Method for removing impurities in sodium silicate solution

Abstract

The invention belongs to the technical field of inorganic chemical separation and purification, and particularly relates to a method for removing impurities in a sodium silicate solution. The method comprises the following steps: stirring a sodium silicate solution at a certain temperature for a period of time, adding hydrogen peroxide and a catalyst into the sodium silicate solution, generating a colloid structure, and aging; and finally, carrying out solid-liquid separation to obtain the impurity-removed high-purity sodium silicate solution. The catalyst is ferrous sulfate heptahydrate or zinc nitrate, can effectively excite decomposition of hydrogen peroxide to form high reactive oxygen free radicals and promote selective precipitation of aluminum oxide into hydrated sodium aluminosilicate colloid, SiO2 in a sodium silicate solution is kept in a dissolved state, and the precipitation rate is 0 or extremely low. The method has the advantages of mild reaction conditions, rapid gel forming, stable structure and simple process, is suitable for industrial continuous operation, and is especially suitable for precursor purification treatment in the preparation process of a high-purity silicon material.

Description

Technical Field

[0001] The invention belongs to the technical field of inorganic chemical separation and purification, and particularly relates to a method for removing impurities from a sodium silicate solution. Background Art

[0002] Sodium silicate is a widely used, important inorganic salt, used in a variety of fields, including papermaking, ceramics, coatings, civil engineering, silica sol preparation, and cleaning agents. The rapid development of high-performance materials and high-purity products has placed higher demands on the purity of sodium silicate products. In industrial production, sodium silicate is typically produced by eutectic or alkali dissolution of silicon sources such as quartz sand, coal gangue, and fly ash with alkaline substances. However, because the raw materials often contain impurities such as aluminum, iron, and titanium, impurities such as aluminum oxide, iron oxide, and titanium oxide are inevitably introduced during the preparation process, resulting in a reduction in the purity of the final sodium silicate solution.

[0003] Alumina is one of the most significant interfering impurities. Under alkaline conditions, alumina often exists as a soluble aluminate or colloidal complex, exhibiting strong stability and making it difficult to remove through traditional physical filtration or sedimentation. Residual alumina not only affects the clarity and stability of sodium silicate products but also, when used as a raw material in downstream applications such as the synthesis of high-purity silica sols and functional silicon materials, can induce impurities, reduce product purity, and even interfere with the material's structural formation.

[0004] Currently, common aluminum removal methods include acid precipitation, complexing agent method, membrane separation technology, and redox method. The acid precipitation method adjusts the pH to precipitate aluminum in the form of aluminum hydroxide, but it also induces partial gelation and precipitation of silica, resulting in silicon loss. The complexing agent method has certain selectivity, but the organic reagents introduced are expensive and prone to secondary pollution. Although membrane separation and ion exchange technologies have high precision, they are complex to operate and have high equipment costs, making them difficult to apply in large-scale continuous production scenarios. Some studies have attempted to use redox treatment, but most require high temperature and high pressure conditions or complex process flows, making them difficult to industrialize.

[0005] Other studies have attempted to use oxidants such as hydrogen peroxide to oxidize impurities, converting them into forms that are easily precipitated or adsorbed. However, due to the low reaction efficiency of oxidation systems, auxiliary agents or special processes are often required, and efficient and controllable impurity removal remains difficult. In particular, the effective separation of aluminum oxide from sodium silicate solutions under normal pressure and mild conditions remains a technical challenge.

[0006] In summary, the existing technology still has the following problems in treating aluminum oxide impurities in sodium silicate solution: first, the treatment efficiency is low and aluminum oxide is difficult to fully remove; second, the operating conditions are complex or the cost is high, which is not conducive to industrial promotion; third, it is often accompanied by the loss of the main component silicon or the generation of by-products, affecting product quality.

