Heap leaching is a large-scale industrial technique designed to recover valuable metals from low-grade ore bodies that would otherwise be uneconomical to process using traditional methods. This process, a form of hydrometallurgy, involves dissolving the target metal using a chemical solution, known as a lixiviant, applied to large piles of crushed rock. The solution percolates through the ore, chemically separating the desired mineral. A metal-rich liquid is collected at the base of the pile, ready for final recovery.
The Engineering Steps of the Leaching Process
The physical process begins with ore preparation, typically crushing it to increase the surface area available for chemical reaction. Particle size is often around 5 millimeters. For ores with many fine particles, agglomeration is performed by mixing the fines with a binding agent, such as Portland cement, and a small amount of lixiviant. This forms larger, porous pellets that improve the ore pile’s permeability, ensuring the solution flows evenly without clogging.
The foundation of the operation is a specially constructed, impermeable leaching pad designed to prevent the chemical solution from contaminating soil or groundwater. The pad is built with a slight slope and lined with a high-density polyethylene (HDPE) geomembrane or compacted clay to create a secure containment barrier. The prepared ore is then stacked onto this liner, forming the massive heap structure, which can sometimes reach heights of hundreds of feet.
Once the ore is stacked, an irrigation system distributes the lixiviant solution evenly over the surface. Drip irrigation systems are preferred over sprinklers because they minimize solution loss due to evaporation and provide uniform distribution. The solution slowly percolates down through the ore, dissolving the metal. This process can take anywhere from a few months for simple oxide ores to several years for more complex materials.
The metal-laden liquid, referred to as the pregnant leach solution (PLS), drains to the base of the heap. It is collected by a network of pipes and trenches situated above the impermeable liner. The PLS is directed into a pregnant solution pond, which acts as a reservoir before being pumped to a processing plant for metal extraction. The entire physical process is a closed-loop system; the solution, once stripped of metal, is often recycled back to the heap after its chemical composition is adjusted.
Chemical Principles of Metal Extraction
The core of heap leaching is the chemical dissolution of the metal from the ore, which is carried out by a specialized lixiviant solution. The choice of lixiviant depends on the specific metal targeted and the ore’s mineralogy. For gold and silver extraction, a dilute, alkaline solution of sodium cyanide is typically used. This solution is maintained at a high pH (9.5 to 11) to prevent the formation of highly toxic hydrogen cyanide gas.
For copper extraction from oxide ores, a highly acidic solution, most commonly sulfuric acid, is used as the lixiviant. The acid reacts with the copper minerals, converting the solid metal compound into a soluble copper sulfate salt. This process selectively isolates the metal using water-based chemistry.
The chemical mechanism involves the lixiviant forming a stable, soluble complex with the target metal ion. During gold cyanidation, the gold metal is oxidized. The resulting gold ion reacts with cyanide to form the stable, water-soluble dicyanoaurate complex, which is carried away by the solution. This selectively releases the valuable metal from the bulk rock material.
Once the PLS is collected, the metal is recovered from the solution using adsorption or solvent extraction methods. For gold, the PLS is typically passed through columns containing activated carbon, which adsorbs the gold-cyanide complex. Alternatively, the metal can be precipitated out of the solution. In copper processing, the copper-rich solution is often sent to a solvent extraction and electrowinning (SX/EW) plant to produce high-purity cathode copper metal.
Primary Metals Recovered and Economic Factors
Heap leaching is primarily used to recover precious metals (gold and silver) and base metals (copper and uranium). The process is selected for low-grade ores characterized by a low concentration of the desired metal, making them unsuitable for the higher costs of traditional milling and smelting. Gold recovery rates from oxide ores often exceed 80%, while copper recovery ranges from 60% to 70%.
The motivation for choosing heap leaching is financial, offering a significantly lower capital investment compared to constructing a conventional mill and flotation plant. Operating expenses are also reduced because the process requires less energy for comminution and avoids the high-temperature stages of smelting. This low-cost structure allows mining companies to profitably exploit vast, low-concentration mineral deposits that would otherwise be classified as waste.
The method’s efficiency is tied to the scale of the operation, processing large volumes of ore over an extended period to maximize total metal recovery. Since the ore does not need to be ground into a fine powder, unlike in milling, the energy savings are substantial. The final product for precious metals is an impure gold-silver alloy, known as doré. Copper operations typically yield pure cathode copper directly from the recovery plant.