Reducing toxic emissions during recycling of e-waste

Contemporary history can be characterised by an ever-increasing dependence on modern gadgets to carry out our daily tasks — be it machines that wash our clothes or a sophisticated personal computer which acts as a window to the world. The growth of distribution networks, regular upgrades and lower prices of these devices have made them more affordable than ever before, and have resulted in a new waste stream: e-waste.

Electrical and electronic devices, discarded at the end of their useful lives, end up as e-waste. Dismantled devices are either recycled, incinerated and/or landfilled. As most of these devices contain some amount of precious metals like gold, several formal and informal e-waste recycling sectors are trying to recover these valuables. However, many of the techniques used are not environmentally sustainable and could release toxic gases during recycling. A recent study by researchers from the Indian Institute of Technology, Roorkee, The University of New South Wales, Australia and Council of Scientific and Industrial Research (CSIR), Odisha, have proposed a novel approach, to capture toxic emissions generated during the recycling of e-waste, with specific focus on the informal sector.

While the recycling of e-waste has economic benefits, inappropriate treatment processes themselves create a fair share of environmentally harmful outcomes, which is substantially high in the informal sector. In the Indian context, “the informal sector consists of kabadi waalas who collect waste from door-to-door, and then sort it in unorganised facilities, without any proper safety procedures and organisation,” points out Professor Rita Khanna, the study’s lead author. Various e-waste recycling methods used in the informal sector include manual dismantling of discarded devices, open burning, chipping or melting them, burning wires to recover copper, acid and cyanide salt leaching, and other inadequate metallurgical treatments. These activities release dust particles loaded with potentially toxic elements (PTEs) into the atmosphere that may re-deposit near the emission site, or be transported over long distances depending on the particulate size. PTEs constitute elements such as zinc, copper, nickel, cadmium, lead, mercury, chromium and arsenic.

Capturing PTEs

This study has focused mainly on capturing such PTEs emitted during the heat treatment of e-waste. While thermal treatment of e-waste in developed countries uses extensive gas cleaning filters to capture PTEs, these are rarely used in the informal sectors or in developing countries due to inadequate legislation, cost factors and technological challenges. In this study, the researchers used a surface phenomenon — adsorption — to capture PTEs in inorganic adsorbents. During adsorption, atoms, ions or molecules from a substance (gas, liquid or dissolved solid) to adhere or stick to the adsorbent surface, for example, the use of silica gel in a desiccator to adsorb moisture.

The process of adsorption involves two components — the adsorbent and the adsorbate. Adsorbent is the substance on the surface of which adsorption takes place and the adsorbate is the substance which is being adsorbed. The researchers chose alumina as the adsorbent medium due to its porosity, surface area and capacity for adsorption. Alumina is also hydrophilic and has low tendency to adsorb organic compounds. It is also thermally stable and does not react or decompose at high temperatures around 600°C. The researchers tested alumina with particle sizes ranging from 20-45µm to greater than 53µm for their adsorption capacity.

The experiments began by mechanically crushing the printed circuit boards from discarded mobile phones to sizes less than 1mm and then carried out experiments on their thermal degradation at about 600°C for different heating times. They observed that metal fumes, dust particles and hazardous constituents could be captured by using adsorbents like alumina. The use of inert gases in the process prevented oxidation during the heat treatment. The adsorption was found to improve with increasing contact time between the gases and the adsorbent.

From a wide spectrum of pollutants, lead and tin were extensively removed, along with trace amounts of copper and zinc. The presence of silicon, magnesium and carbon was also detected in the adsorbent. As compared to other available treatment methods, this method proposes a reduced treatment time. “Our technique takes about 30 minutes, whereas it can be several hours or even days in other methods. We have also used a single step process instead of conventional multi-stage approaches,” says Rita.

Deepthi Nagappa

(The author is with Gubbi Labs, a Bengaluru-based research collective)