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Direct recycling of zinc-contaminated LD filter dusts and slurries

Ziel

The aim of the research is to return a greater proportion of LD than currently achieved directly, ie without prior thermal or other processing in special units, to the steelmaking process by introducing time-controlled separation. This would substantially reduce the cost of processing the remaining zinc-contaminated fines of the LD, as the quantity would be reduced.

Although in theory (mainly thermal) processes are already available, at least as prototypes, for the processing of zinc-contaminated and dusts, the energy input and cost of such processes is enormous. A significant reduction in the quantities of LD which cannot by recycled directly therefore contributes to a reduction in the cost of this steel production process and hence to greater competitiveness and reduced pollution.
Monitoring over time of the zinc contents in the converter primary dust over the course of several heats revealed that even with average zinc quantities of 100 kg per heat, which is 50% higher than today's average zinc input in charge scrap, separation of utilizable sludge fractions by purely mechanical means is possible if:

- in the making of high-quality steels using blue scrap (zinc-free) the entire dust discharge is utilizable,

- using low-zinc mixed scrap, timed separation into a 2/3 high-Zn and a 1/3 low-Zn utilizable sludge fraction is feasible.

When using high-zinc old scrap suspected of containing alloying elements the entire dust charge, including the low-zinc fraction, was always higher than 0.4% Zn and therefore not suitable for use in our sinter plant.

These results suggest that a twin-runner system, controlled by a three-position selector switch depending on scrap grade, could be realized on a production basis. Using the scrap code for the scrap being used the steel mill process computer can determine and set the required switch position when the heat begins.

To verify the results on an industrial scale an additional runner adapted to the existing runner system was put into operation. Tests to separate out low-zinc LD sludges were carried out using the switch at the modification point of the additional runner. The separated sludges were directed to the redesigned sludge processing unit for treatment.

The investigations provided the following findings relative to sludge handling:

- Sludge fractions of any size can be reliably separated using the switch, in the form of a pneumatic slide gate.

- Sludge treatment using magnetic separators for sludge/water separation functions continuously if sedimentation of the LD coarse sludge is prevented by use of compressed air in the runout area of the sludge collector.

Without preliminary drying the separated sludge fractions can be processed together with LD slag into easy-to-handle pellets for sinter plant use.

An overall concept was developed for computerized data capture and analysis of the sludge treatment process. A three-computer network for exchanging test data was installed. This network for continuous quantitative data collection was expanded with the link to the steel mill process computer and now delivers process data from current production on-line. This data is necessary for continuous production operation of the sludge treatment unit.
Final systematic investigations into the conditioning of the LD sludge with fine LD slag (0-10 mm) proved that it is possible over a wide humidity range to produce pellets both in the gravity mixer and in the forced-feed mixer which are readily utilizable in the sinter plant and present no difficulties in terms of agglomeration. Fundamentally and independently of the Zn content this treatment provides a simple and elegant method of conditioning fine steel mill sludges. The LD sludge/slag mixtures tested in the sinter plant laboratory were without exception suitable for sinter plant use and even showed slight advantages over straight LD slag.
In preliminary investigations the primary dust collection system was first examined qualitatively and quantitatively in order to establish the pre-requisites in terms of sampling location, sample quantity and sample number for targeted, systematic operational tests. The solids loading of the wet-cleaned converter dust was examined mechanically and chemically in respect of quantity, consistency and discharge behaviour over time. A fast zinc discharge rate of a dynamic nature preceding the discharge of dust was found. Quantitative sampling was successfully tested using a known zinc input quantity. The discharge behaviour of the dusts entering the secondary dust collection system was examined subsequently. The aim here was to investigate whether a process-dependent discharge rate could be identified, particularly for zinc, which inhibits recycling. This would make it possible to separate out dust fractions from the secondary dust collection system over the course of the process independently of the sludges from primary dedusting.

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Krupp Hoesch Stahl AG
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44147 Dortmund
Deutschland

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