[0007] Therefore, how to achieve efficient removal of alumina in a simple and controllable manner under normal pressure and mild conditions, and thereby improve the purity of the sodium silicate solution, is a technical problem that needs to be urgently solved in this field. Summary of the Invention

[0008] The present invention aims to provide a method for removing impurities from a sodium silicate solution. The method can quickly remove impurities such as aluminum oxide from the sodium silicate solution under normal pressure to obtain a pure sodium silicate solution. The method has a short process flow and is easy to implement in industrial production.

[0009] In order to achieve the above object, the present invention provides the following technical solutions:

[0010] A method for removing impurities from a sodium silicate solution comprises the following steps:

[0011] (1) Stirring the sodium silicate solution at a certain temperature for a period of time to allow the aluminum oxide in the sodium silicate solution to form hydrated sodium aluminosilicate crystal nuclei, and detecting the contents of Al2O3, SiO2, and Na2O therein;

[0012] (2) adding an oxidant and a catalyst to the sodium silicate solution to react, forming a colloid and then allowing the colloid to stand and age for a certain period of time; wherein the oxidant is hydrogen peroxide and the catalyst is ferrous sulfate heptahydrate (FeSO4·7H2O) or zinc nitrate (Zn(NO3)2);

[0013] (3) subjecting the reactant obtained in step (2) to solid-liquid separation to obtain a filter cake containing impurities and a sodium silicate solution after the impurities are removed.

[0014] The sodium silicate solution used in the present invention can be obtained by using calcined coal gangue as raw material and subjecting it to an alkaline cyclic desiliconization reaction. Specifically, after the coal gangue is calcined and activated, the soluble silicon component is extracted under alkaline conditions. Multiple cycles of desiliconization are repeated to produce a secondary desiliconized solution. This solution primarily consists of silicon dioxide (SiO2) and sodium oxide (Na2O), forming a sodium silicate solution with a specific modulus. It also contains small amounts of impurities such as aluminum oxide (Al2O3), requiring further purification to meet the requirements for subsequent high-purity silicon material production.

[0015] Furthermore, the stirring temperature in step (1) is 30° C.-90° C., the stirring speed is 5-100 rpm, and the stirring time is 0.5-48 h.

[0016] Furthermore, the modulus of the sodium silicate solution in step (1) is 0.2-4.

[0017] The present invention sets the modulus of the sodium silicate solution within a suitable range, which can effectively ensure that SiO2 does not precipitate in subsequent reactions, while promoting the coordinated formation of a stable colloid by alumina and silicate, thereby selectively removing impurities while retaining the main components, ensuring that the obtained sodium silicate solution has high purity and stable composition.

[0018] Furthermore, the concentration of the hydrogen peroxide in step (2) is 1-15%.

[0019] Furthermore, after adding hydrogen peroxide in step (2), the molar ratio of H2O2 to SiO2 is (0.5-2):1.

[0020] Furthermore, the amount of ferrous sulfate heptahydrate added in step (2) is calculated based on the mass of the metal element Fe converted into the corresponding oxide FeO, and the mass of FeO accounts for 0.1%-10% of the total mass of SiO2 and Na2O in the reaction solution.

[0021] Furthermore, the amount of zinc nitrate added in step (2) is calculated based on the mass of the metal element Zn converted into the corresponding oxide ZnO, and the mass of ZnO accounts for 0.1%-10% of the total mass of SiO2 and Na2O in the reaction solution.

[0022] Furthermore, the reaction temperature in step (2) is 30° C.-90° C., and the aging time is 1-48 h.

[0023] The present invention first introduces hydrogen peroxide (H2O2) as an oxidant into the pretreated sodium silicate solution. H2O2 can promote the oxidation of Al 3+ Oxidative hydrolysis occurs with SiO4 4- 、Na + The coexisting ions are complexed or polymerized, and finally a colloidal precipitate of hydrated sodium aluminosilicate structure is formed, thereby effectively removing the aluminum oxide impurities from the liquid phase. In order to enhance the rate and selectivity of this oxidation-gel process, a catalyst is further added, namely ferrous sulfate heptahydrate (FeSO4·7H2O) or zinc nitrate (Zn(NO3)2), which provides Fe 2+ or Zn 2+ These metal ions can not only activate the decomposition of H2O2 in an alkaline environment, but also act as bridging ions to promote the polymerization process of Al and Si, thereby significantly improving the precipitation efficiency of aluminum and accelerating the rate of the gelation reaction.

