WEATHERING OF COAL- EXPERIMENTS ......The bituminous coals, which cover the rank from high volatile...

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WEATHERING OF COAL- EXPERIMENTS, MEASUREMENT METHOD S &

PREVENTION: A SURVEY

PRAVEEN K JHA 1, T K DAS1, B N Prasad1 & A. B. SONI2

1RDCIS, SAIL, Ranchi, India 2NIT Raipur, Chhattisgarh, India

ABSTRACT

During transportation and stockpiling, coal is in contact with air for periods of time that may exceed 6 months.

During this time, reaction with oxygen in the presence of water, sunlight and possibly elevated temperatures may take

place. The resulting weathered coal suffers some alteration in its technological properties

This paper describes what is weathering with reference to coking coal, why it is important to understand the coal

weathering. In this paper we try to summaries how different study conducted in lab scale and natural weathering setup.how

they monitor the coal weathering.The different technique to mointor coal weathering also dscussed.some effort also put up

to summarise the ways to reduce coking coal weathering.

KEYWORDS: Weathering with Reference to Coking Coal

INTRODUCTION

There are three types of coal differentiated by rank (maturity):

• Thermal (low-rank steaming coal),

• Bituminous (middle-rank metallurgical coal)

• Smokeless coal (anthracite rank).

The bituminous coals, which cover the rank from high volatile to low-volatile bituminous, are so called because

bitumen can be obtained from them coking coal have unique properties that differentiate from other coal they soften/melt

when heated to high temp (>300) and solidify in to solid mass coke at around 500 0C

The weathering of coal is a natural process, occurring at or near the earth's surface in response to physical,

chemical, or biological attack. This process takes place initially in situ but also occurs subsequent to mining during

stockpiling, processing and transport

From a commercial viewpoint, it is a process of deterioration, adversely affecting those properties of coal which

render it suitable for utilization in industrial processes such as combustion, carbonization and liquefaction. Consequently,

the detection of weathering is very important

In those situations where the properties of the coal when fresh are well documented, weathering may be detected

and approximately quantified by a comparison of analyses of the suspect coal and the fresh equivalent.

International Journal of Chemical & Petrochemical Technology (IJCPT) ISSN(P): 2277-4807; ISSN(E): 2319-4464 Vol. 4, Issue 3, Jun 2014, 17-38 © TJPRC Pvt. Ltd.

18 Praveen K Jha, T K Das, B N Prasad & A. B. Soni

Impact Factor (JCC): 2.9866 Index Copernicus Value (ICV): 3.0

Even during exploration, when it is possible to penetrate the weathered zone, it is not a simple matter to reliably

detect the presence of weathering. The characteristics of the coals in the area are not well documented at this stage and thus

a comparative approach to ascertain the presence and degree of weathering is not possible (21)

At least 3 factors can contribute to change/deterioration of coking properties over time (5)

• Exposure to air(oxygen)

• Moisture (rain/snow) content

• Re handling (size reduction)

Another contributor to weathering is the stresses caused by cyclic sorption and desorption of moisture which

produce fissures and cracks that mechanically weaken the coal This decrepitation phenomenon is referred to a s slackening

Slackening is much more rapid and extensive for lignite s and subbituminous coals than for higher rank coals (3)

Mechanism Of Weathering

Weathering commences once the coal is unearthed and/or removed from the coal seam unless precautions are

taken to prevent its exposure to oxygen (a i r) and changes in temperature and humidity The reaction of th e coal with

oxygen occurs readily at ambient temperature, results primarily in oxidation of the coal' s organic

Constituents’, and leads to an increase in the oxygen content and a decrease in the atomic hydrogen-to-carbon

ratio of the coal

Peroxides have been detected as transient intermediates in the early stages of coal oxidation, and it is generally

thought that decomposition of the peroxides leads to creation of the new oxygen-containing functional groups. carbonyl

and carboxyl groups, and ether linkages have been detected or inferred from wet chemical and spectroscopic data.

In general, the chemical structural changes that accompany

coal' s oxidation at low temperatures (from ambient to 20OoC) by molecular oxygen

are strongly dependent upon coal rank, particle size, oxygen partial pressure, moisture content, and temperature.

Weathering can alter organic and mineral constituents of coal, change its chemical and physical properties and

affect its utilization. In addition to temperature, coal weathering is also known to depend on coal rank, humidity and

oxygen partial pressure The weathering rate showed a strong dependence on temperature and coal rank (13)

Reason to Study Coal Weathering

As an appreciable portion of coal used in the coking industry is stored in largepiles for various periods of time, it

was appropriate to study the deterioration in coal properties due to natural weathering and assess it s effect on coke

properties. especially CSR, and cokemaking (10)

The weathering study is important due to following reasons:

• In general, exposure of the coal to oxygen at ambient temperature can result in a very rapid reduction in the

fluidity that it exhibits when heated and a significant narrowing of its plastic temperature range.The loss of the

Weathering of Coal- Experiments, Measurement Methods & Prevention: A Survey 19


coal's thermoplastic properties suggests that a more highly cross-linked macromolecular structure has been

formed which will not easily melt and flow when heated.

