Science in the Coal Industry

Science in the Coal IndustryAuthor(s): Charles EllisSource: Proceedings of the Royal Society of London. Series B, Biological Sciences, Vol. 139, No.897 (Jul. 10, 1952), pp. 449-463Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/82659 .

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Science in the coal industry

By Sir Charles Ellis, F.R.S.

{Lecture delivered 8 November 1951?Received 10 December 1951)

[Plate 31]

The activities of the coal industry extend over a wide field. There are many varied

aspects of work underground, and on the surface there is the chemical engineering task of preparing the coal for use and distributing it. This wide range of activities

brings with it correspondingly a wide field for the application of scientific methods.

The problems are also the more diverse because of the variety of coals and the

complicated geological stuctures of the coalfields. In this paper I deal primarily with that aspect of science in the industry which is concerned with research, but

to show this in its proper perspective I must refer briefly to the other scientific

fields.

The scientific services

We have found it convenient to group these under the broad titles ' scientific

control' and 'research'. Scientific control comprises in the first place a scientific

service in each of the nine Divisions to deal with the day-to-day work and problems of a local nature as they arise. The first responsibility of this daily service is to

provide the routine test data required by law in order to secure safety against

explosions. It comprises the regular testing of the roadway dusts and the mine air.

Wherever diesel locomotives are used regular tests of the exhaust gases are called

for. In more recent years, in an effort to combat pneumokoniosis, the sampling and

measurement of the airborne dust is carried out to an increasing extent. The

sampling and analysis of the various grades of coal marketed from the 1000 collieries

is now a routine and comprises a wide range of tests on many thousands of com?

mercial sorts of coal. Other routine services common to a large engineering industry have also been developed. On this occasion I am not going to describe these

activities in more detail except to say that there is now at least one good laboratory and sometimes more than one in each of the forty-nine Areas, and in most Divisions

a Divisional laboratory to deal with the more specialized problems. There is also

a laboratory at each of the fifty coke-oven plants belonging to the industry. In addition to this service and also in the Directorate of Scientific Control is the

Coal Survey which is run on a national basis and has the duty of measuring the

physical and chemical properties of the reserves of our coal seams. This service, inherited from D.S.I.R., has been considerably expanded under the Board to meet

the demand for more precise information on the nature and quality of our seams

without which well-founded schemes of reorganization and new sinkings cannot

be undertaken. The Board have embarked on an extended programme of explora? tion of our coal resources by putting down many deep borings from the surface.

Vol. 139. B. (10 July 1952) [ 449 ] 29



450 Sir Charles Ellis

All the rock cores and coal seams recovered from these borings are carefully examined and a detailed analysis of the seams is undertaken. The accumulated

results of the Coal Survey played an important part in the formulation of the

National Plan.

The third branch of this Directorate is what we term the Field Investigation

Group but which perhaps would more usually be called our Operational Research

Group. This work is growing and assuming greater importance. The kind of problems it has tackled are to determine the factors influencing the rate and costs of driving

underground roadways in rock, the comparative costs of various forms of transport

by either diesel locomotive, battery or trolley locomotives, by rubber belts or by

rope haulage systems. Various specialized features of mechanized mining,

particularly the use of power stowing machines, have also been investigated. This

type of work is well known now in industry and is generally recognized as an

essential part of the daily scientific work.

It is against this background of scientific services in the coalfields that the

Research Department must be considered.

The scope of this paper will not permit more than a passing reference to research

on health and human problems in the mining industry. This important activity is now being led by Dr Rogan, formerly of the M.R.C. and now our Chief Medical

Officer. In carrying out this function he is advised and assisted by a number of

independent scientists, grouped into four panels dealing with epidemiology,

physiology, psychology and industrial medicine.

For studying the performance of the various coals the Board depends mainly on

its membership of the British Coal Utilization Research Association (BCURA), but on the preparation of fuels, carbonization and such, the major effort is at

our own research establishment at Stoke Orchard near Cheltenham. This is

supplemented by membership of two research associations, the British Coke

Research Association and the Coal Tar Research Association.

