Occupational health aspects of quartz in

pulverized coal fly ash in Israel

Yaacov Nathan1, Ariel Metzger2, Asher Pardo3, and Michael Dvorachek1 1Geological Survey of Israel, 2Israel Electric Corporation, 3Tel-Aviv University

Abstract

Quartz (α-quartz), is the most common crystalline silica polymorph in nature and is the

only polymorph found in significant concentrations in coal and coal ash. Since exposure to

quartz is liable to cause lung diseases, it is very important to know its concentrations in coal

ash and to evaluate its effects on the workers. Inhalation is the most effective route of entry

for airborne particles of coal ash; therefore the quartz content in the respirable fraction must

be determined since only these quartz particles, when active, can induce respiratory diseases.

Similarly to what was found in other countries (e.g., the Netherlands), we found that

the quartz concentration in the respirable fraction of the coal fly ash produced in Israel is

considerably lower than in the whole ash. While X-ray diffraction is the most accurate way to

measure quartz concentrations in mineral samples, this method cannot distinguish between

coated and uncoated particles. Scanning electron microscopy, though less accurate, is able to

differentiate between coated and uncoated particles. Our experiments show, similarly to those

of other researchers, that most of the quartz particles in the respirable fraction are coated by

amorphous aluminosilicate layers. Coated quartz particles are not considered to have

pneumoconiotic effects.

Nevertheless the Occupational Safety and Health Regulations in Israel still

classify fly ash as silicotic dust due to the presence of quartz. This classification holds for any

percent weight of quartz, even when it is below 1% in the respirable fraction of the fly ash. In

view of the experimental results presented here and results published on this subject in the

literature, it is incorrect to classify fly ash as silicotic only by its quartz content, since it is

proven to be below the accepted epidemiologic non-carcinogenic NOAEL (No Observed

Adverse Effect Level). The results of recent studies, and the present one, strongly suggest

that the inclusion of fly ash in the category of silicotic dust in health and safety

regulations in Israel should be reconsidered.

1 – Introduction Crystalline silica and health: Silica refers to the chemical compound silicon dioxide,

SiO2, which occurs in crystalline and amorphous forms. Crystalline silica occurs as eight

polymorphs (crystals with the same chemistry but with a different crystalline structure): α-

quartz, β-quartz, tridymite, cristobalite, keatite, coesite, stishovite and moganite (Virta,

1993, Guthrie and Heany, 1995 and NIOSH, 2002). Quartz (which is used in this

document, to imply α-quartz) is the most common polymorph in nature and is the only

polymorph which has been found in significant concentrations in coal and coal ash.

Cristobalite has been identified in a few cases in coal (Yossifova, 2007, Türkmenoğlu, and

Yavuz-Işık, 2008). Exposure to quartz is liable to cause lung diseases such as silicosis

(Finkelman et al., 2002). In addition to causing silicosis, quartz is classified by IARC

(International Agency for Research of Cancer) as a human carcinogen at concentrations

above a certain threshold (IARC, 2003).

Quartz is found in coal and coal ash and is the most important toxic component of coal

dust and fly ash in aerosols. Therefore it is important to know its concentrations in coal and fly

ash in these aerosols. Sakurovs et al. (2007) found that the concentrations of quartz in Australian

coals ranged between 0.9 and 3.9%, with an average of 1.8%. Similar results were found in the

Argonne Premium coals, U.S., 0.3 to 3.0% with an average of 1.5% (Ward et al., 2001). In

seven coal samples of the No. 6 Coal of the Junger Coalfield, China, (Dai et al, 2006), the

quartz concentrations ranged from 0.2 to 4.5% with an average of 1.5%. Meij (2003) found

quartz content ranging between 0.5 and 7 % in coals combusted in Dutch power stations with an

average of 2.6%. Similar quartz concentrations were found in Colombian coals, ranging

between 1.4% and 4.2% with an average of 2.5% (Carmona and Ward, 2008). It should be

noted that some of these coals had high ash concentrations (>15%).