• Oxidation reduces the calorific value of coal. Low rank coals will lose about 190 Btu-per-pound for each 1%

increase in oxygen content while high rank coals will lose about 240 Btu-per-pound for each 1% increase in

oxygen content The oxygen content of freshly mined and crushed lignite coals can increase several percent in a

matter of weeks when t h e coal is stored in air at ambient temperature. Such rapid oxidation can lead to

spontaneous combustion of the coal under certain storage conditions

• Oxidation also converts coal to a more hydrophilic material which makes beneficiation more difficult Oxidation

reduces the hydrophobicity-hydrophilicity differences between c o a l ' s organic constituents and mineral matter

which, in turn, makes it more difficult to selectively separate mineral matter particles on the basis of their

hydrophilic surface properties (37-39). This results in a reduction in the efficiency of beneficiation by f l o t a t i o

n, agglomeration and flocculation processes.

• The detrimental effect s of oxidation on a coal' s suitability for producing high-quality metallurgical coke have

been extensively studied and are well understood from a practical viewpoint. the adverse effects include reduction

i n coke s t r e n g t h, coal bulk-density-control problems, overheated charges, carbon deposit s leading to oven

damage, coke handling problems, and reduced coke yield s as a result of increased coke breeze, increased coke

reactivity and decreased coking rate. One important effect of oxidation is to destroy the coal ' s thermoplastic

properties which, in turn, prevents its organic components from adequately fusing and binding together during the

coking process. reduction in coke strength and yield. (3)

• CSR degraded by almost 1 CSR unit for each month the coal were stockpiled or about 3 unit after 90 days (5)

• Increase In oxyegn contents of about 1 % suffice totally destroy the cakig properties of prime metallurgical coal

(8)

Experiments

This paper summaries all efforts done to measure coal weathering till date. study of coal weathering can be

divided in two category.

• Artificial & Lab Scale weathering

• Natural weathering

Previously some work also done to understand the factor like coal rank and temp effects on coal weathering

Natural Weathering

In this case certain amount of coal kept in yard for a fixed tenure. Sample has been taken at different interval for

different analysis. Coal sample analyzed by FSI, Gieseler fluidity, proximate and ultimate analysis etc. In some case coke

also prepared from coal sample collected at different intervals, from pilot oven.coke quality also analyzed

Here we try to summaries the test and their findings



• A pile of about 100 t of the coal blend was stored at INCAR open stockyard for over a year, carbonisations were

carried out at different intervals of time using the 6 t oven of the semi-industrial coke oven battery available at

INCAR. Also, carbonisations were carried out in a movable wall oven of 250 kg capacity.

Four coals of the same blend were stored, without grinding, in a pile of about 50-60 t at INCAR open stockyard.

Carbonisations were carried out during a period of 6 month

Weathering studies were carried with an industrial coal blend and four of the coals that compose the blend; their

main characteristics are shown in Table 1.

Experiment Condition

Atmospheric conditions during the time when the industrial coal blend was stored were very mild.

Temperature varied from 2°C to 22°C, being the mean temperature 12°C.

The amount of rain ranged between 31 and 259 l/m2, the average being 136 l/m2.

Highlights of Findings

Gieseler maximum fluidity is considered to be a very sensitive indicator of the degree of weathering for a coal so

fluidity measures throughout the year and it was found that fluidity decrease from 857 to 160 ddpm.

The quality of the coke produced from the industrial blend is not very much affected by weathering during a

period of a year (in the atmospheric conditions studied)

In the case of individual coals, representing the different components of the industrial blend, the effect depends on

the coal. Indeed, some coals improved in coke strength (coal A), others impaired it (coal C) and yet others did not change

due to weathering (coal D) 1.

• The effect of outdoor storage on the properties of steam coal (High volatile bituminous coal) was studied One

portion of the coal was stored in summer, another portion in winter. Procedures used and results obtained are

described. (7)

Both sets of samples of coal were stored in cloth bags, each holding approximately 100 pounds of coal. The bags

of coal were placed on a concrete base outdoors near the Survey Applied Research building. They were placed in tight

groups with tops open to permit exposure of the coal to the weather. Around the periphery of the groups other bags of coal

were placed as protection against side drafts. At prescribed intervals of elapsed time in storage, samples for analysis were

taken by removing



one bag of sample coal and replacing it with one of the outside bags of coal in order to disturb as little as possible

the weathering conditions of the remaining coal. Fifteen bags of coal (approximately 1500 pounds) were stored in summer

and seventeen (approximately 1700 pounds) were s (7)


IT WAS OBSERVED FROM STUDIES THAT Ash, volatile matter, and total sulfur were not significantly

affected by storage begun in either summer or winter, under the conditions used

Free swelling index values dropped after about 48 weeks of storage. Calorific values dropped 2.1 percent in 52

weeks for the summer-stored coal and 1.4 percent in 20 weeks for the winter-stored coal. (7)

• Six piles, 3 tons each, of each of the coals that were in use at Inland steel US. were made in the open yard at the

Research pilot facility. The coals were Coal A (High Volatile, Coal B (High Volatile), Coal C (High Volatile),

and Coal D (Medium Volatile/

Pile no 1 carbonized on the same day.

