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Efficient Recovery of Combustibles From Coking Coal Fines

Avimanyu Dasa; Biswajit Sarkarb; Vidyadhar Aria; Subrata Roya a Minerals Processing Division, National Metallurgical Laboratory, Council of Scientific and Industrial Research, Jamshedpur, India b Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, New York, USA Online publication date: 29 September 2010

To cite this Article Das, Avimanyu , Sarkar, Biswajit , Ari, Vidyadhar and Roy, Subrata(2010) 'Efficient Recovery of

Combustibles From Coking Coal Fines', Mineral Processing and Extractive Metallurgy Review, 31: 4, 236 — 249 To link to this Article: DOI: 10.1080/08827508.2010.508827 URL: http://dx.doi.org/10.1080/08827508.2010.508827

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Mineral Processing & Extractive Metall. Rev., 31: 236–249, 2010 Copyright # Taylor & Francis Group, LLC ISSN: 0882-7508 print=1547-7401 online DOI: 10.1080/08827508.2010.508827

EFFICIENT RECOVERY OF COMBUSTIBLES FROM COKING COAL FINES Avimanyu Das1, Biswajit Sarkar2, Vidyadhar Ari1, and Subrata Roy1 1

Downloaded By: [Sarkar, Biswajit] At: 16:16 5 October 2010

Minerals Processing Division, National Metallurgical Laboratory, Council of Scientific and Industrial Research, Jamshedpur, India 2 Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, New York, USA The processing of three Indian coking coal fines with feed ash values of 25.12% (S1), 22.97% (S2), and 30.38% (S3) was studied. Substantial improvement in the overall recovery of combustible could be obtained by splitting sample S1, exhibiting good washability but poor release behavior, into a coarser and a finer fraction and treating them by gravity and Jameson cell flotation, respectively. Sample S2 had over 70% of the material below 100 mm and had excellent release characteristics. The Jameson cell flotation indeed resulted in very high recovery of combustibles at the desired target ash values and split processing was not required for this sample. The floatability and washability characteristics of sample S3 indicated that gravity-based methods might improve combustible recovery in terms of theoretical yield at the desired product ash values. A combination of spiral concentration of the coarser fraction and froth flotation of the finer fraction using a Jameson cell showed some improvement in the combustible recovery of this sample. It was established in this study that if the floatability is poor or moderate, then split processing improves coal cleaning performance. Flotation alone may be recommended only when samples exhibit excellent floatability. Keywords: coal preparation, Jameson cell, release analysis, spiral concentration, washability

INTRODUCTION The gravity separation and enhanced gravity separation technologies have globally replaced froth flotation to some extent in fine coal cleaning (Riley and Firth 1993; Honaker et al. 1995; Venkatraman et al. 1995). However, fine coal cleaning in most places is still predominantly done by froth flotation. Mostly, the time-tested mechanical cell is used for this purpose, which has the major disadvantages of poor performance in the ultrafines range (Luttrell, Honaker, and Phillips 1995; McLeavy, Klein, and Grewal 2001). The comparatively larger bubbles are unable to pick up the ultrafines present in the feed material. The ultrafine particles in the mechanical cell lack the momentum required to partially rupture the bubble film resulting in poor recovery in this size range (Knelson Address correspondence to Avimanyu Das, Minerals Processing Division, National Metallurgical Laboratory, Council of Scientific and Industrial Research, Jamshedpur 831007, India. E-mail: das. [email protected] 236


