Metal leaching is a chemical process that dissolves valuable metals from low-grade, complex, or refractory ores. As heap leaching and other modern techniques become widely adopted, they offer an efficient, cost-effective solution for mineral extraction in today’s metallurgical operations. This is particularly true for extracting gold, copper, uranium, silver, zinc, and rare earth metals.
With over twenty years of experience in mineral processing, I've summarized six of the most proven, economical, and industrially viable metal leaching techniques used in actual processing plants. Whether you're new to the industry, an investor, or a mine owner, you'll find valuable insights below.
Metal leaching is a widely used extraction process in metallurgy. It uses chemical reagents to treat ores, converting valuable metals into soluble salts while impurities remain undissolved. These insoluble impurities can then be washed away, and the solution is processed further to obtain pure metal. The leftover material is commonly known as tailings.
Let's analyze the principles, typical equipment, common applications, advantages, and limitations of these six methods.
Principle: Uses a dilute acid (commonly sulfuric acid) or a cyanide system to gradually dissolve target metals from ore in a heap setup. The solution percolates through the ore and collects in a drainage system.
Oxide gold ores often use weak acid or cyanide leaching after pre-treatment.
Oxide copper ores are commonly leached directly with sulfuric acid.
The dissolution reaction depends on temperature, acidity, percolation rate, and ore permeability.
Typical Equipment/Facilities:
Heap leach pad: Impermeable liner, ore stacking system.
Pre-agglomeration drum.
Sprinkler/Drip irrigation system.
Collection trenches, solution storage ponds.
Adsorption or extraction units (e.g., SX-EW, CIC/CIL systems).
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Advantages:
Low CAPEX (capital expenditure).
Suitable for large-scale, low-grade ore processing.
Low operating energy consumption.
Simple operation, easy to scale up.
Disadvantages:
Long leaching cycles.
Recovery rate is generally lower than agitation leaching.
Sensitive to climate and percolation conditions.
Not suitable for fine-grained or clay-rich ores.
Principle: Under high temperature (200–270°C) and high pressure (20–45 bar), acidic solution rapidly breaks down refractory minerals (like sulfides or iron/magnesium/nickel/cobalt in laterites), releasing target metals into solution.
Equipment:
Autoclave.
High-temperature, high-pressure feed system.
Slurry pre-heater.
Cooler.
Solid-liquid separation equipment (filter press).
Advantages:
Fast leaching speed and high efficiency.
Can process highly refractory ores (e.g., nickel laterites).
High recovery rates.
Very high investment costs.
Demanding equipment requirements (pressure-resistant, corrosion-resistant).
High operational technical requirements.
Significant maintenance costs.
3. Alkali Leaching – For alumina extraction from bauxite.
Typical Equipment:
Digester (pressure dissolution vessel).
Settlers, clarifiers.
Washing system.
Precipitation tanks for aluminum hydroxide.
Calcination kiln.
Mature, well-established technology.
High recovery rate.
Suitable for large-scale industrial production.
Relatively high energy consumption.
Sensitive to raw ore mineral composition.
Produces large amounts of red mud, requiring proper disposal.
4. Cyanidation – For gold/silver ores.
Principle: Gold and silver form soluble complexes in the presence of cyanide and oxygen (O₂).
Common forms: Cyanide heap leaching (low-grade), Agitation leaching (CIP/CIL).
Common Equipment:
Agitation tanks, leaching tanks.
CIL/CIP adsorption tanks.
Carbon adsorption equipment.
Carbon elution and electro-winning system.
Heap leaching facilities (for low-grade ore).
Advantages:
High gold/silver recovery rates (often ≥ 95% for CIL).
Mature and reliable process.
Applicable to gold ores of various grades.
Strict cyanide management and environmental controls needed.
Refractory ores require oxidative pre-treatment.
Sensitive to clay-rich ores.
Principle: Uses acidophilic bacteria to oxidize sulfur and iron in sulfide ores, promoting metal dissolution.
Two main types: Bioheap leaching of copper sulfides, and Bacterial oxidation pre-treatment for gold ores.
6. Ammonia Leaching – For high-magnesium nickel laterites and oxide copper ores (e.g., malachite, azurite).
Principle: In an ammonia/ammonium salt system, copper, nickel, and cobalt form stable soluble complexes. Suitable for ores high in calcium and magnesium where acid use is inefficient.
Equipment:
Sealed ammonia leaching reactor.
Aeration system (maintains NH₃ concentration and pressure).
Ammonia absorption tower.
Solid-liquid separation equipment.
Milder than acid leaching for high-Ca/Mg ores.
Good selectivity, fewer impurities dissolved.
Avoids high acid consumption.
Requires specific temperature and ammonia pressure.
Ammonia loss and recovery costs need control.
More complex than standard acid leaching.
The choice of method must be determined by ore testing. Therefore, specific tests and design based on ore samples are essential before finalizing the metal leaching technology.
From what I've seen on site, the main pros and cons of metal leaching are:
| Advantages | Disadvantages |
★Capable of treating low-grade, complex, or waste materials that cannot be economically processed by flotation or smelting. | ‑ Long ramp-up periods (bio-heaps: 6–18 months; pressure acid leaching: 12–24 months). |
| ★Minimal SO₂ emissions in most circuits. | ‑ High acid consumption for carbonate-rich or silicate-rich ores. |
| ★Lower capital expenditure compared with constructing new Cu/Ni smelting facilities—typically 30–50% lower for operations producing less than 50,000 t/y of copper. | ‑ Cold-weather operations require extensive insulation and heat-tracing to maintain solution temperature. |
| ★Suitable for modular construction, especially heap leaching and atmospheric leach circuits. | ‑ Pregnant leach solutions contain only a few grams of metal per liter—handling and pumping large solution volumes significantly increases operating costs. |
Xinhai provided EPC services for this project. Comprehensive processability tests confirmed the ore was amenable to metal leaching. During the leaching stage, eight leaching agitation tanks in series were used, ensuring sufficient reaction time between gold and cyanide. The final gold leaching rate reached approximately 90%.
2. Nicaragua 1,000 t/d Gold Mineral Processing Plant
The ore composition here was complex. After multiple tests, Xinhai confirmed a flowsheet involving crushing, grinding, classification, and cyanidation. Double-impeller agitation leaching tanks were used with a leaching density of 45% and a 48-hour leaching time. The tailings were dewatered to produce a pregnant solution containing gold, silver, and copper. The filter cake was thoroughly washed. The dewatered cake was sent to the tailings dam, with copper to be recovered later via acid leaching.
3. Mongolia 800t/d Gold Processing Plant Project
The ore was identified as a gold ore, with metal minerals mainly being sulfides. After crushing and screening, a heap leaching process was adopted. The original gold grade was 1.16 g/t. The heap leaching rate was 64%, adsorption rate 98.5%, desorption-electrolysis recovery 99.5%, smelting recovery 99.5%, resulting in an overall recovery rate of 62.4%.
Conclusion
The above content summarizes six common metal leaching processes and their typical applications. In actual production, ore properties vary significantly between deposits. It is essential to rely on professional mineral processing testing for systematic ore analysis and scientific data reports to accurately determine the subsequent process flow and investment plan. Contact Xinhai Mining to get your customized ore test report and a one-stop EPC solution, providing robust technical support for your project.
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