High purity quartz: under the spotlight

By Jessica Roberts
Published: Wednesday, 21 December 2011

With an estimated market size of 100,000 tpa, and intrinsic ties to the semiconductor and photovoltaic cell markets, high purity quartz is the downplayed strategic mineral of the green technology boom


Under the spotlight: high-temperature lamp tubing,
such as mercury and halogen lamps, uses a
high purity quartz envelope to contain the lighting
components


When is quartz ‘high purity’? Many players in the quartz and silica sand industries define the grade as starting at the 99.95% SiO2 level (see table- High purity quartz at a glance). This equates to 500 ppm of impurities- including boron, alkalis, and transition metals- which all affect the performance of the final quartz product.

At its Spruce Pine operation in North Carolina, Sibelco subsidiary Unimin Corp. produces the material that has set the purity benchmark for the rest of the market; a high purity quartz, marketed as Iota, which contains 20ppm of impurities as standard- equating to 99.998% SiO2. The group’s purest grade, Iota 8, is 99.9992% SiO2 in composition, containing only 80 parts per billion of impurities (see panel).

High purity quartz is the speciality feedstock for a handful of crucial markets. These include fused quartz crucibles for the manufacture of silicon metal ingots- later processed into silicon wafers for photovoltaic (PV) cell and semiconductor markets- in addition to high-temperature lamp tubing and other quartz glass applications (see panel).

A potential new growth market is high purity quartz for solar silicon production, which is being evaluated by companies seeking an alternative to the costly Siemens process.

In practice, total impurity levels are only a guide- as certain impurities have varying importance, depending on the final market for the quartz.

“In solar silicon markets, the main impurity concerns are from boron and phosphorous levels,” Dr Reiner Haus, managing director of German laboratory Dorfner Anzaplan, told IM. “In the quartz glass market, for instance lamp tubing, the main concern for producers is alkali levels and fluid or gaseous inclusions.”

“A manufacturer of crucibles for monocrystalline silicon production, the material used in semiconductors, needs the highest purity levels of less than 10 ppm,” Haus explained.

Yet a crucible producer for photovoltaic cells manufacture does not require such high specifications, so material containing <20 ppm is acceptable.

“This means the definition of high purity quartz very much depends on its final application. But the general qualification is using the standard specifications of the Iota material grades produced by Unimin,” Anzaplan explained.

Table 1: Quartz developmental products from Kelly’s Basin and Bovill Kaolin (BK) projects

Product

Kelly’s Basin (Q3)

BK Quartz 1

BK Quartz 2

BK Quartz 3

Particle size

-50 mesh

-50 mesh

-50 mesh

-50 mesh

SiO2, % (minimum)*

99.98

99.90

99.95

99.97

Element total, ppm (maximum)

200

1,000

300



* SiO2 calculated from oxides of contaminants

An issue of purity

Unimin’s stringent specifications mean that very few quartz producers worldwide qualify for the high purity quartz (HPQ) market.

“If we take the impurity level as <30ppm, then the global high purity quartz market is very small- no more than 70,000 tpa,” Haus revealed. “There is additional material which could be added to the total HPQ figures, used in the Chinese market, to make this figure up to an estimated maximum of 100,000 tpa.”

Chinese quartz has not yet been qualified to the Iota standard, although producers are working towards this with companies such as Dorfner Anzaplan, to obtain the technologies and processes that can improve quartz purities.

But the purity issue means that at present the market is dominated by two companies- Unimin, and its Spruce Pine neighbour The Quartz Corp.- with smaller companies, such as Russia’s KGOK JSC, competing for market share.

The Quartz Corp. (TQC) is a new joint venture created by two established industrial mineral producers, France’s Imerys SA and Norway’s Norsk Mineral. TQC is a merger of Imerys’ US subsidiaries KT Feldspar and The Feldspar Corp. with quartz producer Norwegian Crystallites, to form a new quartz, feldspar and mica unit.

“It was a natural joint venture to do,” Svein Olerud, CEO of The Quartz Corp., told IM, explaining that the main influencing factors behind the jv’s creation were TQC’s access to large reserves of high purity deposits in Spruce Pine, in addition to Norwegian Crystallites’ “advanced cleaning processes”, where clean, fresh water is the key point.

The next step for the company is expansion, with Olerud revealing that TQC wants to keep its market share in feldspar and mica but increase volumes in the HPQ market.

