Quartz Sand Purification Technology and Preparation Scheme

Quartz sand can be divided into ordinary quartz sand and high-purity quartz sand according to its purity. Ordinary industrial quartz sand refers to quartz sand with a silica content of 96%~99%, an iron oxide content of less than 0.5%, and an alumina content of less than 2.0%.

Currently, the methods for preparing high-purity quartz sand are mainly divided into three categories: chemical synthesis using silicon-containing compounds; grinding and processing of natural crystal powder; and deep purification of quartz minerals. Chemical synthesis methods have high raw material costs, high energy consumption, low output, complex processes, and high equipment requirements. Furthermore, the product particles are too fine and easily agglomerate, making large-scale industrial application difficult.

The processing steps for high-purity quartz refer to the process of removing associated gangue, inclusions, and crystal structure impurities from the raw quartz ore. These typically include calcination, water quenching, grinding, classification, water washing and desliming, scrubbing, electrostatic separation, magnetic separation, flotation, acid leaching, alkali leaching, and high-temperature (atmosphere) roasting. Based on different processing purposes, it is divided into four stages: pre-selection preparation, pre-selection, mineral sorting, and deep purification.

(1) Pre-selection preparation. Quartz is calcined at approximately 1000℃ and water-quenched before entering the mill. Under high-temperature calcination, quartz and gangue undergo crystal transformation. Impurities have different expansion rates than quartz. Under water quenching, the temperature drops sharply, resulting in numerous cracks within the particles. Cracks typically occur at the interface between quartz and gangue, at inclusions, and at crystal structure defects. After crushing and grinding, inclusions and impurities are easily exposed on the surface of the quartz particles, enhancing the separation of quartz from gangue minerals.

(2) Pre-selection. During the grinding process of raw quartz ore, some easily mud-forming minerals form fine-grained slime, which can adversely affect subsequent selection. Pre-washing and desliming can effectively remove clay minerals, with significant impurity removal effects. Scrubbing is a process that uses mechanical force and the abrasive force between sand grains to remove thin films of iron, binders, and clayey impurities from the surface of quartz sand, and to further break up mineral aggregates that have not yet formed individual particles. Methods include mechanical scrubbing, rod mill scrubbing, and ultrasonic scrubbing.

(3) Mineral separation. Magnetic separation and flotation are the most commonly used processes for separating quartz sand from associated gangue. Multi-stage weak-strong magnetic separation removes magnetic impurities such as hematite, limonite, pyrite, ilmenite, and biotite, and can also remove inclusions and intergrowths containing magnetic minerals. Flotation effectively removes non-magnetic associated impurities such as feldspar and mica from quartz sand. It is the most important process in conventional quartz beneficiation and also the most difficult to control. Feldspar and mica, as silicate minerals, are the main sources of aluminum impurities and have poor solubility in both acids and alkalis; incomplete flotation separation directly affects the content of impurities such as Al and Fe in the product.

(4) Deep purification. Conventional mineral processing methods alone cannot remove inclusions and impurities in the crystal lattice, thus failing to yield high-purity quartz sand. By adding chlorine or HCl and calcining at 800–1600°C, the inclusions expand and rupture rapidly, exposing the impurities to the surface. During this process, the impurities migrate to the quartz surface along with the quartz crystal structure transformation; or, under the influence of a high-temperature atmosphere (such as HCl), they react with impurities in the crystal lattice, causing them to transfer to the quartz surface. Finally, chemical leaching removes the impurities.