QUARTZ AS THE MAIN SOURCE OF SILICA AND ITS INDUSTRIAL USES

Krishna P. Kaphle*, Panipokhari, Maharajgunj, Kathmandu, Nepal

kkaphle@hotmail.com

INTRODUCTION

Quartz is a member of Tectosilicate Group. It is a common rock forming mineral which occurs freely as a main mineral constituent in almost all the igneous (except in basic and ultra-basic rocks), metamorphic and sedimentary rocks. It is usually colourless or milky white but frequently coloured due to presence of various impurities. Its hardness is 7 in Moh's hardness scale and specific gravity is 2.65. It shows conchoidal fracture and mainly vitreous luster. The chemical composition is SiO₂. It is characterized by its 7 hardness, vitreous/ glassy luster, conchoidal fracture and crystal form. The optical properties of quartz are colourless, transparent, fresh, weak birefringence, transverse sections are isotropic and shows positive uniaxial interference figure. It possesses strong piezoelectric and pyroelectric properties. Common types of quartz which occur in the rocks are as follows.

Rock Crystal is the purest and most transparent form of quartz crystal (having SiO₂ > 99.8%). They mostly occur in the pegmatite or quartz veins/ dykes and as dog tooth spar in cavity fillings.

Fig1: Quartz crystal from Khejemi, Taplejung, Eastern Nepal

Vein quartz: Massive quartz obtained from the central core of zoned complex pegmatite and from veins, dykes and plugs within igneous and metamorphic host rocks. Most of the vein quartz deposits are believed to have formed by hydrothermal processes. They also occur as fissure and cavity fillings and less commonly as metasomatic replacement bodies.

Quartzite: A common metamorphic rock derived from sandstone/ arenite. Quartz is the chief mineral constituent in it. Its chemical purity can be range from 95 - 99.8% SiO₂. Fine grained quartzite are suitable for the fabrication of millstone, grindstone etc.

Quartz Arenite (Sandstone): It is a sedimentary rock which contains quartz as the chief mineral constituent. Its SiO₂ content is more than 95%.

Novaculite: A very dense, hard, fine grained highly siliceous rock (sp.gr. 2.63 – 2.64). Its chemical composition/ purity can exceeds 99% SiO₂.

QUARTZ OCCURRENCES/ PROSPECTS/ MINES

In Nepal there exists favourable geological environment for quart mineralization. Occurrences of quartz veins and pegmatites are recoded mainly in the Gneiss, Schist (Higher crystalline rocks) in the Higher Himalayan region and in Granite, Schist and Gneiss in the Lesser Himalayan regions. Many quartz bearing pegmatites and hydrothermal quartz veins are known from Taplejung, Ilam, Sakhuwasabha, Nawakot, Rasuwa, Dhadhing, Gorkha, Manang, Jajarkot, Bajhang, Bajura, Baitadi and Dadeldhura districts. Quartzites are the common metamorphic rock that occur both in the Higher and Lesser Himalayan region of Nepal. But they are yet to be well explored and evaluated for industrial use. However, quart crystals are mined from Raluka (Nawakot district), Khejemi/ Sirku (Taplejung district) in small scale by private sectors. Transparent quartz crystals are exported to Singapore, Japan, Switzerland, Germany, UK and USA. Production of Rock crystal/ quartz in the last Five years is given in the table below (Table-1). 2 prospecting licenses and 2 mining licenses for quartz are issued from DEpartment of Mines and Geology (DMG). Only 2 mines (1 in Taplejung, and 1 in Nawakot) are in operation at present. Promising sites for such quartz has yet to be explored in many parts of the Higher Himalayan region. However, preliminary geological investigation indicate that there are better chances to find out high grade/ pure quartz in the pegmatites along with other gem minerals like Tourmaline, Beryl/ Aquamarine, Garnet etc. in Nepal.

Table-1: Quartz production in Nepal

Fiscal Year	Total quartz production (mt.)	Industrial quality (mt.)	Gem quality (kg)	Mines in operation	Remarks
2002	173.720	172	1720	2 Small scale mine	Most of them are exported
2003	178. 265	176.5	1765	2 Small scale mine	Most of them are exported
2004	122.715	121.5	1215	2 Small scale mine	Most of them are exported
2005	110.292	109.2	1092	2 Small scale mine	Most of them are exported
2006	95.889	94.94	949	Small scale mine	Most of them are exported

Source: DMG Newsletter Vol. 4, 5, 6, 7, 8 and 9

Quartz is a common rock forming mineral which occurs freely in almost all the igneous (except in basic and ultra-basic rocks), metamorphic and sedimentary rocks. However, their amount varies considerably in different rock types. Silica is the main constituents in all silicate minerals. Pure transparent quart crystal consists of 99.5 - 99.98% SiO₂. Hydrothermal quartz veins are the most common sources of electronic grade quartz crystal. Pegmatites associated with granite intrusion also yield very pure quartz. The cores of some of the pegmatite bodies are commonly contain abundant fluid inclusions, which render the quartz unsuitable for fusing into clear quartz glass. Massive quartz veins of hydrothermal origin may have zoned structure. Quartz, quartzite, sandstone (arenite), silica sand, novaculite, flint, chert, vein quartz, rock crystals (quartz crystals) are the main sources of silica. They are the special materials used as silica in many industries as mentioned below.

