Distribution of As within Magnetic and Non-Magnetic Fractions of Fluidized-Bed Coal Combustion Ash

Abstract

Separation of coal ash into magnetic and non-magnetic fractions facilitates their utilization when processed separately. Due to desulphurization additives added to coal during the fluidised-bed combustion, non-magnetic fractions often contain elevated CaO levels (while magnetic concentrates are typically rich in Fe₂O₃). Both CaO and Fe₂O₃ are known for their ability to bind As during the combustion, whose distribution is a crucial parameter in terms of proper utilization of these fractions. Therefore, the study deals with the As partitioning within magnetic and non-magnetic fractions of fluidized-bed coal combustion ashes. Two different (successive) procedures of dry magnetic separation were used to separate each ash into strongly magnetic, less magnetic, and a non-magnetic fraction. Due to their optimal utilization, the concentrations of As and other target elements in these fractions were evaluated and compared. Magnetic concentrates from the first separation step (in vibrofluidized state) contained 60–70% Fe₂O₃, magnetic concentrates separated manually out of the residues after the first separation contained 26–41% Fe₂O₃, and the non-magnetic residues contained 2.4–3.5% Fe₂O₃. Arsenic levels were the highest in the non-magnetic residues and gradually decreased with the increasing Fe₂O₃ content in the magnetic fractions. The dominant As association in the studied samples was to CaO (r = +0.909) and with SO₃ (r = +0.906) whereas its joint occurrence with Fe₂O₃ was improbable (r = −0.834).

1. Introduction

Coal has long been a reliable energy source and currently, it is also intensively studied as the source of critical elements, such as REE and Y [1,2]; Ge and Ga [3,4,5]; Li [6]; Nb [5,7]; U, Se, and Re [6]; or Ta, Zr, and Hf [7]. However, due to the presence of minor and trace elements, it can be responsible for serious pollution problems [8,9,10,11]. In general, during coal combustion, minor and trace elements are redistributed among bottom ash (BA) or slag, fly ash (FA), and emissions. Redistribution among these coal combustion products is determined by the operational conditions [12,13], coal characteristics [14,15], co-combustion of coal with wastes [16,17,18], or addition of desulphurization additives [19,20].

Coal combustion is still the main anthropogenic source of As, a toxic and volatile element with potential carcinogenic properties and bioaccumulation tendency in the environment [21]. The average concentration of As in world lignite and bituminous coal is 7.4 and 9.0 ppm, respectively [22]. The shares of As in bottom ash, fly ash, and emissions vary considerably depending on the total ash distribution, combustion temperature, amount and type of desulphurization additive, overall composition of the ashes, and flue gas and type (and working temperature) of the particulate control device. However, since As is quite volatile, its share in emissions is not generally negligible.

The release of As into the environment relates not only to emissions (in the finest fly ash particles or emitted in gaseous form) [11], but is also connected with its presence in coal combustion ashes. Namely, in fly ash, arsenic could be highly enriched [3]. Power stations produce a vast amount of solid wastes annually; therefore, the reuse of these wastes not only produces valuable components but also facilitates mitigation of the problems with waste storage/disposal [23].

Magnetic separation is widely used for ash separation into two basic components: magnetic and non-magnetic fractions [24,25] (the latter can be even further classified by chemical methods [26]), thereby facilitating utilization of these fractions if processed separately (e.g., non-magnetic fraction as a concrete component [27] and the magnetic one as a substitution of natural magnetite in dense medium (coal) cleaning applications) [28,29].

Typical components of non-magnetic fractions of fluidized-bed combustion ashes are Ca-bearing minerals (in addition to aluminosilicates and quartz), while in the magnetic fractions, Fe₂O₃ typically prevails. Since both CaO [30,31,32,33] and Fe₂O₃ [34,35,36,37] can bind (and accumulate) As [38,39,40,41,42,43] during coal combustion, its distribution within these two fractions is crucial in terms of ecological utilization of magnetic concentrates and the non-magnetic residues.

Circulating fluidized-bed combustion is a modern technology. In comparison to PCC, it is advantageous due to its high combustion efficiency and low NO₂ and SO₂ emissions [44,45,46,47]. It is worth mentioning in this context that the presence of Ca-bearing additives (preferentially used for desulphurization) is also beneficial for the retention of As, thereby decreasing its emissions [48]. However, even if the levels of Fe-bearing minerals in fluidized-bed combustion ashes are typically lower (than those of CaO), they can also accumulate As [36].

