Method for preparing high-purity cristobalite

Abstract

The present invention relates to the field of silica sand, particularly to a method for preparing high-purity cristobalite. This method involves drying, calcining, and dispersing amorphous silicon dioxide to obtain high-purity cristobalite. Compared to the prior art, the present invention has the following beneficial effects: 1. Amorphous silicon dioxide has a relatively large specific surface area and is rich in hydroxyl groups on its surface, which makes it prone to the formation of holes in the cristobalite during the conversion process to cristobalite. To eliminate these holes, the present invention employs a surface treatment using a coupling agent followed by calcination. Data show that this significantly reduces the number of hydroxyl groups. 2. For the same mass of coupling agent, the present inventors selected a silane coupling agent with a shorter chain length, which results in fewer holes in the cristobalite and allows for a reduction in the calcination temperature and time. This may be due to the high-temperature decomposition of the silane coupling agent to form silicon oxide. The silicon oxide formed by decomposition has a relatively small particle size, which may preferentially form crystal nuclei and promote the overall crystal transformation.

Description

[Technical Field]

[0001] The present invention relates to the field of silica sand technology, and in particular to a method for preparing high-purity cristobalite. [Background technology]

[0002] Quartz (SiO2) is an oxide of silicon with a shelf-like structure and has many isomorphic forms. At atmospheric pressure, it exists in seven crystalline forms: α-quartz, β-quartz, α-tridymite, β-tridymite, β-tridymite, α-cristobalite, and β-cristobalite. The transition temperatures between these forms at atmospheric pressure are shown in Figure 4. Note that the β-form represents the crystalline form that is stable at high temperatures, while the α-form represents the crystalline form that is stable at low temperatures. It is widely distributed in nature, and unless otherwise specified, the so-called quartz usually refers to α-quartz.

[0003] High-purity quartz sand generally refers to quartz with a silicon dioxide content exceeding 99.9%. Based on SiO2 purity, high-purity quartz products can be divided into four levels: high-end ω(SiO2) ≥ 99.998% (4N8), mid-high-end ω(SiO2) ≥ 99.995% (4N5), mid-end ω(SiO2) ≥ 99.99% (4N), and low-end ω(SiO2) ≥ 99.9% (3N) (see "High-Purity Quartz Concept and Raw Material Classification," Mineral Protection and Utilization, October 2022, No. 5). However, raw natural quartz has difficulty meeting the quality requirements for high-purity quartz. High-purity quartz sand is a silica sand product made from natural quartz ore and obtained through a relatively complex refining process, resulting in extremely high SiO2 purity. In addition, due to limitations in the subsequent product preparation process, high-purity silica sand has strict requirements for the particle size (usually 40-200 mesh) and mineral phase of the product, etc. Therefore, amorphous silicon dioxide such as commonly referred to silicon powder and white carbon black is not high-purity silica sand even if it has a very high purity.

[0004] The impurity elements in natural silica sand primarily include Al, K, Na, Li, Ca, Cu, B, Fe, Mn, Co, Ti, and P. Among these impurity elements, monovalent and divalent ions exist as interstitial atoms in charge-imbalance defects within the quartz crystal lattice as compensating charges, while trivalent, tetravalent, and pentavalent ions (isomorphic impurities) primarily reside within the crystal lattice. To remove impurities from silica sand, engineers have proposed various processes. For example, Zhang Haiqi et al.'s article "Impurity Characteristics in High-Purity Quartz and Research Progress on Deep Chemical Refining Technology" (Mineral Protection and Utilization, August 2022, Issue 4) describes existing silica sand refining techniques. Currently, natural silica sand refining methods primarily include physical and chemical methods. Physical refining methods mainly include color sorting, scrubbing, gravity separation, magnetic separation, and flotation. However, gas-liquid inclusions and isomorphous impurities in the crystal lattice are the main impurity sources and are a significant factor limiting the production of high-purity quartz products. Physical refining methods cannot remove these impurities, so deep chemical refining is required. Deep chemical refining mainly includes acid (alkali, salt) treatment and heat treatment. Acid (alkali, salt) treatment mainly removes impurities present on the surface of silica sand particles or embedded inside the particles in the form of gas-liquid inclusions. Heat treatment mainly uses high temperatures to disrupt the gas-liquid inclusions and reduce the gas-liquid impurities (although it cannot completely remove them).

