The units of the trade

A hydromet circuit speaks four dialects of "how much", and a great deal of avoidable error comes from quoting a number in one when the listener assumes another. The four are weight percent (wt%), grams per litre (g/L), kilograms per tonne (kg/t), and percent solids. They measure different things, and the conversions between them are not optional bookkeeping — they are where order-of-magnitude mistakes hide.

The four units, one at a time

Weight percent is a mass fraction: the mass of a component divided by the mass of the whole, times a hundred. It is how reagents are sold and stored — concentrated sulfuric acid is quoted as a wt%, caustic comes as a wt% solution — because it is a property of the material that does not depend on temperature or how much you have. It is the natural unit for strength.

Grams per litre is a mass concentration: the mass of a component per litre of solution. It is how a process circuit actually meters its solutions, because plants flow volumes — a pregnant liquor is "so many grams per litre of metal", a tankhouse electrolyte is g/L of acid and metal.

The catch is the one slip that matters most: g/L and wt% are not interchangeable without the solution density. To go from wt% to g/L you multiply by the density: g/L = wt% × density(in kg/m³) ÷ 100. A solution that is 20% by mass is not 200 g/L unless it happens to weigh 1000 kg/m³, which strong solutions never do. Carry the density or carry the error.

Kilograms per tonne is a dosing and consumption unit: kilograms of reagent per tonne of dry solids treated. It is how the circuit’s reagent economics are expressed, because consumption scales with ore, not with solution volume. A lime addition of 2 kg/t on a thousand-tonne-a-day plant is a different conversation from 20 kg/t, and the line item is the difference. It is the natural unit for opex.

Percent solids is the slurry unit: the mass of dry solids as a fraction of the total slurry mass (by mass — the usual) or as a fraction of slurry volume (by volume — sometimes). The by-mass and by-volume figures are very different numbers for the same slurry, because the solids are denser than the water, so a given mass of solid takes up little volume. Quoting "30% solids" without saying which basis is a classic ambiguity, and Module 2 is built around resolving it.

Crossing between them

The conversions are simple arithmetic once you respect what each unit divides by. wt% divides by total mass; g/L divides by solution volume and so needs a density to cross to wt%; kg/t divides by solids mass; percent solids divides by slurry mass or volume. The mistakes are almost never the algebra — they are forgetting the density bridge between mass-basis and volume-basis units, assuming a litre of solution weighs a kilogram, or mixing a by-mass and a by-volume figure in the same balance. This page lands you on the two calculators that keep that arithmetic honest, and the rest of Module 1 builds each unit out in turn.