CONCLUSION

ABSTRACT

The REY concentration in coal in the study area ranged from 1.61-68.78 ppm, while the REY concentration in coal ash ranged from

45.41-2108.98 ppm. Samples of K3 on coal and coal ash had a higher REY concentration than the average REY concentration in the

world and the USA. Based on the evaluation of REY enrichment in coal ash, some samples in the study area, such as K3 and K13

have REO values of more than 500 ppm. Meanwhile, based on plotting on the REYdef-Coutl graph, all coal ash samples that have

undergone REY enrichment are classified into area II, which means it is economical to extract.

Coal ash in the study area can be enriched by REY 9 to 30 times the total REY in the feed coal. The mode of occurrence REY in coal

ash spread in all parts of its constituent components, both bound to organic and inorganic components. REY is attached to the

surface of the components that make up coal ash through a physical adsorption mechanism during the coal ashing process.

DISCUSSION

RESULT

INTRODUCTION

REGIONAL GEOLOGY

METHODS

Kalimantan is one of the largest coal-producing islands in Indonesia. Most of the coal from this area is used for power plants through the combustion process, resulting in the enrichment of rare-earth elements and yttrium (REY) in the coal ash. Production of the REY in the coal deposit is mainly from the coal's

fly/bottom ash. The REY produced will be more abundant, if the ash yield resulting from combustion is also higher. This can happen because the coal that undergoes the combustion process will remove the organic material that was burned and leave unburned carbon and mineral matter, including REY. Therefore, future use

of coal will not only focus on electrical energy generated from the combustion process in a coal-fired steam power plant, but also on valuable elements extracted from fly ash and bottom ash (FABA) which have been considered as waste, in which this can increase the selling value and type of coal product. Along with the

increasing demand for REY, research on the potential presence of REY in coal and coal ash is becoming increasingly intense. This research was conducted to give an overview of information about the REY potential of coal and coal ash in Indonesia, and the mode of occurrence of REY in coal ash used to decide the appropriate

extraction method to increase its economic value. The study area is located in Kalimantan, Indonesia, which is represented by coal samples from several clusters, including North Kalimantan (Sub-Tarakan Basin), Central Kalimantan (Upper Kutai Basin), East Kalimantan (Pasir Basin), dan South Kalimantan (Asem-Asem

Basin). This area is one of the best candidates for studying REY in coal because there are tuffaceous layers (tonsteins) reported by previous researchers that may have enriched the coal in REY. For this study, coal beds were sampled randomly and analyzed using polished section, proximate, X-ray diffraction (XRD),

inductively coupled plasma atomic emission spectroscopy (ICP-AES), inductively coupled plasma mass spectrometry (ICP-MS), and spot scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDS). Special attention was given to samples that experienced high REY enrichment. The mineralogy in the coal

ash sample with the dominant concentration average value is 28.9% feldspar group, 28.4% quartz, and 26.6% kaolinite. The results of XRD and petrographic analysis on the coal ash samples did not show the presence of REY carrier minerals in coal such as monazite, xenotime, and phosphate (Finkelman et al., 2018, 2019; Dai et

al., 2020). However, these minerals were not found in the XRD analysis of coal ash samples. This indicates that the presence of REY is not carried by REY carrier minerals, but maybe presently bound to organic material. This can happen because the rock undergoes a leaching process, so REY will be separated from its minerals

and transported by water to become REY rich solution. If the solution is associated with humic acid under certain reductive conditions and pH, REY can be bound to the humic material. Thus, REY can be found in organic materials, but not in REY carrier minerals. Component of coal ash dominated by Fe oxide (42,29%), then

followed by unburned carbon (34,14%), glass (14,11%), quartz (9,32%), and mullite (0,04%). Based on the data collected, it was concluded that the mode of occurrence of REY in coal is associated with organic material (intimate organic association), while the presence of REY in coal ash is tied to all of its constituent components

evenly with the physical adsorption mechanism. But overall, coal ash is still dominated by inorganic components (Fe-oxide), compared to organic components (unburned carbon). REY in coal and coal ash have similar distribution patterns, with LREY being dominant. The average REY concentrations in the coal and coal ash

