1. Introduction to Polyaluminum chloride
Polyaluminum chloride (PAC), also known as basic aluminum chloride, aluminum hydroxychloride, or polymeric aluminum chloride, represents one of the most significant inorganic polymer coagulants in modern water and wastewater treatment. Its general empirical formula is often written as [Al₂(OH)nCl₆–n]m, where m indicates the degree of polymerization and n represents the basicity, typically ranging from 1 to 5.
As a member of the inorganic polymer flocculant (IPF) family, PAC distinguishes itself from traditional aluminum-based salts such as aluminum sulfate (alum) and aluminum chloride due to its pre-hydrolyzed, polymeric structure. This gives it superior coagulation, adsorption, and charge-neutralization capabilities.
The introduction of PAC revolutionized water treatment technology by combining the strong hydrolysis and adsorption potential of aluminum salts with the controllability and stability of polymeric species. From a chemical engineering perspective, its production embodies controlled hydrolysis and polymerization reactions, integrating aspects of reaction kinetics, thermodynamics, pH control, and corrosion management.
This article presents a detailed exploration of the chemical nature, reaction mechanisms, manufacturing technologies, and applications of polyaluminum chloride, focusing on both its industrial chemistry and engineering aspects.
2. Chemical Composition and Structural Characteristics
2.1 Chemical Formula and Composition
The empirical formula of PAC is generally written as:
Aln(OH)mCl3n−m
where nnn represents the number of aluminum atoms in a polymer unit, and mmm represents the number of hydroxyl groups bound to aluminum.
PAC exists as a mixture of various aluminum species, including monomeric, oligomeric, and polymeric hydroxy complexes of aluminum. Common structural species include:
- [Al(H₂O)₆]³⁺ (monomeric)
- [Al₂(OH)₂]⁴⁺, [Al₃(OH)₄]⁵⁺ (dimeric and trimeric)
- Al₁₃O₄(OH)₂₄⁷⁺ (Keggin-type polycation)
- Polymeric chains and networks extending to large cluster structures.
These polycations, especially the Al₁₃ species, are responsible for PAC’s remarkable coagulation performance. The ratio of hydroxyl to aluminum (OH/Al) defines the basicity or degree of polymerization, a key parameter in determining the product’s performance.
2.2 Basicity and Its Significance
The basicity (B) is defined as:
B= [OH−]/3[Al3+]
and expressed as a percentage, where 0% represents pure aluminum chloride (AlCl₃) and 100% represents aluminum hydroxide (Al(OH)₃).
Commercial PAC products typically have basicity values between 30% and 85%:
- Low basicity (30–50%): Higher acidity, more effective at low pH.
- Medium basicity (50–70%): Balanced hydrolysis and charge neutralization.
- High basicity (70–85%): Enhanced floc formation and lower residual aluminum.
Adjusting basicity is central to PAC manufacturing and determines its target application (e.g., potable water, industrial wastewater, paper sizing, etc.).
2.3 Physical Properties
| Property | Description |
| Appearance | Yellow to light yellow solid or solution |
| Molecular weight (approx.) | Variable, dependent on polymerization |
| Solubility | Highly soluble in water |
| Density (liquid, 10–18% Al₂O₃) | 1.2–1.4 g/cm³ |
| pH (1% solution) | 3.5 – 5.0 |
| Odor | Odorless |
| Stability | Stable under acidic to neutral conditions; hydrolyzes in alkaline media |
| Hygroscopicity | Highly hygroscopic in solid form |
The physical form can be liquid (aqueous solution) or solid (powder, granular, or flake). Liquid PAC is preferred for large-scale water treatment operations, whereas solid PAC is suitable for transportation and storage in remote applications.
2.4 Hydrolysis Behavior and Chemical Reactivity
PAC hydrolyzes in water to produce hydroxoaluminum complexes and polymeric species, releasing protons and influencing solution pH:
Aln(OH)mCl3n−m+(3n−m)H2O→nAl(OH)3+(3n−m)HCl
During hydrolysis, species such as Al₁₃O₄(OH)₂₄⁷⁺ and Al₆(OH)₁₅³⁺ are formed, which are strong coagulants due to their high positive charge density. These species act through charge neutralization and sweep flocculation mechanisms, making PAC superior to traditional aluminum sulfate.
