Is hydrofluoric acid necessary for the purification of high-purity quartz?

In the industrial field of high-purity quartz purification, hydrofluoric acid has long occupied an important position, but the risks and hazards it brings have also prompted the industry to continuously explore new paths.

Hydrofluoric acid: a powerful tool for traditional purification, but also a hidden danger

Hydrofluoric acid (HF) is widely used in traditional high-purity quartz purification, its core value lies in its strong ability to remove silicate impurities. In the “fluorine-acid method” flotation, hydrofluoric acid (or fluoride) is combined with inorganic acids (such as hydrochloric acid and sulfuric acid) to ionize active components such as F⁻ and HF₂⁻ in an acidic environment (pH ≈ 2.5). These components corrode the Si-O bonds in the lattice of impurity minerals such as feldspar and mica, exposing Al₃⁺ active sites. Amine collectors are then used to separate the impurities from the quartz. In acid leaching, hydrofluoric acid can destroy the internal inclusion structure of quartz and effectively remove lattice impurities such as Al, Fe, and Ti. For example, after leaching vein quartz from Huanggang, Hubei Province, with a mixture of hydrofluoric acid and hydrochloric acid (1:19), the impurity content was reduced to 79.3 μg/g, and the SiO₂ purity reached 99.992%.

However, the use of hydrofluoric acid has significant limitations: first, the environmental risk is high. If the fluoride ions in flotation or acid leaching wastewater are not handled properly, they will pollute water and soil, and the treatment of fluorine-containing wastewater requires additional investment costs, which increases the production burden; second, the equipment is highly corrosive. Hydrofluoric acid has strict requirements on the material of the reaction vessel, and special anti-corrosion materials need to be used, which increases equipment investment and maintenance costs; third, the operational safety is low, and extremely high requirements are placed on operator protection and production process control, which limits its application in some enterprises.

Hydrofluoric acid-free path: 3 types of technology, covering multiple needs

With the development of green processes and technological breakthroughs, a variety of hydrofluoric acid-free purification technologies have been industrialized or laboratory-verified to meet the needs of high-purity quartz of different purity levels, mainly including the following three categories:

Fluorine-free flotation process

Fluoride-free acid flotation: The pH of the ore pulp is adjusted (usually to 2-3) with inorganic or organic acids such as hydrochloric acid, sulfuric acid, and oxalic acid, keeping the quartz near its zero charge (neutral/weakly acidic) and the surface of impurities such as feldspar negatively charged. Amine collectors or mixed anionic and cationic collectors are then used to selectively adsorb the impurities. For example, a quartz mine in Jiangxi Province used a process combining sulfuric acid pH adjustment with a mixed collector of dodecylamine and sodium petroleum sulfonate to produce a quartz concentrate with a SiO2 grade of 95.04% and a brightness of 94.40%. Furthermore, for the separation of sericite and quartz, at a pH of 3 and a dodecylamine dosage of 2.0×10⁻４ mol/L, the difference in recovery between the two reached 66 percentage points, demonstrating significant separation effectiveness.

“Fluoride-free and acid-free” flotation: Purification is achieved in a neutral or alkaline environment through the synergistic action of activators, inhibitors, and mixed collectors. Under neutral conditions, sodium hexametaphosphate is used to inhibit quartz (forming a hydrophilic adsorption layer), sodium oleate is used to activate feldspar, and dodecylamine hydrochloride is used as a collector, resulting in a quartz recovery rate of nearly 89%. Under alkaline conditions (pH = 9-10), the addition of alkaline earth metal ions such as Ca2+ to activate muscovite and feldspar, followed by the use of collectors such as octadecylamine polyoxyethylene ether diquaternary ammonium salt (OPEBA), allows for efficient separation with a quartz recovery rate of 99.4% and feldspar recovery of only 16.9%. This process is acid-free and fluoride-free, significantly minimizing equipment corrosion and environmental impact.

