Depressant: the chemical that sinks minerals

Outside the mining industry, a depressant is any substance that slows the central nervous system: alcohol, sedatives, tranquilizers. The term describes an effect on mood, alertness or physical energy. In mining and mineral processing, the word names something entirely different, though the metaphor of suppression still applies.

In flotation separation, a depressant is a chemical reagent that prevents certain minerals from floating to the froth surface of a flotation cell. During froth flotation, crushed ore is mixed with water and air bubbles. Collectors coat selected minerals to make them hydrophobic, causing them to attach to air bubbles and rise. A depressant reverses this: it keeps unwanted minerals at the bottom of the cell by strengthening their natural affinity for water, or by blocking collector molecules from binding to the mineral surface.

Depressants are highly specific. Lime, sodium silicate and starch are common depressants used to suppress iron oxides, feldspar or other gangue minerals while allowing copper, zinc or precious metals to float. The choice depends on the mineralogy of the ore. A single mill may use different depressants for different ore types or at different stages of the same separation circuit.

The word entered mining vocabulary through the same impulse that led to 'collector' and 'frother': mineral processing engineers borrowed chemical and pharmacological terminology to name reagents by their functional effect. A substance that depresses the natural tendency of a mineral to float is a depressant, just as a substance that depresses neural activity is a depressant. Both suppress what would otherwise occur.