Dry Slag Granulation Research Articles

Design of a Dry Slag Granulation Plant for Blast Furnace No. 5 of Severstal

Design of a Dry Slag Granulation Plant for Blast Furnace No. 5 of Severstal

Determination of Geometric and Operating Parameters of Device for Spraying Molten Slag in a Dry Slag Granulation Plant

Determination of Geometric and Operating Parameters of Device for Spraying Molten Slag in a Dry Slag Granulation Plant

Experimental investigation on granulation characteristics and waste heat recovery of molten slag in gas quenching dry granulation technique

The gas quenching dry slag granulation technique is one of the most promising slag treatment methods due to realize efficient waste heat recovery as well as high value-added resource utilization. To better understand the granulation characteristics and to achieve high waste heat recovery efficiency in the gas quenching granulation technique, high temperature experiments were performed with different airflow velocity and different molten slag mass flow rate. Particle average diameter, particle size distributions, and fiber mass fraction were in-depth studied for the granulation characteristics. The results indicated that the average diameter decreased with increasing airflow velocity and with decreasing the mass flow rate. The dimensionless diameter empirical formula was presented according to the experimental data. The particle size distributions were mainly concentrated in 1–3 mm. The fiber mass fraction decreased with increasing airflow velocity, but it increased when the airflow velocity exceeds 550 m·s−1 at lower mass flow rate. The higher waste heat recovery rate can be obtained with smaller particle diameter and lower fiber mass fraction. The maximum of waste heat recovery rate can reach 59.55% with the airflow velocity of 550 m·s−1 at the mass flow rates of 4.2 kg·s−1, which is the most ideal working condition.

Numerical investigation of breakup process of molten blast furnace slag through air quenching dry granulation technique

Molten slag is broken up by supersonic air into droplets through the air quenching dry slag granulation technique. The breakup process of blast furnace slag directly determines the droplet diameter and the waste heat recovery. In order to gain deep insight into the granulation mechanism and visualize the breakup process, three-dimensional unsteady numerical simulation based on the k-ω based shear stress transport turbulence model was conducted to simulate the transient breakup process of molten slag (k is the turbulent kinetic energy, and ω is the specific dissipation rate). The coupled level-set and volume-of-fluid method was utilized to capture the sharp air–liquid interface. The results show that a flat film is formed firstly under the effects of air impingement, recirculation zone and pressure gradients. Then, the axial wave and the spanwise wave appear simultaneously and the film is broken up into ligaments owing to the generation of vortex and hole structure at the intersection of axial trough and spanwise trough. Finally, the ligaments are broken up into droplets owing to Rayleigh–Taylor instability at the air–liquid interface. The droplets smaller than 3.00 mm account for 80%, with the average diameter of 1.95 mm.

Simulation of primary breakup of molten slag in the gas quenching dry granulation process

Gas quenching dry slag granulation process is an energy-saving environmental protection method for waste utilization and waste heat recovery of molten slag, and the molten slag is broken up by supersonic airflow into droplets in this process. This paper performed the numerical investigations on the breakup behavior of slag by the coupled level-set and volume of fluid (CLSVOF) methodology and gain a deep insight into the breakup mechanism of the primary breakup and influence of gas Weber number (We) and liquid Reynolds number (Re) on the primary breakup. The results indicated that the primary breakup include flat film breakup and spherical cap film breakup, the both are typical perforation breakup; Aerodynamic force cause K-H (Kelvin Helmholtz) instability, K-H surface waves are the root cause of film breakup; The wavelength and the film thickness decrease with increasing We which is conducive to reducing the ligament diameter; The wavelength and the film thickness decrease first and then increase with increasing Re which is optimum in the range of 1068–1280; The empirical formula of the ligament diameter is obtained. The ligament diameter decrease with increasing airflow velocity and slag density and increase with increasing slag mass flow rate and slag viscosity.

