The New Paradigm Caused by Regulation(EU) 2024/1252 on the Upcycling of the Landfilled Ferrous Slags. Case Study: Iron and Steel Slag Dumps in Romania

1. Introduction

Steel is and will remain for a long time the main metal vector that drives the top branches of the modern economy (automotive industry, metallic construction, food industry, aircraft and defense industries, etc.) [1,2]. Also, cast iron plays an important role in different industrial sectors like machine construction, sewerage construction, but mainly as the precursor of the steel alloys [2,3,4]. Unfortunately, the production a ton of steel in an integrated steel plant implies the generation of approximately 2 tons of waste [5,6,7]. Among the unavoidable wastes and by-products generated by an integrated steel plant are iron and steel slags that play technological beneficial roles during iron oxide reduction in blast furnace or during smelting and refining the composition of a steel grade [3,7]. During the last century and in the first decades of the third millennium huge amount of ferrous slags has been accumulated in Romania as landfilled piles at Galati, Hunedoara, Călan, Târgoviște etc. Though, the production of the iron and steel in Romania diminish in the last decade, ferrous slag still accumulates in dumps [8]. These dumps have created environmental detriments through flying dust pollution, surface and underground water pollution through rainwater leaching and visual discomfort [9]. In addition, some of industrial waste deposits enter under incidence of the Cause 301/17 of the European Court of Justice (ECJ). Cause 301/17 stated that Romania has not fulfilled its obligations under Article 14 letter (b) of Directive 1999/31 concerning the obligation to take all necessary measures to close, as quickly as possible, 68 landfills which, in accordance with Article 8 of the said directive, have not received an authorization to allow them to continue to function [10]. The metallurgical waste dump Călan is included in the list of the deposits of non-hazardous industrial waste subject to Cause 301/17 as being operated by S.C. FOREVER - S.R.L. Călan [11]. This deposit ceased storage waste before July, 2009 [11]. The Călan dump storages mainly blast furnace slag (BFS) and associated waste resulted from pig iron production and from foundry shops that were integrated in the Călan steel plant. [11,12]. A similar status has the steel slag deposit Târgoviste which has accumulated significant amount of steel slag and associated wastes.

The ferrous slags have been subjected under debate regarding its status i.e. waste or not waste [13,14]. Thus, the ferrous slags were classified as waste according to the European Waste Catalog entries [15]: 10 02 01 waste from the processing of slag; 10 02 02 unprocessed slags. The European Waste Catalog was adopted in Romania and the above entries are mentioned in a recent governmental document i.e. in OUG 92/2021, approved by Law 17/2023 art. 8 para. (3) Appendix no. 1 [12]. The revised position paper of EUROFER in 2023 clearly states that “Slag is a by-product of the steelmaking process” [14]. The ferrous slag status of by-product is very important as a waste which ceases to be waste, also ceases to be waste for the purpose of the recovery and recycling targets set out in Directives 94/62/EC and another relevant CE legislation [17].

Recently, the Regulation (EU) 2024/1252, further on referred as Regulation, has turned the paradigm of the historical and ongoing slag dumps from the source of environmental detriment into a potential secondary resource which can mitigate the access risk of the EU to raw materials that are essential for its economy and for critical raw materials (Article 8) [18]. Moreover, Regulation stipulates: “By 24 November 2027, Member States shall adopt and implement measures to promote the recovery of critical raw materials from extractive waste, in particular from closed extractive waste facilities”. Also, Regulation (Art.27) laid down the obligation of the operators of the extractive waste facilities from each EU Member Stare “to provide to the competent authority a preliminary economic assessment study regarding the potential recovery of critical raw materials, from waste stored in their facilities by 24 November 2026. The study shall at least include an estimation of the quantities and concentrations of critical raw materials contained in the extractive waste and in the extracted volume and an assessment of their technical and economic recoverability. Operators shall specify the methods used to estimate those quantities and concentrations.” Ferrous slags enter to the incidence of Regulation as they are the results of the processing of the iron ores that are among the main target of the extractive industry [20,21]. Consequently, the upcycling route of the ferrous slag as cement precursor loses its prevalence. Thus, before the issuing of this regulation, the using of the ferrous slags as precursor in cement industry was considered an upcycling approach, while, in the new context, the new upcycling route for ferrous slags has turned as precursor of the critical raw materials (CRM) [22,23,24]. To fulfill this new trend, it is mandatory to screen for CRMs in the targeted waste deposit to identify which of CRMs has a significant abundance [26]. The presence of CRMs into ferrous slags have been addressed in the literature [26,27]. The were reported that the concentrations of REE in blast furnace slags are several times higher than in those other metallurgical slags [26,27,28].