Research on underground problems is also at the moment carried out at the

Central Research Establishment (CRE) at Stoke Orchard but we are now making

arrangements to set up a separate establishment for this purpose, leaving CRE

primarily for the chemical engineering side. Lastly, and indeed a most important

part of our research effort, are those projects which are carried out in universities.

I shall not attempt a review of the programme of work actually in hand; to deal

with it on broad fines would mean speaking in generalities and to go into details

on all projects would be impossible. I will therefore choose a few of the fields to

indicate the type of work and problems with which we are faced.

Drilling

There is a great deal of work in connexion with the extraction of the coal from

the coal face and arising from the underground work generally. We are just at the

beginning of a scientific approach to a subject which has previously been treated

entirely empirically. The success these empirical methods have had is obvious and

the reason why this type of approach has been adopted is also clear, but I am



Science in the coal industry 451

profoundly convinced that it is time to adopt a different attitude. We are now

beginning to investigate matters like the cutting of coal, the problem of moving material, whether it be air, coal, water or stone, by attempting to start from

a proper understanding of the phenomena and not only by trial and error using

large-scale apparatus. I can illustrate the situation by describing some work that

is in hand on the drilling of holes in stone.

Drilling represents a particular case of the general process of cutting one material

with another, a process which is of great importance in this industry with the wide

use of coal cutting machinery. The forces involved in drilhng are the thrust and the

torque applied to the drill and we are interested in the penetration that can be

T=C(B+juA) j x

o

thrust

grinding

Figtjbe 1 a. Cutting and grinding phases.

pulverized zone

crush test

Figure 1 b. Drilling force and crushing strength.

29-2




achieved per revolution in relation to the thrust that is applied, and also how much

work is required to achieve this. The general relationship between thrust and rate

of penetration which is found broadly for all rotary drills is illustrated by figure 1 a.

At low thrust the grinding phase occurs, only thin slices of material are taken and

there is a tendency for the cutting edges to bounce away from the mineral surface.

In any case the drill surfaces travel a considerable distance in their helical paths to achieve a given amount of advance and they suffer a great deal of wear. In

addition, this tends to produce a great deal of dust. The cutting phase develops

fairly suddenly as soon as the thrust is sufficient. The rate of penetration increases

rapidly and once on this straight line crushing occurs under the foot of the drill

at B and the cutting edges really dig into the rock and tend to sheer off coarse

fragments of debris (see figure 16). After a while however the clogging phase is

reached where there is difficulty in getting rid of the debris from the hole as fast

as it is produced. From the purely empirical standpoint it is clear that a great

improvement would be made if it were possible to defer the clogging phase and

render possible the use of greater thrusts. This has been achieved by Winder with

water

supply

exhaust port for drillings

spring catch

eccentric

cutting tips

driving dogs

drill tube

slot water operating position tubes of water outlets

bit

Figure 2

an ingenious new type of drill which is shown in figure 2. It will be seen that the

drill is so to speak turned inside out, the cutting pieces are fixed pointing inwards

on the edge of a hollow tube and the cut stone is flushed down the centre core of the

tube by water supplied at the cutting tip. Quite apart from anything else this

provides perfect dust suppression. The results obtained with this new drill are

greatly superior and are shown in table 1.

Table L Comparative drill performance

Conventional rotary Hollow rotary

thrust (Lb.)

4000 2000

power (h.p.)

10 5

penetration work/ft. penetration (ft./min) (10-3 ft.Lb./ft.)

82 27

It will be seen that the conventional rotary drill requires 10 h.p. at thrusts of

4000 Lb. to achieve 4 ft./min, whereas the hollow drill with half the h.p. and half

the thrust has 50 % more penetration. At the same thrust, that is, 2000 Lb., the




conventional rotary will only do 3 ft./min. This is an example of a successful

empirical approach, but I venture to hope that it is also the beginning of an attempt to understand the general problem of cutting stone and coal.

It is significant that the simple picture of the process shown in figure 1 a and b

lead to values of the crushing strength only about twice those actually determined

by a crush test. A better agreement could scarcely be expected since, as indicated

in figure 16, the characteristic of the cutting phase is the shearing off of relatively

large pieces by the forces transmitted through a crushed region at the tip of the

drill.

