Immobilization of Chromium in Slags using MgO and Al2O3

Chromium containing slags from stainless steelmaking may be leached by acidic environments, therefore they should be treated before being stockpiled or land filled. In this work, synthetic slags were prepared and the effect of CaO/SiO₂, Cr₂O₃, MgO and Al₂O₃ contents on the stability of the mineralogical species formed was analysed. The morphology and composition of the slags were determined by XRD and SEM‐EDS, whilst their chemical stability was evaluated by leaching with an aqueous acetic acid solution. It was found that CaCr₂O₄ and CaCrO₄ are present in slags prepared with neither MgO nor Al₂O₃. The Al₂O₃‐based slags mainly produced Ca₂Al₂SiO₇ and the Cr(VI)‐containing oxide complex Ca₄Al₆CrO₁₆, whilst MgO‐based slags produced Mg Cr₂O₄ as main mineralogical species. Additionally, Eh‐pH diagrams for the Ca‐Cr‐H₂O and Mg‐Cr‐H₂O systems at 25°C were constructed. The results showed that the lowest chromium concentration levels in the leaching liquors corresponded to MgO‐based slags owing to the stable binding of chromium in spinel with MgO. It was also observed in the Al₂O₃‐based slags that when increasing the slag basicity from 1 to 2, the leachability of the slags was notably increased.     

Thermodynamics of oxygen and nitrogen in liquid iron equilibrated with CaO- SiO2-Al2O3 slags

Nitrogen distribution ratios, L_N ( = (mass pct N)/[mass pct N]), between CaO-SiO₂-Al₂O₃ slags and liquid iron were measured at 1823 and 1873 K as a function of Si (or Al) content in metal, using lime and alumina crucibles. Based on these results and the reported values for activities of SiO₂ (or Al₂O₃), nitride capacities, C_(N), defined by (mass pct N) · PO₂/3/4/PN₂/1/2 were evaluated. Activities of SiO₂ (or Al₂O₃) obtained by using the values for L_N and C_(N) obtained in present and previous gas-slag experiments were compared with previous results. FUMIHIKO TAMURA formerly Graduate Student with the Department of Metallurgical Engineering, Tohoku University.     

Amphoteric behavior of alumina in viscous flow and structure of CaO-SiO<Subscript>2</Subscript> (-MgO)-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> slags

The viscosity of CaO-SiO₂ (-MgO)-Al₂O₃ slags was measured to clarify the effects of Al₂O₃ and MgO on the structure and viscous flow of molten slags at high temperatures. Furthermore, the infrared spectra of the quenched slags were analyzed to understand the structural role of Al₂O₃ in the polymerization or depolymerization of silicate network. The Al₂O₃ behaves as an amphoteric oxide with the composition of slags; that is, the alumina behaves as a network former up to about 10 mass pct Al₂O₃, while it acts as a network modifier, in parts, in the composition greater than 10 mass pct Al₂O₃. This amphoteric role of Al₂O₃ in the viscous flow of molten slags at the Newtonian flow region was diminished by the coexistence of MgO. The effect of Al₂O₃ on the viscosity increase can be understood based on an increase in the degree of polymerization (DOP) by the incorporation of the [AlO₄]-tetrahedra into the [SiO₄]-tetrahedral units, and this was confirmed by the infrared (IR) spectra of the quenched slags. The influence of alumina on the viscosity decrease can be explained on the basis of a decrease in the DOP by the increase in the relative fraction of the [AlO₆]-octahedral units. The relative intensity of the IR bands for the [SiO₄]-tetrahedra with low NBO/Si decreased, while that of the IR bands for [SiO₄]-tetrahedra with high NBO/Si increased with increasing Al₂O₃ content greater than the critical point, i.e., about 10 mass pct in the present systems. The variations of the activity coefficient of slag components with composition indirectly supported those of viscosity and structure of the aluminosilicate melts.     

The effect of Na2O on dephosphorization by CaO-based steelmaking slags

The effect of Na₂O on the equilibrium phosphorous distribution ratio between slag and iron or iron alloys, L_P, has been measured for CaO-SiO₂, CaO-FeOr-SiO₂ (CaO or 2CaO·SiO₂ saturated), and CaO-Al₂-SiO₂ slags. The addition of Na₂O to CaO-SiO₂ slags significantly increases L_P and the phosphate capacity. A 25 pct CaO-25 pct Na₂O-SiO₂ slag has a distribution ratio nearly two orders of magnitude greater than a comparable binary 50 pct CaO-SiO₂ slag. For the CaO-saturated slags containing 40 wt pct FeO_T, the addition of 6 wt pct Na₂O increases L_P by a factor of 5. For the 2CaO·SiO₂-saturated CaO-FeO_T-SiO₂ slag, there is an optimum FeOr content (20 wt pct) for dephosphorization, and 10 wt pct Na₂O increases L_P by a factor of 2. For reducing slags typically used in ladle metallurgy for Al-killed steels (50 pct CaO-40 pct Al₂O₃-10 pct SiO₂), as little as 3 wt pct Na₂O increases L_P by a factor of 100. The present results indicate that small additions of Na₂O to conventional steelmaking slags can greatly improve dephosphorization. Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University.     

