Dry Granulation to Provide a Sustainable Option for Slag Treatment

Transcription

1 Dry Granulation to Provide a Sustainable Option for Slag Treatment D Xie 1, S Jahanshahi 2 and T Norgate 3 ABSTRACT Dry slag granulation is destined to replace conventional water granulation of molten slags in the not too distant future, creating a paradigm shift in the history of slag treatment. In this dry process, molten slag is atomised under centrifugal forces on a spinning disc and the slag droplets generated are quenched and solidi ed quickly using air. The process produces solid slag granules with added value (eg granulated blast furnace slag for cement) and could potentially recover high grade waste heat as hot air at 5-6 C for on-site utilisation (such as drying, preheating, or steam generation). Compared with conventional wet granulation, the dry slag granulation process offers a more sustainable approach for processing molten slags through saving water, reducing pollution, as well as recovering waste heat to reduce energy consumption, hence greenhouse gas (GHG) emissions. INTRODUCTION Metal production by smelting processes 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 millions of tonnes of slags are produced in Australia and hundreds of millions of tonnes globally. Historically the slag is disposed of in air cooled slag pits (see Figure 1), and solidi ed slags may be used for land ll or used for road based materials after crushing and screening. In some modern operations, the molten slag is granulated using water to produce a glassy product that is used as valued feed to replace Portland cement in cement manufacture. However, water granulation consumes a large amount of water (1-1.5 tonne of evaporative loss per tonne of molten slag processed), and may generate acid mist causing air and possible ground water pollution. Apart from issues with sustainable processing requirements, the existing slag processes do not recover a large amount of the waste heat from the molten slag. On cooling from around 15 C (for iron blast furnace slag) to ambient temperature, one tonne of molten slag releases about 1.8 GJ of sensible heat, which could be recovered potentially as a high grade heat source to be used at the plant for various 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 PJ. If this waste heat could be recovered and utilised, it could reduce energy usage and hence greenhouse gas emissions by about 25 to 4 tonne each year depending on the source of the energy being replaced (either coal or natural gas) as shown in Figure 2. Dry slag granulation could provide an ideal solution for the above-mentioned shortcomings of the existing slag processes. In this new approach, molten slag is atomised under centrifugal forces exerted by a spinning disc to produce droplets which are then quenched and solidi ed using air to recover the heat. The process produces solid slag granules that can be used as a valued feed for cement manufacture, as well as hot air at 5-6 C for on-site utilisation (eg drying, preheating, steam generation) at the plant. Compared with conventional water quenching, dry granulation offers a sustainable approach, producing a value-added slag product, recovering waste heat, and minimising environmental impact through saving water and reducing air pollution. 1. Project Leader, Minerals Down Under National Research Flagship, CSIRO, Box 312, Clayton South Vic dongsheng.xie@csiro.au 2. FAusIMM, Theme Leader Driving Sustainability Through Systems Innovation, Minerals Down Under National Research Flagship, CSIRO, Box 312, Clayton South Vic sharif.jahanshahi@csiro.au 3. Research Scientist, Minerals Down Under National Research Flagship, CSIRO, Box 312, Clayton South Vic terry.norgate@csiro.au 22

2 D XIE, S JAHANSHAHI AND T NORGATE FIG 1 - Molten blast furnace slag disposed at air cooled slag pit (Whyalla, OneSteel)..5.4 GHG, Mt BlueScope Total.3 Coal.2 Natural gas.1 OneSteel Energy, PJ FIG 2 - Potential reduction in greenhouse gas (CO2e) emissions if slag heat is recovered and utilised at steel plants. The concept of the dry slag granulation process was proposed as early as the 198s and has since been subjected to extensive studies (Fukuyama Works, 1983; Yoshida et al, 1984; Ando et al, 1985; Development Department, Iron and Steel Plant Division Japan, 1982; Yoshinaga et al, 1982; Pickering et al, 1985; Macauley, 1996; Featherstone, 1998; Mizuochi et al, 21). Molten slag was rst broken up into small droplets through a number of mechanical means such as air blast, rotary drum(s) and spinning disc/cup. The slag droplets were quenched and solidied using air. High-grade heat could be recovered through convection (as well as radiation and conduction) by blowing air to extract 23