PAC also reacts with alkaline compounds to form amorphous Al(OH)₃ precipitates, which can adsorb colloidal and organic impurities from water.
3. Industrial Production Processes
3.1 Overview
Industrial production of PAC involves controlled hydrolysis and polymerization of aluminum salts under specific pH, temperature, and reaction conditions. The most commonly used raw materials are:
- Aluminum hydroxide (Al(OH)₃) or alumina hydrate
- Aluminum metal (Al)
- Aluminum chloride (AlCl₃) or hydrochloric acid (HCl)
- Bauxite or kaolin (as aluminum sources in some regions)
The process design is determined by the desired basicity, purity, and product form (liquid or solid).
3.2 Typical Reaction Routes
Route 1: Reaction of Aluminum Hydroxide with Hydrochloric Acid
This is the most common industrial method.
Al(OH)3+HCl→AlCl3+H2O
Followed by controlled hydrolysis and polymerization:
nAlCl3+mH2O→Aln(OH)mCl3n−m+mHCl Process steps: Preparation of Slurry:
Aluminum hydroxide is dispersed in deionized or softened water to form a uniform slurry.Acid Addition:
Controlled addition of hydrochloric acid at 90–100°C initiates dissolution.Hydrolysis-Polymerization:
The reaction mixture is aged or partially neutralized to form polymeric aluminum species.Adjustment of Basicity:
The OH/Al ratio is tuned by adding more Al(OH)₃ or HCl.Filtration and Concentration:
The product is filtered to remove insolubles and concentrated to the desired Al₂O₃ content (typically 10–18% for liquid PAC).Drying (optional):
For solid PAC, spray drying or drum drying is employed. This route is simple and yields high-purity PAC with adjustable basicity.
Route 2: Reaction of Aluminum Metal with Hydrochloric Acid
A more reactive process uses aluminum metal directly:
2Al+6HCl→2AlCl3+3H2 AlCl3+H2O→Al(OH)Cl2+HCl nAlCl3+mH2O→Aln(OH)mCl3n−m Here, aluminum metal dissolves in acid, generating hydrogen gas. Hydrolysis and polymerization proceed concurrently. Engineering considerations: Gas management: Hydrogen must be vented safely with explosion-proof systems.Heat control: The dissolution is highly exothermic, requiring cooling jackets or reflux condensers.Feedstock purity: Impurities in aluminum metal affect product color and performance. This process produces PAC with medium basicity (40–60%) and is often used in integrated aluminum plants utilizing scrap metal.
Route 3: Co-precipitation of Aluminum Sulfate and Chloride
In this process, aluminum sulfate (Al₂(SO₄)₃) is neutralized with an alkaline reagent (such as aluminum hydroxide or sodium aluminate) in the presence of chloride ions to yield mixed polyaluminum hydroxychlorosulfate coagulants.
Al2(SO4)3+AlCl3+H2O→Aln(OH)mCl3n−m−x(SO4)x This hybrid route enhances floc density and is useful for high-turbidity water treatment.
3.3 Solid PAC Production
Solid PAC is typically produced from liquid feedstock by spray drying, vacuum evaporation, or fluidized bed drying.
- Spray drying yields fine yellow powder with high solubility.
- Drum drying produces flake PAC with excellent storage stability.
- Granular PAC is made via agglomeration and pelletization for dust-free handling.
The solid content (as Al₂O₃) usually ranges from 28–31%, with moisture below 1%.
3.4 Process Control Parameters
From an engineering viewpoint, the following parameters are critical:
| Parameter | Typical Range | Impact |
| Reaction temperature | 85–105°C | Controls hydrolysis rate |
| pH | 3.5–4.2 | Determines basicity and polymerization |
| Reaction time | 1–5 hours | Affects Al₁₃ species formation |
| HCl concentration | 10–20% | Regulates acidity |
| Al(OH)₃ particle size | <100 µm | Ensures uniform dissolution |
Continuous stirred-tank reactors (CSTRs) or batch reactors with acid-resistant linings (PTFE, FRP, or glass-lined steel) are commonly used. Inline pH and temperature sensors allow tight process control.
