By Ananda Bhattacharjee
A major part of the cost of production in electric arc furnace (EAF) steelmaking is that of metallic inputs like scrap, DRI, HBI, pig iron, etc. Given the highly competitive business environment in the steel industry today, saving on metallic inputs has become a major thrust area for reduction of production costs. Similarly, restricting furnace losses can also play an important role. All these can together improve metallic yield in a very effective way, thereby reducing costs.
Consider a steel scrap cost of USD 300 per tonne, when it reaches the nearest port. By the time it lands at the factory site, the costs would have added up to make the price USD 315 per tonne. When it finally reaches the steel melt shop and is charged into the EAF, after the scrap is segregated, sorted and shifted within the scrap yard of the plant, the cost would have moved up further, say to USD 318 per tonne.
If the liquid metal yield in the EAF is about 91%, then the cost of scrap for each tonne of liquid steel produced will be USD 349.45. So, the effective cost is further climbing! As much as USD 31.45 is the yield cost, which is about 9% of the metallic cost for each tonne of liquid steel produced.
When each percent of the metallic yield is spoken of in terms of money, it converts to USD 3.84% yield per tonne of liquid steel. Consider a company that produces 700,000 tonnes per annum of liquid steel and has a yielded metallic cost as mentioned above. If this company increases its liquid steel yield by 0.5%, it saves USD 1,344,000 per annum. Convert that to INR and at today’s rate it is INR 940,80,000 per annum. On liquid tonne basis it is INR 134.40 per tonne, a saving any company should want to address.
Ideas from “Lean” manufacturing can be used. The core idea of Lean manufacturing is simple. It is about continuous work to eliminate wastes in a manufacturing process. Lean manufacturing provides a methodology to systematically reduce the wastes and optimise the use of resources to deliver the value that the customer wants. This article restricts itself to scrap quality and some process aspects of EAF steel making.
The Lean Angle
When an EAF produces more steel than it was doing earlier, with the same iron input, a multiple of other accompanying benefits are likely to follow. This improvement in metallic yield and other benefit streams are actually an outcome of the operations becoming “more” lean. It comes through efforts to optimise the process that progressively removes the unwanted inputs into the process.
At the start, it is important to specify the yield value that is being sought to be achieved. In this case, obviously it is the liquid steel yield value that a company wants to achieve. This value needs to become the “bull’s eye” for the operating team. Yield values are dependent on the charge mix. The proportion of scrap, pig iron, DRI, HBI, hot metal, etc being charged into the furnace decides the Fe value that enters the furnace. This forms the basis for setting the target value for liquid steel yield.
The second step is to map the value stream – all the activities and process steps that convert the scrap, pig iron, DRI, HBI, hot metal, etc, from their storage yard to the tapping of liquid steel into the ladle. A value stream is an end-to-end collection of activities that creates a result for a “customer”, who may be the ultimate customer or an internal “end user” of the value stream. The value stream has a clear goal: to satisfy (or rather, to delight) the customer. Mapping is done of the present state of the value stream along with the estimated future state of the value stream that is likely to result in achieving the target value of liquid steel yield. The future state of the value stream will include a modified set of process parameters as well as a modified set of activities.
Once the above guidelines have been established, the process “flow” can be green-flagged. Gradually, as the modified process parameters and activities are implemented step by step, benefits are likely to be observed.
Bottlenecks in the process and operations need be removed so that the “flow” is maintained. Additionally, facilitating factors, including the offtake of liquid steel by the downstream units (internal customers), need to be ensured.
The value stream mapping may include the following processes. A basic structure is shown in Figure 1:
- Scrap preparation process:
- Scrap cleaning
- Scrap segregation
- Charging making
- Scrap selection
- Charge preparation
- Furnace charging
- Melting and tapping
- Flux addition
- Oxygen injection
- Burner gas
- Carbon injection
- Slag removal
The processes mentioned above are necessary but need optimisation to achieve the desired target.
Figure 1: Basic structure of value stream mapping
The Unclean Scrap
The thought of a lean operation and process in EAF starts with the visualisation of scrap that is as clean as possible. If the procured scrap is not clean enough as in Figures 2 and 3, it is worthwhile to install a system for cleaning the scrap (Figures 6 & 7). Unclean scrap that is charged into the EAF incurs a substantial additional cost, since the unwanted stuff must be melted and then fluxed out of steel. The following are the areas of loss:
- Yield loss: Sterile, gangue and iron oxide do not yield any steel through melting.
- Higher consumption of fluxes to counter their adverse impact on slag. Increased slag volume also leads to increased Fe loss.
- Higher electric power consumption due to unclean scrap and higher flux consumption.
- Increased electrode consumption as a result of the higher electric power consumption.
Clean scrap as in Figures 4 and 5 provide much better value in the melting process in EAFs. As the endeavour to improve liquid steel yield materialises, the benefits are multi-fold. The cascading losses, as indicated above, can be converted to surging benefits.