3 DRY GRANULATION TO PROVIDE A SUSTAINABLE OPTION FOR SLAG TREATMENT heat from ying slag droplets or from solid granulates in a uidised or packed bed to produce hot air. In some 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 proposed and some tested and trialled at pilot and plant scales, but none of them have been successfully commercialised. In comparison with other methods, dry granulation using a spinning disc/cup offers a more ef cient and controlled process for slag granulation (Xie and Jahanshahi, 28). The process, however, has not been commercialised due to some design dif culties, including optimal disc design to suppress the formation of slag wool and a proper granulator design to handle the collected hot granules, which may have a solid shell but could reheat from inside, causing sticking and agglomeration. CSIRO started working on dry slag granulation in 22. Following an extensive investigation, major breakthroughs have been made to overcome the design dif culties described above. A novel disc design has been developed to produce ne granulates without the formation of slag wool as shown in Figure 3. A cyclonic air ow is employed to quench the hot granules to produce a highly glass product. These breakthroughs make it possible to signi cantly reduce the droplet ying distance, contributing to a compact reactor design that can ef ciently recover heat. FIG 3 - Still images of slag atomisation on a spinning disc (152 C and 15 rpm) and solid granules produced. From August 26, CSIRO s dry granulation process was further extended to include waste heat recovery from molten slags. The work was jointly sponsored by CSIRO, Centre for Sustainable Resource Processing (CSRP) (CRC for Sustainable Resources Processing), OneSteel and BlueScope Steel. An integrated dry granulation and heat recovery process has been developed and some of the progress made in the last few years is reported in this paper. NEW INTEGRATED SLAG TREATMENT PROCESS The concept for CSIRO s new integrated slag treatment process is based on a two-step operation involving a dry granulator and a packed-bed counter-current heat exchanger as shown in Figure 4. This concept was developed from focused experimental studies, and process analysis and modelling. The granulator receives and atomises molten slag to produce droplets which are quenched rapidly and solidi ed to become granules. The still hot granules (<9 C) are moved to the second unit, a counter-current packed bed heat exchanger, where they are further cooled and nally discharged at close to ambient temperature to maximise heat recovery. Air is used in both units to recover the heat to produce hot air above 5-6 C. A prototype pilot plant has been designed, built and used to prove the concept. Dry granulation tests were carried out using re-melted industrial iron blast furnace slags. Computational Fluid Dynamic (CFD) modelling techniques were used to simulate disc atomisation, droplet cooling and solidi cation, and heat transfer processes. Based on extensive testing and comprehensive CFD modelling, the process design was optimised with a number of technical challenges and operational dif culties successfully resolved. The compact granulator, as shown in Figure 5, with an external diameter of about 1.4 m and a spinning disc of up to 7 mm in diameter can process molten slags (14-15 C) at up to 1 kg/min, discharge solid granules at 5-1 C and produce off-gas at above 3 C after one minute of slag tapping. This off-gas temperature was limited by the duration of slag tapping. The integrated process can operate with relatively low air ow rates to reach 5-6 C for continuous slag tapping based on heat balance calculations. 24

4 D XIE, S JAHANSHAHI AND T NORGATE Slag 15 C Dry granulation Spinning disc atomisation air 25 C Hot air >6 C Solid granules 9 C Packed bed counter-current Heat Exchanger Drying Preheating Steam Power Desalination Hot air >6 C Cement air 25 C Granules 1 C FIG 4 - Conceptual process flow sheet for integrated dry granulation and heat recovery. FIG 5 - The CSIRO prototype pilot plant for proof of concept of the new integrated dry granulation and heat recovery process. Slag samples taken during the trials and the granulated products collected were characterised with respect to their chemical and physical properties. Sulfur emission during the dry granulation of re-melted blast furnace slag with per cent S was found to be negligible. Off-gas passed through a drop-box to remove a small amount of dust (less than.2 per cent of the products). The integrated process could be operated fully enclosed for containment of any gaseous emissions should the need arise. Dry granulated slag products are compared in Figure 6 with water granulated slag (the feed slag for melting). The dry granulated slag appears dark in colour due to a higher density (about twice that of 25

5 DRY GRANULATION TO PROVIDE A SUSTAINABLE OPTION FOR SLAG TREATMENT FIG 6 - Granules produced from wet and dry granulation processes. the wet granulated slag) but changes to light grey after grinding. Dry granulated slags were mostly elongated balls or spheres. More than 9 per cent of the granules were smaller than 1.5 mm. The granule size was found to be primarily determined by slag tapping rate and disc spinning speed as shown in Figure 7. (A) Tapping rate, kg/min Mean diameter, mm (B) Mean diamter, mm Spinning rate, rpm Dry granulated slags were found to be highly glassy with a glass content above 99 per cent, similar to water granulated slags. Relative mortar strength tests were conducted and the results showed that dry granulated slags have good cementitious properties and are suitable as feed for cement manufacture. Parallel studies were also carried out on the techno-economics of the new integrated process. The evaluation indicated that the integrated dry granulation and heat recovery process could deliver considerable cost savings in both capital and operating expenditures compared with the conventional water granulation process as shown in Figure 8. The assumptions made in carrying out this evaluation were: 8 per cent recovery of slag waste heat to hot air; hot air used to produce steam at 7 per cent boiler ef ciency; natural gas price $7/GJ; electricity price $.8/kWh; maintenance ve per cent of capital cost per year; FIG 7 - (a) Effect of slag tapping rate (slag temperatures C and disc spinning at 178 rpm); and (b) effect disc spinning rate (slag tapping rate kg/min) on granule size (mean diameter). labour requirement of one person per shift for both wet and dry granulation plants; and capital cost of dry granulation plant is 5 per cent of wet granulation plant capital cost (Featherston, 27) plus a further 2 per cent for the heat recovery system. The estimated value for the waste heat utilisation is about $7/t of slag (ie 1.8 GJ/t slag 8 per cent recovery 7 per cent boiler ef ciency $7/GJ) which is more than enough to cover the operating cost for the dry granulation process. 26