4. Product Types and Quality Specifications
PAC products are marketed in various grades and formulations according to basicity, Al₂O₃ concentration, and purity.
| Type | Basicity (%) | Form | Typical Al₂O₃ Content (%) | Main Applications |
| Low-basicity PAC | 30–50 | Liquid | 10–12 | Industrial wastewater, oily water |
| Medium-basicity PAC | 50–70 | Liquid/Solid | 12–18 | Municipal drinking water |
| High-basicity PAC | 70–85 | Solid | 28–31 | Potable water, paper sizing |
| Ultra-pure PAC | 70–85 | Solid/Liquid | ≥30 | Electronic and semiconductor water |
Typical analysis for a commercial PAC (liquid):
- Al₂O₃: 10–18%
- Basicity: 60–75%
- pH (1%): 3.5–5.0
- Insoluble matter: <0.3%
- Fe: <0.01%
- Heavy metals (Pb, As, Cd, Cr): Trace (<0.002%)
5. Mechanism of Action in Coagulation
5.1 Hydrolysis and Charge Neutralization
When PAC is added to water, it undergoes hydrolysis to form Al³⁺ species that neutralize negatively charged colloids and organic particles:
Al3++3H2O⇌Al(OH)3+3H+
This charge neutralization destabilizes colloids, leading to floc formation.
5.2 Adsorption and Bridging
The polymeric aluminum species adsorb onto particles and create bridges between them, forming large, rapidly settling flocs. This mechanism is enhanced in high-basicity PAC with abundant Al₁₃ polycations.
5.3 Sweep Flocculation
At higher doses, amorphous aluminum hydroxide precipitates act as a “sweep” to enmesh suspended solids, effectively clarifying turbid water.
6. Industrial Applications
6.1 Municipal Drinking Water Treatment
PAC is one of the most widely used coagulants in drinking water treatment due to its high efficiency, minimal residual aluminum, and wide effective pH range (4–9).
Advantages over alum (Al₂(SO₄)₃):
- Faster floc formation and sedimentation.
- Less pH depression in treated water.
- Lower dosage required (30–50% less).
- Reduced sludge volume and improved dewaterability.
It effectively removes:
- Suspended solids and turbidity.
- Natural organic matter (NOM).
- Phosphates and color.
- Pathogens by adsorption and sedimentation.
6.2 Wastewater Treatment
PAC is extensively used in industrial wastewater treatment for effluents containing dyes, heavy metals, oil, and suspended solids.
Key sectors include:
- Textiles and dyeing: Removes color and COD by adsorbing dye molecules.
- Food processing: Clarifies effluent and removes organic matter.
- Chemical and petrochemical plants: Reduces oil emulsions and colloids.
- Mining and metallurgical wastewater: Precipitates heavy metals (Cu, Zn, Pb, Cr).
PAC can also be combined with organic polymers (e.g., polyacrylamides) to enhance floc size and sedimentation rates.
6.3 Paper Industry
PAC is a crucial component in paper sizing and retention systems, replacing traditional aluminum sulfate.
Functions include:
- Improving fiber and filler retention.
- Enhancing paper strength and printability.
- Stabilizing pH in neutral/alkaline papermaking.
- Reducing resin and pitch problems.
Its polymeric nature allows stronger bonding with cellulose fibers compared to alum.
6.4 Oil and Gas Sector
In the oil industry, PAC is applied in:
- Produced water treatment: Removes oil-in-water emulsions.
- Drilling mud conditioning: Acts as a flocculant to separate solids.
- Refinery wastewater: Helps remove residual hydrocarbons and heavy metals.
Its high charge density facilitates demulsification and improves water quality for reinjection or discharge.
6.5 Textile and Dyeing Processes
PAC effectively decolorizes dyeing wastewater by breaking chromophore structures and adsorbing dye molecules. Compared to iron-based coagulants, PAC produces lighter-colored sludge, easing disposal.
6.6 Cosmetic and Pharmaceutical Uses
High-purity PAC derivatives, especially aluminum chlorohydrate (ACH), are used in antiperspirant formulations and pharmaceutical applications as astringents. ACH is essentially a refined form of PAC with very high basicity (80–90%) and purity.
