Carbide Slag Decontamination and Mineralization: A Circular Economy Approach to High-Purity CaCO₃ and CO₂ Storage

Abstract

Calcium carbide slag is a highly alkaline solid waste generated during acetylene production, but its long-term accumulation causes land occupation and persistent environmental risks such as soil alkalinization and water pollution. To support circular economy and carbon emission reduction goals, in this study, we develop an integrated physical decontamination–mineralization process combining calcination, magnetic separation, sedimentation, and CO₂ mineralization. After calcination, magnetic separation, and 8 h of gravity sedimentation, the removal efficiency of Si reaches about 67% (residual Si content reduces to 0.43%), while those of Fe and Al are 75.4% and 74.2%, respectively. The purified calcium-rich slurry is then used for CO₂ mineralization. Under a solid-to-liquid ratio of 10% and a CO₂ flow rate of 0.4 L/min, CO₂ is fixed as carbonate solids, yielding calcite-type CaCO₃ with 97.88% ± 0.35% purity. This process is centered on physical separation and uses no acids, alkalis, or ammonium salts, avoiding secondary pollution while achieving waste valorization and permanent CO₂ sequestration. In this study, we provide a scalable, low-impact pathway for alkaline solid waste valorization and carbon emission reduction, contributing to sustainable consumption and production (SDG 12) and climate action (SDG 13).

1. Introduction

Global warming driven by the continuous rise in atmospheric CO₂ concentration has become one of the most serious environmental challenges today. Developing low-cost carbon capture, utilization, and storage (CCUS) technologies is a crucial pathway toward achieving carbon neutrality. Among these, CO₂ mineralization converts CO₂ into stable carbonate solids, enabling permanent carbon sequestration while co-utilizing alkaline industrial solid wastes, thus offering both environmental safety and resource efficiency [1,2]. Alkaline solid wastes such as steel slag, desulfurization by-products, and calcium carbide slag, which are rich in reactive calcium/magnesium components and readily available, are becoming research hotspots for CO₂ mineralization [3,4].

Calcium carbide slag is a strongly alkaline solid waste generated during acetylene production by calcium carbide hydrolysis, and its main component is Ca(OH)₂, along with impurities such as SiO₂, Al₂O₃, Fe₂O₃, MgO, unreacted carbon, and ferrosilicon alloy (FeSi) [5,6,7,8]. The annual output of calcium carbide slag in China is significant; however, long-term stockpiling not only occupies land resources but also leads to prominent environmental issues such as soil alkalinization, elevated groundwater pH, and dust pollution, making its harmless treatment and volume reduction urgently needed. Using carbide slag for CO₂ mineralization sequesters CO₂ while simultaneously recovering value from the solid waste, providing a synergistic “treating waste with waste” pathway that is in line with the core principles of the circular economy and sustainable development [9,10,11]. However, the presence of impurities such as silicon and iron in calcium carbide slag [12,13] severely restricts maximizing the environmental benefits of this technology. On one hand, impurities can lead to residual unreacted polluting components in the mineralized products, increasing the risk of secondary disposal; on the other, impurities may interfere with the stable operation of the mineralization process, reducing CO₂ sequestration efficiency [11,14]. Therefore, efficient impurity removal from carbide slag prior to mineralization is a key step to improving its environmental performance and ensuring process sustainability.

Regarding impurity removal, although chemical leaching methods can improve the purity of the calcium phase, they are often accompanied by the generation of large amounts of acidic/ammonium salt waste liquids, which can easily cause secondary environmental risks such as water eutrophication or heavy metal leaching, contradicting the concept of green governance [15,16,17]. By contrast, physical separation methods (e.g., sieving, magnetic separation, and sedimentation/hydrocycloning) require no addition of chemical reagents or only a small amount of water medium, offering significant environmental advantages such as low secondary pollution, low energy consumption, and a simple process flow [18,19]. When there is a density or magnetic difference between impurities and the main component, physical separation can effectively reduce polluting components. Previous studies have shown that fluidized bed sink-float separation can reduce the impurity content from 5–10% to 0.4% [20]; hydrocyclone separation can enrich Ca(OH)₂ in calcium carbide slag to 94.86% [21]; and “fine grinding dissociation–wet magnetic separation” can achieve a total iron removal rate of 56.82% with a target component recovery as high as 98.53% [22]. However, the silicate impurities in calcium carbide slag have a density similar to that of Ca(OH)₂ and are mostly non-magnetic, and they exist as fine-grained inclusions intergrown with the calcium phase, making it difficult to effectively remove them by single physical separation [23]. Therefore, pretreatment is required to amplify the physical property differences between impurities and the calcium phase, thereby creating conditions for subsequent physical separation with low environmental load.

