Electronic waste, also known as e-waste, WEEE or AEEA, is rapidly becoming a major threat to our environment. With the right processing technology, however, e-waste can become a sustainable source for recovering precious metals. But how is precious metal actually recovered? Allow us to explain.
A typical treatment sequence for e-waste includes the following steps:
– Dismantling or mechanical processing
– Manual or mechanical sorting
Simple starting materials, such as steel and aluminium scrap, are already in a state suitable for final processing. More complex materials, such as printed circuit boards, undergo further treatment in so-called metallurgical processing plants where different melting and refining processes are applied. More than 95% of precious metals are recovered by these processes!
Precious metal refining is the separation of precious metals, such as gold and silver, from materials that also contain non-precious metals. The process of refining precious metals is quite advanced. The most common procedures for separating materials containing precious metals are called pyrolysis or hydrolysis.
This is where it gets more difficult, so it’s not surprising that the recovery of precious metals from e-waste is confined to a select group of specialised companies. With pyrolysis, products containing precious metals are released from the other materials by melting them to form a slag. The precious metals can then be poured off or oxidised.
With hydrolysis, the products containing precious metal are either dissolved in aqua regia (also called royal water, consisting of hydrochloric and nitric acids) or in a solution of hydrochloric acid and chlorine gas. Subsequently, certain metals can be directly precipitated or reduced using a salt, gas, organic and/or nitro hydrate compound. This process creates an impure form of the precious metal. The extracted metals then go through cleaning stages or are crystallised to increase purity. The precious metals are separated from the metal salt by calcination and are then hydrolysed and thermally prepared (pyrolysed).
These pyrolysis and hydrolysis processes produce a gold and silver purity of 99.99%, after which the gold is poured into a 1 kg gold bar that is mainly used to make jewellery and in industrial processes. The silver is poured into 30 kg bars that are used for the production of photographic and electronic materials.
Urban Mining as an alternative
So, recovering precious metals from e-waste is not a simple process! However, good recycling and high recovery rates are becoming increasingly important. Precious metals are scarce and many products cannot be made without them. There is a perception that we have to extract precious metals from mines several kilometres deep. In addition, valuable metals are having to be increasingly mined in ecologically sensitive areas. The alternative is ‘urban mining’, i.e. the recovery of raw materials through recycling. It is expected that by 2050, as much as 120 million tonnes of e-waste will be processed, compared to about 50 million tonnes worldwide today.