The company’s desire to increase its presence in high purity quartz is understandable; for the select few producers involved in this sector, it is a profitable yet strategic business- something which was apparent after Unimin’s quartz facility at Spruce Pine was damaged by a fire in December 2008. The accident essentially cut off the bulk of the world’s HPQ supply overnight, revealing the delicate supply balance that HPQ consumers face.

“HPQ is a niche market but it has a dramatic influence on many other markets, such as semiconductors,” Anzaplan’s Haus commented. “If you are not able to produce any quartz glass crucibles then immediately the semiconductor industry could break down.”

“If you think most of that material has been coming from one or two deposits in North Carolina, then how strategic is this mineral? But nobody realises this, because of the association with silica sand- something which is abundant,” Haus explained.





Processing requirements

Quartz deposits that are naturally high purity are limited, but this very factor- and the high prices the mineral can command- means that numerous juniors are attempting to qualify their material for the market. This can be achieved by upgrading raw materials through chemical and mechanical processing to lower impurity levels.

“We have clients delivering what I would call ‘high quality quartz’, with impurities above the Iota standard material. These companies are attempting to reduce impurity levels but their interest in this market depends on their access to suitable technologies and the level of investment required,” Anzaplan commented.

Iota-purity material is not easy to achieve; even if the numbers on a developer’s chemical analysis initially look promising, they still have to contend with the melting behaviour.

“For high purity quartz we have two subjects to distinguish: chemical purity, and melting behaviour,” Haus revealed.

Melting behaviour is mainly influenced by fluid inclusions within the quartz itself. The fluid inclusions form bubbles in the quartz glass later on, and if the inclusions comprise CO2 then they are unlikely to be detected during the chemical analysis. So while quartz containing inclusions might provide a promising result on the chemical analysis, the first melting test will reveal problems.

“One must find out in advance, by detailed chemical and mineralogical analyses, what kind of impurities are present in the quartz. Then one detects both the fluid inclusions as well as the mineral impurities, and can develop a tailor-made process route for that specific quartz,” Haus told IM.

There is no standard technology route that applies to processing high purity quartz, and the gap between qualifying for lamp tubing over semiconductor crucibles may only be a one-digit ppm difference.

“You apply the technologies which exactly remove the impurities detected. At the beginning of this process you may find that there are impurities which cannot be removed, which stops a project in its early stages,” Haus explained.

Over the last few years Anzaplan has developed new technologies to serve the HPQ market; these include innovations in contamination-free crushing and grinding technology, and technologies for reducing fluid inclusions by thermal treatment or selective fragmentation.

Selective fragmentation uses electrical discharge to fragment rocks through high tensile stress in the areas of crystal boundaries, inclusions or composite interfaces, causing the material to predominantly break at these boundaries. Pressures of up to 1010 Pa are generated by the pulse power, acting similarly to chemical explosives. Consequently, rocks are fragmented into their single mineral constituents with a high degree of selectivity- and once minerals are liberated, it is possible to separate them.

“For selective fragmentation we have a strong relationship with selFrag AG in Switzerland- that technology shows very good results for quartz, as you liberate along grain boundaries which are where most of the impurities are sitting,” Anzaplan’s Haus revealed. “In addition, selective fragmentation does not add any significant contaminations to the quartz during processing, another benefit compared to standard grinding.”

Anzaplan has also developed technologies in-house specifically to target fluid inclusions by opening them and removing the fluid or gaseous content- for instance using thermal processing through continuous heat treatment.

As Haus explained to IM: “The heat treatment of quartz itself has been known about for a long time. But to be able to apply this heat continuously without contaminating the material is the advancement.”

Selective fragmentation can be combined with optical sorting to separate impurities once they have been liberated from the ore. Optical sorting can identify differences between quartz grains- such as clear quartz (pure) and milky quartz (impure)- and separate them for further processing.

Thus a producer may be able to divert the clear quartz stream to high purity applications, and either further process the milky quartz (containing inclusions) to improve purity, or sell it to users with less stringent requirements- such as filler in electronics, where quartz is used in epoxy moulding compounds (EMC).

EMCs are used as an encapsulating layer around the finished silicon chip to protect it from heat, light, dust and other contaminants. The fillers are of lower purity than sand used for quartzware but typically have a smaller particle size- around 5-10 microns.

