Poly Aluminium Chloride

Properties

Density: 1.36 (20℃) Vapor pressure: 0.001 Pa at 20℃ Solubility: Soluble in water Form: Glassy solid Color: Transparent solid Aqueous solubility: Soluble in water, forms slightly turbid colloidal solution Stability: Stable. Incompatible with many metals. CAS No.: 1327-41-9 EINECS No.: 248-670-5 Molecular formula: [Al₂(OH)ₙCl₆₋ₙ·xH₂O]ₘ (m≤10, n=1～5)

Introduction

Poly Aluminium Chloride (PAC) is an inorganic polymer coagulant. It is an inorganic polymer compound, a hydrolysate between aluminium chloride and aluminium hydroxide. Through the bridging effect of hydroxyl ions and polymerization of multivalent anions, it forms an inorganic polymer water treatment agent with relatively high molecular weight and charge.

It can be divided into solid and liquid forms. Solid products vary in color including brown, yellow and white. Different colored PAC differs greatly in application and production technology.

Product Performance

a. PAC water treatment agent has strong adaptability to various water quality, and shows remarkable coagulation and sedimentation effect for high-turbidity water. b. The treated water quality is better than traditional inorganic coagulants such as aluminium sulfate, and the water purification cost is 15-30% lower. Flocs form rapidly and settle fast. c. High alumina content and low dosage help reduce water treatment cost. d. Coagulation can be achieved within raw water pH range of 5.0-9.0. e. Low corrosivity and friendly operating conditions; better solubility than aluminium sulfate. f. Less salt remains in treated water, beneficial for ion exchange and pure water preparation. g. Better adaptability to raw water temperature compared with aluminium sulfate, works well for low-temperature water, minimum precipitation temperature -18℃.

Application

It can remove odor, turbidity, oil, heavy metal salts in water and is widely used in water purification.

1. Purification of domestic drinking water and domestic sewage.
2. Purification of industrial water, industrial wastewater, mine water, oilfield reinjection water, metallurgy, coal washing, leather and various chemical wastewater treatment.
3. Industrial applications: paper sizing, printing and dyeing, cement setting accelerator, hardener for precision casting, binder for refractory materials, glycerin refining, fabric wrinkle resistance and other industries; wastewater can be recycled.
4. Applied in oil-water separation with good performance.

Color Grades

1. White PAC High-purity water purification material, widely used in food, drinking water, urban water supply, precision manufacturing water purification, paper-making, pharmacy, sugar refining, cosmetic additives and fine chemical industries. It features very high purity with relatively high price.
2. Light yellow PAC Mid-to-high grade product. Mainly used for drinking water treatment, with strict limits on heavy metal content. The alumina (Al₂O₃) content is about 30 (±0.5). Fine powder, uniform particles, good flocculation effect, stable and efficient purification, low dosage and low cost. It is widely adopted by water plants for drinking water treatment.
3. Golden yellow PAC Most widely used PAC on the market, high-efficiency flocculant for wastewater treatment. Suitable for industrial water supply, industrial wastewater, industrial water circulation and municipal sewage treatment.
4. Brown PAC Custom water treatment agent with higher iron content. Effective for low-temperature, low-turbidity and high-algae wastewater. Used for drinking water, urban water supply and industrial water purification.

Classification

PAC is generally divided into three grades: food grade, drinking water grade and industrial grade, with obvious differences in indicators during application.

• Food grade PAC (white powder, spray-dried): Al₂O₃ content above 30%. Used for drinking water treatment, pharmaceutical, cosmetic additives and high-tech research.
• Drinking water grade PAC (yellow/golden irregular particles, spray-dried): Al₂O₃ content 26%~30%. Widely used in paper mills, printing & dyeing plants, tanneries, power plants and municipal sewage treatment plants. Available specifications: 26%, 28%, 30%.
• Industrial grade PAC (brown/yellow particles, roller dried): Excellent performance in industrial wastewater treatment. Al₂O₃ content ranges from 22% to 30%.

Coagulant Description

Coagulants can promote aggregation and flocculation of colloidal particles in water, accelerate the formation of coarse particles for fast sedimentation or easier filtration. It includes coagulating agents, flocculants and coagulant aids. Common coagulants include alum, PAC, activated silica, polyacrylamide, bauxite, ferrous sulfate and ferric chloride. Combined use of PAC and alum delivers good performance.

