BM-019 | TCA as a Filter Aid for Polishing Filtration in an Alumina Refinery

Author: Rajendra Kunwar | Bayer Process Expert

1. Executive Overview 

Polishing filtration is a critical clarification step in the Bayer circuit. Its purpose is to remove fine suspended bauxite-residue particles from sodium aluminate liquor before the clarified liquor proceeds to precipitation. Poor clarification can carry iron-bearing and other mineral solids into downstream equipment and product, contribute to operational instability, and compromise hydrate quality. Filter performance therefore needs to be evaluated through filtrate clarity, filtration rate, cycle stability, availability and the reliability of the complete feed-conditioning system—not by throughput alone.

Tricalcium aluminate (TCA) has been used as a continuously dosed filter aid to support polishing filtration in horizontal pressure filters. The operating experience described in the original article identifies a practical starting relationship of approximately 10 parts TCA per part of suspended solids entering the filters, on a mass-concentration basis. This is a plant-trial benchmark, not a universal design constant; each refinery should confirm the optimum dosage against its liquor chemistry, solids characteristics and filter performance.

2. Why Conventional Lime Precoating Can Be Limiting 

Historically, dilute milk of lime—reported in the source article as approximately 4–6% Ca(OH)₂ by volume was applied as a precoat to the leaves of horizontal pressure filters before aluminate-liquor filtration. The precoat formed a barrier intended to retain fine residue particles, including a substantial fraction below 45 μm. However, precoat preparation and application require a batch activity and can increase filter downtime.

The source article also reports that elevated phosphate (expressed as P₂O₅) in the liquor can react with calcium and contribute to disruption of the precoat layer. Cracking or local loss of the precoat can create preferential flow paths, allowing fine solids to pass into the filtrate. The practical consequences are unstable clarity, repeated interventions and reduced filter availability. The magnitude of this effect depends on actual liquor chemistry, precipitate formation and operating conditions; it should be confirmed through plant sampling and trials.

3. TCA: Chemistry and Functional Role 

The TCA phase referred to in the source article is represented as 3CaO·Al₂O₃·6H₂O (often written C₃AH₆ in cement-chemistry notation). In practice, the identity and composition of the calcium-aluminate hydrate produced depend on the preparation conditions and liquor chemistry. For process control, the prepared suspension should therefore be treated as a plant-made filter-aid slurry whose consistency and performance require verification, rather than assuming that every batch is a chemically pure single phase.

When introduced continuously to the decanter-overflow liquor upstream of the pressure filters, the TCA suspension is intended to assist capture of residual fine solids and support a more stable filter cake. Its performance depends on the interaction among particle size and surface properties, liquor composition, mixing intensity, feed distribution, filter-medium condition, pressure profile and cake formation. TCA addition cannot compensate for a damaged filter medium, poor decanter operation or uncontrolled solids surges.

4. Preparation of TCA Suspension 

The preparation route reported in the source article uses lime containing more than 70% CaO. The lime is slaked and then cooked with dilute aluminate liquor at about 90°C for approximately 45 minutes. These values represent the reported operating practice and should be validated for the specific lime quality, liquor composition and plant equipment.

Key preparation controls include:

·       Lime quality: verify available CaO, reactivity, moisture and inert/mineral impurities. A total-CaO assay alone may not adequately describe slaking performance.

·   Slaking and dilution: maintain a reproducible slurry consistency and ensure complete, controlled slaking. Prevent dry pockets, agglomerates and local overheating.

·     Temperature and residence time: monitor the batch temperature and effective hold time; confirm that the selected conditions produce repeatable filter-aid performance.

·   Agitation: use continuous, gentle agitation sufficient to keep solids suspended without excessive air entrainment or damaging changes in particle structure.

·    Suspension quality: inspect for settling, hard deposits, oversized particles and batch-to-batch variability. Where appropriate, establish a plant-specific solids-content or density check.

