High-purity quartz sand is the material basis for the development of high-tech industries, and its application fields involve optical fibers, military, and aerospace industries. These fields have extremely strict requirements on the purity of quartz sand, especially for Fe, Al and other impurities.

Mineral impurities in quartz sand usually exist in the form of non-quartz minerals such as feldspar, mica, garnet, zircon, ilmenite and many others. These impurities mainly exist in the following ways:

(1) As loose associated minerals, they are not chemically combined with quartz crystals;

(2) As mineral fragments, chemically and physically combined with quartz crystals on its surface, such impurities are mainly iron-bearing minerals and aluminum-bearing minerals;

(3) Minerals wrapped by quartz particles or surrounded by quartz crystals combined with each other;

(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 several thousand ppm. Therefore, Al content is an important indicator of the quality of quartz ore.

At present, the purification process of high-purity quartz sand mainly includes mechanical pulverization, magnetic separation, flotation, acid leaching, etc., which can effectively remove metal ion impurities in the quartz lattice.

1.Mechanical pulverization

Mechanical pulverization is a method that uses mechanical force to reduce the particle size of minerals. In the purification process of high-purity quartz, this process is mainly to separate non-structural impurities in quartz minerals from quartz. Non-structural impurities refer to mineral inclusions (mineral impurities) and gas-liquid inclusions (fluid inclusions). Impurities exist in the quartz grain boundaries. After the original quartz minerals are crushed, the particle size is reduced and the specific surface area is increased, 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 pulverization, due to the relatively hard nature of quartz minerals, frequent contact and friction with the equipment will inevitably introduce impurities and cause pollution.

The quartz minerals were ultra-finely pulverized by the wet ceramic ball milling process, and the particle size of the dispersed quartz particles was tested. The surrounding surface is passivated, and the sphericity is obviously increased; and the quartz is purified by water washing and acid leaching, and the whiteness of the obtained quartz is obviously improved, which has a certain reference value for the research on the development and application of quartz.

2.Magnetic separation

In the purification process of high-purity quartz, the purpose of magnetic separation is to remove some magnetic minerals, such as magnetic ilmenite, pyrite, limonite and garnet, in the inclusions of magnetic quartz ore, which has a good effect on the removal and separation of magnetic impurities such as iron and titanium in the raw quartz ore.

The magnetic field strength of the magnetic separator can be adjusted, also known as gradient magnetic separation, use weak magnetism to remove magnetite, and use strong magnetism to remove magnetic minerals such as ilmenite, limonite, hematite, and garnet. For heavy mineral impurities (terrigenous clastic minerals with specific gravity greater than 2.86, such as zircon, epidote, garnet, etc.) existing in the original quartz ore, methods such as gravity separation and high-intensity magnetic separation are generally used. Usually, quartz minerals are scrubbed after magnetic separation, which will improve the purity and whiteness of quartz sand.

3.Flotation

Flotation is the selective separation of hydrophobic substances and hydrophilic substances according to the difference in wettability of the ore body surface, either naturally or after modification. In the purification process of high-purity quartz, flotation is mainly used to remove mica and feldspar minerals that coexist with quartz, and it can also flotate phosphorus- and iron-containing minerals.

According to the different reagents used, quartz sand flotation can be divided into fluorine quartz sand flotation and fluorine-free quartz sand flotation. Fluorine quartz sand flotation uses fluorine-containing agents, such as hydrofluoric acid (HF) as a feldspar activator, and sulfuric acid as a modifier, so that under the strong acid conditions of pH=2-3, dodecylamine used as a collector, and the activated feldspar is adsorbed in advance 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 the corresponding collector to flotate and separate quartz and impurity minerals. In addition, some studies have shown that the flotation effect of mixed collectors is better than that of single collectors and is relatively cost-effective.

Some researchers conducted reverse flotation of vein quartz sand slurry to prepare high-purity quartz sand, and used mixed collectors to purify the fine-coarse combined quartz sand to obtain 4N-grade quartz products. The amount of foaming agent 2# oil is 75g/t, the quartz sand is acidified by sulfuric acid during rough selection, and propylene diamine is used as collector; The dosage is 1:4. In the experimental results, the removal of impurities accounted for more than 50%, the total amount of impurities was 99.01 μg/g, and the removal rates of elemental Al and Fe reached 37.50% and 84.15%, respectively.

4.Acid leaching

Acid leaching is a means of quartz purification according to the different solubility of quartz, mica and feldspar in acidic solutions. Acid leaching can effectively remove the oxide film on the surface and iron ore. For mineral impurities such as mica and feldspar, hydrofluoric acid is generally used for dissolution. Commonly used acid media for acid leaching include hydrochloric acid, sulfuric acid, nitric acid, acetic acid and hydrofluoric acid. Among them, 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 removing Cr and Ti.

Studies have shown that the coexistence of dilute acid and hydrofluoric acid can effectively remove Fe, Al, Mg and other metal impurities, but the amount of hydrofluoric acid should be controlled because hydrofluoric acid can erode quartz particles. The use of different kinds of acids also affects the quality of purification and processing. Among them, the processing effect of HCl and HF mixed acid is the best.

The lab used HCl and HF mixed leaching agent to purify the quartz sand after magnetic separation. Through chemical leaching, the total amount of impurity elements is 40.71μg/g, and the purity of SiO2 is as high as 99.993wt%.

The essence of acid leaching is the interaction between acid solution and impurity minerals. Therefore, in the process of acid leaching, temperature has a great influence on the reaction rate and the final purification effect. The lad used hydrochloric acid and oxalic acid as mixed leaching agents to study the effect of acid leaching temperature, time and concentration on the purification effect of quartz, and finally determined the acid leaching temperature of 60 °C, acid leaching time of 8 h, oxalic acid concentration of 10 g/L, and HCl concentration of 5 %, liquid-solid ratio of 1:5, and stirring speed of 500 rpm are the best conditions for acid leaching. The results show that the removal of iron accounts for 50%.

High temperature and high pressure leaching

It is a relatively mature hydrometallurgical technology in metal ore processing. This technology can effectively reduce the consumption of acid by high temperature and high pressure. The high pressure is provided by the closed environment of the tank reactor composed of stainless-steel jacket and Teflon liner. It can remove symbionts and inclusions more effectively, whose purification effect of stubborn ores is better than that of magnetic separation and flotation.

The researchers found that the high temperature and high pressure acid leaching has better effect than the atmospheric pressure mixed acid leaching. The atmospheric pressure mixed acid leaching uses HF with a concentration of 0.5mol/L and HCl with a concentration of 3.0mol/L as the main leaching agents, and adopts an oxidation leaching process. , the oxidant HNO3 is the auxiliary leaching agent, the concentration is 0.5mol/L, the acid leaching temperature is 80 ℃, the acid leaching time is 8h, the stirring intensity is 300r/min, and the liquid-solid ratio is 3:1; the thermal-pressing mixed acid leaching adopts the HF concentration of 1.2mol /L, HCl concentration 3.0mol/L, HNO3 concentration 1.0mol/L, liquid-solid ratio 5:1, reaction time 6h, reaction temperature 220℃, the results proved that Al, Fe, K, The content of impurities such as Na, Ca and Mg decreased more obviously.