Precipitation: pH control, hydroxides and carbonates

Precipitation is the workhorse of hydromet purification, and the lever it runs on is pH. Most metals are soluble in acid and insoluble as their hydroxides above some pH, so raising the pH of a solution drops metals out of it as solid hydroxides — and because different metals turn insoluble at different pH values, controlling the pH lets you reject one metal while leaving another in solution. This is the one calculation-bearing landing of the module, and the mechanism a crosser will meet more often than any other.

Hydroxides drop at characteristic pH

Each metal hydroxide has a pH at which it begins to precipitate and a slightly higher one by which precipitation is essentially complete, both set by its solubility product. Ferric iron comes down at a low pH, around 3 to 4; aluminium a little higher; the divalent base metals — copper, nickel, cobalt, zinc — come down higher still, spread through the 5 to 9 range; magnesium and manganese higher again. Raise the pH in stages and the metals precipitate in sequence, the low-pH ones first. That is the whole of selective precipitation: staged neutralisation that drops iron and aluminium at a pH low enough to leave the valuable base metal in solution, then either recovers that metal at a higher pH or sends the cleaned liquor forward. The separation is only as sharp as the pH windows are apart, and co-precipitation — the dropping hydroxide carrying some value down with it — sets the practical limit.

Carbonates as the alternative

Some metals are recovered as carbonates rather than hydroxides, by adding a soluble carbonate — soda ash — instead of raising the pH with a hydroxide. Carbonate precipitation often gives a denser, more filterable solid and a product that is itself saleable: nickel and cobalt carbonates as intermediates, lithium carbonate as a final product. The choice between a hydroxide route and a carbonate route turns on the product form wanted and on the filterability and purity each delivers, which is why a precipitation circuit is designed around the product specification rather than the cheapest drop.

The reagent set: lime, caustic, soda ash

The reagent that raises the pH is chosen on cost, on the precipitate it gives, and on the by-product salt it leaves behind. Lime — calcium hydroxide, dosed as milk of lime — is the cheapest base by far and the default for bulk neutralisation and iron removal, at the cost of forming gypsum and a bulky, lower-grade solid. Caustic soda — sodium hydroxide — is dearer but cleaner, used where a high-grade precipitate or a tight pH is wanted. Soda ash — sodium carbonate — supplies the carbonate for carbonate precipitation. The lime make-up, how much lime and water to prepare a milk-of-lime slurry at a target strength, is a committed mass balance the lime-slurry calculator below runs; the dose itself, in reagent per tonne, is what the consumption calculator turns into a rate. The worked thread takes the committed lime batch end to end.