High-purity silica is a class of extremely important optoelectronic materials widely used in high-tech fields such as semiconductors, fiber-optic ports, lasers and aerospace. With the rapid development of these fields, the requirements for impurity content in high-purity silica are more stringent, and the demand for high-purity silica is also increasing. At present, most of the high-purity silica required in China is imported. Therefore, the use of quartz sand purification technology to obtain high-purity silica is an effective way to meet the demand for high-purity silicon in China's high-tech fields, and is of great significance for promoting the development of China's national economy. At present, the main methods for purifying quartz sand can be divided into physical methods and chemical methods.
First, the physical method
Physical methods are mainly water washing and fractional desliming, scrubbing, magnetic separation, flotation and ultrasonic methods.
(1) Washing and grading de-sludge
This method is mainly for quartz sand containing a large amount of clay minerals. Because silica particles as thinner, which decreases the SiO 2 grade, quality and iron mineral impurities and aluminum but increased quality grade, etc., so that the water be selected from quartz sand ore before enrollment, grading is necessary desliming And the effect is also more obvious 18j. It is only used as a pretreatment method for ore candidates. It is applied early and common, but the removal effect of thin film iron and blocking impurity minerals existing on the surface of quartz sand is not significant.
(2) scrubbing
Scrubbing is to remove the thin film iron, the bonding and muddy impurity minerals on the surface of the quartz sand by mechanical force and the grinding and peeling force between the sand grains, and to further smash the unassembled mineral aggregates, and then further purify the quartz sand by the classification operation. effect. At present, there are two main methods of rod scrubbing and mechanical scrubbing. For mechanical scrubbing, the structure and configuration of the relevant mechanical equipment as well as the scrubbing time and scrubbing concentration in the process are the main factors affecting the scrubbing effect. Because of the many factors affecting it, the recovery rate of mechanical scrubbing is very low, only about 40%, so the effect of mechanical scrubbing is not ideal. Compared to mechanical scrubbing, the effect of rod scrubbing is much better than it. In the rod scrubbing process, adding appropriate chemicals, increasing the electric repulsion of the surface of the impurity minerals and quartz particles, enhancing the separation effect between the impurity minerals and the quartz particles, increasing the recovery rate of the scrubbing to 80%, and the rod scrubbing almost It is twice as much as mechanical scrubbing, but it is only a method of pretreatment of ore for the purification of high purity quartz sand.
(3) Magnetic separation
The magnetic separation method can remove impurities contained in the quartz sand particles to the utmost extent, and weak magnetic impurity minerals such as hematite, limonite and biotite , and magnetite-based ferromagnetic minerals. For weak magnetic impurity minerals, a strong magnetic machine of 100,000 e or more is often used. For minerals with strong magnetic impurities, a weak magnetic machine or a medium magnetic machine is often used for magnetic separation. In general, the number of magnetic separations and the strength of the magnetic field have an important influence on the effect of magnetic separation and iron removal. As the number of magnetic separations increases, the amount of iron decreases gradually; while at a certain magnetic field strength, most of the iron is removed, but thereafter Even if the magnetic field strength is much improved, the iron removal rate does not change much. In addition, the finer the grain size of the quartz sand, the better the iron removal effect, because the fine-grained quartz sand contains a high amount of iron-containing impurities. Tian Venus were studied experimentally in the study of the purification process of high purity quartz sand, results showed that, with the magnetic field strength increases, the removal rate of impurities is increased, since the magnetic field strength reaches 100000e, did not significantly increase the removal rate of impurities. Therefore a suitable magnetic field strength should be 100000e. After magnetic separation, 40 mesh Si0 2 grade can reach 99.05%, Fe 2 0 3 content is 0.071%: 40-80 mesh Si0 2 grade is 99.09%, Fe203 content is 0.070%; 80-140 mesh Si0 2 grade 99.14%, Fe 2 0 3 content 0.067%: 140-200 mesh Si02 grade 99.10%, Fe 2 0 3 content 0.069%. However, when there are many impurities in the quartz sand, especially when there are many weak magnetic or non-magnetic impurities, the magnetic separation cannot be purified into high-purity quartz sand.
