High-purity quartz is a critical and high-demand non-metallic mineral material. It has properties of high-temperature resistance, corrosion resistance, thermal stability, and insulation. It is widely used in high-end electric light sources, large-scale and ultra-large-scale integrated circuits, solar photovoltaics, optical fibers, aerospace, and military industries. Given the strategic importance of these sectors for long-term national development, high-purity quartz is classified as a high-tech material under strict protection by governments worldwide. As early as 2010, the European Union listed it as a critical raw material. Currently, only a handful of companies globally are capable of its genuine production.
Table 1 Application Market of High Purity Quartz
| Purity (SiO₂ Content) | >99.99% | >99.997% | >99.999% |
|---|---|---|---|
| Main Products | High-temperature lamp tubes, fused silica tubes, quartz glass, silica powder, optical devices, special quartz optical glass, etc. | Monocrystalline silicon crucibles, polycrystalline silicon crucibles, high-quality quartz glass and products, optical fibers and related optoelectronic components, etc. | Czochralski (CZ) crucibles, ultra-high purity quartz glass for semiconductors, high-end quartz crucibles for semiconductors |
| Application Industry Price (USD/ton) | 600 ~ 1500 | 5,500 ~ 8,500 | 12,000 ~ 15,000 |
Technical Specifications for High-Purity Quartz
There is no universally accepted definition. Müller A. et al. (2007) first attempted to classify quartz based on trace impurity element content, suggesting high-purity quartz should meet (ppm): Al < 30, B < 1, Ca < 5, Fe < 3, K < 8, Li < 5, Na < 8, Ti < 10, P < 2, with total impurities < 50 ppm. Zhang Ye et al. (2010) noted internationally recognized high-purity quartz sand has total content of 15 impurities (Al, K, Na, Li, Ca, Fe, Mg, Mn, Ti, Zr, Cu, Cr, Ni, P, B) less than 22.26 ppm. China’s “First Batch Application Demonstration Guidance Catalog for Key New Materials (2018 Edition)” specifies high-purity quartz should have total content of 12 impurities (Fe, Mg, Cr, Ni, Cu, Mn, Ca, Al, Na, Li, K, B) less than 6 ppm.
Fluid inclusions, common in quartz, are a primary cause of bubbles in high-purity quartz products, severely affecting quality. Therefore, high-purity quartz must contain none or minimal fluid inclusions. Observations suggest it should meet: single-particle fluid inclusion area ratio <1%; under a microscope (10X objective, avg. grain size 0.1mm sample), particles containing fluid inclusions <1%; thermal weight loss <15 ppm.
Table 2 Indicators of High-Purity Quartz US Unimin IOTA Series (ppm) [9]
| Element | Al | B | Ca | Cr | Cu | Fe | K | Li | Mg | Mn | Na | Ni |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| IOTA – Standard | 16.2 | 0.08 | 0.5 | <0.05 | <0.05 | 0.23 | 0.60 | 0.90 | <0.05 | <0.05 | 0.9 | <0.05 |
| IOTA – 4 | 8.0 | 0.04 | 0.6 | <0.05 | <0.05 | 0.3 | 0.35 | 0.15 | <0.05 | <0.05 | 0.9 | <0.05 |
| IOTA – 6 | 8.0 | 0.04 | 0.6 | <0.05 | <0.05 | 0.15 | 0.07 | 0.15 | <0.05 | <0.05 | 0.08 | <0.05 |
| IOTA – 8 | 7.0 | <0.04 | 0.5 | <0.02 | <0.02 | <0.03 | <0.04 | <0.02 | <0.02 | <0.02 | – | <0.02 |
Note: “-” indicates data not reported in the literature.
High-Purity Quartz Raw Materials
Initially, natural crystal (grades I & II) was used. With rapid high-tech industry growth, limited and depleting natural crystal cannot meet demand. Since the 1970s, the USA used granitic pegmatite, Japan used fine-grained pegmatite, while Russia and Germany used metamorphic quartzite and vein quartz. Currently, natural quartz minerals are the primary source.
Mineralogical Characteristics of High-Purity Quartz Raw Materials
The quality of high-purity quartz is not simply inversely related to impurity content in the raw material, but closely tied to the processability of impurities as determined by its process mineralogy. Different quartz ore types have distinct mineralogical features. Detailed analysis is fundamental for determining ore properties, designing beneficiation/purification processes, and setting product targets.
1 Chemical Composition & Occurrence of Impurity Elements
Chemical analysis reveals element types and contents but cannot accurately judge the potential for processing into high-purity quartz. Impurities are diverse in type, content, and occurrence (see Table 3 for common modes).
