Phase transformations during the oxidation of calcium-containing titanium-vanadium slags and their influence on the formation of calcium vanadates

The processes of oxidation of titanium-vanadium slags with the participation of CaO are studied in the temperature range 800–1250°C, and the effect of these processes on the formation of calcium vanadates, which are easily dissolved in weakly acid media, is analyzed. The temperature ranges of the processes related to the oxidation of vanadium-containing phases and the release of vanadium oxides are found. It is shown that anosovite oxides in the temperature range 800–950°C, spinelides oxidize in the range 900–1050°C, and vanadium from the slag phase of calcium aluminotitanate oxidizes at 1050–1250°C. Vanadium-containing perovskite is the most stable phase: it undergoes insignificant structural changes upon roasting. The degree of vanadium extraction from slags reaches 88–92.5% under favorable conditions. The effect of the CaO and FeO contents in a slag on the oxidation of vanadium-containing phases is investigated. A decrease in the FeO content in a slag to 8% or below is shown to negatively affect the oxidation of vanadium in slag phases. At 8.3 or 5.0% FeO in a slag, the maximum degree of vanadium extraction from the slag is 81 or 59%, respectively.     

Titanium-vanadium slags produced upon the direct reduction of iron from titanomagnetite concentrates

The phase compositions of complex titanium-vanadium slags that form upon the processing (using the direct reduction of iron) of titanium-magnetite concentrates of ten CIS deposits are studied by X-ray diffraction and optical and scanning electron microscopies. Depending on the phase composition, these slags can be divided into the following three groups: spinellide, anosovite-spinellide, and anosovite slags. The general laws of element distribution between slag phases are determined. In the titanium-vanadium slags, vanadium is shown not to form individual phases and to enter into the compositions of solid solutions, namely, ansovite and spinellides. In ansovite, vanadium is present in the trivalent state in the form of V₂TiO₅. In the absence of the ansovite phase, vanadium concentrates in spinellide solid solutions, which are partly represented by MTiO₄ solutions. The solubility of vanadium in the spinellide MgAl₂O₄ is controlled by the MgO: Al₂O₃ molar ratio in a slag, and part of the aluminum is substituted by vanadium with the formation of an Mg(Al,V)₂O₄ solid solution only if aluminum is deficient.     

Distribution of vanadium between SiO2 rich slags and carbon saturated liquid iron

In order to determine optimal slag compositions for the extraction of vanadium from hot metal to SiO₂ rich slags, vanadium distributions were determined between slags of the following systems: FeO*–SiO₂ [(%V) ã 1.7] FeO*–SiO₂(sat)-CaO,Na₂O,MgO,Al₂O₃,TiO₂ [(%V) ã 1.7] FeO*–SiO₂–VO_n [(%FeO*)/(%SiO₂) = 1.5–1.7] and carbon saturated liquid iron at 1 300°C using the solid iron foil technique. The distribution increased with FeO* content in the binary FeO*–SiO₂ system, while additions of CaO, Na₂O, MgO and Al₂O₃ to FeO*–SiO₂ based slags under SiO₂ saturation caused the distribution to decrease. A slight decrease in distribution was also observed with increasing vanadium oxide content in FeO*–SiO₂ based slags having a constant (%FeO*)/(%SiO₂) ratio. The highest vanadium distributions were found in SiO₂ saturated slags with high TiO₂ contents. Vanadium valencies in the slags were determined by a wet analytic titration technique and the results showed that V^III+ is predominant. It was suggested that the predominating ionic species of vanadium in SiO₂ saturated slags are V³⁺ and VO⁺ while a change towards VO^?₂ may occur for FeO rich slags.     

Solubility of vanadium from manganese vanadates in aqueous solutions of soda ash

It is shown that, in the course of developing the technology of pure vanadium pentoxide preparation from manganic vanadium-containing metallurgical slags, their oxidative roasting and cinder formation without alkaline additives are accompanied by the decomposition of spinelides with the formation of manganese meta- and pyrovanadates. Concentrated aqueous solutions of soda ash with an Na₂CO₃ concentration of 120–150 g/dm³ are accepted as a selective leaching reagent for vanadium from a cinder. Manganese metaand pyrovanadates are synthesized, and the procedure of their preparation is presented. The solubility of vanadium from manganese vanadates in aqueous solutions of soda ash at \(C_{Na_2 CO_3 } = 150 g/dm^3 \) is studied at 20–95°C for pyrovanadate and at 85–95°C for metavanadate. It is shown that vanadium should be leached from converter manganic slags roasted without alkaline metal additives at a leaching solution temperature higher than 95°C. There is a possibility to increase the vanadium content in a leaching solution to 60–80 g/dm³. The results obtained are used in the development of the technology of vanadium leaching.     

