A new CSLM-based approach to study metallurgical slag solidification

The KU Leuven’s High Temperature Processes and Industrial Ecology research group (SIM2) has developed an innovative experimental setup using confocal scanning laser microscopy (CSLM) to measure the electrical conductivity of slag while simultaneously observing slag crystallisation. A synthetic slag was selected for the study in the first phase. The knowledge obtained with the simple slag system can be partially used for further understanding the industrial slag solidification. This study demonstrates the feasibility of online monitoring of slag crystallisation by electrical conductivity measurement. And a quantitative relationship was established between the slag’s electrical conductivity and its liquid fraction under isothermal conditions. This relationship can be used for the development of online monitoring systems for the precise control of slag crystallisation, to obtain suitable mineralogy for high-added value applications. The results were published in Resources, Conservation and Recycling.

Slag valorisation

Slag is a main by-product of the pyrometallurgical industry. The annual production of iron and steelmaking slag is about 40 million tonnes in Europe, leading to economic and ecological issues. It is therefore of great importance to implement value-added applications for slag.

Slag valorisation requires slag products with specific mechanical, physical and chemical properties, which are directly related to slag composition and mineralogy. Hot stage slag engineering is an effective way to change the properties of the slag in the liquid state in order to control the properties of the solidified slag. Specifically, the slag mineralogy can be manipulated by steering the slag cooling process to obtain the desired slag mineralogy with the designed crystal size and shape, and the ratio of the solid crystalline to amorphous fraction for tailoring the slag properties. This relies on a quantitative understanding of the crystallisation/solidification behaviour of the slag under different hot stage slag engineering conditions.

There is no online technology developed yet to monitor the slag solidification process in the slag yard of metallurgical plants for slag mineralogy control. On the other hand, the electrical conductivity measurement technique is well developed and is characterised by its low installation and operating cost, high efficiency and non-destructive nature. This technique is used in many applications. Since the slag’s electrical conductivity is sensitive to a minor change in its microstructure, conductivity measurements provide a promising method to monitor online slag solidification during hot stage processing.

The relationship between slag crystallization behaviour and electrical conductivity

This paper aims to monitor the slag solidification/crystallisation through electrical conductivity measurements with the newly developed setup under isothermal conditions. The electrical conductivity of a synthetic CaO-Al2O3-SiO2-MgO (CASM) slag was measured; meanwhile, the slag crystallisation behaviour was simultaneously observed in situ at isothermal temperatures ranging from 1250 to 1330 °C.

The effect of the slag’s liquid fraction and composition on the electrical conductivity was quantitatively identified by analysing the microstructure of samples quenched from different temperatures. The results of the slag’s electrical conductivity measurements have shown that the conductivity can be applied to monitor slag crystallisation online, which will be useful to steer the mineralogy of a particular slag for high added-value applications.

Complete reference of the paper

Zhang Ling, Annelies Malfliet, Bart Blanpain, Muxing Guo, Understanding the relationship between slag crystallization behaviour and electrical conductivity under isothermal conditions for online slag solidification monitoring in slag recycling, Resources, Conservation and Recycling, Volume 182, 2022, 106319. https://doi.org/10.1016/j.resconrec.2022.106319

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