andalusiteinsouthafrica* - saimm · pdf fileandalusite reserves,mt identified inferred content...

J. S. At,. Inst. Min. Metal/., vol. 89, no. 6.Jun. 1989. pp. 157-171.

Andalusite in South Africa*by P.W. OVERBEEKt

SYNOPSISThe occurrence, properties, and uses of andalusite, and its advantages over other refractory minerals and materials,

are discussed. Ore reserves are given for the five producing areas, and statistics on production, export sales, andprices are included to illustrate the growth of the industry since the early 1960s.

The mining and beneficiation procedures are described, with special reference to new techniques in concentra-tion and magnetic separation. As fines dumps represent a considerable source of andalusite, possible methodsof recovery are discussed. .

The outlook for the future appears favourable, and possible competition from other sources and substitute refractoryminerals are mentioned. It is concluded that the industry is in a sound position to maintain South Africa's statusas the world's foremost producer and supplier of andalusite.

SAMEVATTINGDie voorkoms, eienskappe en gebruike van andalusiet, en die voordele daarvan vergeleke met ander vuurvaste

minerale en materiale word bespreek. Die ertsreserwes vir die vyf produserende gebiede en statistiek oor produksie,uitvoerverkope en pryse word ingesluit om die groei van die bedryf sedert die vroee sestigerjare te illustreer.

Die mynbou- en veredelingsprosedures word beskryf met spesiale verwysing na nuwe tegnieke in konsentrasieen magnetiese skeiding. Aangesien hope fynmateriaal 'n aansienlike bran van andalusiet verteenwoordig, wordmoontlike herwinningsmetodes bespreek.

Die vooruitsigte vir die toekoms Iyk gunstig en moontlike mededinging uit ander bronne en plaasvervangendev:..urvaste minerale word genoem. Die gevolgtrekking is dat die bedryf in 'n gesonde posisie is om Suid-Afrika sestatus as die were Id se belangrikste produsent en leweransie van andalusiet te handhaaf.

IntroductionAndalusite, sillimanite, and kyanite, generally refer-

red to as the sillimanite-group minerals, constitute trimor-phous forms of Al2SiOs and are important raw materialsfor the production of superior-grade high-alumina refrac-tories for both the ferrous and the non-ferrous industries.The relative proportions in which these sister minerals areused world-wide depends to a large degree on their avail-ability. However, anadalusite has some important advan-tages that are associated with its low relative density, itsrelated lower volume expansion on being heated, and thefact that it can be produced by a relatively simple miningand concentration procedure at its natural grain size.

The increasingly severe service to which refractories arenow subjected in iron and steel plants has given rise toa change from fireclay materials to materials of higheralumina content. Low costs, energy-saving requirements,firebrick production techniques, and the ultimate chemical,physical, and dimensional characteristics of the brickcombine to make andalusite the logical choice as a rawmaterial to provide the additional alumina.

South Africa possesses by far the largest portion of theworld's known deposits of andalusite, and the demon-strated economic reserves are givenl,2 at well over 50 Mt,which represent a reserve life of 250 years at the presentrate of extraction.

It is interesting to note that andalusite is included inthe list of minerals that are exempt from import restric-

. This is another paper in our Mineral Review Series.t Retired. Formerly Assistant Director: Ore-dressing Division, Council

for Mineral Technology (Mintek), Private Bag X3015, Randburg,2125 Transvaal.

@ The South African Institute of Mining and Metallurgy, 1989. SAISSN 0038-223X/3.00 + 0.00. Paper received 12th December, 1988.

tions by the USA. This fact is of great importance whenit is realized that andalusite faces competition, not onlyfrom natural minerals that are produced elsewhere, butalso from potential substitute materials, some of whichare produced on a large scale. It is to the credit of theindustry and to research organizations such as the Councilfor Mineral Technology (Mintek) that South Africa istoday the foremost producer and exporter of andalusite,supplying countries as far afield as Japan, Italy, WestGermany, Great Britain, Turkey, Spain, and the USA.

The Occurrence of Andalusite in South AfricaAndalusite occurs in the metamorphic aureole of the

Bushveld Complex, particularly in the Marico and Thaba-zimbi areas of the western Transvaae and in theChuniespoort-Penge-Lydenburg area of the easternTransvaal, where it occurs in hornfels and schists or asassociated eluvial deposits4,s. These deposits were form-ed by the metamorphism of the aluminous sediments ofthe Pretoria Series as a result of regional and contactmetamorphism. Fig. 1 shows the location of the majordeposits that are being exploited, while Table I lists themajor producing mines, togbether with relevant informa-tion on the holding companies and product specifica-tions6. As can be seen from Fig. 1, there are five distinctareas in which andalusite is mined.

Groot Marico-Zeerust Area, Western TransvaalThe deposits in this area can be grouped into two types:

the metamorphosed shales of the Daspoort Stage, andthe alluvial sands of the rivers that drain these shales. Thisarea is of minor importance at present since the andalusitewith one exception is of low grade (less than 55 per cent

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JUNE 1989 157

.ocation ofGrade, 070

Administrative Trade name of Annual productione and deposit company concentrate Alp, Fe20, t/a x IOJ

ey 109 KT" Annesley Randalusite 59/12 59,5 0,92 75Andalusite (Ply) Lld 60109 60,0 0,86

'roft 99 KT' Verref Mining (Ply) Lld Standard 58,8 1,30 36Made 61,6 0,60

noeg 293 KS' Hoogenoeg Enrosit 59,0 1,00 15sburg Andalusite (PlY) Lld

all Andalusite Weedon's Purusite 59,5 0,70 120tfontein 352 KQ' Minerals (Ply) Lldazimbi

spost Mine Cullinan K57 57,2 0,90 50ntein 400T' Minerals Lld K53 54,0 1,20burg

rax Purity Minerals (Ply) Lld Purity 59,5 0,70 12fontein 260 JP'

Marico

--1-- ,/.~~'(

,/ . Pietersburg '

\) Chuniespoort.++

. Penge, .+ '

)ir:. Thabazimbi L d b +"", ~.O.Middelwit

yenurg.,

;.':ieerust .Pretoria /'~

""/"O..Gr~~~.~~~.i~?,.,..,.,.",, ,, )-'

'-

"::~"".",.",, "':""""".\:..

+ Operating mines0 Under investigation

Fig. 1-Location of andalusite deposits in the Transvaal

Al2O, and more than 1,5 per cent FezOj). No materialof this grade is being produced at present. The exceptionis the shale deposit occurring on the farm Kleinfontein260, where concentrates assaying more than 59 per centAl2O, and less than 1,0 per cent FezOj are produced,Other high-grade shale deposits are likely to be exploitedin the near future.

Thabazimbi Area, Northwestern TransvaalAndalusite is well developed in the Timeball Hill shale

near Thabazimbi, occurring in a soft, highly weatheredmica hornfelsJ. This shale formation is the youngestpart of a huge meta-sedimentary basin called the Trans-vaal system. The heat of the rocks in the middle of thesedimentary basin-known as the Bushveld Complex-was the major factor responsible for the alteration of the

Pretoria shales to andalusite- and chiastolite-bearinghornfelses.

Weedon's Minerals (Pty) Limited operate the andalu-site mine on the Timeball Hill shale, and four processingplants are currently in operation, with an annual produc-tion of close to 120000 t. It is by far the largest andalusitemine in the world. The property runs along a strike inthe Timeball Hill shales for about 6 km but the depositextends much further. The andalusite is marketed underthe trade name Purusite and contains 59 to 60 per centAlP3 and 0,65 to 0,75 per cent Fep3'

Chuniespoort-Penge-Lydenburg Area, NortheasternTransvaalBefore the discovery of the Thabazimbi andalusite, the

deposits that occur in this area were considered to be themost important in South Africa.