A phenomenon which has been noted by many observers and which undoubtedly has a bearing on the problem is that the tungsten carbide tips which are so hard

that they cannot be scratched by the rock or coal, often suffer damage and tiny

pieces flake off leaving small pits. The appearance of a worn cutting edge is often

identical with that of a sand-blasted surface. This must be due to local forces of

very high intensity, but the exact part that these play in the process is not yet understood. We hope to get more information on this by activating the cobalt

which serves as the bonding material in the cemented tungsten carbide and finding out from the radioactivity of the debris how the wear occurs. The sensitivity of

this method should, we hope, permit experiments under interpretable conditions.

I could instance several other problems connected with underground work where

immediate practical results are often achieved without any proper understanding of the underlying phenomena and, welcome as this is, we are fully seized with the

necessity for filling this gap.

Coal preparation

Once the coal has been brought to the surface its preparation and treatment for

the market presents a wide range of problems and to these must be added the

detailed study of the performance of the many different types and grades of fuel

that are produced. From this large choice of subjects I shall choose the cleaning of coal by froth flotation as my next example.

This method of cleaning has been brought to a high degree of perfection in

metallic ore concentration and is founded on a precise understanding of the mutual

surface reactions of the liquid and ore. There are cases when by differential froth

flotation as many as three minerals are separated. The same problems will be

required to be solved for the coal industry but it may not be generally appreciated what a very wide and continuous range of coals has to be treated. Coals in this

country range from anthracite to the high volatile coals. In fact coal is not a sub?

stance but a class of substances with a continuous variation in properties from one

end of the class to the other. At present every member of the class, that is the coal

from any one seam, has to be treated as an individual and separately. Admittedly,

general experience and the relation of the coal to other coals are a great help but

separate investigation is necessary before any specific process such as froth

flotation can be applied to it. A determined attempt is now being made to ascertain

the reason for the very great difference of behaviour shown by British coals to




froth flotation and this will involve not only understanding the exact role of the

surface properties of the coal but also determining how these properties vary

throughout the class of coals.

This will prove a long task, and in the meanwhile, as a matter of practical

expediency, we have constructed an elaborate semi-scale test rig where coals can

be investigated under operating conditions on a scale of up to 2 tons per hour.

By careful design and modern instrumentation it has been possible to speed up the

actual tests and therefore to some extent mitigate the labour of purely ad hoc

empirical investigation. The process of froth flotation consists in essence in suspending the mixture of

fine coal and ash in a very dilute aqueous solution of the 'frothing agent' which is

often a slightly soluble polar organic compound. A stream of fine air bubbles is

passed through the suspension and if the process is successful the air bubbles

attach themselves to the coal but not to the ash. The coal particles therefore float

to the surface in the froth and can be scraped off.

This is largely a matter of surface chemistry, and there is nothing strange in the

fact that different members of this class of coals differ markedly in their behaviour

to froth flotation. Up to the present however it has not proved possible to make

any real progress in sorting out the various factors which are involved, although what I might term most important tactical successes such as separation of witherite

from fine coal are being achieved. However with temperature dependent phenomena such as viscosity and surface tension involved it is obvious that the overall effect

of temperature is worth careful investigation. Some recent results of Whelan at

CRE are shown in figure 3 and are very suggestive. It is not surprising to see the

rise with increasing temperature but the suddenness of the transition to a practical

lOOr-

80

I s f 60

40

St John's Gresford

10 20 30

pulp temperature (? C)

Figure 3

Division

S.W. N.W.

Bedlington N. (N. and C.)

coal rank code

203 502

702/602

carbon d.a.f. (%)

90-91 85

83-84

V.M. d.a.f. (%)

18-19 39

39-40




independence of temperature is striking and may enable the relative importance of surface tension and viscosity to be estimated. The difference of behaviour of the

three coals is well brought out by the different run of these curves.

