Mining With Microbes

MINING WITH MICROBES

Bacteria such as Thiobacillus ferro-oxidans and Thiobacillus thio-oxidans obtain the energy they need to live by oxidising Fe2+ ions and S2- ions respectively. The S ions are present in insoluble minerals of copper, zinc and lead. The oxidation of the S ions by bacteria releases these valuable metal ions into solution (from where the metallic elements can be extracted).

Humans can achieve the same result only by smelting ores at high temperatures, a far more polluting and energy-intensive approach. As we learn more about these biological processes, it is becoming clear that they can be used to process ores - a technology that in the future could transform the metal industries and bring enormous environmental benefits.

Research laboratories around the world are witnessing a marriage between biotechnology and metallurgy, creating a new discipline known as biohydrometallurgy.

By the mid-1980s, the copper industry in the US was on its last legs. Sources of high- grade ores were becoming exhausted, so the industry was having to move, crush and smelt more and more lower-grade rock to end up with the same amount of copper. Coupled with this, the international price of a kilo of copper had dropped from $3.20 in 1980 to $1.30 in 1985.

Also, regulatory agencies were beginning to restrict emissions of sulphur dioxide, a major cause of acid rain, adding to the cost of metal production. To stay in business, firms began to look to technologies that were more cost- effective and less polluting. These proved to be biological and now 30% of the copper produced in the US is extracted in this way.

Smelting copper ore by traditional methods had cost between $130 and $200 per kilo. The introduction of biohydrometallurgy cut the cost to less than $70 per kilo. Production of one tonne of copper by smelting typically results in two tonnes of sulphur dioxide being pumped into the atmosphere. Biological extraction avoids this.

Biohydrometallurgy is straightforward when applied to copper production. First, the low- grade ore, and tailings left from any earlier conventional mining, is piled up in an area where the ground has been made impermeable. It is then sprayed with an acidic leaching solution containing T ferro-oxidans and T thio-oxidans (see Figure 2). These bacteria thrive in an acidic environment. They do not need any organic material on which to feed. They require only a supply of Fe2+ ions, or S2- ions, oxygen and carbon dioxide (plus bacterial nutrients containing nitrogen and phosphorus).

The overall result is that the bacteria convert the insoluble sulphide minerals into a solution containing Cu2+, Fe2+ Fe3+ & and SO42- ions. Because the piles sit on an impermeable base layer, it is easy to drain off the solution carrying the copper ions. The copper ions are then removed from the solution by using another solvent (see Article 2). The remaining leaching solution flows into an open pond, where T ferro-oxidans catalyses oxidation of the remaining Fe2+ ions to Fe3+ ions. This recharges the leaching solution, which is pumped back to the top of the pile for the cycle to begin again.

The copper, meanwhile, is extracted as sheets through an ‘electra-winning’ process, in which an electric current is passed through the copper ion solution. The metal collects on the negative electrodes. This part of the process is still costly in energy but research is under way to develop ‘bioabsorption filters’, such as algae, to collect the copper. The whole process would then be entirely biological.

Biohydrometallurgy may provide a method of underground mining, without the environmental damage associated with conventional techniques. There is now a mine in San Manuel in Arizona, consisting of five holes drilled into an ore deposit, which was fractured by detonating an explosive charge underground. Instead of standard mining practice, an acidic leaching solution containing bacteria is pumped down the central hole where the bacteria do their work. The resulting solution, rich in valuable copper ions, is pumped from the other four holes and processed. The leaching solution is recycled.

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Despite the potential of these methods, the mining industry is reluctant to use them. So far, they have been applied only as a last resort to recover copper from low-grade ores from sites where traditional methods are not profitable. The problem lies in the slowness of the biological process: the bacteria have not yet come to appreciate the importance of rate of return on capital. According to Keith Debus from the Centre for lnterfacial Microbial Engineering at Montana State University, “conventional processes can recover most metal from an ore deposit in a matter of months or years, depending on the size of the deposit and the level of resources applied to production, but biological metal recovery may take decades. Where both techniques have been evaluated, biological approaches have often been found to be cheaper, but delay in cash flow from slower production has hindered adoption”.

Political pressure on the metal industries could force the pace of change. If miners can extract metals from ores of lower grade than is possible with conventional techniques, they can transform old tailings sites from polluting nuisances into valuable sources of raw materials. But research on biological techniques of metal extraction remains rare. Debus sees stricter environmental regulation as the key to encouraging research. “If we were to have left environmental regulations in the US as they were 50 years ago, we would see little use for these biological techniques. In essence, it is an economic issue: the industry has been spilling its waste on the rest of society which imposes a cost on us all. What’s happening is that the cost is now being put back on the mining industry”.

As might be expected, the mining industry rejects this suggestion. It argues that it has always been pushing the research barriers aside in the search for new techniques. Industry has funded much of the research in biohydrometallurgy, which is now beginning to pay back the investment. If society wants to make mining protect the environment, industry’s demand for profit will mean that we may have to accept higher prices. Increasing environmental concern, coupled with the cost-effective techniques of biohydrometallurgy, could change the face of the mining industry for good.

EXTRACTING COPPER IONS FROM

LEACHING SOLUTIONS

Water running out of copper mines and their waste heaps contains dissolved copper ions that have been leached from copper minerals by bacterial action. The dissolved copper ions pose an environmental threat and also represent a loss of revenue for the mining company. Indeed, some mines make use of bacterial leaching solutions to extract copper from low-grade ores and tailings dumps (see Article 1).

Copper ions, Cu2+, can be selectively removed from bacterial leaching solutions by the process of ligand exchange solvent extraction. Other metal ions, such as Fe2+ and Fe3+ are left behind in the aqueous solution.

A compound which is a good ligand for copper ions is dissolved in an organic solvent, such as kerosene, that is immiscible with water. When this solution is shaken with the water containing the copper ions, the following reaction takes place:

Cu2+ (aq) + 2LH (organic) CuL2 (organic) + 2H+ (aq)

Where L represents the ligand.

The effect of the process is to remove the copper ions from the water (where they are at low concentration) and to transfer them to the organic solvent where their concentration is much higher. Some of the ligands (L) that have been used in the process are based upon the structure below.

A single ligand molecule can complex with a Cu2+ ion to form a six-membered chelate ring containing the Cu2+ ion and five of the atoms of the ligand molecule. Two molecules of the ligand react with each copper ion to form a planar complex. The complex has no overall charge and dissolves readily in the kerosene but not in water.

The process in Equation 1 can be reversed by mixing the organic solution with a small volume of concentrated acid. This pushes Cu2+ ions back into the aqueous solution containing the acid and a further increase in concentration of Cu2+ ions is achieved. The two processes, the extraction of Cu2+ ions into the organic solvent and then back into the aqueous phase, both depend on pH (i.e. depend on the concentration of H+ ions in solution).

A GOLDEN OPPORTUNITY

Jack Barrett and Martin Hughes describe how the reorganisation of London University led to a new full-scale commercial process for extracting gold.

‘There’s gold in them thar hills’ - and plenty of volunteers to help dig for it! Few people strike it lucky, however; more fortunes were lost than were ever won in the famous 19th century gold rushes of California, Ballarat and the Klondike.

Gold mining today may not be quite so frenetic but it is still a risky business. Even after finding reliable deposits, extracting the gold from them is no mean feat. Between 15% and 30% of the world’s gold reserves occur as refractory minerals - microscopic particles of gold encapsulated in a mineral matrix. Well known examples of such gold- containing minerals include arsenopyrite (FeAsS), iron pyrites (FeS2) and chalco pyrite (FeCuS2)

The first stage in obtaining the gold is usually to use froth flotation to separate these refractory minerals from any unwanted oxide ores and non-metallic minerals present. This produces a sulphide concentrate, which is then roasted to liberate the gold. The gold is extracted by treating the resulting mixture with an aerated solution of sodium cyanide, a process called cyanidation.

Gold extraction is not without its problems; roasting converts any sulphur in the refractory minerals to sulphur dioxide and any arsenic to arsenic (III) oxide, both of which have undesirable environmental and economic implications. In some cases, roasting traps the gold in fused silicate minerals and fails to liberate all the metal. Cyanidation is also difficult. The mineral matrix acts as an impervious physical barrier and shields most of the gold particles from attack by cyanide ions. Despite being used for over 100 years, cyanidation of refractory ores yields only a fraction of the contained gold.

And so it might have continued, if it had not been for a chance coincidence that resulted in the coming together of the authors in 1984, as part of the reorganisation of the University of London, in which, among other changes, King’s College, Chelsea College and Queen Elizabeth College were merged. At the time, a team of Chelsea undergraduates was carrying out a final-year project on the refractory sulphide concentrate produced from a gold deposit at the Clogau St David’s mine in North Wales. The group quickly hit upon a problem. Since roasting the concentrate to liberate the gold was not permitted on the National Park property where the mine was located, they were only able to extract 10% of the gold by cyanidation. As an alternative to roasting, the undergraduates investigated various ways of converting the gold in the concentrate into soluble compounds, using acidic solutions of thiourea and different oxidants. They showed this could be achieved but not to an extent that would be economic.

Striking it lucky

Instead, the answer to the problem arose from existing interests at Queen Elizabeth College on metal-microbe interactions. For the Chelsea researchers, this work suggested a possible solution and the two groups soon combined forces. The researchers treated the refractory sulphide concentrate with the thermophilic bacterium Suipholobus acidocalderius. These bacteria catalyse the oxidation of the encapsulating sulphide minerals by dioxygen under aqueous conditions at 70°C. Cyanidation of the resulting extract led to a remarkable - and quite unexpected - increase in gold recovery from 10% to 100%!

Figure 1: Examples of the enhancement of gold recovery from a range of refractory sulphide concentrates by the use of bacterial oxidation.

Despite these results, the team was unable to obtain any UK funding to pursue their work. By a lucky coincidence, however, Jack Barrett happened to bump into a former student, Richard Ratcliffe, at about this time. Ratcliffe, who was then working in Australia where he had set up his own mineral export business, was immediately impressed on hearing of the work and returned to Australia with the aim of raising funds.

Golden opportunism

He was soon successful. Besides attracting support for the continuance of the research here in the UK, various Australian businesses were sufficiently interested to provide money to set up a company to exploit the process in Australia. We and Richard retained a modest shareholding in the company, now called BacTech (Australia).

Back in the UK, meanwhile, the Australian funds allowed us to extend our studies of bacterial oxidation to a variety of gold- bearing mineral concentrates, using different bacterial cultures. These studies in turn led to a further bit of good luck - the isolation, by PhD student Au Nobar, of a mixed culture of moderately thermophilic bacteria from samples sent from Australia. This robust and adaptable culture was effective over a wide range of temperatures (30°C to 55°C), pH, water salinity and arsenic concentration. The bacteria worked best at a temperature of 46°C and in solutions with pH between 0.5 and 1.5, greatly enhancing the percentages of gold extracted. Some typical results are shown in Figure 1.

At King’s College, London (as the merged institution was now called), our team went on to work out the basic overall mechanism of the bacterial oxidation (see Box 1). Frustratingly, however, we were unable to get beyond the 1 dm reaction vessel stage before the technology was sold to BacTech in 1988.

Research at BacTech quickly progressed to a 450 dm laboratory plant and then to a 32 m transportable pilot plant. The pilot plant was used for test runs in Australia, Ghana and Bulgaria, and is now in Kazakhstan. It gave excellent results under very varied conditions, ranging from a three-day period in Australia where temperatures of 55°C were recorded, to several weeks in Bulgaria where the temperature was as low as -22°C.

Taking arsenopyrite (idealised formula FeAsS) as the example, the overall oxidation process is represented by the following equation.

2FeAsS+ 7O2 + 4H+ + 2H2O → 2Fe3+ +2H3AsO4 + 2HSO4-

The formal oxidation states of iron, arsenic and sulphur in arsenopyrite are +2, -1 and -1, respectively. The resulting iron (III) is present in complexes such as [Fe (H2AsO4)] 2+ [Fe (HAsO4)] + and [FeAsO4] (omitting any water molecules from these formulae), that contain arsenic in its +5 oxidation state.

Bacterial oxidation occurs in two stages. The first stage involves reactions at the interface between the surface of the bacterial cell and the arsenopyrite. Bacteria catalyse the formation of soluble compounds of iron (II), arsenic (III) and sulphur (VI)

The second stage consists of separate reactions in which iron (II) arid arsenic (III) are oxidised.

No gases are produced during the bacterial oxidation of arsenopyrite. The main products are iron (III) arsenic (V) and sulphuric (VI) acid all of which are water soluble.

Waste water from the process is usually treated with hydrated Calcium hydroxide or crushed limestone in the form of a slurry with water. This neutralises the sulphuric (VI) acid and causes the precipitation of a gelatinous mixture of iron (III) arsenates and iron oxohydroxide (FeO(OH)), together with gypsum (CaSO4.2H2O)

The stable arsenic compound arising from the mixture is iron (III) arsenate (V) dihydrate, it is possible that acidic rain or river water may react slowly with precipitated material and destroy arsenic (V), but the highest concentration of arsenic in such waters does not reach the US Environmental Protection Agency limit of 2 p.p.m.

In September 1994, 11 years after our work on bacterial oxidation began, the first full-scale plant using the patented BacTech process was commissioned and is now producing the first bacterially liberated gold at the Youanmi Mine, 500 km north-east of Perth in Australia. The mine operated as an open-cut until November 1992, when the accessible oxide layers became uneconomic. The deeper sulphide layers were only accessible by underground mining, which began in November 1993. The first sulphide ore, produced in June

1994, was tested at the BacTech laboratory pilot plant in Perth. The gold particles in the

Youanmi deposits are mainly associated with arsenopyrite, and 90% oxidation of the arsenic together with 25% oxidation of the sulphur liberates at least 92% of the gold. The unrefined ore contains about 12g of gold per tonne. A typical concentrate contains 40 g to 65 g of gold per tonne, 2.5% to 3% arsenic and 28% to 32% sulphur.

Good company

Figure 2 shows the various stages of extraction at the Youanmi plant. In 1995, the plant produced over 1 tonne of gold and estimates that there is enough gold in the region to sustain production for at least seven years.

The cost of running the plant is surprisingly low - roughly $30 per tonne of concentrate, including power, chemicals, maintenance and labour (only one person is needed per shift). One reason for this is the use of locally quarried calcrete (1 form of limestone found in cemented superficial gravel) to neutralise the effluent and the bacterially oxidised concentrate. The other reagents are bacterial nutrients containing nitrogen and phosphorus, mainly in the form of ammonium phosphate.

The recovery of base metals during gold extraction is also high. Researchers at BacTech have recovered not less than 89% of copper, nickel, cobalt and zinc, all of which contributes to the profitability of plant operations. In fact, recent work has shown that bacterial recovery of the base metals alone, whether or not gold is present in the concentrate, is feasible and economically competitive with conventional processes.