CHAPTER ONE
Introduction and literature review
1.0 Introduction
1.1 Research Background
A ceramic is an earthy material usually of silicate nature and may be defined as a combination of one or more metals with a non-metallic element usually oxygen. The American ceramic society (1986) had defined ceramics as inorganic non-metallic materials, which are typically crystalline in nature, and are compounds formed between metallic and non-metallic elements. Common examples are; silica – (SiO2) the main ingredient in most glass products; alumina- (Al2O3), used in various applications from abrasives to artificial bones; and more complex compounds such as hydrous aluminium silicate (Al2Si2O5 (OH)4), the main ingredient in most clay product ( Reed, 2001). The evaluation of the potential use of clay deposits in Gwandu town of Kebbi state in ceramic production is the focus of the present study.
1.2 Clay as Minerals
Clays have been known and used by man since ages for varied purposes (Prentice, 1990). clay has come to be defined as a natural earthy, fine-granular material that has a maximum size of about two microns (2µ=0.002mm) that acquires plasticity on being mixed with limited quantity of water (hydro-plasticity) and showing sheet-like crystallographic habit (Velde, 1992; Ideniyi, 2003). From a chemical or mineralogical standpoint, clay is complex aluminosilicate compounds containing attached water molecules, which have their origin in the chemical and mechanical disintegration of rocks, such as granites (Nwajagu, 2005).
The term ‘mineral’ has several connotations. Used and strictly defined by the mineralogist, it refers to a naturally occurring solid inorganic substance with distinctive physical properties and a composition that can be described by a chemical formula (Mc Kelvery, 1986). Similarly, Hurbut and Klein (1977) observed that minerals are inorganic homogenous crystalline solids with chemical composition and an ordered atomic structure/arrangement.
1.3 Clay Mineral Groups
One basic property of clay minerals is the capacity of certain clays to change volume by absorbing water molecules or other polar ions into their structure. This is called the swelling property. Based on this property, Valde (1992) broadly classified all clays into swelling and non-swelling type. Swelling clays are called smectites. The important property according to Valde (1992) is the basic composition and structure of the clays; and this is used to further classify the clay minerals into:
The Kaolinite group (Al2Si2O10 (OH6)):this group has one silica and one alumina unit stacked in alternating fashion (1:1 lattice type). No ion or water molecules can enter the adjacent layers, and only the external surfaces determine their colloidal properties. Owing to its relatively large particles and low specific surface, kaolinite exhibits less plasticity, cohesion and swelling as compared to other clay minerals.
The illite group KAl2 (Si3,Al)O10(OH)2: this group belongs to the hydrated micas within the 2:1 lattice type and consists of two silica and one single alumina unit. Aluminium (Al) replaces the silicon (Si) in the silica units and the negative charges are compensated by potassium (K) atoms which are partially embedded within the units and bind them firmly together, so the expansion of the lattice is effectively prevented.
The vermiculite group: MgAl2 (Si3, Al)O10 (OH)2 this is similar to the illite group except that the magnesium (Mg) rather than K is the exchangeable cation of the interlayer. As the Mg ions are highly hydrated, both an interlayer of exchangeable cations and water hold the units of the 2:1 lattice together.
The montmorillonite or smectite group (Mg, Ca) OAl2O3.5SiO2.H2O: is a 2:1 lattice clay type and contains minerals like montmorillonite, beidellite and nontronite. Some substitution of Al for Si has occurred in the silica unit and of Fe and Mg for Al in the alumina unit. The layers are not tightly bound, so that water, ions and even organic compounds can enter between them. This causes interlayer swelling and expansion of the crystal lattice, contrary to the hydrated micas.
