Methods for extracting silicon oxide from desert sand and preparation methods for nano-silicon oxide, silicon carbide and silicon nitride.

Abstract

This invention discloses a method for extracting silicon oxide from desert sand and a method for preparing nano-silicon oxide, silicon carbide, and silicon nitride, comprising the following steps: Step (1) calcining and pretreating desert sand with a reducing agent followed by water cooling to obtain a calcined product with a loosely structured desert sand particle structure; Step (2) acid leaching the calcined desert sand product, followed by solid-liquid separation after acid leaching to obtain an acidic leachate and solid silicon oxide. Nano-silicon oxide is prepared using hydrochloric acid titration with the prepared solid silicon oxide; silicon carbide and silicon nitride are synthesized using a reduction method with the prepared solid silicon oxide or nano-silicon oxide. This invention solves the problems of low acid leaching efficiency and poor purity when extracting silicon oxide from desert sand using existing silicon oxide extraction methods.

Description

Technical Field

[0001] This invention relates to the field of silicon oxide production technology. Specifically, it relates to methods for extracting silicon oxide from desert sand and methods for preparing nano-silicon oxide, silicon carbide, and silicon nitride. Background Technology

[0002] In recent years, the widespread application of functional materials and devices such as silicon solar cells, memory devices, optical glass, biosensors, and catalysts has led to a rapid increase in demand for high-purity silica raw materials. Furthermore, conventional silica (SiO2) is extensively used in the manufacture of ceramics, glass, cement clinker, and refractory materials, resulting in a high dependence on the mining of high-quality natural quartz minerals. Therefore, some silicate minerals containing silica components, such as feldspar, kaolin, and wollastonite, have also become raw materials for quartz extraction. The large-scale mining of these natural minerals has caused damage to land and the ecological environment. To alleviate these problems, extracting silica from industrial solid wastes such as coal gangue, fly ash, and metallurgical slag has become a new development direction.

[0003] Currently, the extraction of silica mainly focuses on the chemical process of acid leaching, which involves adding various acids to the raw materials to dissolve impurities other than silica. Due to differences in the solubility of acids and the corrosive properties of impurity phases, the purity of the obtained silica varies. To improve acid leaching efficiency and purity, pretreatment processes are employed to activate and modify the raw materials. These include physical methods such as ball milling, magnetic iron removal, sorting, and calcination, as well as chemical methods involving the addition of modifiers such as alkalis, acids, and salts followed by calcination. Chemical pretreatment introduces many additional impurity elements, such as potassium and calcium, which ultimately form new solid waste, placing a significant burden on the subsequent acid leaching process. Furthermore, the addition of carbonates, sulfates, phosphates, and fluorides, which can decompose at high temperatures, releases carbon dioxide and various acidic sulfides, hydrogen chloride, and fluorides, causing severe environmental damage. While absorption and conversion technologies can mitigate pollution, they also significantly increase the complexity of the process and production costs.

[0004] The extracted silica can be further used to synthesize nano-silica, silicon carbide, and silicon nitride powders, all of which are important engineering materials or raw materials. High-purity nano-silica is mainly used to prepare functional materials. Its preparation primarily involves ultrafine grinding of high-purity silica, as well as gas-phase synthesis and wet chemical synthesis using high-purity silicon raw materials (elemental silicon, silica, water glass, silicon tetrachloride, orthosilicic acid, etc.). This method, due to the high cost of the raw materials used, further increases the cost of the synthesized nano-silica.

[0005] SiC and Si3N4 powders are also extremely important engineering materials, mainly used in aerospace, electronic devices, precision instruments, metallurgy and other high-tech equipment technology fields. Representative methods for synthesizing SiC include carbothermal reduction, sol-gel method, polymer thermal decomposition method, and chemical vapor deposition. The main preparation methods for silicon nitride powder include direct silicon powder nitridation, carbothermal reduction, thermal decomposition, sol-gel method, chemical vapor deposition, and self-propagating method. Industrially, SiC / Si3N4 powders are mostly synthesized using quartz, petroleum coke (anthracite), or Si powder as raw materials, employing high-temperature reduction of silicon dioxide with C or N, and direct reaction of C, N, and Si. The reaction process is as follows:

[0006] S iO2(s)+3C(s)→SiC(s)+2CO(g) (1);

[0007] Si+C→SiC (2);

[0008] 3Si + 2N2(g) = Si3N4 (3);

[0009] 3Si+4NH3(g)=Si3N4+6H2(g) (4);

[0010] 3SiO2(s)+6C(s)+2N2(g)=Si3N4(s)+6CO(g) (5).

[0011] Important engineering materials such as silicon dioxide, nano-silicon dioxide, silicon carbide, and silicon nitride require large quantities of silicon dioxide (silicon) as raw materials, which greatly increases the demand for quartz and other silicon dioxide-containing natural minerals. However, the large-scale mining of these minerals not only increases the consumption rate of non-renewable natural minerals but also leads to land and environmental damage, which is inconsistent with the policy of sustainable economic and social development. Therefore, extracting or synthesizing silicon dioxide, silicon carbide, and silicon nitride from low-value and idle resources is a novel method for obtaining these materials.

[0012] On the other hand, my country possesses vast reserves of desert sand resources, but industrial applications are relatively limited, primarily focusing on road construction in remote areas and as fine aggregate in concrete. Desert sand is essentially a low-quality quartz sand, typically with particle sizes ranging from a few micrometers to several hundred micrometers. Its chemical composition includes 75%-85% SiO2, approximately 10% Al2O3, and 5%-15% MgO, CaO, Fe2O3, TiO2, Na2O, and K2O. Therefore, extracting silica from desert sand can alleviate a series of sustainable development problems caused by the over-exploitation of minerals due to the huge demand for silica. However, when existing silica extraction methods are used to extract silica from desert sand, the poor solubility of impurity phases in acid leads to low purity of the extracted silica and low acid leaching efficiency. Therefore, it is necessary to develop a new method for more effectively extracting silica from desert sand. Summary of the Invention

[0013] Therefore, the technical problem to be solved by this invention is to provide a method for efficiently extracting silicon dioxide from desert sand, as well as further methods for preparing nano-silicon dioxide, silicon carbide, and silicon nitride. This invention can solve the problems of low acid leaching efficiency and poor purity when using existing silicon dioxide extraction methods to extract silicon dioxide from desert sand.

[0014] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0015] The method for extracting silica from desert sand includes the following steps:

[0016] Step (1): The desert sand is pretreated by calcination using a reducing agent and then cooled with water to obtain the calcined desert sand product with a loose particle structure.

[0017] Step (2): The desert sand calcination product is acid-leached using the acid leaching method. After acid leaching, solid and liquid are separated to obtain acidic leachate and solid silica.

[0018] The above method for extracting silicon dioxide from desert sand includes a calcination pretreatment method in step (1):

[0019] Step (1-1): Place the solid desert sand raw material in a ball mill, add water and ball mill. After ball milling, dry and pulverize the slurry obtained to obtain slurry powder.

[0020] Steps (1-2): Place the slurry powder in a reducing environment for heating and heat preservation treatment;

[0021] After the heating and heat preservation treatment in steps (1-3) is completed, the powder is taken out and transferred to water for cooling, thus obtaining the desert sand calcination product.

