Plating Sludge at 22 Percent Solids: Dose by Rinse Water, Not Cake Weight

Eight kilograms of powder per tonne of cake sounds generous until you work out that a tonne of cake is 780 kg of water and only 220 kg of hydroxide floc. That was the dose a zinc-nickel plating line was running on 3.5 tonnes a day of filter-press cake at 22 percent solids, and the paint filter test kept failing at four drops in five minutes.

Metal hydroxide sludge is not a uniform solid with water sitting on top of it. It is a loose floc with a huge internal surface, and most of that 78 percent moisture sits inside the floc structure rather than in the free voids. A polyacrylate can only take water it can reach. Dosing by cake weight hides that distinction, and it is the single most common reason a plating shop overpays for polymer and still fails the test.

Separate free water from floc water first

Run a simple centrifugation on a representative sample: 15 minutes at 3,000 g, then read the volume of supernatant. That number, not the total moisture, is what the polymer has to handle. On hydroxide sludges it typically lands at 45 to 60 percent of the total water, so a cake at 22 percent solids behaves like a slurry at 30 to 35 percent solids for dosing purposes.

The working rule that comes out of that is 8 to 15 kg of powder per tonne of free water, which for this shop meant about 5 kg per tonne of cake rather than 8. Half the polymer, a passing test, and a cake that still loads into a skip without sticking. If you dose on total moisture instead, you end up with a cake that looks dry on the surface and releases liquid under the compactor.

Divalent metals cut swelling, and the lab test hides it

Zinc, nickel, copper and calcium all sit in the filtrate at hundreds of milligrams per litre, and divalent cations crosslink the carboxylate groups on the particle surface. The result is a stiff skin that stops the core from expanding. Free swell measured in tap water can overstate the field capacity by 40 to 60 percent on this kind of liquor, which is exactly the gap that turns a bench-approved recipe into a failed truck load.

So test in the filtrate. Take two litres of the liquid that comes off the press, not tap water, and run the absorption test in it for 30 minutes and again at 24 hours. The 30-minute number sizes the dose; the 24-hour number tells you whether the gel will keep re-absorbing leachate in the skip overnight. If the two differ by more than 15 percent, the gel is still creeping and you are better off with a slightly higher dose and a co-binder.

Two acceptance tests, one failure mode

The paint filter test (EPA 9095B, or the local equivalent) is the gate for disposal. The second test is a drop from one metre onto concrete: a properly conditioned cake breaks into lumps, a bad one splashes. Both are cheap and both catch the same underlying problem, which is free liquid that the polymer never reached.

Overdosing has its own signature, and it caught this shop once. A night shift doubled the dose to clear a backlog, and the cake turned tacky. It stuck to the truck bed, had to be scraped out with a shovel, and the leachate reading went up rather than down because the excess powder re-dispersed in the compaction water and carried fines with it. More polymer is not a safety margin. It is a different problem.

For hydroxide sludges with a high metal load, a coarse granule in the 0.5 to 1 mm range disperses better in a paddle mixer than a fine powder, which tends to form skins before the core hydrates. Guidance on picking that cut sits alongside the general polyacrylate granule specifications. Put the free-water figure, the filtrate absorption at 30 minutes, and the paint filter result in your next disposal trial sheet, and the dose stops being a guess.