Quartz, quartzite, and silica sand are the various forms of silica. The chemical composition of silica is SiO2. These forms of silica are used in a number of industries, the important being glass, foundry, sodium silicate, silicon alloys: iron and steel, refractory and ceramic industries. Quartz is mainly used in silicon alloys and ceramic industries, though at times it is also used in the manufacture of glass. Quartzite is used in the manufacture of silica refractories and as a flux in iron and steel industry, silica sand is mostly used in the glass, sodium silicate industries and foundries. The BIS has prescribed specifications for use in glass (IS: 28-1980) and foundry industries (IS: 1987 – 1974) 1. Glass Industry: Four grades of glass making sands are prescribed by the BIS for making glass. Grades:
Chemical: The chemical compositions of the four grades of sand shall be as follows. This does not cover silica sand used for optical and other special glasses.
These requirements shall be as agreed to between the purchaser and the supplier. Physical: The physical specifications of silica sand specifies that silica sand shall be fairly free from contamination like clay material, pebbles and other extraneous matter. The sand should not contain more than 4% moisture. Specifications for by user Industries: The natural silica sand is the most preferred material in a glass industry, but in some cases where the glass plants are located far away from silica deposits, crushed quartz is also used. For use in glass industry the silica sand must be uniform in chemical composition, size and shape of grains. Uniform grain size, promotes even melting in the glass tank. The sand should not be coarser than 20 or 30 mesh nor finer than 100 to 120 mesh. Very fine sand may be carried away by draft in the melting furnace. Both angular and rounded grains are used in the glass trade. As a general rule, the grains should be angular rather than rounded, because they melt more readily than the rounded ones. Silica sand usually contains small amounts of iron oxide, alumina, chromium, magnesium oxide, calcium oxide, potassium oxide because it imparts coloration to the glass. The common permissible limits of iron oxide in silica sand for use in the manufacture of different types of glass are as follows:
Among the other impurities mention may be made of chromium compounds alumina, lime and magnesia. Chromium compounds are undesirable since they have more coloring effect than iron. Alumina trends to decrease transparency and makes the batch more difficult to melt. The maximum quantity permissible in sand is 1.5%. The maximum allowable lime and magnesia is about 0.05% and for alkalies 0.01% or less. 2. Foundry Industry: Indian standards (IS: 3343 – 1965) specification for natural moulding sand for use in foundries. Natural moulding sand contains variable amount of clay, which acts as bond between the sand grains. This sand, therefore, posse strength, plasticity and refractoriness to varying extend depending upon the clay minerals present. When it contains a greater amount of clay, it is blended with river sand, which is relatively clay free so as to get the optimum properties desired in the sand mixture. Washed grains shall be mostly sub-angular to rounded shape. As far as possible, the sand shall be free from gravel. Natural moulding sand for use in foundries shall be of three main grades, namely, A, B and C with respect to clay content. Grade Clay % A 5 to 10 B 10 to 15 C 15 to 20 According to the grain size of moulding sand, each main grade shall be divided in to II sub-grades as indicates below: Grain Size Distribution of Molding S and
For example, natural moulding sand falling under grade ‘A’ with respect to clay content and grade 850 / 425 according to its grain size distribution, shall be designated as grade A 850 / 425. For different grades are discussed below: Clay Content: Clay content for grade ‘A’ shall not exceed 1.0% and for grades ‘B’ and ‘C’ it shall not be more than 2%. If required, clay content for grade ‘A’ may be accepted up to 2% subject to agreement between the supplier and the purchaser. Chemical Composition: High silica sand after washing off the clay matter shall conform to the requirements given below: Chemical Composition of High Silica Sand Requirement percent
Sintering temperatures: The sintering temperature range for grade A of high silica sand shall be 1685 – 1710° C. Grain Shape: Washed sand grains shall be mostly of sub angular to rounded shape. Grain Fineness: High silica sand for foundry purpose shall have a well defined grading with 70% and above of the sand grains retained by 3 adjacent sieves. Sorting coefficient for washed and grades sand shall range from 1-14 to 1-4 whiles for naturally bonded sands, it should be between 1.4 to 2.5. The fineness (grain distribution) of different grades of silica sand shall confirm to the requirements specified below:
(ii) Indian standards (IS: 1987 – 1974) specifications for high silica sand for use in foundries. High silica sand for use in foundries is different from that used in glass and refractory industries. Sand for foundry purpose should have well-developed grading, preferred shape and size, besides meeting the requirements of chemical composition. Sand should be essentially free from mica content. The grain size, shape and its distributions ultimately affect the moulding characteristics of the sand mixture. This standard covers the requirements for both nature and crushed sands. High silica sand for use in foundries shall be of three grades, namely, A, B, and C with respect to silica content of the sand and 6 sub-grades, grades 850/425, 600/300, 425/212, 300/150, 212/106 and 150/75 based on the distribution of sand grains. High silica sand is designated with a symbol, which should be a combination of gradation according to silica content and grain distribution. For example high silica sand falling under grade A with respect to silica content and grade 425/212 according to its grain distribution shall be designated as A 425/212. Specification regarding clay content, chemical composition, sintering temperature and grain fineness. GRAIN FINENESS OF SILICA SAND
(iii) Indian standards (IS: 3339 – 1975) specification for silica flour for use in foundries (First Revision). Silica flour is a good refractory material for moulding work. It is used particularly in the steel foundry in dressing for moulds and cores and also for adding to moulding sand mixtures. Silica flour is used to obtain elevated temperature strength, high density and resistance in metal crushing, washing and grading the high-grade quartz, partite rocks or from white silica sand or other deposits sufficiently pure to get the desired material. Chemical composition: - The silica flour, when analyzed shall confirm to the following requirements.
Fusion Point: - The fusion temperature of silica flour shall bot be below 1700° C. Grain Fineness: - 100% silica flour shall pass through 150 micron I. S. sieve and at least 95% shall pass through 75 micron I. S. sieve. If required silica flour of coarser variety may also be supplied subject to the agreement between purchaser and the manufacturer. 3. Sodium & Potassium Silicate Industry Sodium silicate is proposed by fusing silica sand or quartz powder (20 – 100 mesh) analyzing 99% SiO2 and less than 0.25%; Al2O3, 0.03% Fe2O3, and combined CaO/MgO content of 0.05% with sodium carbonate at 1200° C to 1400° C. The ratio of Na2O3 can be varied to obtain a wide range of products. Similarly, potassium silicate can be manufacture with silica mineral by using potassium carbonate in place of sodium carbonate. 4. Ferro-Silicon Industry Quartz suitable for ferro-silicon production should have more than 98% SiO2, less than 0.4% Al2O3 and not more than 0.2% each of Fe2O3, CaO and MgO. No phosphorous or arsenic should be present in quartz. If affects reduction in the electric furnace. Alkali has a tendency to promote a sticky slag that contaminates the products. If higher iron (more than 0.3%) is present in quartz, then fusion in the furnace takes place at lowest temperature and affects reduction process. Another important factor is that quartz should have good thermal stability at 1200° C or more. 5. Refractory Industry Quartzite is use in the manufacture of acid silica bricks as it was high refractoriness. Refractoriness lowered by the presence of fluxes such as lime, iron oxide, magnesia and alkalies. The purity of raw material i.e., the high silica content is essential with least possible Al2O3. The presence of 0.01% Al2O3 lowers the refractoriness of silica. Silica rock of metamorphic origin is better than that of igneous origin because the silica grains cemented with cristobalite and tridymite are stable phase of silica. Physically, quartzite should be of fine grained, compact and cryptocrystalline type. The raw materials sub-committee of the directorate general of technical development on refractories in its report dated September 1985 has stipulated the following specifications for quartzite.
