A method for producing a cement hexavalent chromium reducing agent by chemical vapor deposition

Abstract

This invention belongs to the field of building materials technology and discloses a method for preparing hexavalent chromium reducing agent for cement by chemical vapor deposition (CVD), comprising the following steps: Step 1, raw material pretreatment: weigh the required raw materials according to the designed dosage, clean to remove impurities, and dry for later use; Step 2, preparation of reducing agent substrate: grind and screen the pretreated raw materials to obtain the reducing agent substrate; Step 3, fluidized bed preparation: load the reducing agent substrate into a fluidized bed, seal the equipment cavity, and introduce inert gas to ensure that the furnace of the fluidized bed is inert; Step 4, FB-CVD coating: at a set temperature and controlled fluidizing gas rate, introduce precursor gas, complete fluidized deposition in the fluidized bed, and discharge after cooling to obtain the hexavalent chromium reducing agent for cement. This invention can better uniformly and densely deposit a protective structure on the surface of the reducing agent powder, and precisely control the reduction and conversion process of hexavalent chromium in clinker; the coating shell is a protective film containing Si or Al elements, which promotes the phase formation of clinker.

Description

Technical Field

[0001] This invention belongs to the field of building materials technology, specifically relating to a method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition. Background Technology

[0002] Existing measures for controlling chromium content in cement, besides raw material control and production process optimization, include the use of cement chromium-reducing agents. Commonly used chromium-reducing agents include ferrous salts, stannous salts, and trivalent antimony salts, all used in the cement clinker grinding stage. While these can reduce the content of water-soluble hexavalent chromium in cement, their drawbacks are also significant. For example, ferrous sulfate is easily oxidized by air and cannot be stored for long periods. The critical temperature for ferrous ions to maintain the reducing properties of water-soluble chromium (VI) in cement is 90℃, while the temperature inside a cement mill can reach 105℃, which will cause Fe... 2+ Oxidized to Fe 3+ It loses its reducing properties. Compared with ferrous salts, stannous sulfate has the advantages of low dosage and high chemical stability, but its higher usage cost limits its widespread application. Trivalent antimony salts also have the advantages of stannous salts, but their solubility in water is low, and they can adversely affect the later strength of cement.

[0003] Invention patent CN112456831A discloses a high-temperature resistant hexavalent chromium reducing agent, mainly composed of stannous sulfate, ferrous sulfate, montmorillonite, bentonite, magnesium fluoride, and humic acid. Although it solves the problem of high-temperature resistance in cement ball mills and requires a small amount, the preservation of the reducing agent itself is not considered. Unprotected stannous sulfate and ferrous sulfate are easily oxidized.

[0004] Invention patent CN104496251A discloses a liquid hexavalent chromium reducing agent and its preparation method, as well as cement and its preparation method, mainly composed of ferrous ions, stannous ions, solvent, dispersant, and stabilizer. Besides the drawbacks of the aforementioned ferrous and stannous salts, increasing the amount of liquid reducing agent added introduces excessive sulfate ions, affecting the cement's setting time. Furthermore, the liquid requires high dispersibility during the clinker grinding stage, and grinding aids such as alkanolamines increase the viscosity of the cement after hydration.

[0005] In summary, adding a reducing agent during the cement grinding stage is a simple process with good chromium reduction effects. However, the mixing and grinding of powders makes it difficult to ensure uniform dispersion of the reducing agent, easily leading to localized incomplete reduction. Furthermore, it cannot control the oxidation and dissolution of chromium at the source, and some additives may adversely affect the cement's fluidity, setting time, and strength. Therefore, how to prepare a hexavalent chromium reducing agent to achieve precise control and protection of hexavalent chromium, thereby controlling the effective components of the reducing agent during the cement clinker firing stage and reducing hexavalent chromium under suitable temperature and environment, has become one of the technical problems that need to be solved. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing hexavalent chromium reducing agent for cement by chemical vapor deposition, so as to solve the technical problem in the prior art that it is difficult to achieve stable and precise control and protection of hexavalent chromium, so as to control the effective components of the reducing agent during the cement clinker firing stage.

[0007] The method for preparing hexavalent chromium reducing agent for cement by chemical vapor deposition includes the following steps: Step 1, Raw material pretreatment: Weigh the required raw materials according to the designed amount, clean them to remove impurities, and then dry them for later use; Step 2, Preparation of reducing agent substrate: The pretreated raw materials are ground and screened to obtain the reducing agent substrate; Step 3, Fluidized Bed Preparation: Load the reducing agent base material into the fluidized bed, seal the equipment cavity, and introduce inert gas to ensure that the furnace of the fluidized bed is inert. Step 4, fluidized bed chemical vapor deposition coating: After step 3, the temperature is continuously increased to the set temperature, the fluidizing gas rate is controlled, the precursor gas is introduced, and oxygen or nitrogen is introduced. Fluidized deposition is completed in the fluidized bed, and the material is discharged after cooling to obtain the cement hexavalent chromium reducing agent.

