Desulfurization and Deashing of Hazro Coal by Selective OilAgglomeration in Various Water Mediums

Halime Abakay Temel* and Fatma Deniz Ayhan

Department of Mining Engineering, Dicle UniVersity, 21280 Diyarbakir, Turkey

ReceiVed February 22, 2006. ReVised Manuscript ReceiVed June 22, 2006

The aim of this study was to study the effects of various water mediums on desulfurization and deashing ofHazro coal by the agglomeration method. For this purpose, three groups of agglomeration experiments weremade. The effects of some parameters that markedly influence the effectiveness of selective oil agglomeration,such as solid concentration, bridging liquid concentration, and pH, on the agglomeration were investigated inthe first group of experiments. The effects of different salts (NaCl, MgCl2, and FeCl3) on the agglomerationwere investigated in the second group of experiments. The effects of lake water and sea water on theagglomeration were investigated in the third group of experiments. The influences of the Mediterranean Seawater and Aegean Sea water on the removal of ash and total sulfur were found to be important.

Introduction

High-ash and high-sulfur coals are unsuitable for efficientuse in carbonization, combustion, gasification, liquefaction, etc.purposes.1,2 Coal usually contains a significant quantity ofdifferent metallic and nonmetallic impurities that cause envi-ronmental or process problems in its usage cycle. Because ofthis fact, many studies have been carried out to reduce thesepolluting impurities in coal, including those of ash, sulfur, andsilicates. The existence of sulfur compounds in coal limits itsindustrial application because of environmental as well astechnical problems. Harmful effects on agricultural products,disruption of the natural equilibrium of the ozone layer,corrosion of metal structures, and respiratory problems ofhumans and animals are undesirable effects of sulfur and itscompounds.3,4

The conventional coal beneficiation methods are inefficientin the cleaning of fine coal particles. Therefore, flotation,selective flocculation, and oil agglomeration methods havegained importance to clean fine particles.5,6

One of the more promising methods for cleaning coal involvessuspending finely ground coal in water and selectively ag-glomerating the more hydrophobic and oleophilic componentswith oil or low-molecular-weight hydrocarbons such as pentaneor heptane as the suspension is agitated vigorously.7-9

The organic macerals tend to be agglomerated in preferenceto the inorganic minerals. Dependent upon their relative sizeand density, the agglomerates can be recovered from thesuspension by floating, skimming, or screening. It was observedthat the effects of pH and ionic strength on coal recovery andseparation efficiency depend upon the surface properties of theparticles.10-13

An increase in the ionic strength was found to favor therecovery of hydrophobic, oleophilic coals and to disfavor therecovery of weakly oleophilic, hydrophilic coals and pyrite.13

It was further observed that the effect of ionic strength on therecovery of a hydrophobic coal depends upon the relativehydrophobicity of the material.14

Thus, a greater effect was observed with a strongly hydro-phobic coal than with a weakly hydrophobic coal. The effectof increasing ionic strength on hydrophobic coals was believedbecause of the compression of the electrical double layersurrounding individual particles, while the effect on hyrophiliccoals was thought because of the adsorption of hydratedcations.10,11,13

In this study, the effects of various water mediums on theagglomeration of Hazro coal were studied, and the experimentalresults are presented here.

Experimental Section

1. Materials. The coal sample used in this work was obtainedfrom Hazro, Turkey. Proximate analysis, ultimate analysis, andpetrographic analysis of the coal sample and major element contentsof the coal ash sample are given in Tables 1-4.

Proximate and ultimate analysis were performed by using Turkishand ASTM standards.16

* To whom correspondence should be addressed. E-mail: habakay@dicle.edu.tr.

(1) Mukherjee, S.; Borthakur, P. C.Fuel 2003, 82, 783-788.(2) Mukherjee, S.; Borthakur, P. C.Fuel 2001, 80, 2037-2040.(3) Abdollayh, M.; Moghaddam, A. Z.; Rami, K.Fuel 2006, 85, 1117-

1124.(4) Demirbas¸, A. Energy ConVers. Manage.2002, 43, 885-895.(5) Capes, C. E.Coal Preparation, 5th ed.; SME: Littleton, CO, 1991;

Vol. 9 (part 4), p 1021.(6) Capes, C. E.; Coleman, R. D. Proceedings of 4th International

Symposium on Agglomeration; Toronto, Canada, 1985, pp 857-866.(7) Wheelock, T. D.; Markuszewski, R.The Science and Technology of

Coal and Coal Utilization; Cooper B. R., Ellingson W. A., Eds.; PlenumPress: New York, 1984; pp 47-123.

