Improving EAF operations through monitoring and control of slag residence time
By Ananda Bhattacharjee
An estimated 30% of today’s world steel production is through the electric arc furnace (EAF) steel-making route. This contribution to world steel production is likely to continue, given that EAF steel-making is more environment friendly and there is availability of a huge stock of scrap steel on earth, waiting to be recycled. It is, therefore, important that EAF steel making improves in terms of efficiency and productivity.
As a chemical reactor in steel making, EAF is quite slow compared to the basic oxygen furnace (BOF) route. The conditions that drive chemical reactions between slag and metal are impaired by insufficient inter-mixing of slag and metal during the process. That’s not all. There is an additional limitation. The design of an EAF is such that there is always a possibility of slag flowing out through the slag-door early in the process.
Early flowing out of slag is a huge problem and is seen in EAF operations where awareness about the resultant losses is poor. It could also be because the operators are not trained well enough to take counter- measures. In some organisations, there is no system of monitoring, analysing and reducing these losses.
While slag is basically a solution of metallic oxides required for refining of steel, it is also a sink for iron – in the form of iron oxide (FeO) – the valuable metal that we strive to recover. Loss of unreduced slag is a direct loss of iron (Fe). In addition, early loss of slag adversely affects arc stability, refractory life, heat insulation of bath, power consumption, electrode consumption, etc. The importance of slag in the EAF process makes it imperative that the approach to EAF steel making should focus on the following:
- Holding slag inside EAF;
- Extracting iron and all other benefits from it; and
- Finally, releasing it through the slag door.
There are technologies that are presently being developed to achieve this end. These technologies are likely to improve consistency of the benefits. However, trained and knowledgeable EAF operators have achieved this type of operation with the help of home-grown interventions.
Every technology that develops, over a period, into a sophisticated electro-mechanical-auto-software combo, has its roots in simple, manual applications. So is the case of the initiative to increase the residence time of slag in EAFs.
The following suggested home-grown methodology involves tweaking the furnace refractory pattern to create extra working volume through the following steps:
- Assessment of the existing working volume of the EAF;
- Measuring and monitoring of the furnace hearth depth at the beginning and end of each refractory campaign:
- This gives a quantification of the actual erosion pattern of the hearth, and
- An assessment of the possibility of operating with a deeper hearth.
- Measuring refractory bank erosion at the end of each refractory campaign:
- This helps to avoid working with thicker banks than required.
- Marginal raising of the door sill height to the extent possible, without adversely affecting the final slag outflow.
The above-mentioned measures help keep a tight control on the working volume of an EAF. The aim is to keep the working volume on the higher side, as practically possible, thereby creating the extra space required to accommodate and hold the slag inside the furnace.
Practices in different steel plants have shown that it is possible to retain the slag in the furnace for as long as 80% of the process time. During this time, the slag is subjected to the required level of fluxing and carbon injection to achieve the basicity (~2.20) and FeO (~20%) levels. The remaining 20% of process time is utilised to drain off the “spent” slag.
The above methodology and the underlying principles are very simple and basic. EAFs tend to deviate from the outlined principles of operations because their inherent design allows the deviation, unless proactively countered. What else would one expect when the point of outflow of slag is so close to the slag-level itself?
Systematic working volume measurement in EAFs and modifying the same with a focus to achieve the target slag residence time is yet to be followed as a strategy in many of the operating EAFs across the world. Developing a strategic focus in this direction will help the techno-economics of EAF steel making positively, particularly in the mini steel mills that operate with less-trained operators.
Technologies have come about in the market that work on the same principle of enhanced duration of slag retention inside EAFs. These technologies also provide added benefits of closed-door operation, use of scrap pusher, etc. These technological benefits are expected to give consistent and higher monetary savings compared to home-grown interventions. The SMS Group and INTECO/PTI have come up with different variants of these technologies.
Closed-door operation: This is an innovative step towards reduction of consumptions in the EAF process. It is becoming a trend in EAFs today. It is stated by Yuri N. Toulouevski and Ilyaz Y. Zinurov in their book, Innovation in Electric Arc Furnaces – Scientific Basis for Selection, that the amount of air that infiltrates into the furnace through the door opening is a function of the roof height, door opening area, and the gas pressure inside the furnace. It is also estimated therein that the air ingress into a 120-ton EAF with H = 2.6 m, h = 1.25 m, and b = 1.0 m, is about 105 m3/min, with gas pressures in the furnace at -5 Pa and the door only 25% open. (H – furnace roof height above the sill level, h – overall height of opening of slag door and b – door width).
A closed-door operation technology will significantly reduce this air ingress and the resultant adverse impacts on electricity and electrode consumption. It will also physically hold the slag inside the furnace and facilitate controlled release.
Retention of slag for a targeted duration inside the EAF before releasing it in a controlled manner gives a few benefits. Slag is a medium which holds a huge amount of iron oxide (FeO) and also helps in the following ways:
- Facilitates chemical reactions in the furnace;
- Facilitates electric arc stability – improves average power input rate in the furnace;
- Facilitates transfer of arc power to the melt by covering the arc;
- Protects the refractories from radiated heat of the arc; and
- Provides thermal insulation over the steel bath.
It is probably clear from the above-mentioned points why conservation and utilisation of slag in the EAF process is a key requirement for efficiency enhancement.
Improvement In Melting Yield
As slag continues to reside inside the furnace for a longer period, it is continuously subjected to carbon injection as well as it interacts with dissolved carbon from the melt. Longer retention of slag in the furnace helps FeO reduction with this carbon. The iron droplets generated by the reduction reaction successfully transfers to the melt, before the slag flows out of the furnace. It has been estimated at various plants that an Fe yield improvement of about 0.5% has been achieved by that. Converting to monetary terms, it means a saving of USD 1.8 per tonne (MT) of liquid steel, at an average scrap price of USD 300/MT.
Reduction In Power, Electrode Consumption
Electric power consumption is a function of a numerous factors, some of which are listed below:
- Charge mix;
- Oxygen consumption;
- Presence of hot heel in the furnace;
- Flux consumption; and
- Foamy slag practice.
Longer slag retention in the furnace reduces flux consumption, as outflow of CaO-rich slag across melting time is minimised. On the other hand, higher slag volume ensures better foaming of slag and arc coverage. Together, these two improvements result in a reduction of power and electrode consumption.
It must be noted here that as the volume of slag in the furnace increases, EAFs can operate with a stable arc at higher voltages and lower currents, at the same time delivering an equal power input rate. Lower electrode current has a direct impact in reducing electrode consumption. Up to 5% reduction in electrode consumption and 1-2% reduction in power consumption can be expected.
An 80-tonne EAF with a 100 MVA transformer in South-East Asia could operate with a stable arc even without the help of a series reactor. This was partly achieved by retaining a higher amount of slag in the furnace by lowering the hearth level and marginally increasing the door sill level, thereby increasing the working volume of the furnace.
The author is a freelance steel technology consultant, based out of Kolkata, India. The views expressed in this article are in line with the current approach to EAF steel making. However, implementation of the suggestions made and information shared in this article have to be done after proper evaluation of the actual conditions and potential of the plant.
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