1. Experiment
The mass ratio of aggregate to powder used in the experiment was 67:33. In order to obtain close packing, the aggregate (8-0mm) 35 adopts four-level particle size gradation in the experiment, that is, 8-5mm: 5-3mm: 3-1mm: 1-0mm=20%: 20%: 15%: 12% , The mass ratio of powder (particle size <0.088mm) is 33%. In the high-alumina bauxite clinker, it is stipulated that K2O≤0.6, and 3-5 times the specified maximum value is taken as the additive addition amount, which is 2.6-4.4% when converted into K₂CO₃, so the addition amount of K₂CO₃ starts from 2%.
During the experiment, it was found that the fluidity of the castable became worse after K₂CO₃ was added, and the material was viscous after adding water and stirring, and hardly flowed after vibration, and it could barely be formed by pressing with fingers, but no air holes were released during the vibration process. Large pores and unevenness can be seen after demoulding. It may be that the water reducer is not suitable after adding K₂CO₃, so different water reducers were added in the experiment to improve its fluidity.
According to the ingredients, add an appropriate amount of water to mix evenly, vibrate to form a crucible, and after curing and drying, the crucible sample is processed at 900°C for 3h. Then put about 120g of 6063# aluminum alloy into the crucible, put it in a muffle furnace and keep it warm at 850°C for 72h to carry out the anti-corrosion test, take it out after cooling with the furnace, cut it along the axis of the crucible, and observe the degree of cross-section erosion.
2. Effect of different water reducing agents on castables
When STP is used as the water reducer, the amount of water added increases with the increase of the amount of K₂CO₃ added. When sodium citrate and FS20 are used as water reducing agents, the sample with 2% K₂CO₃ added is the same as the amount of water added to the M85 sample, and the fluidity is better. This may be because STP complexes with water-soluble K+ ions to form complexes and reduce its water-reducing effect.
After the crucible samples were treated at 900℃×3h, the surfaces of K2, K3 and K4 samples had white flakes peeling off, while the surfaces of K2+N and K2+FS20 samples were smooth. This may be due to the following reaction between K2CO3 and Al2O3 at 900 °C, and the generated CO2 is discharged from the surface of the sample, resulting in exfoliation.
K2CO3+Al2O3=2KAlO2+CO2(g) (1)
At 850°C, the Gibbs free energy change in formula (4.1) is ΔG=-15.5kJ.
3. Erosion result of K2CO3 additive
The castable becomes black after being eroded/infiltrated by molten aluminum, and the pores in the eroded part become denser after being filled. After K2CO3 was added, the corrosion resistance of the sample deteriorated, all the aluminum alloy in the crucible entered into the refractory material, and the crucible sample produced 1-3mm large cracks. K2, K3, and K4 samples were peeled off after being treated at 900°C, the apparent porosity of the sample surface increased, the permeation channels of the molten aluminum increased, and reacted with the refractory material, accompanied by volume shrinkage, further promoting the melting of the molten aluminum. Penetration, eventually causing material cracking. Although the surface of K2+N and K2+FS20 is smooth after heat treatment at 900°C and no white peeling occurs, in an air atmosphere of 850°C, K2CO3 reacts with metal Al and O2 in the air as follows:
2K2CO3+4Al+3O2(g)=4KAlO2+CO2(g) (2)
At 850℃, the Gibbs free energy change of reaction (2) is ΔG=-2671.26KJ. Judging from ΔG, the reaction (2) is more likely to occur than (1), and the reaction rate is faster, and the CO2 produced accelerates the corrosion of the aluminum alloy melt to the refractory material at high temperature. Therefore, the addition of K2CO3 additives reduces the corrosion resistance of refractory materials to aluminum alloy melts.
After adding K2CO3 as a sintering aid, when STP is used as a water reducer, the sample has a large amount of water and poor fluidity. When sodium citrate and FS20 are used as a water reducer, the sample has a small amount of water and good fluidity. After the crucible sample was treated at 900°C, K₂CO₃ reacted with Al2O3 to generate KAlO2 and CO2, and the surface produced layered exfoliation. During the corrosion test at 850°C, K2CO3 reacted with metal Al and O2 to form KAlO2 and CO2. The reaction was rapid and the corrosion resistance of the sample deteriorated.