[0022] In the above method for extracting silica from desert sand, in step (1-1), the solid desert sand raw material is either a single type of desert sand or a mixture of a solid reducing agent and desert sand. The solid reducing agent is one or a combination of two or more of coal powder, carbon powder, and wood chips. The ratio of water added during ball milling to the mass of the solid desert sand raw material is 1-1.5:1. The ball milling time is 3-5 hours. The slurry drying temperature is 90-120°C, and the drying time is 24-48 hours. Under these drying conditions, the slurry can be completely dried, and the oxidation of the solid reducing agent in the slurry and severe agglomeration after drying can be avoided. The slurry powder obtained after ball milling passes through a 650-mesh sieve. In step (1-2), the heating and holding temperature is 650-750°C, and the heating and holding time is 0.5-3 hours.

[0023] In the above method for extracting silica from desert sand, in step (1-1), the solid desert sand raw material is a mixture of a solid reducing agent and desert sand, with a mass ratio of solid reducing agent to desert sand of (0.5-1.5):1; the particle size of the solid reducing agent is 50-120 μm, and the solid reducing agent is composed of 30-50 parts by weight of coal powder, 10-20 parts by weight of wood chips, and 50-70 parts by weight of carbon powder. The solid desert sand raw material obtained after mixing the solid reducing agent with the desert sand in the above proportion has moderate density, which is beneficial for the wood chips, coal powder, and carbon powder in the solid reducing agent at this dosage to be gasified sequentially and fully exert their reducing effect during the heating and heat preservation process. This effectively weakens the crystal structure of impurities in the desert sand, laying the foundation for subsequent acid extraction of silicon. In step (1-2), during the heating and heat preservation treatment: first, the temperature is raised to 500℃ at a heating rate of 5-10℃ / min, and then raised to 650-750℃ at a heating rate of 15-20℃ / min, and kept at 650-750℃ for 0.5-3 hours. In step (1-3), after the heating and heat preservation treatment is completed, the powder is quickly transferred to water for cooling, and the water temperature is room temperature. After the powder cools to room temperature, the solid and liquid are separated, and the powder is naturally dried in the air for 48-72 hours until it is semi-dry, which is the desert sand calcination product.

[0024] The method for extracting silicon dioxide from desert sand described above, step (1-2) is as follows:

[0025] The slurry powder is pressed at 5–15 MPa for 1–3 minutes to obtain powder blocks. Under these pressing conditions, a well-formed powder block is obtained without excessive density that would affect the heating and heat preservation effect. Furthermore, the powder block naturally disperses into powder after the heating and heat preservation process. The powder block is then placed in a stainless steel container, and a layer of carbon powder is evenly covered on top before the container is sealed. Covering with carbon powder prevents the solid reducing agent from oxidizing too quickly. Ventilation holes are provided on the container lid to ensure the smooth discharge of gases such as carbon dioxide generated during the heating and heat preservation process. The container is then subjected to heating and heat preservation treatment. The mass ratio of the covering carbon powder to the powder block is 1:5–10, and the particle size of the carbon powder is 40–70 μm. Covering the powder block with carbon powder and controlling its particle size range of 40–70 μm effectively prevents the solid reducing agent from oxidizing too quickly and ensures that no carbon powder residue remains after the heating and heat preservation process.

[0026] Alternatively, the slurry powder can be placed in a stainless steel container with an air inlet and an exhaust outlet at each end. During heating and heat preservation, the reducing gas is introduced through the air inlet, and the reacted gas is discharged through the exhaust outlet. The reducing gas is one or a mixture of two or more of the following: natural gas, carbon monoxide, hydrogen, gaseous alcohols, or gaseous aldehydes. The flow rate of the reducing gas is 1–2 L / min. Under this reducing atmosphere, the reduction effect on the slurry powder is most ideal.

[0027] In the above method for extracting silicon dioxide from desert sand, step (2) involves acid leaching as follows:

[0028] Step (2-1): Disperse the desert sand calcination product in water at a mass ratio of 1:(1~3), stir and mix, and remove the suspended particles in the dispersion system, mainly unreacted reducing agent carbon and other impurities.

[0029] Step (2-2): Add inorganic acid solution to the dispersion system, stir again, and remove suspended particles from the dispersion system; the mass ratio of inorganic acid solution to the mass of desert sand calcination product in step (2-1) is (1-2):1; the inorganic acid solution is a mixture of hydrochloric acid with a mass concentration of 36wt%, sulfuric acid with a mass concentration of 75wt%, and hydrofluoric acid with a mass concentration of 36wt% in a mass ratio of (10-30):(0-10):(0-10);

[0030] Steps (2-3): The dispersion system is subjected to reflux heating and acid leaching at a temperature of 100-150℃ for 30-120 min. After the acid leaching reaction is completed, liquid-solid separation is performed to obtain solid powder.

[0031] Steps (2-4): After repeatedly washing the solid powder until it is neutral, dry it at 120°C for 24-48 hours to obtain solid silicon oxide.

[0032] The above-mentioned method for extracting silicon dioxide from desert sand also includes the following steps: heating the acidic leachate to boiling and evaporating it while boiling until colorless and transparent crystals appear; separating the solid and liquid and drying the crystals to obtain chlorides. During chloride separation, the acidic leachate is heated to boiling and kept boiling until no more water vapor is produced, thus obtaining mixed chlorides. Aluminum chloride, potassium chloride, and sodium chloride are then crystallized in the order of crystallization.

[0033] The method for preparing nano-silica, which utilizes the solid silica prepared by the above-described method for extracting silica from desert sand to prepare nano-silica, includes the following steps:

[0034] Step (301): Solid silicon oxide and solid sodium hydroxide are thoroughly mixed in a mass ratio of 1:(1-3), compacted, and calcined at 500-700℃ for 30-90 min to obtain silicon oxide alkaline-treated product; the silicon oxide alkaline-treated product obtained by the calcination method in step (301) is conducive to the complete reaction of silicon and will not generate other insoluble substances.

[0035] Step (302): Add water to the silicon dioxide alkaline treatment product, stir to dissolve and filter to remove insoluble impurities to obtain a clear solution; while stirring, add hydrochloric acid solution with a concentration of 2-4 mol / L dropwise to the clear solution; under these reaction conditions, it is beneficial to obtain silicon dioxide with high purity;

[0036] Step (303): When the pH value of the above mixed system drops to 3.5-3.0, remove all the precipitate in the mixed system, and continue to add hydrochloric acid solution with a concentration of 2-4 mol / L to lower the pH of the mixed system to the range of 3.0-1.5, and maintain it for 10-20 min; collect the white precipitate generated in the mixed system and wash it until neutral, and finally place the washed white precipitate at 100-120℃ to dry for 12-24 h, and grind and disperse it to obtain nano-silica with a particle size of less than 100 nm.

[0037] The method for preparing silicon carbide involves using solid silicon oxide prepared by the method described above for extracting silicon oxide from desert sand, or using the method described above for preparing nano-silicon oxide; the method for synthesizing silicon carbide is as follows:

[0038] Step (101): Mix the solid silicon dioxide obtained in step (2) or the nano silicon dioxide obtained in step (3) with carbon powder and wood chips in a mass ratio of (50-60):(30-40):(10-20) to obtain mixed raw material powder;

[0039] Step (102): Add water to the mixed raw material powder at a mass ratio of 1:1 and grind for 5 to 12 hours. After grinding, dry at 90°C for 24 to 48 hours to obtain a dry mixed powder with a particle size of less than or equal to 10 μm.

[0040] Step (103): Place the dry mixed powder into an alumina crucible, compact it, and cover it with carbon powder. After covering the crucible, leave a vent hole. The mass ratio of carbon powder to dry mixed powder is 1:5 to 10.