samples that have undergone the ashing process at a temperature of 1000C for 1 hour are 12.98 ppm and 230.9 ppm, these values are smaller than the REY average concentrations in coal and coal ash in the world ranges from 68.5 ppm and 404 ppm, and the US ranges from 62.1 ppm and 517 ppm, but there is one sample of coal

and coal ash with the highest concentration in the K3 sample of 68.78 ppm and 2108.9 ppm which is higher of the average REY concentrations in coal in the world and the US (Dai et al., 2016). These data show that there is a chance to extract REY from FABA that the REY substance of the ash is enriched by more than 10 times,

compared to the REY content of the coal feed to power plants. REY concentration data can be used to evaluate the economic potential of REY on coal and coal ash for industrial purposes by using parameters, namely REOash value, critical REY percentage, and Coutl value. The average REOash concentration is 278 ppm, with

the highest REOash concentration in the K3 sample, which is 2526.86 ppm. In general, the REOash value in the study area is smaller than the REOash value in the world at 483 ppm and in America at 621 ppm (Seredin & Dai, 2012). Evaluation of the economic potential of REY can be reviewed by comparing the REOash with a

cut-off grade value of >500 ppm (Blissett et al., 2014), the REOash value that is close to a cut-off grade value of >500 ppm still has the potential for the REY extraction process economically. The average critical REY concentration in coal ash is 91.3 ppm, which is higher than the average critical REY concentration in the study of

Blissett et al., (2014) which was only 69 ppm. The value of the critical REY concentration in coal ash will be directly proportional to the average value of the critical REY percentage of 39.6% and the average Coutl value of 1.17. In general, the critical REY percentage value and the Coutl value in coal ash are higher, when compared

to the critical REY percentage value in the research of Blissett et al., (2014) which only ranges from 32.8 – 35.6%, and the Coutl value ranges from 0, 84 – 0.96. If the REYdef - Coutl graph is plotted for REY-rich coal ashes (Seredin & Dai, 2012), then the results show that the coal ash sample belongs to cluster II, where this cluster

has the potential for an economical REY extraction process which is characterized by an abundance of critical REY ranging from 30% - 51% and Coutl values ranging from 0.7 to 1.9. From these data, it can be concluded that the coal ash sample in the study area has the potential for an economical REY extraction process.

Enrichment of Rare Earth Elements and Yttrium in Kalimantan Coal Ash

Rare earth elements and yttrium (REY) are commodities for which demand has increased

significantly in the last few years (Fig. 1). The abbreviation “REY” includes the lanthanides plus

scandium and yttrium. Seredin and Dai (2012) explained the significance of coal deposits as potential

sources for REY because the abundances of those elements in coal are similar or even higher to their

concentrations in conventional REY deposits. Research on REY from coal has been studied by many

authors (Ezkenazy, 1987; Seredin, 1996; Seredin and Finkelman, 2008; Dai et al., 2011; Dai et al., 2012;

Seredin and Dai, 2012; Dai et al., 2016; Dai et al., 2017; Zheng et al., 2017; Dai and Finkelman, 2018, and

others). According to Seredin and Dai (2012), there are four types of REY enrichment in coal basins.

These types are (1) terrigenous, (2) tuffaceous, (3) infiltration, and (4) hydrothermal.

The increasing needs of REY is driven by the important commercial usage of REY for electronic

and optical industries, oil extraction and refining, automobile industries, information and nano-

technologies, nuclear reactors, and green energy developments (Seredin and Dai, 2012). One of the

alternative source of REY is coming from coal. The abundances of REY in coal deposits are similar or

even higher to their concentrations in conventional REY deposits. Thus, the concentration and mode

of occurrence REY in coal/coal ash is becoming very important parameters to determine the

extraction method.