The chlorite group (Mg,Fe)4 Al4Si2O4(OH)8: has a 2:2 lattice type wherein Mg rather than Al is the predominant ion in the alumina unit. Essentially, the chlorite structure consists of two silica, one alumina and one magnesia unit. Substitution of Al for Si in the silica unit and of Al and Fe for Mg in the magnesia unit results in electrostatic binding of the structural units. (Raymond, 1990)
The attapulgite or palygorskite group: has a 2:1 lattice that extent in only one direction, thus resulting in needle or chain-like minerals. Two silica units enclose the magnesia unit because Mg predominates over Al. Isomorphous substitution of Al for Si in the silica unit is rather small but isomorphous substitution of Al and some other cations such as iron (Fe) and manganese (Mn) in the magnesia unit is high. (Borode, 2000)
1.4 Clay transformation on Heating
According to Velde (1992) there are four ranges of temperature which produces characteristic transformations in clay materials. The free-water dehydration range (dryingrange) (50-120oC); the clay stability range (120-600oC); the anhydrous clay range (600-
900oC); and the re-crystallization range (above 900oC)”. In the manufacture of ceramics (bricks and tile inclusive), the 600-1000oC zone is of greatest importance in transforming the dried clay into a new, more rigid substance. In this range the interaction of the clay and nonclay additives occurs to form new materials or physical states (glass or new crystalline phases).
In clays an important volume change (Shrinkage) takes place as they recrystallize into other phases, losing their crystalline water above 1000oC (Velde, 1992). This loss is important to the ceramic process. When firing brings the material into this thermal region, the shrinkage effect must be modified by the addition of sufficient non-clay materials, called temper or grits, sands of various types, pure quartz, alkali feldspar, grog (ground-up burned refractory materials) chamotte (calcined and ground-up kaolinite or fire clay) are used depending on the quality and use of the ceramic product (Nwajagu, 2005)
1.5 Characteristics of Clay Minerals
Plasticity
This is the most important characteristic of clays. The word clay comes from the German verb kleben, “to sticks” to and that is of course, what immediately comes to mind when we think of clay. It is a material that “sticks to” the hand and “sticks to” itself in respond to the touch of the hand. In other words, the identifying characteristic of plasticity is intrinsic to the meaning of the word clay.
Plasticity refers to the ability of materials to form and retain the shape directed by an outside force. “This is one of the most important of the properties of clay and one which is least understood”. The plasticity of most clay minerals derives from the unique crystal structure of their molecules, which are minute in size and platelike in shape. These crystals form flat, two dimensional sheets that touch each other on only two sides. There is a disproportionately large ratio of surface area to mass in these platy crystals. When water floods these two-dimensional sheets, it creates a strong bond between them in much the same way that a wet, flat plate bonds to a table surface. The water also acts as a lubricant and causes the platy crystals to slide over one another. The strongly bonded, sliding sheets will take on whatever shape they are directed toward by an outside force.
Particle Sizes
Particle size is an all-important characteristic of clay minerals and is of crucial importance in the identification of clay minerals. All clay minerals possess a fine grained, minute particle size. “Clays are characterized primarily by their small particle size, which is usually taken as less than 2µm. Coarse, medium and fine clays have ranges about 2.0-0.5, 0.5-0.2, and below 0.2µm respectively.
Subject to the exception noted above, the fineness of the particle size determines the plasticity of the clay mineral. The minute clay crystal contains more surface area and therefore greater water-bonding capacity than the larger particle size crystals of other materials, such as feldspar. Consequently, the bonds between the clay particles and the sliding power produced by water increase in strength as the particles become smaller and minute. Hence, clay with smaller particle sizes is more plastic and takes on more water than clays with a larger particle size.