Ma et al. [24] reported a cyclone-based physical impurity removal process for purifying carbide slag. In that study, calcination at 900 °C transformed silicon impurities into high-density calcium silicate (Ca₂SiO₄); subsequent cyclone separation removed silicates based on density differences, followed by magnetic separation to eliminate ferromagnetic impurities, ultimately yielding high-purity CaCO₃ via mineralization. However, the turbulent flow field generated by high-speed rotation during cyclone separation may re-entrain the ultrafine Ca₂SiO₄ particles (typically <10 μm) formed during calcination, thereby compromising separation efficiency. To overcome this limitation, in the present study, we adopt gravity sedimentation as the core solid–liquid separation unit instead of cyclone separation. High-temperature calcination converts silicon impurities into high-density Ca₂SiO₄ (≈3.3 g/cm³), producing a significant density difference relative to Ca(OH)₂ (≈1.2 g/cm³). According to Stokes’ law, the particle settling velocity is proportional to the density difference, which in this case is approximately 2.1 g/cm³. Under static conditions, the dense Ca₂SiO₄ particles settle spontaneously, while the lighter Ca(OH)₂ remains suspended in the middle layer. This mechanism requires no additional energy input during sedimentation and avoids turbulence-induced re-entrainment, thereby offering good process stability and scale-up potential, consistent with the requirements of sustainable development: low energy and low environmental impact.

Accordingly, we first identify in this study the selective enrichment patterns of impurities in different particle size fractions using particle size classification and X-ray diffraction analysis. Then, the effects of key parameters (calcination temperature, calcination time, sedimentation time, solid-to-liquid ratio, and CO₂ flow rate) on impurity removal efficiency and product quality are systematically investigated. Finally, the optimal process conditions are determined. We aim to provide a scalable engineering solution that has low environmental impact for purifying carbide slag, thereby supporting the synergistic realization of industrial solid waste valorization and carbon emission reduction goals.

2. Materials and Methods

The raw carbide slag used in this experiment was obtained from a chemical company in Changzhi City, Shanxi Province, China. It appeared grayish-white with a fine particle size and a water content of approximately 3%. Deionized water was provided by the laboratory, and the CO₂ used for mineralization had a purity of 99.9%. All reagents were of analytical grade. The raw carbide slag was placed in an electric blast drying oven (Model 101, Beijing Yong Guangming Medical Instrument Co., Beijing, China) and dried at 105 °C for 12 h until constant weight. A 10 g portion of the dried and sieved carbide slag was placed in a corundum crucible and calcined in a muffle furnace (Model KSL-1200X-5L, Hefei Kejing Materials Technology Co., Ltd., Hefei, China) in air at a heating rate of 10 °C/min to target temperatures of 500, 600, 700, 800, and 900 °C. The effect of calcination time (30, 60, 120, 180, and 300 min) was also investigated. After calcination, the samples were cooled to room temperature inside the furnace and then transferred to a desiccator for storage.

The calcined product was mixed with deionized water at preset solid-to-liquid ratios (5%, 10%, 15%, and 20%, w/v) and stirred at 600 r/min for 30 min using a constant-temperature heating magnetic stirrer (Model DF-101S, Hangzhou Ruijia Precision Instrument Co., Hangzhou, China) to prepare carbide slag slurry. After digestion, magnetic separation was performed using a magnetic rod (surface magnetic field intensity ≥ 0.5 T; diameter, 25 mm; length, 300 mm), which was immersed into the slurry and gently stirred for 5 min to adsorb ferromagnetic impurities such as FeSi. After magnetic separation, the rod was removed, and the adsorbed magnetic impurities were dried for further use. The non-magnetic slurry was then poured into a 500 mL graduated cylinder and allowed to settle at room temperature for specified periods (2, 4, 6, 8, 10, and 12 h), the slurry separated into three layers: a bottom layer of high-density impurities, a middle layer of Ca(OH)₂-rich suspension, and a top clear liquid layer. The middle-upper layer (approximately at three-quarters the cylinder height) was collected as the calcium source for subsequent carbonation reactions. All experiments were repeated three times.