Heat Transfer in Fluidized Beds in a Dry Slag Granulation Unit

The world’s leading countries develop dry slag granulation units that are capable of producing solid slag granules and recovering the sensible heat of slag. The main element of such units is a granulation chamber where molten slag is atomized and solidified. In this chamber, up to 30% of the sensible heat of the molten slag is extracted. To recover more heat, the solid granules from the granulation chamber should be fed to fluidized beds. The more there are such beds, the better the granulated slag can be cooled and, therefore, the more heat can be recovered. Foreign publications on dry slag granulation do not provide methods for designing fluidized beds for dry slag granulation units. A method for analyzing the heat transfer in fluidized beds during cooling of slag granules by counter airflow is presented. The method allows determining the heat flow from the slag to the air in fluidized beds and the temperatures of the slag and air exiting the fluidized beds. Recommendations on choosing the airflow velocity, the rational mass of slag in one fluidized bed, the number and dimensions of beds and evaluating air pressure losses in fluidized beds are formulated. It is shown that three fluidized beds in a dry slag granulation unit are sufficient to recover up to 92% of the sensible heat of slag.

RecoDust\u2014An Efficient Way of Processing Steel Mill Dusts

RecoDust is believed to be an efficient way of processing steel mill dusts to produce reusable secondary raw material resources. The rising costs of the steel production process and the increasing public attention towards environmental protection have caused steel plants worldwide to focus on emission-reduction processes and circular economy. As a reusable secondary raw material resource, the steel mill dust contains a high amount of iron and zinc. Nowadays, most of the steel mill dusts are processed under the Waelz-Process. However, this process is limited or impossible without additional dust treatment and can only be operated economically with zinc contents over 20 wt%. In the future, RecoDust in combination with dry slag granulation will become an efficient technique for processing lower zinc-containing materials of a steel mill. A challenging area in the field of RecoDust is the melting/reduction phase to separate volatile components and the slag treatment. A previous study was limited to the recovery of zinc. This paper compares the energy consumptions of the primary metal production to the recycling and of the RecoDust process to the Waelz-Process and outlines a new approach to RecoDust slag treatment.

Design and Operation of Dry Slag Granulation Pilot Plant

CSIRO has been working on a dry granulation process, integrated with heat recovery, since 2002. It involves a rotary disc that atomizes molten slag to produce liquid droplets, which are rapidly quenched to become solid granules. The hot granules are fed to a counter-current moving packed bed heat exchanger, where they are further cooled and finally discharged at close to ambient temperature. Air is used in both units to recover the heat. Development has proceeded through proof-of-concept tests, a prototype and now a pilot plant, capable of processing 100 kg/min of slag. Extensive CFD modeling was used to predict disc and granulator performance as a function of design and operating parameters. Experimental results on the dry slag granulator pilot plant have demonstrated that the process can effectively produce glassy slag granules from molten iron blast furnace slag, and recover significant heat, and that the CFD model can be used to predict process performance. Work continues to scale-up the process and extend the operation to other metallurgical materials, such as non-ferrous slags and mattes.

Life cycle and economic assessment of multi-stage blast furnace slag waste heat recovery system

Abstract In order to utilize the waste heat of blast furnace slag more effectively, the multi-stage slag waste heat recovery system was proposed. This study employed the life cycle assessment to evaluate the environmental performance of the proposed system. Meanwhile, the economic feasibility of the system was analyzed in terms of capital cost and return analysis. Based on the current status of the environment, CO2 tax was also referred in the net explicit profit and net implicit profit calculation of the system. The energy consumption and resource consumption of the system were 268.84 kg coal-eq/tslag and 1213.60 kg/tslag, respectively. The main environmental impact came from the slag production process. Under the consideration of CO2 tax, the net explicit profit and net implicit profit of the proposed method were 13.40 $/tslag and 135.14 $/tslag when the CO2 tax were 33.00 $ per ton. In this process, the evaluations of the proposed system were done in comparison with the conventional water quenching method-open circuit process and latest dry slag granulation method by the main energy, environmental and economic indicators. As results suggested, the multi-stage slag waste heat recovery system had a good application potential in the aspect of energy conservation and emission reduction for iron and steel industry.