Physical characteristics:
The thermal characteristic of quartzite should be such that (i) there is uniformity during thermal conversion; (ii) specify gravity of fired quartzite lumps in conventional kilns at 1430° C with proper firing schedule should be 1 less than 2.46, and (iii) fired quartzite lumps should be clean, white and spot free. 6. Iron and steel industry Quartzite is added to the blast furnace charge to correct silica alumina ration. The size of quartzite chips ranges from 10 to 80mm. The silica content should be 94% minimum and alumina 1% maximum. Some steel plants also use banded hematite quartzite in place of quartzite. 7. Cement Industry In cement industry silica sanded crushed sandstone are used as an additive to make for silica deficiencies in the raw mill. The silica content should be 65 – 98%, Fe2O3 –0.5- 6% Al2O3: 3.58 a0%, MgO: 1-105%. Standard sand as fine aggregate of particular specification is used for assessing the quality of cement in the laboratory by means of comprehensive strength test. The BIS specifications for standard sand are given below: - Indian Standard specification for standard sand for testing of cement (IS: 650, 1996) The standard sands play a very important role in the cement industry for testing the quality of cement. The standard lays down requirement for Indian standard sand used in test of cement. The standard sand shall be obtained from Ennore, Tamil Nadu. The sand with particle size greater than 1mm may also be obtained from Mudaliarkuppam, Tamil Nadu. Physical Characteristics: The standard sand shall be of quartz, of light gray or whitish variety and shall be free from silt. The sand grains shall be angular, the shape of the grains approximating to the special form, elongated and flattened grains being present only in very small or negligible quantities. The standard sand shall (100%) pass through 2mm I. S. sieve and shall be (100%) retained on 90 micron I.S. sieve with the following particle size.
Chemical Requirements: The standard sand shall be free from organic impurities. The loss of weight on exaction with not hydrochloric acid of specific gravity 1.16 shall be more than 0.25%. The minimum compressive strength requirements in the case of standard sand for ordinary Portland cement are as follows: 1.
For 3 Days not less than 100kg / square cm. 8. Fertilizer Industry Silica sand is used in fertilizer industry as filler to balance the fertilizer grades. There is no rigid specification for this industry. The size of silica sand should be 18 – 100mesh, with SiO2 content of 80% minimum and moisture 1% maximum. In some case even river sand is used. 9. Ceramic Industry Quartz is used in ceramic industry to reduce shrinkage in drying and firing and to impart rigidity to the body. Iron is most deleterious constitute and it should be as less as possible if Fe2O3 is more than 0.1% dot will form on the ceramic body and in the case of electrical insulator current mat pass. Most of the major ceramic units in the country consume high silica quartz with low iron content. In general silica should be 97 – 99.9% Fe2O3: 0.05% to 1%, Al2O3: 0.1% - 2%. 10. Silicon carbide and other abrasives Silica sand and quartz are used in the manufacture of silicon carbide grains. The sand should for this purpose contain more than 99% SiO2, less than 0.1% each of Fe2O3 and Al2O3, no lime, magnesia and phosphorous. Similarly, quartz should contain 99.50 – 99.75% SiO2, 0.04 – 0.05% Al2O3, 0.05 – 0.10% Fe2O3, 1.10% CaO and MgO. Quartz is also used for the manufacture of sand paper / cloth. For making this quartz (flint) should have high silica (99%) and low iron content as far as possible. Another abrasive use of silica sand is sand blasting where by sand is fired under pressure usually in the presence of water to suppress the dust against the material to be polished. For this purpose the chemical composition of the sand is relatively unimportant but size frequency distribution and the absence of clay clogging materials are very important. 11. Chemical Industry Quartz with +96% SiO2 is used in the manufacture of phosphorous-pent sulfide. Silica sand with +99% SiO2 and free of iron is used in the manufacture of ultramarine blue. 12. Asbestos Products Industry Silica powder with +97.5% SiO2, 0.8% Fe2O3 and 1.5% CaO is used as one of the constituents in the manufacture of asbestos – cement sheets and pipes. 