BM-040 |BURNT LIME SPECIFICATIONS FOR TRI-CALCIUM ALUMINATE (TCA) SYSNTHESIS IN SECURITY FILTRATION SYSTEM Author: Rajendra Kunwar, Bayer Process Expert

 

1. Executive Summary & Chemical Process Context

In modern alumina refinery operations utilizing the Bayer Process, achieving ultra-low suspended solids (typically < 10 mg/L, targeted at < 5 mg/L) in the pregnant liquor prior to precipitation is vital for maintaining product purity and hydrate quality. Security filtration (or polish filtration), conducted via pressure filters or Kelly filters, relies heavily on a pre-coat and body-feed filter aid. Traditional diatomaceous earth and cellulose media suffer from rapid silica dissolution in high-temperature and caustic alkalinity. To eliminate silica contamination, Tri-Calcium Aluminate (TCA) of hydrocalumite-type structures is synthesized in-situ as an insoluble, highly crystalline inorganic filter aid.

The performance, filterability, morphologic structure and chemical stability of TCA directly depend on the quality and chemical reactivity of the starting raw material: Burnt Lime (Quicklime / Calcium Oxide, CaO). This specification establishes the rigorous chemical, physical, structural and reactivity criteria required for quicklime to ensure optimal slaking kinetics, rapid formation of well-crystallized TCA prisms and minimal cake resistance across pressure filter leaves.

■ CRITICAL ENGINEERING IMPERATIVE Sub-optimal burnt lime directly causes low TCA yield, amorphous calcium carbonate precipitation, unreacted lime carryover, and severe cake blinding on filter leaves. This standard operating procedure (SOP) defines strict operating limits for Available CaO (≥ 90.0%), Reactivity (slaking temperature rise ΔT ≥ 40°C in 2 min) and impurity caps (SiO2 ≤ 1.0%, MgO ≤ 1.2%) to guarantee maximum security filtration efficiency.

 

2. Tri-Calcium Aluminate (TCA) Synthesis Mechanism

The synthesis of TCA occurs through a controlled two-stage reaction involving the slaking of burnt lime followed by hydrothermal reaction with sodium aluminate in Bayer liquor:

2.1 Lime Slaking Reaction:
    CaO (s) + H₂O (l)  ➔  Ca(OH)₂ (aq/slurry) + ΔH  (-65 kJ/mol)

2.2 Hydrothermal TCA Precipitation Reaction:
    3 Ca(OH)₂ (s) + 2 NaAl(OH)₄ (aq)  ➔  3CaO · Al₂O₃ · 6H₂O (s) + 2 NaOH (aq)

To function as an effective filter aid, TCA must precipitate as distinct cubic or tabular micro-crystals (10 – 30 µm) with high aspect ratio and uniform grain size distribution. If the burnt lime exhibits slow slaking or excessive unburnt limestone grit ($CaCO_3$), the reaction forms micro-crystalline or needle-like metastable phases ($C_4AH_{13}$) which yield a highly compressible filter cake, resulting in rapid pressure drop, severe cake compaction, and reduced cycle length.

3. Technical Specification Requirements

Burnt Lime supplied under specification must comply with the mandatory parameters outlined below. Testing and sampling shall be conducted in accordance with ASTM C25 / ASTM C110 standards.

Parameter / Property	BM-040 Requirement	Target / Preferred	Test Method Standard
Available Lime (Active CaO)	≥ 90.0 wt%	92.5 wt%	ASTM C25 (Sugar Method)
Total Calcium Oxide (Total CaO)	≥ 92.5 wt%	94.0 wt%	XRF / ASTM C25
Magnesium Oxide (MgO)	≤ 1.20 wt%	< 0.80 wt%	ICP-OES / ASTM C25
Silicon Dioxide (SiO₂)	≤ 1.00 wt%	< 0.50 wt%	XRF / Wet Chemical
Ferric Oxide & Alumina (Fe₂O₃ + Al₂O₃)	≤ 0.80 wt%	< 0.40 wt%	XRF Analysis
Loss on Ignition (LOI @ 950°C)	≤ 3.00 wt%	< 1.50 wt%	Gravimetric ASTM C25
Unburnt Carbonate (CaCO₃ as CO₂)	≤ 2.00 wt%	< 1.00 wt%	Evolved Gas / Titration
Slaking Reactivity: Temperature Rise (ΔT 2 min)	≥ 40.0 °C	45.0 - 52.0 °C	ASTM C110 Slaking Test
Total Temperature Rise (ΔT Max)	≥ 45.0 °C	50.0 - 58.0 °C	ASTM C110 Slaking Test
Time to Reach Max Temp (t_max)	≤ 3.0 minutes	1.5 - 2.5 minutes	ASTM C110 Calorimetry
Particle Size (Lump / Crushed)	10 mm – 50 mm	20 mm – 40 mm	Sieve Analysis ISO 3310
Fines Content (< 2 mm at delivery)	≤ 10.0 wt%	< 5.0 wt%	Dry Sieve Analysis
Residue on 150 µm Sieve (Post-Slaking Grit)	≤ 1.50 wt%	< 0.50 wt%	ASTM C110 Wet Screen