EFFICIENT RECOVERY OF COMBUSTIBLES FROM COKING COAL FINES

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and Jones 1994; Honaker and Das 2004). It is well known that because of weathering and surface oxidation, the washability and floatability characteristics of the fine feed material deteriorate sooner than anticipated (Osborne 1988; Adel, Wang, and Yoon 1991). This puts burden on the washing circuit to perform at preset efficiency level in spite of the deterioration in the raw material quality by changing reagent dosage and other operating variables. Such changes are often carried out by experience and a set of thumb rules. A couple of aspects require special attention in view of the above. Thorough characterization of the raw material has never been a high-priority area in coal preparation plants. This paper demonstrates the importance of characterization data toward fine coal cleaning. The primary drawback of the mechanical cell toward ultrafine coal cleaning may be overcome by the use of Jameson cell (Mohanty and Honaker 1999; Harbort et al. 2003). This cell has been proven for its efficiency in recovering ultrafine particles by generating tiny air bubbles that are capable of picking them up. Also, because of very large velocities near the entry, substantial momentum is imparted to the fine particles to facilitate partial rupture of the bubble film by the ultrafines and thereby enhancing their likelihood of attachment (Cowburn et al. 2006; Das et al. 2006). Gravity separation is definitely the first choice because of its simplicity of operation, provided it gives the requisite output. However, flotation cannot be completely done away with (Das et al. 2008). A combination of gravity separation and Jameson cell separation may be a novel process route for cleaning fine coal that does not exhibit good floatability, as has been demonstrated in this work. Indian coals usually have high ash levels in which sulphur content is not really considered to be a problem. However, because of the high ash content, elaborate beneficiation is required where a clean coal grade of 15–17% ash is considered acceptable for the present-day blast-furnace applications. However, in order to conserve this nonrenewable resource and also because of a higher productivity requirement on the steel plants, a lower ash in the clean coal is being increasingly demanded by the industry. A target of 12–14% ash in the clean coal has been set by many industries. The coal preparation plants using conventional methods are finding it difficult to meet this product specification. In this work, efforts are made to generate a product with 12% ash from 0.5-mm coal fines using a novel beneficiation route. Three samples, S1, S2, and S3, are tested using Jameson flotation cell and spiral concentrator. We present the results in two parts. In the first part, kinetics of froth flotation of samples in Jameson flotation cell is examined. The roles of airflow rate, pulp density, and reagent dose are established. In the second part, the beneficiation of S1, S2, and S3 samples using Jameson flotation cell and combination of Jameson flotation cell and spiral concentrator are discussed. RAW MATERIALS AND THEIR CHARACTERISTICS In order to establish a strategy for efficient combustible recovery, three different coking coal fines were taken up for investigation. The samples were collected from operating plants in the eastern region of India, namely Bhojudih and Munidih. These were designated as S1 (Bhojudih coal), S2 (Munidih coal-1), and S3 (Munidih coal-2). These samples were nominally of 0.5-mm size, the feed


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for conventional froth flotation in the plant. These were characterized in terms of their size distribution, size-wise ash distribution, washability, and release analyses. The size and size-wise ash distributions of the three samples are shown in Table 1. The notable observation from this table is that sample S2 has a very fine size distribution. Nearly 70% of the materials are observed to be smaller than 100 mm in size in this coal. Froth flotation involves complex interfacial phenomena. Several complex interfacial interactions, such as particle–particle, particle–reagent, and particle–air interactions are involved. In order to fully understand these phenomena, it is required to characterize coal fines surfaces in terms of their surface charge, surface tension, and hydrophobicity. The adsorption characteristics of collectors and frothers on the coal surface also play prominent roles in froth flotation of fine coal. Moreover, agglomeration of fine particles in the presence of collectors makes this process more complex. Therefore, complete understanding of such complex phenomena is very difficult. On the other hand, release analysis is an established method that can average out the above-mentioned physical phenomena and provide a theoretical limit for flotation performance. Therefore, release analyses are performed for all the three coal fines to evaluate the floatability of these coal fines (Dell 1964). The release analysis was performed in our laboratory by floating as much coal as possible in the first stage after adding reagents. In the second stage, further flotation of this float material was carried out to obtain various float fractions at different time intervals without adding any more collectors. However, a little frother was added in this second stage. The yield measurement and ash analysis results of the float fractions of the second stage were used to develop the release analysis curve. The release analyses data of the three samples are shown in Figure 1. It is evident from this Figure that sample S2 has the best floatability among the three samples, while sample S1 has the poorest floatability in the desired clean coal ash range. Sample S2 contains not only large amount of ultrafines but also has excellent floatability. Hence, it is envisaged that flotation alone may result in acceptable beneficiation performance. Because of its finer size distribution, the Jameson cell

Table 1 Size and size-wise ash distribution of the three samples Coal S1 Size (mm) þ500 500 þ 300 300 þ 210 210 þ 150 150 þ 100 100 þ 75 75 þ 45 45 Head

Coal S2

Coal S3

Wt (%)

Ash (%)

Wt (%)

Ash (%)

Wt (%)

Ash (%)