[0005] Compared with physical refining methods, chemical refining is more complicated in operation and relatively expensive, but chemical treatment is the most effective and essential method for producing high-purity quartz. Summary of the Invention [Problem to be solved by the invention]

[0006] However, the conventional technology has the following problems.

[0007] 1) Before silica stone can be processed into high-purity silica sand, it must undergo steps such as pickling, flotation, magnetic separation, gravity separation, high-temperature water quenching, and chlorination roasting. This makes the process long, the pickling process is inefficient, and the process is complicated. In addition, foreign impurities such as metal elements such as iron, sodium, and aluminum are easily introduced during the impurity removal process.

[0008] 2) The hydrofluoric acid, hydrochloric acid, and nitric acid used in pickling are highly concentrated and used in large quantities, so a lot of waste containing fluorine and chlorine is generated during pickling, which increases disposal costs.

[0009] In a paper by Jiang Xuexin et al. entitled "Thermodynamic Desorption of Gas-Liquid Impurities in Natural Quartz" (Silicate Journal, October 2004), they further investigated the effect of impurities in quartz on quartz products. They found that silica sand contains gas-liquid inclusions and a relatively high content of hydroxyl groups (usually over 80 ppm) on the surface, which makes it easy for bubbles to form during the production of quartz products, affecting the quality of the products. [Means for solving the problem]

[0010] In order to solve the problems in the background art mentioned above, the present invention provides a method for preparing high-purity cristobalite, which comprises drying, calcining, and dispersing the high-purity cristobalite in amorphous silicon dioxide to obtain the high-purity cristobalite.

[0011] Preferably, the amorphous silicon dioxide includes, but is not limited to, that obtained by oxidation of silicon metal.

[0012] The oxidation of the silicon metal includes either combustion of the silicon metal, or high-temperature reaction of the silicon metal with high-purity water, or conversion of the silicon metal into a silicon-containing organic compound such as silane, followed by calcination to convert it into amorphous nano-silicon oxide.

[0013] Preferably, the particle size of the amorphous silicon dioxide is between 5 nanometers and 1 micron.

[0014] Preferably, the drying conditions are a temperature of 100°C to 150°C for 1 to 2 hours.

[0015] Preferably, the firing conditions are a temperature of 1100°C to 1700°C for 2 to 10 hours.

[0016] Preferably, the particle size of the high-purity cristobalite is 120 to 450 microns.

[0017] Preferably, the total content of the elements Al, B, Ca, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, P, Ti, and Zn in the high-purity cristobalite is less than 20 ppm.

[0018] Preferably, the method for preparing high-purity cristobalite further comprises the step of treating with a coupling agent after drying.

[0019] Preferably, the coupling agent is any one selected from a silane coupling agent and a titanate coupling agent.

[0020] Preferably, in order to reduce the inclusion of impurities, the silane coupling agent is a silane coupling agent containing only the elements carbon, silicon, hydrogen, and oxygen.

[0021] Preferably, the silane coupling agent containing only the elements carbon, silicon, hydrogen and oxygen has a carbon chain length of 5 or less. [Effects of the Invention]

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] 1. Amorphous silicon dioxide has a relatively large specific surface area and is rich in hydroxyl groups on its surface. This makes it easy for holes to form in the cristobalite during the conversion process. To eliminate these holes, the present invention uses a coupling agent to treat the surface, followed by calcination. Data shows that the hydroxyl groups are significantly reduced.

[0024] 2. For the same mass of coupling agent, the inventors of the present application selected a silane coupling agent with a shorter chain length, which resulted in fewer holes in the cristobalite and allowed the firing temperature and time to be reduced to some extent. This is because the silane coupling agent decomposes at high temperatures to form silicon oxide, and the particle size of the silicon oxide formed by decomposition is relatively small, which may preferentially form crystal nuclei and promote the transformation of the overall crystal form.

[0025] 3. The high-purity cristobalite obtained by this invention contains elements such as Al, B, Ca, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, P, Ti, and Zn, as well as relatively few hydroxyl groups, compared to existing high-purity quartz.

[0026] 4. The present invention uses silane coupling agent treatment to reduce the pores in cristobalite and the baking temperature and time, thereby saving energy consumption. [Brief explanation of the drawings]

[0027] [Figure 1] FIG. 1 is an XRD diagram of high-purity cristobalite of Example 7. [Figure 2] 1 is an infrared spectrum of amorphous silicon dioxide A. [Figure 3] 1 is an infrared spectrum of the high-purity cristobalite of Example 7. [Figure 4] FIG. 1 is a diagram of atmospheric pressure transition temperatures between various crystalline forms of silicon oxide. DETAILED DESCRIPTION OF THE INVENTION

[0028] Unless otherwise specified, all raw materials and reagents used in the following examples are commercially available or can be prepared by conventional methods.