(4) Flotation
Flotation is to remove non-magnetic associated impurity minerals such as feldspar and mica in quartz sand. At present, there are mainly two methods of fluorine flotation and fluorine-free flotation. Fluorine flotation is carried out using a cationic collector and a hydrofluoric acid activator at an acidic pH range. However, considering the serious environmental impact of fluorine-containing wastewater, people began to turn to fluorine-free flotation. Using the difference in the structure of quartz and feldspar, rationally withering the ratio and dosage of the anion-cation mixed collector, and using their different zeta potentials, preferentially flotation of feldspar to achieve the separation of the two. It has been reported in the literature that under neutral conditions, the addition of a fluorine-free flotation agent increases the Si0 2 content of the silica fine powder from 99.1% to 99.77%, and accordingly the Fe 2 0 3 content decreases from 0.081% to 0.023%. The yield is between 83% and 85%. This indicates that the fluorine-free flotation can significantly improve the quality of the silica fine powder. Tangya Fei et employed hexametaphosphate as a dispersant, and sodium phosphate flotation adjusting agent, dodecylamine as collector can remove iron impurities, Fe 2 O 3 content decreases from the quartz fine powder by 0.09 to 0.02%, yield The rate reached 85%.
(5) Ultrasonic method
Ultrasonic method is an acoustic wave that relies on a medium to propagate. It has mechanical energy, which will cause interaction with the medium during propagation, producing various effects (mechanical effect, thermal effect and cavitation effect). When the ultrasonic pulverizing head acts on the liquid, the inside of the liquid changes, generating pressure or pulling force. When the tensile force reaches a certain strength, cavitation occurs, causing numerous small bubbles, and the bubbles are compressed with the ultrasonic vibration to reduce the pressure; When the bubble reaches a critical dimension (which determines the frequency of the ultrasonic wave), these bubbles will rupture, creating a huge pressure, violent impact on the solid particles in the liquid, under this violent impact, the surface of the particle The impurities or water-repellent film rapidly peel off from the surface of the particles, become a fine suspension under the action of the dispersing agent, and are separated from the quartz sand, and after washing and separating, the purity of the quartz sand is greatly improved. Liao Qing et al. In the medium of water and a small amount of dispersant, the 0-15 mm deposited quartz sandstone granule powder was ultrasonically treated to make the quartz sand containing Fe 2 0 3 0.12% and Si0 2 99.42% reach Fe 2 0 . 3 0.01%, 8i0299.8%, the recovery rate is above 99%, reaching the standard of optical glass sand.
Second, the chemical method
The chemical method is mainly acid leaching method and complex method. The acid leaching method is characterized in that quartz is insoluble in acid (except HF), and other impurity minerals can be dissolved by acid, so that further purification of quartz can be realized. The complex method uses the quartz powder to form a coordination compound with the impurity ions in the solution after acid leaching, so that the impurity ions in the solution are further removed.
The acid leaching method is further divided into a single acid leaching method and a mixed acid leaching method. Acid leaching method often uses acids such as sulfuric acid, hydrochloric acid, nitric acid and hydrofluoric acid. The acids commonly used in complexation are mainly oxalic acid and acetic acid. The above acids have a good removal effect on metal impurity minerals in quartz. The removal of Pe and A1 by various dilute acids is obvious, while the removal of Ti and Cr is mainly treated with concentrated sulfuric acid, aqua regia and hydrofluoric acid. The main factors affecting the acid treatment effect are acid concentration, temperature, time, and washing process.
(1) Single acid leaching method
A certain amount of quartz sand is placed in a certain concentration of acid solution, heated to a certain temperature, heated for a suitable time, the acid solution is recovered, and the quartz sand is washed and dried. Silty clay diatomaceous earth in a certain area of ​​Sichuan Province, using sulfuric acid as the acid leaching agent, through the study of temperature, sulfuric acid concentration and liquid-solid ratio, it is found that when the temperature is 90 ° C, the sulfuric acid concentration is 40%, the liquid-solid ratio 10: 1, diatomaceous earth iron leaching rate optimal, Fe 2 0 3 content is reduced from 3% -4% to 0.86%, A1 2 0 3 content was also reduced from 9.55% to 7.08%, celite The content of SiO 2 in the medium is increased to 80% or more. Zhou Yongheng in the acid leaching purification study of quartz, through the study of the acid immersion temperature, concentration and time of hydrofluoric acid, the results show that the pulse quartz raw material powder at a temperature of 120 ° C, the ratio of HF to water is 0.4 -0.5 solution acid leaching for 0.5-6h, its purity can reach the standard of medium and high-grade quartz glass.