7.9 9.9 12.6 7.9 11.9 6.3 13.1 30.4 100

26.97 18.04 18.38 16.71 23.33 20.57 19.44 36.02 25.12

7.5 9.8 5.0 3.0 4.8 5.3 11.3 53.3 100

25.33 23.36 20.01 20.24 20.25 24.16 19.30 23.90 22.97

2.3 19.6 17.2 9.4 12.5 7.3 9.3 22.4 100

24.10 24.15 27.02 31.88 32.64 35.06 39.90 31.69 30.38


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Figure 1 The release analysis data of the three coking coal samples.

flotation was deemed appropriate. For the other two samples the floatability was relatively poorer and the possibility of beneficiating them using gravity methods was also investigated. In this regard, the washability analyses of these samples were carried out to understand their nature and develop a proper strategy for improved coal cleaning performance. The washability data of all the three samples are shown in Figure 2. A comparison between the washability and release data reveals the true character of the samples from coal cleaning point of view. Such understanding is made use of in the formulation of a strategy for proper cleaning of these coals toward obtaining a better performance than what may be achieved in the conventional flotation route. It can be seen from Figures 1 and 2 that yield of combustibles in the clean coal from sample 2 would be superior in using flotation (release data) to that of using gravity-based techniques (washability data). For the other two samples, gravity-based techniques may help in improving the beneficiation performance of these coals.

Figure 2 The washability data of the three coking coal samples.

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REAGENTS High-speed diesel was used as collector, and methyl isobutyl carbinol (MIBC) was used as frother in the flotation experiments. METHODS


Jameson Flotation Cell Jameson Flotation Cell model J150=1 supplied by the Roche Mining (MT), Australia, was used for the present work. The pulp level was controlled by the chain rings and the level was varied from 21 to 26.5 cm from the top. As the requirements of reagents in a Jameson cell are generally higher than in a conventional cell, the collector dosage was varied from 0.6 to 3.0 kg=t depending upon the requirement. The frother level was kept at 0.3 kg=t. The airflow rate was controlled by the air inlet valve and was varied from 3 to 9 pm. The solids content of the feed was maintained at 10%. For the kinetic experiments, the tailings stream was circulated back into the feed sump. For the coal cleaning experiments, the tailings stream was a free stream. Spiral Concentrator A spiral concentrator (Model LC 3000) supplied by Humphreys, a division of Carpco, Inc., USA, was used for the studies. The concentrator had a feed sump fitted with a bypass valve for controlling the feed pulp flow. A feed rate of 500 kg=h was maintained during the experiments. The solids content of the feed pulp was kept at 10%. The splitter positions were adjusted to obtain the best results. Random Error in Experimentation The experiments in the Jameson cell were carried out according to a design of experiments in which 11 experiments were performed. The experiment at the base-level condition for the operating variables was performed thrice on different days to estimate the variability. The standard deviation (r) was observed to be 1.08 for the yield and 0.27 for the ash values in the Jameson cell experiments. The feed rate and pulp density for the experiments in the spiral concentrator was unchanged, and the best results were identified by trials adjusting the splitter positions. At the desired splitter position only two experiments were carried out in each case and the average value was reported. Again r was observed to be 0.67 for the yield and 0.18 for the ash values in the spiral experiments. Considering the small values of r, it may be assumed that the random errors in the experiments presented here are within the acceptable limits. EXPERIMENTAL RESULTS AND DISCUSSION The combustible recovery from coal fines by froth flotation is a strong function of the degree of hydrophobicity of fine particles, size distribution of the particles, and mineral (ash) matter distribution in the feed materials. In the present work,


241

the applicability of the Jameson cell flotation is investigated for all the three samples, as it is known to enhance fine particle recovery (Cowburn et al. 2006). Kinetics Studies in Jameson Cell In the evaluation of experimental results, combustible recovery (%) and ash reduction (%) of the concentrates are considered. The cumulative combustible recovery (CR) in the concentrates is calculated for 300 s as follows:


CR% ¼ ½CðtÞ ð100 XC ðtÞ=½F ð100 XF Þ;

ð1Þ

where F is the feed mass, C(t) is the concentrate mass, and XF, XC(t) are the gravimetric mass percentage of ash in the feed and concentrate, respectively. Several experiments are carried out to investigate the influence of airflow rate, feed pulp density, and pulp flow rate on the flotation kinetics. The experimental conditions are shown in Table 2 along with the observed rate constant values. Figure 3 shows the cumulative recovery of combustibles for the three types of coals with time under various experimental conditions. The recovery was certainly not very good for coal S1 (Figure 3a). An intermediate level of airflow rate, pulp level, and collector dosage gives the best recovery of combustibles. Under favorable kinetic conditions nearly 80% recovery of the combustibles is achievable for coal S2, as shown in Figure 3b. A lower pulp level with a high airflow facilitates the recovery. Collector requirement for this coal is less. The combustible recovery for coal S3 is somewhat intermediate. A recovery of about 60% was achievable (Figure 3c). Table 2 Conditions for the kinetic experiments and the corresponding rate constants Experiment no. Coal S1 1 2 3 4 5 6 Coal S2 1 2 3 4 5 6 Coal S3 1 2 3 4 5 6