[0041] Step (104): Heat the crucible to 1300-1600℃ and hold for 2-4 hours, then cool it naturally to 650-700℃; then transfer it to an air atmosphere and hold at 650℃ for 30 minutes to obtain the silicon carbide calcined product.

[0042] Step (105): Soak the silicon carbide calcination product in a 10wt% hydrofluoric acid solution for 1-2 hours, then separate the solid and liquid components. Wash the separated solid product with water until it is neutral, and finally dry it to obtain silicon carbide powder. The drying temperature is 100-120℃ and the drying time is 24-48 hours.

[0043] The method for preparing silicon nitride involves using solid silicon oxide prepared by the method described above for extracting silicon oxide from desert sand, or using the method described above for preparing nano-silicon oxide; the method for synthesizing silicon nitride is as follows:

[0044] Step (201): Mix the solid silicon dioxide obtained in step (2) or the nano silicon dioxide obtained in step (3) with carbon powder and wood chips in a mass ratio of (50-60):(30-40):(10-20) to obtain a mixed raw material powder;

[0045] Step (202): Add water to the mixed raw material powder at a mass ratio of 1:1 and grind for 5 to 12 hours. After grinding, dry at 90°C for 24 to 48 hours to obtain a dry mixed powder with a particle size of less than or equal to 10 μm.

[0046] Step (203): Place the dried mixed powder into a tube furnace with a rotatable alumina inner cylinder. The alumina inner cylinder rotates at a speed of 10-20 rpm. This rotation speed can ensure that the mixed powder reacts evenly and fully. Introduce a reducing gas with a flow rate of 1-2 L / min through the gas inlet. The reducing gas is one or a mixture of two or more of the following: natural gas, carbon monoxide, hydrogen, gaseous alcohols, or gaseous aldehydes. Heat the tube furnace to 1250-1450℃ and hold for 1-2 hours. If the temperature is higher than 1450℃, it may lead to the formation of elemental silicon.

[0047] Step (204): After the heat preservation is completed, stop the introduction of reducing gas, and then introduce a mixture of ammonia and nitrogen into the gas inlet of the alumina inner cylinder. The volume ratio of ammonia to nitrogen in the mixture is 1:1. Ammonia or nitrogen can also be used alone, but the nitriding effect is poor. The total flow rate of the mixture is 1-2 L / min. After heat preservation for 1-2 hours, stop the introduction of ammonia. Hydrogen in ammonia may have an adverse effect on the microstructure of silicon nitride. Therefore, it is necessary to stop the introduction of ammonia first and allow it to cool naturally. After the temperature cools naturally to 600℃, stop the introduction of nitrogen and continue to cool to 300℃. Stop the rotation of the alumina inner cylinder and allow it to cool naturally to room temperature to obtain the silicon nitride calcination product.

[0048] Step (205): Soak the silicon nitride calcination product in a 10wt% hydrofluoric acid solution for 1-2 hours, then separate the solid and liquid components. Wash the separated solid product with water until it is neutral, and finally dry it to obtain silicon nitride powder.

[0049] The principle of this invention, which uses reducing substances to pretreat desert sand, is as follows:

[0050] Besides differing from quartz sand in chemical composition and particle size, desert sand also differs in phase composition and microstructure. Desert sand is mainly composed of quartz, feldspar, iron-titanium oxides, and a glassy phase. Only a small portion of the particles in desert sand are single-phase quartz; the vast majority are multiphase mixtures, such as... Figures 1a to 1d As shown, this phase mixing mostly occurs at the micrometer or nanometer scale, making it difficult to break desert sand particles to the micrometer or nanometer scale and separate them into single-phase particles using conventional industrial ball milling methods. Physical methods such as magnetic iron removal, sorting, and calcination also have extremely limited effectiveness in removing impurity phases. These characteristics also determine its poor solubility in acidic media.

[0051] This invention addresses the problem of severely impacting the extraction efficiency and purity of silica by proposing a pretreatment process using calcination in a reducing atmosphere. This process not only completely avoids the drawbacks of secondary solid waste and acidic gas emissions caused by the aforementioned addition of acidic substances and mineralizers, but also weakens the crystal structure of impurity compounds while forming microcracks inside desert sand particles. This improves the dissolution rate of inclusions in desert sand and the purity of silica, reduces the production cost of silica, and increases production efficiency.

[0052] The theoretical basis for improving the acid dissolution efficiency of desert sand by calcining under a reducing environment in this invention is as follows: The C or H in the reducing agent undergoes a physicochemical reaction with impurity compounds at high temperatures. C or H forms oxygen vacancies by abstracting oxygen from the compound structure. The formation of oxygen vacancies weakens the chemical bonding at the phase interface and the crystal structure of the impurity compounds, increasing the interfacial energy and the free energy of the impurity compounds. This facilitates the dissolution of the weakened impurity compounds by the acidic medium entering the sand particles along the cracked phase interface, thereby improving the efficiency and effectiveness of subsequent acid dissolution. Furthermore, the significant expansion caused by the α-quartz to β-quartz crystal transformation at 573℃ leads to phase interface rupture and also increases the surface area of ​​the impurity phase particles. This explains the use of a higher heating rate in the initial calcination stage and a higher water cooling rate after calcination to achieve a higher cooling rate.

[0053] Reports indicate that the temperatures at which carbon and hydrogen directly reduce silicon oxide to elemental silicon are above 1400℃ and 1000℃, respectively. The desert sand pretreatment calcination temperature used in this invention is far below these temperatures, thus avoiding silicon oxide reduction.

[0054] Since acids (except hydrofluoric acid) are almost insoluble in silicon dioxide, the removal of impurity ions dissolved in the silicon dioxide lattice is poor without the addition of hydrofluoric acid, making it difficult to obtain ultra-high purity silicon dioxide. The relatively insoluble substances in desert sand are feldspar phases, including calcium feldspar (CaAl2Si2O8), potassium feldspar (KAlSi3O8), sodium feldspar (NaAlSi3O8), and feldspar with mixed K, Na, and Ca solutions. Their basic structural unit is the tetrahedron ([SiO]4 and [AlO]4), with each tetrahedron sharing an oxygen atom, forming a three-dimensional framework structure. Potassium, sodium, and calcium ions are located in the voids within this framework. Additionally, the iron oxide phase also exhibits a [FeO]4 tetrahedral structure. Therefore, by utilizing the diffusion of C or H ions to the surface of the feldspar and iron oxide phases, oxygen ions shared (common points, common edges) in the tetrahedrons ([SiO]4, [AlO]4, [FeO]4) or even oxygen ions within the tetrahedrons can be captured at high temperatures. This breaks the three-dimensional framework structure, produces a dissociated phase interface, and weakens the feldspar and iron oxide structures, thereby increasing the surface energy, bulk free energy, and chemical activity of the reactants.

[0055] The decomposition temperature of feldspar substances begins at approximately 900°C; the direct reduction of iron oxide by coke, natural gas, and hydrogen occurs at temperatures above approximately 800°C. Within the calcination temperature range of this invention, the decomposition of the feldspar phase and the reduction of iron oxide are unlikely, and of course, the decomposition of the feldspar phase and the reduction of iron oxide are not the technical routes of this invention. Furthermore, while increasing the calcination temperature certainly promotes chemical reactions, it also leads to increased energy consumption.

[0056] The technical solution of the present invention achieves the following beneficial technical effects:

[0057] 1. This invention utilizes a solid reducing agent or reducing gas to pre-treat desert sand by calcination, which enables the formation of microcracks inside the desert sand particles and weakens the structure of the difficult-to-dissolve impurity crystalline phases in the desert sand. This increases the dissolution rate of inclusions in the desert sand and improves the purity of the solid silicon oxide obtained after acid dissolution, thereby achieving the technical objective of extracting high-quality silicon oxide from desert sand. This is beneficial for reducing the production cost of silicon oxide and improving production efficiency.