[0008] Preferably, the precursor is silane, dibutylamine silane, or trimethylaluminum.

[0009] Preferably, the raw material is any one or a mixture of any of ferrosilicon, silicon carbide, and boron carbide.

[0010] Preferably, in step 4, the set temperature for fluidized bed deposition is 300℃~540℃, the fluidizing gas velocity is 0.3~0.5m / s, and the deposition time is 20min~180min.

[0011] Preferably, when the precursor is dibutylamine silane or trimethylaluminum, the precursor gas is introduced in step 4 in the following manner: Simultaneously introduce precursor gas and oxygen; Alternatively, oxygen can be introduced beforehand, and then precursor gas can be introduced simultaneously, with the ratio of precursor gas to oxygen adjusted. In the aforementioned introduction method, the introduction ratio of precursor gas and oxygen ranges from 1:20 to 30 by volume.

[0012] Preferably, when the precursor is silane, in step 4, the precursor gas is introduced by introducing a mixture of precursor gas and nitrogen, with the ratio of precursor gas to nitrogen being 1:12.5 by volume.

[0013] Preferably, after the precursor deposition is completed, the introduction of precursor gas is stopped, and the mixture is purged with inert gas for a certain period of time.

[0014] Preferably, in step 1, the raw material is soaked in ethanol, ultrasonically cleaned, and dried to a constant weight.

[0015] Preferably, in step 2, the grinding time is 10 minutes, the rotation speed of the ball mill is no more than 400 r / min, and the powder obtained by grinding passes through a 200-mesh screen.

[0016] Preferably, in step 3, an inert gas is continuously introduced at a rate of 2 L / min, and the pressure in the fluidized bed is first increased to 0.15 MPa and then maintained for 10 min; the fluidized bed is heated to 60-100℃ at a heating rate of 3-5℃ / min and held for 1 h.

[0017] The technical advantages of this invention are as follows: This invention uses fluidized bed chemical vapor deposition (FB-CVD) to prepare a hexavalent chromium reducing agent with a core-shell coated structure. A specific inorganic coating layer is constructed on the surface of the reducing agent substrate through the CVD process. This coating layer can isolate and protect the reducing agent during the preheating and decomposition stages of cement raw materials (below 900℃). The modified reducing agent is then fed into a specific high-temperature zone within the kiln at 1200℃~1450℃, precisely controlling the reduction and conversion process of hexavalent chromium in the clinker, ultimately achieving the goal of reducing chromium content in the clinker.

[0018] Compared to existing coating technologies such as sol-gel and liquid phase deposition, this method, using fluidized bed chemical vapor deposition, achieves a more uniform and dense deposition of reaction products on the powder surface, forming a protective structure. This avoids the drawbacks of other coating methods, such as amorphous coating structures and powder agglomeration of the reducing agent. Furthermore, the coating shell used in this method is a protective film containing Si or Al elements, which not only does not affect the mineral composition of cement clinker and introduce other harmful components, but also promotes the formation of clinker phases. Attached Figure Description

[0019] Figure 1 This is a fluidized bed deposition principle diagram of a chemical vapor deposition method for preparing hexavalent chromium reducing agent for cement according to the present invention; Figure 2 The graph shows the chromium reduction effect of adding the hexavalent chromium reducing agent obtained in Example 6 of this invention to different steel slag raw material formulations compared to not adding the hexavalent chromium reducing agent. Detailed Implementation

[0020] The following detailed description of the embodiments, with reference to the accompanying drawings, will further illustrate the specific implementation of the present invention, in order to help those skilled in the art to have a more complete, accurate, and in-depth understanding of the inventive concept and technical solution of the present invention.

[0021] like Figures 1-2 As shown, this invention provides a method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition, comprising the following steps: Step 1: Raw material pretreatment: Weigh the required raw materials according to the designed amount, clean them to remove impurities, and then dry them for later use.

[0022] Specifically, the raw materials are first soaked in ethanol and ultrasonically cleaned to remove oil and other impurities, and then dried and dehydrated to a constant weight.

[0023] Step 2: Preparation of reducing agent substrate: The pretreated raw materials are ground and screened to obtain the reducing agent substrate.