In the frame of the new paradigm of slag upcycling, this paper addresses a preliminary screening for CRMs in the iron slag from Călan dump and in the steel slag from Târgoviște dump. The screening for CRM in a stockpile implies a complex serial process consisting of preliminary investigation of the site, construction of the site conceptual model based on available information, site inspection, planning the sampling campaign, sample collection, on-site sample preparation, sample transport to laboratories, laboratory measurements, data analysis and, finally, construction of the source model regarding targeted analytes [29,30,31]. Such a complex process needs financial and human considerable efforts. In our case, there is no available information of the CRM contents in the Călan and Târgoviște dumps, neither on the main composition of the substances deposited in these piles. All the knowledge about Călan dump is that it was used for deposition of the blast furnace slag (BFS) coming out of Călan iron plant during 1886-2007[2]. Most probably, waste associated to pig iron production and casting like blast furnace dust, used sand, waste linings etc. were co-deposited on Călan dump. Also, limited data are available on Electric Arc Furnace (EAF) slag deposited nearby Târgoviște, Colanu village [32]. Being aware of the information lack about CRMs abundance in these dumps, a preliminary study was conducted to ground a more pertinent study in case where the results of this study are positive. Also, this study aims to establish the performance characteristics of the XRFS technique, which is the cheapest and the most accessible technique for measuring the composition of the solid wastes [33,34]. XRFS reported compositions of the blast furnace slags encompassed both oxides and elemental mass concentrations. [22,31,33]. The characteristics of the analytical performances of the XRFS technique can be assessed through the limit of quantifications, the accuracy and the relative expanded uncertainty for each constituent [34,35].

Taking into account the above considerations, we addressed two synergic objectives in this study i.e. establishing the analytical performances of our XRFS laboratory procedure dedicated to ferrous slag investigation and screening for CRM in dumped ferrous slags. Both objectives are critical for complying with the requirements of the Regulation because the lack of accuracy of the measurement will compromise the screening for CRM, while non-screening for CRM contravenes to the requirements of Regulation. The study was conducted on a certified reference material made from steel slag aiming assessing the characteristics of analytical performance of the XRFS procedure. Subsequently, XRFS measurement were conducted on four ferrous slags aliquots (2 of EAF slag and 2 of air-cooled BFS , aka ACBFS). The main novelty of the paper is the way of ensuring the reliability of the XRFS results. Another important novelty of the paper is the way in which the performance characteristics of the XRFS procedure were established based on weighted arithmetic mean and on the maximum likelihood approach. The XRFS measurements carried on ferrous slags demonstrate that they contain significant amounts of CRM like Ba, Bi, Sr, Ti, W, Y, etc. Our preliminary LIBS measurements on these slags disclosed the presence of light CRM like Li and Be, that cannot be detected by XRFS. Further researches are envisaged to develop an integrated XRFS, LIBS and XRD procedure for comprehensive and reliable CRM screening in ferrous slag dumps.

2. Materials and Methods

The first XRFS measurements were carried on the certified reference material, Certificate No. 050616-101-Slag, provided by Brammer Standard Company, Inc. The material is a fine powder that was dried for one hour at 105 ^oC and mix well before using. The steel slag samples were obtained from the Târgovişte (Colanu) slag dump. The aliquots were prepared from slag fraction 8-63 mm (Figure 1.a).