[0058] 2. In this invention, when calcining pretreated desert sand in a reducing environment, the temperature is first raised to 500°C at a relatively low heating rate (5-10°C / min), and then raised to the calcination temperature (650-750°C) at a relatively high heating rate (15-20°C / min) for calcination at 15-20°C / min. This is because this heating method and calcination conditions allow the quartz crystals in the desert sand to undergo a significant expansion when the calcination temperature reaches 573°C or higher, resulting in a complete transformation from α-quartz to β-quartz. This causes the phase interface to break down, thereby increasing the surface area of ​​the impurity phase particles and providing favorable conditions for subsequent acid extraction of silica. After calcination, the powder is rapidly cooled to room temperature in water because this cooling method further weakens and cracks the impurity crystal phase structure in the calcined desert sand, making the impurity crystal phase easier to dissolve with acid.

[0059] 3. In this invention, the calcination temperature during the pretreatment of desert sand in a reducing environment is only 650-750℃, which is far lower than the temperature at which silicon oxide is reduced to elemental silicon as reported in the literature. This will not cause the reduction of silicon oxide and will help ensure a high yield of solid silicon oxide.

[0060] 4. The synthesis temperature for preparing silicon carbide and silicon nitride using the solid silicon oxide or nano-silica prepared by this invention is relatively low compared to the traditional synthesis temperatures of silicon carbide and silicon nitride. This is due to the characteristics of the silica phase in desert sand. The silica phase in desert sand consists of micron- and nano-scale α-quartz and amorphous silica; nano-scale α-quartz has a high specific surface area and surface energy; while amorphous silica has a non-dense network structure and high free energy. Therefore, they can undergo chemical reactions at lower temperatures, achieving phase transformation and crystal structure transformation. Furthermore, the use of sawdust as a reducing agent in the preparation of silicon carbide and silicon nitride may also contribute to the accelerated reaction. Because sawdust is porous, carbon is easily volatilized and provides a large reaction contact area. Especially in the synthesis of silicon nitride, the use of a rotating mode increases the contact between gas and solid materials, all of which contribute to lowering the synthesis temperature. Attached Figure Description

[0061] Figure 1aMicrostructure diagram of desert particles in this invention (2μm, quartz-feldspar-titanium iron oxide mixed phase);

[0062] Figure 1b Microstructure diagram of desert particles in this invention (2μm, feldspar-titanium iron oxide mixed phase);

[0063] Figure 1c Microstructure diagram of desert particles in this invention (2μm, quartz-titanium iron oxide mixed phase);

[0064] Figure 1d Microstructure diagram of desert particles in this invention (2μm, quartz-feldspar mixed phase);

[0065] Figure 2a Microstructure diagram (2 μm) of desert sand after pretreatment in Comparative Example 1 of this invention;

[0066] Figure 2b Microstructure diagram (2 μm) of desert sand after pretreatment in Comparative Example 2 of this invention;

[0067] Figure 3 Microstructure diagram (2μm) of desert sand after pretreatment in Example 1 of this invention;

[0068] Figure 4 Microstructure diagram (2μm) of desert sand after pretreatment in Example 3 of this invention. Detailed Implementation

[0069] Example 1

[0070] In this embodiment, the method for extracting silicon dioxide from desert sand includes the following steps:

[0071] Step (1): The desert sand is pretreated by calcination using a reducing agent and then cooled with water to obtain the calcined desert sand product with a loose particle structure.

[0072] In this embodiment, carbon powder is used as a solid reducing agent and mixed with solid desert sand through ball milling. The specific method for calcination pretreatment is as follows:

[0073] Step (1-1): Place the solid desert sand raw material in a ball mill, add water, and ball mill. After ball milling, dry and pulverize the slurry obtained to obtain slurry powder. The solid desert sand raw material is a mixture of solid reducing agent and desert sand, with a mass ratio of solid reducing agent to desert sand of 0.5:1. The solid reducing agent is carbon powder with a particle size of 50-120μm. The mass ratio of water added during ball milling to the mass of solid desert sand raw material is 1:1. The ball milling time is 3 hours. The slurry drying temperature is 100℃, and the drying time is 24 hours. The slurry powder obtained after ball milling is passed through a 650-mesh sieve.

[0074] Steps (1-2): Place the slurry powder in a reducing environment for heating and heat preservation treatment;

[0075] The slurry powder was pressed at 5 MPa for 3 minutes to obtain powder blocks. The powder blocks were then placed in a stainless steel container, and a layer of carbon powder was evenly covered on the powder blocks before the container was covered. A vent hole was left on the container lid. The container was then heated and kept warm. The mass ratio of carbon powder to powder blocks was 1:5, and the particle size of the carbon powder was 40-70 μm. During the heating and keeping warm treatment, the temperature was first increased to 500℃ at a heating rate of 5℃ / min, and then increased to 650℃ at a heating rate of 15℃ / min. The temperature was kept at 650℃ for 3 hours.

[0076] After the heating and heat preservation treatment is completed in steps (1-3), the powder is quickly transferred to water for cooling. The water temperature is room temperature. After the powder cools to room temperature, the solid and liquid are separated and the powder is naturally dried in the air for 48 hours until it is semi-dry, thus obtaining the desert sand calcination product.

[0077] Step (2): The desert sand calcination product is subjected to acid leaching treatment. After acid leaching, solid and liquid are separated to obtain acidic leachate and solid silica. The specific method is as follows:

[0078] Step (2-1): Add the desert sand calcination product to a stainless steel container with a polytetrafluoroethylene coating, and add water at a mass ratio of 1:1. After stirring and mixing, remove the suspended particles in the dispersion system.

[0079] Step (2-2): Add inorganic acid solution to the dispersion system, stir again, and remove suspended particles from the dispersion system; the mass ratio of inorganic acid solution to the mass of desert sand calcination product in step (2-1) is 1:1; the inorganic acid solution is a mixture of hydrochloric acid with a mass concentration of 36wt%, sulfuric acid with a mass concentration of 75wt%, and hydrofluoric acid with a mass concentration of 36wt% in a mass ratio of 20:4:2.

[0080] Steps (2-3): The dispersion system is subjected to heating and acid leaching treatment at a temperature of 150℃ for a reaction time of 30 min. The reaction is carried out with stirring. After the acid leaching reaction is completed and the mixture is cooled to room temperature, liquid-solid separation is performed to obtain solid powder.

[0081] Steps (2-4): The solid powder was repeatedly washed until neutral, and the resulting white powder was dried at 120°C for 24 hours to obtain solid silicon oxide; the content of silicon oxide in the solid silicon oxide was found to be 95.6 wt%.

[0082] The acidic leaching solution was heated to boiling, and the evaporated acid gas and water vapor were collected while boiling until colorless and transparent crystals appeared. When trace elements other than aluminum were detected in the crystals, the aluminum chloride crystals were separated. Evaporation continued until a mixed chloride crystal containing other elements was formed. The purity of the aluminum chloride obtained in this example was 92.3 wt%.

[0083] Example 2

[0084] In this embodiment, the method for extracting silicon dioxide from desert sand includes the following steps:

[0085] Step (1): The desert sand is pretreated by calcination using a reducing agent and then cooled with water to obtain the calcined desert sand product with a loose particle structure.

