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Article

Effect of Selective Milling on the Concentration Process of Critical Raw Materials from MSW Incinerator Bottom Ash

by
Ida B. G. S. Adhiwiguna
1,*,
S. Humaira Sahbudin
1,
Winfried Ruhkamp
2,
Ragnar Warnecke
3 and
Rüdiger Deike
1
1
Chair of Metallurgy—Institute for Technologies of Metals, University of Duisburg-Essen, Friedrich-Ebert-Str. 12, 47119 Duisburg, Germany
2
LOESCHE GmbH, Hansaallee 243, 40549 Düsseldorf, Germany
3
GKS-Gemeinschaftskraftwerk Schweinfurt GmbH, Hafenstraße 30, 97424 Schweinfurt, Germany
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(11), 1174; https://doi.org/10.3390/min14111174
Submission received: 22 October 2024 / Revised: 18 November 2024 / Accepted: 18 November 2024 / Published: 19 November 2024
(This article belongs to the Special Issue Recent Advances in Separation Techniques for Critical Minerals/Metals, Circular Economy, and Sustainability)
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Abstract

:
This research introduces selective milling as a reliable and effective initial concentration process to enable efficient separation and ensure high recovery rates of valuable and critical materials (minerals and metals) from processed incinerator bottom ash (pr.IBA), a treated mineral fraction originating from the conventional municipal solid waste (MSW) incinerator bottom ash (IBA) processing steps. Four different types of pr.IBA (each sample weighing up to three tons) were selectively milled using a demonstration-scale vertical roller mill to produce three distinct products: fine, middle, and coarse fractions. Chemical analysis demonstrated that a concentration step after selective milling could be reliably achieved regardless of the variation in the sources and qualities of the input materials. Specifically, calcium-containing compounds can be enriched in the fine fraction, potentially containing Ca2SiO4, CaSO4, and CaCO3. Complementary to its particle size equivalent to the raw mix, this calcium segregation could be valuable as an alternative material in cement clinker production. Conversely, the segregation of metal-bearing substances, particularly iron and copper, was detected in the coarse fraction. Such segregation is comparable to specific ore grades and enhances the possibility of metal recovery from pr.IBA.

Graphical Abstract

1. Introduction

According to the latest survey conducted in 2021 by ITAD (German Association of Waste-to-Energy Plants) and IGAM (German Association of Incinerator Bottom Ash Processing Plants) [1], around six million tons of raw municipal solid waste (MSW) incinerator bottom ash (IBA) were produced in Germany, whereas its further valuable utilization alternatives are still exceptionally narrow. It is publicly reported in the abovementioned survey that only approximately 10% and less than 20% of the total IBA in Germany could be recovered in metal recycling and deployed in the construction sector, respectively. Consequently, the rest will commonly land at disposal sites despite possessing high potential in terms of material composition, where 85% of its mass accounts for highly demanded minerals and valuable metal fractions, including critical and precious metals. Considering the increasing tendency of MSW generation [2], followed by tight disposal restrictions and limited disposal site numbers in Germany, the need for alternative applicable processes to valorize the IBA has transformed into a demanding urgency.
The main challenge in adding value and utilizing IBA at the industrial scale is its characteristics as a highly heterogeneous material, including a relatively high concentration of contaminants [3]. Although IBA has been proven to contain a significant amount of minerals, its application in the construction sector is limited by the behavior of those undesired impurities that can degrade product quality and threaten the environment. On the other hand, solely depending on metal recovery is also considered not economically feasible [4], given the exponential counterproductive expansion in resulting cost as the recycling rate increases [5]. Therefore, a synergy is necessary to combine all available alternatives to generate various streams of valorization for each fraction from IBA.
Mechanical-based treatment could be a promising method for delivering the necessary concentration process to overcome the abovementioned challenge. Furthermore, this alternative should also be attributed with the lowest barrier to future industrial applicability compared to thermal and chemical processes, at least considering a short-term strategy. Specifically, in addition to well-established separation processes, including screening, magnetic, eddy current, and density separation operations, emerging selective milling could be a promising method. This perspective is gaining more attention in the mining industry because of the increasing demand for metals, which require further beneficiation of low-grade ores [6]. Unfortunately, this method is currently overlooked in mineral recycling, including during IBA processing. As represented in studies involving the milling process [7,8,9], incorporating a milling operation for IBA is equivalent to the pulverization step for direct employment as a cement or concrete component.
Specifically, as recently published by Malaiškienė et al. [10], the IBA was thoroughly ground using a ball mill until reaching a particle size of 13.7 µm for cement production. Despite considering the different particle size classifications, Cheng et al. [11] also took a similar approach of fine grinding to a particle size of less than 250 µm for a comparable application. Furthermore, an interesting approach was taken by Cheng et al. [12] by considering different particle sizes of IBA but eventually pulverizing to less than 75 µm for cement application. Nonetheless, it is worth mentioning that using a ball mill to process the IBA before further application in the cement industry requires a certain level of cleanliness to avoid any disparity in the cement quality. The reason is based on the fact that the pulverizing process using a ball mill might only increase the liberation degree but without any separation process. Additionally, regardless of local legal quality standards, such a high-quality IBA that can match the cement industry requirements is available in small volumes, thus delaying the scaling up of those research results.
Consequently, given that even the most recent publication incorporating a milling process for IBA always involves complete transformation into powder, a selective milling approach is relatively attractive to provide additional perspective. In contrast to the ball milling process, selective milling does not entirely pulverize the IBA. Instead, it selectively mills the input materials based on their hardness characteristic and subsequently separates the product into different fractions. Considering that the selective milling process using vertical roller mills is currently an operating approach in the cement industry, it is convincing to explore selective milling as an alternative concentration method to ensure fewer barriers during future industrial implementation. Therefore, given that selective milling for IBA has not yet been deeply studied and has unrevealed potential, this research aims to provide an additional perspective and investigate the effect of selective milling on the concentration process to extend the industrial utilization opportunity of IBA.