(2) Mixed acid leaching method
Since each acid has different effects on the removal of impurities in the quartz sand, different acids are mixed together to produce a synergistic effect, so that the removal rate of impurities in the quartz sand is higher, and quartz sand with higher purity can be obtained. The washed quartz sand is added to the mixed acid solution, and the mixture is stirred and leached at normal temperature for 24 hours. If it is heated under the condition of stirring, the time is generally 2-6 hours, and the washing is dry. Zhang Wei et al. used a liquid-solid ratio of 18% hydrochloric acid and silicon micropowder at room temperature, followed by 25% sulfuric acid, and a liquid-solid ratio of sulfuric acid to silicon micropowder of 2:1 for the second acid leaching. The acid leaching time was 12h, and the iron content in the silicon micropowder after two acid leaching treatments was <60ug/g. The washed quartz sand is slurried into a container containing a mechanical stirrer according to a solid mass fraction of 20%-80%, and then a hydrochloric acid solution (1% to 10%) and a fluorosilicic acid solution (1) are added. %-10%), the quartz sand and the solution are stirred at 75-100 ° C for 2-3 h, then the solution in the slurry is removed, and then washed several times with water until the pH of the washing liquid is near neutral. By this method, the iron content of the quartz sand can be reduced from 0.0059% to 0.0002%-0.005%. The ratio of mixed acid used in Shenjiu is sulfuric acid: hydrochloric acid: nitric acid: hydrofluoric acid two 50%: 25%: 15%: 10%, heated to 80 ° C, the concentration of leaching pulp is 50% -55%, and its silica The content of iron and the content of iron impurities reach the standard of high-purity quartz sand (SiO 2 ≥99.98%, Fe 2 O 3 ≤0.001%).
(3) Network law
The complex method is to react a medium-strength organic acid with impurities on the surface of quartz sand, and also form a stable coordination compound with the impurity ions after the reaction, thereby reducing the concentration of impurity ions on the surface of the particles, and also Prevents the precipitation of ions during the washing process, and further reduces the impurity content in the quartz sand. Panias et al. 120, a quartz sand with an average particle size of 20 ltm and a iron content of 110×10°, weighed a certain amount into an oxalic acid solution, and heated to 80 ° C for a treatment time of 3 h, which can dissolve iron-containing minerals in acidity. The solution can only exist in the form of Fe3+, and Fe3+ forms a stable chelate with oxalic acid. The iron removal rate is between 80% and 100%. After treatment, the iron content in the quartz sand is less than 10x10. Hunan Liuyang quartz sand, after the oxalic acid treatment, the SiO 2 content in the sample increased from 98.26% to 99.81%, A1 2 0 3 content is reduced from 0.18% to 0.15%, Fe 2 0 3 content is reduced to 0.10% .
The above-mentioned acids can be regenerated, reused by evaporation, coagulation or other methods. When the purity of Si0 2 is very high, the water for cleaning the acid solution must be distilled water or deionized water to prevent impurities such as iron contained in the tap water from contaminating the high-purity SiO 2 .
(4) Other methods
The application of SiO 2 is very extensive, and its purity requirements are different for different applications. Therefore, some other purification methods are sometimes used for further purification. For example, the electrical selection method utilizes the slight difference in electrical properties between quartz and impurity minerals. , select a trace of metal impurity minerals: thermal burst method is to heat the silica to a certain temperature after the inclusions in the mineral burst, so that impurities in the inclusions can be removed. The thermal chlorination method removes impurity minerals and metal inclusions in the bubble phase of the quartz.
Third, the conclusion
Whether it is physical or chemical, it has an irreplaceable role in the entire process of quartz sand purification. In practical applications, physical methods are often used for pretreatment, such as water washing and fractional deliming, scrubbing, magnetic separation, flotation and ultrasonic methods, to remove most of the impurities, and then chemical methods, namely acid leaching and complexation. The method further removes trace impurities in the quartz sand. However, the acid used in chemical methods has serious environmental pollution, which requires us to seek a new, non-polluting purification method. Combining the expertise of chemistry, physics, mechanical chemistry, electromagnetic (wave) chemistry, etc., research on the purification technology of high-purity quartz sand is an important development direction in the future. With the development of the economy, the beneficiation and purification of quartz sand has important economic and social benefits. Especially with the development of microelectronics, optoelectronics and other industries, the excellent performance of high-purity quartz sand is irreplaceable by other powders, and the market prospect is extremely broad.
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