Evaluation of the washability characteristics of Khushab coal (Pakistani) by heavy media separation process

Abstract

The use of high-sulfur Pakistani coals can cause serious problems of slagging and fouling in thermochemical conversion reactors along with environmental issues like acid rain, etc. In this study, a pre-combustion technique, namely heavy media separation, is employed for the cleaning of low-grade Pakistani coal. Six crushed coal samples of different particle sizes were individually subjected to heavy media solutions of ZnCl₂ of different specific gravities. It was found that the sample with a particle size of −6.25+4 mm at specific gravity of 1.4 produced the optimum float product as clean coal, showing 83.53% yield of clean coal with 1.24% ash and 1.0% sulfur contents. An overall reduction of 91.68% in ash and 86.11% sulfur contents was obtained. Moreover, up to 19.3% enhancement of gross calorific value was achieved. The resultant clean coal can be used in various energy recovery schemes in Pakistan such as coal-fired power plants and cement industries.

Keywords

Coal washing Pakistani coal ash sulfur proximate analysis gross calorific value

Introduction

It is a common practice today that most of the modern coal cleaning plants in the world use specific gravity-based processes, of which heavy media separation is the widely used process due to its simplicity and high degree of efficiency. The process significantly enhances the purity of coal by reducing the level of impurities. The heavy media separation process exploits the specific gravity difference between coal and the associated extraneous mineral matter. For undertaking the cleaning tests, low-grade coal is crushed to a predetermined particle size and is subjected to heavy medium of known specific gravity where lighter clean coal floats and the heavier dirt, containing gangue minerals, sinks. By this coal cleaning procedure, washability or beneficiation characteristics of low-grade coals are established. The float–sink coal cleaning process, due to its outstanding merits, is used worldwide and its popularity is gaining momentum day by day.^1–3

To study the washability characteristics of low-grade coals in the laboratory, in general, organic liquids are used as heavy media, because organic liquids are relatively inert towards all coal types. But for all practical purposes, and to replace the expensive, volatile and toxic organic liquids, zinc chloride (ZnCl₂) solutions of various specific gravities are used to carry out the test work on coal samples.⁴ Important parameters such as specific gravity of heavy medium and the size of raw coal, that produce the best results, are taken as guidelines for designing a coal cleaning plant.⁵ The present study represents 213 million tonnes of coal deposits of Khushab mine in the Salt Range area of the Punjab province of Pakistan. It may be pointed out that most of the coal resources of Pakistan belong to lignite to sub-bituminous rank, containing 4 to 10%of pyritic sulfur and a significant amount of mineral matter, with low heating value.⁶

Currently, Pakistan is in a grip of severe energy crisis of its history due to the increased demand of energy. The existing oil and gas resources of the country are depleting fast and the energy needs are heavily dependent on imported energy sources. The government of Pakistan is now committed to utilize the indigenous coal resources for the economic and social development of the country.⁷ There are a few cement plants in the vicinity of Khushab coal mine which are using imported coals. There are huge prospects for installation of coal cleaning plants in Khushab area if relevant washability data are available. Realizing the need for clean coal for this energy potential area, an attempt has been made to upgrade the coal by using float–sink cleaning process. The process is expected to reduce sulfur and mineral matter in the raw coal and will help meeting the energy needs of the nearby industrial zone. Moreover, the research findings through this study will provide guidelines for the establishment of industrial coal cleaning plants based on Khushab coal, leading to the establishment of industrial coal cleaning plants.

Experimental

A bulk representative coal sample, weighing about 50 kg, was arranged through courtesy of the Pakistan Mineral Development Corporation (PMDC) that owns the Khushab coal mine. The sample was collected in accordance with the sampling procedure as described in ASTM D2234M-08, keeping in view the thickness and continuity of the coal seam which represented the entire coal deposits. The gross coal sample, having top sized lumps of 100 mm, was stage crushed by a roll crusher down to the size of 25 mm. The crushed material was reduced to 1/4th of its weight by using coning and quartering method ASTM D346-08. The big lots, each weighing about 12 kg, were further reduced into smaller lots of 1 kg each by using riffle splitter for float–sink test.

