Microscopic Miners
In a process that is well established in copper and gold mining, naturally occurring bacteria separate a metal from its ore in a process that requires almost no energy. Such bacteria could soon be helping to recycle industrial waste.
Axel Schippers hoards his treasures in Germany’s Federal Institute for Geosciences and Natural Resources in Hanover. There, some 1,500 strains of bacteria are frozen at -140° Celsius. When required, Schippers, a geomicrobiologist, can retrieve these organisms from their icy repose. They include the species Acidimicrobium, Acidithiobacillus, Leptospirillum and Sulfobacillus. “These bacteria are all acidophilic – i.e., they like an acidic environment – and can oxidize metal sulfides,” Schippers explains. “That’s why they all play a key role in biomining.”
Biomining is a metal extraction method that is based on the use of bacteria. “This technology now accounts for around 15 percent of all copper mining worldwide,” says Schippers. “It accounts for around three percent of all gold mining and is used to a lesser degree in nickel, cobalt, and zinc mining.” Centers of biomining are in the copper mines of Chile and the gold mines of Ghana, South Africa, Central Asia, and Australia.
Axel Schippers hoards his treasures in Germany’s Federal Institute for Geosciences and Natural Resources in Hanover. There, some 1,500 strains of bacteria are frozen at -140° Celsius. When required, Schippers, a geomicrobiologist, can retrieve these organisms from their icy repose. They include the species Acidimicrobium, Acidithiobacillus, Leptospirillum and Sulfobacillus. “These bacteria are all acidophilic – i.e., they like an acidic environment – and can oxidize metal sulfides,” Schippers explains. “That’s why they all play a key role in biomining.”
Biomining is a metal extraction method that is based on the use of bacteria. “This technology now accounts for around 15 percent of all copper mining worldwide,” says Schippers. “It accounts for around three percent of all gold mining and is used to a lesser degree in nickel, cobalt, and zinc mining.” Centers of biomining are in the copper mines of Chile and the gold mines of Ghana, South Africa, Central Asia, and Australia.
Colonizing Ores and Oxidizing Metal Sulfides
The metalliferous ores containing copper, zinc, and nickel are largely made up of metal-sulfur compounds (metal sulfides). As these compounds are insoluble, the only way to extract their metal content is to heat the ore in a smelting furnace, a process requiring huge amounts of energy. “Bacteria, on the other hand, dissolve these metals, but without the need for high temperatures, as in a smelting plant,” Schippers explains.
One particularly effective biomining method is known as bioleaching. “Here, crushed ore is sprayed with dilute sulfuric acid. This stimulates the growth of acidophilic bacteria that occur naturally in the rock. The bacteria colonize the ores and oxidize the insoluble metal sulfides, thereby converting them into soluble metal sulfates,” says Schippers. The liquid that runs out of the bottom of the heap contains the dissolved metals. By subjecting this solution to an electrolytic process, the metal is extracted in its pure and solid form.
Schippers knows of over 30 strains of bacteria that are suitable for biomining, and this figure is growing steadily. “Obviously, this extends the range of applications. Today we know which bacteria are suitable for specific minerals and, most importantly, what conditions they need to work most effectively.” For a long time, biomining was viewed as a green alternative to traditional smelting, which not only requires a lot of energy but also produces sulfur dioxide, which in turn leads to acid rain. “But that’s not a problem in modern smelting plants, where over 99 percent of sulfur dioxide is converted into sulfuric acid, which is now an important byproduct of copper extraction,” Schippers explains.
The metalliferous ores containing copper, zinc, and nickel are largely made up of metal-sulfur compounds (metal sulfides). As these compounds are insoluble, the only way to extract their metal content is to heat the ore in a smelting furnace, a process requiring huge amounts of energy. “Bacteria, on the other hand, dissolve these metals, but without the need for high temperatures, as in a smelting plant,” Schippers explains.
