Technical Field of the Invention

The present invention relates to methods of recovering a precious metal from an article such as electronic waste, and to bioreactors relating thereto.

Background to the Invention

Electronic waste, which includes discarded electrical or electronic devices, is produced in massive and ever-increasing quantities worldwide, due in part to advances in technology and growing demand therefor.

Electronic waste often includes precious metals, such as gold, which are economically valuable and highly desirable to recover. Various methods for recovering precious metals from electronic waste are known. For example, gold is typically recovered from electronic waste using highly reactive chemical reagents, such as aqua regia, cyanide and thiourea. These reagents, however, are toxic and can cause safety issues. In addition, many known methods involve direct bioleaching of gold which results in the gold being dissolved in solution, meaning that additional processing steps (e.g. carbon activation and cementation) are required to provide the gold in a recoverable solid form. Accordingly, known methods can have a negative impact on the environment and not be cost effective.

Therefore, the provision of an improved method of recovering a precious metal from an article, for example, electronic waste, would be desirable.

It is an object of the present invention to address or ameliorate one or more of the above-mentioned or other problems.

Summary of the Invention

According to a first aspect of the present invention, there is provided a method of recovering a precious metal in solid form from an article, the article comprising a base metal attached to the precious metal, the method comprising the steps of: (a) bioleaching of the base metal by bacterial oxidation so as to detach the precious metal in solid form from the base metal; and (b) recovering the precious metal. Through bioleaching of the base metal by bacterial oxidation, the base metal forms a salt (and is solubilised) and the precious metal detaches in solid form from the base metal, facilitating recovery of the precious metal. Advantageously, this obviates the need for additional processing steps for conversion of gold to a recoverable solid form, and for reagents which can be highly toxic. Thus, the present method can be more cost and time effective, and less environmentally damaging, than methods of the prior art.

In some embodiments, the bioleaching step is performed in the presence of an added acid generator.

In some embodiments, the acid generator is elemental sulfur.

The acid generated by the added acid generator can assist in the oxidation and bioleaching of the base metal to allow for the precious metal to detach in solid form. The acid may be generated by bacterial oxidation of the added acid generator. In the case of sulfur, the added sulfur can be oxidised by bacteria to generate sulfuric acid, which in turn can assist base metal oxidation.

The precious metal may be selected from the group comprising: gold, silver, platinum, palladium, iridium, osmium, rhodium and ruthenium. In some embodiments, the precious metal may be gold.

The article may comprise a substrate on which the base metal is attached. For example, the article may comprise a plastics (e.g. fibreglass), silicon, ceramic or metal substrate.

The article may be an electronic article. The electronic article may be electronic waste. The electronic article may comprise a printed circuit board (PCB) or a hard drive. The electronic article (e.g. PCB or hard drive) may comprise a silicon or plastics substrate on which the base metal is connected. The electronic article may be any other suitable electronic article as are known to those skilled in the art.

The article may be a non-electronic article, e.g. a catalytic converter or jewellery. In particular the method can be used to process electronic waste, such as printed circuit boards, such that gold fingers comprising gold connected to copper or other base metal on a silicon or plastics substrate, can be removed in solid form, for easy recovery of the gold.

The bacterial oxidation is achieved using bacteria. The bacteria may be acidophilic bacteria. Those skilled in the art will appreciate that any suitable bacteria capable of base metal recovery via indirect oxidation may be used.

The bacterial oxidation may be indirect bacterial oxidation. This involves using bacteria to indirectly oxidise the base metal, wherein the bacteria acts as a catalyst to continuously generate an oxidising agent which oxidises the base metal. For example, where the oxidising agent for the base metal is Fe ³⁺ ions, those ions oxidise the base metal and are reduced to Fe ²⁺ ions, which are oxidised back to Fe ³⁺ ions by the bacteria, and the cycle continues.

