Short communication

Ash composition of the main Albanian coals

S.A. Loljaa,* , H. Haxhia, Dh. Gjylib,1

aChemical Technology Sector, Faculty of Natural Sciences, University of Tirana, Tirana, AlbaniabB. Vasil Shanto, Central Laboratory of Geology, Tirana, Albania

Received 18 January 1999; received in revised form 20 May 1999; accepted 27 May 1999

Abstract

Coal ashes from 17 mines were analysed in terms of ash yield and percentages of SiO2, Al2O3, Fe2O3, CaO, MgO, K2O, Na2O, TiO2, P2O5,MnO, NiO. The composition has been analysed in terms of: (1) contribution to ash contents; (2) acidic, amphoteric and basic oxides; (3) co-ordination number in solid state; (4) fluxing components; (5) cement-constituting minerals; (6) periodical table. Significant correlations wereobserved on expressing the percentage in mol% or ion% and not in wt%. The decrease in basic oxides was associated with an increase inacidic ones.q 1999 Elsevier Science Ltd. All rights reserved.

Keywords:Chemical composition; Ash; Coal

1. Introduction

As the ash yield and composition vary considerably andas ash properties change with the composition, both meltingand hydraulic properties of coal ash also vary considerably[1–4]. Therefore, the contribution and relation of macro-components in coal ashes in a group of 17 main coalmines in Albania will be considered. The oxides determinedin this paper are: SiO2, Al2O3, Fe2O3, CaO, MgO, K2O,Na2O, TiO2, P2O5, MnO, NiO. The relations will beexpected to have an empirical equationY � AX 1 B, aswe shall look for any real important relations existing irre-spective of their cause. The purpose of this paper is tosuggest the use of any reasonable concept involving thechemical aspects of ash species whenever appropriate.The respective coal mines are located in three regionsabout 100 km by air away from each other. The chemicalcomplexes of ash compounds are partly built both uponmolecular and ionic units. In addition, the number of ionsor cations may vary within a certain group of oxides, but thenumber of oxide ions remain constant [4]. Therefore, the ashcomposition will be expressed in wt%, mol% and ion%.

The weight percentages of ash in the representative coalsamples were determined by the conventional method ISO1171. In order to reach the ultimate ash by removing all theremaining CO2 of carbonates, and H2O of hydrated silicates

and not ignited carbon, the ash obtained by the determina-tion ISO 1171 was also calcinated at 1000–11008C. Allexperimental data are given in Table 1, where the firstseven mines belongs to the Tirana region, second sevenones to the Korc¸a region and the last three to the Memaliajregion.

2. Discussions

2.1. Contribution of oxides to ash contents

Table 1 shows that ash contents of the Albanian coals fallbetween 3.55 and 23.37 wt%. Coals of Tirana and Korc¸aregions vary more in their ash contents, although each ofthese regions cover an area of about 40 km. The wide varia-tion of weight loss due to calcination (called re-loss in Table1) originates from the complexity of ash compounds stillexisting before calcination and involving carbonates,sulphates, hydrated silicates and not ignited carbon. Thelast cause is emphasised when the weight re-lost after calci-nation looks to be high.

In average, we consider the descending order of thecontents of oxides as in Table 1 to be the following: SiO2,Al 2O3, Fe2O3, CaO, MgO, K2O, Na2O, TiO2, P2O5, MnO,NiO. The same order remains if we would not regard theweight loss due to calcination. As to mol% normalised to100%, the descending order of the contents of oxidesbecame the following: SiO2, CaO, Al2O3, MgO, Fe2O3,Na2O, K2O, TiO2, P2O5, MnO, NiO. Expressing the contentsin ion% and normalised to 100%, the descending order is the

Fuel 79 (2000) 207–209

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* Corresponding author. Tel.:1 355-4227669(w).E-mail address:slolja@fshn.tirana.al (S.A. Lolja)1 Tel.: 1 355-42-23466.

