Effects Of Mild-calcined Bauxite And Coal Gangue Additions On Properties Of Castables In The Al₂O₃-SiO₂ System

With many advantages, refractory castables have presently become the most active and attractive part in monolithic refractories. Lowering raw material cost and improving properties are of important aspects of R & D and application of castables. In some specific systems, some natural minerals in the Al₂O₃-SiO₂ system like bauxite and coal gangue produce primary mullitization and secondary mullitization effects when subjected to heating. Such beneficial in situ effects can be taken use to improve properties of castables. Concerning the sufficient structural water in the raw minerals, which may destroy castables structure due to harsh decomposition during heating, this work has tried to add bauxite and coal gangue in castables after mild calcination of them to remove the structural water. The mild-calcined bauxite and coal gangue were respectively added in ultra-low cement (ULC) castables of the Al₂O₃-SiO₂ system and their influences on properties were investigated, exploring a new approach to using raw bauxite and coal gangue as mild-calcined materials in castables. Few published works in the respect have been found.This work use the raw bauxite with 60.29% Al₂O₃ and raw coal gangue with 39.12% Al₂O₃. Based on thermal analysis by TG and DSC, the mild-calcination for bauxite and coal gangue was determined at 600℃for 6h and 700℃for 6h respectively. According to PSD control,the respective wet grinding time for the mild-calcined bauxite and coal gangue to be turned into fine powders was set as 15min and 30min. In this way, two types of powders for this investigation were obtained. The aggregate portion of the ULC Al₂O₃-SiO₂ castables consisted of white fused alumina with top size of 8 mm and the matrix portion was composed of white fused alumina, fused mullite, microsilica,α-alumina ultrafines and CA cement (3%). There were two series for the investigation, one being respectively added with 5%, 10%, 15% and 20% mild-calcined bauxite and the other added respectively with 5%, 10% and 15% mild-calcined coal gangue, substituting accordingly fused mullite and white fused alumina in both cases.Adding mild-calcined bauxite can enhance the hot modulus of rupture (HMOR) at 1400℃of the castable after drying. After thermal shock by quenching from 1100℃to flowing water, the residual cold modulus of rupture (CMOR) ratio of the specimens pre-heated at 1500℃for 3h are improved. The explosion temperature can be increased by 100℃after mild-calcined bauxite addition. The in situ formed needle- or column-like and interlaced mullite in the matrix derived from the added bauxite is contributive to the properties improvement. However, the added bauxite introduced impurity in the castables to yield higher liquid content.Explosion resistance of the castables can be significantly improved by the coal gangue addition, as evidenced by the fact that at 15% of the coal gangue addition, the explosion temperature in increased by 250℃, comparing to no mild-calcined coal gangue addition. Adding the coal gangue can obviously improve the HMOR at 1400℃of the castables when first heating up, and the reheated HMOR at 1400℃of castables pre-heated at 1500℃for 3h remains the same tendency of improvement. The TSR characterized by residual CMOR ratio, after the quenching from 1100℃to flowing water for one and three cycles, of the specimens pre-heated at 1500℃for 3h gets significantly improved by the coal gangue addition. The refractoiness under load of the castables is also enhanced by the coal gangue addition. The matrix of the coal gangue added castables is featured by the in situ formed needle- or column-like and interlaced mullite, which beneficiates the properties improvement. Such a favorable microstructure has a durable effect, without deterioration when subjecting to reheating.Based on the result from this work, it is feasible to add appropriate amount of mild-calcined bauxite and coal gangue in Al₂O₃-SiO₂ castables, as an economical and properties favorable new way to use naturally occurred raw materials. The achievements by this work deserve to be a reference and guidance to further work for related applications.