MEVE 013: Unit 16 - Mining and Bioleaching

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UNIT 16: MINING AND BIOLEACHING

16.1 Introduction

Mining involves extracting valuable minerals or other geological materials from the Earth. Traditional mining techniques often involve environmentally destructive processes like smelting, chemical leaching, and the use of cyanide. Bioleaching (biomining) offers a sustainable alternative by using microorganisms to extract metals from ores and waste.

Bioleaching utilizes natural metabolic processes of microbes, particularly certain bacteria and archaea, to solubilize metals from their ores. This environmentally friendly method is especially effective for low-grade ores that are otherwise not economically viable for extraction.

16.2 Beginning of Bioleaching Process

Bioleaching began as an incidental discovery when mine drainage water was found to contain dissolved metals due to microbial activity. In the 1950s, researchers identified that bacteria such as Acidithiobacillus ferrooxidans play a role in oxidizing metal sulfides.

Over time, bioleaching was developed into a deliberate method to recover metals like copper, gold, uranium, and nickel. The process gained traction due to its lower environmental impact and cost-effectiveness compared to conventional metallurgical methods.

16.3 Microorganisms in Bioleaching

Certain extremophilic microorganisms thrive in acidic and metal-rich environments. They derive energy by oxidizing ferrous ions, elemental sulfur, or sulfide minerals. Major microbes used in bioleaching include:

  • Acidithiobacillus ferrooxidans – oxidizes ferrous iron and sulfide minerals.
  • Acidithiobacillus thiooxidans – oxidizes elemental sulfur and sulfur compounds.
  • Leptospirillum ferrooxidans – specializes in ferrous iron oxidation.
  • Sulfobacillus spp. – thermophilic bacteria used for refractory ores.
  • Ferroplasma spp. – acidophilic archaea used in high-temperature leaching.

These microorganisms accelerate the breakdown of metal sulfides, releasing the target metals into solution.

16.4 Operating Factors That Affect Bioleaching

Several environmental and operational factors influence the efficiency of the bioleaching process:

  • pH: Most bioleaching bacteria thrive in acidic conditions (pH 1.5–3.0).
  • Temperature: Thermophilic species require higher temperatures (45–80°C) for optimal activity.
  • Oxygen: Acts as a terminal electron acceptor in the microbial oxidation process.
  • Carbon dioxide: Needed for autotrophic microbial growth.
  • Nutrients: Nitrogen, phosphorus, and trace elements must be present for microbial metabolism.
  • Pulp density: Affects oxygen diffusion and microbial access to ore particles.

Maintaining these parameters ensures efficient leaching and microbial growth.

16.5 Recovery of Metals

After solubilization, the metal-rich solution (pregnant leach solution) is processed for metal recovery. Common recovery methods include:

  • Solvent extraction and electrowinning (SX/EW) for copper.
  • Precipitation using reducing agents or pH adjustments.
  • Ion exchange techniques for selective recovery.
  • Activated carbon adsorption for precious metals like gold.

These processes isolate and purify the target metals for commercial use.

16.6 Methods in Mineral Recovery

Bioleaching can be conducted using various engineering methods, depending on ore type and scale:

  1. Heap Leaching: Crushed ores are piled onto a pad and irrigated with acidic solutions. Suitable for low-grade ores and large-scale operations.
  2. Dump Leaching: Similar to heap leaching but uses uncrushed ore in massive heaps. Common for copper extraction.
  3. Tank Leaching (Bioreactors): Finely ground ores are mixed with microbial cultures in controlled tanks. Offers better control and faster processing.
  4. In situ Leaching: Leaching solutions are pumped into ore bodies underground without mining the ore. Environmentally safer but less common.

16.7 Commercial Processes of Bioleaching

Bioleaching is commercially viable for several metals:

  • Copper: Most widely bioleached metal, especially from chalcopyrite and chalcocite.
  • Gold: Often bioleached from arsenopyrite-containing ores after bacterial oxidation.
  • Nickel: Lateritic and sulfide ores can be bioleached.
  • Zinc and cobalt: Bioleaching under development.

Major mining companies such as BHP and Rio Tinto have adopted bioleaching in their operations.