[0029] The amorphous silicon dioxide of the present invention has a particle size of 5 nanometers to 1 micron and a specific surface area of ​​90 to 200 m 2 / g. Sources can be purchased commercially, e.g., Aladdin, or you can make your own using the following method.

[0030] Photovoltaic grade polycrystalline silicon is combusted in oxygen under conditions controlled by conventional means to produce high purity amorphous silicon dioxide of different particle sizes.

[0031] In the present invention, amorphous silicon dioxide (particle size 5 nanometers, specific surface area 200 m 2 / g) is referred to as amorphous silicon dioxide A.

[0032] Amorphous silicon dioxide (particle size 10 nanometers, specific surface area 150 m 2 / g) is referred to as amorphous silicon dioxide B.

[0033] Amorphous silicon dioxide (particle size 1 micron, specific surface area 90m 2 / g) is expressed as amorphous silicon dioxide C.

[0034] Example 1 Amorphous silicon dioxide A is dried at 100°C for 2 hours, calcined at a high temperature of 1170°C for 10 hours, cooled naturally, and dispersed in an air stream to obtain high-purity cristobalite with a particle size of 120 microns.

[0035] <Example 2> Amorphous silicon dioxide A is dried at 150°C for 1 hour, calcined at a high temperature of 1700°C for 2 hours, naturally cooled, and dispersed in an air stream to obtain high-purity cristobalite with a particle size of 200 microns.

[0036] Example 3 Amorphous silicon dioxide B is dried at 100°C for 2 hours, calcined at a high temperature of 1170°C for 10 hours, naturally cooled, and dispersed in an air stream to obtain high-purity cristobalite with a particle size of 380 microns.

[0037] Example 4 (Reflects the results of producing high-purity cristobalite from micron-sized raw materials) Amorphous silicon dioxide C is dried at 100°C for 2 hours, calcined at a high temperature of 1170°C for 10 hours, naturally cooled, and dispersed in air to obtain high-purity cristobalite with a particle size of 630 microns.

[0038] <Example 5> 3 kg of amorphous silicon dioxide A was dried at 100 °C for 2 hours and 30 g of coupling agent (CHO)Si(CH) 10 A modified liquid is obtained by mixing CH3 with 100g of ethanol, and the modified liquid is uniformly mixed with dried amorphous silicon dioxide, dried at 100°C for 2 hours, fired at a high temperature of 1170°C for 10 hours, naturally cooled, and dispersed in an air stream to obtain high-purity cristobalite with a particle size of 180 microns.

[0039] Example 6 3 kg of amorphous silicon dioxide A is dried at 100°C for 2 hours, and 30 g of the coupling agent (CHO)Si(CH)CH is mixed with 100 g of ethanol to obtain a modified liquid. The modified liquid is then uniformly mixed with the dried amorphous silicon dioxide, dried at 100°C for 2 hours, fired at a high temperature of 1170°C for 10 hours, naturally cooled, and dispersed in an air current to obtain high-purity cristobalite with a particle size of 157 microns.

[0040] Example 7 3 kg of amorphous silicon dioxide A is dried at 100°C for 2 hours, and 30 g of the coupling agent (CHO)Si(CH)CH is mixed with 100 g of ethanol to obtain a modified liquid. The modified liquid is then uniformly mixed with the dried amorphous silicon dioxide, dried at 100°C for 2 hours, fired at a high temperature of 1170°C for 6 hours, naturally cooled, and dispersed in an air current to obtain high-purity cristobalite with a particle size of 135 microns.

[0041] (Comparative Example 1) 3 kg of amorphous silicon dioxide A is dried at 100°C for 2 hours, and 30 g of the coupling agent (CHO)Si(CH)CH is mixed with 100 g of ethanol to obtain a modified liquid. The modified liquid is then uniformly mixed with the dried amorphous silicon dioxide, dried at 100°C for 2 hours, fired at a high temperature of 1170°C for 8 hours, naturally cooled, and dispersed in an air current to obtain high-purity cristobalite with a particle size of 163 microns.

