Extraction of Titanium from Blast Furnace Slag: Research on the Crushing Process of TiC-Bearing Slag

Abstract

TiC-bearing slag is an intermediate product in the titanium extraction process following the “high-temperature carbonization and low-temperature chlorination” method. It exhibits complex grinding characteristics, and traditional grinding methods often yield issues such as broad particle size distribution (PSD) and overgrinding, adversely affecting process efficiency. In this study, TiC-bearing slag was investigated as the raw material. Through methods such as SEM-EDS, MLA, and scientific experiments, we established quantitative relationships between mechanical inputs and product granulometry. The results indicate that the coexistence of TiC and pyroxene-group minerals leads to poor grinding selectivity, causing TiC to resist fracturing while pyroxene minerals are prone to overgrinding. Furthermore, the experiments demonstrated that high circulating-load grinding combined with staged sieving is most effective for producing narrow PSD products. This research explores the methods and paths for achieving controlled narrow PSD of TiC-containing slag in the crushing process, and providing data support for the selection of industrial-scale grinding processes.

1. Introduction

The vanadium–titanium magnetite is a complex polymetallic ore containing economically valuable elements including iron, titanium, vanadium, and chromium. In China, over 90% of these mineral reserves are concentrated in the Panxi region of Sichuan Province, with secondary deposits distributed across Hebei and Anhui Provinces [1]. The predominant processing method remains blast furnace smelting, yielding high-titanium blast furnace slag with a TiO₂ content between 20% and 25% by weight [2,3]. To valorize this slag, numerous research efforts have explored multiple recovery pathways: the production of titanium–silicon alloys from titanium-containing slag [4,5], enrichment and separation of perovskite [6,7], beneficiation and separation of titanium carbide after carbothermal reduction [8,9], leaching for TiO₂ extraction from titanium-rich slag [10,11,12,13], the generation of TiCl₄ through high-temperature carbonization followed by low-temperature chlorination [14,15,16,17], and some other methods [18]. Notably, the TiCl₄ production route achieves over 65% titanium recovery efficiency, positioning it as the most industrially viable option. The complete industrial process comprises five critical phases: blast furnace slag hot charging, carbonization smelting, TiC-bearing slag comminution, fluidized-bed chlorination, and residual treatment, as shown in Figure 1 specifically.

TiC-bearing slag is a type of slag containing TiC. It is produced by smelting blast furnace slag with carbon in an electric furnace. During the smelting process, TiO₂ in the slag undergoes progressive conversion into TiC. By the end of the smelting stage, the TiC content in the slag reaches approximately 13% to 15%. Pre-chlorination processing mandates precise particle size control, with optimal fluidization observed within the 0.045–0.180 mm (80–325 mesh) range [19,20]. Suboptimal particle sizing (<0.045 mm) induces particle entrainment during chlorination, compromising operational continuity through electrostatic precipitator fouling and process instability.

While conventional mineral processing employs ball milling for size reduction—exemplified by the cement industry standards requiring 95% smaller than 0.045 mm (−325 mesh) particles—advanced size-selective comminution remains understudied. Zou et al. [21] conducted dry grinding experiments using a self-developed impeller-type crusher, systematically investigating the influence of parameters such as screen cage-impeller clearance, rotational speed, induced draft fan flow rate, and screen aperture on powder mass distribution. Their work successfully maximized the yield of expanded black rice flour within the 0.15–0.75 mm particle size range. Li et al. [22] examined the effects of different crushing methods on coal PSD and proposed a combined approach involving “rod mill coarse crushing followed by jet mill fine crushing”. They found that operating the rod mill for 35 min resulted in a narrower PSD, indicating selective crushing behavior. Zhou et al. [23] applied jet milling technology to achieve controlled crushing of glass beads in the 0.10–0.40 mm range, achieving a product yield of 78–83%. Additionally, Xu et al. [24] conducted in-depth investigations into factors influencing the classification of narrow PSD powders and proposed a novel classification device based on their findings. Although recent studies have shown that the comminution method could exert some control over the PSD of the crushed product, it remains challenging to achieve narrow PSD crushing of the carbide slag with distinctive characteristics.

