In alumina refineries operating under the Bayer process, high-solids red mud thickeners and residue disposal preparation systems represent the final and most demanding stages of red mud handling. At this point, the objective is no longer only clarification or washing efficiency, but the production of a high-density, stable residue suitable for safe transport, stacking, and long-term storage. Anionic polyacrylamide (A-PAM) is a critical flocculant used at this stage to enable effective dewatering, improve mud compaction, and ensure stable residue disposal performance.

1. Role of High-Solids Red Mud Thickeners

High-solids thickeners receive red mud that has already passed through primary settling and CCD washing circuits. Although most recoverable liquor has been removed, the mud still contains a significant amount of free water and entrained liquor. The purpose of high-solids thickeners is to maximize solids concentration, reduce slurry volume, and produce a pumpable but highly dense underflow. This step is essential for minimizing disposal area requirements and improving environmental safety.

2. Challenges of High-Solids Red Mud Handling

Red mud at this stage contains ultra-fine particles with strong surface charges and high surface area. The slurry typically operates at very high pH and ionic strength, and solids concentrations are already elevated. Under these conditions, particle–particle interactions become complex, natural settling is extremely slow, and mud beds can become unstable or difficult to compress. Mechanical dewatering alone is insufficient, making chemical flocculation with anionic polyacrylamide indispensable.

3. Flocculation Mechanism of Anionic Polyacrylamide

Anionic polyacrylamide used for high-solids red mud applications has very high molecular weight and optimized anionic charge. When added to the thickener feed or dilution zone, the polymer chains adsorb onto multiple red mud particles simultaneously, creating strong polymer bridges. This transforms dispersed fine particles into large, structured flocs capable of releasing interstitial water under compression. The resulting flocs are robust enough to withstand shear during pumping and raking while remaining compressible in the thickener bed.

4. Increase of Underflow Solids Concentration

The primary application benefit of anionic polyacrylamide in high-solids thickeners is the significant increase in underflow density. Well-formed flocs settle quickly and compact efficiently, allowing water to be released upward and removed as overflow. Higher underflow solids reduce the volume of red mud transported to disposal areas, lowering pumping energy, pipeline wear, and operating costs. Dense underflow is also critical for modern residue management methods such as dry stacking or thickened tailings disposal.

5. Improvement of Mud Bed Stability

In high-solids thickeners, maintaining a stable mud bed is essential to avoid rake overload, torque spikes, and bed lifting. Anionic polyacrylamide produces uniform, compressible flocs that form a stable and controllable mud bed. This stability allows thickeners to operate at higher solids loading without operational disruptions, increasing overall plant capacity and reliability.

6. Enhancement of Overflow Clarity and Water Recovery

Although the focus at this stage is on solids concentration, clear overflow remains important for water recovery and reuse. Anionic polyacrylamide captures fine particles that would otherwise escape with the overflow, producing low-turbidity water suitable for recycling to washing circuits or plant water systems. Improved water recovery reduces freshwater intake and supports sustainable water management in alumina refineries.

7. Support for Residue Disposal Preparation

Before red mud is sent to residue disposal areas, it must exhibit predictable rheology and low free-water content. Anionic polyacrylamide helps produce a residue with improved yield stress and reduced bleed water, which enhances stacking stability and reduces the risk of seepage. This is particularly important for residue storage facilities operating under strict environmental and safety regulations.

8. Reduction of Environmental Impact

Effective use of anionic polyacrylamide in high-solids thickening reduces the overall environmental footprint of red mud disposal. Higher solids content means smaller disposal footprints, lower dam heights, and reduced risk of alkaline liquor release. Better water recovery also minimizes the generation of contaminated wastewater, contributing to improved environmental compliance.

9. Operational and Economic Benefits

From an operational perspective, anionic polyacrylamide allows high-solids thickeners to function efficiently under variable feed conditions. It reduces rake torque, improves bed control, and minimizes downtime. Economically, improved dewatering and water recovery lead to lower energy consumption, reduced chemical losses, and decreased costs associated with residue transport and storage.

10. Conclusion

The application of anionic polyacrylamide in high-solids red mud thickeners and residue disposal preparation is essential for modern, high-capacity alumina refineries. By enabling strong flocculation, efficient compaction, and stable mud beds, A-PAM supports the production of dense, manageable red mud residue. This not only improves operational efficiency and cost control but also plays a critical role in meeting environmental and safety requirements associated with red mud disposal.