[0024] During the reaction, SiO2 remains dissolved, but Al 3+The reaction rapidly undergoes oxidative hydrolysis to form positively charged Al(OH)3 intermediates. These intermediates, under the action of the catalyst and oxidant, further combine with silicate and sodium ions to form a macromolecular hydrated sodium aluminosilicate complex colloid. The solution quickly transitions from a transparent to an emulsified state, forming a colloidal structure. The reaction temperature for this colloidation process is controlled between 30-90°C, which helps regulate the rate and size of colloid formation, preventing irregular precipitates or excessively large particles that would make separation difficult. The added hydrogen peroxide concentration is set between 1-15%, and the molar ratio of H2O2 to SiO2 is controlled within a range of (0.5-2:1). This ensures sufficient oxidizing power to drive the reaction while preventing overoxidation or uncontrolled free radicals from triggering undesirable side reactions, thereby improving system stability. The catalyst addition is calculated based on the mass of Fe or Zn element converted to FeO or ZnO, and is controlled at 0.1-10% of the total mass of SiO2 and Na2O in the reaction system. This ensures a high concentration of catalytically active sites while preventing interference with the solution composition caused by excess metal ions.

[0025] After colloid formation, it is not immediately separated but rather enters an aging phase, which involves allowing the nascent colloid particles to settle for 1-48 hours. This aging process further aggregates, bridges, and dehydrates the nascent colloid particles, increasing their particle size and making their structure more compact, significantly improving their sedimentation and filterability. The present invention achieves efficient and selective precipitation of alumina through meticulous control of the oxidant, catalyst, temperature, and reaction time. The resulting colloid structure is both stable and facilitates subsequent separation, a key step in achieving the present invention's excellent impurity removal performance.

[0026] Furthermore, the solid-liquid separation method in step (4) is filter press, suction filtration, gravity sedimentation or centrifugal separation.

[0027] Compared with the prior art, the advantages and beneficial effects of the present invention are:

[0028] The method of the present invention can achieve rapid, efficient and selective removal of aluminum oxide impurities under normal pressure conditions, while effectively retaining the main component SiO2 in the sodium silicate solution, with a SiO2 precipitation rate of 0 or very low. By introducing hydrogen peroxide and synergistically acting with the catalyst, aluminum oxide is stably precipitated in the form of hydrated sodium aluminosilicate colloid, forming a dense structure that is conducive to subsequent filtration and washing, and the filtrate is clear and stable. Compared with traditional acid precipitation or complexing agent aluminum removal methods, this method has a short reaction time, mild conditions, and does not require complex equipment or high-cost additives. It has good environmental friendliness and industrial promotion value, and is particularly suitable for application scenarios with high silicon purity requirements, such as high-purity silica sols and high-end coatings. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The XRD patterns of the solid phase precipitated under different catalyst addition amounts in Example 1 are shown.

[0030] Figure 2The XRD patterns of the solid phase precipitated under different catalyst addition amounts in Example 2 are shown. DETAILED DESCRIPTION

[0031] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0032] Unless otherwise specified, the raw materials used in the examples are common commercially available products.

[0033] Example 1

[0034] This embodiment provides a method for removing impurities from a sodium silicate solution, wherein the raw material used is an industrial sodium silicate solution obtained by alkaline cyclic desiliconization treatment using roasted coal gangue as the raw material;

[0035] The method comprises the following steps:

[0036] (1) 150 mL of industrial sodium silicate solution was stirred at 50 rpm for 3 hours at 50°C to allow the aluminum oxide in the sodium silicate solution to form hydrated sodium aluminosilicate crystal nuclei. The sodium silicate solution was prepared to have a modulus of 1.36, and its composition was detected as follows: Al2O3 concentration was 10.48 g / L, SiO2 concentration was 184.29 g / L, and Na2O concentration was 140.17 g / L.

