How Anionic Polyacrylamide Works in Red Mud Treatment

Anionic polyacrylamide (anionic PAM) is the key flocculant used in the clarification and washing stages of red mud treatment in aluminum refineries. Its role is to accelerate solid–liquid separation, improve mud compaction, and yield clear sodium aluminate liquor. Red mud consists of extremely fine, negatively charged mineral particles (often <10 microns), which naturally settle very slowly. Anionic PAM modifies the behavior of these particles so they settle rapidly and compact efficiently.

Here is how anionic polyacrylamide works, step by step:

1. Adsorption onto Red Mud Particle Surfaces

Although red mud surfaces are typically negatively charged, the harsh Bayer liquor (high caustic concentration, high ionic strength, presence of iron and aluminum oxides) reduces electrostatic repulsion, allowing the anionic PAM molecules to adsorb onto particle surfaces through:

• hydrogen bonding,

• Van der Waals attraction,

• ligand-type interaction with surface metal ions,

• physical entanglement.

The long polymer chain lays across many particles.

2. Polymer Bridging – The Core Mechanism

Once adsorbed, the long-chain anionic PAM acts like a bridge linking multiple particles together.

How bridging works:

• A section of the polymer attaches to one particle.

• The free part of the chain extends into the slurry.

• That free segment attaches to another particle.

• Repeated many times → clusters of particles form.

These clusters are called flocs.

Because anionic PAM has extremely high molecular weight (often 10–20+ million daltons), it can link hundreds to thousands of tiny red mud particles together.

3. Formation of Large, Dense Flocs

As bridging continues, individual tiny particles convert into large, heavier flocs that can settle much more quickly due to increased mass and reduced drag.

Properties of flocs formed by anionic PAM:

• large size (100–1000+ microns),

• strong internal structure,

• good permeability (liquor can escape easily),

• stable under shear.

This transformation is the key to fast clarification.

4. Rapid Settling in Thickeners (Primary Clarification)

In the primary settler:

• large flocs settle quickly to form a dense mud layer,

• a clear interface develops between mud and liquor,

• the overflow liquor becomes clear and low in suspended solids,

• the settler can handle higher throughput.

Settling rate increases 10–20 times compared with untreated mud.

5. Producing Clear Overflow Liquor

Anionic PAM pulls fine particles down into the floc structure. This dramatically improves overflow clarity.

Clear liquor is essential for:

• efficient alumina precipitation,

• avoiding scale build-up,

• maintaining product quality,

• reducing filtration load.

Without PAM, the overflow would be cloudy and contain unacceptably high mud fines.

6. Increasing Red Mud Underflow Density

Anionic PAM helps red mud compact strongly at the bottom of the thickener.

Benefits of higher underflow density:

• lower mud volume,

• easier pumping and disposal,

• improved washer efficiency,

• reduced soda loss.

The polymer creates flocs that pack more tightly because of inter-particle bonding and good permeability.

7. Improving Washing Efficiency in CCD Washers

In the washing circuit, anionic PAM is used to help red mud settle quickly in each stage, enabling better separation between wash liquor and solids.

Results:

• higher caustic soda recovery,

• less alumina lost in mud,

• reduced chemical consumption.

Efficient wash settling depends directly on PAM flocculation quality.

8. Enhancing Tailings & Waste Management

Well-flocculated mud:

• dewaters more efficiently,

• yields lower-moisture filter cake,

• improves stability of tailings storage,

• reduces environmental hazards and pond size.

Thicker mud allows refineries to adopt advanced disposal methods like paste thickening or dry stacking.

In Summary: How Anionic PAM Works

Mechanism:
✔ Adsorbs onto red mud particles
✔ Creates polymer bridges
✔ Forms large, dense, fast-settling flocs

Effects:
✔ Rapid solid–liquid separation
✔ Clear overflow liquor
✔ Higher mud underflow density
✔ Improved washing efficiency
✔ Better filtration and tailings management

Without anionic polyacrylamide, modern red-mud handling would be slow, inefficient, and extremely expensive.