Effects Of Adding Mild-calcined Bauxite And Coal Kaolinite Aggregates On Properties Of Al₂O₃-SiO₂Castables

Naturally occurred minerals for refractory raw materials in the Al₂O₃-SiO₂systemneed to be calcined at high temperature to become grogs, before being used inmonolithic refractories. Calcination of raw material at high temperature is of an energyconsuming process, in which the mullitization occurs completely, and the favorable in-situ effect by the mullitization can not be further made use of in the subsequent heatingup. A new concept is put forwarded by this work to calcine bauxite and coal kaoliniteat relatively lower temperatures to remove the compounded water, and then to addthem as aggregates into Al₂O₃-SiO₂castables for certain applications, for the purposeof lower consumption. It has not been reported to investigate the effects of such mild-calcined aggregates on properties of castables where they are added.Raw bauxite with70.97%Al₂O₃and raw coal kaolinite with38.60%Al₂O₃wereadopted as starting materials for this work, and their thermal behaviors wereinvestigated by means of DSC-TG. They were calcined at700℃,900℃,1100℃and1300℃respectively as mild-calcined aggregates （MCA）. The effects of calcinationtemperature on bulk density, apparent porosity, particle strength and mineralogicalcomposition of the MCA were comparatively investigated, and microstructure of somesamples was also investigated by means of SEM. When calcined at700℃, crystalstructure of the bauxite and the coal kaolinite are destroyed due to the removal ofstructural water, leading to poor strength of their grains. When calcined at900-1100℃,however, the volumetric change becomes small, and their grain strength becomes closeto that of their grogs. The micro-pore structure in the MCA is helpful to improve someproperties, e.g. enhancing the bonding between the aggregate and the matrix, loweringthermal conductivity and bulk density. Accordingly, for this work, the mild-calcination temperatures of the bauxite and coal kaolinite were set at900℃and1100℃respectively.Taking Al₂O₃-SiO₂ultra-low cement castables as reference, the bauxite and coalkaolinite MCA were added by15%,30%and45%respectively to accordingly replacethe used special grade bauxite and flint clay grogs, and the effects of adding suchMCA on properties of the castables were investigated, in terms of bulk density,apparent porosity, permanent linear change, cold crushing strength, cold modulus ofrupture, hot modulus of rupture （HMOR）, refractoriness under load （RUL） and thermalshock resistance （TSR）. Microstructural analysis on some samples was also carried outby means of XRD and SEM.Cold strengths of the castables with increased addition of the mild-calcinedbauxite aggregates remain almost unchanged, with satisfactory volumetric stability andRUL, while HMOR and TSR slightly decrease. Bulk density of the castable getssignificantly reduced by the mild-calcined bauxite addition, beneficial to achievingreduced weight. Compared to the added coal kaolinite calcined at900℃, the castableswith the added coal kaolinite calcined at1100℃show better volumetric stability. Atthe addition of15%, RUL and HMOR of the castables get improved, in stead ofdecreasing. With further additions of the mild-calcined bauxite, TSR of the castablescan also be improved.Based on the results achieved by this work, it is feasible to use mild-calcinedbauxite and coal kaolinite aggregates in Al₂O₃-SiO₂castables for service temperaturesaround1300℃. Adding MCA can lead to lower cost and consumption of raw materialsunder the precondition of ensuring the material properties, helpful to the developmentof low consumption type refractories.