Redemption of Microscale Mill Waste into Commercial Nanoscale Asset

Abstract

Mill-scale is a porous, hard and brittle coating of several distinct layers of iron oxides (predominantly Fe3O4) formed during the fabrication of steel structures. It is magnetic in nature with iron content up to as high as 93%. About 1240 million metric tons of steel was produced in 2006 globally, 1.5 % of which by weight accounts for the mill-scale waste. Thus, 18.6 million metric ton of mill scale waste was produced in one year alone. Most of the steel mill-scale waste (almost 80%) end ups in a landfill; a small fraction of it is also used to make reinforced concrete in Russia and some Asian countries. A purer commercial form of this oxide in combination with nickel and zinc oxide is used in making ceramic magnets (soft ferrites) which are an integral part of all the audio-visual and telecommunication media on this planet as well those in the space. The mill-scale waste could be a valuable technological resource if properly processed and converted into nanoscale species, in particular nanoscale iron particles for hydrogen fuel cell, medical imaging and water remediation applications. In order to achieve the much-discussed and sought-after hydrogen economy via an ‘econo’ viable and ‘enviro’ friendly route, a roadmap for utilizing the mill-scale waste has been developed. The method consists of reacting heated iron with steam, also appropriately called metal-steam reforming (a route well-known to the metallurgists for centuries) generating high purity hydrogen, with a twist. The innovation lies in the conversion of the coarse oxide scale into nanoscale iron by a novel solution-based technique. This produces highly uniform zerovalent iron particles as small as 5 nm. The scope of utilizing the mill-scale waste is broadened several folds as nanoscale iron and nanomagnetite find potential applications in de-arsenification of drinking water, destruction of perchlorate and reduction of hexavalent chromium ions in water sources. In addition, nanoscale iron and magnetite are finding increasing application as the preferred contrasting agents in magnetic resonance imaging - MRI.