A deep purification method for removing trace silicon impurities in high-purity boric acid

Abstract

A deep purification method for removing trace silicon impurities in high-purity boric acid belongs to the technical field of inorganic chemical purification. The method sequentially performs pre-oxidation, complex shielding and selective precipitation on the crude boric acid solution for the transformation and preliminary separation of different forms of silicon impurities; then uses a composite adsorbent of layered double hydroxide and amino-functionalized mesoporous silica for deep adsorption; and finally obtains a high-purity boric acid solution through precise filtration separation including microfiltration, ultrafiltration and nanofiltration. Through the synergistic effect of multiple steps, the method can systematically solve the removal problem of colloidal silicon and dissolved silicon, stably reduce the silicon impurity content in boric acid to the ppb level, meet the ultra-high purity requirements of raw materials in the field of semiconductors and the like, and has the advantages of regenerable adsorbent and environmental friendliness.

Description

Technical Field

[0001] This invention belongs to the field of inorganic chemical purification technology, specifically relating to a deep purification method for removing trace silicon impurities from high-purity boric acid. Background Technology

[0002] High-purity boric acid is a crucial basic material indispensable in cutting-edge fields such as semiconductor manufacturing, precision optics, and nuclear energy. Its purity directly determines the performance and reliability of the final product. The presence of trace silicon impurities is particularly critical, as it can cause lattice defects, light scattering, and degradation of electrical properties. With the feature size of microelectronic devices continuously shrinking to the nanometer scale, the tolerance for silicon impurities in boric acid has decreased to the ppb level, posing unprecedented challenges to purification technologies. Traditional industrial-grade boric acid purification mainly relies on recrystallization technology, which can effectively reduce most ionic impurities. However, it has limited effectiveness and low efficiency in removing silicon impurities that exist in solution primarily as colloidal silica or complex silicates, making it difficult to meet the standards for current ultra-high purity applications.

[0003] To overcome this bottleneck, the industry has explored various technological approaches for deep silica removal. For example, adsorption methods use materials such as activated alumina or functionalized silica gel, which capture silicon species through surface interaction. However, when treating complex industrial liquids, these methods generally suffer from poor selectivity, easy saturation of adsorption capacity, and difficulty in regeneration. They may even introduce secondary pollution due to adsorbent leaching. Ion exchange methods utilize the exchange of silicate ions with resin, which is effective for dissolved silica. However, when dealing with colloidal silica, resin pores are easily blocked, leading to increased operating pressure, a sharp drop in exchange capacity, and the large amount of regeneration wastewater generated during operation, resulting in additional treatment costs and environmental burden. Flocculation and sedimentation methods use inorganic or polymeric flocculants to cause colloidal silica to agglomerate and settle. This method is effective for removing colloidal silica but almost ineffective for dissolved silica. Furthermore, the flocculation process is easily affected by factors such as pH and temperature, and residual flocculant metal ions may constitute a new source of impurities. Membrane separation technology, especially ultrafiltration and nanofiltration, has shown advantages in retaining colloidal and macromolecular silica species due to its precise sieving characteristics. However, boric acid is prone to crystallization on the membrane surface during the concentration process, forming a dense scale layer with pollutants, which leads to rapid decline in membrane flux, frequent cleaning, and seriously affects the economy and continuity of the process.

[0004] To overcome the limitations of single technologies, recent research has tended to combine multiple methods, such as the "flocculation-adsorption-filtration" combined process. While this combined process improves silicon removal efficiency to some extent, it is essentially a simple superposition of unit operations and fails to fundamentally address the differences in the physicochemical behavior of different forms of silicon impurities, such as colloidal, dissolved, and polymerized states. Existing technologies typically lack targeted pretreatment for silicon species and fail to optimize their removal kinetics through chemical means; each step is often performed independently, lacking synergistic design based on solution chemistry principles, resulting in a lengthy overall process, high reagent consumption, and bottlenecks when pursuing extremely low silicon content. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a deep purification method for removing trace silicon impurities from high-purity boric acid, which has a high removal rate, good selectivity, stable operation and can effectively avoid secondary pollution.

