Research Status of Purification Process of High Purity Ultrafine Silicon Micropowder

Nowadays, the preparation of high-purity ultrafine silicon micropowder generally includes chemical synthesis and natural mineral purification. However, long-term research has found that the chemical synthesis method has high cost and low yield and cannot meet the demand. In this paper, the research progress and application of mineral purification method are introduced from the preparation of high purity ultrafine silicon micropowder using natural vein quartz .

With the development of modern high-tech and new materials, ultra-fine pulverization technology has played an important role in microelectronics, aviation, special ceramics, refractory materials, composite materials, new energy and bio-chemical industries, and has become an important The means of processing raw materials. Ultrafine pulverization is achieved by means of impact, collision, shearing, grinding and dispersion of materials. After more than 20 years of development, China's ultra-fine pulverization technology has also been greatly improved, but most of them are introduced through absorption.
The current ultra-fine pulverizing equipment includes agitating mill, vibrating mill, ball mill , jet mill, pressure roller mill, high-speed mechanical impact mill, colloid mill, planetary mill, and the like. At home and abroad, we have been working on micron-scale mineral materials. Most of them are air-milling machines that use natural quartz minerals as raw materials to prepare ultra-fine powders to meet market demand and to better reduce harmful impurities in powders. Natural quartz minerals contain a large number of inclusions and cracks. The use of ultra-fine pulverization technology can greatly reduce the number of cracks and defects, and combined with the purification process can better reduce the content of harmful impurities.
In terms of current crushing technology, natural quartz minerals can be used in ball mills, agitating mills, jet mills, vibrating mills, and the like. However, since these devices are easy to introduce harmful impurities during the pulverization process and it is generally difficult to pulverize the particle size of the powder to the micron order, it is difficult to satisfy the preparation of the high-purity ultrafine powder. Among them, jet mill is a better crushing technology, such as the most widely used JOM circulating jet mill and fluidized bed jet mill.
In order to solve these problems, Zhang Xiaozhong et al. tested the ultrafine pulverization particle size control of quartz sand, and used the vibrating mill to process ultra-fine powder quartz for comparison of dry and wet processes. It is considered that the feed amount and its uniformity and the feed size ratio match the product. The influence of the particle size distribution is large; the wet grinding process has high efficiency but large dielectric loss. Yang Huifen et al studied the ultrafine pulverization of powdered quartz and the modification of its surface. The effects of abrasive time, ball-to-batch ratio, solid-liquid ratio and stirring intensity on sample quality and performance were studied.
Hao Baohong studied the ultrafine pulverization of powdered quartz. The test showed that the particle size limit of powder quartz under dry grinding was 1.28 μm, while the particle size limit under wet milling was 1.01 μm. Li Huajian and others conducted a process study on high-quality quartz preparation of high-purity ultra-fine silicon micropowder, using a vibration grinding and grading system to produce products that meet the requirements of the electronic and electrical grades and coatings industry.

As early as in the 1970s, foreign countries began to study the technology of using quartz minerals to prepare high-purity quartz sand. At present, the United States is at the leading level, characterized by large industrial output, specialization in preparation, high degree of automation, high detection level and stable product quality. The direct purification of quartz from natural rock minerals is the most advanced technology in the world for the production of high-purity quartz powder. By the 1990s, Kemmochi and Stato were processed from ordinary quartz into quartz powder for high-grade quartz glass.
Russia, Japan and Germany are basically self-sufficient. In addition to Brazil's export of unprocessed crystal ore, the world's high-purity quartz sand market is basically controlled by UNIMIN Corporation. Unimite's “IOTA” trademark is recognized by the world's quartz glass manufacturing companies as the most famous trademark. Its purity is regarded as the international standard purity, and it is also the measure of the quality of other manufacturers in the world. Its products have been developed to the sixth generation, the transparency is optical grade, and the content of silica is currently developing from 99.9992% to 99.9994%.
China's high-purity quartz powder is basically made of crystal or crystal scrap. The crystal reserves in China are limited, the price is expensive, and the texture is not uniform. Some mineral impurities and impurities mixed in the process cannot be removed, resulting in high production by crystal. Pure quartz powder has small output and unstable quality. Moreover, so far, the domestic technical problems of extracting high-purity quartz powder from natural rocks have not been completely solved in China, and industrialization cannot be achieved. Therefore, on the one hand, the demand for high-purity quartz powder is increasing, and on the other hand, it is the short board of silicic material purification technology, which makes the research on the preparation process of high-purity silicon micropowder urgent.

For natural minerals, the conditions of the minerals vary greatly, and the defects in the inclusions and cracks are particularly obvious. The above description is not repeated here. In the purification of natural minerals, the research in the literature mostly uses the water-quenching method to induce a huge volume change by the crystal transformation between quartz crystals, and through the rapid cooling, the defects inside the mineral are expanded, resulting in inclusions and fissures. The contained impurity minerals are exposed to provide conditions for further purification.