2. Materials and Methods

2.1. pr.IBA as the Primary Investigated Sample

The sample considered in this study is the processed IBA (further named pr.IBA), which is only a specific fraction of the original raw IBA that stands for a residual material generated by the state-of-the-art recovery process commonly carried out in Germany, as shown in Figure 1. In this instance, the raw IBA represents the fresh and wet bottom ash produced from the MSW incineration process, with an expected annual volume of approximately 6 Mt in Germany. This raw IBA was naturally weathered (atmospheric aging) for around two to three months before the slag processing plants underwent a specific sequence of screening and separation processes.
The operation begins with a classification process where the oversized (>32 mm) metal is separated as a metal scrap for further recycling; meanwhile, the incomplete-incinerated waste is transferred back into the incineration process. As a result, there will be a residual fraction known as unprocessed IBA, with an annual expected volume of nearly 5.9 Mt to act as the input material for the subsequent separation operations carried out by the IBA processing companies. The following classification conducted by the respective processing plants included a series of sieving, magnetic, and eddy current separation processes. As a result, three different fractions are yielded: ferrous, nonferrous, and pr.IBA, as indicated in Figure 1. In the case of pr.IBA, this fraction can be subsequently categorized into two different particle size ranges of 0–10 mm and 10–32 mm, with theoretical volumes of 2.7 Mt and 1.5 Mt, respectively. Conclusively, this dried pr.IBA 0–10 mm is considered the primary investigated sample in this research, whereas another fraction of 10–32 mm is used as a comparative sample during milling.

2.2. Selective Milling Procedure Using a Vertical Roller Mill

Three tons of each pr.IBA samples were considered in this milling trial for each size fraction of 0–10 mm (3 sources) and 10–32 mm (1 source) to ensure that the sample was representative. The milling process was performed in a demonstration machine using a vertical milling technique operated by LOESCHE in their test center in Neuss, Germany. The working mechanism of this milling machine is illustrated in Figure 2. During the operation, the sample of IBA (either pr.IBA 0–10 mm or 10–32 mm—not mixed) is charged into the mill and falls into the center of the milling machine (1). Once the IBA lands on the rotating milling table (2), it is transferred to the milling roller (3) due to a centrifugal force (5). This adjustable milling roller will eventually grind the IBA selectively (4), responding to a combination of set parameters.
As the milled IBA reaches the end of the milling table, an upward gas flow intercepts the material ejection, where the lighter fraction will be carried up (6), and the heavier fraction will drop down (8) and be collected as a coarse fraction. The carried-up light fraction encounters a classifier (7) at the upper part of the milling machine, producing two additional milling products. The fraction that passes the classifier is transported out from the top of the milling machine as a fine fraction; meanwhile, the oversized fractions (9) are conveyed out from the middle part of the milling machine as a middle fraction.
In the whole set of experiments, the milling trials for pr.IBA 0–10 mm were repeated eleven times (with a supplementary four milling trials for other variants), and the trials for pr.IBA 10–32 mm were reproduced nine times. Regarding the milling parameters, the charging rate of the input material was 250 kg/h, and the working pressure of the milling was adjusted in the range of 20–30 bar. Based on the forthcoming results, this range in working pressure has a relatively minor effect on the product quality, especially with the fine fraction as the essential product and the tendency in the chemical composition, which is relatively constant. Moreover, these starting parameters were selected based on the experience considering the optimal milling process and the proportion of milling products. It is worth mentioning that the potential to refine the process for desired product quality can still be carried out, yet it is currently not the main focus of the present study.

2.3. Characterization Methods

As mentioned earlier, the primary sample in this research is the pr.IBA 0–10 mm due to its higher expected material volume and potentially higher content of contaminants. Accordingly, the analyses were conducted more intensively on the pr.IBA 0–10 mm before and after milling using X-ray diffraction (XRD) and X-ray fluorescence (XRF) methods. In this case, especially for the middle and coarse fraction, the analyzed samples were first divided using a rotary sample divider and pulverized using a vibratory disc mill to the particle size of approximately less than 90 µm to ensure the analysis was representative.
Sieve analysis was also carried out for all the fractions (fine, middle, and coarse) to identify the particle distributions of all the milling products. Comparatively, only the XRF method was used to analyze the chemical composition of pr.IBA 10–32 mm before and after the milling process as a comparative sample to evaluate the tendency found in comparable analysis results of pr.IBA 0–10 mm. Moreover, two additional samples (different sources) of pr.IBA 0–10 mm (coded as var.Af and var.Bf) were also tested to determine the effects of the input materials on the fine fraction.
If a detailed analysis of certain particles’ macrostructure and chemical composition was necessary, the metallography technique was employed by employing a light microscope and scanning electron microscope with energy-dispersive X-ray spectroscopy (SEM/EDS). In this case, the studied sample was mounted in resin, followed by a sample preparation involving grinding and polishing. During the metallography analysis using a light microscope, a stitching approach was also carried out in addition to a single take of one image. Specifically, several metallography images were captured and compiled automatically into an overview image to provide a broader perspective of the sample structure.