One particularly effective biomining method is known as bioleaching. “Here, crushed ore is sprayed with dilute sulfuric acid. This stimulates the growth of acidophilic bacteria that occur naturally in the rock. The bacteria colonize the ores and oxidize the insoluble metal sulfides, thereby converting them into soluble metal sulfates,” says Schippers. The liquid that runs out of the bottom of the heap contains the dissolved metals. By subjecting this solution to an electrolytic process, the metal is extracted in its pure and solid form.
Schippers knows of over 30 strains of bacteria that are suitable for biomining, and this figure is growing steadily. “Obviously, this extends the range of applications. Today we know which bacteria are suitable for specific minerals and, most importantly, what conditions they need to work most effectively.” For a long time, biomining was viewed as a green alternative to traditional smelting, which not only requires a lot of energy but also produces sulfur dioxide, which in turn leads to acid rain. “But that’s not a problem in modern smelting plants, where over 99 percent of sulfur dioxide is converted into sulfuric acid, which is now an important byproduct of copper extraction,” Schippers explains.
Biomining: An energy-saving alternative to smelting.
Nevertheless, biomining makes sense because of its low energy requirements, particularly in the case of ores characterized by low metal content. “Biomining is not about to replace traditional smelting methods,” says Schippers. “Despite their high energy demand, these methods are still much more efficient for metal-rich ores. But when commodity prices are high and it becomes economical to exploit low-grade reserves, biomining can be a genuine alternative.”
Biomining has also been introduced in nickel, cobalt, and zinc extraction. Operators using such techniques include Finland’s Talvivaara Mining Company, which has developed its own bioleaching process with bacteria that can simultaneously extract several different metals from the same mineral deposit. This process is used at the company’s mine in the Finnish town of Sotkamo, where deposits were long regarded as too low-grade to be commercially viable. Thanks to biomining, Talvivaara has become Europe’s largest nickel producer. According to Schippers, there is one area where a lot of research is still required: “Biomining for rare earths is very much at the experimental stage,” he says. However, in industrial waste reprocessing and recycling, the use of bacteria could well make sound commercial sense. “Lab and pilot studies have shown that bacteria can be used to reprocess slag that would otherwise end up as landfill because of its high heavy-metal content,” says Schippers. Given the concentration of residual metals in such slag, this could even turn out to be financially attractive. At present, as Schippers explains, the processes piloted in the lab are not yet commercially viable. But this might quickly change, should commodity prices rise steeply enough. In that case, it could well become profitable to bio-leach slag and electronic scrap for the valuable metals they contain.
Nils Ehrenberg
Picture credits: Talvivaara Mining Company
Nevertheless, biomining makes sense because of its low energy requirements, particularly in the case of ores characterized by low metal content. “Biomining is not about to replace traditional smelting methods,” says Schippers. “Despite their high energy demand, these methods are still much more efficient for metal-rich ores. But when commodity prices are high and it becomes economical to exploit low-grade reserves, biomining can be a genuine alternative.”
Biomining has also been introduced in nickel, cobalt, and zinc extraction. Operators using such techniques include Finland’s Talvivaara Mining Company, which has developed its own bioleaching process with bacteria that can simultaneously extract several different metals from the same mineral deposit. This process is used at the company’s mine in the Finnish town of Sotkamo, where deposits were long regarded as too low-grade to be commercially viable. Thanks to biomining, Talvivaara has become Europe’s largest nickel producer. According to Schippers, there is one area where a lot of research is still required: “Biomining for rare earths is very much at the experimental stage,” he says. However, in industrial waste reprocessing and recycling, the use of bacteria could well make sound commercial sense. “Lab and pilot studies have shown that bacteria can be used to reprocess slag that would otherwise end up as landfill because of its high heavy-metal content,” says Schippers. Given the concentration of residual metals in such slag, this could even turn out to be financially attractive. At present, as Schippers explains, the processes piloted in the lab are not yet commercially viable. But this might quickly change, should commodity prices rise steeply enough. In that case, it could well become profitable to bio-leach slag and electronic scrap for the valuable metals they contain.
Nils Ehrenberg
Picture credits: Talvivaara Mining Company
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