The quality of quartz is not a simple correspondence with the content of impurity elements in the raw material, but is closely related to the selectivity of impurities determined by the mineralogical characteristics of the raw material process.
The impurity elements contained in quartz mainly include Al, Fe, K, Na, Ca, Mg, Ti, Li, Cr, Ni, Cu, B, Mn, P, etc. Quartz can be classified according to the size, distribution, existence form and other characteristics of the impurities. Medium impurity elements are divided into three categories: independent gangue mineral impurities, inclusion impurities and lattice homogeneous isomorphic substitution impurities.
1. Gangue minerals
Mica, feldspar, hematite, tourmaline, chlorite and clay minerals are one of the main sources of impurity elements in quartz. They can easily become mineral inclusions in quartz during the geological mineralization process. Quartz gangue minerals can be effectively separated through technical means such as mineral processing and chemical purification.
The embedding characteristics of gangue minerals in quartz have a significant impact on the quartz purification effect. The greater the intensity of quartz transformation by diagenesis and metamorphism, the more obvious the embedding difference between quartz and gangue minerals, and the embedding characteristics gradually change from adjacent type to Slit-shaped or even wrapped-shaped, etc., significantly increase the difficulty of separation of quartz and gangue minerals.
2. Inclusion impurities
Fluid inclusions are widely present in minerals or rocks. The number of fluid inclusions per cubic centimeter is about 102 to 109, and the diameter is generally less than 50 μm. Fluid inclusions in quartz can be divided into pure gas, pure liquid, gas-liquid mixed inclusions and three-phase inclusions according to the state of the contained materials.
The fluid captured during the formation process of the fluid inclusion is a supersaturated solution. When the temperature is lowered, it will crystallize from the solution to form daughter minerals including halite, potassium salt and some silicate minerals. Therefore, the fluid inclusion contains Na , K, Ca and other impurities are one of the main sources of impurity elements in high-purity quartz.
The main methods for separating fluid inclusions in quartz sand include: mechanical crushing method, differential corrosion method, high-temperature bursting method, high-temperature thermal chlorination method, etc.
The size of fluid inclusions in quartz is generally between 1 and 50 μm. As long as the quartz is crushed to a fine enough size, most fluid inclusions in quartz can be separated. However, considering the particle size requirements of quartz sand, this method is not universally applicable. properties; fluid inclusions distributed in quartz cracks are easier to separate from quartz through mechanical crushing.
The differential corrosion method uses fluid inclusions to exist at defects in quartz, making the fluid inclusions more easily destroyed and dissolved by acid solutions, which plays a certain role in separating fluid inclusions in quartz.
The high-temperature bursting method uses the difference in thermal expansion coefficients between quartz and fluid inclusions to separate quartz and fluid inclusions. During the high-temperature roasting process of quartz, when the internal pressure of the fluid inclusion is greater than the binding pressure of the quartz on the inclusion, the fluid inclusion will burst suddenly to release the internal impurities, and then acid cleaning can effectively separate the fluid inclusion impurities in the quartz.
Research shows that the composition, size, location and shape of fluid inclusions all affect the high-temperature bursting behavior of fluid inclusions. The burst temperature of liquid-rich inclusions is generally slightly higher than the uniform temperature, while the gas-rich inclusions can remain at higher temperatures without bursting; the burst internal pressure of fluid inclusions is closely related to the size of the fluid inclusions, and for large quartz For most fluid inclusions, the burst internal pressure of 5 μm ~ 10 μm fluid inclusions is generally less than 300Mpa, while for some smaller fluid inclusions, the burst internal pressure is as high as 500Mpa or more. The burst temperature of large fluid inclusions is generally lower. , and for some very small fluid inclusions, they will not burst even when heated to very high temperatures; for fluid inclusions of the same size, inclusions located on the surface of quartz particles are more likely to burst under low pressure than inclusions inside. The burst internal pressure of irregularly shaped inclusions in quartz is lower than that of regular-shaped inclusions. When fluid inclusions explode at high temperature, the increase in internal pressure is mainly achieved through temperature increase. Appropriate selection of higher roasting temperatures is beneficial to the separation of fluid inclusions in quartz.
3. Lattice isomorphism replaces impurities
During the formation process of quartz crystal, some elements will replace the silicon element and enter the quartz crystal, forming structural impurities in quartz. There are three main ways of existence:
(1) Equivalent substitution, such as isomorphic substitution of Ti4+, Ge4+, etc. with Si4+;
(2) Ion group substitution, such as Al3+ and adjacent P5+ replacing Si4+;
(3) Charge compensation substitution, such as Al3+, Fe3+ replacing Si4+ to form [AlO4/M+]0 or [FeO4/M+]0 structural center, M+ acts as the price compensation ion balance charge, here M+ is mainly H+, Li+, Na+, K+.
At present, technologies such as acid leaching and atmosphere roasting are mainly used to separate lattice structure impurities in quartz. Although the content of these impurity elements in quartz is very low, it is very difficult to separate and remove them from quartz, which is a highly restrictive The most critical factor in the quality of pure quartz sand.
During the acid leaching process, the bond energy and properties of the metal-oxygen (Me-O) bonds in the quartz lattice determine the ease of leaching. Me (Li+, Na+, K+)-O has the smallest bond energy and is the easiest Destroyed, but because alkali metal ions play a role in balancing charges in quartz, they cannot be effectively leached and separated; Me (Fe3+, Cu2+, Ca2+, Mn2+, etc.)-O bond energy is second, and it is the lattice in quartz that is easier to leached and separated. Impurity elements; Me (Al3+, Ti4+)-O bond energy is large, Al and Ti replace Si in the quartz lattice to form new [AlO4], [TiO4], which is the most difficult lattice impurity element in quartz to be leached and separated.