What are the chemical purification technologies for high-purity quartz?

After preliminary purification of quartz ore through physical methods such as mineral processing, quartz with a SiO₂ content of 99.3%–99.9% can be obtained. To achieve higher purity, chemical purification is required. Chemical purification technologies include acid leaching, alkali leaching, and thermal treatment. Acid and alkali leaching use acids or bases to remove impurities from quartz, sometimes combined with ultrasonic-assisted leaching to achieve target purity. Thermal treatment removes inclusion impurities or isomorphous impurities in quartz through high temperatures.

1. Acid Leaching
Acid leaching is the most widely studied chemical purification process. The type of acid significantly affects leaching efficiency. Commonly used acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), hydrofluoric acid (HF), oxalic acid (H₂C₂O₄), and nitric acid (HNO₃). Mixed acids are often employed, with other factors such as temperature, time, and stirring intensity also influencing results.

1.1 Fluorinated Leaching
Hydrofluoric acid (HF) effectively removes surface impurities from quartz and dissolves quartz, mica, and feldspar. The dimer (HF)₂ and HF₂⁻ in HF solution nucleophilically attack Si atoms, breaking Si–O bonds and promoting metal impurity leaching.

HCl + HF: Xiong Kang et al. used a mixed HCl-HF system to reduce total impurities to 4.07×10⁻⁵ (91.11% removal) and achieve 99.993% SiO₂.

H₂SO₄ + HF: Xia Zhangjie et al. achieved Fe, K, and Al removal rates of 97.31%, 94.87%, and 86.47%, respectively, using 3 mol/L H₂SO₄ + 0.5 mol/L HF at 80°C for 8 h.

Oxalic Acid (H₂C₂O₄): Li Xianrong et al. combined HCl, H₂C₂O₄, and HF to achieve 93.97% Fe removal, reducing Fe content to ~4×10⁻⁶.

HNO₃: Lei Shaomin et al. used HF-HCl-HNO₃ (0.5, 2.5, 1.0 mol/L) to achieve 99.9% SiO₂.

1.2 Fluorine-Free Leaching
To avoid environmental and yield issues caused by HF, fluorine-free methods are emerging:

H₂SO₄ + H₂C₂O₄: Tuncuk et al. reduced Fe₂O₃ content to 1×10⁻⁶ (98.9% removal).

NH₄Cl-Assisted Leaching: Lin Min achieved total impurity removal rates of 85.2% (NH₄Cl-HCl) and 84.0% (NH₄Cl-H₂SO₄).

Complexing Agents: Zhong et al. found oxalic and acetic acids most effective, with citric, humic, EDTA, and thiourea also enhancing impurity removal.

High-Temperature Oxalic Acid: At >90°C and pH 2.5–3.0, iron oxide dissolution accelerates.

1.3 Ultrasonic-Assisted Leaching
Ultrasonic cavitation enhances leaching by breaking liquid inclusions on quartz surfaces:

H₃PO₄: Zhang et al. achieved 77.1% Fe removal at 80°C with 100-mesh quartz.

H₂C₂O₄: Du et al. reached 75.4% Fe removal at 95°C and 150 W ultrasound.

HF + H₂C₂O₄: Yang et al. increased SiO₂ from 99.583% to 99.905% under optimized conditions.

2. Alkali Leaching
Alkali solutions (e.g., NaOH) dissolve impurities like muscovite and widen surface cracks, aiding subsequent acid leaching:

Lei Shaomin et al. combined roasting, NaOH leaching, and mixed acid leaching to achieve 99.991% SiO₂.

Shao et al. used 12% NaOH at 200°C for 100 min, followed by HCl-HNO₃-HF leaching, reducing impurities by 47.21% and achieving 99.994% SiO₂.

3. Thermal Treatment
3.1 High-Temperature Roasting
Heating quartz to induce inclusion rupture (e.g., at 900°C):

Yang improved SiO₂ to 99.905% (Fe₂O₃ reduced to 0.022%) using 900°C roasting + ultrasonic-assisted H₂C₂O₄-HCl.

Xia Zhangjie combined hot-pressure H₃PO₄ leaching and Na₂CO₃ roasting (1,000°C) to achieve 99.995% SiO₂.

3.2 Microwave Heating
Internal heating via microwaves removes inclusions efficiently:

Hou et al. removed most fluid inclusions at 900°C.

Li et al. combined microwave heating (1,000°C) with HNO₃ leaching to reduce Fe from 2.85×10⁻⁴ to 1.67×10⁻⁷ (99.94% removal).

3.3 Chlorination Roasting
Used by Unimin Corporation (U.S.) to remove refractory Ti:

NH₄Cl or HCl gas reacts with impurities (e.g., Al, Fe, K, Na) to form volatile chlorides. At 1,200°C, K and Na impurities can drop to 1×10⁻⁷ and 3×10⁻⁸.

Conclusion
Acid leaching dominates current industrial practices, while fluorine-free and ultrasonic-assisted methods show promise. Chlorination roasting remains restricted by Unimin’s patents. Future research should focus on optimizing fluorine-free systems, understanding chlorination mechanisms, and tailoring thermal treatments for specific quartz sources.

Source: Adapted from Progress in High-Purity Quartz Applications and Chemical Purification Technologies by Wu Zhichao et al. (Quartz Industry).
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