[0006] The technical solution adopted by this invention to solve its technical problem is: a deep purification method for removing trace silicon impurities from high-purity boric acid, comprising the following steps:

[0007] (1) The crude boric acid solution containing trace silicon impurities was subjected to pre-oxidation treatment, complexation shielding treatment and selective precipitation treatment in sequence to selectively convert and separate silicon impurities;

[0008] (2) The liquid phase after step (1) is subjected to adsorption treatment to further remove dissolved silicon impurities;

[0009] (3) The liquid phase after step (2) is subjected to precision filtration and separation to obtain a high-purity boric acid solution.

[0010] This invention employs a multi-step synergistic purification strategy of "pretreatment-adsorption-fine filtration". First, through a series of pretreatments in step (1), different forms of silicon impurities are chemically transformed and initially enriched and separated, especially colloidal silicon that is difficult to remove directly, which is transformed into a form that is easy to process later. Then, the adsorption treatment in step (2) is specifically designed to deeply capture the silicon species remaining after pretreatment, especially those in dissolved state. Finally, the fine filtration in step (3) serves as a safeguard step, which can completely trap the fine flocs and adsorbent particles formed during pretreatment, ensuring the cleanliness of the product. The synergistic design of these three steps enables efficient and stable removal of silicon impurities at the ppb level or even lower concentrations, while effectively avoiding the limitations of a single technical path. The process is highly adaptable, and the purification effect is significantly better than the traditional simple combination process of "flocculation-adsorption-filtration".

[0011] Specifically, in step (1):

[0012] The pre-oxidation treatment includes: adding an oxidant to a crude boric acid solution and carrying out an oxidation reaction at 50°C to 60°C;

[0013] The complexation shielding treatment includes: adding an organic acid complexing agent to the pre-oxidized solution and adjusting the pH of the solution to 7.5-8.5;

[0014] The selective precipitation treatment includes: adding a flocculant to the solution under the specified pH conditions and performing programmed cooling to induce the selective precipitation of silicon impurities and the formation of easily separable flocs.

[0015] Pre-oxidation treatment utilizes oxidants to disrupt the organic coating layer on the surface of colloidal silica or partially alter its surface chemical state, increasing the density of silanol groups and thus significantly enhancing its hydrophilicity and reactivity, creating favorable conditions for subsequent flocculation and adsorption. Complexation shielding treatment, through the addition of organic acid complexing agents and pH control, allows the organic acid complexing agents to preferentially complex Fe in the solution. 3+ Al 3+ Metallic cation impurities are prevented from co-precipitating with silicate ions or competing for adsorption sites, thus improving the selectivity of the silicon removal process. Furthermore, this pH range is conducive to the stable existence of silicate ions and provides a suitable environment for the flocculant to exert its optimal effectiveness. Selective precipitation treatment, under the above optimized conditions, involves adding a flocculant and combining it with programmed cooling. Utilizing the difference in the effect of temperature changes on the solubility of boric acid and silicate (salts), silicon impurities are induced to preferentially precipitate and aggregate, and are then captured and swept by the flocculant to form large and dense flocs, thereby achieving efficient preliminary separation of silicon impurities from the liquid phase. These three pretreatment steps are interconnected, laying a solid foundation for subsequent deep adsorption.

[0016] Preferably, the oxidant is hydrogen peroxide, and the amount added is such that the final concentration of hydrogen peroxide in the crude boric acid solution is 20ppm to 30ppm.

[0017] Hydrogen peroxide (H2O2) is a "green" oxidant, whose decomposition products are water and oxygen. It does not introduce new anionic impurities into the system, avoiding the risk of secondary pollution, making it particularly suitable for the preparation of high-purity substances. Controlling its concentration within a low range is sufficient to produce effective and mild oxidative modification of colloidal silicon surfaces, enhancing their surface polarity, while avoiding instability in subsequent processes or unnecessary side reactions caused by the presence of excessive H2O2.

[0018] Preferably, the organic acid complexing agent is a salt of citric acid or ethylenediaminetetraacetic acid, and its addition amount is 0.01% to 0.05% of the mass of the crude boric acid solution.

[0019] The addition of citric acid or EDTA salts can effectively complex trace metal impurities commonly found in boric acid. This concentration range effectively shields the silicon removal process from interference by metal ions, preventing the formation of insoluble silicates that consume flocculants or clog adsorbent pores. It also avoids the potential for subsequent separation difficulties or increased organic carbon residues due to excessive complexing agent addition. Using salts such as sodium citrate or disodium EDTA avoids the drastic impact of direct acid addition on the system's pH, facilitating precise pH control.

