A method for preparing low-calcium calcium silicate by using fly ash and coal gangue

Abstract

The application provides a method for preparing low-calcium calcium silicate from fly ash and coal gangue, and relates to the technical field of building materials, which comprises the following steps: taking a sodium silicate solution as raw material, increasing the SiO2 concentration of the sodium silicate solution to 130-200 g / L, and adjusting the modulus to 2-3.5; diluting the solution, preheating, and reacting with lime milk; controlling the molar ratio of SiO2 / CaO, stirring for 1-6 hours, and standing for 0.5-12 hours; carbonizing by introducing CO2 gas until the pH value is reduced to 8.5-9.8; and filtering, washing, and drying to obtain the low-calcium calcium silicate; and optionally, adding an additive composed of hydroxyapatite nanowires, mesoporous silicon dioxide, nano cerium oxide and the like before carbonization, so as to buffer the pH drop and improve the structure; the obtained low-calcium calcium silicate has a low molar ratio of Ca / Si, a good water content, excellent performance, and is suitable for the fields of papermaking filler and the like.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, and in particular to a method for preparing low-calcium calcium silicate using fly ash and coal gangue. Background Technology

[0002] Fly ash and coal gangue are bulk solid wastes generated during coal mining and utilization. Their storage not only occupies land but can also cause dust and groundwater pollution. In the resource utilization of solid waste, alkaline treatment of fly ash or coal gangue can extract sodium silicate solution, which can then be causticized with lime slurry to recover sodium hydroxide, while simultaneously generating calcium silicate as a byproduct. However, the calcium silicate produced by traditional processes is mainly a high-calcium product with a SiO2 / CaO molar ratio of approximately 1:1. It has low bulk density, high moisture content, coarse particle size, average whiteness, and high pH value. In practical applications, such as a power plant solid waste treatment center, the low bulk density of this calcium silicate means that transportation and storage costs account for more than 30% of the product value, and the economical transportation radius is usually no more than 100 kilometers. At the same time, the high pH value requires additional acid to neutralize to pH <10 when used as a paper filler or polymer material additive, increasing the treatment cost by about 15-20%, which seriously limits its high-value application. Therefore, developing a method to directly produce low-calcium calcium silicate to increase product density, reduce moisture content and pH value has become an urgent need in the industry. Summary of the Invention

[0003] The purpose of this invention is to provide a method for preparing low-calcium calcium silicate using fly ash and coal gangue, thereby solving the problems of low bulk density, high moisture content, large particle size, and excessively high pH value caused by the high-calcium calcium silicate product in existing technologies. These problems result in high transportation and storage costs, a short economic application radius, and increased costs and limited applicability in high-value application fields (such as papermaking and high-grade fillers) due to the need for additional alkali reduction treatment. The specific technical solution of this invention is as follows:

[0004] This invention provides a method for preparing low-calcium calcium silicate using fly ash and coal gangue, comprising the following steps:

[0005] (1) Provide sodium silicate stock solution and silicon supplementing raw material, wherein the sodium silicate stock solution is derived from the alkaline desilication process of fly ash and / or coal gangue, and the silicon supplementing raw material is fly ash and / or coal gangue;

[0006] (2) Mix the sodium silicate stock solution with silicon supplementing raw material, add alkali, heat to 95-100°C and react for 2-6 hours, filter to obtain solution A, wherein the SiO2 concentration of solution A is 130-200 g / L, the Na2O concentration is 50-75 g / L, and the modulus is 1.79-4.13;

[0007] (3) Take solution A, add deionized water to dilute to a SiO2 concentration of 50-160 g / L, and preheat to 75-95°C;

[0008] (4) Prepare lime milk with a concentration of 40-150 g / L;

[0009] (5) Mix the preheated solution A with lime milk, wherein the molar ratio of SiO2 to CaO is 2:1 to 12:1, and stir the mixture at 75-95°C for 1-6 hours to obtain a reaction slurry;

[0010] (6) Let the reaction slurry stand at 75-95°C for 0.5-12 hours to mature, and obtain matured slurry;

[0011] (7) Introduce CO2 gas into the aging slurry until the pH of the system drops to 8.5-9.8 to obtain carbonized slurry;

[0012] (8) Filter the carbonized slurry, wash the filter cake with hot water, and then dry it to obtain low-calcium calcium silicate.

