BM-006: Systematic Design of 3,000 m3 Pre-Desilication Tanks for Massive Single-Stream Alumina Refineries

Executive Summary: Designing ultra-large scale desilication circuits for a 6 Million tonnes per annum (6 Mtpa) Alumina refinery requires transitioning from single-vessel concepts to an optimized multi-stage series train. This article outlines the critical process-to-mechanical design sequence covering mass balance, high reactive silica management, tank geometry (H/D = 1.20 to 1.30) and rigorous full-suspension agitation (Degree of Agitation = 10) to prevent catastrophic solids settling, mitigate desilication product scaling (DSP) and minimize caustic soda losses. 

1. Process Mass Balance & Slurry Volumetric Flow Rate

For a world-scale 6 Mtpa Alumina refinery operating at an availability of ~8,200 hours per year, processing bauxite with an assumed bauxite factor of 3 yields a total annual throughput of 18,000,000 tonnes.

• Dry Bauxite Feed Rate: 2,195 tph dry bauxite
• Wet-Grinding Slurry Density: 45% (w/w) solids assuming liquor density of 1.25 t/m³ and bauxite density of 2.4 t/m³
• Total Volumetric Flow Rate: Enters the desilication circuit at 3,060 m³/h.

2. Staging Configuration & Residence Time Kinetics

With each individual vessel providing a working volume of 3,000 m³, a single tank provides exactly 1 hour of residence time per parallel processing stream.

• The High-Silica Challenge: Operating with a total Silica content of 2.9% vastly exceeds the critical 1.5% reactive silica decision boundary. This high loading demands a tightly controlled pre-desilication strategy to restrict caustic soda loss and scaling in downstream heat exchangers and pipelines.
• Train Configuration: High-silica bauxites mandate 8 to 10 hours of total pre-desilication residence time. A single-stream layout requires 8 to 10 tanks of 3,000 m³ capacity configured in a continuous series train to achieve desired desilication kinetics while eliminating hydraulic short-circuiting.

Engineering Insight: In multi-stage series trains, maintaining precise level control across tanks is critical to prevent cascading surge conditions that starve downstream flash vessels or pressure digestion streams.

3. Vessel Geometrical & Mechanical Design Limits

To handle a 3,000 m³ fluid volume without destructive dead zones or progressive particle settling, vessel geometry must adhere to strict design constraints:

 

Design Parameter	Target Specification	Operational Impact
Aspect Ratio ($H/D$)	1.20 to 1.30 : 1	Maximizes top-to-bottom plug flow behavior and uniform thermal distribution.
Wall Baffling	4 standard baffles (1/12 times D)	Converts destructive rotational swirling into high-velocity axial turnover.
Bottom Cone Slope	15-30o conical bottom	Directs heavy settled solids efficiently toward the central bottom discharge point.

 4. Full-Suspension Agitation System (Agitation Degree = 10)

Handling abrasive bauxite slurries in a massive 3,000 m³ vessel requires an agitation degree of 10 to guarantee complete off-bottom suspension of coarse mineral fractions.

Mechanical Configuration & Drive Train

• Drive Mechanics: Top-entering, high-torque industrial gearbox coupled with an extra-thick solid overhung shaft are required for this huge capacity vessel. No steady-rest or bottom internal bearings are installed, eliminating premature mechanical failure from abrasive bauxite wear.
• Impeller Arrangement: Dual-impeller configuration on a single solid shaft:
• Lower Impeller: Positioned close to the tank floor (typically $0.5 \times D$ off-bottom) to handle dense particle lifting.
• Upper Impeller: Placed mid-way up the liquid column to maintain homogeneous vertical solids distribution up to the overflow weir. 

• Impeller Profile: High-efficiency axial-flow hydrofoils (A310 or equivalent) to maximize volumetric pumping rate per kilowatt.
• Power Density Estimates: An agitation scale of 10 for high-density slurries dictates an absorbed power density of about 0.15 to 0.20 kW/m³, translating into a 450 kW to 600 kW drive motor rating per tank. 