The bacterial oxidation may be direct bacterial oxidation. This involves using the bacteria to oxidise the base metal directly, i.e. the bacteria acts as the oxidising agent for the base metal.

The bioleaching step may use one or more bacteria selected from the group comprising: Lentosprillum spp. such as Leptosprillum ferriphilum or Leptosprillum ferrooxidans; Acidothiobacillus spp. such as Acidothiobacillus ferroxidans, Acidothiobacillus ferridurans, Acidothiobacillus ferriphilus Acidothiobacillus caldus, Acidothiobacillus thioxidans, or Acidothiobacillus ambivalens; Sulfobacillus spp. such as Sulfobacillus acidophilus or Sulfobacillus thermosulfidooxidans; and (archaea) Ferroplasma spp.

In some embodiments, the bioleaching step may use Acidothiobacillus ferroxidans.

In some embodiments the bioleaching step may use a consortium of bacteria comprising at least one Leptosprillum spp., at least one Acidothiobacillus spp. and at least one Sulfobacilus spp. In some embodiments, the bioleaching step may use a consortium comprising Acidothiobacillus ferroxidans, at least one Leptosprillum spp . ; optionally at least one further A cidothiobacillus spp. and at least one Sulfobacilus spp.

In some embodiments, the bioleaching step may be using a consortium of bacteria comprising: Leptosprillum ferriphilum, Leptosprillum ferrooxidans, Acidothiobacillus ferroxidans, Acidothiobacillus ferridurans, Acidothiobacillus ferriphilus, Sulfobacillus acidophilus, Sulfobacillus thermosulfidooxidans, Acidothiobacillus caldus, Acidothiobacillus thioxidans, Acidothiobacillus ambivalens and (archaea) Ferroplasma spp.

Those skilled in the art will appreciate that any base metal may be used which can be indirectly or directly oxidised by the bacteria and thereby detach from the precious metal.

The base metal may be selected from one or more of: copper, tin and aluminium. Any other suitable base metal may be used as are known to those skilled in the art.

The bioleaching step may be in the presence of ferrous (Fe ²⁺ ) ions. The ferrous ions may be provided as iron (II) sulfate (FeS0 ₄ ). The bacteria may oxidise the ferrous (Fe ²⁺ ) ions to form ferric (Fe ³⁺ ) ions which oxidise the base metal.

The bioleaching step may be in the presence of a culture medium.

The culture medium may comprise the ferrous ions.

The culture medium may be a liquid.

The culture medium may comprise one or more of: 9k, 4.5 k, heterotrophic basal salt (HBS), acidophilic basal salt (ABS) and trace element medium.

Each of the 4.5k and the 9k may independently comprise: (NFLOiSC , KC1, K2HPO4, MgS0 ₄ -7H20, Ca(N0 ), FeS0 ₄ -7H ₂ 0 and distilled water.

The 4.5k may be the same as 9k in all respects except that it comprises half the amount of iron as 9k.

The culture medium may have a pH no greater than about 5, suitably no greater than about 4.5, suitably no greater than about 4, suitably no greater than about 3.5, suitably no greater than about 3, suitably no greater than about 2.5, suitably no greater than about 2. The culture medium may have a pH of about 2 to about 5. The culture medium may have a pH of about 2.

The culture medium may comprise one or more acids. A non-limiting example of a suitable acid is sulphuric acid. Those skilled in the art will appreciate that the amount of acid present can vary depending on the target pH.

The culture medium may comprise sulfur. Advantageously, the presence of sulfur can result in an increased concentration of sulphuric acid, which in turn can reduce or prevent formation of any unwanted precipitates, e.g. iron precipitate.

In the bioleaching step, the article may be exposed to the bacteria and, where present, the culture medium, for at least about 12 hours, suitably at least about 1 day, suitably at least about 2 days, suitably at least about 3 days, suitably at least about 4 days, suitably at least about 5 days, suitably at least about 6 days, suitably at least about 7 days, suitably at least about 8 days.

