Introduction

Iron and steel casting is still a pivotal sector within the metallurgical industry that plays significant roles in various industrial applications. According to recent data from the European Foundry Association (CAEF) [1], the production of iron and steel casting in Europe in 2021 reached a volume of 10.7 million tons. As one of the most prominent casting producers globally, Germany contributes approximately 3.2 million tons. Among those numerous iron and steel casting products, a constantly growing tendency was observed in ductile cast iron (SGI) production since it is known for its mechanical properties and versatility. Specifically in Europe, SGI products constitute a substantial portion of the total iron and steel casting production volume. In 2021, around 4.7 million tons of SGI were produced in CAEF country members, and at least 20% originated from Germany. Supplementary to its industrial necessity, the cast iron foundry has been constantly practicing the concept of circularity through its recycling approach. However, as one energy-intensive operation related to a high-temperature process, the cast iron foundries registered in the European Pollutant Release and Transfer Register (PRTR) System are responsible for around 0.2 Mt CO2 emissions in Germany in 2021 [2].

Given that the foundries are dominated by small and medium-sized enterprises (SMEs), the issue of emission reduction gains relatively less priority compared to the iron and steel industries. According to Trianni et al. [3], the reasons are strongly associated with the insufficiency of available resources followed by the more demanding urgency to sustain the business, which is entirely understandable from the perspective of industrial practice. Based on the investigation conducted by Zhu et al. [4], three factors contribute significantly to the specific emission in the production process of cast iron, namely the production of initial charge materials, the melting operation to prepare the liquid metal and the further necessary treatments for the molten cast iron. From the perspective of charge materials, the increase in specific emissions correlates to pig iron utilization [5, 6]. However, increasing the proportion of steel scrap to pig iron is not without a limit. Excessive steel scrap consumption requires further adjustment by introducing certain ferroalloys and carburizers to maintain the chemical composition of cast iron. Based on the comprehensive life cycle perspective, this practice would tendentially be counterproductive, as previously investigated by Abdelshafy et al. [7].

Comparatively, as amplified by Mitterpach et al. [8], higher negative environmental impact is expected to be influenced mainly by the melting operation, specifically by the melting furnace utilized during the process. The observation further strengthens the results from Finkewirth et al. [9] that suggest the employment of an induction furnace can deliver a reduced amount of specific emission compared to the cupola furnace. However, it is essential to note that induction furnaces depend highly on power generation. Therefore, unless the grid emission factor can be controlled under a specific limit as described in [4, 10], induction furnaces are not always eco-friendlier than cupola furnaces [11]. Furthermore, the selected indicators would strongly influence the justifications of the life cycle analysis [12]. The idea of shifting the melting technology also tends to be the highest possible barrier for the foundry industry, and hence, it is unlikely to become a short-term strategy.

Considering the challenges connected to the other factors, the treatment process of molten cast iron should become the starting point. Particularly during the production of SGI, it is known that a perfect combination of various materials and treatment parameters is necessary. One of those materials is magnesium, which has been used for years to influence the SGI microstructure and cast iron with vermicular graphite (CGI), specifically during desulfurization. However, the production process of magnesium itself is related to unneglectable environmental challenges that indirectly affect the carbon footprint of the cast iron product. On the other side, high dependency due to dominant sources in the global supply of magnesium raises concerns regarding supply chain and market competitiveness [13, 14]. Therefore, it is crucial to challenge the status quo of utilizing magnesium and explore another reliable alternative to assure economic and ecological sustainability.

The calcium-based desulfurization process, which has been transformed into a standard practice among European iron and steelmakers [15], could also be one promising sustainable alternative for the cast iron industry. Extensive research has been carried out to optimize the application of the lime-based desulfurization process due to its cost and emission advantages. The concept was early explored by Niedringhaus and Fruehan [16], which proved that the CaO could be utilized as a desulfurization agent through a surface reaction mechanism. This finding is supported by Inoue and Suito [17] and Takahashi et al. [18], who verified that increased CaO in slag could enhance the sulfide capacity and that the liquid slag composition controls CaO-based desulfurization rate. The performance of CaO to desulfurize hot metal was subsequently examined by Lindström and Sichen [19] by carefully comparing the desulfurization abilities of several common desulfurization agents in laboratory conditions. The results documented that certain qualities of CaO could deliver a comparable outcome to the magnesium process. Contrary, compared to the investigation results reviewed by Manachin and Shevchenko [20] using industrial scale calculations, the single injection of CaO, regardless of its quality, is still undesirable compared to the magnesium process due to higher reagent consumption, higher loss of hot metal, and decrease in hot metal temperature. Therefore, a co-injection with magnesium is industrially more practicable, considering the provenly effective capability of CaO to reduce the resulfurization phenomena by forming a more stable sulfide than magnesium [21].