Coal petrography was done according to ASTM D2799-05a, 2005, maceral classification followed ICCP System 1994 (ICCP, 2001; Sykorova et al., 2005; Pickel

et al., 2017), and fly ash classification followed Hower et al (2017). Proximate analysis for 35 coal samples were conducted by following ASTM Standards

D3173-03 (2005), D3174-04 (2005), and D3175-02 (2005). For all samples, ICP- MS and ICP-AES analyses were performed by ALS Canada Ltd. (Vancouver,

Canada) using the fused bead method prior to acid digestion. XRD analysis for 10 coal samples was done according to Chen (1997) and SEM-EDS analysis for

6 coal ash polished section.

Figure 1. REO production chart globally (USGS,2016)

Figure 2. Coal producing basin in Kalimantan

The Tertiary basins of Kalimantan (Barito Basin, Kutai

Basin, and Tarakan Basin) are underlain by a variety of

amalgamated terranes: continental basement in the southwest,

accreted zone of Mesozoic age, and some continental fragments

and suture zones of unknown age and origin (van de Weerd and

Armin, 1992). The Tarakan Basin is a basin located in the

northernmost part of the island of Kalimantan, in the north it is

bounded by the Semporna High, in the south it is bounded by

the Mangkalihat Mountains which separates the Tarakan Basin

from the Kutai Basin, in the west it is bounded by the Kuching

High, and to the east, it is bounded by the Makassar Strait

(Ardinata, 2019). Several coal-producing formations in the

Tarakan Basin are the Naintopo Formation, Meragoh Formation,

and Tabul Formation. Coal in this basin generally has lignite to

subbituminous rank coal.

The Kutai Basin is divided into two sub-basins, namely

Upper Kutai and Lower Kutai. The boundaries of the Upper

Kutai sub-basin are difficult to define, while Lower Kutai is

bounded by two faults with a NW-SE direction, namely the

Adang Fault in the south and the Sangkurilang Fault, Bangalon

Fault, and Tinggian Mangkalihat Fault in the north. The western

part of the basin is bounded by the Kuching High. Upper Kutai

is part of the basin that experienced uplift due to tectonic

processes in the form of an inversion in the Early Miocene,

while Lower Kutai is dominated by Neogene rocks (Moss and

Chambers, 1999). In the study area, the formation that

producing coal is Batu Ayau Formation with the coal rank is

bituminous.

The Barito Basin before the Miocene was a very large basin.

However, in the Middle Miocene, a tectonic event occurred,

Meratus Mountains were uplift, which caused the basin to be

divided into the Barito Basin, Pasir Basin, and Asem-Asem Basin.

Therefore, the three basins have almost the same stratigraphic

sequence.

Pasir Basin is bounded by the Meratus Mountains in the

west, the Asem-Asem Basin in the south, and the Adang Fault in

the north which separates it from the Kutai Basin. The formation

of this basin began in the Late Cretaceous, marked by a collision

between the Paternoster microcontinent and Southwest

Kalimantan which caused extensional deformation in the Early

Tertiary due to oblique convergence (Satyana et al., 1999). As a

result of this deformation, a fracture with the direction of the

NW-SE trend is formed.

The Asem-Asem Basin is separated from the Barito Basin by

the Meratus Mountains. The western part of the basin is bounded

by the Sunda Shelf. The Asem-Asem Basin was deposited in a

deltaic environment or a transitional depositional environment

which becomes a fluvial sediment accumulation zone at the mouth

of the river.

In the research area, the coal-producing formation in the

Pasir Basin is the Warukin Formation, while in the Asem-Asem

Basin there are two coal-producing formations, namely the

Warukin Formation and the Dahor Formation. Coal produced by

the two basins is included in lignite - subbituminous coal.

Coal samples from the study area have an average REY concentration of 12.95 ppm, much lower than the average of REY in

world coal (68.5 ppm) (Seredin and Dai, 2012). The exception is the K3 sample from North Kalimantan Cluster, the sample has the

same REY content as the world average REY of coal, the concentration is 68.78 ppm. Meanwhile, the REY concentration in coal ash

could be higher and enriched more than 10 times than the total REY in the feed coal. For example, the coal samples from East

Kalimantan Cluster (SA14-SA18) enriched 10-13 times, and the coal samples from South and North Kalimantan Cluster can be enriched

9 to 30 times of the total REY in the feed coal. Although REY in coal does not experience enrichment, REY in coal ash still has the

potential for extraction. Before extraction, it is necessary to evaluate the economics of REY.

