Objectif
The aim of this research is to improve the processing of the LD slag after the steel teeming without disturbing the classical treatment of the LD slag: cooling in the slag pits, scrap removal in a plant by magnetic separation and size classification by screening.
By combining the careful selection of slags with lower free lime content and the application of an extra weathering of the hot slag in the pits, it is possible to produce LD aggregates of higher stability. The German standards (Nordrhein-Westfalen) of < 5% expansion is respected and the Belgian (French part of Belgium) limit of 3% is almost reached.
An improvement of the environmental conditions (ie a reduction of the dust emission) was also observed by applying this improved water cooling.
This valorization of the LD slag by improvement of its quality proposed by SIDMAR in collaboration with CRM, is linked with economical, technical, social and environmental views of the problem.
The economical view leads us to the civil construction. This sector needs great quantities of aggregates and LD slag can be utilized without the need of an expensive special treatment. Nevertheless the application of the quality assurance concept to the aggregates and the competition on their market in the civil construction oblige the steel industry to produce better quality by-products.
The first objective of the steelplant is to produce steel. During the last decade the quality specifications of the steel increased. One of the ways to produce a steel of better quality is to use more lime during the blowing of the steel in the BOF vessel. This method leads also to a higher ratio of free lime in the LD slag. In order to obtain LD slag aggregates of better quality, the technical view of this research, it is necessary to reduce the free lime and/or its effects.
This project has also a social objective, namely to improve the working conditions during handling of the LD slag by reducing dust emissions.
The environmental point of view is also taken into account because the project aims to better valorize by-products instead of landfilling them while consuming natural products.
In order to obtain to a quality assured aggregates from the LD slag, it is necessary to improve not only the weathering conditions, but also to select and to control the concerning slags:
- the selection of slags with a lower content of free lime is based:
a) on a quick measurement of the free lime on content effectuated on a sample of liquid slag taken during teeming from the converter and
b) on the communication, as soon as possible, by radio to the driver of the transport car, who is responsible for the shipment of the liquid slag from the steelplant to the pits, of the order in which section of the slag pit the slag should be teemed.
- the slag pit area offers a sufficient number of pouring sections in order to receive 2 or 3 different classes of slag;
- the control of the weathered slags is effectuated by use of an equipment for measuring its expansion under the action of water steam (FEhS method).
The modified water cooling processing is carried out in the slag pit as follows:
1) after the first (and usual) water sprinkling in the pit, the slag is broken by mobile machines;
2) then a second and intensive sprinkling is applied on the still hot slag in the pit without generating sludge.
The result of this treatment is an improvement of the stability of the slag: the volume expansion measured by the steam test is divided by a factor of 2 and is maintained under 5%.
If a further artificial sprinkling is applied later on the slag yard, an additional reduction of the expansion of 1 to 2% is observed.
Another advantage of the application of the second sprinkling in the pits is the decreased level of dust emissions during the slag handling in the pits and in the plant for scrap removal.
After the weathering period on the slag yard, a part of these processed slags was used for the construction of an internal road in the area of SIDMAR. This road is conceived for heavy trucks. They were no problems during the construction of the road foundation and the compacting results of the slag are similar to those obtained when using natural aggregates. After 6 months, no cracks are observed on this road.
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Coordinateur
9042 GENT
Belgique
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