6.7 Construction and Cement Additives
PAC can be used as a setting accelerator in cement formulations, improving early strength and water reduction. Its acidic character also enhances dispersion in concrete additives.
7. Environmental, Safety, and Handling Aspects
7.1 Safety and Toxicology
PAC is considered non-toxic and environmentally benign when handled properly. However, it is acidic and may cause mild irritation upon contact.
Safety parameters:
- Corrosive to metals and skin (pH < 4).
- Avoid inhalation of dust or mist.
- Use PPE (gloves, goggles, acid-resistant clothing).
LD₅₀ (oral, rat): >5000 mg/kg (low toxicity).
7.2 Storage and Transport
- Liquid PAC: Store in lined polyethylene or FRP tanks at ambient temperature; avoid freezing.
- Solid PAC: Store in dry, ventilated conditions; hygroscopic materials should be sealed.
- Shelf life: 6–12 months depending on formulation.
- Classified as non-hazardous for transport (no UN number required for most grades).
7.3 Environmental Benefits
PAC contributes significantly to environmental protection:
- Reduces organic and nutrient load in effluents.
- Produces lower sludge volumes, reducing disposal burden.
- Does not introduce sulfates (unlike alum), minimizing corrosion in distribution systems.
Neutralized aluminum hydroxide sludge can be safely landfilled or reused in cementitious materials.
8. Advances in Manufacturing Technology
The evolution of PAC production reflects trends toward sustainability and process intensification. Major developments include:
- Continuous Production Systems:
Transition from batch to continuous stirred-tank reactors (CSTRs) for uniform product quality. - Membrane Separation Purification:
Removes colloidal impurities and improves color and stability. - Green Feedstocks:
Utilization of recycled aluminum or bauxite residues for cost and waste reduction. - pH-Controlled Hydrolysis Reactors:
Employ automatic titration systems to maintain stable polymerization conditions. - Spray Drying Optimization:
Improves solid PAC solubility and flowability using controlled inlet temperatures and atomization techniques. - Composite Coagulants:
Formulation of PAC with polyacrylamide, silicate, or ferric salts for synergistic coagulation effects (e.g., PAFC, PASiC).
9. Economic and Market Considerations
PAC is a globally traded commodity with rising demand driven by:
- Increasing environmental regulations for clean water.
- Urbanization and industrial expansion.
- Shift away from alum due to superior performance.
Cost drivers:
- Aluminum raw materials (hydroxide or metal).
- Hydrochloric acid prices.
- Energy consumption (especially for drying).
- Waste acid recovery systems.
The market is dominated by Asia-Pacific, particularly China and India, with Europe and North America following in municipal and industrial applications.
10. Process Engineering Challenges
Chemical engineers face several technical challenges in PAC production and utilization:
- Corrosion Control:
High chloride content requires acid-resistant reactors and piping. - Reaction Heat Management:
Hydrolysis and acid dissolution are exothermic; proper cooling prevents decomposition. - Basicity Adjustment Precision:
Small pH deviations cause drastic changes in polymer structure and performance. - Hydrogen Safety (in Al metal route):
Proper venting and gas monitoring systems are mandatory. - Quality Consistency:
Online monitoring of basicity and Al³⁺ concentration ensures product uniformity.
11. Comparison with Other Coagulants
| Coagulant | Principal Ion | pH Range | Coagulation Efficiency | Sludge Volume | Key Features |
| Aluminum sulfate (Alum) | Al³⁺ | 5–7 | Moderate | High | Low cost, pH sensitive |
| Ferric chloride | Fe³⁺ | 4–9 | High | Moderate | Effective for phosphates |
| Polyaluminum chloride (PAC) | Al³⁺ (polymeric) | 4–9 | Very high | Low | Fast floc formation, low residual Al |
| Polyferric sulfate | Fe³⁺ (polymeric) | 4–9 | Very high | Moderate | Strong oxidation ability |
PAC generally provides the best balance between cost, efficiency, and sludge management, making it the preferred choice for modern coagulation systems.