3. Experimental Results

3.1. Initial Characterization of pr.IBA

The representation of pr.IBA 0–10 mm and 10–32 mm, both in moist and dry conditions, are shown in Figure 3 and Figure 4, respectively. Depending on the circumstances during the weathering process, the moisture content could be up to 15 wt.%. Furthermore, the samples macroscopically consisted of mineral gravel, although some rusted pebbles were also captured, indicating potential residual iron-bearing materials. The indication is further substantiated in Figure 5, as the pr.IBA 0–10 mm was observed under a light microscope. Based on the analysis, brighter areas engulfed in an inhomogeneous structure can be observed, amplifying the existence of a metallic fraction in a heterogeneous mixture with other particles possessing specific mineral composition configurations.
Despite the possibility of forming specific compounds, the residual metallic components in pr.IBA 0–10 mm and 10–32 mm can also be traced based on the chemical composition analysis in Table 1. According to these results, both samples contain approximately 7–8 wt.% Fe. This Fe content should partially represent the metallic particles documented in Figure 5. In another case of mineral particles, the chemical analysis also indicates that the main components are composed of Si, Ca, and Al, which probably form specific chemical compounds containing other major and minor elements, as also listed in Table 1.
In addition to the results in Table 1, two additional samples (var.Af and var.Bf) of pr.IBA were also analyzed, as provided in Table 2. Based on this result, the major components are also composed of Si, Ca, and Al at comparable contents, as in Table 1. In contrast, these two samples contain significantly lower Fe concentrations, ranging from 1 to 2 wt.%. Interestingly, some trace elements were also detected to be lower, including Mn, Cr, Ni, Sb, Co, and Mo, which are related to the iron content in terms of metallic alloys bound in a silicate matrix or associated with a spinel structure as reported by Wei et al. [15]. However, it is also worth noting that notably higher concentrations of Pb and Sn were recorded in Table 2 instead, which is essentially undesirable.

3.2. Mass Balance and Size Distribution After the Milling Process

As mentioned, three distinct products are considered after the selective milling process, and their appearance is shown in Figure 6. Generally, the fine (F) fraction represents a completely pulverized product. In contrast, the coarse (C) fraction is slightly distorted, and the middle (M) fraction is roughly between fine and coarse fractions.
Complementary to the physical appearance of the fine, middle, and coarse fractions in Figure 6, sieve analysis was performed to describe the size distribution of each milling product. These results documented that the fine fraction has a predominant particle size of <200 µm (Figure 7), whereas its dominant fraction has a particle size of <20 µm. The particle size of the middle fraction (Figure 8) ranges in between the fine and coarse fractions with a relatively uniform distribution of sieve classification from 0.2 to 0.5 mm. In contrast, the coarse fraction (Figure 9) consists of a particle size of >0.5 mm yet is dominated by a grain size range of 1–2 mm, indicating that the original pr.IBA input materials are ground to a certain degree before yielding a coarse fraction.
A further intriguing observation is also indicated by the total mass balance of all the selective milling products considered in this study. Table 3 shows that the main fraction yielded from the milling process is the middle fraction for almost all input materials compared with the fine and coarse fractions, which are relatively balanced in mass proportion. Considering the pr.IBA, for both 0–10 mm and 10–32 mm, the middle fraction accounts for approximately half of the proportion (41–55 wt.%); meanwhile, the fine and coarse fractions account for only a maximum of 40 wt.%. Comparatively, different behaviors are recorded in var.Af and var.Bf, where a fluctuation in mass proportion occurs. Specifically, despite relatively comparable proportions in the fine fraction, the middle and coarse fractions distributions are notably distinguishable from pr.IBA 0–10 mm and 10–32 mm.