[0086] In this embodiment, no solid reducing agent is added during ball milling. Reduction calcination is carried out by introducing a reducing gas. The specific method of calcination pretreatment is as follows:

[0087] Step (1-1): Place the solid desert sand raw material in a ball mill, add water, and ball mill. After ball milling, dry and pulverize the slurry to obtain slurry powder. The solid desert sand raw material is a single type of desert sand. The ratio of water added during ball milling to the mass of the solid desert sand raw material is 1:1. The ball milling time is 3 hours. The slurry drying temperature is 100℃, and the drying time is 48 hours. The slurry powder obtained after ball milling is passed through a 650-mesh sieve.

[0088] Steps (1-2): Place the slurry powder in a reducing environment for heating and heat preservation treatment;

[0089] The slurry powder is placed in a heatable, rotatable stainless steel container with an internal stainless steel mesh. An inlet and an outlet are provided at both ends of the container. During the heating and heat preservation process, a reducing gas is introduced through the inlet, and the reacted gas is discharged through the outlet. The reducing gas is carbon monoxide, and its flow rate is 1.5 L / min. The slurry powder is loaded into the stainless steel container, which is then rotated and heated. Specifically, the temperature is first increased to 500°C at a rate of 10°C / min, then increased to 650°C at a rate of 20°C / min, followed by the introduction of the reducing gas carbon monoxide, and the temperature is maintained at 650°C for 2.5 hours.

[0090] After the heating and heat preservation treatment is completed in steps (1-3), the powder is quickly transferred to water for cooling. The water temperature is room temperature. After the powder cools to room temperature, the solid and liquid are separated and the powder is naturally dried in the air for 60 hours until it is semi-dry, thus obtaining the desert sand calcination product.

[0091] Step (2): The desert sand calcination product is subjected to acid leaching treatment. After acid leaching, solid and liquid are separated to obtain acidic leachate and solid silica. The specific method is as follows:

[0092] Step (2-1): Add the desert sand calcination product to a stainless steel container with a polytetrafluoroethylene coating, and add water at a mass ratio of desert sand calcination product to water of 1:1.5. After stirring and mixing, remove the suspended particles in the dispersion system.

[0093] Step (2-2): Add inorganic acid solution to the dispersion system, stir again and remove suspended particles from the dispersion system; the mass ratio of inorganic acid solution to the mass of desert sand calcination product in step (2-1) is 2:1; the inorganic acid solution is a mixture of hydrochloric acid with a mass concentration of 36wt% and hydrofluoric acid with a mass concentration of 36wt% in a mass ratio of 25:5.

[0094] Steps (2-3): The dispersion system is subjected to heating and acid leaching treatment at a temperature of 100℃ for a reaction time of 90 min. The reaction is carried out with stirring. After the acid leaching reaction is completed and the mixture is cooled to room temperature, liquid-solid separation is performed to obtain solid powder.

[0095] Step (2-4): The white powder obtained by repeatedly washing the solid powder until it is neutral is dried at 120°C for 24 hours to obtain solid silicon oxide. The content of silicon oxide in the solid silicon oxide is 96.8 wt%. The extraction rate of silicon oxide from desert sand in this example (90.5%) is slightly higher than the silicon oxide extraction rate (85.6%) in Example 1.

[0096] The acidic leaching solution was heated to boiling, and the evaporated acid gas and water vapor were collected while boiling until colorless and transparent crystals appeared. When trace elements other than aluminum were detected in the crystals, the aluminum chloride crystals were separated. Evaporation continued until a mixed chloride crystal containing other elements was formed. The purity of the aluminum chloride obtained in this example was 94.2 wt%.

[0097] Example 3

[0098] In this embodiment, the method for extracting silicon dioxide from desert sand includes the following steps:

[0099] Step (1): The desert sand is pretreated by calcination using a reducing agent and then cooled with water to obtain the calcined desert sand product with a loose particle structure.

[0100] In this embodiment, a solid reducing agent is prepared by mixing 30 parts by weight of coal powder, 10 parts by weight of wood chips, and 70 parts by weight of carbon powder. The particle size of the solid reducing agent is 50–120 μm. The specific method of calcination pretreatment is as follows:

[0101] Step (1-1): Place the solid desert sand raw material in a ball mill, add water, and ball mill. After ball milling, dry and pulverize the slurry obtained to obtain slurry powder. The solid desert sand raw material is a mixture of desert sand and solid reducing agent. The ratio of water added during ball milling to the mass of solid desert sand raw material is 1:1. The ball milling time is 4 hours. The slurry drying temperature is 90℃, and the drying time is 48 hours. The slurry powder obtained after ball milling is passed through a 650-mesh sieve.

[0102] Steps (1-2): Place the slurry powder in a reducing environment for heating and heat preservation treatment;

[0103] The slurry powder was pressurized at 15 MPa for 1 min to obtain powder blocks. The powder blocks were then placed in a stainless steel container, and a layer of carbon powder was evenly covered on the powder blocks before the container was covered. A vent hole was left on the container lid. The container was then heated and kept warm. The mass ratio of carbon powder to powder blocks was 1:10, and the particle size of the carbon powder was 40-70 μm. During the heating and keeping warm treatment, the temperature was first increased to 500℃ at a heating rate of 10℃ / min, and then increased to 750℃ at a heating rate of 20℃ / min. The temperature was then kept at 750℃ for 0.5 h.

[0104] After the heating and heat preservation treatment is completed in steps (1-3), the powder is quickly transferred to water for cooling. The water temperature is room temperature. After the powder cools to room temperature, the solid and liquid are separated and the powder is naturally dried in the air for 72 hours until it is semi-dry, thus obtaining the desert sand calcination product.

[0105] Step (2): The desert sand calcination product is subjected to acid leaching treatment. After acid leaching, solid and liquid are separated to obtain acidic leachate and solid silica. The specific method is as follows:

[0106] Step (2-1): Add the desert sand calcination product to a stainless steel container with a cooling reflux device, and add water at a mass ratio of desert sand calcination product to water of 1:3. After stirring and mixing, remove the suspended particles in the dispersion system.

[0107] Step (2-2): Add inorganic acid solution to the dispersion system, stir again, and remove suspended particles from the dispersion system; the mass ratio of inorganic acid solution to the mass of desert sand calcination product in step (2-1) is 2:1; the inorganic acid solution is hydrochloric acid with a mass concentration of 36wt%.

[0108] Steps (2-3): The dispersion system is subjected to reflux heating and acid leaching. The mixture is heated to boiling and the reflux reaction time is 120 min. After the reflux reaction is completed and the mixture is cooled to room temperature, liquid-solid separation is performed to obtain solid powder.

[0109] Steps (2-4): The solid powder was repeatedly washed until neutral, and the resulting white powder was dried at 120°C for 24 hours to obtain solid silica. The silica content of the solid silica was found to be 97.7 wt%. The silica extraction rate (95.6%) from desert sand in this example was significantly higher than that in Example 2.

[0110] The acidic leaching solution was heated to boiling, and the evaporated acid gas and water vapor were collected while boiling until colorless and transparent crystals appeared. When trace elements other than aluminum were detected in the crystals, the aluminum chloride crystals were separated. Evaporation continued until a mixed chloride crystal containing other elements was formed. The purity of the aluminum chloride obtained in this example was 91.3 wt%.

[0111] Example 4

[0112] In this embodiment, the method for extracting silicon dioxide from desert sand includes the following steps:

[0113] Step (1): The desert sand is pretreated by calcination using a reducing agent and then cooled with water to obtain the calcined desert sand product with a loose particle structure.