Although EMC filler prices are not in the same range as true HPQ material- which is around $5,000/tonne and upwards- the grade is not as sensitive to impurities such as alumina (it is sensitive to uranium and thorium, however) and the investment in processing technology is somewhat lower.

Exploration targets

HPQ’s premium price of >$5,000/tonne has made it the focus of numerous exploration targets. As The Quartz Corp’s Svein Olerud commented: “A lot of projects are trying to get into this market, as it obviously looks attractive from the outside.”

Projects that are trying to build up production and qualify for the HPQ market are from Australia, Belgium, Canada, Chile, China, France, Germany, Greece, India, Mauritius, Norway, Russia, Thailand, Turkey, and the US- to name a few.

“The time it takes to develop a deposit depends on many things, not only purity. The licences, environmental permits, logistics, all play a role,” explained Anzaplan. “We have overseen almost 90 HPQ projects worldwide, some stumbled at first hurdle, or second or third, it’s hard to get them into production.”

“Over the next few years we should see approvals for projects that can serve HPQ markets,” Haus revealed to IM. “At the moment we have six customers on development projects, one of these being Nordic Mining.”

Nordic, which is developing the 2.7m-tonne Nesodden quartz project in Kvinnherad, Hordaland in Norway, recently obtained positive results for high purity quartz grades after testing by Anzaplan.

“We have been successful in producing quartz grades that meet market specifications for several products in the HPQ market segment such as for crucible, semiconductor and microelectronics,” Mona Schanche, Nordic’s exploration manager, told IM. “We are still optimising the process to further improve quality.”

“At this point we view all HPQ markets as interesting. We see that we might be able to serve the solar market with our raw quartz and that we need to process the quartz to meet the higher value markets,” Schanche added.

Also currently testing material is Canadian junior i-minerals Inc., which is developing the kaolin-halloysite-quartz-feldspar mineral suite found at two deposits from its Helmer-Bovill project in the US state of Idaho.

i-minerals recently finished a high purity quartz pilot plant at North Carolina State University’s minerals research laboratory, obtaining bench scale results for Kelly’s Basin and Bovill Kaolin material (Table 1).

“The pilot plant produced samples so that companies interested in our quartz can test the product,” Linda Koep, i-minerals’ market development manager, told IM. “We have several companies agreeing to conduct a melt test on Kelly’s Basin Q3 and Bovill Kaolin Q2 and Q3 [samples]. The quartz will be available in January 2012 for those tests.”

New entrants

This niche market has seen two new entrants in 2011- both based in Russia.

Polar Quartz, based in Yugra, western Siberia, was established as a state enterprise in 2000, and three years later transformed into an open joint-stock company. Its project has faced delays starting production but in June 2011 it signed an agreement with RUSNANO, Khanty-Mansiysk, Corporation Ural Industrial, Ural Polar, and Khanty-Mansiysk Bank to develop a vertically-integrated quartz operation to produce quartz micro- and nanopowders and high purity quartz concentrates.

RUSNANO has invested $37.8m in the project, with Polar Quartz expected to reach production volumes of 10,200 tpa of high purity quartz concentrate and other quartz materials by 2016.

The investment group, a result of the reorganisation of state corporation Russian Corp. of Nanotechnologies, has also signed an agreement with KGOK JSC in Kyshtym, Chelyabinsk, to develop and expand the company’s existing high purity quartz production. The project, worth $75m, is expected to be finalised in 2014- with KGOK’s high purity quartz output increasing to 10,000 tpa.

RUSNANO has also been a key player in the establishment of Russian Quarz LLC, a joint venture between RUSNANO, Quartz-VIK LLC and KGOK JSC. Russian Quartz was created in August 2011 to focus on the production of high purity quartz and fused quartz crucibles.

Market drivers

Russia’s focus on the crucible market is understandable; this sector is the main driver for HPQ at present, owing to healthy consumption of silicon ingots for semiconductor and photovoltaic (PV) cell applications.

For companies with HPQ not yet classified to the Iota benchmark, crucibles can also provide a market for out-of-spec material. Chinese producers, for example, have been able to use some of their own quartz in combination with high purity grades to supply material for the lower purity crucible layers, Anzaplan’s Haus explained.

“On the outer crucible layers a lower quality material is used, and for the inner layer they apply the highest quality quartz- in some applications this works well,” Haus told IM.