Chemical and Crystalline Structure
All clays contain significant amount of silica (SiO2), alumina (Al2O3) and water (H2O). More important is the fact that the atoms are bonded together in flat, two-dimensional sheets of two or more layers. Sheets of silica, alumina, and other metallic oxides are interspersed with sheets of hydrogen-oxygen molecules (HOH). The top layer consists of silicon and oxygen atoms; the second layer is alumina and (in the case of clay minerals other than perfectly ordered kaolinite) other metallic ions such as magnesium, calcium, iron, sodium, and potassium. The third layer contains the hydrogen and oxygen atoms. It is the second layer of aluminium and other metallic atoms that holds the most interest. This is the layer in which exchanges of aluminium are made with other metallic atoms. The exchange property of the aluminum layers create the complexity and disorder of some clay molecules, for here, many different atoms can appear. This fascinating exchange property includes the ability to store, exchange, and transfer energy, and it is the reason certain clays are thought to be a possible link between inorganic and organic matter. (Callister, 2003)
Metamorphosis by Fire
A unique feature of clay is their metamorphosis by fire into a stronger material. Most earth materials are weakened and broken by the heat of the fire. Clays, on the other hand, exchange their fragile and perishable existence for a hard, durable form that is capable of lasting for thousands of years. They become hard, water- impermeable materials with a new crystalline structure and different physical properties. (Ijagbemi, 2005)
Shrinkage
Shrinkage is the loss in volume of clay when it dries or when it is fired. Both drying and firing shrinkages are important properties of clay used for structured clay products. Drying shrinkage is dependent on water content, the character of the clay minerals, and the particle size of the constituents. Drying shrinkages is high in almost every plastic clay and tends to produce a week, porous body. Montmorillonite in relatively large amounts (10-25%) cause excessive shrinkage, cracking, and slow drying. Firing shrinkage on the other hand depends on the volatile materials preset, the types of crystalline phase changes that take place during firing, and the dehydration characteristic of the clay minerals (Parker, 1982).
Colour
Colour is important in most structural clay products, particularly the maintenance of uniform colour. The colour of a product is influenced by firing temperature and degree of vitrification, the proportion of alumina, lime, and magnesia in the clay material, and the composition of the fire gases during the burning operation. The best white-burning clays contain less than 1% Fe2O3. Buff burning clays contain 1-5% Fe2O3 and redburning clays contain 5% of more Fe2O3 (Parker, 1982).
1.6 Categories of Ceramics
Ceramics are classified into three basic categories; these are:
Traditional Ceramics:
These are ceramics based on mineral silicate, silica, and mineral oxides that are found in nature. The primary products are fired clay (pottery, tableware, brick and tile), cement, and natural abrasives such as alumina. The mineral silicates, such as clays of various compositions, and silica, such as quartz, are among the most abundant substances in nature and constitute the principal raw-materials for traditional ceramics.
(Basoum, 1997).
New Ceramics
The term new ceramics refers to improvements in processing techniques that provide greater control over the structures and properties of ceramic materials. In general, new ceramics are based on compounds other than variations of aluminum silicate, which form most of the traditional ceramic materials. New ceramics are ceramic materials developed synthetically over the last several decades, usually simpler chemically than traditional ceramics for example oxides, carbides, nitrides, and borides. (Barsoum, 1997)
Glass
As a type of ceramic, glass is an inorganic, non-metallic compound (or mixture of compounds) that cools to a rigid condition without crystallizing, or is essentially an amorphous solid made by fusing silica (silicon dioxide) with basic oxides which has been cooled to a hard condition without crystallization (Burgmn, 1970). Glasses are based primarily on silica and distinguished by their non-crystalline structure. Other ingredients in glass include: Sodium oxide (Na2O), calcium oxide (CaO), aluminum oxide (Al2O3),
Magnesium oxide (MgO), Potassium oxide (K2O), Lead oxide (PbO), and Boron Oxide (B2O3). The importances of these ingredients are: They act as flux (promoting fusion) during heating, increase fluidity in molten glass for processing; improve chemical resistance against attack by acids, basic substances, or water, add color to the glass.
