In-depth Analysis: What is High-Purity Quartz Technology?

High-purity quartz refers to quartz series products with a SiO2 purity greater than 99.9%. It is the material foundation of high-end products in the silicon industry, widely used in industries such as photovoltaics, electronic information, optical communication, and electroluminescent sources. It holds an important position and role in the strategic emerging industries of new materials and new energy.

Based on SiO2 purity, it can be classified into:

• Low-end with SiO2 ≥ 99.9% (3N)
• Mid-end with SiO2 ≥ 99.99% (4N)
• High-end with SiO2 ≥ 99.998% (4N8)

It can also be classified based on the total amount of impurity elements like Al, B, Li, K, Na, Ca, Mg, Ti, Fe, Mn, Cu, Cr, Ni, etc., into:

• Low-end ≤ 1000×10^-6
• Mid-end ≤ 100×10^-6
• High-end ≤ 20×10^-6

Each purity grade of high-purity quartz can be divided into varieties such as 40-80 mesh, 80-140 mesh, 80-200 mesh, 80-300 mesh, and so on.

High-purity quartz technology is a systematic engineering project that includes high-purity quartz raw material selection technology, processing technology, processing equipment technology, and quality inspection technology. These aspects are both independent and interrelated, forming a comprehensive technological whole.

Initially, high-purity quartz was processed from first and second-grade natural crystals. Natural crystals are usually formed in crystal cavity environments under certain geological conditions. The specificity of their genesis results in two inherent deficiencies:

1. Small reserves and poor mining conditions, which, after years of development and utilization, inevitably lead to resource scarcity, high prices, and an inability to meet the needs of large-scale industrial production.

2. The chemical composition of mineral crystals is unstable and influenced by changes in the crystalline environment. This leads to significant fluctuations in raw material chemical composition in large-scale industrial applications, making raw material standardization difficult and unable to meet the needs of high-end, high-purity quartz product production.

Thus, it is necessary to start with other quartz mineral resources to fundamentally solve the high-purity quartz raw material problem, which is the basic technical approach domestically and internationally.

In the 1990s, Japan processed transparent high-purity quartz using fine-grained quartzite as raw material.

Russia and Germany processed high-purity quartz using vein quartz and metamorphic quartzite as raw materials.

In the 1980s, the American company PPCC processed high-purity quartz using granite from the Foxdale area of the northwest coast of England as the raw material for Western European quartz glass. The product’s SiO2 purity was 4N, Fe content < 1×10^-6, and other impurity element contents < 5×10^-6.

Starting in the 1990s, the American company Unimin began to develop and utilize the pegmatite granite in the Spruce Pine area of North Carolina effectively. It has developed high-purity quartz series products such as IOTA-STD (standard grade), IOTA-4, IOTA-6, and IOTA-8, almost monopolizing the international market and becoming the international standard.

It is evident that apart from natural crystal, vein quartz, and granite quartz among the six genesis above, quartz mineral resources are ideal raw materials for processing mid-end and high-end high-purity quartz products.

Not all vein quartz and granite quartz can be processed into high-purity quartz, given the current level of processing technology. Only a very few, even exceptionally rare, can be processed into high-end products.

That is, choosing vein quartz or granite quartz is only the right general direction; it does not solve the key issue of specific raw material selection.

The main reason is the existence of various subdivided genesis types of vein quartz and granite, influenced by the ore-forming geological conditions. There are also significant differences in the mineralogy, petrology, and ore deposit characteristics of vein quartz and granite of the same genesis type.

According to reports, the American company Unimin is very selective about high-purity quartz raw materials and has strict requirements.

Unimin Quartz Raw Material Selection Criteria: One is quartz with the least impurities in the crystal structure, such as IOTA-STD aluminum content (14-18)×10^-6, IOTA-4 aluminum content (8-10)×10^-6; the other is quartz with fewer gas-liquid inclusions, such as pegmatite granite and crystal.

It has been shown that the content of impurity elements in the raw material does not simply correspond to its quality. Instead, it relates to the selectivity of impurities determined by the raw material’s process mineralogical characteristics. For example, despite the high impurity element content in the Spruce Pine pegmatite rock samples in the United States, they are used as raw materials for IOTA’s high-end products.

Currently, the main processing technologies for high-purity quartz include grading, scrubbing, chemical acid leaching, flotation (both fluorine-containing and non-fluorine flotation), gravity separation, magnetic separation, chlorination roasting, and microbial leaching. The raw materials used include vein quartz, pegmatite granite, quartzite, and quartz sandstone.

Vein quartz is a magmatic-hydrothermal vein related to granite, mostly in irregular vein forms. Vein quartz is pure white with a greasy luster and high purity, with its SiO2 content exceeding 99%. In China, vein quartz mines are mainly located in areas such as Jiangsu Donghai, Sichuan, Heilongjiang, Hubei, etc. Hubei Province’s Qichun County has quartz stone reserves exceeding 100 million tons with a silicon content of more than 99.98%, ranking first in the country.

