The main application of Hydroxamated Polyacrylamide (HPAAM) in aluminum plants—specifically within alumina refineries that use the Bayer process—is as a high-efficiency flocculant for red mud settling and separation.

 What is HPAAM?

Hydroxamated Polyacrylamide (HPAAM) is a modified polyacrylamide polymer, incorporating hydroxamic acid functional groups. These hydroxamate groups have a strong chelating affinity for multivalent metal ions, especially iron (Fe³⁺) and aluminum (Al³⁺) oxides. This selective metal-binding property makes HPAAM especially suitable for interacting with red mud particles, which are primarily composed of iron oxides, aluminum oxides, silicates, and residual bauxite minerals.

Main Application: Red Mud Flocculation and Settling

In the clarification stage of the Bayer process, the digestion of bauxite in caustic soda produces a slurry of dissolved sodium aluminate and solid impurities called red mud. Efficient separation of red mud from the pregnant liquor is critical. Here, HPAAM is dosed into primary thickeners (or settlers) to flocculate fine red mud particles and speed up settling.

Mechanism:

• Hydroxamate groups on HPAAM bind selectively to metal oxides (Fe, Al) on red mud particles.

• High molecular weight polymer chains bridge particles together to form large, dense flocs.

• These flocs settle rapidly, producing:

  • Clear overflow liquor (low in suspended solids)

  • Dense underflow mud (easier to handle and dewater)

Operational Benefits in Aluminum Plants

1. Improved Red Mud Settling Rates

   • Enhances thickener throughput and allows higher solids loading.

2. Clearer Overflow Liquor

   • Reduces turbidity and suspended solids in sodium aluminate solution.

   • Improves downstream precipitation and filtration efficiency.

3. Higher Underflow Density

   • Produces thicker red mud that requires less washing and disposal volume.

4. Reduced Caustic Soda Loss

   • Better separation minimizes the loss of valuable NaOH entrained in mud.

5. Shear-Stable Flocs

   • HPAAM forms robust flocs that resist degradation under mixing or flow turbulence.

6. Lower Dosage Compared to Conventional Flocculants

   • More effective due to strong chemical binding with red mud.

Other Application Points in Alumina Refineries

• Secondary thickeners

• Washer circuits (CCDs) – to reduce caustic loss and improve mud compaction

• Red mud disposal systems – to pre-thicken slurry before discharge

Summary

In conclusion, the main application of Hydroxamated Polyacrylamide (HPAAM) in an aluminum plant is to enhance red mud settling in the Bayer clarification process, leading to more efficient solid-liquid separation, better caustic recovery, and improved process economics.