·   Safe handling: apply site procedures for hot caustic liquor, lime slaking, dust, splash protection, equipment isolation and cleaning.

5. Continuous Dosing and Dosage Basis 

The prepared TCA suspension is continuously dosed into the decanter-overflow liquor feeding the polishing filters. Dosing should be linked to measured filter-feed flow and suspended-solids concentration wherever reliable measurements are available. A fixed pump setting can underdose during a solids excursion and overdose during cleaner operation.

The original article reports that trials at various refineries found a TCA addition equivalent to about ten times the suspended-solids concentration to be adequate for smooth polishing filtration. The example below preserves that reported rule of thumb: 

Parameter	Reported example	Interpretation
Suspended solids in decanter overflow	100 mg/L (100 mgpl)	Measured or representative filter-feed solids concentration
TCA dosage under 10:1 rule	1,000 mg/L = 1 g/L (1 gpl)	Ten times the suspended-solids concentration, expressed on a compatible mass basis

Important dosage clarification: the 10:1 relationship is meaningful only when the suspended-solids concentration and TCA dose are expressed on a consistent mass basis. If the TCA dosing pump delivers a slurry, its slurry concentration and density must be used to convert the required active-solids dose into a volumetric pump rate. Do not interpret 1 g/L of TCA solids as 1 L of TCA slurry per cubic meter without this conversion.

A practical mass-balance expression is:

TCA solids rate (kg/h) = target TCA dose (kg/m³) × filter-feed flow (m³/h)

The actual slurry-pump setting is then calculated from the TCA suspension’s measured solids mass fraction or concentration. Confirm pump calibration at the operating back-pressure and check that the injection point provides adequate mixing without creating localised over-concentration.

6. Critical Operating Parameters and Monitoring

A reliable TCA system needs coordinated monitoring of liquor, filter feed, preparation and filtration performance. The following parameters are recommended for a site-specific operating-control sheet:

Parameter	Why it matters	Recommended control approach
Filter-feed suspended solids	Drives the incoming solids load and dosage demand.	Trend concentration and flow together; investigate sudden excursions and decanter upsets.
TCA slurry solids/density	Converts required active dose to a reproducible pump rate.	Check each batch and recalibrate the dose calculation when slurry strength changes.
Preparation temperature and hold time	Influence repeatability of the prepared suspension.	Record actual temperature profile and residence time; use the reported 90°C/45 min as a trial starting point, not an unverified universal optimum.
Agitator and tank condition	Poor suspension uniformity causes variable dosing and settling.	Maintain continuous gentle agitation; inspect tank bottom, suction line and recirculation arrangement.
Filtrate clarity / residual solids	Direct indicator of polishing performance.	Trend with consistent sampling and a validated analytical method; investigate sudden increases immediately.
Filtration rate and cycle duration	Show whether the aid is improving throughput or creating excessive cake resistance.	Compare at similar feed conditions and track cycle-to-cycle variability.
Differential pressure and cake behaviour	Can reveal blinding, abnormal cake build-up or hydraulic instability.	Trend pressure profile against the filter manufacturer’s operating limits.
Filter-medium integrity and wash condition	A damaged or blinded medium can dominate performance regardless of dose.	Inspect cloth/leaves, seals and cleaning effectiveness during planned maintenance.
Liquor chemistry, including phosphate	May influence calcium-bearing precipitates and filter-aid behaviour.	Track relevant chemistry and correlate changes with clarity and cake performance.

7. Optimization: Avoid Both Under-Dosing and Over-Dosing

Under-dosing can leave fine particles insufficiently captured, leading to poor filtrate clarity or unstable performance during changes in feed solids. Excessive dosing is not automatically beneficial: it can increase solids loading, alter cake resistance, raise material consumption and create additional sludge or cleaning burden. The target should be the lowest stable dose that meets the refinery’s clarity and throughput requirements with an adequate operating margin.