A lot of bulk sample was crushed by hammer mill to the size of 4.75 mm (4 mesh) and ground to 250 µm (60 mesh) by using pulverizer, for the determination of proximate analysis, sulfur content and gross calorific value (GCV), according to ASTM standards: total moisture (D3173), volatile matter (D3175), ash (D3174), fixed carbon (D3712), sulfur (D4239) and GCV (D5865). However, for the float/sink test-work, six 1 kg samples were individually crushed by a hammer mill into the following six particle sizes: −25 mm+ 18.75 mm, −18.75+12.50 mm, −12.5+8.00 mm, −8.00+ 6.25 mm, −6.25+4.00 mm and −4.00+2.00 mm.

Prior to undertaking the float–sink tests, all the six coal samples of different sizes were air dried (D-3302-07 a) and sieved to remove fines (74 µm) to avoid the disturbance due to their adherence on the coarser coal particles (ASTM D4749-08) during the cleaning process. The stock solution of zinc chloride (ZnCl₂), having specific gravity of 2.0 was prepared using distilled water. This solution was used to prepare five heavy media solutions of specific gravities: 1.4, 1.5, 1.6, 1.7 and 1.8 with the help of calibrated hydrometer.

In the first set of float–sink experiments, a coal sample of −25+18.75 mm in size, weighing 500 g, was put in a 500 ml beaker containing enough ZnCl₂ solution having specific gravity 1.4. The coal sample was stirred for 5 min and sufficient time was given for the separation of float and sink products. The first float product in the form of clean coal was collected and the sink product of this specific gravity was floated again at relatively denser specific gravity of ZnCl₂. Similarly, its float was collected and the sink was transferred to a higher specific gravity beaker (i.e. 1.6). The same process was repeated till specific gravity of 1.8. For this set of float–sink, with size fraction of −25+18.75 mm, one sink product and five float products were obtained. This cleaning process was adopted for all other five size fractions (i.e. −18.75+12.5 mm, −12.5+8 mm, −8+6.25 mm, −6.25+4 mm and −4+2 mm).

The float products of all the six test samples and the final sink products were filtered and washed with distilled water till the last washing, and when tested with AgNO₃ solution, they did not show any chloride ions. For all the float–sink tests, the procedure as mentioned in ASTM D-4371-08 was precisely followed. Reproducibility of the test findings was also checked in many float–sink batch experiments. All the products (30 float and 6 sink products) were dried, weighed and placed in air tight bags for analysis and assessment of the washability characteristics. The results have been shown graphically in Figures 1 to 6. The composition of untreated sample (as shown in Table 1) indicates that the Khushab coal belongs to the sub-bituminous rank.

Figure 1.

Percent yield of float products from coal samples of different sizes at different specific gravities.

Table 1.

Proximate analysis of Khushab coal.

Moisture (%)	Volatile matter (%)	Fixed carbon (%)	Ash (%)	Sulfur (%)	GCV
3.92	36.80	44.96	14.32	7.15	6049 Kcal/kg

