Second life for slag - Processing dry slag granulates
Minerals Down Under National Research Flagship Project Leader at the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia, Dr Dongsheng Xie, reports on dry slag granulation, a process of using metal smelting waste by-products.
Metal production by smelting results in the production of a molten by-product called slag, which consists of a solution of mainly oxide impurities in the ore or concentrate. Each year hundreds of millions of tonnes of slags are produced globally.
Historically, the slag was discarded in air-cooled slag pits and often used for landfill or road making material after crushing and screening. In some modern operations, the molten slag is granulated using water to produce a glassy product that can be employed as valuable cementitious material to replace Portland cement. However, water granulation takes one to one and a half tonnes of evaporative loss of water per tonne of molten slag processed, and may generate acid mist causing air and possible ground water pollution. It also fails to recover a large amount of process heat. On cooling from around 1,500°C to ambient temperature, one tonne of molten iron blast furnace slag releases about 1.8GJ of sensible heat, that could be recovered for use in plant applications.
The Australian steel industry (BlueScope Steel and OneSteel) produces more than two million tonnes of slags each year with waste heat totalling up to 3.6 petajoules. If reused, it could reduce greenhouse gas emissions by about 250,000-400,000t each year depending on the source of the energy being replaced – coal or natural gas.
The concept of dry slag granulation was proposed in the 1980s and has since been subjected to extensive experimentation. Dry granulation processes produce molten droplets through mechanical means, such as air blast, rotary drum(s) and spinning discs/cups, that are quenched and solidified using air. High-grade heat is recovered through convection (as well as radiation and conduction) by air from flying slag droplets or from solid granulates in a fluidised or packed bed. In some proposed processes, a proportion of the waste heat may be recovered by passing a liquid coolant through boiler tubes embedded in the granulator to produce steam. A number of methods have been tested at pilot and plant scales, but none have been successfully commercialised.
The Commonwealth Scientific and Industrial Research Organisation (CSIRO) started working on dry slag granulation in 2002 and has continued this R&D project through CSIRO’s Minerals Down Under National Research Flagship. A novel disc design was developed that produces fine droplets without the formation of slag wool and a cyclonic airflow to quench and produce a high-glass product. This work led to a compact reactor design that can be extended to include waste heat recovery from molten slags.
The new integrated dry granulation and heat recovery process is based on a two-step operation involving a dry granulator and a packed-bed counter-current heat exchanger (see diagram above). The granulator receives and atomises molten slag to produce droplets, which are rapidly quenched and solidified to become granules. The still-hot granules (<900°C) are moved to a counter-current packed bed heat exchanger, where they are further cooled and finally discharged at close to ambient temperature to maximise heat recovery. Air is used in both units to recover the heat, providing hot air at 500-600°C.
A prototype pilot plant has been built and used for the proof-of-concept with re-melted industrial iron blast furnace slags (see image above). The plant has a compact granulator, with an external diameter of about 1.4m and a spinning disc of up to 70mm in diameter, it can process molten slags (1,400-1,500°C) at up to 10kg/min and discharge solid granules at 50-100°C. The granulator can produce off-gas above 300°C after one minute of slag tapping. This temperature was limited by the duration of slag tapping. The integrated process can operate with relatively low air-flow rates to reach 500-600°C for continuous slag tapping based on heat balance calculations.
Slag samples taken during the trials, and the granulated products collected, were characterised with respect to their chemical and physical properties. Sulphur emission during the dry granulation of re-melted blast furnace slag with 0.52-0.73% sulphur was found to be negligible. Off-gas was passed through a drop-box to remove a small amount of entrained dust (less than 0.2% of the products). The integrated process can be operated fully enclosed for containment of any gaseous emissions should the need arise.
The granule size was found to be primarily determined by slag tapping rate and disc spinning speed with more than 90% less than 1.5mm. Slag granules produced are about twice the density of water granulated slag.
Similar to water granulated slag, dry granulated slags have high glass content with good
cementitious properties suitable for cement manufacture. A preliminary techno-economic analysis has been carried out for the new process, it could deliver significant cost savings in terms of both capital and operating.
Plant investigations into slag tapping, as well as options for on-site use of the recovered waste heat, were also carried out at OneSteel’s Whyalla steelworks and BlueScope’s Port Kembla site, both in Australia. The slag temperature at the end of the slag runner (1,430-1,510°C) was found to be suitable for dry granulation. Considerable fluctuations in slag temperature and casting rates (one half to two tonnes per minute) would need to be addressed in future plant trials. Steam generation appears to be the favourable option.
A larger, semi-industrial scale dry granulation plant has been built for further demonstration of the new process and validation of Computational Fluid Dynamics process modeling in preparation for further scale up to commercial scale plant to be trialled at one of the Australian steel plants.
Further information: Dr Dongsheng Xie