A general review of coal preparation inSouth Africa

by D. W. HORSFALL*, M.S.A.C.P.S.SYNOPSIS

The coal-preparation techniques used in South Africa over the past ten years are reviewed, the term preparationbeing taken to refer to processes involving beneflciation techniques.

The success of the preparation methods adopted can be gauged from the fact that, although South African coals aregenerally of a fairly Iow quality, the country is producing coals both that are used locally for the raising of steam andfor metallurgical purposes, and that compete successfully on world export markets.

SAMEVATTINGDie steenkoolbereidingstegnieke wat die afgelope tien jaar in Suid-Afrika toegepas is, word in oenskou geneem.

Die term bereidingword geag te verwys na die prosesse wat veredelingstegnieke insluit.Die sukses van die bereidingsmetodes wat toegepas word, kan geyk word aan die feit dat. hoewel Suid-Afrikaanse

steenkool oor die algemeen van 'n redelik lae gehalte is, die land steenkool produseer wat plaaslik vir die opwekkingvan stoom en vir metallurgiese doeleindes gebruik word en suksesvol op die wereld se uitvoermarkte meeding.

Introduction

South Africa, it has often been observed, is almostuniquely dependent upon coal for energy; it is estimatedthat 75 per cent of the energy used is coal based. How-ever, like other Gondwanaland coal seams, the SouthAfrican seams are generally associated with large per-centages of closely intergrown matter, which renders theproduction of coals of low ash both costly and difficult,

As a result, special preparation technologies have beendeveloped, and certain operating plants can claim to beconducting some of the most efficient separations in theworld, Two other features have stimulated the develop-ment of coal-preparation technologies, namely, theshortage of coking coal, and the desire to make as muchhigh-grade material as possible from the predominantlylow-grade reserves of the country. The former has ledto the design of plants for the processing of raw coalsfrom which exceptionally low yields of useful productsare obtained; and the latter to an emphasis on tech.niques involving double-stage washing and fines bene-fication to take advantage of the liberation occurringin the smaller sizes. As the need to produce coal ofhigh calorific value (for example, for export) fromessentially low-grade reserves tends to increase, thefurther development of advanced coal-preparationtechnologies can be expected,

This paper gives some account of the major develop-ments in coal-preparation techniques over the past tenyears and an indication of what the future may bring.Although preparation in its widest sense includes screen-ing and crushing, the term is used here only whenbeneficiation techniques are employed;. Future develop-ments may include greater washing of power-stationcoals, a greater emphasis on the beneficiation of fines,and a major increase in installed plant capacity to meetexport needs.

Coal Reserves

Quantity, Quality, and LocationEstimating the extent of South Africa's coal reserves

*Coal Division, Angl0 American Corporation of South AfricaLimited, P,D. Box 61587, Marshalltown 2107, South Africa.~ 1980.

is something of a popular pastime. The Petrick Com-missionl gave the mineable in situ reserves as 92 billion(109) tons, grouped in the quality bands shown in Fig.l,but suggested that, under the prevailing economiccircumstances, 26 billion tons were extractable. ij:ow-ever, it has been argued 2 that exploration since' thereport was published has added 10 billion tons toPetrick's 92 billion in situ; moreover, the increased use

BLOCK DIAGRAM

SHOWING 91767 MILLION lONNES RAW BITUMINOUS COAL (LOCALLY KNOWN

AS 'LOW GRADE STEAM COAL~MINEABLE IN SITU DISTRIBUTED ACCORDING

TO ASH CONTENT

40000 - -.i!R~S-400 METRESOEPT~-

V1UJZZ0...

UJ

::>

~uu:a:0~u

~«a:

'"g

'"a:UJa.

LL0

,<f)

~:::>0

,;;:

'"'UJ~

V1Z0

~~

~

J()35

ASH

Fig. I-Diagram, reproduced from the Petrick Reportlshowing the quality distribution of in situ coal reserves.

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY AUGUST 1980 257

",' zo' 30' 3"

COALFI Er.

REPUBLIC OF

S OF THE

SOUTH AFRICA~-'

..r'-,-f'-,

.L UIS

24 I)

,f'~

---r -,.

J:

,f"""'..-"--

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BLOEMFONT IN (

/-./"~~/" "I '

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NAME OF COALFIELD. (

---

.1 LI MPOPO

2 WATERBEPd\

3'SOUTPANSBERG, PAFURI5 SPRINGBOK FLATS6, WE STERN AREA

7 SPRINGS - WIT BANK.

8 KOMATI POORT

9 O. F. S. - VIERFONTE'IN

10 OLO SPRINGFIELD

11 VEREENI61NG

12 SOUTH RAND

13 HIGHVELD

I'EASTERN TRANSVAAL

IS KLlP RIVER

16 UTRECHT

17 VRYHEID

18 ZULULAND

19 MOL T ENO - INDWE

J"

:t,

34'

'0'. 2.' JO' 32',-------- - --------

u-- --- ---~-

Fig,2- The Petrick Report shows reserves of 81 274 million tons to a depth of 300 metres, This diagram uses 91767 milliontons, derived from the inclusion in the calculation of an additional 100 metres of depth.

258 AUGUST 1980 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

Yield Ash Yield Ash,

Floats 1,35 12,4 7,5 13,2 5,31,35 to 1,40 21,1 8,8 28,8 6,81,40 to 1,4fi 36,6 9,5 52,4 8,81,45 to 1,50 62,4 10,5 71,5 10,61,50 to 1,55 75,0 12,1 80,5 11,71,55 to 1,60 82,4 12,7 89,5 13,01,60 to 1,6,~ 87,0 13,3 91,5 13,41,65 to 1,70 89,8 13,7 93,2 13,8Raw coal 100,0 17,3 100,0 16,3

TABLE 1WASHABILITY DATA FOR COAL FROM WITBANK NUMBER 2 SEAM AND WATERBERG (CUMULATIVE VALUES)