Pile Nos. 2 to 6 were carbonized after 35 days, 70 days, 105 days, 180 days, and 420 days of natural oxidation,

respectively coals from each pile were subjected to the following analyses:

rheological, proximate, ultimate, alkali solubility, petrography, pH (methanol/water soak), FTIR-PAS. and

sole-heated oven (SHO) analysis (10)


Develop Ph (methanol/water soak) as a technique to monitor coal weathering. Develop co-relation between Ph

and coke quality (CSR), this co-relation used by Inland steel US to find suitability of incoming coal for coke making.

• The process of natural oxidation of two low-rank coals exposed to the atmosphere for 11 months has been studied

by Fourier transform infrared (FT-IR) spectroscopy. The study was carried out on samples taken at different time

intervals and additionally from zones where signs of high oxidation and self-ignition were detected.

The aliphatic hydrogen (3000-2800 cm-1) and oxygen-containing structures (1800-1500 cm-1) regions of the

spectra were examined by curve-fitting analysis and a series of structural parameters based on ratios of integrated

absorbance areas of curve-fitted bands were established.


The aliphatic hydrogen content of samples tended to decrease with increasing time of storage and carboxyl groups

only increased slightly under conditions of low pile activity.

When oxidation and self-ignition processes took place, the structural changes were more significant. Aliphatic

structures decreased drastically and net production of oxygen-containing structures was observed. Aliphatic hydrogen

content evaluated from integrated absorbance measurements of normalized spectra and the CO/aliphatic hydrogen ratio

seemed to be very sensitive in detecting signs of weathering even at very low levels of activity CO/Hal ratio seems to be

very sensitive in detecting signs of weathering even at very low levels of activity (19)



• About 50 t of two medium volatile bituminous coals were stored in two piles (2.5 m height) at the INCAR open

stockyard for several months. During the study the proximate and ultimate analyses of the coals were performed

at fixed periods of time. The Gieseler maximum fluidity (MF) and the free-swelling index (FSI) tests of both coals

were carried out according to the ASTM D2639-74 and ASTM D720-91 procedures, respectively

In all cases, natural coal particles (particle size less than 0.15 mm) were used for PA-FT-IR spectroscopy




Tables 2 and 3 show that proximate and ultimate analyses do not provide sufficient information for studying the

effects of the weathering process on these coals, suggesting that the analyses performed are not sensitive enough to detect

variations in weathered coals. In fact, the only significant changes are the reduction in sulphur and the increase in oxygen

content in coal B. There is also a slight tendency for the volatile matter content to decrease in both coals.

it is clear that the weathering process produces considerable changes in the plastic properties of the coals

change in fluidity of coal PA-FT-IR analysis of weathered coals

A series of parameters defined as ratios of integrated absorbance were used in an attempt to quantify some of the

structural changes which take place during the weathering process

For Aliphatic Structures

Oxygen-Containing Structures



The ratio of aliphatic hydrogen content to aromatic carbon (Hal/Car) in Eq. (1) provides a measure of the

evolution of aliphatic structures during weathering, while the methyl to methylene ratio (CH3/CH2) in Eq. (2) can be

considered as an estimation of the length of the aliphatic chains.

The ratio of the CO groups to the aromatic carbon content of the samples (CO/Car) is given by Eq. (3), while the

evolution of the carboxylate groups (COO/Car) was followed by the ratio shown in Eq. (4)


PA-FT-IR has revealed that the aliphatic hydrogen content of both coals tends to decrease with increasing time of

weathering, as does the average length of the alkyl chains. The shortening of the alkyl chains and the considerable

reduction in the aliphatic hydrogen content are the cause of the decrease in the plastic properties of the coals with

weathering time

A different evolution of the CO groups was also detected depending on the parent coal. While this parameter

increases in one of them, in the other it decreases, probably due to chemical reactions of the carboxylic acids with the basic

components of the coal mineral matter (20)

Lab Scale Weathering

In lab scale study differebt coal sampled crushed to certain size (-60, -28, 1-2 mm).The crushed sample kept in

flat container which is subjected to normal Lab conditon or oxidised under static air in a electric oven.In some case glass

reactor, fixed bed reactor was also used to study the coal weatering at different temprature.

• Four pristine coals were obtained for this pilot study.The coals were crushed to 1-2 mm and split into four

representative samples. One sample was retained as the datum whereas the other samples were evenly distributed

on open flat containers and oxidized under static air in a thermostatically controlled electric oven in the following

manner: sample A was oxidized at 70, 140 and 210 ° C for 24 h, whereas samples B, C and D were oxidized at 70

° C for 1, 10 and 100 h.