• Establish a baseline using representative feed-flow, suspended-solids, filtrate-clarity, filtration-rate and cycle-time data.
• Verify the TCA slurry concentration and calibrate the dosing pump before changing the dose.
• Conduct controlled stepwise trials around the current dose while keeping other operating conditions as stable as practicable.
• Evaluate each trial over sufficient filter cycles to capture variability; avoid selecting a dose from a single favorable sample.
• Select the operating window that meets the site’s filtrate-solids specification while maintaining acceptable filtration rate, cycle stability and consumption.
• Revalidate the dose after changes in bauxite source, decanter performance, liquor chemistry, filter medium, filter configuration or TCA preparation method.

8. Troubleshooting Guide 

Observed symptom	Potential causes to check	First response
Filtrate clarity deteriorates suddenly	Feed-solids surge; inadequate dose; poor mixing; damaged cloth or seals; pre-existing filter upset.	Confirm feed solids and flow, verify actual TCA delivery, inspect filter integrity and sample the filtrate.
Dose varies despite constant pump setting	TCA settling; changing slurry concentration; suction blockage; pump wear or calibration error.	Check tank agitation, slurry density, suction line and delivered volume/mass.
Filtration rate falls	Excessive or unsuitable filter-aid dose; high feed solids; filter-medium blinding; altered cake properties.	Compare feed conditions and dose history; inspect cloth and assess controlled dose changes.
Deposits form in preparation/dosing equipment	Inadequate agitation; long stagnant periods; poor slurry handling or unsuitable line velocities.	Review tank geometry, agitation, flushing and line layout under site engineering procedures.
Performance changes with liquor chemistry	Changes in phosphate or other liquor constituents may affect calcium-bearing solids and filtration behaviour.	Trend chemistry with filter data and carry out a controlled plant or laboratory evaluation.

9. Plant Performance Indicators

TCA optimization should be judged using a balanced set of indicators rather than a single filtration-rate number. At minimum, record:

·       Filtrate suspended solids or turbidity, using a method correlated with the refinery’s product and process requirements.

·       Filter throughput, filtration rate, cycle duration and frequency of filter intervention.

·    TCA consumption per unit volume of liquor and, where useful, per unit mass of incoming suspended solids.

·       Filter availability, cleaning frequency, cloth life and maintenance hours.

·     Downstream consequences, including any evidence of solids carryover or adverse impact on precipitation and hydrate quality.

The source article does not provide a universal filtrate-solids limit, optimum filtration rate, or guaranteed saving from TCA use. These targets should be set from the refinery’s product-quality requirements, filter design, historical baseline and validated plant trials.

10. Engineering Conclusions

Continuous TCA addition offers an alternative to relying solely on batch lime precoat application for polishing filtration. The reported preparation basis—lime containing more than 70% CaO, cooking with dilute aluminate liquor at about 90°C for roughly 45 minutes—and the approximate 10:1 TCA-to-suspended-solids dosage relationship provide useful starting points for plant evaluation. They should not be adopted as universal design guarantees without confirming chemistry, suspension properties and filter response.

The strongest results come from integrating consistent TCA preparation, calibrated flow-proportional dosing, reliable feed-solids measurement, sound filter-medium maintenance and routine filtrate-quality monitoring. The objective is stable clarification at the minimum effective dosage, with improved filter availability and no compromise to downstream Bayer-process performance. 

About CETI Enterprises

CETI Enterprises is a premier Indian consultancy and engineering firm dedicated to empowering Bayer process professionals through specialized online and onsite technical training programs designed to unlock advanced metallurgical expertise. The organization delivers high-level engineering consultancy focused on efficiency enhancement, systematic process optimization, advanced troubleshooting, and operational debottlenecking for active alumina refineries worldwide. 

Contact & Professional Services: For technical inquiries, efficiency audits and plant optimization requirements, reach out directly to Rajendra Kunwar (Bayer Process Expert) at rajendra@ceti.co.in