Distribution of Phosphorus between FeSi/Si and CaO-SiO2 Slags at 1600o C
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The aim of this thesis was to determine data for the phosphorus(P) distributioncoefficients between ferrosilicon(FeSi) or silicon(Si) and calcia-silica(CaO SiO2)slags. The work was carried out due to inconsistencies in literature on the phosphorusdistribution between Si and slag and because phosphorus distribution betweenFeSi and CaO SiO2 slag has not been previously been determined for silicon richFeSi alloys. Calcia-silica slags of compositions 43:57,50:50 and 65:35 in weight% (wt%) dopedwith approximately 300 parts per million(ppm) of P and Fe:Si alloys of compositions20:80,50:50, and 80:20 (wt%) have been prepared in induction furnaces. Equilibriumexperiments were performed by holding metal and slag in a graphite crucibleinside a furnace with resistance heating for 3 hours at 1600 °C in argon atmosphere.The samples were then quenched. After sample preparation, slag and metal wereanalyzed by Inductive Coupled Plasma Mass Spectroscopy. One experiment wasalso conducted with 20:80 wt% Fe:Si and CaO SiO2 containing 5.4 wt% P2O5.After a holding time of 3 hours at 1600 °C, the sample was cooled at a rate of 1°C per minute down to the solidification temperature of the metal (1360°C). Thissample was analyzed by Electron Probe Micro Analyzer.Equilibrium experiments with quenching showed that the phosphorus distributioncoefficients between Fe50:Si50 (wt%) and SiO2 CaO slags were 0.08 ± 0.06,0.14 ± 0.15 and 0.53 ± 0.42 for CaO contents of slag of 35, 50 and 57 wt% respectively.Experiments with Fe20:Si80 and SiO2 CaO slags gave values for thephosphorus distribution coefficient of 0.14 ± 0.15, 0.13 ± 0.3 and 0.52 ± 0.41 forCaO concentration in slag of 35,50 and 57 wt% respectively. Between Si andCaO SiO2 slags the phosphorus distribution coefficients were 0.12 ± 0.09 and 0.4for CaO concentration in slag of 50 and 57% respectively.The main findings were that the phosphorus distribution coefficient did notchange significantly with alloy composition. Slag composition on the other handhad a large impact on the phosphorus distribution coefficient. The phosphorusdistribution coefficient increases with increasing basicity or CaO/SiO2 ratio ofthe slag. Increasing the basicity also lead to increased calcium pick up in themetal which was consistent with published data. Thermodynamic modeling of thesystem should be conducted by including phosphorus in slag in the form of bothphosphate and phosphide. The average loss of phosphorus after the experiment was100 ppm. The use of graphite crucible most likely leads to the loss of phosphorus asP4 in gas phase. The experiment with controlled cooling gave equal concentrationof phosphorus in FeSi and Si. Phosphorus was not detected in the slag matrix.Some minor areas in the slag which also contained alumina and magnesia had aphosphorus concentration double of what was detected in the metals.