Oven-Dewatering of Otamiri Clays Designated for Production of Porcelain and Multi-Factorial Analysis of Periodic Water Loss

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2.1. Clay Sizing and Moulding

The dried clay was homogenized and sieved using an assembly of sieves running from 0.075 to 2 mm. The sieve assembly was placed on a mechanical sieve shaker and power supply switched on. The set-up was allowed to function for 5 minutes. The sieving process was repeated severally until the quantity of clay required for the moulding process was available. Hundred grams (100g) of the 0.425mm sieved sized clay sample and 10g of bentonite powder were weighed out and thoroughly mixed. The bentonite powder was used to increase plasticity and strength during firing. Six percent (6%) of total weight (clay and Bentonite) of water was added and it was then mixed until complete homogenity was achieved. The mixed samples were poured into the rectangular metal mould of internal dimension 50 x 18 x 10 mm. Just before air drying, the clay samples were weighed.

2.2. Air Drying

The moulded specimens were carefully placed in a plastic tray and kept outside the laboratory to loose some water and become strengthened. The reason for air drying includes (1) to prevent the samples from being defective as a result of evaporation during oven drying and firing (2) to give the specimens adequate strength during oven drying and firing. The weights of the specimens were measured prior to oven drying.

2.3. Oven Drying

An electrically heated oven of internal dimension 500 x 500mm was used in the drying operation. The oven was sourced from Erosion Research Center of the Federal University of Technology, Owerri (FUTO). Each set of specimens was dried at a temperature of 80°C for 30, 50, 70, 90, 110 and 130 mins., after which their respective weights were measured. The oven drying process was repeated for other sets of specimens at temperatures 90, 100 and 110°C, also each set at drying times 50, 70, 90, 110 and 130 mins., and their corresponding weights measured and recorded accordingly.

3. Results and Discussion

The result of chemical analysis of Otamiri clay is shown in Table 1. The table shows that the clay is most constituted by SiO₂ while Na₂O is the poorest constituent.

Table 1. Chemical composition of Otamiri clays

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Table 2. Result of sieve analysis of Otamiri clays

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Where

S. S = Sieve sizes (mm)

M.R = Mass retained (g)

M.P = Mass passing (g)

%P = Percent passing (%)

Results of the sieve analysis indicate that the weights of clay before and after washing and drying are 50 and 15.7g respectively. This implies that the weight of fines is 34.3g, while the process residue is 15.7g. Table 2 shows that M.R and M.P varied with sieve sizes. This resulted in 100% passing of the clay. Between 0.600 and 0.150 mm sieve sizes, the M.P dropped resulting in the decrease of the %P.

Figure 2. Comparison of quantities of water evaporated from a rectangular surface at different temperature

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Figure 3. Coefficient of determination between quantity of water evaporated and drying time as obtained from experiment

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Figure 4. Coefficient of determination between quantity of water evaporated and drying time as obtained from derived model

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Figure 5. Comparison of quantities of water evaporated as obtained from experiment and derived model

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Figure 6. Comparison of quantities of water evaporated as obtained from experiment, derived model and regression model

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Analysis of Figure 2 shows that the weight of evaporated water increases with increase in the drying time of the clay materials until maximum shrinkage occurs, providing the materials are of the same shape. At this stage (maximum shrinkage), the inter-particle voids containing water are displaced, permitting close contact of the clay particles. This is because shrinkage begins with the inception of water evaporation.

The quantity of water evaporated was least on oven drying the clay material at 100°C irrespective of the drying time employed, providing the evaporating surface is flat. At 100°C, water in the clay evaporates as steam and part of the steam condenses to water without any temperature change and falls back to the evaporating surface. This results to an overall low level of water evaporation at the end of the drying process. This is inline with previous research ^[8]. The flat evaporating surface ensures uniform evaporation of water per second during the drying process.

3.1. Analysis of water loss

Analysis of water loss during oven drying of the clay was carried out using a derived model. Experimental results generated from oven drying rectangular shaped clay materials at 80°C (as shown Figure 2 and Table 3) was used for the model formulation. These results were randomly considered amongst results obtained from drying temperatures 90, 100 and 110°C.

Table 3. Variation of evaporated water from Otamiri clay with drying time

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Computational analysis of results in Figure 3 for a drying temperature of 80°C, represented in Table 3 indicate that;

(1)

Introducing the values of K, S, N and Ne into equation (1) reduces it to;

(2)

Where

= Model-predicted evaporated water (g)

(ϑ)= Drying time (mins.)

(β) = Evaporating surface area (mm²)

(θ) = Drying temperature (°C)

K = 0.0733, S = 1.02 x 10^-6^, N = 7.7x10^-3 and

Ne = 0.4462; where K, S, N and Ne are empirical constants determined using C-NIKBRAN ^[9].

3.2. Boundary and Initial Conditions

Consider a rectangular shaped clay product of length 49mm, width 17mm, and breadth 9mm exposed to drying in the oven while it was in slight wet condition. Initially, atmospheric levels of oxygen are assumed. Atmospheric pressure was assumed to be acting on the clay samples during the drying process (since the furnace is not air-tight). The grain size of clay particles used is 425µm, weight of clay and binder (bentonite) used (for each rectangular product); 100g and 10g respectively, quantity of water used for mixing; 6% (of total weight), drying temperature used; 80°C, area of evaporating surface; 833mm²and range of drying time used; (30-130 mins.).

The boundary conditions are: atmospheric levels of oxygen at the top and bottom of the clay samples since they are dried under the atmospheric condition. No external force due to compression or tension was applied to the drying clays. The sides of the particles are taken to be symmetries.

Figure 3-Figure 5 indicates that a plot of experimental results generated from the application of a drying temperature of 80°C (represented as Table 3) and those predicted from the derived model indicate close alignment of curves (Figure 5). Figure 3 and Figure 4 show that model-predicted results (Figure 4) proffered a higher correlation compared to results obtained from the experiment (Figure 3).

3.3. Comparison of Derived Model with Standard Model

The validity of the derived model was further verified through application of the regression model (Least Square Method (ReG)) in predicting the trend of the experimental results. Comparative analysis of Figure 6 show very close alignment of curves and significantly similar trend of data point’s distribution for experimental (ExD), derived model-predicted (MoD) and regression model predicted (ReG) results of evaporated water. These values are in proximate agreement.

The standard error incurred in predicting water loss for each value of the drying times as obtained from experiment, derived model and regression model were 0.8051, 2.1 x 10^-4 and 3.45 x 10^-5 % respectively.

3.4. Deviation of Model-Predicted Results

Table 4 shows that the maximum deviation of model-predicted quantity of evaporated water (from that of the experiment) is less than 20%.

Table 4. Variation of experimental results of water loss with resultant correction factors and deviations of model-predicted results

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