Formation of aluminum titanate-mullite composite from bauxite red mud
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Formation of Aluminum Titanate-Mullite Composite from Bauxite Red Mud T. MAHATA, B.P. SHARMA, S.R. NAIR, and D. PRAKASH Aluminum titanate ceramics have a low thermal expansion coefficient, which results in excellent thermal shock resistance, low Young’s modulus, moderate strength, and low wettability by liquid metals.[1,2] The former three properties are related to the presence of extensive microcracks in the sintered body.[2] Crystallographic anisotropy in the thermal expansion coefficient[3] is mainly responsible for the development of microcracks during cooling from the sintering temperature. Mechanical strength can be improved by incorporation of a second phase such as mullite or zirconia.[4,5] The dispersed second phase acts as a barrier against microcrack coalescence. Due to the combination of the aforementioned physical and mechanical properties, aluminum titanate and its composites are potential materials for applications as liquid metal flow regulators, risers, thermocouple sleeves, burner nozzles, ceramic filters, etc.[1,6,7] The present article deals with the preparation of an aluminum titanatemullite composite material from bauxite red mud. Red mud, the waste by-product of Bayer’s process in aluminum extraction, contains mainly oxides of aluminum, titanium, iron, and silicon. Most of the Indian red muds are characterized by high titania content (16 to 25 wt pct).[8] In this article, experimental results are based on red mud obtained from M/s BALCO (Kobra, MP, India). The composition of BALCO red mud after calcination at 800 8C is given in Table I. In the table, the unaccounted for balance 12.3 pct may be attributed to the presence of molecular water (mainly in AlOOH) and oxides of several other elements such as V, Cr, Zr, Ga, Ba, Pb, Mn, and Mg. The red mud was first leached with water to remove water soluble sodium compounds. This was followed by leaching with
T. MAHATA and D. PRAKASH, Scientific Officers, D, B.P. SHARMA, Scientific Officer, H, and Head of Division, and S.R. NAIR, Scientific Officer, C, are with the Powder Metallurgy Division, Bhabha Atomic Research Centre, Navi-Mumbai - 400 705, India. Manuscript submitted February 17, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
dilute hydrochloric acid to preferentially remove Fe2O3, leaving the residue rich in Al2O3, TiO2, and SiO2. The details of the leaching experiments with three different acid concentrations are shown in Table II. With a low (1:3) HCl concentration, only 13.9 pct iron was removed. More than 90 pct iron was leached out with either 1:1 or 3:1 HCl. However, the removal of iron with 3:1 HCl was marginally higher than that with 1:1 HCl. On the other hand, the amount of residue obtained after leaching with 3:1 HCl was considerably less than that obtained after leaching with 1:1 HCl. It was confirmed that with 3:1 HCl, a substantial amount of Al2O3 was also leached out along with the Fe2O3. Considering the objective of preferentially leaching away Fe2O3, while retaining most of the Al2O3, TiO2, and SiO2 values, 1:1 HCl concentra
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