At low temperatures (e.g. < 70 ° C), the rate of reaction is low and oxidation may be described in terms of a

"chemically controlled" reaction (Karsner and Peflmutter, 1982), whereas at higher temperatures (e.g. 210 ° C) the rate of

reaction is higher and oxygen reacts readily with the molecular structure of coal; at this temperature it is the availability of

the gaseous reactant that limits the reaction (i.e. diffusion control). (22)

at temperatures higher than 70 °C there is, in most cases, an increase in reflectance, whereas at temperatures

below 70 °C (e.g. weathering) reflectance often decreases

• In lab western canadian prime coking coal, sized to -60 mesh and stored in fairly thin layers under normal

laboratory conditions


Result show an average reduction of the FSI by 1 unit per 8 month of storage and a loss Of 10-20% in Gieseler

fluidity after 1 week.(8)



• Twenty gram aliquots (portion) (-60 mesh) of all eight PCSP coals were exposed to a dry air flow (10 ml/min) in a

100 ml glass reactor, at 100 0C for 8 days.(9)

Canyon seam coal was weathered for 2, 4, 6 and 8 days in separate glass reactors a t 100°C and the Pittsburgh #8

seam coal was weathered a t 150OC for 1 and 3 days in order to determine the effect s of weathering time and temperature,

respectively (9)


It uses re- hydration of weathered coal sample for Ph determination. and found result are more accurate then

simple Ph method.

The Illinois 6 coal natural -28 mesh portion of fresh run-of-mine ROM) used in this study Illinois 6 coal (hvBb)

was weathered for up to 330 days at 25 and 8OoC with humid air. It s behavior was compared with Pittsburgh Seam

(hvAb) and Horsepen Seam (mvb) coals weathered under the same conditions

Changes in elemental composition (H.C.0) observed upon weathering occurred more rapidly for the Illinois 6 coal

(hvBb) than for the Pittsburgh Seam (hvAb) and Horsepen Seam (mvb) coals, These values indicate that rates of change in

elemental compositions with weathering time are dependent on coal rank Factor which contribute weathering

Thermal (nonoxidative) treatment at 80°C has little effect on the chemical and physical properties of a fresh hvAb

coal.

(13)

• Medium and high volatile bituminous coals were weathered at temperatures 25OC. 5OoC and 80°C in flowing

humid air for as long as 369 days. Absolute humidity, which was constant for all experiments, was equal to 80%

relative humidity at 2OOC (14)

Figure 1: Coal Weathering Unit



A schematic diagram of the weathering unit is shown in Figure. The coal (1.2 kg, -28 mesh) used in the study was

dispersed in a fixed-bed reactor (3" ID1 using 3/8" lntalox ceramic saddles (ca. 1.2 k g) to prevent air channeling.

The temperature of the coal bed was controlled by circulating water from a thermostated bath through the reactor's

outer jacket. The water circulator maintained the coal bed a t temperatures up to 80°C to within 1OC. Two thermocouples

were located at 1/3 and 2/3 of the coal bed height to monitor bed temperature.

Air was introduced to the bottom plenum of the reactor at 1.7 SCFH. A third thermocouple was inserted in the

reactor bottom to monitor the temperature of the incoming air. Air humidity (80% relative humidity at 2OoC) was

controlled by dividing the air into two controlled-flow parallel streams with one stream passing through two water

saturators in series. Feed air and reactor off-gas, sampled several times for gas chromatographic analysis, showed no

significant difference in oxygen composition, thus indicating that these reactors are being operated at differential

conditions and that the air flow is sufficient for uniform coal weathering. Periodic samples were taken from the weathering

units as follows. Bed back-pressure, typically 0.5 psig, was monitored to ascertain the absence of channelling (14)

Weathered coals were sampled periodically and characterized by a variety of relevant techniques,

Ultimate and proximate analyses, forms of sulfur, Gieseler plastometer, free swelling index (FSI), Audibert-Arnu

dilatometer, heat of combustion, slurry pH, alkaline extraction and petrography.

The froth flotation performance of the fresh and weathered coals was measured as a function of weathering time

at several collector dosages.

The weathered coals were also characterized by Fourier Transform infrared spectroscopy (FTIR) and X-ray

photoelectron spectroscopy (XPS) to study the chemical nature of weathering.


Properties of laboratory weathered coals were compared with those of a naturally weathered coal

Coal recovery in the froth flotation tests using frother (9.6 mg MlBClL slurry, 0.38 Ib MlBClton coal) but no

collector showed a more gradual decrease with weathering time Clearly, coking caking properties are lost more rapidly

than flotation recovery

FSI is obviously less responsive to early weathering then Gieseler plastometer and Audibert-Arnu dilatometer

measurements

The relative importance of the effects of organic matrix and pyrite oxidation on flotation recovery appear to be

dependent on the weathering temperature. If this is the case, it would appear imperative to use realistically low

temperatures to model natural weathering (14)

• In this study sample were collected from an outcrop (most oxidized) and the mine highwall (least oxidized), and

at an intermediate location to obtain an oxidation profile across the strip pit.

Two high-volatile bituminous coals (Pittsburgh and Harlan seams) were used in long-term, room-temperature

oxidation treatments. The -60 mesh coals were put into several 100-ml beakers and stored separately in dry, ambient and

humid air atmospheres at room temperature in the laboratory for 950 days.



The dry environment was simulated by placing the coals in a desiccator containing Drierite, whereas the humid

environment was simulated by placing the coal in a similar desiccator vessel, but with water present. The ambient samples

were stored in beakers open to the laboratory atmosphere. (15)

57Fe Mossbauer spectroscopy and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy were used

to investigate the oxidation of minerals and macerals, respectively, and measurements of the Gieseler plasticity were made

as representative of an important technological property, coal fluidity, that is affected by oxidation.