In recent years China has opened a number of new crucible furnaces, but less than 30% of this capacity is understood to be active- as producers face problems with their quartz raw material.

Despite this, Chinese manufacturers have seen good demand from their customers, and China is now the fastest-growing market in the crucible sector.

“The US producers are trying to follow as well, building their capacities, but generally the quartz-consuming companies in Europe or the US are quite stable in their demand, with small improvements, while in Asia we see lots of new installations. And that’s driven, finally, by the photovoltaic market,” Haus explained.

Demand for photovoltaic (PV) cell installations has increased yearly by double-digits in recent times, although regional uptake of the solar technology has differed considerably.

Countries such as India, Russia and Saudi Arabia continue to implement extensive PV expansion plans despite the global economic uncertainty. South Korea’s KCC Corp. recently agreed to jointly invest $380m with Mutajadedah Energy Co. to build a polysilicon plant in Saudi Arabia by 2014. The plant is expected to have an initial nameplate capacity of 3,350 tpa of solar grade polysilicon, produced by 50:50 joint venture Polysilicon Technology Co. (PTC). The jv also intends to expand the plant to 12,000 tpa and investigate downstream manufacturing of ingot and wafers.

But growth in some regions has been balanced by a slowdown in others, with traditionally strong regions like Europe showing declining PV investment. SiC Processing GmbH, a leading supplier of recovered silicon carbide slurry used for the wire sawing of silicon metal wafers, announced in December 2011 that it had reduced working hours and production at its site in Bautzen, Germany owing to falling demand from the European market.

Yet SiC Processing is maintaining its interest and expansion activities in the Asian PV market. In November 2011, the company commissioned the first of six production lines at its new plant in Zhenjiang in China. Production lines no.2 and no.3 were expected to be complete in end-December 2011. After completion of all six lines in Zhenjiang, there will be approximately 90,000 tpa capacity of SiC recycled slurry available by end of 2012.

Semiconductor wafer producers have also been hit by European PV decline. A recent report on a leading global supplier, Wacker Chemie AG, by private equity research organisation Jeffries Group Inc. commented in late-2011: “It has become increasingly clear that second half demand pick up [for PV] is not as strong as we or the industry had expected.”

Jeffries points to the PV industry suffering from overcapacity and demand constraints, something that Anzaplan concurs.

“PV has been a genuine growth industry although we see them struggling by worldwide overcapacities these days,” Haus told IM. “While some more advanced countries have reduced their in-feed tariffs they still have growing demand, but growth is becoming smaller, while new growth markets in India, Asia and the US will pick up demand by 2012+.”

Haus forecasts that consolidation in the PV industry will continue over the next few years to compensate for the significant investments made into the Siemens process three to four years ago, prompted by a shortage in solar silicon supply.

Solar silicon alternatives

The Siemens process is a costly technology, using high amounts of energy and aggressive chemicals. Owing to this, other routes for the production of solar silicon have been evaluated- namely upgraded metallurgical silicon (UMG) and direct reduction (DR) processes- but each uses specific feedstocks.

“For the Siemens process one can use a standard metallurgical grade silicon which was manufactured from standard quartz,” Haus told IM, adding that technologies which need less investment, like UMG or direct reduction, require a purer material- as the purer the quartz, the purer the silicon.

In the direct reduction process, silica is reduced to silicon by making the silica the cathode in a bath of molten calcium chloride.

“Essentially what happens, in this electrochemical process, is the oxygen and some other impurities are removed from the silica, leaving silicon behind,” explained Professor Derek Fray, director of research at the University of Cambridge’s Department of Materials Science and Metallurgy. “The purity of the product depends upon the quality of the feedstock and may need to be electrorefined.”

The energy requirement and the CO2 footprint of DR is far lower than from the Siemens process. Fray told IM: “An independent company carried out an economic assessment and came to the conclusion that the cost for direct reduction/electrorefining should be about $8/kg and figure significantly less than for the carbothermic reduction followed by the Siemens process.”

DR requires a high purity quartz- not defined as the Iota material- but with very low boron and phosphorous content. Anzaplan’s Haus said: “You could live with a total impurity level of 100 ppm, but boron and phosphorous levels must be at the lowest point.”

Although DR has not yet been installed in a commercial operation, at least one pilot plant has been developed. The commercialisation of the project is managed by JR Jericho Resources Pty Ltd, an Australian silica resource and research private company, with the pilot plant planned to be established in Australia in association with MHM Metals Ltd, an ASX-listed resources company with high purity silica reserves, access to a competitive renewable energy source and experience in the manufacture of metals.

