Heap Leaching for Gold: A Low-Cost Route for Low-Grade Ore
Key takeaways
- Heap leaching trades recovery for cost. Expect lower recovery than a tank leach, but far lower capital and operating cost per tonne.
- Permeability is everything. The heap must let solution percolate evenly; clay and fines destroy that.
- Agglomeration solves sticky ore. Binding fines to coarse particles keeps the heap permeable.
- It suits low-grade, oxidized, free-milling ore. Sulphide or refractory ore will not leach well in a heap.
- Containment is non-negotiable. A lined pad and closed solution circuit are the basis of the environmental permit.
Heap leaching is the reason many low-grade gold deposits are economic at all. Instead of grinding ore and leaching it in tanks, you stack it on a lined pad and trickle cyanide solution through it over weeks or months. The trade is straightforward: you accept lower recovery in exchange for a fraction of the capital and operating cost.
How heap leaching works
Crushed ore is stacked on an impermeable lined pad to form a heap. A drip irrigation system applies dilute cyanide solution to the top, and the solution percolates down through the heap by gravity, dissolving gold as it travels. Pregnant solution — the gold-bearing liquor — drains from the base of the heap into a collection pond, and the gold is then recovered by adsorption onto activated carbon or by zinc precipitation.
The barren solution left after recovery is recharged with cyanide and returned to the top of the heap. The circuit is closed, which is essential both for reagent economy and for environmental control.
What makes ore suitable
| Ore characteristic | Suits heap leach? | Reason |
|---|---|---|
| Low grade, large tonnage | Yes | Low cost per tonne is the whole point |
| Oxidized, free-milling | Yes | Gold dissolves readily, no pre-treatment needed |
| Coarse, competent rock | Yes | Good permeability and stable heap structure |
| High clay content | Only with agglomeration | Clay blocks percolation and channels the solution |
| Sulphide or refractory gold | No | Gold is locked or the ore consumes cyanide |
| Very fine gold | Poor | Fine gold dissolves slowly and may not be reached by solution |
Permeability and agglomeration
The heap depends on solution moving evenly through the whole mass of ore. If some zones are more permeable than others, solution takes the easy path and bypasses much of the ore entirely, and no amount of extra leaching time will recover the gold in those bypassed zones.
Clay and fines are the usual culprits. The standard fix is agglomeration: mixing the ore with cement or lime and water so that fine particles stick to coarser ones, producing a more uniform, permeable heap. Agglomeration also helps stabilize the heap physically and can raise pH to reduce cyanide loss.
Heap leaching compared with tank leaching
| Factor | Heap leach | Agitated tank leach (CIL/CIP) |
|---|---|---|
| Capital cost | Low | High |
| Operating cost per tonne | Low | Higher |
| Typical recovery | 50–80% | 90%+ on free-milling ore |
| Leach time | Weeks to months | Hours |
| Grind size required | Coarse crush only | Fine grind |
| Suits low grade? | Yes | Only if grade is high enough |
| Suits refractory ore? | No | Only after pre-treatment |
The comparison comes down to grade. Because heap leaching costs so much less per tonne, it can make a 0.5 g/t deposit viable — but it will recover only part of the gold. Where the grade is high enough to justify grinding and tanks, a CIL circuit will recover far more gold, and the extra revenue usually outweighs the extra cost.
Design and operating essentials
A heap leach facility rests on a few things being done properly. The pad must be lined with an impermeable membrane and engineered to collect every drop of solution, with a leak detection layer beneath. The heap is built in lifts, and the application rate of solution is controlled so that it percolates rather than ponds on the surface.
Aeration matters more than many operators expect. The dissolution reaction needs oxygen, and the base of a deep heap can become oxygen-starved, which is one reason recovery from deeper lifts is often lower. Irrigation scheduling — alternating application and rest periods — helps draw air into the heap and improves recovery of the lower lifts.
Environmental control
Heap leaching is closely regulated because cyanide solution sits on an open pad for months. The controls are well established: a lined and contained pad, a closed solution circuit with no discharge, a cyanide detoxification step before any water leaves the site, and a closure plan that accounts for rinsing and stabilizing the heap at the end of its life.
These are permit conditions in most jurisdictions rather than optional extras. Planning for them at the design stage is far cheaper than retrofitting containment later — and the same closed circuit that satisfies the regulator also saves reagent, since cyanide is recovered and reused rather than lost.
Rule of thumb: heap leaching is a low-cost, low-recovery route. Choose it when the deposit is too low-grade to grind, not when you want to save money on a deposit that would pay for tanks.
Frequently asked questions
What recovery can heap leaching achieve?
Typically 50% to 80% for gold, depending on ore type, crush size and heap height. Coarse, well-oxidized, free-milling ore performs best; clay-rich or partially refractory ore performs poorly and may not be suitable at all.
Why is heap leach recovery lower than tank leaching?
Because the solution does not contact every particle. Percolation follows the most permeable paths, leaving some ore bypassed. Tank leaching agitates the pulp so every particle meets the solution, which is why it recovers more gold.
What is ore agglomeration and why is it used?
Agglomeration mixes ore with cement or lime and water so fine particles bind to coarser ones. It is used to restore permeability in clay-rich or fine ore, and it also helps stabilize the heap and control pH.
How long does heap leaching take?
Weeks to months, compared with hours in a tank leach. The long contact time is part of why heap leaching is cheap — no grinding, no agitation, no tanks — but it also means the working capital tied up in a heap is significant.
Is heap leaching environmentally safe?
It can be, when properly designed and operated. The essential controls are a lined containment pad, a closed solution circuit, cyanide detoxification before discharge and a closure plan. These are standard permit conditions in most jurisdictions.
Considering heap leaching for a low-grade deposit? Send us your ore characteristics and we will assess whether it fits.
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