Pulverising, baking and dissolving

It was only supposed to be a temporary solution. Twenty years ago, a German company unloaded 5,000 tonnes of waste containing zinc ferrite - a compound of zinc and iron - at the port of Rotterdam. The company went bankrupt and the mountain of waste is still there. Zinc is a valuable substance; separate the zinc atoms from the iron atoms and you are onto a winner. Unfortunately, a standard procedure using acids did not really work on this specific molecule.

Group leader of the metals production, refining and recycling research group, Dr Yongxiang Yang (ME), has been working at TU Delft on the recovery of metals from waste since 2005. “We recently developed a very new technique for this particular waste problem in Rotterdam,” he says. “We mix the material with sodium carbonate and bake the mixture at about 800°C. This dissolves the molecules.” Yang can recover the metal by taking a number of extra steps, whereby the mixture is dissolved in sodium hydroxide (NaOH) and undergoes an electrolytic reaction.

"We mix the material with sodium carbonate and bake the mixture at about 800°C" 

Yang hopes that this technique will interest Tata Steel. For some time now, he has been cooperating with the steel manufacturer and the company Nyrstar on technologies to recover zinc and iron from waste. He recently received funding to hire two post-docs and a PhD candidate to research Tata Steel’s iron processing technique, HIsarna.

Yang’s team is also working on recovering the rare earth metals neodymium, praseodymium and dysprosium from electronic waste. In the lab where PhD candidates Prakash Venkatsen and Sebastiaan Peelman work, cardboard boxes are filled to the brim with cut-up magnets from hard disk drives and a mixture of finely shredded e-waste (pieces of motherboard and numerous electronic components).

The PhD candidates pulverise this material into powder and extract the earth metals via various steps of electrolysis and leaching.Venkatsen shows an Erlenmeyer flask full of pink liquid. “That pink colour comes from the neodymium.We precipitate this molecule and are left with this powder,” says the researcher while he fetches a beaker containing a pinch – a few grams – of the valuable material.The harvest from a few kilos of magnets. Four PhD candidates are working on the recovery of rare earth metals. They will be defending their dissertations this summer.