In this paper, we systematically evaluated the mineralogical properties, comminution challenges, and optimal size reduction strategies of the carbide slag generated during the high-temperature carbonization and low-temperature chlorination process. Through comparative analysis of five grinding methodologies—planetary ball milling, rod milling, high-pressure roll compaction, wet grinding, and impact crushing—we established quantitative relationships between mechanical inputs and product granulometry. The paper explores the methods and paths for achieving controlled narrow PSD of TiC-containing slag during the crushing process, providing data support for the selection and optimization of industrial-scale grinding processes; at the same time, it also offers reference solutions for common problems encountered in product crushing, such as the severe overgrinding of the particle size, difficulty in achieving controlled particle with narrow PSD, and low production capacity for specific narrow particle size products.

2. Materials and Methods

2.1. Experimental Material

TiC-bearing slag was sourced from a specific company, with its chemical composition detailed in

Table 1. Composition of TiC-bearing slag/wt.%.

Compositions	CaO	SiO2	Al2O3	TiC	MgO	Ti2O3	Cf	Fe	Other
Mass fraction	27.4	24.69	13.25	14.14	8.16	4.46	2.98	1.41	3.51

. As indicated in the table, the TiC-bearing slag contains 14.14% TiC, while the remaining constituents are comparable to those found in blast furnace slag, including CaO, SiO₂, Al₂O₃, and MgO. The dry bulk density of the slag is 0.95 t/m³, and the particle size ranges from 0 to 6 mm. Further details regarding the specific PSD are provided in

Table 2. PSD of slag containing titanium carbide/wt.%.

Particle Size/mm	<0.038	0.038~0.15	0.15~0.25	0.25~0.83	0.83~1.4	>1.4
Mass fraction	1.35	6.79	3.77	32.64	33.32	22.13

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2.2. Experimental Method

TiC-bearing slag was used as the raw material, and grinding experiments were conducted using various methods, including ball milling, rod milling, high-pressure roller milling, wet ball milling, and high-speed impact crushing. The specific equipment and parameter settings for each experiment are summarized in

Table 3. Experimental equipment and methods.

No.	Crushing Method	Experimental Equipment Parameters	Experimental Process Parameters
1	Ball milling	The dimensions of the ball mill cylinder are φ305 × 305 mm. The ball size distribution is as follows: 43 balls of φ36.5 mm, 67 balls of φ30.2 mm, 10 balls of φ25.4 mm, 71 balls of φ19.1 mm, and 94 balls of φ15.9 mm, totaling 285 balls with a combined mass of 20.13 kg.	Closed-circuit grinding experiments were conducted under the conditions of a 250% and 500% circulating load, with a charging quantity of 15 kg, a mill rotation speed of 70 rpm, and a control sieve of 80 mesh.
2	Rod milling	The rod mill has barrel dimensions of φ305 × 610 mm. It is loaded with six steel rods of φ31.75 mm and two steel rods of φ44.45 mm, each measuring 533.4 mm in length, resulting in a total steel rod mass of 33.38 kg.	The experimental setup consisted of a feed quantity of 20 kg, with the mill operating at 46 rpm and an 80-mesh screen. Closed-circuit grinding tests were carried out under circulating load conditions of 100%, 300%, and 500%. The specific experimental methods are similar to the closed-loop cycle experiment of ball milling.
3	High-pressure roller milling	The high-pressure roller mill testing machine is equipped with rollers having a diameter of φ420 mm and a width of 100 mm. The rotational speed of the compression rollers is 15 r/min, while the hydraulic station operates at a pressure of 0.5 MPa.	The material loading capacity is 15 kg per batch, with a moisture content of 6.4%. The experiment was conducted using a high-pressure roller mill operating in an open-circuit configuration. After the slag is crushed once, a particle size analysis is conducted on the entire sample.
4	Wet ball milling	The wet ball mill cylinder has dimensions of φ420 × 450 mm and is loaded with a grinding media mass of 55 kg. Among the steel balls, 60% of them have a diameter of 10 mm, while the rest have a diameter of 20 mm.	The batch loading capacity is 7 kg, with a pulp concentration of 50%. The mill operates at a rotational speed of 59 r/min, and the grinding durations for the open-circuit experiments are set at 5 min, 10 min, and 50 min, respectively.
5	High-speed impact crushing	Rated power: 2.2 kW; cutter head diameter: φ150 mm; no-load speed: 6000 rpm.	The raw material was subjected to sieving pretreatment and classified into three particle size ranges: 0–4 mm, 0.25–4 mm, and 0.18–4 mm, each with a mass of 1 kg, for an initial open-circuit crushing experiment. Subsequently, the powder obtained from the first crushing of the 0.18–4 mm fraction was sieved, and particles with a size of ≥0.18 mm underwent secondary crushing, followed by tertiary crushing using the same procedure.