Fluoride-free acid leaching process

Organic acid leaching: Organic acids such as oxalic acid, citric acid, and acetic acid remove metal impurities through complexation, with low toxicity and minimal pollution. For example, vein quartz from Guangxi was leached for 12 hours using a mixture of 15% oxalic acid and 0.8% nonionic surfactant at 70°C and a liquid-to-solid ratio of 4:1. The result was an increase in SiO2 purity from 98.86% to 99.80%, while the Fe and Al contents dropped to 0.01% and 0.02%, respectively, achieving a total impurity removal rate of 81.7%. Citric acid can simultaneously complex Fe and Al impurities and prevent their redeposition, while acetic acid can remove weakly alkaline impurities such as Ca and Mg, achieving a balanced purification effect and environmental performance.

ZhengZhou Quartz Master New Materials Co., Ltd.’s “A Fluorine-Free and Nitric Acid-Free Leaching Process for Preparing High-Purity Quartz Sand” relates to the field of high-purity quartz sand purification. The process includes ore selection, cleaning, crushing, screening, magnetic separation, flotation, high-temperature microwave degassing of liquid inclusions, fluorine-free and nitric acid-free leaching (hydrochloric acid + oxalic acid), drying, and magnetic separation. This process uses an environmentally friendly fluorine-free and nitric acid-free leaching method for pickling and impurity removal. By controlling the pickling solution ratio, pickling time, and temperature, it effectively reduces the content of impurities such as Fe, Al, and Mg in the quartz sand. The final product purity can reach 5N (higher than 4N8), while reducing wastewater treatment and significantly reducing environmental pollution.

To further enhance the effectiveness of acid leaching in removing impurities, the compounds are often treated with hot pressing or ultrasound during acid leaching. Ultrasonic or hot pressing techniques can enhance the penetration and reactivity of non-fluorinated acids. Jingsheng LI et al. used ultrasonically assisted mixed acid chemical leaching of HCl and H₂C₂O₄ to remove aluminum impurities from quartz minerals. The experimental results showed that the aluminum content of the quartz sand concentrate obtained by fluoride-free leaching was lower than that of some fluoride-containing chemical leaching methods, and the reaction time was shorter, eliminating the formation of fluoride ions and conserving energy.

Heat treatment combined with fluorine-free process

Heat treatment (high temperature roasting, microwave calcination) destroys the internal inclusions of quartz through physical action, creating conditions for fluorine-free purification and reducing dependence on hydrofluoric acid:

High-temperature roasting: Quartz sand is heated to 900°C and water quenched, using thermal stress to create microcracks that cause internal inclusions to burst. Subsequent fluoride-free acid leaching allows for efficient impurity removal. For example, high-temperature roasting followed by fluoride-free acid leaching significantly reduces the content of impurities such as Al, Fe, P, and K in quartz, destroys the crystal structure of gangue minerals such as muscovite, and achieves deep purification without the need for hydrofluoric acid.

Microwave calcination: Microwaves selectively heat iron-containing impurities and gas-liquid inclusions (quartz barely absorbs microwaves), creating microcracks at temperatures below 573°C (quartz’s α/β phase transition temperature), assisting in fluoride-free acid leaching. Research by Li et al. showed that microwave treatment of quartz sand at 400°C for 30 minutes reduced the iron content from 285 ppmw to 0.383 ppmw, a removal rate of 99.87%. This method outperforms traditional high-temperature calcination and overcomes the difficulty of removing inclusion impurities without the need for hydrofluoric acid.

Conclusion

In general, hydrofluoric acid is not necessary for the purification of high-purity quartz.

From a technical perspective, several hydrofluoric acid-free purification technologies, including fluorine-free flotation, fluorine-free acid leaching, and thermal treatment-assisted fluorine removal, have been demonstrated in laboratory settings. Some even achieve highly efficient impurity removal, reaching high purity standards. However, large-scale industrial application of these hydrofluoric acid-free technologies remains limited, with most still in the laboratory research and development stage.

With the country’s increasing environmental protection requirements and the advancement of its “dual carbon” goals, hydrofluoric acid-free high-purity quartz purification technology has become a research and development focus. Fluorine-free flotation, fluorine-free acid leaching, and heat treatment-assisted fluorine-free processes are considered the future mainstream due to their efficiency, environmental protection, and cost advantages. In the future, the use of hydrofluoric acid in high-purity quartz purification may gradually be limited to high-end niche applications, no longer being a “must-have” in this field.