The actual silica sand requirements in glass making process fall into two groups namely: the degree of purity, indicated by the chemical composition of the sand and the natural physical state of the sand. (Waudby, 1994). Irrespective of glass to be manufacture, the degree of purity required varies. According to Tooley (1987) iron content is perhaps the most critical chemical component for all glass making raw materials. Iron control is important because of its effect on glass colour particularly white and flint glasses (Reed, 2001)
1.7 Clay types used in Ceramic Industries
Kaolin or China Clay
Though relatively scarce in nature, kaolin is of particular interest to potter. It is indispensable in the making of pure white porcelain or china clay. The whitest-burning kaolin clays, and hence the most pure, are primary clay that were weathered at the site of the feldspar. They are coarse in particle size and are therefore less plastic compared to most sedimentary clays. In chemical composition kaolins approach the formula of the mineral kaolinite (Al2O3.2SiO2.2H2O). Kaolin is highly refractory clay and has a melting point above 18000C. Used by itself, kaolin is difficult to shape into objects because of its poor plasticity, and also, because of its refractoriness, it is difficult to mature by firing to a hard, dense object. In practice, therefore, kaolin is seldom used by itself; other materials are added to it to increase it workability and to lower the kiln temperature necessary to produce a hard; dense product. As would be expected, the shrinkage of the kaolin is low because of its relatively coarse grain structure, and it has a little dry strength (Omowumi, 2000)
Ball clay
Ball clay are somewhat the opposite; of kaolin in their properties. They are high in iron content, more fusible, much more plastic, and fine in particle size. Ball clay and kaolin clay are really complementary in character and are often combine in clay bodies to adjust the mixture toward practical, workable clay.
Ball clay is highly plastic, although not so pure as kaolin; it is relatively free from iron and other mineral impurities and burns to a light gray or light buff colour. It tightens into a dense structure when fired to about 18000C. Different ball clays vary considerably in composition.
Ball clays are impossible to be use alone in pottery because of their excessive shrinkage, which may be as high as 20 percent when fired to maturity. They are usually used as an admixture to other clays to gain increase plasticity and workability. In manufacturing whitewares, ball clay is indispensable as an addition to the body to overcome the nonplastic properties of kaolin. However, if whitness is desired, not more than about 15 percent of ball clay can be added to a clay body; more than this amount in a whiteware body results in a gray, off-white, or buff colour. The presence of ball clay in a porcelain body decreases its translucence.
Fire clay
Fire clay is not so well defined as a type of clay as either ball clay or kaolin. The term
‘fire clay’ refers to refractoriness or resistance to heat and clays which vary widely in other properties may be called fire clays if they are refractory. Some fire clays are very plastic and some lack plasticity, and the fired colour may vary. Any clay which resists fusion or deformation up to about 15000C may be called fire clay. Such refractoriness or resistance to heat means that the clay is relatively pure and free from iron, although most fire clays burn to a buff or brownish colour. Sometimes with darker splotches due to the concentrations of iron-bearing minerals. (John, 2003)
Fire clays are useful for a great variety of products, principally in the manufacture of fire brick and other refractory parts of kilns, furnaces, boilers, and melting pots.
Industries such as steel, copper, and other metallurgical industries could not operate without fire brick furnaces in which high-temperature smelting is done.
Stoneware Clay
Stoneware clays are plastic clays which mature or become vitreous at 12000C to 13000C. Their fired colour ranges from a light gray or buff to a darker gray or brown. Stoneware clays are secondary or sedimentary clays. They vary widely in colour, plasticity, and firing range, and there is no sharp distinction between what might be called a fire clay or stoneware clay. The classification really hinges upon the possible use of the clay in ceramics, rather than upon the actual chemical or physical nature of the clay or its geological. One clay, for instance, might be successfully used both as a fire in the making of bricks and refractories and as a stoneware clay in the making of high-fired stoneware. Many types of clay are quite suitable for making stoneware without any additions. Such clays may have just the right plasticity for wheel and may have desirable drying and firing characteristics.
Earthenware Clay
Most of the usable clay found in nature might be called ‘earthenware’ clay or common clay. These clays contain iron and other mineral impurities in sufficient quantity to cause the clay to become tight and hard-fired at 9500C-11000C. In the raw such clay is red, brown, greenish, or gray, as a result of the presence of iron oxide. Fired, the clay may vary in colour from pink to buff to tan, red, brown, or black, depending on the clay and the condition of the firing. Most of the pottery the world over has been made of