[0013] Furthermore, in step (2), the alkali concentration is adjusted by adding sodium hydroxide or sodium hydroxide solution.

[0014] Furthermore, in step (2), the SiO2 content of the silicon-supplementing raw material is 45%-52%, and it is dried and ground; in step (7), the flow rate of the CO2 gas is 0.4-0.5 L / min, and the stirring speed is 150 rpm.

[0015] Furthermore, in step (7), the carbonization process monitors the pH value using an online pH meter, and stops ventilation when the pH drops to 9.2, 9.8, or 8.5.

[0016] Furthermore, the washing in step (8) involves repeated washing with hot deionized water at 60°C until the washing solution is acidified with dilute nitric acid and then 0.1M silver nitrate solution is added dropwise without producing a white precipitate, indicating that chloride ions have been completely removed; the drying in step (8) involves drying at 105°C for 12 hours and grinding through a 200-mesh sieve.

[0017] Furthermore, between steps (6) and (7), an auxiliary agent is added, wherein the amount of the auxiliary agent added is 3%-5% of the mass of solution A, and the mixture is stirred at 150 rpm for 30 minutes.

[0018] Furthermore, the raw materials for preparing the additives include hydroxyapatite nanowires, mesoporous silica, nano-cerium oxide, sodium polyacrylate, and polyvinylpyrrolidone.

[0019] Furthermore, the raw materials for preparing the additive include, by weight, 10 parts hydroxyapatite nanowires, 3 parts mesoporous silica, 1 part nano-cerium oxide, 2 parts sodium polyacrylate and 1 part polyvinylpyrrolidone.

[0020] Furthermore, the preparation method of the additive includes heating hydroxyapatite nanowire powder with a mixture A containing mesoporous silica and nano-cerium oxide at pH 10.0, 130°C, and 0.3 MPa for 6 hours, followed by cooling, shear dispersion, and spray drying.

[0021] Furthermore, the inlet air temperature of the spray dryer is 200°C, the outlet air temperature is 90°C, the feed pump speed is 6 mL / min, and the compressed air flow rate is 600 L / h.

[0022] The beneficial effects of this invention are as follows: By synergistically controlling the concentration and modulus of the sodium silicate raw material solution and matching a specific lime slurry feeding ratio and carbonization process, this invention successfully transforms the high-calcium calcium silicate (Ca / Si≈1) product, which is difficult to avoid in traditional processes, into a series of low-calcium calcium silicates (Ca / Si=0.25-0.5) with controllable structures. This transformation results in a significant and synergistic improvement in several key physicochemical properties of the product: the bulk density increases from the traditional 0.15-0.2 g / cm³ to 0.42-0.63 g / cm³. The product's g / cm³ significantly reduces transportation and storage costs; the moisture content is reduced from 75-80% to 64-72%, reducing drying energy consumption; simultaneously, the product's whiteness is increased to 90.5-92.0, and the pH value of the system can be stably reduced to below 10 through mild carbonization; these comprehensive improvements in performance directly broaden the product's application prospects in high-value-added fields such as papermaking and polymer fillers, which have strict requirements for pH value and whiteness, and realize the high-value utilization of industrial solid waste; the principle behind the above-mentioned beneficial effects lies in the precise guidance of the thermodynamics and kinetics of calcium silicate nucleation and growth processes; firstly, the modulus of the sodium silicate solution is increased to 2.0-3.5, which essentially reduces the concentration of free alkali in the system, promoting the transformation of silicate ions from monomers or oligomers to chain-like and network-like high-polymerization morphologies. When these highly polymerized silicate groups react with a limited amount of calcium ions, they tend to form low-calcium calcium silicate gels or hydrates with a more developed silicon-oxygen network structure and calcium ions filling the network gaps, rather than a calcium-rich phase encapsulated by Ca(OH)2. The subsequent CO2 carbonization step neutralizes residual alkaline sodium salts in the product, reducing its alkalinity; furthermore, the generated calcium carbonate microparticles may refine the product particles and improve particle size distribution. Optional special additives, such as hydroxyapatite nanowires which act as heterogeneous nuclei to promote uniform nucleation, mesoporous silica which provides spatial support to prevent excessive gel shrinkage, and nano-cerium oxide which may buffer the sudden pH drop during carbonization through its redox properties, work synergistically to further optimize the product's microstructure and final properties. The entire process conforms to the basic principles of colloidal chemistry and precipitation reactions. Detailed Implementation

[0023] The technical solutions in the embodiments of this application are described clearly and completely below. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0024] Preparation Example 1

[0025] This preparation example provides an additive for a method of preparing low-calcium calcium silicate using fly ash and coal gangue. The raw materials for preparing the additive include, by weight, 10 parts hydroxyapatite nanowires, 3 parts mesoporous silica, 1 part nano-cerium oxide, 2 parts sodium polyacrylate and 1 part polyvinylpyrrolidone.