Reports of quartz concentration in coal ash are usually based on quartz analyses

conducted on fly ash samples collected in the silos of the power stations and not on the

airborne fly ash dust or on respirable fly ash particles, which are able to penetrate into the

lungs, although these are the particles which actually affect the health of the workers. For

example, Meij (2003) found concentrations ranging from 3 to 14% in coal fly ash produced in

the Netherlands and taken from silos, with an average of 9%. It should be noted that Meij

mentions in the same paper that the concentration of quartz in the respirable fraction of coal

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ash is considerably lower than its concentration in the bulk coal ash; and furthermore, about

half of this quartz is embedded in vitreous material. Exposure to this kind of ash does not

cause silicosis (Meij, 2003). In a preliminary study we found quartz levels of 1.5 to 7% in

coal fly ash taken from silos in Israeli power plants and, similarly to Meij (2003), found a

decrease in the quartz content in the respirable fraction compared to the content in whole ash;

furthermore, a large part of this quartz was coated by amorphous aluminosilicates. Inhalation

into the lungs is the most important mechanism of exposure to airborne particles of coal fly

ash; therefore it is extremely important to determine the quartz content in the respirable

fraction of the suspended airborne particles. These particles are very fine so they are able to

penetrate into the air exchange zone of the respiratory system, deep into the lungs. Only these

quartz particles, when active, can induce respiratory diseases.

The NIOSH review (2002) classified industries, in which workers are potentially

exposed to respirable quartz. While workers in coal mining are included among those who

are in this category (NIOSH, 2002, Shatzel, 2006), workers in coal ash production and

utilization industries are not classified as potentially exposed to respirable quartz (NIOSH,

2002). Similarly, the IARC review (1997) includes workers in coal mines as potentially

exposed to respirable quartz, while workers in coal ash production and utilization industries

are not included. Borm (1997) published a review of data on toxicity and occupational health

hazards of coal fly ash which states that workers exposed to fly ash are not affected by

quartz-induced diseases. Furthermore it has been shown in several studies (Raash and

Schilling, 1980, Gulumian et al, 2006) that while quartz is present in coal fly ash, this quartz

does not lead to the published recognized health effects of exposed fresh quartz surfaces. The

absence of such effects of quartz in fly ash has been explained by several reasons: 1- the

quartz content in the respirable particles is considerably lower than that in the larger ones

(Meij, 2003). 2 – the majority of the few quartz particles in the finer-size particle fractions of

the fly ash is either embedded in an amorphous phase or coated by amorphous

aluminosilicate layers. The presence of coating has potential to lessen quartz toxicity.

(NIOSH 2002, Cpreck et al, 2007, Wendlant et al. 2007), and 3- the toxicity of quartz

particles is also related to reactive sites present on the surfaces of the quartz particles and

such sites are lacking in fly ash quartz (Bolsaitis and Wallace, 1996, Castranova et al, 1996,

Fubini, 1998, Fubini et al, 2004).

3

Nevertheless, according to the Israeli Occupational Safety and Health Regulations

issued in 1984 and updated in 2000, fly ash is still classified in Israel as a hazardous and

silicotic dust due to the presence of crystalline silica as quartz. This is in contradiction to scientific

observations which firmly established that, as a rule, fly ash does not cause crystalline silica-induced

diseases such as silicosis. This definition holds regardless of the percent weight of quartz, even when

it is below 1% in the respirable fraction of the fly ash, in contrast to the definition of silicotic dust in

widely accepted occupational exposure standards around the world. One of the implications of this

definition is that fly ash dust is considered as silicotic regardless of its quartz content and

even if it is below the accepted epidemiologic non-carcinogenic level NOAEL (No observed

adverse effect level).

2 – Occupational health standards in Israel and in the world:

In compliance with the above regulations, workers in the power stations in Israel are

classified as workers exposed to free crystalline silica. Accordingly, the dust levels to which

the relevant workers are exposed are semi-annually monitored for crystalline silica by the

Israel Electric Corporation. According to the instructions issued by the Ministry of Industry,

Trade and Labor (MOITAL), the results of these measurements should be compared to the

TLVs' – Threshold level Limit Values – applied to free crystalline silica concentrations in the

air. In order to perform this comparison, the measurements of silica concentration in the dust

must be conducted by a certified laboratory. The TLVs' used for silica in Israel are 0.3 mg/m3

for total dust, and 0.1 mg/m3 for respirable dust as a time weighted average (TWA) over 8

hours (MOITAL, 1999).

