Pregnant leach solution
- Abbreviation
- PLS
- Also known as
- PLS, pregnant solution
Pregnant leach solution (PLS) is the metal-bearing solution that drains from a heap leach or other leaching process after it has dissolved metal from the ore. It is collected and sent for metal recovery, such as solvent extraction and electrowinning for copper.
What is pregnant leach solution?
In heap leaching, leach solution is applied to the top of a pile of ore and percolates down through it, dissolving the target metal. By the time it drains from the base of the heap it is “pregnant” with metal. This pregnant leach solution, or PLS, flows to collection ponds and then to a recovery plant.
After the metal is removed, the depleted solution, called barren solution or, in copper circuits, raffinate, is adjusted and recycled back to the heap.
What PLS contains
- The target metal, such as copper in sulfuric acid solution or gold as a cyanide complex. Copper PLS typically carries a few grams of copper per liter.
- Impurities, such as iron, aluminum, magnesium and other elements dissolved from the gangue.
- Suspended solids, fine particles washed from the heap.
- Residual reagents and, in copper operations, acid.
How metal is recovered from PLS
- Copper is usually recovered by solvent extraction and electrowinning: an organic extractant selectively takes copper from the PLS and transfers it to a clean electrolyte, from which copper is plated as cathode.
- Gold and silver are recovered by adsorption on activated carbon or by zinc precipitation.
Why PLS quality matters
The metal concentration in PLS reflects how well the heap is leaching; low or falling grades can signal channeling, poor solution distribution or slow leach kinetics. Solids and some surface-active substances in PLS can cause problems downstream, such as stable emulsions and “crud” in solvent extraction, so reagents used on the heap must be compatible with the recovery circuit.
Frequently asked questions
Because it is “pregnant” with dissolved metal after passing through the ore, in contrast to the barren solution that remains after the metal is recovered.
In copper solvent extraction, raffinate is the aqueous solution left after copper has been extracted from the PLS. It is returned to the heap as leach solution.
Sources
- SME Mineral Processing and Extractive Metallurgy Handbook, Society for Mining, Metallurgy and Exploration, 2019
- Copper solvent extraction and electrowinning references, U.S. Geological Survey and industry literature