1. Introduction
Mixing raw materials is critical in scrap-based plants, such as electric arc furnaces (EAFs), where meeting chemical composition targets and a cost-efficient mix of raw materials should be simultaneously taken into account. Mathematical models developed to study the mixing of raw materials can be classified into the following groups: (i) mass–energy balance, (ii) optimization and (iii) statistical models. The mass and energy balance models are divided into static and dynamic models and used to calculate energy consumption, melt chemical composition and tapping time [
1,
2,
3,
4]. In the calculations, the scrap composition is assumed to be known, and uncertainties are not taken into account.
The raw material optimization models can be classified into deterministic and stochastic models [
5,
6,
7]. In a linear deterministic optimization model developed by Lahdelma et al. [
5], safety margins are considered to account for uncertainties in the scrap composition. The element concentration and its variance were estimated by applying the ordinary and recursive least squares and maximum likelihood methods. Additionally, several stochastic models have been developed to include uncertainties in scrap materials by using chance constraint (CC) programming. In chance constraint optimization, the constraint fulfillment under uncertainties is expressed by probability estimations. Gaustad et al. [
6] developed a chance constraint model in which the production cost is minimized subject to probability constraints for lower and upper limits of a target product. It is assumed that element considerations are normally distributed in each scrap material. The constraints are written for the normally distributed element concentration, and the variance of the chemical composition of the different types of scrap is known. The variances can be estimated by methods such as the random sampling analysis (RSA), which is followed by optical emission spectroscopy determinations [
8].
Some statistical models use process data to predict scrap properties [
9,
10]. Birat et al. [
9] developed a model to estimate the average concentration of tramp elements in scrap by solving linear regression equations. The scrap compositions were related to the melt compositions through the raw material recipe data and yields of the heats. The partial least squares-based method was applied in a model developed by Sandberg et al. [
10], in which the scrap properties were estimated by using EAF process data. A scrap chemical composition was calculated for a “pure heat”, which corresponds to a heat charged by 100% of that scrap type. The Cu content in different types of scrap was estimated by a 95% confidence interval. The reliability of the results was studied by checking if the scrap portion in the mix is close to one in a pure heat simulation of a specific scrap type. The results showed that the method is applicable when there is a correlation between a scrap and other scrap types and the target product chemical composition.
Uncertainties are not limited to scrap composition as described above, but they involve all measurements from loading scrap to tapping the liquid steel. In this study, the aim is to develop a statistical model as a calculation tool that can mainly be used in an EAF plant to deal with the uncertainties in steel scrap analysis. Furthermore, it should deal with lack of scrap analysis and facilitate production planning of a heat to meet the target composition. In doing this, the tramp element copper in scrap is taken into account. This element is difficult to remove from steel, and exceeding the maximum allowed concentration can result in detrimental effects on final products, such as reducing ductility and causing hot cracks [
11]. Thus, it is important to evaluate the copper concentration and its dispersion in different types of charged scrap. This importance is described by Gyllenram et al. [
12] by obtaining a relationship between the quality cost, confidence interval and percent of low-quality scrap with high Cu content (with an average of 0.3 and standard deviation of 0.03), which can be used to determine the risk percent lying outside the target product limits. Moreover, identifying the Cu content can lead to a lower charging of high-purity raw materials. This is clearly demonstrated by charge optimizations conducted for two hypothetical scenarios by using the web-based software RAWMATMIX® (Kobolde & Partners AB, Stockholm, Sweden) [
13,
14]. The main input data for these are listed in the
Appendix A. The real copper content in a scrap type is considered to be 0.1%, and the content considered by a hypothetical steel plant is 0.3%. Therefore, in the optimizations, the copper content is assumed to be 0.1% and 0.3% in the first and second scenario, respectively. The results show that pig iron is added to dilute the melt in order to not exceed the maximum concentration of copper in the target product, as shown in
Table 1. This results in an increase in material cost and scope 3 carbon footprint by 74 USD/tm and 638 kgCO
2eq/tm, respectively.
Moreover, the uncertainties with respect to the alloying element chromium are also investigated. Determining the content of alloying elements in scraps contributes to a planned material recipe that makes use of such elements. This leads to a lower usage of primary alloys, with high upstream carbon footprint values, which consequently reduces the costs and the carbon footprint of products.
The developed statistical model simulates the melt composition by taking into account different uncertainties involved in the EAF process. The model uses the maximum likelihood method to estimate variances in scrap chemical composition. This method was briefly mentioned by Lahdelma et al. [
5], while it is described here in detail. Additionally, the uncertainties involved in weighing of scrap and in element distribution factors are included.
Author Contributions
Methodology, N.A. and R.G.; mathematical model, M.A. and N.A.; programming and software development, N.A. supervision, M.A., R.G. and P.G.J.; writing—original draft, N.A.; writing—review and editing, M.A., R.G. and P.G.J. All authors have read and agreed to the published version of the manuscript.
Funding
This research was partly funded by the Sweden’s innovation agency, Vinnova, grant number 2019-05653.
Acknowledgments
The authors wish to thank the Sweden’s innovation agency, Vinnova, for the financial support for parts of this work and also Mikael Hansson and Peio Etxebeste for their contributions to the programming of the charge program simulation.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1.