[0024] The raw material is any one or a mixture of several of ferrosilicon, silicon carbide, and boron carbide. Here, the raw material is the active ingredient of the reducing agent.

[0025] If the raw material is not a single substance but a mixture of two or more, it should be mixed first, and then the mixed raw material should be loaded into a ball mill for grinding. The grinding time should be set to 10 minutes, and the speed of the ball mill should not exceed 400 r / min. The resulting powder should be passed through a 200-mesh sieve, and the raw material remaining on the sieve should be used as a reducing agent base material.

[0026] Step 3, Fluidized Bed Preparation: Load the reducing agent base material into the fluidized bed, seal the equipment cavity, introduce inert gas, increase and regulate the pressure, and simultaneously heat up and maintain the temperature to ensure that the inside of the fluidized bed furnace is inert.

[0027] The inert gas used is N2, Ar, or another inert gas. The pressure is initially increased to 0.15 MPa, then held for 10 minutes, after which the pressure drop is ensured to be <0.005 MPa. During the pressure holding process, high-purity N2 is continuously introduced at a rate of 2 L / min. Then, the fluidized bed is heated from room temperature to 100 °C (the temperature of a tubular furnace for fluidized beds) at a rate of 3–5 °C / min and held for 1 hour. This ensures that the fluidized bed is in an inert atmosphere.

[0028] Step 4, FB-CVD coating: After step 3, the temperature is continuously increased to the set temperature, the fluidizing gas rate is controlled, the precursor gas is introduced, and oxygen or nitrogen is introduced. Fluidized deposition is completed in the fluidized bed, and the material is discharged after cooling to obtain the cement hexavalent chromium reducing agent.

[0029] The precursors are silane (SiH4) and dibutylamine silane (C8H4). 22 N2Si or trimethylaluminum (Al(CH3)3). The precursor is introduced into the fluidized bed in gaseous form and forms a shell coating the active ingredient of the reducing agent through fluidized deposition.

[0030] The set temperature for fluidized bed deposition is 300℃~540℃, the fluidizing gas velocity is 0.3~0.5m / s, and the deposition time is 20min~180min.

[0031] The precursor gas and oxygen can be introduced in any of the following ways: 1) Introduce a mixture of precursor gas and nitrogen.

[0032] 2) First, vaporize the precursor in a constant temperature water bath, and then simultaneously introduce the precursor gas and oxygen in a certain proportion.

[0033] 3) First, vaporize the precursor in a constant temperature water bath, pre-introduce oxygen, then simultaneously introduce the precursor gas and adjust the ratio of precursor gas to oxygen.

[0034] The ratio of precursor gas to oxygen is 1:20 to 30 by volume.

[0035] In step 4, after the precursor deposition is complete, the introduction of precursor gas is stopped, and the mixture is purged with inert gas for a certain period of time. The method for stopping the introduction of precursor gas and oxygen and purging with inert gas can be any of the following: 1) Stop the introduction of precursor gas directly and purge with inert gas for a certain period of time.

[0036] 2) First, stop the introduction of precursor gas, then introduce oxygen and inert gas to purge for a certain period of time, then stop the introduction of oxygen and purge with inert gas only for a certain period of time.

[0037] In the above steps, fluidization refers to the process where the reactant gas is introduced from the bottom of the fluidized bed, suspending and tumbling the powder particles to form a fluid-like suspension state.

[0038] Deposition refers to the process of introducing a reactive gas containing precursors into a high-temperature bed, where a pyrolysis reaction occurs on the particle surface to generate a solid film that grows in situ.

[0039] The core principle of this method includes: using fluidized bed chemical vapor deposition (FB-CVD) technology to controllably coat and densely form films on the surface of powder in batches. The effective components of the reducing agent and the deposited precursor materials form a core-shell coating structure that protects the effective components of the reducing agent. When the reducing agent system is heated to a specific temperature (900℃~1250℃), the core-shell coating structure (mainly the SiO2 and Al2O3 substances in the shell) will undergo solid-phase / liquid-phase reactions with some mineral components in cement clinker, such as CaO + SiO2 to generate C2S / C3S, and CaO + Al2O3 to generate C3A. During the core-shell structure reaction, the reducing agent components are released, reducing hexavalent chromium in the clinker to trivalent chromium, forming targeted reduction intervention. At the same time, the raw material components used in the hexavalent chromium reducing agent of this invention are inexpensive, do not introduce harmful impurity ions, and can improve the later strength of cement.