Alkali extraction test is used in the metallurgical coal industry to detect coal oxidation prior to the coking

operation. It is noted that the badly oxidized coal taken from the outcrop of the pit has a transmission value of only 30%,

while the least oxidized highwall coal has a value of 97%.

• The coal was oxidized at room temperature after being crushed to 70 mesh and spread thinly in Pyrex trays. No

attempt was made to control the ambient conditions where

the temperature remained at 200C and, 50% relative humidity during the course of the experiments. The samples

were then used ‘as is’ for the TG-FTIR analysis

one fresh western Canadian coking coal as well as samples of the coal weathered at room temperature for varying

times (68, 95 and 115 weeks)

(18)


Most of the oxygen incorporated into the coal structure during extended periods of oxidation produces water as a

result of pyrolysis.

• The early stages of low temperature oxidation/weathering involve the formation of organic peroxides.

These peroxides decompose at low temperature below the softening point of bituminous coals producing water,

which may be used as a marker for the onset of oxidation.

• Most of the oxygen incorporated into the coal structure during extended periods of oxidation produces water as a

result of pyrolysis.

• TG-FTIR may be used to detect and follow the progression of coal oxidation processes with ease.

Effect of Coal Weathering on Mineral Matter

Except for the iron bearing mineral pyrite which is readily air-oxidized, the coal inorganic constituents apparently

remain unchanged by weathering.

Methods for Measurement of Oxidations

Since coal is a very heterogeneous material and its properties differ according to rank and seam, it is very difficult

to define reliable standard values for the degree of weathering. (9)



Therefore, most methods for determining the degree of weathering provide relative Values have practical

usefulness only if measured values can be calibrated against coal samples weathered under carefully standardized

conditions.(9)

There are many empirical methods used to detect oxidation that essentially compare "fresh" coal to their oxidized

counterpart, with the assumption that an in-situ coal seam sample is unweathered, which may not be the case

(Marchioni, 1983).

In the past, many different attempts have been made to measure the degree of

weathering of a given coal sample.(9)

• Determine the oxygen content by oxidative, reductive, or pyrolytic methods

• Change in caking properties of coal (swelling prop of coal)

• Alkali extraction test

• Fourier Transform Infrared Spectroscopy (FTIR, PA-FTIR, TG-FTIR)

• Petrography

• Thermal analysis methods, e.g., thermogravimetry (TG)

• The Zeta potential of coal particle suspensions in H20

• coal slurry pH measurement

• XPS

• Froth flotation

Determine the Oxygen Content

Combustion techniques are commonly used in the analysis of coal and organic compounds in general for the

determination of elemental hydrogen, carbon and nitrogen according to ASTM D3176-89 [1]. For oxygen, the method in

common practice involves the determination Fby-difference_ from directly determined values for moisture, ash, sulfur,

hydrogen, carbon and nitrogen.

In spite of the inherent errors of this approach, which may be significant, it must be recognized that, in many

cases, oxygen values obtained Fby-difference_ are adequate; for others, such as studies of coal weathering, more accurate

values are required (16)

Most oxygen measurement techniques are time-consuming and unreliable.

Hydrogen~carbon and oxygen/carbon atomic ratios

The atomic ratios, H/C and O/C (calculated from ultimate analyses), for each suite of samples are plotted on

Figure 9a, b. Also shown on these diagrams is the approximate position of Seyler's "band of normal coals". There is

generally a marked decrease in O/C and an increase in H/C with increasing sample depth, under decreasing effects of

weathering



Near-surface coals plot well away from the normal band on the sub-hydrous side. Plots of less weathered coals

show a trend toward the normal band. Without any knowledge of the properties of the fresh coal it is clear that G21 to 14

m (Figure 9a), all samples in $2 and the shallowest samples in Adit 2 and Val d'Or (Figure 9b) are altered by weathering.

Use of the H/C--O/C diagram clearly delineates samples of high and intermediate degrees of weathering without

any prior knowledge of the coal's rank or composition and this is very useful. If data is available on fresh equivalents, the

method can be further refined; however, it doesn't appear any more sensitive to weathering than other chemical tests and is

less sensitive than rheology and staining (21)

As a result of oxidation, carbon and hydrogen content has been found to decrease whilst oxygen content increases

Calorific value undergoes a fairly rapid decrease with oxidation

Caking Properties of Coal

Swelling properties of coal can be measured by means of Free Swelling Index (FSI), dilatation, Gieseler fluidity.

In the early stage of weathering, Gieseler fluidity appears to be the most sensitive of the three methods. Under moderate

and severe weathering conditions, however, FSI is a good index of weathering,

coking caking properties are lost more rapidly than flotation recovery. FSI is obviously less responsive to early

weathering then Cieseler plastometer and Audibert-Arnu dilatometer measurements (14)



Fluidity is measured using the Gieseler plastomete (ISO) The imp results are maximum fluidity, DDPM, Intial

softening point and resolidification temp.

Where properties of the fresh coal are known, rheologic tests should be sensitive to mild levels of weathering (21)

Fluidity of coal is controlled by:

• petrography and rank.