[0037] (2) adding 7.5% hydrogen peroxide to the sodium silicate solution of step (1) to control the molar ratio of H2O2 to SiO2 to be 1.8:1, and adding ferrous sulfate heptahydrate (FeSO4·7H2O) with an equivalent mass of FeO as a catalyst at 1%, 3%, and 5% of the total mass of SiO2 and Na2O in the solution, respectively, and stirring the reaction at 40°C, recording the time when the system begins to form a colloid, and then continuing to stir the reaction for 10 minutes, stopping stirring, and entering an aging stage, wherein the aging condition is to stand at 25°C for 18 hours to further mature and stabilize the colloidal network structure;

[0038] (4) After the reaction is completed, the reaction system is subjected to solid-liquid separation using vacuum filtration to obtain a filter cake and a supernatant.

[0039] The solid phase experimental data under different catalyst addition amounts are shown in Table 1. The obtained solid phase was analyzed and the results are as follows: Figure 1 shown.

[0040] Table 1 Solid phase experimental data with different catalyst addition amounts

[0041]

[0042]

[0043] As can be seen from Table 1, with the increase of catalyst addition, the colloid precipitation time is slightly prolonged, but compared with the traditional separation method, the overall precipitation time is shorter. The precipitated solid phase contains a large amount of aluminum oxide and sodium oxide. The precipitated sodium aluminosilicate hydrate is analyzed. Figure 1 Further confirmation of the physical phase, such as Figure 1 As shown, after adding ferrous sulfate as a catalyst, there is an obvious diffraction peak of hydrated sodium aluminosilicate in the XRD spectrum of the precipitate, indicating that the precipitate contains hydrated sodium aluminosilicate.

[0044] The components of the supernatant after the solid-liquid separation were analyzed, and the results are shown in Table 2.

[0045] Table 2 Liquid phase composition analysis

[0046]

[0047] It can be seen from Table 2 that with the increase in the amount of catalyst added, the Al2O3 content in the liquid phase decreases significantly, the alumina in the liquid phase precipitates with the solid phase, SiO2 precipitates in the form of hydrated sodium aluminosilicate, and the amount precipitated in the form of silicon dioxide is 0.

[0048] The filter cake after the solid-liquid separation was washed with 500 mL of 50°C pure water. The composition of the washing water is shown in Table 3:

[0049] Table 3 Analysis of washing water components

[0050]

[0051]

[0052] Table 3 shows that the Al₂O₃ content in the wash water is extremely low, reaching 0.04% at 1% catalyst addition, rising slightly to 0.17% at 3%, and dropping to zero at 5%. This indicates that the majority of the alumina is firmly fixed in the colloidal structure during the reaction and is not lost with the filtrate or wash water. This demonstrates the high selectivity and stability of the precipitation process, and the Al component in the filter cake is not easily re-dissolved. This also confirms the tight bonding of Al and Si in the filter cake, forming a relatively stable aluminosilicate structure.

[0053] On the other hand, the SiO2 content in the wash water is around 5%, mainly due to the residual sodium silicate solution on the surface of the filter cake, especially before sufficient filtration or cleaning, which easily carries liquid SiO2. As the catalyst addition increases, the SiO2 washout amount decreases slightly (from 7.54% to 5.00%). This is related to the tighter colloid structure and less residual liquid, reflecting that the gel density is increased and the elution is more thorough at high catalyst addition.

[0054] The washing water data further verified the structural stability of the precipitated product in the method of the present invention and its high fixation ability for alumina. It also suggests that through reasonable washing steps, residual SiO2 can be effectively recovered and silicon loss can be avoided, which helps to improve the recovery rate of sodium silicate liquid, reflecting the green environmental protection and resource utilization value of the process.