[0040] In the following embodiments, the fluidized bed uses an ultrasonic atomizing fluidized bed tube furnace provided by Hefei Kejing Company, product model OTF-1200X-S-FBUS, with a maximum operating temperature of 1200℃. A porous quartz plate is embedded inside the furnace tube, with a pore size of 5-15μm.

[0041] Specific embodiments of the present invention are as follows: Example 1 This embodiment provides a method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition, including the following steps.

[0042] Step 1, Raw material pretreatment: The coarse solid particles of ferrosilicon and silicon carbide are soaked in ethanol and ultrasonically cleaned to remove oil and other impurities, and then placed in a vacuum drying oven to dry and dehydrate to constant weight.

[0043] The ferrosilicon uses FeSi45, with a Si content of 40%~47% and an Fe content of 45%~50% by mass percentage; the silicon carbide has a SiC content of ≥99%.

[0044] Step 2: Preparation of reducing agent substrate: Mix ferrosilicon and silicon carbide at a mass ratio of 1:1 and pour into a ball mill for grinding. The ball mill speed should not exceed 400 r / min and the grinding time should be 10 min. The resulting powder should be passed through a 200-mesh sieve, and the raw material on the sieve should be retained to obtain the reducing agent substrate.

[0045] Step 3: Fluidized bed preparation: Take 10g of pretreated substrate and spread it evenly on the distribution plate of the fluidized bed reactor. Introduce high-purity nitrogen gas and pressurize it to 0.15MPa, maintaining the pressure for 10min. Then, continuously introduce high-purity nitrogen gas at a rate of 2L / min. The fluidized bed temperature is continuously increased from room temperature to 100℃ at a rate of 5℃ / min, and then held at that temperature for 1h to ensure that the fluidized bed is in an inert atmosphere.

[0046] Step 4, FB-CVD coating: After the heat preservation is completed, the fluidized bed temperature is continuously raised to 540℃, the fluidization gas velocity is kept constant, and a mixture of silane gas (precursor) and nitrogen gas is introduced at a volume ratio of 1:12.5. During the fluidized deposition process, the fluidization gas velocity is 0.3m / s and the deposition time is 20min.

[0047] After deposition, the silane gas supply was stopped, and the carrier gas (high-purity nitrogen) was continuously purged for 10 minutes. The furnace was then cooled to room temperature before being discharged, yielding a hexavalent chromium reducing agent with a coated structure. The resulting product was designated FC1.

[0048] Example 2 This embodiment provides a method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition. The steps are basically the same as in Example 1, except that the deposition time in step 4 is 40 minutes. The resulting product is designated FC2.

[0049] Example 3 This embodiment provides a method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition. The steps are basically the same as in Example 1, except that the deposition time in step 4 is 60 minutes. The resulting product is designated FC3.

[0050] Example 4 This embodiment provides a method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition, including the following steps.

[0051] Step 1, Raw material pretreatment: The coarse solid particles of silicon carbide and boron carbide are soaked in ethanol and ultrasonically cleaned to remove oil and other impurities, and then placed in a vacuum drying oven to dry and dehydrate to constant weight.

[0052] By mass percentage, silicon carbide contains ≥99% SiC; boron carbide contains ≥95% B4C.

[0053] Step 2: Preparation of reducing agent substrate: Mix silicon carbide and boron carbide at a mass ratio of 3:7, and pour into a ball mill for grinding. The ball mill speed should not exceed 400 r / min, and the grinding time should be 10 min. The resulting powder is passed through a 200-mesh sieve, and the raw material on the sieve is retained to obtain the reducing agent substrate.

[0054] Step 3, Fluidized Bed Preparation: Take 10g of pretreated substrate and evenly spread it onto the distribution plate of the fluidized bed reactor. Introduce high-purity nitrogen gas and pressurize it to 0.15MPa, maintaining the pressure for 10min. Then, continuously introduce high-purity nitrogen gas at a rate of 2L / min. The fluidized bed temperature is continuously increased from room temperature to 60℃ at a rate of 5℃ / min, and then maintained at this temperature for 1h to ensure that the fluidized bed is inert.

[0055] Step 4, FB-CVD coating: First, the precursor dibutylamine silane (C8H) is coated... 22 N2Si) is vaporized in a constant-temperature water bath at a temperature of 50°C. After the temperature is maintained, the fluidized bed temperature is continuously raised to 350°C, and dibutylamine silane and oxygen are simultaneously introduced at a volume ratio of 1:20. During the fluidized deposition process, the fluidization gas velocity is 0.5 m / s, and the deposition time is 60 min.