• grainsize.

• chemistry.

• the amount, grain size and chemistry of ash.

• aging and oxidation 2

Alkali Extraction Test

Lowenhaupt and Gray applied this akali-extraction method using light transmittance as an index of weathering for

high to low volatile bituminous coals.(9)

U.S. Steel has developed the alkali-extraction test as a means to identify oxidized (weathered) coal in coal blends.

In the test, coal is boiled in a caustic solution so that oxidized coal dissolves and darkens the solution. The solution is then

tested for light transmittance, with the solution transmittance decreasing as the coal oxidation increases. A transmission

value less than 80% indicates that the coal is too oxidized for metallurgical use. This rejection limit corresponds to 8 to

12% microscopically recognizable oxidized coal. (12)

Petrographic determinations of oxidized coal in four ranks of metallurgical coal show a linear relationship

(0.964 correlation coefficient) with corresponding transmission values and a detection limit of three percent oxidized coal

for the test.

In addition, the oxygen content and infrared-band intensities of these coals also show a linear trend with

transmission values. The study indicates that the alkali-extraction test is a reliable test for detecting oxidized coal in



metallurgical coal blends and is superior to the Free-Swelling Index for setting mining limits in stripping operations. U.S.

Steel is presently using the test to monitor captive and purchased coals for acceptance as metallurgical coal (12)

alkali extraction test is used in the metallurgical coal industry to detect coal oxidation prior to the coking

operation. Usually, a coal with a light transmission value less than 80% is regarded as too oxidized for metallurgical usage

(7). It is noted that the badly oxidized coal taken from the outcrop of the pit has a transmission value of only 30%, while

the least oxidized highwall coal has a value of 97%. (15)

Fourier Transform Infrared Spectroscopy (FTIR, PA-FTIR, TG-FTIR)

FTIR in some cases Fourier Transform infrared (FTIR) spectra were collected of the neat samples

(not ground further after removal from weathering unit) (14)

FTlR oxidation index is defined as the r a t i o of the integrated intensity of the carbonyl band (1635-1850 cm-') to

that of the C-H stretching band (2745- 3194 cm-’) in the diffuse reflectance FTlR spectrum of coal. Increases in the

oxidation index upon weathering can be attributed to progressive oxidation of C-H groups to carbonyl groups as the coal

weathers (13)

The oxidation index of Illinois 6 coal not only increases more rapidly with weathering time, but it also has a

higher initial value than Pittsburgh Seam and Horsepen Seam coals. This indicates that both the initial value and the rate of

change with time of the oxidation index are rank dependent.(13)

There is a general linear relationship between the oxidation index and FSI of Illinois 6 coal Weathered at both 25

and 80°C. The alkali extraction test also shows a general linear relationship with oxidation index for the Feathered Illinois

6 coal (Figure 5). (13)



TG-FTIR

During rapid pyrolysis of coal, TG–FTIR (thermogravimetry – Fourier transform infrared) technique can be

effectively used to simultaneously detect and measure the three main O-containing gases, namely H2O, CO and CO2.

Their sum corresponds to the quantitative amount of oxygen in the coal and is, in general, inherently more accurate than

the Fby-difference_ values (16)

Pyrolysis techniques are commonly used in the analysis of coal and organic compounds in general for the

determination of elemental hydrogen, carbon and nitrogen according to ASTM D3176-89 [1]. For oxygen, the method in

common practice involves the determination by difference from directly determined values for moisture, ash, sulfur,

hydrogen, carbon and nitrogen In spite of the inherent errors of this approach, which may be significant, it must be

recognized that, in many cases, oxygen values obtained by difference are adequate; for others, such as studies of coal

weathering, more accurate values are required.'

For a number of years we have used this technique routinely in our work on coal oxidation where small changes

in oxygen content are important [2]. However, although we have found this technique both useful and reliable, it does not

provide information about the chemical speciation of oxygen in coal. From a scientific point of view, the variation of

functional group composition with rank is of great interest. This has led us to consider the use of thermogravimetry

coupled to gas analysis by infrared spectroscopy (TG-FTIR) to measure organic oxygen in coal directly (17)

Petrography

Petrographically observed variations in coal which have been reported in the literature as attributable to oxidation

and weathering include (21)

• the formation of rims along grain boundaries and microfractures,

• the formation of micropores and microfissures,

• increased relief and variation in vitrinite reflectance

Artificial oxidation caused the formation of bright rims whilst natural weathering processes resulted in dull rims.

Low-rank coals were unlikely to produce rims



When oxidized for durations ranging from 1 to 100 h at low temperature (70°C), the character of the oxidation rim

varies depending upon the rank of coal. The development of darkened rims around the outer edges of particles of low rank

coal is contrasted against the development of bright particle-rims that exhibit a higher reflectance in the higher rank coals.

(22)

Other petrographic techniques utilised in studies of coal weathering and oxidation include

• Differential staining technique (Gray et al., 1976),

• Microhardness variation {Nandi et al., 1977) and

• The reflectance of vitrinite in char {Pearson and Creaney, 1981).