.

The closed-loop cycle experiment of ball milling is as follows: 15 kg of materials are loaded initially. After one stage of crushing, a particle size screening is conducted. The coarse particles (re-suspended materials) are mixed with the new materials (totaling 15 kg), and returned to the mill for next grinding. Only the qualified products (the fine particles screened out) are discharged as the final product. The above steps are repeated until the system reaches a stable state: that is, the cycle load is stable within the range of −2% to +2% the target cycle load. When the experiment is concluded, a particle size analysis of the overall sample is finally conducted. In addition, the high-pressure roller grinding, wet ball grinding, and high-speed impact crushing all employ open-loop experiments. Additionally, in the open-circuit experiment, the experiment concludes after the materials are loaded and undergo one-time crushing, followed by a particle size analysis of the entire sample.

Following the grinding process, particle size analysis was performed on the resulting powder to evaluate the effect of different grinding techniques on the PSD of the final product.

2.3. Characterization and Analysis

The microscopic morphology of TiC-bearing slag was analyzed using a scanning electron microscope (Sigma 500, Carl Zeiss AG, Jena, Germany) equipped with an energy dispersive spectrometer (EDS) and a mineral liberation analyzer (MLA650F). Before conducting the analysis of the TiC-bearing slag samples, they were first crushed to a particle size of ≤0.074 mm. Then, the samples were impregnated with resin, followed by grinding and polishing using standard techniques, and gold spraying was applied. The Bond (F.C. Bond) work index (W_ib) and abrasion index of TiC-bearing slag were evaluated in accordance with JC/T 734, the Chinese industry standard for grindability testing of cement raw materials [25]. The PSD of the product powder was determined through sieve analysis, following GB/T 1345-2005 [26], the Chinese national standard for cement fineness testing.

3. Result Analysis and Discussion

3.1. SEM Analysis

The microstructural morphology of TiC-bearing slag is presented in Figure 2. As shown in Figure 2a,b, the slag primarily comprises gray and light gray spherical particles, along with distinct bright white phases. Based on the data summarized in Figure 2d and

Table 4. EDS analysis in Figure 2c.

Point	Ti	V	Fe	Mn	Si	C	O	Ca	Al	Mg	Na	Possible Phase
1	85.44	1.34	—	—	—	6.68	5.34	1.20	—	—	—	TiC
2	82.84	5.41	—	—	—	7.44	4.31	—	—	—	—	TiC
3	81.65	4.84	—	—	1.17	9.08	—	2.02	1.24	—	—	TiC
4	—	—	81.06	4.11	3.32	11.51	—	—	—	—	—	Fe
5	3.35	0.28	—	—	17.2	—	34.93	26.19	11.16	6.89	—	Augite
6	2.62	—	—	2.07	17.38	—	33.1	25.48	11.63	7.2	0.52	Augite
7	78.44	11.58	—	—	—	7.01	2.97	—	—	—	—	TiC (with Fe)

in Figure 2c, the gray regions are predominantly composed of silicate phases, as indicated by points 5# and 6#. The gray spherical particles are mainly constituted of the TiC phase, with most diameters measuring less than 10 μm, as demonstrated by points 1#, 2#, 3#, and 7#. Additionally, small bright white regions embedded within the silicate matrix are identified as metallic iron (Fe), as confirmed by point 4#.

MLA analysis was performed on the TiC-bearing slag shown in Figure 2a, and the compositions of the constituent phases are summarized in

Table 5. Mineralogical analysis by MLA.