Regardless of the general knowledge generated in the iron and steel industries, no similar lime-based process has been reported in cast iron production to a certain extent of the author’s knowledge. Considering all the prospective advantages for the future of foundry industries, this study aims to provide a comprehensive ecotechnological examination for implementing a lime-based desulfurization process in cast iron industries. Referring to the barriers explored in [3], the progressive adoption of this exceptional operation would secure an eco-friendly process and increase market competitiveness, thus holding a promising chance of sustainable growth for the cast iron industries.

Materials and Methods

Technological Approach

The operation analyzed in this study involves a newly developed injection process in the cast iron foundry in Germany. The technological concept was based on positive results on a laboratory scale that would be provided in a separate dedicated report. This alternative desulfurization process involves an introduction of a lime powder desulfurization agent into 2.5-ton molten cast iron by employing inert nitrogen gas as transport media through a refractory-coated lance. The complete construction of the injection machine is provided in Fig. 1.

Fig. 1
figure 1

Construction of an automatic injection machine for lime-based desulfurization of cast iron. [22, 23]

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Considering the future coupled integration defined by working synchronously in one integrated industrial cast iron production system, the injection machine was designed to be comparable to the magnesium-based desulfurization process. In this regard, the lime injection operation should also be conducted using a batch ladle-based approach, starting by tapping one ladle of molten cast iron from a cupola furnace, which has a typical composition provided in Table 1. The molten iron would subsequently be transported to the injection station and desulfurized at a starting temperature of around 1500 °C for five to seven minutes. Furthermore, the slag skimming process is carried out, followed by chemical composition and temperature measurements, before being transferred to the further production sequence of the CGI manufacturing process following the routine standard operational procedures in the foundry involved in this study.

Table 1 Initial chemical composition of cast iron before desulfurization
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Following the establishment of the lime injection desulfurization process, the development encompassed three sequential phases. The first step was the experimenting stage, which involved a set of trials for different mixtures of lime-based desulfurization agents and lance designs to identify the optimal process. Two distinct designs of refractory-coated lance were tested: one (In-Tip) and two (To-Tip) holes in the nozzle system. In addition, the desulfurization ability of four mixtures was also examined, namely CaO-Na2CO3-CaF2 (KNF), CaO-Na2CO3 (KN), CaO-C-Na2CO3 (KCN), and CaO-C (KC) including an evaluation to the generated slag for respective mixtures through a leaching test. Subsequently, the second sequence was the injection optimization phase, which explored different configurations of injection parameters and used a coarser lime particle size of < 2 mm (was < 100 µm during the previous stage). Conclusively, the final leap was the integration process, where the optimum result from the preceding phases was employed to integrate the injection method for desulfurizing the molten cast iron into the operating production system in the cast iron foundry.

Ecological Approach

Supplementary to the technical analysis, a straightforward life cycle study was conducted following the general framework provided in DIN EN ISO 14040. This study analyzes the expected environmental benefits of substituting the magnesium-based operation with the lime-based desulfurization process. By referring to a set of primary industrial data, the evaluation of possible emission reduction during the desulfurization of 1 kg cast iron was carried out and subsequently used as the functional unit. However, the analysis provided in this ecological approach involves only a direct juxtaposition by considering solely the desulfurization operation. Hence, the process sequences before and after the desulfurization operation are settled to be identical, which is also required to support the concept of coupled integration mentioned earlier. The study will be carried out under a cradle-to-gate approach based on contextual scenarios in the foundry participated in this study. Considering only the final development phase (integration process) of lime injection technology, the material and energy balance both for magnesium-based designated as treatment 1 (current) and lime-based desulfurization indicated as treatment 2 (alternative) are, respectively, provided in Figs. 2 and 3. It is worth noting accordingly, that some prior limiting circumstances should be taken into account to refine the subject of this comparative study.

Fig. 2
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Material flow diagrams of the current magnesium-based desulfurization process

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Fig. 3
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Material flow diagrams of the alternative lime-based desulfurization process

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By starting from the perspective of emitted emission for both process options, the air contamination in the form of gas, fume, and dust is hypothesized to be resultantly comparable. Therefore, they are not considered in the life cycle inventory. A similar approach was also adopted to exclude the slag discharge since the slag from the lime-based process could be recycled back into the cupola furnace instead of landfilled [14]. This interchange between slag amount and recyclability is expected to neutralize their contribution from the perspective of a comparative study. Despite this limitation, the result of this study should be negligibly affected since forthcoming results will elucidate that the indirect emissions (Scope III) [24] considered during the calculation contribute enormously dominant than these direct emissions (Scope I) components.

Regarding energy consumption, electricity is considered the primary energy source supplied by the industrial power mix in Germany unless explicitly mentioned. Furthermore, regarding the input materials, all consumables are considered qualitatively pure or available in one component system except for the FeSiMg-wire and injection lance. Consequently, the life cycle inventory for FeSiMg-wire and injection lance would comprehensively include a separate consideration for each building component and proportionally calculated based on their contribution. Given the restriction of publishing internal company data, the composition in this regard will not be explicitly revealed, though it could be traced through the calculation process.

Considering all the predefined boundaries, the life cycle inventory in terms of specific emission ranges is provided in Tables 2 and 3. The data were mainly gathered from secondary sources, such as peer-reviewed literature studies and official reports, followed by necessary calculations. The involvement of range values instead of one single number as the calculation approach is appraised to be beneficial since it could represent a holistic comparative CO2eq. Practically, by using minimum and maximum emissions, the calculation results can accommodate sensitivity associated with the technology alternatives and energy source combinations, which depend highly on the supplier's circumstances.

Table 2 List of estimated specific emission for materials in magnesium-based desulfurization process
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Table 3 List of estimated specific emission for materials in lime-based desulfurization process
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Experimental Results

Lime-Based Desulfurization by Injection Process on an Industrial Scale

The change in chemical composition following the desulfurization process was recorded and analyzed in every development phase, as recorded in Fig. 4. It is observable that the first two development phases exhibited high fluctuation in end sulfur content due to the experimentation of different lime-based desulfurization mixtures as well as possible parameter combinations. In contrast, the production phases showcased stable and consistently good-quality desulfurization treatment where the end sulfur contents are mainly lower than 0.015% as required.

Fig. 4
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Sulfur content in molten cast iron before (initial) and after (final) desulfurization process

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Supplementary to the results summarized in Fig. 4, details of final sulfur concentration during the first two development phases are comparatively provided in Fig. 5, where the To-Tip KC comprises both KC-1 and KC-2 (lime particle size of < 100 µm and < 2 mm, respectively). Accordingly, it is elucidated that the To-Tip lance design is better than the In-Tip in terms of desulfurization degree. It is also suggested that introducing the CaF2 and Na2CO3 into the lime-based desulfurization mixture could slightly improve the desulfurization process, as represented by a lower final sulfur content than the KC (without CaF2 and Na2CO3). In the case of KC, the results in Fig. 4 indicate that a comparable desulfurization degree could also be achieved using a coarser lime particle.

Fig. 5
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Effect of different lance design and desulfurization mixtures on the final sulfur content

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Considering the better performance of the desulfurization agent containing CaF2 and Na2CO3, the slag analysis indicated a counterbalancing effect. According to the leaching test of desulfurization slags in Table 4, the operation involving CaF2 is expected to be hard to take care of due to its contamination and requires an underground landfill. Improvement in landfill classes to DK 1 is observed in cases where no CaF2 was incorporated in the mixture, which practically means it is less hazardous. Furthermore, the mixture that was utterly free from CaF2 and Na2CO3 can even reach the DK 0, which indicates high stability, less contaminant, and higher recyclability, as well as possesses comparable quality to the slag from the magnesium-based (FeSiMg) process.

Table 4 Effect of different desulfurization mixtures on the landfill classes of the desulfurization slag
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Besides exploring the process development and parameter configuration, the iron desulfurized using optimum parameters (Fig. 3 using To-Tip KC-2) was subsequently utilized to produce industrial cast iron products during the last development phase. Following the positive results in chemical composition, specifically final sulfur content, the lime-based desulfurized cast iron was transferred to the post-desulfurization process, including examination sequences following the standard operational procedures. Table 5 and Fig. 6 show that the CGI produced from lime-based desulfurized cast iron exhibited comparable mechanical properties and desirable graphite morphology, suggesting no disparity in achieving comparable material characteristics.

Table 5 Average mechanical properties of CGI made from FeSiMg- and lime-based desulfurized cast iron
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Fig. 6
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Microstructure of CGI made from a FeSiMg- and b lime-based desulfurized cast iron

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Ecological Impact Assessment in Comparative CO2eq

As previously indicated, an essential ecological analysis complemented the perspective after exploring the technological feasibility. Table 6 presents the results of the CO2eq emission comparison between the FeSiMg-wire desulfurization process and the lime-based operation according to the respective mass and energy balance provided in Figs. 2 and 3 both for one treatment as well as for the designated functional unit.

Table 6 Comparative impact assessment (estimated CO2eq) for desulfurization process of cast iron
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The calculations reveal that, depending on the technology employed in the manufacturing process of raw materials, the expected CO2eq emissions for treating one ladle of molten iron range from 190 to almost 590 kg (highlighted in yellow shade) for the FeSiMg-wire process. In contrast, the lime-based process demonstrates significantly lower estimated emissions ranging from 52 to 107 kg (highlighted in green shade) for the same quantity of molten cast iron. It is also indicated from this calculation result that the raw materials play a significantly critical role, considering their massive contribution to the whole life cycle of emission regardless of the process alternative. Table 6 also suggests that the manufacturing process of raw materials accounts for approximately 90% of the total emission.

Complementary to the ecological advantage explored earlier, technological acquisition could also provide a capital gain. As indicated in Table 7, a certain saving amount should also be expected for one treatment process upon employing a lime-based desulfurization process. It is worth mentioning that this saving, which could range from 10 to 50 € for one treatment, should first cover the initial investment cost of around 1.5 Mio. € before delivering an actual profit. Considering a linear breakeven point (BEP), the number of treatments in one year and the price fluctuation of raw materials would highly determine the extent of BEP.

Table 7 Cost estimation for the desulfurization process of cast iron (including possible slag disposal)
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Discussion

As summarized earlier in Fig. 4, a distinct behavior in terms of significant fluctuation in desulfurization performance is recorded, highlighting the challenges faced during the first two development phases. During these periods, some prospective mixtures were examined, including incorporating additives and fluxes to control dissolved oxygen [58] and to enhance the proportion of liquid slag on the surface of lime particles [59]. This course of action was aimed to drive the process toward the highest desulfurization degree possible coupled with an optimum level of lime utilization to counterbalance the desulfurization duration of five to seven minutes, which lags the magnesium-based operation (three to five minutes).

Adding aluminum to control the dissolved oxygen based on the investigation results from Matousek [60] was one of the first considerations during the optimization phase. However, given the critical limit of 1000 ppm [61] to avoid any undesired casting defects, intentionally bringing the aluminum into molten cast iron was considered a long-term strategy. On the other hand, as indicated in Fig. 5, increasing the proportion of liquid slag by utilizing Na2CO3 and CaF2 proved effective and comparable to previous studies provided in [62]. Unfortunately, the slag leaching test results in Table 4 suggest that these modifications lead to highly contaminated slags, which is potentially counterproductive. In addition, violent reactions followed by intensive fume generation were observed during the desulfurization process involving Na2CO3. Amplifying the consideration of safety procedure, uncontrolled turbulence in molten metal is also not preferable as it can reduce the refractory lifespan both on the process ladle and the injection lance.

Interesting to observe is the utilization of KC as a desulfurization agent, where comparable desulfurization performance is surprisingly observed. This introduction of carbon was initially aimed to compensate for the depletion in dissolved carbon following the intensive atmospheric high-temperature treatment. According to the correlation proposed by Orths and Weis [63, 64] and recently supported by Grachev [65], this discrepancy is chemically related to the dissolved oxygen level. Considering the treatment temperature of 1500 °C and the chemical composition of the molten cast iron, carbothermic reduction of oxidized dissolved silicon is favorable, thus inducing a decrease in carbon content. The effect of involving carbon is seemingly not just limited to the alloying process but, to a certain extent, eases the desulfurization process as once explored by Boyd et al. [66], Fruehan [67], and Turkdogan and Martonik [68]. Their correlation is circumstantially rational since a higher carbon content will influence the dissolved oxygen level [69] in molten iron, which eventually will positively facilitate the reaction between lime and dissolved sulfur. Furthermore, complementary to its effectiveness in reducing the sulfur content, utilization of KC is followed by less contamination on the desulfurization slag until it is comparable to the magnesium-based process provided in Table 4. This favorable combination is the main reason for selecting KC as the standard desulfurization agent considered during the integration process and the life cycle assessment.

It is worth noting that the comparable desulfurization degree without incorporating fluxes, as mentioned earlier, was theoretically appraised as an anomaly. Contrary to the numerous investigation results, liquid slag is expected to deliver a better desulfurization degree since it is crucial to facilitate sulfur diffusion, particularly if forming a solid reaction product due to high silicon content is anticipated. One plausible explanation associated with this phenomenon is the dynamic resultant from the kinetic perspective. Given the size of the ladle in a cast iron foundry, which is 100 times smaller than in the iron and steel industry, the available upward transport period of lime to the surface of molten metal is significantly narrowed. Even though the desulfurization process could continue at the slag-metal interface on the top surface of molten cast iron, the observations indicate that the desulfurization reaction was surficial and limited to the pure lime particle. In other words, the further diffusion of sulfur through desulfurization product on the surface of reacted lime is limited because of its lower rate compared to the floating duration and additional availability of fresh lime due to the continuous injection process. This dynamic can also explain the occurrence observed in the boundary of this study that using finer particles (KC-1) is not necessarily better than coarser lime (KC-2) since a comparable final sulfur content was measured as provided in Fig. 4. This limitation is yet wholly acceptable from an industrial perspective due to other technical considerations including material handling, stable injection rate, desulfurization batch duration, reliability of final sulfur content, and final slag mineralogy. Accordingly, as indicated in Fig. 4, desulfurization employing To-Tip KC-2 is considered optimum results for the integration process.

Additional crucial enhancing factors can also be generated from the perspective of ecological advantages. According to the calculation carried out in this study, considering the whole life cycle as summarized in Fig. 7, the result indicates that the lime-based desulfurization alternative offers less carbon footprint operation than the magnesium-based process. Specifically, the calculation demonstrated that the least environmental-friendly technology currently available for the lime-based approach (43 g CO2eq) could still deliver more sustainable operation compared to the cleanest (76 g CO2eq) accessible technology required to desulfurize one-kilogram molten cast iron using FeSiMg-wire. As mentioned previously, the production process of raw materials (FeSiMg and lime) contributes significantly to the total reported emissions. In addition, the results in Table 6 also suggest that the transportation emission associated with the FeSiMg process is up to six times higher than the lime-based alternative. This disparity represents that the required materials for the lime-based desulfurization process are readily available domestically, indicating a plausible opportunity to reduce dependency on the global supply chain or high-risk monopolistic market situation. However, it is essential to note on the other side that the injection machine for the lime-desulfurization process consumes more energy, although its contribution to the overall emissions is considered negligible.

Fig. 7
figure 7

Summary of comparative carbon footprint potential (CO2eq) for magnesium-based and lime-based desulfurization process in cast iron

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Another challenge in implementing the lime-based desulfurization process is the significant amount of slag generated, which is also associated with higher expected metal loss. This occurrence results from high molten bad turbulence during the injection process, including the molten metal trapped in lime particles due to agglomeration (sintered) mechanism or pore penetration. The disadvantage of this high slag volume can also be observed in the cost estimation in Table 7, which shows that the slag disposal could account for up to 25% of the total treatment cost and thus reduce the promising capital gain of the lime-based desulfurization process. Consequently, a recycling practice was taken as an alternative to overcome this challenge. Specifically, the foundry in this study charged back the lime-based desulfurization slag into the cupola furnace for the lamellar cast iron (LGI) production process, where the sulfur should not be a concern, and the rest of the iron could be recovered. Nonetheless, this action depends entirely on slag disposal and energy price fluctuation. Hence, it is contextual to each iron foundry, which always holds distinct circumstances.

Conclusion

Based on the experimental results conducted on industrial scales coupled with an ecological consideration, the following conclusions could be extracted from this study:

  1. (1)

    Lime-based desulfurization through injection mechanism in the cast iron industry can be successfully implemented, as proven by the reliable and reproducible final sulfur content lower than 0.015%, followed by the comparable quality of the final cast product.

  2. (2)

    Given the technological consideration of adopting the lime powder injection approach, it offers additional opportunities for further improvement, including an extended alloying function for fine alloying element materials, which is challenging in foundry practice.

  3. (3)

    The ecological impact assessment indicates that the lime-based desulfurization process holds significantly lower theoretical CO2eq (43 g/kg cast iron) than its counterpart (76 g/kg cast iron), which is mainly associated with the production process of raw materials.

  4. (4)

    The substitution of magnesium with lime reduces the dependency and consumption of an expensive resource since the magnesium can no longer be economically recycled from sulfide or oxide form in the slag once utilized in the desulfurization process.

  5. (5)

    The adaptation of lime as a desulfurization agent in cast iron enhances a secure supply chain, higher market competitiveness, and sustainable economic advantages due to lower desulfurization costs.