Results and discussion

Figure 1 shows the float yields that are affected by the particle sizes and the specific gravities. It was found that the float, product with maximum yield of 83.53%, was obtained at a specific gravity of 1.4, using the sample with particle size of −6.25+4 mm. Subsequently, the yield started decreasing at higher specific gravities, beyond 1.4. It may be pointed out that the coal under investigation contained pyrite, shale and other clay minerals as major components of mineral matter.⁸ The mesh of liberation study under optical microscope (ASTM-D-2799-05) indicated that at the particle size of −6.25+4 mm, both pyrite and gangue minerals existed as coarsely distributed in coal matrix which were largely liberated during crushing and grinding. It was further observed that when the sample with relatively finer particle size of −4+2 mm was used for float–sink test at a specific gravity of 1.4, the yield reduced to 68.31%; this could be due to the fact that HMS is more effective for the separation of coarser particle sizes if they have the liberated form of mineral matter including pyrite. It is also obvious that the particles of clay minerals produced excessive slimes which adhere on coal particles and, being hydrophilic in nature, depressed the floatability of coal and as a result the recovery of combustibles is reduced as evident in Figure 1. It is also supported by the research findings reported in literature. The float obtained at −6.25+4 mm and specific gravity 1.4, having very low ash and low sulfur contents may conveniently be used in cement and power plants. Due to the poor grade and negligible recovery of floats at specific gravity of 1.6 to 1.8 (with the sample of −6.25+4 mm), they were not added to cumulative float yield (Figure 1). The results reported in Figure 2 indicate the effect of specific gravities and particle sizes on ash reduction in float products. It was found that the sample of −6.25+4 mm, at a specific gravity of 1.4 produced the float containing maximum ash reduction of 91.68%. It was found that ash reduction subsequently decreases in floats as the specific gravity increases; this is in agreement with the findings of Zhang et al.⁹ At a specific gravity of 1.8, the reduction in ash is 60.12% (Figure 2) which could be an indication of liberation of the mineral matter like shale and clay minerals. For the coarser sizes above −6.25+4 mm, the ash reduction further decreases; it could be due to the fact that coarser coal particles (above −6.25+4 mm) still have finely disseminated mineral matter which existed in un-liberated form, and have less exposure to the separation media.^10,11 The sample with the particle size of −8+6.25 mm at a specific gravity of 1.4 showed ash reduction of 78.61% in floats and the reduction gradually reduced with increasing specific gravities. At the highest specific gravity of 1.8, the ash reduction was 27.17%. When tests were carried out at specific gravities of 1.4 to 1.8, at the particle size of −25+18.75 mm, a similar tendency was experienced. The ash content in the float reduced from 40.88 to 21.65%. Such trend is in agreement with the research findings reported in literature, indicating that as the particle size decreases, the degree of demineralization increases.¹² The reason may be that in coarser particle sizes than −6.25+4 mm, the degree of liberation of mineral matter was poor or not enough. As far as the particle size finer than −6.25+4 mm is concerned, the liberated coal particles are contaminated with adhered slimes of clay minerals, and as a result the grade of floats reduced. It may be the reason that in the finest particle size of −4+2 mm, the ash reduction was as low as 8.1% at the specific gravity of 1.8.¹²

Figure 2.

Percent reduction in ash of float products from coal samples of different sizes at different specific gravities.

Figure 3 deals with sulfur reduction at various particle sizes and at different specific gravities. It was found that particle size of −6.25+4 mm and specific gravity of 1.4 were the optimum parameters at which the better results were obtained. The maximum sulfur reduction achieved was 86.1%. It is also noted and shown in Figure 3 that as the particle size decreased from −6.25+4 mm to the finer sizes, the sulfur reduction progressively decreased from 45 to 30%. It was in concurrence with the results reported by various research workers.^9,12 It is anticipated that the sulfur in coal under investigation, largely present in the form of pyrite, was mostly liberated at the particle size of −6.25+4 mm. Pyrite being heavier (specific gravity 4.85) than water settled down and negligibly reported in float products (specific gravity 1.4). However, in finer particle sizes, the reduction in sulfur content progressively decreased as the specific gravity increased beyond 1.4. Such observations have also been reported in literature.¹³ It is summarized that the maximum of sulfur largely appeared in sink products for particle size of 6.25 + 4mm. The pyrite particles present as coarse inclusions are weakly bound to coal macerals and were easily liberated on crushing.¹⁴ Volatile matter in float (Figure 4) obtained at a specific gravity of 1.4 and particle size of −6.25+4 mm, increased to 27%, whereas the percent increase of fixed carbon in floats (Figure 5) at the same specific gravity and particle size increased to 15.65%. At this specific gravity and particle size, the recovery of both volatile matter and fixed carbon was high, indicating the fair release of coal particles on crushing. But comparatively at coarser sizes, the recovery of volatile matter and fixed carbon, at the same specific gravity and particle size, reduced due to lesser liberation of coal particles. This behavior of combustibles at lower density media and coarser particle sizes has also been reported in literature.^9,15 In finer particle size of −4+2 mm, the recovery of volatile matter and fixed carbon was negligible because the maximum coal particles reported in floats had been recovered at −6.25+4 mm. Figure 6 shows the percent increase of GCVs at different specific gravities and coal samples of different sizes. It was found that percent increase in GCV declines as the specific gravity increases. Among all particle sizes, the maximum percent increase of GCV is at the specific gravity of 1.4 which shows better grade of float product.⁹ The best grade of clean coal, obtained at a specific gravity of 1.4 with the particle size of −6.25+4 mm, shows 19.28% increase in GCV as the maximum value of 7382 kcal/kg. The reduced values of GCV at larger particle sizes and at higher specific gravities are due to the poor grade of float products which were contaminated with greater amount of mineral matter. At the lowest particle size of −4+2 mm, the contamination of mineral matter with coal is still high and thus the GCV value is also at the lowest limit.