Primary quartz deposit can occur in the form of veins, pipes, pockets, stock-works and composite lodes confined to siliceous host rock. They are composed mostly of very large transparent crystals or milky or gray coloured quartz that grew inward from the walls of druses cavities to form comb structure (Dog tooth spar). Vugs are abundant and some lined with wall formed quartz crystals (Fig.1). Most of the clear and electronic quality quartz crystals are especially smaller that weigh less than 250g. Most Piezoelectric grade quartz, however, occurs as clear or translucent pyramidal terminations on milky quartz crystals that can extend one ton in weight.

 

INDUSTRIAL AND OTHER USES OF QUARTZ (SILICA)

Quartz has many uses. Some crystalline varieties like water clear Rock crystal, Amethyst, Rose quartz, Citrine, Smokey quartz, Milkey quartz and other cryptocrystalline verities like Chalcedony/ Agate, Flint, and Jasper can be cut into gems and used in jewelllery. Quartz is also used as an abrasive material, in mortar, sand paper, toothpaste, soaps, filter, paints, porcelain, foundries, and to manufacture of glass, idol craft material, sodium silicate, ceramics, pottery, silica bricks, fused silica ware etc. High purity metallurgical high grade silica (Quartz 99.5 - 99.82% SiO₂ avg.) is used in silicon metal and ferrosilicon alloys production. High grade quartz is also used for refractory, metallurgical and other uses. Depending upon its purity the best quality water clear crystal with polished faces quartz is used in optical, electronic, and scientific apparatus. And similarly depending on the piezoelectric properties quartz is used to control the frequency of radio circuits. In many countries high grade quartz is used predominantly in the manufacture of fused quartz ware for the electronic industries and other metallurgical uses. Quartz is the chief source of silica. Impure quartzite and sandstone are used as building stone and paving stones. Some of the major uses are briefly mentioned below.

 

Electrometallurgy: Metallurgical quality quartz is used in the production of silicon metals. Silicon alloys and as a flux in the smelting of elemental iron, nickel, zinc, copper, lead etc.

Steel making: In electric furnace quart/ silica helps to remelt scrap, in blast furnaces it helps to reduce iron ore pellets to pig iron and in basic oxygen furnace it helps to convert iron into steel. The principal use of silica rock in steel making is as acid flux and slag conditioner at blast furnace operation.

Abrasives: Quartz or novaculite cut into whetstone, oilstone, files and other special shapes for craft uses.

Acid Tanks and Towers: Acid towers can be packed with quartz gravel. It helps in drying and purification of gases e.g. chlorine.

Refractories: Quartzite and quartz pebbles are crushed and used as granister in manufacturing refractory bricks, tiles, kiln furnishing, mortars, Bessemer converters etc.

Electronics and optics: Natural electronic grade quartz crystal can be used in electronics and optics. Now a day, in the market synthetic crystals are replacing the natural crystal.

 

Other Uses:

Non whetstone grade novaculite is ground and used as filler/ extender in latex paints, silicon rubber and plastic casting, molding and potting compounds such as for large electrical insulators.  

High purity quartz river gravel is used in 3 to 4 layers in Air preheaters. It operates continuesly at temperature 1165^o-1294^oC and is used in the thermal destruction of volatile organic compounds present in the off gases from industrial facilities.

Rapid gravity filters at municipal water treatment contain quartz gravel of 2.54cm at the base and 3.2 at the top (for total/ thickness 30.5 to 45.7cm). 

Quartz suitable for refractory purpose must have 99.19% SiO₂, 0.02 – 0.03% Fe₂O₃, 0.1 to 0.25% Al₂O₃ and 0.045% Na₂O + K₂O

EXPLORATION

In Nepal exploration of quartz as such is not done. However, during exploration of gemstones especially tourmaline and beryl lots of pegmaties were studied and evaluated in different parts of the country. Some of the promising pegmatites for tourmaline and beryl are known from Sankhuwasabha, Ilam, Taplejung, Manang and Jajarkot area. Two private companies are engaged to mine crystal quartz in Raluka (Nawakot) and in Khejemi/ Sirku (Taplejung), Table-1. Two other Prospecting license holders are also exploring quarts in Nepal.

During the exploration of quartz a reconnaissance geological study is must to select the possible sites for large Pegmatite bodies, Vein quartz, Quartzite, Novaculite, Sandstone etc. Once the site is identified follow up detail investigation is required to calculate the tentative reserve of the deposit and its grade i.e. amount of silica content in it. After that the evaluation of commercial potential normally requires:

-          Drilling, drill core sampling, testing to calculate the reserve/ tonnage and grade (possible diamond drill) and prove the deposit.