5. Conclusion & Circuit Architecture

To process a single-stream 6 Mtpa plant throughput safely, a train of 8 to 10 tanks connected in series is mandatory, though twin parallel trains of 4 to 5 tanks each can be evaluated during detailed engineering stage of the project. 

We will welcome peer review, operational feedback and technical remarks from fellow process engineers, equipment suppliers, agitator designers and plant operators. 

Rajendra Kunwar

Bayer Process Expert, India

 

 

BM-005: Desilication - The First Chemical Control Point in Bayer Process

1. Introduction

After crushing and grinding of bauxite, the Bayer process moves from predominantly physical preparation to its first major chemical control operation called pre-desilication. For a gibbsitic bauxite refinery, this operation is particularly important because the reactive silica in bauxite principally associated with kaolinite and other reactive aluminosilicates is attacked by caustic liquor. If this silica is allowed to remain in solution and travel into digestion, it subsequently participates in DSP formation at locations where its presence is undesirable.

The objective of pre-desilication is therefore not simply to “Remove silica.” The real objective is to Convert reactive silica into a controlled, crystalline desilication product (DSP), while minimizing the associated losses of Na₂O and Al₂O₃ and discharge that DSP with the residue (Red mud). This makes desilication a chemical control point, soda-loss control point, equipment-protection point and ultimately an alumina-quality control point. 

As per Industrial Experts, properly operated pre-desilication can convert approximately 90% of reactive silica into DSP, although the achievable value is strongly dependent on bauxite mineralogy, liquor chemistry, temperature, residence time and available DSP surface area. 

2. Is Reactive Silica same as Total Silica?

This distinction is fundamental. Bauxite silica can broadly be considered as: 

• Reactive silica (R. SiO₂): Mainly kaolinite and other caustic-reactive silicate minerals. 

• Non-reactive silica: Principally quartz under normal low-temperature conditions.

Reactive silica is the silica that creates the major Bayer-process chemical burden. For example, kaolinite can react with caustic liquor according to the simplified reaction. Thus, a mineral originally presents as a solid in bauxite becomes soluble silicate and aluminate species. The soluble silicate must then be removed from solution. 

3. Formation of DSP

The dissolved silicate does not remain indefinitely in Bayer liquor. It reacts with sodium aluminate and other liquor species and precipitates as desilication product (DSP).

The DSP family principally includes the following: 

• Bayer sodalite
• Bayer cancrinite
• Sodium aluminosilicate phases
• Calcium-containing DSP phases where Ca is available. 

For conventional low-temperature gibbsitic Bayer refining, sodalite-type DSP is particularly important. The actual DSP composition therefore changes with liquor chemistry. 

4. The DSP Formation Mechanism

DSP formation should not be regarded as an instantaneous single chemical reaction.

The actual crystallization sequence depends on following parameters: 

• temperature
• residence time
• caustic concentration
• aluminate concentration
• silica concentration
• carbonate
• sulphate
• chloride
• calcium
• seed surface
• liquor supersaturation. 

This has an important plant implication as “Desilication is a kinetic operation of chemical-equilibrium operation.” Simply providing a large tank volume is not sufficient. The system must provide the correct temperature–time–chemistry–solids-surface-area combination. 

5. Why Pre-Desilication Is Necessary

If reactive silica is not substantially removed before digestion, several undesirable phenomena occur as Without adequate pre-desilication, it causes following negative results: 

• higher caustic soda consumption;
• higher alumina loss;
• increased DSP generation;
• greater red-mud soda loading;
• silica remaining in pregnant liquor;
• heat-exchanger scaling;
• reduced heat-transfer coefficient;
• increased cleaning frequency;
• possible silica contamination of hydrate/alumina;
• deterioration of overall refinery recovery.

