Reduction kinetics and recovery of valuable metals from metallurgical slags based on high temperature physical chemistry

Abstract

A massive amount of ferrous (Electric Arc Furnace, EAF) and non-ferrous (Copper) slags produced by pyrometallurgical processes are generated. EAF- and Cu-slags have typically been disposed of by means of landfilling or by using it in several fields. Landfill treatment of the slags are problematic because of environmental contamination by means of leaching; also, the large amounts of slag produced can quickly fill landfill sites. Accordingly, there is a need for a recycling solution for EAF- and Cu-slags. In addition, efficient MnO reduction in FeMn slag is important issue to increase Mn recovery in ferroalloys industry. Therefore, the goal of the recent work is to quantitatively investigate the recovery of valuable metals such as Fe and Mn from EAF- and Cu-slag (or FeMn slag) by means of the pyrometallurgical process using various reducing agent such as carbon, Al(-dross), with emphasis on understanding the reduction behavior of FeO and MnO and elucidating the reaction mechanism. Furthermore, understanding industrial EAF slag in view of slag foamability and macro simulation using FactSageTM software were considered.

In chapter IV, silicothermic reduction behavior of MnO in CaO-SiO2 and BaO-SiO2 based slag system was investigated. The activation energy of the silicothermic reduction process was determined to be about 217.9 kJ/mol, which was very close to the activation energy for mass transfer in the slag phase. The mass transfer coefficient of SiO2 exhibited a maximum value at 5 mass pct CaF2, which originated from an increase in the apparent viscosity of the slag due to the precipitation of solid cuspidine at CaF2 content greater than 5 pct. Similarly, 90 pct yield of Mn recovery in BaO-SiO2 based slag was obtained at 5 mass pct CaF2, while no further increase in Mn recovery was found at CaF2 > 5 pct. This originated from an increase in viscosity of slag due to the precipitation of high melting point compounds. Consequently, the addition of fluxing additive CaF2 should be carefully determined, because an excess of CaF2 results in the formation of cuspidine or Ba2SiO4.

In chapter V, the author investigated carbothermic and aluminothermic smelting reduction (ASR) behavior of FeO and iron recovery in Cu slag by solid carbon and aluminum. In particular, we quantified the recovery of iron by performing typical kinetic analysis and considering slag foaming, which is strongly affected by the thermophysical properties of slags such as apparent viscosity. Iron recovery was maximal (about 60 pct) in the 20 mass pct CaO system and Al/FeO molar ratio = 0.53 because no solid compounds such as Mg2SiO4, Ca2SiO4, MgAl2O4 formed in this system, resulting in a highly fluid slag. Furthermore, carbothermic reduction of FeO in Cu slag (chapter IV) was predicted using Macro processing in FactSageTM 7.1 software. The calculated results for change of the slag composition was in well accordance with experimental results as function of reaction time. Especially, recovery of iron is good agreement with experimental data in 20 pct CaO system excluding rest of conditions. It is clear evidence that present model are well simulated in view of iron recovery. Furthermore, even though there are slight discrepancy between the calculated results and experimental results of solid-liquid fraction and reduction pathway, the reaction model constructed in this study exhibited a good predictability of reduction of FeO in the slag and iron recovery.

In chapter VI, the ASR of FeO (and MnO) and Fe (and Mn) recovery from EAF slag by aluminum (-dross). The aluminothermic reduction of FeO appeared to proceed rapidly and in good stoichiometric balance. Adding an optimal amount of Al (Al/FeO molar ratio~0.8) and CaO addition in case of Al-dross (40g CaO addition) yielded a Fe recovery of about 90% because of high fluidity of slags. Furthermore, the Mn could also be reduced from the EAF slag in the case of excess Al addition (Al/FeO≥0.8). Consequently, to maximize Fe recovery from EAF slag, it is crucial to control the slag composition, namely to ensure high fluidity by suppressing the formation of solid compounds.

In chapter VII, industrial EAF slag composition in commercial melt shop in view of slag foamability. Basic behavior of FeO in EAF slag was confirmed in view of fundamental thermodynamics. Monoxide ([Mg,Fe]·O) and spinel ([Mg,Fe]Al2O4) phase in EAF slag are confirmed by XRD analysis and these results can be understood equilibrium cooling by FactSageTM software. Especially, relationship between MgO and FeO at fixed C/S was considered in view of phase equilibria. Foam height was affected by slag viscosity and gas generation based on the results of foam height trendy in change of C/S and FeO content. Foaming index (Σ) decreased with increasing C/S and FeO content. Foam height has linear relationship with (η·aFeO) value, indicating that thermodynamic and physical property of slag simultaneously affects slag foaming. Therefore, optimum slag chemistry are highly required in view of thermodynamics and physical properties.