Metallurgical-Grade Alumina (Smelter-Grade Alumina, SGA)

Overview

Metallurgical-grade alumina (MGA, also called smelter-grade alumina or SGA) is the bulk Bayer-process Al₂O₃ feedstock for primary aluminum electrolysis via the Hall-Héroult process. It accounts for ~85% of global Al₂O₃ tonnage — global supply runs ~140 million t/yr with China supplying ~80 million t/yr from refineries clustered in Shanxi, Shandong, Henan (Sanmenxia bauxite belt), Guizhou, and Guangxi.

The specification regime is tight on physical properties (sandy form, attrition resistance, fines control) optimized for modern dry-scrubbing cell technology that captures HF emissions from the electrolyte, but loose on chemical purity (Na₂O 0.30–0.50% is acceptable because the Hall-Héroult bath is itself sodium-based). Al₂O₃ content ≥98.5%, with the balance being a mix of Bayer-process unavoidable contaminants (Na₂O, SiO₂, Fe₂O₃, CaO, TiO₂, V₂O₅).

Sandy MGA dissolves into the cell crust through a dispersion-and-dissolution mechanism whose kinetics depend on particle size (D50 80–110 μm), fines content (below 45 μm fraction ≤12%), and specific surface area (BET 60–80 m²/g). Modern point-fed prebake cell designs require sandy form factor — floury MGA (older Bayer practice) doesn't dissolve fast enough and is no longer commercially traded outside legacy plants.

Trade pricing tracks the LME aluminum benchmark with a regional FOB adjustment (Australia / Brazil / China origin FOB price + LME-aluminum-linked sliding-scale). Long-term off-take contracts (3–5 year duration) dominate transactional volume; the spot market is shallow and not where independent traders add value.

Technical Specifications

Applications

Primary Aluminum Electrolysis (Hall-Héroult Process)

The single largest application by tonnage — about 92% of MGA goes here. The Hall-Héroult process electrolyzes Al₂O₃ dissolved in molten cryolite (Na₃AlF₆) at 950–970°C; sandy MGA is dosed point-by-point onto the electrolyte crust, dissolves into the bath, and is reduced to aluminum metal at the carbon cathode. ~1.92 t of Al₂O₃ produces 1 t of aluminum metal. The dry-scrubbing system captures HF emissions on the incoming alumina (which is why specific surface area matters — too low BET and the scrubbing efficiency drops).

Aluminum Fluoride (AlF₃) Manufacture

About 4% of MGA. AlF₃ is the make-up flux added to Hall-Héroult cells to maintain bath chemistry; it is produced by reacting MGA with HF gas (the dry-scrubbing route) or with fluosilicic acid (the wet route). HF-route AlF₃ is the dominant industry choice today, and MGA is the natural co-located feedstock at integrated smelter-refinery sites.

Synthetic Corundum (White Fused Alumina)

About 2% of MGA. Fusion of MGA in an arc furnace at >2000°C produces white fused alumina ingot, which is crushed and graded into FEPA-standard abrasive grits for grinding wheels, refractory aggregate, and sandblasting media. White fused alumina is preferred over brown fused alumina (made from bauxite) for cleaner cutting performance and higher purity (Al₂O₃ ≥99%).

Refractory Aggregate for High-Alumina Bricks

About 1% of MGA. Used as the alumina source in high-alumina firebrick (Al₂O₃ 50–80%) for steel furnace lining, glass tank crown, kiln furniture. Tabular alumina (a separate engineered grade from MGA) is the higher-spec choice for premium refractory applications.

Welding Flux Raw Material

Small but specialized — MGA contributes Al₂O₃ to the slag-forming flux system used in submerged-arc welding of steel structures and pressure vessels. Together with CaO, SiO₂, MgO, and fluoride salts, MGA tunes the slag basicity, viscosity, and inclusion-removal performance.

Selection Guide vs Calcined / Activated / High-Purity Alumina

Choose MGA for: primary aluminum smelting (the only economically sensible feedstock at scale); AlF₃ manufacture; white-fused-alumina abrasive production; low-end refractory aggregate. The differentiator is price — MGA at $400–600/t is the cheapest Al₂O₃ available, and applications that don't need higher purity / different particle morphology should default to it.

Choose calcined alumina instead for: technical ceramic sintering, refractory shapes for steel furnace tundish (low-soda requirement), polishing slurry. Choose activated alumina for: desiccant and catalyst support applications. Choose HPA for: LED sapphire, battery separator coating, IGBT substrate, anywhere ≥99.99% purity is needed.

The price differential ratios — relative to MGA at 1× baseline — are approximately: standard calcined 2×, low-soda calcined 4×, reactive calcined 8×, tabular alumina 6×, activated alumina 5–8×, HPA 4N 50×, HPA 5N 150×.

Equivalent Grades

• Alcoa SmartCal / Smart Alumina — North American MGA, traded primarily through Western Alumina co-located smelter pipeline
• Rio Tinto / Pacific Aluminium MGA — Australian sandy alumina, integrated bauxite-to-alumina-to-aluminum
• Hydro Alunorte (Brazil) — South American MGA, world's largest single refinery
• Aluminum Corporation of China (Chalco) MGA — multi-refinery network across Shandong / Shanxi / Henan / Guangxi
• China Hongqiao MGA — Shandong-based, integrated smelter feedstock
• Generic Chinese MGA — commodity grade ≥98.5%, multi-refinery origin