X-ray fluorescence spectrometry (XRF, Panalytical Axios, Malvern Panalytical, Almelo, The Netherlands) was used to determine the major chemical components (e.g., CaO, SiO₂, Al₂O₃, Fe₂O₃, MgO) of raw carbide slag, calcined products, and the supernatants from the middle and upper layers after sedimentation. Inductively coupled plasma optical emission spectrometry (ICP-OES, model Agilent 5800, Agilent Technologies, Inc., Santa Clara, CA, USA) was employed for quantitative elemental analysis of both solid and liquid samples. The phase composition of the samples was characterized by X-ray diffraction (XRD, D2 PHASER, Bruker AXS GmbH, Karlsruhe, Germany), and the instrument was operated at a tube voltage of 40 kV and a tube current of 40 mA, using Cu Kα radiation (λ = 0.154 nm). Scanning was performed over the 2θ range of 10–80° with a step size of 0.02° and a scanning speed of 10°/min. Phase identification was carried out using Jade 6.0 software. The microscopic morphology of the samples was observed via scanning electron microscopy (SEM, JSM-IT500HR, JEOL, Tokyo, Japan). Prior to imaging, the dried samples were mounted on conductive adhesive tape and sputter-coated with a thin gold layer, and observations were performed at an accelerating voltage of 10 kV and a working distance of 10 mm. Energy-dispersive X-ray spectroscopy (EDS) coupled to the SEM was used for micro-area elemental analysis, and Fourier transform infrared spectroscopy (FT-IR, Perkin-Elmer Frontier, Waltham, MA, USA) was utilized to characterize the functional groups of the samples. Samples were prepared as KBr pellets with a sample-to-KBr mass ratio of approximately 1:100. Spectra were recorded in the wavenumber range of 4000–400 cm⁻¹ at a resolution of 0.9 cm⁻¹ at 25 °C. The whiteness of the calcium carbonate product was measured using a whiteness meter (WSL-B, Shanghai Inesa Physical and Optical Instruments Co., Ltd., Shanghai, China) in accordance with the Chinese national standard GB/T 23774-2009 [25], whereas its purity was determined by EDTA titration, following the Chinese national standard GB/T 19281-2014 [26].

3. Results and Discussion

3.1. Distribution Patterns of Impurities in Calcium Carbide Slag

To investigate the size distribution characteristics of impurities in carbide slag, the raw material was subjected to dry sieving and separated into ten size fractions. The range and mass distribution of each size fraction are presented in

Table 1. Mass distribution of different particle sizes.

Particle Size (μm)	Mass Fraction (%)	Cumulative Mass (%)
–55	1.68	1.68
55–74	7.8	9.48
74–88	14.28	23.76
88–96	16.64	40.4
96–105	12.5	52.9
105–125	21.72	74.62
125–150	10.2	84.82
150–250	4.14	88.93
250–450	8.04	96.97
>450	2.82	99.79

. The elemental content and phase composition of each size fraction were quantitatively analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES) and X-ray diffraction (XRD), respectively, and the results are shown in Figure 1.

Figure 1a shows that silicon (Si) is the most abundant impurity element in carbide slag, exhibiting a bimodal distribution: two peaks appear in the 55–74 and 105–125 μm size fractions (with contents of 4.8 wt% and 5.6 wt%, respectively), indicating that Si tends to concentrate in the medium-fine fractions. The aluminum (Al) content generally increases with increasing particle size, reaching a maximum value (0.5 wt%) in the 200–450 μm fraction. By contrast, the iron (Fe) and magnesium (Mg) contents remain around 0.1 wt% across all size fractions, showing no clear particle-size dependence. The XRD patterns in Figure 1b further reveal the occurrence modes of various impurities in different particle sizes. All size fractions display clear diffraction peaks of Ca(OH)₂, indicating that it is the dominant matrix phase, and diffraction peaks of CaCO₃ are also clearly present in all fractions > 55 μm. For the key impurity components, the XRD results are consistent with the elemental analysis: diffraction peaks of SiO₂ are detected in all fractions > 74 μm, corroborating the higher Si content observed in the medium size fractions. For iron impurities, their occurrence modes show a distinct size-dependent differentiation: diffraction peaks of ferric oxide (Fe₂O₃) are mainly concentrated in the small-to-medium size range of 55–250 μm, while characteristic peaks of iron silicide (FeSi) appear only in the coarse fraction > 450 μm.