Therefore, this study deals with the As partitioning within magnetic and non-magnetic fractions of fluidized-bed coal combustion ashes. For detailed elucidation, two different (successive) procedures of dry magnetic separation were used to separate each sample into strongly magnetic, less magnetic, and non-magnetic fractions. With the view of their optimal utilization, the concentrations of As and other target elements in these fractions were evaluated and compared in order to reveal As associations.

2. Materials and Methods

Bottom ash (BA) was collected at an atmospheric circulating fluidized-bed power station, where bituminous coal was combusted with dry desulphurization additive at an approximate temperature of 850 °C. Ash samples were collected at regular time intervals and after mixing, ca. 3–5 kg samples were set apart for laboratory study by quartering. From these bulk ash samples, 3 particle-size fractions were prepared on sieves (when dry): <0.2, 0.2–0.4, and >0.4 mm.

In order to simulate industrial separation into highly magnetic, less magnetic, and non-magnetic fractions, magnetic fractions were separated by a two-step procedure from these 3 particle-size fractions: (i) first, in a vibrofluidized state and then (from the residue after the first separation), (ii) manually using a hand-held magnet. Separation in the vibrofluidized state removed highly magnetic particles while the separation with the hand-held magnet removed less magnetic grains (that was not retained during vibrofluidized separation). Both separations were conducted from the dry sample to avoid leaching of the studied elements.

The concentrations of the target elements in the aforementioned samples were determined by a wavelength-dispersive X-ray fluorescence spectrometer ARL PERFORM’ X with 4200 W X-ray tube (Thermo Fisher Scientific, Ecublens, Switzerland) using 12 certified reference materials from China, the USA, and the EU. Mineral phases were identified by means of a Rigaku SmartLab X-ray diffractometer (with a CoKα X-ray tube) (Rigaku, Tokyo, Japan).

Correlation coefficients (and their statistical significance) were calculated using QC. Expert 3.3.0.7. program (TriloByte Ltd., Pardubice, Czech Republic).

3. Results and Discussion

3.1. Composition of Ashes and Their Magnetic/Non-Magnetic Fractions

3.1.1. Chemical Composition of the Studied Fractions

Two-step (dry) magnetic separation of BA was conducted to differentiate between Ca- and Fe-associated As. The results of the chemical analysis of the three particle-size fractions of the studied BA and their magnetic concentrates and non-magnetic residues are summarized in

Table 1. Concentrations of the studied elements in magnetic concentrates and non-magnetic residues prepared from three particle-size fractions of BA (MVS = magnetic vibrofluidized state, MM = magnetic manually, NM = non-magnetic).

Particle Size	BA < 0.2			BA 0.2–0.4			BA > 0.4
Proceeding of Separation	MVS	MM	NM	MVS	MM	NM	MVS	MM	NM
Fraction (wt.%)	0.88	0.89	98.23	0.53	0.64	98.83	1.72	2.48	95.80
As (ppm)	26	39	75	13	56	90	13	25	36
CaO (wt.%)	13.99	26.77	63.62	13.24	30.93	61.48	8.96	10.64	45.02
Fe2O3 (wt.%)	60.36	33.32	2.42	62.82	26.70	2.82	70.37	40.31	3.45
SiO2 (wt.%)	11.16	17.31	4.50	10.46	19.22	7.29	7.78	25.84	23.12
Al2O3 (wt.%)	6.73	10.08	2.51	6.87	11.80	4.02	5.16	13.44	12.10
MgO (wt.%)	1.75	1.52	0.83	1.98	1.59	0.89	1.95	1.60	0.79
K2O (wt.%)	0.37	0.58	0.08	0.29	0.55	0.15	0.20	0.93	0.68
SO3 (wt.%)	2.08	6.28	23.97	1.62	6.52	19.99	1.10	2.66	10.62
TiO2 (wt.%)	0.53	0.72	0.20	0.56	0.85	0.30	0.46	0.97	0.76

as well as the percentage yields of these fractions. The concentrations of As, CaO, and Fe₂O₃ are plotted (for easier evaluation) in Figure 1.

