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.

SUMMARYAn economically viable and environmental-friendly method of generating PEM grade hydrogen has been proposed and is by the reaction of certain metals with steam, appropriately called 'metal-steam reforming'-MSR. The drawbacks of conventional processes (hydrogen and carbothermic reduction schemes) are overcome by resorting to solution-based reduction schemes and are made economically feasible using iron oxides from steel industry's mill-scale waste. A novel aqueous-based room temperature technique using sodium borohydride (NaBH 4 ) as the reducing agent has been developed that produces highly active nanoscale iron particles ($40 nm). By using hydrazine as an inexpensive and, compared with NaBH 4 , more stable reductant, body centered cubic iron particles with $5 nm edges were obtained via solvothermal process under mild conditions from acid digested mill-scale waste. The nanoscale zerovalent iron (nZVI) powder showed improved kinetics and greater propensity for hydrogen generation than the coarser microscale iron. The rate constants for the MSR were obtained for all the reduction schemes employed in this work and are given by k hydrogen 5 0.0158 min À1 k carbon 5 0.0248 min À1 k sodiumborohydride 5 0.0521 min À1 and k hydrazine 50:1454 min À1 , assuming first order kinetics. Another innovative effort converted the magnetite waste directly into nZVI under solvothermal conditions, thus obviating the sluggish and time-consuming acid dissolution step. This particular aspect has significant ramification in terms of time and cost of making the iron precursor. To initiate and sustain the somewhat endothermic MSR process, a solar concentrator consisting of a convex polyacrylic bowl with reflective aluminum coating was fabricated and evaluated. This unique combination of mill-scale waste as iron source, hydrazine as reductant, mild process conditions and solar energy as the MSR actuator obviates several drawbacks plaguing the grand scheme of producing and delivering pure and humidified H 2 to a PEMFC stack.

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Mill-scale wastes from steel industry have been successfully reduced to pure metallic iron via hydrogen and carbothermic reduction. Use of high temperature and H 2 gas has been eliminated by a novel solution-based room temperature reduction technique whereby nanoscale iron powder is produced. This new scheme totally obviates the issue of sintering of the iron/iron oxide and hence the deactivation during the cyclic operation of metal-steam reforming. Some of the preliminary results of this investigation are discussed.