Petrography studies were carried out, taking a representative sample of washery middling for the investigation. The polished sections were prepared using epoxy, resin and hardener for the coal sample. The result of modal analysis is given in Table 1. From the microscopy observations (modal distribution analysis), it has been found that the given coal sample consists mainly of vitrinite maceral and the proportion of the inertinite macerals is comparatively lower than that of the vitrinites. The volume% of the mineral matters was found to be 38.62, which signifies the enrichment of the mineral matters in the given coal. The random reflectance (Rr%) is 1.25, which indicates that the coal sample is medium volatile coking coal in nature. Figure 1 shows the distribution of the organic and inorganic phases of the coal sample. Most of the vitrinite and inertinite macerals have been observed to be interlocked with some mineral matters. The mineral matters have been seen as pore-filled, fracture-filled and interlocking in nature.

Table 1. Coal macerals and mineral matters distribution (vol%).

Figure 1. Distribution of different coal macerals and mineral matters present in middling sample. (a) Vitrinite maceral (V); (b) Vitrinite (V) and inertinite (In) macerals and some pore filled interlocked mineral matters (M); (c) Vitrinite (V) and inertinite (In) macerals; (d) Mineral matter (M) enriched grain containing some inertinite (In) macerals; (e) Some interlocked mineral matters (M) within vitrinite (V) maceral; (f) Inertinite (In) maceral.

The middling sample was crushed to three different top sizes, viz. −6 mm, −3 mm and −1 mm. The size analyses of the crushed samples are given in Table 2. It shows that, with the decrease in the top size of the sample, the ash content in the −0.5 mm or −0.15 mm fraction increases. Washability studies with all three different top sizes of middling sample were carried out.

Table 2. Size analyses of crushed middling sample.

3.2.1. Crushed to −6 mm and −3 mm

Usually, the washery middlings samples (Washed power coal) are interlocked coal particles that failed to respond to the gravity separation while treated in heavy media cyclone. They need adequate liberation to unlock the non-combustible substances (mineral matter) for generating the product with low ash. Therefore, it is very important to select the optimum feed size while processing of such samples will be carried out. In this case, top size of the middling sample received was −13 mm. Initially, the middling sample was crushed to top size of 6 mm and 3 mm. A washability study of −6 mm and −3 mm was carried out by varying the density of the medium from 1.4 to 2.0 gm/cm³. Ash content of all the products was analyzed based on dry-moisture-free basis. The washing characteristics of the crushed coal could be obtained from the weight% and ash% of the floats collected at different densities, as presented in Figure 2. It is found from Figure 2 that washing characteristics of −3 mm feed size are marginally better than those of the −6 mm sample due to improved liberation. The product yield at 18% ash level is 3% higher for the −3 mm feed size compared to the −6 mm feed size.

3.2.2. Crushed to −1 mm

From the washability study, it is found that the product yield and ash content are not significant even at −3 mm feed size. Therefore, in view to improve the product yield and ash content, sample needs to crush further to increase the liberation characteristics. Therefore, it was crushed to −1 mm. In this case, washability studies were carried out for the fractions, viz. −1 + 0.5 mm and −1 + 0.15 mm to assess the maximum yield achievable at the targeted ash level from these fractions. The results are shown in Figure 2. The study shows that reducing the top size of middling sample to −1 mm is more effective than the other two top sizes (−6 mm and −3 mm) as it produces higher product yield at 18% - 19% ash level. Table 3 shows that the product yield is less than 20% by weight at an 18% ash level when the top size of feed is above 1 mm. The higher yield is found with −1 + 0.15 mm fraction, between the two different size fractions of −1 mm. The product yield is 28% by weight for the feed size of −1 + 0.15 mm, which is 13% more than that of the −6 + 0.5 mm feed size at 18% ash, and 10% more than the −3 + 0.5 mm feed size. Figure 2 also shows that the −1 + 0.15 mm size fraction produces encouraging results at 18% and 19% ash levels among all the size fractions.

Figure 2.Washing characteristics of −6 mm, −3 mm and −1 mm crushed middling sample.

Table 3. Washability results at 18% - 19% ash level for different feed sizes.

Based on the washability study, top size of the middling sample (washed power coal) was fixed at −1 mm to conduct the beneficiation studies for ash reduction. An attempt was made to process the sample by gravity separation, as it is one of the cheapest wet processing methods. Therefore, processing studies were conducted using spiral concentrator using −1 + 0.15 mm fraction to produce a clean coal product.

