Grade-recovery curve
- Also known as
- grade-recovery trade-off
A grade-recovery curve is a plot of concentrate grade against metallurgical recovery for a mineral separation process. It shows the trade-off between the two: recovering more of the valuable mineral usually means accepting a lower grade.
What is a grade-recovery curve?
A separation process such as froth flotation can be run to produce a small amount of very clean concentrate or a larger amount of less clean concentrate. The grade-recovery curve captures that range of possible outcomes. It plots concentrate grade on one axis and metallurgical recovery on the other; the curve typically slopes downward, because pushing recovery higher pulls in more gangue and lowers grade.
How the curve is built
- Timed flotation tests. In a laboratory flotation test, concentrate is collected in several increments over time. The earliest froth is richest; later increments add recovery at lower grade. Plotting cumulative grade against cumulative recovery gives the curve.
- Plant data. Operating data across different conditions can be plotted the same way.
- Release analysis, a specialized test procedure, estimates the best curve achievable for a given ore.
How the curve is used
- Choosing an operating point. A plant chooses where to operate on the curve based on metal prices, smelter terms and costs. Higher grade may be worth giving up some recovery, or the reverse.
- Comparing reagents and conditions. A new reagent, grind size or circuit change is judged by whether it moves the curve up and to the right: higher grade at the same recovery, or higher recovery at the same grade. Comparing single grade and recovery numbers without the curve can be misleading, because they may simply reflect different points on the same curve.
Frequently asked questions
Because the easiest-floating particles are usually the richest. Recovering the harder-to-float valuable particles, often fine or poorly liberated, also brings more gangue into the concentrate.
It means the process now achieves better combinations of grade and recovery than before, through improvements in flotation selectivity, liberation or reduced entrainment, rather than simply trading one for the other.
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