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Contenido archivado el 2022-11-21

Innovatory techniques for intense dewatering of iron metallurgy sludge for upgrading

Objetivo



Many processes in the steel industry produce effluent laden with matter in suspension which produces sludge when purified for the purposes of dumping in a natural environment or recycling.

These sludges, which are initially in a liquid state, such as when extracted by desludging settling basins, are generally thickened and concentrated by prolonged residence in settling tanks or lagoons or dewatered mechanically by centrifuging or filtration.

Dumping these sludges is becoming a less and less acceptable solution as many sites are saturated and there are environmental constraints and administrative requirements. Moreover, regulations will soon only authorise dumping of wastes which are proven to be incapable of upgrading, which is not the case for the steel industry sludges considered here.

As these wastes are rich in iron and possibly alloying metals, the upgradable fraction ought to be recycled by agglomeration or pyrometallurgy. In general, this requires intense dewatering of the sludge, as do some vital preliminary treatment processes (dezincification of sludge by heat treatment). This involves a very high energy drying process which has to be kept to a minimum by dewatering the sludge as much as possible beforehand.

The aim of the research project is to use innovatory techniques or combinations of known methods to obtain intense dewatering of steel industry sludge which will minimise or cut out altogether thermal drying processes which are expensive in terms of energy.

This will also enable new techniques for treating or upgrading sludge (thermal or pyrometallurgical processes) to be improved or made technically and economically viable.

The following categories of sludge will be considered as they cause major waste disposal problems in the steel industry and represent the type of waste that the industry is very keen to upgrade.

- Sludge from scrubbing blast-furnace gas

- produced in current wet scrubbing installation (raw sludge or sludge which has been hydrocycloned industrially as the case may be),
- stocked in old lagoons which have to be reused.

- Sludge from melting shops

In both cases steel making using phosphorous and hematite ore will be considered.

Choice of techniques

These techniques will either be innovatory in themselves or in the way they are applied:

- In the case of old sludges constituting the sediment of lagoons (of which there are hundreds of thousands of tonnes in all integrated steelworks whether active or disused), the possibility of using electro-osmosis as a means of on-site draining prior to mechanical removal will be assessed. This technique has already been used for drainage and consolidation of argillaceous soils in geotechnology. The principle lies in applying a continuous electrical field between electrodes placed in the humid porous mass. As a result of superficial electrical phenomena, the interstitial water migrates to the system cathode where it can be drained and disposed of. This technique has already been used successfully by IRH on other sludges and wastes.

- In the case of sludges treated at source the innovatory aspect of the research will lie in:

- optimum combination of two techniques, one conventional (continuous centrifugation) and the other under development (continuous or possibly discontinuous filtration of a very thin layer several millimetres thick under very high pressure),
- a search for original methods of preliminary conditioning of the sludge (new flocculants producing agglomerates which are resistant to high compressive and shear loads) which will be assessed by an original rheological technique specially developed by IRH, a variant of which has recently been introduced on an industrial scale.

-In all cases the assessment of the degree of dewatering will be based not only on the residual humidity in the treated material but also on the concept of water-to-solid binding, which can be measured by determining water activity.

Conventional methods of mechancial dewatering will only be used to provide performance reference standards (dry content, dewatering speeds).

Convocatoria de propuestas

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Régimen de financiación

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Coordinador

INSTITUT DE RECHERCHES HYDROLOGIQUES
Aportación de la UE
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Dirección
Rue Gabriel Péri 11 BIS
54500 Vandoeuvre-les-Nancy
Francia

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Coste total
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Participantes (3)