Technology

A detailed description of the Bayer Process is provided at the 'Technology Type' level. 

Process Steps, Conditions & Parameters (PFD-Based)

Figure 1 — Bayer Process Block Flow Diagram | Source: Environmental Protection Agency (EPA) of Ireland (Apr 2019)

The Bayer process follows a continuous loop of six principal stages:

Step 1 — Ore Preparation (Crushing & Grinding)

Bauxite is crushed at the mine and conveyed to the refinery, where it is ground in SAG (semi-autogenous) and/or ball mills to a particle size of ≤1.5 mm. Hot recycled caustic liquor is added during milling to produce a pumpable slurry (~40–55% solids). The slurry is transferred to holding/slurry storage tanks to buffer supply interruptions and begin desilication — the early precipitation of reactive silica as sodium aluminium silicate before the main digestion circuit.

Parameter	Value
Target particle size	≤1.5 mm
Slurry solids concentration	~40–55 wt%
NaOH addition	From recycled spent liquor

 

Step 2 — Digestion

The bauxite slurry is pumped through preheaters and into pressurised autoclave digesters, where additional concentrated NaOH is added. Temperature and pressure depend on bauxite mineralogy:

Mineral Phase	Temperature	Pressure
Gibbsite	135–150°C	Near-atmospheric
Boehmite	205–245°C	Elevated (~1.5–3.5 MPa)
Diaspore	>250°C	~3.5 MPa (~35 atm)

The alumina dissolves selectively to form sodium tetrahydroxoaluminate (green liquor/pregnant liquor), while iron oxides, titanium oxide and calcium compounds remain undissolved. Lime (CaO) may be added to precipitate silica as calcium silicate, protecting yield. Residence time in digesters is typically 30–60 minutes.

Step 3 — Clarification (Solid–Liquid Separation)

The hot slurry is cooled and transferred through flash tanks (which recover steam/heat) to large gravity settling thickeners (mud thickeners). The undissolved solids — red mud (predominantly iron oxide, silica, titania, unreacted alumina) — settle and are separated from the clarified pregnant liquor. Flocculants such as starch are added to improve settling of fine particles. The pregnant liquor is polished through security filters (leaf or pressure filters) to eliminate fine solids before precipitation.

The red mud underflow is washed in a counter-current decantation (CCD) washing train to recover caustic soda, which is recycled. Washed residue is pumped to residue disposal areas (RDA).

Parameter	Value
Thickener diameter	Up to 60 m+
Flocculant	Starch or synthetic polymer
Red mud generation	~1.0–2.0 t dry/t alumina (varies by bauxite)
Caustic recovery	>95% by CCD washing

 

Step 4 — Precipitation (Crystallisation)

The clarified pregnant liquor (supersaturated in NaAlO2) is cooled progressively through plate heat exchangers, transferring heat to the cold spent liquor return stream. The cooled liquor is seeded with fine Al(OH)3 crystals from previous cycles in large precipitation tanks (precipitators) operating in series. Crystals nucleate on the seeds, grow and agglomerate over a residence time of 24–72 hours. The slurry is then classified:

• Coarse fraction (product hydrate) → calcination

• Fine fraction (seed crystals) → recycled back to precipitation tank feed

• Spent liquor → evaporation and return to digestion

Parameter	Value
Precipitation temperature	55–75°C (decreasing through chain)
Seed ratio	300–600 g seed/L liquor
Residence time	24–72 hours
Al(OH)3 recovery	~90% of dissolved alumina
Precipitator tank volume	Up to 10,000 m³ each

 

Step 5 — Evaporation (Liquor Reconcentration)

Spent liquor from precipitation is dilute in NaOH and must be reconcentrated before returning to digestion. This is achieved in multi-effect evaporator trains using steam heat. Evaporation also concentrates organic impurities (primarily sodium oxalate) that build up in the circuit; these are managed by oxalate seeding and precipitation or liquor burning in a rotary kiln.

