Effects of Additives on Residual Expansion of Aluminum Spinel Castables after Firing

The aluminum spinel castable has good volume stability, but it is found that there is a phenomenon of large expansion caused by different types of dispersants added. So far, it has been thought that the expansion of aluminum spinel is caused by the increase in the amount of CaO·6Al2O3 produced. However, the test showed that: adding sodium polymetaphosphate to generate Ca3(PO4)2, swelling will occur.

Aluminum-magnesium and aluminum-spinel castables are widely used in various kilns mainly based on ladle. In particular, the aluminum spinel castable is composed of alumina raw materials and spinel raw materials, so it does not generate spinel and expands like the aluminum-magnesium castable, and has better volume stability. There have been many previous reports on the expansion of magnesia and aluminospinel castables with heating. It is generally believed that alumina reacts with CaO in high alumina cement to form CaO·6Al2O3 (hereinafter referred to as CA6) at high temperature, which leads to thermal expansion. In addition, there are also reports that: if quartz powder coexists, it will promote the formation of CA6, and the expansion will increase.

In the process of studying the effect on the expansion of aluminum spinel castables, it was found that abnormal expansion occurred when sodium polymetaphosphate was added. So far there is no report on the effect of sodium polymetaphosphate on the expansion of aluminum spinel castables. Therefore, this paper studies the effect of sodium polymetaphosphate on residual expansion of aluminum spinel castables after firing.

2.1 Sample composition

The composition of ingredients used in the test is listed in Table 1. The main raw materials are fused alumina, spinel and high alumina cement. In order to more clearly detect the effect on expansion, the size of the raw material is determined to be 1 mm or less, and the reactivity is higher. No. 1, No. 2 and No. 3 samples, the amount of sodium polymetaphosphate added is divided into 3 levels; No. 4 sample is added with phosphoric acid without sodium; No. 5 sample is used for comparison, and polyacrylic acid is added.

2.2 Test method

Add 11.5% water to the ingredients and mix with a universal mixer for 3 minutes. The mixture was poured into a mold (25mm×25mm×120mm) and incubated at room temperature for 24h. After curing, demould, measure the size, then dry at 110°C for 24h, and then heat-treat at 1500°C for 3h. The linear change rate was calculated|required based on JISR2554 (the test method of the linear change rate of a refractory castable). The resulting mineral phases were identified by XRD. The microstructure after firing was observed by SEM, and elemental analysis was performed by EPMA at the same time. In addition, the equilibrium formation phase was simulated by the thermodynamic equilibrium calculation software FactSage.

3.1 Line rate of change

Sample No. 2 to which sodium polymetaphosphate was added and sample No. 4 to which an aqueous phosphoric acid solution was added swelled, while sample No. 5 to which polylacenoic acid was added shrunk.

The sample No. 1 without addition had shrinkage, while the change rate of the residual line of the samples No. 2 and No. 3 added with sodium polymetaphosphate became larger as the addition amount increased.

3.2 Generated mineral phases

There was little difference in the CA6 peak intensity between the two, and no difference was found in the resulting minerals. Figure 5 shows the relationship between the amount of sodium polymetaphosphate added and the intensity of the first peak of CA6. Even if the amount of sodium polymetaphosphate added was increased, the CA6 production amount hardly changed.

3.3 Microstructure

Needle-like crystals were found in the matrix for both, but there was no obvious difference in the microstructure.

In samples No. 2 and No. 3 to which sodium polymetaphosphate was added, localized areas of Ca and P aggregation were found. The portion represented by the ellipse has a composition of CaO:P2O5=2.4:1. According to the composition ratio, it can be determined that it is tricalcium phosphate Ca3(PO4)2. An enlarged view of the place where Ca and P are aggregated in sample No. 2 is shown in FIG. 8 . A void of about 10 to 20 μm was observed around the particles with high Ca and P concentrations.

3.4 Simulation of Equilibrium Spawn Phase

The amount of phosphoric acid was varied and the equilibrium formation phase was simulated at 1500°C. If the amount of phosphoric acid increases, the production of CA6 decreases, while the production of Ca3(PO4)2 increases.

The problem of swelling caused by the addition of sodium polymetaphosphate to aluminum spinel castables was investigated. It was previously thought that the expansion of the aluminum spinel castable was affected by the generated CA6, but according to the results in Figure 5, the expansion caused by the addition of sodium polymetaphosphate was not affected by the CA6. The sample that expands greatly after heating is due to the formation of Ca3(PO4)2. According to literature reports, the thermal expansion coefficient of Ca3(PO4)2 is 14.8×10-6/K, which is larger than 8.8×10-6/K of Al2O3 and 7.6×10-6/K of spinel. In addition, voids were created around the Ca3(PO4)2 particles, indicating that the expansion of Ca3(PO4)2 was greater than that of the surrounding matrix. Therefore, it is generally considered that the more sodium polymetaphosphate is added, the more Ca3(PO4)2 is generated, and the more the swelling increases.

In addition, the results obtained by the equilibrium phase simulation are consistent with the experimental results, which also proves that the swelling caused by the addition of sodium polymetaphosphate is not caused by the increase of CA6 production, but by the generated Ca3(PO4)2. On the one hand, the swellability shown in Figure 1 is abnormal. Compared with this result, samples No. 2 and No. 3, which are swelled by Ca3(PO4)2, only use raw materials below 1mm, and the swelling is obviously small. , it is generally believed that there are factors that further affect the expansion.

The effect of sodium polymetaphosphate on the residual expansion of aluminum spinel castables after firing was studied, and the following results were obtained.

(1) Adding sodium polymetaphosphate, the residual line change rate after heating at 1500°C increases. In addition, the residual line change rate increases with the increase of sodium polymetaphosphate addition amount.

(2) Even if the amount of sodium polymetaphosphate added was increased, the amount of CA6 production hardly changed.