A. W. Cramb

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

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.

Separation and dissolution of Al2O3 inclusions at slag/metal interfaces

Abstract To achieve better process control and product quality in clean steel manufacturing, it is necessary to understand the underlying processes behind (i) separation of inclusions across slag/metal interfaces and (ii) dissolution of the inclusions in the slag. This paper presents results on both these processes obtained through in situ observations with a confocal scanning laser microscope (CSLM) at steelmaking temperatures. In the case of inclusion separation, it was found that liquid inclusions could be observed to separate across the slag/metal interface and dissolve. Solid Al2O3 inclusions, however, were observed at the slag side of the interface and could be seen to agglomerate. The dissolution rate of slag particles was found to be a function of slag chemistry and temperature and to be controlled by diffusion in the liquid slag.

Interfacial tensions of liquid Fe-Ni alloys and stainless steels in contact with CaO-SiO2″AI2O3-based slags at 1550 °C

In the present work, the interfacial tensions of Fe-Ni alloys in contact with slags of the CaO-Al2O3-SiO2 system were measured at 1550 °C. Nickel additions to the alloy were found to decrease interfacial tension. The effects of alumina and titania additions to the slag on the interfacial tension of the Fe-20 wt pct Ni alloy were determined: alumina was found to increase the interfacial tension by a small amount, while titania was found to decrease it drastically. Using the present interfacial tension data for the CaO-Al2O3-SiO2 system and the ones measured by Jimbo and Cramb, Girifalco and Good’s interaction coefficient (ϕ) was determined as a function of the slag composition using regression analysis and was found to be a useful means of correlating interfacial tension data. The interfacial tension of an Fe-20 wt pct Ni-2.39 wt pct Al alloy in contact with a CaO-Al2O3-SiO2 slag was found to decrease drastically in the first 60 to 75 minutes of the experiment due to the dynamic effects of mass transfer. Slight lowering of interfacial tensions of industrial stainless steels due to sulfur transfer from liquid metal to slag was also observed. The equilibrium interfacial tensions of type 304 stainless steels were found to be more dependent on the slag chemistry than on the nickel and chromium content of the alloy.

DIRECT OBSERVATION OF SPONTANEOUS EMULSIFICATION AND ASSOCIATED INTERFACIAL PHENOMENA AT THE SLAG-STEEL INTERFACE

Interfacial tension decreases drastically when an intense chemical reaction occurs at a steel–slag interface. This phenomenon results in spontaneous droplet spreading during the initial period of reaction and droplet recovery as the rate of reaction decreases. In the present work, spreading tendency was found to be associated with spontaneous emulsification of both steel in slag and slag in steel. Spontaneous emulsification was observed at 1550°C when a liquid Fe–3.28%Al alloy droplet was placed in contact with liquid CaO–SiO2–Al2O3 (40:40:20 by weight) slag. Surface turbulence induced metal emulsification and droplet spreading was observed by X–ray photography. Spontaneous droplet spreading was noted at alloy aluminium contents as low as 0.25%. Spontaneous emulsification was steel into slag was documented at aluminium contents greater than 3%. From the observation of quenched Fe–3.28%Al alloy droplets by optical and scanning electron microscopy, slag entrapment and metal emulsification were documented and the metal–slag interface was shown to be extremely perturbed during the reaction of aluminium with silica.

Surface tensions of liquid Fe-Cr and Fe-Cr-N alloys

In the present work, surface tensions of liquid Fe-Cr and Fe-Cr-N alloys were determined. Previous studies on surface tensions of liquid Fe-base alloys show a large variation between investigations that could be due to the presence of surface-active impurities such as O or S in the alloy samples or due to inaccurate experimental techniques. A figure, which is a plot of the surface tension of liquid Fe-Cr alloys determined by previous investigators, shows the typical disagreement. The purpose of the present work is to determine accurate surface-tension values of liquid Fe-Cr and Fe-Cr-N alloys, using ultrapure samples and modern image processing techniques.

Levitation of liquid sodium droplets

Droplets of liquid sodium ranging from 1.2 to 2.1 g, immersed in mineral oil, were levitated in an electromagnetic field. The experimental setup was designed and constructed to levitate small metal droplets at audio frequencies. The levitated droplet was found to be very stable inside the inductor, and the equilibrium shape attained by the droplet in the electromagnetic field was measured during the experiment. A surface coupled mathematical model was used to calculate the self-consistent equilibrium droplet shape of liquid sodium under the influence of an electromagnetic field. The predicted shapes of the metal droplet and the position of the droplet inside the inductor compare well with the experimental data.

Clean Steel: Advancing the State of the Art (TRP 0003)

This project had 3 objectives: (1) to determine the kinetic factors governing inclusion removal from liquid steels at a slag metal interface; (2) to develop a methodology to enable steels of less than 1 ppm total oxygen to be produced with an average inclusion diameter of less than 5 {micro}m; and, (3) to determine the slag-metal interface conditions necessary for ultra clean steels. In objectives 1, and 3, the major finding was that dissolution rates of solid particles in slags were found to be significant in both ladle and tundish slags and must be included in a model to predict steel cleanliness. The work towards objective 2 indicated that liquid steel temperature was a very significant factor in our understanding of clean steel potential and that undercooled steels equilibrated with low oxygen potential inert gases have the potential to be significantly cleaner than current steels. Other work indicated that solidification front velocity could be used to push particles to produce clean steels and that reoxidation must be severely curtailed to allow the potential for clean steels to be realized.