[0026] The preparation method of the additive, by weight, includes dissolving calcium nitrate and diammonium hydrogen phosphate in deionized water at a calcium-to-phosphorus molar ratio of 1.67 to prepare a calcium salt solution with a concentration of 0.1 mol / L and a phosphate solution with a concentration of 0.06 mol / L. 120 mL of the phosphate solution is added to a reaction vessel, and 200 mL of the calcium salt solution is added dropwise at a rate of 2 mL / min. During the dropwise addition, the pH value is maintained at 9.5 with a 1 mol / L ammonia solution. The mixture is stirred for 30 minutes and subjected to a hydrothermal reaction at 180℃ for 18 hours. After cooling to room temperature, the product is washed alternately by centrifugation with deionized water and anhydrous ethanol for 10 minutes each, three times each. The product is then placed in a vacuum freeze dryer and dried at -50℃ and 10 Pa for 24 hours to obtain hydroxyapatite nanowire powder.

[0027] Mixture A was prepared by shearing 60 parts of deionized water, 2 parts of dispersant sodium polyacrylate and 1 part of stabilizer polyvinylpyrrolidone at 5000 rpm for 30 minutes, adding 3 parts of mesoporous silica and 1 part of nano-cerium oxide, and then dispersing at 12000 rpm for 30 minutes to obtain mixture A.

[0028] Ten parts of hydroxyapatite nanowire powder were added to mixture A and stirred at 5000 rpm for 10 minutes. The pH was adjusted to 10.0 with a 0.5 mol / L sodium hydroxide solution. The temperature was increased to 130℃ at a rate of 2℃ / min and maintained at 0.3 MPa for 6 hours. After cooling to 60℃, the mixture was sheared and dispersed at 8000 rpm for 60 minutes. The resulting product was dried using a spray dryer to obtain the additive. The drying inlet air temperature was set to 200℃, the outlet air temperature to 90℃, the feed pump speed to 6 mL / min (corresponding to a nozzle orifice diameter of 0.7 mm), and the compressed air flow rate to 600 L / h.

[0029] Example 1

[0030] This embodiment provides a method for preparing low-calcium calcium silicate using fly ash and coal gangue, the method comprising the following steps:

[0031] Step 1: Take 2.0 L of sodium silicate raw solution produced by alkaline desilication of coal gangue in the mining area. The concentration of SiO2 in the sodium silicate raw solution is 55 g / L and the concentration of Na2O is 102 g / L. Take fly ash from the power plant with a SiO2 content of 52%, dry it and ball mill it to a particle size D90 of 45 μm, and use it as a silicon supplementing raw material.

[0032] Step 2: Stir the coal gangue desilication liquid and 80g of fly ash at 350 rpm, add solid sodium hydroxide, heat to 100℃ and react for 4 hours, filter to obtain solution A, the SiO2 concentration in solution A is increased to 165 g / L, and the modulus is 2.8;

[0033] Step 3: Take 1.0 L of solution A, add 0.65 L of deionized water to dilute and stir until the SiO2 concentration drops to 100 g / L, modulus is 2.8, and preheat to 85℃; prepare a 100 g / L lime slurry (prepared from analytical grade CaO), and calculate the amount of lime slurry to add based on the SiO2 / CaO molar ratio of 5:1 under stirring at 85℃ and 400 rpm. Add the lime slurry to solution A, and stir at 85℃ for 3 hours to obtain a reaction slurry. Let the reaction slurry stand at 85℃ for 2 hours to mature; introduce CO2 gas at a flow rate of 0.5 L / min under stirring at 150 rpm, and stop the gas flow when the pH of the system drops to 9.2. The filter cake was vacuum filtered at MPa for 15 min. The filter cake was repeatedly washed with hot deionized water at 60℃ until a small amount of washing liquid was taken. After acidification with dilute nitric acid, 0.1M silver nitrate solution was added to test the filter cake. No white precipitate was produced, indicating that chloride ions had been completely removed. The filter cake was dried at 105℃ for 12 hours and ground through a 200-mesh sieve to obtain low-calcium calcium silicate.