Andalusite in the form of chiastolite is widely spreadin metamorphosed shales of the Pretoria Series5, and isrecovered from both bedrock and surficial deposits. Thedeposit is characterized by its coarse crystal size comparedwith the Marico and Thabazimbi deposits, and by thepresence of staurolite, an iron-bearing aluminium silicate.A remarkable feature of the andalusite from these areas(apart from its high alumina content and the coarsecrystal size) is its consistent relationship between gradeand density. The most important deposits in this areaoccur on the farm Annesley 109 KT, Streatham 100 KT,Havercroft 99 KT, Holfontein 126 KT, and Morgenzon125 KT. The deposit has been proved over at least 16 kmalong strike in the Daspoort shales. The average andalu-site content is 8 per cent.

Two mines are operating in the Penge area: HavercroftAndalusite Mine, owned by Vereeniging Refractories, andAnnesley Andalusite Mine, owned in part by AnnesleyAndalusite (Pty) Limited. This mine was previouslyoperated by Rand London Corporation, which purchas-ed the mine from Zimro (Pty) Limited. The originalowners, however, were Hudson Mining Co. Limited,

TABLE IANDALUSITE PRODUCERS IN SOUTH AFRICA

min

AnneslPenge

H avercPenge

Hooge

Pieter

TimebGroo

Thab

Kruger

KlipfoLyden

AndafKleinGroot

,Identification of the farm on which the mine is situated

158 JUNE 1989 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

--

Andalusite Reserves, MtIdentified Inferred

Content AlzO3 resourcest resourcestAreas % 070 High grade Low grade Total Mt Mt

stern Transvaalale 10 52-59 0,5 20,0 21 60,0 60,0

lIuvium 50 53 0,5

rthwestern Transvaalale 10-15 58-60 0,5 0,5 2,5 2,5

rtheastern Transvaalale 8 56-59 28,5 29,0 29,0 44,0

lIuvium 10-15 58 0,5

al 30,0 20,5 50,5 91,5 106,5

which started the mine in 1960. It was the first mine inthe Free WorId to produce a coarse-crystal, high-alumina,low-iron andalusite, and it represents a good exampleof the co-operation that exists between industry andMintek7. The first dense-medium separation plant in thisarea was installed at this mine.

The third mine to produce a coarse-crystal product isthe Hoogenoeg Mine, which is owned and operated byHoogenoeg Andalusite (Pty) Limited on the farm Uitkyk,some 30 km from Chuniespoort. The ore is exposed overa strike length of close to 10 km. The bedrock andalusite,which is the only source mined, occurs in a zone some30 m thick. The degree of weathering is not constant, andthere are some hard zones that make recovery difficult.The ore dips at about 30 degrees to the south, which in-creases the overburden as the face is developed. Atpresent, development is along the strike, rather than intothe dip. The orebody is unique in that it is high up onthe side of a mountain, and hence has a large amountof overburden. A road 7 km in length had to be con-structed up the side of the mountain from the village togive access to the deposit.

The andalusite content varies from 8 to 15 per cent,with an average of 10 per cent, and is characterized bythe presence of large amounts of staurolite, which, whenconcentrated with the andalusite, can amount to over 20per cent of the dense-medium concentrate.

Cullinan Minerals Limited operate the KrugerspostAndalusite Mine, which is situated about 10 km to thenorth of LydenburgB. The ore at this mine differs fromthat at the other three mines in the area in that the crystalsize is similar to that of the Marico- Thabazimbi deposits.The deposit is located at the eastern extremity of theBushveld Complex, in which the invading lava of theComplex metamorphosed the highly aluminous shales ofthe Magaliesberg Stage of the Pretoria Series, resultingin the formation of andalusite. Erosion and weatheringsoftened the hard metamorphic rocks, and no blastingis required in the mining operation. The mining method

used is opencast bench mining on hill slopes with benchheights of about 4 m. The andalusite content of the shaleis 8 to 12 per cent.

There are major differences in the physical propertiesof the andalusite and host rock from the various areasdescribed above. These differences affect to a large degreethe beneficiation procedures that are used to recover theandalusite, as will be shown later in this review.

Ore ReservesPrimary or in situ andalusite makes up the bulk of the

ore reserves. The confirmed reserves and resources ofrecoverable andalusite from the various areas are givenin Table 11, which shows that some 30 Mt of high-gradeandalusite are immediately available for recovery.

Production and ExportWhen the growth of the andalusite industry is assess-

ed in terms of production, local consumption, and ex-port sales, several factors are readily apparent as havinginfluenced the beneficiation process, the rate of exporta-tion, and the value of the material. Table III shows theproduction, exports, and export values for andalusitefrom 1960 to 1988, and Table IV gives the total exportsof andalusite from South Africa to various world con-sumers.

As Table III shows, in 1966 there was a sharp increasein export tonnage, which was m.ore than double the 1962tonnage. This coincided with the coming into operationof the first mine in the eastern Transvaal, which is nowthe Annesley Andalusite Mine. This deposit, the first toexploit the coarse-crystal variety of andalusite, wascharacterized, not only by the coarse size of the crystalsbut, more importantly, by the substantial increase ingrade compared with andalusite from other sources. Theconcentrate grades increased from 54 per cent ~O3 and1,5 per cent FezO3 to between 59 and 60 per cent AlzO3and less than 1,0 per cent FeP3' Before 1965, andalusite

TABLE IIANDALUSITE RESERVES AND RESOURCES IN SOUTH AFRICA'

WeShA

NoSh

NoShA

Tot

,Source: Minerals Bureau

t Identified resources: that portion of a natural concentration of solid of which the location, quality, and extent are known to various levelsof assurance, ranging from measured to inference from geological projection

t Inferred resources: That portion of the identified resource which is unexplored and estimates of quality and size are based mainly on geologicalprojection


Export valuetTotal

Significant production Exports Ryear t t f.o.b. R/t

1960 9320 3701 82880 231962 18350 6082 142234 231966 21 530 12632 334 565 261968 22430 14850 412300 281970 42 530 21500 642 150 301973 60200 16650 572 468 341975 77150 23800 1241 187 521978 112000 54 500 5 088 775 931980 189909 116180 11435247 981982 153231 75 429 9 005 948 1191983 116576 69264 7 875 964 1141984 143 300 91 574 12250520 1341985 194693 142143 25002746 1761986 181624 113514 22284597 1961987 194373 117622 26503981 225

J an.-Oct. 262 358 79 064 20605046 255

TABLE IIIPRODUCTION AND EXPORT SALES.

. Source: Minerals Bureau

t Average value; high-grade concentrates can command prices of upto R320 per tonne

TABLE IVDESTINATION OF ANDALUSITE EXPORTS (1987).

DestinationAmount exported

t 070

Western Europe

AsiaNorth AmericaCentral and South America

Oceana

5894623 2051857427921805

56221822

.Source: Minerals Bureau

was produced only in the Marico area, where the gradeswere low and the supplies inconsistene. Although theexport tonnages and the grade of the concentrate increas-ed sharply, the increase initially had no significant effecton the monetary value per ton, which remained fairly con-stant up to 1975. However, the coming on-stream of theeastern Transvaal mine had the significant effect ofassuring consistent supplies of a better-quality product.

The year 1978 showed another dramatic increase inproduction- 77 000 tons in 1975 to 112000 tons in 1978.More significant, however, was the virtual doubling inthe export value of the concentrates. The total exportvalue increased from Rl,2 million to R5, 1 million. Thisincrease, which coincided with the opening of the Time-ball Andalusite Mine in the Thabazimbi area, is indicativeof the more responsible attitude that producers haveadopted towards ensuring consistently high grades andregular supplies. This, together with the efforts of Mintekin providing technical advice and assistance, is the majorreason for South Africa's prominent position as a reliablesource of andalusite.