[0020] More specifically, the programmed cooling includes cooling from 50℃~55℃ to 30℃~35℃ at a rate of 0.3℃ / min~0.8℃ / min.

[0021] Controlling this cooling rate is beneficial for forming silicate (salt) microcrystals with uniform particle size and dense structure, avoiding the problem of excessively fine grains and difficulty in flocculation and separation caused by excessive crystal nuclei generated by rapid cooling. This specific temperature range and cooling rate are optimized based on the difference in solubility changes between boric acid and silicate (salt) at different temperatures. This maximizes the supersaturation of silicon impurities and promotes their preferential precipitation, while maintaining a high solubility of boric acid bulk. This achieves efficient separation of silicon and boron during floc separation, improving the recovery rate of boric acid.

[0022] Specifically, the flocculant includes inorganic flocculants and organic polymeric flocculants; preferably, the inorganic flocculant is polyaluminum chloride or polyaluminum ferric silicate, and the organic polymeric flocculant is quaternized chitosan. More preferably, the mass ratio of the inorganic flocculant to the organic polymeric flocculant, based on the effective components, is 2~10:1, more preferably 3~5:1.

[0023] The combination of inorganic and organic flocculants achieves a synergistic effect. The inorganic flocculant primarily destabilizes colloidal silica by compressing its electric double layer through charge neutralization. The organic polymeric flocculant, with its long-chain structure, adsorption bridging and netting / sweeping action, aggregates the destabilized fine particles into large flocs. This combination allows for destabilization first by the inorganic flocculant, followed by efficient aggregation by the organic flocculant, resulting in larger, denser flocs with faster settling speeds. Polyaluminum chloride / polyaluminum ferric silicate and quaternized chitosan are preferred because the former forms a high-valent aluminum / iron hydroxide complex in water with strong charge neutralization ability for negatively charged colloidal silica, while the latter, as a natural modified polymer, has high cationicity and is environmentally friendly. Maintaining a mass ratio of 3-5:1 ensures optimal flocculation with minimal organic flocculant dosage while guaranteeing sufficient charge neutralization, balancing efficiency and economy, and reducing the introduction of organic matter.

[0024] Preferably, in step (2), the adsorbent used in the adsorption treatment is a composite adsorbent material of layered bimetallic hydroxide and amino-functionalized mesoporous silica.

[0025] A bifunctional adsorption system was constructed using a composite adsorbent of layered double hydroxide (LDH) and amino-functionalized mesoporous silica. LDH, due to the positive charge on its layers and the exchangeability of anions between them, can efficiently capture silicate anions in solution through ion exchange and electrostatic adsorption. Amino-functionalized mesoporous silica, with its high specific surface area, regular mesoporous channels, and abundant amino groups, can effectively adsorb neutral or weakly acidic silicon species through physical adsorption, hydrogen bonding, or coordination. Combining the two not only leverages their respective adsorption advantages to achieve full-spectrum capture of dissolved silicon in different forms, but also effectively disperses and supports the layered structure of LDH, preventing its stacking and exposing more active sites, thereby significantly improving the overall adsorption capacity and adsorption kinetics.

[0026] More preferably, the layered bimetallic hydroxide comprises magnesium and aluminum, and the molar ratio of magnesium to aluminum is 2-4:1; the amino density of the amino-functionalized mesoporous silica is 3.0 mmol / g-3.6 mmol / g. Even more preferably, the mass ratio of the layered bimetallic hydroxide to the amino-functionalized mesoporous silica, on a dry basis, is 1-5:1, more preferably 2-3:1.

[0027] The LDH is a magnesium-aluminum system with a Mg / Al molar ratio between 2:1 and 4:1. This ratio provides suitable lamination charge density, good crystallinity, and chemical stability, resulting in a large anion exchange capacity and excellent kinetic performance. The amino-functionalized mesoporous silica has an amino density of not less than 3 mmol / g, ensuring sufficient chemisorption sites on its surface and strong chemisorption of silicon species. The optimal mass ratio of LDH to amino silica in the composite adsorbent is 2-3:1. This ensures that LDH provides sufficient ion exchange capacity as the main adsorbent, while an appropriate amount of amino silica serves as a functional additive and dispersion matrix. This significantly improves the overall adsorption performance and selectivity of the composite material for silicon, while avoiding excessive costs due to excessive use of expensive functionalized silica, thus achieving an optimal balance between adsorption efficiency and economic benefits.