Staining Technique

A petrographic staining technique showed the most sensitive response to weathering. This test is considered the

most suitable of these for the detection of weathering, as it is independent of prior knowledge of the properties of the fresh

coal and of the petrographic composition (21)

The staining technique is outlined by Gray et al. (1976) and has been revised (R.J. Gray, pets. commun., 1980).

The polished petrographic sample is etched with potassium hydroxide, immersed in a solution of saffranin "0" in alcohol

and examined petrographically.

Fresh, relatively high-rank coals are unaffected by the stain, however, zones of oxygen ingress (grain margins and

fractures) are highlighted by staining, becoming light-green (mildly oxidized) through yellow-green to yellow when highly

oxidized.

In low-rank coals the surface is destroyed by etching (as they are alkali soluble when fresh) and this part of the

test is excluded. In addition, because of their chemistry, fresh coals take on a light green stain but those affected by

weathering may still be detected by the intensification of the staining on grain margins and fractures

In this study, samples were stained and the proportion of grains which had taken-up the stain was determined.

Staining with Saffranin-O {after Gray et al., 1976) revealed all grains affected by the stain to a depth of 14 m,

thence a decreasing proportion to 10% at 23 m, indicating minor weathering to this point. Even with extremely long

etching time (up to 1 hour) samples beyond 23 m showed no affect of staining and are apparently unweathered. This

technique is a more sensitive parameter of weathering than reflectance analyses or textural observations (21)

Staining (with Saffranin-O} appears to be the most sensitive petrographic parameter. This method can detect very

low levels of weathering and is as sensitive The main advantage of this test is that it can detect weathering without any

knowledge of fresh coal characteristics and is thus suitable to detect the weathered zone in relatively unexplored areas and

to detect weathering in single samples. In addition, this test is not influenced by variation in petrographic composition (21)

Reflectance

Reflectance cannot define weathering in individual samples unless an "expected value" for fresh coal is available

Bireflectance also increases in weathered samples (as in laboratory experiments; Benedict and Berry, 1964) but is not

considered suitable as a weathering indicator.,



Me- Hugh et al. (1991) propose the use of fluorescence microscopy in the detection of low level oxidation in

bituminous coals by establishing a prof'de of fluorescence intensity levels for "fresh" coal of all rank, then essentially using

fluorescence as a comparative technique, (22)

The proposed oxidation quotient (O/Q) is a ratio that combines the mean fluorescence intensity value measured at

a fixed wavelength (e.g. 550 nm) and vitfinite/huminite reflectance, determined on the same coal, under standardized

conditions, and calculated by:

O/Q = Fluor. intensity/% Ro max

where O/Q = the oxidation quotient,

intensity=the mean maximum fluorescence intensity of % Ro max -~" the mean maximum reflectance.

Vitrinite elasticity tests (23)

Nandi et al. 3 found that different of types of microhardness impressions were obtained on coals of different rank,

and that oxidation could affect the physical state of fresh vitrinite so that a plastic state could, depending on the rank of the

coal, be transformed into an elastic state.

Based on these findings, impression types were used to evaluate the changes in the physical nature of vitrinite in

coals brought about by weathering

The impressions obtained were categorized according to their nature: plastic, plastic brittle, brittle, elastic brittle

or elastic. This information was used to develop an empirical formula for calculating an elasticity index (El) for each

sample.

An EI of 0-20 indicated that the majority of vitrinite particles exhibited predominantly plastic properties, 20-40,

brittle properties, 40-60, elastic-brittle properties, 60-80, elastic properties, and 80-100, highly elastic properties

Fluorescence measurements and elasticity tests showed very good agreement in the evaluation of oxidation levels.

They appear to be more reliable than chemical or rheological methods. This may be because the determinations of

chemical, swelling and caking properties are bulk-oriented techniques, whereas fluorescence and elasticity measurements

are taken exclusively on vitrinite The two methods are based on different physical properties of coal. However, their

sensitivity in detecting changes caused by oxidation is equally high. The practical applications of both techniques may be

wide-ranging. They have already been successfully used for the detection of oxidation of coals in stockpiles

• Thermal analysis methods,

Thermal analysis methods, e.g., thermogravimetry (TG), also show a strong response to weathering

• Zeta potential of coal particle suspensions in H20

The Zeta potential of coal particle suspensions i n H20 can also be a good indicator of weathering. weathered coal

shows lower Zeta potential values than fresh coal

• Coal slurry pH measurement

A simple pH titration method based on rehydrating coal s l u r r i e s with water a t 150OC was proved to be a



successful way o f monitoring weathering effects in all ANL-PCSP coals (9)

Coal slurry pH values are used to characterize samples. It has been suggested as a means to indicate the degree of

oxidation or weathering (11)

CSR in all cases decreases with an increase in weathering time The CSR is primarily dependent on the plastic

properties of coal which are known to deteriorate with oxidation of coa1

Coke Strength After Reaction with CO2 (CSR) is an important measure of coke quality for blast furnace operation

This study was undertaken to predict changes in CSR values of coke caused by weathering of coals during storage.

CSR values of coke were compared with a variety of feed coal properties, including pH of a methanol/water soak.