[0042] (Comparative Example 2) ITOA-6 high-purity quartz from Unimin, USA, is dried at 100°C for 2 hours, fired at a high temperature of 1170°C for 10 hours, naturally cooled, and dispersed in air to obtain high-purity cristobalite with a particle size of 230 microns.

[0043] <Results and Detection> The obtained samples were tested for impurity ions using ICP-OSE (detection limit is 1 ppb), and the results are shown in Table 1.

[0044] [Table 1]

[0045] The particle size, porosity, and crystallinity of the obtained samples were measured. The pore size was measured by nitrogen adsorption / desorption, the particle size by a particle size analyzer, and the crystallinity by XRD. The results are shown in Table 2.

[0046] [Table 2]

[0047] Table 3 shows the hydroxyl content (unit: ppm) in the samples before and after treatment with the coupling agent in Examples 5-7, calculated based on infrared spectrum. The results are shown in Table 3.

[0048] [Table 3]

[0049] Data analysis: As can be seen from Table 1, compared with the raw material, the impurity content of the product is increased to some extent, which may be due to the inevitable contamination in the preparation process.

[0050] The samples in Table 2 treated with the coupling agent showed a significant decrease in porosity when fired at high temperatures. Example 6, using a long-chain silane coupling agent, had a relatively high porosity compared to Example 7. On the other hand, the use of a short-chain silane coupling agent allowed for a relatively high degree of crystallinity to be achieved in a relatively short time, resulting in greater energy savings.

[0051] After the samples in Table 3 were treated with a coupling agent, the hydroxyl group content was significantly reduced, and the hydroxyl group content in the cristobalite obtained after calcination was further reduced.

[0052] Figure 2 shows the infrared spectrum of raw high-purity silicon dioxide, with a peak at 3410 cm -1 and 1642 cm -1 Since there is a vibration peak at , it is clear that there are hydroxyl groups on the surface.

[0053] Figure 3 shows the infrared spectrum of cristobalite, with a peak at 3410 cm -1 and 1642 cm -1 Since there is no vibration peak at , it is clear that no hydroxyl groups can be detected from the surface.

[0054] The above examples are merely preferred specific embodiments of the present invention and do not limit the scope of protection of the present invention. Those skilled in the art will recognize that any equivalent replacement or modification made based on the technical solutions and inventive concepts of the present invention within the technical scope disclosed in the present invention should be covered by the claims of the present invention.

Claims

1. A method for preparing high-purity cristobalite, comprising drying, calcining, and dispersing amorphous silicon dioxide to obtain high-purity cristobalite.

2. 2. The method for preparing high-purity cristobalite according to claim 1, wherein the amorphous silicon dioxide includes, but is not limited to, that obtained by oxidation of metallic silicon.

3. 3. The method for preparing high-purity cristobalite according to claim 2, wherein the oxidation of the metal silicon includes any one of burning the metal silicon, reacting the metal silicon with high-purity water at high temperature, or converting the metal silicon into a silicon-containing organic substance such as silane, followed by calcining the organic substance to convert it into amorphous nano-silicon oxide.

4. 2. The method for preparing high-purity cristobalite according to claim 1, wherein the particle size of the amorphous silicon dioxide is 5 nanometers to 1 micron.

5. 2. The method for preparing high-purity cristobalite according to claim 1, wherein the drying conditions are a temperature of 100 to 150°C for 1 to 2 hours.

6. 2. The method for preparing high-purity cristobalite according to claim 1, wherein the firing conditions are a temperature of 1100°C to 1700°C for 2 to 10 hours.

7. 2. The method for preparing high-purity cristobalite according to claim 1, wherein the particle size of the high-purity cristobalite is 120 to 450 microns.

8. 2. The method for preparing high-purity cristobalite according to claim 1, wherein the total content of the elements Al, B, Ca, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, P, Ti, and Zn in the high-purity cristobalite is less than 20 ppm.

9. 2. The method for preparing high-purity cristobalite according to claim 1, further comprising the step of treating with a coupling agent after drying.

10. 9. The method for preparing high-purity cristobalite according to claim 8, wherein the coupling agent is any one selected from the group consisting of a silane coupling agent and a titanate coupling agent.

11. 11. The method for preparing high-purity cristobalite according to claim 10, wherein the silane coupling agent contains only the elements carbon, silicon, hydrogen, and oxygen.

12. 12. The method for preparing high-purity cristobalite according to claim 11, wherein the silane coupling agent containing only carbon, silicon, hydrogen, and oxygen elements has a carbon chain length of 5 or less.

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