Phase composition of the vanadium-containing titanium slags forming upon the reduction smelting of the titanomagnetite concentrate from the Kuranakhsk deposit

The phase composition of the vanadium-containing titanium slags that form upon the reduction smelting of the titanomagnetite concentrate from the Kuranakhsk deposit with an added CaCO₃ flux is studied by optical microscopy, X-ray diffraction, and scanning electron microscopy. The laws of formation of the phase composition and the interphase distribution of vanadium and other elements are revealed as a function the CaO and FeO contents in the slags. It is shown that, at low CaO contents (up to 5%), the phase composition of the slags containing 15–30% FeO is mainly represented by spinelides (Al-V-Cr and Al-Ti-V spinels and (Fe,Mg)₂TiO₄ ulvospinel), anosovite, and glass. When the CaO content in slag increases, titanium is fixed into perovskite. At 17–20% CaO and ≤8.3% FeO in slag, a new crystalline phase, i.e., Ca-Al-V titanate of a complex composition, forms along with perovskite, the Al-V-Cr spinel, anosovite, and glass. Vanadium in the slags is mainly distributed between anosovite, the spinelides, and the Ca-Al-V titanate, and vanadium is absent in the glassy phase.     

The Thermodynamics of Some Transition Metals in Molten Slags

In the present paper, the thermodynamic behaviour of transition metals, such as Cr, Ti, Nb and V in molten slags is systematically analysed based on a literature survey. These metals exist in molten slags with multi valences. Oxygen partial pressure, slag basicity, total content of each transition metal, content of each component in slag, and temperature are the influential factors on their thermodynamic properties in molten slags. Higher basicity and strong oxidative atmosphere are generally favourable for the stable existence of transition‐metal ions with higher oxidation states. Temperature is a factor that is less influential than the above‐mentioned ones. For a transition metal in molten slag, the concentration ratio of ions of different valences depends on the activity coefficient ratio of their oxides. The present paper summarizes the activity studies regarding the transition metal oxides in various molten slags. For chromium and titanium oxides, information on CaO? SiO₂ based systems is involved. For titanium oxides, its thermodynamic behaviour in MnO? SiO₂ based slags is introduced. For niobium and vanadium, the information in Na₂O? SiO₂, CaO? CaF₂? SiO₂ systems is provided. Thermodynamic studies are described for Nb₂O₅? MnO? SiO₂ molten slag equilibrated with liquid iron at 1828 K.     

The sulfur partition ratio with Fe-CSAT melts and the sulfide capacity of CaO-SiO2-Na2O- (Ai2O3) slags

The sulfur partition ratio between slag and carbon saturated iron and the sulfide capacity of CaO-Na₂O-SiO₂ slags and a 48 pet CaO-45 pet Al₂O₃-7 pet SiO₂-(Na₂O) slag have been mea-sured at 1400 °C. The addition of Na₂O to a CaO-SiO₂ slag increases the sulfur partition ratio and the sulfide capacity; however, Na₂O at low concentrations has no measurable effect on the sulfide capacity of a CaO-Al₂O₃-SiO₂ slag. To convert the sulfur partition ratio to the sulfide capacity, the oxygen potential was calculated assuming equilibrium between iron in the alloy and FeO in the slag with the activity of FeO calculated via a regular solution model. The optical basicity may be used to correlate the data, but at high Na₂O contents the data do not adhere to the correlation previously developed for CaO-based slags. Formerly Graduate Student at Carnegie Mellon University     

The Influence of SiO2 on the Extraction of Ti Element from Ti‐bearing Blast Furnace Slag

The present paper investigated the crystallization behavior of Ti‐concentrating phases in titanium‐bearing blast furnace slags influenced by the silica content. The objective was to recycle the titanium from titanium‐bearing blast furnace slags by enriching Ti element in anosovite. The effect of SiO₂ on the formation of anosovite in the titanium‐bearing blast furnace slags was studied under C/CO equilibrated atmospheres using a combination of X‐ray Diffraction (XRD) and Energy Dispersive X‐ray Spectroscopy (EDX). It was found that, under the C/CO equilibrated atmospheres, the formed primary phases transformed from perovskite to anosovite by adding SiO₂ up to 35 wt% in the slags. The related mechanism was investigated according to the theory of molten slags structure and the principle of thermodynamics.     

