Electrical Porcelain Production From Selected Kaolin Deposit in South Western Nigeria Using Slip Casting

The production of Electrical porcelain by slip casting using Ikere-Ekiti Kaolin and clay from South Western Nigeria was the focus of this work. Atomic Absorbtion Spectrometric(AAS) analyses of samples of Ikere -Ekit i Kaolin and clay, Ile-Ife clay and Iwo Kaolin was done. AAS analyses showed Iwo Kaolin and Ile -ife clay having low alumina and high impurity contents making them low refractory. Init ial tests confirmed its low refractoriness as Iwo Kaolin could not withstand the bisque firing at 900°C cracking extensively. Thus Ile-ife clay and Iwo Kaolin were disqualified as candidate materials for electrical porcelain production. Ikere-Ekiti kao lin and clay found to be high in alumina content and having low impurity contents were used in the experimental production. Results showed Ikere –Ekiti kaolin and clay suitable for porcelain production. Values of standard refractory tests fell with in standard values for porcelain production.


Introduction
Kaolin is commercial clay co mposed principally of the hydrated aluminosilicate clay mineral kaolinite. The co mmercial value of kaolin is based on the mineral"s whiteness and fineness, but controllable part icle size wh ich may be optimized during processing. Particle size affects fluid ity, strength, plasticity, colour, abrasiveness and ease of dispersion. Other important properties include the flat particle shape, which increases opacity and hiding power, its soft and non abrasive texture, due to the absence of coarser impurities, an its chemical inertness. These key properties distinguish kaolin fro m other kaolin itic clays like ball clay and fireclay. The kaolinite content of processed grades of kaolin varies, but is generally in the range of 75% to 94%. Associated minerals may have considerable influence on the suitability of the clay for a particular application. Kao lin fro m different parts of the world have markedly different properties.
Kaolin has a chemical formu la of Al 2 Si 2 O 5 (OH) 4 . It is non plastic with a Mohr scale hardness of 2 to 2.5. It has a dull and earthy luster and a refractive index o f α 1.553 -1.565, β 1.559 -1.569, γ 1.569 -1.570, with a specific gravity of 2.16 -2.68. The melting temperature is as h igh as over 1700℃ and it is triclin ic in its crystal system [1].
On the other hand, ball clay is an earth material of very fine part icle size which forms as an end result of the the res idu e due to weat hering o r by hyd ro thermal act ion which is a result of sedimentary deposit. Clay is cohesive and usually plastic when wet. It serves as a primary binder and fires in different co lours depending on the types and co mpositions. It shrinks when dry and expands when wet. It is a poor conductor and that is why clay materials are used as thermal insulators. The chief clay minerals are kaolinites and mont morilonite and the impurities that are usually present are; silica, ferric o xide, chro miu m, magnesium, lime potash, berylliu m, vanadium, tantalu m and they mostly occur in oxide form. These impurities that are present in clay usually impact aesthetic qualities to raw and fired clays, wh ich is of importance in Agriculture, Geology and Engineering. These clay materials, are used for making furnace linings, kilns, nozzles, stoves crucibles and ladles for pouring mo lten metal, heat exchangers and driers. There is ongoing research as to its suitability for car engine blocks. The presence of some of the impurit ies mentioned above makes the industrial application of clay possible, since these impurities possess the needed property, refractoriness; which is the temperature at which a material softens, melt or fuses. Clay materials are usually stable at high temperatures and this makes them have good thermal shock, i.e. ability to retain their original forms without cracking, spalling or flaking under sudden thermal changes and to have good resistance to environmental attack. It is their refractoriness and other properties like fusibility, poor heat and electrical conductivity, porosity, permeability, plasticity, slag resistance, which are expected for a clay material are determined by the type and quality of impurities and mineral contents of such a clay, which in turn depends on the deposit and mode of formation of that particular clay. There has been concerted effort at assessing local clays with the intention of proffering possible industrial usage and possibility of b lending [7,8,9]. The objective of th is work therefore was to see the possibility of developing standard electrical porcelain fro m a blend of local raw clay materials.

Materials
Kaolin and potter"s clay samples fro m Ikere-Ekiti in Ekiti State, Kaolin fro m Iwo in Osun State and clay fro m Ile -Ife also in Osun state in Nigeria were co llected for laboratory analyses. The chemical analyses of the clays were done using Atomic Absorption Spectrometer (AAS), and the results are presented in Table 1.

Method
Close proximity of Iwo Kaolin and Ife clay to the Obafemi Awolowo Un iversity campus prompted the testing of the materials for possibility of their being useful for electrical porcelain production. Initial co mposition tests however showed that Iwo kaolin could not withstand the bisque firing, which was done at 900℃, as the sample had lots of cracks. Thus it was abandoned as a candidate material for Kaolin production. Ekiti clay, known for h igh alu minosilicate co ntent [5,7], hence high refractoriness [10,11], was used instead.
The samples used were of four different co mpositions. Each of the four samp les was subjected to standard refractory tests [12,13]. The tests performed were; dry and fired shrinkage, porosity, compressive strength and bulk density tests.