Witbank Number 2 Seam

Relative density 100 x 32 mm 32 x 6 mm 6 x 0,5 mmYield Ash

19,433,357,275,583,686,691,393,715,1

4,55,97,99,6

10,711,412,012,6

Relativedensity

0,5 mm x 75 p.mYield Ash

1,301,30 to 1,401,40 to 1,501,50 to 1,601,60 to 1,701,70 to 1,80

Raw coal

10,832,165,482,483,190,3

100,0

2,64,57,6

10,011,211,817,1

Waterberg coal

Relative density YieldCum. %

AshCum. %

Floats 1,501,551,601,651,70

150 x 15 mm raw coal

18,627,934,138,944,7

100,0

20,924-,126,127,629,755,4Raw coal

150 x 1.5 mm after 1,7 wagh and crushing to -10 mm

Floats 1,3.5 14-,3 7,91,40 21,9 10,61,45 34,3 14,2

Raw coal after de-shaling 100,0 30,8

Floats 1,351,401,45

Natural arisings 15 x 0,5 mm

39,154,068,6

100,0

6,49,0

11,418,3Raw coal

of opencast methods with their higher extraction ratehas increased the economically extractable tonnage to61 billion. Essentially, the recoverable tonnage is afunction of price: the scarcity of coking coal, for example,means that the extraction of blend coking coal from theWaterberg region becomes cost-competitive with coalimports. Ten years ago, the mining of 15 million tons ofrun-of-mine coal for only 1,8 million tons of metallur.gical product, plus perhaps 2,5 to 3,0 million tons ofhigh-ash power-station middlings, would have seemedunthinkable: today the mine to execute such a projectis in an advanced state of development. The net effectis to increase the 'recoverable' category of coal reserves.

Fig. I shows that the greater part of the reserves areof low grade, i.e. at least over 20 per cent raw ash. Thelocation of major coal reserves is shown in Fig. 2. Themain mining area of the country is the Witbank/Middel-burg coalfield, from which some 80 per cent of thecountry's coal is mined. 3

WashabilityCoal seams vary widely in washability but, with a few

exceptions, are mainly characterized by high per-centages of near-density coal at most separating gravi-ties. If high-density washing is undertaken simply forthe removal of adventitious dirt, the amount of near-density coal may be as little as 10 per cent; but, if thesame coal is washed to give a low-ash (7 per cent)metallurgical coal, the amount of near-density materialcan rise to 70 per cent or more. It is the developmentof markets for such premium-grade coals that led to theinstallation of highly efficient plants in the past decade.

Washability data are presented in Table I for twocoal seams: a moderately easy coal (number 2 seam,Witbank area), which is the major source of steam andexport coal; and the Waterberg coal of the northernTransvaal.

Liberation EffectsThe removal of gross contaminants, such as discrete

stone and nodular pyrite, is often possible at fairly largeparticle sizes, e.g. 50mm or more. However, for the moreselective removal of ash-forming constituents, or simplyfor an increase in the yield of low-ash coal withoutmiddlings, crushing to small or fine sizes shows promise.

Coals containing a large proportion of inorganic con-stituents generally exhibit some degree of liberation onbeing crushed to smaller sizes. The phenomenon hasnot been studied in great detail, but data for the number2 seam (Fig. 3)4 clearly show that there is merit in aclose examination of the size at which a given coal isto be washed. However, even at fine sizes, 50 per centor more near-density material may be present, andhence the need for efficiency even at sizes smaller thanImm. The most remarkable improvements in the yieldof low-ash coal occur when the particle sizes are reduced tosmaller than 1O0p.m, and at such sizes, it has beensuggested5, even the selective removal of specific ash.forming constituents may be possible.

MarketsQualities Required

The different categories of end-uses for South Africancoals and the qualities supplied to them are broadlyshown in Table no The ash ranges are related to actualtonnages as supplied (1977 figures) in the series ofhistograms given in Fig. 4. These demonstrate theemphasis on the use of relatively high-grade productsin comparison with the reserve availability, which is alsoplotted in Fig. 4. The reserve availability is the Petrickreport histogram of Fig. 1 converted to percentages.

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY AUGUST 1980 259

Consumer Product and size Ash Volatile Calorific Swellingmm % matter value number

% MJ/kg

{

SAR and { Rounds +31,5 10 to 18 25 to 33 25 to 30 Generally 0 to 1General Cobbles 100 x 31,5 10 to 18 22 to 34 24 to 30 Generally 0 to 1

Also to Nuts 40 x 22,4 11 to 20 22 to 33

I

24 to 29 Generally 0 to 1general Peas 25 x 6,3 11 to 20 22 to 30 23 to 29 Generally 0 to 1trade

E.S.C.O.M. { Mixed smalls -25 12 to 21 22 to 31 22 to 29 Generally 0 to 1Duff -6, 3 12 to 23 21 to 31 22 to 29 Generally 0 to 1

Captive power station -25 20 to 27 21 to 26 20 to 25 0 to 1Blend coking coal Normally -32 10 to 13 30 to 34 28 to 30 2 to 4Straight coking coal Normally -32 10 to 14 21 to 30 30 to 32 4!to8tLow-ash export coal Normally -32 7 to 7,.5 31 to 32 29 to 32 +2tSasol -38 +30 ca 21 18 to 19 0

,\c"

0..... '\" 7", ~K'" 12"\

"\ '" "" 1\"'\ "" "" "\'\ \

\ \ \

\ \ \ \

\ \\

\ \ \'" \ \

I " "" "\

~'\ '"'-..... '\

Category 1978 1985 2000

Escom .. .. .. .. 39,5 63,0 130,0Sasol ..

".. .. .. 5,8 21,0 2],0

!scor .."

.. .. .. 6,5 7,4 10,5Commercial (includingmulllcipal power, othermetallurgical, general trade,and anthracite) .. .. .. 19,3 28,8 39,8Local sales.. .. .. .. 71,1 120,0 201,0Exports

"

.. .. .. 15,5 40,0 55,0Total sales tons .. .. 86,6 160,0 256,0

TABLE IIRANGE OF COAL QUALITIES SUPPLIED TO SOUTH AFRICAN AND EXPORT MARKETS'

202,2

152,4

19,1

101 ;6

50,8

31,8

12,7

9{>

E 6"E~~ 3,2:.....