REY economic evaluation can be done through two parameters. The first parameter uses the REO value and the second uses the

critical REY and Coutlook percentages than plotting it on the REYdef-Coutl graph. REO calculations are carried out because in nature

REY is not found as a free element but binds to other elements and forms oxide compounds. Coal ash in the study area has an average

REO of 231.69 ppm, but in some samples such as K3 (2,526.86 ppm), K12 (474.22 ppm), and K13 (540.32 ppm) it can be almost the same

or even exceed the REY average in world coal ash (483 ppm). Seredin and Dai (2012) in their research set the minimum cut-off grade

for REO to be economically extracted is 1000 ppm. However, Blissett et al., (2014), used a lower cut-off grade of 500 ppm. The

determination of the lower limit value is based on REY commodity prices in line with the demand for REY, so it is necessary to

readjust the limit value. As a result, only samples K3 and K13 exceeded Blissett's cut off grade. The second parameter, all the enriched

REY coal ash samples belongs to area II which means it is promising and economical for extraction. Cluster II is characterized by 30% <

REYdef < 51 and the Coutlook value is < 0.7 Coutl < 1.9.

SEM-EDS analysis has a detection limit of 0.01% or 100 ppm, so this analysis cannot detect the presence of REY in coal ash and

the mode of REY presence in coal ash cannot be determined. REY which is bound to organic material in coal will be released during

the ashing process and the REY (non-volatilize elements) will be spread throughout the constituent parts of coal ash and REY attaches

to the surface of the coal ash constituent components through the physical adsorption mechanism during the coal ashing process.

REY in coal ash is likely to be spread throughout its constituent components, both bound to organic and inorganic components. REY

can be bound to the organic components that make up coal ash in the form of unburned carbon (UC) which is indicated by the

abundance of element C. Meanwhile, REY to the inorganic components that make up coal ash can be bound to silicate, aluminasilicate,

and ferrite minerals which are composed of a combination of Si, Al, and Fe elements, which are present dominantly in the inorganic

material that makes up coal ash.

Central Kalimantan Cluster

East Kalimantan Cluster

North Kalimantan Cluster

South Kalimantan Cluster

cut-off grade

K1-K17

12

3

SA15

K3

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Anggara, F., Cikasimi, M., Rahmat, B., Wibisono, S.A., dan Susilawati, R., 2019, Karakteristik dan Genesa Pengayaan Unsur-Unsur Tanah Tarang pada Batubara Lapangan Batubara Muara Tiga Besar Utara, Tanjung Enim, Cekungan

Sumatra Selatan: Buletin Sumber Daya Geologi, v. 14, p. 198– 212.

Anggara, F., T.B.M. Petrus, H., Patria, A.A. & Bangun, A.S. 2020. Preliminary Study of Rare Earth Element and Yttrium (REY) Content of Coal In Sangatta Coalfield, East Kalimantan, Indonesia. Indonesian Journal on Geoscience, 7, 305-

314, https://doi.org/10.17014/ijog.7.3.305-314.

Besari, D.A. & Anggara, F. 2018. Komponen Penyusun Fly Ash dan Bottom Ash Beberapa Pulau Jawa. Proceeding, Seminar Nasional Kebumian Ke-11, 493-504.

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Chen, P.Y. 1977. Table of Key Lines in X-ray Powder Diffraction Patterns of Minerals in Clays and Associated Rocks. Authority of The State of Indiana, 77.

Courteney, S., Cockcroft, P., Lorenz, R., Miller, R., Ott, H.L., Prijosoesilo, Wright, A.W.R. & Wiman, S.K. 1991. Indonesian Oil and Gas Field Atlas. Kalimantan, Indonesian Petroleum Association, v. 5.