The structure of coals

It will now be abundantly obvious that if these two examples are fairly typical of where we stand in investigations into coal matters then in this particular industry, in contrast to many others, there is not a great mass of fundamental knowledge

awaiting application and it is basic research that we need. I am convinced that the

difficulty in doing basic research arises partly from the wide variety of coals. Until

we understand in more detail how the different members of this class of substance

are related to one another, results obtained on one coal will have little application to those of another and we shall never be able to get away from the tremendous

burden of almost infinitely repeated experiments. There is undoubted evidence that a class exists and that a generic relation exists

between the members of the class. If any two properties of coal are taken and

plotted against one another the resulting points He in a broad band. This generic relation is due to a common origin and to the fact that the coals we have today have

progressed to varying stages along a more or less similar process of coalification by heat and pressure. This is often summed up by use of the broad term 'rank'; coals

which are oldest and have suffered most being called 'high rank', the younger ones, still with a lot of volatile material in them 'lower rank'.

The question is whether this coherence in the class can be related to some common

basic structure. I shall describe a physical approach to this problem although

many people would maintain that, since under the microscope every minute portion of a coal is almost invariably a highly complex mixture of different materials, this

is rather like attempting to study the chemistry of plants on the basis of chemical

examination of the whole plant. I will excuse my artificial simplification of the

problem by saying at once that I recognize the limitations but yet I believe it is

the way to make some advance and find out something about the structure of coals.

H. I*. Riley has studied in great detail the X-ray diffraction lines from coal and

by measuring their width has endeavoured to form an idea of the size of the

diffracting units. He thus concluded that in a wide range of bituminous coals there

were domains of ordered structure some 20 A in size. He thought some consisted

almost exclusively of layers of carbon atoms spaced at approximately 3-5 A

together with others which he characterized as 'bitumin\ essentially less flat and

less aromatic and with a layer plane spacing of up to 4-5 A.

Broadening of spectrum lines is a difficult technique to use at any time since

instrumental errors are liable to enter directly into the result, but Riley's views

were of such interest that I decided they must be followed, up We are fortunate

in having the support of the Cavendish Laboratory and P B. Hirsch is now

re-examining this whole question using a variety of methods.

His results to date can only be considered as preliminary but with that reserva?

tion I should like to refer to them. He has taken X-ray powder photographs using




Cu Koc radiation monochromatized by reflexion from a plane Hthium fluoride crystal, and with hydrogen in the camera to avoid scattering. So far he has onlyused one coal.

The films were photometered and the corrected intensity put on an approximate absolute scale by assuming that at high angles the scattering intensity was equal to that scattered by the same material in a truly amorphous state. After subtracting the incoherent scattering the resulting intensity curve {a) is shown in figure 4.

An attempt was then made to compare this curve with that to be expected from

2-5i? 002

0-4 0-8

2 sin 0/A

Figure 4

randomly-orientated layers of the graphite type consisting of carbon atoms at the

corners of linked hexagons, the distance between the centres of neighbouring

hexagons being 2-46 A. Reasonable agreement (see curve b) was found using the

Warren formulae if 90 % of the carbon was assumed to be in such layers and that

these were about 13 A in diameter. The fit for the 11, 20 and beginning of the 21

bands is promising. The disagreement in absolute intensity of the 10 band is found

in all substances of the graphite type and is probably due to a non-spherical sym? metric scattering factor of the carbons in these substances. The strong peak in the

002 band however is not reproduced by this calculation and it is most interesting that, to give this peak, the scattering unit has to be assumed to be more complex and to consist of at least three of the graphite type layers. The calculated curve

(c) is also shown and for this the layers are assumed to be randomly displaced

parallel to themselves but at a uniform spacing of 3-45 A. This result confirms

Riley's views. Although the work is at an early stage I think we are justified in

accepting Riley's other observation that this same type of sub-structure occurs in

a wide range of coals from low to high rank.




Riley also investigated cokes and found evidence there for the existence of

a somewhat similar unit. Very detailed information on this has recently been

reported by Franklin. Using the complete X-ray spectrum she has studied the

dependence of the structure of cokes on the temperature to which they were heated.