According to Stokes’ law, the free-settling velocity of a spherical particle in a quiescent fluid is proportional to its density and the square of its diameter. Consequently, particles of higher density and larger size settle faster. The observed particle-size and phase distributions provide a basis for designing subsequent physical purification steps: FeSi in the >450 μm coarse fraction can be preliminarily removed by sieving or low-intensity magnetic separation; the <55 μm fine fraction, with its low impurity content and high reactivity, can be directly used for subsequent carbonation. Conventional sieving is ineffective for the 55–250 μm medium-fine fraction, where Fe₂O₃ and SiO₂ coexist. Therefore, a pretreatment based on thermally induced phase transformation [27] (e.g., calcination to convert silicon into high-density calcium silicate and to promote magnetic transformation of iron) is required, followed by deep separation using a coupled gravity–magnetic field.

3.2. Optimization of Impurity Removal Process by Sedimentation Pretreatment

In carbide slag, amorphous silica and fine aluminosilicates are mutually encapsulated with calcium-based components, making it difficult to separate them using conventional physical screening. As shown by the SEM-EDS results in Supplementary Figure S1, high-temperature calcination converts the fine silicon impurities into high-density calcium silicate through reaction with calcium (the figure shows co-localization of Ca, Si, and O in the same micro-areas, and the particle surface forms a dense structure), while other impurity phases also undergo corresponding transformations. This relieves the encapsulation and enlarges the density difference between the calcium phase and the impurities, creating conditions for their removal by gravity sedimentation [28]. Based on this, magnetic separation and sedimentation experiments were conducted on the calcined product, and the content and removal efficiency of each impurity element in the middle-upper layer of the sediment were analyzed.

Gravity sedimentation is a key step for removing silicon impurities from calcium carbide slag [29]. Figure 2 shows the purification effect of the supernatant after sedimentation under different conditions. As shown in Figure 2a,b, when the treatment temperature increased from 500 to 900 °C, the silicon content after sedimentation decreased from 1.26% to 0.47%, and the iron content decreased from 0.09% to 0.02%. This is because at 500 °C, the impurities have not yet formed a heavy phase, resulting in an insufficient density difference and difficulty in sedimentation; by contrast, at 900 °C, the impurities transformed into heavy products, which can rapidly settle to the bottom in the fluid, achieving deep purification. The effect of treatment time is shown in Figure 2c,d: at 900 °C, with increasing treatment time, the silicon content after sedimentation gradually decreased, reaching its minimum at 180 min and then remaining unchanged; the iron content first decreased and then increased. When the treatment time was less than 180 min, iron was removed together with the heavy phase due to its fast settling velocity, leading to a decrease in iron content in the supernatant. However, when the treatment time was too long (exceeding 180 min), the calcined product particles underwent clear agglomeration and surface densification (see Supplementary Figure S2). This increased the interparticle interaction forces and sedimentation resistance, causing some Fe-bearing particles to fail in settling effectively to the bottom, and leading to a renewed increase in Fe content [29,30]. Therefore, the optimal pretreatment conditions were determined to be 900 °C and 180 min.

Under the above optimal thermal treatment conditions, after the product was slurried and subjected to magnetic separation, a kinetic analysis of the gravity sedimentation process was performed. According to Stokes’ law, the terminal settling velocity $v_s$ of a spherical particle in a quiescent fluid is given by [31]

$$v_s={\displaystyle \frac{({\rho }_p-{\rho }_f)g{d}^2}{18\mu }}$$

where $v_s$ is the terminal settling velocity; ${\rho }_p$ and ${\rho }_f$ are the densities of the particle and the fluid, respectively; $g$ is the gravitational acceleration; $d$ is the particle diameter; and $\mu$ is the dynamic viscosity of the fluid. In the present system, the density of hydrated Ca(OH)₂ is approximately 1200 kg/m³, while that of Ca₂SiO₄ formed after calcination is approximately 3300 kg/m³. Assuming similar particle sizes for the two phases, the settling velocity of Ca₂SiO₄ is about 11.5 times that of Ca(OH)₂. It should be noted that Stokes’ law strictly applies to spherical particles, and its use for non-spherical particles introduces some deviation [30]. Experimental observations (Figure S1) show that Ca₂SiO₄ particles are regular in shape and relatively coarse, whereas Ca(OH)₂ exists as fine, flocculent aggregates. These differences in morphology and particle size further amplify the difference in settling velocity. Consequently, under the action of gravity, the dense impurity Ca₂SiO₄ rapidly settles to the bottom, while the light Ca(OH)₂ remains suspended in the middle layer, providing a kinetic basis for efficient separation.