Pulp level from top (mm)

Airflow (l pm)

Collector (kg=t)

k (per s)

26.5 26.5 26.5 21.0 21.0 23.8

3 9 9 3 3 6

3.0 3.0 1.0 3.0 1.0 2

0.0069 0.0093 0.0088 0.0077 0.0062 0.0099

26.5 26.5 26.5 21.0 21.0 23.8

3 3 9 3 3 6

1.5 0.5 1.5 1.5 0.5 1.0

0.0032 0.0053 0.019 0.010 0.0111 0.0139

26.5 26.5 26.5 26.5 21.0 21.0

3 3 9 9 3 3

1 0.6 1 0.6 1 0.6

0.0133 0.0094 0.0115 0.0076 0.0086 0.0125


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Figure 3 Cumulative recovery of combustibles for (a) coal S1, (b) coal S2, and (c) coal S3.

A lower airflow rate appears to be favorable for combustible recovery. In addition, the collector requirement is relatively low in this case. Flotation has been treated as a rate process, and first-order kinetics has been shown to be reasonably accurate (Kelly and Spottiswood 1989; Polat and Chander 2000). ln Rm =ðRm RÞ ¼ kt;

ð2Þ

where Rm is the maximum cumulative recovery of combustibles, k is the rate constant, and R is the recovery at time t. Of course, Equation (2) is applicable to systems where all particles have similar flotation rate constants (similar degree of hydrophobicity, size, density, and shape). A detailed description of flotation kinetics of coal fines is available elsewhere (Kawatra and Eisele 2001). In this work, this equation is used to obtain the average k values of a mixture of particles having different degrees of hydrophobicity, size, density, and shape. Thus, the k values reported here are associated with averaging error. However, these average k values are advantageous because these are easy to estimate and straightforward to use for having an idea of the overall performance of the flotation cell. These k values have been used in this work only as an indicative of flotation kinetics and not for predicting the actual rate.



243

The cumulative mass fraction recovered in the float fraction for all conditions was fitted to the classical first-order kinetic model, as shown in Figure 4. From this figure, the rate constant values were obtained as the slope of the straight lines. The rate constants are shown in Table 2 for various experimental conditions. It was found that the rate constant is affected by airflow rate, pulp level, collector dose, and feed type. Higher values of airflow rate, collector dose, and pulp level help in achieving higher rate constant. However, feeds with larger particles do not help in achieving a higher rate constant. Thus, a higher airflow rate, higher collector dose, higher pulp level, and a feed having finer particle would be a good candidate for flotation in Jameson cell. The froth flotation rate constant for S2 is 0.019=s at an airflow rate of 9 pm, 26.5 pulp level, and 1.5 kg=t of collector dose. Under similar conditions, a lower rate constant was found for S1 (0.0088=s) and S3 (0.0155=s). The higher rate constant of S2 suggested that an economic combustible recovery could be achieved using froth flotation alone. The froth flotation of the feed S1 was not able to give a higher combustible recovery. However, an intermediate rate constant did not clearly suggest using froth flotation for a better combustible recovery. It can also be seen from this table that, in general, the rate constant values are reasonably high under favorable conditions. These values are higher in the case of coal S2, which has a finer size distribution. For a better combustible recovery through froth flotation, a higher flotation rate constant is a prerequisite.

Figure 4 First-order fitting of the Jameson cell flotation responses under various kinetic conditions for (a) coal S1, (b) coal S2, and (c) coal S3.

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It was thus envisaged that removal of coarser fraction would further enhance the kinetics, and the Jameson cell would be ideally suited for froth flotation of the fines with superior flotation kinetics. The coarser fraction is likely to respond better in a gravity concentrator, paving the way for a better overall recovery of the combustibles. The main advantages of the Jameson flotation cell are the rapid collection of particles in the downcomer and the relatively small footprint. The cell is simple and easy to operate and draws air from the atmosphere, eliminating the need for compressors or blowers.