Magnetic separation in the vibrofluidized state provides the highest Fe₂O₃ contents (60–70% in all magnetic concentrates); the manual separation applied on the residue after the first separation provided a concentrate with lower purity (26–41% Fe₂O₃) because the particles with a higher Fe content were already removed during the first separation step. The non-magnetic residues contained only 2.4–3.5% Fe₂O₃ after these two separation steps.

Figure 1 (and Table 1) clearly documents the lowest levels of As in the coarsest BA fraction, which is a similar distribution to CaO. In contrast, the levels of Fe₂O₃ increase with increasing particle size, which is also the case of Si. This trend corresponds with the finely ground desulphurization additive (for efficient interaction with gaseous SO₂/SO₃) and quite large grains with a high SiO₂ content (e.g., quartz particles).

The concentrates originating from the first magnetic separation (in the vibrofluidized state) contain the lowest As and CaO contents while the Fe₂O₃ percentage is the highest in the case of all particle-size fractions. The second magnetic separation (applied to the residue after the first one) provides the magnetic concentrate with a somewhat lower Fe level, which contains moderately higher concentrations of As and CaO. After magnetic separation, the residues (dominantly composed of CaO and SO₃) are enriched in As.

3.1.2. Mineral Composition of the Studied Fractions

Minerals identified in the studied samples are listed (according to their abundance) in

Table 2. Mineral phases identified in the studied magnetic and non-magnetic fractions (MVS = magnetic vibrofluidized state, MM = magnetic manually, NM = non-magnetic).

Particle Size	Sample	Mineral Phases	Particle Size	Sample	Mineral Phases	Particle Size	Sample	Mineral Phases
<0.2 mm	MVS	Hematite/Fe2O3	0.2–0.4 mm	MVS	Hematite/Fe2O3	>0.4 mm	MVS	Hematite/Fe2O3
		Magnesioferrite/MgFe2O4			Magnesioferrite/MgFe2O4			Magnesioferrite/MgFe2O4
		Maghemite-Q/Fe2O3			Maghemite-Q/Fe2O3			Maghemite-Q/Fe2O3
		Quartz/SiO2			Quartz/SiO2			Quartz/SiO2
		Anhydrite/CaSO4			Anhydrite/CaSO4			Anhydrite/CaSO4
		Gehlenite/Ca2Al2SiO7			Gehlenite/Ca2Al2SiO7			Gehlenite/Ca2Al2SiO7
	MM	Hematite/Fe2O3		MM	Hematite/Fe2O3		MM	Hematite/Fe2O3
		Magnesioferrite/MgFe2O4			Magnesioferrite/MgFe2O4			Magnesioferrite/MgFe2O4
		Quartz/SiO2			Quartz/SiO2			Quartz/SiO2
		Anhydrite/CaSO4			Anhydrite/CaSO4			Anhydrite/CaSO4
		Gehlenite/CaAl2SiO7			Gehlenite/CaAl2SiO7			Gehlenite/CaAl2SiO7
		Monticellite/Ca(Mg0.93Fe0.07)SiO4			Portlandite/Ca(OH)2			Rutile/TiO2
	NM	Portlandite/Ca(OH)2		NM	Portlandite/Ca(OH)2		NM	Portlandite Ca(OH)2
		Anhydrite/CaSO4			Anhydrite/CaSO4			Anhydrite/CaSO4
		Quartz/SiO2			Quartz/SiO2			Quartz/SiO2
		Lime/CaO			Lime/CaO			Lime/CaO
		Gehlenite/Ca2Al2SiO7			Gehlenite/Ca2Al2SiO7			Gehlenite/Ca2Al2SiO7
		Calcite/CaCO3			Rutile/TiO2			Rutile/TiO2

. X-ray diffraction patters of the magnetic vibrofluidized state, magnetic manual, and non-magnetic fractions of the finest (<0.2 mm) and the coarsest (>0.4 mm) ash samples are depicted in Figure 2 and Figure 3, respectively.

The minerals in Table 2, ordered according to their relevance, suggest similar (nearly the same) mineral compositions in all three magnetic vibrofluidized state fractions, which contain hematite, magnesioferrite, and maghemite (with some admixture of quartz, anhydrite, or gehlenite). Such a composition also corresponds with ca 60–70% Fe₂O₃ in these fractions.