Incomplete desilication is specifically associated with silicate scaling and potential alumina-product contamination. 

6. Soda Loss — The Economic Penalty of Reactive Silica

This is one of the most important aspects of desilication.DSP does not consist only of silica. It incorporates: 

• SiO₂
• Al₂O₃
• Na₂O
• water
• anions such as carbonate, sulphate, chloride and hydroxide. 

Since DSP leaves the circuit with red mud, the soda and alumina incorporated in it are effectively removed from the active Bayer liquor circuit. Literature consistently identifies DSP formation as a major source of caustic and alumina. 

7. The “Sodalite Factor” : A Practical Plant Concept

There is an important distinction between the stoichiometric composition of an ideal DSP phase and the plant sodalite factor used for estimating actual chemical soda loss. The actual DSP generated in an operating refinery is not necessarily a pure theoretical mineral. Its composition changes with:

• liquor chemistry,
• temperature,
• residence time,
• sulphate,
• carbonate,
• chloride,
• calcium,
• DSP phase transformation,
• digestion condition.

Therefore, for plant calculations, a soda to reactive silica ratio factor is normally established from laboratory test work of bauxite at established process conditions. Stoichiometric number are only used by Bayer Process Experts based on their rich experience with similar bauxite.

Sodalite factor of approximately 0.70 (Na₂O/R.SiO₂) is adopted as an empirical / theoretical design sodalite factor which has been observed close to actual operating sodalite factor of Alumina refinery. Therefore, one can use “sodalite factor = 0.70” in a design calculation with clear understanding of the numerator, denominator, analytical basis and explicitly defined units in order to have accurate results. 

8. Recommended Method for Steady Control of g/l SiO₂

This is one of the most important practical recommendations. Do not control desilication only by:

• temperature;
• residence time;
• tank level;
• operator experience. 

Instead establish a closed-loop silica control system by measuring feed silica, outlet silica, silica saturation, silica removal and then arrive at equilibrium silica. 

9. Desilication Control Philosophy

A robust desilication control philosophy is developed purely based on outlet dissolved SiO₂. 

10. What Happens When Desilication Is Insufficient?

If sufficient desilication is not done then residual reactive silica enters digestion circuit causing following consequences: 

           10.1 At digestion temperature, it may cause

• higher soda loss;
• higher alumina loss;
• more DSP;
• increased residue volume/load. 

High-temperature digestion can also increase silica dissolution from minerals such as quartz, particularly where the digestion temperature is high. 

10.2 Impact on Digester Heat-Transfer Equipment

Residual silica can subsequently precipitate as sodium aluminosilicate causing scale formation in:

• heaters;
• heat exchangers;
• liquor lines;
• digestion equipment.

Scale reduces the overall heat-transfer coefficient causing high steam demand.

10.3 Impact on Clarification

Higher DSP loading affects the residue system. Potential effects include:

• increased mud solids;
• altered particle-size distribution;
• changed settling behavior;
• increased liquor entrainment;
• higher filtration/washing load;
• increased soda loss with residue.

The actual impact is refinery-specific because settling depends on the complete residue mineralogy and flocculation system.  

 1        10.4 Impact on Precipitation

The most dangerous consequence of inadequate desilication is allowing excessive dissolved silica to reach the pregnant liquor ultimately increasing the silica impurity in product ATH and alumina.

10.5 Impact on Evaporation

The evaporator circuit is particularly sensitive to impurities and scaling. If silica-bearing liquor enters the evaporation system: 

• liquor concentration increases;
• silica supersaturation may increase;
• DSP/silicate scale may form;
• heat-transfer coefficient decreases;
• circulation becomes more difficult.

Thus, insufficient desilication indirectly affects the refinery's steam economy. 