[0055] Example 2

[0056] This embodiment provides a method for removing impurities from a sodium silicate solution. The difference from Example 1 is that in step (2), zinc nitrate with an equivalent mass of ZnO is added as a catalyst at 1%, 3%, and 5% of the total mass of SiO2 and Na2O in the solution, respectively. The rest is the same as Example 1.

[0057] The solid phase experimental data under different catalyst addition amounts are shown in Table 4. The obtained solid phase was analyzed and the results are as follows: Figure 2 shown.

[0058] Table 4 Solid phase experimental data of different catalyst addition amounts

[0059]

[0060] As can be seen from Table 4, with the increase of catalyst addition, the catalytic reaction is more intense and the colloid precipitation time is shorter. There is no obvious change pattern in the wet weight and dry weight of the precipitated filter cake, and the change in the moisture content of the filter cake is also small. At the same time, the Al2O3 and Na2O contents in each solid phase are high, and the analysis shows that hydrated sodium aluminosilicate is precipitated. Figure 2 Further confirmation of the physical phase, such as Figure 2 As shown, after adding zinc nitrate as a catalyst, there is an obvious diffraction peak of hydrated sodium aluminosilicate in the XRD spectrum of the precipitate, indicating that the precipitate contains hydrated sodium aluminosilicate.

[0061] The components of the supernatant after the solid-liquid separation were analyzed, and the results are shown in Table 5.

[0062] Table 5 Liquid phase composition analysis

[0063]

[0064] It can be seen from Table 5 that with the increase of the amount of catalyst added, the concentration of Al2O3 in the liquid phase is extremely low, SiO2 is precipitated in the form of hydrated sodium aluminosilicate, and the amount precipitated in the form of silicon dioxide is extremely low.

[0065] The filter cake after the solid-liquid separation was washed with 500 mL of 50°C pure water. The composition of the washing water is shown in Table 6:

[0066] Table 6 Analysis of washing water components

[0067]

[0068] Table 6 demonstrates that the precipitate structure formed under the zinc nitrate catalytic system exhibits excellent stability and compactness. The Al2O3 content in the wash water is extremely low, reaching as low as 0.01%, indicating that the aluminum oxide impurity is efficiently precipitated during the reaction, with virtually no re-dissolution. The SiO2 and Na2O content in the wash water remains within reasonable limits, primarily due to a small amount of residual solution in the filter cake, with no free SiO2 precipitation observed. The overall washing time is short, and the filtrate is clear, demonstrating a uniform colloidal structure and excellent filterability, facilitating rapid separation and impurity removal, further validating the efficiency and applicability of the zinc nitrate catalytic system.

[0069] Comparative Example 1

[0070] The difference between this comparative example and Example 1 is that step (1) is not performed, that is, the sodium silicate solution is not stirred.

[0071] Compared to Example 1, which achieved an Al2O3 removal rate of 63.08% under 1% FeO catalyst conditions and reduced the Al2O3 concentration in the filtrate to 1.59 g / L, the Al2O3 concentration in the filtrate of Comparative Example 1 remained as high as 2.90 g / L, with a removal rate of less than 45%. This is because the alumina in the original solution did not initially form hydrated sodium aluminosilicate nuclei. As a result, while a colloidal structure was still formed during the subsequent catalytic reaction, the dealumination efficiency was significantly reduced. Furthermore, the colloidal structure formed in the precipitate was unstable, resulting in SiO2 co-precipitation. The SiO2 content in the filtrate dropped from 55.07 g / L in the Example to 46.83 g / L, corresponding to a SiO2 precipitation rate exceeding 10%.

[0072] Comparative Example 2

[0073] The difference between this comparative example and Example 1 is that no catalyst is added in step (2).