The results indicated that an increase in coal oxidation resulted in a drop in pH of methanol/water soak. CSR generally

dropped with a drop in pH for all the coals. However, good correlation existed between CSR and pH for lower rank

(high volatile) coals.(10)

Correlation of Change in pH to CSR

The changes in coal quality were correlated to coke quality. The increase in coal weathering resulted in

deterioration in CSR and was accompanied by a drop in pH of methanol/water soak

On the basis of results from this study, a coal oxidation monitoring plan for the lower rank (high volatile) coal has

been devised as follows:

• Obtain the pH of the incoming coals from the respective mines.

• Identify the placement of incoming coals in the coke plant yard.

• Monitor the drop in the pH of coal in the coal piles.

• Estimate the loss in CSR by using a set of graphs that depict a drop in pH versus a drop in CSR. Figures 4 show

one such example.

• Once the coal oxidation has affected CSR in such a way that the target CSR is not met, as indicated through the

drop in pH. then the usage of oxidized coal in the blend should be redefined.

• If new high volatile coals are brought in. the graphs of a drop in pH versus CSR could be developed while

monitoring the new coal pile and verifying the results through pilot oven carbonization (10)

XPS

XPS shows that the surface organic O/C r a t i o increased more rapidly than the bulk O/C r a t i o obtained by

ultimate analysis, indicating the sensitivity of the coal surface to oxidation (weathering).(14)

Froth Flotation

Froth flotation recovery of the weathered coals deteriorated with increasing degree of weathering and showed a

different dependence on temperature than maximum fluidity. Flotation recovery can be largely restored by increasing the

collector dosage used. (14)



Techniques such as the Gieseler plastometer and the Free Swelling Index test (e.g. Huffman et al., 1985), the

alkali-extraction test (Lowenhaupt and Gray, 1980) or cationic staining (Gray et al., 1976; Axelson, 1987) are all extremely

sensitive to the onset of oxidation, even before changes in chemical composition become apparent;

however, low rank "fresh" coal has both a higher oxygen content and lacks sufficient thermoplasticity for such

tests to be fully effective. The measurement of huminite and vitrinite fluorescence, at a fixed wavelength (e.g. 550 nm), is a

potentially useful technique for the detection and evaluation of oxidation in low rank coal (Bend et al., 1989; Quick et al.,

1989; (22)

Prevention from Weathering

There are many study to minimize the self burring of coal (Boiler grade) using certain chemicals, but for coking

coal, very few study has been done. Suitability of these chemicals can be studied for coking coal also

The one of the major limitation, will be what will be effect of coke quality and how it behave in blast furnace this

must be studied before finalizing that chemical.

• Laboratory studies conducted by the Bureau of Mines to evaluate the effectiveness of 10 additives to inhibit

the self~heating of coal. (6)

The relative effectiveness of the additives was determined by the observed changes in the minimum SHT's of the

mixtures, or by the time required for the sample temperature to reach

150"C, compared with the untreated coal and a coal-water blank.

Sodium nitrate, sodium chloride, and calcium carbonate were found to be the most effective inhibitors, followed

by ammonium dihydrogen phosphate, calcium chloride, ammonium chloride, sodium acetate, and potassium chloride.-

• To prevent oxidation of coal sample, it is suggeseted that tempratures during storage be kept as low as possible

Antioxidants (hydroquinones amine etc) added to coal have little effect, it will not offer protectio against

oxidation for more then a few month. in case of small piles resaonable protection obtained y spraying the

surface of pile with petroleum products (or aqueous emulsion of hydrocarbon) fatty acid. (8)

REFERENCES

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5162(80)90006-3

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Giroux a, J.T. Price a, M.A. Khan b aCANMET Energy Technology Centre, 1 Haanel Drive, Nepean, Ontario,

Canada K1A 1M1 bFording Coal Ltd., 205 Ninth Avenue S.E., Calgary, Alberta, Canada Accepted 30 November

2002

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J.-P. Charland, J.F. Gransden, J.T. Price CANMET Energy Technology Centre-Ottawa, NRCan, 1 Haanel Drive,



Ottawa, Ont., Canada K1A 1M1 Received 9 October 2003; revised 5 February 2004; accepted 28 February 2004;

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Miranda Instituto de Carboqulmica, C.S.I.C., P.O. Box 589, 50080 Zaragoza, Spain Received 7 July 1995

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Instituto Nacional del Carbo´n (INCAR), CSIC, Apartado 73, 33080 Oviedo, Spain Received 24 May 2002;

accepted 4 October 2002

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METHODS, DAVID L. MARCHIONI*, Geological Survey of Canada, Institute of Sedimentary and Petroleum

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accepted September 2, 1982)

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The University of Regina, Regina, Sask. $4S 0.42, Canada (Received October 27, 1992; revised version accepted

December 29, 1992)

23. Detection of the incipient oxidation of coal by petrographic techniques Krystyna J. Kruszewska and Vivien M. du

Cann* Faculty of Earth Science, University of Silesia, ul. B~dzi6ska 60, 41-200 Sosnowiec, Poland *Division of

Materials Science and Technology, CSlR, PO Box 395, Pretoria 0001, South Africa(Received 5 July 1995)

WEATHERING OF COAL- EXPERIMENTS ......The bituminous coals, which cover the rank from high volatile...

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