3.3. Chemical Composition Development of Milling Products

Several chemical composition analyses were subsequently conducted to characterize the effect of the selective milling operation. The first approach was an XRD analysis of the primary sample considered in this study (pr.IBA 0–10 mm) for both the as-received condition and all its milling products. As shown in Figure 10, numerous indivisible peaks are recorded, representing various unquantifiable phases in those selected samples. In this instance, it is substantiated that the pr.IBA is still a heterogeneous material, whether before or after the selective milling process has been carried out.
However, considering the main component of pr.IBA, some distinct segregation could already be observed. Taking the as-received pr.IBA 0–10 mm into account, indicated by the red line in Figure 10, several primary phases were detected (major phase identification was aided by an automatic detection system embedded in the XRD software) coherently with the XRF analysis results in Table 1. These phases include quartz (SiO2), akermanite (Ca2MgSi2O7), calcite (CaCO3), feldspar, belite (Ca2SiO4), anhydrite (CaSO4), hematite (Fe2O3), magnetite (Fe3O4), some minor indications of wustite (FeO), metallic iron, and aluminum. Additionally, a notable content of the ettringite phase can be identified only in the as-received pr.IBA 0–10 mm, which is before the milling process. Interestingly, despite already passing through a sequence of the separation process, these phase compositions of pr.IBA still align with the typical raw IBA, as reported in [15,16,17,18].
After the milling process, except for ettringite detection, those other typical phases found in the as-received pr.IBA 0–10 mm are also detected in the milling products, yet they are associated with specific enrichment. As indicated by the black line in Figure 10, enrichment in the grindable and brittle phases was measured. The phases include CaCO3, CaSO4, and Ca2SiO4, in addition to the possible decomposition products of ettringite, including portlandite (Ca(OH)2), gypsum (CaSO4·2H2O), hemicarbonate, sodium sulfate, a minor indication of Fe3O4 and almost no metallic particles. Comparatively, in the case of the middle fraction indicated by the green line in Figure 10, an enrichment of SiO2 is notably detected, followed by a decreasing proportion of CaCO3 and CaSO4. Conclusively, significant segregation of metallic particles and iron oxides is recorded for the coarse fraction represented by the blue line, followed by a lower indication of CaCO3, CaSO4, SiO2, and feldspar contents.
The enrichment pattern measured in the milling product of pr.IBA 0–10 mm was further amplified by the chemical composition analysis performed using the XRF method. As documented in Figure 11, calcium enrichment is observed in the fine (F) fraction, and the highest silicon concentration is recorded in the middle (M) fraction. Notably, the construction of the gray boxes follows a similar approach indicated by the legend in Figure 7, Figure 8 and Figure 9. The difference is that the gray boxes in this chemical composition diagram represent a normalized value (scale on the left Y-axis) relative to the as-received concentration provided in Table 1; meanwhile, the average measured content (wt.% or ppm) in all products is provided by the orange dots (scale on the right Y-axis). In addition, as a comparison, the results for all milling products for pr.IBA 10–32 mm are also provided. In this specific case, a similar tendency is elucidated for calcium and silicon in Figure 11.
Other elements also presented a comparable tendency in the chemical composition of all the milling products between the pr.IBA 0–10 mm and 10–32 mm. In the case of the coarse (C) fraction, iron enrichment is further substantiated in Figure 12a, as previously mentioned in the XRD results. This significantly higher iron content than that of the fine and middle fractions, which are relatively close to each other, is incongruent with the pattern in calcium. Interestingly, this segregation pattern of iron also applies to the copper content in Figure 12b, where notable concentrations can be detected in the coarse fraction. However, in the case of copper, the lowest content was measured in the middle fraction instead of in the fine fraction. A lower gap in the concentration difference for both pr.IBA 0–10 mm and 10–32 mm can be observed in aluminum and magnesium, the two other major components of the milling products considered in this study. As documented in Figure 13, a modest enrichment in the coarse fraction is represented, particularly for aluminum. In the case of magnesium, the disparity is narrower. However, the lowest magnesium content is measured in the middle fraction, parallel to the recorded tendency for aluminum and copper, but it contradicts the silicon pattern.
In addition to the analysis results for the abovementioned major elements, various patterns in the concentration disparities of the other considered components in Table 1 are also recorded for all the milling products of pr.IBA 0–10 mm and 10–32 mm. However, this complete analysis is not provided in the main text of this present study but is available in Appendix A and Appendix B, except for the fine fraction. Considering this fraction as one of the essential direct milling products that can be utilized in clinker production (explored in detail in a dedicated report), information regarding the range in the chemical composition of the fine fraction is critical. Accordingly, Table 4 provides the analysis results of the fine fractions from different pr.IBA (different sources and sizes), where calcium enrichment or increasing Ca/Si ratio in the fine fraction is recorded, compared to the as-received or before milling in Table 1 and Table 2. Specifically, a calcium concentration of 11–20 wt.% can be expected, followed by comparable approximate silicon content of 11–20 wt.%.
Furthermore, the residual iron content on the other side could still be associated with the original available concentration under the as-received conditions. In contrast, a relatively constant concentration range was observed for the other major components in all the studied fine fractions, particularly aluminum and magnesium. Unfortunately, this justification of a comparable range is becoming more problematic for different elements, such as those with concentrations < 1 wt.%. In this case, the appraisal should be based on future applications with respective concentration limits, which are not covered in detail in the present study.