[0028] Preferably, the adsorption treatment is performed under ultrasonic assistance, with an ultrasonic frequency of 20kHz~60kHz and a power density of 30W / m³. 3 ~100W / m 3 .

[0029] Applying appropriate ultrasound generates cavitation, microfluidics, and microjets in the liquid. These effects effectively break down the liquid-solid boundary layer, enhancing the mass transfer and diffusion of silica adsorbates to the adsorbent particle surface. Simultaneously, the cavitation effect of ultrasound may slightly "clean" the adsorbent surface, exposing more fresh active sites. This allows the adsorption process to reach equilibrium more quickly, improving the processing efficiency per unit time, especially when treating low-concentration impurities, significantly shortening the contact time required to achieve deep purification.

[0030] Preferably, in step (3), the precision filtration separation includes microfiltration, ultrafiltration and nanofiltration steps performed sequentially.

[0031] A three-stage precision filtration separation process—microfiltration, ultrafiltration, and nanofiltration—is employed, creating a precision retention barrier with progressively smaller pore sizes. Microfiltration, as the first barrier, effectively removes large particulate flocs and adsorbent aggregates generated during pretreatment and adsorption. Ultrafiltration further retains colloidal silica, large molecular organic impurities, and even finer suspended solids. Nanofiltration, as the final membrane barrier, typically has a pore size of around 1 nm, sufficient to effectively retain remaining tiny silica particles, dimer or oligomeric silicate ions, and even some monovalent ions. This multi-stage filtration design not only ensures extremely high purity of the final filtrate, thoroughly removing silica and other particulate impurities, but also reduces the fouling load of a single membrane process through staged filtration, extending membrane lifespan and improving the stability and economy of the entire filtration system.

[0032] More preferably, the microfiltration uses a ceramic membrane with an average pore size of 0.1 μm to 0.5 μm; the ultrafiltration uses a hollow fiber ultrafiltration membrane with a molecular weight cutoff of 5 kDa to 20 kDa; and the nanofiltration uses a nanofiltration membrane with an average pore size of 0.5 nm to 2.0 nm, preferably a spiral wound polyamide composite nanofiltration membrane or a sulfonated polyethersulfone nanofiltration membrane.

[0033] Ceramic membranes with an average pore size of 0.1μm to 0.5μm were chosen for microfiltration because they possess excellent chemical stability, high temperature resistance, high mechanical strength, and are easy to clean and regenerate. They are ideal for treating complex chemical solutions and can stably retain large particles generated in the preceding steps. Hollow fiber ultrafiltration membranes with a molecular weight cutoff of 5kDa to 20kDa were used. This cutoff range is specifically matched to the size of nanoscale colloidal silica and organic macromolecules that may remain after pretreatment, ensuring effective removal. Simultaneously, the hollow fiber membrane module has a high packing density and a small footprint. Nanofiltration membranes were specifically defined as having an average pore size of 0.5nm to 2.0nm. This pore size range matches the nanofiltration process's molecular weight cutoff of approximately 200Da to 1000Da. Through precise size exclusion, it can efficiently retain silicate ions and their oligomers with a hydration radius of approximately 0.4nm to 0.5nm, physically ensuring near-complete removal and thus guaranteeing that the dissolved silica content in the final product is reduced to below ppb levels.

[0034] Spiral-wound polyamide composite membranes have high desalination rates, high throughput, and good pressure resistance; sulfonated polyethersulfone membranes have excellent solvent resistance and hydrophilicity.

[0035] Preferably, after step (3), the high-purity boric acid solution is further subjected to a temperature-controlled crystallization step.

[0036] Compared to traditional natural cooling or rapid cooling crystallization, programmed temperature-controlled crystallization can effectively regulate the nucleus generation and crystal growth process by precisely controlling the cooling rate and crystallization path. This is beneficial for growing large boric acid crystals with uniform particle size, complete crystal shape, and few encapsulated impurities. Uniform particle size facilitates subsequent solid-liquid separation, washing, and drying, while complete crystal shape means smooth crystal surface and less adsorption of mother liquor, thereby further improving product purity.

[0037] More preferably, the programmed temperature-controlled crystallization includes:

[0038] The first cooling stage: the temperature is reduced from 30℃~40℃ to 25℃~30℃ at a rate of 0.1℃ / min~0.5℃ / min.

