Aggregate has a very important influence on the performance of castables, and the study of the influence of the type of aggregate on the performance of castables is of great significance to improve the service life of refractory materials and reduce the cost of refractory materials. In this paper, from the perspective of replacing slightly high-grade alumina aggregate or even brown corundum aggregate with 86-grade homogenized alumina aggregate, the influence of different alumina aggregates on the performance of aluminium and magnesium castables under cement bonding system is studied and compared with the specimen with brown corundum as aggregate, while the reaction of slag and refractory material is simulated by using the slag-resistant test of the induction furnace and combined with the thermo-chemistry software FactSage6.2 to analyse the reaction of slag and refractory material, and the reaction of slag and refractory material is further analysed. The anti-slag erosion mechanism of homogenised bauxite-based aluminium-magnesium castables is of great significance in understanding the performance advantages of homogenised bauxite as well as its scope of use.
Raw materials for testing
Tests were conducted with homogenised bauxite (particle sizes 5~3mm, 3~1mm, 1~0mm), rotary kiln bauxite (particle sizes 5~3mm, 3~1mm, 1~0mm), inverted flame kiln bauxite (particle sizes 5~3mm, 3~1mm, 1~0mm), and brown corundum (particle sizes 5~3mm, 3~1mm, 1~0mm) as aggregates. Brown corundum fines, magnesia fines, Secar71 cement, activated α-Al2O3 micropowder as well as ElkemSiO2 micropowder were used as matrix, and sodium tripolyphosphate and sodium hexametaphosphate were used as water reducing agents. The chemical composition of the raw materials used in the tests is listed in Table 1.
Table 1 Main chemical composition of raw materials (wt%)
Specimen Preparation
The test formulations are shown in Table 2. The homogenised bauxite, rotary kiln bauxite, inverted flame kiln bauxite, brown corundum as aggregate samples were named as sample HC, sample GC, sample DC and sample BC respectively. the raw materials were weighed according to the test formula, added with an appropriate amount of water, and fully stirred, then vibration moulded into a 40×40×160mm long strip specimen, and then the moulded specimen was removed from the mould after being maintained for 24h in the room temperature, and then it was put into an oven and dried for 24h in the oven at 110°C. Finally, the baked specimen was put into a CSL high temperature sintering furnace for heat treatment at 1100°C and 1600°C for 3h. The moulded specimens were dried in the oven at 110℃ for 24h, and finally the baked specimens were placed in the CSL high-temperature sintering furnace for heat treatment at 1100℃ and 1600℃ for 3h. After the temperature of the furnace cooled down to room temperature, the specimens were removed and the performance tests were carried out.
Table 2 Test formulations (wt%)
Pilot Performance Test
(1) Room temperature physical properties
According to GB/T2997-2000, GB/T5072-2008, GB/T3001-2000, GB/T5988-2004, respectively, the apparent porosity and bulk density of the dried specimen, room temperature compressive strength, room temperature flexural strength and the rate of change of the line of the specimen after burning are detected.
(2) Thermal shock resistance
The specimen after heat treatment at 1100 ℃ for 3h is placed in an electric furnace preheated to 1100 ℃ (under air atmosphere), after holding for 30min, the specimen is removed and quickly immersed in circulating water, after which the specimen is placed in air for 5min. the test is repeated for 3 times, and at the end of the test, the thermal shock stability is evaluated according to the breakage of the specimen after water-cooling at 1100 ℃ for 3 times or the residual strength.
(3) Slag resistance
Induction furnace method is used to evaluate the slag resistance of the specimen, the schematic diagram of the test setup is shown in Fig. 1. the long specimen after drying at 110℃ for 24h is poured into the crucible and loaded into the induction melting furnace. The cast crucible is shown in Fig. 2. The steps of dynamic induction furnace slag resistance test are as follows: firstly, put about 6kg of common steel into the crucible first, heat it with electricity, and add 308g of ladle slag after all the steel is melted, the chemical composition of the slag is as shown in Table 3, and the alkalinity of the slag is n(CaO)/n(SiO2)=4.56. When the steel piece is completely fused with the slag, the timing is started, the temperature is controlled to be at 1600°C, and the furnace is stopped after 0.5h. After the test is finished, a long strip of specimen is poured into the crucible, and it is dried at 110°C for 24h, then the specimen is poured into the induction furnace as shown in Fig. 2. At the end of the test, the residual specimen was removed from the crucible, and after removing the hanging slag, the specimen was cut from the middle position along the direction of the longitudinal axis of the specimen, and the erosion area (or erosion rate) and the penetration area were used to characterise the degree of erosion of the specimen and the degree of specimen penetration, respectively, where the erosion and penetration areas of the specimen were analysed and measured with the help of AdobeAcrobatPro software.
Left 1 Schematic cross-section of crucible after slag erosion test
Right 2 Picture of crucible after drying after casting
Table 3 Chemical composition of ladle slag (wt%)
The depth of erosion of each specimen was measured at the end of the slag resistance test, and the erosion rate as well as the erosion area and penetration area of the specimen were calculated. The erosion rate is calculated as follows: Figure 3 shows the side diagram of the specimen after induction slag erosion. As shown in the figure, h0 is the original height of the specimen, measure the residual height of the specimen h1, then the maximum depth of erosion of the specimen after slag erosion is h2 = h0-h1, where the erosion rate is calculated according to the following formula:
Where: υ is the erosion rate, mm-h-1; h is the maximum erosion depth of the specimen after slagging, mm; t is the erosion time, h.
Fig. 3 Side view of specimen after induction furnace test
The erosion area and penetration area are calculated as follows: the cross-section of the specimen after induction slag erosion is shown in Fig. 4. As shown in the figure, the statistical area of the specimen is selected first (the selected area is the same for each group of comparison specimens, and the method of selecting the statistical area is as follows: a certain length is taken along the lengthwise direction starting from the position of the slag line, which is determined according to the erosion situation, but the same length is taken lengthwise by each group of specimens), and the statistical area is measured. The erosion area S1 and infiltration area S2 in the area.
Fig. 4 Schematic cross-section of specimen after induction furnace test
Conclude
(1) Aluminium-magnesium castable specimens containing different alumina aggregates have a large difference in the room temperature performance of the specimens due to the difference in the generated spinel content. After heat treatment at 1600°C, the volume shrinkage of cement-bound homogenised alumina-based aluminium-magnesium castables after high-temperature treatment, compared with the specimens containing inverted-flame kiln alumina aggregates, showed a larger rate of change in the line, and the specimens had a lower compressive strength and a poorer resistance to thermal shock.
(2) Through the induction furnace slag resistance test, the results show that: using cement-bonded aluminium-magnesium castables, the slag erosion resistance of the specimens with 86 grade homogenised alumina as the aggregate is not much different from that of the specimens with 88 grade rotary kiln alumina and 88 grade inverted flame kiln alumina as the aggregate, but the resistance to slag infiltration of the homogenised alumina based castables specimens is poorer.
(3) The homogenised alumina-based aluminium-magnesium castables will generate large cracks around the aggregate near the hot surface after slag etching, which is unfavourable to the slag penetration resistance of the specimens.
(4) Combined with the microstructure analysis and the thermodynamic simulation results of the reaction between three kinds of alumina aggregate and slag, it can be concluded that the products of alumina aggregate specimens reacted with the slag are corundum, spinel, CA6 and low melting point phases (C2AS and CAS2), in which the reaction between homogenised alumina aggregate and slag will generate CA6, and the reaction between the matrix and the slag is the direct solubilisation of spinel into the slag.