The low-temperature crushing reduced e-waste into metals, oxides, polymer without using chemicals

Indian Institute of Science (IISc) researchers have found a novel way to recycle the mounting pile of electronic waste more efficiently and in an environmentally friendly manner. According to the United National Environmental Programme, about 50 million tonnes of e-waste is generated annually across the world.

The new approach is based on the idea of crushing e-waste into nanosize particles using a ball mill at very low temperature ranging from -50 to -150 degree C.

Reduction into constituents

When crushed to nanosize particles for about 30 minutes, different classes of materials — metals, oxides and polymer — that go into the making of electronic items get physically reduced into their constituent phases, which can then be separated without using any chemicals. The use of low-temperature grinding eliminates noxious emission. The results of the study were published in the journal Materials Today.

“The behaviour of individual materials is different when they are pulverised at room temperature. While metal and oxides get mixed, the local temperature of polymer increases during grinding and so the polymer melts instead of breaking,” says Dr. Chandra Sekhar Tiwary from Materials Engineering Department at IISc and the first author of the paper. “The polymer starts reacting with the rest of the components and forms a chunk. So we can’t separate the individual components.”

“The deformation behaviour at low temperature is very different from room temperature. There are two processes that happen when milling. The polymer material breaks but metals get welded, some sort of solid-state welding resulting in mixing; the welded metals again get broken during milling. At low temperature mixing does not happen,” says Prof. K. Chattopadhyay from the Materials Engineering Department at IISc and the corresponding author of the paper. There is also a lower limit to which materials can be broken into when e-waste is milled at room temperature. The maximum size reduction that can be achieved is about of 200 nanometre. But in the case of low temperature ball milling the size can be reduced to 20-150 nanometres.

Novel design

The low-temperature ball mill was designed by Dr. Tiwary. The cryo-mill grinding chamber is cooled using liquid nitrogen and a small hardened steel ball is used for grinding the material in a controlled inert atmosphere using argon gas. “The interface remains clean when broken in an inert atmosphere,” says Prof. Chattopadhyay.

“One of the main purposes of ball milling [at room temperature] is to mix materials. But in the case of ball milling at low temperature we did not observe any mixing; the individual components separate out really well. We wanted to use this property more constructively. So we took two printed circuit boards from optical mouse and milled them for 30 minutes,” recalls Dr. Tiwary.

The polymer becomes brittle when cooled to -120 degree C and ball milling easily breaks it into a fine power. Metals and oxides too get broken but are a bit bigger in size.

The crushed powder was then mixed with water to separate the components into individual classes of materials using gravity. The powder separated into two layers — the polymer floats at the top due to lower density, while metals and oxides of similar size and different density settle at the bottom. The bottom layer when diluted further separated into oxides at the top and metals at the bottom. The oxides and metals were present as individual elements.

“Our low-temperature milling separates the components into single phase components without using any chemicals, which is not possible using other techniques,” says Prof. Chattopadhyay. “Our process is scalable and is environment friendly though it uses higher energy.”