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Kaolin & Fire ClayContentsClay Clay Minerals Groups of clay minerals Kaolin Kaolin and its deposits in Egypt Fire Clay Properties and tests of fire clay Industrial applications of fire clay Manufacturing Process of fire clayClay:Clay is a naturally occurring material composed primarily of finegrained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried or fired. Clay deposits are mostly composed of clay minerals (phyllosilicate m
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ClayClay MineralsGroups of clay mineralsKaolinKaolin and its deposits in EgyptFire ClayProperties and tests of fire clayIndustrial applications of fire clayManufacturing Process of fire clay
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Clay:Clay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range ofwater content, and which can be hardened when dried or fired. Claydeposits are mostly composed of clay minerals (phyllosilicateminerals), minerals which impart plasticity and harden when fired ordried, and variable amounts of water trapped in the mineral structureby polar attraction. Organic materials which do not impart plasticity mayalso be a part of clay deposits.
Clay minerals are typically formed over long periods of time by thegradual chemical weathering of rocks (usually silicate-bearing) by lowconcentrations of carbonic acid and other diluted solvents. Thesesolvents (usually acidic) migrate through the weathering rock afterleaching through upper weathered layers. In addition to the weatheringprocess, some clay minerals are formed by hydrothermal activity. Claydeposits may be formed in place as residual deposits, but thickdeposits usually are formed as the result of a secondary sedimentarydeposition process after they have been eroded and transported fromtheir original location of formation. Clay deposits are typicallyassociated with very low energy depositional environments such aslarge lake and marine deposits.
Clay Minerals:Clay minerals are hydrous aluminium phyllosilicates, sometimes withvariable amounts of iron, magnesium, alkali metals, alkaline earths andother cations. Clays have structures similar to the micas and thereforeform flat hexagonal sheets. Clay minerals are common weatheringproducts (including weathering of feldspar) and low temperaturehydrothermal alteration products. Clay minerals are very common infine grained sedimentary rocks such as shale, mudstone and siltstoneand in fine grained metamorphic slate and phyllite.
Clay minerals include the following groups:
1. Kaolin group which includes the minerals kaolinite, dickite,halloysite and nacrite.
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Some sources include the serpentine group due to structuralsimilarities (Bailey 1980).
2. Smectite group which includes dioctahedral smectites such asmontmorillonite and nontronite and trioctahedral smectites forexample saponite.[1]
3. Illite group which includes the clay-micas. Illite is the onlycommon mineral.
4. Chlorite group includes a wide variety of similar minerals withconsiderable chemical variation.
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Kaolin [ Kaolinite ]:Kaolinite is a clay mineral with the chemical composition Al2Si2O5(OH)4.It is a layered silicate mineral, with one tetrahedral sheet linked throughoxygen atoms to one octahedral sheet of alumina octahedra (Deer etal., 1992). Rocks that are rich in kaolinite are known as china clay orkaolin.
The name is derived from Gaolin高岭 ("High Hill") in Jingdezhen,Jiangxi province, China. Kaolinite was first described as a mineralspecies in 1867 for an occurrence in the Jari River basin of Brazil.[3]
Kaolinite is one of the most common minerals; it is mined, as kaolin, inBrazil, France, United Kingdom, Germany, India, Australia, Korea , thePeople's Republic of China, and the USA.
Chemical formula : Al2Si2O5(OH)4
Color: White, sometimes red, blue or brown tints from impurities
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Crystal System: Triclinic
Cleavage: Perfect
Moh's Scale of hardness : 2-2.5
Luster: dull and earthy
Streak: white
Specific Gravity: 2.16 - 2.68
Kaolin deposits are found in three main areas in Egypt :
1- Sinai [ Nitch's Valley, Mesba' Salama, and Farsh El Ghezlan.]2- Western Coast of Suez [ Abo El Darag, and El Galala El
Ba7areya.]3- Aswan [ kalabsha]
These kaolins can satisfy the local demand for filler, paper and ceramicindustries 10 times for at least 50 years, in comparison to the currentdemand. However, the Egyptian kaolin is hard and massive. It is alsolow grade so that it needs beneficiation to be suitable for paper andceramic industries, which import their needs at the present time.Beneficiation of the Egyptian kaolin from different localities indicatedtheir technical viability for ceramic production and as paper filler. In themeantime, the Egyptian government has created a new healthy climateto encourage investment in Egypt. For example, the governmentprovided many facilities to investors in the new cities, increased theperiod of free taxes to 10–20 years (depending on the project site) andreduced the interest of borrowing money from banks. All thesefacilities, among others, will have a positive impact on the economicviability of projects.
Kaolin reserve in Egypt is more than 200 million ton . it's used insome industries like:
1- Ceramics2- Rubber3- Paper
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Fire Clay :Fire clay is a specific kind of clay used in the manufacture of ceramics,especially fire brick. The fire attribution is given for its refractorycharacteristics. There are two types of fire clay: flint clay and plastic fireclay.It is resistant to high temperatures, has a fusion point higher than1,600°C, and therefore it is suitable for lining furnaces, as fire brick,and manufacture of utensils used in the metalworking industries, suchas crucibles, saggars, retorts, and glassware. Because of its stabilityduring firing in the kiln, it can be used to make complex items of potterysuch as pipes and sanitary ware. Its chemical composition consists of ahigh percentage of silicon and aluminium oxides, and a low percentageof the oxides of sodium, potassium, and calcium. Unlike conventionalbrick-making clay, it is mined at depth, usually found as a seatearthassociated with coal measures.