10.6 Recommended Daily Desilication Dashboard 

Parameter	Unit	Purpose
Bauxite R.SiO₂	%	Feed-forward control
Bauxite total SiO₂	%	Mineral balance
Bauxite moisture	%	Slurry control
Slurry density	t/m³	Solids control
PDS temperature	°C	Reaction kinetics
Residence time	h	Reaction completion
Na₂O	g/L	Liquor chemistry
Al₂O₃	g/L	Liquor chemistry
RP	—	Liquor balance
Feed SiO₂	g/L	Silica load
Outlet SiO₂	g/L	Primary KPI
Equilibrium SiO₂	g/L	Thermodynamic reference
Silica ratio	—	Degree of saturation
Silica supersaturation	g/L	Process control
DSP concentration	g/L	Seed inventory
DSP bleed	t/day	Solids balance
Soda loss	kg/t ATH	Economics
Mud soda	% Na₂O	Residue loss
Heat exchanger ΔT	°C	Scaling indicator
Heat-transfer coefficient	W/m²K	Equipment health

11.  Key Technical Takeaway

For gibbsitic bauxite, It is always recommend that the desilication section be treated as the first formal chemical control point of the refinery. The key philosophy for Alumina Refinery is highlighted below: 

“Know the reactive silica entering the refinery, predict the silica entering desilication, control the temperature, residence-time, DSP environment, continuously measure dissolved SiO₂ leaving desilication, calculate silica ratio/supersaturation and prevent the silica problem from being transferred to digestion, heat exchangers, precipitation and evaporation circuits.” 

Trust, our Engineers are enriching their knowledgebase through Bayer Process Masterclass. So far, we have published BM-001 to BM-005 and BM-006 is under preparation which will be published shortly. If you wish to learn on any specific aspects of Alumina production, please feel free to interact with us.

Rajendra Kunwar

Bayer Process Expert.

CETI Enterprises, India.

www.ceti.co.in

BM-004: RUN-OF-MINE BAUXITE TO OPTIMISED BAYER FEED

1. Introduction

In the previous Bayer Process Masterclasses, we established that the economic value of bauxite is determined not merely by its Al₂O₃ content, but by its available alumina, reactive silica, mineralogy, moisture, impurities and ultimate alumina recovery. The Bayer Process Masterclass article, BM-004, now moves from the quality of bauxite to its physical and chemical preparation for the Bayer Process.

The journey begins with: Run-of-Mine Bauxite → Crushing → Screening → Blending → Grinding → Slurry Preparation → Desilication → Digestion Feed

The objective is not simply to reduce the particle size of bauxite or remove reactive silica. The real objective is: To produce a homogeneous, correctly ground and appropriately desilicated bauxite slurry that can be digested efficiently, with maximum alumina recovery, minimum caustic and energy consumption, and minimum total cost per tonne of alumina.

This distinction is fundamental to good refinery design and operation.

2. From ROM Bauxite to Bayer Feed

ROM bauxite arriving from the mine is generally unsuitable for direct feeding to the digestion circuit. It may contain:

• Large lumps
• Clay and fines
• Oversize material
• Tree roots
• Organic matter
• Stones and foreign material
• Variable moisture
• Significant short-term variation in Al₂O₃
• Variable reactive SiO₂
• Different mineralogical domains
• Metallic foreign materials.

 

A modern refinery therefore requires a properly designed bauxite preparation system before the ore enters grinding and digestion. A typical arrangement is given below:

Mine → ROM Hopper → Crushing → Screening → Stockpile/Blending → Reclaiming → Grinding → Slurry Preparation → Desilication → Digestion

The exact configuration depends upon:

• Mine characteristics
• Bauxite hardness
• Moisture
• Lump size
• Available alumina
• Reactive silica
• Refinery capacity
• Required slurry concentration
• Grinding target
• Digestion technology
• Degree of feed homogenisation required.

