Molecular Sorting – Recovery of Metals

The recycling of raw materials, in particular special metals, because of their value (precious metals), their availability (rare earths) or their toxicity (heavy metals) is of great importance, both for industrial production and for the environment. Process and wastewater streams, for example from alkaline solution baths in the electroplating industry, and also landfill leachate, may contain significant quantities of dissolved metals. Also in the recycling of solids such as waste electronic equipment or ashes from combustion processes, the dissolution of metals (leaching) in bioreactors (bioleaching) can be an efficient method of transferring these metals to an aqueous solution. After that, further processes such as “enrichment”, “separation” and “deposition” are necessary to recover a utilizable metal. The design of these process steps is of decisive importance for the efficiency and sustainability of the whole process.

Need for new technologies

Economical and ecological efficiency on the industrial scale is only possible to a limited extent with the technologies available today, especially when there are only low concentrations of the metal ions in a solution. Of course, there are technologies for the selective separation of individual metals from a solution. However, these technologies are generally very cost-intensive, and neither environmentally friendly nor universally applicable. Furthermore, the precision of separation is not sufficient to achieve a quality equal to that of the primary material. To close the material cycles within production processes and in recycling there is therefore a great need to develop new technologies that are efficient, easy to integrate and can be applied flexibly to various groups of metals.

In the project “Molecular Sorting”, funded as part of the Fraunhofer-Gesellschaft’s "Markets Beyond Tomorrow" research program, Fraunhofer IGB is developing new metal-recovery technologies based on the microbiological, separation [1, 2] and electrophysical technology know-how available at the IGB. For this purpose the technologies bioleaching (to dissolve the metals), adsorption and membrane filtration (to concentrate the dissolved metal ions), electrophoretic separation and the use of ionic liquids for electrolytic processes (for fractionation and galvanic deposition of metals) are being studied, further developed and incorporated in an integrated process.

Selected reference substances

For the development work, the following substances were chosen as reference substances:

• Precious metals: gold, silver, copper, palladium
• Rare earths: neodymium
• Toxic metals: lead, mercury.

Here, both economic criteria (economic significance, range) and ecological reasons (toxicity, prevalence) were decisive.

Interactions of metallic surfaces with microorganisms are generally not noticed until they cause damage as a result of biocorrosion. The same processes can be used to dissolve metal ions from materials and to make them recyclable. Without knowing the scientific background, people used microbiological processes centuries ago to obtain metals such as copper from natural deposits. The field of bioleaching includes enriching microbial populations with the aim of recovering metals from industrial waste materials, consumer goods or various types of process water by using technical processes.

Two procedures for bioleaching were established at the Fraunhofer IGB:

• An anaerobic process
• An aerobic process

The bioleaching processes were tested on metallic recyclable materials as well as on waste wood and rail sleepers and were initially set up on the laboratory scale. In the first process step suitable microbial mixed populations were enriched and then metal ions were successfully solubilized from particulate source material. Fig. 2A shows an example of a micropopulation of various types of bacteria that had colonized the source material (metal shavings). The process of metal solubilization was evaluated analytically by means of ICP spectroscopy. Considerable quantities of manganese, nickel, iron, copper, zinc and titanium were solubilized especially from waste wood and rail sleepers. There was also evidence of a precipitation of the metals in the suspension (Fig. 2B).

The conception for a technical process was developed on the basis of these results. For the bioleaching process the design includes a fixed-bed reactor that ensures a sufficiently high catalyst density by means of biomass retention.

In order to achieve efficient separation of metal ions from low-concentration metal solutions, an adsorption process for concentrating the ions is required. In this project, adsorber materials are being developed that permit the selective adsorption of metal ions from aqueous solutions. For this, we are pursuing two different concepts:

Adsorbers on polymer basis

So as to find suitable functional groups for the specific adsorption of metal ions, a screening was carried out with various functional groups. On the basis of these results polymers were synthesized with the functional groups that demonstrated the best adsorption characteristics with regard to certain metals. Fig. 4 shows examples of two of the monomers used (top, middle) as well as the crosslinker (bottom).

