Heap Leaching Gold: Process and Where It Fits

Heap leaching is the low-cost workhorse for gold ores that don't justify grinding and tank agitation. You'll see it on low-grade oxide and transitional deposits where the gold is fine but the rock is permeable. The process isn't complicated: stack the ore, irrigate it with a weak cyanide solution, and collect the loaded solution. If you're comparing options, start with gold recovery methods.

What Is Heap Leaching?

Heap leaching is a hydrometallurgical process in which ore is stacked on an impermeable liner and irrigated with a dilute cyanide solution. The solution percolates by gravity through the ore, dissolves gold, and drains to a collection system. It's a bulk leaching method, not a per-ton intensive milling route. You don't need fine grinding or slurry pumping for the whole tonnage—just enough preparation to keep the heap permeable and the solution flowing.

Most heap leach operations process crushed or run-of-mine ore placed in lifts. The pad beneath can be a single-use pad or a reusable pad with multiple lifts. A typical cycle runs for weeks to months, depending on ore mineralogy, crush size, and solution application rate. Xinhai includes heap leaching among its gold recovery routes, alongside gravity, flotation, and tank cyanidation.

The process has a low barrier in simple oxide terrain but can fail quickly if permeability is poor. That's why every heap leach project starts with column leach testwork, not pad drawings.

The Pad, Irrigation, and ADR Chain

Heap leaching has three physical subsystems: the pad, the irrigation network, and the ADR plant. ADR stands for adsorption, desorption, and recovery.

The pad starts with a compacted subgrade and a geomembrane liner, usually HDPE or LLDPE. On top of that sits an overliner drainage layer and a network of perforated pipes. All pregnant solution flows to a lined pond or directly into the ADR circuit. You can't skimp on liner welding and leak detection—cyanide solution is hazardous, and any leak becomes a reportable event.

The irrigation network uses drip emitters or wobblers to apply solution at rates that match the ore's permeability. You'll often see drip lines buried just below the surface to reduce evaporation. The solution percolates through the heap and collects in the pad drainage system.

In the ADR plant, pregnant solution passes through carbon columns where gold loads onto activated carbon. The barren solution returns to the heap. Loaded carbon is stripped with hot caustic cyanide solution, and the eluate is recovered by electrowinning or zinc precipitation. It's a closed loop, at least in design.

Solution chemistry matters. Heap leach solutions are typically much weaker than CIL tank solutions because contact time is long and ore grade low. Operators monitor pH and free cyanide concentration daily. Lime or caustic is added to keep pH high enough to avoid hydrogen cyanide gas. High copper or mercury in the ore can load carbon and complicate recovery, so testwork checks for those metals early.

Crush Size and Agglomeration: Why They Control Drainage

Crush size determines how fast solution moves through the heap. If you crush too fine, the heap seals and solution ponds on the surface. If you leave ore too coarse, gold trapped inside large particles won't dissolve before the cycle ends. Most oxide heaps target a coarse crush, far coarser than a CIL mill grind. Gold ore milling for tank leaching grinds to a fine slurry, which heap leaching cannot accept.

For ores with high clay or fines, agglomeration becomes essential. You mix crushed ore with cement or lime and water in a drum or on a belt. The cement binds fine particles to coarse ones, forming porous agglomerates. That keeps the heap permeable and prevents channelling. Without agglomeration, a clay-rich heap can turn into a pond within days. Xinhai's test programs evaluate crush size, agglomeration dosage, and column leach response before any pad design is frozen.

The pad and heap design, not the ADR plant, usually control throughput. You can add more carbon columns, but you can't easily fix a compacted heap. So testwork focuses on percolation rate and fines migration as much as gold recovery.

You'll follow a standard sequence: crush the ore to a target size, add binder, agglomerate in a rotating drum, cure the agglomerates, and then stack. Column tests simulate the heap with a pipe of ore and monitor pregnant solution grade over time. Xinhai's laboratory and pilot facilities run these tests before designing the full pad and ADR plant.

Ore Types and Grades That Favour Heaps Over Tank Leaching

Heap leaching suits low-grade oxide, saprolite, and some transitional ores. The target grade is typically lower than what a CIL or CIP plant would treat economically. Gold leaching options change with ore type.

Oxide ores don't consume much cyanide and dissolve gold quickly. Sulfide ores, especially refractory pyrite or arsenopyrite, usually don't leach well in a heap unless they are oxidized first or bio-oxidized. That's why heap leach pads often process weathered near-surface material, while fresh sulfide ore goes to flotation or whole-ore CIL.

Permeability is the second filter. A heap needs gravity flow through the full lift. If the ore is clay-rich, tight, or has a high fines content, you'll either need agglomeration or you'll reject heap leaching entirely. A competent oxide ore with good permeability and a grade too low for milling is the classic heap leach candidate.

Some projects use a hybrid: heap leach for near-surface oxide, then a CIL or flotation plant for deeper sulfide. That keeps the mill smaller and uses the oxide resource that would otherwise be waste.

Recovery and Cycle Time: Heap Leach vs CIL

Recovery expectations differ sharply. A heap leach typically recovers less of the contained gold than an agitated CIL circuit. The reasons are physical: large particles, uneven solution contact, and slow diffusion. Xinhai's internal data indicates CIL/CIP recovery can reach 99% on suitable ores, but that figure comes from fine grinding and full agitation. A heap leach on the same ore would fall well short, often by tens of percentage points. The trade-off is capital and operating cost per tonne stacked.

Cycle time also separates the two. CIL leaches much faster because the ore is milled to a pulp and mixed with carbon. A heap leach runs for weeks to months, sometimes longer for high lifts or cold climates. You'll need more working capital for the longer leach cycle, but you avoid the cost of milling and tailings disposal at pulp density.

You'll also need to manage gold inventory. In a heap, gold sits in the pile for weeks before it reports to the ADR plant. In a CIL circuit, gold moves through the tanks quickly. That inventory difference affects project finance and working capital.

When you evaluate a low-grade oxide, a heap leach is often the only route that clears the hurdle rate. For a higher-grade ore, the lost recovery and long cycle time make CIL the better fit. For a direct tank comparison, see CIL vs CIP. There isn't a universal answer—it's an economic comparison against the ore's leach response.

Containment and Cyanide Management Under the ICMI Code

Heap leaching uses cyanide at concentrations far below CIL tanks, but the volume and pad area create different risks. The liner, collection pond, and solution channels must be engineered to prevent releases. Everything sits inside a containment system. Groundwater monitoring wells around the pad detect any excursion early. You don't want cyanide migrating off-site; the regulatory and reputational cost is never worth the shortcut.

The International Cyanide Management Code is the industry benchmark for cyanide transport, storage, use, and decommissioning. It's a voluntary code, but many major lenders and governments require it for heap leach operations. The Code covers pad construction, liner integrity, emergency response, and worker safety. Signatory companies undergo third-party audits. Xinhai designs heap leach and other cyanide-bearing plants with these obligations in mind, including liner protection and solution management.

Decommissioning is part of the code. A heap after closure is rinsed to reduce residual cyanide and metals, then capped. You'll need financial assurance for that work from the start, not after the last ounce is poured. The USGS Mineral Commodity Summaries is a standard reference for gold supply data that lenders and owners use during feasibility work.

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