Pregnant leach solution (PLS) is the cyanide solution that has percolated through a heap or agitated leach circuit and is now "pregnant" with dissolved gold and silver. PLS is collected from the heap pad or leach tank and routed to the recovery plant (ADR or Merrill-Crowe) where the metals are extracted before the now-barren solution returns to the heap or leach.
Composition
Typical PLS: 200–500 ppm NaCN free cyanide, pH 10.5–11, dissolved Au at 0.5–10 ppm depending on ore grade and leach age, dissolved Ag at 1–100+ ppm depending on ore character, and dissolved base metals (Cu, Zn, Fe) varying by deposit. Solution density: 1.00–1.03 g/mL.
Heap Leach Flow
Solution irrigated to the top of the heap percolates down through ore for hours to days, dissolving Au and Ag along the way. The PLS is collected at the impermeable pad base into a lined PLS pond. From there it is pumped to the ADR or Merrill-Crowe plant. After recovery, the "barren" solution returns to the heap for re-irrigation.
Why Pond Size Matters
The PLS pond is both a buffer (smoothing flow rate variation between irrigation and recovery) and an upset-volume reservoir (e.g. holding solution during a plant shutdown). Pond design is part of the heap-leach pad geotechnical and water-balance engineering.
Solution Management
Through the project life, PLS volume and tenor change — generally rising tenor at early heap age and declining as more easily-leached gold is recovered. KCA engineering accounts for tenor variability when sizing the recovery plant.
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