Thermal Conductivity Measurements of Some Synthetic Al2O3-CaO-SiO2 Slags by Means of a Front-Heating and Front-Detection Laser-Flash Method

The thermal conductivities of some synthetic slags containing Al₂O₃, CaO, and SiO₂ have been determined in the temperature range between 1623?K (1350?°C) and 1823?K (1550?°C) by applying a front-heating front-detection laser-flash method. In this method, the temperature response curve is measured in the short initial time period immediately after irradiating a laser pulse. The resultant values obtained by this method are unaffected by the radiative heat transfer. The temperature dependence of the thermal conductivity values were found not to be significant for all slag samples currently investigated. The thermal conductivity (??) of samples is represented with standard deviation less than 2?pct of its value as a function of compositions as follows: $$ \lambda = - 0.48\left[ {{\text{Al}}_{ 2} {\text{O}}_{ 3} } \right] - 0.57\left[ {\text{CaO}} \right] - 0.55\left[ {{\text{SiO}}_{2} } \right] + 57.1{\text{ W m}}^{ - 1} {\text{ K}}^{ - 1} $$ where [Al₂O₃], [CaO], and [SiO₂] are molar percent of Al₂O₃, CaO and SiO₂, respectively. The equation is suggested to cover the region of the following compositions: 8.0?mol pct < Al₂O₃ <21.0?mol pct, 31.5?mol pct < CaO <41.5?mol pct and 43.0?mol pct < SiO₂ <58.1?mol pct. The addition of Al₂O₃ to slags with a constant CaO/SiO₂ molar ratio resulted in an increase in thermal conductivity. In contrast, the effects of addition of SiO₂ and CaO are found to be insignificant.     

Activities of MnO in CaO-SiO2-Al2O3-MnO (<10 Pct)-FetO(<3 pct) slags saturated with liquid iron

Activity coefficients of MnO and Fe,0 in CaO-SiO₂-Al₂O₃-MnO(<10 mass pct)-Fe,O(<3 mass pct) slags were determined at 1873 K in an Al₂O₃ or CaO crucible by using the reported values for the activities of Al₂O₃ and SiO₂ or the analyzed contents of oxygen. The activity coefficients of MnO and Fe_tO were found to be constant in the studied concentration range of MnO and Fe_tO. The former increased with an increase in the CaO content, while the latter increased with an increase in the SiO₂ content.     

Influence of MgO addition on the properties of porous Al2O3 refractory containing TiO2 and Al2O3 in the matrix

The properties of porous Al₂O₃ refractory containing TiO₂/Al₂O₃ with a molar ratio of 0.96 in the matrix were studied with different amounts of MgO additives. Bright glossy areas were observed around and inside the Al₂O₃ particles. These were determined as Al₂O₃ · TiO₂ from the results of X-ray diffraction and EPMA analyses. The addition of MgO generated the dark glossy areas the compound of which was Na₂O · 4 MgO · 15 Al₂O₃. But for high MgO contents, Al₂O₃ · TiO₂, MgO · Al₂O₃ and Na₂O · 4 MgO · 15 Al₂O₃ coexisted in the dark glossy areas. These products prevented TiO₂ from invading the grain boundary, consequently grain-boundary cracks in the particles decreased and the physical properties, especially compressive and bending strength, were extremely improved. Thermal shock resistance was also improved by the rise of strength and fracture energy due to the addition of MgO. Corrosion resistance increased as the content of MgO increased. This was considered to be generated by the occlusion of FeO into MgO. Particularly, the fact that the penetration thickness decreased by the addition of MgO will contribute not only to the stable operation in actual use but also eliminating the procedure during which the porous plugs are cleaned by O₂ after every usage.     