Cokes prepared from a variety of organic substances, pitch, petroleum and coal

were investigated and in all cases Miss Franklin detected the existence of crystal? lites consisting of a number of parallel graphite-lake layers together with a certain

amount of non-organized carbon. On gradually raising the temperature of pre?

paration from 1000 to 3000? C the non-organized carbon disappeared, the crystal? lites grew both in size and number of layers and in the graphitizing substances

three-dimensional order began to appear. Now, the important point in the present connexion is that the crystallites which Franklin detected and whose growth she

was able to follow in such detail seem to be in principle the same units that exist

in coal. There is the same random-layer structure and Franklin's relation between

layer diameter and number of layers when extrapolated to small values agrees with those found by Hirsch.

I shall assume that these subunits have a permanent and independent existence,

and, in fact, I regard them as the basic bricks of which coals are made.

However, before examining this possibility I should like to complete the review of

our knowledge in this field and examine the evidence for a grouping of these sub-

units into larger units. This was a view first put forward by Bangham, working at

BCURA, and there is considerable evidence for it. He thought that certainly among the vitrines the subunits were grouped together into something much larger which

he called micelles. Bangham arrived at this conclusion from studying the porosity of coal. Coal is a porous solid and by the normal methods of measuring the density in a gas like helium and also the particle density and the adsorption of the vapours of condensable liquids it is possible to obtain values for both the internal volume

and the internal surface. These are average properties of the coal and quite general

arguments can show that from them we can calculate average descriptions of the

internal pore structure. The values so obtained are consistent with the view that

coal is a compaction of micelles of about 200 A. The structure is similar to what

would be obtained by slightly pressing together glass beads at their softening point. It must also be mentioned that there is clear evidence that the diameters of the

pores vary along their length in a manner characteristic of the particular coal as

would indeed be expected on this view.

This simple and attractive picture, that the subunits demonstrated by the X-ray measurements are grouped together into types of macro-molecules or micelles, was

generally adopted until in the summer of this year first Lecky, Hall and Anderson

and then Malherbe described experiments on the adsorption of nitrogen and argon at low temperatures which suggested far lower values for the accessible surface.

These would give an average pore diameter of some 1000 A which would be quite

incompatible with the fine pores shown by the behaviour of coal as a molecular

sieve. Lecky fully appreciated this situation and suggested that coals might indeed possess very little permanent internal surface. He drew a parallel with the

behaviour of polymers and proteins and concluded that it might be the polar nature




of methyl alcohol or water which enabled them to penetrate into the structure

of coal itself. The great importance of Lecky's views are that they bear directly on the structure of coal and would, if substantiated, force us to abandon the

attractive micellar model. A direct way of settling this question, and one that

would use the coal in its virgin state, would be to measure how it scatters X-rays. If these micelles do exist it might be possible to detect their effect on the scattering at very small angles and distinguish it from the scattering due to the much smaller

pores. Measurements have been made by D. H. Riley and by Nelson but to obtain

evidence that can be used to settle such a vital point of principle requires apparatus and technique which was not then available. The experiment is being undertaken

by Hirsch in the Cavendish Laboratory but the apparatus is still under construction.

I must therefore turn to other, slightly indirect, evidence.

Dry den working at BCURA has studied in great detail the behaviour of coal with

certain so-called solvents. He found that when bright coal is extracted with

ethylene diamine, or other closely related solvents such as monoethanolamine and

diethylene triamine, the yield of extract varies continuously with temperature over a wide range from 20 to 200? C and depends on the type of coal. There seems

little difference between the extract and the residue, and they both have ultimate

analyses very similar to that of the parent coal. It would seem therefore that what

is happening is a dispersion of the coal by the action of the liquid, rather than the

solution of any constituent. This point is important because the dispersed material

is easier to investigate than the raw coal and we wish to apply evidence on the size

of the dispersed particles to the original coal. I shall therefore consider some further

arguments on this point advanced by Dry den.

The amount of extract obtained at a given temperature is small for the high- rank coals, rises rapidly with decreasing rank and begins to level off at the lower

ranks. This variation with rank can be compared with the heat evolved on treating the coal with ethylene diamine. There is in any case a marked change with rank of

the heat of wetting of a liquid like methyl alcohol but the effects with ethylene diamine are distinctive. The point is best brought out by plotting the difference

in the heats of wetting of these two liquids against the carbon content of the coal.

