Light Weight Insulating Bricks from Coal and Clay

Transcription

1 Page 150 Light Weight Insulating Bricks from Coal and Clay M.A. Gafur 1*, Sudip Shaha 2, M. R. Qadir 1, J. Ferdaus 3, S. A. Urmi 3 and F. N. Robel 2 1. PP and PDC, Bangladesh Council of Scientific and Industrial Research, Dhaka-1205, Bangladesh 2. Department of ACCE, NSTU, Nkakhali, Bangladesh 3. Department of Materials and Metallurgical Engineering, BUET, Dhaka-1000, Bangladesh Abstract: *Corresponding author: d_r_magafur@yahoo.com Light weight insulating bricks were produced by incorporation of coal into clay followed by firing at C. Two different sizes of coal content (63 micron and 125 micron) were used in different percentages. Then the effect of coal addition and size of coal on thermal, mechanical and physical properties of the insulating bricks were investigated. It is observed that thermal conductivity, cold crushing strength and bulk density decreases with increasing percentages of coal content. Thermal conductivity decreases from 0.54 to W m -1 K -1 for the coal size of 125 micron and from 0.51 W m -1 K -1 to 0.39 W m -1 K -1 for the coal size of 63 micron. Effect of coal size on bulk density and cold crushing strength is also evident. Finer coals lower the bulk density but increase the cold crushing strength. Keywords: Insulating Brick, Coal, Clay, Thermal Conductivity. 1. INTRODUCTION Refractories are inorganic, non-metallic, porous and heterogeneous materials composed of thermally stable mineral aggregates, a binder phase and additives. The main types include fire-clay bricks, ceramic fiber and insulating bricks that are made in varying combinations and shapes for diverse applications. Atmosphere, temperature, and the materials in contact are some of the operating factors that determine the composition of refractory materials. In the production of light weight insulating bricks, two approaches are generally followed: one is to incorporate carbon rich material and the other is to add organic materials. The latter approach has attracted more attention due to easy availability of the organics, and is known as an effective method of producing light weight insulating bricks. On firing the carbon rich material burns out and leave both open and closed pores within the clay body. Another technique for light weight insulating material production is to add liquid organic material. Expanded polystyrene beads are incorporated into clay mixtures which are then fabricated into products by firing at high temperatures. The incineration of polystyrene beads results in the formation of small vacant cells in the clay(1). The mechanical and thermal properties of light weight insulating bricks partly depend on the percentage of porosity and the size of the pores in the brick. This research was undertaken to find out the optimum size and percentage of coal content in the clay-coal system for producing light weight insulating bricks from local clay having a good combination of thermal and mechanical properties.

2 Page MATERIALS AND METHOD 2.1 Raw Materials The raw materials used for the manufacture of the specimens of light weight insulating bricks for this work were china Clay, Coal, Grog and Molasses. The percentage of raw materials used are shown in table 1. Coal content Molases Grog Clay balance Table 1: Percentages of raw materials used for manufacturing light weight insulating brick specimens. Fig 1: XRD analysis of white china clay White china clay was used for this experiment. Chemical and mineralogical compositions were determined by standard chemical analysis process and X-ray diffraction. Fig. 1 shows the X-rd pattern of white china clay indicating the presence of Kaolinite. Coal was procured from Aliganj, Pagla, Narayanganj, which was imported by the local dealers from Meghalaya. The type of coal is anthracite coal. The carbon content is between %.The coal was crushed by hammer and then grinded. The size of the coal to prepare the specimens varies from 63 micron to 125 micron. Molasses was used as a binder in the green product. It was burned at 1100 C and was found to contain 7% residue. Fire clay goods are composed of fire clays or china clays, with the addition of grog or free silica. Grog is nothing but broken granulated fired refractory clay and is made from rejected fire clay works, broken crucibles etc. For very plastic clay, the proportion of opening material may be as great as two parts by weight of grog to one part of clay. The use of silica as an opening agent changes the chemical composition and reduces the refractoriness. 2.2 Preparations of specimens The specimens were prepared by mixing, molding, drying and firing. Four different percentages (7, 15, 25, and 30) of Coal were used. Hand mixing was employed to prepare the specimens. The clay and coal were thoroughly mixed. To facilitate the powder compaction process a small amount (4%) of molasses in water was added to the mixture and then the mixture was again thoroughly mixed. Specimens were prepared by hand molding. Two different sizes of specimens were made. For bulk density and compressive strength samples in the form of bars of dimension 9 cm. 4.5 cm. 4.5 cm. were prepared. For measuring thermal conductivity, cylindrical disc of dimension 9 cm. Dia. and 0.5 cm. After molding the compact green specimens were naturally dried for 7-10 days. The dried specimens were charged in a furnace at 25 C. and heated slowly to 600 C to make the siliceous content fused. Then the specimens were heated to 1100 C and soaked for 24 hours and then slowly cooled.