[0034] Example 2

[0035] This embodiment provides a method for preparing low-calcium calcium silicate using fly ash and coal gangue, the method comprising the following steps:

[0036] Step 1: Take 1.5 L of sodium silicate raw solution produced by alkaline desilication of fly ash. The concentration of SiO2 in the sodium silicate raw solution is 40 g / L and the concentration of Na2O is 55.3 g / L. Take coal gangue (SiO2 content is 45%) that has been crushed, ground and passed through a 200-mesh sieve as a silicon supplementing raw material.

[0037] Step 2: Stir the fly ash desilication liquid and 60g of coal gangue powder at 350 rpm, add sodium hydroxide solution, heat to 95℃ and react for 2 hours, filter to obtain solution A, the SiO2 concentration in solution A is increased to 130 g / L, and the modulus is 2.0;

[0038] Step 3: Take 1.0 L of solution A, add 1.6 L of deionized water to dilute and stir until the SiO2 concentration drops to 50 g / L and the modulus is 2.0. Preheat to 75℃. Prepare a lime slurry with a concentration of 40 g / L (prepared from analytical grade CaO). Calculate the amount of lime slurry to be added based on the SiO2 / CaO molar ratio of 12:1, while stirring at 75℃ and 400 rpm. Slowly pour solution A into the lime slurry and stir at 75℃ for 1 hour to obtain a reaction slurry. Let the reaction slurry stand at 75℃ for 0.5 hours to mature. Introduce CO2 gas while stirring at 150 rpm. Stop the gas flow when the pH of the system drops to 9.8. Vacuum filter at -0.08 MPa for 15 minutes. Wash the filter cake repeatedly with hot deionized water at 60℃ until a small amount of washing liquid is collected. Acidify with dilute nitric acid and add 0.1 M... The absence of white precipitate in the silver nitrate solution test indicates that chloride ions have been completely removed. The filter cake was dried at 105℃ for 12 hours and then ground through a 200-mesh sieve to obtain low-calcium calcium silicate.

[0039] Example 3

[0040] This embodiment provides a method for preparing low-calcium calcium silicate using fly ash and coal gangue, the method comprising the following steps:

[0041] Step 1: Take 1.0 L of sodium silicate stock solution produced by alkaline desilication after mixing fly ash and coal gangue at a mass ratio of 1:1. The concentration of SiO2 in the sodium silicate stock solution is 80 g / L and the concentration of Na2O is 45.7 g / L. Separately, take fly ash with a SiO2 content of 48% and coal gangue at a mass ratio of 1:1, dry them, and grind them until D90 is less than 20μm, as the silicon supplementing raw material.

[0042] Step 2: Add 300g of silicon supplementing material and solid sodium hydroxide to the above sodium silicate stock solution. Stir at 600 rpm for 6 hours at 95℃. Filter to obtain solution A. The SiO2 concentration in solution A is 200 g / L and the modulus is 3.5.

[0043] Step 3: Take 0.5 L of solution A, add 0.125 L of deionized water to dilute and stir until the SiO2 concentration drops to 160 g / L and the modulus is 3.5. Preheat to 95℃. Prepare a lime slurry with a concentration of 150 g / L (prepared from analytical grade CaO). Under stirring at 95℃ and 600 rpm, calculate the amount of lime slurry to be added based on the SiO2 / CaO molar ratio of 2:1. Add solution A and lime slurry simultaneously to the reactor over 2 hours using a double-dropping method. Stir and react at 95℃ for 6 hours to obtain a reaction slurry. Let the reaction slurry stand at 95℃ for 12 hours to mature. Introduce CO2 gas under stirring at 150 rpm. Stop the gas flow when the pH of the system drops to 8.5. Vacuum filter at -0.08 MPa for 15 min. Wash the filter cake repeatedly with hot deionized water at 60℃ until a small amount of washing liquid is collected. Acidify with dilute nitric acid and add 0.1 M... The filter cake, which showed no white precipitate when tested with silver nitrate solution, indicated that chloride ions had been completely removed. The filter cake was dried at 105°C for 12 hours and then ground through a 200-mesh sieve to obtain low-calcium calcium silicate.

