Gangue

What is gangue?

Ore is rock that contains enough valuable mineral to be mined and processed at a profit, but most of it is not the valuable mineral. The rest is gangue: the minerals and rock that have no commercial value in that operation and must be separated and discarded. In a copper ore, for example, the copper sulfide minerals may make up only a small percentage of the rock, and the quartz, feldspar, micas, carbonates and iron sulfides around them are gangue.

Gangue is relative

Whether a mineral counts as gangue depends on the operation. Pyrite is gangue in most copper and zinc plants, but in some gold operations it carries gold and is recovered. A mineral discarded in one era can become valuable later as prices or technologies change, which is one reason old tailings are sometimes reprocessed.

Common gangue minerals

• Silicates: quartz, feldspars, micas and clays.
• Carbonates: calcite and dolomite.
• Iron sulfides: pyrite and pyrrhotite, in many base metal ores.
• Iron oxides, in some ores.

Clays are a particular problem, because they break into very fine particles that raise slurry viscosity, coat valuable minerals, and are easily carried into concentrates.

Why gangue matters in processing

Separating gangue efficiently is the purpose of mineral processing. It matters for several reasons:

• Concentrate quality. Gangue in the concentrate lowers concentrate grade and can bring smelter penalties for elements such as arsenic or silica.
• Cost. Every metric ton of gangue that travels with the concentrate adds transport and smelting costs.
• Tailings. Almost all the gangue ends up as tailings, which must be dewatered and stored safely.
• Declining ore grades. As ore grades fall, plants process more gangue for each unit of metal, and separating finely intergrown minerals becomes harder.

How gangue is separated

The first step is grinding to free the valuable mineral from the gangue, called mineral liberation. Separation then exploits differences in properties:

• Surface chemistry, in froth flotation. Reagents make the valuable mineral float and leave the gangue behind, often with depressants to keep gangue from floating.
• Density, in gravity separation.
• Magnetic properties, in magnetic separation.
• Chemical dissolution, in leaching processes such as heap leaching, where the metal is dissolved and the gangue stays as solid residue.

Gangue that is not separated by attachment but carried with water into the flotation froth, called entrainment, is a major source of concentrate contamination.

Gangue and Locus biosurfactants

Locus Mining describes its biosurfactant-based flotation reagents as improving selectivity, separating value from gangue, and reducing gangue entrainment in flotation concentrates.

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