The quality requirements of high-purity quartz powder are extremely strict on the total content of impurities, but the content of iron and aluminum is particularly harsh. There are basically five forms of iron in the natural minerals: 1 the state of the particles is present in the feldspar of clay or kaolin ; 2 the state of the iron oxide film exists on the surface of the quartz particles; 3 the iron or iron-bearing mineral 4 diffused state exists inside the quartz mineral; 5 exists in the solid crystal state inside the quartz crystal. According to the different forms of iron impurities, there are generally several ways to remove them:
(1) Screening and grading
Different application fields have different particle size requirements for quartz powder, and the particle size index can be controlled by screening and classification. At the same time, the classification process can play the role of de-sludge and reduce the iron content. It is found that the content of silica decreases with the decrease of the particle size of quartz powder, and the iron content is opposite. At present, in the developed countries such as the United States and Japan, due to the high degree of automation of the beneficiation process, the hydrocyclone is used for the classification of quartz sand, and good results have been achieved.
(2) scrubbing
Scrubbing is the purpose of preliminarily purifying the iron-containing film and impurity minerals on the mineral surface by mechanical force and friction between mineral particles. There are mainly mechanical scrubbing, ultrasonic scrubbing and rod scrubbing. In the study of scrubbing, the content of iron in the minerals after grinding and scrubbing is reduced from 0.19% to 0.10%, and the removal rate is as high as 47.4%. The research of the Soviet Raman concentrator shows that the ultrasonic-flotation-magnetic separation process can reduce the iron content to 0.1%, which has better impurity removal effect.
(3) Magnetic separation
The magnetic separation is divided into dry type and wet type. The main principle is to remove the iron-containing impurities and the magnetic impurity minerals by high-strength magnetic field to achieve the purpose of purification. It can be known from the related literature that as the strength of the magnetic field increases, the impurity removal rate increases, but the impurity removal rate no longer increases after the magnetic field strength increases to a certain intensity; the content of iron-containing impurities increases with the number of magnetic separations. Reduced; the smaller the particle size of the mineral powder, the better the effect of removing impurities.
(4) pickling
Since SiO ₂ is insoluble in an acid other than hydrofluoric acid, and other mineral impurities can be dissolved by the acid solution, the purpose of further purifying the quartz mineral can be achieved by utilizing this feature. When natural quartz minerals contain iron films or adhere to the surface of quartz particles in the form of inclusions, it is necessary to use acid to dissolve these harmful components. Other methods are generally difficult to remove, so pickling is irreplaceable in the existing purification process. Purification means and can get good impurity removal effect. Usually, pickling uses sulfuric acid, oxalic acid, hydrochloric acid, hydrofluoric acid, and the like.
According to the data, the effect of pickling and impurity removal is related to the concentration of acid solution, temperature and time, and other factors have less influence. For domestic pickling, it is still in the stage of trial and small-scale trial production, and the acid leaching theory of the system has already been formed in foreign countries, and the prepared products are also very stable in quality. In recent years, some emerging purification processes have emerged, such as ultrasonic pickling and microwave pickling, which have achieved good experimental results in laboratory research.
(5) Microbial treatment
There are many kinds of microorganisms, and one or more species are found in many microorganisms. The use of the secreted metabolites to dissolve the harmful impurities on the surface of quartz minerals to achieve purification has been mentioned many times in foreign related literature. It shows that when the iron-containing film on the surface of the quartz particles is soaked with a culture solution such as polymyxin, aflatoxin, penicillium or pseudomonas, a good removal effect is obtained, wherein the iron oxide of the aflatoxin is obtained. The removal rate is as high as 75% or more, which also provides a new idea for the preparation of high-purity quartz micropowder.

The feldspar, mica or clay minerals contained in quartz minerals are the main source of aluminum. The clay minerals can be removed by the aforementioned scrubbing method, but the flotation process can be used for the removal of feldspar and mica minerals. Flotation is divided into two ways: fluorine and fluorine-free flotation. Commonly used fluorine flotation is the use of sulfuric acid as a pH adjuster, DWBE and SHN as mixed collectors or hydrofluoric acid using hydrofluoric acid as activator, amine cation collector and YS or SHN for harvesting. Anionic flotation of the agent.
Many studies have found that fluorine flotation can remove feldspar from quartz minerals well. Schaper.E reported that quartz ore in Germany is treated with sulfuric acid and hydrofluoric acid as regulators, and two stages of reverse flotation remove heavy minerals and long Stone and other impurities. However, environmental problems have emerged in recent years. In order to reduce the use of HF, the fluorine-free process has been greatly developed. In 1976, the United States SG Myrham first proposed fluorine-free flotation. According to research by Tomita Kenji, it is known that high-grade aliphatic ammonium salts and sodium petroleum sulfonate are used as mixed collectors in sulfuric acid or hydrochloric acid medium, which can separate quartz and feldspar well.
The fluorine-free flotation experiment of the Glass- lined Design Institute has also achieved success and made a high contribution to the development of China's mineral processing industry.

There are many kinds of impurity elements in natural quartz minerals, and different methods are needed to remove different impurities. The slight difference in electrostatic separation method using a quartz mineral impurities on the electrical properties of the selected trace metal impurity minerals; thermal decrepitation is the inclusion of silica is heated to a certain temperature minerals explosion, wrapped The impurities in the body are removed; the thermal chlorination method removes the metal inclusions in the quartz mineral bubbles.

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