Slurry Dilution Explained

Definition

Slurry dilution is adding water to a slurry to lower its percent solids by mass — for example to drop a thickener feed to a flocculation-optimum concentration, to thin a slurry for pumping or sampling, or to hit a target density downstream. The defining feature of a slurry dilution calculation is that it is done on a dry-solids basis: the mass of dry solids does not change when you add water, so the whole calculation pivots on how much water accompanies each tonne of solids at the initial and target percent solids. A slurry at percent solids by mass Cw carries liquid mass = dry solids × (1/Cw − 1). The dilution water you must add is simply the extra water needed to go from the initial water content to the (larger) target water content: ΔW = dry solids × (1/Cw,target − 1/Cw,initial). Because the solids are constant, lowering the percent solids always increases the water, which is why the target must be lower than the initial concentration for dilution to make physical sense.

Why it matters

Slurry dilution looks like ordinary dilution but it is not the same as the generic liquid C1V1 = C2V2 mixing you would use for a dissolved reagent. In a slurry the 'concentration' is percent solids by mass and the conserved quantity is the dry solids, not a dissolved species in a volume — so the calculation runs through the dry-solids mass balance, not a volume-ratio. Getting that basis right matters because the added-water number sizes dilution-water supply, feedwell or launder capacity, and the resulting slurry flow that downstream pumps and thickeners must handle, and because mixing the two bases (mass percent vs a volume dilution ratio) gives badly wrong water figures. It also matters to know what the balance deliberately does not tell you: it assumes water density and ideal mixing, and it says nothing about whether the diluted slurry will settle, stay in suspension, pump cleanly, or behave the way the downstream thickener needs. Those depend on rheology, particle size, and settling behaviour from testwork — the dilution sum only gets the water and the resulting flow right.

Formula

Units involved

• •ṁ_solids — dry solids flow in t/h (or batch mass in t)
• •Cw,initial, Cw,target — percent solids by mass as a fraction (target < initial)
• •ṁ_slurry — slurry mass flow in t/h
• •ΔW — dilution water in t/h (≈ m³/h at ~1000 kg/m³)
• •ρ_liquid — liquid (water) density in kg/m³

Concept diagram

Worked example

Common mistakes

• •Using a generic liquid dilution (C1V1 = C2V2) instead of the dry-solids mass balance.
• •Setting a target percent solids that is not lower than the initial — dilution then has no meaning.
• •Confusing percent solids by mass with percent solids by volume.
• •Treating the added-water number as proof the diluted slurry will pump or settle correctly.
• •Ignoring the larger resulting slurry flow that downstream equipment must handle.

When to use the calculator

Use the slurry dilution water calculator to find the water to add to take a slurry from an initial to a lower target percent solids on a dry-solids basis, and the thickener feed dilution calculator when that dilution is specifically a thickener or clarifier feed being brought to a flocculation target. Pair them with the slurry density and percent solids (mass ↔ volume) calculators to convert between density, percent solids, and the two bases. For the settling and pumping behaviour of the diluted slurry, use site testwork and vendor data.

FAQ