Another important source of information on the health aspects of coal ash is found in

the MSDS - Material Safety Data Sheets – attached to shipments of building materials in

developed countries all over the world. In European countries such as the UK and Germany, it

is mentioned in the MSDSs prepared for coal fly ash and bottom ash that this material contains

crystalline silica, but that most of it is embedded within glassy phases, and therefore cannot be

considered as hazardous dust. For example, the MSDSs issued for coal ash in the UK

(UKQAA, 2000), and in Germany (VGB, 2000), recommend that the occupational exposure

level to which workers may be exposed while using coal ash should be kept lower than the

standard values employed in these countries for nuisance dust and not a hazardous dust.

4

3 – Samples and methods: 3.1 – Dust monitoring and sampling.

3.1.1 Dust sampling at the power station: a first series of dust samples was collected

during the period October-November 2007, at the Orot-Rabin power station, in order to check

the quartz content in total and respirable dust particles. The Orot-Rabin power station is one

of the two coal power stations in Israel. They both use the same kind of coals and produce

about 65% of the electricity in Israel. Unfortunately, the weight of most samples collected

was too small to enable an accurate determination of the quartz content, because the amount

of quartz in the sample did not reach the detection limit (5 µg per sample). Other dust

samples were collected at the same location during the period November-December 2008.

Air was sampled using calibrated "personal sampler" pumps (SKC Inc.). Stationary samples

were collected at two different locations inside the building containing the truck-loading

lanes, under the fly ash silos, at a height of 2 m above the ground. Inhalable dust samples

were collected on pre-weighted PVC membrane filters (5μ pore size, 25 mm diameter), using

an IOM dust sampler (sampling particles smaller than 200 μm aerodynamic diameter, and

having a 50% cut point at 100 μm). It ran at a calibrated flow rate of 2.0 l/min. For respirable

dust, the samples were collected on pre-weighted PVC filters (5μ pore size, 37 mm diameter),

each attached to an aluminum cyclone at a calibrated flow rate of 2.5 l/min, according to the

standard method used for collecting particles smaller than 10 μm aerodynamic diameters,

with a cut point at 4.0 μm. Four dust samplers were placed at each of the two sampling

locations, two with IOM sampler for inhalable dust sampling and two with cyclones for

respirable dust sampling. In total, 8 dust samples were collected, 4 comprising inhalable dust,

and 4 comprising respirable dust. The sampling work was conducted by Tevet; a certified

laboratory specializing in occupational health monitoring.

3.1.2 - Sampling from the silo ash: When dust was sampled in 2007, about 30 kg of

fly ash were collected fresh from the silo at the power plant; the sampling was done only

when the ash provenance was from burning a single (not blended) coal. Particular care was

given to acquiring a representative sample by mixing 10 samples taken at half hour intervals.

3.2 – Analytical methods

3.2.1 XRD – X-Ray diffraction (XRD) is today the most accurate and the method

most used to analyze quartz concentrations. This is also true for coal and coal ash samples

5

(HSE, 1988, Ward et al., 2001, NIOSH, 2003, Dai et al., 2006, and, Carmona and Ward,

2008.). A problem arises from the fact that many quartz particles in coal ash are coated by an

amorphous material composed of aluminosilicates, rendering the particles inactive, but are

identified by XRD as active quartz and therefore included in the reported quartz

concentration. The crystalline silica concentration was determined in all the dust samples

mentioned in section C1.1 at the EMSL Analytical Inc. laboratory in Westmont, NJ, USA.

Silica was analyzed according to the standard method NIOSH 7500 (NIOSH, 2003).

According to this method the crystalline silica concentration is determined by XRD after

appropriate sample preparation. A bulk material sample (fly ash from the silo) was included in

the sample set for comparison purposes.

3.2.2 I.R. – Infra Red spectroscopy (IR) has also been used to measure quartz in coal

(HSE, 1987). The advantage of this method is that it is able to measure quartz in very small

samples. However even when using Fourier Transform Infra Red spectroscopy (FTIR), the

results of this method are less accurate than those obtained using XRD.