[0114] In this embodiment, a solid reducing agent consisting of 50 parts by weight of coal powder, 20 parts by weight of wood chips, and 50 parts by weight of carbon powder is added during ball milling. The particle size of the solid reducing agent is 50–120 μm. Reduction calcination is carried out by introducing a reducing gas. The specific method for calcination pretreatment is as follows:

[0115] Step (1-1): Place the solid desert sand raw material in a ball mill, add water, and ball mill. After ball milling, dry and pulverize the slurry to obtain slurry powder. The solid desert sand raw material consists of desert sand and a solid reducing agent. The mass ratio of desert sand to solid reducing agent is 1:0.5, and the mass ratio of water added during ball milling to the mass ratio of solid desert sand raw material is 1:1. The ball milling time is 5 hours. The slurry drying temperature is 100℃, and the drying time is 28 hours. The slurry powder obtained after ball milling is passed through a 650-mesh sieve.

[0116] Steps (1-2): Place the slurry powder in a reducing environment for heating and heat preservation treatment;

[0117] The slurry powder is placed in a heatable, rotatable stainless steel container with an internal stainless steel mesh. An inlet and an outlet are provided at both ends of the container. During the heating and heat preservation process, reducing gas is introduced through the inlet, and the reacted gas is discharged through the outlet. The reducing gas is natural gas, and its flow rate is 1.0 L / min. The slurry powder is loaded into the stainless steel container, which is then rotated and heated. Specifically, the temperature is first increased to 500°C at a rate of 5°C / min, then increased to 700°C at a rate of 15°C / min. Natural gas is then introduced, and the mixture is kept at 700°C for 1.5 hours.

[0118] After the heating and heat preservation treatment is completed in steps (1-3), the powder is quickly transferred to water for cooling. The water temperature is room temperature. After the powder cools to room temperature, the solid and liquid are separated and the powder is naturally dried in the air for 48-72 hours until it is semi-dry, thus obtaining the desert sand calcination product.

[0119] Step (2): The desert sand calcination product is subjected to acid leaching treatment. After acid leaching, solid and liquid are separated to obtain acidic leachate and solid silica. The specific method is as follows:

[0120] Step (2-1): Add the desert sand calcination product to a stainless steel container with a cooling reflux device, and add water at a mass ratio of desert sand calcination product to water of 1:1.5. After stirring and mixing, remove the suspended particles in the dispersion system.

[0121] Step (2-2): Add inorganic acid solution to the dispersion system, stir again, and remove suspended particles from the dispersion system; the mass ratio of inorganic acid solution to the mass of desert sand calcination product in step (2-1) is 1.5:1; the inorganic acid solution is a mixture of hydrochloric acid with a mass concentration of 36wt%, sulfuric acid with a mass concentration of 75wt%, and hydrofluoric acid with a mass concentration of 36wt% in a mass ratio of 30:3:5.

[0122] Steps (2-3): The dispersion system is heated under reflux until the reaction system boils, and the reaction time is 60 minutes. The reaction is carried out with stirring. After the acid leaching reaction is completed and cooled to room temperature, liquid-solid separation is performed to obtain solid powder.

[0123] Steps (2-4): The solid powder was repeatedly washed until neutral, and the resulting white powder was dried at 120°C for 48 hours to obtain solid silica. The silica content of the solid silica was found to be 98.0 wt%. The silica extraction rate (97.6%) from desert sand in this example was significantly higher than that in Example 2.

[0124] The acidic leaching solution was heated to boiling, and the evaporated acid gas and water vapor were collected while boiling until colorless and transparent crystals appeared. When trace elements other than aluminum were detected in the crystals, the aluminum chloride crystals were separated. Evaporation continued until a mixed chloride crystal containing other elements was formed. The purity of the aluminum chloride obtained in this example was 90.9 wt%.

[0125] Comparative Example 1

[0126] The method for extracting silica from desert sand includes the following steps:

[0127] Step (1): The desert sand is pretreated by calcination to obtain the calcined desert sand product;

[0128] Step (1-1): In this comparative example, no reducing agent was used during the calcination treatment. The desert sand was directly placed in a ball mill, and then water with a mass ratio of 1:1 to the desert sand was added to the ball mill. After ball milling for 2 hours, the slurry was taken out and dried at 120°C, and then pulverized into powder. The slurry powder obtained after ball milling was passed through a 650-mesh sieve.

[0129] Steps (1-2): Add the slurry powder to a stainless steel container, heat it to 600°C at a heating rate of 5°C, and keep it at that temperature for 1 hour.

[0130] Steps (1-3): After the heat preservation is completed, the product is quickly transferred to water for cooling. The water temperature is room temperature. After the powder cools to room temperature, the solid and liquid are separated and the powder is naturally dried for 48 hours until it is semi-dry, thus obtaining the desert sand calcination product.

[0131] Step (2): The calcined desert sand product is subjected to acid leaching treatment. After acid leaching, solid and liquid are separated to obtain acidic leachate and solid silica. Specifically:

[0132] Step (2-1): Mix the desert sand calcination product and water in a stainless steel container at a mass ratio of 1:1. After stirring and mixing, remove the suspended particles in the dispersion system.

[0133] Step (2-2): Add inorganic acid solution to the dispersion system, stir again and remove suspended particles from the dispersion system; the mass ratio of inorganic acid solution to the mass of desert sand calcination product in step (2-1) is 1:1; the inorganic acid solution is made by mixing hydrochloric acid with a mass concentration of 36wt% and hydrofluoric acid with a mass concentration of 36wt% in a mass ratio of 20:2.

[0134] Steps (2-3): Heat the stainless steel container until the dispersion system boils, and continue the reaction at boiling point for 60 minutes. The reaction is carried out with stirring. After the reaction is completed and cooled to room temperature, filter the mixture to achieve liquid-solid separation and obtain solid powder.

[0135] Steps (2-4): The solid powder was repeatedly washed until neutral to obtain a white powder, and the white powder was dried at 120°C for 48 hours to obtain solid silicon oxide. The mass fraction of silicon oxide in the obtained solid silicon oxide was 89.6 wt%. The extraction rate of silicon oxide from desert sand in this comparative example (82.3%) was not as good as the extraction efficiency of Examples 1-4 above.

[0136] The acidic leaching solution was heated to boiling, and the evaporated acid gas and water vapor were collected while boiling until colorless and transparent crystals appeared. When trace elements other than aluminum were detected in the crystals, the aluminum chloride crystals were separated. Evaporation continued until a mixed chloride crystal containing other elements was formed. The purity of the aluminum chloride obtained in this comparative example was 91.5 wt%.

[0137] Comparative Example 2

[0138] The only difference between this comparative example and Comparative Example 1 is that in steps (1-2), the slurry powder is added to a stainless steel container and heated to 700°C at a rate of 10°C, and held at that temperature for 1 hour. The other steps are exactly the same as those in Comparative Example 1.

[0139] The solid silica prepared by the method of this comparative example has a silica mass fraction of 92.2 wt%; and the silica extraction rate (84.8%) of this comparative example from desert sand is not as good as the extraction efficiency of Examples 1-4 above; the aluminum chloride obtained by this comparative example has a purity of 93.0 wt%.