1,7z....:E Oil

0,43

0;'

0,15

0,11

0,02020- 30 .0 50

YIOlO ",10 60 10 80

Fig. 3-Llberation curves showing ash versus yield at differentparticle sizes.

Quantities RequiredThe tonnages sold in various categories are given in

Table Ill.It should be noted that most of the Escom coal is

supplied unwashed, all the Sasol coal is unwashed,virtually all the metallurgical coal is washed, most ofthe commercial coal is washed, and (with minor excep-tions) all the export coal is washed. Hence, the coalthat has passed through a beneficiation stage amountsto about 40 million tons per annum; that is, of thecountry's saleable output, some 45 to 50 per cent isbeneficiated. On the figures shown in Table Ill, thepercentage of coal in South Africa that is beneficiatedmay tend to decrease (to about 40 per cent) by theyear 2000 as a result of the rapid growth of Escom.This is contrary to international trends. However,increasing environmental and coal-conservation measures

TABLE IIITONNAGES OF COAL SOLD (AS SUPPLIED BY THE MINERALS BUREAU)

9J,

1

may lead to the beneficiation of large amounts ofpower-station coal. This prospect is considered below.

Coal Preparation

Past Developments in N atalSCoal washing was first employed in South Africa in

1905, in Natal, when a plunger jig was installed to treatcoking coal. Between 1905 and 1910, five such jigswere installed with a total capacity of 200 000 tons perannum, i.e. some 8 per cent of the 2 500 000 tons ofsaleable coal produced in Natal in 1910. There wassteady but slow progress from that date; by 1926 therewere 300 000 tons of washed coal from a total output of5 160000 tons. By that time, washing was being appliedto steam coal; of the beneficiated 300 000 tons, only150000 tons were coking coal. By 1946, the total outputwas 5 342 000 tons of washed coal, being about 1100000tons, i.e. 20 per cent (15 per cent coking, 5 per centnon-coking).

Past Developments in the Transvaal and Orange FreeStateS

Coal washing was first employed in the Witbank areain 1909, when a six-unit plunger jig washer was installedfor the treatment of nut coal. The capacity of 120 000tons per annum represented about 21 per cent of thetotal Transvaal and Orange Free State output in 1910(just under 5 million tons). The plant was closed down,in about 1918, and until 1934 coal washing was notemployed on a full scale in the coalfield. A pilot Draperwasher was built at Landau Colliery in 1932 to supplycoal for coking tests.

In 1934, Navigation washery, with a capacity of90 000 tons per month, was erected to prepare blendcoking coal. The washery was of the trough type and

260 AUGUST 1980 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

Export me r kC2tTotal tonnagC2

01010....C01UL-01D..

0 5

170

T 6001

g' 5'E: 401

~ 301D.. 2

1

10 15 20 25Ash-

30

DomC2stic/lndustriol

Total tonnogll 20793092

100

10 15 20 25 30Ash-

Mlltoll urgicolTotal tonnogC2 3620720

0 5 10 15 20 25

Tons Mineobl<2 Ash -in sutu I( 109

5432-1

40

30

6992%

703°'"

30

37.4

t t TotalInfllrrlld

Indicotlldt01

g' 20....C01U

<-If

10

0

18,6

1,5

g,so0....~ 40L-If

35

100

t6~01

g' 50

'E: 40010 30~

D.. 2

10

35

~01

~~100

5!c

;;;; 70:g 6~ 5'E 4g, 300'E: 2001~ 10If

40

SosolTotal tonnog<25116426

0 5 10 15 20 25 30 35Ash-

Escom

Total tonnogll 34 231 735

10 20 30Ash (unwoshlld) "/.

54,1"10

0 5 10 30 35

0 5 3510 15 20 25Ash rong<2"1o

30

Fig.4-Histograms showing the qualities of coal used in various sectors of the coal trade (1976 figures)?(Note. The figures show the quality range in each sector of the market: the dark shaded part of each histogram shows thepercentage of total coal consumed by all markets. The bottom right diagram is the Petrick histogram of Fig. I, which

has been converted, in the bottom left, to percentages of the reserves.)

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY AUGUST 1980 261

50

"U

">-. 60-.70

treated minus 2-inch coal. In 1935, in the Orange FreeState, a dense-medium washer of the Chance type wascommissioned, having a capacity of 100000 tons permonth. From that time onwards there was a steadyincrease in the number of plants, both jigs and dense-medium being employed.

A major step forward in preparation technologyoccurred in 1972, when the first plant to supply 'lowash coal' to the Japanese market commenced operations9.Since that time, a further five low-ash plants have comeinto operation.

The cleaning of fine coal has tended to be neglected.The first froth-flotation plant was installed at a coking-coal mine in about 1960; the first autogenous cycloneunit at a Transvaal mine producing blend coking coalin 1966. The first froth-flotation plant to treat exportthermal coal came on stream in 1979.

The extension of dense-medium cleaning to fine sizes,

Relative Density

0

10

20

30 1:>G;

"'-Cl);.

~O ,,'L:

cl.-u

";::

80

90

lOO0 2 ~ 6 8 10 12 I~ I'; 18

.,. Ash

Fig. S-Washability curves for the 32 x 6mm fraction of Wit.bank number 2 coal, based on the data of Table I.

262 AUGUST 1980 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

1 3

say O,lmm, shows great promise, and a plant is currentlyunder construction to treat Imm by O,lmm fines bydense-medium separation.

Present Preparation TechniquesIt has been observed that most South African coals

are difficult to prepare throughout the range of separatingdenRities employed, though the difficulty tends to in-crease as the separating density decreases. The dataof Table I for Wit bank coal plotted as a graph (Fig. 5)clearly show that the same coal might be classed as'moderately difficult' if it is washed at a high densitysimply to remove shale or nodular pyrite, and verydifficult when a low-ash coal, and hence a low-densityseparation, are required. This, in turn, means that theequipment used to treat that seam will vary accordingto the beneficiation demands if a consistently highefficiency in the separation of organic matter, known asorganic efficiency, is to be obtained.