 

3. ROM Bauxite Handling

The ROM hopper receives bauxite directly from mine transportation or mine crushing facilities. The hopper should provide sufficient surge capacity to decouple:

Mine operation ↔ Crushing operation ↔ Refinery operation

This is important because a refinery should not experience repeated process disturbances merely because of short-duration interruptions in mine truck movement.

Important design considerations

• Maximum ROM lump size
• Bulk density
• Moisture
• Angle of repose
• Hopper geometry
• Wear lining
• Bridging tendency
• Clay content
• Required live capacity
• Apron/belt feeder selection.

For sticky or high-moisture bauxite, hopper geometry and feeder selection become particularly important.

 

4. Crushing of Bauxite

The purpose of crushing is to reduce ROM bauxite to a size suitable for subsequent handling, screening, blending and grinding. Crushing should not be confused with grinding.

Crushing objective: Large ROM lumps → manageable feed size

Grinding objective: Manageable feed→ optimum particle-size distribution for digestion

The crusher should therefore not be operated with the philosophy of producing the final grinding size. Over-crushing may increase:

• Electrical consumption
• Dust generation
• Fines generation
• Material handling problems
• Equipment wear.

The optimum crushing product should be established from the downstream grinding requirement.

5. Typical Crushing Equipment

Depending on ROM characteristics, the following equipment may be considered.

5.1 Jaw Crusher

Advantages:

• Simple
• Robust
• Suitable for large lumps
• Relatively easy maintenance

 Limitations:

• Batch-like crushing action
• Higher vibration
• Less suitable for very sticky feed.

5.2 Gyratory Crusher

 Suitable for:

• Very high throughput
• Large-scale mining operations
• Continuous operation

Its high capacity makes it attractive for very large refinery projects where mine-to-refinery logistics justify the investment.

 

5.3 Impact Crusher

Can be attractive where the bauxite is relatively soft and friable.

Advantages:

• Good reduction ratio
• Compact arrangement
• Suitable for softer material.

However, wear and fines generation must be evaluated.

 

5.4 Double-Roll Crusher

Can provide controlled reduction and is particularly useful for relatively soft materials.

The final selection should be based on actual:

ROM size + moisture + hardness + abrasiveness + throughput + required product size.

 

6. Screening

The basic object of screening is used to separate:

• Oversize
• Acceptable feed
• Fines

The screen arrangement should prevent unnecessary material from entering the grinding circuit. A well-designed screening system can therefore reduce the grinding energy requirement.

Important parameters

• Screen aperture
• Feed rate
• Moisture
• Bed depth
• Screening efficiency
• Oversize recycle
• Fines bypass.

The screening system should be designed together with the crushing circuit rather than as an isolated piece of equipment.

 

7. Bauxite Stockpiling and Blending

One of the most underestimated operations in alumina refining is bauxite homogenisation.

A refinery may receive bauxite having excellent average quality but significant short-term variations.

Feeding these materials without adequate blending can create large fluctuations in digestion and liquor chemistry.

Therefore, mine planning and refinery process control must be connected through a properly designed blending strategy.

 

8. Stockpile and Reclaimer Selection

Common systems include:

• Longitudinal stockpiles
• Chevron stacking
• Windrow stacking
• Circular stockpiles
• Bridge reclaimers
• Bucket-wheel reclaimers
• Front-end-loader systems.

For large refineries, automated stacking and reclaiming can provide significantly better homogenisation. However, the economics must consider:

CAPEX + operating cost + blending efficiency + reliability + footprint.

The cheapest stockpile system is not necessarily the lowest-cost system when downstream process stability is considered.

 

9. Bauxite Grinding – Why Is It Required?

Grinding is one of the most important operations between the mine and digestion.

The principal objectives are:

1. Reduction of particle size
2. Liberation of alumina-bearing minerals
3. Improvement of digestion kinetics
4. Production of consistent slurry
5. Reduction of undigested coarse particles
6. Achievement of the required particle-size distribution.