The collected gross sample of EAF slag weighed approximately 4 kg. The gross sample were crushed, grinded and milled until achieving a powdered state. The sieved fraction through a 1mm mesh was coned and quartering and a subsampling of circa 0.2 kg were delivered to an advanced milling into a ball mill Retsch PM100. The milled process lasted until the residue on the sieve (65 μm mesh) was less than 10%. The powder passed through 65 μm mesh sieve undergoes a subsampling through coned and quartering and circa 50 g of powder was selected for XRFS measurements. The XRFS measurements were carried with a Xepos ED(P)-XRF spectrometer (SPECTRO Analytical GmbH) which uses three secondary scattering targets in order to diminish the background intensity of the X-ray fluorescence spectra, which help increasing the sensitivity of the instrument in detection the light elements from Na to Ti. The aliquot for XRFS measurement were prepared as Pressed Pellet. The pellet contains 6.25 g of powdered slag and 1.75 g of lithium tetraborate. The XRF spectra were processed with the Turboquant Pellets analytical program which can delivered elemental composition or a combination of oxide and elemental composition aka bond composition. The same aliquot preparation and XRFS measurement processes were applied to the ACBFS samples (Figure1.b).

The accuracy of the XRFS measurement procedure for an analyte was estimated as the absolute relative difference between the certified value of the reference sample and the weighted mean obtained through repeated measurements on the aliquots prepared from reference material. The weighted arithmetic mean was calculated as:

$$\overline{\mathit{c}}={\displaystyle \frac{\sum _{\mathit{i}=1}^{\mathit{n}}{\mathit{w}}_{\mathit{i}}·{\mathit{c}}_{\mathit{i}}}{\sum _{\mathit{i}=1}^{\mathit{n}}{\mathit{w}}_{\mathit{i}}}}$$

(1)

where $\overline{\mathit{c}}$ is the weighted arithmetic mean of a suite of measurements; ${\mathit{c}}_{\mathit{i}}$ is the measured concentration in i^th round; ${\mathit{w}}_{\mathit{i}}$ is the weight of the C_i variable;

The common practice in mathematical statistics is to consider the measurand as a random variable, written with capitals, while its value is written in small letters. The term ${\displaystyle \frac{{\mathit{w}}_{\mathit{i}}}{\sum _{\mathit{i}=1}^{\mathit{n}}{\mathit{w}}_{\mathit{i}}}}={\mathit{p}}_{\mathit{i}}$ in Eq.(1) can be considered as the occurrence probability of the c_i value aka weight of C_i variable.

The variance of the weighted mean, V($\overline{\mathit{C}})$, is calculated as:

$$\mathbf{V}\left(\overline{\mathit{C}}\right)=\sum _{\mathit{i}=1}^{\mathit{n}}{\mathit{p}}_{\mathit{i}}^2·{\mathit{\sigma }}_{\mathit{i}}^2$$

(2)

where ${\mathit{\sigma }}_{\mathit{i}}^2$ is the variance of the ${\mathit{C}}_{\mathit{i}}$ measurand. ${\mathit{C}}_{\mathit{i}}$ is consider an independent random variable for every i=1...n.

The maximum likelihood approach imposes the minimization of the $\mathbf{V}\left(\overline{\mathit{C}}\right)$ value, which, in turn, implies that derivatives:

$${\displaystyle \frac{\partial \mathbf{V}\left(\overline{\mathit{c}}\right)}{\partial {\mathit{w}}_{\mathit{i}}}}=0,\mathrm{i}=1\dots \mathrm{n},$$

(3)

The solution of the system made of n equations of the form Eq. (3), with the unknowns w_i, 1=1..n, is:

$${\mathit{w}}_{\mathit{i}}={\displaystyle \frac{1}{{\mathit{\sigma }}_{\mathit{i}}^2}}$$

(4)

In the frame of the maximum likelihood approach the weight of C_i is:

$${\mathit{p}}_{\mathit{i}}={\displaystyle \frac{\frac{1}{{\mathit{\sigma }}_{\mathit{i}}^2}}{\sum _{\mathit{i}=1}^{\mathit{n}}\frac{1}{{\mathit{\sigma }}_{\mathit{i}}^2}}}$$

(5)

Substituting ${\mathit{p}}_{\mathit{i}}$ given by Eq.(5) in Eq.(2), one obtains:

$$\mathbf{V}\left(\overline{\mathit{C}}\right)={\displaystyle \frac{\sum _{\mathit{i}=1}^{\mathit{n}}\frac{1}{{\mathit{\sigma }}_{\mathit{i}}^2}}{{\left(\sum _{\mathit{i}=1}^{\mathit{n}}\frac{1}{{\mathit{\sigma }}_{\mathit{i}}^2}\right)}^2}}={\displaystyle \frac{1}{\sum _{\mathit{i}=1}^{\mathit{n}}\frac{1}{{\mathit{\sigma }}_{\mathit{i}}^2}}}$$