 

4. Impact of Impurities & Physical Properties on Filtration

4.1 Available CaO and Slaking Kinetics

A high concentration of Available CaO (≥ 90.0%) ensures complete conversion to supersaturated Ca(OH)₂ upon slaking. Soft-burnt quicklime produced in modern parallel-flow regenerative (PFR) kilns exhibits high specific surface area (>2.0 m²/g)) and small crystallite size. High reactivity allows instant slaking, generating sub-micron Ca(OH)₂ Ca(OH)₂ particles that react uniformly with aluminate species to form homogeneous, highly porous TCA agglomerates.

4.2 Silica (SiO₂) and Magnesium Oxide (MgO) Penalties

• Silica (SiO₂ ≤ 1.0%): Excess reactive silica reacts with caustic liquor to form Desilication Products (DSP - Hydrogarnet/Cancrinite). DSP scales foul filter cloths, reduces permeability, and re-contaminates clarified liquor with dissolved silica, compromising alumina product purity.
• Magnesium Oxide (MgO ≤ 1.2%): MgO undergoes slow hydraulic slaking, forming colloidal Mg(OH)₂ gels. These gelatinous precipitates coat TCA crystals, blinding the filter media and causing an exponential rise in filter differential pressure (ΔP) ultimately affecting the fitration rate and filtrate quality.

4.3 Post-Slaking Grit and Unburnt CaCO₃

Unburnt core limestone (CaCO3) acts as an inert mass that settles rapidly in synthesis tanks, leading to slurry inhomogeneity and abrasive wear on slurry pumps and filter leaf dosing valves. Sieve residue more than 150 micron size must remain below 1.5 wt% to avoid localized filter cake erosion.

5. Quality Assurance, Sampling & Compliance Protocol

To ensure batch-to-batch consistency for security filtration operations, suppliers must follow the QA/QC protocol defined below:

■ Sampling Protocol: Representative composite samples shall be collected from every 250 metric tons of lime delivered, following ISO 5658 / ASTM C50 sampling standards. Samples must be sealed in moisture-proof container bags immediately to prevent atmospheric slaking/carbonation.

■ Slaking Reactivity Test: Reactivity shall be verified using an insulated reaction vessel according to ASTM C110 (40g quicklime added to 400ml distilled water at 40°C). The temperature rise at 2 minutes must meet or exceed 40.0°C.

■ Rejection Criteria: Any shipment exhibiting Available CaO < 90.0%, LOI > 3.0%, or ΔT (2 min) < 40.0°C shall be subject to immediate rejection and quarantine at the supplier's expense.

■ Storage & Transport: Burnt lime must be transported in dry, sealed pneumatic tanker trucks or bulk bags with polyethylene liners. Maximum storage duration in refinery silos shall not exceed 14 days to prevent hydration.

6. Standard Approval & Authorization

Specification Bis a mandatory engineering standard for all quicklime procurement intended for Bayer liquor security filtration systems. Adherence to these limits guarantees optimal TCA crystal morphology, extended filtration cycle lifetime, and superior filtrate clarity.

Prepared By	Reviewed By	Approved By
Lead Process Engineer Bayer Liquor Operations Sign: ___________________ Date: ____ / ____ / 2026	Manager - QA/QC & Process Control Sign: ___________________ Date: ____ / ____ / 2026	VP Technical Services & Engineering Sign: ___________________ Date: ____ / ____ / 2026

 

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.