It is then found that the differential heat evolution starts quite suddenly at

88 to 90 % carbon, levelling off after 82 to 83 % carbon. There is a great similarity between these curves showing the heat and the extraction yield and there can be

little doubt that the extra heat evolved is associated with the phenomenon of

dispersion. For convenience I will use a suggestion of Dryden's and refer to the

extra heat shown by ethylene diamine as if it were due to chemi-sorption. We

must then be clear that the measured chemi-sorption heat is at most 40 cal/g, about twice that of physical adsorption heat. Further, if the ethylene diamine is

added slowly, most of this heat is evolved before the coal is actually dispersed, the

energy of interaction showing itself in a very large swelling. Solution only becomes

apparent after a considerable amount of solvent has been taken up, and at this

stage little heat is evolved. It seems a reasonable assumption that the energy

necessary actually to separate the particles which form the dispersion can only be

a fraction of the chemi- or adsorption heats, that is a few calories per gram.



MUs Proc. Roy. Soc. B, volume 139, plate 31

***?- ?.^V v V

1 V ^

-* ?*1%?

SaRW ;'^^SV

O-Ii

Figure 5

(Facing p. 459)




These arguments have been presented to support the view that whatever units

are subsequently found in the dispersion existed as such in the coal and were held

together by forces comparable with adsorption forces. They can scarcely have been

created by the mild action of the dispersing liquid. Two methods have been used to investigate the particles in these dispersions.

Kann, working with Dryden, used optical methods and found no noticeable

difference, except gradual increasing particle size, in the scattering from ten

successive stages of extraction, thus confirming the view that the action of the

liquid is to disperse the coal, not dissolve constituents from it. While there was

a background of scattered light which might have been due to particles smaller

than 200 A there was indubitably evidence of particles in a range of size up to

700 A. In fact, using a method equivalent to that of an ultra-microscope, it was

possible to see the diffraction discs directly. In the second place Nagelschmidt, working at the Safety in Mines Research

Establishment, has examined an ethylene diamine extract* with the electron micro?

scope. The extract, provided by Dryden of BCURA, was diluted with distilled water

to various values between 1:5000 and 1:25000, and microdrops evaporated on to

three Formvar films. One of the photographs is shown in figure 5, plate 31, which is

of a specimen that had been lightly shadowed with gold-palladium to give shadow

lengths of some twice the height of the particles. There are some large particles visible occasionally of sizes up to 0-1 to 0*3/^ and it is not known at present whether

these are aggregates of smaller particles or a genuine constituent of the extract.

What is really important is that there are many particles, apparently rounded, of

the order of 150 to 600 A in diameter and most of them, are between 200 and

300 A.

It thus seems reasonably certain that large micelles are present in the ethylene diamine dispersions and further, for the reasons I have given, that they are present as such in the original coal. This appears to confirm the view that the subunits

shown by the X-ray work are grouped together into larger units. Much more work

will be required before a matter of this importance can be considered settled and

above all to find how widely this holds for all members of the class of coals, but

it is permissible to adopt the existence of micelles as a hypothesis and to see where

it leads us.

We first return to consider the crystallite subunits. The X-ray evidence is quite definite but it only shows the existence of certain domains of order and there are

different physical arrangements which could produce this order. For example, there

might be quite large flat layers of carbon and other atoms, arranged in some two-

dimensional pattern, but the axis of the pattern could only be ordered throughout a relatively small domain. The boundary of the domain would constitute a dis?

location between it and the neighbouring domain with another orientation. A picture of this kind could be developed and yet not go beyond the facts but it would

inevitably remain very general in character. While it might not be incorrect it

would be of little help as a guide to research. My own view is that in this subject * Ellington High Main (82% carbon). Extracted one hour at room temperature and

filtered through sintered glass disk, no. 1 porosity.




we have long suffered from the lack of a simple model because of the near certainty that any model would be proved wrong. Personally I feel there is a lot to be said

for a model so definite that it can be proved wrong by experiment. Usually it leads

on to a better model.