6 D XIE, S JAHANSHAHI AND T NORGATE Capital cost (A$/annual t slag) Wet granulation Dry granulation Operating cost (A$/t slag) Fuel (nat gas) Wet granulation Dry granulation Electricity Water Maintenance Labour Total FIG 8 - Comparison of estimated capital and operating costs for the integrated dry slag granulation and heat recovery process with that of conventional wet granulation. Plant investigations were also carried out at OneSteel s Whyalla steelworks and BlueScope s Port Kembla steelworks in preparation for future plant trials of the new slag process. The range of the slag temperature at the end of the runner ( C) was found to be suitable for dry granulation operation, while considerable uctuations in slag temperature and casting rates (.5-2 tonnes per minute) would need to be addressed in future plant trials. Opportunities for on-site utilisation of the waste heat recovered have also been explored. A number of options have been assessed and ranked based on a set of technical and economic criteria. Use of hot air to generate steam appears to be the favoured option (Norgate et al, 27) which will be studied further in due course. SUMMARY AND FUTURE PLAN A new integrated dry granulation and heat recovery process has been developed and the concept has been proved through pilot tests with re-melted industrial blast furnace slags at up to 1 kg/min production rate. Several technical challenges associated with operating a compact granulator and treating molten slags in an intensive process have been successfully resolved. The new process can dry granulate molten blast furnace slags at C and discharge solid granules at 5-1 C, while simultaneously recovering heat from the slag using air. The dry granulated slags were found to have high glass content and good cementitious properties suitable for cement manufacture. A preliminary techno-economic evaluation indicated that the new process may also deliver considerable savings in capital and operating cost. The process is now being scaled up for further development and demonstration. A semi-industrial scale dry granulation plant is being built at CSIRO. An advanced CFD modelling tool has been developed and used for scale up and optimisation of the process. The scale up to a full size dry granulation plant for industrial plant trials is planned to take place at one of Australian blast furnaces in one to two year s time. While the development work has been focused on iron blast furnace slags, the new process may also treat slags from non-ferrous smelters. So far, CSIRO s work has attracted considerable interest from slag producers, slag handlers and cement manufacturers, engineering companies and equipment manufacturers in Australia and around the world. A business and commercialisation plan is being developed. ACKNOWLEDGEMENT This project was carried out under the auspice of the Centre for Sustainable Resource Processing (CSRP), which is established and supported under the Australian Government s Cooperative Research Centres Program. Financial support from CSIRO Minerals Down Under Flagship, CSRP, OneSteel and BlueScope Steel towards this project is acknowledged. The authors are grateful to contributions made by Jason Donnelly, Robert Flann, Benny Kuan, Yuhua Pan, Steve Sanetsis, Bernie Washington from CSIRO; John Mathieson and Mark Biasutti from BlueScope Steel; Phil Ridgeway, Francois Verdoorn and Gregg Winson from OneSteel. 27

7 DRY GRANULATION TO PROVIDE A SUSTAINABLE OPTION FOR SLAG TREATMENT REFERENCES Ando, J, Nakahara, T, Onous, H, Ichimura, S and Kondo, M, Development of slag blast granulation plant characterised by innovation of the slag treatment method, heat recovery and recovery of slag as resources, Mitsubishi Heavy Industries technical review, pp Development Department, Iron and Steel Plant Division Japan, Dry granulation and heat recovery of molten blast furnace slag, Iron and Steel Plant Division, Japan, 22(3): Featherstone, W B, Slag treatment improvement by dry granulation, Iron and Steel Engineer, July, p 42. Featherstone, W B, 27. Slag processing technologies, VA TECH, Melbourne, 9-1 August. Fukuyama Works, NKK Japan, Blast granulation system of BOF Slag and its products, Nippon Kokan technical report, Overseas No 38, p 87. Macauley, D, Slag treatment Time for an improvement, Steel Times International, pp S Mizuochi, T, Akiyama, T, Shimada, T, Kasai, E and Yagi, J, 21. Feasibility of rotary cup atomizer for slag granulation, ISIJ International, 41(12): Norgate, T, Xie, D, Jahanshuhi, S and Russell, M, 27. Assessment of utilisation of slag waste heat in steel plants, Proceedings of First CSRP Conference, Melbourne, November, pp Pickering, S J, Hay, N Roylance, T F and Thomas, G H, New process for dry granulation and heating recovery from molten blast furnace slag, Ironmaking and Steelmaking, 12(1): Xie, D and Jahanshahi, S, 28. Waste Heat Recovery from Molten Slags, in Proceedings International Congress on Steel (ICS28), Gifu, Japan, 6-8 October 28. Yoshida, H, Nara, Y, Nakatani, G, Anzai, T and Sato, H The technology of slag heat recovery at NKK, Technical Research Center, NKK, Japan, paper 21. Yoshinaga, M, Fujii, K, Shigematsu, T and Nakata, T, Dry granulation and solidi cation of molten blast furnace slag, Tran ISIJ, 22: