9.2 Leaching Technologies (Heap, Agitation & Tank)
Key Takeaways
- Shrinking-core equations can test whether film transfer, product-layer diffusion, or surface reaction is rate-controlling, but apparent activation-energy ranges are diagnostic evidence rather than hard universal boundaries.
- Heap-leach containment, drainage, lift height, agglomeration, irrigation, water balance, and leak detection are designed from ore tests, climate, foundation, chemistry, consequence, and current permit requirements; liner and application-rate details are site-specific.
- High-Pressure Acid Leach (HPAL) extracts nickel and cobalt from tropical limonite laterites at \(240^\circ\text{C} - 270^\circ\text{C}\) and \(4.0 - 5.5 \text{ MPa}\) operating pressure using sulfuric acid in titanium-lined autoclaves.
- Coral Bay Nickel Corporation (CBNC) in Palawan and Taganito HPAL Corporation (THPAL) in Surigao del Norte are landmark Philippine HPAL plants producing Mixed Sulfide Precipitate (MSP).
- Bio-oxidation and bioleaching commonly use acidophilic microbial consortia, which may include Acidithiobacillus ferrooxidans, to regenerate ferric iron and oxidize reduced sulfur; performance depends on mineralogy, temperature, acidity, oxygen, nutrients, and inhibition.
Extractive hydrometallurgy involves the selective dissolution of valuable metals from crushed or ground ores using aqueous lixiviant solutions, followed by purification and metal recovery. Hydrometallurgical processing is increasingly applied to low-grade, complex, or oxide ores that are unsuited for conventional pyrometallurgical smelting or mineral flotation.
Thermodynamics, Mass Transfer & Kinetic Rate Models
The dissolution of a solid mineral particle in an aqueous lixiviant is a heterogeneous solid-liquid reaction. The process is commonly described by the Shrinking Core Model, which involves five sequential steps:
- Diffusion of lixiviant reactants through the liquid boundary layer surrounding the particle.
- Diffusion of reactants through any porous solid product layer (leached ash shell).
- Chemical reaction at the unreacted mineral core interface.
- Diffusion of soluble reaction products outward through the product layer.
- Diffusion of reaction products back into the bulk solution.
The overall leaching rate is governed by either diffusion control (where mass transfer across the fluid film or porous shell limits the rate) or chemical reaction control (where the interface reaction rate is limiting). The temperature dependence of the reaction rate constant ($k$) follows the Arrhenius equation:
Where $E_a$ is the activation energy ($\text{kJ/mol}$), $T$ is absolute temperature ($\text{K}$), $R$ is the universal gas constant ($8.314 \text{ J/mol}\cdot\text{K}$), and $A$ is the frequency factor. Low apparent activation energy can support a diffusion-control hypothesis and higher values can support a surface-reaction hypothesis, but mixed control, evolving surfaces, transport, and experimental range prevent hard universal cutoffs. Other key kinetic drivers include lixiviant concentration, solution pH, oxidation-reduction potential (Eh), mineral surface area, and dissolved gas mass transfer (such as oxygen dissolution).
Heap Leaching Design & Engineering Fundamentals
Heap leaching is a low-cost hydrometallurgical method for extracting gold, silver, and copper oxide ores from low-grade ROM (run-of-mine) or coarsely crushed deposits ($10 - 25 \text{ mm}$ top size).
1. Pad Liner Infrastructure & Construction
To prevent environmental contamination and pregnant solution loss, heap leach pads are constructed over engineered containment liners. A composite clay or geosynthetic layer with an HDPE geomembrane is one common containment concept. Material, thickness, hydraulic performance, redundancy, leak detection, and drainage must follow site design and permit criteria. Perforated polyethylene drain pipes embedded in a protective gravel layer collect the draining solution above the liner.
2. Solution Distribution & Irrigation
Lixiviant (e.g., dilute sodium cyanide solution at $\text{pH } 10 - 10.5$ for gold, or sulfuric acid $\text{H}_2\text{SO}_4$ for copper oxide) is applied across the top of the heap using low-pressure drip emitters. Drip irrigation minimizes evaporation losses, aerosol drift, and heap surface crusting compared to impact sprinklers. Application rate is established by column tests, permeability, climate, solution inventory, oxygen demand, stability, and recovery response; any quoted range is project-specific.
