Introduction

Poly(hydroxamic acid)-based flocculants are specialized anionic polymers used extensively in solid-liquid separation processes due to their high selectivity and complexation ability with metal ions. In the context of the aluminum industry, particularly in Bayer process-based alumina production, these anionic flocculants have shown significant advantages in improving productivity, reducing waste, and enhancing the overall efficiency of red mud separation. This detailed technical article explores the main application and function of poly(hydroxamic acid)-based anionic flocculants used in aluminum plants, focusing primarily on their role in red mud settling, liquor clarification, and process optimization.

Understanding Poly(Hydroxamic Acid)

Poly(hydroxamic acid) (PHA) is a water-soluble anionic polymer containing hydroxamic acid functional groups (-CONHOH). These groups have a strong ability to chelate metal ions, especially trivalent ions like aluminum, iron, and other heavy metals. The polymer can be synthesized via partial hydrolysis of polyacrylamide or other acrylate-based precursors and subsequent amidoximation reactions.

Key characteristics of poly(hydroxamic acid):

• Strong complexation ability with metal ions

• High anionic charge density

• Water-soluble and stable under alkaline conditions

• Effective under high temperature and high pH, as found in aluminum production

Main Application in Aluminum Plants: Red Mud Settling

The primary application of poly(hydroxamic acid)-based flocculants in aluminum plants is in the settling and clarification of red mud—a key step in the Bayer process, the most widely used method for refining bauxite to produce alumina.

Bayer Process Overview

In the Bayer process:

1. Bauxite ore is digested in hot concentrated sodium hydroxide.

2. This forms a slurry where alumina dissolves to form sodium aluminate, and the insoluble residues (mainly iron oxides, silica, and titanium oxides) form red mud.

3. The red mud must be separated from the aluminate liquor before the precipitation of aluminum hydroxide can proceed.

This separation step is where poly(hydroxamic acid)-based flocculants are crucial.

Function of Poly(Hydroxamic Acid) in Red Mud Settling

Poly(hydroxamic acid)-based anionic flocculants play a central role in enhancing the sedimentation of red mud by promoting faster and more efficient floc formation. Their functions include:

1. Rapid Flocculation of Fine Particles

Red mud contains extremely fine particles (sub-micron size) that are difficult to settle naturally. Poly(hydroxamic acid) facilitates:

• Electrostatic bridging of negatively charged fine solids

• Complexation with trivalent ions (Al³⁺, Fe³⁺) in the mud, forming large, dense flocs

• Increased particle aggregation, enabling gravity-driven settling

2. Improved Mud Settling Rate

The flocculant accelerates the settling of red mud in thickener tanks, resulting in:

• Faster clarification of pregnant liquor

• Higher throughput in the clarification stage

• Reduced retention time in settlers or decanters

3. Enhanced Supernatant Clarity

By efficiently binding ultrafine solids, poly(hydroxamic acid) improves the clarity of the overflow liquor, reducing residual solids that could interfere with downstream processes such as aluminum hydroxide precipitation.

4. Reduced Polymer Dosage

Compared to traditional flocculants (such as anionic polyacrylamide), poly(hydroxamic acid) offers:

• Higher efficiency at lower dosages

• Better performance under extreme alkaline and thermal conditions

• More selective interaction with metal oxides common in red mud

5. Lowered Sodium Loss

PHA flocculants may also contribute to:

• Reduced sodium losses in the red mud due to better phase separation

• Higher soda recovery, improving the overall economics of the Bayer process

Advantages in Alumina Refining

Implementing poly(hydroxamic acid)-based flocculants in aluminum refineries provides the following advantages:

• Higher productivity: Faster red mud settling means quicker liquor recovery and greater plant throughput.

• Reduced mud volume: Denser flocs reduce the mud volume, improving storage and disposal efficiency.

• Cleaner liquor: Fewer impurities in the recovered liquor reduce scaling and increase the efficiency of precipitation tanks.

• Improved washing efficiency: Red mud with better floc characteristics is easier to wash, leading to better soda and alumina recovery.

• Compatibility with Bayer conditions: Stable at high pH (11–13) and temperatures up to 100°C, where traditional flocculants degrade.

Application Method

In an aluminum refinery, poly(hydroxamic acid)-based flocculants are typically dosed into the primary settler or thickener feedwell where red mud slurry enters. Key parameters include:

• Dosage range: Typically 50–200 g/ton bauxite depending on mud composition

• Form: Generally provided as a liquid emulsion or water-soluble powder

• Dilution: Often diluted to 0.1–0.5% active concentration before application

• Mixing and contact time: Adequate turbulence ensures uniform mixing with the slurry to enable full flocculant performance

Process optimization through jar testing is commonly used to determine the exact dosage and conditions required for a given bauxite or mud composition.

Performance Evaluation

Performance indicators for poly(hydroxamic acid) in alumina refining include:

• Mud settling rate (cm/min)

• Overflow turbidity (NTU or mg/L solids)

• Mud density and underflow solids content

• Floc size and strength

• Chemical oxygen demand (COD) of filtrate

These parameters help quantify how effectively the flocculant is separating solids from the liquor, and guide process adjustments.

Comparison with Other Flocculants

Compared to conventional flocculants like polyacrylamide or synthetic anionic copolymers, poly(hydroxamic acid) provides:

• Better selectivity toward alumina and iron-containing solids

• Enhanced flocculation under high pH and temperature

• Stronger resistance to degradation in caustic environments

• Superior clarification in highly loaded mud systems

It is especially advantageous when dealing with bauxites that yield fine red mud, or when higher clarification standards are necessary.

Environmental and Safety Considerations

Poly(hydroxamic acid)-based flocculants are typically:

• Low in toxicity to operators and aquatic life when used appropriately

• Not bioaccumulative

• Designed to be used at minimal effective concentrations

• Biodegradable over time, depending on molecular structure

Proper handling procedures, including PPE and spill control, should be followed. Waste treatment systems should be evaluated for the presence of residual polymer.

Conclusion

The main application of anionic flocculant based on poly(hydroxamic acid) in aluminum plants is the settling and separation of red mud in the Bayer process. These flocculants serve a critical function in improving mud settling rate, enhancing supernatant clarity, and ensuring efficient liquor recovery under the high-temperature and caustic conditions characteristic of alumina production. Through complexation with metal oxides and strong flocculating action, they deliver superior performance compared to traditional polymers, optimizing the operational efficiency and sustainability of aluminum refineries. With proper application, they contribute significantly to reducing processing costs, minimizing environmental impact, and increasing alumina yield.