This effect is quantified in Fig. 6, in which unitefficiency, measured by Ecart Probable6, is plottedagainst organic efficiency. Three washing cases areshown: de-shaling, reducing the ash to between 13 and14 per cent, and making a 7 per cent ash product. Itcan be seen that different standards, and hence equip-ment, can be required for each case. The graph alsoquantifies the relationship between economic recovery andplant performance, and gives a strong justification forregular performance testing.

The axis showing 'organic efficiency' also correlateswith revenue, and, in the case of the low-ash products,it can be seen that a serious decline in revenue resultsfrom very small changes in plant efficiency. Suchchanges would not necessarily be accompanied by achange in separating density, i.e. they are not readilydetec~able, and careful plant operation may be necessaryto prevent unsuspected coal losses creeping away withexpected revenue. The Ep axis may itself be related toinstallation and operating costs of equipment; hence,such a diagram can give a clear picture of the investmentrequirements to produce a particular efficiency level atdifferent ash levels. The diagram shows that, for theproduction of low-ash coal, exceptionally efficient pro-cesses must be used if high organic efficiencies (andhence revenues) are to be obtained.

With regard to the types of washer actually in use,details are given elsewhere8. Statistics of relative usageare given in the proceedings of the Seventh InternationalCoal Preparation Congress (1976) and are shown inTable IV. The figures have changed in the past threeyears and would now show, essentially, the greater use ofdense-medium washing at the expense of jig washing.No statistics were presented at the Eighth InternationalCoal Preparation Congress, which was held in theU.S.S.R.

Use of Double-stage WashingPerhaps the most marked feature in the development

of preparation techniques is the spread of double-stageprocessing. The first such unit was installed at Naviga-tion in 1934, i.e. at the first 'serious' washing plantbuilt in the Transvaal. Old report books of the era showthat the plant superintendent attempted to make coalof about 7 per cent ash, but the imperfect nature of

20

Dense-medium Dense mediumbaths centrifugal

Jig Magnetic Frothwashers medium Other Cyclones Others flotation

% % % % % %23,0 48,5 14,0 8,0 4,0 2,5

o,o~Very poor operot Ion

badly run Jig or

dense medium

o~ --good jigf fair

dense medium

Revenue --- capitalcostfton

\de-shallng

operation only(l,ttle or no

- - near density

material.)

r~~~~i5-~

0,04

good dense medium

-----------

ijz

"'uiL

"-"'

:gc~0~

rn

n

I-Z:0

0;02 - - - - - ~ - - - - - -excellent dense medium

~"~

O,~for:ll-:u=ai=- - - -

-~7'1' 0:- U~Ddense medium separation ~~~ri:~ar

density

0,005 - - - - - - - - - - -virtually perfect

Organ"lc Efficiency

:u.2c

~Co

Fig, 6-Effect of the ash level sought on the efficiency required'",

TABLE IVUSAGE OF DIFFERENT TYPES OF WASHER

the Draper washer's separation made his task impossible,as shown by Fig. 6. The compromise was about 11 percent ash for the metallurgical coal and 25 to 30 per centash for the middlings. The wheel has come full circlein the development of techniques for the production oflow-ash coal, as shown later. However, the trend isobvious: there are now eight double-stage plants inoperation, of which seven were installed during the pastsix years. The use of such technology is obvious whenthe requirements of the market are related to the qualityof the reserves.

Equipment for the Beneficiation of Small and Coarse CoalBy coarse coal is meant coal of a lower size limit of

about 6mm, which can be washed in dense-mediumbaths, while small coal refers to coal of an upper sizeof about 15mm (such as can be treated in cyclones)and a lower size limit of 0,5mm wedge-wire mesh. Smallcoal is the correct English technical terminology for suchsizes, and the use of fines to describe them is deploredas being slipshod, imprecise, and incorrect. Germanterminology is feinkohl for what is smalls in English,feinstkohl for what is in English fines. It seems to beinviting misunderstanding to use the term fines to coveranything with a top size of from 0,5mm to 32mm.

Chance washers, Wemco drums, and shallow DSMtype baths are employed for coarse coal. A unit developedin South Africa, the Norwalt vessel, is also widely em-ployed. The above types are all in use for the productionof low-ash coal, save for the Chance, but they incor-porate special design features in order to attain therequired levels of efficiency. For small coals, both DSMcyclones and Dynawhirlpool washers are used.

Jigs are used over the range 150mm by o for 'easy'separations, but, as pointed out, few coals fall into thiscategory, with the result that the general tendency hasbeen to adopt dense-medium processes. However,changes in preparation practices for power-station coalsmay alter this situation and lead to a revival of interestin jigs.

Some operating results, both from routine plantoperations and from special low-ash washers, are givenin Table V.

Beneficiation of Fine CoalFine coal refers to the underflow of a screen with

0,5mm wedge-wire apert;ures. In fact, such material,when re-screened on a standard laboratory sieve, canhave up to 50 per cent plus 0,5mm material, dependingupon the state of the screen decks in the plant. Thefines are better referred to as minus Imm, which gives amore accurate picture of the upper size limit.

