Methods for removing impurities from quartz raw materials.
Calcination - water quenching
Quartz is calcined and water quenched. In the process of calcination of quartz at high temperature, as the temperature increases, the quartz crystals will expand, prompting a large number of cracks on the surface of quartz. Cracks usually occur at interfaces and defects in the crystal structure, and by changing the temperature and time of calcination, the inclusions are cracked and impurities are exposed to new surfaces, so that they can be effectively removed in subsequent processes [1,4,6].
After roasting the quartz sand raw materials, the removal rate of impurity iron reached 41% as the temperature increased to 900 °C and the holding time was 2 h, and the water absorption peak of the quartz raw materials was greatly reduced, indicating that the inclusions inside the quartz sand were removed after roasting and achieved the purification effect [7]. Washing-grading-desilting-scrubbing
During the grinding process of quartz ore, some easily muddled minerals form fine-grained sludge, which will have an adverse effect on subsequent separation[4], and water washing and graded desliming have a good effect on the removal of soil minerals attached to the surface of quartz ore [1].
Scrubbing is to remove the film iron, viscous and turbid impurity minerals on the surface of quartz sand under the action of mechanical external force and abrasive grain peeling force between sand grains, and then achieve the impurity removal effect of quartz sand through the desliming process. Process factors such as the structure of the scrubbing machine, the scrubbing time, the concentration of the scrubbing sample, and the number of scrubbing all affect the final scrubbing effect. Scrubbing techniques are mainly divided into rod grinding scrubbing and mechanical scrubbing. Mechanical scrubbing can be applied to enterprise processing, can produce a large number of products, and is simple to operate, low cost, the disadvantage is that the iron removal rate is low. In the rod grinding and scrubbing process, appropriate chemicals are added to increase the electrorepulsion force on the surface of impurity minerals and quartz particles, and enhance the separation between impurity minerals and quartz particles. Compared with mechanical scrubbing, rod abrasive scrubbing is more effective than mechanical scrubbing, but it cannot be applied in mass production[2].
Gravity separation-magnetic separation-flotation
Gravity separation is the screening of minerals according to the different gravity of the minerals. Due to the different densities of mineral grains, they are affected by different fluid and mechanical forces in the medium, resulting in loose layering, so that the ore particles are separated [8].
Magnetic separation is to use the difference in the magnetic permeability of impurity minerals in quartz to separate magnetic impurity minerals under the action of magnetic field. The quartz in quartz sand is a diamagnetic substance, which cannot be magnetized in the magnetic field, and most of the impurities containing Fe and Ti are paramagnetic substances and can be magnetized, so that the impurities containing Fe and Ti can be removed through magnetic separation, and a high quartz sand content can be obtained. The factors affecting the effect of magnetic separation include magnetic field strength, slurry concentration, ore particle size, etc. [8].
Flotation is the use of the natural or hydrophobic nature of the mineral surface to stir the sample to create bubbles, which then bring the ore particles to the bubble layer for purification. At present, there are three methods of quartz flotation: fluorine-free acid method, fluorine-free acid-free method, and fluorine-acid method. According to the types of impurity minerals, it can be divided into positive flotation and reverse flotation. Due to the presence of a large amount of mica and feldspar in quartz, their surface properties are the same as those of quartz, which is more difficult in the flotation process. Although it can be removed, it is only removed on the surface[2]. The main factors affecting the flotation effect are the selection of flotation reagent, the concentration of the slurry, the pH value of the slurry, the particle size of quartz, etc.
The magnetic separation and flotation processes have high requirements for the dissociation degree of the monomer, and have a good purification effect on the crushed to medium-fine grains of quartz, which is generally used for the production of quartz powder or as a pre-treatment process for the production of quartz sand, but is not suitable for quartz ores with fine embedded grain size and complex embedding relationships [9]. Acid leaching
Mineral inclusions mixed acid dissolution uses the characteristics that quartz can only be dissolved in hydrofluoric acid, and other mineral inclusions impurities can be dissolved by acid to achieve the separation of quartz and impurities, and the commonly used acids are sulfuric acid, hydrochloric acid, nitric acid, hydrofluoric acid, etc. Sulfuric acid has a good solubility effect on sulfide ore and pyrite, hydrochloric acid has a good solubility effect on calcite, dolomite and galena, nitric acid has a good solubility on pyrite, marcasite and arsenic, pyrite, etc., and hydrofluoric acid has good solubility over silicate minerals. There are many types of impurities in natural quartz minerals and their existence forms are complex, and the use of mixed acid to dissolve the impurities of mineral inclusions in quartz is the best effect on the purification of quartz sand [5].
It has been found that the use of mixed acids in the acid leaching process is more effective in removing impurities, and quartz will be dissolved by hydrofluoric acid, so the concentration of hydrofluoric acid in the mixed acid should not be too high [8]. The factors that affect the effect of acid leaching are: acid leaching time, acid concentration, washing process and temperature, etc.
High-temperature chlorination degassing method
The high-temperature chlorination degassing method has a certain effect on the removal of gas-liquid inclusions, but the chlorination mechanism of some metal oxides is still relatively complex, especially the initial temperature of high-temperature chlorination is still a problem, and Unimin Company of the United States mainly uses this technology to produce high-purity quartz sand, and Unimin has been in the leading position in the world in this technology [10].
Sato and Kemmochi et al. [11] combined high-temperature chlorination, magnetic separation, and electric separation with the current technology by combining high-temperature chlorination, magnetic separation, and electric separation, so that the impurity content in quartz sand was greatly reduced, and the standard for producing high-grade quartz materials was reached.
High-temperature burst method
In the process of high-temperature roasting of quartz, with the increase of temperature, the internal pressure of the fluid inclusions is greater than the binding pressure of the quartz on the inclusions, and the internal impurities of the fluid inclusions can be released by sudden bursting, and then the internal impurities of the fluid inclusions can be dissolved by subsequent acid cleaning. The vast majority of fluid inclusions can burst when they reach a uniform temperature, but not all fluid inclusions can burst at high temperatures. The increase of the internal pressure of the fluid inclusion is mainly achieved by the temperature increase when the fluid inclusion bursts at high temperature, and the appropriate selection of higher temperature may produce a better effect on the burst of the fluid inclusion. During high-temperature vacuum roasting, the internal pressure of the fluid inclusions inside the quartz expands dramatically, and at the same time, due to the high vacuum in the furnace, a huge pressure difference forms between the inside and outside of the fluid inclusions, which increases the possibility of the fluid inclusions bursting [5].
The 2nd Anhui International Quartz Industry Conference and Exhibition 2024.