(8) Steedman, W. G.; Krishnan, s. V.Fine Coal Processing; Mishra S.K., Klimpel R. R., Eds.; Noyes Publications: Park Ridge, NJ, 1987; pp179-205.

(9) Keller, D. V.; Burry, W. M., Jr.Coal Prep.1990, 8, 1-17.

(10) Fan, C. W.; Markuszewski, R.; Wheelock, T. D.Fizykochem. Probl.Mineralurgii 1987, 19, 17-26.

(11) Yang, G. C. C.; Markuszewski, R.; Wheelock, T. D.Coal Prep.1988, 5, 133-146.

(12) Sadowski, R.; Venkatadri, J. M.; Druding, R.; Markuszewski, R.;Wheelock, T. D.Coal Prep.1988, 6, 17-34.

(13) Fan, C. W.; Hu, Y. C.; Markuszewski, R.; Wheelock, T. D.EnergyFuels1989, 3, 376-381.

(14) Wheelock, T. D.; Markuszewski R.; Fan C. W.; Hu Y. C.; TysonD. Fossil Energy Quarterly Report for April 1, Ames, IA, 1988, IS-4975.

2052 Energy & Fuels2006,20, 2052-2055

10.1021/ef060079e CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 08/09/2006

It was determinated that the rank of the coal sample was sub-bituminous coal according to petrographic analysis results.17

The coal sample was ground (dry) to a nominal top size of-0.1mm in a ball mill (Denver type) for agglomeration tests. The screenanalysis of the coal sample is given in Table 5.

2. General Method. The agglomeration experiments wereperformed in a 250 mL beaker with four baffles at the borders tocreate turbulence using a FRAMO-Geratetechnik, LR20-type me-chanical stirrer. The agitation was provided by a centrally locatedflat blade turbine impeller (consisting of four blades, 50 mm indiameter and 10 mm in width) at a fixed distance from the bottomof the vessel.

The coal-water suspension of a given solid content (wt %) wasprepared. The suspension was conditioned at 1800 rpm for 35 min.Afterward, an appropriate amount of oil (mL of oil/g of coal) wasadded, and mixing was continued for another 10 min. Afteragglomeration, the suspension was transferred to a 0.106 mm (140mesh, U.S. Standard) screen. The agglomerates were taken asoverscreen products, and the tailings were taken as underscreenproducts. The agglomerates and tailings were filtered and, afterward,dried in an oven at 100-105°C. Ash and total sulfur contents weredetermined for both the agglomerates and tailings.

A total of 50% kerosene plus 50% fuel oil (number 4) was usedas bridging liquid. The density of 50% kerosene plus 50% fuel oil(number 4) was determined as 0.84 g/cm3. Tap water was used asthe water for the suspension (pH∼ 7.5) in the first group ofexperiments. NaOH and H2SO4 were used as pH modifiers in thethree group experiments.

NaCl, MgCl2, and FeCl3 were used in the second group ofexperiments. The salts were of reagent-grade chemicals. TheMediterranean Sea water, Aegean Sea water, and soda lake watertaken from the Mediterranean Sea (Mersin, Turkey), Aegean Sea(Izmir, Turkey), and soda lake (Van, Turkey), respectively, andwere used in the three group experiments.

In the evaluation of experimental results, total sulfur and ashcontents of products were considered.

Results and Discussion

1. The First Group of Agglomeration Experiments. Theeffects of the solid concentration, bridging liquid concentration,and pH on the agglomeration were investigated in the first groupof experiments. The operating conditions of the agglomerationtests were as follows: stirring speed, 1800 rpm; bridging liquid,50% kerosene plus 50% fuel oil; bridging liquid concentration,40% (mL of oil/g of coal); pH 6.

The operating conditions were determined according to pre-experiments.

1.1. The Effect of the Solid Concentration.The effect ofthe solid concentration was established. Test results are givenin Figure 1.