Magnetic fractions separated manually from the residue after the first magnetic separation contain a lower percentage of Fe₂O₃ (26–41%), which is in line with the lower occurrence of Fe-bearing minerals (hematite and magnesioferrite) and greater abundance of other minerals (quartz, anhydrite, gehlenite, rutile, monticellite, and kaolinite). All the residues after the magnetic separations exhibited almost the same mineral composition as well. Since the Fe₂O₃ levels are very low (only a few %), the dominant minerals are those of Ca, Si, or Al (or Ti): portlandite, anhydrite, quartz, calcium oxide, gehlenite, rutile, and calcite.

3.2. Distribution of As and Target Elements within Magnetic and Non-Magnetic Fractions

3.2.1. Enrichment Factor

For easier evaluation of the overall distribution trends of As and other target elements (expressed in the form of oxides), the Enrichment factor (EF) in the magnetic fractions related to the corresponding non-magnetic residues was calculated according to the formula:

$$E{F}_{\left(i\right)}=\frac{w_i\left(magnetic fraction\right)}{w_i\left(non-magnetic fraction\right)}$$

(1)

where the w_i is the weight fractions of the i-th element (or its oxide) in the magnetic or non-magnetic ash fraction. The EF values (rounded to two decimals) are summarized in

Table 3. Enrichment factors of the studied elements in magnetic fractions (vs. non-magnetic residues).

Particle Size	BA < 0.2		BA 0.2–0.4		BA > 0.4
Proceeding of Separation	MVS	MM	MVS	MM	MVS	MM
As	0.35	0.52	0.14	0.62	0.36	0.69
CaO	0.22	0.44	0.22	0.50	0.20	0.24
Fe2O3	24.94	13.77	22.28	9.47	20.40	11.68
SiO2	2.48	3.85	1.43	2.64	0.34	1.12
Al2O3	2.68	4.02	1.71	2.94	0.43	1.11
MgO	2.11	1.83	2.22	1.79	2.47	2.03
K2O	4.63	7.25	1.93	3.67	0.29	1.37
SO3	0.09	0.26	0.08	0.33	0.10	0.25
TiO2	2.65	3.60	1.87	2.83	0.61	1.28

.

Enrichment factors (calculated according to Equation (1)) are plotted for As, CaO, and S in Figure 4a and Fe₂O₃ in Figure 4b. The same concentrations of the studied element are shown in the magnetic and non-magnetic fractions. Figure 4 suggests that As and CaO (and S) are strongly depleted in all magnetic concentrates, which is even more pronounced in the first (vibrofluidized) separation. As expected, the Fe₂O₃ content is always strongly enriched in the first magnetic concentrates and its enrichment in the concentrate originating from the second separation step is somewhat lower.

3.2.2. Correlations between the Concentrations of the Target Elements

The correlation calculations are used frequently when predicting the content and distribution of elements in coals or combustion residues and for the assessment of their environmental toxicity [48,49]. In order to gain insight into elemental relationships and to evaluate As associations, a correlation matrix was calculated (

Table 4. Correlation matrix calculated from the concentrations of the target elements in 9 fractions (statistically significant values are highlighted in bold).

Elements	As	CaO	Fe2O3	SiO2	Al2O3	MgO	K2O	SO3	TiO2
As	1	0.909	−0.834	−0.306	−0.362	−0.769	−0.365	0.906	−0.462
CaO		1	−0.923	−0.302	−0.384	−0.924	−0.388	0.974	−0.527
Fe2O3			1	−0.084	0.010	0.960	0.005	−0.866	0.172
SiO2				1	0.986	0.019	0.984	−0.401	0.950
Al2O3					1	0.137	0.965	−0.492	0.983
MgO						1	0.097	−0.882	0.308
K2O							1	−0.456	0.949
SO3								1	−0.615
TiO2									1

). The only statistically significant relationships of As are those with CaO and SO₃ (r = 0.909 and 0.906). The correlation coefficient between the contents of As and Fe₂O₃ is negative (r = −0.834), revealing a strong inverse relationship. It is interesting in this context that Fe₂O₃ exhibits a strong positive relationship with MgO (r = + 0.960). The association of As to Al₂O₃ is improbable as well because r (As−Al₂O₃) = −0.362. The critical value of the correlation coefficient in this case (for 9 measured values, α = 0.05, and two-tailed probability) is r = 0.666.