The best-established method of fines beneficiation isfroth flotation, and six units are in operation: five inNatal and one in the Transvaal. The basic problemrestricting the use of flotation in, say, low-ash separationis the large amount of intergrown material present, evenat fine sizes. To make a low-ash coal, a flotation collectorwould have to distinguish between a particle of, say,18 per cent ash, which could be acceptable in the clean

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY AUGUST 1980 263

Plant type and Type %near EP Imperfection Organiccoal size doo density efficiency

Low-ash plant A %75 by 12 mm

0,006 0,004 93,4(bath) 1,34 8212 by 0,5 mm

0,014 0,010 94,3(centrifugal) 1,38 70Low-ash plant B32 by 6 mm

0,009 0,065 96,0(bath) 1,39 786 by 0,5 mm

84 0,033 0,024 82,7(centrifugal) 1,40

Dense-medinm plant treating metallurgical coal from Witbanknumber 5 seam

TABLE VOPERATING RESULTS OF DENSE-MEDIUM PLANTS FROM PER-

FORMANCE TRIALS CONDUCTED BY THE FUEL RESEARCH INSTITUTE

32 by 6 mm(bath)6 by 0,5 mm(centrifugal)

I,M 5 to 6 % 0,016

1,60 5to6% 0,047

0,016

0,029

99,9

99,0

Bath-type dense-medium washer treating nominally plus 6 mm coalalready de-shaled in a baumjig

+ 30 mm 1,46 55 0,007 0,005 99,9530 by 12 mm 1,45 49 0,009 0,006 99,7612 by 0,5 mm 1,45 44 0,010 0,007 97,29

coal, and one of 25 per cent ash, which would have tobe rejected. The situation is similar to that shown inFig. 6: if Iow-ash contents are required, a highly dis-criminating process must be used; if products of mediumto high ash content are required, a less discriminatingprocess, such as flotation, is adequate. Natal coals tendto have good liberation characteristics; the washabilitydata would show little near-density material (i.e. inter-grown material) present if, say, 10 to 12 per cent ashis required: hence the applicability of flotation in thatProvince. The single Transvaal unit, which came onstream in 1979, is a similar case in that a product of15 to 16 per cent is required, at which level the incor-poration or rejection of intergrown material is not socritical since there is less such material present.

For both Natal and Transvaal coals, water-onlycyclones have been employed but with indifferentsuccess, even on coals that contain little near-densitymaterial. The latest unit is at Rietspruit Mines, butperformance data have not yet been released. Theseunits never seem to fulfil the promise given by perfor-mance figures and small-scale tests, and only in themost exceptional circumstances could they be used asthe sole means of fines beneficiation. They could beuseful as a clean-up stage on a coal stream alreadycleaned by some other means, e.g. fines cleaned by ajig; but, if used as the sole means of beneficiation, theygive neither a clean product nor a coal-free discard.Their basic probleniis"a lack of selectivity, even thoughthey work on differences in density. Thus, the remarkson flotation are equally appropriate to water-onlycyclones.

Developments in Coal-Preparation Techniques

It might be argued that the production of Iow-ashcoal is now part of the accepted practice, but, as newplants are stilI something of a novelty, they are describedhere.

264 AUGUST 1980 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

Production of Low-ash Coal

Tests on the production of Iow-ash coal were initiatedat Landau in 1965. By 1969 the Landau scheme wassufficiently advanced, having involved trials up to thefull-scale plant stage, for it to bc used as the basis for'Heads of Agreement' negotiations (by which stageother mining companies had become involved) on thesale of Iow-ash coal to the Japanese steel industry.

Eight mines were involved in the first discussions, butfurther investigation rationalized the scheme so thatthe contract was met by only four of them. This wasthe situation, therefore, when the contract was signed in1971. The contract called for up to 2,3m tons of low-ash coal per annum to be exported for ten years, com-mencing 1976; but limited port facilities through Maputoprior to that time led to the construction of a plant atLandau. Landau began exporting in 1973, while thethree other mines involved came fully on stream in1976, when the new port of Richards Bay becameoperational. The contract specification requires t~ecombined four-mine output to have an average analysIsof 7,0 per cent ash (maximum 7,5 per cent) and a mini-

mum swelling index of 2,1 to 2,2. The coal is currentlyexported, as well as being used in the South Africaniron and steel industry, as blend coking coal. In 1977, afifth Iow-ash plant, Bank, came on stream, producingup to 0,9 million tons per annum of coal of similar ashcontent but lower swelling index. This coal does notaugment the original contract for blend coking coal,but is being used in special coke-manufacturing opera-tions. The preparation plants are, with one exception,basically similar and follow the flowsheet evolved f~rLandau. Raw coal from Witbank number 2 seam ISpre-screened before being washed into two or threesize ranges, the number and range being determined bythe coal-washability characteristics. At present, thelower size limit is 0,5mm (ww), though a 20 t/h dense-medium cyclone unit to treat 0,5mm (ww) by 75 f-tm(sq) is being constructed at Greenside. In all the plants,the coarse coal is washed in bath-type vessels (Teska,Wemco-type drum, and Norwalt are in use), and thesmall coal in centrifugal separators of the DWP orcyclone type. The high-density cut is usually carriedout first, so that the second-stage products are Iow-ashcoal and power-station coal (also sold on the exportmarket). In one case, Haasfontein, only single-stagewashing is adopted, the entire discards being fed to aminemouth power station.

The qualities of the coal, reported on an air-driedbasis, are given in Table VI, the data being those ofthe Fuel Research Institute, who carry out the con-tractual quality determinations. A typical flowsheet ofa Iow-ash plant (double stage) is given in Fig. 7.

The initiation and completion by 1977 of five Iow-ashplants, with a combined productive capacity of 3,2 ~il-lion tons of this special product, represents the majoradvance in coal preparation in South Africa in the pastdecade. From the initial conceptual studies in 1965 tothe completion of the Bank plant took 12 years, and, aspointed out, the contract for blend coking coal providedthe basis for the building of the port of Richards Bayand its associated coal terminal, and hence laid the

TABLE VI

QUALITY OF PREPARED COAL"

Mine Grade* Moisture Ash Calorific value Volatile Total Swelling

% % MJ/kg matter % number%

2,4 7,5 31,0 32,0 0,6 32,3 15,9 27,3 23,2 0,8

2,5 7,0 31,1 32,5 0,5 32,4 14,8 27,8 25,5 0,8

2,3 7,5 31,0 34,1 0,6 32,4 15,9 27,0 24,2 0,5

2,4 6,3 31,5 33,0 0,5 212,3 21,3 24,7 26,3 1,3

2,7 7,2 30,8 28,4 0,42,6 15,7 27,1 23,9 0,6

2,5 6,9 31,2 32,9 0,6 32,7 12,9 28,4 25,1 0,6 0

Landau LACPSS

Van Dyks Drift LACPSS

Greenside LACPSS

Haasfontein LACPSS

Bank LACPSS

Kleinkopje LACPSS

Mine Calorific Total Sulphurvalue emission

MJ /kg g/MJ

Arnot 21,5 0,8 0,37Kriel 22,8 0,9 0,39Matla 22,9 1,2 0,52Haasfontein 24,7 1,3 0,53(Komati)Optimum 24,4 1,1 0,45Coal Brook A 16,7 0,2 0,12Coal Brook B 18,8 0,5 0,27Cornelia No. 2 18,3 0,6 0,33

