Influence of Sintering Temperatures on Physico-Mechanical Properties and Microstructure of Refractory Fireclay Bricks

This research emphasized on the influence of sintering temperatures on the microstructure and physico-mechanical properties of Kpata fireclay brick and Qua'an Panfireclay brick were assessed for suitability and application as refractory bricks. The clay samples were sintered at varied temperatures of 900 C, 1000 C, 1100 C and 1200 C. The clay samples were subjected to X-ray diffractometer to analyze the phase transformation after being sintered. The XRD results indicated the finger prints of the phase changes inthe Kpata and Qua’an Pan fireclay bricks.At the best sintering temperature of 1200 C, phase changes were quartz, mullite, rutile, corundum and cristobalite phases, while Qua'an Panfireclay brick had phase changes of quartz, cristobalite and mullite. SEManalysiswas carried out to observe the surface morphology.The refractoriness of Kpata and Qua’an Pan bricks were 1621 C and 1564 C respectively. Their cold crushing strength and apparent porosity were investigated. Keyword-SinteringTemperatures, Phase Transformation, Fireclay Brick, Refractoriness.

Deplorably, $2.9 billion was being invested and depleted annually on the importation of these refractories [13]. It was on these premises that necessitated and informed the choice of investigation on these clays mentioned.

A. Materials
The clay samples used in this research were collected from Kpata in Bassa local government area of Kogi state and Qua'an Pan in Qua'an Pan local government area of Plateau state, all in North central geo-political zone of Nigeria. The clay samples deposit sites were dogged 2 meters deep into the earth using an iron digger; 20 kg of the clay samples each were packed with shovel into a plastic bag. The clay samples lumps had an average size of 12-25 cm. The collection and transportation of all the clay samples were according to ASTM-D4220/D4220M-14 [14].
B. Methods 1) Sample preparation: Clay samples were ball milled to form fine powder particles. Each of the specimen powder was pressed into pallets using Caver hydraulic pressing machine. A force of 4 tonnes was applied with a holding time of 60 seconds. The samples were then transferred into furnace and subjected to thermal treatment by sintering process at varied sintering temperatures of 900 0 C, 1000 0 C, 1100 0 C and 1200 0 C for 8 hours as soaking time with heating rate of 2.5 °C/min.The apparent porosity test was performed using the sintered test samplesand the test was conducted according to ASTM C20-2000 [15].The refractoriness test was performed according to ASTM C24-09 [16].
2) Specimen characterization: The X-ray diffraction (XRD) patterns were achieved using a BRUKER D8 ADVANCE machine. The pattern was scanned in series of angles diffraction from 10 0 to 80 0 (2θ) in steps of 0.034" designed for clay using copper (CuKɑ) with a wavelength of 1.5406Å as X-ray source. At various angle, the intensity diffracted are measured and recorded instantly on a chart where the suited (θ) and (d) values were generated and received. The chemical composition was determined using the Oxford X-Supreme 8000 (XRF) technique. This analytical method is non-destructive, employed to identify the concentration of elements existing in the clay samples. The Scanning Electron Microscopy (SEM), JEOL JSM-6380LA and energy dispersive X-ray spectroscopy (EDX) were used to determine the elements dominate in the fireclay brick specimens and to observe the surface morphology of the Kpata and Qua'an Pan fireclay bricks.

III. RESULTS AND DISCUSSION
In the analyzed results of chemical composition of the two clay samples in Table 1, the dominating oxides for the two specimens were SiO 2 and Al 2 O 3 . Kpata fireclay brick had SiO 2 and Al 2 O 3 with weight values of 74.8% and 22.599 % respectively, while Qua'an Panfireclay brick had SiO 3 and Al 2 O 3 with weight values of 63.437 % and 32.673 % respectively. The sintering of refractory bricks can impact strength to a large extent in the bricks products as they become strongerwhen the pose are closed due to rise or increase in sintering temperature [17].The Kpata and Qua'an Pan fireclay bricks fall within the range of temperature 1500-1750 0 C for fireclays according to ASTM classification of refractories by temperature ranges [5].The SEM micrographs were taken on X1500 magnification. Both clay samples showed a remarkable improvement on densification. As sintering temperatures increases, there was progressive improvement on the surface morphology of Kpata and Qua'an Panfireclay bricksas presented in Fig. 1 and Fig. 2. Percentage porosity reduced as sintering temperatureswere increased.    In the evaluation of physical and mechanical chattels, the evaluated results of percentage porosity and cold crushing strength in Fig. 3 and Fig. 4 [13]. In terms of mechanical properties, the Kpata fireclay brick had the high cold crushing strength of 23.36 MPa at optimum sintering temperature of 1200 0 C as against the lowest strength of 19.12 MPa at 900 0 C sintering temperature. The Qua'an Pan fireclay brick had the highest cold crushing strength of 21.97 MPa at optimal sintered temperature of 1200 0 C and the lowest strength of 17.64 MPa at 900 0 C sintering temperature. The cold crushing strength also improved and increased as the sintering temperature was increased [17].The cold crushing strength (mechanical strength) of the Kpata and Qua'an Pan fireclay bricks fell within the standard values of 15-59 MPa for refractories [4].  The X-ray diffraction (XRD) results of the two clay deposits showed the crystallization and phase transformation at varied sintering temperatures.A very close observation on the Kpata clay deposit showed phase changes of quartz, corundum, anatase, rutile, mullite and cristobalite presented in Fig. 5, while Qua'an Panclay manifestedphase transformation of quartz, corundum, hamatite, mullite and cristobaliteshown in Fig. 6. The refractoriness of the clay samples were done one after the other by shaping the sample into a conical form. The standard pyrometric Segar cones of numbers 28, 29 and 30 of known softening temperatures were arranged on a standard circular shaped plaque with the aid of a binder. The cones at an angle of 82 0 to the horizontal were fixed on the plaque. It was then transferred to the refractoriness testing equipment and fired. The observation with the aid of a mirror attached to the refractoriness heating equipment, the PCE cone bent at firing temperature of 1621 0 C which corresponded to the equivalent of segar cone No: 29, hence, the refractoriness for Kpata fireclay bricks and this qualifiedthe sample as high duty (Siliceous) brick. Similarlly, the same PCE test was conducted for the Qua'an Pan fireclay brick using segar cones 19, 20 and 23. Its refractoriness was 1564 0 C which corresponded to segar cone No: 20 and qulified as low heat duty brick. Both refractory bricks qualified as fireclay bricks according to ASTM C27-98 [5]. The refractoriness of the two fireclay bricks fell within the standard refractoriness of 1500-1750 0 C for fireclay refractories [13] [18].

IV. CONCLUSION
The stability and suitability of Kpata and the Qua'an Pan claysarebased on their results of refractoriness, apparent porosity, cold crushing strength, SEM and XRD are adequate and appropriate to be used for production of refractory bricks.