following: O22, Si41, Al 31, Ca21, Fe31, Mg21, Na1, K1,Ti 41, P51, Mn21, Ni21. Table 1 shows that SiO2, Al2O3,Fe2O3, CaO, MgO consist about 86 wt% or 95 mol% ofthe coal ash mass, where almost half belongs to SiO2. Theoxide ion claims about 60% of the ionic contents of ashmass, whereas Si41, Al 31, Ca21, Fe31, Mg21 cover about36 and 17% belong to Si41 alone. The coal ashes from themines Krrabe¨-2, Mushqeta-2, M-Drenove¨-1/2, Dardhas-6have nickel percentages higher than 0.1 wt%, which maybe attractive to use these ashes as raw materials for nickelextraction. The variation was calculated in Table 1 as aquotient of the difference between the upper and lowervalues and the average value of a certain column. Thedescending order of the variation (wt%) is the following:P2O5, MgO, MnO, CaO, Na2O, K2O, NiO, Fe2O3, TiO2,Al 2O3, SiO2. If the weight loss due to calcination wouldbe omitted, then the descending order of the variationwould change to: P2O5, CaO, MnO, Na2O, MgO, NiO,K2O, Fe2O3, TiO2, Al2O3, SiO2, (either wt% or mol%normalised to 100%). Referring to ionic percentagesnormalised to 100%, the descending order of the variationis the following: P51, Ca21, Na1, Mn21, Mg21, Ni21, K1,Fe31, Ti41, Al 31, Si41, O2. The oxide ion had a very smallvariation of its contents round its average value of60.80 ion%. It was found that the contents (mol%) of bothCaO and Na2O trend to decrease with the contents of SiO2.The contents of Si41 moves in accordance with the totalcontents of oxide anions, but at the same time it is incontrary to the contents of Na1, Ca21, Ca21 1 Mg21

:

2.2. Oxides as acids and bases

The oxides may be divided into three groups as follows[5]: acids to be SiO2, TiO2, P2O5; amphoteric oxides to be

Al2O3 and Fe2O3; bases to be Na2O, K2O, CaO, MgO, MnO,NiO. Else, if divided into two groups Al2O3 would be inacids and Fe2O3 in bases [2,3]. Being divided into bothtwo and three groups, the respective groups’ contents(wt% or mol%) and their respective ratios did not correlateto the coal ash contents (wt%). A correlation coefficientR�0:962 exists between the contents of acid and basic oxides(mol%), but both these have a low correlation to thecontents of amphoteric oxides. As the contents of acidiccations increase�R� 0:950�; basic cations decrease�R�0:980� and amphoteric ones do not correlate with the totalcontents of the oxide anion.

2.3. Oxides as crystals

From Table 1, the largest quantity 31.54–80.67 mol%)belongs to oxides of the co-ordination number of 4 SiO2,Al 2O3, a smaller quantity (16.18–67.08 mol%) belongs tooxides holding a co-ordination number of 6 Fe2O3, MgO,TiO2, Fe2O3, NiO, MnO, Na2O, CaO and the smallestquantity (0.58–3.66 mol%) belongs to oxides holding aco-ordination number of 8 P2O5, K2O.

2.4. Oxides as fluxing agents

As ash fusion tests have been carried out in a reducingatmosphere (20 vol% H2, 80 vol% O2) in our laboratory, theoxides were grouped [2,5] as the following: fluxing acids tobe SiO2, TiO2, P2O5; non-fluxing acids to be Al2O3; fluxingbases to be Na2O, K2O, MnO, Fe2O3, NiO; and non-fluxingbases to be CaO, MgO. Table 1 shows that coal ashes covera range of 26.74–67.17 mol% fluxing acids, 8.06–17.97 mol% non-fluxing acids, 5.64–15.89 mol% fluxingbases and 8.96–36.89 mol% non-fluxing bases. It wasfound that: (1) the contents of fluxing acids and non-fluxing

S.A. Lolja et al. / Fuel 79 (2000) 207–209208

Table 1Experimental data on macro-components of coal ashes (wt%)

Number Mine-Stratum Ash SiO2 Al 2O3 Fe2O3 CaO MgO K2O Na2O TiO2 P2O5 MnO NiO Re-Loss Total

1 Krrabe-2 6.71 23.85 13.3 15.3 14.75 5.6 1.4 2.45 0.47 0.29 0.05 0.1 22.443 1002 Mushqeta-1 8.3 50.75 15.96 11.7 10.25 3.5 3 0.79 0.82 0.17 0.04 0.06 2.953 1003 Mushqeta-2 7.91 34 11.4 20 9.35 5 2.23 2.09 0.6 0.25 0.06 0.1 14.922 1004 Mezez-10 10.9 40.5 17.66 10.05 15.15 3.64 2.58 1.13 0.71 0.23 0.06 0.06 8.233 1005 Valias-28 22 40.08 15.57 20.36 7.01 5.54 2.25 2.87 0.73 0.14 0.1 0.05 5.3 1006 Manze-4 9.54 47 22.07 11.1 4.2 5.16 2.58 0.87 0.63 0.2 0.11 0.06 6.017 1007 Manze-5 9.25 47.4 22.12 10 3.2 4.8 3.29 0.7 0.73 0.15 0.09 0.06 7.461 1008 M-Drenove-1 4.1 47.2 12.39 17.1 3.35 15.12 0.7 0.91 0.33 0.18 0.08 0.11 2.535 1009 M-Drenove-2 4.03 48.4 12.63 14.9 1.6 18.4 0.88 0.74 0.26 0.21 0.09 0.1 1.784 10010 Dardhas-6 3.55 55.38 20.4 10.08 3.5 2.52 4.11 1.44 0.96 0.06 0.08 0.03 1.438 10011 Petrushe¨-1 4.22 52 19.56 8.8 6.65 3.42 4.72 2 0.24 0.17 0.08 0.04 2.316 10012 Alarup-1 22.9 50.64 12.95 12.38 11.74 4.66 1.2 0.91 0.58 0.29 0.14 0.01 4.504 10013 Alarup-2 23.4 38.28 18 11.98 17.17 2.02 1.3 1.03 0.86 0.86 0.22 0.01 8.274 10014 Bezhan-1 16.4 45.6 24.41 5.2 8.05 4.2 2.33 1.03 0.69 0.38 0.05 0.06 8.008 10015 Memaliaj-5 10.1 16.94 5.93 7.59 24.88 6.48 1.3 4.08 0.42 0.16 0.01 0.05 32.156 10016 Memaliaj-6 9.02 39.64 13.56 12.78 14.89 4.16 2.5 2.33 0.58 0.08 0.01 0.05 9.52 10017 Memaliaj-7 8.01 47.78 15.79 8.38 12.27 4.54 2.65 2.57 0.58 0.14 0.03 0.05 5.218 100Average 10.6 42.67 16.1 12.22 9.88 5.81 2.29 1.64 0.6 0.23 0.08 0.06 8.42Variation 1.87 0.9 1.15 1.24 2.35 2.82 1.75 2.06 1.2 3.48 2.62 1.73 3.65

ones tend to increase simultaneously; (2) the contents ofnon-fluxing bases looks to be in contrary to that of non-fluxing acids; (3) the ratio between non-fluxing acids andnon-fluxing bases appear to quickly increase when fluxingacids increase their contents; (4) the increase in ratiobetween fluxing acids and fluxing bases appear to increasethe contents of non-fluxing acids and decrease of thecontents of non-fluxing bases. The last finding may beexplained by accepting that the increase in the ratio betweenfluxing acids and fluxing bases goes through a decrease offluxing bases that cause a simultaneous decrease in non-fluxing bases. The last ones open more space for non-fluxingacids to contribute by increasing their contents. This issupported by the aforementioned findings (1)–(3) and thefair correlation �R� 0:944� between non-fluxing basicoxides and fluxing acidic ones. As to ion%, the contentsof fluxing acidic cations increase�R� 0:943� and non-flux-ing basic ones decrease�R� 0:928�; their respectivecontents versus the total contents of oxide anions whereasfluxing and non-fluxing basic cations appeared to decreasetheir contents versus fluxing acidic cations.