However, an important engineering principle must be remembered:

“The objective is optimum grinding, not maximum grinding.” Grinding bauxite excessively fine does not necessarily increase overall refinery performance.

10. The Critical Relationship Between Grinding and Digestion

The relationship can be represented conceptually as: “Lower particle size increases surface area and digestion kinetics” but at the same time, Lower particle size will cause high grinding energy and excessive fines may adversely affect the following:

• Slurry rheology
• Clarification
• Settling
• Filtration
• Red-mud washing
• Pumping
• Energy consumption.

Therefore, the refinery designer must identify the economic optimum particle-size distribution rather than simply specify the smallest possible P80.

 

11. Grinding Technologies

The principal grinding technologies that may be considered include:

 

Ball Mill

A proven technology for producing controlled fine grinding.

 

Rod Mill

More suitable for relatively coarse grinding and specific feed characteristics.

 

SAG/AG Mill

Potentially attractive for large-scale operations depending upon ore suitability and circuit design.

 

HPGR

High-pressure grinding rolls may offer energy advantages in suitable applications, but their suitability must be established through ore testing and pilot evaluation.

 

Vertical/Alternative Grinding Systems

These may be considered where:

• Space is limited
• Specific energy reduction is important
• Ore characteristics are favorable.

For a large alumina refinery, selection should be based on life-cycle economics, not equipment preference alone.

 

12. Major Grinding Equipment Selection Criteria

The following parameters should be established before selecting the grinding system:

Parameter	Importance
Bauxite hardness	Determines grinding duty
Abrasiveness	Determines wear
Feed size	Determines crusher/mill configuration
Moisture	Influences slurry preparation
Required P80	Determines mill duty
Throughput	Determines mill size
Slurry concentration	Influences mill operation
Specific energy	Determines power consumption
Wear rate	Influences OPEX
Availability	Influences production cost
Maintenance	Influences life-cycle cost

The final selection should preferably be supported by:

laboratory testing, pilot testing, vendor guarantee and plant-scale validation.

 

13. Wet Grinding vs Dry Grinding

For a Bayer refinery, wet grinding is generally attractive because the downstream process is already based on aqueous slurry.

Advantages include:

• Direct slurry production
• Lower dust generation
• Improved working environment
• Better integration with digestion
• Reduced intermediate material handling
• Potentially simpler downstream arrangement.

However, wet grinding introduces:

• Pumping requirements
• Slurry rheology considerations
• Water balance requirements
• Corrosion/wear considerations.

The correct choice must therefore be based on the complete refinery flowsheet.

14. Grinding Circuit Configuration

A typical grinding circuit may consist of Bauxite Feed, Milling, Classification, Fine Slurry, Desilication/Digestion

with coarse material returned to the grinding mill.

Hydroclones / Tailormade screens are commonly used for bauxite slurry classification.

Key control parameters

• Mill feed rate
• Mill power
• Mill speed
• Slurry density
• Water addition
• Grinding media loading
• Cyclone pressure
• Cyclone feed density
• Overflow particle size
• Underflow density
• Circulating load.

 

15. Particle Size – P80

P80 is a useful parameter for controlling grinding performance. It represents the particle size below which approximately 80% of the material passes. However, P80 alone does not completely describe the grinding product.

Two slurries may have identical P80 values but significantly different:

• Fine fraction
• Coarse fraction
• Surface area
• Particle-size distribution.

Therefore, plant control should monitor the complete PSD for effective digestion.

 

16. Grinding Energy Consumption

Grinding can be a significant electrical-energy consumer. A basic performance indicator is Specific Grinding Energy. This parameter should be continuously monitored.

“The plant objective is not minimum kWh/t at any cost but minimum total refinery cost per tonne of alumina while achieving the required digestion performance.” This distinction is very critical.

 

17. Energy Conservation in Grinding

Major opportunities include

• Avoid Over-Grinding
• Optimize Mill Loading
• Optimize Classification
• Maintain Grinding Media
• High-Efficiency Motors
• Maintenance of Equipment

Worn liners, poor lubrication, misalignment and mechanical losses directly increase specific energy and continuous steady operation of the plant.

