Siemens Optimizes Blast Furnace Top-Gas Dust Recycling Process
08/14/2007 - Siemens Metals Technologies will install its new cyclone separator for dedusting blast furnace top gases at JSW Steel Ltd in India.
Siemens Metals Technologies has developed a new cyclone separator design for the dedusting the top gas exiting from a blast furnace.
With this solution, the dust-separation efficiency can be flexibly adjusted to recycle the highest possible quantity of dust (and hence the highest amount of iron) to the ironmaking process without exceeding limitations in permissible concentrations of zinc and other heavy metal components. The first industrial prototype will be installed in a new Indian blast furnace for which Siemens is providing the engineering, technology, and equipment.
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The first installation of a full-sized cyclone designed on the basis of the model work will be in the new No. 3 Blast Furnace at JSW Steel Ltd in India.
Start-up of this blast furnace, which will be India's largest, is scheduled for mid-2008.
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In order to use blast furnace top gases for downstream heating applications, the gas is cleaned in a two-stage dedusting process comprised of a dry-type dust-catcher or cyclone as the first stage and a wet scrubber as the second stage. Dust trapped in the first stage is usually recycled to the ironmaking process, while the dust (sludge) from the wet scrubber is normally discarded, incurring extra costs for disposal or dumping. For example, blast furnaces with hearth diameters from 8.5 to 14 meters would generate between 250,000 and 700,000 Nm3 of top gas per hour that would have to be treated. Dust contained in these gas volumes would range from 5 to 14 tonnes, and would typically contain an iron content of 15–35%, representing a valuable by-product.
Increasing numbers of blast furnace operators are using higher-efficiency dust-separation cyclones (instead of low-efficiency dust catchers) in order to maximize iron recovery. However, not all top-gas dusts are appropriate for recycling. For example, high concentrations of heavy metals in the blast-furnace burden, especially zinc, could potentially affect process parameters. Results could include higher coke-consumption rates, the formation of skulls in the upper shaft area, or a reduction of lifetime of the furnace refractories.
Cyclones, which have better separation efficiency, can remove a higher portion of the fine zinc-bearing and other heavy-metal-bearing dusts, which are then recycled to the process. There is some danger that concentrations of unwanted metallic compounds may accumulate due to the repeated sequences of evaporation, separation and recycling. Therefore, optimized and flexible dust-separation efficiency in the first stage of top-gas treatment is vitally important, both from cost and operational perspectives.
On the basis of CFD (computer fluid dynamics), cyclone performance was modeled, and varying geometric changes were investigated. A physical 1/10-scale Perspex model incorporating a number of features (for testing various geometries) was then constructed. Testing variables included inlet bends with and without guide vanes, variable barrel lengths, and the position and geometry of the vortex finder. Fine sawdust and hollow glass micro-spheres were used for testing.
In order to enable the dust-separation efficiency to be varied to prevent unacceptable quantities of fine, zinc-bearing dust from being recycled to the process, several design modifications were carried out on the model. Four bypass pipes were installed to allow dust and gas from the top of the cyclone to be diverted to an outlet pipe, which reduced efficiency mostly in the desired dust-size range.
The industrial version would be additionally equipped with a blank flange installed at some position along the length of each duct. Depending on the content of zinc in the recycled dust, the number of blanked-off bypass ducts can be changed during a planned furnace shutdown to vary the dust-collection efficiency. In this way, a maximum dust-recycling ratio could be achieved, without exceeding acceptable zinc concentrations.
The first installation of a full-sized cyclone designed on the basis of the model work will be in the new No. 3 Blast Furnace at JSW Steel Ltd in India. The start-up of this blast furnace, which will be India's largest, is scheduled for mid-2008.
Metals Technologies (MT), a Division of the Siemens Group Industrial Solutions and Services (I&S), is one of the world's leading engineering and plant-building companies for the iron and steel industry as well as for the flat-rolling sector of the aluminum industry and for open-cast mining. MT, which was created from the integration of Voest-Alpine Industrieanlagenbau, Austria, and the electrical engineering product business and automation solutions of Siemens, provides a comprehensive range of supplies and services for all related technological processes and integrated automation solutions for the entire life-cycle of metallurgical plants.
The Siemens Industrial Solutions and Services Group (I&S) is the integrator of systems and solutions for industrial and infrastructure facilities and global service provider for the plant and projects business covering planning, installation, operation and the entire life cycle. I&S uses its own products and systems and process technologies in order to enhance productivity and improve competitiveness of companies in the sectors of metallurgy, water treatment, pulp and paper, oil and gas, marine engineering, open-cast mining, airport logistics, postal automation, intelligent traffic systems and industrial services. In fiscal 2006 (to September 30) I&S employed a total of 36,200 people worldwide and achieved total sales of EUR 8.819 billion, according to U.S. GAAP.
