COMPENSATION FOR THERMAL SHOCK AND FIRING TIME MINIMIZATION IN SILICA REFRACTORY PRODUCTION

Abstract

This research seeks to investigate how a minimized firing time and natural clay used as binder (materialize) in silica production compensate for strength and thermal shock from a choice of the locally available materials for silica refractory production. The natural raw clay got from Ode-Aye (Efira) (Latitude: 6.5879E; Longitude: 4.74358N) and silica sand from (Ebute) Igbokoda (Latitude: 6.3496E; Longitude: 4.80326N), in Ondo State, were the materials options considered for this research. The silica sand obtained was poured in a large bowl of water and the suspended particles were repeatedly decanted to further ensure cleaner silica sand. After washing, it was then sun-dried for two days. The weight of the dried silica sand was 30.308 kg; sieve analysis was carried out using sieve mesh sizes of 600 μm, 425 μm, 300 μm, 212 μm, 150 μm, 75 μm and base pan. It was then ready for mixing and consequently moulding. In the same vein, the clay from Ode-Aye, a site in Ondo State, (Nigeria) was soaked into a large volume of water of a bathe bowl of 100 litres for 2 days, as to soften the dissolved the clay lumps. The clay mixture was stirred to form slurry, then pulped to remove the undissolved clay residue. The dissolved clay was filtered, drained and sun dried. The dried clay which was in dried-lump form was crushed and grinded using laboratory mortar and pestle, then the ball mills respectively. Screening was done using 60 μm sieve size to have the clay in powdered form and weighed. The extracted clay which was plastic in nature was used in preparing several silica refractory samples. Each refractory sample produced from the silica sand and clay mixture weighed 50 g, while the clay proportion of the samples varied from 10%, 20%, 40%, to 100% which were represented by C1, C2, C3 and C4 respectively, for each of the silica grain sizes used that is 150 μm, 212 μm, 300 μm, and 600 μm which were represented by S1, S2, S3, and S4 respectively. Compression moulding machine was used to mould the samples to diameter 32 mm ± 2 mm. the numbers of samples produced per each composition mixture of clay and silica sand were in triplicate. Silica refractory samples were produced from varying percentage of clay blends in the silica refractory mix and fired in the furnace, maintaining some carefully planned time-temperature schedules. Three firing approaches adopted and developed for this research were; the Step Firing (SF), Continuous Firing (CF) and Reduced Continuous Firing (CRF). The maximum firing temperature attainable in the furnace used was 11000C for firing the moulded silica refractory samples. The refractory samples were labeled and arranged with notations for easy identification. The digital calipers was used to measure the height and diameter of each sample both before and after firing to ascertain the various dimensional changes which occurred in term of contractions and expansions in the volume of each sample and relevant data were obtained for stress, deformation and Young Modulus from the cold crushing strength test analysis. The objectives of this study were to investigate the effect of : a) varying proportion of raw clay in silica and refractory mix on size changes and firing cracking of the product silica refractory under same firing condition; b) firing rate variation on crack susceptibility of the silica refractory mixes having varying proportions of raw clay; and c) silica sand grain sizes, on firing cracking and size changes in the silica refractory production while maintaining a constant raw clay proportion in the refractory mix. The result obtained showed that the sample bearing 40% clay blend in its refractory mix could withstand faster firing with less pronounced thermal cracks and improved strength.