The production figures for 1987 were 194373 t, ofwhich 117622 t were exported at a total value of R26,5million. As can be seen from Table Ill, there were some


---------

fluctuations in the amount exported and the total value.The value per tonne showed a steady increase, however,and the present (1988) value per tonne is in the regionof R300. Factors that affected exports, as well as localsales, are the world recession and the decrease in steelproduction world-wide. According to present indications,demand is exceeding supply, and several mines are en-larging their plants to cope with this increased demand.

Specifications of AndalusiteThe name andalusite is derived from the Spanish pro-

vince of Andalusia, where it was first noted. Andalusiteis a refractory aluminium silicate with the chemical com-position AlzSiOs or AlzO3SiOz' Ideally, andalusiteshould contain 62,9 per cent AlzO3 and 37,1 per centSiOz, but it hardly ever occurs in the pure form andcontains a number of impurities that substantially reducethese values. The major variety of andalusite is chiasto-lite, which exhibits a regular arrangement of inclusionsin the shape of a cross, as shown in Fig. 2. These inclu-sions are the major source of impurities that reduce theAlzO3 content, although other sources of impurities canalso be present. Mineralogically, the impurities are asfollows:

(i) hydrous iron oxides and carbon contained in thechiastolite cross,

(ii) micaceous impurities (sericite, biotite, and phlogo-pite) occurring mainly on crystal surfaces,

(iii) inclusions of ilmenite in the crystal lattice,(iv) inclusions of FeO in the crystal lattice,(v) quartz blebs intimately intergrown with the andalu-

site, and(vi) extraneous impurities such as staurolite and shale.

Fig. 2-Andalusite crystals after concentration, eastern Transvaal

With the exception of the extraneous impurities, theimpurities are not easily removed owing to their finelydivided nature and intimate association with the andalu-site. In spite of their apparent detrimental effects, theyare absorbed into the glass phase when the material isfired. Phlogopite, which is present on the crystal surfaces,has a fluxing action that improves the body by aiding thebonding mechanism. Andalusite can be altered to sericite,a process that, in the case of chiastolite, begins along thelines of the carbonaceous inclusions or on the outercrystal surfaces.

Purusite Randalusite K57Constituent Weedon's Annesley Cullinan

Alp, 59,50 59,50 52,20Fe,O, 0,70 0,90 0,91SiO, 39,00 39,20 40,78TiO, 0,20 0,14 0,26CaO 0,20 0,06 0,06MgO 0,10 0,12 0,11Nap 0,10 0,10 0,10K,O 0,25 0,20 0,20

Loss on ignition* 0,40 0,50 0,47Relative density, g/cm' 3,10 3,10Floats on TBEt, % 0,20 0,50 1,0Particle size, %

> 6.0 mm 0,00 27,00> 3.3 mm 1,50 72,30

> 1.7 mm 75,00 0,70> 0.6 mm 17,00 0,00< 0.6 mm 6,50 0,00

*At IOOO°C

t Tetra-bromo-ethane: relative density 2,96

Marico, % Thabazimbi, % Lydenburg, %

ve density Mass AI,O/ Fe,O] Mass Al,O] Fe,O] Mass Alp,t Fe,O,

3,10 3,7 58,4 1,27 15,4 61,2 0,30 43,6 59,9 0,60to 3,10 60,0 55,2 1,31 67,0 59,0 0,60 47,2 56,3 0,90to 3,05 20,6 50,5 1,94 15,0 58,4 0,80 5,4 42,5 1,15to 3,00 11,5 47,3 2,16 2,6 57,6 1,20 0,9 47,8 1,482,95 4,2 38,8 2,76 Nil Nil Nil 2,9 43,7 2,62

verage 100,0 52,8 1,60 100,0 59,7 0,59 100,0 57,2 0,84

Properties of AndalusiteSome of the relevant properties of pure andalusite are

given below.

Chemical composition

Crystal systemCleavageHardness (Moh)Habit

Colour

Relative densityVolume expansion on

being fired

Al2O3SiO3 containing62,9070AlP3 and 37,1070SiO2OrthorhombicDistinct on 1.1.06,5 to 7,5Prismatic, often coarse,well-formed crystalsGrey to black, brown toreddish pink3,13 to 3,163 to 6070

Decomposition At 1300 to 1600°C, form-ing mullite and silica.

Weathering and alteration have a marked effect on thequality of the andalusite crystals and their relative density.Table V gives a typical chemical analysis of andalusiteconcentrates from three different mines. There are alsolarge variations in the size of the crystals, and Table Vgives the particle-size distributions of concentrates fromthe various areas. Table VI shows the relationship betweenthe alumina and ferric oxide contents on the one hand,and the relative density on the other, resulting fromweathering. It can thus be seen that the physical proper-ties vary from deposit to deposit, and these propertiesto a large degree determine the precise nature of the bestbeneficiation process to be used.

TABLE VTYPICAL CHEMICAL AND PHYSICAL ANALYSISOF CONCENTRATES

Analysis, 070

K53Cullinan

54,001,20

41,800,330,060,200,150,20

0,55

3,0

0,01,5

47,048,5

3,0

TABLE VITHE RELATIONSHIP BETWEEN ALUMINA AND FERRIC OXIDE CONTENTS AND RELATIVE DENSITY IN ANDALUSITE CONCENTRATES

Relati

>3,053,002,95

<

A

*This type of material is not being produced at present but the higher-grade material shows similar tendencies

t Represents a fine-grained, high-grade andalusite; the coarse-grained crystals show little variation in density


Calcining temperature Energy for calcinationMaterial QC BTU x 10 per ton

Magnesite 1900-2000 6-52Mullite 0 19Bauxite (85-900/0) 1700-1800 13Bauxite (70%) 1700 9Bauxite kadin 1650-1700 8Kaolin 1500-1600 7Fireday 1350 4Kyanite 1380-1380 4Andalusite 0 0

Specifications for Andalusite ConcentratesGenerally, specifications for andalusite concentrates

are a matter of agreement between producer and con-sumer. In the early years up to 1965, concentrates assay-ing 54 per cent Al2O3and more than 1,2 per cent Fe2O3were accepted readily, but present demands are for high-alumina, low-iron material. The fine crystal size was alsotolerated before, but present demands are for coarsercrystals. Fine-grained concentrates are 'sweetened' by theintroduction of coarser crystals from other sources. Someproducers market separate grades according to theiralumina and iron contents, and their crystal size. All theproducers aim to provide concentrates containing themaximum amount of alumina possible and the minimumamount of impurities, notably iron oxides, and severalproducers are engaged in special retreatment processes,such as two-stage separations, multiple scrubbing, roast-ing and magnetic separation, and chemical treatment, toimprove their grades.

The major requirements for andalusite concentratescan best be gauged from an examination of typical chemi-cal analyses of the concentrates from various sources(Table V). This shows that there are major differencesin the particle-size distribution of the concentrates. TheLydenburg material, for example, contains less alumina,and the lower-grade material is comparable with that pro-duced in the Marico district in the early 1960s.

The nature of the impurities is apparent in both thebeneficiation process used and the end-use of the con-centrate. Iron that has replaced alumina within the crystalstructure or that occurs in the chiastolite cross is notdetrimental, the only effect being one of discoloration.However, iron present as relatively large discrete grainsor adhesions to the crystal surfaces is very undesirableand must be removed by further beneficiation. Such ironshows up in fired bricks as black spots or blow-holes,which act as incipient fusion points when the bricks arein service.

Total alkalis in concentrations greater than 0,5 per centlower the fusion temperature of the bricks, and weakenthe strength under load, while the higher glass and silicalevels due to alkalis make the refractory more prone tochemical attack. The grain size affects the strength underload and, for this reason, a certain proportion of coarsegrains is required. Andalusite is recovered at its naturalgrain size and, although a maximum size is not generallyspecified, the ratio of particle sizes is analogous to thatin concrete mixes. Sizes smaller than 0,5 mm are not ac-cepted, which, conveniently, is also the lower size thatcan be treated effectively in the dense-medium separa-tion process. Some fines, however, are added to the mix-ture when the brick is formed. Coarse crystals impart ahigher physical strength under load and give a brick ofhigher apparent density. However, bricks made withmaterial containing a large proportion of fines requireseveral stages of compression in the mould if they are tomatch the apparent density of bricks made from coarsercrystals. Fig. 3 compares grain sizes of South Africanandalusites with those of andalusites produced else-wherelO. Small grain sizes restrict the use of andalusitein the manufacture of firebricks, and fines below 0,5 mmare used mainly in monolithic applications.

