What are Metal Scavengers?
To some the words ‘metal scavenger’ may conjure up images of lonely figures combing an empty beach for treasure, but to those in the clean-tech industry metal scavengers are cleverly engineered materials which remove traces of precious metals from solutions. The solutions in question may be difficult to treat waste streams, intermediates in a multi-step synthesis or the remnants of the electronics industry dissolved in acids and solvents.
Metal scavengers typically consist of reactive ligands functionalised onto an inert backbone, i.e. polystyrene resins or silica gels. For example, the metal scavengers developed by PhosphonicS are made from inorganic silica, a readily available starting material, with multifunctional organic groups attached to the silica’s surface.
How do metal scavengers work?
When a stream interacts with the scavenger, whether in a column or in slurry, the ligands ‘grab’ the metal, sticking it to the surface of the silica whilst allowing everything else to pass through untouched. For the metal to be removed effectively the scavenger ligands must bind the metal more strongly than the ligands in the stream. The next step of the process depends on the requirement of the user; whether they want to recover the metal to regain value from the process or whether they need to remove the metal and obtain a clean solution and product.
Why use metal scavengers?
There are three main reasons for scavenging precious metals. These are either to the improve purity of products, to recover value or to clean up waste.
The use of cross-coupling reactions has revolutionised modern chemistry and many pharmaceutical companies now rely on this technique, catalysed by platinum group metals (PGMs), to synthesise complex chemical structures which act as APIs (active pharmaceutical ingredients). Whilst the high yields and mild reaction conditions are great advantages of this chemistry, there remains the issue of dealing with the catalyst used during the process which often remains in solution or even coordinated to the API. These metal traces can cause problems with subsequent reaction steps and the formation of impurities. Critically, since these products will eventually enter the human body, the levels of such metals must be reduced to <10 ppm in order for the drug to be granted approval. Traditional techniques for purification result in product loss, thus the use of metal scavengers to remove the metal provides a more favourable option without compromising yield. Additionally, due to the benign nature of the scavengers, they can be introduced at a variety of stages within the manufacturing process. This can lead to a decrease in unwanted side reactions and consequently fewer impurities being generated.
The use of silica-based metal scavengers to recover value from what was previously considered waste can be beneficial to the chemical, mining refining and electronics industries. Barrens streams (the by-product of the refining process) with their high base metal and low precious metal content are not economically worth refining further using standard techniques, but these can be selectively treated to remove both types of metals. Two different scavengers can be employed, one to remove the precious metals and another to remove the base metals.
Increasingly stringent environmental guidelines for the disposal and shipping of hazardous waste mean that the cost of treatment is increasing. In order for companies to sustain their profitability they need new methods of hazardous waste treatment which do not involve shipping these dangerous liquid waste streams overseas for specialised treatment. Scavengers offer this, changing the nature of the waste from liquid to solid and making it non-hazardous which consequently can then be transported and refined much more easily. On top of this, new legislation coupled with societal pressure to be environmentally responsible is driving many companies to use green chemistry techniques and the responsible disposal of waste and recycling of materials are a key part of this. Metal scavengers can assist in both these areas and since currently around 90% of chemical processes involve the use of metal catalysts, often to reduce the economic and environmental impact of the process, recovering the metal is the next logical step.
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