The most interesting relationships are plotted in Figure 5. They are As–CaO, As–Fe₂O₃, As–SO₃, S–CaO, MgO–Fe₂O₃, and Fe₂O₃–CaO. The data listed in Table 1 and Table 3 and plotted in Figure 5 indicate a similar distribution of As, CaO, and SO₃ while the distribution of Fe₂O₃ seems to be substantially different (and similar to that of MgO). The aforementioned direct/inverse relationships are in agreement with the mineral analysis results (Section 3.1.2.) documenting an abundant joint occurrence of CaO–SO₃ in anhydrite and MgO–Fe₂O₃ in magnesioferrite. Joint occurrence of CaO–Fe₂O₃ minerals was not identified by X-ray diffraction, which is in line with the inverse relationship.

Arsenic association with calcareous minerals is in agreement with other fluidized-bed coal combustion results [47,50,51,52,53,54]. Most researchers reported efficient As retention using CaO [52,53,54]; however, CaSO₄ can also capture As [43,55]. Since the correlation coefficient r(As–SO₃) is very high as well (r = +0.906), the association of As to calcareous sulphates can be expected.

The samples evaluated in this study originated from the coal combustion at ca. 850 °C. According to Zhang et al. [36], ferrospheres separated from coal ash exhibited the best As retention efficiency at 600 °C (ca. 0.55 mg As/g) while at 800 °C, it was only ca. 0.20 mg As/g (i.e., nearly one third) [36]. In the case of CaO, there is still no exact consensus in the literature; nevertheless, higher temperatures seem to promote As retention on Ca-bearing minerals. Li et al. [56] observed an increasing retention efficiency within the temperature range of 400–1000 °C while Chen et al. [55] concluded that the most effective As retention on CaO was at 750 °C. Sterling and Helble [32] reported maximum As capture on CaO at 1000 °C. However, CaSO₄ can also retain As [57] and when CaSO₄ was tested for As retention [55], the retention efficiency increased with an increasing temperature from 750 to 1050 °C. Therefore, high temperatures in the fluidized bed (850 °C) might be more favorable for As capture on CaO and notably on CaSO₄ (while the efficiency of Fe-minerals is lower at higher temperatures). Moreover, the SO₃ concentration in non-magnetic residues is higher (10–24%) than that in magnetic fractions (1–2% in the first-step concentrate and 3–7% in manually separated concentrate).

4. Conclusions

Magnetic separation is a widely accepted method used to separate coal ash into two basic components: magnetic and non-magnetic fractions, thereby facilitating their utilization when processed separately. Due to the desulphurization additives added to coal during the fluidized-bed combustion, non-magnetic fractions often contain elevated CaO contents while magnetic concentrates are rich in Fe₂O₃. Both these components are known for their ability to bind As during combustion, whose distribution is a crucial parameter in terms of the utilization of both these fractions. Therefore, this study investigated the As partitioning within magnetic and non-magnetic fractions of fluidized-bed coal combustion ashes.

For a detailed elucidation, two different (successive) procedures of dry magnetic separation were used to separate each sample into strongly magnetic, less magnetic, and non-magnetic fractions. Magnetic concentrates originating from the first separation step (in the vibrofluidized state) contained 60–70% Fe₂O₃, magnetic concentrates separated manually from the residues after the first separation contained 26–41% Fe₂O₃, and the non-magnetic residues contained 2.4–3.5% Fe₂O₃.

Arsenic levels were the highest in the non-magnetic residues (and gradually decreased with an increasing Fe₂O₃ content in the magnetic fractions). The dominant arsenic association in the studied samples was to CaO (r = +0.909) and with SO₃ (r = +0.906), which is in line with the abundant occurrence of anhydrite identified by XRD analysis. The distribution of Fe₂O₃ and CaO was substantially different (r = −0.923), which agrees with the absence of the joint mineral of Fe₂O₃ and CaO by XRD. Instead, the association of Fe₂O₃ and MgO was documented both by a high correlation coefficient (r = +0.960) and by identification of magnesioferrite.

Magnetic separation of the ashes is usually conducted with the aim to facilitate the utilization of magnetic and non-magnetic fractions separately. In the case of the samples studied herein, As was depleted in magnetic fractions in relation to non-magnetic ones. It means that if strongly magnetic concentrates are separated from such ash, their As concentrations are decreased to one-third of the As levels in the non-magnetic fractions. Less magnetic fractions contain As contents that are two-thirds of the levels in non-magnetic fractions, which enhances their potential for further technological use.