Figure 1. Sources of light in Electric Arc Furnace.
The Electric Arc Furnace (EAF) process is extremely energy-intensive and emits massive amounts of light. These light emissions provide a lot of information about the EAF process and can therefore be used as a control parameter in the process (Aula 2016). This short summary gives a better understanding of how to gather this information from light and what factors affect it. The text is based on research done on Optical Emission Spectroscopy (OES) and high-temperature metallurgy.
The main light sources during flat bath conditions are molten slag and the arc. Combustible material produces flames that also emit light in the EAF atmosphere. The electric arc emits mainly ultraviolet and visible light. However, infrared radiation dominates the optical emission spectrum emitted in an EAF.
Optical conditions in the EAF
The understanding of optical conditions is important since they affect the optical emission spectrum, especially when the conditions are as harsh as they are in the EAF. The extreme environment inside the furnace affects the optical conditions in many ways. Large amounts of gas molecules and dust particles are formed during the heat. In addition, a thick layer of fumes and gases prevails inside the furnace. These factors create an optically thick atmosphere inside the furnace that absorbs the optical radiation from the arc and the bath. (Aula 2016)
The optical thickness of the EAF process varies in different process periods. In the early process stages, the gas formation is high, which affects the optical thickness of the EAF atmosphere. When scrap melting progresses, the volatile compounds and small particles are removed and the atmosphere clears. During flat bath conditions or when the arc is partly covered, the dust particles are mainly caused by the splashing of metal and slag. In the foaming slag phase, the bursting of bubbles generates dust particles. However, in these conditions, it has been observed that the fume density is the lowest. (Aula 2016) By tracking the changes in light emissions we can observe the melting progress and other information from the furnace
The particle sizes affect the optical properties of the EAF dust. Small particles scatter and absorb the light more effectively than larger ones. Therefore, the optical conditions inside the EAF depends both on the size and amount of the dust particles. (Aula 2016)
Optical emission spectrum
The optical emission spectrum determines from which point the furnace emits the measured optical radiation. Spectrum types in an EAF during a melt are dark current, thermal radiation, alkali peaks and atomic emissions. (Aula et al. 2014)
Figure 2. Spectrum types in an EAF during melting (modified from Aula et al. 2014).
At the beginning of the melt when the arc is covered completely by the steel scrap, only the dark current is observed. It can also be seen when new material is added. Dark current has a very low intensity. (Aula et al. 2014)
When the scrap in the viewing cone has melted the thermal radiation emerges. Observation of thermal radiation in the earlier stages is also possible due to the hot fumes coming from the melt and reflections from the steel scrap. The optical thickness of the EAF process affects thermal radiation. (Aula et al. 2014)
Alkali peaks are generated by the atomic emissions of alkali metals contained in the hot slag or gas. (Aula et al. 2014)
An electron returning from a higher energy level to a lower energy level emits atomic emission radiation. Atoms have a unique orbital configuration; Therefore, the wavelengths of atomic emissions are also unique. The atomic emission spectrum emerges when the measurement head sees the completely exposed electric arc. (Aula et al. 2014)
Optical emissions as a part of EAF control
Gathering of on-line data from the EAF process is extremely difficult due to the harsh conditions inside the furnace. Developing a dynamic control model for the EAF process is not possible without real-time data. Measuring values such as temperature of the side panel cooling-water, arc voltage and off-gas temperature and composition can provide information of the EAF process. However, these parameters do not provide accurate real-time data on the most critical metallurgical parameters, i.e. melting of the scrap, temperature of the steel and composition of the slag. (Aula 2016)
Optical emission analysis can provide on-line information from the inside of the electric arc furnace. The technology behind ArcSpec is based on OES, which allows real-time monitoring of the EAF process and the light it emits. This allows controlling the electric arc furnace with more accuracy than ever before leading to significant cost savings and production capacity improvements.
References
Aula, M., 2016. Optical Emissions from Electric Arc Furnaces. Doctoral thesis. Oulu: University of Oulu.
Aula, M., Leppänen, A., Roininen, J., Heikkinen, E., Vallo, K., Fabritius, T., and Huttula, M., 2014. Characterization of Process Conditions in Industrial Stainless Steelmaking Electric Arc Furnace Using Optical Emission Spectrum Measurements. Metallurgical and Materials Transactions B, 45(3), pp. 839-849.
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