Continuous Flotation in Jameson Cell Owing to the large amount of ultrafines content in sample S2, its excellent floatability and poor washability characteristics, and superior kinetics, the Jameson cell flotation was used for this sample for continuous operation. These flotation data are presented in Figure 5. This figure indicates that a clean coal with about 12% ash is produced at over 75% yield. The theoretical limit as indicated by the release curve could nearly be achieved by Jameson cell flotation. The Jameson cell flotation performance for S2 and S3 samples revealed that the yield values as indicated by the release analysis data could not be achieved, although the performance was quite good for sample S3 vis-a`-vis its release analysis. Figures 6 and 7 show the Jameson cell flotation performance with samples S1 and S3, respectively. For sample S1, 45% yield at about 12% clean coal ash and for sample S3, 54% yield at 12% clean coal ash are obtained using Jameson cell flotation. The data show that for sample S1, the Jameson cell flotation performance is much poorer than what is theoretically achievable by flotation. In addition, the washability data indicate that higher yield values can theoretically be achieved using gravitybased techniques. On the other hand, flotation performance for sample S3 is somewhat poorer than what is indicated by the release curve. Also, the washability data indicate that a higher recovery of combustibles is theoretically possible if gravitybased techniques are used. Partial rupture of the bubble film is the key issue in flotation. Ultrafine particles do not have sufficient momentum to partially rupture the air bubbles and attach on that bubble. Therefore, it is not possible to have a good combustible recovery for

Figure 5 The Jameson cell flotation data vis-a`-vis release analysis of sample S2.


245


Figure 6 The Jameson cell flotation performance against washability and release data for sample S1.

these coal particles using conventional mechanical cells. As the particle size increases, the momentum of the particle also increases that helps in achieving the desired rupture of the bubble film and subsequent attachment of particles on the bubble. Above a critical size of the particles, however, the momentum of the particles becomes so large that air bubbles are destroyed because of particle–bubble collision resulting in poor flotation performance. In the case of attachment also the bubble–particle assembly becomes unstable with a large particle. Bubble size also has significant influence in the process. As bubble size decreases, the interfacial surface area increases, providing larger available sites for attachment. Jameson flotation cell has the ability to generate smaller size bubbles vis-a`-vis the larger size bubbles produced in conventional flotation cells. In addition, Jameson cells impart a greater momentum to the particles. Hence, these cells are suitable for the flotation of smaller size particles (Jameson and Manlapig 1991). From Table 1 it may be seen that feed S2 contains about 55% of 45-mm particles, whereas feed S1 and S3 contain about 40% and 50% þ 100-mm particles, respectively. Owing to the presence of higher amounts of 45-mm particles, better performance of Jameson flotation cell is achieved. On the other hand, the presence of higher amount of larger size particles in feed S1 and S3 reduces the combustible recovery from fine coal using Jameson flotation cell. From Table 1 it may also be seen that feed S1 and S3 contain higher mineral matter (ash) than feed S2. Therefore, the

Figure 7 The Jameson cell flotation performance against washability and release data for sample S3.

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specific gravity values of feed S1 and S3 are higher. This results in a higher momentum of these particles leading to the destruction of bubbles during bubble–particle collision. The gravity-based separation techniques are also not very efficient in ultrafine particle processing. Although the enhanced gravity separators, such as Knelson, Falcon, and MGS concentrators were developed to cater this need, their industrial applications are still limited. In view of the experimental observations, findings from kinetics studies, and considering the applicability of the Jameson cell flotation for ultrafines, it was decided to split the two samples into a coarser and a finer fraction. The coarser fraction was to be treated using gravity concentration, while the finer fraction was to be treated by flotation in Jameson cell.


Split Processing As mentioned above, gravity separation also needs to be considered toward improving the overall recovery of combustibles from samples S1 and S3. These two samples were split into þ0.15-mm and 0.15-mm fractions. The coarser fraction (0.5 0.15 mm) is treated in a spiral concentrator, and the finer fraction (0.15 mm) is treated by froth flotation using a Jameson cell. The split processing flow diagram is shown in Figure 8. Processing of Coarser (þ0.15-mm) Fraction. Several tests were carried out by changing the feed pulp density, feed flow rate, and splitter positions in spiral concentrator. The best results of spiral concentration of this sample’s þ 0.15-mm fraction are shown in Table 3. It may be seen that about 62% yield (24% with respect to original) at 13% clean coal ash is obtained using gravity concentration of this fraction.

Figure 8 Flow diagram for split processing of the two coals, S1 and S3.


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Table 3 Spiral concentration of þ 0.15-mm fraction of coals S1 and S3 Stream


Coal S1 Concentrate Middling Tailings Coal S3 Concentrate Middling Tailings

Yield (%)

Ash (%)

% Yield (with respect to original)

62.4 15.9 21.7

13.32 21.24 59.63

23.9 6.1 8.3

52.6 29.2 18.2

12.11 28.15 69.16

25.5 14.2 8.8

The middling of the spiral concentration stage had an ash content of 21% for S1 and 28% for S3. Efforts were also made to recover the combustibles from these middling fractions using froth flotation in Jameson cell. The flotation data for the middling product of the spiral stage for both these samples are shown in Table 4. About 3% and 7% combustibles were recovered from the middling streams at nearly 11% product ash value for samples S1 and S3, respectively. Processing of Finer (0.15-mm) Fraction. The 0.15-mm fraction of both S1 and S3 were separately subjected to flotation in Jameson cell. The results of these tests are shown in Table 5. Nearly 54% yield of the clean coal is achieved in both cases at 12% product ash. The yield values with respect to original were 33% for S1 and 28% for S3. In both cases rejectable tailings with 51% ash in the case of S1 and 58% ash in the case of S3 were generated. Split Process vis-a`-vis Jameson Cell Flotation Now, the final product in split processing consists of two flotation concentrates and the spiral concentrate. If they are combined, the overall concentrate is found to contain 12.4% ash with the overall yield of 60.14% for S1. It is interesting to compare the beneficiation performance of split process with the flotation results of this sample that yielded a clean coal with 12% ash at 45% yield. It can be seen that a substantial improvement in the overall yield is achieved by split processing of this sample compared with the conventional route (flotation alone) at the same overall target ash of the clean coal. Table 4 Jameson cell flotation of middlings from spiral concentration of coals S1 and S3 Stream Coal S1 Concentrate Tailings Coal S3 Concentrate Tailings

Yield (%)

Ash (%)


48.6 51.4

11.48 30.89

2.9 3.2

52.5 47.5

11.78 45.68

7.4 6.7

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A. DAS ET AL. Table 5 Jameson cell flotation of 0.15-mm fraction Stream Coal S1 Concentrate Tailings Coal S3 Concentrate Tailings

Yield (%)

Ash (%)


53.9 46.1

11.9 51.1

33.3 28.4

53.8 46.2

12.05 58.49

27.70 23.78

Table 6 Overall comparison of flotation and split processing performance


Coal S1

Coal S3

Yield (%)

Ash (%)

Yield (%)

Ash (%)

45 60

12 12

55 61

12 12

Jameson cell flotation Split processing

In the case of sample S3, by split processing an overall concentrate at 12.04% ash was obtained at 60.63% yield. In contrast, using only flotation a clean coal having 12% ash was generated at 54% yield. Thus, it is evident that some improvement in the overall yield is achieved by split processing of this sample over flotation at the same overall target ash of the clean coal. The advantage of split processing over flotation of these fine coals is summarized in Table 6. SUMMARY AND CONCLUSION Close investigation of the release and washability analyses of the samples revealed that sample S1 has poorer floatability compared with sample S3. The release curve for S1 was much below its washability curve vis-a`-vis the same for S3. The kinetics studies indicated that Jameson flotation cell might be effective in the flotation of these three coal samples in general and feed S2 in particular. The split processing of S1 resulted in substantial improvement in the yield (60%) at 12% target ash when compared with flotation alone (45% yield at the same target ash). In the case of S3, split processing resulted in 6–7% improvement in the yield of clean coal over conventional flotation. It can be concluded from this work that if the floatability of the sample were poor (release curve much below the washability curve), split processing would lead to substantial improvement in combustible recovery. The improvement in split processing for a moderately floatable coal (release curve somewhat close to the washability curve) cannot be as much as for a poorly floatable coal. However, some improvement is still possible. Besides, split processing will also reduce the load on flotation and thereby cut down on the reagent expenditure. A fine coal sample having excellent floatability may be treated by flotation alone if the size is fine enough. The study also establishes the need for thorough characterization of the fine coal sample before it is treated to achieve the desired specifications of the product.


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