BM-042 | PREPARATION OF TRICALCIUM ALUMINATE FOR SECURITY FILTRATION IN ALUMINA REFINERY

Author: Rajendra Kunwar, Bayer Process Expert

1. EXECUTIVE SUMMARY & CHEMICAL PRINCIPLE

In high-purity alumina refining (Bayer Process), security polishing filtration of green liquor before precipitation is critical to achieve ultra-low silica and iron oxide contamination in the final hydrate product.

Tricalcium Aluminate (TCA), specifically the hexahydrate form (3CaO · Al₂O₃ · 6H₂O or C₃AH₆), acts as a targeted filter aid and silica-scavenging agent. Pre-coating leaf filters with freshly synthesized TCA reduces liquor turbidity to less than 10 mg/L solids while capturing dissolved reactive silica (SiO₂) into insoluble hydrogarnet structures (3CaO · Al₂O₃ · xSiO₂ · (6-2x) H₂O).

2. CHEMICAL STOICHIOMETRY & REACTION KINETICS

3 Ca (OH)₂ (slurry) + 2 NaAlO₂ (aq) + 4 H₂O ──> 3CaO · Al₂O₃ · 6H₂O ↓ +  2NaOH (aq)

Key Thermodynamic & Kinetic Parameters

·       Precipitation Temperature: 92°C – 98°C (maintains crystal stability in the cubic C₃AH₆ hydrogarnet phase).

·       Target Ca: Al Molar Ratio: 3.0: 1.0 (controlled by lime dosing against liquor alumina concentration).

·       Slurry Residence Time: 45 – 60 minutes in continuous stirred tank reactors (CSTRs).

·       Specific Surface Area Target: 4.5 – 6.2 m²/g to ensure high filter cake permeability and optimal pre-coat thickness.

3. MATERIALS & EQUIPMENT SPECIFICATIONS

Parameter / Reagent	Grade / Spec	Function / Control Target
Quicklime (CaO)	High-calcium active lime (>88% Available CaO)	Reactant; slaked at 85°C to form fine particle Ca (OH)₂
Spent / Green Liquor	140–160 g/L Na₂Oeq, A/C ≈ 0.60	Aluminum source and alkaline reaction medium
Reaction Tank	Carbon Steel with internal heating coils	Primary synthesis reactor with dual-impeller agitation
Polishing Filter Vessel	Vertical pressure leaf filter (e.g., Kelly / Niagara type)	Pre-coat matrix host for final liquor clarification

4. STEP-BY-STEP OPERATING PROCEDURE

Phase I: Slaked Lime Slurry Preparation

a)   Charge the lime slaker with demineralized condensate water heated to 80°C – 85°C.

b)   Dose high-calcium quicklime continuously to produce a Slaked Lime [Ca (OH)₂] milk at 180 – 220 g/L solids density.

c)    Pass the slaked lime through a 250 µm vibrating screen to reject grit (CaCO₃ unburnt core).

Phase II: TCA Synthesis Reaction

a)   Meter green/spent liquor into Reactor T-4101 at a controlled flow rate.

b)   Heat reactor contents to 95°C (±2°C) under continuous agitation (120 RPM).

c)    Inject the screened Ca(OH)₂ slurry into the turbulent zone of T-4101 over 20 minutes to prevent localized unreacted lime nucleation.

d)   Maintain digestion at 95°C for 50 minutes. Sample slurry at 40 min mark for XRD/titration check to verify complete conversion to C₃AH₆.

Phase III: Filter Pre-Coat Application

a)   Recirculate synthesized TCA slurry through the Polishing Filter leaves at a velocity of 1.2 – 1.5 m/s.

b)   Monitor differential pressure (ΔP) across filter screens until a uniform pre-coat cake thickness of 3.0 – 5.0 mm is deposited.

c)    Divert filtrate to recirculation until turbidity drops below 5 ppm.

d)   Switch filter to main process stream for high-security green liquor polishing.

5. CRITICAL CONTROL PARAMETERS & TROUBLESHOOTING

Lime Slaking: 85°C ──>Reactor: 95°C / 50 min ──> Pre-Coat Filter Leaves: 3-5mm Cake.

·       Temperature Dropping < 88°C: Risks formation of needle-like metastables (C₄AH₁₃), leading to high compressibility and rapid filter cake blinding.
Corrective Action: Immediately engage auxiliary high-pressure steam coils to restore temperature to 95°C.

·        Excessive Silica in Filtrate (>15 mg/L): Indicates insufficient TCA surface area or unreacted free lime.
Corrective Action: Increase reaction holding time by 15 minutes and verify lime quality (>88% active CaO).

 

CETI ENTERPRISES, INDIA

CETI Enterprises supports alumina industry professionals through technical training, process audits, calcination performance assessment, energy optimization, process troubleshooting, equipment design review, debottlenecking and refinery improvement programmes. For further details please visit our website at https://ceti.co.in 

About the Author: Rajendra Kunwar is Bayer Process & Alumina Technology Specialist. Through CETI Enterprises, he offers expert technical consultancy in customized online / onsite team training, process for production of specialty grade ATH & Alumina, efficiency enhancement, quality improvement, energy reduction, cost reduction, troubleshooting and plant debottlenecking. To explore how we can support your plant operations or upcoming expansion, please contact us at rajendra@ceti.com.