Mineral	Wt%	Area%	Area (Micron)	Particle Count	Grain Count
Augite (Ca(Ti,Mg,Al)(Si,Al)2O6)	62.07	61.11	4,826,893.72	9539	12,852
Carbon (C)	0.85	0.84	66,203.7	1235	1477
Magnesium aluminate spinel	2.03	2	157,669.3	691	1232
(MgO-Al2O3)
Magnesium-titanium spinel (MgO-TiO2)	0.75	0.73	58,014.28	1522	4699
Titanium carbide (TiCxOy)	6.72	6.61	522,427.83	4222	17,750
Iron (Fe)	0.13	0.13	10,129.17	212	260
Vitric (SiO2)	2.68	2.64	208,220.71	4118	23,938
Titanaugite ((Ca(Ti,Mg,Al)(Si,Al)2O6), high Ti)	23.92	23.55	1,859,934.68	15,752	40,836
Fe-Si alloy (Fe-Si)	0.14	0.14	11,106.22	30	37
Forsterite (Ca2Mg(Si2O7))	0.35	0.34	26,960.18	149	251
Calcium aluminate (CaO-6Al2O3)	0.06	0.06	4733.95	68	83
Perovskite (CaO-TiO2)	0.19	0.18	14,408.84	204	336
Other	0.11	1.67	132,306.04	2098	5437
Total	100	100	7,899,008.62	39,840	109,188

. According to the data in the table, the TiC-bearing slag primarily consists of augite, titanaugite, and TiC (or TiC_xO_y) phases. Based on this phase analysis, it is inferred that the presence of TiC and pyroxene phases contributes significantly to the difficulty in crushing and the tendency for over-grinding during the processing of TiC-bearing slag. The Mohs hardness of titanium carbide ranges from 8.5 to 9.5, with a Vickers hardness between 28 and 35 GPa [27], indicating its extremely high hardness. In comparison, pyroxene exhibits a Mohs hardness ranging from 3.5 to 6.5 and a Vickers hardness between 3 and 10 GPa. Consequently, under identical grinding conditions, over-grinding is more likely to occur.

3.2. Bond Milling Power Index and Abrasion Index

The standard Bond ball mill and rod mill work index test results are presented in

Table 6. Test results of the Bond power index.

Sample	Particle Size (mm)	Ball Milling Power Index Wib (250% Cycle Load) kWh/t	Rod Mill Work Index Wib (100% Cycle Load) kWh/t	The Hardgrove Grindability Index Hg
S-1#	13~19	34.61	24.17	17.94
S-2#	0~6	23.84	18.96	27.01
Cement clinker	-	13.4~14.87	-	38~50

. It can be observed that when the feed particle size is reduced from 19–13 mm to 6.00–3.35 mm under a circulating load of 250%, the ball mill work index (W_ib) decreases from 34.61 kWh/t to 23.84 kWh/t, and the rod mill work index under a 100% circulating load also decreases correspondingly. Compared with the typical ball mill work index of cement clinker, which ranges from 13.4 to 14.87 kWh/t and corresponds to a Hardgrove grindability index of 38–50 H_g [28], it is evident that TiC-bearing slag is more resistant to comminution and requires greater energy consumption to achieve a comparable particle size.

The test results of the standard Bond abrasion index are summarized in

Table 7. Test results of the Bond abrasion index.

Sample	Particle Size (mm)	The Mass of the Blade Before Testing (g)	The Mass of the Blade After the Test (g)	Abrasion Index
S-1#	13~19	92.7158	92.5116	0.2042
S-2#	0~6	92.8674	92.7931	0.0743

. It can be observed that when the feed particle size of TiC-bearing slag is reduced from 13–19 mm to 0–6 mm, the Bond abrasion index decreases from 0.204 g to 0.074 g. This indicates that adjusting the feed particle size effectively reduces equipment wear and contributes to cost reduction.

3.3. Ball Milling

The experimental results of closed-circuit ball milling under circulating load conditions of 250% and 500% are presented in Figure 3. As shown in the figures, the grinding process reached a state of stability after eight experimental iterations under both load conditions, as illustrated in Figure 3a,c, respectively. Moreover, as the number of closed-circuit cycles increased from P1 to P8, the mass percentages of particles with sizes ≤0.075 mm and ≤0.045 mm gradually declined and eventually stabilized in each test scenario, as displayed in Figure 3b,d. A comparative analysis of the different circulating loads, as depicted in Figure 3e,f, indicates that increasing the circulating load from 250% to 500% resulted in a reduction in the mass percentages of particles sizes ≤0.075 mm and ≤0.045 mm in the final product—from 33.29% to 31.62%, and from 25.05% to 17.09%, respectively—during the stable grinding phase. This suggests that an increased circulating load can moderately decrease the proportion of fine particles in the final powder product. However, it is important to note that such an increase also significantly diminishes grinding efficiency and leads to higher energy consumption.