Desulphurisation ability of blast furnace slag containing high Al2O3 and 5 mass% TiO2 at 1773 K

The sulphide capacities (C_S) of CaO–SiO₂–Al₂O₃–MgO–TiO₂ blast furnace slags were experimentally measured at 1773?K, and the percentages of free oxygen ions (O^2?) and bridge oxygen (O⁰) of molten slag were calculated using molecular dynamics at 1773?K. The measured and calculated results were closely correlated with the change in CaO/SiO₂, MgO, and Al₂O₃ contents in the slag. The results show that C_S increases with increasing CaO/SiO₂ and MgO contents in the slag, but decreases with increasing Al₂O₃ content. The effects of O^2? and O⁰ on C_S were analysed using multiple linear regression, and results show that sulphide increases with the increase in the mass percentage of free oxygen and decreases with the increase in the mass percentage of bridge oxygen.     

The Activities of FexO in {CaO‐SiO2‐Al2O3‐MgO‐FexO} Slags at 1723 K

In Japanese steelworks, hot metal is now produced by scrap melting process. With this process removal of sulphur is very much handicapped because of very high sulphur levels (0.04 to 0.09 pct by weight) and relatively low tapping temperatures (1623 to 1723 K). In order to overcome such disadvantages, the authors explored on the phase diagrams of {CaO‐SiO₂‐Al₂O₃‐MgO} slags, and this research revealed that those slags at 35 wt%‐Al₂O₃ would be good candidates as reagents for the removal of sulphur from high sulphur hot metal at relatively low temperatures. For better understanding of the thermodynamic properties of the candidate slags, in this study, activities of Fe_xO were determined by using solid‐state electrochemical cells incorporating MgO‐stabilized zirconia and Mo + MoO₂ reference electrode.     

A Viscosity Estimation Model for Molten Slags in Al2O3‐CaO‐MgO‐SiO2 System

A model for viscosity estimation of molten slags in the Al₂O₂‐CaO‐MgO‐SiO₂ system is presented in this work. The model is an extension to the viscosity estimation model of molten slags in the CaO‐FeO‐MgO‐MnO‐SiO₂ system developed before by the present author. The present model has explicitly taken charge compensation into consideration. It is postulated that Al exists in a structural unit MAl₂O₄ when MO/ Al₂O₃ >1 for the Al₂O₃‐MO‐SiO₂ system (MO=CaO, MgO). MAl₂O₄ has a similar behaviour as SiO₂, i.e. it can form an Al‐O‐Al network and be depolymerised by network modifying oxides (CaO, MgO). The present model is applied in viscosity estimation of some slags within the Al₂O₃‐CaO‐MgO‐SiO₂ system. A mean deviation of less than 25% is achieved for the present model.     

Phase equilibrium data and liquidus for the system “MnO”-CaO-(Al2O3 + SiO2) at Al2O3/SiO2 = 0.41

Phase-equilibrium data and the liquidus for the system “MnO”-CaO-(Al₂O₃-SiO₂) at a manganese-rich alloy saturation have been determined in the temperature range from 1423 to 1723 K. The results are presented in the form of a pseudoternary section “MnO”-CaO-(Al₂O₃ + SiO₂) with an Al₂O₃/SiO₂ weight ratio of 0.41. The following primary phases are present in the range of conditions investigated: 3Al₂O₃·2SiO₂; SiO₂; MnO·Al₂O₃·2SiO₂; (Mn,Ca)O·SiO₂; 2(Mn,Ca)O·SiO₂; MnO·Al₂O₃; (Mn,Ca)O; α-2CaO·SiO₂; α′-2CaO·SiO₂; 2CaO·Al₂O₃·SiO₂; CaO·SiO₂, and CaO·Al₂O₃·2SiO₂. The presence of alumina in this system is shown to have a significant effect on the liquidus compared to the system “MnO”-CaO-SiO₂, leading to the stabilization of the anorthite and gehlenite phases.     

Activities of FetO in CaO-Al2O3-SiO2-FetO (<5 pct) slags saturated with liquid iron

The activity coefficients of Fe_tO in CaO-Al₂O₃ and CaO-Al₂O₃-SiO₂ slags with 0.01 to 5 mass pct Fe_tO were determined at 1873 K from the data obtained in the present and previous slag-metal experiments, using an alumina or lime crucible. It was found that the activity coefficients of Fe_tO obeyed a dilute solution law and increased with increasing the content of SiO₂. Based on the findings pertaining to the activity coefficient, the values for the activities of SiO₂ and Al₂O₃ in CaO-Al₂O₃-SiO₂ slags were assessed.     