3. Heap Height & Cycle Times
Ore is stacked in horizontal lifts ranging from $6 \text{ to } 10 \text{ meters}$ in height using conveyor stacking systems or low-ground-pressure dozers to avoid over-compaction. Total heap heights on permanent multi-lift pads can exceed $50 - 100 \text{ meters}$. Leach cycle times vary from $60 \text{ days}$ for fast-leaching gold oxides to over $180 - 360 \text{ days}$ for copper sulfide heaps. Draining solution enriched with dissolved metal (Pregnant Leach Solution, PLS) flows to collection ponds, while barren solution from downstream recovery is reconstituted and recycled.
Vat Leaching, Agitation Tank Leaching & Autoclave HPAL Technology
When ores are high grade, fine-grained, or slow-leaching, reactor containment and intense agitation are necessary to accelerate recovery rates.
1. Agitation Tank Leaching
Finely ground ore pulp ($P_{80} < 75 \text{ }\mu\text{m}$, $40 - 50%$ solids by weight) is leached in a series of Continuously Stirred Tank Reactors (CSTR). Mechanical impellers maintain solids in uniform suspension, while spargers inject air or pure oxygen. Agitation-leach residence time and extraction depend on mineralogy, grind, chemistry, oxygen or oxidant transfer, mixing, temperature, and testwork; generic values do not define design performance.
2. High-Pressure Acid Leach (HPAL) for Nickel Laterites
High-Pressure Acid Leach (HPAL) is an advanced autoclave technology developed to extract nickel and cobalt from low-grade tropical nickel laterite ores—specifically the iron-rich limonite upper zone (goethite, $\text{FeO(OH)}$).
In the HPAL process, ore slurry is fed into multi-compartment horizontal autoclaves constructed with titanium cladding or lead/acid-resistant brick linings. Concentrated sulfuric acid ($\text{H}_2\text{SO}_4$) is injected at elevated temperatures ($240^\circ\text{C} - 270^\circ\text{C}$) and high operating pressures ($4.0 - 5.5 \text{ MPa}$, or $40 - 55 \text{ bar}$). Under these hydrothermal conditions, sulfuric acid rapidly dissolves nickel and cobalt, while iron selectively hydrolyzes and precipitates out of solution as insoluble hematite ($\text{Fe}_2\text{O}_3$):
This selective precipitation regenerates acid in situ and leaves a low-iron, nickel-cobalt-rich pregnant solution. Prominent industrial examples in the Philippines include the Coral Bay Nickel Corporation (CBNC) plant at Rio Tuba, Palawan, and the Taganito HPAL Corporation (THPAL) plant in Surigao del Norte. Both facilities process limonite laterite ores to produce Mixed Sulfide Precipitate (MSP) containing ~55% Ni and ~5% Co for global refining.
Bioleaching & Biomining Kinetics
Bioleaching (microbial hydrometallurgy) utilizes specialized acidophilic chemolithoautotrophic microorganisms to catalyze the oxidation of insoluble sulfide minerals.
- Microbial consortia: Acidophilic iron- and sulfur-oxidizers may include Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, and Leptospirillum species. The named organisms are principally mesophilic or moderately thermotolerant under process-dependent conditions; separate thermophilic archaea or bacteria are selected for high-temperature circuits. Species, pH, temperature, aeration, nutrients, and tolerance must be established by testwork rather than assigned one 30–80°C range.
- Mechanism: The bacteria oxidize ferrous iron ($\text{Fe}^{2+}$) to ferric iron ($\text{Fe}^{3+}$) and elemental sulfur ($\text{S}^0$) to sulfuric acid ($\text{H}_2\text{SO}_4$). Ferric ions act as a powerful chemical oxidant that attacks sulfide minerals such as pyrite ($\text{FeS}_2$), arsenopyrite ($\text{FeAsS}$), and chalcocite ($\text{Cu}_2\text{S}$), breaking down the sulfide matrix and releasing trapped gold or soluble copper into solution. Bioleaching is widely applied to pre-treat refractory gold concentrates prior to cyanidation.
High-Pressure Acid Leach (HPAL) plants such as Coral Bay Nickel Corporation (CBNC) in Palawan and Taganito HPAL Corporation (THPAL) in Surigao del Norte process tropical limonite laterite ores under severe hydrothermal conditions. What are the typical operating temperature, pressure, and primary chemical reagent utilized in HPAL autoclaves?
In modern heap leach pad engineering for gold and copper oxide ores, what is the primary structural function of combining a compacted low-permeability clay sub-base with an overlying high-density polyethylene (HDPE) geomembrane?
Which microorganism is widely utilized in bio-hydrometallurgical operations to catalyze the bio-oxidation of refractory sulfide gold ores and low-grade copper sulfides?