The Application of Polyamine in Alumina and Red Mud Processing

1. Background: Alumina and Red Mud Separation

In alumina refineries, bauxite is digested in concentrated sodium hydroxide under high temperature and pressure to dissolve alumina. This process, known as the Bayer process, produces an insoluble residue called red mud. Red mud consists of extremely fine particles such as iron oxides, titanium oxides, silica, and unreacted aluminosilicates.

These red mud particles are highly negatively charged, very fine in size, and present at high solids concentrations. Without chemical treatment, red mud settles slowly and forms poorly compacted sediments, leading to low clarification efficiency and significant losses of valuable caustic liquor. Efficient solid–liquid separation is therefore critical, and polyamine plays an important role in this stage of alumina production.

2. Role of Polyamine in Alumina Processing

Polyamine is a cationic polymer coagulant characterized by high positive charge density and relatively low molecular weight. In alumina and red mud processing, polyamine is primarily used as a coagulant or coagulant aid, rather than a high-molecular-weight flocculant.

The main functions of polyamine in alumina refineries include:

• Destabilization of fine red mud particles

• Improvement of settling and clarification

• Enhancement of mud compaction

• Recovery of caustic soda and process liquor

3. Working Mechanism of Polyamine in Red Mud Systems

Red mud particles carry strong negative surface charges due to hydroxylated oxide surfaces and high alkalinity. This causes electrostatic repulsion between particles, preventing natural agglomeration.

When polyamine is added:

• Positively charged polyamine molecules adsorb rapidly onto red mud particle surfaces

• Surface charge is neutralized or reduced

• Electrostatic repulsion between particles is minimized

• Fine particles collide and form micro-aggregates

Because polyamine has short polymer chains, it produces small, dense micro-flocs rather than large, fragile flocs. These micro-flocs settle quickly and serve as ideal nuclei for further floc growth when combined with anionic polyacrylamide flocculants.

4. Application Points in Alumina Refineries

4.1 Primary Clarification (Mud Settlers)

One of the most common applications of polyamine is in primary mud settlers or thickeners. Here, polyamine is dosed into the slurry feed to rapidly destabilize fine red mud particles before sedimentation.

Benefits include:

• Faster settling rates

• Improved overflow clarity

• Increased throughput of settlers

• Reduced solids carryover to liquor circuits

By improving clarification efficiency, polyamine helps protect downstream heat exchangers, evaporators, and precipitation circuits from fouling.

4.2 Red Mud Washing Circuits

In multi-stage counter-current decantation (CCD) systems, efficient washing of red mud is essential for recovering caustic soda and dissolved alumina. Polyamine improves washing performance by enhancing floc strength and reducing fines entrainment.

Key advantages:

• Better separation between mud and wash liquor

• Higher caustic soda recovery

• Reduced soda losses in final residue

• Improved stability of washing thickeners

Polyamine ensures that fine particles remain in the underflow, preventing carryover into overflow streams.

4.3 High-Solids Mud Thickening

Modern alumina refineries often operate at very high solids concentrations to reduce water consumption and residue storage volume. Under such conditions, red mud particles are difficult to aggregate.

Polyamine performs well in these harsh environments because:

• It acts rapidly even at high solids loadings

• It is stable under strong alkaline conditions

• It improves mud bed compaction

This leads to higher underflow density and improved residue disposal efficiency.

5. Synergy Between Polyamine and Polyacrylamide

In most alumina operations, polyamine is not used alone. Instead, it is applied in combination with anionic polyacrylamide flocculants.

Typical treatment sequence:

1. Polyamine dosing – charge neutralization and particle destabilization

2. Anionic polyacrylamide addition – polymer bridging and floc enlargement

This synergistic approach offers several benefits:

• Reduced flocculant consumption

• Stronger, more shear-resistant flocs

• Faster settling and clearer overflow

• Improved mud washing efficiency

Polyamine creates optimal conditions for anionic polyacrylamide to perform effectively.

6. Dosage and Operating Considerations

The dosage of polyamine in alumina and red mud processing depends on:

• Bauxite mineralogy

• Particle size distribution of red mud

• Solids concentration

• Temperature and alkalinity

Typical polyamine dosages range from 50 to 300 g/t of dry solids, although precise optimization requires site-specific testing. Polyamine is usually diluted with process water and dosed at points of high turbulence to ensure uniform dispersion.

7. Advantages of Using Polyamine in Alumina Refineries

Key advantages include:

• High efficiency under extreme alkalinity

• Rapid coagulation of ultra-fine particles

• Improved clarification and settling

• Enhanced caustic liquor recovery

• Reduced chemical consumption when combined with PAM

• Improved residue handling and disposal