Item Waterberg I No. 4 Seam No. 2 SeamBlend

I

Formed Formedcoking coal .~~ coke

.--Ash % 10 9 7,5Volatile matter % 35 30 35Yield coking coal % 12 20 33Yield midlings % 18 65 53Yield discard middlings % 70 15 14

Run-of-mine for 1,8million t/a coal t 15,Om 9,Om 6,Om

Coal for cokingaccount t 15,Om 3,2m 2,8m

Coal for power station t Nil 5,8m 3,2m

Mining cost at R15 perannual ton R 225m 48m 42m

Plant capacity(6000 h) t/h 2500 1500 1000Plant cost at

R15 000 t/h R 37m 23m 15mTotal capital R 262m 71m 57m

Yield coke % 65 70 65Cost per annual ton of

coke R 224 56 49Ash in coke % 15,4 13,6 11,5Fixed carbon in coke % 84,6 86,4 88,5

Capital cost per a!lnualton of fixed carbon R 265 65 55

*LAC = low-ash coalPSS = power-station smans

TABLE VIISULPHUR EMITTED BY VARIOUS COALS (BASED ON FIGURES

SUPPLIED BY POWER STATIONS)

foundation for today's buoyant South African coal-export scene. From exports of about 1 million tons perannum prior to 1973, exports topped 15 million tons in1978, are planned to reach 20 million tons in 1980, andcould be running at 40 million tons by 1985.

Production of Power-station CoalsAs already pointed out, although most power-station

coals are supplied unwashed, power plants remotefrom mines are generally supplied with washed coal(these account for some 13 per cent of the electricity gene-rated). For a station in the Cape, for example, railagecosts account for about 75 per cent of the deliveredcost, so that the railage of coals of high calorific valueis an obvious economy. Coals consumed in pitsmouthpower stations can, and do, have much lower calorificvalues, as evidenced by Table 11. Consequently, bene-ficiation is rarely carried out. However, two factorsmay lead to the increased cleaning of power-stationcoals: the increase in strip mining, and environmentalconcern.

Coals mined by strip methods tend to contain moreadventitious stone than coals mined underground.Mineral matter reduces the calorific value, but even more

TABLE VIIICAPITAL COST OF OBTAINING FIXED CARBON FOR THE BLASTFURNACE (INCLUDING MINING AND PREPARATION, BUT EXCLUDING

CARBONIZING COSTS)12

important is its effect on abrasive properties. Highmaintenance costs both on mills and boiler tubes resultfrom stone inclusions. At the same time, the concen-tration of major power stations in the fairly confinedWitbank(Middelburg area could lead to thermal andair pollution, even though the sulphur content of mostpower-station coals seems to be low.

At the present time, regulations have not been pro-

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY AUGUST 1980 265

ASH'. CV(MJfkg Abr(mg' 5Raw Coal

19,5 25,72 392 D,92

Songle wash16,2product 25,2D 197 D,.7

Discord .9,7 12,82 16.7 4,14

ASH' CV/MJ kg) Ab,. mgle ,5

1<Jw Coal 19,8", 23,72 392 0,92

P.Stat;on19,3", 23, 7~ 242 0,54

Coalemium produd 10,0", 28,0 108 0,32

Discard 4g;1", 12,82. 1647 ,14

..

RAWCOAL

(a)

'°0',.RawCoal

(b)

100",RawCoal

- O.SmmF;nes

6,OSmm [)scard

Combined Discar ds\a Dump

High Density Wash Low Density Was"

7,S,6mm

6, Smm

CombinedLow Ash

Coal

Com!>nedFOwer Slat""Coal

Fig. 7-A flowsheet of a typical Iow-ash coal washery8.

11,""D;scard

.. includ'ng the heat control 01 thepyrite

30,D",Prem,umProduct

11.1",Discard

Fig. 8-Effect of washing on power-station coals.(a) single-stage washing (b) double-stage washing

266 AUGUST 1980

mulgated regarding sulphur emissions from powerstations. However, an advisory figure of some 0,17 gof sulphur per megajoule has been suggested. Virtuallyall existing power stations use coal with a sulphuremission in excess of that figure (Table VII).

Because the sulphur is generally pyritic and nodular,high-density washing can remove much of the sulphur(as well as the abrasive stone) without excessive loss ofcalorific values in the discard; most of these values areprobably, in any event, in the pyrite. This is showndiagrammatically in Fig. 8 (a). At such a high densitythere is little near-density material, and the densitydifference between the potential discard and the cleanedcoal is also large enough to allow for a very high organicefficiency with a process such as a jig. The latter hasthe merit that it can readily operate at densities evenhigher than 2,0 if the coal has the right constitution.Of the power station contracts recently awarded, twowill entail the combustion of washed or partly washedcoal.

Widespread washing of power-station coals is still inits infancy, although the foregoing rationale has beenconfirmed on the full scale by results from SpringfieldColliery (1968 onwards) and Kilbarchan (from 1976):in both cases, washing gives a coal that is substantiallybetter from the point of view of its use as a pulverizedfuel. If a single-stage wash is to be embarked upon,much of the groundwork is laid for double-stage washingin order to 'top and tail' the coal, i.e. take off a low.density, low-ash fraction for metallurgical use or ex.port, and a high-density discard fraction. The middlingproduct may have roughly the same calorific value asthe original raw coal, but will be enhanced as power-station feedstock by virtue of lower sulphur content,lower abrasiveness, and marginally increased Hardgroveindex as shown in Fig. 8 (b).