(6)

The performance characteristics of the XRFS procedure considered in this paper are: relative accuracy and uncertainty ratio, defined as follows:

➢

Relative accuracy (RA) is the ration between accuracy and the certified value:

$$\mathit{R}\mathit{A}\left(\mathit{\%}\right)={\displaystyle \frac{\mathit{a}\mathit{b}\mathit{s}({\mathit{c}}_{\mathit{m}\mathit{e}\mathit{a}\mathit{s}}-{\mathit{c}}_{\mathit{c}\mathit{e}\mathit{r}\mathit{t}\mathit{i}\mathit{f}\mathit{i}\mathit{e}\mathit{d}})}{{\mathit{c}}_{\mathit{c}\mathit{e}\mathit{r}\mathit{t}\mathit{i}\mathit{f}\mathit{i}\mathit{e}\mathit{d}}}}\mathit{x}100\mathit{\%}$$

(7)

where C_meas is the measured value of the analyte, while C_certified is its certified value.

➢

Uncertainty ratio (RU) is the ratio between measurement uncertainty and certified uncertainty where they are of the same type i.e. compound uncertainty or expanded uncertainty. The RU is expressed as rational number.

The characteristics are qualitatively assessed according to Table 1.

Once having evidences that our XRFS procedure provides reliable outcomes then we proceed with slag characterization as a potential source of positive minerals and possible as CRM secondary resources. Also, the chemical character of a slag was estimated according to the conventional practice using the basicity index, denoted BI, which is the ratio between the content of basic oxides and acidic oxides [21]:

$$BI={\displaystyle \frac{c_{CaO}+c_{MgO}+c_{FeO}+c_{MnO}+c_{CrO}+c_{{Na}_{2}O}+c_{K_{2}O}}{c_{Si{O}_2}+c_{P_2{O}_5}+c_{Ti{O}_2}+c_{V_2{O}_5}}}$$

(8)

where the concentrations of the oxides are given in %wt.

The strongest basic oxides ranked in descendent order are: Na₂O, K₂O, CaO, MgO, BaO, FeO, MnO, Cu₂O, NiO, ZnO, Fe₂O₃, Cr₂O₃, PbO [21]. Al₂O₃ is consider as amphoteric oxide [21]. The strongest acid oxides are SiO₂, V₂O₅, Cr₂O₃, MoO₃, WO₃, Mn₂O₇ [21].

The basic ferrous slags are many times sub-divided based on the CaO/SiO₂ concentration ratio as follows [21]: a) weak basic slag for CaO/SiO₂ < 1.5; b) average basic slags for CaO/SiO₂ = 1.6 -:- 2.5; c) strongly basic slag for CaO/SiO₂ > 2.5.

3. Results and Discussions

The results of 4 repeated measurements carried on aliquots prepared from powdered BS 101-3 reference material are posted in Table 2.

In the Table 2 are posted only the components that can be compared with the certified composition values given in the Certificate No. 050616-101-SLAGS (Table 3). The synthetic analysis of the XRFS measuring performances are shown in Table 3 where are posted the weighted arithmetic mean of the 4 repeated measurements ($\overline{C})$, the expanded measurement uncertainty with 95% degree of confidence, denoted U(95%), calculated for the coverage factor k=2, RA, RU and QMPC parameters.

As could be seen in Table 3 the measurement uncertainties calculated in the frame of the maximum likelihood are less than the certified uncertainties i.e. RU<1. Also, the RA exceeds 5% only in two cases whose characteristics of performance were considered as being of good quality. In all other cases, the performance characteristics of the measurements are very good and excellent.

The BI of the certified steel slag calculated with certified values has a BI=3.98 while BI calculated with the measured values is 3.81. The BI values are close each other and differ with circa 4.4%. The ratio $c_{CaO}$/$c_{SiO2}$ calculated based on certified values is 2.86 while that calculated based on measured values is 2.90 i.e. the ratios differ with circa 1.4%. Both ratios let one qualifies the slag as a strongly basic slag. The values of the BI and of the ratio $c_{CaO}$/$c_{SiO2}$ obtained on certified slag show also that the XRFS procedure we use is robust and reliable. Thus, we have been confident that the measurement values we obtained on 4 slag aliquots are reliable and could serve as a preliminary surveying for CRM in the above-mentioned slag piles. The XRFS measurement results obtained on aliquots prepared from ACBFS and EAF slags are posted in Table 4.

The data presented in Table 4 are as they are delivered by the Turboquant software, therefore the common law of the same number of digits cannot be respected in Table 4. This is also due to the large differences between the values of the measured concentrations. However, the concentrations of major constituents (values measured in %wt.) are comparable with literature data and can be considered as reliable [26,33].

The BI of the investigated slags and their CaO/SiO₂ ratios are given in Table 5.

The screening for CRM implies data analysis at ppm level therefore a selection of the CRM measured by XRFS and their concentrations in ppm (wt.) is given in Table 6. The maximum value of a CRM concentration (C_max) is shown in the 10^th column of the table, while in 11^th is posted its abundance in Earth crust [37]. The 12^th column contains the 1-digit rounded ratio between the measured concentration and its known concentration in Earth crust (C_EC). The reader should be aware that the concentration of a CRM in Table 6 refers to its oxide while the reference value of C_EC is for element. Thus, the ratio in column 12^th is informative. For a comparison element to element, the composition in Table 6 must be recalculated. This is not an impossible task, but for the present case, it is beyond the scope of this study.

The CRM like Ti and Mn are abundant in EAF slags. Also, EAF can be suspected as a secondary source for Ta, W and Bi that are abundant in case of W and Bi (hundred times) and about 10 times in case of Ta. BFS slags can be considered as abundant in V and quite rich in As, Ba and Hf. Though, Y and Sr are not on the CRM list, it worth noting that these elements are abundant in the slag we investigated. The presence of the very important light CRM like Li, Be, B and F cannot be investigated by XRFS technique.

To overcome this drawback, preliminary LIBS measurements were carried on some ACBFS lumps at SciAps Laboratory in Germany, using SciAps Z903 LIBS Analyzer instrument. One of the LIBS spectra is shown in Figure 2, where the images of the sample and of 3 laser spots are shown as insets.

Figure 2 clearly shows the presence of the characteristic lines of light elements as H, Li, O, Mg etc. A semi-quantitative estimation of the elemental composition of the sample is given in Table 7.

The data in Table 7 attest the potential of LIBS technique to measure elemental concentration in a large range i.e. from ppm to %wt., but LIBS needs proper calibration on ferrous slags to provide reliable quantitative results. In this direction, we consider that LIBS in combination with XRFS and XRD measurements on the same aliquots can provide a more thoroughly analysis of the CRM into ferrous slags. Accordingly, further researches are foreseen to get an integrated XRFS, LIBS and XRD procedure for CRM trustworthy screening as Regulation requires

4. Conclusions

This study is a first step in grounding the screening process for CRM in ferrous slag dumps in Romania, at least, to come in line with the requirements of the REGULATION (EU) 2024/1252.

The performance characteristics of the XRFS procedure we used to measure the chemical composition of the ferrous slags (ACBFS, EAF) are fitted to the purpose of this study. Hence, the posted results in the paper are deemed as reliable.

This study demonstrates that the Călan and Târgoviște ferrous slag dumps contain significant amounts CRM like Ti, V, Mn, Bi even W incorporated in their oxides. Also, important elements like Y, Sr, Ba can be deemed as being in significative amounts in these slag dump.

The preliminary LIBS measurements demonstrate that light CRM like Li and Be are contained in ferrous slags.

Noteworthy, the XRFS, even XRD cannot detect light CRM, hence they cannot ensure a trustworthy screening for entire list of CRM. Therefore, the paper emphasizes the need for an integrated XRFS, LIBS and XRD procedure for reliable screening of the CRM in secondary resources, as Regulation requires.

The authors are aware of the importance of the sampling plan and statistical data processing as critical factors for the decision on harvesting CRM from these secondary resources. Hence, further researches are foreseen on this topic.