Under the above optimal heat treatment conditions, after the product was slurried and subjected to magnetic separation, the influence of gravity sedimentation time on the impurity removal efficiency was systematically investigated, and the results are shown in Figure 3. With increasing sedimentation time, the silicon content first decreased and then increased. At a sedimentation time of 8 h, the silicon content in the middle-upper layer reached its minimum (0.43%), and the removal efficiency reached its maximum. The aluminum content and removal efficiency gradually stabilized with increasing sedimentation time; insufficient sedimentation in a short time led to a high aluminum content, whereas after sufficient sedimentation, the removal efficiency reached its maximum and remained unchanged. The contents of iron and magnesium continuously increased with prolonged sedimentation time, which may be attributed either to the unclear boundary between the middle layer and the sediment layer due to insufficient sedimentation, or to the tight packing of the bottom sediment and even particle resuspension caused by excessively long sedimentation, thereby allowing impurities to re-enter the middle-upper layer. Considering both separation purity and process cost, the optimal sedimentation time was determined to be 8 h, under which the removal efficiency of silicon impurities was optimal, and the separation states of aluminum, iron, magnesium, and other elements were also relatively favorable.

3.3. Influence of Pretreatment Impurity Removal on Mineralization Efficiency

When evaluating the influence of pretreatment and mineralization parameters on product quality, purity and whiteness serve as the most direct macroscopic performance indicators [32]. With the optimization of sedimentation time, solid-to-liquid ratio, and CO₂ flow rate, both the purity and whiteness of the resulting calcium carbonate product initially increased and then leveled off or decreased slightly, reaching their peak values (purity ~97.88% ± 0.35%, whiteness of 92) under the optimal combination of conditions, as shown in Figure 4. To reveal the underlying mechanism of this macroscopic performance evolution, a step-by-step analysis was conducted from the perspectives of phase transformation and crystal morphology.

Gravity sedimentation is a core step for removing impurities such as silicon, aluminum, and iron from calcium carbide slag [33], and its thoroughness directly determines the microscopic crystal morphology of the subsequent mineralization products (Figure 5) [34]. To achieve efficient sedimentation, the raw material must be pretreated by appropriate thermal treatment (e.g., 900 °C, 180 min) to convert the impurities into a calcium silicate phase with higher density and larger particle size. As shown in Figure 5a–d, when the pretreatment is insufficient, the transformation of impurity phases is incomplete, and the residual fine particles are difficult to capture by sedimentation; these unremoved impurities later act as heterogeneous nucleation sites in the carbonation step, becoming incorporated into the CaCO₃ lattice, which leads to low crystallinity and clear agglomeration of the product [35]. Under appropriate thermal treatment conditions, the impurities are converted into coarse, heavy forms that settle readily; after thorough gravity sedimentation, they are deeply removed, resulting in a final carbonation product that is a pure calcite phase with no visible impurity attachment and high crystallinity, and with uniform grain size. The effect of sedimentation time on solid–liquid separation is shown in Figure 5e,f. As the sedimentation time increased from 4 to 8 h, the diffraction intensity of the calcite main peak (2θ ≈ 29.4°) increased from 2102 to 2453 counts (see Supplementary Material Table S1), the peak sharpened, and the crystallinity improved significantly; simultaneously, the SEM images showed that the particle morphology became more regular and well-dispersed. By contrast, when the sedimentation time was insufficient (4 h), even though the impurity phases had been sufficiently transformed by the prior heat treatment, suspended impurity particles still re-entered the calcium-rich precursor, severely damaging the product morphology. These results indicate that, based on the heat treatment rendering the impurities settleable, the adequacy of gravity sedimentation is a critical step that determines the purity of the carbonation product.