[0044] Example 4

[0045] This embodiment provides a method for preparing low-calcium calcium silicate using fly ash and coal gangue. Based on Example 1, the additive used in Example 1 is introduced before carbonization. The preparation steps 1-3 are the same as in Example 1. Step 4 is as follows: after aging, 3% by mass of the additive in solution A is added and stirred for 30 minutes. CO2 gas is introduced at a flow rate of 0.4 L / min while stirring at 150 rpm. Online pH meter monitoring shows that it took 15 minutes from the start of gas introduction to the pH value of the system dropping to 9.2. During this period, the pH decrease rate ΔpH / Δt was 0.33 / min, which achieved buffer control of the pH decrease process.

[0046] The added additives act as pH buffers and structure guides during carbonization, effectively alleviating local acidification and protecting the integrity of the silica-oxygen network in the calcium silicate gel.

[0047] Example 5

[0048] This embodiment provides a method for preparing low-calcium calcium silicate using fly ash and coal gangue. Based on Example 1, the additive used in Example 1 is introduced before carbonization. The preparation steps 1-3 are the same as in Example 1. Step 4 is as follows: after aging, 5% (by mass) of the additive from solution A is added and stirred for 30 minutes. CO2 gas is introduced at a flow rate of 0.4 L / min while stirring at 150 rpm. Online pH meter monitoring shows that it took 22 minutes from the start of gas introduction to the pH value of the system dropping to 9.0. During this period, the pH decrease rate ΔpH / Δt was 0.27 / min. The addition of the additive achieved buffer control of the pH decrease process.

[0049] Comparative Example 1

[0050] The difference between Comparative Example 1 and Example 1 is that after the silicate adjustment in step 2, the SiO2 concentration in the sodium silicate solution was not increased to the range of 130-200 g / L. Instead, the initial raw material solution (SiO2 concentration 55 g / L, modulus 0.4) was directly diluted. Specifically, the operation of "adding fly ash and adjusting the alkali concentration, reacting at 100°C for 4 hours" was not performed. Instead, 2.0 L of the initial raw material solution was directly diluted with water to a SiO2 concentration of 100 g / L (modulus still 0.4), and then the reaction in step 3 was carried out. Other contents are the same as in Example 1.

[0051] Comparative Example 2

[0052] The difference between Comparative Example 2 and Example 1 is that in step 2, excess solid sodium hydroxide was added to adjust the alkali concentration so that the modulus of the sodium silicate solution after the reaction was 1.2 (below the range of the present invention), but its SiO2 concentration was still adjusted to 165 g / L. All other aspects are the same as in Example 1.

[0053] Comparative Example 3

[0054] The difference between Comparative Example 3 and Example 1 is that in step 3, the amount of lime slurry added is calculated and added based on a SiO2 / CaO molar ratio of 1.5:1 (lower than the lower limit of the present invention). All other aspects are the same as in Example 1.

[0055] Comparative Example 4

[0056] The difference between Comparative Example 4 and Example 1 is that the carbonization step of "introducing CO2 gas until the pH is less than 10" in step 3 is omitted. That is, after aging for 2 hours, filtration, washing and drying are performed directly. The rest is the same as Example 1.

[0057] Comparative Example 5

[0058] The difference between Comparative Example 5 and Example 2 is that Comparative Example 5 does not use fly ash desilication liquid to dissolve silicon in coal gangue to adjust the modulus and concentration. Specifically, industrial water glass with a modulus of 2.0 and a SiO2 concentration of 130 g / L is used directly, diluted with water to a SiO2 concentration of 50 g / L, while keeping the modulus unchanged, and then the reaction in step 3 is carried out. Other contents are the same as in Example 2.

[0059] Comparative Example 6

[0060] The difference between Comparative Example 6 and Example 4 is that in step 4, an auxiliary agent B, which is a simple physical mixture of nano-calcium carbonate and mesoporous silica in a 1:1 mass ratio, is used instead of the auxiliary agent prepared in Preparation Example 1. The amount added is also 3% of the mass of solution A. All other contents are completely consistent with Example 4.

[0061] Comparative Example 7

[0062] The difference between Comparative Example 7 and Example 4 is that the additives added in step 4 do not contain nano-cerium oxide in their raw materials. Otherwise, they are the same as in Example 4.

[0063] Comparative Example 8

[0064] The difference between Comparative Example 8 and Example 4 is that the additives added in step 4 do not contain mesoporous silica in their raw materials; otherwise, they are the same as in Example 4.

[0065] Comparative Example 9

[0066] The difference between Comparative Example 9 and Example 4 is that the additives added in step 4 do not contain hydroxyapatite nanowires in the raw materials for its preparation. Otherwise, they are the same as in Example 4.

[0067] Performance testing

[0068] All examples and comparative examples were tested using the following methods: X-ray fluorescence spectroscopy (XRF) combined with chemical titration was used to determine the calcium-silicon molar ratio (Ca / Si) and chemical composition of the products; the specific surface area was determined using nitrogen adsorption-desorption (BET) method, and the average pore size was calculated using the BJH method; the particle size distribution (D50 is the median diameter) was determined using a laser particle size analyzer; the whiteness of the powder was determined using a Hunter whiteness meter; the adsorption rate constant (K value) was calculated by measuring the calcium ion adsorption kinetic curve of the product in saturated calcium hydroxide solution, referring to the building materials industry standard, to evaluate its pozzolanic activity; the bulk density was determined according to GB / T 16913-2008; the moisture content was calculated by drying at 105℃ to constant weight, and the results are shown in Table 1.

[0069] Table 1

[0070]

[0071] Table 1 shows that by precisely adjusting the SiO2 concentration and modulus of the sodium silicate solution to a specific range, and controlling the molar ratio, time, and carbonization process of the synthesis reaction, low-calcium calcium silicate with good performance can be prepared in a controllable manner. The unique composite additive further enhances performance through the synergistic effect of the components. Comparative Example 1 directly used a low-concentration, low-modulus raw material solution. The silicate ions in the system had a low degree of polymerization and insufficient concentration, failing to effectively form stable low-calcium calcium silicate crystal nuclei with calcium ions, resulting in incomplete reaction. The product contained a large amount of residual Ca(OH)2, a high Ca / Si molar ratio, and extremely poor purity and whiteness. This demonstrates that pretreating the raw material solution to a high concentration (≥130 g / L) and a suitable modulus (2-3.5) is the thermodynamic and kinetic basis for the reaction. The excessively low modulus in Comparative Example 2 indicates that the system's alkalinity (OH-) is too low. - Excessive lime slurry concentration in Comparative Example 3 promoted the formation of thermodynamically more stable but denser crystalline phases with lower specific surface areas, such as calcium silicate, instead of anhydrous calcium silicate or highly active amorphous structures. This resulted in a sharp drop in the specific surface area of ​​the product, limiting its application performance. This demonstrates that the modulus is crucial for controlling the crystalline phase of the product and obtaining high activity. Excessive lime slurry in Comparative Example 4 disrupted the stoichiometric equilibrium for the formation of low-calcium products, also leading to a large amount of residual Ca(OH)2, excessively alkaline product, and a Ca / Si molar ratio deviating from the target range. This proves that controlling the SiO2 / CaO molar ratio is key to obtaining low-calcium products. The absence of a carbonization step in Comparative Example 4 resulted in the inability to neutralize the residual strong alkaline sodium salts and free alkali on the product surface and within the pores. This not only made the product itself highly alkaline, limiting its application range, but also significantly increased the difficulty of washing and water consumption. This demonstrates that the carbonization step is indispensable for reducing product alkalinity and improving economic efficiency and applicability. Comparative Example 5, while capable of generating amorphous CSH gel, lacks the stabilizing effect of trace metal ions potentially present in the industrial solid waste-derived solution, resulting in relatively poor thermal stability. Comparative Example 6 (a simple physical mixture) lacks a stable three-dimensional network structure formed through hydrothermal reaction. Nano-calcium carbonate dissolves rapidly under acidic conditions, failing to provide sustained buffering, and its instantaneous release of large amounts of Ca²⁺... + This disrupts the local reaction equilibrium, resulting in limited improvement in product bulk density and other indicators. This underscores the necessity of the specific composition and process of the additives in this invention. Comparative Example 7, lacking nano-cerium oxide, loses its crucial redox buffering capacity and cannot react with CO2 / HCO3. -Effective coupling of the system led to a rapid pH drop and an increased risk of localized acidification, thus affecting the integrity of the product structure and resulting in poor whiteness improvement and impurity removal. In Comparative Example 8, the absence of mesoporous silica deprived the additive of its high specific surface area spatial support and dispersion effect, causing the hydroxyapatite nanowires to easily aggregate and weakening their structure-directing function. The final product had a coarser particle size, and the improvement in bulk density and specific surface area was significantly lower than in Example 4. In Comparative Example 9, the absence of hydroxyapatite nanowires caused the additive to lose its function as a heterogeneous nucleus and selective adsorbent. Although the product underwent carbonization, the whiteness improvement was limited, and the effect on Al³⁺ was poor. + If the ability to fix impurities is weak, then the core mechanism for improving product purity will fail.