As shown in Figure 1, the best solid concentration fordesulfurization and deashing was found to be 10% solids. Ashcontent and combustible yield of the agglomerate obtained at10% solids were 19.10 and 74.45%, respectively. The total sulfurcontent of the coal was reduced from 6.90 to 3.53% at 10%solids.

1.2. The Effect of the Bridging Liquid Concentration. Theeffect of bridging liquid was established. The bridging liquidconcentration was varried between 10 and 50%. Test resultsare given in Figure 2.

As shown in Figure 2, increasing the amount of 50% keroseneplus 50% fuel oil decreased both ash and total sulfur contentsof the agglomerates. A bridging liquid concentration of 45%was established as being the best, because of the low ash content(18.45%) and high total sulfur reduction (53.33%) of theagglomerate obtained. It was clear that the combustible yield

(15) Ayhan, F. D.; Abakay, H.; Saydut, A.Energy Fuels2005, 19, 1003-1007.

(16) Sevinc¸ , M. Yurt Madenciligini Gelistirme Vakfı, 1997.(17) Abakay, H. Master’s Thesis, Dicle University, Turkey, 2001.

Table 1. Proximate Analysis Results of the Coal Sample15

component as received air dried drieda

moisture (%) 2.76 1.99ash (%) 24.57 24.77 25.27volatile matter (%) 34.90 35.18 35.89fixed carbon (%) 37.76 38.06 38.84upper heating value (kcal/kg) 5890 5937 6058total sulfur (%)pyritic sulfur (%) 6.90 6.90 7.00sulfate sulfur (%) 4.95organic sulfur (%) 0.10organic sulfur (%) 1.85

a The sample was dried to constant mass at 105°C.

Table 2. Ultimate Analysis Results of the Coal Sample15

ultimate analysis(daf) ultimate analysis(daf)

C 70.24 N 0.66H 5.67 O (diff.) 16.43S (total) 7.00

Table 3. Petrographic Analysis Results of the Coal Sample17

maceral group percent by volume

huminite 72liptinite 6inertinite 6pyrite 6clay and silicate minerals 10

Table 4. Major Element Contents of the Coal Ash Sample15

components composition (%) components composition (%)

SiO2 42.0 MgO 0.5Fe2O3 16.5 Na2O 0.2Al2O3 33.9 K2O 0.9CaO 0.9 SO3 0.8

Table 5. Size Analysis of the Coal Sample15

size fraction(mm)

amount(wt %, dry)

cumulative amount under size(wt %)

-0.106+ 0.075 12.70 100.00-0.075+ 0.053 17.80 87.30-0.053+ 0.045 14.90 69.50-0.045+ 0.038 18.40 54.60-0.038 36.20 36.20total 100.00

Figure 1. Effect of the solid concentration on agglomeration (bridgingliquid concentration, 40%).

Desulfurization and Deashing of Hazro Coal Energy & Fuels, Vol. 20, No. 5, 20062053

increased with an increasing oil concentration. For S¸ ırnakasphaltite-containing high-ash content, similiar results werereported.18

1.3. The effect of pH.The effect of pH was established. Testresults are given in Figure 3. As shown in Figure 3, the totalsulfur contents of the agglomerates obtained at pH 6, 7, and 8were 3.22, 3.09, and 2.91%, respectively, in which the sulfur-bearing minerals were agglomerated at minimum levels. Thetotal sulfur contents of the agglomerates obtained at pH 3, 4,and 5 were 4.03, 3.81, and 3.52%, respectively, in which thetotal sulfur reductions were higher than the other pH valuesbecause the agglomeration of sulfur-bearing minerals was high.

In an aqueous suspension, the surface of pyrite can oxidizeunder acidic conditions to form both elemental sulfur and sulfate,whereas under basic conditions at low temperature, the surfacecan oxidize to produce a surface layer of iron oxides.19 Whenthe oxidized pyrite was suspended in an acidic solution, itbehaved like a hydrophobic material and agglomerated readilywith heptane, but when it was suspended in a basic solution, itbehaved like a hydrophilic material and did not agglomerate.