Table 3 Common Occurrence States of Impurity Elements in Quartz Raw Materials [1,12]
| Element | Occurrence State | Existing Form | Element | Occurrence State | Existing Form |
|---|---|---|---|---|---|
| Al | Isomorphous Independent Mineral | Lattice defects in quartz, mica, feldspar, clay minerals | Ti | Isomorphous Independent Mineral | Lattice defects in quartz, rutile |
| Fe | Isomorphous Independent Mineral | Lattice defects in quartz, iron oxides, solid mineral inclusions | Ge | Isomorphous Defect | Lattice defects in quartz |
| Li | Isomorphous Inclusion | Lattice defects in quartz, liquid phase in fluid inclusions | Mg | Isomorphous Inclusion | Quartz crystals, mica admixtures |
| K | Isomorphous Inclusion | Lattice defects in quartz, mica, clay minerals, liquid phase in fluid inclusions | Ca | Independent Mineral Inclusion | Minerals such as fluorite, liquid phase in fluid inclusions |
| Na | Isomorphous Inclusion | Lattice defects in quartz, mica admixtures, liquid phase in fluid inclusions | -OH | Isomorphous Defect | Lattice defects in quartz |
2 Mineral Composition & Texture
To select the right raw material and design optimal purification, identifying the mode of occurrence of impurities is essential. Associated independent gangue minerals (e.g., mica, feldspar, hematite, tourmaline, chlorite, clay minerals) are major carriers of impurities and easily form mineral inclusions during mineralization, significantly impacting final product quality. The intergrowth texture between quartz and gangue directly affects liberation degree and thus purification efficiency. Stronger diagenetic/metamorphic alteration leads to more distinct intergrowth differences, evolving from adjacent to sutured or even encapsulated types, increasing liberation difficulty and reducing processability potential.
- Figure 1(a): Granitic pegmatite from Spruce Pine, USA—complex mineralogy but quartz easily liberates, contains minimal inclusions.
- Figure 1(b): Vein quartz from Qinghai, China—coarse, pure grains with minor muscovite at grain boundaries/borders, liberates easily, can yield >99.99% SiO₂, Al <10 ppm after processing.
- Figure 1(c): Purified concentrate showing persistent mineral inclusions—under current tech, inclusions trapped within quartz grains cannot be effectively separated. Quartz with poor liberation and abundant inclusions is difficult to process into high-purity quartz.
3 Fluid Inclusions
Ubiquitous in minerals/rocks (10²–10⁹ per cm³, typically <50 µm). Their type, size, and content significantly impact quality. Classified by content: gas, liquid, gas-liquid, three-phase. They contain impurities (Na, K, Ca, etc.) and severely negatively affect melting behavior. Studies show removing fluid inclusions is more challenging than removing elemental impurities and is a key limiting factor. Selecting quartz with极少 or no fluid inclusions is crucial.
4 Lattice Impurities
Elements substituting for Si⁴⁺ during crystal formation become structural impurities. Though low in content, they are extremely difficult to remove and are the most critical factor constraining quality. Modes: 1) Isovalent substitution (Ti⁴⁺, Ge⁴⁺); 2) Coupled substitution (Al³⁺ + P⁵⁺ for 2Si⁴⁺); 3) Charge-compensated substitution (Al³⁺, Fe³⁺ for Si⁴⁺ balanced by Li⁺, Na⁺, K⁺, H⁺). Al is typically highest. Its substitution creates charge imbalance, often correlating with higher Li, K, Na content. Al content can indicate raw material quality. Under current technology, lattice impurities are virtually unremovable.
Figure 2 shows SEM-EDS mapping of a vein quartz particle after heat-pressure leaching. Al is evenly distributed (Figure 2b), matching the particle shape, indicating lattice-bound Al. Mg distribution is indistinct (Figure 2c). GFAAS/AAS analysis showed Al=13.92 ppm, Mg=0.59 ppm post-leaching, highlighting the extreme difficulty of removing lattice Al.
Global surveys indicate that processable natural quartz must possess: chemically pure grains, low/no lattice impurities, large grain size, few mineral/fluid inclusions, and low associated gangue mineral content.
Typical High-Purity Quartz Raw Materials
Though quartz is abundant, high-purity raw material forms only under specific geologic conditions. Very few deposits are suitable, with extremely complex processing. Based on genesis: magmatic, metamorphic, hydrothermal (see Table 4 for characteristics/examples).
The classic example is the Spruce Pine (USA) granitic pegmatite, formed under Alleghanian greenschist-facies metamorphism promoting dynamic recrystallization, plastic deformation, and impurity migration to new grain boundaries via fluids, resulting in high-purity quartz with few inclusions.
Magmatic pegmatitic quartz crystallizes slowly from high-T magma, allowing impurity exsolution, yielding high purity and few fluid inclusions.