Formation of hexavalent chromium by reaction between slag and magnesite-chrome refractory

The goal of this work was to understand how Cr⁶⁺ formation is affected by the interaction between chromite phases present in magnesite-chrome refractory and different slag compositions. In addition, the formation of Cr⁶⁺ as a function of chromite particle size and cooling rate due to the chromite phase/slag interaction was investigated. The following slag compositions were studied: calcium aluminate, calcium aluminum silicate, and calcium silicate. It was found that the presence of uncombined CaO in the calcium aluminate slags is responsible for a higher yield of Cr⁶⁺. However, the replacement of Al₂O₃ by SiO₂ in calcium aluminate slags decreases the formation of Cr⁶⁺. SiO₂ reacts with CaO to form stable 2CaO·Al₂O₃·SiO₂ and CaO·SiO₂ phases, consequently decreasing the amount of uncombined CaO available to react with the chromite phase to form Cr⁶⁺. Moreover, the content of Cr⁶⁺ is decreased by increasing chromite particle size and increasing the cooling rate of magnesite-chrome refractory.     

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.     

Effect of composition on the crystallisation behaviour of blast furnace slag using single hot thermocouple technique

When the content of glass in blast furnace slag is over 95%, it can be used as a raw material in the manufacture of cement. The critical cooling rate required for the formation of glassy slag is one of the important characteristics for molten BF slag. The crystallisation behaviour of molten BF slag has been studied by in situ observation with the single hot thermocouple technique. The isothermal and non-isothermal experiments were performed to construct time–temperature-transformation and continuous-cooling-transformation diagrams. The effect of MgO, Al₂O₃ and binary basicity (CaO/SiO₂) on the critical cooling rate of the CaO–SiO₂–MgO–Al₂O₃ slags were studied under conditions of CaO/SiO₂?=?1.1–1.4, 6–12?mass% MgO and 10–16?mass% Al₂O₃. The following finding are reported in the present paper: (i) Higher MgO content increased the critical cooling rate; higher Al₂O₃ content decreased the critical cooling rate; higher CaO/SiO₂ increased the critical cooling rate. (ii) The crystallisation temperature of molten BF slag lowers as the cooling rate increases, the slag have larger critical cooling rate, higher crystallisation temperature. The results could be used to design proper cooling path of molten BF slag for the formation of glassy.     

Experimental and Theoretical Studies on the Viscosity–Structure Correlation for High Alumina-Silicate Melts

Blast furnaces are encountering high Alumina (Al₂O₃ > 25 pct) in the final slag due to the charging of low-grade ores. To study the viscosity behavior of such high alumina slags, synthetic slags are prepared in the laboratory scale by maintaining a chemical composition of Al₂O₃ (25 to 30 wt pct) CaO/SiO₂ ratio (0.8 to 1.6) and MgO (8 to 16 wt pct). A chemical thermodynamic software FactSage 7.0 is used to predict liquidus temperature and viscosity of the above slags. Experimental viscosity measurements are performed above the liquidus temperature in the range of 1748 K to 1848 K (1475 °C to 1575 °C). The viscosity values obtained from FactSage closely fit with the experimental values. The viscosity and the slag structure properties are intent by Fourier Transform Infrared (FTIR) and Raman spectroscopy. It is observed that increase in CaO/SiO₂ ratio and MgO content in the slag depolymerizes the silicate structure. This leads to decrease in viscosity and activation energy (167 to 149 kJ/mol) of the slag. Also, an addition of Al₂O₃ content increases the viscosity of slag by polymerization of alumino-silicate structure and activation energy from 154 to 161 kJ/mol. It is witnessed that the activation energy values obtained from experiment closely fit with the Shankar model based on Arrhenius equation.     

Distribution of Bi Between Slags and Liquid Copper

The distribution of Bi between liquid copper and calcium ferrite slag containing 24 wt pct CaO, iron silicate slag with 25 wt pct SiO₂, and calcium iron silicate slags was measured at 1573 K (1300 °C) under controlled CO-CO₂ atmosphere. The experimental results showed that bismuth distribution is affected by the oxygen partial pressure, and bismuth is likely to exist in slags in the 2+ oxidation state, i.e., as BiO. The distribution ratio between calcium ferrite slag and metal was found to be close to that of iron silicate slag. The Bi distribution ratio was found to decrease with increasing SiO₂ and Al₂O₃ content in slag. Increasing temperature was found to decrease the Bi distribution ratio between slag and metal. Using the measured equilibrium data on Bi content of the metal and slag and composition dependence of the activity of Bi in liquid copper, the activity and hence activity coefficient of BiO in the slag was calculated. The close value of activity coefficient of BiO in both slags at the same oxygen partial pressure indicates that the CaO-BiO and SiO₂-BiO interactions are likely to be at the same level, or the FeO _x -BiO interaction is the predominant interaction for BiO in the slag. Therefore at a constant FeO _x content in the slag, the CaO-BiO and SiO₂-BiO interactions doesn’t affect \( \gamma_{\text{BiO}} \) significantly.     