[0039] The second cooling stage: the temperature is reduced from 25℃~30℃ to 15℃~20℃ at a rate of 0.5℃ / min~1.0℃ / min.

[0040] The first stage employs a very slow cooling rate. This stage aims to precisely control the supersaturation of the solution to a low level, promoting the formation of a moderate number of uniformly sized crystal nuclei and preventing excessive nucleation explosions that produce too many fine crystals. The second stage, based on the existing seed crystals, appropriately accelerates the cooling rate to provide sufficient impetus for crystal growth, allowing the crystals to grow fully around the nuclei, thereby obtaining a product with a concentrated particle size distribution and good crystal integrity.

[0041] Preferably, the method further includes a step of regenerating the adsorbent after adsorption treatment, wherein the regeneration step includes desorbing the adsorbent with an alkaline solution and recovering silicon resources from the desorption solution.

[0042] This invention utilizes an alkaline solution, such as sodium hydroxide, to desorb the saturated composite adsorbent after adsorption. The alkaline solution desorbs the adsorbed silicon species, allowing the adsorbent to recover most of its adsorption capacity and be reused. This significantly reduces the cost per use of the adsorbent and improves the economics of the process. More importantly, the silicon-rich desorption solution can be used as a raw material for further processing, such as acidification, aging, and drying, to recover valuable silicon products, thus achieving waste resource utilization.

[0043] Compared with existing technologies, this invention has the following advantages: The deep purification method provided by this invention precisely targets and transforms silicon impurities of different forms through a synergistic pretreatment of "pre-oxidation-complexation shielding-selective precipitation"; combined with a specially formulated bifunctional composite adsorbent for deep adsorption; and finally, through multi-stage precision filtration and safe crystallization, a complete, efficient, and controllable purification system is formed. This method can systematically solve the problem of removing colloidal and dissolved silicon, effectively avoiding the problems of secondary pollution, efficiency bottlenecks, and unstable operation existing in traditional methods. It can stably reduce the trace silicon impurity content in high-purity boric acid to the ppb level, meeting the quality requirements of ultra-high purity boric acid in high-end fields such as semiconductors and optics. At the same time, this method has the advantages of regenerable adsorbent and recyclable silicon resources, making it environmentally friendly and highly economical. Detailed Implementation

[0044] The present invention will be specifically described below through examples. Example 1 is the basic example. The process conditions in the other examples and comparative examples that are not explicitly described are the same as those in Example 1. The industrial boric acid used in the examples and comparative examples contains H3BO3 ≥ 99.5% and Si impurity content of 45 ppm.

[0045] Example 1

[0046] First, 1.0 kg of industrial boric acid was dissolved in 6.0 L of ultrapure water at 70 °C. Pretreatment followed: hydrogen peroxide was added to a final concentration of 25 ppm, and the mixture was stirred at 55 °C for 40 min to complete pre-oxidation. Then, trisodium citrate (0.03 wt%) was added, and the pH was adjusted to 8.0 with ammonia to achieve complexation shielding. Next, polyaluminum chloride (15 ppm as Al2O3) was added and stirred rapidly, followed by quaternized chitosan (3 ppm) with a 5:1 mass ratio of active ingredients. Finally, the temperature was reduced from 55 °C to 35 °C at a rate of 0.5 °C / min, and the mixture was allowed to stand for 90 min to induce selective precipitation of silica impurities, forming flocs.

[0047] The pretreated supernatant was subjected to adsorption treatment at 40℃. A composite adsorbent consisting of magnesium aluminum hydrotalcite and amino-functionalized mesoporous silica at a dry weight ratio of 3:1 was used, with a total dosage of 18g. The adsorption process was carried out with stirring at 120rpm and at 40kHz and 50W / m³. 3 The process was performed under ultrasonic assistance for 60 minutes. Subsequently, the adsorbed liquid underwent three-stage precision filtration: microfiltration, ultrafiltration, and nanofiltration were carried out sequentially using a ceramic membrane with an average pore size of 0.2 μm (operating pressure 0.10 MPa), a hollow fiber ultrafiltration membrane with a molecular weight cutoff of 10 kDa, and a spiral-wound polyamide composite nanofiltration membrane with an average pore size of 1.0 nm (operating pressure 1.2 MPa).

