Percent solids and slurry density

Almost every stream in a hydromet circuit is a slurry — solids suspended in liquor — and two numbers describe it: how much of it is solids, and how heavy the mixture is. Percent solids and slurry density are not independent; given the solids density and the liquor density, each determines the other. Getting the relation right is the foundation of every mass balance and every pump and pipe decision downstream.

Percent solids: by mass or by volume

Percent solids is the fraction that is solid, and it comes in two flavours that must never be confused. Percent solids by mass is the dry-solids mass over the total slurry mass — the usual plant figure, the one a moisture or filtration measurement gives. Percent solids by volume is the solids volume over the slurry volume. For the same slurry these are very different numbers, because the solids are denser than the liquor: a given mass of dense mineral occupies little volume, so a slurry that is, say, 30% solids by mass can be only around 14% by volume. Quoting a percent solids without its basis is a standing ambiguity; the first question to ask of any such figure is "by mass or by volume?".

The house relation

Slurry density follows from the percent solids by mass and the two component densities through one relation, the same one the calculators on this site use: 1 ÷ ρ_slurry = X_s ÷ ρ_solids + (1 − X_s) ÷ ρ_liquor, where X_s is the solids mass fraction. It is just the statement that the slurry’s volume is the sum of the solids volume and the liquor volume, written per unit mass. Rearranged, it gives slurry density from mass fraction, or mass fraction from a measured slurry density — which is why an online density gauge can read out percent solids in real time. Solids specific gravity (SG) is the solids density relative to water, so an SG of 2.70 is 2700 kg/m³; liquor is often taken near 1000 kg/m³ unless it is a strong leach solution, in which case its own density belongs in the relation.

This relation is the spine that the rest of Module 2 and a good deal of the plant hangs on. The leach-tank-sizing work, for instance, derives its slurry density from exactly this equation before it sizes a single vessel — the worked thread below quotes that committed derivation. Once you can move between percent solids and slurry density with confidence, the slurry mass balance and the line-sizing that follow are bookkeeping on top of it.

The two calculators below are the working tools: one gives slurry density and solids volume fraction from the percent solids and the component densities; the other crosses between percent solids by mass and by volume. Both run the house relation, so the numbers they return are the numbers the rest of the path is built on.