4. Applicability of Selective Milling as a Concentration Process

Based on the chemical analysis provided in Table 1 and Table 2, the untapped potential of all considered pr.IBA is substantiated in the present study. Although the fact that the pr.IBA has already been separated using several sequences of state-of-the-art screening, magnetic, and eddy current separation processes, as represented in Figure 1, some fractions of metallic iron and aluminum can still be detected in the as-received condition. In addition, chemical composition analysis revealed that opportunities for other valuable metals and mineral recoveries are widely available. Therefore, it is unfortunate to recognize that the pr.IBA is only used as less-valuable construction material at disposal sites, as is practiced in Germany [1].
In the abovementioned case, those insufficient valorization approaches for the pr.IBA can be justified from two perspectives. From the standpoint of the metal industry, the first opportunity directly perceived from the chemical analysis of pr.IBA (before the milling process) is the copper content and other trace elements measured at lower concentrations. The fact that copper is categorized as a strategic raw material according to the EU Critical Raw Material Act [19,20], the recovery of copper from pr.IBA should be considered. The argument is based on the chemical analysis results for copper concentrations in Table 1 and Table 2, where a range of 0.4–0.6 wt.% can be expected. This measured concentration should be encouraging since a comparable content is also considered the mined grade in exploited copper ore, as summarized by Flores et al. [21]. A similar copper concentration is also currently valued in Escondida, one of the largest copper mines in the world, which is operated by the BHP [22]. Therefore, considering the importance of copper, exploring a profitable method to recover it from pr.IBA is theoretically reasonable.
On the other hand, from the mineral recovery perspective, another chance worth addressing is the chance of charging pr.IBA as alternative construction materials. One possible alternative is cement clinker production because of the high silicon, calcium, iron, and aluminum contents in pr.IBA, which are reported to be relatively close to the cement composition [23,24,25]. As supported by the XRD analysis documented in Figure 10, the fraction of silicon and calcium contents should also be in a configuration of SiO2, Ca2SiO4, CaSO4, and CaCO3 systems, which are traditionally the essential components of the raw mix during the production of the cement clinker. In this instance, their employment as alternative secondary materials would save a certain amount of natural resources, representing a double positive impact regarding lower emissions and resource efficiency.
The fact that calcium also exists in CaSO4 and Ca2SiO4 could even enhance this effect since less CO2 would be emitted during the clinker production process because of the reduced amount of required CaCO3. Interestingly, those calcium-bearing substances in sulfate and silicate could be significantly present in the fine fraction. Deike et al. [26] reported that a convincing correlation was measured between the sulfur content in the fine fraction and the calcium concentration, indicating the possible formation of CaSO4. On the other hand, in addition to the availability of Ca2SiO4, variations in calcium-containing silicates, including wollastonite (CaSiO3) and akermanite (Ca2MgSi2O7), were also detected using XRD analysis in the fine fraction. Finally, in addition to the silicon- and calcium-bearing phases, even the presence of iron oxide components in pr.IBA could be beneficial since it is also necessary to produce a liquid phase during the clinker-burning process.
Considering the advantages of both perspectives simultaneously, a specific concentration process is necessary to optimize the recovery process of both the metal and mineral fractions. Such an approach could also deliver a greater opportunity to generate a profitable operation than solely focusing on yielding one product (either metal or mineral only). Consequently, a comminution process is required to liberate and separate the expected products because of the morphology of pr.IBA particle as a heterogeneous mixture, as indicated in Figure 5. Based on this argument, a selective milling operation is carried out, and its applicability in processing pr.IBA is substantiated in the present study. As shown in Figure 11 and Figure 13, a concentration tendency can already be observed, and this generated tendency persists despite different input materials being considered (Table 1 and Table 2). Practically, this result is essential considering the characteristics of pr.IBA are never identical in chemical composition. In this case, the results show that as long as the milling parameter can be kept in a tolerable fluctuation, the concentration tendency between all the milling products will remain relatively stable (including the fine fraction in Table 4). Moreover, it is also proven that the result is unattached to the chemical composition of IBA before milling since only the mass proportion (Table 3) among the fine, middle, and coarse fractions changed accordingly, indicating the effectiveness of the selective milling.
These documented concentration phenomena in the chemical analysis for all the milling products of both pr.IBA 0–10 mm and 10–32 mm are associated with the hardness properties of different mineral configurations, which is the core and determining aspect of the selective milling process. In this case, the mechanism is based on the well-known correlation between the Mohs scale, hardness characteristics, and grindability indices of different minerals, as recently confirmed in [27,28,29]. Based on this concept, calcium enrichment (Figure 11a) is expected to be measured in the fine fraction since calcium-rich minerals tend to have a high grindability index (easy to grind). This enrichment is considered positive since a high calcium content would be more beneficial for the cement industry, especially if the concentrated calcium is a mixture of CaSO4 and Ca2SiO4, as demonstrated by the XRD analysis in Figure 10. This particular fraction also possesses a fine grain size distribution comparable to raw mix, as documented in Figure 7, which adds more value to the fine fraction for the cement industry.
In contrast to calcium segregation, aluminum enrichment (Figure 13a) is detected in the coarse fraction, which should be associated with the hardness properties of Al2O3. It is known that Al2O3 has a relatively low grindability index with an approximate Mohs scale of nine. However, it is also worth mentioning that given the concentration of aluminum lower than 5%, it is already challenging to assess the chemical composition development by solely comparing the tendency to the as-received composition. As shown in Figure 13a, it is revealed that the relative value of fine and middle fractions fluctuates near the value of 1.00, which can be considered constant; meanwhile, the coarse fraction is above 1.00, indicating an enrichment. This discrepancy can be explained by including the effect of the chemical compositional range of pre-milling conditions, which generally will have a more notable impact on the component with lower concentration. Considering this perspective, a direct concentration comparison of all milling products is included in each respective diagram (orange dots). It is based on the fact that the generation of fine, middle, and coarse fractions are always dependent on each other; hence, their relation becomes more reliable, including for the major component with lower concentration. In this instance, as also provided in Figure 13a, an aluminum enrichment in the coarse fraction is supported by the highest average aluminum content compared to fine and middle fractions.