-          Chemical tests of the samples

-          Economic evaluation and feasibility studies

-          Industrial testing of samples

If all these detail studies help to confirm the deposit as an economic one and appears feasible for mining then only select the suitable type and environment friendly mining method with a view to obtain maximum amount of mineable quartz reserve.

MINING AND PROCESSING

Vein type quartz deposits are generally small in size and irregular in shape. Therefore, in most cases they are mined by simple method in open pits. Where as the large deposits with enormous quantities of quartz are extracted either by opencast mining or underground. To recover only a few kg of piezoelectric crystals (which can be used for piezoelectric and optical application) huge amount of quart has to be mined. The rest other than piezoelectric crystals are used for other purposes. For Example in World War II, 40,000m3 of vein quartz was mined out of that only 200t of rock crystal obtained. Out of which only 2 tons of the crystals were suitable for piezoelectric application (Radio components). Cloudiness, cracks, fluid bubbles, other inclusions and impurities etc. degrade the quality. Proper processing is highly important. Processing of metallurgical grade quartzite consists only of crushing, washing, and screening.

Metallurgical Application/ Use

Depending on the purity/ quality of quartz they are treated differently for specific use. High quality crushed quartz or quartzite pebbles are used in the electric furnaces for the production of silicon metal, ferrosilicon (FeSi) and other silicon alloys. Silica metal producers prefer to use quartz or quartzite lump that exceeds 2.54cm diameter; and have a minimum softening point of 1700^oC and do not decrepitates below 950^oC. The rock/ mineral should contain at least 98.5 to 99% SiO₂ (preferably 99.3 to 99.8% SiO₂) and less than 0.01% Fe₂O₃, 0.15% Al₂O₃, 0.2% CaO, 0.2% MgO, 0.2% LOI. Fe₂O₃ and CaO are the critical impurities for metallurgical grade.

For Chemical Grade Silicon Production: Silica must have high reactivity and very low alumina 0.1% Al₂O₃ content and < 0.05% FeO₂, 0.005% CaO and 0.002% TiO₂. There should not be any phosphorus, sulphur, arsenic impurities at all.

For Ferrosilicon Production: The producer can accommodate smaller lumps of silica rock ranging from 0.32 to 10.16 cm in diameter. It should contain > 98% SiO₂, <0.4% Al₂O₃ (preferably < 0.25%), <0.2% Fe₂O_3. Its alumina content influences the consumption of electric energy during smelting.

For Abrasive Application: In grinding mills, Flint/ quartz/ jasper must have high crushing strength/ high hardness, high specific gravity, toughness/ durability high purity, homogeneity, resistance to staining, fracturing, cheeping even in high temperature.

Electronics: For electronic and optics quartz crystals should weigh no less than 50 to 100g. Piezoelectric grade crystals should contain no fracture, no liquid bubble inclusion, no rutile fibers, and no intergrowth with other minerals. The defective ones can be used in producing synthetic quartz crystals. Non piezoelectric grade pieces of ultra pure quartz that weight 10 to 30g can also be fused and fabricated into optical fiber performs (e.g. rods and tubings) precision optical and electro-optical blanks, billets and prisms, quartz ware apparatus etc.

CONCLUSION AND RECOMMENDATION

There is a high possibility to find pure quartz crystals in quart veins and pegmatites in Granites, Schists, Gneisses in the Lesser and Higher Himalayan region of Nepal. Since the quartz can be used in many industries the international market is very good. Therefore, it is recommended that DMG and private sectors should explore these resources and try to exploit/ mine them in environment friendly manner.

Reference:

Donald D. Carr (Edited), Industrial Minerals and rocks 6th Edition, 1994. 

H.H. Read, Rutleys Elements of Mineralogy, 25th Edition ,Published by Thomas Murby & Co. London.

James D. Dan, Danas  Manual of Mineralogy, 17th Edition, Published by John Wiley & Sons, Inc. London 1959.

Paul F. Keer Optical Mineralogy, 3rd Edition, Published by McGra Hill Book Company, Inc.,  Japan (Asian edition).

K.P. Kaphle (edited), DMG Newsletter Volume 5, 6, 7, 8 and 9 (year 2002 to 2006), Published by Department of Mines and Geology.

 Mineral Resources of Nepal (2004) published by Department of Mines and Geology.

ECONOMIC FACTORS / PRICE

Table: Average unit value of specially silica material products in the USA ($/Ton)

Year	1986	1987	1988	1989	1990	2001	2002	2003	2004	2005
1. Special silica stone
1.1 Crude	514	354	299	163	506
1.2 Finished products	14,082	12,319	14,595	14,480	14,062
2. Silica Gravel
2.1 Silicon and ferrosilicon	10.19	10.77	11.45	12.06	11.60
2.2 Filtration	25.72	18.50	5.56	13.57	16.26
2.3 Flux	-	-	8.82	-	-
2.4 Other uses	16.25	11.50	10.66	-	-
Overall Average	11.00	11.22	10.22	11.45	11.62

 (Reference: Industrial Minerals and rocks 6th Edition Donald D. Carr, Senior Editor 1994  and ……….