Blast Furnace Ironmaking – A View on Future Developments

Within many industries there is a constant drive to reduce environmental impact and energy costs thereby ensuring maximum energy re-use. Within the iron and steel industry the ironmaking processes are acknowledged to be the focus area when the topic of energy saving arises.A lot of attention has been applied to the ironmaking processes with regards to environment and energy saving, with established technologies being in place to reach these goals such that the process per se, is now at or very close to theoretical limits.The following areas of the blast furnace process are currently the focus of Primetals activity with regards the target to further improve energy saving with incremental improvements:•Flue gas recycling within blast furnace stoves•MERIM dry gas cleaning process•Dry slag granulation including heat recoveryThis paper will present the concepts and latest status of the above technologies. The paper will review the progress to date of these development activities and thus present ideas on options for energy saving and re-use within the ironmaking system.

Experimental study of rotating dry slag granulation unit: Operating regimes, particle size analysis and scale up

Blast furnace slag is a high-value by-product of the iron and steel industry. It leaves the plant at a very high temperature and possesses a large quantity of high-grade energy. One of the promising methods to extract this energy is dry slag granulation. In this study, the effectiveness of dry slag granulation was studied using a mixture of rosin and paraffin wax as an analogue for blast furnace slag. The effects of various parameters such as rotational speed of the disc, diameter of the disc and flow rate of the molten liquid have been studied. Different ranges of operating conditions in terms of non-dimensional numbers for fiber formation and particle formation were determined. This helps us determine the operating conditions under which particle formation is ensured. The study shows that with an increase in the rotational speed or disc diameter the average particle diameter decreases whereas with an increase in the flow rate the average particle diameter increases. The design of a granulation unit for a commercial plant utilizing the data from lab scale experiments is discussed.

Energy–environment–economy evaluations of commercial scale systems for blast furnace slag treatment: Dry slag granulation vs. water quenching

The high-temperature blast furnace slag is conventionally treated by water quenching (WQ) method with enormous waste heat unrecovered. To address this issue, dry slag granulation (DSG) technology has been proposed to recover the waste heat from molten slag. Before commercial implementation, the sustainability and feasibility of the DSG should be well assessed. In this study, life cycle assessment (LCA) is conducted on a designed DSG prototype system in commercial scale. The environmental sustainability and economic benefit of DSG are evaluated and compared with the WQ systems. The LCA results reveal that the DSG can potentially reduce the energy and resource consumptions by 150kg-coal-eq/t-slag and 1547kg/t-slag, respectively. The analysis on environment impact also clarifies that the DSG is an environmentally friendly method for slag treatment. Furthermore, the DSG method represents a possibility to recover the heat from high-temperature blast furnace slag and turn it into a valuable material with a profit of 92.9$/t-slag.

Blast furnace ironmaking: view on future developments

Within many industries, there is a constant drive to reduce environmental impact and energy costs thereby ensuring maximum energy re-use. Within the iron and steel industry, the ironmaking processes are acknowledged to be the focus area when the topic of energy saving arises. A lot of attention has been applied to the ironmaking processes with regards to environment and energy saving, with established technologies being in place to reach these goals such that the process per se is now at or very close to theoretical limits. The following areas of the blast furnace process are currently the focus of Primetals activity with regard the target to further improve energy saving with incremental improvements: flue gas recycling within blast furnace stoves, MERIM dry gas cleaning process and dry slag granulation including heat recovery. This paper will present the concepts and latest status of the above technologies. The paper will review the progress to date of these development activities and thus present ideas on options for energy saving and reuse within the ironmaking system.

Thermodynamic analysis and optimization of an air Brayton cycle for recovering waste heat of blast furnace slag

The efficient utilization of waste heat of blast furnace (BF) slag can reduce energy consumption of iron and steel industry. Based on dry slag granulation (DSG) technology which is a text facility dealing with 1.2 ton molten slag per hour, an air Brayton cycle for recovering waste heat of BF slag is proposed. A finite time thermodynamic (FTT) model of the air Brayton cycle is established and its waste heat recovery performance is analyzed. Moreover, by taking power output as objective, the compressor's pressure ratio and inlet relative pressure drop are optimized. Results show that the maximum power output is 51.46 kW the maximum heat recovery efficiency is 11.98%, and the corresponding thermal efficiency is 20.70% when the compressor pressure ratio is 4.29 and compressor inlet relative pressure drop is 0.023.

