Principles of Titanium Slag Smelting
The electric furnace smelting of titanium slag involves the carbothermic reduction of ilmenite. The process entails mixing ilmenite ore with a solid carbonaceous reductant—such as anthracite, petroleum coke, or metallurgical coke—and feeding the mixture into an electric furnace for reduction smelting. The objective is the selective reduction of iron oxides within the ore to metallic iron, while titanium oxides are enriched in the slag phase. Subsequent separation yields a titanium-rich slag as the primary product and metallic iron as a by-product.
The primary constituents of titanium concentrate are TiO₂ and FeO, accompanied by impurities like SiO₂, CaO, MgO, Al₂O₃, and V₂O₅. The smelting process facilitates the reaction between iron oxides and carbon under high-temperature, strongly reducing conditions within a molten bath. The resultant titanium slag and metallic iron are effectively separated due to differences in specific gravity and melting point. Key chemical reactions include:
Fe₂O₃ + C → 2FeO + CO (1)
FeO + C → Fe + CO (2)
Titanium slag is classified as a high-melting-point slag. Its melt exhibits distinctive properties: high corrosiveness, elevated electrical conductivity, and a sharp increase in viscosity as the temperature approaches its solidification point. These characteristics undergo significant changes throughout the smelting process due to compositional evolution.
2.1 High Conductivity and Open-Arc Melting Characteristics
High Conductivity: Molten ilmenite possesses high electrical conductivity (~2.0–2.5 kS/m at 1500°C, rising to ~5.5–6.0 kS/m at 1800°C). As reduction proceeds, the FeO content decreases while TiO₂ and lower titanium oxides (Ti₂O₃, Ti₃O₅) increase, causing conductivity to rise dramatically. For example, Sorel slag exhibits a conductivity of 15–20 kS/m at 1750°C, which is orders of magnitude higher than conventional metallurgical slags (~0.1 kS/m) and even exceeds that of molten ionic salts like NaCl. This indicates electronic conduction behavior, with conductivity being relatively insensitive to temperature changes.
Open-Arc Melting Regime: This exceptional conductivity dictates the furnace's operational mode. Heat is generated primarily by the open electric arc between the electrode tip and the molten bath surface, a characteristic "open-arc smelting" regime. This contrasts with "submerged-arc" operations used for high-resistance slags, where electrodes are buried, and heat comes from resistive heating within the slag itself. While a brief submerged-arc phase may occur initially, the process quickly transitions to a predominantly open-arc mode. In the later stages, the arc can contribute over 90–97% of the total heat input.
2.2 Influence of Melting Point and Viscosity
High Melting Point: The strong Ti-O bond results in high melting points for titanium oxides. Titanium slag, being predominantly titanium oxides, has a melting point typically between 1580–1700°C, increasing with higher TiO₂ content. This necessitates a highly concentrated heat source in the smelting zone.
"Short Slag" Viscosity Behavior: Titanium slag exhibits "short slag" characteristics. When fully molten above its melting point, its viscosity is very low. However, as the temperature approaches the melting point, viscosity increases precipitously due to the narrow crystallization range. The rapid precipitation of solid crystals suspended in the melt severely impairs fluidity, complicating tapping operations.
2.3 High Chemical Reactivity of the Melt
Titanium slag melt, containing significant amounts of low-valency titanium oxides (Ti³⁺), is highly chemically active and aggressively attacks most conventional refractory materials. Consequently, the standard protective measure in reduction furnaces is to maintain a frozen layer of titanium slag ("slag skull") lining the furnace walls.
2.4 Effect of Slag Boiling
The primary reduction reactions occur at the melt surface. However, violent localized reactions and CO gas generation can be triggered by events such as the sudden collapse of solid charge into the melt or the descent of high-carbon iron droplets through the slag. This causes violent slag boiling and splashing, which can submerge electrodes, cause current surges and short circuits, and destabilize the entire smelting process. Continuous feeding and closed furnace designs are employed to mitigate this issue and stabilize operations.
2.5 Influence of Impurity Elements
Impurity oxides (FeO, MgO, CaO, MnO, Al₂O₃) form binary compounds or eutectics with TiO₂, generally lowering the slag's melting point and acting as effective fluxing agents. However, excessive impurities reduce the final TiO₂ grade of the slag. The slag's melting point is also influenced by its reduction degree (O/Ti ratio). A minimum eutectic exists near O/Ti ≈ 1.76. Increasing reduction (lower O/Ti ratio) raises the melting point, indicating an optimal smelting endpoint around O/Ti = 1.76.
3.1.1 Representative Process Conditions and Operation (Example)
Carbon Addition: Based on stoichiometry for reducing 96% of FeO to Fe and 30% of TiO₂ to Ti₃O₅, the theoretical carbon requirement is ~7.98% of ore mass (equivalent to ~9.85% coke). In practice, carbon addition is often around 12%.
Electrical Parameters: Operating voltage is often constrained by transformer-furnace matching. A secondary voltage of ~100V might be used.
Batch Operation Example: A batch might use 1.49 tons of concentrate. A portion (e.g., 0.78 tons) is mixed with reductant and charged initially. The remainder (0.71 tons) is added intermittently during smelting to control slag composition and prevent crusting/splashing. A typical smelting cycle lasts ~180 minutes. Tapping involves burning open the tap-hole, allowing mixed slag and iron to flow into a slag pot with a bottom tap-hole for subsequent iron separation. Post-tapping, the furnace is prepared for the next cycle by sealing the tap-hole, adding a base charge, and restarting power.
4.1 Phase Structure of Ilmenite
Natural ilmenite (FeTiO₃, theoretical 52.6% TiO₂) is often a solid solution, represented generally as m((Fe,Mg,Mn)·TiO₂)·n((Fe,Cr,Al)₂O₃), where m+n=1. Weathered deposits (placers) can contain phases like pseudorutile (Fe₂Ti₃O₉), altered pseudorutile, leucoxene, and rutile (TiO₂), yielding higher TiO₂ content (up to 95-100% for rutile) and lower impurity levels compared to primary rock ilmenite.
4.2 Phase Composition of Titanium Slag
Titanium slag is primarily a two-phase material:
Black Titanate Solid Solution Phase (90-95%): A pseudobrookite solid solution with the general formula (M²⁺Ti₂O₅), where M²⁺ represents Fe²⁺, Mg²⁺, Mn²⁺, etc., and also incorporating Ti³⁺ (as Ti₃O₅) and other cations (Al³⁺, V³⁺). A typical example is Sorel slag: (FeTi₂O₅)₀.₃₁(MgTi₂O₅)₀.₃₀(Al₂TiO₅)₀.₀₆(MnTi₂O₅)₀.₀₀₈(V₂TiO₅)₀.₀₁₂(Ti₃O₅)₀.₃₁.
Silicate Glass Phase (5-10%): A glassy matrix with a typical composition of ~60% SiO₂, 18-20% Al₂O₃, 9-10% CaO, 1-4% MgO, 2-4% FeO, and 3-4% TiO₂.
Product Specifications:
Acid-Soluble Slag (for Sulfate Process TiO₂): Requires good acid solubility (acid hydrolysis rate ≥94%), appropriate levels of FeO and MgO to facilitate the acid digestion reaction, controlled low-valency titanium content, and limited harmful impurities (S, P, Cr, V).
Chloride-Grade Slag (for Chloride Process TiO₂): Requires high TiO₂ content (typically ≥92%), very low levels of CaO+MgO (generally ≤1.0%) to prevent adhesive formations during fluidized-bed chlorination, and a suitable particle size distribution.
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