Understanding the gold heap leaching process

In gold ore beneficiation technology, heap leaching is widely employed due to its simplicity, low energy consumption, minimal equipment requirements, and reduced infrastructure investment and production costs. It is particularly suited for processing low-grade gold ores such as oxidised ores, by-product ores extracted during mine development, or surface-rejected ores.

In production, the typical heap leaching process for gold ore comprises six major stages: ore preparation, leach pad construction, heap formation, leach solution preparation, solution distribution and collection, and processing of the leachate.

01 Ore Preparation

The ore preparation stage primarily involves crushing, pelletisation, and pre-treatment operations. These aim to enhance ore leachability and heap permeability, prevent uneven fluid flow, blockages, and channeling, thereby improving gold recovery rates and reducing leachate consumption. Crushing employs different processes based on ore characteristics and technical requirements.

Pelletisation is necessary for fine ore and ore with high clay content. A specified quantity (3–6 kg/t) of cement or lime binder is added to the ore, supplemented with an appropriate amount of sodium cyanide solution. The ore is maintained at 8–12% moisture content and cured for 8–12 hours to form agglomerates. Pelletisation improves heap permeability, ensures adequate oxygen supply, and facilitates pre-cyanidation of gold, significantly enhancing gold leaching rates.

02 Leach Pad Construction

To collect leachate and prevent solution leakage, the heap site requires base preparation and lining prior to construction. Primary materials for leach pad construction include clay, sand, gravel, asphalt, reinforced concrete, plastic sheeting, or rubber-plastic sheets. Longitudinal and transverse drainage channels are laid with lump ore atop the pad layer. Finally, a layer of coarse ore is spread across the entire base to protect the underlying pad.

03 Heap Construction

The purpose of heap construction is to ensure the ore pile possesses good, uniform permeability while guaranteeing slope stability. Pile height primarily depends on permeability characteristics, leaching agent concentration in the solution, and the equipment and methods employed. Typically, pile heights range from 3 to 9 metres, though heaps for heap leaching of raw ore can reach up to 46 metres. Construction methods include multiple-heap, multi-layer, and slope techniques. Most heap leaching plants for gold ore employ lorries, front-end loaders, and bulldozers for transporting and stacking ore, whilst promptly loosening the heap to prevent mechanical compaction.

04 Leach Solution Preparation

Water and leaching agents are blended according to specific formulations to create solutions or leach liquids for extracting valuable components from gold ore. The composition and dosage of the leach solution must be determined based on the ore type, mineral composition, and chemical properties. Alkaline sodium cyanide solutions are commonly employed as leach liquids in heap leaching of gold ore.

05 Solution Distribution and Collection

Liquid distribution involves uniformly spraying the leaching solution onto the surface of the ore heap. The distribution system comprises a solution preparation tank, pumps, delivery pipes, and a network of spray pipes and nozzles laid across the heap. Taking spray distribution as an example, the pipe network uses high-strength polyethylene plastic pipes, while the nozzles employ rotating oscillating types. These offer a large spray radius, produce large, uniform droplets that resist atomisation, and are easy to install.

During spraying, two objectives must be met: firstly, ensuring uniform distribution of the leachate across the ore heap; secondly, achieving the required spray intensity, defined as the volume of liquid applied per unit area within a given timeframe.

06 Enriched Liquid Processing

Gold extraction employs carbon adsorption followed by desorption electrolysis (or zinc powder displacement precipitation). Activated carbon becomes gold-bearing carbon by adsorbing gold from the pulp. After adsorption, the gold-bearing pulp is conveyed via an air lift to a carbon recovery screen for separation. Following screening and washing, the pulp proceeds to desorption electrolysis. Under high-temperature and high-pressure conditions, anions readily adsorbed by activated carbon are introduced into the desorption system to displace Au(CN)₂⁻, yielding gold slime and depleted carbon, thereby achieving gold desorption. The precious liquid recovered from desorbed gold-bearing carbon undergoes electrowinning to obtain solid gold.

These constitute the six primary stages of conventional heap leaching for gold ore processing. Occasionally, heap leaching may be combined with carbon-in-pulp (CIP), carbon-in-pulp (CIP) or zinc displacement methods. This involves directing high-grade gold ore to cyanidation plants for processing, whilst employing heap leaching for low-grade ore, surface waste rock or tailings.