Effect of Atmosphere and Slag Composition on the Evolution of PH3 Gas during Cooling of Reducing Dephosphorization Slags

Abstract

The distribution ratio of phosphorous between the CaO-CaF2 (-SiO2) flux and SiMn alloy melts at 1 823 K was measured under strongly reducing atmosphere. Furthermore, thermodynamic and kinetics analyses were carried out for the environmental stability of reducing refining slags containing Ca3P2 under wet cooling conditions from the effect of slag composition on the evolution of PH3 (phosphine) gas. The distribution ratio of phosphorous between the CaO-CaF2 (-SiO2) flux and SiMn metal phases increased with increasing CaO concentration in the flux, followed by a constant value. The composition for the saturating distribution ratio of phosphorous was in good accordance to the saturation content of CaO in the CaOCaF2 flux at 1 823 K. When the Vee ratio (= CaO/SiO2) of the dephosphorization slag was greater than about 1.35, the lime and dicalcium silicate phases precipitated during solidification, resulting in an increase in the evolution rate of PH3 gas under wet and dry conditions due to an increase in the reaction area. However, when the Vee ratio of the slag was lower than about 1.35, fluorite, cuspidine, and wollastonite phases precipitated from the phase diagram, resulting in less amount of PH3 evolution during cooling because the reaction between Ca3P2 and H2O was restricted to the surface of bulk slag.

Reducing Dephosphorization of SiMn Alloy Melt and Suppression of Phosphine (PH3) Gas Emission

“…The experimental composition of the slag in the CaO-CaF2 system ranged from 5 to 25% CaO. The schematic diagram of the experimental apparatus is shown in previous articles [3,5,6]. The slag and SiMn alloy were held in graphite crucibles under a CO atmosphere to equilibrate.…”

Conversion of Calcium Phosphide to Calcium Phosphate in Reducing Dephosphorization Slags by Oxygen Injection

“…15) It is very interesting in Figs. 2 and 3 that the calcium phosphide (Ca3P2) was identified in the as-received RDP slag, 21) whereas the β-tricalcium phosphate (β-3CaO·P2O5) which is the most stable phase among the tricalcium phosphate compounds at room temperature and tetracalcium phosphate (4CaO·P2O5) were detected at the expense of Ca3P2 phase in the oxygen-treated slag.…”

“…However, when the basicity of RDP slag is lower than about 1.35, the fluorite, cuspidine, and wollastonite compounds precipitated during cooling, resulting in a negligible amount of PH3 evolution because the reaction between Ca3P2 and H2O was confined to the surface of bulk slag. 15) Even though the above results are very useful to restrain the emission of phosphine gas from RDP slag during cooling, the problem due to phosphine emission at room temperature is still remained. Actually, the safety treatment of RDP slag by oxygen blowing was originally reported by Arato et al 16) and Katayama et al 17) They primarily investigated the dephosphorization efficiency of molten stainless steel by applying calcium alloy and CaC2-CaF2 flux, respectively, and proposed that oxygen blowing was effective for reducing PH3 emission from RDP slag.…”

“…15) When the basicity of RDP slag is greater than about 1.35, the lime and dicalcium silicate compounds precipitated during solidification, resulting in an increase in the emission rate of PH3 gas due to a dramatic increase in the reaction area occurred from a disintegration of slags. However, when the basicity of RDP slag is lower than about 1.35, the fluorite, cuspidine, and wollastonite compounds precipitated during cooling, resulting in a negligible amount of PH3 evolution because the reaction between Ca3P2 and H2O was confined to the surface of bulk slag.…”