Thermodynamic Simulation of the Influence of the Temperature and the Basicity of a Boron-Containing Slag on Steel Desulfurization

The results of thermodynamic simulation of the desulfurization of a medium-carbon steel by slags of the CaO–SiO₂–MgO–Al₂O₃–B₂O₃ system are presented. The HSC Chemistry 6.12 software package is used for the simulation. The thermodynamic simulation is performed for 20 various chemical compositions of slags with various B₂O₃ contents (1–4%)¹ and basicities ((CaO)/(SiO₂) = 2–5). The computations are performed using the Equilibrium Compositions module in the temperature range from 1500 to 1700°C with an increment of 50°C at a gas phase pressure of 0.1 MPa. The main results of the calculations are presented as the dependences of the change in the sulfur content in steel [S] on the temperature, the content of B₂O₃, and the slag basicity. An increase in the temperature of metal desulfurization from 1500 to 1700°C exerts a favorable effect on the sulfur content for the studied range of slag basicities. In particular, the sulfur content in steel decreases from 0.012 to 0.009% when steel is processed with the slag having 3% B₂O₃ and a basicity (CaO)/(SiO₂) = 2. A positive effect of an increase in the slag basicity from 2 to 5 on metal desulfurization is observed: the degree of desulfurization increases from 61.1 to 97.2% at 1600°C and 3% B₂O₃ content in the slag. As the B₂O₃ content in a slag increases from 1 to 4%, its refining properties decrease significantly in the range of basicity not higher than 2. In the range of high slag basicities (3–4), the negative effect of acidic oxide B₂O₃ on the refining properties of the slag decreases, providing low sulfur contents (which do not exceed [S] = 0.003–0.004% at 4% B₂O₃). At a slag basicity of 5, the sulfur content in steel decreases to 0.001%, all other things being equal. The simulation results can be used for the calculation of steel desulfurization processed by slags containing B₂O₃.     

A Clean and Efficient Method for Recovery of Vanadium from Vanadium Slag: Nonsalt Roasting and Ammonium Carbonate Leaching Processes

A cleaner method has been developed for the extraction of vanadium from vanadium slag. Compared to the traditional alkaline salts roasting followed by the water leaching process, in the nonsalt roasting process because no additives are added, the chromium spinel in the raw vanadium slag will not be converted to carcinogenic chromate salts and exhaust gas will not be produced. The ammonium metavanadate is precipitated from the water leach solution. The wastewater from the vanadate precipitation process can be recycled into the leaching process. The leaching residue can be comprehensively utilized in conjunction with an iron-making process using blast furnace. The nonsalt roasting mechanism was systematically investigated in a laboratory study. The XRD and morphology analysis of roasted vanadium slag showed that the oxidation of vanadium spinel occurred in the following steps: (1) the destruction of vanadium spinel and the formation of solid solution of Fe₂O₃·V₂O₃; (2) the oxidation of solid solution of Fe₂O₃·V₂O₃ to Fe₂O₃·V₂O₄ and a portion of the V(IV) in the Fe₂O₃·V₂O₄ was reacted with basic oxide such as MgO to generate the low-valence vanadate Mg₂VO₄; (3) the formation by further oxidation of highest-valence vanadates Mn₂V₂O₇ and Mg₂V₂O₇. The effects of particle size, oxygen concentration, gas flow rate, and temperature on vanadium recovery were investigated. Simultaneously, the effects of leaching variables, including ammonium carbonate concentration and temperature, were examined. The thermodynamics of the system are also reported.     

Evaluation of Secondary Steelmaking Slags and Their Relation with Steel Cleanliness

Based on data provided from an industrial plant and FactSage commercial software use, a study of secondary refining slags and inclusion cleanliness was performed. Six heats of two slag series, namely, A and B, with average chemical composition (wt pct) of 43.00CaO-25.90SiO₂-12.96Al₂O₃-18.13MgO for series A and 49.98CaO-23.88SiO₂-10.11Al₂O₃-11.99MgO-4.03CaF₂ for series B, were used for the study. Both series used DIN 38MnS6 modified steel. The effective viscosity, solid fraction, composition of the liquid fraction, and slag saturation degree in MgO (calculated through thermodynamic software) were related to the experimental results obtained for the inclusion cleanliness. The B slags showed lower effective viscosity than the A slags, due to their high liquid fraction. Regarding the capacity of slags in the inclusion removal, slag B5 resulted in the lowest inclusion density and was considered as the best choice among the slags studied. The inclusion species formed using B slags are constituted especially of CaO-Al₂O₃-SiO₂ and MgO-Al₂O₃ and are Al₂O₃ rich. The presence of sulfide-type inclusions (AlMnS and CaS) were more pronounced among A slags.     

