Polyacrylamide (PAM) plays a critical role in the treatment and management of red mud—the highly alkaline waste residue generated during the Bayer process for aluminum (alumina) production. In this context, polyacrylamide is primarily used as a flocculant and settling aid to accelerate solid–liquid separation between the red mud solids and the sodium aluminate liquor. The goal is to recover as much process liquor as possible while producing dense, compact red-mud underflow with minimal suspended solids in the overflow.

Because red mud contains extremely fine particles (mainly iron oxides, aluminosilicates, titanium dioxide, and residual caustic), it exhibits poor settling characteristics and forms stable suspensions. Anionic polyacrylamide and, in some cases, amphoteric polyacrylamide are used to promote aggregation of these particles into large, fast-settling flocs that can be efficiently separated in thickeners and washers.

1. Overview of Red Mud Formation in Aluminum Processing

In the Bayer process, bauxite ore is digested under high temperature and pressure in a concentrated caustic soda (NaOH) solution. Aluminum hydroxide dissolves as sodium aluminate, while the insoluble impurities—iron oxides, silica, titania, and other minerals—remain as a thick, highly alkaline residue known as red mud.

This red mud slurry contains a large proportion of water (often > 60%) and ultrafine solids (usually < 10 µm), which makes separation and disposal difficult. To enable the recycling of caustic liquor back into the digestion circuit and to recover alumina more efficiently, red mud must be rapidly settled and thickened.

Without chemical assistance, the settling rate is extremely slow, leading to poor clarity of overflow liquor and low mud solids in the underflow. That is where polyacrylamide flocculants come in—transforming the slow-settling colloidal suspension into large, dense flocs that settle quickly and compact well.

2. Function of Polyacrylamide in Red Mud Separation

The primary function of polyacrylamide in red-mud treatment is flocculation—the aggregation of fine particles into larger flocs through charge neutralization and polymer bridging.

Red-mud particles carry a negative surface charge due to hydroxyl ions and the high-pH environment (pH > 11). Therefore, an anionic polyacrylamide with appropriate molecular weight and charge density is most effective for bridging between particles and forming strong, shear-resistant flocs.

When PAM is added to the red-mud slurry:

1. Its long molecular chains adsorb onto multiple particle surfaces.

2. The polymer bridges adjacent particles, linking them together.

3. The small particles form aggregates (flocs) that are heavier and settle rapidly.

4. The clarified sodium aluminate liquor is recovered with minimal suspended solids.

Thus, polyacrylamide ensures efficient solid–liquid separation, higher mud compaction, and better process-liquor clarity, directly influencing alumina yield and caustic recovery.

3. Types of Polyacrylamide Used in Red-Mud Settling

Different grades of polyacrylamide are used depending on the plant configuration, mud characteristics, and washing stages:

• Anionic polyacrylamide (APAM): The most common type for red-mud treatment. It has high molecular weight and moderate to high anionic charge density (typically 20–40%). It provides strong bridging between fine negatively charged particles and forms dense flocs.

• Amphoteric polyacrylamide: Contains both cationic and anionic groups; sometimes used when the red-mud chemistry varies or the mud contains specific mineral impurities that require balanced charge interactions.

• Low-ionic-strength PAM formulations: Used in the final washing stages to maintain clear overflow and optimize soda recovery.

4. Major Applications of Polyacrylamide in Red-Mud Circuits

Polyacrylamide is employed at multiple points in the alumina refinery’s mud-handling and liquor-recovery system:

a. Primary Red-Mud Settling

After digestion and flash cooling, the slurry enters large thickeners. PAM is dosed to promote rapid settling of the red-mud solids. The resulting underflow becomes dense and compact (50–60 wt% solids), while the overflow liquor is clear and low in suspended solids (< 20 mg/L).

b. Washer Circuit (Counter-Current Decantation)

The settled red mud is washed in several stages (typically 4–6 washers) to recover entrained sodium aluminate liquor and caustic soda. Polyacrylamide aids floc formation and enhances settling efficiency in each washer, ensuring minimal alumina loss and maximum soda recovery.

c. Filtration and Mud Disposal

In some plants, polyacrylamide-treated mud is further thickened or filtered before disposal to reduce moisture content. This helps produce drier mud cakes and decreases the volume of waste stored in red-mud ponds.

d. Liquor Clarification

PAM can also be applied in overflow-polishing clarifiers or security filters to ensure the recovered sodium aluminate liquor meets process-clarity standards before precipitation.

5. Benefits of Polyacrylamide in Red-Mud Management

The use of polyacrylamide in red-mud treatment provides several major operational and environmental advantages:

• Faster Settling Rate: Increases mud-settling velocity by several times compared to unassisted settling.

• Improved Overflow Clarity: Reduces suspended solids in the recovered liquor, ensuring high-quality feed for precipitation.

• Higher Mud Solids: Produces compact, easily handled underflow with higher solids concentration.

• Reduced Chemical Loss: Minimizes caustic and alumina loss in the mud, improving process efficiency.

• Lower Water Consumption: Enhances wash efficiency, reducing fresh-water demand.

• Reduced Red-Mud Volume: Improves dewatering, which lowers the volume of mud requiring storage or disposal.

• Operational Stability: Enhances thickener performance, reducing overflow upsets and improving process reliability.

6. Selection Criteria for Effective PAM Performance

The efficiency of polyacrylamide in red-mud applications depends on several factors:

• Molecular weight and charge density: High-molecular-weight PAMs provide better bridging but may require careful dosing to avoid overdosing and floc breakage.

• Dosage rate: Typically 20–200 g per ton of dry solids, depending on mud characteristics.

• pH and temperature stability: PAM formulations must withstand the high-pH (> 11) and elevated temperature (60–90 °C) environment of Bayer circuits.

• Mixing and shear conditions: Proper dispersion and low-shear mixing are essential to prevent polymer degradation.

Modern refiners often conduct jar tests and plant trials to optimize PAM type and dosage for maximum clarity and throughput.

7. Environmental and Process Sustainability

The use of polyacrylamide contributes to sustainable red-mud management. By improving settling and dewatering, PAM reduces the footprint of red-mud disposal areas, facilitates dry stacking, and lowers the environmental risk associated with red-mud ponds. Furthermore, because PAM is used in very low dosages and is largely inert, it has minimal impact on the chemistry of the recovered process liquor or the environment.