Open slag bath furnace (OSBF): Pig iron production using reductive smelting

The Open Slag Bath Furnace (OSBF), commercialised as iBlue® and developed by Tenova, is an AC electric furnace designed for melting DRI to produce pig iron. The intent of combining a direct reduction plant with a reducing furnace technology like the OSBF is primarily driven by the need to reduce carbon emissions associated with conventional pig iron production (the blast furnace route).

TECHNICAL DESCRIPTION
The OSBF is designed for primary DRI feed. A secondary feed system is designed to feed a variety of reverts (examples include ladle slag, mill scale, fume dust, BOF sludge, etc.), fluxes and reductants. The OSBF solution is based on 2 x 90 MW OSBF rated to process 2.5 million tonnes DRI per annum. Higher electrical resistances are reached in an OSBF (compared with other conventional furnaces operating with the electrodes immersed in the liquid slag or submerged in the furnace charge) resulting in a higher power factor and better transformer efficiencies.

Key operational steps include:
• DRI supply: The metallised DRI is transferred to the OSBF feed bins (via gravity). A separate furnace feed system with smaller bins is also included for adding reductant and fluxes, as well as to make minor corrections in the carbon balance and adjust the slag chemistry when necessary. Anthracite is typically considered as a reductant.
• Electric Melting: The furnace equipped with three Söderberg electrodes is operated in an open arc (or so-called brush arc) mode, where the power input becomes less dependent on the slag conductivity and eliminates the need for maintaining a coke bed in the furnace.
• Hot Metal Tapping: Hot metal tapping is expected to be semi-continuous or as determined by the downstream process requirements and the size of the torpedo car or ladle. The electric melter does not operate under the same high pressure as a blast Furnace, therefore tap-ping is less complex.
• Slag Granulation: In a treatment station, air is blown from a high-speed fan to granulate the OSBF slag. This treatment is characterised by an ultrafast dry cooling of the OSBF slag to avoid slag disintegration that typically occurs during natural cooling. The result of the treat-ment is a granulated material with a granulometry of 0.5 – 3 mm.
• Off-gas cleaning system: The dust-laden gas emitted from the furnace enters a patented hot gas filtration system, passing through the filter element bundle and exiting the clean gas section through a top outlet nozzle. The dust in the gas that passes through the filter elements remains on the outside of the filter element and is regularly removed to prevent blinding of the elements. The filter elements are cleaned online by a blowback system, which is activated/deactivated based on the measured differential pressure across the filter tube sheet.

DEGREE OF MATURITY
For high carbon DRI, this process has been well proven and in commercial operation for decades, it has reached a TRL>9. For instance, Iron Dynamics (USA, established in 1996), New Zealand Steel (NZ, first production early 1970’s), and Highveld Steel and Vanadium (South Africa, first production late 1960’s) have successfully used reductive melting at a commercial scale.

For low carbon DRI applications (e.g. when hydrogen is used in the direct reduction plant), the estimated TRL is 6/7. The biggest challenge is associated with process designs where the melter has to recarburise hydrogen-based DRI with very low carbon content.

CROSS-MEDIA EFFECTS
No significant cross-media effects are reported.

BARRIERS TO IMPLEMENTATION
This solution is CAPEX intensive, however the CAPEX investment be potentially recovered from the revenues generated by the metal and slag.

Coupling a direct reduction plant with an OSBF makes sense for low-grade DRI and high-carbon DRI (4%+C), which, at the moment, is not a high-volume product in the commodity market (HBI is still the industry reference, which is normally around 1.5%C).

Basic information about the technique

Participant Companies