From this standpoint I think it is worth while adopting the temporary hypothesis that there are in coals actual discrete basic units, the crystallites, consisting of

a small number of randomly orientated, parallel layers of graphite-type linked

hexagons of carbon atoms. These will be considered to be stable units of great internal strength which persist even when the coal is heated.

In coal only some 80 to 90 % of the carbon is in these crystallite units, the re?

mainder is unorganized in the sense that the atoms scatter X-rays independently as if they were a gas. This carbon, with the hydrogen and oxygen, will be regarded as attached to the outside of the crystallite and forming a cross-linkage holding the crystallites together. The strength of these cross-linkages is far less than that

of the internal bonds of the crystallite. In fact temperatures of just above 400? C

are capable of supplying energies sufficient to rupture them, and this is shown by the commencement of pyrolysis, and the evolution of tar and lighter hydrocarbons.

The volatile matter, in the ordinary sense of the term, is thus identified as the

binding material holding together the crystallites. This hypothesis is crude but it

has the advantage of being so definite that it should not be difficult to see just how

near the truth it is. The volatile matter is susceptible of detailed investigation and

also X-ray measurements on any particular coal will give information on the size

and average number of layers of the crystallites and also on the amount of un?

organized carbon. On heating to a moderate temperature, say 800 to 1000? C, volatile matter, which could be identified, would be given off and, as Miss Franklin

has shown, the coke that remains could be analyzed into the same or larger crystal? lites with a certain remainder of unorganized carbon. If these measurements all

fitted together quantitatively it would give some support to this simple model.

The micelles have a special place in this picture because, while they occur in

virgin coal, they appear to be rather a result of the way the coal has been formed

than a characteristic manifestation of the coal substance. Coal melts on heating but this is usually obscured by the pyrolysis or thermal breakdown of the matrix

substance at about the same temperature. However, the melting point can be

lowered by pressure so as to occur before any thermal decomposition takes place and in that case a dense solid is obtained of the true density of coal. In other

words the micelles have lost their identity and we have just the coal substance

consisting of the crystallites embedded in the matrix.

On this view the micelles owe their existence to surface forces which preserve their separate identity at ordinary temperatures. An analogy is to liken coal in its

virgin state to a collection of particles of a wax. As long as this is kept below the

melting point of the wax the interstices and porous structure of the mass will be

preserved, but on heating the wax melts and the particles run together. In coal, of course, the 'wax' begins to decompose at about this temperature.

It is a consequence of this picture that the micelles in any one coal will have a

range of sizes and we cannot expect to find a precise dimension associated with




each coal. The range of micellar sizes, the pore structure and the van der Waals

type forces holding them together are all characteristic of each coal but they will

depend not only on the properties of the true coal substance but also how the coal

was formed. It should in principle be possible to have two coals with different

micellar sizes and pore structure but yet consisting of the same coal substance.

Formation of coke

Finally, I propose to revert to a purely phenomenological treatment of the

formation of coke. The views I shall describe have no claim to originality, in fact

that would be difficult in a subject on which so much has been written, but it will

be clear how much we should be helped by some such hypothesis as the one I have

just described.

It is, of course, clear that the formation of a solid like coke from a collection of

coal particles can only occur if at some stage the coal particles fuse and form

a liquid or at least a plastic state. On the other hand there is the thermal decom?

position of the coal leading to the evolution of tar and gases. It is the relation of

these two processes which determines whether coke is formed and if so its character.

A coking coal is one that melts at a temperature before much thermal decomposi? tion occurs. In terms of the model the micelles melt and fuse together before the

material, holding the crystallites together begins to decompose.

> 25 50

time (min)

Figure 6. Gieseler test?the fluidity of coal maintained at constant temperatures. ---, 406? C;--, 437? C.

This is well illustrated by the results shown in figure 6 due to Rhys Jones.