3.3.1. Spiral Concentrator

Spiral concentrators are devices used to separate solid components in a slurry form. The separation process is based on the combination of the density of solid particles and hydrodynamic properties, especially, drag force. In spiral concentrator, different forces like gravitational, centrifugal, drag, lift and friction forces act on the particle during its passage down the curvilinear path of the spiral. The resultant of the forces governs the separation of particles on the spiral deck depending on their size and density. Spiralling of −1 + 0.5 mm and −1 + 0.15 mm were performed at 10% solid concentration. From the results of spiralling (Table 4), it is found that the response of −1 + 0.15 mm is better than that of −1 + 0.5 mm feed size. The product yield is 4.3% (w.r.o.) less for the −1 + 0.5 mm feed size, as the weight% in the narrow size range (−1 + 0.5 mm) is less, resulting in a lower product yield with respect to the feed content. The product yield for −1 + 0.15 mm feed size is 11.7% with the respect to the original feed with ash content of 19.34%. The tailing generated from spiral concentrator is 31.4% by weight with 51.95% ash. It shows that effective rejection is achievable through gravity separation using spiral concentrator. The spiral middling obtained from −1 + 0.15 mm feed size is 31.0% by weight with ash content of 31.45% and used for column flotation studies with the fines of −0.15 mm (38.07% ash), removed from the crushed to −1 mm sample. The fines were also processed separately to study their response with different types of frother.

Table 4. Results of spiral concentrator using feed sizes of −1 + 0.5 mm and −1 + 0.15 mm.

WRO: With respect to original feed.

3.3.2. Processing of Fines (−0.15 mm) Removed from −1 mm Crushed Coal

The −0.15 mm fraction separated out from −1 mm coal during the crushing of the middling sample contains 38.07% ash. To reduce the ash content, it was processed using column flotation under varied conditions. The feeding and tailing flow rate were kept at 1.5 lpm and 3 lpm, respectively. Diesel oil was added as collector along with the sodium silicate as depressant. While conducting the tests, variables kept constant are pulp density of solid at 10%, sodium silicate at 3.5 kg/t, diesel oil at 210 g/t and wash water flow at 0.25 lpm.

1)Effect of FrothHeight

Column flotation studies were carried out with the fines (−0.15 mm) varying the froth height between 15 cm and 35 cm. The effect of MIBC concentration was studied at 65 g/t and 80 g/t with the froth heights. Figure 3 shows that higher dosage of 80 g/t MIBC increases the product yield (36.9%) significantly with high ash content (31.33%) at lower froth height of 15 cm. This signifies that increasing froth height reduces the floatability of less hydrophobic particles (gangue) than the more hydrophobic coal particles due to the weaker bonding. At higher concentration of MIBC (80 g/t), the product ash was found to be as ash as 22.51% even at higher froth height of 35 cm. This indicates that increased frother dosage not only increases the hydrophobicity of the desired particles but also increases the entrainment of the undesirable particles (gangue); thus, leading to lower product quality. The lower dosage of MIBC (65 g/t) produces low ash product at all point of froth height when compared with higher dosage. At froth height of 35 cm and 65 g/t of MIBC, the product ash level significantly reduces to 18.4% which is quite encouraging. The high froth height in flotation column reduces the tendency for unselective flotation, minimizing ash content in the product and reduces the tendency for unselective flotation. This reveals that a lower concentration of frother leads to less entrainment of gangue materials into the froth, due to relatively less stability of the froth [10]-[12].

Figure 3. Effect of concentration of MIBC and froth height on product recovery.

2)Effect of FrotherTypes

The response of −0.15 mm fraction in a column flotation was compared using two different types of frother, like Methyl Iso-Butyl Carbinol (MIBC) and Synthetic Frother (SF) from Nalco (Figure 4). Flotation tests were also conducted at different Superficial Air Velocity (SAV), keeping the froth height constant at 20 cm.

MIBC: The performances of MIBC and SF under varied conditions are presented in Figure 4. It shows that the flotation performance of MIBC is superior, even at higher SAVs. The results from Figure 4 indicate that MIBC produces very low yields at higher froth depths, attributed to the decreased stability of the froth. MIBC offers better selectivity, leading to lower ash content even at higher superficial air velocities. The findings suggest that MIBC is preferable to use where both product recovery and quality are predominant.

Synthetic Frother (SF): In contrast, while SF produces a high yield, it is associated with higher ash content. This disparity indicates a potential issue with selectivity; SF might enhance the floatability of both the target minerals (coal) and undesirable gangue minerals, leading to increased ash content in the final product. The use of Synthetic frother provides high yields but at the expense of increased ash content.