[0056] After deposition, the dibutylamine silane gas was stopped, while oxygen and high-purity nitrogen were continuously introduced and purged for 10 minutes. Then, the oxygen supply was stopped, and the furnace was purged with high-purity nitrogen for 20 minutes. The furnace was then cooled to room temperature before being discharged to obtain a hexavalent chromium reducing agent with a coated structure. The resulting product was designated SB1.

[0057] Example 5 This embodiment provides a method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition. The steps are basically the same as in Example 4, except that the deposition time in step 4 is 90 minutes. The resulting product is designated SB2.

[0058] Example 6 This embodiment provides a method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition. The steps are basically the same as in Example 4, except that the deposition time in step 4 is 120 min. The resulting product is designated SB3.

[0059] Example 7 This embodiment provides a method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition, including the following steps.

[0060] Step 1, Raw material pretreatment: The coarse solid particles of ferrosilicon and boron carbide are soaked in ethanol and ultrasonically cleaned to remove oil and other impurities, and then placed in a vacuum drying oven to dry and dehydrate to constant weight.

[0061] The ferrosilicon uses FeSi45, with Si content ranging from 40% to 47% and Fe content ranging from 45% to 50% by mass percentage; the boron carbide has a B4C content of ≥95%.

[0062] Step 2: Preparation of reducing agent substrate: Mix ferrosilicon and boron carbide at a mass ratio of 6:4, and pour into a ball mill for grinding. The ball mill speed should not exceed 400 r / min, and the grinding time should be 10 min. The resulting powder is passed through a 200-mesh sieve, and the raw material on the sieve is retained to obtain the reducing agent substrate.

[0063] Step 3: Fluidized bed preparation: Take 10g of pretreated substrate and spread it evenly on the distribution plate of the fluidized bed reactor. Introduce high-purity argon gas and pressurize it to 0.15MPa, holding the pressure for 10min. Then, continuously introduce high-purity argon gas at a rate of 2L / min. The fluidized bed temperature is continuously increased from room temperature to 60℃ at a rate of 3℃ / min, and then held at that temperature for 1h to ensure that the fluidized bed is in an inert atmosphere.

[0064] Step 4, FB-CVD coating: First, the precursor trimethylaluminum (Al(CH3)3) is vaporized in a constant-temperature water bath at 30°C. After the water bath is kept at this temperature, the fluidized bed temperature is continuously raised to 300°C, and oxygen is pre-introduced for 10 minutes at a flow rate of 0.1 L / min. Then, trimethylaluminum gas is introduced, and the oxygen flow rate is gradually increased until the ratio of trimethylaluminum to oxygen introduced simultaneously is 1:20 by volume. During the fluidized deposition process, the fluidization gas velocity is 0.3 m / s, and the deposition time is 60 minutes.

[0065] After deposition, the flow of trimethylaluminum gas was stopped, while oxygen and high-purity argon were continuously introduced and purged for 10 minutes. Then, the oxygen flow was stopped, and the furnace was purged with high-purity argon for 20 minutes. The furnace was then cooled to room temperature before being discharged to obtain a hexavalent chromium reducing agent with a coated structure. The resulting product was designated FB1.

[0066] Example 8 This embodiment provides a method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition. The steps are basically the same as in Example 7, except that the deposition time in step 4 is 120 min. The resulting product is designated FB2.

[0067] Example 9 This embodiment provides a method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition. The steps are basically the same as in Example 7, except that the deposition time in step 4 is 180 min. The resulting product is designated FB3.

[0068] Example 10 This embodiment provides a method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition, including the following steps.

[0069] Step 1, Raw material pretreatment: The coarse boron carbide solid particles are soaked in ethanol and ultrasonically cleaned to remove oil and other impurities, and then placed in a vacuum drying oven to dry and dehydrate to constant weight.

[0070] The B4C content in boron carbide is ≥95% by mass percentage.

[0071] Step 2: Preparation of reducing agent substrate: Boron carbide, which is used as the reducing agent substrate, is poured into a ball mill and ground. The ball mill speed is no more than 400 r / min and the grinding time is 10 min. The resulting powder is passed through a 200-mesh sieve, and the raw material on the sieve is retained to obtain the reducing agent substrate.

[0072] Step 3, Fluidized Bed Preparation: Take 10g of pretreated substrate and evenly spread it onto the distribution plate of the fluidized bed reactor. Introduce high-purity nitrogen gas and pressurize it to 0.15MPa, maintaining the pressure for 10min. Then, continuously introduce high-purity nitrogen gas at a rate of 2L / min. The fluidized bed temperature is continuously increased from room temperature to 60℃ at a rate of 5℃ / min, and then maintained at this temperature for 1h to ensure that the fluidized bed is inert.