The bioleaching step may be performed in a vessel (e.g. a bioreactor) at a mixing speed of at least 30 RPM, suitably at least 40 RPM, suitably at least 50 RPM.

The recovering step may involve recovering the solid precious metal using any suitable means. For example, the recovering step may comprise filtering out the solid precious metal.

According to a second aspect of the present invention, there is provided a bioreactor comprising: an article comprising a precious metal attached to a base metal; and one or more species of bacteria operable to bioleach the base metal by bacterial oxidation so as to detach the precious metal in solid form from the base metal.

The bacterial may be operable to bioleach the base metal by indirect bacterial oxidation.

According to a third aspect of the present invention, there is provided a method of recovering a precious metal in solid form from an article, the article comprising a base metal attached to the precious metal, the method comprising the steps of: (a) bioleaching of the base metal by bacterial oxidation in the presence of an added acid generator so as to detach the precious metal in solid form from the base metal; and (b) recovering the precious metal.

The acid generator may comprise elemental sulfur.

According to a fourth aspect of the present invention, there is provided a method of recovering a precious metal in solid form from an article, the article comprising a base metal attached to the precious metal, the method comprising the steps of: (a) bioleaching of the base metal by bacterial oxidation using a consortium of bacteria so as to detach the precious metal in solid form from the base metal; and (b) recovering the precious metal.

The consortium of bacteria may comprise any consortium of the first aspect of the invention.

According to a fifth aspect of the present invention, there is provided a method of recovering a precious metal in solid form from an article, the article comprising a base metal attached to the precious metal, the method comprising the steps of: (a) bioleaching of the base metal by bacterial oxidation using a consortium of bacteria in the presence of an added acid generator so as to detach the precious metal in solid form from the base metal; and (b) recovering the precious metal.

The acid generator may comprise elemental sulfur.

The consortium of bacteria may comprise any consortium of the first aspect of the invention.

Any features of any aspect of the present invention can be combined with any features of any other aspect of the present invention. For example, those skilled in the art will appreciate that the optional features in respect of the first aspect of the present invention may apply in respect of the second, third, fourth, and fifth aspects of the present invention.

Detailed Description of the Invention

In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawing, of which: Figure 1 shows a schematic illustration of a bacteria-catalysed reaction in relation to an example of the present invention.

Figure 2 shows an illustration of the mechanism by which both iron (II) and elemental sulfur are oxidised by bacteria to form iron (III) and sulfuric acid, respectively. The role of the generated acid in assisting the oxidation of base metals (M) to release solid precious metal is also displayed.

Figure 3 shows a graph of bacterial culture medium pH versus time (days) for media containing Acidothiobacillus ferroxidans bacteria and various concentrations of elemental sulfur (0, 2, and 5 g/L).

Figure 4 shows a graph of bacterial culture medium pH versus time (days) for media containing a consortium of bacteria (composition of the consortium: Leptosprillum ferriphilum, Leptosprillum ferrooxidans, Acidothiobacillus ferroxidans, Acidothiobacillus ferridurans, Acidothiobacillus ferriphilus, Sulfobacillus acidophilus, Sulfobacillus thermosulfidooxidans, Acidothiobacillus caldus, Acidothiobacillus thioxidans, Acidothiobacillus ambivalens and (archaea) Ferroplasma spp) and various concentrations of elemental sulfur (0, 2, and 5 g/L).

With reference to Fig. 1, a method of recovering a precious metal in solid form from an article, the article comprising a base metal (copper in Fig. 1) attached to a precious metal (gold in Fig. 1), involves a bacteria-catalysed reaction. In particular, the reaction involves using bacteria in a medium to oxidise ferrous salts (Fe ²⁺ , which can be provided by iron (II) sulfate) to provide ferric ions (Fe ³⁺ ). The ferric ions then oxidise the copper base metal and thereby the ferric ions are reduced to ferrous ions. As the copper base metal is oxidised, the copper base metal forms a salt (i.e. copper (II) sulfate) and is in solution in the medium, and the precious metal detaches in solid form from the copper base metal.

In this way, the precious metal can be conveniently and efficiently recovered in solid form (by detaching from the article as the base metal is solubilised as a soluble salt, for example), without further processing steps and without using highly toxic reagents, as are often used in methods of the prior art.

Without being bound by theory, base metal recovery is believed to be enhanced by the oxidation/reduction reactions in the continuous presence of Fe ³⁺ , which is continuously regenerated by bacterial oxidation of Fe ²⁺ to Fe ³⁺ .

Example 1: Indirect bioleaching for solid gold extraction from printed circuit boards comprising gold fingers, using Acidothiobacillus ferroxidans

Bacterial Inoculation:

Acidothiobacillus ferroxidans was inoculated to 9k medium which had the following composition (unit: g/1): (NH ₄ ) ₂ S0 ₄ 3.0, KC1 0.1, K ₂ HP0 ₄ 0.5, MgS0 ₄ -7H ₂ 0 0.5, Ca(N0 ₃ ) 20.01, FeS0 ₄ -7H ₂ 0 44.2, in distilled water. Sulfuric acid 1 M was used to adjust the pH to 2. This operation was performed in a 250 ml Erlenmeyer flask with a total liquid volume of 100 ml of medium. Following inoculation, bacterial growth was performed in a shaker incubator at 30°C and 100 RPM.

Addition to gold fingers:

Gold fingers were mechanically separated from PCBs so that the gold concentration was higher than in whole PCBs. The gold fingers contained 42% organics, 10.86% calcium, 10.30% copper and 4.35% aluminium and 0.82% gold. Gold plated on the surface of both sides of the gold fingers had a thickness of 3.5 micron.

Bacterial growth was confirmed and monitored by colour change from transparent to orange/red. When the oxidation/reduction potential reached 500 RmV and for [Fe ³⁺ ] equal or superior to 5 g/1, 0.2 g of non-shredded gold finger was added to the flask, and the flask was returned to the shaker incubator in the previous condition.

Gold leaves separation:

Following 5 days of incubation, the gold leaves, which remained solid, were separated from the PCB support as the base-metals, which were holding gold to the PCB, were dissolved, due to the bioleaching activity of the bacteria on the base metals. Dissolution of the metals was confirmed by further content analysis of the leachate solution by inductively coupled plasma atomic emission spectroscopy (ICP-OES). The solid gold leaves remained floating in the solution, in a collectable form.

Example 2: Indirect bioleaching for solid gold extraction form gold fingers using a consortium of acidophilic bacteria

Composition of the consortium:

Leptosprillum ferriphilum, Leptosprillumferrooxidans, Acidothiobacillus j err oxidans, Acidothiobacillus ferridurans, Acidothiobacillus ferriphilus, Sulfobacillus acidophilus, Sulfobacillus thermosulfidooxidans, Acidothiobacillus caldus, Acidothiobacillus thioxidans, Acidothiobacillus ambivalens and (archaea) Ferroplasma spp

Bacterial Inoculation:

The bacterial consortium was inoculated in the medium containing acidophilic basal salt, trace elements, 50 mM FeS0 ₄ , with a small amount (approx. 2g/L) of sulfur added to the flask. Sulfuric acid (1 M; approx a few drops) was used to adjust the pH to 2. This was performed in a 250 ml Erlenmeyer flask, with a total liquid volume of 100 ml of medium. Following inoculation, bacterial growth was performed in a shaker incubator at 30 °C and 100 RPM.

