Concentrate grade
- Also known as
- concentrate, con grade
Concentrate grade is the concentration of the valuable metal or mineral in the concentrate produced by a mineral processing plant, usually expressed as a percentage by weight, such as percent copper. Higher grade means less gangue travels with each metric ton of metal.
What is concentrate grade?
A mineral processing plant takes in ore with a low content of valuable mineral and produces a concentrate with a much higher content. Concentrate grade is that content, most often stated as the percentage of the target metal by weight: percent copper in a copper concentrate, percent zinc in a zinc concentrate, grams per metric ton for precious metals, or percent iron in an iron ore concentrate.
Grade measures purity. The more gangue that reports to the concentrate, the lower its grade.
Grade depends on mineralogy
The highest possible grade is limited by the mineral itself. Chalcopyrite, the most common copper mineral, contains about 35 percent copper by weight, so even a perfectly clean chalcopyrite concentrate cannot exceed that. Concentrates of richer minerals, such as chalcocite, can reach higher grades. In practice, grades are lower than the theoretical limit because some gangue and other sulfides always accompany the valuable mineral.
Why concentrate grade matters
- Payment terms. Smelters pay for the metal contained in a concentrate, minus deductions and treatment charges. Lower grade means more metric tons to treat for the same metal.
- Penalties. Gangue and certain elements, such as arsenic, bismuth, mercury, silica or magnesium, attract penalties or can make a concentrate hard to sell.
- Transport. Every metric ton of gangue in the concentrate is shipped, often over long distances, at full cost.
- Downstream processing. Cleaner concentrates are easier and cheaper to smelt or refine.
Grade and recovery
Grade is always judged alongside metallurgical recovery, the share of the valuable mineral captured in the concentrate. The two tend to move in opposite directions: pushing to recover more of the valuable mineral pulls in more gangue and lowers grade, while cleaning the concentrate harder loses some valuable mineral to the tailings. The relationship is shown by the grade-recovery curve. Plants choose an operating point on that curve, and the real gain from better chemistry or equipment is moving the whole curve, getting higher grade at the same recovery or the reverse.
What lowers concentrate grade
- Entrainment of fine gangue carried into the froth with water.
- Poor mineral liberation, so particles containing both valuable mineral and gangue float together.
- Poor flotation selectivity, where gangue minerals take up collector and float.
- Slimes and clays that coat particles or are carried into the froth.
Cleaner flotation stages, froth washing, regrinding, depressants and reagent changes are the usual levers for raising grade.
Concentrate grade and Locus biosurfactants
Locus Mining describes its biosurfactant-based flotation reagents as improving recovery without sacrificing concentrate grade, mainly by improving selectivity and reducing entrainment.
Frequently asked questions
It varies with mineralogy. Concentrates made mostly of chalcopyrite are limited by that mineral’s copper content of about 35 percent and are typically well below it; concentrates of richer copper minerals can be higher.
Ore grade is the metal content of the rock fed to the plant. Concentrate grade is the metal content of the product that leaves it. Processing raises grade by removing gangue.
By removing more gangue: additional cleaning stages, regrinding to improve liberation, reducing entrainment, and using depressants and selective reagents so that less gangue floats.
Sources
- Wills' Mineral Processing Technology, 8th edition, B. A. Wills and J. A. Finch, Butterworth-Heinemann, 2016
- SME Mineral Processing and Extractive Metallurgy Handbook, Society for Mining, Metallurgy and Exploration, 2019