Sulphide First, Polymer Second: Order Matters in Metal-Bearing Sludge

Zinc at 4,800 milligrams per kilogram, lead at 900, and a plant supervisor who wanted to know why the gelled material still failed the leachate test when the same recipe worked perfectly on the sump two hundred metres away. The gel was fine. It held water, it passed the free-liquid check, it stacked. What it also did was lock the metals into a matrix that kept releasing them, because the chemistry that binds metals had never been given a chance to happen.

Sequestration and gelation are two different jobs, and they compete for the same water. Get the order wrong and the second job prevents the first.

What Gelation Does to a Metal

A superabsorbent pulls liquid into a network and holds it there. That is excellent for stopping free liquid and useless for immobilising a dissolved metal ion, because the ion is still dissolved — just inside a gel instead of inside a puddle. If the surrounding matrix later shifts in pH, the metal comes back out.

Worse, the gel slows diffusion so effectively that any reagent added afterwards cannot reach the metal. A batch that is gelled and then dosed with a sulphide will show almost no reaction in the middle of the mass. The reagent sits on the surface, the metal stays where it was, and a composite sample taken from the outside will look like a success.

That is the failure mode to watch for: a batch that passes on a grab sample from the surface and fails on a core from the centre. If your sampling protocol does not include a core, you will not see it until a laboratory does.

The Sequence That Works

Treat the liquor as a chemical problem first and a physical one second.

• Screen and settle. Grit and refuse fines consume reagent and give false readings.
• Adjust pH to the range where the chosen precipitant works — typically 8.5 to 9.5 for sulphide precipitation, and it must be measured, not assumed.
• Add the precipitant with mixing, at 1.5 to 2.5 times the stoichiometric requirement for the sum of the metals present. Sludge is not a pure solution; competing ions take their share.
• Check the result before proceeding. A field ORP reading in the range of minus 50 to minus 150 millivolts is a reasonable indicator that reducing conditions have been reached; if it is still positive, the dose was short.
• Mix for a further 8 to 10 minutes to let the precipitate flocculate, then add the polymer.
• Gel, and note that a metal-laden batch typically needs 4 to 8 percent more polymer than the same liquor without the precipitate, because the solid particles compete for water and coat the granules.

The whole sequence adds roughly twenty minutes to a batch. On a 20 cubic metre batch that is twenty minutes of one operator’s time and one mixer, which is cheap against a re-worked load.

A Case Where the Order Was Right and the Mixing Was Wrong

A contractor did everything in the correct order but added the sulphide solution through a 25 millimetre hose into a 5 cubic metre tank with a paddle running at 40 rpm. Sulphide is dense and sinks; the paddle was an axial-flow type that pushed it to the wall and down. Half the reagent went to the bottom and stayed there. The ORP probe, mounted 400 millimetres below the surface, read fine. Four days later the core samples were 30 percent over the limit on nickel.

The fix was not more reagent. It was an eductor and a slower addition over six minutes, plus an ORP probe on a pole that could be lowered to three depths. Reagent consumption went down, and the result became consistent.

Testing the Result Honestly

The test your regulator accepts is almost certainly a leachate extraction on a subsample of the treated material rather than a total metals digest. That matters, because a total digest tells you what is in the material and the extraction tells you what will come out of it. Both have their place; only one of them answers the question at the gate.

Take the subsample from a core, not a scoop, and take it after the material has cured for the period your acceptance arrangement specifies. A gel that is two hours old and a gel that is seven days old are different materials, and the metals result can move in either direction as the matrix matures.

Retain a second portion in a sealed jar for the same period. If the two disagree, you have a sampling problem rather than a chemistry problem and the answer is a better core, not more sulphide.

Cost and Dose Reality

On a sludge at 12 percent solids with mixed metals, expect a sulphide or organosulphide dose in the range of 3 to 8 kilograms per cubic metre, and a polymer dose 4 to 8 percent above the unsequestered baseline. If the metals are mostly zinc and lead, the upper end; if it is a light contamination at a few hundred milligrams per kilogram, the lower.

The solidification sequence guidance covers the reagent order for metal-bearing streams alongside the gel dosing steps, which is the part most trial protocols leave out.