3.4. Rod Milling

The experimental results of closed-circuit rod milling under circulating loads of 100%, 300%, and 500% are presented in Figure 4. As illustrated in the figures, the PSD of the product reached a stable state after 10, 11, and 10 grinding cycles under 100%, 300%, and 500% circulating load conditions, respectively, as shown in Figure 4a,c,e. Moreover, with an increase in the number of grinding cycles, the mass fractions of particles with sizes ≤0.075 mm and ≤0.045 mm gradually stabilized in each test scenario, as depicted in Figure 4b,d,f. A comparative analysis of different circulating loads, as shown in Figure 4g,h, indicates that increasing the circulating load from 100% to 500% resulted in a decrease in the content of particle sizes ≤0.075 mm from 44.54% to 36.53%, and a reduction in the content of particle sizes ≤0.045 mm from 28.10% to 20.10%. It is evident that, during the rod milling process, an increase in circulating load leads to a certain degree of reduction in the mass proportion of fine particles, a phenomenon similar to that observed in ball milling.

3.5. High-Pressure Roller Milling

The experimental results of high-pressure roller grinding are presented in Figure 5. The data indicate that after a single crushing cycle, the central region of the roller exhibits a higher throughput than the peripheral regions, with mass percentages of particles ≤0.180 mm reaching 37.66% and 14.47% in the central and peripheral areas, respectively. Moreover, the PSD of the high-pressure roller grinding products is non-uniform, showing finer particles in the central region compared to the periphery. Specifically, the content of particles ≤0.075 mm is 61.08% in the central region and 53.89% in the peripheral areas, while the proportion of particles ≤0.045 mm is 39.31% and 30.83%, respectively. The dominant mechanism of high-pressure roller grinding is layer compression crushing. Experimental observations suggest that this method produces a considerable amount of fine particles, indicating that the layer compression approach may not be effective in controlling excessive grinding of TiC-bearing slag during the crushing process.

3.6. Wet Ball Milling

The results of the wet ball milling experiment are presented in Figure 6. As shown, the efficiency of wet ball milling is relatively high. At grinding durations of 5 min, 10 min, and 50 min, the mass fractions of particles with a size ≤ 0.18 mm in the product are 56.45%, 78.23%, and 99.83%, respectively. Moreover, after 5 min of grinding, a considerable amount of fine powder has been generated in the TiC-bearing slag, with the contents of particles ≤ 0.075 mm and ≤ 0.045 mm reaching 44.38% and 28.47%, respectively. After 10 min of grinding, these values increase to 48.52% and 28.62%, respectively. By 50 min, the proportions of particles ≤ 0.075 mm and ≤ 0.045 mm reach as high as 94.69% and 77.67%, indicating significant overgrinding. Overall, as the grinding time increases, the degree of overgrinding becomes more pronounced. However, with shorter grinding times, the extent of particle size reduction remains insufficient. Therefore, wet ball milling may not be an optimal method for processing TiC-bearing slag.

3.7. High-Speed Impact Crushing

Figure 7 presents the results of high-speed impact crushing experiments. As illustrated in Figure 7a, the fine particle content in the product is influenced by the particle size of the raw material. By removing the fine particles initially present in the raw material, the fine particle content in the final product can be effectively reduced. Figure 7b displays the outcomes of three consecutive crushing cycles applied to TiC-bearing slag particles with an initial particle size greater than 0.18 mm. In each cycle, particles with a size of 0.18 mm or smaller were sieved out after crushing, while the coarser particles were subjected to further crushing. The results indicate that with an increasing number of crushing cycles, the mass fraction of particles ≤0.045 mm gradually decreases to 20.20%, 15.24%, and 13.60%, respectively. This suggests that the amount of fine powder generated per cycle diminishes progressively, indicating that the material becomes increasingly resistant to further crushing. The observed trend can be attributed to the relatively low hardness of pyroxene, which facilitates its breakage during the early stages of crushing. As the proportion of pyroxene decreases and the relative content of TiC increases, the overall hardness of the particles rises. Consequently, the generation of fine particles is reduced under identical crushing conditions.