[0072] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for preparing low-calcium calcium silicate using fly ash and / or coal gangue, characterized in that, The method includes the following steps: (1) Provide sodium silicate stock solution and silicon supplementing raw material, wherein the sodium silicate stock solution is derived from the alkaline desilication process of fly ash and / or coal gangue, and the silicon supplementing raw material is fly ash and / or coal gangue; (2) Mix the sodium silicate stock solution with silicon supplementing raw material, add alkali, heat to 95-100℃ and react for 2-6 hours, filter to obtain solution A, wherein the SiO2 concentration of solution A is 130-200 g / L, the Na2O concentration is 50-75 g / L, and the modulus is 2.0-3.5; (3) Take solution A, add deionized water to dilute to a SiO2 concentration of 50-160 g / L, and preheat to 75-95℃; (4) Prepare lime slurry with a concentration of 40-150 g / L; (5) Mix the preheated solution A with lime milk, wherein the molar ratio of SiO2 to CaO is 2:1 to 12:1, and stir the mixture at 75-95℃ for 1-6 hours to obtain a reaction slurry; (6) Let the reaction slurry stand at 75-95℃ for 0.5-12 hours to mature it, and obtain matured slurry; (7) Introduce CO2 gas into the aging slurry until the pH of the system drops to 8.5-9.8 to obtain carbonized slurry; (8) Filter the carbonized slurry, wash the filter cake with hot water, and then dry it to obtain low-calcium calcium silicate.

2. The method for preparing low-calcium calcium silicate using fly ash and / or coal gangue as described in claim 1, characterized in that, In step (2), the alkali concentration is adjusted by adding sodium hydroxide or sodium hydroxide solution.

3. The method for preparing low-calcium calcium silicate using fly ash and / or coal gangue as described in claim 1, characterized in that, In step (2), the SiO2 content of the silicon-supplementing raw material is 45%-52%, and it is dried and ground. In step (7), the flow rate of CO2 gas is 0.4-0.5 L / min, and the stirring speed is 150 rpm.

4. The method for preparing low-calcium calcium silicate using fly ash and / or coal gangue as described in claim 1, characterized in that, In step (7), the pH value of the carbonization process is monitored by an online pH meter, and the ventilation is stopped when the pH drops to 9.2, 9.8 or 8.

5.

5. The method for preparing low-calcium calcium silicate using fly ash and / or coal gangue as described in claim 1, characterized in that, The washing in step (8) involves repeated washing with hot deionized water at 60°C until the washing solution is acidified with dilute nitric acid and 0.1M silver nitrate solution is added dropwise without producing a white precipitate, indicating that chloride ions have been completely removed; the drying in step (8) involves drying at 105°C for 12 hours and grinding through a 200-mesh sieve.

6. The method for preparing low-calcium calcium silicate using fly ash and / or coal gangue as described in claim 1, characterized in that, Between steps (6) and (7), there is also a step of adding an auxiliary agent, wherein the amount of the auxiliary agent added is 3%-5% of the mass of solution A, and the mixture is stirred at 150 rpm for 30 minutes; the raw materials for preparing the auxiliary agent include 10 parts by weight of hydroxyapatite nanowires, 3 parts of mesoporous silica, 1 part of nano-cerium oxide, 2 parts of sodium polyacrylate and 1 part of polyvinylpyrrolidone; the preparation method of the auxiliary agent includes heating hydroxyapatite nanowire powder with a mixture A containing mesoporous silica and nano-cerium oxide at pH 10.0, 130℃ and 0.3 MPa for 6 hours, cooling, shearing dispersion and spray drying.

7. The method for preparing low-calcium calcium silicate using fly ash and / or coal gangue as described in claim 6, characterized in that, The spray drying process has an inlet air temperature of 200℃, an outlet air temperature of 90℃, a feed pump speed of 6 mL / min, and a compressed air flow rate of 600 L / h.

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