Engineering design of direct contact counter current moving bed heat exchangers

A heat transfer model was applied to simulate heat exchange in a direct contact counter current moving bed (CCMB) heat exchanger associated with a dry slag granulation process developed by CSIRO. Cold air is used to recover heat from hot solid slag granules in the heat exchanger. Experimental data was used to calibrate the heat transfer model. The model was then used to predict the solid and gas temperature distributions along the height of the bed, the heat loss, and the operating and design conditions for scale-up of the CCMB to semi-industrial and industrial operations.Two scenarios were examined using the model to determine conditions for i) minimising the solid outlet temperature and ii) maximising the gas outlet temperature. The model can also be used to determine the nominal bed height to ensure that the designed gas outlet temperature is achieved. The modelling results suggest that a relatively small bed height (below 0.5m) will be sufficient for heat exchange. The model also suggests that a solid to gas mass flow rate ratio of 0.8 should be used to minimise the solid outlet temperature, while a ratio of 1.0 should be used to maximise the gas outlet temperature. Therefore an appropriate operating window for a CCMB heat exchanger for recovering heat from hot slag was found with a solid to gas mass flow rate ratio between 0.8 and 1.

Development of Low-Emission Integrated Steelmaking Process

This paper provides a summary of the progress made over the 8 years of an R&D program that focused on the development of know-how and processes that could result in substantial reduction in net CO2 emission by the steel industry. The processes that were developed covered introduction of renewable carbon and energy sources as well as minimising waste heat from processes. The current status of each of the processes and application areas is provided. The use of biomass-derived fuels and reductants in the ironmaking and steelmaking industry provides a sustainable option for reducing net CO2 emissions at a lower capital cost and technological risk than other breakthrough technologies under development. A key focus of this program has been to partially substitute these fossil-based fuels with renewable carbon (charcoal) from sustainable sources such as plantations of biomass species or forest wastes. Raw biomass is unsuitable for applications in ironmaking and steelmaking and should be converted into charcoal (char) through a pyrolysis process before use. A new pyrolysis process which operates continuously and autogenously has been developed and piloted. The biomass-derived chars and hydrocarbon fuels have great potential in lowering the net CO2 emissions of integrated (BF-BOF route) steel plants. Life cycle assessment has quantified the potential reduction in net CO2 emissions and covers cradle to gate, including plantation, harvesting, transport, pyrolysis and use of chars and bio-oil products. The properties of chars produced by biomass pyrolysis can be tailored to each of the several applications proposed (sintering solid fuel, cokemaking blend component, blast furnace tuyere injectant, liquid steel recarburiser, etc.), thus resulting in optimal performance and greater value-in-use of the char. Our economic analysis has made allowance for such value-in-use in applications, particularly as a replacement for BF pulverised coal injection. This analysis shows that key factors influencing the economics are the net cost of producing charcoal from biomass, selection of pyrolysis technology, value of the pyrolysis by-products, as well as the value-in-use for the charcoal. Dry slag granulation (DSG) has the potential to make a fundamental change in slag treatment and deliver a more sustainable alternative compared with the conventional water granulation process. The DSG process not only saves valuable water resources and reduces sulphurous emissions, but it may also recover a large amount of the high-grade heat in molten slag so to reduce greenhouse gas emission. CSIRO has been working on the development of a novel DSG process, integrated with heat recovery, since 2002 and has made significant progress in process design and optimisation based on process modelling, laboratory investigations, extensive pilot plant trials and characterisation of the solidified product granules.

Exergy and Its Efficiency Calculations in Ferrochrome Production

Ferrochrome production is a high energy intensive process consuming around 3000 to 3500 kWh/t of electrical energy. Ferrochrome is produced by smelting of different grades of chromite ore with coke and fluxing agents such as lime, dolomite, and quartz in a submerged arc furnace (SAF). Apart from the production of ferrochrome, co-products that are produced during the process include carbon monoxide rich off-gas and slag. The slag is cooled with high pressure jet water which results in the formation of slag granules. In the present practice, off-gas generated from the SAF is used for ladle preheating, as fuel in sinter plant, while the remaining is unutilized. Approximately 34 to 40 pct of heat from off-gas is utilized, while the remaining 60 to 66 pct of the off-gas can be utilized for generating electricity by gas combustion turbine. The concept of exergy is applied to monitor the existing process and to understand the feasibility of modification. In the present study, comparison of exergy efficiency for existing process and two adapted case studies has been performed, involving utilization of off-gas for sintering and power generation and waste heat utilization from dry slag granulation. It is observed that there was considerable increase in exergy efficiency for waste heat utilization by dry granulation case study when compared with other two case studies.