Grain size in millimetres (log scale)

0,015, 0,5, 1,0I

1050,05 0,1, I

ITIIJ US Kyanite4 grain sizes

French ~nd~lusite

I

I gram size

South A~rican DIDPuruslte4 grain sizes

South African

I

Randalusite2 grain sizes D

Fig. 3-Comparison of grain sizes in four andalusites

Advantages of AndalusiteAndalusite, together with its sister minerals kyanite and

sillimanite, is consumed predominantly within the refrac-tory industry because of its ability to form the refractoryhigh-performance mullite phase (3 AlP3' 2 SiO2) at hightemperatures. This phase confers a high, hot strength withhigh resistance to chemical and physical erosion-proper-ties that are desirable at high temperatures and underchemically corrosive conditions.

Andalusite has several advantages over kyanite, silli-manite, and other high-alumina and fireclay materials.It differs from kyanite and sillimanite in that it can beused in the manufacture of fired refractory products with-out being calcined. This is possible because of its muchlower volume expansion on being fired-l to 1,5 per centmaximum at 1500°C.

Since andalusite needs no calcining, it offers signifi-cant economies in that it saves energy, an advantage thatis certain to be of importance in the light of increasedenergy costs. Table VII compares the energy requirementsfor the calcining of various refractory materials.

TABLE VIIENERGY REQUIREMENTS FOR THE CALCINING OF VARIOUS

REFRACTORY MATERIALS

Andalusite has excellent resistance to slag penetrationand attack, especially by alkalis. This is due to its dense,homogeneous single-crystal structure, in which there are


virtually no channels of weakness along which the slagcan travel. Consequently, andalusite is superior in thisrespect to chamottes, bauxitic materials, and bauxite, inwhich even the smallest piece of material is still a com-pound of tiny crystals of kaolin and corundum betweenwhich the slag can penetrate. Fig. 4 illustrates the effectof slag att,ack on andalusite and a bauxitic material.

AndalusiteBauxiticmaterial

Slag penetratesbetween small

crystals within theparticle

Fig. 4-Effect of slag penetration as shown through apetrographic microscope

Slag attacks onlyedges of particle

In the manufacture of bricks, particularly of com-plicated shapes, andalusite has advantages over otheralumina-bearing refractories. There are no large volumechanges, and the changes during firing are slight, ensuringbetter dimensional tolerance and warpage characteristics.Andalusite is also the only mineral that can be used inits natural grain size as an aggregate in the manufactureof bricks.

Uses of AndalusiteMost of the andalusite that is produced is used in the

manufacture of high-alumina refractories, a small pro-portion being used for ceramic purposes. Andalusiterefractories are favoured under abrasive conditions wherehigh loads and high temperatures are encountered. In hot-blast stoves, they are used as checker bricks in the upperzone, in the dome, and in the combustion shaft. In blastfurnaces, andalusite refractories are used in the lining ofthe stack and bosh areas, in tilting and swivelling troughs,as linings for iron and slag runners, and as tap-hole muds.Andalusite also finds application in ladles that are usedunder a variety of conditions.

Other applications are as soaking-pot covers, nozzlesfor the blowing of argon gas, refractory shapes such asburner bodies and combustion chambers in cement kilns,linings for glass furnaces, kiln furniture for the firing ofceramic products, and in copper-roasting furnaces. It isalso used in unshaped refractory applications, such ascastable or gunning mixes, ramming mixes, foundry sand,roofs of reheat furnaces, and heat-treatment furnaces.Special-purpose products include sanitaryware and white-ware, and high-temperature applications such as spark-plug bodies, insulators, and pyrometer tubes.

Nature of Andalusite DepositsAs mentioned earlier, all the andalusite currently pro-

duced is derived from shale deposits. These deposits arebasically hornfelses, consisting of a micaceous clayeymatrix in which the andalusite crystals are distributedrandomly, as shown in Fig. 5. Depending on locality, theshale is soft and highly weathered, and the crystals arereadily liberated from the shale on processing. Then,again, the shales can be hard and compact, and moresophisticated methods are required to release the crystalsfrom the host rock. The crystal sizes also vary and, sincethey are required to be recovered at their natural grainsize, both the mining methods and the subsequent libera-tion and beneficiation procedures have to be adapted toensure minimum breakage of the crystals and maximumrecovery.

Fig. 5-Andalusite crystals in shale, showing random orienta-tion and fracture

It can thus be seen that, although these procedures aregenerally standard throughout the industry, variationsexist that allow for the inherent differences in ore charac-teristics that are found from deposit to deposit.

Mining of Andalusite OreAll the deposits currently exploited are mined by open-

cast methods, using both face development and bench-mining procedures. Fig. 6 shows a typical opencast minein the northwestern Transvaal.

Generally, mining consists in the removal of over-burden, which is basically a bulldozer-loader-truckingoperation. The overburden can range in thickness froma few metres to up to 60 m, as found at one of the minesin the northeastern Transvaal.

Once the ore has been exposed, the mining consistsbasically in an ore-loosening operation with the use ofrippers, scrapers, or face-shovels but, as the mining pro-ceeds and the ore becomes harder at depth, blasting hasto be carried out to loosen the ore. All the mines treatingthe coarse-crystal variety are now resorting to controlledblasting to loosen the ore. At two of the mines, the depth


Coarse-crystal ore

+Run-of-mine ore

+Vibrating grizzly

Feeder, 50mm aperture

+Jaw crusher

+Vibrating screen12mm aperture

+Gyratory or rollscrusher to 12 mm

+Attrition scrubber

+Wet vibrating screen

12mm and 1,0mm

tII I

>12 <12>1,0 <1,0

Fig. 6-An opencast andalusite mine in the Thabazimbi area

to which the ore extends and the thickness of the over-burden make underground mining a distinct possibility.

In general, the deposits have strike lengths of between2 and 8 km, and ore depths or thicknesses of up to 100 m.The andalusite content of the shales is not consistent, butcan vary from 8 to 20 per cent with an average contentof 10 per cent. Mining costs constitute only a small por-tion of the overall costs, and mining and transport cangenerally be carried out at a cost of R2 to R3 per tonne.In situ values of ore vary according to head grade andrecovery, but usually amount to about R18 per tonne(1988), based on a recovery rate of 65 per cent.

In contrast to the South African situation is the miningprocedure used at Glomel, France, which is at presentthe only andalusite producer of significance outside SouthAfrica. The ore is loosened by vertical blasting, and isfurther broken by hydraulic-percussion lump breakers.Several stages of jaw and gyratory crushing are followedby semi-autogenous milling using steel balls. By the timeliberation has been achieved, the andalusite crystals arevery much finer than in the South African product. ForSouth African producers to follow this procedure wouldseriously affect their viability, and it is fortunate that mostof the deposits are amenable to relatively simple and in-expensive mining and liberation procedures.

BeneficiationThe beneficiation process generally relies on three

physical characteristics of the ore:(i) the friable nature of the shales and the natural

cleavage between crystal and shale,(ii) the differences in relative density between the anda-

lusite and the shale, and(iii) differences in magnetic susceptibility between the

andalusite (non-magnetic) and the iron-bearing im-purities, which are usually only feebly magnetic.

Impure andalusite has a relative density of 3,05 to 3,10,compared with a relative density of 2,70 to 2,85 for theshale component. This difference in relative densityallows separation by gravity means, but is not sufficientlygreat for simple procedures such as jigging, tabling, orspiralling to be effective. Shape is also a negative factorwhen jigs are considered, and the crystal size rules outthe use of tabling of other forms of thin-film concentra-tion.

Dense-medium separation has been found to be a very

efficient means of concentration, and is the only methodof concentration currently used in the production ofandalusitell. Before dense-medium separation can beapplied, however, the material has to be properly pre-pared, and it is in the preparation of the feed to thisseparation that variations are found.

A standard beneficiation procedure consists of thestages shown in Fig. 7. Since no two ores behave similar-ly when treated by the standard procedure, producershave incorporated variations in these procedures. Thesevariations are indicative of differences in approach to theproblems of beneficiation by producers, which is ahealthy sign since it shows that they are not satisfied withthe standard procedure but are looking for ways andmeans to improve recovery, reduce costs, and provide stillhigher-grade products.

Fine-crystal ore

+Run-of-mine ore

+Vibrating feeder

with grizzly

+Jaw crusher to 50 mm

+Attrition scrubber

+Screening

r--+---,Coarse Fines

+DMS

tSink

IFloat

Waste+

DMS

+r-- ---,Sink Float

Waste

+Dryer

+Magnetic separation

+Dryer

+Screen

+Magnetic separation

Fig. 7-Schematic flowsheets for plants operating on coarse- andfine-crystal andalusite

In the application of the various unit operations shownin Fig. 7, it must be borne in mind that the object is, first-ly, to release the andalusite crystal with minimum break-age and, secondly, to clean the liberated crystal from theadhering impurities while at the same time dispersing theparticles of soft shale that can adversely affect the sub-sequent concentration processes. The final object is theproduction of a pure concentrate that will conform tothe specifications.


Primary CrushingRun-of-mine ores usually have a maximum size of 350

to 400 mm, and will, because of their friable nature, con-. tain a considerable amount of fines. It has been shownthat up to 20 per cent of the andalusite content of an orecan be lost as fines smaller than 1,0 mm and, for thisreason, it is advisable to remove the fines prior to crush-ing.

Primary crushing is done in jaw crushers, although arolls crusher is the only stage of crushing used at theKrugerspost Mine owing to the friable nature of the ore.The size to which the ore is crushed depends largely onthe nature of the ore and its subsequent treatment. Forfine-grained andalusite (smaller than 4,0 mm), a singlestage of jaw crushing provides feed to the attrition scrub-bers (usually smaller than 50 mm), which is the majormethod of liberation for crystals of this size.

At mines treating shales with coarse crystals (smallerthan 10,0 mm), jaw crushing serves to prepare the ore forsubsequent secondary crushing. In these examples, thepractice is to screen out material of the correct size fromthe run-of-mine ore by means of a vibrating grizzlyfeeder. The fines and the crushed product are then com-bined to form the feed to the secondary stage.

Secondary CrushingSecondary crushing is generally used only for ores that

have coarse crystals and are generally not as friable, Le.shales that will not readily disperse in an attrition mill.Secondary crushing is thus the major means of achievingthe liberation of the coarse crystals from the shale. It hasbeen shown that a shale particle that has been crushedto below 12,0 mm will, in the case of ores with coarsecrystals, be virtually barren of andalusite, whereas asimilarly sized particle of shale, in the case of ores withfine-grained crystals, can contain up to 5 per cent andalu-site. These particles will then need to be crushed to below6,0 mm for complete release of the andalusite crystals.Crushing to this size in conventional crushers such asgyratory crushers or rolls will result in the crystals beingbroken to sizes too fine to be recovered; hence, the dif-ference in procedure for ores containing coarse and finecrystals. Secondary crushing can be done in a gyratoryor cone crusher or in a rolls crusher.

Horizontal-impact crushers are also coming into pro-minence, and are now being used at the Andafrax Minein the Marico district. Rolls crushers are used at Haver-croft and Annesley in the northeastern Transvaal, whilethe Hoogenoeg Mine uses a gyratory crusher.

ScreeningScreening is used at various stages in the process, as

shown in Fig. 7, and serves to scalp out the fines andremove the slimes. For the efficient removal of slimes,wet screening is used.

Vibrating screens are favoured, and the use of poly-urethane panels to minimize wear is now standard onmost mines. Trommels are still used, particularly wherethey form an integral part of the scrubbing mill. Prob-ability screens, such as the Morgenson Sizer, are used onsome mines where sticky ores are encountered and blind-ing could be a problem.

Screening is also used further down the beneficiation

procedure. In dense-medium separation, it is used todewater the ore prior to concentration and to drain andrecover the medium from the ore particles. Magneticseparation is more efficient on sized material, and thedense-medium concentrate after being dried is screenedby means of double-deck vibrating screens or trommelsinto several sizes for processing on a magnet.

Attrition ScrubbingThe process of attrition scrubbing is usually the stage

in the beneficiation process that is subject to more varia-tions than any of the other unit operations. Attritionscrubbing fulfils several functions, depending on the typeof ore that is being treated. Its major function is in thecleaning of adhering surface coatings from the andalusitecrystals. This is important since crystals that are inade-quately scrubbed not only have a lower alumina and ahigher iron content, but surface coatings can lower thefusion temperature and weaken the strength of the brick.An equally important function of the attrition scrubberis the dispersal of particles of soft shale, which, if notdispersed, can disintegrate in the dense-medium separa-tion stage and contaminate the medium.

As mentioned previously, the attrition scrubber canalso serve to liberate the andalusite from the shale and,for mines treating fine-grained andalusite shales, this isthe only effective method of liberation.

Attrition scrubbers operating on fine-grained crystalsthus serve the treble purpose of crystal liberation, crystalcleaning, and shale dispersal. They also substantially in-crease the concentration of al1dalusite and, from a run-of-mine ore with an andalusite content of 10 per cent,scrubbing can provide a feed material containing up to50 per cent andalusite, with a commensurate reductionin the mass to be treated in the subsequent stages.

Multiple stages of attrition scrubbing are often neces-sary when harder shales are treated, and a producer inthe northwestern Transvaal has included a cyclone de-sliming stage between the primary and the secondary at-trition milling. This avoids the use of an additional screen-ing stage, and removes light particles of shale togetherwith the slimes.

The deficiencies of attrition milling are often overcomeby the use of varied operations or even alternative pro-cedures. In France, semi-autogenous milling with steelballs is practised, whereas a local producer resorts to hardrubber lugs in the second stage of milling as a means ofobtaining good disintegration of the shale and cleaningof the crystal surfaces. Mintek developed a procedure thatinvolved pre-screening of the feed to the scrubber for theremoval of liberated crystals. The screen oversize was thenfed to a modified attrition mill (shown in Fig. 8) fittedwith discharge grates in the mill shell. Hollow steel pipesin the mill break down the harder particles of shale toa size that will pass through the grate. Liberated andalu-site crystals also passes through the grate, thus minimiz-ing further breakage as would have happened in a con-ventional mill. This procedure increased the andalusiterecovery by as much as 30 per centl2. However, this millwas not as effective with shales containing coarse crystals,in which the mechanism of liberation is different fromthat of fine-crystal shales.

JUNE 1989 165JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

"--

Fig. 8-Mintek-modified mill, with discharge grates in the shelland a prescreening trammel

,Diamond pans have been found to be an effective

means of attrition scrubbing in addition to acting as aconcentrator, and are used exclusively at the HavercroftMine. The ore is prepared by two-stage crushing in jaw-and-rolls crushers. No fines removal is carried out (thefines acting as a dense medium analogous to puddle indiamond treatment), and the ore passes direct into thepans. Slimes and light particles of shale discharge overthe central weir, while the heavier andalusite crystalsmigrate to the bottom of the pan, where the raking ac-tion creates sufficient attrition to clean the surfaces ofthe andalusite crystals and to disperse the softer particlesof shale. The water required is claimed to be much lessthan for drum-type scrubbers. The enriched underflowpasses to screw classifiers, where the slimes are removedby the use of additional wash water.

Attrition scrubbers are now being used further downthe process route on finished and semi-finished concen-trates to further improve the quality of the final product.These scrubbers are smaller versions of the primary scrub-bers, and are probably more effective in reducing the sur-face impurities since there is a more intimate contactbetween the andalusite particles than would be the casein primary scrubbers, where particles of shale and slimescan dampen or inhibit the contact between the andalusiteparticles.

Dense-medium SeparationAs mentioned previously, the differences in relative

density between andalusite and the other constituents ofthe ore were thought to be sufficiently high for the ap-plication of conventional gravity-concentration pro-

cedures such as jigging, pneumatic (dry-air) tabling, andwater-film concentrators9, and in the early years, whenproduction was confined to the Zeerust-Groot Maricoarea, there were several examples of plants using thesetypes of concentrators. However, these methods werevery inefficient because of the shape factor and the factthat the differences in relative density were not sufficientfor good separation. This was particularly so with andalu-sites that had been subjected to extensive weathering orthat had a large range of densities (Table VI). Further-more, these methods were not able to treat the high ton-nages required to make the deposit economically viable.

Dense-medium separation eventually proved to be theonly effective method of concentration to attain thegrades of andalusite required by the consumer. Thismethod is capable of separating particles with differencesin relative density that are as little as 0,1 density unit athigh-tonnage throughputs and at a large range of par-ticle sizes.

The major factor influencing efficient separation indense-medium separators is the quality of the medium.Ferrosilicon is the only medium used in the concentra-tion of andalusite, and various grades are available to pro-vide suspensions with relative densities of up to 3,8without seriously affecting the viscosityl3. The proper-ties, advantages, and uses of ferrosilicon powders aredescribed by Collins et af.14, which is an excellent guideto the selection of the correct medium.

Various types of separating vessels are used in dense-medium separation, and Fig. 9 shows details of three ofthe separators most commonly used in the andalusiteindustry.

Cone Separators. This type of separator was initiallyfavoured because of its simplicity of operation andbecause a relative density of 2,95 for the medium ensureda gangue-free andalusite concentrate. The major dis-advantages of cone separators are the low throughput interms of the size of the vessel and the large amount ofmedium in circulation. Also, the viscosity of the mediumaffects the efficiency of the method and limits its use toparticles above 2 mm in size. There is only one exampleof a cone separator at present in use in the processingof andalusite. In this example, the cone is used as a secon-dary concentrator to up-grade a low-grade concentratefrom a cyclone separator.

Cylindrical Separators. A typical example of a cylindricalseparator is the Dyna-Whirlpool and, at the KrugerspostMine near Lydenburg, Dyna-Whirlpool separators areused to treat a fine-grained andalusite ore8. A newer ver-sion of the Dyna-Whirlpool is the Tri-Flo separator,which is a two-stage separator that can produce twoseparate concentrates and a tailing or, alternatively, amiddling product that can be recycledl5. Although thistype of separator is not being used at present, the increas-ing tendency towards two-stage separations warrants aninvestigation of its use.

Cyclone Separators. With one exception, all the andalu-site mines in South Africa use the dense-medium cycloneas the means of separating andalusite from gangue, andit has been shown to be a highly efficient separator whenoperated correctlyl6. Cyclones with an included coneangle of 20 degrees are standard in mineral separations.


TailingFee:g.

Feed ~and medium

Cone

~ Medium

Concentrate

Cyclone Dynawhirlpool

Fig. 9-Examples of heavy-medium separators

At the Timeball Hill Mine, a cyclone with an includedcone angle of 10 degrees has proved highly effective.Cyclones with larger cone angles (40 to 60 degrees) havea tendency to reject coarse, heavy particles and are notfavoured.

Operation of a Dense-medium Separation PlantThe major factors that influence the efficient opera-

tion of a dense-medium separation plant are, firstly, thepreparation of the feed to the cyclone and, secondly, thechoice of the correct medium. Poor feed preparation willaffect the quality of the medium, leading to inefficientseparations, while incorrect grade or poor quality of themedium will negate the effect of a properly prepared feed.

Other factors that affect the efficiency are variationsin feed pressure, the ratio of medium to ore, and therecovery, cleaning, and densification of the medium9,17,18.Modern practice in medium recovery, cleaning, and den-sification involves the use of cylindrical densifiers, two-stage magnetic separation, and automatic density con-trol. The cost of ferrosilicon is at present close to R800per ton, and an efficient recovery and cleaning systemis of paramount importance in reducing operating costs.An efficiently operated plant will have a ferrosilicon lossof between 100 and 200 g/t of feed to the dense-mediumplant. Fig. 10 show the principle of operation of dense-medium separation and the various components of theplant.

Determination of EfficiencySince there are no practical methods of analysing an

ore for its andalusite content (the host rock and othergangue minerals also contain alumina and iron), thedetermination of the head grade of ores or other andalu-site-bearing materials other than concentrates requires theuse of physical methods. Heavy-liquid separation is themethod most commonly used, and the results are repro-

ducible. The heavy liquid most frequently used is tetra-bromo-ethane (TBE), which has a relative density of 2,96.The determination of the andalusite content of oresrequires careful crushing to liberate the crystals from thehost rock. A relatively simple test of the efficiency of theseparation is thus the treatment of the products of dense-medium separation by heavy-liquid separation, which in-dicates the amount of misplaced particles. Chemicalanalysis is then used to determine the quality of the con-centrates, as shown in Table V.

A more reliable method of determining efficiency,which allows a comparison of the effect of changes inoperating conditions, is the use of the Tromp EfficiencyCurve19. From this curve, both the density of separation(D, or Dso) and the efficiency or Probable Error (Ep)can be obtained. A typical Tromp Curve is shownin Fig. 11.

Allied to the use of the Tromp Curve is the washabilitycurve, which is used extensively in the coal industry. Thewashability curve shows the amenability of the ore todense-medium separation and, when used in conjunctionwith the Tromp Curve, gives an accurate prediction ofthe performance of the concentrator.Two-stage Concentration

Currently, there is a tendency for producers to usemultiple concentration stages as they strive, not only toimprove the quality of their product, but also to obtainhigher recoveries. Recoveries at the various mines aregenerally Iow, and over half of the andalusite content ofthe ore is lost as fines and as misplaced particles in thevarious stages of concentration.

At the Annesley Mine, a Iow-grade concentrate pro-duced in the primary cyclone is further scrubbed and thensubjected to a second stage of dense-medium separation.At the Andafrax Mine, a cone separator is used to up-grade the cyclone concentrate. At the Hoogenoeg Mine,the dense-medium concentrate is screened at 6,0 mm, and


-- ~

Feed

Fresh water~ M 8

.

U

.l 0 Magnetic

I separator

Mixing tank

I"..

I'I' I' '" ", ~- ~~

-r==:=-~-~-

the particles above 6,0 mm in size are carefully crushedto below 6,0 mm and are then recycled to the scrubbingstage. Since the particles break preferentially along thelines of the chiastolite cross, liberation of the impuritiespresent in the cross results in a product of higher grade.

Magnetic SeparationThe final stage in the concentration procedure is

magnetic separation. Specifications for andalusite con-centrates require the FeZO3 content to be not greaterthan 1,0 per cent. Concentrates with lower values willcommand a premium price while penalties are imposedfor every 0,1 per cent that the FeZO3content exceeds thespecified value.

Since most of the iron-bearing impurities are onlyfeebly magnetic, high-intensity magnetic separation isrequired to reduce the FeZO3 content to acceptable

Fig. 10-Details of a dense-medium cyclone plant

levels. Mintek initially used a four-pole Crossbelt sepa-rator when evaluating ores. Induced roll (IR) separatorswere used with success at the various mines, but thesemachines had the disadvantage that the rolls were sub-ject to high wear as a result of particles rubbing betweenthe roll and the pole piece. The need for maximum fieldstrength requires that the air gap be kept as small aspossiblezo. This is shown graphically in Fig. 12, whichalso illustrates the principle of the IR separator. Otherdisadvantages of IR separators are the high power re-quirements and the high mass in relation to size.

A new development in dry high-intensity magneticseparation is the Permroll separator, and at present fiveof the six mines use Permrolls in their circuits. TimeballAndalusite uses both IR and Permroll separators, and theKrugerspost Mine uses only IR separators. Permroll-typeseparators are claimed to have achieved markedly lower


.g.

~50....i!)

oD40E

;::3r:: 30r::

,820;;

oD';: 10t;is 0

2,80

0..:..:..:

3"0....

3r::0

,~i!)

'"0r:: 10Fig. 12-Variation..c::

of field strength bfjwith air gap in an r::i!)induced-roll ....

magnetic separatort;

'0

:2 100'r::';;; 90

'"«$~ 80....i!)

1; 70ui!)

60

Ep

E p =d75-dz5 =00582 '

(efficiency or probable error)

2,90 3,00 3,10

Density units

Fig. 11- The Tramp efficiency curve for a dense-mediumcyclone

iron values in andalusite concentrates than other typesof separator. The Permroll separator, details of whichare given in Fig. 13, has the advantage of minimal powerrequirements, low mass, light construction, the absenceof an air gap, and no direct contact between particle androll, which completely eliminates roll wear. Fig. 14 showsa single-stage Permroll separator in operation.

Table VIII compares the operation of Permroll, IR,and Cross belt separators. A new magnetic alloy neo-dymium-iron-boron will soon be available, which willincrease the magnetic induction by a substantial amount.

The Havercroft Mine has introduced a roasting stageprior to magnetic separation. Roasting under controlled

20

15

5

05

conditions increases the magnetic susceptibility of feeblymagnetic particles, and gives a superior-grade productwith an FeZOJ content of less than 0,6 per cent.

Recovery of Fine AndalusiteAndalusite particles below 0,6 mm are not recovered

by dense-medium separation, the major reason beingthat, at sizes below 0,6 mm, recovery of the medium byscreening becomes very inefficient and excessive screenareas are required in addition to the huge quantities ofwater that are needed to wash particles of this size freefrom the adhering medium. In the early days of the in-dustry, specifications for andalusite concentrates requireda minimum of fines below 0,6 mm, and this requirementis still adhered to at present.

The demand for fine andalusite down to 0,1 mm is in-creasing, and several producers are investigating therecovery of andalusite from their slimes dams, whichrepresents a vast source of high-grade material if it canbe recovered.

Several methods of concentration are available intheory. Dense-medium separation is the obvious choicesince it has been proved on fine materials such as coaland cassiterite. The plant required for fine material dif-fers in several respects from that required for coarsematerial. The major difference is in the recovery of themedium from the separation products and, since screenscannot be used for this purpose, wet-drum magneticseparators are the obvious alternative. This technologyis well established in coal mines treating fines down to0,1 mm.

Tests at Mintek on fines below 0,6 mm showed dense-medium separation to be successful, and concentratesassaying 61,0 per cent Alz°J and 0,5 per cent FezOJ were

Air gap 3,2 mm

Air gap 4,8 mm

I10

I15

I20

d,c, field supply, A

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

------

JUNE 1989 169

Grade, "70Non-magnetics

Separator Product AI,O, Fe,O3 "70

Crossbelt Feed 56,3 2,58Non-magnetics 57,4 1,03 91,0

Induced roll Feed 56,2 2,58N on-magnetics 58,6 0,95 87,0

Permroll Feed 56,2 2,58Non-magnetics 59,6 0,71 83,0

I i:~!J !J

Strong Weak Non-mags mags mags

t , tLJ~\d0Product split as it passes over the Permroll

Fig. 13- The product split in a single-stagePerm roll separator

Fig. 14-A Perm roll separator treating andalusite concentrate

T ABLE VIIIPERFORMANCE OF MAGNETIC SEPARATORS IN THE TREATMENT

OF ANDALUSITE CONCENTRATE

Note: In all cases four passes over the separator were required

produced. The low FezO3 value was obtained in a wethigh-intensity magnetic separator. However, no attemptshave been made to establish whether a dense-mediumplant using magnetic separation to recover the mediumand clean the andalusite crystals of adhering medium iseconomically viable.

Flotation is an optional method for the recovery of fineandalusite. However, it is limited at present to a maxi-mum particle size of 0,3 mm, although new techniques

such as column flotation can treat coarser sizes. Somesuccess has been achieved at Mintek with flotation, butthe recoveries have been very low and the grades are notyet comparable with those obtained by dense-mediumseparation. Gravity concentration by spirals or tables ispossible provided closely sized feeds are used, but insuf-ficient work has been done on the evaluation of thistechnique. The water requirements are obviously a seriousfactor and must be considered in the evaluation of theseprocedures.

The Role of MintekA review of the andalusite industry would not be com-

plete without an acknowledgement of the role that Mintekand its predecessors have played in the processing ofandalusitez1. In fact, the first work on andalusite wasdone about 50 years ago in the Minerals ResearchLaboratory, which eventually became Mintekzz. Sincethat time there has been a close association between theindustry and Mintek, and many of the developments andimprovements that have been introduced are a directresult of research carried out at Mintek. This includedwork on dense-medium separation, washability, the useof the Tromp Curve in the prediction of results and thedetermination of efficiencies, and improvements inattrition-scrubbing techniques, as well as extensive testson various types of magnetic separators.

This close co-operation with the industry has resultedin the successive implementation of more efficientmethods in South African andalusite plants. Mintek'sservice continues, not only in improving mineral-proces-sing methods, but also in developing analytical techniquesand certified analytical reference materials.

Future OutlookThe present demand for andalusite is strong and is like-

ly to continue so for the foreseeable future. At present,producers are hard-pushed to fulfil the demands fromconsumers world-wide, and all the plants are operatinga t full capacity. Several producers are increasing theirplants to cope with the demand, and it is likely that oneor two new producers will be in operation soon.

The major reasons for the increased demand lie, firstly,in the improved economic conditions world-wide alliedto a shortage of zircon and, secondly, in the high pricesbeing asked for bauxite. The South African producershave improved considerably both in their consistency ofsupply and in the quality of their products, and premiumquality concentrates are being sold at between R320 andR330 per tonne f.o.b. Richards Bay. Material of lowerquality commands a price of R250 to R260 per tonne.

The demand for fine andalusite (smaller than 0,6 mm)is also increasing. It is unfortunate that natural fines arenot being produced at present, but dump material repre-sents a vast source of fine andalusite if it can be recovered.At present, coarse concentrates are being milled to supplythe demand.

Future competition is most likely to come from newsources located in other countries and from substitutematerials. Extensive andalusite deposits in China are un-der investigation, and both Australia and Brazil arereported to have andalusite deposits that can be exploited.However, for the foreseeable future, it is evident that


South Africa will be the major supplier of this vital com-modity. It is to the credit of the industry in general thatmuch effort is being put into improving, not only the ef-ficiency of the plants, but also the quality of the products,by the application of new technology, and that SouthAfrica's dominant position in this particular area is beingmaintained.

AcknowledgementsThe author thanks the Minerals Bureau, Mintek, and

andalusite producers at the various mines in South Africafor supplying the information and statistics included inthis review.

References1. DEPARTMENT OF MINERAL AND ENERGY AFFAIRS. Mineral produc-

tion and sales statistics 1985 (provisional). Johannesburg, MineralsBureau, 1986.

2. Andalusite Mining Magazine, May 1988. pp. 417-420.3. BLAIN, M.R. The occurrence of andalusite in the metamorphosed

Pretoria Group Sediments along the North Western margin of theBushve1d Complex. Pretoria, Geological Survey, Report Eg. 2114:,Nov. 1975 (unpublished).