Viscosity of blast furnace type slags

The effect of MgO, TiO₂, or Fe₂O₃ on the viscosity of 40CaO-40SiO₂-20Al₂O₃ (mass pct) slags has been measured by the rotating crucible viscometer. Viscosity of these quaternary slags decreased with an increase in the content of additive oxide. At the same content of additive oxide, the viscosity decreases from MgO, TiO₂ to Fe₂O₃. In addition, the effect of SiO₂ or Al₂O₃ on the viscosity of 26.1CaO-73.9Fe₂O₃ (mass pct) (CF) and 14.9CaO-85.1Fe₂O₃ (mass pct) (CF₂) slags has been measured. Viscosity of calcium ferrite slags increased with increasing SiO₂ or Al₂O₃ content. Al₂O₃ was found to be more effective for increasing the viscosity at the same content of the additive oxide. This article is based on a presentation given in the Mills Symposium entitled “Metals, Slags, Glasses: High Temperature Properties & Phenomena,” which took place at The Institute of Materials in London, England, on August 22–23, 2002.     

Rate of interfacial reaction between molten CaO-SiO2-Al2O3-Fe x O and CO-CO2

Measurements of the rates of reduction of iron oxide from molten CaO-SiO₂-Al₂O₃-Fe _x O slags by Ar-CO mixtures have been made using a thermogravimetric method. The apparent first-order rate constant, with respect to the partial pressure of CO, of the gas/slag interfacial reaction was deduced from the measured rates, where the effects of the mass transfer in the gas and slag phases were minimized. It was found that the apparent first-order rate constant decreased with the concentration of ‘FeO’ from 100 to 20 wt pct, whereas it remained essentially constant in the range from 5 to 20 wt pct ‘FeO’. At a given iron oxide concentration, the reduction-rate constant increased significantly with an increase in the CaO/SiO₂ ratio. For fixed slag compositions, the reduction rate increased slightly with the oxidation state of the slags. When the rate constant is expressed in the form of k=k′(Fe³⁺/²⁺)^α, the values of α range from 0.15 to 0.25. The effect of temperature in the range from 1673 to 1873 K on the reduction rate of iron oxide in a 40.4CaO-40.4SiO₂-14.2Al₂O₃-5‘FeO’ (wt pct) slag was studied. The calculated activation energy, based on these results, is 165 kJ/mol.     

Activity of chromic oxide in the CaF2−CaO−Cr2O3 and the CaF2−Al2O3−Cr2O3 systems

The values of the activity of Cr₂O₃ in the slags based on the CaF₂−CaO−Cr₂O₃ and the CaF₂−Al₂O₃−Cr₂O₃ systems which may be used in the electroslag remelting (E.S.R.) process have been determined at 1450, 1500 and 1550°C by equilibrating the slags with Pt−Cr alloys of known chromium activity under known oxygen partial pressure and studying the equilibrium 2[Cr] alloy+3/2 O₂(g)=(Cr₂O₃)_slag. It was found that activity of Cr₂O₃ decreases with the addition of CaO and Al₂O₃ in the respective systems. In slags containing less than about 20 wt pct CaO and in the Al₂O₃ bearing slags, solutions of Cr₂O₃ showed a positive deviation from ideality and in slags containing more than 20 wt pct CaO, they showed a negative deviation. Both the authors were formerly with the Department of Metallurgy, University of Sheffield, England     

Iron redox equilibria in CaO-Al2O3-SiO2 and MgO-CaO-Al2O3-SiO2 slags

Measurements have been made of the ratio of ferric to ferrous iron in CaO-Al₂O₃-SiO₂ and MgO-CaO-Al₂O₃-SiO₂ slags at oxygen activities ranging from equilibrium with pCO₂/pCO≈0.01 to as high as air at temperatures of 1573 to 1773 K. At 1773 K, values are given by $\begin{gathered} \log {\text{ }}\left( {\frac{{Fe^{3 + } }}{{Fe^{2 + } }}} \right) = 0.3( \pm {\text{ }}0.02){\text{ }}Y + {\text{ }}0.45( \pm {\text{ }}0.01){\text{ }}\log \hfill \\ \left( {\frac{{pCO_2 }}{{pCO}}} \right) - 1.24( \pm {\text{ }}0.01) \hfill \\ \end{gathered} $ where Y=(CaO+MgO)/SiO₂, for melts with the molar ratio of CaO/SiO₂=0.45 to 1.52, 10 to 15 mol pct Al₂O₃, up to 12 mol pct MgO (at CaO/SiO₂≈1.5), and with 3 to 10 wt pct total Fe. Available evidence suggests that, to a good approximation, these redox equilibria are independent of temperature when expressed with respect to pCO₂/pCO, probably from about 1573 to 1873 K. Limited studies have also been carried out on melts containing about 40 mol pct Al₂O₃, up to 12 mol pct MgO (at CaO/SiO₂≈1.5), and 3.6 to 4.7 wt pct Fe. These show a strongly nonideal behavior for the iron redox equilibrium, with $\frac{{Fe^{3 + } }}{{Fe^{2 + } }} \propto \left( {\frac{{pCO_2 }}{{pCO}}} \right)^{0.37} $ The nonideal behavior and the effects of basicity and Al₂O₃ concentration on the redox equilibria are discussed in terms of the charge balance model of alumino-silicates and the published structural information from Mössbauer and NMR (Nuclear Magnetic Resonance) spectroscopy of quenched melts.     