Table 4 Characteristics of Quartz of Different Genetic Types and Typical Application Examples [18]
| Different Genetic Types of Quartz | Characteristics | Typical Application Examples |
|---|---|---|
| Magmatic Type | Thạch anh granit | Formation temperature (700 °C – 1,000 °C), quartz grains are pure with almost no fluid inclusions. |
| Metamorphic Type | High-Grade Metamorphic Rock Quartz | Formation temperature (750 – 900 °C), low fluid inclusion content. |
| Metamorphic Quartzite Quartz | The longer the formation time and the more intense the thermal events it has undergone, the purer the quartz and the fewer the fluid inclusions. | |
| Hydrothermal Growth Type | Early-stage Pegmatite Quartz | Formation temperature 600 – 700 °C, grain size generally 2 – 6 mm, transparent, single crystal pure, few fluid inclusions. |
| Mid to Late-stage Pegmatite Crystal | Formation temperature 500 – 600 °C. | |
| Hydrothermal Vein Quartz | Formation temperature 400 – 500 °C, transparent – semi-transparent, relatively low fluid inclusion content. | |
| Hydrothermal Vein Quartz | Formation temperature 50 – 400 °C, white – milky white, contains a large number of tiny fluid inclusions. |
Selecting the right raw material requires detailed mineralogical study using multiple techniques, focusing on: 1) Texture and mineral inclusion content; 2) Fluid inclusion content; 3) Lattice impurity content.
High-Purity Quartz Processing Technology
The goal is to separate various impurities. Steps: 1) Comminution and classification to liberate quartz from gangue and achieve target size; 2) Targeted techniques to separate independent minerals, inclusions, and lattice impurities.
1 Comminution & Classification Pretreatment
Aims for effective liberation and fluid inclusion release, and provides suitably sized feed. To avoid iron contamination, use ZrO₂ or agate media. Thermal comminution (heating followed by quenching) reduces hardness/energy, minimizes contamination, and creates microcracks aiding chemical purification. High-Voltage Pulse Fragmentation uses shockwaves to break quartz along impurity-rich grain boundaries.
2 Separation of Associated Independent Minerals
Effective methods include color sorting, scrubbing, gravity separation, tách từ, Và flotation (see Table 5).
Table 5 Separation Technology of Associated Independent Minerals and Quartz [23-25]
| Separation Method | Principle | Main Impurities Removed | Characteristics |
|---|---|---|---|
| Color Sorting | Optical properties of minerals | Dark-colored impurity minerals, milky quartz, etc. | Highly effective for coarse particles. |
| Chà rửa | Friction between mineral particles | Fine mud and oxide films adhering to quartz particle surfaces. | Mechanical scrubbing, chemical scrubbing, ultrasonic scrubbing. |
| Gravity Separation | Mineral density | Mica, zircon, rutile, etc. Hematite, magnetite, tourmaline, mica, and other magnetic minerals. | Significant loss of concentrate. |
| Magnetic Separation | Mineral magnetism | Multi-stage high-intensity magnetic separation. | |
| Tuyển nổi | Mineral surface properties | Mica, feldspar, apatite, etc. | Reverse flotation, multiple cleaning stages. |
Multi-stage high-intensity magnetic separation removes magnetic minerals and inclusions. Flotation separates silicate minerals like mica and feldspar. Multiple cleaning stages are essential. Combined flowsheets are often needed based on ore characteristics.
After pretreatment and physical separation, SiO₂ content can reach ~99.9%, but not high-purity specs, as these methods are ineffective against inclusion and lattice impurities.
3 Separation of Inclusion Impurities
3.1 Mixed Acid Leaching of Mineral Inclusions
Utilizes quartz’s solubility only in HF, while other mineral inclusions dissolve in acids (H₂SO₄, HCl, HNO₃, HF). Mixed acids are most effective for complex impurities. Thermodynamic studies (Table 6) show common impurities can dissolve in HF-containing acids, but reaction rates are slow (equilibrium constants ~1.0–1.5).
