Titanium sponge is the primary metallic titanium intermediate — the porous, silver-grey solid produced when TiCl₄ is reduced by magnesium. Virtually all wrought titanium mill products (sheet, bar, tube, forgings) and titanium alloys trace back to sponge quality. Understanding Kroll process chemistry and GB/T 2524 grade distinctions is essential for alloy producers, aerospace forgemasters, and chemical equipment fabricators selecting raw feed.
Kroll Process Overview
The dominant commercial production route is the Kroll process, a two-stage batch reaction:
Chlorination — ilmenite (FeTiO₃) or rutile (TiO₂) reacts with chlorine gas at 800–1000 °C to produce titanium tetrachloride vapour:
> TiO₂ + 2 Cl₂ → TiCl₄ + O₂
TiCl₄ is then purified by fractional distillation to remove FeCl₃, SiCl₄, and VOCl₃ impurities to ≥99.9% purity.
Magnesium reduction — purified TiCl₄ is fed into a sealed steel retort charged with liquid magnesium at 800–850 °C:
> TiCl₄ + 2 Mg → Ti + 2 MgCl₂
The reaction is exothermic and self-sustaining once initiated. Metallic titanium deposits as a porous sponge mass; by-product MgCl₂ is tapped or drained continuously. After reduction, the retort undergoes vacuum distillation at 900–1000 °C to remove residual Mg and MgCl₂ from the sponge pores. The recovered Mg and Cl₂ are recycled, making the Kroll process a partially closed loop.
Cycle time per retort is typically 7–14 days. The sponge is then crushed, blended for homogeneity, and sampled against GB/T 2524 specification.
Grade Comparison per GB/T 2524
China's national standard GB/T 2524 defines titanium sponge grades by impurity caps on Fe, Si, Cl, C, N, O, and H.
| Grade | Fe max (%) | Si max (%) | Cl max (%) | O max (%) | N max (%) | H max (%) | Hardness HB |
|---|---|---|---|---|---|---|---|
| Grade 0 | 0.06 | 0.03 | 0.06 | 0.06 | 0.012 | 0.003 | ≤ 90 |
| Grade 1 | 0.10 | 0.04 | 0.08 | 0.10 | 0.015 | 0.003 | ≤ 100 |
| Grade 2 | 0.15 | 0.05 | 0.10 | 0.12 | 0.020 | 0.004 | ≤ 110 |
Hardness is the proxy for total interstitial content (O + N + C); lower grade numbers = lower interstitials = softer, more ductile alloy.
Grade Selection by End-Use
Grade 0 — Aerospace & Defence The strictest interstitial caps make Grade 0 the standard feedstock for aerospace alloys such as Ti-6Al-4V (Grade 5) and Ti-6Al-4V ELI (Grade 23). Turbine blades, airframe structural forgings, and medical implants all require sponge hardness HB ≤ 90 to meet ASTM B265 / AMS 4928 downstream mechanical property floors. Oxygen content above ~0.08 % in Grade 0 sponge will push the final alloy above ELI limits.
Grade 1 — General Industrial Intermediate purity for non-aerospace structural uses: sporting goods, marine hardware, automotive exhaust, architectural cladding. Grade 1 is blended with recycled scrap and alloying additions before melting in vacuum arc remelting (VAR) furnaces. Cost premium over Grade 2 is modest; it is the most widely traded export grade from Chinese producers.
Grade 2 — Chemical Processing Commercially pure titanium (Grades CP-1 to CP-4 per ASTM) used in heat exchangers, reactor vessels, and desalination equipment tolerates higher interstitials because fabrication forming requirements (ductility, weldability) are met even at Grade 2 chemistry. Process economics favour the lower-cost Grade 2 feed when downstream corrosion resistance — not mechanical strength — is the design driver.