3.2.3 – Microscopic observations and chemical elements mapping. Scanning Electron

Microscopy (SEM) including Computer Controlled Scanning Electron Microscopy (CCSEM)

together with Energy Disperse Spectrometry (EDS) is increasingly used to measure quartz in

coal fly ash (Meij et al., 2000, Kutchko and Kim 2006, Goodarzi, 2006 and Cprek et al.,

2007). Although it is less accurate than XRD, its results are more significant because they can

differentiate between coated and uncoated quartz particles and at the same time determine

their size. We used a Jeol JSM 840 SEM together with a Link 10000 analytical attachment

(EDS). The samples were analyzed as dispersed powders, previously separated into large and

small particles. We took Secondary Electron micrographs (SE) which enable observing the

morphology and size of the particles and Back-Scattered micrographs which differentiate

between particles with "heavy" and "light" elements. The "dijimap" software was also used.

This software simultaneously gives a SE and a BS micrograph and semi-quantitative maps of

the elements which are chosen. Finally, selected particles were quantitatively analyzed (SEM

X-ray non-dispersive analysis) for O, Na, Mg, Al, Si, K, Ca, Ti, and Fe.

4 – Results: 4.1 Monitoring results

6

Table 1 presents the results of the quartz concentration (as weight percent) in the dust

samples collected during the period of November-December 2008, according to the analyses

performed by the EMSL Analytical Inc. laboratory. In these airborne samples composed of

fly ash, the quartz content is in the range of 2.3-4.3%. It is established clearly by these results

that the quartz concentration analyzed by XRD in the dust samples is lower than the

concentration analyzed by the same method for a bulk fly ash sample (5.0 %). In addition, the

results show that the average quartz concentration is slightly lower in the small fly ash

particles sampled in the air (respirable dust – 2.7%) than in the slightly larger fly ash particles

sampled at the same location (inhalable dust – 3.3%). These results, found on the basis of our

monitoring work on fly ash dust sampled in Israel, are in agreement with the results of studies

reported in the literature noted in the Introduction. The low quartz levels found in the

respirable ash particles, which include a major part of quartz coated with aluminosilicates not

known to have any silicotic effects, strongly indicate that like in other industrial countries, fly

coal ash dust can be regarded and classified in Israel as nuisance dust and not as a hazardous

dust.

Table 1: Concentration of quartz in coal fly ash dust samples

Sample No. Quartz Concentration (%)

Inhalable dust 1 3.3 2 3.2 3 2.3 4 4.3

Respirable dust5 2.5 6 2.8 7 2.7 8 2.9

Bulk fly ash9 5.0

4.2 Experimental results

4.2.1 - XRD results - Two fractions (from 20 to 40 μm and less than 10 μm) were

separated from fly ash collected from the silo during the combustion of a Colombian coal (La

Loma coal), which has a particularly high quartz content (~ 8%). For comparison, one

fraction (less than 10 μm) was separated from the fly ash of a South African coal, AKD coal,

which has the usual quartz content (~ 4%).

7

Figure 1 - Diffractograms of three size fractions of fly ash obtained from burning coal

from two mines, La Loma, Colombia and AKD, South Africa. La Loma I is a fraction

of particles less than 10 μm; La Loma II is a fraction of particles between 20 and 40

μm; AKD is from particles less than 10 μm. Legend: M – mullite, Q – quartz, C –

Calcite.

It can be clearly seen from Figure 1 that the concentration of minerals

(crystalline) in the fraction of La Loma fly ash containing particles smaller than 10μm,

and especially that of quartz, the most common mineral, is significantly lower than that

in the fraction between 20 and 40 μm. A semi-quantitative approximation of the quartz

concentration in the two fractions is: 7 ± 2 % for the large particles fraction and 1.5 ±

1% for the smaller particles fraction. Furthermore, no quartz could be detected in the

small particle fraction of AKD ash; this means that its concentration is less than 0.5%.

The wide extended peak in AKD shows that it contains much amorphous material.

