Hundreds of millions of tons of metals are mined every year. Over a hundred thousand square kilometers of land are dedicated to mining uses (Maus, et al., 2022). As the mining industry approaches the trillion-dollar value mark (Garside, 2024), we want to draw attention to the microscopic life playing a supersized role in the future of mining.
The microbially-mediated process of metal mobilization and recovery is called bioleaching, or sometimes biomining. It is the exploitation of natural processes by which microorganisms transform metals, such as the direct oxidation of metal-bearing compounds or the reaction between those compounds and microbial by-products, such as acids and chelators (Saldana, et al., 2023).
Bioleaching can be an appealing primary and secondary ore processing strategy. First, it can be a lower-cost processing strategy because its input requirement, including decreased use of solvents and reagents as well as a lower overall energy demand, are far lower than traditional pyrometallurgical methods. As a secondary process, biomining can also increase yield because it is more effective and economical at recovering metals from low grade ores, thus diverting materials for further processing that would otherwise become waste (Saldana, et al., 2023). As an added benefit, by increasing the extraction of metals from processed materials before they are discarded in waste piles, in addition to diminished processing chemical usage, the adoption of these strategies could minimize the heavy metal contamination by leaching associated with mining wastes.
Ever since ancient civilizations first began mining, microorganisms (including bacteria, archaea, and even fungi) have contributed to metal recovery and bioremediation, unbeknownst to the engineers of the day. In the mid-20th century, microbiological technologies began to be deliberately applied. As early as the 1970s, a U.S. mine was producing 200 tons of copper daily with the help of microbial metal recovery on an industrial scale. Over the past fifty years, bioleaching techniques have contributed to the global production of copper, gold, and even uranium (Bosecker, 1997).
While biomining has long been recognized, the recent commercial availability of the molecular biological tools (MBTs) marks a turning point in market uptake. These tools now empower mine operators to optimize biomining strategies with greater precision. Figure 1 below highlights several key applications of MBTs, which are discussed in detail in this post.
Indigenous Genetic Potential for Bioleaching
When implementing a bioleaching program, an early decision facing stakeholders is whether to buy a commercially available microbial culture or utilize their site’s indigenous microbiota (Rawlings & Johnson, 2007).
Figure 2 below illustrates the weighing of pros and cons that goes into making this decision.
As illustrated in Figure 2, the effectiveness of using indigenous consortia hinges on the presence of the correct microorganisms to mobilize the target metals. If site managers choose to employ their native consortium without first verifying the community is capable of bioleaching, the cost of the necessary infrastructure as well as the lost profits could be severe. Conversely, prematurely choosing a commercial culture before testing the indigenous community could overlook the resources already present at the site and potentially cost more in the long run. For these reasons, site managers can utilize molecular biological testing to investigate their site’s genetic potential for bioleaching and therefore make the most cost-effective decision.
Culture Infiltration Testing
Once the culture has been selected, mine operators can utilize MBTs to optimize their culture dosage and ensure the consortium is penetrating through the heap. Ideally, engineers want to find the culture dosage that maximizes recovery while minimizing cost.
In heap leaching set-ups, such as those illustrated in Figure 3 below, if the applied culture is only treating the top layers of the heap, metal recovery can be diminished as the metals in the bottom layers are not reached and as metals leached from the top layers are be immobilized by other reactions below. On the other hand, applying too much of a culture can reach diminishing marginal returns and be needlessly expensive. Therefore, the collection of samples from strategic locations within the heap and/or collection reservoir and the analysis of those samples for bioleaching microbes can help site managers find the sweet spot in culture application.
Microbial Consortium Monitoring
Monitoring the microbial community is essential for the program’s long-term success in both heap and tank leaching set ups.
Even in controlled processing plants which use commercial cultures, continual monitoring is necessary due to one simple fact: mined material is not sterile. As material is introduced to the tank bioreactor, it brings with it indigenous microorganisms. These introduced microbes can either improve the applied culture by supplementing underrepresented processes or harm the culture by promoting conditions (i.e., redox, pH) counterproductive to metal recovery.
In heap leaching, consortium monitoring is particularly important due to the heterogeneity inherent to engineered heaps. These constructions are rich in microenvironments that necessitate biodiversity for optimal bioleaching capability.
In either case, utilizing molecular biological testing for bioleaching microbes and overall community health can promote metal recovery and predict the need for treatment or intervention.
qPCR Assay Development
MBTs provide substantial added value to the vendors of cultures as much as to the end user. MI’s quantitative polymerase chain reaction (qPCR) assay development experience is unmatched in the commercial industry. By developing a qPCR assay for a given culture, vendors gain access to unbiased, third-party data that is indispensable for internal quality assurance and quality control monitoring and for external validation that is appealing for end users. Additionally, once the culture is applied, the newly designed qPCR assay can be employed for an even more targeted culture infiltration testing regime as described above.
Conclusion
Biomining is poised to play a pivotal role in the future of enhanced metal recovery. By adopting MBT-supported optimization strategies now, mine operators position themselves as early adopters of innovative, sustainable and economically advantageous technologies. Reach out to our team today to discuss utilizing MBTs at your site!