Table 6 Decomposition Reaction Gibbs Free Energy and Equilibrium Constant of Common Mineral Impurities in Quartz in HF-Containing Mixed Acids at Different Temperatures [13]
| Temperature/°C | 25 | 75 | 100 | 150 | 175 | 200 | 225 |
|---|---|---|---|---|---|---|---|
| Potassium Feldspar ΔrGT | –403.2 | –451.86 | –474.98 | –517.36 | –536.08 | –548.97 | –583.72 |
| K | 1.18 | 1.17 | 1.17 | 1.16 | 1.15 | 1.15 | 1.15 |
| Albite ΔrGT | –409.22 | –455.41 | –477.36 | –517.61 | –535.37 | –547.16 | –581.23 |
| K | 1.18 | 1.17 | 1.18 | 1.16 | 1.15 | 1.15 | 1.15 |
| Anorthite ΔrGT | –539.45 | –571.71 | –586.77 | –614.89 | –628.75 | –635.14 | –660.45 |
| K | 1.24 | 1.22 | 1.21 | 1.19 | 1.18 | 1.18 | 1.17 |
| Diopside ΔrGT | –676.42 | –663.01 | –637.67 | –619.14 | –595.78 | –556.82 | –556.92 |
| K | 1.31 | 1.26 | 1.23 | 1.19 | 1.17 | 1.16 | 1.14 |
| Muscovite ΔrGT | –704.97 | –768.97 | –799.71 | –858.27 | –886.61 | –905.49 | –954.7 |
| K | 1.33 | 1.3 | 1.29 | 1.28 | 1.27 | 1.27 | 1.26 |
| Spodumene ΔrGT | –1015.6 | –1060.2 | –1078.3 | –1108.4 | –1123 | –1124.5 | –1145.1 |
| K | 1.51 | 1.44 | 1.42 | 1.37 | 1.35 | 1.33 | 1.32 |
| Hematite ΔrGT | –86.59 | –80.44 | –77.14 | –72.39 | –72.92 | –68.75 | –67.38 |
| K | 1.04 | 1.03 | 1.03 | 1.02 | 1.02 | 1.02 | 1.02 |
| FeO ΔrGT | –111.77 | –110.56 | –110.09 | –103.65 | –105.58 | –105.2 | –106.15 |
| K | 1.05 | 1.04 | 1.04 | 1.03 | 1.03 | 1.03 | 1.03 |
| Magnetite ΔrGT | –179.71 | –172.36 | –168.59 | –164.2 | –166.58 | –161.95 | –161.44 |
| K | 1.08 | 1.06 | 1.06 | 1.05 | 1.05 | 1.04 | 1.04 |
| Pyrite ΔrGT | –161.69 | –187.38 | –203.87 | –241.16 | –260.4 | –285.45 | –307.56 |
| K | 1.07 | 1.07 | 1.07 | 1.07 | 1.07 | 1.08 | 1.08 |
Effective exposure of inclusions is prerequisite. Phase-Transition Thermal Treatment (heating to ~573°C α-β transition or 1470°C for cristobalite, then rapid cooling) uses volume expansion to create cracks, exposing inclusions. Drawbacks: possible vitrification at high T, formation of stable oxides/nitrides.
3.2 High-Temperature Bursting of Fluid Inclusions
Heating causes internal pressure to exceed confinement, bursting inclusions and releasing impurities for subsequent acid washing. Not all burst: liquid-rich inclusions burst near homogenization temperature; vapor-rich ones can withstand higher T. Thermodynamic modeling (Table 7) shows higher internal pressure for liquid-homogenized inclusions vs. vapor-homogenized at same T.
3.3 Chlorination (Chloride Volatilization)
Heating quartz to 1000–1500°C under Cl₂, HCl, or mixed gases volatilizes metal impurities as chlorides and helps remove fluid inclusions/hydroxyl groups. A chemical potential gradient drives inclusion diffusion. Research (e.g., Mao Lingwen et al.) showed [OH⁻] reduced from 35 ppm to 20.5 ppm at 1250°C in Cl₂.
Removal of Lattice Impurities
Involves breaking Me-O bonds introduced via substitution. Bond energies vary (see Table 8):
Table 8 Bond Energy of Me–O Bonds in Silicates [13]
| Me | Si⁴⁺ | Mn²⁺ | Cu²⁺ | Ca²⁺ | Mn²⁺ | Pb²⁺ | Ti⁴⁺ |
| Bond Energy KJ/mol | 10,312 – 13,146 | 3,745 | 3,598 | 3,510 | 3,816 | 3,469 | 12,058 |
| Me | Al³⁺ | Zn²⁺ | Fe³⁺ | Li⁺ | Na⁺ | K⁺ | Ba²⁺ |
| Bond Energy KJ/mol | 7,201 – 7,858 | 3,037 | 3,845 | 1,469 | 1,347 | 1,251 | 3,213 |
Alkali metal (Li, Na, K)-O bonds are weakest but hard to remove due to charge balance role. Fe, Cu, Ca, Mn-O bonds are moderately removable. Al, Ti-O bonds are strongest, making Al and Ti the most recalcitrant lattice impurities.
During crystal structure change (e.g., quartz → cristobalite at ~1500°C), bonds break/reform, lattice expands (c-axis: 5.404Å → 6.971Å), potentially allowing impurity migration to surfaces. N₂ atmosphere may promote higher conversion than vacuum.
Below quartz melting point, impurities react with chlorinating agents (HCl, NH₄Cl, Cl₂) to form volatile chlorides. The accompanying phase transformation may aid impurity migration to the surface for reaction and prevent re-incorporation upon cooling.
Written by Ma Chao: Mineralogical Characteristics and Processing Technology Advances of High-Purity Quartz Raw Materials, Mineral Resources Protection and Utilization
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