Polyaluminum Chloride: Reading Basicity, Colour and Grade Before Signing the Order

Polyaluminum chloride is the default coagulant in a very large share of the world's water and wastewater plants, and that ubiquity is exactly why it gets bought badly. Two products with the same name can differ in basicity, in iron content, in insoluble matter and in the raw materials they came from, and those differences show up as dose, sludge volume and residual metal in the finished water. Understanding three variables - basicity, colour and synthesis route - is enough to make a defensible purchase.

Basicity: the number that quietly drives cost

Basicity - the degree to which the aluminium has been pre-hydrolysed before it ever reaches your tank - is the single most important specification on a drinking-water-grade product. Lower basicity generally means a higher price, which is why buyers are tempted to shop on price alone and then pay for it in dose.

The economics run the other way once you look at total cost. Raising basicity from 65% to 92% has been reported to cut production raw-material cost by 20% and use cost by 40%. That is not a marginal difference; it is the difference between a coagulant programme that scales and one that eats the operating budget. The catch is that basicity is set by raw material and process, so a manufacturer has to tune it deliberately and a buyer has to specify it. Different feedstocks and different processes land on different basicity values, and the two-step route - aluminium hydroxide heating-pressurising acid dissolution followed by calcium aluminate ore powder neutralisation - exists precisely because the single-step version comes out too low.

Colour tells you the feedstock

Solid polyaluminum chloride comes as yellow, pale yellow or white powder or granule, and the shade is not cosmetic. Colour tracks the iron-ion impurities in the product: higher purity, lighter colour.

White material is high-purity iron-free, sometimes sold as food grade. Beyond water treatment it goes into paper sizing, sugar decolourisation and clarification, tanning, pharmaceuticals, cosmetics and precision casting. Yellow material is made from calcium aluminate powder, hydrochloric acid and bauxite, and serves sewage and drinking-water duty; when the destination is drinking water the feed switches to aluminium hydroxide powder and hydrochloric acid with a little calcium aluminate powder, finished by plate-frame pressure filtration or spray drying. Brown material adds iron powder to that recipe. Liquid product is normally a pale-yellow to colourless transparent liquid at 10-20% concentration, with shade and strength varying by polymerisation degree and acidity.

Five routes in, five impurity profiles out

• Metallic aluminium method. Runs on aluminium scrap - chips, ash and slag. Ash is fed slowly into hydrochloric acid under agitation, then aged, polymerised and settled to a liquid product that is diluted, filtered, concentrated and dried. The acid variant uses HCl and is hard to hold on quality; the alkali variant is more difficult to run, needs more equipment and more alkali, and burns raw material on pH control; the neutralisation variant is the most widely used and meets national standards as long as the ratio is held.
• Aluminium hydroxide method. The feed is high-purity, so heavy metals and other toxic substances come out low. Heating-and-pressurising acid dissolution is simple and has little effect on water quality, but the product basicity is low, which is why the two-step version with calcium aluminate neutralisation is standard.
• Aluminium oxide method. Feeds on gibbsite, bauxite, kaolin or coal gangue, in two steps: crystallise aluminium chloride first, then reach polyaluminum chloride by thermal decomposition or neutralisation.
• Aluminium chloride method. The most widely used route of the five. Crystalline aluminium chloride is boiled and thermally decomposed at 170°C, then aged and polymerised with water, solidified and dried.
• Alkali dissolution method. Aluminium ash reacts with sodium hydroxide to sodium aluminate solution, then hydrochloric acid adjusts pH. Colour and appearance are good and insolubles are low, but sodium chloride content is high, raw-material consumption is high, alumina content in solution is low, and industrial production cost is significant.

How it actually destabilises a colloid

Four mechanisms are in play, and knowing which one is doing the work explains most dosing puzzles.

Double-layer compression. Counter-ion concentration is highest at the micelle surface and decays outward until it matches bulk solution. Add electrolyte and the diffuse layer thins; two approaching micelles then repel less, collide from a shorter distance, and the net force flips from repulsion to attraction. This is the mechanism behind harbour siltation - fresh water carrying clay colloids hits seawater, ionic strength jumps, and the clay drops out.