PAC is a new inorganic polymer coagulant. Different from traditional low-molecular crystalline salt coagulants, PAC has a structure composed of multivariate hydroxyl complexes with variable forms. It features fast flocculation and sedimentation, wide applicable pH range, low corrosion to pipeline equipment and obvious water purification effect. It can effectively remove chroma, SS, COD, BOD and heavy metal ions such as arsenic and mercury in water. Widely applied in drinking water, industrial water and wastewater treatment.

Main Quality Indicators

Three core quality indicators of PAC: basicity, pH value and alumina content.

1. Basicity (Salt basicity) Refers to the hydroxylation degree of hydroxyl forms in PAC, expressed as percentage of molar ratio B=[OH]/[Al]. It is one of the most important indicators closely related to flocculation performance. Within raw water turbidity 86~10000 mg/L, the optimal basicity of PAC is 40~85%. Many characteristics of PAC are related to basicity.
2. pH value Represents free OH⁻ quantity in PAC solution. Generally pH rises with higher basicity, while there is no fixed corresponding relation for different liquid compositions. For liquid with same basicity, pH varies with concentration; pH decreases as concentration increases.
3. Alumina content Measurement of effective component. Higher relative density usually means higher alumina content. The viscosity of PAC increases with alumina content. Under same conditions and alumina concentration, PAC has lower viscosity than aluminium sulfate, facilitating transportation and application.

Application as Refractory Binder

PAC can be used as binder for refractory materials. Produced from aluminium-containing raw materials or metallic aluminium through acid dissolution, hydrolysis and polymerization. PAC solution shows acidity. Also named aluminium hydroxychloride or basic aluminium chloride, with general formula [Al₂(OH)ₙCl₆₋ₙ]ₘ.

When used as binder for bulk refractory materials, PAC will not reduce refractoriness. During heating, PAC dehydrates and decomposes to generate highly dispersed active alumina which benefits sintering. It is suitable for unburned or fired refractory products, refractory plastics, ramming mixes and castables.

For unshaped refractory materials, suitable basicity range is 46%～72% and density range is 1.17～1.23 kg/m³ for good bonding strength. When used for refractory castables, synthetic magnesium aluminate spinel, fused MgO and bauxite cement can be used as setting accelerator.

Since PAC solution is acidic (pH<5), it may react with iron-containing compounds in refractory materials and release hydrogen gas, leading to swelling. A material aging step is required in production process to avoid swelling and cracking of finished molded products.

Production Methods

1. Production from aluminium hydroxide via acid dissolution High-purity aluminium hydroxide is adopted, with low heavy metal content. Aluminium hydroxide has poor acid solubility, so heating and pressurization in reactor are required for leaching. After separation and maturation, PAC liquid is obtained. Aluminium leaching rate can reach over 90%. The process is relatively simple, while the basicity of the product is relatively low at about 40%. To improve basicity without introducing heavy metal impurities, aluminium scrap and sodium aluminate can be added to low-basicity products, raising basicity above 70%.
2. Production from aluminium chloride via boiling pyrolysis Solid PAC can be prepared by boiling pyrolysis of crystalline aluminium chloride. Crystalline aluminium chloride is thermally decomposed at 170℃ to release hydrogen chloride. Then water is added for maturation polymerization, solidification, drying and crushing to get solid PAC. This method is rarely used now.

Alternative route: under heating condition, crystalline aluminium chloride solution reacts with calcium aluminate to get PAC with basicity 80%~90%. 1 ton of crystalline aluminium chloride can produce about 5 tons of PAC (10% Al₂O₃ mass fraction).

Usage Method

Direct adding solid PAC into sewage will make suspended substances flocculate and precipitate rapidly, causing equipment blockage. Solid dosage is 2-3 times of aqueous PAC solution, which increases operation cost. Correct operation steps:

1. Conduct lab test first to select optimal dosage with good flocculation effect. Prepare solution before use. Dosage can be confirmed according to water quality for non-drinking water treatment.
2. For solid PAC: dissolve with water to prepare 10-25% solution first, then dilute to target concentration. Add solid slowly into water under continuous stirring during dissolution.
3. Do not mix PAC with other brands or types of water treatment chemicals, and store separately from other chemicals.
4. Stock solution and diluted solution have slight corrosivity, lower than other inorganic flocculants.
5. Shelf life: liquid PAC for 6 months, solid PAC for 2 years. Solid PAC can still be used after moisture absorption.
6. The treated water can meet domestic drinking water sanitary standards with proper dosage.

Chemical Properties

Colorless or yellow resinous solid. Its aqueous solution is colorless or yellowish-brown transparent liquid, sometimes grey-black mucus due to impurities. Soluble in water.