Dai, S., Hower, J.C. & Finkelman, R.B. 2020. Organic associations of non-mineral elements in coal: A review. International Journal of Coal Geology, 218, 103347, https://doi.org/10.1016/j.coal.2019.103347.

Finkelman, R.B., Palmer, C.A. & Wang, P. 2018a. Quantification of the modes of occurrence of 42 elements in coal. International Journal of Coal Geology, 185, 138-160, https://doi.org/10.1016/j.coal.2017.09.005.

Finkelman, R.B., Palmer, C.A. & Wang, P. 2018b. Quantification of the modes of occurrence of 42 elements in coal. International Journal of Coal Geology, 185, 138-160, https://doi.org/10.1016/j.coal.2017.09.005.

Finkelman, R.B., Dai, S. & French, D. 2019. The importance of minerals in coal as the hosts of chemical elements: A review. International Journal of Coal Geology, 212, 103251, https://doi.org/10.1016/j.coal.2019.103251.

Manurung, H., Rosita, W., Anggara, F., Petrus, H.B.T.M., Bendiyasa, I.M. 2020. Leaching of REY from Non-magnetic Coal Fly Ash with Acetic Acid.: IOP Conf. Series: Materials Science and Engineering.

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Prihutami, P., Prasetya, A., Sediawan, W.B., Petrus, H.TBM., Anggara, F. 2021. Study on Rare Earth Elements Leaching from Magnetic Coal Fly Ash by Citric Acid: Journal of Sustainable Metallurgy, https://doi.org/10.1007/s40831-021-

00414-7

Figure 3. Stratigraphic of Barito, Kutai, and Tarakan Basin (Satyana et al., 1999)

Indonesia has huge coal resources, around 140.47 × 10^9 tons (Handbook of Energy and Economic Statistics of Indonesia, 2020), however there is very limited

study on economic mineral in coal and coal by product including REY. Anggara et al. (2018;2019) studied the effect of tonstein layer in enrichment process of REY in

coal from south Sumatra basin, Indonesia and showed that the coal is enriched beneath the tonstein layer. They proposed the enrichment processes of REY

beneath the tonstein layer, however mode of occurrence of the REY in coal hasn’t determine yet. Recent study on the study of REY in coal and coal ash are

conducted by Anggara et al, 2020, 2021; Prihutami et al., 20201; Rosita et al., 2020; Manurung et al., 2020 and others.

There are many coal basin in Indonesia (see Figure 2) with different geological setting. Coal bearing formation in Barito, Kutai, and Tarakan Basin, Indonesia is

selected as research area. The most significant economic potential coal eastern and central Kalimantan in which Miocene coal is widespread and more economical

to be mined while in Southeast Kalimantan is Eocene coal (Fig. 3). Thus, the objective of the study are (1) to characterize coal samples; (2) to examine the REY

concentration in coal and, coal ash; (3) to examine REY’s mode of occurrence in coal ash.

Figure 5. Total REO in coal ash

Figure 6. The Comparison of total REY in feed coal and coal ash

Figure 7. Plotting on REYdef-Coutl Graph (Seredin and Dai, 2012)

Figure 8. SEM-EDS Analysis in sample K3

Figure 9. SEM-EDS Analysis in sample SA15

Ferian Anggara

1,2

,Kevin

3

,Shola Aulia Wahyudina

3

,Basuki Rahmat

4

,S.S. Rita Susilawati

4

,D. Hendra Amijaya

1,2

,Himawan T.B.M. Petrus

2,5

Department of Geological Engineering, Universitas Gadjah MadaUnconventional Geo-resources Research Group, Faculty of Engineering, Universitas Gadjah Mada;

Undergraduate Program, Department of Geological Engineering, Universitas Gadjah Mada; Center for Mineral, Coal, and Geothermal Resources, Geological Agency;

Department of Chemical Engineering (Sustainable Mineral Processing Research Group), Universitas Gadjah Mada

1

2

3

4

5

Figure 3. Ash yield

Figure 4. Petrographic of coal ash combustion

Figure 5. XRD analysis result data