3.8. Comprehensive Analysis

Regarding the controlled grinding method for the narrow PSD of TiC-bearing slag, investigations were carried out on several grinding techniques, including ball milling, rod milling, high-pressure roller grinding, wet ball milling in large-scale mills, and high-speed impact crushing. The PSDs of the product powder under various grinding parameter conditions were analyzed, and the results are presented in

Table 8. Comparative analysis of various crushing methods.

No.	Grinding Method	Experimental Conditions	Acting Force	Particle Size ≤ 0.075 mm in the Product (%)	Particle Size ≤ 0.045 mm in the Product (%)	Strengths	Weaknesses
1	Ball milling	C = 250%	Compression, shearing, and impact	33.29	25.05	The equipment is mature and has a large processing capacity.	Requires a very high grinding circulation load; high energy consumption.
		C = 500%		31.62	17.09
2	Rod milling	C = 100%	Compression and impact	44.54	28.1
		C = 300%		37.6	23.48
		C = 500%		36.53	20.1
3	High-pressure roller milling	/	Compression	61.08	39.31	High grinding efficiency.	The product is severely overgrounded; the quality of the product fails to meet the process requirements.
4	Wet ball milling	5 min	Compression, shearing, and impact	44.38	28.47	The equipment is mature.	The work environment is noisy and dirty; the product is severely overgrounded.
		10 min		48.52	28.62
		50 min		94.69	77.67
5	High-speed impact crushing	The first time	Crushing	33.75	20.2	High quality of products.	Low equipment processing capacity and low efficiency.
		The second time		30	15.24
		The third time		26.37	13.6

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As indicated by the results, each grinding approach has its own merits and demerits. For methods like ball milling and rod milling, the overgrinding amount of the powder can be mitigated by increasing the circulating load. When the circulating load is increased to 500%, the content of particles with a size of ≤0.045 mm in the product can be decreased to 20% or lower. However, it is necessary to strike a balance between efficiency and energy consumption. High-pressure roller grinding and wet ball milling exhibit high grinding efficiency. Nevertheless, a substantial quantity of fine-grained powder is generated during a single operation or within a short time, making these methods less suitable for meeting the quality control requirements of TiC-bearing slag powder. High-speed impact crushing results in less overgrinding, and the particle size can generally meet the requirements of the subsequent low-temperature chlorination process. However, issues such as low equipment processing capacity and low efficiency exist.

Therefore, through comprehensive analysis, we have found that the existence of pyroxene, which is relatively easy to crush, and the titanium carbide phase, which is relatively difficult to crush, in the mineral phase of TiC-bearing slag is the main cause of the difficulties in crushing and the tendency to overgrind during the comminution process. To control the over-comminution of the product powder, it is necessary to adopt a lower comminution intensity (such as shortening the single grinding time and reducing the impact force), and increase the number of screening operations (such as increasing the circulating load). By removing the qualified products from the system as early as possible, we can prevent the particles from being comminuted again. In this way, powder with a narrow PSD can be prepared.

4. Conclusions

(1) The challenges associated with fragmentation and over-grinding during the comminution of TiC-bearing slag were fundamentally attributed to its mineralogical composition. The slag primarily comprises augite, titanaugite, and TiC (or TiC_xO_y) phases. The TiC phase possesses high hardness, which makes titanium carbide-enriched particles highly resistant to breakage. In contrast, the relatively lower hardness of augite results in a greater tendency for fine particle generation during mixed grinding, thereby contributing to over-grinding phenomena.

(2) Pre-reducing the particle size of TiC-bearing slag prior to crushing could significantly alleviate the difficulty of material fragmentation. Experimental data show that when the feed size is reduced from 13–19 mm to 0–6 mm, the Bond ball mill work index (W_ib) decreases from 34.61 kWh/t to 23.84 kWh/t, and the Bond abrasion index drops from 0.204 g to 0.074 g.

(3) The selection of grinding method and process parameters had a significant impact on the PSD of the final product. To achieve a narrow and well-controlled PSD, it is essential to implement integrated control over comminution intensity and classification efficiency. This requires the application of low-intensity comminution techniques (such as shortened grinding duration and reduced impact force), combined with enhanced screening practices (including increased circulating load), to ensure timely removal of particles that have reached the target fineness from the system. This approach effectively minimizes re-comminution and facilitates the production of powder with a narrow and controlled PSD.