Test Method: The test simulates the actual use of a tundish and ladle to create a composite crucible. The outer layer of the crucible 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 were utilized. After baking the crucible at 110°C for 24 hours, about 100 g of iron powder with different phosphorus contents was added. This iron powder was prepared by uniformly mixing phosphorus-rich and reducing iron powders in various proportions. A layer of about 10 g of steel slag, with a basicity of 1, was placed on top of the crucible to reduce the dephosphorization of the slag. The crucible was 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 were removed for chemical analysis, X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. At least three sampling points were selected on the metal test block to ensure uniformity, while the alkaline castables were sampled after being crushed in layers.
Test Results and Discussion: The relationship between phosphorus content and CaO content in the alkaline castables was analyzed. When the castable contained 25% CaO, the phosphorus content in the metal test block significantly decreased. Further increasing the CaO content only slightly improved the dephosphorization effect. The study also examined the original phosphorus content in the metal sample and its impact on the phosphorus content and dephosphorization rate after the melting test. When the original phosphorus content in the metal sample was 0.03%, the phosphorus level after the test was low, resulting in a low dephosphorization rate. However, when the original phosphorus content was higher, such as 0.1% or 0.5%, the dephosphorization rate could reach up to 90%. Notably, the difference between the two levels became minimal at higher phosphorus concentrations. The phosphorus content in the alkaline castable after the test was found to increase with the original phosphorus content in the metal test block. Scanning electron microscopy (SEM) observations combined with electron probe microanalysis revealed that phosphorus levels reached 0.94% at one point and 0.48% at another. This confirmed the presence of phosphorus in the lining. Although phosphorus has a higher affinity for oxygen than iron, at 1600°C, it cannot be removed through oxidation because the reaction forming gaseous P₂O₅ is thermodynamically unfavorable: 2P + 5O → P₂O₅(g), ΔG° = -742,032 + 532.71T > 0. Instead, phosphorus must form stable phosphates with basic oxides to be removed. The dephosphorization reactions involving MgO and CaO in basic refractories are as follows: 3MgO(s) + 2P + 5O → Mg₃P₂O₈(s) ΔG°Mg = -284,600 + 142.45T lgKMg = 62,210/T - 31.14 4CaO(s) + 2P + 5O → Ca₄P₂O₉(s) ΔG°Ca = -343,000 + 143.35T lgKCa = 74,970/T - 31.33 At the same oxygen potential, combining equations (3) and (6) gives: lgKMg - lgKCa = 2lg(CaO/MgO) = -12,760/T + 0.19 At the steelmaking temperature (1873 K): lg(CaO)/lg(MgO) = -3.3113 This indicates that the amount of phosphorus in equilibrium with Ca₄P₂O₉ is four orders of magnitude lower than that in the Mg₃P₂O₈-MgO system. Therefore, from a thermodynamic perspective, dephosphorization via MgO is negligible compared to that via CaO. Based on the test results, when the original phosphorus content was 0.1%, the dephosphorization rate using a castable containing 25% CaO reached 89%. This highlights the critical role of CaO in the dephosphorization process within basic refractories.
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