Influence of MgO,Al2O3 and CaO/SiO2 on the viscosity of blast furnace type slag with high Al2O3 and 5 wt-% TiO2

The effect of MgO, Al₂O₃ and CaO/SiO₂ on the viscosity of CaO–SiO₂–Al₂O₃–MgO–5 wt-% TiO₂ slag was studied in the temperature range of 1673–1773?K. At a fixed CaO/SiO₂ ratio of 1·17 and 12 wt-% Al₂O₃, the viscosity of the slag decreased with increasing MgO content because of depolymerisation of the silicate structures. At a fixed CaO/SiO₂ ratio of 1·17 and 8 wt-% MgO, the viscosity of the slag increased with increasing Al₂O₃ content. At 8 wt-% MgO and 12 wt-% Al₂O₃ wt-%, increasing the CaO/SiO₂ ratio from 1·07 to 1·50 resulted in lower slag viscosity. The temperature dependencies of the viscosity on MgO addition, Al₂O₃ addition, and CaO/SiO₂ ratio were analyzed, and the apparent activation energies of each system were found to be between 178 and 232?kJ/mol, 273 and 360?kJ/mol, and 204 and 233?kJ/mol, respectively. Five different viscosity models were employed to predict slag viscosity, and the Riboud model was found to be the best for predicting this parameter.     

Viscosity of copper slags from chalcocite concentrate smelting

The viscosity of smelting slags from the Glogow copper plant in Poland was measured using a concentric cylinder viscometer. These slags contain typically 45 pct SiO₂, 16 pct CaO, 8 pct MgO, 11 pct Al₂O₃, and only 5 to 7 pct total iron. The viscosity was measured as a function of the CaO, MgO, SiO₂, Cu₂O, Cr₂O₃, and Fe₃O₄ contents in the temperature range from 1473 to 1623 K. Silica and chromium oxide additions increased the viscosity, while small additions of the other oxides decreased the viscosity. However, at large additions of CaO or MgO, cooling resulted in a rapid increase in the viscosity upon reaching the transition temperature. This critical transition temperature increased with increasing additions of CaO and MgO. This was explained by the precipitation of solid particles upon reaching the saturation limit. Depending on the slag composition, the activation energy for viscous flow was found to be in the range from 200 to 370 kJ/mol.     

Calcium deoxidation and nitrogen distribution in liquid nickel equilibrated with CaO-Al2O3 slags

The Wagner’s first-ordere _o ^Ca and second-order (r _o ^Ca ) interaction parameters between Ca and O in liquid nickel were determined as −1220 and 1.35 x 105, respectively, at 1873 K in the equilibrium experiments between liquid nickel and CaO-Al₂O₃ slags in an A1₂O₃ or CaO crucible. The values fore _Ca ^O , (−3060), r _Ca ^o (8.47 × 105), r _Ca ^O,Ca (6.76 x 10⁵), and r _Ca ^O,Ca (6.76 x 105) were also estimated. Nitride capacities, C(_N), defined by (mass pct N)·P ₂ ^3/4 PskN₂1/2 were obtained by using the present results for nitrogen distribution ratio, L_N =(mass pct N)/[mass pct N], and the reported values for the activity of Al₂O₃.     

Thermodynamics of oxygen and nitrogen in liquid nickel equilibrated with CaO-TiO x and CaO-TiO x -Al2O3 melts

Equilibrium between CaO-TiO _x or CaO-TiO _x -Al₂O₃ slags and liquid nickel with respect to oxygen and nitrogen has been studied at 1873 K as a function of Ti (or Al) content in metal, using lime and alumina crucibles. The Al-O, Ti-O, and Ti-Al relations were obtained for liquid nickel equilibrated with these slags. Nitride capacity,_{C (N)}, defined by (mass pct N) · , was evaluated from nitrogen distribution ratios,L _N {= (mass pct N)/[mass pct N], coupled with oxygen content in liquid nickel. Activities of Al₂O₃ and activity coefficients of TiO₂ and TiO_1.5 were estimated either from the content of Al, Ti, and O, using the compiled data for the free energy of formation of oxide and the respective interaction parameters or from the values forL _N andC _(N).