Complete Guide to Quartz Sand Purification Processes

Quartz sand, also known as silica sand, is a common non-metallic mineral raw material with extensive applications. Quartz sand purification is a highly challenging separation technique that removes minor or trace impurities to obtain refined quartz sand or high-purity quartz sand.

1. Water Washing and Graded Desliming

The silica content in quartz sand decreases as particle size becomes finer, whereas impurity minerals such as iron and aluminium content increase conversely. This phenomenon is particularly pronounced in quartz sand containing substantial clay minerals. Therefore, water washing and graded desliming of raw quartz sand prior to processing are essential and yield significant results.

2. Scouring

Scouring employs mechanical force and abrasive action between sand grains to remove surface-bound iron, cementation, and clayey impurities from quartz sand. It further pulverises unbroken mineral aggregates, followed by classification to achieve enhanced quartz sand purification.

Currently, two primary methods exist: rod mill scouring and mechanical scouring. For mechanical scrubbing, it is generally recognised that the primary factors influencing scrubbing efficiency stem from the structural characteristics and configuration of the scrubbing machine, followed by process parameters including scrubbing duration and concentration. Research indicates that scrubbing concentrations between 50% and 60% yield optimal results for sand ore. Excessively high or low concentrations diminish the removal of impurity minerals and, to some extent, increase the difficulty of purifying the quartz sand. Scouring duration should, in principle, be based on achieving preliminary product quality requirements and should not be excessively prolonged. Prolonged duration increases equipment wear, elevates energy consumption, and raises mineral processing and purification costs.

3 Magnetic Separation of Quartz Sand

Magnetic separation maximises the removal of weakly magnetic impurity minerals such as hematite, limonite, and biotite, including intergrown particles. Strong magnetic separation typically employs wet strong magnetic separators or high-gradient magnetic separators. Generally, for quartz sand containing predominantly weakly magnetic impurities like goethite, hematite, and biotite, wet strong magnetic separation at 10,000 Oersted or above is effective. For quartz sand containing predominantly ferrite-type strongly magnetic impurities, weaker or medium-strength magnetic separators yield superior separation results.

4 Acid Leaching of Quartz Sand

Acid leaching exploits quartz's insolubility in acids (except HF), while other impurity minerals dissolve in acid solutions, enabling further purification of quartz. Commonly employed acids include sulphuric, hydrochloric, nitric, and hydrofluoric acids; reducing agents comprise sulphurous acid and its salts. Research indicates these acids effectively remove non-metallic impurities from quartz, though acid type and concentration significantly influence removal of metallic impurities. It is generally accepted that various dilute acids exhibit significant efficacy in removing Al, whereas the removal of O necessitates acid leaching using more concentrated H₂SO₄, aqua regia, or HF acid. Typically, mixed acids composed of the aforementioned acids are employed for the acid leaching removal of impurity minerals. Considering the dissolving effect of HF acid on quartz, its concentration is generally maintained below 10%. Beyond acid concentration, factors such as acid dosage, leaching duration, temperature, and pulp agitation all influence quartz acid leaching efficacy. Control of these variables should prioritise the final quartz grade requirement, striving to minimise acid concentration, temperature, and dosage while reducing leaching time to achieve quartz purification at lower processing costs.

5. Quartz Sand Flotation

Flotation methods are employed to remove non-magnetic associated impurity minerals such as feldspar and mica from quartz sand. The separation of quartz and feldspar has been extensively researched both domestically and internationally. Conventional processes utilise cationic collectors and hydrofluoric acid activators within an acidic pH range. Given the severe environmental impact of fluoride-containing wastewater, the ‘fluoride-free acidic flotation method’ emerged internationally in the 1970s. For instance, Japan successfully separated feldspar from quartz using sulphuric or hydrochloric acid (pH=2) for pulp conditioning, combined with a mixed collector comprising higher aliphatic amine salts and sodium petroleum sulphonate. Fluoride-free, acid-free flotation represents a newly developed separation technique for quartz and feldspar, extensively researched in recent years. Since 1984, Tang Jiaying and colleagues have investigated mixed cationic-anionic collector flotation separation. This method operates in a naturally neutral medium, exploiting structural differences between quartz and feldspar to optimally adjust the ratio of mixed collectors, thereby preferentially floating feldspar and achieving separation. However, fluorine-free and acid-free flotation remains less mature than HF or acid-based methods. Mica and quartz share similar isoelectric points, complicating separation. Employing anionic collectors under acidic conditions or mixed anionic-cationic collectors under alkaline conditions yields excellent results. Generally, following washing, desliming, magnetic separation, and flotation, quartz sand purity can reach 99.3% to 99.9%, largely meeting industrial sand requirements.