Technology : Ironing out industry's problems with waste

2019-02-27 09:12:08

By David Bradley MOLTEN iron could be used to recycle everything from toxic industrial waste to beer cans, according to American researchers. They say that their prototype system can convert waste into reusable products, with no poisonous by-products. Christopher Nagel and his team at Molten Metal Technology in Waltham, Massachusetts, working with Robert Bach, a chemist at Wayne State University in Detroit, Michigan, use the power of molten iron at 750 °C to tear apart waste into its constituent elements. The iron then acts as a catalyst, combining the elements to produce useful gases, metal alloys and ceramics. Recycling highly toxic industrial effluents—for example, the toluene diisocyanate solutions from the manufacture of polyurethane—is usually not cost-effective. But incineration can release nitrogen and sulphur oxides, which cause acid rain, and even generate poisons such as dioxins. Nagel and his colleagues realised that breaking up the waste into its component atoms and then allowing them to form stable products was the key to getting rid of waste without by-products. The researchers have demonstrated their recycling system on several industrial waste streams in a pilot plant in Massachusetts. They found that the solvent chlorobenzene contaminated with cobalt oxide—a common waste product of chlorinated compound manufacture—was broken down by molten iron into hydrogen, carbon, chlorine, oxygen and cobalt atoms. In the latest issue of Environmental Science and Technology (vol 30, p 2155), the researchers explain that by adding certain chemicals, such as hydrogen, and varying the operating conditions they were able to switch reaction paths and end up with different products. In the case of the chlorobenzene and cobalt oxide mixture, the main product is synthesis gas—a mixture of mainly carbon monoxide and hydrogen that can be recycled as a chemical feedstock. Chlorine is recovered as hydrogen chloride gas. The process also produces an iron-cobalt alloy, which can be used for making steel. The system is not limited to chemical waste. The team has successfully converted mixed metal and plastic waste from missile guidance systems into synthesis gas and iron alloys. The researchers have patented the technology. They say it produces levels of nitrogen and sulphur oxides below the detection limits of between 1 and 100 parts per million,