Application of Anionic Polyacrylamide in Primary Settlers / Red Mud Thickeners

In alumina refineries operating under the Bayer process, anionic polyacrylamide (A-PAM) is a critical flocculant used in primary settlers and red mud thickeners. These units are responsible for separating solid red mud residue from the pregnant sodium aluminate liquor after bauxite digestion. Without chemical flocculation, red mud particles—extremely fine, highly charged, and suspended in strong alkaline liquor—would settle too slowly for industrial-scale operation. Anionic polyacrylamide provides the essential mechanism for fast, stable, and efficient solid–liquid separation.

1. Role in Red Mud Settling

Red mud consists of iron oxides, aluminosilicates, titania, calcium compounds, and unreacted bauxite, typically with particle sizes below a few microns. These particles remain dispersed due to surface charge and high liquor viscosity. When anionic polyacrylamide is added to the feed of the primary settler, its high molecular weight polymer chains adsorb onto multiple particles, forming bridges between them. This polymer-bridging action converts fine dispersed solids into large, dense flocs that settle rapidly under gravity.

2. Improvement of Settling Rate and Thickener Capacity

The most important application benefit is the dramatic increase in settling velocity. Fast floc formation shortens the time required to form a clear interface between solids and liquor, allowing primary settlers and red mud thickeners to operate at higher throughput. This directly increases refinery productivity and reduces the number or size of thickeners required for a given capacity.

3. Enhancement of Overflow Clarity

Anionic polyacrylamide ensures that fine solids are effectively captured in the settling flocs, producing very clear overflow liquor. Clear pregnant liquor is essential for downstream processes such as flash cooling, filtration, and alumina precipitation. Reduced suspended solids help prevent scaling, fouling of heat exchangers, and contamination of aluminum hydroxide product.

4. Increase of Underflow Density

In red mud thickeners, anionic polyacrylamide helps form compact, compressible flocs that release water efficiently at the bottom of the thickener. This results in higher underflow solids concentration, reducing the volume of red mud that must be pumped to washing or disposal stages. Higher underflow density also improves the efficiency of counter-current decantation (CCD) washing, lowering caustic soda and alumina losses.

5. Stability in Highly Alkaline Conditions

Primary settlers operate at high pH (often >13) and elevated temperature. Anionic polyacrylamide used for red mud applications is specially designed to maintain molecular integrity and flocculation performance under these harsh conditions. Its chemical stability ensures consistent settling behavior despite variations in bauxite quality or digestion conditions.

6. Support for Mud Washing and Residue Management

Effective flocculation in primary settlers sets the foundation for downstream residue handling. Well-flocculated red mud enters washing thickeners with improved structure, enabling better liquor displacement and more efficient washing. This reduces soda loss, improves residue dryness, and lowers environmental impact at disposal areas.

Summary

The application of anionic polyacrylamide in primary settlers and red mud thickeners is essential for:

• Rapid red mud settling

• High thickener throughput

• Clear pregnant liquor overflow

• High-density mud underflow

• Improved washing efficiency and residue handling

In short, anionic polyacrylamide is a key process chemical that enables stable, economical, and high-capacity operation of primary settlers and red mud thickeners in modern alumina refineries.