The DR market is clearly advancing, and in the future is expected to compete with the Siemens process on a lower-cost basis.

“That will be the future: the Siemens process on the one hand for the highest quality wafer solar modules, where you only have limited space to install, in competition with the DR and UMG processes where the overall costs are lower,” a European wafer producer told IM.

UMG upgrades metallurgical silicon on a chemical basis, unlike the Siemens process which uses the gaseous phase for purification. But to reduce the costs the UMG processors are looking for purer material that they can feed into the process, meaning pure quartz and pure carbon sources to reduce post-treatment costs.

“If we just assume that 20-30% of the future market will be based on high purity and high quality quartz, this would use up today’s production of high purity quartz,” Haus cautioned.

If UMG and DR solar silicon technologies take off, the other markets that consume high purity quartz will face additional competition- even traditional markets such as fused quartz crucibles. Clearly, there will be a supply strain on existing HPQ sources.

“The next 3-5 years will be key for the PV market, with companies watching how it grows- and also whether enough new quartz deposits to serve it are being discovered,” Haus told IM.

“I think there are enough deposits to serve the future market,” he added.

Supply security is now a chief concern for solar silicon producers, exhibited through a growing trend to secure upstream supplies. Those producing silicon are now looking for new deposits and exclusivity contracts with high purity quartz producers; so even when new deposits come on stream, there may not be enough high purity quartz available for each and every consumer.

Selected high-purity quartz supply and market movements

Company

Location

Comments

Creswick Quartz Pty Ltd

Creswick, Victoria, Australia

·         Creswick has secured quartz tailings located in the Creswick district of Victoria, being remnants from the gold-rush of the 1800s. Investigations confirm that this ‘industrial waste’ contains no gold within the quartz at ppb levels, and is low in impurities, especially boron and phosphorous

·         volume of above-ground quartz is in excess of 1m tonnes, and is claimed to be of uniform quality, while the company’s licensed area for below-ground minerals in the Creswick district is over 300km2

·         physical and chemical treatment has revealed that the tailings can produce various particle sizes that are 99.995% SiO2, a step towards the goal of 99.999% SiO2

·         quartz will be marketed for semiconductor and quartzware applications, including the manufacture of fused quartz crucibles

 

i-minerals Inc.

Helmer-Bovill, north-western Idaho, US

·         Vancouver-based junior is developing two deposits at its Helmer-Bovill project in Idaho; Kelly’s Basin and Bovill Kaolin, where extensive kaolin-halloysite-quartz-potassium feldspar resources exist

·         recently finished the high purity quartz pilot plant at North Carolina State University’s minerals research laboratory, obtaining bench scale results (see ‘Exploration’ chapter for details)

·         several companies have agreed to conduct a melt test on i-minerals’ highest purity quartz samples, beginning in January 2012

 

Mauritanian Minerals Co.

Oum Agueneina, Mauritania

·         Mauritanian Minerals Co. (MMC) is planning to produce high purity quartz at its deposit located at Oum Agueneina in Mauritania, 150km from Nouadibou port on the border with Western Sahara

·         MMC is targeting a production of 300,000 tpa quartz gravel for electrometallurgy and fused silica from the project’s 2,000km2 open-pit mine

·         Samples taken from the site and analysed under inductively coupled plasma mass spectrometry (ICP-MS) indicated total impurities ranging 0.6-44.3 ppm; many samples were within the high purity quartz specifications of Unimin’s Iota product

 

Momentive Performance Materials Inc.

Geesthacht, Germany; Hebron, Ohio, US

·         in September Momentive announced it would double the production of fused quartz crucibles at its facility in Geesthacht through a $14m expansion

·         Expansion aimed at fused quartz crucibles for silicon ingots, large-diameter fused quartz tubing, rods, and solid ingot in which silicon wafers are processed to make microchips

·         Geesthacht expansion will enable the site to increase production of large diameter fused quartz crucibles by approximately 100%, for monosilicon ingot production

·         in April 2011 the group also revealed it would expand operations at its speciality fused quartz facility at Hebron in Ohio, US, through a $5.4m investment to build a 2.6km2 production area

·         the expansion allows for a 30% increase in production capacity for the company’s largest diameter fused quartz tubing, which is sold to producers of leading-edge 300mm wafer processing furnaces