Step 6 — Calcination

Washed and filtered Al(OH)3 hydrate is fed into high-temperature calciners to drive off chemically bound water and produce anhydrous α-Al2O3:

• At 400–600°C: γ-Al₂O₃ forms (chemically active)
• At 1,000°C (typical SGA production): transition aluminas
• Above 1,150°C: α-Al₂O₃ (corundum — inert ceramic grade)

Smelter-grade alumina (SGA) is produced at approximately 1,000°C. The final product is a white crystalline powder with particle size 0.5–10 μm and Na2O content of 300–7,000 ppm.

Parameter	Value
Calcination temperature (SGA)	950–1,050°C
Equipment	Rotary kilns or CFB flash calciners
Calcination energy	~1.4 GJ/t alumina (theoretical)
Product phase	α-Al2O3 (SGA)

 

Key Equipment & Devices

Figure 2 — Bayer Process Flow Diagram - Wagerup refinery expansion | Source: Alcoa (Aug 2003)

Equipment	Function	Key Specifications
SAG/Ball Mills	Bauxite grinding to slurry	Particle size target ≤1.5 mm
Slurry Storage Tanks	Buffer storage & pre-desilication	Agitated; lined steel vessels
Preheaters (Shell & Tube HX)	Slurry preheat before digester	Steam heated; recovers flash steam
Digesters / Autoclaves	High-T/P dissolution of alumina	135–250°C, up to 35 atm; series-connected
Flash Tanks	Pressure let-down + heat recovery	Multi-stage; steam recovered to preheaters
Rotary Sand Trap	Remove coarse undissolved solids	Before thickeners
Mud Thickeners (Settlers)	Red mud gravity settling	Rake thickeners, diameter up to 60 m+
CCD Washing Train	Caustic recovery from red mud	Counter-current decantation; multiple stages
Security/Leaf Filters	Final polishing of pregnant liquor	Pressure leaf or vacuum drum filters
Plate Heat Exchangers	Cool pregnant liquor before precipitation	Transfers heat to spent liquor return
Precipitator Tanks	Al(OH)3 crystallisation	Up to 10,000 m³ each; 8–20 tanks in series
Classifying Thickeners	Separate seed from product hydrate	Gravity settling; conical bottom
Hydrate Filters	Dewater Al(OH)3 before calcination	Vacuum or pressure drum filters
Multi-Effect Evaporators	Reconcentrate spent caustic liquor	Steam-driven; 3–7 effects
Rotary Kilns / CFB Calciners	Calcine Al(OH)3 to Al2O3	950–1050°C; gas-fired
Electrostatic Precipitators (ESP)	Capture alumina dust from calciners	On each calciner stack
Boilers / Steam System	Generate process steam	High-pressure and low-pressure steam mains
Oxalate Kilns	Destroy organic impurities	Rotary kiln; oxalate → carbonate

 

References

1. International Aluminium Institute. The Aluminium Story > Mining and Refining > Refining Process (Feb 15, 2022)
2. IDC Technologies. Bayer Process (Document date Nov 18, 2014)
3. Alcoa Corporation. Environmental Review and Management Programme: Wagerup Refinery Unit Three (2005, May). Alcoa World Alumina Australia
4. Wikipedia. Bayer process (Page version Feb 13, 2026)
5. Environmental Protection Agency (EPA) of Ireland. Aughinish Aluminium Limited (AAL) Environmental Licence Review Application, Section 4.8 Operational Report (Document date Apr 29, 2019)
6. Learn Metallurgy. Bayer Process (Accessed Feb 24, 2026)
7. Science Direct. Bayer Process (Accessed Feb 25, 2026)
8. Industrial News Service. Metso Outotec to deliver two energy-efficient flash evaporation plants to NALCO’s alumina refinery in India (Dec 8, 2024)
9. Outotec. Alumina and aluminium technologies brochure (2011)