[0140] Figures 2a to 2b The microstructures of desert sand particles after pretreatment under the conditions of Comparative Example 1 and Comparative Example 2 are shown respectively. Figure 3 and Figure 4 The figures show the microstructures of desert sand particles treated under the pretreatment conditions of Examples 1 and 3, respectively. It can be seen from the figures that high-temperature calcination alone is insufficient for effective separation of the phases. Figure 2a and Figure 2b High-temperature calcination and reduction treatment produced a significant number of microcracks. Figure 3 )and( Figure 4 Some impurities are distributed along the phase interface, while others penetrate the bulk of each phase. This pretreatment-induced change in microstructure greatly reduces the difficulty for acidic media to enter desert sand, significantly increases the contact area between acidic media and desert sand components, thereby improving the dissolution rate of impurity phases.

[0141] Example 5

[0142] This embodiment uses the solid silica (purity 97.7 wt%) prepared in Example 3 as a raw material to prepare nano-silica, specifically including the following steps:

[0143] Step (301): Solid silicon oxide and solid sodium hydroxide are thoroughly mixed at a mass ratio of 1:12, compacted, and calcined at 650°C for 60 min to obtain the silicon oxide alkaline-treated product.

[0144] Step (302): Add water to the silicon dioxide alkaline treatment product, stir to dissolve and filter to remove insoluble impurities to obtain a clear solution; while stirring, slowly add a 3 mol / L hydrochloric acid solution to the clear solution.

[0145] Step (303): When the pH of the above mixed system drops to 3.5–3.0, all the precipitate in the mixed system is removed, and a 3 mol / L hydrochloric acid solution is added dropwise to lower the pH of the mixed system to the range of 3.0–1.5, and this is maintained for at least 10 minutes. The white precipitate generated in the mixed system is collected and washed until neutral. Finally, the washed white precipitate is dried at 120°C for 24 hours and ground until the particle size is less than 100 nm, thus obtaining nano-silica. The average particle size of the nano-silica prepared in this embodiment is 86.2 nm, and the purity is 99.3 wt%.

[0146] The waste liquid in step (303) is further evaporated to crystallize sodium chloride crystals, and the acidic solution is collected during the evaporation process.

[0147] Example 6

[0148] This embodiment uses the solid silicon oxide prepared in Example 1 as a raw material to prepare silicon carbide. The specific method is as follows:

[0149] Step (101): Mix solid silicon dioxide with carbon powder and wood chips in a mass ratio of 55:30:15 to obtain mixed raw material powder;

[0150] Step (102): Add water to the mixed raw material powder at a mass ratio of 1:1 and grind for 6 hours. After grinding, dry at 90°C for 24 hours to obtain a dry mixed powder with a particle size of less than or equal to 10 μm.

[0151] Step (103): Place the dry mixed powder into an alumina crucible, compact it, and cover it with carbon powder. After covering the crucible, leave a vent hole. The mass ratio of carbon powder to dry mixed powder is 1:10.

[0152] Step (104): Heat the crucible to 1400℃ and hold for 1.5h, then cool it naturally to 650-700℃; then transfer it to an air atmosphere and calcine at 650℃ for 30min to remove residual carbon powder, and obtain silicon carbide calcined product;

[0153] Step (105): The calcined silicon carbide product is soaked in a 10 wt% hydrofluoric acid solution for 2 hours to remove residual silicon oxide and other impurities, and then subjected to solid-liquid separation. The separated solid product is washed with water until neutral, and finally dried to obtain silicon carbide powder. The obtained silicon carbide powder is β-SiC type silicon carbide powder. The mass fraction of β-SiC type silicon carbide in this silicon carbide powder is 93.8 wt%.

[0154] Using the method of this embodiment, the silicon carbide powder prepared from the solid silicon oxide prepared in Examples 2 to 4 contained 94.7 wt%, 95.5 wt%, and 96.3 wt% β-SiC type silicon carbide, respectively.

[0155] Example 7

[0156] This embodiment uses the solid silicon oxide prepared in Example 1 as a raw material to prepare silicon nitride. The specific method is as follows:

[0157] Step (201): Mix solid silicon dioxide with carbon powder and wood chips in a mass ratio of 50:40:10 to obtain mixed raw material powder;

[0158] Step (202): Add water to the mixed raw material powder at a mass ratio of 1:1 and grind for 12 hours. After grinding, dry at 90°C for 48 hours to obtain a dry mixed powder with a particle size of less than or equal to 10 μm.

[0159] Step (203): Place the dried mixed powder into a tube furnace with a rotatable alumina inner cylinder. The alumina inner cylinder rotates at a speed of 10 rpm. A reducing gas with a flow rate of 2 L / min is introduced through the air inlet. The reducing gas is natural gas. The tube furnace is heated to 1300℃ and kept at that temperature for 1.5 h.

[0160] Step (204): After holding the temperature for 1.5 hours, a mixture of ammonia and nitrogen is introduced into the air inlet of the alumina inner cylinder. The volume ratio of ammonia to nitrogen in the mixture is 1:1. The total flow rate of the mixture is 1.5 L / min. After holding the temperature for another 1.5 hours, the ammonia is stopped and the cylinder is allowed to cool naturally. After the temperature has cooled naturally to 700°C, the nitrogen is stopped and the cylinder is allowed to cool to 300°C. The inner cylinder is then stopped from rotating and allowed to cool naturally to room temperature to obtain the silicon nitride calcination product.

[0161] Step (205): The calcined silicon nitride product is soaked in a 10 wt% hydrofluoric acid solution for 1.5 h to remove residual silicon oxide and other impurities. Solid-liquid separation is then performed, and the separated solid product is washed with water until neutral. Finally, it is dried to obtain silicon nitride powder. The silicon nitride powder contains 52.2 wt% α-Si3N4 type powder and 40.3 wt% β-Si3N4 type powder, with a total mass fraction of 92.5 wt% for both crystal types.

[0162] Using the method of this embodiment, the total mass fractions of α-Si3N4 type and β-Si3N4 type silicon carbide in the silicon carbide powder prepared using the solid silicon oxide prepared in Examples 2 to 4 as raw materials are 93.8 wt%, 95.2 wt%, and 95.6 wt%, respectively.

[0163] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of the claims of this patent application.

Claims

1. A method for extracting silicon dioxide from desert sand, characterized in that, Includes the following steps: Step (1): Pre-treat desert sand by calcination with a reducing agent and then water-cool it to obtain the calcined desert sand product; Step (2): The desert sand calcination product is acid-leached using the acid leaching method. After acid leaching, solid and liquid are separated to obtain acidic leachate and solid silica. The calcination pretreatment method in step (1) is as follows: Step (1-1): Place the solid desert sand raw material in a ball mill, add water and ball mill. After ball milling, dry and pulverize the slurry obtained to obtain slurry powder. Steps (1-2): Place the slurry powder in a reducing environment for heating and heat preservation treatment; After the heating and heat preservation treatment is completed in steps (1-3), the powder is taken out and transferred to water for cooling, thus obtaining the desert sand calcination product; In step (1-1), the solid desert sand raw material is a single desert sand, or the solid desert sand raw material is a mixture of solid reducing agent and desert sand, and the solid reducing agent is one or a combination of two or more of coal powder, carbon powder and wood chips; the ratio of water added during ball milling to the mass of solid desert sand raw material is 1 to 1.5:1; the ball milling time is 3 to 5 hours; the slurry drying temperature is 90 to 120°C, and the drying time is 24 to 48 hours; the slurry powder obtained after ball milling passes through a 650-mesh sieve; in step (1-2), the heating and holding temperature is 650 to 750°C, and the heating and holding time is 0.5 to 3 hours.

