤20 mg / L.

[0108] The implementation results of the process in this embodiment show that the total salt recovery rate reaches 97.3%, the purity of the recovered potassium aluminum sulfate reaches 98.9%, the overall process energy consumption is reduced by 29%, the filtrate circulation rate reaches 84%, the dosage of the customized flocculant (PAM graft) is reduced by 24% compared with the conventional flocculant, and the suspended solids removal rate reaches 98.8%.

[0109] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention. Clearly, those skilled in the art can make various alterations and modifications to the invention without departing from its spirit and scope. Thus, if these modifications and modifications of the invention fall within the scope of the claims and their equivalents, the invention is also intended to include these modifications and modifications.

Claims

1. A method for efficient recovery and recycling of salts in the production of titanium dioxide using the sulfuric acid process, characterized in that, Includes the following steps: S1. Pre-treat the saline wastewater generated during the sulfuric acid process for titanium dioxide production to remove solid particles from the wastewater; S2. Concentrate the salts in the wastewater and adjust the ion ratio and pH of the concentrated solution to ensure the K+ concentration is high. + Al 3+ SO4 2- The ion molar concentration ratio is between 98% and 102% of the ion molar ratio in the KAl(SO4)2 compound, and the solution pH is 1 to 1.5; S3. Concentrate and crystallize to obtain solid potassium aluminum sulfate dodecahydrate; S4. Dehydrate the solid potassium aluminum sulfate dodecahydrate to obtain potassium aluminum sulfate hemihydrate; S5. Graft the potassium aluminum sulfate hemihydrate with a water-soluble polymer compound containing active functional groups to obtain a flocculant.

2. The method for efficient salt recovery and recycling in the sulfuric acid process for titanium dioxide production as described in claim 1, characterized in that, The pretreatment described in step S1 employs multi-stage ceramic membrane filtration.

3. The method for efficient salt recovery and recycling in the sulfuric acid process for titanium dioxide production as described in claim 1, characterized in that, The concentration in step S2 is achieved by reverse osmosis concentration, and the mass percentage concentration of salt in the concentrated solution is 10-12%.

4. The method for efficient salt recovery and recycling in the sulfuric acid process for titanium dioxide production as described in claim 1, characterized in that, Step S3 further includes: S31. Employing MVR gradient concentration; in the first stage, concentration is carried out at 65-75℃ and 0.03-0.07MPa until the solid content reaches 12-18%; in the second stage, concentration is carried out at 75-85℃ and 0.06-0.1MPa until the solid content reaches 22-28%; S32. Employing two-stage flash evaporation; in the first stage flash evaporation, potassium aluminum sulfate dodecahydrate seed crystals are added at a rate of 0.1-0.3% of the solution mass; in the first stage flash evaporation, the temperature is reduced to 40-50℃ at a cooling rate of 5-8℃ / min. The pressure is reduced to 0.015~0.025 MPa; the second-stage flash evaporation uses a cooling rate of 1~2℃ / min to reduce the temperature to 25~35℃ and the pressure to 0.008~0.012 MPa; S33. Freeze crystallization is carried out by step cooling; in the first stage, the temperature is reduced to 15~25℃ at a cooling rate of 4~6℃ / h, and then stirred for 1~3h; in the second stage, the temperature is reduced to 3~7℃ at a cooling rate of 1~3℃ / h, and then stirred for 3~5h; S34. Solid-liquid separation is performed to obtain the potassium aluminum sulfate dodecahydrate solid.

5. The method for efficient salt recovery and recycling in the sulfuric acid process for titanium dioxide production as described in claim 1, characterized in that, When the concentrated crystallization in step S3 yields solid potassium aluminum sulfate dodecahydrate, a concentrated mother liquor is also obtained. The concentrated mother liquor is purified by a modified diatomaceous earth adsorption column. The solid content of the purified concentrated mother liquor is tested. When the solid content is ≥10%, it is sent to step S3 to be mixed with the solution to be concentrated and crystallized. When the solid content is <10%, it is mixed with the concentrated solution in step S2.

6. The method for efficient salt recovery and recycling in the sulfuric acid process for titanium dioxide production as described in claim 5, characterized in that, When the modified diatomaceous earth adsorption column outlet solution Fe 3+ When the concentration is >50ppm, a sulfuric acid solution with a mass percentage concentration of 10-30% is used for elution. After elution, the solution is rinsed with water until the pH of the outlet solution is 2-3. The eluent is then transported to the sulfuric acid titanium dioxide hydrolysis stage and mixed with the hydrolyzed titanium dioxide material. The amount of eluent transported is 1-2% of the mass of the hydrolyzed titanium dioxide material.

7. The method for efficient salt recovery and recycling in the sulfuric acid process for titanium dioxide production as described in claim 1, characterized in that, The dehydration method described in step S4 is a microwave method, wherein the microwave frequency is 2400~2500MHz, the power is 800~1000W, and the time is 6~10min.

8. The method for efficient salt recovery and recycling in the sulfuric acid process for titanium dioxide production as described in claim 1, characterized in that, The water-soluble polymer compound containing active functional groups is selected from at least one of polyacrylamide, acrylamide-sodium acrylate copolymer, chitosan, or polydiallyldimethylammonium chloride.

9. The method for efficient salt recovery and recycling in the sulfuric acid process for titanium dioxide production as described in claim 1, characterized in that, Step S5 further includes: adjusting the solid content of the hemihydrated potassium aluminum sulfate to 20-30% by adding water, then adding a solution of water-soluble polymeric compound containing active functional groups, and reacting at 60-70°C for 20-40 minutes; the mass percentage concentration of the water-soluble polymeric compound containing active functional groups in the solution is 4-6‰, and the amount of the solution added is 0.6-1.0% of the mass of the hemihydrated potassium aluminum sulfate, based on the amount of the water-soluble polymeric compound containing active functional groups therein.

10. The method for efficient salt recovery and recycling in the sulfuric acid process for titanium dioxide production as described in claim 9, characterized in that, The solution after the reaction is spray-dried to obtain the flocculant, the flocculant having an average particle size of 100~200μm.

11. A method for applying a flocculant obtained using the efficient salt recovery and recycling method in the sulfuric acid process for titanium dioxide production as described in any one of claims 1 to 10, characterized in that, After adding 0.05-0.1% flocculant to titanium dioxide production wastewater with a pH of 2-3 and reacting, the pH of the wastewater is adjusted to 6-7. Then, another 0.05-0.1% flocculant is added and reacted again. Finally, solid-liquid separation is performed to obtain a supernatant with suspended solids removed.

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