Dry Granulation of Blast Furnace Slag for Heat Recovery

Blast furnace slag is known as by-product from the production of hot metal at integrated steel and iron mills. Approximately 300 kg of this liquid slag with a temperature of about 1500 °C is tapped together with each ton hot metal. This means a worldwide annual production of about 400 million tons of blast furnace slag. Further processed, water granulated blast furnace slag can be produced, if the liquid slag is cooled down rapidly for solidification. State of the art is a closed system which uses lots of water for rapid cooling and solidification to produce a granulated blast furnace slag which has an amorphous structure. This product normally is dried and grinded for use as a binder in the cement production due to its latent hydraulic properties. The disadvantage of wet systems is on the one hand, the recooling of the circulating water and on the other hand the need for drying of the granulated blast furnace slag, due to the residual moisture up to 20 mass percent. Furthermore the sensible heat of about 1.5 GJ/t of slag is lost. [1]

Therefore a new lab rig has been installed by Siemens VAI in cooperation with Montanuniversität Leoben at the Chair of Thermal Processing Technology for research and development. In this project there are the co-partners voestalpine Stahl GmbH, ThyssenKrupp Steel Europe AG and FEhS-Institut für Baustoff-Forschung e.V. This project is funded by the German Federal Ministry of Economics and Technology (BMWi).

Figure 1: Lab rig at the University of Leoben, Chair of Thermal Processing Technology (Picture: Siemens VAI)

The lab rig is based on the “Rotating Cup” principle. The liquid slag will be atomized by a cup and small particles spread away due to the occurring forces. First tests involving the “Rotating Cup” principle were performed in Redcar at British Steel in the 90's. Further investigations took place at Vitkovice in the Czech Republic between 2002 and 2004. These tests were part of the ZeroWaste project of Siemens VAI. Both research activities did not focus on heat recovery due to the lack of political pressure at this time. It could be shown that dry granulation for producing glassy blast furnace slag is also feasible with the “Rotating Cup”. [1]

Figure 2: Trials at Redcar (left) and Vitkovice (right) [1]

The new lab rig at the University of Leoben has been designed on basis of the results of the lab tests at FEhS-Institut and the modelling of Siemens VAI to find the best operating parameters for producing high quality granulated blast furnace slag and highest possible air outlet temperatures. Therefore about 300 kg of slag is molten in the “Flash-Reactor” for each trial at the University's workshop. The liquid slag is tapped into special slag pots which are lined with refractory to prevent heat losses. Afterwards the slag pot is manipulated into a tilting unit, installed close to the granulator. The liquid slag feed into the granulator can be measured and controlled via load cells. The liquid slag will be delivered to the centre of the granulator passing a slag runner and a vertical refractory pipe. After atomizing the slag droplets cool down rapidly during their flight towards the granulator wall once spread away by the fast spinning cup. The surface of the particle has to solidify during this short flight time before falling into a developed fluidized bed made by granulated particles. [1]

Figure 3: “Rotating Cup” principle [1]

Furthermore the granulator wall is indirect cooled by a water jacket. Air will be introduced at the bottom of the device. A special air distribution insures a proper cooling of the particles, an agitated bed and no agglomeration. So a maximum heat exchange rate can be reached. Hot off gas can be added to raise the input air temperature for the granulation process. Hence a recirculation system can be simulated which should be used in industrial application. Flow straighteners at the top hinder any carryover of the particles, where the hot air leaves the granulator. [1]

Afterwards the air is cooled down by a spray tower before leaving the lab rig passing a droplet separator and the suction fan. The granulated slag particles can be discharged at the bottom of the granulator after each trial.

Figure 4: Isometric drawing of the lab rig

The whole lab rig is fully automated. More than 90 sensors are recording the process data like slag mass flow, different pressures, temperatures and other measurands. This helps analyzing each trial to find the best operating parameters for getting a proper product while having a maximum air outlet temperature. The main challenge considers ensuring rapid slag freezing to reach the transformation point of the slag very fast. Otherwise the product will not get fully glassy but partially crystalline which is unsuitable for further use in the cement industry. Cooling with air is much more difficult due to a substantially lower heat capacity compared with water used in conventional devices. Furthermore the air heats up while streaming through the agitated bed. So additionally the temperature difference worsens the setting process.

First campaigns showed promising results regarding glass content, grain size distribution and particle shape. Also a significant off gas temperature increase could be measured. With the obtained data, process optimization can be done and a scale up for further investigations can be realized in the future. Using dry slag granulation at integrated steel and iron mills will gain access to the sensible heat of the blast furnace slag. The recovered energy can be used for the production of steam or electricity or for other useful preheating processes. An energy potential for recovering 20 MW thermal energy or alternatively of about 6 MW electric power generation from a slag feed of one ton per minute can be calculated. Without the need of an upstream drying system for the blast furnace slag energy and as in consequence CO₂ can be saved. In summary dry granulation of blast furnace slag allows an environmental friendly and sustainable subsequent processing for one of the last big heat recovery potentials at a modern blast furnace. [1]

References

[1]        McDonald, I., Long, E., Werner, A. & Most, D. (2010) Dry slag granulation - The environmental friendly way to making cement. In: IOM3 conference on Waste Recovery in Ironmaking and Steelmaking Processes. London: United Kingdom