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

2,0 3,0 6,01,0 1,5 6,01,0 1,5 5,53,0 4,5 5,5

2,59,6 Nil 10,01,2 2,01,2 2,51,0 1,5 2,5Nil 2,5 NilNil Nil Nil

1,520,0 44,0

AUGUST 1980 267

It is the author's personal view that, necessarythough certain developments may be in metallurgicalcoal to meet the continuing requirements of existingcoke ovens, the production of formed coal and formedcoke may be more economical for any expanded con-sumption of reductants. For either of these develop-ments12, non- or weakly coking coal can be used. Out-line figures on the relative capital investment to pro-duce carbon (which is what, ultimately, the steel pro-ducer wants) by different routes are presented in TableVIII. This table compares the cost of carbon fromnumber 2 and number 4 seam sources linked to a powerstation, with the investment costs in the northernTransvaal. The latter have been somewhat amelioratedby Escom's recent award of a power-station contract toIscor. Nonetheless, the comparative investment costsper ton of carbon per annum make interesting reading.Some endorsement of this concept was given in theCoalcom Report13.Production of Metallurgical Coals14

Of singular interest are the mining ventures in thenorthern Transvaal - in the Waterberg and the Sout-pansberg coalfields. The coal measures in these areasare characterized by thin, highly interbanded seams,in which the dirt bands far exceed, in a mining section,the amount of coal. In the Waterberg, no selectivity iscarried out in mining, the whole mass of coal seamsand shale partings being sent to the preparation plant.

!scor's Waterberg mine, Grootgeluk, is already in anadvanced stage of construction and due to produce coalin 1980. Designed for a strip-mining operation, thepreparation plant will treat some 15 million tons ofrun-of-mine coal per annum to produce 1,8 million tonsof blend coking coal per annum. Some 2,5 to 3,0 milliontons per annum of middlings (35 per cent ash) willeventually be supplied to an Escom power station. Theblend coal will have 10 per cent ash and 35 per centvolatile matter, and a swelling index of 3 to 3t. Over-burden will amount to about 9 million tons per annum.

The plant will employ primary baths to scalp high-density (plus 1,7) dirt from the 150 by 15mm raw coal,with a cyclone rewash for the deshaled plus 15mm coalafter crushing to minus 1Omm, as well as for the naturalminus 15mm. Froth flotation will be employed for theminus 0,5mm fines. The following are some salient featuresof the plant, which will possibly be the largest pre-paration plant in the world:Nominal plant capacity 3000 t/h.Primary crushing 3 rotary breakers, each of 1000 t/h.Scalping baths handling 2000 t/h 150 by 15mm in fivestreams, each of 400 t/h (Teska vessels envisaged).(The washability of this coal is given in Table I.)Primary dense-medium cyclones Treating natural minus15mm raw coal in five streams, each of 200 t/h (1000t/h average throughput).Secondary dense-medium cyclones Treating crushed pro-ducts from the scalping baths: 1000 t/h capacity infive strealllS, each of 200 t/h.Froth-flotation plant Treating combined minus 0,5mm(ww) material, 375 t/h capacity, served by five streamseach served by a 40 mm diameter thickener; and frothcells of 75 t/h.

Over the past two years, another northern Transvaaldeposit, known as Venda or Soutpansberg coal, has been

intensively investigated by Iscor and certain mininghouses. At the present stage, no decision has been takenon whether a mine will be established in the region.The tests indicated that the coal has exceptionallywell-developed coking properties, which appear toimprove towards the west; hence, the current degree ofinterest in the coal deposits in the Kruger NationalPark. It is believed that, although the coal would bedeep mined, beneficiation would not pose such for-midable problems as in the western regions of the field.

Dense-medium Beneficiation of FinesAlthough enhanced yields of low-ash coal are obtained

as the particle size of the coal is reduced, large amountsof near-density material are present even at fine sizes,say, up to 60 per cent. As pointed out, the establishedmethod of froth flotation is not really applicable sincethe presence of large amounts of intergrown material(evidenced by near-density values) baffles the mostselective collectors. Tables, although approaching therequired efficiency parameters, can do so only at whollyuneconomical throughputs. A considerable amount oftestwork within Anglo American Corporation led to theconclusion that only dense-medium cycloning, de-veloped to 75 p.m, could effectively wash the fines fromSouth African coal to low-ash contents.

At the same time, the Fuel Research Institute wasthoroughly testing other well-established systems for thebeneficiation of fines (froth flotation, tables, and water-only cyclones) to establish their limits and potentialsin the production of low-ash coal. The Anglo Americangroup's work on dense-medium beneficiation encouragedthe Fuel Research Institute to build a 5 t/h pilot plantto take the process to the commercial stage. The In-stitute has operated the plant for some three years withgreat success, and the construction of a full-scale unithas been commenced at Greenside Colliery.

Overseas, dense-medium cyclones have already beenoperated successfully down to zero size. Such plants havegenerally washed, say, minus 12mm coal, which greatlyeases handling problems. However, greater efficiencyappears to result if fines are treated on their own ratherthan with small coal. Fig. 6 implies that, even at finesizes, efficient separations contribute materially to highorganic efficiencies (because much near-density materialis still present), so that the approach being tried inSouth Africa (i.e. the treatment of fines alone) is prob-ably the best route technically. An additional advantageof having such facilities for the beneficiation of fines isa reduction in overall coal losses. The dense-medium

TABLE IXCOAl, EXPORTS FROM RICHARDS BAY (IN MILLION TONS PER ANNUM)

Company Nov. '74 Phase 2 Unutilized Totalallocation participation portion allocation

Anglo AmericanGeneral MiningBPShellRand MinesTCOANACAPATotalIDC & AssociatesSouth CapeKwaNgomaTOTALS

5,02,52,57,5

10,0

2,52,57,5

40,0

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

/

"

facilities installed to treat the plus 75 p,m fraction willalso treat 0,5mm wedge-wire oversize in the fines andreduce coal losses substantially. In this respect, densityseparation is superior to froth-flotation processes, whichcannot handle oversize material: particles greater than,say, Imm tend to report as tailings whatever their ashcontent.

