High-purity quartz sand is the material basis for the development of high-tech industries, and its application fields involve optical fiber, military, and aerospace industries. These fields have extremely strict requirements on the purity of quartz sand raw materials, requiring very low impurity content in quartz sand, especially for Fe, Al and other impurities. Therefore, quartz sand purification technology is related to the long-term development of my country’s high-tech field. At present, the purification technology of high-purity quartz sand is monopolized by the United States, Germany and other countries, and the export of purification technology and products is restricted. Therefore, it is of great significance to master the purification technology of high-purity quartz sand.
Mineral impurities in quartz sand usually exist in the form of non-quartz minerals, such as feldspar, mica, garnet, zircon, ilmenite and many other minerals. These impurities mainly exist in the following ways:
(1) As a loose associated mineral, it is not chemically combined with the quartz crystal;
(2) As mineral fragments, they are chemically and physically combined with quartz crystals on their surface, such impurities are mainly iron-containing minerals and aluminum-containing minerals;
(3) Minerals encased in quartz grains or surrounded by interbonded quartz crystals;
(4) As interstitial ions to replace silicon, these impurities mainly include: Al3+, Fe2+, Fe3+, B3+, Ti4+, Ge4+, P5+, etc. These ions replace Si4+ to form covalent bonds. When this happens, it is usually accompanied by doping of elements such as Li1+, K1+, Na1+ and H1+ to maintain the electrical neutrality of the SiO2 lattice. Al element is one of the main impurity elements in quartz ore, and Al3+ and Si4+ have similar radii, which can easily replace Si4+, and its content is usually as high as thousands of ppm, so Al content is an important indicator of quartz ore quality.
At present, the purification process of high-purity quartz sand mainly includes mechanical crushing, magnetic separation, flotation, acid leaching, calcination, water quenching, high-temperature chlorination roasting, etc. The method can effectively remove the metal ion impurities in the quartz lattice.
1. Mechanical crushing
Mechanical pulverization is a method of reducing the particle size of minerals by using mechanical force. In the high-purity quartz purification process, this process is mainly to separate the non-structural impurities in the quartz mineral from the quartz. The non-structural impurities refer to mineral inclusions (mineral impurities) and gas-liquid inclusions (fluid inclusions). Impurities exist in the quartz grain boundaries. After the raw quartz minerals are crushed, the particle size decreases and the specific surface area increases, so that the impurities between the grain boundaries are exposed on the outer surface of the quartz particles, thereby improving the purification efficiency of the subsequent process.
In the process of mechanical crushing, due to the relatively hard ground of quartz minerals, frequent contact, impact and friction with equipment will inevitably introduce impurities and cause pollution. The most effective solutions to this problem are: self-grinding and high-pressure pulse crushing technology.
2. Magnetic separation
The principle of magnetic separation is based on the different magnetic properties of quartz minerals, and it is a means of sorting impurities from quartz ore to separate magnetic mineral impurities. In the purification process of high-purity quartz, the use and purpose of magnetic separation is to remove some magnetic minerals in the inclusions of quartz raw ore with magnetism, such as magnetic ilmenite, pyrite, limonite and garnet, etc. Some particles with magnetic mineral inclusions can also be removed by magnetic separation. Iron-containing impurities, especially magnetic substances, are easily magnetized effectively, while quartz minerals, especially non-metallic magnetic substances, cannot be effectively magnetized. Using this differential magnetic separation method, a large amount of magnetic quartz impurities can be separated from high-purity quartz metal Separation and removal of minerals.
3. Flotation
Flotation is the selective separation of hydrophobic and hydrophilic substances based on the difference in wettability of the ore body surface, whether natural or modified. In the purification process of high-purity quartz, flotation is mainly used to remove mica and feldspar minerals that are symbiotic with quartz, and can also float phosphorus- and iron-containing minerals. There are many types of flotation agents, which can be divided into collectors, regulators, and foaming agents according to their properties and functions in the medium and operating environment of quartz sand flotation.
According to the different reagents used, quartz sand flotation can be divided into fluorine-containing quartz sand flotation and fluorine-free quartz sand flotation. Flotation of fluorine-containing quartz sand uses fluorine-containing agents, such as hydrofluoric acid (HF) as feldspar activator, sulfuric acid as regulator, so that under strong acidic conditions of pH = 2 to 3, dodecylamine, etc. The cation is used as a collector, and the activated feldspar is pre-adsorbed and then separated. Similarly, fluorine-free quartz flotation is to use sulfuric acid or hydrochloric acid as the activator of impurity minerals in quartz without using fluorine-containing agents, and then use corresponding collectors to separate quartz and impurity minerals by flotation. In addition, studies have shown that the flotation effect of mixed collectors is better than that of single collectors and is relatively cost-effective.
4. Pickling
Acid leaching is a means of purifying quartz according to the different dissolving abilities of quartz, mica and feldspar in acidic solution. Acid leaching can effectively remove oxide films on the surface of iron ore and quartz particles in quartz. For mineral impurities such as mica and feldspar, hydrofluoric acid is generally used for dissolution. Commonly used acidic media for pickling include hydrochloric acid, sulfuric acid, nitric acid, acetic acid and hydrofluoric acid, among which dilute acid has a better effect on removing Al and Fe, and more acidic concentrated sulfuric acid, aqua regia and hydrofluoric acid are used for the removal of Cr and Ti .
5. Heat treatment
In the purification process of high-purity quartz, heat treatment refers to high-temperature calcination of quartz (800-900°C). Heat treatment has two effects in the quartz purification process. First, the quartz mineral is hard, with a Mohs hardness of 7, which is a refractory mineral. However, there is anisotropy in the internal crystal structure of quartz minerals, and the expansion coefficient of impurities such as muscovite and silicate minerals in the minerals is different from that of quartz. Therefore, after high-temperature calcination and water quenching, quartz minerals will expand rapidly when heated, and stress concentration will be caused by rapid cooling and emergency contraction, resulting in cracks in the minerals, thereby realizing the crushing of quartz. In addition, high-temperature calcination can effectively destroy the structure of muscovite and silicate quartz minerals, and further promote the fragmentation of quartz and the exposure of internal impurities, which is beneficial to the subsequent acid leaching and other processes.