[0048] The high-purity boric acid filtrate obtained after filtration was subjected to temperature-controlled crystallization. First, the temperature was lowered from 40°C to 28°C at a rate of 0.3°C / min, then further lowered to 18°C ​​at a rate of 0.8°C / min, and allowed to mature for 60 minutes. After crystallization, the crystals were centrifuged and washed with cold water. The resulting wet crystals were then subjected to two-stage vacuum drying at 40°C and 60°C to obtain the final high-purity boric acid product.

[0049] ICP-MS analysis showed that the product contained 0.8 ppb of silicon and 1.9 ppb of other key metallic impurities, with uniform crystal particle size distribution. Furthermore, the saturated composite adsorbent was regenerated with 0.5 M NaOH solution at 50°C for 60 min, achieving recycling.

[0050] Example 2

[0051] 1.0 kg of industrial boric acid of the same specification was dissolved in 6.0 L of ultrapure water. In the pretreatment stage, hydrogen peroxide was added to control its final concentration to 20 ppm, and the reaction was carried out at 50 °C for 40 min. Subsequently, trisodium citrate was added at a rate of 0.01 wt%, and the pH of the solution was precisely adjusted to 7.5 and stabilized with ammonia. In the flocculation and sedimentation stage, polyaluminum chloride (calculated as Al2O3, 15 ppm) was added first, followed by quaternized chitosan, with the effective component mass ratio controlled at 10:1. The temperature was then programmed to decrease from 50 °C to 30 °C at a rate of 0.3 °C / min, and allowed to stand for aging. Subsequent adsorption treatment used a composite adsorbent, with a Mg-Al LDH to amino silica mass ratio of 2:1, and adsorption was carried out for 60 min without ultrasonic assistance. The precision filtration and crystallization steps were the same as in Example 1. The final product was found to have a silicon content of 1.8 ppb.

[0052] Example 3

[0053] The first half of the pretreatment was the same as in Example 1. During the selective precipitation stage, polyaluminum ferric silicate (15 ppm as Al+Fe) was added first, followed by quaternized chitosan (3 ppm), controlling the effective component mass ratio of the two to be 3:1. Subsequently, a rapid cooling program of 0.8 °C / min was implemented, reducing the temperature from 55 °C to 35 °C, followed by static aging. This faster cooling rate was intended to test the floc formation ability and density of the flocs at a higher supersaturation rate. The adsorption treatment used the same composite adsorbent as in Example 1, but at a 60 kHz ultrasonic frequency and 100 W / m³. 3 The process was carried out at high power density to enhance mass transfer. The filtration and crystallization processes remained unchanged. The product was found to contain 0.9 ppb of silicon.

[0054] Example 4

[0055] In the pretreatment, the final concentration of hydrogen peroxide was set at 30 ppm, and the reaction temperature was 60℃. During the complexation and shielding stage, disodium EDTA was added at a rate of 0.05 wt.%, and the solution pH was adjusted to 8.5. The flocculant used was a mixture of PAC and quaternized chitosan at a mass ratio of 5:1; precipitation was achieved by cooling at a rate of 0.5℃ / min. In the adsorption treatment, the composite adsorbent consisted of magnesium aluminum hydrotalcite (Mg / Al molar ratio 4:1) and amino-functionalized mesoporous silica (amino density 3.6 mmol / g) at a dry basis mass ratio of 5:1, with the same total dosage. Adsorption was carried out at 40℃ under ultrasound-free conditions. Subsequent treatment involved three-stage membrane filtration (using sulfonated polyethersulfone membrane for nanofiltration) and crystallization. The product was found to have a silicon content of 1.5 ppb.

[0056] Example 5

[0057] The pretreatment process parameters were the same as in Example 1. In the adsorption stage, an adsorbent composed of magnesium aluminum hydrotalcite and amino-functionalized mesoporous silica at a dry weight ratio of 1:1 was used. No ultrasonic assistance was applied throughout the adsorption process; mechanical stirring was performed at 120 rpm for 60 min at 40°C. In the subsequent precision filtration, the nanofiltration membrane operating pressure was increased to 1.5 MPa. The programmed temperature-controlled crystallization used milder conditions: the temperature was reduced from 40°C to 25°C at a rate of 0.1°C / min, and then further reduced to 15°C at a rate of 0.5°C / min. The product was found to have a silicon content of 4.6 ppb.

