Present situation and development trend of quartz sand purification process

With the rapid development of high-tech industries such as optical power supply, electronic industry, optical communication, SiO2 film materials, large-scale and super large-scale integrated circuits, laser, aerospace and military industry, the demand for high-grade quartz raw materials is increasing. However, due to the high quality requirements of these special quartz raw materials, the required content is usually more than 99.9%, or even 99.99%, and the allowable impurity content is very low. Only high-grade natural grade I and II crystals can meet the requirements. Natural crystal resources are drying up day by day, especially high-grade natural crystal resources are scarce, and their distribution is extremely uneven all over the world. 95% of crystals are concentrated in Brazil and Madagascar, making other countries strive to find alternatives.

Quartz sand Weifang guote mining equipment Co., Ltd

At present, there are three ways to solve crystal substitute materials: artificial crystal, sol-gel and four silica gas phase precipitation method. Due to the low output, high energy consumption and high cost of artificial crystal and synthetic method, it is difficult to produce on a large scale. Using a large number of silica in nature (including quartzite, quartz sandstone and vein quartz as the main minerals) to prepare high-purity or ultra-high-purity quartz raw materials after purification has the advantages of wide source, low production cost and large batch. It is the most effective way to replace natural crystals.

01

Processing status of high purity quartz raw materials replacing crystal at home and abroad

China began to study the purification method and process of ordinary quartz raw materials for quartz glass in the late 1980s, and has made some progress. The products studied can reach the level of grade II, III and IV crystals. However, due to the limited reserves, high price and uneven texture of crystals in China, some mineral impurities and mixed substances in the process cannot be removed.

At present, Lianyungang Area of Jiangsu Province is at the highest level in China to produce ultra-pure quartz sand with crystal as raw material, but a large number of industrial production, the uniformity of mineral structure and the chemical content of internal quality are unstable. It is only suitable for medium and low-grade quartz glass. The technical difficulties of high-purity and low hydroxyl quartz raw materials have not been overcome. China still needs to import a large number of large-diameter quartz tubes, high-quality quartz rods and quartz ingots, as well as quartz glass used in optical communication.

As early as the 1970s, foreign countries began to study the technology of preparing high-purity quartz sand from quartz sand. In the 1980s, American PPCC company purified quartz from granite in foxdale area on the northwest coast of England, and the content of SiO2 in the product reached 99.5% 99%, Fe impurity less than 1 × 10-6, other transition elements less than 5 × 10-6； From the 1990s to now, unimin company, the largest supplier of high-purity quartz in the world, sorted and purified high-purity quartz from granite in spruce pine area, North Carolina. After purification, the content of SiO2 reached 99.5% More than 99%, and the content of SiO2 with the highest purity is 99 More than 999%, and its high-purity quartz occupies a monopoly position in the raw material market at home and abroad. The purity of silicon oxide is currently increasing from 99 9992% to 99.9994%.

On the basis of traditional processes, kemmochi and Sato in Japan comprehensively use physical and chemical purification technologies such as flotation, magnetic separation, electric separation and high-temperature chlorination to process vein quartz into ultra-high-purity quartz glass raw materials, so as to meet the needs of high-grade quartz glass production for optical, optical fiber casing and electronic industry.

02

Occurrence state of silica impurities

In addition to the main mineral quartz, silica is usually accompanied by impurity minerals such as feldspar, mica, clay and iron. The prepared high-purity and ultra-high-purity quartz raw materials are all impurities except silica, and the main harmful impurities are iron and aluminum impurities. Therefore, the progress and improvement of the purification method and process flow of silicon raw materials are mainly reflected in the effective removal of iron and aluminum impurities.

Iron often exists in silica in the following forms: it exists in clay or kaolinized feldspar in the state of fine particles; Attached to the surface of quartz particles in the form of iron oxide film; Contained in particles such as heavy minerals and iron minerals; It is in the state of immersion or lens in the interior of quartz particles or in the interior of quartz crystal in solid solution state. In addition, a certain amount of mechanical iron will be mixed in the processing process.

Aluminum containing impurities mainly come from feldspar, mica and clay minerals, and Al3 + replaces si4 + in quartz lattice. This substitution of heterovalent isomorphism often causes alkali metal cations to enter the structural gap to maintain the balance of electrons and form structural impurities.

In addition, fluid inclusions are common in silica, which can be divided into primary inclusions, pseudo secondary inclusions and secondary inclusions according to their genesis. Primary inclusions are inclusions formed before or at the same time with the main minerals, which is characterized by no spatial movement after the formation of inclusions. The primary inclusions occupy the crystal structure of the main minerals and are evenly distributed in the crystals.

Pseudosecondary inclusions are inclusions formed when the crystallized minerals are broken and cracked due to stress and tectonic action in the crystallization process of main minerals. In these fractures, the ore-forming solution re enters and produces recrystallization. It is characterized by displacement in space after formation. The outer end of pseudosecondary inclusion terminates at a growth surface in the crystal, and there is an obvious arrangement surface.

Secondary inclusions are inclusions formed in any process after the crystallization of main minerals is basically completed. After the formation of crystals, they break due to the influence of external forces and produce cracks. At this time, the ore bearing solution active in the environment may penetrate into the crystals and become inclusions. Secondary inclusions are generally distributed along fractures at the position of late structural healing, and several groups of inclusions can intersect, with complex shapes.

The fluid volume is very small, generally with a diameter of about microns. When crushing quartz ore, the secondary inclusions are easy to be mechanically broken, but the primary inclusions are difficult to be broken and eliminated. That is, the use of high-temperature roll firing can only burst the local gas inclusions on the surface, which is not enough to change the state of internal micro bubbles. The small molecule gas in the fluid inclusion can be discharged through high temperature and prolonging the exhaust time. However, Co, CO2 and other gases are very difficult to be discharged from the solid or melt, resulting in defects in the molten products.

03 beneficiation and purification process

According to the occurrence state of impurities and inclusions in silica mineral raw materials, the beneficiation and purification process is mainly divided into scrubbing magnetic separation flotation acid leaching and other process flows. With the deepening of beneficiation process research, electric separation and biological beneficiation are introduced.

The beneficiation and purification process flow of quartz sand is determined according to the occurrence state of impurity minerals in raw material ore, beneficiation cost and the requirements of industrial use of products, and is jointly formulated according to certain processes.

At present, the most mature and widely used industrial method is the "fluorine-free acidic" method. The medium, alkaline and other methods are only limited to laboratory research, but due to its own advantages, it has a good application prospect. Therefore, its industrial application research will be the direction of development in the future.

Although great progress has been made in the research on the beneficiation and purification process of silica minerals, the development is very slow, and less than 10 papers in this field are published every year. The main factors restricting the development of the beneficiation and purification process of silica instead of crystal are: first, the cost and demand accuracy of the assay and analysis of SiO2 and trace element content are too high to be widely studied; Secondly, the removal technology of inclusions is not ideal. Only a few silica minerals with a small amount of inclusions may replace natural crystals to produce high-purity or ultra-high-purity quartz powder after beneficiation and purification. The vast majority of silica minerals with more inclusions can not be used for deep processing to produce high-purity or ultra-high-purity quartz powder.