Dust formation in electric arc furnace: birth of the particles



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INTRODUCTION

The Electric Arc Furnace (EAF), designed for steelmaking from recycled scrap iron (see figure 1), also co-produces between 15 to 25 kg of dust per ton of steel. Dust formation is strongly linked to the process which can be divided into five steps:



  • furnace charging: the scrap and the additives (lime, coal…) are loaded into special charging buckets which are then emptied into the furnace;

  • melting: an electric arc is created between the graphite electrodes and the scrap which entails the charge melting and the formation of a steel bath covered by a slag layer, volatile solute species (e.g. zinc) begin to be removed;

  • refining: in this step of the process, phosphorus is removed from the steel bath by interfacial reactions between the slag and the liquid metal, injection of oxygen promotes the decarburization reaction with dissolved carbon and bubbles of carbon monoxide (CO) are formed, which helps to remove other dissolved gases;

  • slag foaming: the CO-bubbles crossing the slag layer make it foam, the foaming process being enhanced by the addition of coal powder;

  • casting: after the composition and the temperature of the bath have been controlled, the liquid steel is cast.

During the process, the fumes are extracted through an aperture in the furnace roof. These are post-combusted, cooled, and cleaned from the transported dust, which is collected in large bag filters. This dust contains hazardous, leachable elements such as zinc, lead or cadmium which require EAF dust to be stored in specific landfills.

In order to propose economically feasible solutions for both recycling and/or reducing EAF dust, the understanding of the dust formation is necessary. The present paper describes the different mechanisms of formation identified thanks to a morphological and mineralogical characterization of various dust samples, and then focuses on the study of the main source of emission, i.e. bubble burst at the surface of the liquid bath. An original experimental device was designed in order to understand and quantify precisely this phenomenon. The results of the experimental study are presented and discussed.


Figure 1. Schematic representation of an Electric Arc Furnace




CHARACTERIZATION OF ELECTRIC ARC FURNACE DUST

The investigations regarding the morphology and mineralogy of the particles contained in EAF dust give useful information for the identification of the dust formation mechanisms. Several dust samples coming from different industrial furnaces were observed by SEM (Scanning Electron Microscopy) and analyzed by EDS (Energy Dispersive Spectrometry). As shown in figure 2, EAF dust particles cover a wide range of sizes. To simplify the survey of the morphologies, we distinguished two categories of particles: large particles from a few dozen to a few thousand micrometres, and finer particles lower than 20 μm.


Figure 2. EAF dust






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