Iron And Steel Loss
As the process in EAF progresses, there is a stream of losses that take place. These losses are inevitable with the technology at the disposal of the steel makers in the world today. However, the losses can be reduced with various efforts. Losses are many, in the form of steel, heat, slag, water, electrode, power, etc. This article focuses on the major losses in the form of iron and steel loss. This loss has two main pathways in an EAF:
- Loss of liquid steel flowing out through the slag door; and
- Oxidation loss of Fe, into the slag.
Loss of liquid steel flowing out through the slag door: Loss of liquid steel flowing out through the slag door is something that can be minimised by exercising control on the furnace working volume in relation to the weight of scrap and other Fe sources being charged into the furnace. The furnace’s working volume should be adequate to accommodate the liquid steel, slag and the steel level jumps due to boiling.
Steel boiling happens due existence of steep gradients in carbon concentration, as well as in temperature, between different zones of the steel bath. Both the situations are an outcome of insufficient stirring of the liquid bath. It can be countered with an optimised charge-mix layering in the charging basket and supported by an optimised oxygen blowing process. The layering usually involves adequate dispersion of high carbon materials like pig iron and cast iron in the charge basket. Heavy scraps like ingot cuttings are usually located in the lower layers. The optimisation parameters vary from one EAF to another and depend on the design of the furnace.
Oxidation loss of Fe, into the slag: An optimised oxygen blow achieves the following:
- Steady and uniform decarburisation of the bath (subject to bath temperature and carbon concentration).
- Generation of sufficient stirring in the bath that equalises the oxygen and carbon concentration in the bath.
- Generation of adequate stirring of the bath that will facilitate melting of submerged steel scrap.
- An end point of oxygen blowing that happens immediately on achieving the target tapping carbon in steel.
The stirring energy generated in the liquid steel by the blowing process is of vital importance. Sufficient stirring of the bath during oxygen blowing ensures efficient usage of oxygen for decarburisation. EAFs are at a disadvantage in this regard compared to BOFs. The following data on stirring in oxygen converters, taken from the book, Innovation in Electric Arc Furnaces – Scientific Basis for Selection, by Yuri N. Toulouevski and Ilyaz Y. Zinurov, illustrates clearly the impact of stirring energy on mixing time:
|The effect of bottom blowing on the stirring intensity of oxygen converters (average values)|
|Type of converter||Only top blowing – LD||With combined blowing: top and bottom||Only with bottom blowing Q-BOP|
|Gas blown via bottom||–||N2 and Ar||O2||O2|
|Intensity of bottom blowing (m3/tonne-min)||–||0.15||0.9||5|
|Time of complete bath stirring(s)||100||40||20||4-5|
Unfortunately, similar data on EAF stirring are not available. It is reported that the above data were obtained with the use of copper additives as the tracers. It shows that despite the high intensity of oxygen top-blowing in oxygen converters, about 4.5–6.0 m3/tonne-min, the additional injecting of even very small amounts, 0.15–0.9 m3/tonne-min of gases through the bottom tuyeres increases the stirring intensity by 2.5–5 times, and, with replacing the top-blowing with bottom-blowing, approximately by 20–25 times.
The EAF slag has been seen to have higher FeO% compared to oxygen converters. This is a direct consequence of lower stirring energy inherent in the EAF process (Figure 8)
Figure 8: Relation between FeO% and C% in steel at tap in EAF compared to BOF and Q-BOP (Source: E. T. Turkdogan)
The above data should be taken merely for understanding the impact of different types of gas flows and their stirring efficiencies in the steel bath. Each type of blowing technique comes with its advantages and disadvantages. A combined and optimal blowing is probably the best solution for the future of EAFs. Many EAFs around the world have now adopted use of Porous Plugs for gas purging through the furnace bottom, in combination with oxygen blown from the top.
Despite the deficiency of stirring energy in EAFs, they have an advantageous carbon injection facility which can be judiciously used to lower the FeO% in slag.
When looked at from the Lean angle, higher stirring energy will bring about the following positives:
- More efficient use of oxygen for de-carburising: This results in lowering of excess oxidation of Fe, thereby improving liquid steel yield.
- It will reduce the non-equilibrium concentration of oxygen and carbon in the steel bath; something that will make it easier for the operating personnel to achieve that targeted carbon content and dissolved oxygen parts per million (ppm) in steel, at the time of tapping. This results in elimination of over-oxidation of steel, particularly in case of low carbon steels.
The effectiveness of all the steps taken to control oxidation of Fe, and its loss into the slag in the form of FeO, is reflected in the content of FeO in the final slag of EAF, and it should be in the range 18-23%.
Application of the Lean methodology in the value stream for converting metallics to liquid steel in EAF is an effective way of not only improving the way we visualise the process, but also to bring about cost reductions systematically. In the steel industry, this methodology can be applied to a gamut of operations.
The author is a freelance steel technology consultant based out of Kolkata, India. He may be contacted at [email protected] or +919763777846)
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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