The oxidized surface appeared to be coated with both ahydrophilic material such as basic ferric sulfate and a hydro-phobic material such as elemental sulfur. In an acidic solution,the hydrophilic material was dissolved, leaving the hydrophobicmaterial, while in a basic solution, the hydrophilic materialbecame hydrated and dominated the surface characteristics.20

The practical application of pH control in suppressing theagglomeration of pyrite by oil was demonstrated by Leonard et

al.,21 who showed that the maximum reduction of the pyriticsulfur content of an Iowa bituminous coal was achieved by usinga pH of 9-11 during agglomeration. The best pH value fordesulfurization was 8.

The ash contents of agglomerates obtained at pH 3 and 4 were20.95 and 20.50%, respectively, in which the ash contents ofthe agglomerates were higher than those at the other pH valuesbecause ash-forming minerals were agglomerated at high ratios.

Ayhan et al.15 found that the zero point of charge of Hazrocoal is located at pH 7.0. The greatest combustible yield wasrealized at the isoelectric point for the coal, where the hydro-phobicity of the coal surface should have been a maximum.Similarly, in this study, the response of Hazro coal to agglom-eration at pH 7 was good depending upon the effect of the iso-electric point. Therefore, the best pH value for deashing was 7.

The best agglomeration conditions were as follows: solidconcentration, 10%; bridging liquid concentration, 45%; pH 7.

It was found that, when Hazro coal was subjected to threecleaning flotation processes, a clean coal that contained 1.50%pyritic sulfur and 14.16% ash with 69.70% pyritic sulfurreduction was obtained.15 However, when Hazro coal wassubjected to agglomeration at the best conditions, an agglomerateproduct that contained 3.09% total sulfur and 17.14% ash with55.22% total sulfur reduction was obtained. It was clear thatthe agglomeration method did not discriminate well betweencoal- and sulfur-bearing mineral in coal. The results areconsistent with those reported by Leonard et al.21

2. The Second Group of Agglomeration Experiments.Theeffects of NaCl, MgCl2, and FeCl3 on the agglomeration wereestablished. The best agglomeration conditions were used inthe second group of experiments. Test results are given inFigures 4-6.

2.1. The Effect of NaCl. As shown in Figure 4, the ashcontent of agglomerates decreased to 300 mg/L and then

(18) Abakay, H.; Ayhan, F. D.; Kahraman, F.Fuel 2004, 83, 2081-2086.

(19) Hiskey, J. B.; Schlitt, W. J.Interfacing Technologies in SolutionMining; AIME: New York, 1982; pp 55-74.

(20) Drzymala, J.; Wheelock, T. D.Coal Prep.1992, 10, 189-201.(21) Leonard, W. G.; Greer, R. T.; Markuszewski, R, Wheelock, T. D.

Sep. Sci. Technol.1981, 16, 1589-1609.

Figure 2. Effect of the bridging liquid concentration on agglomeration(solid concentration, 10% solids).

Figure 3. Effect of pH on agglomeration (solid concentration, 10%solids; bridging liquid concentration, 45%).

Figure 4. Effect of NaCl on agglomeration.

Figure 5. Effect of MgCl2 on agglomeration.

2054 Energy & Fuels, Vol. 20, No. 5, 2006 Temel and Ayhan

increased. Increasing the amount of NaCl decreased the totalsulfur content of agglomerates. The best results for desulfur-ization and deashing were obtained at 450 and 300 mg/L,respectively. The total sulfur reduction of agglomerates obtainedat 450 mg/L was 65.07%. At 300 mg/L, the ash content ofagglomerates was 16.23%.

2.2. The Effect of MgCl2. As shown in Figure 5, increasingthe amount of MgCl2 decreased the ash content of agglomeratesand increased the total sulfur content of agglomerates. The bestresults for desulfurization and deashing were obtained at 200and 450 mg/L, respectively. The total sulfur reduction and totalsulfur content of the agglomerate obtained at 200 mg/L were58.70 and 2.85%, respectively. The ash content and combustibleyield of the agglomerate obtained at 450 mg/L were 17.10 and73.47%, respectively.

2.3. The Effect of FeCl3. FeCl3 was shown to be an effectivedepressant for sulfurized pyrite.20 Also, FeCl3 was used by Bakeret al.22 as a pyrite depressant for coal pyrite. As shown in Figure6, the best results for desulfurization and deashing were obtainedat 450 and 300 mg/L, respectively. The total sulfur reductionand total sulfur content of the agglomerate obtained at 450 mg/Lwere 60.43 and 2.73%, respectively. The ash content andcombustible yield of the agglomerate obtained at 300 mg/L were19.66 and 70.48%, respectively.

