During the formation of quartz crystal, some elements will replace silicon into the quartz crystal, forming the structural impurities of quartz. Although the content of these impurities is very low, it is difficult to separate from quartz, which is the most critical factor restricting the quality of high-purity quartz.
Among the quartz structural impurities, the content of Al impurity element is generally the highest. Since Al exists in the form of Al3+ instead of Si4+, the internal charge of the quartz lattice is unbalanced. When there is a large amount of Al impurities in the quartz, the content of Li, K, Na and other impurity elements will increase. Therefore, the content of Al in natural quartz can be used to judge the quality of quartz raw materials.
Under the existing processing technology, the lattice impurities in the quartz raw material can hardly be removed. Although the content of Al in the form of lattice impurities is extremely low, it is extremely difficult to remove, which is the key to restricting the final quality of high-purity quartz.
In the whole process of purification, the impurity elements Fe, Cr, Ni, Na, K, Ca, Mg, Cu, etc. in quartz can be greatly reduced after roasting, water quenching, magnetic separation, and acid leaching. However, after a series of purification processes, the removal effect of Al is limited, mainly because Al3+ enters the lattice to replace Si4+, and the ionic radius is relatively close, so it is not easy to purify.
There are similar impurity elements such as Ti4+, B3+, and P3+. It can be seen that the impurities in the natural quartz, especially the impurities in the state of the same quality, directly restrict the production of high-purity quartz products. High-purity quartz.
A typical example is the quartzite in the Norway area. Although it is associated with various minerals such as kyanite, it has few lattice impurities and almost no fluid inclusions, and is considered to be likely to be processed into high-purity quartz; while Sierra de Comechigones ( Although the pegmatite in Argentina) has a high content of SiO2, it is difficult to be processed into high-purity quartz due to the high lattice impurities in fine-grained quartz.
In summary, lattice impurities are the most critical factor restricting the quality of high-purity quartz.
The (Al3+, Ti4+)-O bond energy in the lattice impurities is relatively large, and Al and Ti replace the Si in the quartz lattice to form new [AlO4] and [TiO4], which are the most difficult lattice impurity elements in quartz to remove. .
Lattice structure impurities are the ultimate bottleneck that is difficult to break through in the processing of high-purity quartz products. To select the correct high-purity quartz raw material and formulate the best quartz purification scheme, the occurrence state of impurity elements in the quartz must be clarified. The current methods for studying the occurrence state of impurity elements mainly include:
(1) X-ray diffraction data fitting and refinement analysis;
(2) LA-ICP-MS surface scan image analysis;
(3) Infrared absorption spectrum analysis;
(4) Electron probe and energy spectrum analysis;
(5) Analysis of cathodoluminescence characteristics.
At present, the chlorination roasting process is a more effective method to remove quartz lattice impurities. Chlorination roasting is that at a temperature lower than the melting point of quartz, the impurity components in quartz and the chlorinating agent are converted into chlorides and volatilized. During the high temperature chlorination roasting process, there is a crystal transformation of quartz, which makes the crystal lattice in the quartz crystal change. Metal ions such as Al3+, Ti4+, etc. may migrate and diffuse to the quartz surface, and chemically react with HCl, NH4Cl and Cl2 to become volatile components to achieve separation from quartz, and also prevent impurity elements from re-migrating and diffusing during the cooling process. into the quartz lattice.