Adsorption charge neutralisation. The micelle surface adsorbs counter-ions, oppositely charged micelles, or the charged portion of a chain macromolecule, and that adsorption neutralises part of its charge. The classic demonstration compares Na with dodecylammonium ions (C12H25NH) in removing turbidity from a negatively charged silver iodide sol: the monovalent organic amine ion destabilises far more strongly than Na, and past a certain dose it re-stabilises the sol by flipping the surface from negative to positive. Na at any dose does not. Aluminium and iron salts re-stabilise and reverse charge at high dose for exactly the same reason.

Adsorption bridging. Linear polymer flocculants carry groups that react with the micelle surface; the rest of the molecule extends into solution and grabs a second particle. Too few particles and the extended tail folds back onto the original particle, so nothing bridges. Too much polymer and every surface saturates, and the suspension re-stabilises. Bridged flocs also break under prolonged intense stirring - the bridge pulls off and curls back, and the colloid returns.

Sweep flocculation. At high enough dose the metal hydroxide precipitates - Al(OH)3, Fe(OH)3, Mg(OH)2 - and physically enwebs the colloids as it forms. In neutral and acidic pH, where those precipitates are positively charged, sulfate in the water speeds settling. Notably, the optimum dose is inversely proportional to the concentration of material to be removed: more particles, less metal coagulant needed.

Dosing practice that holds up

Dissolve the solid at 1:3 with water, then dilute 10-30 times with clean water to working concentration. The best dosing pH is 3.5-5.0; hitting that window matters more than most operators expect, because it is where coagulation benefit peaks. For raw water at 100-500 mg/L turbidity, 10-20 kg per 1,000 tonnes is the usual band; some references put the figure at 5-10 kg per 1,000 tonnes for the same turbidity range, which is a good illustration of why a small trial against your own water beats any table.

The jar test sequence is worth doing properly. For the flocculation stage, stir at 150 rpm for about 6 minutes, then 60 rpm for about 4 minutes, with total floc growth time held to 10-15 min - enough turbulence to grow and coarsen floc, not enough to shear it. For settling, drop to 20-30 rpm for 5 minutes then let it stand 10 minutes before reading residual turbidity. In the plant, inclined-tube or inclined-plate settlers do this better than a plain tank, and dissolved-air flotation is better still for light floc.

For rural or small systems, roughly 1 g per 50 kg of water in a settling vat works, with the supernatant drawn off after standing. If PAC is paired with a high-molecular flocculant, dissolve anionic or cationic polyacrylamide with it as a composite, or add PAC first to build floc and then add anionic polyacrylamide for bridging. In domestic and production sewage, about 30 g of PAC per tonne followed by diluted polyacrylamide is a common starting point.

Liquid or solid

Liquid needs no make-down, loads easily and is cheaper per unit of active matter, but it has to move by tank truck, and each tonne of solid is equivalent to 2-3 tonnes of liquid. That makes liquid attractive mainly within about 100 km of the supplier. Solid is the dried form, ships without tankers, and costs labour on the front end because it has to be dissolved before use.

Where PAC is the wrong call

Three limitations are worth stating plainly. PAC has adsorption, coagulation and precipitation behaviour but is poor in stability and is corrosive - skin contact needs immediate water flushing. Its settling performance is far behind polyacrylamide, so where settling is the constraint rather than charge neutralisation, a polymer is the better tool. And for high-colour, high-COD, high-BOD raw water it usually needs an auxiliary agent to reach an acceptable result.

Used with those boundaries in mind, PAC remains the most cost-effective broad-spectrum coagulant available: effective at low dose across a wide pH range, fast to hydrolyse, strong in adsorption, producing large dense floc that settles quickly and leaves low effluent turbidity, and compatible with spray drying that keeps the product stable and safe for drinking-water duty.