4.2.2 - SEM results - We took Secondary Electrons (SE) microphotographs to study

the morphology of the particles and Back-Scattered (BS) microphotographs to get a

qualitative chemistry of the particles. This method is based on the relative weight of the

elements; the heavier elements scatter more electrons than the lighter and are therefore

8

brighter in the microphotographs. The bright particles usually contain iron (Fe) and/or

calcium (Ca). An example of a SE microphotograph is shown in Figure 2 and the BS

microphotograph of the same sample is shown in Figure 3.

We used the Dijimap software to obtain a semi-quantitative chemistry of the particles.

Dijimaps of particles smaller than 10 μm and between 20 and 40 μm are shown in Figures 4

and 6, respectively. Secondary Electron (SE) microphotographs of a few enlarged particles in

these dijimaps are shown in Figures 5, 7, and 8. Tables 2 and 3 present results of the

quantitative chemical relative abundance of O, Na, Mg, Al, Si, K, Ca, Ti, and Fe in selected

single particles smaller than 10 μm and between 20 an 40 μm, respectively.

Figure 2 - Secondary Electron (SE) microphotograph of relatively small coal fly ash particles.

Most of the particles in this size fraction are spherical (cenospheres).

9

Figure 3 - Back-Scattered Electron (BS) microphotographs of the particles shown in

Figure 2. The very bright particles contain iron. The darker ones are aluminum-

silicates and the in-between are calcium aluminum-silicates.

10

Figure 4 - Dijimap of particles smaller than 10 μm. Dijimap of fly ash particles contains

an SE microphotograph, a BS microphotograph, and 7 element maps of the concentration

of Na, Mg, Al, Si, S, K and Ca. The concentrations in each element are represented by

colors and their intensity: blue (negligible), green (medium) and red (high).

11

.

Figure 5 – Enlarged Secondary Electrons (SE, 255) microphotograph of the

particles shown in Figure 4

Table 2

Analyses of selected particles smaller than 10 μm; the total of the elements analyzed is

considered to be 100%. The sample number relates to the particles in Figure 5. Results are

given in %.

Element/Sample 1 2 3 4

O 61.4 60.7 64.4 58.8Na 4.7 4.1 2.8 3.9 Mg 0.6 1.0 1.0 0.6 Al 14.4 10.8 3.6 10.0Si 15.6 18.2 25.5 23.0K 0.8 0.7 0.7 1.6 Ca 0.5 2.6 0.6 0.5 Ti 0.1 0.3 0.2 0.2 Fe 1.8 1.6 1.3 1.4

Total 100 100 100 100

In Table 2, only particle 3 is almost pure quartz, all the other particles contain 10% Al

or more; and even particle 3 has 3.6% Al, which shows that even this quartz is coated by a

thin aluminosilicate phase. The numbers appearing in the columns headlines refer to the particles

number in Figure 5.

12

Figure 6 – Dijimap of particles between 20 and 40 μm. This dijimap of fly ash

particles contains a SE microphotograph, a BS microphotograph, and 7 element

maps of the concentration of Na, Mg, Al, Si, S, K and Ca. The concentrations are

represented by colors and their intensity: blue (negligible), green (medium) and

red (high).

13

Figure 7 – Enlarged Secondary Electrons microphotograph of the particles shown in

Figure 6 (BSE 255). The sample numbers on the particles are the numbers of the chemical

analyses in Table 3.

Table 3

Analyses of selected particles between 20 and 40 μm; the total of the elements analyzed is

considered to be 100%. The sample number relates to the particle number in Figure 7. The

results are given in %.

Element/Sample 1 2 3 4

O 64.40 60.68 64.35 58.79Na b.d.l. 4.07 2.80 3.94 Mg b.d.l. 0.99 0.95 0.63 Al 0.30 10.80 3.59 9.96 Si 35.40 18.17 25.46 22.96K b.d.l. 0.71 0.72 1.63 Ca b.d.l. 2.64 0.56 0.46 Ti b.d.l. 0.30 0.20 0.21 Fe b.d.l. 1.62 1.34 1.44

Total 100 100 100 100

b.d.l. – below detection limit

14

In Table 3, only particle 1 is a quartz particle, particle 3 probably contains

quartz, but in this case, the particle is coated by amorphous aluminosilicate. The

morphology of particle 1 is unusual for a fly ash particle, see enlargement in

Figure 8.