A flowchart of the procedure to estimate the accuracy of the scrap chemical composition.
Figure 2.
Estimated mean standard deviations for concentration of Cu () with a 95% confidence interval in scrap types, x1–x6 for cases 1–4, (a–d).
Figure 3.
Estimated mean standard deviations for concentration of Cr () with a 95% confidence interval in six scrap types, x1–x6 for cases 5 and 6, (a,b).
Figure 4.
Density plot of Cu concentration in target product A.
Figure 5.
Comparison between the reported Cu concentration and calculated values in scrap (x1–x10).
Figure 6.
Comparison between the concentrations of Cu calculated using the reported scrap compositions (R) and the calculated scrap compositions using the NNLS and ML methods. Data are presented for three target products: A, B and C, (a–c).
Figure 7.
Comparison between the reported Cr concentration and calculated ones in scrap (x1–x10).
Figure 8.
Comparison between the concentration of Cr calculated using the reported scrap composition (R) and the calculated scrap composition using the NNLS and ML methods. Data are given for three target products: A, B and C, (a–c).
Table 1.
Cost and scope 3 carbon footprint results for two scenarios of copper content assumptions in a scrap type.
Scenario | Actual Cu in Scrap (%) | Assumed Cu in Scrap (%) | Scrap Amount (kg) | Pig Iron Amount (kg) | Material Cost (USD/tm) | Carbon Footprint Scope 3 (kgCO2eq/tm) |
---|
1 | 0.1 | 0.1 | 102,301 | 0 | 238 | 9 |
2 | 0.1 | 0.3 | 68,027 | 34,561 | 300 | 647 |
Table 2.
Average concentration of Cu and Cr in scrap, and in six scrap types ().
j | 1 | 2 | 3 | 4 | 5 | 6 |
---|
CCu,j (%) | 0.05 | 0.05 | 0.14 | 0.3 | 0.2 | 0.05 |
CCr,j (%) | 0.15 | 0.2 | 1.43 | 0.2 | 0.1 | 1 |
Table 3.
Standard deviations for Cu and Cr in each scrap type, standard deviations for weighing and standard deviation for Cr distribution factor.
Case | | | | | | | | |
---|
1 | 0.02 | 0.02 | 0.02 | 0.02 | 0.02 | 0.02 | 0 | 0 |
2 | 0.02 | 0.015 | 0.04 | 0.01 | 0.03 | 0.025 | 0 | 0 |
3 | 0.02 | 0.015 | 0.04 | 0.01 | 0.03 | 0.025 | 10 | 0 |
4 | 0.02 | 0.015 | 0.04 | 0.01 | 0.03 | 0.025 | 50 | 0 |
Case | | | | | | | | |
5 | 0.01 | 0.04 | 0.03 | 0.04 | 0.02 | 0.3 | 0 | 0 |
6 | 0.01 | 0.04 | 0.03 | 0.04 | 0.02 | 0.3 | 0 | 0.05 |
Table 4.
Calculated average concentration of Cu in scrap, , in six scrap types () using the NNLS method.
Cases | x1 | x2 | x3 | x4 | x5 | x6 |
---|
1 | 0.049 | 0.051 | 0.142 | 0.302 | 0.201 | 0.042 |
2 | 0.048 | 0.048 | 0.145 | 0.303 | 0.202 | 0.055 |
3 | 0.049 | 0.049 | 0.142 | 0.301 | 0.204 | 0.046 |
4 | 0.049 | 0.049 | 0.140 | 0.302 | 0.199 | 0.065 |
5 | 0.151 | 0.195 | 1.427 | 0.196 | 0.104 | 0.993 |
6 | 0.147 | 0.204 | 1.435 | 0.209 | 0.104 | 0.943 |
Table 5.
Calculated mean concentration of Cu in scrap, , and its standard deviation, , in six scrap types () using the maximum likelihood method for nine cases.
Cases | Variable | x1 | x2 | x3 | x4 | x5 | x6 |
---|
1 | | 0.05 | 0.051 | 0.139 | 0.303 | 0.201 | 0.043 |
| 0.02 | 0.023 | 0.017 | 0.017 | 0.021 | 0.019 |
2 | | 0.048 | 0.048 | 0.145 | 0.303 | 0.201 | 0.055 |
| 0.021 | 0.011 | 0.035 | 0.010 | 0.029 | 0.027 |
3 | | 0.049 | 0.052 | 0.143 | 0.302 | 0.204 | 0.046 |
| 0.021 | 0.018 | 0.036 | 0.002 | 0.027 | 0.021 |
4 | | 0.050 | 0.048 | 0.141 | 0.301 | 0.197 | 0.064 |
| 0.019 | 0.008 | 0.041 | 0.013 | 0.028 | 0.016 |
5 | | 0.151 | 0.195 | 1.426 | 0.196 | 0.103 | 1.011 |
| 0.01 | 0.04 | 0.031 | 0.038 | 0.018 | 0.299 |
6 | | 0.150 | 0.202 | 1.429 | 0.204 | 0.097 | 0.994 |
| 0.010 | 0.041 | 0.011 | 0.018 | 0.012 | 0.297 |
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