13. Insecticide Industry Silica sand with +95% SiO2 is used as a carrier in the insecticide industry. The size of silica sand granules needs to be –15 +30 mesh. However, the specifications for this industry are not very rigid. 14. Electrode Industry Silica sand or quartz powder is used as one of the components of welding flux mixture in electrode industry, and for this purpose the silica minerals should contain 97.5 – 99.5% SiO2, 0.05% (max.) Fe2O3, 0.03% (max.) S, having + 100 – 200 mesh size. 15. Coal Washery Industry In coal washery silica sand is used as one of the ingredients in the flotation process of coal. Sand with a constant density of more than 2.64 is used. Grains must be sub-angular to round. Clay content is limited to 0.5% and no organic matter is tolerated size distribution range between 30 mesh and 100 mesh. 16. Paint Industry According to the BIS specifications (IS: 67 – 1979) the material should be of two types, namely, crystalline and amorphous. The material shall contain not less than 98% SiO2 and it shall be in dry powder form. Lead constant is SiO2 shall not exceed 0.03%. Color and particle shape shall match with the approved sample. 17. Rubber Industry Silica sand and quartz further are used as reinforcing filer to improve physical properties of rubber. Generally, silica mineral with + 98% SiO2 and less than 0.1% metallic impurities is preferred. 18. Water Filtration Sand and gravel are usually used as filter media for filtration of water. The BIS specification (IS: 8419 – 1977) for this further is as follows: Effective Size: - The particle diameter shall correspond to 10% finer on the grain size curve. The uniformity coefficient ratio shall be D60/D10 where D60 is the particles diameter corresponding to 60% finer on the grain size curve and D10 is the particle diameter corresponding to 10% finer on the grain size curve. i. Filter Sand: - Rounded grains are preferred to angular one for the removal of turbidity. It shall consist of hard and durable grains of silica sand and shall have a specific gravity not less than 2.5. The minimum SiO2 content shall be 90% and impurities like clay, silt, etc. shall not exceed more than 5% by volume. The acid soluble matter shall not be more than 5%. The loss on ignition shall not exceed 0.7%. ii. Filter Gravel: - It shall consist of hard and preferably rounded stones with an average specific gravity of not less than 2.5. It shall be free from clay, sand, loan and organic impurities. It shall contain not more than 2% by mass of thin, flat or elongated pieces determined by hand picking. The acid solubility for gravel sizes 10mm or larger should not exceed 10%, and for sizes smaller than 10mm it should be less than 5%. Gravel should be free from excessive amount of limestone and shall. 19. Sand / Lime Bricks Industry Large quantities of silica sand are used making sand-lime building bricks. For this purpose sand may contain up to 2% of clay but it should not at any rate exceed 5%. It should be evenly grades. At least 10% should be sharp and angular, reasonably clean and free from organic matter. 20. Plastering Mortars Natural and coarse sands are used in mortars for internal wall and ceiling plastering ans external plastering using mixes of lime, cement, gypsum and sand. The BIS specifications (IS: 1542 – 1977) for sand plaster are as under. The sand shall be hard, durable, clean and free from disoriented coating and organic matter. It shall not contain any appreciable amount of clay halls. Clay and dust-contents should not exceed 5% by weight. The sand should not contain impurities such as iron pyrite, alkalies, salts, coal, mica, shall organic impurities, etc. these impurities adversely affect the hardening, the strength, the durability and the appearance of plaster. In the inventory as on 01.01.1980, the reserves in some deposits were classified considering Fe2O3 as the key constituent and also other parameters, namely SiO2 and Al2O3 however, in many deposits the average figures for chemical analysis data were not reported. In some of these deposits the reserves were classified based on actual reported use while in the remaining deposits, the reserves were grouped under "Unclassified" grade. The classification was as under:
After considering the specifications of the user industries the exact group recommends the following classification for adoption in estimation of reserves according to end use.
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