The solid-to-liquid ratio affects the crystallinity and morphology of the product. XRD (Figure 6a) shows that when this ratio decreased from 20% to 10%, the intensity of the calcite main peak (2θ ≈ 29.4°) increased from 2134 to 2544 counts, and the peak narrowed; further reduction to 5% caused a slight decrease in intensity. In the FT-IR spectra (Figure 6b), as the ratio increased, the peak at approximately 3436 cm⁻¹ (O–H stretching of crystal water) first weakened and then strengthened, while the absorption band at approximately 1444 cm⁻¹ (C–O bond) first increased and then decreased. These variations indicate that at low solid-to-liquid ratios, the system has good fluidity, allowing sufficient contact between Ca²⁺ and CO₃²⁻; at 10%, the reaction rate is moderate, leading to high crystallinity and minimal impurity peaks; and above 10%, the viscosity increases, Ca²⁺ diffusion becomes restricted, and the reaction efficiency declines. SEM (Figure 6c) shows that at 10%, the particles are well-defined cubes with a narrow size distribution. As indicated by the letters in Figure 4b (where different letters denote significant differences at p < 0.05), the purity and whiteness at a solid-to-liquid ratio of 10% are significantly higher than those of the other groups. Therefore, the optimal ratio is 10%.

CO₂ flow rate also affects the crystallinity and morphology of the product. XRD and FT-IR analyses (Figure 6d,e) show that as this rate increased from 0.2 to 0.4 L/min, the intensity of the calcite main peak gradually increased, and the absorption band at approximately 1444 cm⁻¹ (C–O bond) also increased in intensity, indicating improved crystallinity. When the flow rate was further increased to 0.8 L/min, the main peak intensity decreased slightly, and the C–O bond absorption band no longer increased. At a low flow rate, the reaction rate is slow, leading to insufficient contact between Ca²⁺ and CO₃²⁻; at a high flow rate, local supersaturation induces secondary nucleation and particle agglomeration [36]. The fine crystals produced by secondary nucleation have low crystallinity, resulting in peak broadening and decreased intensity in the XRD patterns. SEM (Figure 6f) shows that at 0.4 L/min, the product exhibits regular morphology and a uniform particle size distribution. The significance letters in Figure 4c further confirm that the purity and whiteness at 0.4 L/min are significantly higher than those at 0.2 and 0.8 L/min (p < 0.01). Therefore, the optimal CO₂ flow rate is 0.4 L/min. Under these conditions, the system is free from interference by impurity ions, and both the combination of Ca²⁺ with CO₃²⁻ and the ordered growth of crystal faces reach an ideal state, ultimately yielding a high-quality calcium carbonate product with regular morphology, high purity, and high whiteness.

3.4. Full-Process Verification and Performance Analysis of Decontamination–Mineralization

Based on the impurity distribution characteristics and the optimized pretreatment parameters established above, the integrated “calcination–magnetic separation–sedimentation–mineralization” decontamination–mineralization process was validated, thereby establishing a reproducible and scalable high-efficiency technological system. By systematically integrating all unit operations, the process ensures efficient impurity removal and optimal utilization of the calcium-rich precursor, laying a foundation for subsequent scale-up and industrial application.

First, considering impurity targetability, equipment availability, and mineralization compatibility, the process sequence was defined as calcination activation, wet magnetic separation, gravity sedimentation, and then extraction of the calcium-rich middle layer for mineralization to produce nano-CaCO₃ (Figure 7). This route effectively uses calcination to activate Ca(OH)₂ in carbide slag, and precisely removes magnetic and high-density silicate impurities through magnetic separation and sedimentation, achieving deep purification of the calcium-rich precursor phase.

In the process technology validation step, XRD and FT-IR were used to characterize the phase composition and functional groups of the final product (Figure 8). The results show that the diffraction peaks of the product are completely consistent with calcite CaCO₃ (Figure 8a), with characteristic main vibration bands at 1436.0, 875.4, and 711.5 cm⁻¹ (Figure 8c). The product achieves a purity of 97.88% and a whiteness of 92 (Figure 8b), meeting the highest grade requirements of the HG/T 2226-2010 standard [37]. The mass balance calculation shows that 1.502 t of high-purity CaCO₃ can be produced from 1 t of dry carbide slag, with a calcium recovery of 86.8% (Table S2). Moreover, the material flows of by-products such as magnetic waste and sedimentation sludge are clearly defined, and overall mass conservation is well satisfied, demonstrating the scientific design and green, high-efficiency characteristics of the process configuration.

