Vanadium recovery in the ferrovanadium production process

Abstract

Ferrovanadium can be produced in a DC arc furnace by reducing V2O3 by means of an aluminothermic reaction. Vanadium recovery is decreased by the formation of vanadium oxides within the slag, along with ferrovanadium droplet entrainment in the slag. High refractory wear rates increase the operational cost of the process, which is mainly a result of the high operating temperature (slag temperature of 2100˚C) and incompatibility between the slag and the refractory. The effect of slag composition on the extent of refractory wear was investigated by heating slags of different MgO contents to 1800˚C in the contact with magnesia refractory. The chemical interaction between the slag and refractory was investigated, along with the dimensional change of a piece of refractory which was reacted with slag to determine the extent of slag-refractory interaction. Changing the slag composition to favour an increase in vanadium recovery and implementing a two stage melting process were both investigated. The (CaO+MgO): Al2O3 ratio was varied to alter the amount of spinel (MgAl2O4) that forms in the slag in order to establish the effect of spinel formation on vanadium recovery. The two stage process included adding excess aluminium to the first stage to increase V2O3 reduction and adding Fe2O3 to the second stage to produce a ferrovanadium product with an aluminium content below 1.5 wt%. The slag-refractory test results indicated that the extent of refractory wear can be decreased by increasing the MgO content of the slag in contact with the MgO refractory. The results also indicated that refractory wear was driven by the formation of MgAl2O4, which is the product of a chemical reaction between the slag and the refractory. The amount of spinel formed with a change in (CaO+MgO): Al2O3 ratio did not influence the vanadium recovery. The change in (CaO+MgO): Al2O3 ratio of the slag did influence the vanadium recovery due to the effect it has on the activity coefficient of V2O3. The vanadium recovery decreased linearly from 75 wt% to 25 wt% as the (CaO+MgO): Al2O3 ratio increased from 0.30 to 1.43. The vanadium recovery was also strongly influenced by the calculated slag volume, with the metal recovery decreasing with an increase in slag volume. A two stage process was simulated whereby excess aluminium was added in the first stage to increase vanadium recovery and hematite was added in the second stage to decrease the aluminium content of the ferrovanadium. The two stage process tests done on a laboratory scale indicated that it is possible to increase the overall vanadium recovery by increasing the aluminium used to reduce the V2O3, followed by the addition of Fe2O3 to produce a metal product with an aluminium content below 1.5 wt%. The optimum aluminium addition for the first stage was determined to be 3.5 wt% excess, which resulted in a vanadium recovery of 73.9 wt%. The vanadium recovery of the two stage process was 72.6 wt%, which is higher than the 71.4 wt% obtained for the single stage process. The use of Fe2O3 did however decrease the vanadium content of the metal from 68.6 wt% to 63.5 wt%.