Introduction
PAC (Poly Aluminium Chloride) is manufactured through two main drying processes: spray drying and drum drying. The production method significantly affects product quality, dissolution speed, impurity levels, and price. Understanding these differences helps buyers make informed procurement decisions and avoid overpaying for specifications they don’t need — or under-specifying for critical applications.
Manufacturing Process Overview
Spray Drying Process
- Liquid PAC solution (after reaction and polymerization) is pumped to the top of a spray tower
- The solution is atomized into fine droplets through high-pressure nozzles or a rotary atomizer
- Hot air (200-300°C) is blown counter-current or co-current, instantly evaporating water
- Fine, hollow spherical powder particles fall to the collection cone
- Product is sieved and packaged
Key characteristic: Extremely rapid drying (seconds) at controlled temperature preserves the polymer structure. The hollow sphere morphology creates excellent dissolution properties.
Drum Drying Process
- Liquid PAC solution is fed onto the surface of a rotating, steam-heated drum (140-180°C)
- A thin layer of PAC dries on the drum surface over 30-60 seconds
- A scraper blade removes the dried PAC as flakes or irregular particles
- Flakes are crushed and sieved to desired particle size
Key characteristic: Longer contact time with hot metal surface can degrade some polymer species. The flake morphology requires crushing, creating irregular particles with slower dissolution.
Detailed Comparison
| Parameter | Spray Dried PAC | Drum Dried PAC |
|---|---|---|
| Particle morphology | Hollow microspheres (0.1-0.5mm) | Irregular crushed flakes (0.5-2mm) |
| Bulk density | 0.55-0.70 g/cm3 | 0.65-0.85 g/cm3 |
| Dissolution time (1% solution, 25°C) | 3-8 minutes (fast, complete) | 15-45 minutes (slower, may leave residue) |
| Water insolubles | 0.1-0.5% (very low) | 0.5-2.0% (higher) |
| Iron (Fe) content | <50-100 mg/kg (low) | 100-500 mg/kg (higher — drum wear) |
| Heavy metal (As, Pb, Cd, Cr) | Very low — meets drinking water standards | Variable — depends on raw material and equipment |
| Basicity preservation | Excellent — rapid drying preserves polymer species | Moderate — some polymer degradation on hot drum surface |
| Product uniformity (batch to batch) | Excellent — continuous, controlled process | Moderate — batch variation more common |
| Appearance | Fine, free-flowing, light yellow to white powder | Granular/flake, darker yellow to brown |
| Hygroscopicity (moisture absorption) | Moderate — hollow spheres have less surface area | Higher — irregular particles absorb moisture faster |
| Shelf life (sealed, cool, dry) | 12-24 months | 6-12 months |
| Unit price (relative) | Higher (15-30% premium) | Lower (base price) |
When Spray Dried PAC Is Worth the Premium
- Drinking water treatment: Low heavy metals (<50 ppb As, <100 ppb Pb) and low iron are non-negotiable for WHO/NSF compliance. Spray dried PAC is the standard for potable water.
- Automatic dosing systems: Fast, complete dissolution prevents clogging of dosing pumps, filters, and inline mixers. Spray dried PAC’s 3-8 minute dissolution is essential for automated plants.
- High-purity applications: Electronics manufacturing, pharmaceutical wastewater, food/beverage processing — low insolubles and metals matter.
- Cold water treatment (<5°C): Drum dried PAC dissolution slows dramatically in cold water; spray dried PAC is less affected due to higher surface area and porosity.
- Where consistency matters: Large municipal plants treating 100,000+ m3/day — batch-to-batch variation in drum dried PAC creates operational headaches. Spray dried consistency simplifies plant operation.
When Drum Dried PAC Is Sufficient
- Industrial wastewater (non-potable): Steel, mining, construction — heavy metal traces in the PAC are negligible compared to the wastewater being treated.
- Manual batch treatment: Small ETPs where operators manually prepare solutions with mixing tanks — the longer dissolution time is not a constraint.
- Cost-sensitive projects: 15-30% price difference matters for bulk consumption (500+ tons/year). If drum dried meets treatment targets, the savings are real.
- Applications with sedimentation after PAC dosing: Any undissolved PAC settles with the sludge and doesn’t affect treated water quality.
How to Verify Drying Method (Without Visiting the Factory)
- Visual inspection: Spray dried PAC is fine, uniform powder (like flour). Drum dried PAC has visible irregular granules/flakes.
- Dissolution test: Add 10g to 1000mL water, stir at 200 rpm. Spray dried dissolves completely in <10 minutes leaving clear to slightly hazy solution. Drum dried takes 20-40+ minutes, may leave visible sediment.
- Bulk density measurement: Weigh 100mL of PAC powder. Spray dried: 55-70g. Drum dried: 65-85g.
- Water insolubles test: Dissolve 10g in 1000mL, filter through 0.45um membrane, dry and weigh residue. Spray dried: <0.05g. Drum dried: 0.05-0.2g.
- Iron content (quick field test): Dissolve 1g in 100mL, add a few drops of potassium thiocyanate solution. Red color indicates iron. Compare with known standards. Spray dried is typically colorless to faint pink.
Economic Analysis — Cost per m3 Treated
Scenario: Industrial wastewater ETP treating 500 m3/day.
- Drum dried PAC: $250/ton, dosage 300 mg/L = 150 kg/day = $37.50/day
- Spray dried PAC: $320/ton (28% premium), dosage 250 mg/L = 125 kg/day = $40.00/day
Spray dried costs only $2.50/day more due to 17% lower dosage requirement. For drinking water plants where dosage is lower (10-30 mg/L), the price difference per m3 is negligible. Factor in reduced downtime from clogged dosing lines, consistent jar test results, and fewer operator adjustments — spray dried often wins on total cost of ownership.
HydroChemix supplies both spray dried and drum dried PAC with transparent quality specifications. Contact jingshuicc@gmail.com to discuss your application requirements and request samples of both types for side-by-side evaluation.