[0073] Step 4, FB-CVD coating: First, the precursor dibutylamine silane (C8H) is coated... 22 N2Si) is vaporized in a constant-temperature water bath at a temperature of 50°C. After the temperature is maintained, the fluidized bed temperature is continuously raised to 320°C, and dibutylamine silane and oxygen are simultaneously introduced at a volume ratio of 1:20. During the fluidized deposition process, the fluidization gas velocity is 0.5 m / s, and the deposition time is 40 min.

[0074] After deposition, the dibutylamine silane gas was stopped, while oxygen and high-purity nitrogen were continuously introduced and purged for 10 minutes. Then, the oxygen supply was stopped, and the furnace was purged with high-purity nitrogen for 20 minutes. The furnace was then cooled to room temperature before being discharged to obtain a hexavalent chromium reducing agent with a coated structure. The resulting product was designated W1.

[0075] Example 11 This embodiment provides a method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition. The steps are basically the same as in Example 10, except that in step 4, dibutylamine silane and oxygen are simultaneously introduced at a volume ratio of 1:25, the fluidization gas velocity is 0.4 m / s, and the deposition time is 60 min. The resulting product is designated W2.

[0076] Example 12 This embodiment provides a method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition. The steps are basically the same as in Example 10, except that in step 4, dibutylamine silane and oxygen are simultaneously introduced at a volume ratio of 1:30, the fluidization gas velocity is 0.3 m / s, and the deposition time is 80 min. The resulting product is designated W3.

[0077] To verify the technical effects of the present invention, the following comparative examples were also provided: Comparative Example 1 This comparative example provides a method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition, comprising the following steps.

[0078] Step 1, Raw material pretreatment: The coarse solid particles of ferrosilicon and silicon carbide are soaked in ethanol and ultrasonically cleaned to remove oil and other impurities, and then placed in a vacuum drying oven to dry and dehydrate to constant weight.

[0079] The ferrosilicon uses FeSi45, with a Si content of 40%~47% and an Fe content of 45%~50% by mass percentage; the silicon carbide has a SiC content of ≥99%.

[0080] Step 2: Preparation of reducing agent: Mix ferrosilicon and silicon carbide at a mass ratio of 1:1, and grind them in a ball mill. The ball mill speed should not exceed 400 r / min, and the grinding time should be 10 min. Pass the obtained powder through a 200-mesh sieve, and keep the raw material on the sieve to obtain the reducing agent. The obtained product is numbered Q1.

[0081] Comparative Example 2 This comparative example provides a method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition. Compared with Comparative Example 1, the steps are basically the same, except that the raw materials are silicon carbide and boron carbide. By mass percentage, the SiC content in silicon carbide is ≥99%; and the B4C content in boron carbide is ≥95%. In step 2, silicon carbide and boron carbide are mixed at a mass ratio of 3:7. The resulting product is designated Q2.

[0082] Comparative Example 3 This comparative example provides a method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition. Compared with Comparative Example 1, the steps are basically the same, the difference being that the raw materials are ferrosilicon and boron carbide, wherein the ferrosilicon uses FeSi45, with a Si content of 40%~47% and an Fe content of 45%~50% by mass percentage; and the boron carbide has a B4C content ≥95%. In step 2, ferrosilicon and boron carbide are mixed at a mass ratio of 6:4. The resulting product is designated Q3.

[0083] To verify the protective effect of the coating modification technology proposed in this invention on the reducing agent during the preheating and decomposition stage of cement raw materials (room temperature to 900℃), the chromium reduction effects of 12 reducing agents prepared in Examples 1-9 and Comparative Examples 1-3 were compared. The coating layer thickness of the hexavalent chromium reducing agent with coating structure obtained in Examples 1-9 is shown in Table 1.

[0084] Table 1 Thickness of reducing agent coating layer

[0085] The cement raw materials used in the experiment used hot-quenched steel slag instead of iron tailings as iron raw materials, and the dosage of reducing agent was fixed at 0.5% of the mass of cement raw material powder. The specific chemical composition of raw materials and raw material formula are shown in Tables 2 and 3.

[0086] Table 2 Chemical composition of experimental raw materials

[0087] Table 3 Raw material proportions and ratios (lime saturation coefficient KH, silica ratio SM, aluminum ratio IM)

[0088] The 12 reducing agents were respectively incorporated into the raw meal powder and calcined in a muffle furnace at a maximum temperature of 1450℃ and a heating rate of 10℃ / min to obtain 12 groups of clinker. A separate group of reference clinker without reducing agents was also prepared under the same calcination conditions and designated Blank. The resulting clinker was then ground together with 5% desulfurized gypsum to finally produce P•Ⅰ52.5 cement.

[0089] The water-soluble hexavalent chromium (VI) content and mechanical strength in the above 13 groups of cement were determined according to GB 31893-2015 "Limits and Determination Methods for Water-Soluble Hexavalent Chromium (VI) in Cement". The results are shown in Table 4.

[0090] Table 4 Cement strength and water-soluble hexavalent chromium content of different embodiments

[0091] As shown in Table 4, the hexavalent chromium reducing agents with coating structures prepared in Examples 1-9 of this invention can effectively reduce the content of water-soluble hexavalent chromium in cement, with an optimal chromium reduction rate of 65%. Furthermore, the effect improves with increasing coating thickness, demonstrating the key role of coating modification technology in enhancing the chromium reduction effect of the reducing agent. Simultaneously, it can also enhance the later-stage strength of cement clinker, increasing the 28-day compressive strength by 3.4 MPa.

[0092] To demonstrate the technical advantages of the coating modification technology proposed in this invention, the hexavalent chromium reducing agent (SB3) prepared in Example 6 was added to different steel slag raw material formulations to compare the chromium reduction effect.

[0093] The cement raw materials used in the experiment used different steel slags instead of iron tailings as iron raw materials, and the dosage of reducing agent was fixed at 0.5% of the mass of cement raw meal powder. The specific chemical composition of raw materials and raw meal formula are shown in Tables 5-7.

[0094] Table 5 Chemical composition of experimental raw materials

[0095] Table 6 Raw material proportions and ratios (lime saturation coefficient KH, silica ratio SM, aluminum ratio IM)

[0096] Table 7 Total Chromium Content of Raw Materials

[0097] The reducing agent SB3 from Example 6 was sequentially incorporated into the raw meal powder and calcined in a muffle furnace at a maximum temperature of 1450℃ and a heating rate of 10℃ / min, yielding four groups of clinker, numbered RM-SB, JL-SB, ZL-SB, and DG-SB. Four control groups of clinker without the reducing agent were also prepared using the same calcination method, numbered RM, JL, ZL, and DG. The resulting clinker was then ground together with 5% desulfurized gypsum to finally produce P·Ⅰ52.5 cement.

[0098] The content of water-soluble hexavalent chromium in the above 13 groups of cement was determined according to GB 31893-2015 "Limits and Determination Methods for Water-Soluble Hexavalent Chromium (VI) in Cement". The results are shown in Table 8 and... Figure 2 As shown.

[0099] Table 8. Content of water-soluble hexavalent chromium in different steel slag clinker

[0100] From Table 8 and Figure 2 It is evident that the hexavalent chromium reducing agent with a coating structure proposed in this invention exhibits stable and efficient chromium reduction effects in clinker systems with different steel slag, verifying its broad applicability to complex industrial solid waste cement systems. Among these, the chromium reduction effect is optimal in converter steel slag-substituted clinker systems, with a chromium reduction rate as high as 74%, significantly outperforming the performance of traditional reducing agents in similar systems. Furthermore, the chromium reduction effect of the reducing agent also increases with the increase of the total hexavalent chromium content in the steel slag.

[0101] Three groups of hexavalent chromium reducing agents (W1, W2, and W3) prepared in Examples 10-12 were added to a multi-element solid waste raw meal formulation for comparison of chromium reduction effects (Blank represents the comparative proportion without the addition of hexavalent chromium reducing agents from the examples). The cement raw meal used in the experiment employed converter steel slag instead of iron tailings as the iron-based raw material, while also incorporating solid waste magnesium slag and phosphorus slag. The reducing agent dosage was fixed at 0.5% of the cement raw meal powder mass. The raw meal formulation is shown in Table 9.

[0102] Table 9 Raw material proportions and ratios (lime saturation coefficient KH, silica ratio SM, aluminum ratio IM)

[0103] The above raw material formula was fed into a muffle furnace for calcination at a maximum temperature of 1450℃ and a heating rate of 10℃ / min to obtain 4 groups of clinker. The obtained clinker was ground together with 5% desulfurized gypsum to finally prepare P·Ⅰ52.5 cement product.

[0104] The water-soluble hexavalent chromium (VI) content and mechanical strength in the above four groups of cement were determined according to GB 31893-2015 "Limits and Determination Methods for Water-Soluble Hexavalent Chromium (VI) in Cement". The results are shown in Table 10.