 

18. Slurry Preparation

After grinding, bauxite is converted into a controlled slurry suitable for the Bayer circuit.

Important parameters include:

• Solid consistency
• Particle-size distribution
• Temperature
• Na₂O concentration
• Al₂O₃ concentration
• Water balance
• Viscosity
• Flow rate.

The slurry must be sufficiently fluid for:

• Pumping
• Mixing
• Heating
• Desilication
• Digestion.

But excessive dilution increases:

• Water circulation
• Heating duty
• Evaporation requirement
• Pumping
• Equipment size.

Therefore, Optimum slurry density is an important refinery-energy parameter.

 

19. Desilication – The Next Critical Step

Reactive silica is one of the most economically important impurities in bauxite. During Bayer processing, reactive silica can dissolve in caustic liquor and subsequently participate in the formation of desilication products (DSP). This results in:

• Caustic soda consumption
• Alumina loss
• Additional residue generation
• Increased soda in red mud
• Increased refinery operating cost.

Therefore, desilication is not merely an impurity-removal operation.

It is an alumina recovery and soda-loss control operation.

 

20. Why Desilication Is Important

The simplified desilication process sequence is:

Reactive SiO₂ → Dissolution in Bayer Liquor → Silicate Species → DSP Formation

DSP incorporates sodium and alumina-bearing species and reports largely with the residue.

Consequently, Reactive silica ↑ → DSP ↑ → Soda loss ↑ + Alumina loss ↑ + Residue ↑

This is why reactive silica control has a direct relationship with refinery economics.

 

21. Pre-Desilication and Post-Desilication

Two broad approaches are possible:

Pre-Desilication:

Bauxite slurry is subjected to controlled desilication before the main digestion step.

Potential advantages:

• Reactive silica conversion before digestion
• Better control of silica entering high-temperature digestion
• Potential improvement in liquor quality.

Post-Desilication:

Desilication occurs after digestion or as an integrated downstream operation depending on the refinery flowsheet. The optimum arrangement depends on: 

• Reactive silica
• Bauxite mineralogy
• Digestion temperature
• Caustic concentration
• Residence time
• DSP behaviour
• Overall refinery configuration.

For gibbsitic bauxite, the decision should be made based on actual ore mineralogy and reactive silica rather than applying a universal flowsheet.

 

22. Practical Reactive-Silica Boundary

For the type of gibbsitic bauxite considered in this Bayer Process Masterclass, approximately 1.5% reactive SiO₂ is a useful practical decision boundary for flowsheet evaluation. It should not, however, be treated as an absolute universal limit.

Indicatively: Reactive SiO₂ < ~1.5%

Post desilication strategy is considered.

However, Reactive SiO₂ > ~1.5%

Stronger consideration is given to controlled pre-desilication and its impact on caustic, alumina recovery, residence time and residue generation. The final decision is always established through characterization of representative bauxite and complete testing in laboratory.

 

23. Desilication Operating Parameters

The major parameters requiring control are:

Temperature: Higher temperature generally accelerates reactions, but excessive temperature may alter the desired process chemistry and energy balance.

Residence Time: Adequate residence time is required to achieve the intended degree of desilication.

Caustic Concentration: Controls silica dissolution and reaction kinetics.

Slurry Density: It influences:

• Reaction kinetics
• Heat transfer
• Agitation
• Pumping
• Equipment capacity.

Agitation: Adequate agitation is required to maintain:

• Uniform solids suspension
• Heat transfer
• Consistent reaction conditions.

Reactive Silica Loading: This is one of the most important feed-forward control parameters.

 

24. Desilication Equipment

Agitated tanks are commonly used where controlled residence time and reaction conditions are required. Important design parameters widely considered by Alumina Experts are listed below:

• Tank volume
• Number of stages
• Residence time
• Diameter/height ratio
• Agitator type
• Impeller diameter
• Power input
• Baffle arrangement
• Heating system
• Instrumentation.

For large refineries, multiple tanks in series can provide better control of residence-time distribution than one very large tank.

 

25. Agitator Selection

Agitator selection should consider:

• Slurry solids concentration
• Particle size
• Settling velocity
• Viscosity
• Tank geometry
• Required suspension quality.

The agitator should provide adequate mixing without excessive power consumption. A common mistake is to specify agitator power only on the basis of kW/m³ without considering the actual slurry suspension requirement. For Desilication tank agitator design, degree of agitation is always recommended as 10 to have full tank suspension of solids. Hence the correct recommended engineering approach is:

Slurry properties → Suspension requirement → Impeller selection → Power requirement → Mechanical design.

 

26. Process Control Philosophy

The grinding and desilication section should be controlled as an integrated system.

Feed-forward control

Bauxite laboratory/online data:

Al₂O₃ + Reactive SiO₂ + Moisture + PSD

should influence:

• Bauxite blending
• Mill feed
• Water addition
• Caustic addition
• Desilication residence time.

 

Feedback control

Actual process measurements should then correct the operation.

Important measurements include:

• Feed rate
• Slurry density
• Temperature
• PSD
• Reactive silica
• Caustic concentration
• Tank level
• Flow rate
• Mill power
• Cyclone pressure.

 

27. Recommended Operating Dashboard

A practical operator dashboard may contain:

Parameter	Unit	Purpose
Bauxite feed	t/h	Throughput
Available Al₂O₃	%	Recovery basis
Reactive SiO₂	%	Desilication requirement
Moisture	%	Water balance
Mill power	kW	Grinding efficiency
Specific grinding energy	kWh/t	Energy KPI
P80	µm	Grinding control
Slurry density	% solids	Process control
Desilication temperature	°C	Reaction control
Residence time	min/h	Reaction control
Caustic concentration	g/L Na₂O	Chemistry
Residual reactive SiO₂	%	Desilication performance
Alumina recovery	%	Overall performance
Soda loss	kg/t Al₂O₃	Economic KPI

 

28. From Process Parameters to Cost of Alumina

The economics of bauxite preparation should not stop at the grinding plant. Consider the Grinding cost and Desilication cost. But the most important component is the economic value of alumina and soda lost through DSP.

Therefore, this is the correct basis for comparing alternative process configurations.

 

29. Why Lowest CAPEX Is Not Necessarily Lowest Cost

Suppose two options of grinding system are available.

System A: Lower CAPEX but

• Higher power consumption
• Higher wear
• Poorer classification
• Greater variation in P80.

System B: Higher CAPEX but

• Lower kWh/t
• Better classification
• Stable P80
• Lower maintenance
• Better digestion performance.

For a large refinery operating 330–350 days/year, the additional CAPEX of System B may be recovered through lower operating cost. Therefore, equipment selection should be based on:

Life-Cycle Cost = CAPEX + Lifetime OPEX + Production Loss + Maintenance Cost

rather than CAPEX alone.

 

30. Integrated Optimisation

The real optimization to include the Bauxite Quality, Blending, Crushing, Grinding, Optimum PSD, Slurry Density, Desilication, Controlled Reactive SiO₂, Digestion, Alumina Recovery, Cost/t Alumina

Every section affects the next. A decision made in the grinding circuit therefore cannot be evaluated independently from digestion and refinery economics.

Hope, the young engineers will find this article technically informative. If you feel that any critical information requires clarifications / elaboration, Please place your valued comments / remarks. Your suggestions will add value in further improvements in quality of articles of Bayer Process Masterclass series in future.

Rajendra Kunwar

Bayer Process Expert,

Delhi NCR, India.

Mobile: +91-83800 43065.