Test Method: The test simulates the actual use of a tundish and ladle by creating a composite crucible. The outer layer is made of graphite, while the inner layer consists of an alkaline castable with a thickness of approximately 10 mm and varying CaO content. The casting material used has a composition of (MgO + CaO) at 96%. No phosphorus-containing water reducers are employed. After baking the crucible at 110°C for 24 hours, about 100 g of iron powder with different phosphorus contents is added. This iron powder is prepared by mixing phosphorus-rich and reducing iron powders in various proportions. A layer of about 10 g of steel slag, with a basicity of 1, is placed on top of the crucible to help reduce phosphorus in the slag. The crucible is then heated to 1600°C in an electric furnace and held at that temperature for 30 minutes. After cooling, the metal test block and the alkaline castable test block are removed for chemical analysis, X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. At least three sampling points are selected on the metal test block to ensure uniformity. For the alkaline castables, samples are taken after layered crushing.
Test Results and Discussion: The relationship between phosphorus content and CaO content in the alkaline castables was analyzed. When the castable contains 25% CaO, the phosphorus content in the metal test block is significantly reduced. However, increasing the CaO content further only slightly improves the dephosphorization effect. The relationship between the original phosphorus content in the metal sample and the phosphorus content and dephosphorization rate after the melting test was also studied. When the crucible with 40% CaO was melted at 1600°C for 30 minutes, the dephosphorization rate was calculated as the ratio of the difference between the original phosphorus content and the post-test phosphorus content to the original value. If the original phosphorus content was 0.03%, the reduction was minimal, resulting in a low dephosphorization rate. In contrast, when the original phosphorus content was high, such as 0.1% or 0.5%, the dephosphorization rate could reach up to 90%. The difference between these two levels becomes negligible at higher phosphorus concentrations.
The phosphorus content in the alkaline castable after testing was found to increase with the original phosphorus content in the metal test block. SEM observations and electron probe analysis of the lining confirmed the presence of phosphorus, with values of 0.94% and 0.48% recorded at two different points. This suggests that phosphorus from the molten metal has been absorbed into the refractory lining. Although phosphorus has a greater affinity for oxygen than iron, at 1600°C, direct oxidation of phosphorus to form gaseous P₂O₅ is not thermodynamically favorable, as shown by the Gibbs free energy equation: 2P + 5O = P₂O₅(g), ΔG° = -742,032 + 532.71T > 0. Therefore, phosphorus cannot be removed directly through oxidation. Instead, it must form a stable phosphate compound with a basic oxide to be effectively removed.
The dephosphorization reactions involving MgO and CaO in the basic refractories are as follows:
3MgO(s) + 2P + 5O = Mg₃P₂O₈(s) (ΔG° = -284,600 + 142.45T)
4CaO(s) + 2P + 5O = Ca₄P₂O₉(s) (ΔG° = -343,000 + 143.35T)
At the same oxygen potential, the equilibrium constants for these reactions can be compared. By solving the equations, we find that the logarithm of the equilibrium constant for MgO is much smaller than that for CaO, indicating that CaO is far more effective in removing phosphorus. At a steelmaking temperature of 1873 K, the phosphorus content in equilibrium with Ca₄P₂O₉ is four orders of magnitude lower than that in the Mg₃P₂O₈-MgO system. Thus, from a thermodynamic perspective, the dephosphorization effect of MgO is negligible compared to that of CaO. Based on the test results, when the original phosphorus content was 0.1%, the dephosphorization rate using a castable with 25% CaO reached as high as 89%. This demonstrates that CaO plays a critical role in the dephosphorization process within basic refractories.
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