Removal of Boron and Phosphorus from Silicon Using CaO-SiO2-Na2O-Al2O3 Flux

A combination of solvent refining and flux treatment was employed to remove boron and phosphorus from crude silicon to acceptable levels for solar applications. Metallurgical grade silicon (MG-Si) was alloyed with pure copper, and the alloy was subjected to refining by liquid CaO-SiO₂-Na₂O-Al₂O₃ slags at 1773 K (1500 °C). The distribution of B and P between the slags and the alloy was examined under a range of slag compositions, varying in CaO:SiO₂ and SiO₂:Al₂O₃ ratios and the amount of Na₂O. The results showed that both basicity and oxygen potential have a strong influence on the distributions of B and P. With silica affecting both parameters in these slags, a critical $ P_{{{\text{O}}_{2} }} $ could be identified that yields the highest impurity pick-up. The addition of Na₂O to the slag system was found to increase the distributions of boron and phosphorus. A thermodynamic evaluation of the system showed that alloying copper with MG-Si leads to substantial increase of boron distribution coefficient. The highest boron and phosphorus distribution coefficients are 47 and 1.1, respectively. Using these optimum slags to reduce boron and phosphorus in MG-Si to solar grade level, a slag mass about 0.3 times and 17 times mass of alloy would be required, respectively.     

On the Formation of Vanadium Ferrites in CaO–SiO2–FeO–V2O5 Slags

Selective extraction of valuable elements (such as V, Cr, Mn, etc.) and their compounds from metallurgical slag is in a focus of many researchers. Although vanadium may be present in slag as oxides and/or complex spinels with Fe, Mn, etc. during an alloyed steel production, the majority of vanadium in metallurgical slags typically exists as V₂O₅, which comprises up to 3–5 wt% of the slag in some cases. Due to the vanadium toxicity, these slags are forbidden in many civil engineering applications. As a result, hundreds of thousand tonnes of V₂O₅‐bearing slags are landfilled every year. In the present work, the formation of vanadium ferrites (FeV₂O₄ and Fe₂VO₄) in synthesized CaO–SiO₂–FeO–V₂O₅ slags containing 5 wt% V₂O₅ was examined under different partial pressures of oxygen. For the current slag chemistry range, an XRD analysis confirmed the presence of vanadium ferrite in slag samples treated at 1773 K in an argon atmosphere (PO₂ = 10^?1–10^?2 Pa) while no solid was noted in samples treated in air. Results were discussed based on thermodynamic consistency.     

Viscosities of the Quaternary Al2O3‐CaO‐MgO‐SiO2 Slags

Viscosities of some quaternary slags in the Al₂O₃‐CaO‐MgO‐SiO₂ system were measured using the rotating cylinder method. Eight different slag compositions were selected. These slag compositions ranging in the high basicity region were directly related to the secondary steel making operations. The measurements were carried out in the temperature range of 1720 to 1910 K. Viscosities in this system and its sub‐systems were expressed as a function of temperature and composition based on the viscosity model developed earlier at KTH. The iso‐viscosity contours in the Al₂O₃‐CaO‐MgO‐SiO₂ system relevant to ladle slags were calculated at 1823 K and 1873 K for 5 mass% MgO and 10 mass% MgO sections. The predicted results showed good agreement with experimental values and the literature data.     

Experimental Studies of the Alkali Behaviour in Blast Furnace Type Slags

In a preceding paper, existing data on the chemical behaviour of alkali compounds and their influence on physical properties of BF slags as well as on materials circulation in the BF operations were critically reviewed. In the present work, new investigations based on fundamental experiments and thermochemical modelling are introduced. They have been focused on the determination of both chemical activity of Na₂O and kinetics of Na₂O and K₂O evaporation from BF‐type slags. The results for the activities a_Na2O agree fairly well with data obtained from calculation based on the slag model of H. Gaye which seems generally suitable to predict chemical activities in molten slags. Kinetics of alkali evaporation from BF‐type slags follow a first‐order reaction. The rate constants for K₂O are remarkably higher than those for Na₂O. Alkali evaporation is enhanced with decreased %(CaO)/%(SiO₂) ratio and lower Al₂O₃ content and is increased with temperature. It is further favoured under reducing conditions in the absence of iron oxide in the slag.