[0058] Example 6

[0059] The pretreatment, adsorption, and three-stage filtration purification steps were performed exactly as described in Example 1. In the post-crystallization stage, a two-stage temperature-controlled crystallization process was implemented: the first stage involved cooling from 30°C to 25°C at a rate of 0.5°C / min; the second stage involved cooling from 25°C to 20°C at a rate of 1.0°C / min. After centrifugation and washing, the resulting wet crystals and mother liquor were collected separately. The key step was adsorbent regeneration: the saturated composite adsorbents collected after adsorption treatment in Example 1 and this example were combined and desorbed using 1.0M NaOH solution at 60°C with stirring for 90 min. The desorbed adsorbent was thoroughly washed with ultrapure water until neutral and dried at 80°C. The regenerated adsorbent's adsorption capacity for a solution containing borosilicate was tested and found to be approximately 85% of that of the fresh adsorbent.

[0060] In this embodiment, the final boric acid product has a silicon content of 0.8 ppb, and the crystal particle size is slightly finer but the yield is improved.

[0061] Example 7

[0062] 1.0 kg of industrial boric acid was dissolved in 6.0 L of ultrapure water. In the pretreatment stage, hydrogen peroxide was added to control the final concentration to 25 ppm, and the reaction was carried out at 55 °C. Subsequently, trisodium citrate was added at a controlled amount of 0.03 wt%, and the pH of the solution was precisely adjusted and stabilized at 8.0. Flocculation and precipitation employed a composite system of polyaluminum chloride and quaternized chitosan, with a strict mass ratio of 4:1 based on the effective components, followed by a programmed cooling rate of 0.5 °C / min. In the adsorption treatment, magnesium aluminum hydrotalcite (Mg / Al molar ratio 3:1) and amino-functionalized mesoporous silica (amino density 3.2 mmol / g) were combined in a dry basis mass ratio of 2.5:1. The adsorption process was conducted using an ultrasonic frequency of 40 kHz and a concentration of 60 W / m³. 3 The process was carried out with the assistance of power density. The temperature-controlled crystallization employed a two-stage cooling process: the first stage reduced the temperature from 40℃ to 28℃ at a rate of 0.3℃ / min, and the second stage reduced the temperature from 28℃ to 18℃ at a rate of 0.8℃ / min. Testing revealed a silicon content of 0.5 ppb and a total of 0.8 ppb for other metallic impurities, with regular crystal morphology and uniform particle size.

[0063] Example 8

[0064] The parameters for the pretreatment and filtration steps were as described in Example 7. In the adsorption process, the dry weight ratio of magnesium aluminum hydrotalcite to amino-functionalized mesoporous silica in the composite adsorbent was 3:1. This adsorbent was used to treat three batches of the same pretreated boric acid solution. After each adsorption, the adsorbent was not discarded but immediately regenerated. The regeneration step involved dynamically circulating the saturated adsorbent with 0.8M NaOH solution at 55°C for 70 min, followed by thorough washing with hot ultrapure water until neutral. The adsorption efficiency of the adsorbent on fresh feed solution was measured after each regeneration. The results showed that the silicon content of the final product was <0.8 ppb during the first use; <1.2 ppb after the second cycle; and remained at 1.8 ppb after the third cycle. Simultaneously, amorphous silica powder was successfully recovered from the desorption solution after the third regeneration.

[0065] Example 9

[0066] The pretreatment steps, including pre-oxidation, complexation shielding, and selective precipitation, were performed exactly according to the preferred parameters of Example 1. During the adsorption stage, instead of using a composite adsorbent of magnesium aluminum hydrotalcite and amino-functionalized mesoporous silica, 18g of amino-functionalized mesoporous silica was used alone. The adsorption conditions were the same as in Example 1. Subsequent filtration and crystallization steps remained unchanged. Test results showed that the silicon content of the final product was 8 ppb.

[0067] Example 10

[0068] The pretreatment steps, including pre-oxidation, complexation shielding, and selective precipitation, were performed exactly according to the preferred parameters of Example 1. In the adsorption stage, instead of using a composite adsorbent of magnesium aluminum hydrotalcite and amino-functionalized mesoporous silica, 18g of magnesium aluminum hydrotalcite was used. The adsorption conditions were the same as in Example 1. Subsequent filtration and crystallization steps remained unchanged. Test results showed that the silicon content of the final product was 12ppb.