The concentration process becomes more attractive if elements in the middle of the Mohs scale band are considered. One of those elements is silicon, which has a Mohs scale of around seven. In this case, an enrichment of silicon can be detected in the middle fraction, as shown in Figure 11b. However, compared with calcium and aluminum segregations, this silicon enrichment in the middle fraction is also followed by a relatively notable concentration in the coarse fraction (the relative values of silicon content are above 1.00, representing an enrichment in both fractions compared with the as-received condition). Given the dominant concentration of Si in Table 1 and Table 2, the formation of a SiO2-based complex compound is expected to be supplementary to the simplest form of SiO2, which could result in harder substances and eventually segregate in the coarse fraction. In addition, the formation of this complex also explains the narrow concentration gap or fluctuation in some minor components of pr.IBA (provided in Appendix A and Appendix B), including the magnesium in Figure 13b. Depending on the other components, the Mohs scale of the magnesium-containing compounds could span from one for talc, four for dolomite, six for periclase, and up to eight for spinel.
Complementary to the fluctuation associated with the formation of complex compounds, an evident segregation of iron and copper in the milling product can be elucidated in Figure 12. This result is intriguing because the highest concentration is measured in the coarse fraction despite the hardness level of iron and copper oxides being relatively in the middle of the Mohs scale (like silicon). In this instance, as also mentioned in the XRD results in Figure 10, the effect of the residual metallic fraction could come into play. As documented in Figure 14, some metallic particles are captured in the coarse fraction, which is commonly more challenging to grind than nonmetallic compounds. Another possibility is that the metallic particles can only be flattened during the milling process instead of pulverized [30], thus causing them to land on the coarse fraction.
Furthermore, the relation of iron as a metallic component with the enrichment in coarse fraction can be substantiated by the disparity detected in the pr.IBA var.Af and var.Bf. In both cases, an increase in iron content is observed in Table 4 for the fine fraction compared to their as-received composition in Table 2. It is also worth reminding that both pr.IBA var.Af and var.Bf already possessed a lower iron concentration before selective milling than the primary studied sample pr.IBA 0–10 mm in Table 1, followed by lower Mn, Cr, Ni, Sb, Co, and Mo, which can be related to the iron content as metallic alloys. According to those results, it is convincing to presume that the pr.IBA var.Af and var.Bf are more liberated, or cleaner, from metallic iron. Consequently, it is suggested that the residual iron content in pr.IBA var.Af and var.Bf should be present in the form of mineral substances compounding with particular high grindability components, which eventually accumulate in the fine fraction upon selective milling and are comparable with iron contents of other fine fractions in Table 4 for pr.IBA 0–10 mm and 10–32 mm.
Further disparity, possibly involving metallic particles, is also indicated in the behavior of copper. Considering the mentioned response of a metallic phase during selective milling, it could also explain why the copper concentration in the fine fraction was higher than that in the middle fraction. The first possibility might involve the deformation of copper particles and the initiation of surface erosion. This surface erosion results in the dispersion of copper-containing dust-like particles that are eventually transported up and contained in the fine fraction during and after milling. The second option is the formation of copper as a sulfide substance with a relatively low melting point. As captured in Figure 15 by SEM/EDS, a dendrite with a possible Cu-S configuration is elucidated, which indicates that this copper sulfide should be established due to or during the incineration process. In this case, coupled with a low Mohs scale (only around two) of copper sulfide, such a solidified structure is easy to mill; hence, the copper enrichment in the fine fraction can be justified.
However, although there is an anomaly of copper content in the fine fraction, the highest possible metal content is still clearly measured in the coarse fraction. This concentration in the coarse fraction can be valued as an advantage, since the separation of the metallic fraction (presumably dominated by iron-, aluminum-, and copper-based compounds, as listed in the chemical analysis) is critical for enhancing the valorization feasibility of the mineral fraction in the construction sector. Similarly, producing a concentrated fraction with a relatively high metal content (in this case, the coarse fraction) can also increase its value during further metal recycling. Referring to the copper content in Figure 13b, the measured copper content after selective milling in the coarse fraction is more than 0.75 wt.% on average and could reach 1.0 wt.%, which increases its value as an alternative copper source.
Furthermore, as listed in Table 4, despite relatively significant enrichment in calcium and a decreasing content of the metallic phase as an effect of enrichment in the coarse fraction, the fine fraction could still contain some residual metal content, as also captured in Figure 15. Based on the report of Kolovos et al. [31], a particular concentration of metallic cations could influence the behavior of cement clinker even though, as once reported in [32,33], it should be proportional to the input amount compared with the raw mix. Therefore, research into how this fine fraction is utilized directly in clinker production is necessary to explain its effect comprehensively.
In the case of the middle fraction, a high silicon content followed by the lowest copper concentration should also be a positive indication of further utilization. However, additional milling energy is required since the particle size of the middle fraction is not equal to the particle size of the raw mix used for cement clinker production. On the other hand, the particle size of the middle fraction (as well as the coarse fraction) shown in Figure 8 and Figure 9 could be beneficial in opening further separation opportunities to increase its cleanliness. In this instance, it is possible to charge the middle and coarse fractions back into the metal separation process using once again an additional screening, magnetic, eddy current, and (if necessary) density separation process sequence to produce additional alternative mineral fractions as well as highly concentrated metallic fractions, which will be addressed in a separate report.

5. Conclusions

Based on the observed results, this study highlights the effectiveness of the selective milling approach as an alternative concentration process for enhancing the cleanliness of the mineral fraction by separating the metallic constituents from the pr.IBA. These separation results justify the advantages of employing selective milling, since only completely pulverizing it using a ball mill as a commonly conducted method will not improve the quality of pr.IBA since its chemical composition will remain constant. In this instance, the following outcomes can be drawn from the present study:
  • Despite already undergoing separation processes using state-of-the-art methods, the pr.IBA still holds immense and significant potential for metal and mineral recovery. In this case, a concentration process is successfully performed by selective milling to produce three different products with distinct chemical compositions: fine, middle, and coarse fractions. This outcome is the primary benefit of selective milling compared to conventional fine grinding, which can only reduce the particle size without any concentration mechanism and separation process.
  • It is substantiated that the chemical composition of the resulting fine fraction and the concentration tendency between all milling products can remain relatively constant regardless of the input materials if the milling parameter can be adjusted within a tolerable fluctuation, indicating the reliability of the selective milling.
  • This study demonstrated that by taking advantage of the hardness characteristics of various compounds in pr.IBA, an enrichment of calcium-bearing materials can be produced in the fine fraction. Based on the analysis results, the phases include CaSO4, Ca2SiO4, and CaCO3, which are chemically necessary and, thus, possess potential as alternative materials for cement clinker production, both from the perspective of substituting natural resources and reducing carbon emissions.
  • A similar application in cement production can also be foreseen from the middle fraction, considering its silicon enrichment and, interestingly, a lower copper content. In this instance, in contrast to the fine fraction, an enhancement in quality is still possible for the middle fraction. Specifically, due to its particle size, another separation process can still be carried out, which opens an opportunity to increase its value and produce an additional material stream.
  • A significant segregation of metallic particles or metal-containing fractions (Fe and Cu) is expected within the coarse fraction, which is separated from the mineral fraction. Besides enhancing the valorizing potential of the fine and middle fractions in the cement industry, this coarse fraction can be further processed to recover the metal for conventional metal recycling operations and, as a byproduct, yield a coarse mineral fraction that might benefit another purpose in the construction sector.

Author Contributions

I.B.G.S.A.: Conceptualization, Methodology, Validation, Investigation, Formal Analysis, Data Curation, Writing—Original Draft, Visualization. S.H.S.: Formal Analysis, Investigation, Data Curation, Writing—Original Draft, Visualization. W.R.: Methodology, Data Curation, Validation, Formal Analysis, Investigation, Resources, Writing—Review and Editing. R.W.: Conceptualization. Methodology, Resources, Writing—Review and Editing, Supervision, Project Administration, Funding Acquisition. R.D.: Conceptualization, Methodology, Validation, Formal Analysis, Resources, Writing—Review and Editing, Supervision, Project Administration, Funding Acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the funding program ReMin: Resource Efficient Circular Economy—Construction and Mineral Material Cycles (Project EMSARZEM: FKZ 033R265) under the Research for Sustainable Development Program (FONA) initiated by the Federal Ministry of Education and Research (BMBF), Federal Republic of Germany. The authors also acknowledge support from the Open Access Publication Fund of the University of Duisburg-Essen.

Data Availability Statement

The original contributions presented in the study are included in the article. Further inquiries can be directed to the corresponding authors.

Acknowledgments

The authors acknowledge the support of Aneta Knöpfelmacher and Stefan Seemann from VDZ Technology GmbH for the thorough chemical analyses, including XRD and XRF, for the studied samples before and after the selective milling process.

Conflicts of Interest

I.B.G.S.A., S.H.S. and R.D. certify they have no conflicts of interest to declare. W.R. is employed by the company LOESCHE GmbH; R.W. is employed by the company GKS-Gemeinschaftskraftwerk Schweinfurt GmbH. These remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Appendix A

The Concentrations of the Major Elements in All the Milling Products

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Figure A1. (a) Na and (b) K contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
Figure A1. (a) Na and (b) K contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
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Figure A2. (a) Zn and (b) P contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
Figure A2. (a) Zn and (b) P contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
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Figure A3. (a) Ba and (b) Mn contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
Figure A3. (a) Ba and (b) Mn contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
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Figure A4. (a) Pb and (b) Cr contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
Figure A4. (a) Pb and (b) Cr contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
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Appendix B

The Concentration of the Minor Elements in All the Milling Products

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Figure A5. (a) Sr and (b) Ni contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
Figure A5. (a) Sr and (b) Ni contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
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Figure A6. (a) Sn and (b) Sb contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
Figure A6. (a) Sn and (b) Sb contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
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Figure A7. (a) Co and (b) Mo contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
Figure A7. (a) Co and (b) Mo contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
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Figure A8. (a) V and (b) As contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
Figure A8. (a) V and (b) As contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
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Figure 1. The theoretical annual mass balance (in tons) of IBA in Germany according to information provided in references [1,13].
Figure 1. The theoretical annual mass balance (in tons) of IBA in Germany according to information provided in references [1,13].
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Figure 2. Schematic of the LOESCHE demonstration-scale vertical roller mill [14] used for selective milling of pr.IBA with a representation of input materials and output products.
Figure 2. Schematic of the LOESCHE demonstration-scale vertical roller mill [14] used for selective milling of pr.IBA with a representation of input materials and output products.
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Figure 3. Samples of pr.IBA 0–10 mm in (a) moist and (b) dry conditions.
Figure 3. Samples of pr.IBA 0–10 mm in (a) moist and (b) dry conditions.
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Figure 4. Samples of pr.IBA 10–32 mm in (a) moist and (b) dry conditions.
Figure 4. Samples of pr.IBA 10–32 mm in (a) moist and (b) dry conditions.
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Figure 5. Metallography analysis (stitching approach) on the pr.IBA 0–10 mm after the observation under a light microscope—a brighter area indicates a metal particle.
Figure 5. Metallography analysis (stitching approach) on the pr.IBA 0–10 mm after the observation under a light microscope—a brighter area indicates a metal particle.
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Figure 6. Typical representation of milling products: (a) fine, (b) middle, and (c) coarse fractions.
Figure 6. Typical representation of milling products: (a) fine, (b) middle, and (c) coarse fractions.
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Figure 7. Typical particle size distribution of the fine (F) fraction.
Figure 7. Typical particle size distribution of the fine (F) fraction.
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Figure 8. Typical particle size distribution of the middle (M) fraction.
Figure 8. Typical particle size distribution of the middle (M) fraction.
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Figure 9. Typical particle size distribution of the coarse (C) fraction.
Figure 9. Typical particle size distribution of the coarse (C) fraction.
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Figure 10. XRD results of the as-received sample and all the milling products of pr.IBA 0–10 mm.
Figure 10. XRD results of the as-received sample and all the milling products of pr.IBA 0–10 mm.
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Figure 11. (a) Ca and (b) Si contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
Figure 11. (a) Ca and (b) Si contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
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Figure 12. (a) Fe and (b) Cu contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
Figure 12. (a) Fe and (b) Cu contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
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Figure 13. (a) Al and (b) Mg contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
Figure 13. (a) Al and (b) Mg contents in all the milling products of pr.IBA 0–10 mm and 10–32 mm.
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Figure 14. Metal particles found in the coarse fraction after the milling process of pr.IBA 0–10 mm.
Figure 14. Metal particles found in the coarse fraction after the milling process of pr.IBA 0–10 mm.
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Figure 15. Particle containing an indication of copper sulfide dendrite in the fine fraction.
Figure 15. Particle containing an indication of copper sulfide dendrite in the fine fraction.
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Table 1. Average chemical composition (XRF) of pr.IBA 0–10 mm and pr.IBA 10–32 mm before the selective milling process.
Table 1. Average chemical composition (XRF) of pr.IBA 0–10 mm and pr.IBA 10–32 mm before the selective milling process.
Major Components (wt.%) of pr.IBAMinor Components (ppm) of pr.IBA
Elements0–10 mm10–32 mmElements0–10 mm10–32 mm
Silicon (Si)22.5519.96Strontium (Sr)390396
Calcium (Ca)13.8710.99Nickel (Ni)292485
Iron (Fe)7.666.92Tin (Sn)235146
Aluminum (Al)3.864.17Antimony (Sb)174130
Sodium (Na)2.362.43Cobalt (Co)7545
Magnesium (Mg)1.311.04Molybdenum (Mo)4537
Potassium (K)0.830.93Vanadium (V)4375
Copper (Cu)0.560.47Arsenic (As)1124
Zinc (Zn)0.540.27
Phosphorus (P)0.330.26
Barium (Ba)0.260.17
Manganese (Mn)0.150.11
Lead (Pb)0.140.34
Chromium (Cr)0.080.11
Table 2. Average chemical composition (XRF) for other variants (from different sources) of pr.IBA 0–10 mm: pr.IBA var.Af and pr.IBA var.Bf before the selective milling process.
Table 2. Average chemical composition (XRF) for other variants (from different sources) of pr.IBA 0–10 mm: pr.IBA var.Af and pr.IBA var.Bf before the selective milling process.
Major Components (wt.%) of pr.IBAMinor Components (ppm) of pr.IBA
Elementsvar.Afvar.BfElementsvar.Afvar.Bf
Silicon (Si)29.4027.17Strontium (Sr)268320
Calcium (Ca)9.7710.13Nickel (Ni)8882
Iron (Fe)1.601.88Tin (Sn)423128
Aluminum (Al)3.284.28Antimony (Sb)7469
Sodium (Na)3.414.22Cobalt (Co)4429
Magnesium (Mg)0.951.05Molybdenum (Mo)57
Potassium (K)1.230.97Vanadium (V)3942
Copper (Cu)0.470.35Arsenic (As)1313
Zinc (Zn)0.450.35
Phosphorus (P)0.480.30
Barium (Ba)0.150.19
Manganese (Mn)0.080.08
Lead (Pb)0.380.38
Chromium (Cr)0.030.04
Table 3. Mass distribution (wt.%) of all the milling products from different input materials.
Table 3. Mass distribution (wt.%) of all the milling products from different input materials.
Selective Milling Product Variationpr.IBA
0–10 mm		pr.IBA
10–32 mm		pr.IBA
var.Af		pr.IBA
var.Bf
F: Fine Fraction20–3920–3013–1522–25
M: Middle Fraction41–6145–5537–3862–66
C: Coarse Fraction16–2518–34 47–5012–13
Table 4. Variation in the chemical composition (XRF) of the fine fractions from different pr.IBA.
Table 4. Variation in the chemical composition (XRF) of the fine fractions from different pr.IBA.
Elements
(wt.% Unless Stated)		pr.IBA
0–10 mm		pr.IBA
10–32 mm		pr.IBA
var.Af		pr.IBA
var.Bf
Silicon (Si)14.92–17.2115.41–19.7211.65–14.0312.71–13.24
Calcium (Ca)17.60–20.1413.60–14.7511.15–11.5817.94–18.16
Iron (Fe)5.94–6.324.97–5.342.61–2.783.76–3.81
Aluminum (Al)3.66–4.104.35–4.773.64–4.103.65–4.27
Sodium (Na)1.72–2.021.89–2.021.50–1.701.70–1.78
Magnesium (Mg)1.30–1.401.01–1.121.13–1.271.13–1.24
Potassium (K)0.92–0.981.05–1.100.90–1.360.89–1.31
Copper (Cu)0.33–0.380.20–0.360.15–0.170.27–0.28
Zinc (Zn)0.54–0.580.28–0.430.22–0.250.45–0.46
Phosphorus (P)0.40–0.450.24–0.300.42–1.000.42–0.97
Barium (Ba)0.31–0.330.19–0.380.15–0.160.20–0.21
Manganese (Mn)0.17–0.190.11–0.280.04–0.050.03–0.04
Lead (Pb)0.11–0.140.07–0.140.12–0.280.19–0.20
Chromium (Cr)0.08–0.090.05–0.140.03–0.040.04–0.05
Sulfur (S)1.93–2.341.07–1.490.75–0.861.82–2.13
ppm–Strontium (Sr)494–576362–742389–409581–595
ppm–Nickel (Ni)304–315222–43688–119115–119
ppm–Tin (Sn)163–18099–206112–114240–246
ppm–Antimony (Sb)216–242131–20176–81139–145
ppm–Cobalt (Co)45–5545–15969–8099–103
ppm–Molybdenum (Mo)59–6630–6316–1821–22
ppm–Vanadium (V)48–5467–13354–5670–72
ppm–Arsenic (As)3–2415–351–181–2
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Adhiwiguna, I.B.G.S.; Sahbudin, S.H.; Ruhkamp, W.; Warnecke, R.; Deike, R. Effect of Selective Milling on the Concentration Process of Critical Raw Materials from MSW Incinerator Bottom Ash. Minerals 2024, 14, 1174. https://doi.org/10.3390/min14111174

AMA Style

Adhiwiguna IBGS, Sahbudin SH, Ruhkamp W, Warnecke R, Deike R. Effect of Selective Milling on the Concentration Process of Critical Raw Materials from MSW Incinerator Bottom Ash. Minerals. 2024; 14(11):1174. https://doi.org/10.3390/min14111174

Chicago/Turabian Style

Adhiwiguna, Ida B. G. S., S. Humaira Sahbudin, Winfried Ruhkamp, Ragnar Warnecke, and Rüdiger Deike. 2024. "Effect of Selective Milling on the Concentration Process of Critical Raw Materials from MSW Incinerator Bottom Ash" Minerals 14, no. 11: 1174. https://doi.org/10.3390/min14111174

APA Style

Adhiwiguna, I. B. G. S., Sahbudin, S. H., Ruhkamp, W., Warnecke, R., & Deike, R. (2024). Effect of Selective Milling on the Concentration Process of Critical Raw Materials from MSW Incinerator Bottom Ash. Minerals, 14(11), 1174. https://doi.org/10.3390/min14111174

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