3 Bulk Density (Kg./m3) thermal conductivity*10ˆ-3 (kw m -1 K -1 ) Page Measurement of Physical Properties Bulk density of the specimens was measured according to ASTM Standard Method C-134(2). The cold rushing strength was determined according to AST method C 93-67(3). Thermal conductivity of the sample was measured by using Lees and Cholton s apparatus (4-6). 3. RESULTS AND DISCUSSION Effects of coal on mechanical, thermal and physical properties of clay insulating bricks are discussed below: Bulk density: Fig. 2 shows the effect of coal content on the bulk density of the insulating brick. The bulk density decreases with the increase of coal addition due two factors addition of lighter coal which converted to porosity after firing (7). A marked effect of the size of the coal content on the bulk density was found. The bulk density decreases with the decrease of the size of the coal in specimens. It is noticed that the percentage of coal incorporated into the clay body was generally increased from 7 to 30 percent by weight, the bulk density was found to decrease linearly coal size 125 coal size coal content (wt%) coal size 125 micron coal size 63 micron coal (wt %) Fig 2: Effect of coal on bulk density Fig 3: Effect of coal on thermal conductivity Thermal Conductivity Fig. 3 shows the effect of coal addition of thermal conductivity of insulating brick. It reveals that with the increase of coal addition the thermal conductivity decreases. The higher the coal addition percentage, the more is the porosity. As air is better insulator than any other solid materials the large amount of porosity in the coal-clay bricks results the overall decrease of thermal conductivity. The decrease in thermal conductivity with the increase of coal content is in agreement to Budnikov (8) and Gafur and Haque (7). It is also found that lager the size higher the thermal conductivity. As the bulk

4 compressive strength (MPa) (compressive strength (MPa) Page 153 density increases with the increases of coal size, the thermal conductivity increases with the increases of coal size. Cold Crushing Strength Fig. 4 shows the effect of coal addition on cold crushing strength (CCS) of insulating bricks. It reveals that CCS decreases with increase of coal addition. With the increases of coal addition the amount of porosity and load bearing area decreases. As a result CCS decreases with the increase coal addition. Similar results were obtained by Gafur and Haque (7). The cold crushing strength of the specimen was found to be reduced from MPa to MPa for the coal size of 125 micron and for the coal size of 63 micron it was found to be reduced from 3.26 MPa to MPa coal size 63 micron coal size 125 micron coal (wt %) bulk density (kg/m3) Fig 4: Effect of coal addition on cold crushing strength Fig 5: Relation between bulk density and CCS. A very clear effect of the size of the coal was also observed. The finer the size of the coal, the higher the cold crushing strength and vice versa. Finer coal resulted in the fine pores having smaller crack lengths than those obtained than those obtained by coarser coal particles. Consequently CCS of the specimen having finer pores was found to be higher than those of specimens having larger coal particles. Fig. 5 shows the effect of bulk density on CCS of insulating bricks. It reveals that with the increase of BD the CCS increases. With the increase of BD, the porosity decreases and the load bearing area increases resulting in higher CCS. Finer coal resulted in fine pores having small crack length than those created by the coarse coal particles. As a result cold crushing strength of the specimen having finer pore was found to be higher than that of the specimen having less fine pores. The stress contraction factor is larger for specimens containing coarser clay particles and thus strength is higher for specimens containing finer coal than those containing less fine coal. Again as mentioned before the coarse coal was relatively more elongated and has more sharp corners than the finer one. Thus the stress contraction factor of the coarser pores will be multiplied and the resulting strength of those specimens having large size coal is expected to be lower than that of the specimen having finer coal. The relationship between compressive strength and thermal conductivity of the prepared insulating bricks is shown in the following figure (Fig. 6).

5 CCS (MPa) Page coal size 63 micron coal size 125 micron thermal conductivity (W m -1 K -1 ) Fig. 6 Relation between cold crushing strength and thermal conductivity of the insulating brick It shows that with the increase of thermal conductivity, the CCS of the insulating bricks increases for both the coal size. It is also found that the finer the coal the higher CCS and the lower TC. This figure may be used for easy selection of the insulating bricks for some specific purpose. It can also be seen from the figure that the specimen prepared by using finer coal gives a good combination of strength and insulating property. It has been observed that pores of larger size contribute to increasingly high conductivity, while small porosity remains a good barrier to heat flow. Therefore at furnace temperature the bricks prepared by using finer coal will be very effective for heat insulation. 4. CONCLUSION For the same size of coal the bulk density decreases linearly with the increase of coal content. For the same percentage of coal content bulk density is higher in specimen containing finer coal and vice versa. The thermal conductivity and cold crushing strength decreases with the increase of coal content. Also cold crushing strength is affected by the size of coal. So, the bricks produced by using finer coal are suitable for a good combination of strength and insulation ACKNOWLEDGEMENTS The authors are thankful to PP&PDC, BCSIR, Dhaka-1205, Bangladesh for extending experimental facilities.

6 Page 155 REFERENCES 1. Hibel, R., Products alleges de terre cuite ajout de poly styrene expanse, terre Cuite, 4, 21 (1969). 2. ASTM Designation C , standard test methods for size and bulk density of refractory brick and insulating firebricks. 3. ASTM Designation C 93-67, standard test method for cold crushing strength and modulus of rupture of insulating firebrick. 4. Muncaster, Roger. A-Level Physics. New York: Hyperion Books, Poynting, JH. A Text Book of Physics. City: Munshi Press, M.A. Gafur and M. N. Haque, Effect of Particle Size and Percentage of Sawdust, Transaction of Indian Ceramic Society, Vol 52(5), Sp-Oct 1993, p Budnikov, P.P., The Technology of Ceramics and Refractories, p.365, Edward Arnold, London(1964)