4. FRENCH, N.R., and HECKROODT, R.O. Andalusite from the East-ern Transvaal as a refractory raw material. Annals Geol. Surv. S.Afr., vol. 14, no. 1. 1980. pp. 101-103

5. BLAIN, R.M., COETZEE, CB., and LOMIJARD, B.Y. Andalusite,sillimanite and kyanite. Mineral resources of the Republic of SouthAfrica. Coetzee, CB. (ed.). Handbook 7, Geological Survey ofSouth Africa. pp. 225-260.

6. DEPARTMENTOF MINERAL ANDENERGY AFFAIRS. Minerals Bureau,private communication.

7. OVERBEEK, P.W., and LEVIN, J. Mintek, private communication.8. CULLlNAN MINERALS LIMITED. Information brochure on Krugers-

post andalusite.

9. LEVIN, J., and OVERBEEK, P.W. A summary of work done on theconcentration of andalusite from deposits in the western Transvaal.Johannesburg, National Institute for Metallurgy, Report no. 1779,Jan. 1976. 44 pp.

10. SIl.LlMANITE MINERALS. Europe places demand on andalusite. In-dustrial Minerals, Jan. 1985. pp. 41-53.

11. OVERBEEK, P.W. The beneficiation of andalusite. 14th AnnualSymposium, South African Ceramic Society, Pretoria, 1982.

12. OVERBEEK, P.W. The development of a modified milling techniquefor the treatment of andalusite-bearing shales. Randburg, Coun-cil for Mineral Technology, Report no. MI41, 1984.

13. WELLBELOVED, D., CRAVEN, P.M., and GERICKE, W.A. The bene-ficiation by dense-medium separation of Mamatwan ore. The ThirdSamancor Symposium on Dense-medium Separation, Feb.lMar.1988.

14. COLLlNS, B., NAPIER-MuNN, T.J., and SCIARONE, M. The produc-tion, properties, and selection of ferrosilicon powders for heavy-medium separation. J. S. Afr. Inst. Min. Metal/., vol. 75, no. 5.pp. 103-105.

15. FALCON, LM. Conversion of Rooiberg plant from Dyna-whirl-pool to Tri-flo operation. The Third Samancor Symposium onDense-medium Separation, Feb.lMar. 1988.

16. POPPLEWELL, G.M., and BuRKs, F.B.J .M. A review of the develop-ment of dense-medium plant design in the diamond industry. Ibid.

17. HAWKER, T.G. Recovery of magnetically susceptible dense-mediamaterials. Min. Miner. Engng, Dec. 1971. pp. 17-8, 23.

18. CHASTON, I.R.M. Heavy-medium plant design and practice fordiamond recovery in Africa. Tenth International Mineral ProcessingCongress, London, Institution of Mining and Metallurgy, Apr.]973.

19. TROMP, K.F. New methods of computing the washability of coals.Colliery Guard., vol. 154. 21st May, 1937. pp. 955-959.

20. ANON. Magnetic separation. Ind. Min., May 1988. pp. 21-33.21. The production of andalusite and other refractory minerals. Rand-

burg, Council for Mineral Technology, Application Report no. 5,1988.

22. LEVIN, J. The story of Mintek 1934-1984. Randburg, Council forMineral Technology, 1985.

Reference works on advanced materials

Each of the following seven studies is a comprehen-sive state-of-the-art reference work with executive sum-mary and conclusions. Alumina, the general workhorseceramic, has been used commercially since the 1930s. Thevarious forms of boron nitride find specialist uses.Aluminium nitride is of particular current interest as anelectronic substrate and packaging material for VLSIs.Titania is no longer a minor appendage of the pigmentbusiness but now constitutes a major range of advancedmaterials. Rapid development of the silicon ceramics andzirconia has been spurred by the ultimate goal of the all-ceramic engine that will run unlubricated and uncooled,and will save energy. This goal has not been realized buthas resulted in the development of new, stronger, tougher,and more temperature- and corrosion-resistant forms ofthe materials-and also fibres, whiskers, and composites.These materials are all being used by industry to save fuel,time, and money. The sections include: The different

forms of the materials; Methods of production; Worldproduction in the major countries; World trade; Worldconsumption; End-uses; Prices; Activities by companiesand research organizations; Exchange rates and termsused. All seven of the reports, which are world-wide inscope, were prepared by MMR (Mitchell Market Reports)of Harrow, England.

. Zirconia. 285 pp. (2 vols.) 06/88 $1050 (£600). Alumina. 389pp. (2 vols.) 05/87 $1315 (£750). Silicon carbide. 300 pp; 09/88 $1050 (£600). Boron carbide and boron nitride. 163 pp. 12/87 $875(£500). Silicon nitride and the sialons. 230 pp. (New edition).07/89 $1050 (£600). Aluminium nitride. 150 pp. 05/89 $875 (£500)

. Titania and the inorganic titanates. 150 pp. 06/89$1050 (£600).

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

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JUNE 1989 171

Safety at Genmin minesSeven out of the ten gold mines that had the lowest

fatality rates in 1988 were in the Genmin group. Accord-ing to statistics released by the Chamber of Mines, theWinkelhaak Gold Mine, situated close to Evander in theeastern Transvaal, was at the top of the list, with thelowest fatality rate of 0,198 per thousand workers. Thisfigure compares favourably with the average fatality rateof 1,1 per 1000 workers of the 39 gold mines that aremembers of the Chamber of Mines.

Besides the Winkelhaak Mine, which received theLynne van den Bosch Award for the lowest fatality ratein the Chamber of Mines Gold Division, the followingmines turned in low fatality figures per thousand workers:West Rand Cons. (0,410), situated near Krugersdorp;Grootvlei (0,411), near Springs; Leslie (0,440) and Kinross(0,680), both in the Evander area; and Stilfontein (0,716)and Buffelsfontein (0,720) in the Stilfontein area of thewestern Transvaal. The average fatality figures for thesemines were more than 50 per cent better than the averageof the 39 mines for which the Chamber kept statistics.

Two of the collieries in the Genmin stable, TransNatal's Kilbarchan Mine, near Newcastle, and the DelmasColliery, near Delmas, had the third- and sixth-lowestfatality figures out of the 31 coal mines that are members

of the Chamber of Mines. These two mines were alsoawarded the c.S. McLean Shield for the lowest repor-table accident figures per 1000 workers over a three-yearperiod.

On top of this, Winkelhaak, Kilbarchan, and BafokengSouth (a platinum mine near Rustenburg) also achieveddouble-millionaire status for working two million shiftswithout any fatal accidents.

Three other mines managed by Genmin are currentlymillionaires. They are Delmas, Samancor's Wessels-Hotazel Mine, and Barberton Gold Mines, which werealso awarded the St Barbara trophy. Another memberof the group, Impala's platinum refinery at Springs hasalso become a millionaire.

Genmin's success in the field of mine safety can largelybe attributed to the work done by the highly specializedmine-safety unit that was created about two years ago.Genmin's Senior Consultant, Health and Safety, MrGeorge Krafft, says the unit has done intensive researchthat has, in turn, led to improved and updated safety pro-cedures and better training programmes. These steps, aswell as the attitude of the employees and managementtowards safety, have led to higher safety standards andlower accident figures.

The photograph, which was taken at the launching of General Mining Metals & Minerals (Genmin), shows the Chair.man of Genmin, the General Managers of five of the mines, and the General Manager of the Platinum Refinery at Springs,who received awards for the Iow fatality and accident rates during 1988. Left to right: Derrick Parfitt (Barberton GoldMines), Humphrey Q'Keefe (Impala Platinum Refineries), Steve Ellis (Chairman of Genmin), John Smithies (Bafokeng

South), Cyril Warburton (Winkelhaak), Harry Kruger (Wessels.Hotazel), and Dutch Botes (Delmas Colliery)


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