In traditional blast furnace slag handling — whether granulation by water quenching or pit dumping — the substantial amount of energy contained in the liquid slag is lost. Mindful of the environmental and energy issues the steel industry is facing, Paul Wurth took up the challenge to develop an alternative slag treatment process in order to recover the energy contained in the slag while maintaining unchanged the physical properties of the granulated slag. This process is called dry slag granulation with energy recovery.
 
Steel spheres are injected evenly into the liquid slag providing a large contact surface for efficient heat transfer. The slag cools down rapidly from about 1 450°C to 650°C, thus ensuring vitreous solidification required for further use of the slag in the cement industry. The resulting mixture of steel spheres and solidified slag is then subject to heat recovery within a counter-current heat exchanger. The energy is recovered in form of hot air at a temperature of approximately 600°C. It can be used directly as thermal energy, or it can be converted to steam and subsequently to electrical energy.
 
Moreover, this innovative process brings about important water savings compared with wet granulation (700 liters per tonne of slag), a reduction of the sulfur and CO2 emissions, and reduced costs for transportation and further treatment (drying) in the cement production due to the fact that the product is dry.
 
After different series of trials, the construction of an industrial pilot plant started in October 2012 on the site of Dillinger Hütte in Germany. Designed for a daily production of 80 tonnes of slag, the plant has been commissioned in November 2013. The first operating results are very promising and confirm the technical, energetic and environmental viability of this new development, which can also been applied to steelmaking, EAF or ferronickel slags.