CFD Modelling of the Effects of Operating Parameters on the Spreading of Liquids on a Spinning Disc

A novel dry slag granulation process based on a spinning disc is being developed by CSIRO. This process utilises centrifugal force to break up molten slag into droplets, which are then quenched into solidified granules by a flow of cold air. In this process the sensible heat of slag is recovered as hot air. In the present work, a previously developed steady-state, two-dimensional and multiphase CFD model was applied to perform parametric numerical experiments to investigate the effects of a number of parameters on the liquid film thickness at the disc edge, which included liquid mass feeding (pouring) rate, disc spinning speed, disc radius, liquid viscosity, density and surface tension. The modelling results were compared with experimental data and were found to be in good agreement. To reduce the number of simulations needed, Box and Behnken's fractional factorial design of numerical experiment was adopted. Furthermore, in order for the modelling results to be applicable to atomisation of different liquids using spinning discs of different sizes, a dimensionless correlation was developed based on dimensional analysis of the numerical simulation data. The modelling results indicate that the liquid film thickness can be significantly influenced by the disc radius and spinning speed, the liquid mass feeding rate, viscosity and density, whereas the liquid surface tension has a negligible effect.

The formation and breakup of molten oxide jets

Experimental investigations on the capillary breakup of jets of molten oxides (slags) at high temperatures in an inert atmosphere are presented in this paper. The (in)stability of ligaments, threads, or jets of metallurgical slags is of importance in many heat and mass transfer processes related to high-temperature metal production, and aftertreatment of slags as currently investigated in the dry slag granulation process. In order to quantify the dynamic disintegration process of slag jets into droplets, a three-zone high-temperature furnace with maximum temperatures of 1750°C and optical access was built. In the present study, a molten synthetic calcia/alumina slag at 1600°C was used to form jets of 1mm in diameter at different flow rates. The various phenomena of jet formation, appearance of instability waves, jet disintegration and droplet detachment were captured using a high-speed camera. The jet length distribution was calculated and compared with predictions. A Fast Fourier Transform of the temporal development of the jet length was also performed. The jet length showed good agreement with empirical correlations if the instability wave velocity is used in the definition for Reynolds and Weber number. The size distribution of the formed droplets was investigated and compared to theoretical predictions. For higher flow rates, the main peak agrees very well with theoretical equations. For low flow rates near to transition to the dripping regime, short jets formed in a highly repetitive manner. The drop size distribution was found to be very narrow with a mean diameter according to Tate's law taking into account a Harkins and Brown correction factor.

The Flash-Reactor as Special Melting Unit for Powdery Materials in DSG (Dry Slag Granulation) Application

The Flash-Reactor is a new facility for recycling of zinc containing dusts from the steel industry. Therefore the facility must have the capabilities to process a wide range of dusts with varying zinc content. The Flash-Reactor pilot plant was successfully set into operation at the Montanuniversitat in Leoben in 2010 and another series of experiments was conducted in February 2011. Because of the various operating conditions adjustable in the facility and the compact design the Flash-Reactor can be used as a special melting unit for powdery goods. Siemens VAI wants to build a pilot plant for dry slag granulation for testing the possibility of integrated heat recovery, needing small amounts of liquid slag for the test performance. This is where the Flash-Reactor enters the project as upstream melting unit. Heat recovery during slag granulation provides energy recovery of 20 MWth or 6 MWel regarded to a slag mass flow of 1 t/min. Tests in the beginning of the last decade have proven the functionality of the compact dry granulation process, industrial application has failed due to uncertain economic lookout for the steel industry in that time.