Addition to gold fingers:

As described in Example 1, 0.6 g of the gold finger was added to the flask from the first day. Colour change confirming bacterial growth was observed after 48 hrs, and as in example 1, after 5 days the solid gold leaves were found floating at the surface of the solution, in a collectable form. Due to the lower amount of Fe (II) introduced in this medium (in comparison to 9k medium in example 1) and the presence of sulfur which resulted in sulfuric acid, no iron precipitate was observed therefore avoiding mixing iron with gold. On day 6, following removal of the gold leaves and the remained plastics of PCBs from the flask, another 0.6 gr of gold finger was added to the flask for which only 3 days were necessary for the gold leaves to be separated from PCB support. This successive gold leaf extraction demonstrated the continuous ability of the consortia to separate leaves. The leachate was analysed and showed a high concentration of Cu and A1 demonstrating bioleaching activity on the base metals. Example 3: Indirect bioleaching for solid gold extraction from gold fingers - effect of adding sulfur as an acid generator

As demonstrated in Example 2, addition of an acid generator, such as sulfur, is beneficial to the gold extraction mechanism. In the case of sulfur, it is converted by bacteria to form sulfuric acid and results in reduction of precipitate formation, which can otherwise act as a passivation layer on the surface of the waste.

In order to study the beneficial effects of sulfur addition in more detail, tests were performed in 250 mL Erlenmeyer flasks with a total liquid volume of 100 mL of medium. Bacteria (Acidothiobacillus ferroxidans or consortium of Example 2) were inoculated in a medium containing acidophilic basal salts and varying concentrations of FeS0 ₄ (30, 40, or 50 mM) and sulfur (0, 2, or 5 g/L). Sulfuric acid 1 M was used to adjust the pH to 2. Following inoculation, bacterial growth was performed in a shaker incubator at 30°C and 160 RPM.

Varying numbers of gold fingers (13, 15, or 17 fingers) were added to the flask from the first day. Colour change confirming bacterial growth was observed, and after varying periods of time, solid gold leaves were found floating at the surface of the solution, in a collectable form.

Results:

The results are summarised in Table 1 below. Table 1

The results show that the addition of sulfur, even in a small amount (2 g/L), allowed for a significant reduction in the processing time required to extract solid gold from gold fingers compared to the corresponding runs in which no sulfur was added. The effect was seen regardless of whether Acidothiobacillus ferroxidans alone or a consortium of bacteria was used.

A reduction in processing time was also seen upon using the consortium of bacteria compared to using Acidothiobacillus ferroxidans, in runs performed in the absence of added sulfur.

Increasing the ferrous ion concentration was found to reduce the processing time for runs where no sulfur was added, which is likely attributed to a greater concentration of iron (II) ions available to participate in the bacterial redox cycle leading to faster base metal oxidation. However, the same effect was not seen where sulfur was added, with the increased ferrous iron concentration appearing to somewhat increase processing time. This may be due to the formation of increased amounts of iron precipitate, forming a passivation layer on the gold finger surface. Even at high ferrous concentrations, however, the effect of sulfur at reducing processing times compared to the equivalent runs performed in its absence was still visible.

Without being bound by theory, addition of sulfur is believed to improve process efficiency in several ways. As displayed in Figure 2, bacteria can act as both sulfur and iron oxidants. The added sulfur is oxidised by the bacteria to form sulfuric acid, which can thereafter assist iron (III) in base metal oxidation, as shown in Figure 2. The sulfuric acid is also able to reduce the formation of precipitates forming a passivation layer on the gold finger surface to further increase efficiency.

The ability of the bacteria to oxidise sulfur to sulfuric acid, leading to a lowering of pH, was compared between Acidothiobacillus ferroxidans (single species) and the consortium of Example 2 (mixed species). The results in Figures 3 and 4 show that in both cases, addition of sulfur resulted in a reduced pH of the medium, which was generally maintained/further reduced with time, signifying sulfuric acid formation. The effect was more pronounced when the sulfur concentration was increased. Whilst the effect was seen in the presence of both the single species and mixed species, the lowest pH values were reached with the single species.

The one or more embodiments are described above by way of example only. Many variations are possible without departing from the scope of protection afforded by the appended claims.