 

Nordic Mining

Kvinnherad, Hordaland, Norway

·         developing the 2.7m-tonne Nesodden quartz deposit in Norway, a hydrothermal quartz situated in Proterozoic basement rocks south of the Hardanger Fault Zone (HFZ). The HFZ is a 600km-long Caledonian ductile shear zone

·         in August 2011, Nordic obtained positive results for high purity quartz grades after testing by German laboratory Dorfner Anzaplan

·         two quartz types were tested; a 50 pmm sample and 100 pmm, yielding a mix with 16 ppm total impurities

 

Polar Quartz OJSC; RUSNANO

Yugra, western Siberia, Russia

·         in mid-June 2011, RUSNANO, Khanty-Mansiysk, Corporation Ural Industrial, Ural Polar, and Khanty-Mansiysk Bank signed an agreement for the development of a vertically-integrated quartz operation headed by Polar Quartz, to produce quartz micro- and nanopowders and high purity quartz concentrates

·         the investment from RUSNANO totals $37.8m, with Polar Quartz expected to reach production volumes of 10,200 tpa of high purity quartz in 2016

 

The Quartz Corp.

Spruce Pine, North Carolina, US

·         in March 2011, French minerals group Imerys combined its US-based Spruce Pine companies with Norsk Mineral’s Norwegian Crystallites to form a new quartz, feldspar and mica joint venture

·         the companies, KT Feldspar and The Feldspar Corp. (TFC), and Norwegian Crystallites are now owned and operated by 50:50 joint venture The Quartz Corp.

·         TQC will invest in improving the flotation processes at Spruce Pine to improve the quality of the quartz, and plans to significantly expand processing capacity in Norway, although TQC’s total capacity is unknown

 

Russian Quarz LLC; RUSNANO; KGOK JSC

Kyshtym, Chelyabinsk, Russia

·         in early August 2011, Russian Quartz LLC was established as a joint venture between KGOK JSC, Quartz-VIK LLC and RUSNANO JSC to focus on the production of  high purity quartz

·         RUSNANO and KGOK have also signed an agreement to develop and expand KGOK’s existing high purity quartz production

·         the deal, which will use quartz from KGOK’s deposit, is worth $75m with around a third of this supplied by RUSNANO

·         the project is expected to be finalised in 2014, with KGOK’s high purity quartz output growing to 10,000 tpa

·         in addition to raw materials supply, Russian Quartz LLC is understood to be working with RUSNANO on the establishment of a fused quartz crucible business

 

Unimin Corp.

Spruce Pine, North Carolina, US

·         in mid-2010 Unimin announced it would double high purity quartz feedstock capacity at its operations in Spruce Pine and install a new flotation plant to complement its existing Schoolhouse facility

·         the new plant feeds Unimin’s two refinery plants – Red Hill and Crystal – to produce its branded Iota quartz, ensuring stable supply globally

 




High purity quartz- at a glance

High purity quartz is defined as containing <30 ppm of impurities, or a grade of >99.997% silicon dioxide (SiO2). It is primarily found in the Spruce Pine deposits of North Carolina, US, where Unimin Corp. and The Quartz Corp. both have operations. Producers in countries such as China and Russia have quartz very close in purity to the Spruce Pine material, although it has not yet been qualified to Unimin standards. The global market is around 70,000 tpa for ‘on-spec’ material, but can be increased to around 100,000 tpa if off-spec production is included.

There are generally three markets for HPQ: 1) lamp tubing, which is very steady; 2) fused quartz crucibles for silicon production used in semiconductors, typically a cyclical market; and 3) fused quartz crucibles for photovoltaic (PV) cell-grade silicon production, which is generally a well-performing market although this differs regionally.

For HPQ producers it is important to be able to balance these different markets; a quartz producer depending solely on the semiconductor business is exposed to every upturn and downturn in demand, whereas a company also serving the steadier lamp tubing and PV markets has more stability.

The importance of purity

Alkali content:
devitrification and sagging are the primary limiting factors in high-temperature glass performance. Trace alkalis such as lithium, potassium and sodium can have a significant negative impact on the thermal stability of quartzware such as crucibles, by accelerating the dissolution of quartz vessels into the pure silicon melt. The alkalis act as a flux, lowering the melting temperature of the quartzware.