Quartzite is formed from siliceous rocks or quartz sandstones through a series of metamorphosis and thermal contact, with quartz mineral content generally exceeding 85%. It is often associated with tourmaline, zircon, mica, feldspar, and clay minerals, with hardness and density higher than that of quartz sandstone. Quartzite mines are distributed in Qinghai, Anhui, Liaoning, Shaanxi, etc., and are one of the main sources of siliceous mineral raw materials in China.

Quartz sandstone is a consolidated clastic rock with a quartz fragment content of more than 95%. It is often associated with tourmaline, rutile, magnetite, mica, feldspar, and clay minerals. In China, quartz sandstone mines are distributed in Sichuan, Hunan, Jiangsu, Zhejiang, Yunnan, Shandong, etc. They are the main raw materials for processing glass, ceramics, casting, and other quartz industrial minerals and materials.

The raw material for the American Unimin TOTA series high-purity quartz sand is pegmatite granite. However, research in this area could be stronger in China, and no reported achievements have been made in processing high-purity quartz sand from pegmatite granite.

Compared to general mineral processing engineering, high-purity quartz sand processing equipment has the following characteristics:

Acid leaching and water washing are important links in the high-purity quartz sand processing technology. Due to the extremely high SiO2 purity requirements and low content of impurity elements in high-purity quartz, the purity of acids and water used must meet corresponding requirements; otherwise, it is difficult to produce qualified products.

Hot acid leaching plays a key role in the purification processing of high-purity quartz. One of quartz’s important chemical properties is excellent acid resistance (except for HF), while other metal impurity components in the ore generally have poor acid resistance. This effect is more pronounced under certain temperature conditions.

The high-purity quartz processing acid leaching technology utilizes this principle to achieve chemical purification. Studies have shown that using the appropriate acid formula according to the characteristics of the mineral raw materials can better remove metal minerals, iron-containing minerals, carbonate minerals, and thin film iron between quartz particles in the raw materials.

If a certain amount of HF acid is added to the acid formula combination, it has a better effect on removing trace mica and feldspar impurities in the raw materials. Therefore, strong corrosive reagents such as hot acid and HF acid are often used.

Practice has proven that in the purification processing of high-purity quartz, any materials in contact with the raw materials, such as containers, significantly impact the quality of the samples. Strict control of the material standards in all processing links of high-purity quartz sand is the key to ensuring quality.

The characteristics of high-purity quartz SiO2 purity ensure that there can be no pollution during the production process. However, due to the long processing flow and complex technology of high-purity quartz sand, it isn’t easy to fully seal the production process.

To prevent air dust pollution, strict requirements must be imposed on the air environment for production, packaging, storage, etc.

High Safety Requirements: The production line composed of strong corrosive reagents, toxic gases (if chlorination roasting is used), high temperatures, etc., must have higher production safety guarantees.

The special nature of the above process conditions determines the high requirements for high-purity quartz processing production equipment. Developing safe, environmentally friendly, energy-saving, and efficient production equipment is the key condition for realizing scale and industrialization.

The total content of impurity elements such as Al, B, Li, K, Na, Ca, Mg, Ti, Fe, Mn, Cu, Cr, Ni, etc., in the American Unimin IOTA-STD products is usually < 20×10^-6, with a maximum value < 22×10^-6. For such high-purity substances, chemical analysis methods and X-ray fluorescence spectroscopy (XRF) are difficult to meet their quality inspection requirements.

For the detection of metal elements, especially trace metal elements, inductively coupled plasma optical emission spectrometry (ICP-OES) has the most advantages, with good detection limits, high detection accuracy, short time consumption, and high sensitivity. Currently, ICP-OES has become an effective method for detecting trace chemical components of high-purity materials.

ICP detection technology is an important support and component of high-purity quartz technology, which has practical and theoretical significance for promoting the development of China’s high-purity quartz technology.

High-purity quartz has stable physical and chemical properties, with characteristics such as low impurity content and difficult ore dissolution. In the process of dissolving and leaching high-purity quartz detection samples, the basic factors involved include sample weight, reagent combination, reagent dosage, reagent purity, etc.

This technology includes sample preparation and instrument detection, which are two major parts. The key technology is the sample’s dissolution and leaching preparation.

Experiments have shown that in the sample preparation process, the sample weight, reagent combination, reagent dosage, and reagent purity used will have an important impact on the ICP detection results.

The amount of high-purity quartz used ≥2000mg; reagent purity is a high-purity grade (MOS or BV-III), reagent combination is HF+HNO3; concentrated HNO3 is used in three times, with a total amount ≥5mL; HF dosage is 25mL.

According to the processing technology characteristics and purity requirements of high-purity quartz sand, steel sieves must not be used throughout the entire sample preparation process to avoid iron contamination.

Additionally, conducting high-purity quartz sample dissolution and leaching preparation in ultra-clean laboratory conditions will help avoid air impurity pollution and reduce detection errors.

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