[0105] Table 10 Cement strength and water-soluble hexavalent chromium content in a multi-element solid waste system

[0106] As shown in Table 10, the hexavalent chromium reducing agent with a coating structure proposed in this invention still exhibits excellent and stable hexavalent chromium reduction performance in complex cement systems containing multiple solid wastes (such as steel slag, magnesium slag, and phosphorus slag), with the optimal chromium reduction rate reaching up to 85%. Simultaneously, it also has a positive promoting effect on the later-stage strength development of cement clinker, increasing the 28-day compressive strength by up to 4.5 MPa.

[0107] This enhanced effect indirectly confirms the controllable release mechanism achieved by the coating structure: the coating structure prevents the reducing agent from being oxidized or consumed too quickly in the early stage of clinker calcination (room temperature to 900°C), while continuously releasing the effective components in the high-temperature zone (1200°C to 1450°C), achieving not only deep reduction of hexavalent chromium but also improving mechanical properties. In summary, the reducing agent of this invention has significant engineering application prospects in the resource utilization of diverse solid wastes and the preparation of green and low-carbon building materials.

[0108] The present invention has been described above by way of example with reference to the accompanying drawings. Obviously, the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvements made using the inventive concept and technical solution of the present invention, or the direct application of the inventive concept and technical solution of the present invention to other occasions without modification, are all within the protection scope of the present invention.

Claims

1. A method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition, characterized in that, Includes the following steps: Step 1, Raw material pretreatment: Weigh the required raw materials according to the designed amount, clean them to remove impurities, and then dry them for later use; Step 2, Preparation of reducing agent substrate: The pretreated raw materials are ground and screened to obtain the reducing agent substrate; Step 3, Fluidized Bed Preparation: Load the reducing agent base material into the fluidized bed, seal the equipment cavity, and introduce inert gas to ensure that the furnace of the fluidized bed is inert. Step 4, fluidized bed chemical vapor deposition coating: After step 3, the temperature is continuously increased to the set temperature, the fluidizing gas rate is controlled, the precursor gas is introduced, and oxygen or nitrogen is introduced. Fluidized deposition is completed in the fluidized bed, and the material is discharged after cooling to obtain the cement hexavalent chromium reducing agent.

2. The method for preparing hexavalent chromium reducing agent for cement by chemical vapor deposition according to claim 1, characterized in that, The precursor is silane, dibutylamine silane, or trimethylaluminum.

3. The method for preparing hexavalent chromium reducing agent for cement by chemical vapor deposition according to claim 1, characterized in that, The raw materials are any one or a mixture of any of the following: ferrosilicon, silicon carbide, and boron carbide.

4. The method for preparing hexavalent chromium reducing agent for cement by chemical vapor deposition according to claim 1, characterized in that, In step 4, the set temperature for fluidized bed deposition is 300℃~540℃, the fluidization gas velocity is 0.3~0.5m / s, and the deposition time is 20min~180min.

5. The method for preparing hexavalent chromium reducing agent for cement by chemical vapor deposition according to claim 2, characterized in that, When the precursor is dibutylamine silane or trimethylaluminum, the precursor gas is introduced in step 4 as follows: Simultaneously introduce precursor gas and oxygen; Alternatively, oxygen can be introduced beforehand, and then precursor gas can be introduced simultaneously, with the ratio of precursor gas to oxygen adjusted. In the aforementioned introduction method, the introduction ratio of precursor gas and oxygen ranges from 1:20 to 30 by volume.

6. The method for preparing hexavalent chromium reducing agent for cement by chemical vapor deposition according to claim 2, characterized in that, When the precursor is silane, in step 4, the precursor gas is introduced as a mixture of precursor gas and nitrogen, with a volume ratio of 1:12.

5.

7. The method for preparing hexavalent chromium reducing agent for cement by chemical vapor deposition according to claim 1, characterized in that, After the precursor deposition is complete, the precursor gas supply is stopped, and the mixture is purged with inert gas for a certain period of time.

8. The method for preparing hexavalent chromium reducing agent for cement by chemical vapor deposition according to claim 1, characterized in that, In step 1, the raw materials are soaked in ethanol, ultrasonically cleaned, and dried to constant weight.

9. A method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition according to claim 1, characterized in that, In step 2, the grinding time is 10 minutes, and the powder obtained from grinding is passed through a 200-mesh sieve.

10. The method for preparing a cement hexavalent chromium reducing agent by chemical vapor deposition according to claim 1, characterized in that, In step 3, inert gas is continuously introduced at a rate of 2 L / min. The pressure in the fluidized bed is first increased to 0.15 MPa and then maintained for 10 min. The fluidized bed is then heated to 60-100℃ at a heating rate of 3-5℃ / min and held for 1 h.

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