Determination of Dry and Fired Shrinkage
The samples were marked along a line, in order to maintain the same position after heat treat ment. A distance of 2cm was measured with the vernier caliper as the init ial length of the sample. The samples were air-dried for 24 hours and oven dried at 100℃ for another 24 hours. The length at this stage was taken and was recorded as dry length. The samples were then fired to 1200℃ for 6 hours. The samples were cooled to room temperature and the length measurements recorded.

Determination of Porosity
After firing, the samples were soaked in a desiccator vacuum which was filled with water. However, the dry weights of the samples were taken before soaking in water. They were p laced in a dessicator vacuum and evacuated and were left to soak for 2 hours. The samples were then removed and the soaked weights (W s ) were taken. The e xpression for porosity is given in equation (3) %Porosity = (W s -W d )/W d x 100% (3) Where W s = soaked weight; W d = dry weight

Determination of Co mpressive Strength
Test pieces of the samp les were prepared to a standard size of 633.47mm 2 cross sectional area on a flat surface. The samples were fired in a furnace at 1200℃ and the temperature maintained for 5 hours. The samples were then cooled to room temperature. The specimens were placed on a co mpressive tester and load was applied axially by turning the hand wheel at a uniform rate until failure occurs. The manometer read ings were recorded and comp ressive strength was calculated using equation (4).

Determination of Bulk Density
Prepared samp les were air dried for 24 hours and oven dried at 100℃, cooled in a dessiccator and weighed to the accuracy of 0.001g, after wh ich the specimen was transferred to the beaker and heated for 30 minutes to assist in releasing the trapped air. The specimen was cooled and soaked weight (W s ) taken. The specimen was then suspended in water using beaker placed on a balance. The suspended weight (S) was taken. The bulk density was calculated fro m equation (5) Bulk Density = (W d x D w )/(W s -S) (5) Where W d =Dry weight, D w =Density of water W s =Soaked weight, S=Suspended weight 2.2.5. Determination of Loss on Ignition 50g of the samp le was dried at 100℃ and cooled in the desiccator. A porcelain crucib le was cleaned, dried and weighed (m 1 ) to the nearest 0.001g. The dried sample was introduced into the crucible and the crucible together with the clay sample ions weighed (m 2 ) to an accuracy of 0.001g. Deposit in South Western Nigeria Using Slip Casting The crucible containing the clay sample was placed in a muffle furnace and heated to a temperature of 100℃ for three hours. The crucible and its contents were cooled in a desiccators and then weighed (m 3 ) to the nearest 0.001g. The Loss on Ignition (LO1) was calculated using equation (6) Where m 1 = mass of porcelain crucible (6) m 1 = mass of porcelain crucible m 2 = mass of sample and proclaim crucib le m 3 = mass of fired clay sample and proclaim crucib le The method used in the production of this electrical po rcelain is slip casting. The slurry slip was cast into the plaster moulds of the insulator shape. After about 20 minutes, the excess slip in this case was drained off. The mould was opened and the insulator removed and allowed to d ry at room temperature.
The electrical porcelain was dried at 100℃ in the oven with accurate temperature control, after which it was fired to a bisque temperature of 900℃. The porcelain was d ipped in a glaze, after which it was dried and glost fired to 1300℃. The process flow chart is as shown in Fig.1. Table 2 shows the values of some standard refractory tests performed on the four compoundments of Ikere -Ekit i kaolin and clay. Table 3 shows the comparative impurity content in Iwo and Ikere-Ekiti Kaolins,Ife and Ikere -Ekit i clays. Fig.2 shows the glazed and glost fired porcelain produced and Fig.  3 the biscuit fired, unglazed porcelain.

Result of chemical Analysis
Co mparing the analysis of Iwo Kaolin with that of Ikere -Ekiti Kao lin, it could be seen that the percentage of imp urit ies present in Iwo Kaolin is higher than that in Ikere-Ekiti Kaolin (Table 3). Ife clay could also be seen to have a higher impurity content compared with that of Ikere-Ekit i.
It could be inferred fro m the chemical analysis that the inability of Iwo Kaolin mixed with Ife Clay to withstand high temperature without cracking was due to the high proportion of impurit ies present. Also, the percentages of A1 2 0 3 in Ife clay (22.36) and Iwo Kao lin (28.69) are s maller than that of Ikere Ekiti Kaolin (36.10) and clay (32.40). It is established that the amount of Al 2 0 3 present in clays affect refractoriness [8,9]. Iwo kaolin and Ile-ife clay were first used, but did not withstand the bisque firing, which was done at 900℃, as the sample had lots of cracks.

Bu lk Density of the body
Bulk density is an important property in porcelain wares. Bulk densities of the mixed samples lie within the range of 1.7 to 2.1 g/cm 3 wh ich fall in standard requirements for porcelain body [2].

Total shrinkage of the body
It was observed that average total shrinkage for each of the samples was within the reco mmended value for porcelain production [14]. Higher shrin kage values result in warping and cracking of the porcelain wares resulting in los s or reduction in its strength.

Conclusions
Ikere -Ekiti kaolin and clay were found suitable for porcelain production. Values of standard refractory tests fell within standard values for porcelain production. The higher the percentage of impurit ies present in kaolin clay, the h igher the tendency for the samp le to crack while firing at high temperatures.