3. The Third Group of Agglomeration Experiments. Theeffects of the Mediterranean Sea water, Aegean Sea water, andsoda lake water on agglomeration were established. The bestagglomeration conditions were used in the third group ofexperiments.

3.1. The Effect of Soda Lake Water.For this experiment,the soda lake water was mixed with tap water at different ratios.The soda lake water ratio was varied between 1 and 100%. Testresults are given in Figure 7.

As shown in Figure 7, the ash content of agglomeratesdecreased to a 10% ratio and then increased. Increasing the ratioof soda lake water increased the total sulfur content ofagglomerates.

The best results for desulfurization and deashing wereobtained at 1 and 10% ratios, respectively. The total sulfurreduction and total sulfur content of the agglomerate obtainedat a 1% ratio were 61.74 and 2.64%, respectively. The ashcontent and combustible yield of the agglomerate obtained at a10% ratio were 16.82 and 75.93%, respectively.

3.2. The Effects of the Mediterranean Sea Water andAegean Sea Water.Test results are given in Table 6. The bestagglomeration conditions were used in the experiments. Ex-perimental conditions were as follows: stirring speed, 1800 rpm;

bridging liquid, 50% kerosene plus 50% fuel oil; solid concen-tration, 10%; bridging liquid concentration, 45%; pH 7.

As shown in Table 5, the total sulfur contents of agglomeratesachieved with the Mediterranean Sea water and Aegean Seawater were 2.19 and 2.23%, respectively. The combustible yieldsobtained in the usage of the sea waters were higher than thoseobtained at the other mediums. Also, the combustible yieldsobtained in the agglomeration of S¸ ırnak asphaltite were foundto be high when the Mediterranean Sea water and soda lakewater were used in agglomeration medium.15

Conclusions

The results obtained from this study are as follows: (1) Threegroup of agglomeration experiments were conducted on variouswater mediums. (2) The effects of the solid concentration,bridging liquid concentration, and pH on agglomeration wereinvestigated in the first group of experiments. The best ag-glomeration conditions were as follows: pH 7; solid concentra-tion, 10%; bridging liquid concentration, 45%. (3) The effectsof three salts (NaCl, MgCl2, and FeCl3) on agglomeration wereinvestigated in the second group of experiments. The usage ofNaCl, MgCl2, and FeCl3 in the agglomeration medium had apositive effect on the reduction of ash and sulfur content ofagglomerates. NaCl was the most effective of the investigatedsalts, in regard to removing total sulfur and ash from the coalsample. (4) The effects of the soda lake water, MediterraneanSea water, and Aegean Sea water on agglomeration wereinvestigated in the third group of experiments. (5) Agglomera-tion results indicated that, when compared to various watermediums, the following order for the ash content was ob-tained: Mediterranean Sea water< Aegean Sea water< NaCl< soda lake water< FeCl3 < MgCl2 < tap water, and thefollowing order for the reduction of total sulfur was obtained:Mediterranean Sea water> Aegean Sea water> NaCl > sodalake water> FeCl3 > MgCl2 > tap water. When the Mediter-ranean Sea water was used as an agglomeration medium, anagglomerate product containing 2.19% total sulfur and 14.24%ash with a total sulfur reduction of 68.26% was obtained froma feed that contained 6.90% total sulfur and 24.77% ash.

EF060079E(22) Baker, A. F.; Miller, K. J. U.S. Bureau of Mines Report of

Investigations RI 7518, 1971.

Figure 6. Effect of FeCl3 on agglomeration. Figure 7. Effect of the soda lake water agglomeration.

Table 6. Effects of the Mediterranean Sea Water, Aegean SeaWater, and Tap Water on Agglomeration

agglomeratesMediterranean

Sea waterAegean

Sea watertap

water

ash (%) 14.24 15.71 17.14total sulfur (%) 2.19 2.23 3.09total sulfur reduction (%) 68.26 67.68 55.22combustible yield (%) 81.87 78.26 78.83

Desulfurization and Deashing of Hazro Coal Energy & Fuels, Vol. 20, No. 5, 20062055