16.8 Recovery of Copper by Dump Leaching

In dump leaching, large heaps of low-grade copper ore are exposed to acidic leach solutions. Bacteria like A. ferrooxidans oxidize iron and sulfur compounds, liberating copper ions into solution. The copper is then recovered by solvent extraction and electrowinning (SX/EW).

Dump leaching is economical and suitable for previously mined waste dumps and tailings.

16.9 Uranium Bioleaching

Uranium exists in ores as UO₂ (uraninite), which is insoluble. Certain microbes oxidize U(IV) to U(VI), making it soluble in acidic solutions:

  • Bacteria such as A. ferrooxidans facilitate uranium solubilization by oxidizing Fe²⁺ to Fe³⁺, which then oxidizes UO₂.
  • Bioleaching is applied to low-grade uranium ores where conventional extraction is uneconomical.

16.10 Microbial Sorption in Metal Recovery

Besides bioleaching, microbes can adsorb and accumulate metals on their cell surfaces or intracellularly — a process known as biosorption.

Applications include:

  • Removal of heavy metals (e.g., lead, cadmium, mercury) from wastewater.
  • Use of dead biomass (e.g., fungal or algal cells) as biofilters.
  • Immobilized microbial systems in column reactors for metal recovery.

16.11 Oil Recovery

Microorganisms can enhance oil recovery by:

  • Producing biosurfactants that reduce interfacial tension.
  • Generating gases (e.g., CO₂, CH₄) that increase pressure in reservoirs.
  • Biodegrading heavy hydrocarbons to improve fluidity.

This approach, known as Microbial Enhanced Oil Recovery (MEOR), is under research and field trials for economic feasibility.

16.12 Petroleum Prospecting

Some microbes selectively colonize oil-bearing formations. Their presence or metabolic byproducts (e.g., hydrocarbons, gases) in surface samples can indicate underground petroleum deposits.

Biogeochemical prospecting uses microbial data as an indirect method to identify promising sites for drilling.

16.13 Let Us Sum Up

  • Bioleaching offers a sustainable, eco-friendly alternative to traditional mining.
  • Microorganisms like Acidithiobacillus spp. play central roles in oxidizing metal sulfides.
  • Metals like copper, uranium, nickel, and gold can be effectively recovered.
  • Processes such as heap, dump, and tank leaching allow large-scale application.
  • Microbial biosorption and enhanced oil recovery highlight the diverse applications of environmental microbiology in resource management.

16.14 Keywords

  • Bioleaching: The process of using microorganisms to extract metals from ores or waste materials through biochemical oxidation.
  • Biomining: A broader term encompassing all microbial processes, including bioleaching and biosorption, used to extract and recover metals.
  • Acidithiobacillus ferrooxidans: A key acidophilic bacterium that oxidizes iron and sulfur compounds, facilitating the breakdown of metal sulfides.
  • Heap Leaching: A technique where crushed ore is piled in heaps and irrigated with microbial leach solutions to extract metals.
  • Dump Leaching: A method similar to heap leaching but uses uncrushed, low-grade ore piles; commonly used for copper extraction.
  • Tank Leaching (Bioreactor Leaching): A controlled leaching method where finely ground ore is processed with microbial cultures in enclosed reactors.
  • In Situ Leaching: The process of injecting leaching solutions directly into ore bodies underground to dissolve and extract metals without excavation.
  • Biosorption: The passive binding of metal ions to the surface of microbial cells (living or dead), used for metal removal and recovery from solutions.
  • Microbial Enhanced Oil Recovery (MEOR): A method of improving oil recovery by using microbes or their metabolic products to alter the oil reservoir's properties.
  • Sulfur-Oxidizing Bacteria: Bacteria that gain energy by oxidizing elemental sulfur or sulfide compounds, crucial in the breakdown of metal sulfides.
  • Metal Recovery: The process of extracting purified metal from leach solutions using methods like precipitation, ion exchange, or electrowinning.
  • Uranium Bioleaching: The microbial oxidation of uranium ores, converting insoluble U(IV) to soluble U(VI), making it recoverable from low-grade ores.
  • Copper Extraction: The recovery of copper from ores, especially using bioleaching methods like dump and heap leaching.
  • Environmental Biotechnology: The use of biological systems and organisms to solve environmental problems, including sustainable mining and metal recovery.
  • Microbial Prospecting: The use of microbial presence or metabolic indicators to identify underground petroleum or mineral deposits.

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