2. The method for extracting silicon dioxide from desert sand according to claim 1, characterized in that, In step (1-1), the solid desert sand raw material is a mixture of solid reducing agent and desert sand, with a mass ratio of solid reducing agent to desert sand of (0.5-1.5):1; the particle size of the solid reducing agent is 50-120 μm, and the solid reducing agent is composed of 30-50 parts by weight of coal powder, 10-20 parts by weight of wood chips and 50-70 parts by weight of carbon powder. In steps (1-2), during the heating and heat preservation treatment: first, heat the temperature to 500℃ at a heating rate of 5-10℃ / min, then heat the temperature to 650-750℃ at a heating rate of 15-20℃ / min, and keep it at 650-750℃ for 0.5-3 hours. In steps (1-3), after the heating and heat preservation treatment is completed, the powder is quickly transferred to water for cooling. The water temperature is room temperature. After the powder cools to room temperature, the solid and liquid are separated and the powder is naturally dried in the air for 48-72 hours until it is semi-dry, which is the desert sand calcination product.

3. The method for extracting silicon dioxide from desert sand according to any one of claims 1-2, characterized in that, The method for step (1-2) is as follows: The slurry powder is pressed at 5-15 MPa for 1-3 minutes to obtain powder blocks. Then, the powder blocks are placed in a stainless steel container, and a layer of carbon powder is evenly covered on the powder blocks before the container is covered. Ventilation holes are left on the container cover. Then, the container is heated and kept warm. The ratio of the mass of the covering toner to the mass of the powder block is 1:5 to 10, and the particle size of the toner is 40 to 70 μm; Alternatively, the slurry powder can be placed in a stainless steel container with an air inlet and an air outlet at each end. During the heating and heat preservation process, the reducing gas is introduced through the air inlet, and the gas after the reaction is discharged through the air outlet. The reducing gas is one or a mixture of two or more of the following: natural gas, carbon monoxide, hydrogen, gaseous alcohols, or gaseous aldehydes. The flow rate of the reducing gas is 1 to 2 L / min.

4. The method for extracting silicon dioxide from desert sand according to claim 3, characterized in that, In step (2), the acid leaching treatment method is as follows: Step (2-1): Disperse the desert sand calcination product in water at a mass ratio of 1:(1~3), stir and mix, and then remove the suspended particles in the dispersion system; Step (2-2): Add inorganic acid solution to the dispersion system, stir again, and remove suspended particles from the dispersion system; the mass ratio of inorganic acid solution to the mass of desert sand calcination product in step (2-1) is (1-2):1; the inorganic acid solution is a mixture of hydrochloric acid with a mass concentration of 36wt%, sulfuric acid with a mass concentration of 75wt%, and hydrofluoric acid with a mass concentration of 36wt% in a mass ratio of (10-30):(0-10):(0-10); Steps (2-3): The dispersion system is subjected to heating and acid leaching treatment at a temperature of 100-150℃ for a reaction time of 30-120 min; after the acid leaching reaction is completed, liquid-solid separation is performed to obtain solid powder. Steps (2-4): After repeatedly washing the solid powder until it is neutral, dry it at 120°C for 24-48 hours to obtain solid silicon oxide.

5. The method for extracting silicon dioxide from desert sand according to claim 1, characterized in that, It also includes the following steps: heating the acidic extract to boiling, and evaporating it in the boiling state until colorless and transparent crystals appear, separating the solid and liquid and drying the crystals to obtain chloride.

6. A method for preparing nano-silica, characterized in that, Includes the method for extracting silica from desert sand as described in claim 1 and the following steps: Step (301): Solid silicon oxide and solid sodium hydroxide are thoroughly mixed in a mass ratio of 1: (1~3), compacted, and calcined at 500~700℃ for 30~90min to obtain silicon oxide alkaline treatment product; Step (302): Add water to the silicon dioxide alkaline treatment product, stir to dissolve and filter to remove insoluble impurities to obtain a clear solution; while stirring, add hydrochloric acid solution with a concentration of 2-4 mol / L dropwise to the clear solution; Step (303): When the pH value of the above mixed system drops to 3.5-3.0, remove all the precipitate in the mixed system, and continue to add hydrochloric acid solution with a concentration of 2-4 mol / L to lower the pH of the mixed system to the range of 3.0-1.5 and maintain it for 10-20 min; collect the white precipitate generated in the mixed system and wash it until neutral, and finally place the washed white precipitate at 100-120℃ to dry for 12-24 h, and grind and disperse it to obtain nano-silica with a particle size of less than 100 nm.

7. A method for preparing silicon carbide, characterized in that, The method for extracting silica from desert sand as described in claim 1, or the method for preparing nano-silica as described in claim 6, includes the following steps: Step (101): Mix solid silica or nano silica with carbon powder and wood chips in a mass ratio of (50-60):(30-40):(10-20) to obtain mixed raw material powder; Step (102): Add water to the mixed raw material powder at a mass ratio of 1:1 and grind for 5 to 12 hours. After grinding, dry at 90°C for 24 to 48 hours to obtain a dry mixed powder with a particle size of less than or equal to 10 μm. Step (103): Place the dry mixed powder into an alumina crucible, compact it, and cover it with carbon powder. After covering the crucible, leave a vent hole. The mass ratio of carbon powder to dry mixed powder is 1:5 to 10. Step (104): Heat the crucible to 1300-1600℃ and hold for 2-4 hours, then cool it naturally to 650-700℃; then transfer it to an air atmosphere and hold at 650℃ for 30 minutes to obtain the silicon carbide calcined product. Step (105): Soak the silicon carbide calcination product in a 10wt% hydrofluoric acid solution for 1-2 hours, then separate the solid and liquid components. Wash the separated solid product with water until it is neutral, and finally dry it to obtain silicon carbide powder. The drying temperature is 100-120℃ and the drying time is 24-48 hours.

8. A method for preparing silicon nitride, characterized in that, The method for extracting silica from desert sand as described in claim 1, or the method for preparing nano-silica as described in claim 6, includes the following steps: Step (201): Mix solid silica or nano silica with carbon powder and wood chips in a mass ratio of (50-60):(30-40):(10-20) to obtain mixed raw material powder; Step (202): Add water to the mixed raw material powder at a mass ratio of 1:1 and grind for 5 to 12 hours. After grinding, dry at 90°C for 24 to 48 hours to obtain a dry mixed powder with a particle size of less than or equal to 10 μm. Step (203): Place the dried mixed powder into a tubular furnace with a rotatable alumina inner cylinder. The alumina inner cylinder rotates at a speed of 10-20 rpm. A reducing gas with a flow rate of 1-2 L / min is introduced through the gas inlet. The reducing gas is one or a mixture of two or more of the following: natural gas, carbon monoxide, hydrogen, gaseous alcohols, or gaseous aldehydes. The tubular furnace is then heated to 1250-1450℃ and held for 1-2 hours. Step (204): After the heat preservation is completed, stop the introduction of reducing gas, and then introduce a mixture of ammonia and nitrogen into the gas inlet of the alumina inner cylinder. The volume ratio of ammonia to nitrogen in the mixture is 1:

1. The total flow rate of the mixture is 1-2 L / min. After heat preservation for 1-2 hours, stop the introduction of ammonia and allow it to cool naturally. After the temperature cools naturally to 600℃, stop the introduction of nitrogen and continue cooling to 300℃. Stop the rotation of the alumina inner cylinder and allow it to cool naturally to room temperature to obtain the silicon nitride calcination product. Step (205): Soak the silicon nitride calcination product in a 10wt% hydrofluoric acid solution for 1-2 hours, then separate the solid and liquid components. Wash the separated solid product with water until it is neutral, and finally dry it to obtain silicon nitride powder.

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