Figure 3.

Percent reduction of sulfur in float products from coal samples of different sizes at different specific gravities.

Figure 4.

Percent increase of volatile matter in float products from coal samples of different sizes at different specific gravities.

Figure 5.

Percent increase of fixed carbon in float products from coal samples of different sizes at different specific gravities.

Figure 6.

Percent increase of gross calorific value in float products from coal samples of different sizes at different specific gravities.

Conclusions

Heavy media separation technique was applied on indigenous low-grade coal found in Khushab district. For upgradation of the coal, an extensive experimental work was undertaken using wide range of particle sizes and specific gravities of heavy medium. The particle size of −6.25+4 mm and specific gravity of 1.4 were found to be the optimum parameters to successfully upgrade the coal. The clean coal product with 83.53% yield and reduction of 91.68% in ash and 86.11% sulfur contents was achieved in this study. Moreover, up to 19.3% enhancement of GCV in clean coal was achieved. The finally recovered clean coal containing 1.24% ash and 1.0% sulfur with a GCV of 7382 kcal/kg is a good marketable and environment friendly product that can be safely used as an energy source for all coal-fired utilities including cement and power plants.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Hafiz Sana did her MSc in Coal Technology in 2010 and MPhil in Coal Technology in 2016 from Centre for Coal Technology, University of the Punjab. She joined Centre for Coal Technology in 2013 as research officer. She has published 3 journal papers. She has supervised 12 research thesis of MSc Coal Technology students.

Sumaira Kanwal did her MSc in Coal Technology from Centre for Coal Technology, University of the Punjab in 2008 and secured a Gold Medal and MPhil in Environmental Sciences in 2009 from College of Earth & Environmental Sciences, University of the Punjab. She joined Centre for Coal Technology in 2009 as a research scholar and was later promoted as lecturer in 2011 based upon her exceptional performance. She is currently undertaking her PhD studies. She has published 6 journal papers.

Javaid Akhtar completed his PhD in Chemical Engineering from University Teknologi Malaysia, Malaysia in 2011. Earlier, he completed his MPhil in Chemical Engineering in 2005 and BSc in Chemical Engineering in 2002. He is working as an assistant professor at Centre for Coal Technology since 2006. He taught subjects on computer applications, coal mining, coal mine safety, and fluidized bed combustion. His major interests include coal technology, energy and water treatment. He has published various research articles on water treatment, coal technology, and biomass conversion in journals of international repute.

Naseer Sheikh is a legendary professor serving at Centre for Coal Technology since 2003. At the centre, he taught subjects on coal chemistry, and coal processing technologies for more than a decade. He retired as member of technology from Pakistan Council of Scientific & Industrial Research (PCSIR) Laboratories, Lahore. He worked on mineral processing, ore mining, and coal cleaning processes. He supervised many thesis on froth flotation, agglomeration, heavy media separation process for cleaning of low grade Pakistani coals.

Shahid Munir received his PhD from University of Leeds, UK in the area of clean coal technology. He has participated in many international/national conferences and workshops. His area of research is biomass-coal co-combustion and his research work has been extensively published in international and national journals of high impact factor. He has published a book titled Co-Combustion of Coal with Biomass, Energy for a Clean Environment. He is working as Professor and Director at the Centre for Coal Technology, University of the Punjab. So far, he has done the technical auditing of eight coal fired power plants as an expert of the Government.

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