.

2 1

Figure 8 –Enlargement of particles 1 and 2 in Fig. 7.

15

Figure 9 – Higher resolution of the surface of a quartz particle. It is probable that the small

cenospheres are vitreous silica. This possibly explains the lack of active surface in many

quartz particles.

SEM with dispersive X-ray chemical analysis of the filters collected in November-December

2008.

The particles that were found on the filters were fly ash particles (Figure 10) and as

can be clearly seen, most of them are cenospheres. The filters, including a blank filter, which

were analyzed in the SEM showed the presence of Cl (Figure 11), and its origin is probably

seawater.

16

1

Figure 10 – Microphotograph of the surface of filter I-336. Almost all the particles in this size fraction

are fly ash cenospheres. This filter was sampled in the silo building where the trucks come to

be filled with fly ash.

Figure 11 – Chemical elements spectrum of particle (1) on filter I-336 (Figure 10), an

amorphous aluminosilicate phase, with a few other cations. The Cl content is probably due to

seawater spray on the filter.

17

E – Discussion: The power plants in Israel are similar to those used in other countries and the coals

bought by Israel are similar to those bought by countries which do not have coal mines (e.g. the

Netherlands). Therefore, there is nothing specific or different about the coal ash produced in

Israel.

Obviously, the concentrations of quartz, similarly to the other mineral elements

(Kutchko and Kim, 2006) are higher in the ash than in the coal, since the ash represents the

inorganic constituent of the coal and all the quartz in coal is in the inorganic part. The quartz

concentration also varies for different coal sources. Nevertheless, the content of quartz in coal

ash is considerably lower than that calculated from its concentration in the inorganic fraction

of the coal. This is due to the formations of new silicates during combustion, mainly with

aluminum and sometime also with calcium and/or iron oxides. The fine quartz particles,

because of their relative larger surfaces, have a higher reactivity; and a larger fraction of these

particles reacts with the amorphous aluminosilicates phases together with the CaO or

Ca(OH)2 particles to form new phases in the boiler. This appears to be the explanation for the

differences in the mineralogy (mainly the quartz content) for the different size fractions of the

coal fly ash which have the same chemical analysis. In general, the natural particles from

geological sources such as dust from coal mines or volcanic ash are mostly angular to sub-

angular; in contrast, the fly ash particles are mostly spherical with some input of sub-angular

particles. According to the results presented here, most of the very fine respirable particles in

coal ash are spherical, whereas a fraction of the larger particles is angular or sub-angular.

Furthermore, similarly to that found in other studies, most of the fine-sized particles (less

than 10 μm) are coated by aluminosilicate layers. Finally, Figure 9 gives a possible explanation

for the lack of active quartz surfaces. This is probably why the workers in coal mines are

considered as having an occupational exposure to respirable crystalline silica, whereas workers

engaged in coal ash handling in electric power plants and elsewhere are not included in this

category (IARC, 1997, NIOSH, 2002). Borm in his review (1997) concludes that

epidemiological studies on working populations exposed to fly ash have found no evidence

for effects commonly seen in coal workers (pneumoconiosis, emphysema) with the exception

of airway obstruction at high exposure. It should be noted that there has been no case of

silicosis among the workers of the two coal power plants in Israel.

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F - Conclusions The extensive literature review and our results show clearly that coal fly ash produced

in Israel is identical to coal fly ash produced in other countries. This highly suggests that the

definition of coal fly ash as a silicotic dust and its inclusion in the category of silicotic dust in

health and safety regulations in Israel should be reconsidered, and should be similar to that in

the regulations of most other countries (e.g., the Netherlands, U.K. and the U.S.A.), where

coal fly ash is considered to be a nuisance dust and not a hazardous dust.

Acknowledgements This work was supported by the Geological Survey and the National Coal Ash Board.

We thank Ms. Nili Almog for her help with the figures, Dr. Henry Foner for his critical

reading, and Mrs. Beverly Katz for her careful editorial help.

19

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