Biomining and bioleaching

Biomining and bioleaching are innovative biotechnological processes that utilise microorganisms to extract metals and minerals from ores, waste, or contaminated environments. These environmentally friendly methods have emerged as sustainable alternatives to traditional mining and metallurgical practices, which often involve high energy consumption and the use of harmful chemicals. By employing naturally occurring bacteria, archaea, or fungi, biomining facilitates metal recovery in an eco-efficient manner.

Background and Concept

Conventional mining and smelting methods for metal extraction have historically contributed to environmental pollution through the release of toxic gases, heavy metals, and acidic effluents. In contrast, biomining relies on the metabolic activities of microorganisms that can dissolve or mobilise metals from solid materials.
The concept of microbial extraction dates back to observations in the 1940s when scientists discovered that bacteria thriving in mine drainage environments could accelerate the oxidation of metal sulphides. Since then, biomining has developed into a well-established field within industrial biotechnology and environmental microbiology.
Biomining encompasses two major processes — bioleaching and biooxidation. While both involve microbial activity, bioleaching specifically refers to the microbial solubilisation of metals, whereas biooxidation refers to microbial oxidation of ores to facilitate subsequent chemical extraction.

Principles of Biomining

Biomining works by exploiting the natural metabolic pathways of certain microorganisms capable of oxidising metal compounds. These microbes derive energy by oxidising inorganic substances such as iron (Fe²⁺ to Fe³⁺) or sulphur compounds (S⁰ to SO₄²⁻). The resulting reactions generate acidic and oxidising conditions that dissolve metal ions from ores or waste materials into solution.
The general process involves:

1. Microbial Attachment: Microorganisms attach to the surface of the ore particles.
2. Oxidation Reactions: The microbes catalyse oxidation of metal sulphides (such as pyrite or chalcopyrite), releasing metal ions.
3. Leaching Solution Formation: The dissolved metal ions enter solution, from which they can be recovered through precipitation or solvent extraction.

For example, the overall reaction for copper extraction from chalcopyrite (CuFeS₂) can be summarised as:CuFeS₂ + 4O₂ → Cu²⁺ + Fe²⁺ + 2SO₄²⁻
Microorganisms then further oxidise Fe²⁺ to Fe³⁺, which in turn continues to attack more ore particles, sustaining the leaching cycle.

Microorganisms Involved

Several types of acidophilic and chemolithotrophic microorganisms are commonly used in biomining and bioleaching:

• Bacteria: Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, Leptospirillum ferrooxidans, Thiobacillus caldus.
• Archaea: Sulfolobus metallicus, Ferroplasma acidiphilum, which are effective at higher temperatures.
• Fungi: Aspergillus niger and Penicillium simplicissimum are used in some bioleaching processes for non-sulphide ores or e-waste.

These microorganisms thrive in extreme conditions such as low pH and high metal concentrations, making them particularly suited for industrial applications.

Types of Biomining

Biomining processes are broadly classified based on the method of microbial application:

1. In-situ Biomining:
   • Microorganisms are introduced directly into underground ore deposits through boreholes.
   • Leach solutions containing microbes are circulated to dissolve metals, which are later pumped to the surface for recovery.
   • Used mainly for low-grade ores and inaccessible deposits.
2. Heap or Dump Leaching:
   • Crushed ore is piled into heaps, irrigated with an acidic microbial solution, and allowed to percolate through the heap.
   • The leachate collected at the bottom contains dissolved metals.
   • Widely used for copper and uranium recovery.
3. Tank (Bioreactor) Leaching:
   • Finely ground ores are mixed with microbial cultures in controlled bioreactors.
   • Allows precise control over temperature, aeration, and pH for enhanced metal recovery.
   • Common for gold, cobalt, and nickel biooxidation processes.

Bioleaching: Definition and Process

Bioleaching is a subset of biomining in which specific microorganisms extract metals from ores by converting insoluble metal compounds into soluble forms. It is particularly effective for sulphide ores and low-grade minerals that are uneconomical to process using conventional methods.
The process generally involves three main mechanisms:

• Direct Bioleaching: Microbes attach directly to the ore surface and catalyse the oxidation of metal sulphides.
• Indirect Bioleaching: Microbial metabolic products (such as Fe³⁺ ions or sulphuric acid) chemically oxidise the metal compounds without direct contact.
• Contact and Non-contact Mechanisms: Depending on whether the microorganisms form biofilms on the ore surface or act through diffusible oxidants in solution.

Metals commonly extracted through bioleaching include copper, zinc, cobalt, nickel, and uranium.

Applications of Biomining and Bioleaching

Biomining and bioleaching have several important industrial and environmental applications:

• Copper Extraction: Large-scale bioleaching of chalcopyrite and other copper sulphide ores in Chile, Peru, and South Africa.
• Gold Recovery: Microbial biooxidation of refractory gold ores containing pyrite or arsenopyrite to liberate encapsulated gold for further cyanidation.
• Uranium Recovery: Use of Acidithiobacillus ferrooxidans in uranium leaching from low-grade ores.
• Nickel and Cobalt Extraction: Biomining of laterite and sulphide ores using thermophilic archaea.
• E-waste Recycling: Bioleaching of electronic waste to recover metals such as copper, zinc, and precious metals.
• Environmental Remediation: Microbial treatment of mine tailings and acid mine drainage to recover metals and reduce pollution.

Advantages of Biomining and Bioleaching

• Environmentally Friendly: Minimises air and water pollution compared to smelting or chemical leaching.
• Energy Efficient: Operates at ambient temperature and pressure, reducing energy requirements.
• Cost-effective for Low-Grade Ores: Enables extraction from ores that are uneconomical for conventional methods.
• Reduced Chemical Usage: Relies on microbial metabolism instead of harsh reagents.
• Sustainability: Facilitates metal recycling and resource conservation.

Limitations and Challenges

Despite its advantages, biomining faces several practical and technological constraints:

• Slow Kinetics: Microbial processes are slower compared to traditional chemical methods.
• Sensitivity to Environmental Conditions: Microbial activity depends on optimal pH, temperature, and oxygen levels.
• Metal Specificity: Not all metals can be efficiently extracted through microbial action.
• Scale-up Challenges: Difficulties in maintaining microbial viability and uniformity in large industrial heaps.
• Biohazard Risks: Potential release of acidic effluents if not properly managed.

Future Prospects

With increasing emphasis on sustainable resource management, biomining is gaining global attention as a green mining technology. Research is focused on improving microbial efficiency through genetic engineering and synthetic biology, developing thermophilic and acid-tolerant strains, and integrating bioelectrochemical systems for faster recovery.
Emerging applications include:

• Biomining of rare earth elements and lithium from complex ores.
• Bioleaching of industrial waste and metallurgical residues.
• Use of consortia of microorganisms (multi-species systems) for enhanced leaching efficiency.