Quartz can be refined from a variety of pure natural quartz (such as crystal, quartz sand, etc.), or it can be synthesized by silicon-containing materials such as silicon tetrachloride through hydrolysis. According to the purity of quartz, it can be divided into two types: transparent quartz and opaque quartz.
Natural quartz raw materials: high-purity quartz sand, purification technology is the key
Natural quartz sand is made from quartz ore. Quartz ore needs to be crushed, screened, pickled, washed, and calcined to obtain quartz sand. The quartz sand is then made into quartz and its products through a smelting process. Quartz sand can be divided into four grades: ordinary quartz sand, refined quartz sand, high-purity quartz sand, fused silica sand, etc. Among them, high-purity quartz sand usually requires SiO2 ≥ 99.998% and Fe ≤ 1ppm.
Quartz ore is widely distributed in China. It has been proven that the reserves of high-grade quartz ore in 2015 are about 3 billion tons. According to China Powder Technology Network, in 2015, the domestic proven high-grade quartz mine reserves were about 3 billion tons. Domestic quartz ore is mainly distributed in Guangdong, Guangxi, Hainan, Jiangsu, Fujian and other places. From a global perspective, the East China Sea in Jiangsu is the main production area of high-grade quartz ore raw materials, which can stably produce more than 99.99% of quartz sand. Some high-purity quartz sands are usually extracted using quartz ore such as silica, which is cheaper and more widely distributed.
A few companies, such as the United States and domestic quartz stocks, have mastered production technology suitable for production. The production of high-purity quartz sand is a systematic project, which not only requires related purification production equipment, but also requires a complex process that is compatible with China's unique quartz mineral resources. Pure quartz sand has high requirements for the quality of the mine; at present, only a few companies, such as the United States and domestic quartz shares, have the ability to produce high-purity quartz sand on a large scale. United States of America has high-quality white granite minerals, large ore body size, low fluid impurities, and stable quality. Its IOTA-STANDARD grade quartz sand is widely recognized as the standard product of high-purity quartz sand worldwide, and its product purity index is recognized as For "International Standard Purity". The quartz diagenetic conditions in China are different from those in the United States. Quartz ore has the disadvantages of more fluid impurities, smaller ore body size, and unstable ore quality. Compared with the United States, domestic quartz sand purification technology is more complicated.
Preparation of Quartz: Controlling Purity and Hydroxyl Content
At present, the traditional quartz preparation methods include electric melting method, gas refining method, direct synthesis method, indirect synthesis method, plasma chemical vapor deposition and sol-gel method. The electrofusion and gas smelting methods refer to the use of natural crystalline quartz (crystal or pure silica) to make quartz through melting; the remaining four are synthetic methods, which are made of silicide (SiCl4) through chemical vapor deposition to make quartz. Due to the different preparation methods of quartz, quartz behaves differently in terms of SiO2 purity, hydroxyl content and other indicators, resulting in different material characteristics.
Preparation method of natural quartz:
1) Electrofusion method
Quartz raw materials are fused in a vacuum (5 × 10-2mmHg) or low-pressure inert (N2) medium.
Continuous electric melting: quartz sand is melted in a tungsten crucible with electric heating elements; the molten material flows from the furnace through the bottom mold to form a quartz tube, a quartz plate and a quartz rod. Through this production method and the selection of suitable raw materials, high purity, good mechanical properties, temperature resistance, transmittance and low hydroxyl content can be obtained;
Vacuum electrofusion: Put quartz powder in a graphite crucible, heat it in a vacuum electrofusion furnace, and melt under vacuum to generate quartz ingots. Then the smelted quartz gadolinium is reheated and processed into various quartz products such as rods, tubes or plates;
An arc method was used to make a quartz crucible.
2) Gas refining method
The flammable gas such as oxyhydrogen flame or other hydrocarbon (acetylene) is used to generate high heat to melt the material.
Two-step gas refining: Quartz ingots are heated by melting with hydrogen and oxygen flames. These quartz ingots can be processed by electrofusion to produce quartz tubes, quartz rods, quartz wafers and squares. These quartz tubes have high light transmittance, which guarantees that there are basically no bubbles in the product.
Preparation method of synthetic quartz:
3) Direct Synthesis
Because it is prepared by the hydrolysis process, quartz contains more water, hydroxyl (OH-) is about 500-1500ppm, and it contains H2 dissolved in the glass structure. Because of the presence of hydroxyl groups, the heat resistance of the glass becomes poor, and its absorption ability in the infrared region is strong. This type of glass is not suitable for semiconductor heat treatment equipment and optical fibers. However, due to the presence of hydroxyl groups, the glass has a high ultraviolet light transmittance and can be irradiated with ultraviolet laser light for a long time, so it is suitable for making substrates for lithographic masks and optical lenses in laser lithography systems.
Hydrogen-oxygen flame hydrolysis: Bubbling method is usually used to bring silicon tetrachloride liquid into the burner through the charging gas, and then the hydrogen-oxygen gas mixed combustion generates a high-temperature flame to hydrolyze and melt the gaseous silicon tetrachloride, which is generated after the reaction. Silica is deposited on the substrate.
4) Indirect synthesis
The raw materials used in this method are also SiCl4, H2, and O2 gas, but this method first allows SiO2 particles to accumulate into rods, and the resulting rods have a porous and opaque structure, and can be made into transparent glass rods after sintering.
Compared with the direct method, the biggest advantage of the indirect method is that it can control the content of -OH in the glass, which is suitable for preparing synthetic quartz without -OH. Because the hydroxyl group has strong absorption in the infrared region, the quartz optical fiber preform as the optical fiber substrate cannot contain water, that is, the content of -OH is required to be less than 1 ppm.
Chemical vapor deposition (CVD): SiCl4 gas is used as a raw material to deposit SiO2 particles in the tube to form a porous rod, which becomes a transparent glass rod after sintering;
Outside vapor deposition method (OVD): SiO2 particles are accumulated outside the tube, and the quartz rod in the center is removed to form a porous rod, which becomes a transparent glass rod after sintering
Vapor-phase axial method (VAD): H2, O2 and SiCl4 are introduced into the reaction chamber to form a large-diameter solid SiO2 particulate porous rod vertically. After Cl2 removal of water and sintering, a large-size optical fiber preform is formed.
5) Plasma melting method
Plasma method is also one of the methods for the synthesis of -OH-free quartz. The plasma torch is generated by the inductive coupling of a high-frequency electromagnetic field. The core temperature is as high as 15000K, and the melt is melted. This kind of glass contains oxygen and chlorine. It is not resistant to ultraviolet light and is not suitable for ultraviolet optical materials in the semiconductor industry. However, because it contains almost no -OH, it can be used for glass tubes used in the CVD method of internal deposition.