Water balance: where water enters and leaves

A hydrometallurgical plant is, among other things, a water-handling plant. Water carries the chemistry, the solids and the heat, and a plant that cannot account for its water cannot account for anything dissolved or suspended in it. The water balance is the bookkeeping that says where every stream of water comes from and goes to — and whether, over a steady period, what comes in equals what goes out. When it does not, the circuit either floods or runs dry, and both stop the plant.

Where water enters

Water comes into the circuit at several points. Leach make-up and reagent solutions bring it; wash water added on the solid–liquid separation circuit brings a great deal of it; gland service, flocculant make-up and reagent dilution add more; and in an open circuit, rainfall and run-off onto the residue facility can be a large and uncontrolled input. Each of these is metered or estimated, and together they are the supply side of the ledger.

Where water leaves

Water leaves in fewer but larger streams. It departs as moisture in the product and, far more, as moisture in the residue or tailings — the liquor the washed solids carry away, which is also a soluble-value loss as the washing module showed. It leaves by evaporation, from ponds, thickeners and hot streams, a loss that can dominate in an arid climate. And it leaves through a deliberate bleed — a stream taken out of a recycle loop to stop a dissolved impurity building up without limit. The bleed is the relief valve of the water and impurity balance together.

Why it closes, or accumulates

A circuit recycles water aggressively, because fresh water costs money and every litre recycled is a litre not bought and not discharged: thickener overflow returns as process water, filtrate returns to leach, pond water is reused. But recycling concentrates whatever does not leave with the water — dissolved salts, fine solids, heat — so a closed loop without a bleed accumulates them until something fails. The art of the water balance is closing it tightly enough to minimise fresh-water draw while bleeding enough to keep the recycled streams usable. The balance is built from the same single-stream mass balances the slurry module taught, summed over the plant: the slurry mass balance splits each stream into solids and liquor, the thickener water-recovery calculation says how much water a thickener returns to the loop, and the dilution relation sizes the make-up water a stream needs. The worked thread runs the thickener case, the single largest water-recovery step in most circuits.