Samples of coal are brought as quickly as possible to predetermined temperatures and the viscosity measured as a function of time. The coal only just melts at the

lower temperature 406? C and there is a considerable delay in reaching the maximum

fluidity. Since thermal decomposition is only slight at this temperature a large measure of fluidity is maintained for a long while before, finally, a low temperature coke is formed. At the higher temperature thermal decomposition is more rapid




and is complete in 50 min. While the rising portions of the curves are complicated

by rate of heat transfer and thermal decomposition it is clear that the variation

of viscosity with temperature is rapid as would be expected. These simple experiments bring out the essential points bearing on the coking

process and on blending. In any practical treatment the coal is subjected to a regime of varying temperatures over a considerable time. The coal itself, as a substance, has a definite melting-point, and a characteristic variation of viscosity with

temperature. However, in this same region of temperature it undergoes thermal

decomposition at a rate of reaction increasing sharply with temperature. The

relation of temperature with time in the mass of coal in a coke oven is complicated

by the fact that the progress of the heat transfer from the hot walls occurs in turn

through three different states, the solid coal, the melted coal, and the final coke.

i

c3

100 75 50 25 0 25 50 75

Figure 7. Properties of a two-component coking blend: -

-, maximum fluidity of coal.

o 100

3

i

% coal A

% coal B

, shatter index of coke;

However, I suggest that we can begin to see at least the possibility of ordering the

practical results in terms of rather more basic phenomena. I will give Rhys Jones's

preliminary views on this matter. The strength of the resultant coke determines its

value for blast furnaces and foundry work, and is an important practical feature

to understand. He looks at this strength as being determined by the behaviour of

three separate domains. The first domain is small compared to any bubble in

the solidified froth, the second is large compared to bubbles but small com?

pared to the piece of coke produced by fissures, and the third is that of the lump coke. The reason for this division is that the mechanical properties of the carbon

material (first domain) should be dependent on the type of coal and the final

temperature of carbonization. Franklin's work on the crystallite growth in carbons

foreshadows a rational understanding of the physics of this process, and suggests the possibility of relating the crystallite structure of the coke to the crystallite structure of the coal it comes from.

The second domain is that of the bubble structure. Even were the mechanical

properties of the carbon material constant, the strength of pieces of coke in this




domain would depend on the sizes of the bubbles, the thicknesses of the bubble

walls and the general mutual arrangement of the bubbles.

The strength of the third domain, that of the lump coke, is determined by the

faults and fissures in the mass which arise from macroscopic unevenness in tem?

perature gradients both in heating and cooling. The bubble structure can be modified in many ways, in practice by the type of

oven or heating regime. Rhys Jones has carried out some experiments on the

properties of a two component mixture which bear on this point. His results are

shown in figure 7 where the fluidity and strength of coke (resistance to shatter

or 'shatter index') are plotted against the composition of the blend. Coal A alone

melts easily and the gas evolution in this highly fluid mass leads to too frothy a coke and a low shatter index. Some 15 to 20 % addition of B has a marked effect

on reducing the fluidity and there is a corresponding increase in the shatter index

due to stronger sponge structure.

Conclusion

My theme in giving this description of how I view the present state of research

in the coal industry has been to show by some practical examples that advances

are being made in several fields. These advances are important for the industry and are suitable for early application but in each example I have stressed the need

to strive for a more fundamental understanding of the processes involved. The

outstanding example is the need for a better understanding of what coal is.

In all the problems that arise from cutting the coal, from stopping the dust, from preventing breakage, from cleaning it, from briquetting it, from coking it

and from burning it, everywhere we are faced with a multiplicity of empirical results because of the great variety of coals. I am profoundly convinced that what

we need most at the moment are quantitative parameters to define coals so that

a reasonable number of experiments can by interpolation for intermediate coals

cover a large field.

References

Bangham, D. H., Franklin, R. E., Hirst, W. & Maggs, F. A. P. 1949 Fuel, 28, 231.

Dryden, I. G. C. 1951 Fuel, 30, 47. Franklin, R. E. 1951 Proc. Roy. Soc. A, 209, 196. Kann, L. 1951 Fuel, 30, 47.

Lecky, J. A., Hall, W. K. & Anderson, R. B. 1951 Nature, Lond., 168, 124. Malherbe, P. le R. 1951 Fuel, 30, 54.

Riley, H. L. 1944 Conference on the ultra-fine structure of coals and cokes. London: British Coal Utilization Research Association.



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Science in the Coal Industry