Effect of Oxygen Potential on De-Si and De-P during Hot Metal Pretreatment

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The effect of oxygen potential on De-Si and De-P, and initial [Si] content on De-P during hot metal pretreatment were studied by Fe2O3-CaO-CaF2 slag. The results showed that the final [Si] content of 0.144 %-0. 090 % can be reached if the oxygen potential is controlled in the range 1. 64×10-4% to 3. 26×10-4%; when initial [Si] content in hot metal is lower than 0.15 %, the De-P ratio of 85 % and 80 % were obtained by controlling oxygen potential higher than 1. 7×10-4% at 1 573 K and 5. 5×10-4% at 1 623 K respectively, but the final [P] content was a little higher in later case. The optimum initial [Si] content is 0.10 % to 0.15 %.

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The change law of oxygen activity was studied by using Fe 2O 3-CaO-CaF 2 and Na 2CO 3/Na 2SO 4 fluxes under laboratory conditions. At the same time, the range of oxygen activity which is in favour of removing phosphorus and sulphur simultaneously was also investigated. The results showed that the oxygen activity in the iron melt during de-phosphorization can reach up to 5.91×10 -6-10.03×10 -6 in about 5 min and then only varies within a narrow range. A final phosphorus content of 0.008% and a final sulphur content of 0.003% can be obtained if the oxygen activity is controlled within the range of 4.5×10 -6-8.0×10 -6.
The substitution effect of Al2O3 for CaF2 on hot metal dephosphorisation has been studied at 1723 K. The initial composition of the slag was 40%CaO (mass percentage)+52% Fe2O3+8%CaF2. Then, Al2O3 was substituted for CaF2 up to 8%. The hot metal initial silicon was 0.61%, and the P contents were 0.063, 0.073 and 0.091%. Experimental results showed that with increasing Al2O3 content, the melting point of the slag slightly increased, and the oxidising power and phosphate capacity for the slag were enhanced, which led to an increase in dephosphorisation. The range of phosphorus content at the end point was between 0.009 and 0.014%, and the corresponding dephosphorisation varied from 82.5 to 89.0%. This result may be acceptable for industrial use.
The purpose of the study is to decrease content of CaF2 in dephosphorization slag. The dephosphorization effects of CaF2 replaced by B2 O3 were investigated. The dephosphorization experiments were carried out at 1450 °C in air atmosphere. The results show that the melting points and dephosphorization ratios change little when CaF2 was replaced by B2O3. The dephosphorization ratios are all greater than 80% and melting points can meet the requirement of hot metal pretreatment. Because of the change of alkalinity caused by B2O3, the oxidation of slag increases with increasing of B2O3 contents. It is beneficial to pre-dephosphorization. As a result, for decreasing its pollution, CaF2 can be replaced by B2O3 to dispose fluoride-free pre-dephosphorization slag.
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