3)Impact of SuperficialAir Velocity

The impact of superficial air velocity on product yield and ash content at a 20 cm froth height for MIBC, and at 20 cm and 30 cm froth heights for SF, is shown in Figure 4. A strong frother produces stable froth, which hinders the drainage of water and fine particles, leading to less gangue rejection and consequently lower product quality. At lower superficial air velocity, MIBC shows to be quite effective in optimizing both yield and ash content of the products. This efficiency indicates that at reduced SAV, MIBC provides better particle-bubble attachment and flotation selectivity, allowing for a more concentrated product, which is a critical role for achieving a good recovery.

SF, being a stronger frother, produces a more stable froth than MIBC [13][14]. The effect of SF at increased froth height (30 cm) revealed that at low Superficial Air Velocity (SAV), the product ash content could be reduced to 5% compared to that at a low froth height of 20 cm. However, the product yield increases significantly with an increase in SAV. This signifies that increasing SAV can enhance the floatability of less hydrophobic particles (gangue) and, therefore, causes more entrainment in the product [15].

Figure 4. Effect of superficial air velocity on product yield and ash of fines.

3.3.3. Column Flotation with Spiral Middling and Fines

To increase the product yield, the middling product generated from the spiralling of the −1 + 0.15 mm fraction was treated with the fines. The spiral middling constitutes 31.0% by weight, with an ash content of 31.45%. This amount is significant and also contains carbon values. This product could be further processed after grinding it to 0.15 mm. The composition of the feed used for the column flotation is provided in Table 5. MIBC was used in the subsequent tests to achieve a product with low ash content due to its suitability, especially in the present study. Column flotation tests were carried out using reagents such as diesel oil (210 g/t) as a collector, sodium silicate (3.5 kg/t) as a depressant, and MIBC (50 g/t) as a frother.

Table 5. Composition of feed for column flotation.

The experiments were carried out by varying the superficial air velocity for two different froth heights (20 cm and 30 cm) to get the desired product with good recovery. The effect of superficial air velocity on product yield (presented with respect to original feed) and their ash content is given in Figure 5. Similar trends are observed for two froth heights. It is found that higher air flow rate increases the product yield with increase of ash content due to the entrainment of the impurities [16]. It could be possible to get product with 17.48% ash with yield of 13.5% at 30 cm at high froth depth. The very low SAV and high froth height are not favourable for the mineralised bubble to reach the froth level. Thus, reduces the product yield.

Figure 5. Effect of superficial air velocity and froth height on composite feed of spiral middling and −0.15 mm fines.

Table 6. Processing results using spiral concentrator and column flotation.

The outcome of the processing of middling sample using a spiral concentrator and column flotation is summarised in Table 6. The product yield of 11.7% with 19.34% ash could be obtained from the gravity separation using −1 + 0.15 mm. Flotation of composite feed of −0.15 mm generates 13.5% product with 17.48% ash. Thus, final product becomes 22.2% by weight, with 18.46% ash that could be produced.

3.3.4. Petrography Study of Washed Coal Product

The final product thus generated from processing of middling sample was characterised for its coking property. The quality of washed product was measured by petrographic study. The phase distribution, i.e., modal analysis of coal macerals, mineral matters, reflectance measurement and photomicroscopy were carried out under both reflected and fluorescent light using a conventional polarized microscope. From the microscopy observations, it has been found that, vitrinite group of macerals dominates in the respective sample. Their proportion (45.17 vol%) is received to be more than the inertinite group of macerals (29.29 vol%). The relative concentration of the mineral matters has been found to be lower (25.54 vol%) than the macerals. The macerals are seemed to be free from interlocking characteristics (Figure 6(a), Figure 6(b)). However, some pore space filled with alumino-silicate minerals (Figure 6(c), Figure 6(d)) is observed in the washed sample. From the random reflectance measurement, the rank of the given coal was observed to be medium volatile coking coal (Rr% of 1.30). The result of modal analysis and mean random reflectance measurement is given in Table 7. The product quality was also measured in terms of calorific value and found to be 6989.5 kcal/kg.

Figure 6. Photomicrographs showing distribution of macerals and mineral matters in washed coal sample. (a) A typical vitrinite maceral (Vt) with some fine layers of inertinite maceral and pore filled mineral matters; (b) Inertinite (In) and vitrinite (Vt) macerals; (c) A typical interlocked grain of vitrinite (Vt), inertinite (In) and mineral matters (Mm); (d) Bands of vitrinite (Vt), inertinite (In) macerals and pore filled interlocked mineral matters (Mm).

Table 7. Modal analysis (macerals and mineral matters distribution) (vol%) and mean vitrinite reflectance (Rr%).