Export CoalsCurrent South African coal exports through Richards

Bay amount to about 15 million tons per annum, ofwhich some 20 per cent is low-ash coal, the remainderbeing thermal power-station coal and anthracite. It isexpected that exports will have risen to 20 million tonsper annum by 1980 and three mines opened in 1979 tosupply most of this extra tonnage.

Ermelo Mines (a joint venture of General Mining, BP,and Total) will supply some 3 million tons of coal perannum, as will Rietspruit Colliery (Shell and Barand).The third mine, Kleinkopje (Amcoal), will supply 2,7million sales tons for the first four years (1979 to 1983),composed of 2,2 million tons of power-station coal and0,5 million tons of low-ash coal for the local metal-lurgical market, rising to 4,3 million tons (2 million tonsof export power coal, 1,6 million tons of domestic-powercoal, and 0,7 million tons of domestic low-ash coal)for the remainder of its life. Of the three mines, Klein-kopje is the only one with double-stage washing facilities.The other two, serving only the export thermal mu'rket,are straightforward single-stage washing plants.

Depending upon the success achieved by the dense-medium beneficiation of fines, Kleinkopje may use thatmethod of treatment to augment its production of bothlow-ash and power-station coal.

Planning for the next 'great leap forward', namelyphase 3 of Richards Bay, is a full-time occupation inthe mining houses involved. It has been suggested 2that South Africa may be supplying 25 to 30 per centof the international steam-coal trade by 1985. The pre-sent stated limit on coal exports from Richards Bay isgiven as 44 million tons per annum, but various authori-ties have stated, with some conviction, that the figurecould be increased, the estimates vary from an increase of50 per cent (i.e. to 66 million tons) to 100 per cent or200 per cent.

A breakdown of the allocations in phase 3 are givenin Table IX, which shows that Anglo American, GeneralMining, Barand, BP, and Shell will supply some 22,5million tons of new output per annum between now and1985/86. Most of these tons will be beneficiated: SouthAfrica has precious little high-grade coal left and, if anaverage yield of 70 per cent is assumed, some 30 milliontons of preparation capacity will be needed per annum.At an average operating time of, say, 5000 hours, thisis at least 6000 t/h. Preparation-plant costs of at leastR15000 per ton per hour can be expected, so that theindustry will be paying about 90 million rands fornew equipment in the next five or six years. This ismunificent by any standards, and that is without anybeneficiation of power-station coals. One hopes thatrationalization will prevail and that, as suggestedearlier, double-stage washing, which could result inastonishing benefits, will be adopted.

ConclusionSouth Africa, perhaps as a result of having coals that

268

are of fairly poor quality by world standards, now has ahighly developed and efficient coal-preparation tech-nology. Coals have been produced that compete success-fully in the export markets for both metallurgical andthermal coals, that are used on the domestic marketfor steam raising and metallurgical purposes, and thathave contributed materially to the economic well-beingof the country. As the demand for coal rises and speci-fications become ever more stringent, the technologiesavailable will develop further to satisfy the new needs.

Coal is South Africa's only major, indigenous sourceof fossil energy, and it is to be hoped that sincere andsuccessful attempts will be made to improve its extrac-tion, beneficiation, and utilization. There is muchgroundwork to be done to ensure that uses are rational.ized, and types of coal are identified that may havespecially valuable properties.

Forty years ago, two farsighted and sagacious scien-tists, Drs Vogel and Quass, investigated the amena-bility of South African coals to hydrogenation tech-niques, which could give perhaps two or three times asmuch petrol from coal as Sasol produces for a similarcoal consumption and capital requirement. Of the threeseams identified as being particularly suitable, two areeither exhausted or committed for combustion use. Itis therefore extremely important that, on the threshold of(perhaps even half into) the New Coal Age, accountshould be taken of the different types of coal availableand the potential uses of those types. Clearly, reservescannot be locked up indefinitely against some annusmirabilis of coal conversion, but a little caution couldprofitably be exercised.

Acknowledgement

The author thanks the Management of the CoalDivision, Anglo American Corporation, for permissionto publish this paper, and stresses that the views ex-pressed are his own and not necessarily those of theCorporation.

References

1. Report of the Commission of Enquiry into South Africa'sCoal Reserves (Petrick Report), 1975.

2. Coal, Special Supplement to Finance Week, 19th Jan., 1979.3. SMITH, D. A. M. The location and nature of South Africa's

coal reserves. Spring School in Coal Processing (JointSACPS/RAU venture), Sep., 1978.

4. HORSFALL,D. W. Coal- key to energy: a personal viewpoint.Rhodesian Science .Journal, Feb. 1977.

5. BoRro, R. W., and NARCISO, R. N. The use of gravityfractionation techniques for assessing slagging and foulingpotential of coal ash. American Society of MechanicalEngineering, Winter Meeting, San Francisco, 1978.

6. Coal preparation course, vo!. 1.7. HORSFALL, D. W. Towards a policy for South African coa!.

De Beers Metallurgical Symposium, 1977 (published byR. Falcon in 'Coal in South Africa', Miner.Sci. Engng,Apr.1978).

8. HORSFAJ~L.D. W. Basic coal preparation. Spring School,1978. ibid.

9. BAIN, A., and HORSFALL, D. W. Landau's Iow ash prepara-tion plant. Coal, Gold, Base Miner., Aug. 1972.

10. HORSFALL, D. W. Dense medium processes in the productionof Iow ash coa!. South African Institute of Mining andMetallurgy Colloquium 1974.

11. SAVAGE, W. H. Average analysis of product samples.Pretoria, Fuel Research Institute, Bulletin no. 90. 1978.

12. HORSFALL, D. W. Coal as a metallurgical reductant. S.Afr.Min. Engng .J., Jan. 1978.

13. INDUSTRIAL DEVELOPMENT CORPORATION. Report of theCoalcom Study Committee, Ju!. 1978.

14. Gouws, P. Beneficiation aspects of exploiting of the Water-berg Coalfield. Coal RC8earch in South Africa, Aug. 1976.

AUGUST 1980 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY