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