Transition metals: of particular importance in the production of high purity quartz glass, transition metals can dissolve or diffuse into the silicon and negatively impact the predictability and reliability of the device. Advanced processing methods can reduce transition metals, including Cr, Cu, Fe, Mn, Ni, and Zn, to the ppb level.

Boron content: in solar silicon applications, it is particularly important that low and consistent boron levels are maintained in fused quartz crucibles, including Cz-type crucibles used for monocrystalline silicon production. Unimin states that boron does not respond to conventional chemical refining, as the boron ion is “strongly bonded into the silicate network” rather than occurring as boron-containing minerals. Thus it is necessary to source PV-grade crucible quartz from deposits with a naturally low boron content.

Typical silica sand and quartz specifications, by market

Type or application

SiO2 minimum (%)

Other elements (maximum %)

Other elements (maximum ppm)

Market size (m tpa)

Typical price ($/tonne)

Clear glass-grade sand

99.5

0.5

5,000

>70

30

Semiconductor filler, LCD and optical glass

99.8

0.2

2,000

2

150

‘Low grade’ high purity quartz

99.95

0.05

500

0.75

300

‘Medium grade’ high purity quartz

99.99

0.01

100

0.25

500

‘High grade’ high purity quartz*

99.997

0.003

30

<0.1

~5,000


*‘High grade’ high purity quartz, with <30 ppm, is the standard high purity material (Iota) produced by Unimin Corp. at Spruce Pine

 

Note 1: Specific other elements may be limited by application. E.g. Fe2O3 <100 ppm for float glass and <40 ppm for low-iron float glass

 

Note 2: Generally ‘high purity’ quartz has Fe2O3 <15 ppm, Al2O3 <300 ppm, and alkali and alkali earth oxides <150 ppm

 

Note 3: In some applications Al2O3 can substitute for some SiO2, e.g. up to 1.5% Al2O3 in float glass

 

Note 4: Limits can vary according to the composition of other raw materials in the application

 

Source: Richard Flook


Spotlight on Unimin Corp’s Spruce Pine operations

A subsidiary of Belgian industrial minerals group Sibelco, Unimin is the world’s leading source of high purity quartz from its operations in North Carolina. The quartz, sold under the Iota brand, is the purity benchmark for numerous juniors attempting to enter this speciality sector.

In the 1980s Unimin acquired Spruce Pine-based Industrial Minerals and Chemicals Corp. (IMC), the original developer of the Iota material. It followed this with the purchase of Feldspar Corp’s quartz business in 1991 (Imerys went on to buy Feldspar Corp. from Zemex in 2007; the business is now part of The Quartz Corp. joint-venture between Imerys and Norsk Mineral’s Norwegian Crystallites).

The Spruce Pine alaskite ore is the starting point for Unimin’s Iota products, formed in the Carboniferous period around 300-350m years ago during the Alleghenian (or Appalachian) orogeny. The ore is a result of localised geological processes, including temperature, pressure and hydrothermal alteration during greenschist metamorphism. The greenschist overprint is most observed at Unimin’s highest quality desposit, Harris, where Iota 4, 6, and 8 material is sourced.

Unimin high purity quartz grades

Iota standard (99.998% SiO2): suitable for a clear fused quartz product with a low coefficient of thermal expansion. Quartz glass applications include mercury and halogen lamps, and semiconductor quartzware, where a lower cost material is preferred.

Iota 4 (99.999% SiO2): produces fused quartz with good viscosity and vitrification properties. Raw material is processed to reduce total K, Li, and Na content to 1.4 ppm. Grade is primarily used in process tubing, wafer handling systems, quartz block, and semiconductor grade crucibles for growing monocrystalline silicon ingots.

Iota 6 (99.9991% SiO2): suitable for a fused quartz with a combined K, Li, and Na content of 0.5 ppm and Fe content of 0.15 ppm. Applications include quartzware with low-alkali requirements, diffusion tubing, and Czochralski (Cz) type crucibles where diffusion of contaminants in the solid state cannot be tolerated.

Iota 8 (99.9992% SiO2): raw material for ultra-high purity quartz glass for >12-inch wafers. Used in demanding applications, the total K, Li, and Na content of Iota 8 is reduced to 80 ppb and critical transition metals are <50 ppb. Primarily used in larger diameter crucibles where improved viscosity provides wall stability and high purity eliminates the need for synthetic liners.