[0069] Comparative Example 1

[0070] 1.0 kg of industrial boric acid raw material of the same specifications as in Example 1 was dissolved in 6.0 L of ultrapure water. Pre-oxidation and complexation shielding treatments were completely skipped, and the pH was not adjusted (initial pH approximately 4-5). Polyaluminum chloride (PAC) was directly added to the solution at a dosage of 30 ppm (Al₂O₃), and the solution was rapidly stirred and allowed to settle. Subsequently, the supernatant after flocculation was directly adsorbed using the same composite adsorbent as in Example 1, along with the same three-stage membrane filtration and crystallization process. The final product, analyzed by ICP-MS, showed a silicon content of 15 ppm, a removal rate of only about 66%, and an aluminum impurity content >50 ppb.

[0071] Comparative Example 2

[0072] All steps and parameters from pretreatment to adsorption were strictly performed according to Example 1 to ensure that the initial state of the post-adsorption liquid was the same as in Example 1. In the solid-liquid separation step, instead of using a precision filtration process of "ceramic microfiltration-hollow fiber ultrafiltration-nanofiltration," a standard plate and frame filter with a 0.45μm mixed cellulose ester membrane was used for a single filtration. The filtered clarified liquid was also subjected to programmed temperature-controlled crystallization. Final product testing revealed a silicon content of 25 ppb.

Claims

1. A deep purification method for removing trace silicon impurities from high-purity boric acid, characterized in that, Includes the following steps: (1) The crude boric acid solution containing trace silicon impurities was subjected to pre-oxidation treatment, complexation shielding treatment and selective precipitation treatment in sequence to selectively convert and separate silicon impurities; (2) The liquid phase after step (1) is subjected to adsorption treatment to further remove dissolved silicon impurities; (3) The liquid phase after step (2) is subjected to precision filtration and separation to obtain a high-purity boric acid solution; In step (1): The pre-oxidation treatment includes: adding an oxidant to a crude boric acid solution and carrying out an oxidation reaction at 50°C to 60°C; The complexation shielding treatment includes: adding an organic acid complexing agent to the pre-oxidized solution and adjusting the pH of the solution to 7.5-8.5; The selective precipitation treatment includes: under the pH conditions, adding a flocculant to the solution and performing programmed cooling to induce the selective precipitation of silicon impurities and the formation of easily separable flocs; the oxidant is hydrogen peroxide, and the organic acid complexing agent is a salt of citric acid or a salt of ethylenediaminetetraacetic acid; The programmed cooling includes cooling from 50℃~55℃ to 30℃~35℃ at a rate of 0.3℃ / min~0.8℃ / min; The flocculant is a compound of inorganic flocculant and organic polymeric flocculant; the inorganic flocculant is polyaluminum chloride or polyaluminum ferric silicate, and the organic polymeric flocculant is quaternized chitosan. The adsorbent used in the adsorption treatment in step (2) is a composite adsorbent material of layered bimetallic hydroxide and amino-functionalized mesoporous silica; The precision filtration separation described in step (3) includes microfiltration, ultrafiltration and nanofiltration steps performed sequentially; The microfiltration process uses a ceramic membrane with an average pore size of 0.1 μm to 0.5 μm. The ultrafiltration uses a hollow fiber ultrafiltration membrane with a molecular weight cutoff of 5kDa to 20kDa; The nanofiltration process uses spiral-wound polyamide composite nanofiltration membranes or sulfonated polyethersulfone nanofiltration membranes with an average pore size of 0.5 nm to 2.0 nm.

2. The deep purification method for removing trace silicon impurities from high-purity boric acid according to claim 1, characterized in that, The amount of oxidant added is such that the final concentration of hydrogen peroxide in the crude boric acid solution is 20 ppm to 30 ppm.

3. The deep purification method for removing trace silicon impurities from high-purity boric acid according to claim 1, characterized in that, The amount of organic acid complexing agent added is 0.01% to 0.05% of the mass of the crude boric acid solution.

4. The deep purification method for removing trace silicon impurities from high-purity boric acid according to claim 1, characterized in that, The layered bimetallic hydroxide contains magnesium and aluminum, with a magnesium to aluminum molar ratio of 2 to 4:1; the amino density of the amino-functionalized mesoporous silica is 3.0 mmol / g to 3.6 mmol / g.

5. The deep purification method for removing trace silicon impurities from high-purity boric acid according to claim 1, characterized in that, The adsorption treatment was performed under ultrasonic assistance, with an ultrasonic frequency of 20kHz~60kHz and a power density of 30W / m³. 3 ~100W / m 3 .