[0074] Compared with the gelation time of only 2'13" under the condition of 1% FeO in the embodiment, in comparative example 2, no obvious emulsification state was observed after 10 minutes of reaction, and the unstable gel could only be initially formed after being extended to more than 20 minutes. The filtrate detection results showed that the Al2O3 concentration only dropped from 10.48 g / L to 5.72 g / L, and the removal rate was 45.4%, which was far lower than the level of more than 63% in the embodiment. Moreover, due to the lack of bridging and complexing ability of metal ions, the colloidal structure was loose, and part of SiO2 was co-precipitated. The SiO2 in the filtrate dropped to 48.92 g / L, corresponding to a precipitation rate of about 11%. The colloidal components of the overall system were disordered, filtration was difficult, and the separation effect was poor, making it difficult to meet the dual requirements of industrial application for decontamination efficiency and stability.

[0075] Comparative Example 3

[0076] The difference between this comparative example and Example 1 is that the catalyst in step (2) is replaced by an equimolar amount of sodium nitrate (NaNO3).

[0077] The gelation rate of the system in Comparative Example 3 was significantly slowed, requiring over 6 minutes for turbidity to appear. Furthermore, the gelation was uneven and exhibited poor agglomeration. Analysis of the filtrate after the reaction showed that the Al2O3 concentration had dropped to 3.87 g / L, resulting in a removal rate of only approximately 37%, far lower than the 63.08% in the example. Simultaneously, the SiO2 concentration had dropped to 47.61 g / L, with a precipitation rate exceeding 13%, indicating significant SiO2 loss, indicating that sodium nitrate was unable to stabilize the main component. In the absence of bridging ions, the reaction product exhibited a loose structure, hindering sedimentation and filtration, resulting in turbidity in the final filtrate and poor impurity removal.

[0078] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for removing impurities from a sodium silicate solution, comprising the following steps: (1) Stirring the sodium silicate solution at a certain temperature for a period of time to allow the aluminum oxide in the sodium silicate solution to form hydrated sodium aluminosilicate crystal nuclei, and detecting the contents of Al2O3, SiO2, and Na2O therein; (2) adding an oxidant and a catalyst to the sodium silicate solution to react, forming a colloid and then allowing it to stand and age for a certain period of time; wherein the oxidant is hydrogen peroxide and the catalyst is ferrous sulfate heptahydrate or zinc nitrate; (3) subjecting the reactant obtained in step (2) to solid-liquid separation to obtain a filter cake containing impurities and a sodium silicate solution after the impurities are removed.

2. The method for removing impurities from a sodium silicate solution according to claim 1, wherein The stirring temperature in step (1) is 30° C.-90° C., the stirring speed is 5-100 rpm, and the stirring time is 0.5-48 h.

3. The method for removing impurities from sodium silicate solution according to claim 1, wherein The modulus of the sodium silicate solution in step (1) is 0.2-4.

4. The method for removing impurities from sodium silicate solution according to claim 1, wherein The concentration of the hydrogen peroxide in step (2) is 1-15%.

5. The method for removing impurities from sodium silicate solution according to claim 1, wherein After adding hydrogen peroxide in step (2), the molar ratio of H2O2 to SiO2 is (0.5-2):

1.

6. The method for removing impurities from a sodium silicate solution according to claim 1, wherein The amount of ferrous sulfate heptahydrate added in step (2) is calculated based on the mass of the metal element Fe converted into the corresponding oxide FeO, and the mass of FeO accounts for 0.1%-10% of the total mass of SiO2 and Na2O in the reaction solution.

7. The method for removing impurities from a sodium silicate solution according to claim 1, wherein The amount of zinc nitrate added in step (2) is calculated based on the mass of the metal element Zn converted into the corresponding oxide ZnO, and the mass of ZnO accounts for 0.1%-10% of the total mass of SiO2 and Na2O in the reaction solution.

8. The method for removing impurities from a sodium silicate solution according to claim 1, wherein The reaction temperature in step (2) is 30° C.-90° C., and the aging time is 1-48 h.

9. The method for removing impurities from a sodium silicate solution according to claim 1, wherein The solid-liquid separation method in step (3) is pressure filtration, suction filtration, gravity sedimentation or centrifugal separation.

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