Adsorbers from renewable resources

Lignin and sheep's wool were selected as renewable resources. On the one hand, the two classes of substances lignin and keratin already possess a high density of functional groups, on the other they tend to be waste materials and are therefore easily available. These materials were used untreated and also in a modified state for the adsorption experiments.

Fig. 5 shows results of adsorption experiments in which the selectivity of the polymer adsorbers is determined. For this purpose, the adsorbers are brought into contact with an aqueous solution containing various metals of the same molar concentration. It can be seen clearly in the diagram at the top of Fig. 5 that the synthesized polymers exhibit greatly differing selectivities. The P(VPS-co-MBA) polymer favors the adsorption of neodymium, followed by lead. However, the P(N-ATU-co-MBA) polymer favors the adsorption of silver, followed by copper. The diagram at the bottom of Fig. 5 shows the adsorption characteristics of sheep's wool and lignin. It can be seen that after modification the sheep's wool adsorbs significantly more gold and mercury. Lignin favors the adsorption of gold.

To achieve an intensification of the process, adsorption will be combined with a membrane separation in the project. For the polymer P(N-ATU-co-MBA) a so-called polymer-enhanced ultrafiltration (PEUF) was carried out in which a silver solution was filtered by means of a UF. The result of this experiment is shown in Fig. 6. For comparison, the filtration process was carried out once again with the addition of the polymer. The results indicate clearly that due to the addition of the polymer (red bar), even when the silver concentration in the solution is increased, more silver was retained by the adsorption on the polymer than compared with the UF membrane (blue bar). The adsorbers are also to be incorporated direct in the membrane. For example, lignin was used in the production of membranes by mixing it directly into an initial polymer solution (Fig. 7). In addition, particles of the polymer adsorbers were also integrated into membranes (Figs. 8, 9). We could demonstrate that the adsorption properties of the polymer materials are preserved when incorporated in the membrane.

For the fractionation and subsequent deposition of the various metal ions as a metallic solid we give preference to further developing electrophysical processes such as electrophoresis and galvanic deposition. For the separation of various metal ions in solution we have developed a laboratory prototype that works on the principle of free-flow electrophoresis (Fig. 10). This procedure also permits the separation of metal ions which are very similar due to their chemical and physical properties and which can therefore only be separated to a limited extent using conventional technologies (e.g. rare earth ions).

The experiments to date confirm the feasibility of a fractionation with a high degree of selectivity of the metal ions. Here, for example, the metal ion mixtures copper-iron, neodymium-iron and the three-component mixture iron-copper-neodymium were separated (Fig. 11). In the case of the two-component mixtures we were able to increase the purity of the fractions to over 90 percent by means of a single cycle. In order to increase the efficiency even further, complexing agents were used, resulting in an almost complete separation of the substance mixtures. A transfer of the proven successful separation to other systems of substances is planned for subsequent experiments.

For the deposition of rare earths and metals following the fractionation, suitable ionic liquids were chosen as electrochemically stable electrolytes. In order to test the application, a trial setup including a reactor system was developed. First experiments to investigate the stability of the selected ionic liquids were carried out successfully and, compared with water, they show a very broad electrochemical window (Fig. 12).

The aim of recycling substances is to provide an efficient supply of secondary resources in the same quality as the original raw material. For industrial implementation, it is not sufficient just to make available the separate stages of the process. Rather, what is required is a well-thought-out process chain for an integrated metal-recycling concept. These concepts are transferred into pilot scale, tested and validated in practice and verified with investigations into environmental compatibility by means of life-cycle assessments.

[1] Bach, M.; Niedergall, K.; Schiestel, T.; Tovar, G. (2013) Nanostructured composite adsorber membranes for the reduction of trace substances in water: The example of bisphenol A, Industrial & Engineering Chemistry Research (submitted)

[2] Niedergall, K.; Hänel, C. et al. (2012) Recovery of metal ions from high diluted solutions or complex mixtures by membrane processes, EUROMEMBRANE, London, United Kingdom