In terms of energy consumption and environmental performance, the energy demand of this process is mainly concentrated in the calcination unit. Owing to the lower thermal decomposition enthalpy of Ca(OH)₂ in carbide slag, its calcination energy consumption is significantly lower than that of conventional limestone. The flue gas contains abundant sensible heat, offering considerable potential for waste heat recovery; the recovered heat can be used for upstream raw material preheating or power generation, thereby improving overall system energy efficiency. Meanwhile, the CO₂ in the calcination off-gas can be directly recycled to the subsequent mineralization process (see the carbon loop path in Figure 7), achieving carbon recycling. Compared with conventional chemical leaching processes (CO₂ emissions of approximately 300–308 kg per tonne of product) and cement clinker production (approximately 669 kg CO₂ per tonne), this process uses no external chemical reagents throughout the flow, resulting in outstanding carbon reduction benefits and exhibiting a net-negative carbon potential over its life cycle [38,39]. Both the sedimentation and mineralization steps are physical operations; process water is recycled and by-products are readily suitable for further harmless utilization. Overall, the process meets the requirements of sustainable development.

From a mechanistic design perspective, this process can be summarized as a continuous coupling of “thermochemical conversion–magnetic separation–density classification–controlled mineralization crystallization”. First, calcination induces phase transformation of impurities such as silicon in the carbide slag. Subsequently, magnetic separation removes magnetic and weakly magnetic impurities, e.g., iron. The gravity sedimentation step further promotes the graded enrichment of high-density impurities, yielding a clarified Ca(OH)₂-rich slurry from the middle layer, which provides a relatively favorable reaction environment for subsequent mineralization crystallization. The mineralization step is carried out in the purified Ca(OH)₂ system, which enhances the purity and crystalline quality of the calcium carbonate product. Thus, a synergistic effect between impurity removal and CO₂ mineralization is achieved, forming a closed-loop process.

This process aligns well with the distribution and physicochemical characteristics of impurities in carbide slag and is generally consistent with the optimal operating parameters determined in previous experiments. The process shows potential for engineering scale-up and can serve as a technical reference for the high-value utilization of calcium resources from industrial by-products and for the green carbon cycle.

4. Conclusions

To address the limitations posed by complex impurity distributions that inhibit direct carbonation of carbide slag, in this study, we develop an integrated “calcination–magnetic separation–settling–mineralization” process coupling physical impurity removal with CO₂ sequestration. Gravity settling was identified as the pivotal step for deep impurity removal. Under optimized laboratory batch conditions—including calcination at 900 °C, wet magnetic separation, and 8 h of gravity settling—high-density silicate and other heavy impurities were efficiently enriched at the bottom, thereby eliminating insufficient separation from short settle times and re-mixing effects associated with overlong settling. Extraction of the remediated Ca(OH)₂ solution from the middle layer reduced the characteristic silicon impurity content to 0.43% (removal efficiency ~67%), with aluminum and iron removal rates of 74.2% and 75.4%, respectively. Effective pretreatment eliminated residual impurities, ensuring unimpeded mineralization. Consequently, under a solid–liquid ratio of 10% and CO₂ flow rate of 0.4 L/min, well-dispersed cubic calcite crystals, meeting a purity of 97.88% ± 0.35% and whiteness of 92.0% ± 0.42% (mean ± SD, n = 3), were obtained. These parameters satisfy the top-grade requirements of HG/T 2226-2010, supporting application as high-value fillers in rubber, plastics, papermaking, and coatings. Based on the above experimental results, a preliminary assessment of the engineering scale up feasibility of the process was conducted. The waste heat and flue gas generated during calcination can be recovered and utilized: the waste heat can be used for raw material preheating or power generation, while the flue gas can be directly fed into the mineralization stage as a CO₂ source, thereby reducing net energy consumption. In the sedimentation step, an inclined plate settler can be employed; based on the shallow settling principle, it reduces the hydraulic retention time to minutes while maintaining the density difference separation efficiency. The entire process does not introduce conventional chemical reagents such as acids, bases, or ammonium salts, thus avoiding such chemical pollution at the source. Moreover, the process water can be recycled. Therefore, the process exhibits good potential for scale-up.