2.5. Oxides of cement minerals

Referring to the wt% normalised to 100%, calculations[2] showed that the values of slag coefficient were 0.587–0.707; of silica module 0.834–1.999; of alumina module0.570–4.694; of hydraulic module 0.021–0.817; of satura-tion coefficient20.277 to 0.262. All mines possess lowhydraulic capability and Memaliaj-5 only possesses a posi-tive saturation coefficient. The coal ashes of mines Dardhas-6 and Petrushe¨-1 simultaneously appear to have satisfactoryvalues of the above parameters that define the possible exis-tence of cement-constituting compounds. We may also notehere that coal ashes of mines Krrabe¨-2, Mushqeta-1/2,Mezes-10, Alarup-1/2 and Memaliaj-5/6/7 contain moreCaO than Al2O3 (mol%), indicating a possible formationof 2CaO·Al2O3·SiO2 and 2CaO·MgO·2SiO2 in slag. In addi-tion, all mines possess values higher than 0.2 for the activityindex and mine Memaliaj-5 possesses a value 1.371 higherthan 0.65 for the basic module required both by the slagcement receipt. From the aspect of correlating the cement-distinguishing oxides, it was found that both the contents(mol%) of SiO2 and SiO2 1 Al2O3 appeared to decreasewith an increasing CaO content. The same was observedbetween the contents (mol%) of SiO2 and CaO1 Al2O3:

2.6. Oxides according to Periodical Table

Staying in contrary, a better correlation�R� 0:973� isreached versus the contents of non-metal oxides SiO2,P2O5 by adding together the contents of the oxides of alka-line metals Na2O, K2O, alkaline–earth metals CaO, MgOand metals Al2O3. The same correlation coefficientremained when the contents of transition metal oxidesreplaced that of Al2O3. Considering the amphoteric beha-viour of Aluminium and Silicon, it was found that the

contents of Al2O3 1 SiO2 1 P2O5 decreased with thecontents of Na2O 1 K2O 1 CaO1 MgO �R� 0:982�.Whereas, a lower correlation exists for this relationshipwhen referring to ionic percentages. The contents of SiO2 1P2O5 followed the trend to decrease with the contents ofalkaline and alkaline-earth metaloxides. In other words,the decrease in basic oxides is associated with an increasein acidic oxides. This is in support of what we foundpreviously by viewing the oxides simply as acids, amphotersand bases.

3. Conclusions

The ashes of Albanian coals come closer to the quarter ofthe coal mass. No significant correlations existed betweenthe contents of oxides and the contents of ash in terms ofwt%. The contribution order of macro-elements to coalashes changed when expressing them from wt% to mol%or to ion%. SiO2, Al2O3, Fe2O3, CaO, MgO are the mainoxides and SiO2 can call for more than half of coal ashes.The variation in contents of oxides in coal ashes changed itsorder a little when referring the percentages from wt% tomol% or ion%. O22, SiO2 and Al2O3 have the smallestvariation in contents, while CaO is one of the most variableoxide. No significant correlation was found between the twooxides irrespective of percentage expression. The oxidesbeing assembled as acidic, amphoteric and basic ones, orsolely as acids and basics, did not correlate with coal ashcontents. The contents of cations with acidic behaviourincreased and of those with basic behaviour decreasedversus the total contents of oxide anions. The contents ofnon-fluxing basic oxides stays in contrary to the contents offluxing acidic oxides. The coal ashes of mines Dardhas-6and Petrushe¨-1 may easily join the Portland cement produc-tion and the coal ash of Memaliaj-5 may serve as a raw mate-rial for slag cement production. Having a good correlation, thecontents of oxides pertaining to elements of chemical groups1A, 2A were in contrary with the contents of oxides pertainingto elements of chemical groups 3A, 4A, 5A.

Acknowledgements

The authors wish to thank the staff at the Organic Chemi-cal Technology Sector in the Faculty of Natural Sciences ofthe University of Tirana and at the Central Laboratory ofGeology.

References

[1] Vassilev SV, Kitano K, Vassileva CG. Fuel 1997;76:3.[2] Pinguli R. Inorganic chemical technology. Part III, Tirana: University

Press, 1991 (in Albanian).[3] Manz OE. Fuel 1997;76:691.[4] Thompson D, Argent BB. Fuel 1999;78:629.[5] Gray VR. Fuel 1987;66:1230.

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