A good fireclay should have 24-26% plasticity and shrinkage after firingshould be within 6-8% maximum. It should also not contain more than25% Fe2O3.Because of the abundant supply of fireclay and its comparativecheapness, the refractory bricks made out of it are the most commonand extensively used in all places of heat generationA group of refractory clays which can stand temperatures abovePyrometric Cone Equivalent (PCE) 19 are called fireclay. The claywhich fuses below PCE 19 is not included under refractory. Fireclay isessentially of kaolinite group and has a composition similar to that ofchina-clay. In nature it is usually found to contain 24-32 per centAl2O3, 50-60% SiO2 and LOI between 9 to 12%. Impurities like oxidesof calcium, iron, titanium and magnesium and alkalies are invariablypresent, making it white, grey and black in colour.
Properties and TestsRefractoriness and plasticity are the two main properties needed infireclay for its suitability in the manufacture of refractory bricks. A goodfireclay, should have a high fusion point and good plasticity. Dependingupon their capacity to withstand high temperatures before melting, thefireclays are graded into the following:Low duty - withstand temperatures between 1515-1615°C (PCE 19 to28)Intermediate duty - 1650°C (PCE 30)
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High duty - 1700°C (PCE 32)Super duty - 1775°C (PCE 35)The pyrometric cone equivalent (PCE) of a particular fireclay asdesigned by Edward Orton, Jr., is determined by testing against aseries of standardised test pieces, cone shaped and having ceramiccomposition with different softening points, one withstanding a littlehigher temperature than the other.
The test pieces are generally made to form triangular pyramids havinga height 4 times the base. The softening point is reached dependingupon the temperature and the rate of rise of heat. Cones are numberedfrom 022, 021, 020, 02, 01, 1, 2 to 42. Where the softening range incones is too close, for example, in 21, 22, 24 and 25, they are omittedfrom the series and where the temperature range is widely spaced,extra cones like 311/2, 321/2 etc. are added. At the rate of 20ºC riseper hour in temperature the cones numbering 022 to 01 have softeningpoints between 585ºC to 1110ºC and those numbered 1 to 35 havesoftening points between 1125ºC to 1775ºC. Thus, the predeterminedpyrometric cone equivalents of standard test-pieces are placed alongwith cone made of the samples to be tested in the furnace and thePCE's of the samples are determined by comparison. The softeningpoint is noticed when the tip of the cone starts bending with the rise ofthe temperature. In practise it has been observed that the higher thealumina content in the fireclay, the higher is the fusion point. Allfireclays are not necessarily plastic clays. In such cases, some plasticclay, like ball clay is added to increase platicity to a suitable degree. Agood fireclay should have 24-26% plasticity and shrinkage after firingshould be within 6-8% maximum. It should also not contain more than25% Fe2O3. It has been observed that some clays lacking plasticitywhen allowed to 'weather', i.e. left in the open for a few months,become plastic due to the formation of humic acid in the clay. Non-plastic fireclay is also known as flint-clay. It may be called semi-flintand semi-plastic depending upon the degree of plasticity.
Industrial ApplicationsBecause of the abundant supply of fireclay and its comparativecheapness, the refractory bricks made out of it are the most commonand extensively used in all places of heat generation, like:in boiler furnacesglass melting furnaceschimney liningspottery kilnsblast furnaces
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reheating furnacesFireclay is classified under acid refractories. Acid refractories are thosewhich are not attacked by acid slag. In blast furnaces, the lining is donealmost entirely with fireclay bricks. Pouring refractories like sleeves,nozzles, stoppers and tuyers are made of fireclay.
Manufacturing ProcessManufacturing of refractory bricks from fire-clay is an interestingfeature. The clay mined is stacked in the factory yard and allowed toweather for about a year. For daily production of different types ofrefractories, this weathered clay is taken and mixed in differentpercentages with grog.
The mixture is sent to the grinding mill from where it is transferred tothe pug mill. In the pug mill a suitable proportion of water is added soas to give it proper plasticity. The mould is supplied to differentmachines for making standard bricks or shapes. Intricate shapes aremade by hand. The bricks thus made are then dried in hot floor driersand after drying they are loaded in kilns for firing. The firing ranges are,of course, different for different grades of refractories. After firing, thekilns are allowed to cool; then the bricks are unloaded. By burningfireclay is converted into a stone-like material, highly resistant to acid,water and most other solutions. While manufacturing high aluminousfire-bricks bauxite is added along with grog in suitable proportions.
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References:
1- Nagui A. Abdel-Khalek, The Egyptian kaolin: an outlook in theview of the new climate of investment
2- http://en.wikipedia.org/wiki/Clay
3- http://en.wikipedia.org/wiki/Clay_mineral
4- http://en.wikipedia.org/wiki/Kaolinite
5- http://www.sis.gov.eg/Ar/Pub/egyptmagazine/issue44/110407000000000010.htm
6- http://en.wikipedia.org/wiki/Fire_clay
7- http://www.mineralszone.com/minerals/fire-clay.html
