We investigated the effect of flux (lime) addition on the reduction behavior of iron oxide in copper slag by solid carbon at 1773 K (1500 °C). 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. The iron oxide in the copper slag was consistently reduced by solid carbon over time. In the kinetic analysis, we determined mass transfer coefficients with and without considering slag foaming using a gas holdup factor. The mass transfer of FeO was not significantly changed by CaO addition when slag foaming was ignored, whereas the mass transfer of FeO when slag foaming was considered was at a minimum in the 20 mass pct CaO system. Iron recovery, defined as the ratio of the amount of iron clearly transferred to the base metal ingot to the initial amount of iron in the slag phase before reduction, was maximal (about 90 pct) in the 20 mass pct CaO system. Various types of solid compounds, including Mg2SiO4 and Ca2SiO4, were precipitated in slags during the FeO reduction process, and these compounds strongly affected the reduction kinetics of FeO as well as iron recovery. Iron recovery was the greatest in the 20 mass pct CaO system because no solid compounds formed in this system, resulting in a highly fluid slag. This fluid slag allowed iron droplets to fall rapidly with high terminal velocity to the bottom of the crucible. A linear relationship between the mass transfer coefficient of FeO considering slag foaming and foam stability was obtained, from which we concluded that the mass transfer of FeO in slag was effectively promoted not only by gas evolution due to reduction reactions but also by foamy slag containing solid compounds. However, the reduced iron droplets were finely dispersed in foamy and viscous slags, making actual iron recovery a challenge.

Effect of CaO Addition on Iron Recovery from Copper Smelting Slags by Solid Carbon

The reduction of CaO-SiO2-Al2O3-FeO slags containing less than 10 wt pct FeO by solid carbonaceous materials such as graphite, coke, and coal char was investigated at reaction temperatures of 1400 °C to 1450 °C. The carbon monoxide evolution rate from the system was measured using stationary and rotating carbon rods, stationary horizontal carbon surfaces, and pinned stationary spheres as the reductants. The measured reaction rate ranged from 3.25 × 10−7 mol cm−2 s−1 at 2.1 pct FeO under static conditions to 3.6 × 10−6 mol cm−2 s−1 at 9.5 pct FeO for a rotating rod experiment. Visualization of the experiment using X-ray fluoroscopy showed that gas evolution from the reduction reaction caused the slag to foam during the experiment and that a gas film formed between the carbon surface and the slag at all times during experimentation. The reaction rate increased with increased slag FeO contents under all experimental conditions; however, this variation was not linear with FeO content. The reaction rate also increased with the rotation speed of the carbon rod at a given FeO content. A small increase in the reaction rate, at a given FeO content, was found when horizontal coke surfaces and coke spheres were used as the reductant as compared to graphite and coal char. The results of these experiments do not fit the traditional mass transfer correlations due to the evolution of gas during the experiment. The experimental results are consistent, however, with the hypothesis that liquid phase mass transfer of iron oxide is a major factor in the rate of reduction of iron oxide from slags by carbonaceous materials. In a second article, the individual rates of the possible limiting steps will be compared and a mixed control model will be used to explain the measured reaction rates.

Reduction of FeO in smelting slags by solid carbon: Experimental results

Recovery of iron and removal of hazardous elements from waste copper slag via a novel aluminothermic smelting reduction (ASR) process

Interfacial kinetics on the hydrogen reduction of liquid Fe
 t
 O in Fe
 t
 O-M
 x
 O
 y
 slag (M
 x
 O
 y
 = CaO, SiO2, Al2O3, and TiO2) has been studied at 1673 K. Because the rate of hydrogen reduction was very fast, the rate was controlled by gas-phase mass transfer under most of the experimental conditions. The effect of CaO or SiO2 addition on the interfacial chemical reaction rate of hydrogen reduction was empirically evaluated as a function of the ferrous-ferric ratio in the slag. The observed interfacial chemical reaction rates in Fe
 t
 O-CaO and Fe
 t
 O-SiO2 slags showed reasonable agreement with the estimated values. Most of the available literature data on the reduction rate of liquid iron oxide by solid carbon, hot metal, and reducing gases were also reviewed and compared with the results of the present work. It was found that the rate of hydrogen reduction of liquid iron oxide slag is much faster than that with other reducing agents such as solid carbon, carbon dissolved in the liquid iron, and CO gas.

Interfacial kinetics of hydrogen with liquid slag containing iron oxide

Data from the Gamma-Ray Spectrometer (GRS) that flew on the MESSENGER spacecraft indicate that the O/Si weight ratio of Mercury's surface is 1.2 ± 0.1. This value is lower than any other celestial surface that has been measured by GRS and suggests that 12–20% of the surface materials on Mercury are composed of Si-rich, Si-Fe alloys. The origin of the metal is best explained by a combination of space weathering and graphite-induced smelting. The smelting process would have been facilitated by interaction of graphite with boninitic and komatiitic parental liquids. Graphite entrained at depth would have reacted with FeO components dissolved in silicate melt, resulting in the production of up to 0.4–0.9 wt.% CO from the reduction of FeO to Fe0—CO production that could have facilitated explosive volcanic processes on Mercury. Once the graphite-entrained magmas erupted, the tenuous atmosphere on Mercury prevented the buildup of CO over the lavas. The partial pressure of CO would have been sufficiently low to facilitate reaction between graphite and SiO2 components in silicate melts to produce CO and metallic Si. Although exotic, Si-rich metal as a primary smelting product is hypothesized on Mercury for three primary reasons: (1) low FeO abundances of parental magmas, (2) elevated abundances of graphite in the crust and regolith, and (3) the presence of only a tenuous atmosphere at the surface of the planet within the 3.5–4.1 Ga timespan over which the planet was resurfaced through volcanic processes.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2017JE005367

A Low O/Si Ratio on the Surface of Mercury: Evidence for Silicon Smelting?

A kinetic study has been made on the reduction of iron oxide in molten slag, held in an alumina crucible under argon gas atmosphere, with graphite, cylindrical in shape. The composition of the primary slag was 20.5%Li2O-38.4%CaO-41.1%SiO2 (molar ratio=1:1:1). The initial concentration of iron oxide was changed between 3% and 12.5%. The experimental temperature was 1300°C. The rotation speed of the graphite cylinder was varied from zero to 1900 rpm. The reaction rate was largely affected by slag foaming. At higher FeO concentrations, the apparent rate constant was almost independent of the mechanical stirring. At lower concentrations, the effect of the rotation on the apparent rate constant was found to be small at lower speeds but tended to become larger at higher speeds. This was an indication that the melt was strongly agitated by CO gas bubble evolution. The effect of foaming is taken into account by introducing gas holdup factor into ordinary (foam-free) kinetic equations. Values of the mass transfer coefficient of oxygen in the molten slag, calculated by applying the penetration theory, are close to those of the apparent rate constant, measured experimentally. This implies that the rate of the reduction is controlled predominantly by the mass transfer in the slag phase.

Hirohide Kurimoto

Papers

Effect of Slag Foaming on the Reduction of Iron Oxide in Molten Slag by Graphite