PARTICLE SIZE DISTRIBUTION IN DIFFERENT GRINDI'NG ___ .,..- -:· ~

SYSTEMS

by Professor P G Kihlstedt, Division of Mineral

Processing, The Royal Institute of Technology,

Stockholm, Sweden.

Synopsis

Particle size distributions for different ores and

grinding systems have been studied with me_as.urements

down to ca 2 ~m. When mineral aggregates are ground,

two maxima are normally observed in the mass frequency

curve for the ground product, a coarse grinding maxi­

mum which becomes higher the finer the grinding and

an abrasion maximum.

Three grinding systems have been included in the in­

vestigation, a rod mill- ball mill, a 'ball mill. and

an autogenou~ mill in a closed c~rcuit. Parallel with

the full-scale sampling, grinding has been carried out

in a laboratory rod mill - ball mil-l to permit a compa­

rison between the different grinding systems. The

quantities studied have been the energy consumption,

k 80 and the specific s urface . k 80 is t h e t heor etica l

mesh through which 80% by weight of the material ccan

pass.

Another study has comprised two different grindi~g

systems with autogenous - pebble mills and has been

designed to show how an open grinding system -with an autogenous mill influences the mass frequency curves

for the material and for the mineral content of the

ground product.

2

Partikelgro·:isenverteilung bei verschiedenen Vermahlungs­

systemen

Man hat Untersuchungen der Partikelgrossenverteilung

bei verschiedenen Erzen und Vermahlungssystemen durch­

geftihrt mit Messungen bis hinab zu ca 2 pro. Bei Zer­

kleinerung von Mineralaggregaten erhalt man gewohnlich

zwei Maxima auf der Massenfrequenzkurve des gemahlenen

Produktes. Einesteils ein groberes mahltechnisches

Maximum, das hoher ist, je feiner die Vermahlung ge­

trieben wird, und anderenteils ein Abntitzungsmaximum.

In einer Untersuchungsserie haben wir drei Vermahlungs­

systeme, St abmtihle-Kugelmtihle, Kugelmlihle und einfache

Autogenmtihle in geschlossenen Kreisen studiert. Parallel

mit Probeentnahme in vollgrosser Skala haben wir in

dem System S.tabmtihle-Kugelmtihle Vermahlung in Labora­

torieskala durchgeftihrt und damit einen Vergleich

zwischen den verschiedenen Vermahlungssystemen er­

moglicht. Die undersuchten Grossen sind Energiever­

brauch, k80 , sowie die spezifische Oberflache. k80 ist

die gedachte Maschenweite, durch die 80 Gewichts-% des

Gutes passieren.

Eine andere Untersuchung umfasst zwei verschiedene

Vermahlungssysteme mit Autogenmlihle-Steinmtihle und soll

beleuchten, wie ein offenes Vermahlungssystem mit

Autogenmtihle die Massenfrequenzkurven des gemahlenen

Produktes in Bezug auf Gut resp. Mineralgehalt beein­

flusst.

Repartition de· ·1a ·t ·aille des particules pour les

different·s typ·es· de ·broyages

Des observations sur la repartition de la taille des

particules ont ete effectuees pour des minerais

differents et pour d~s procedes de broyage differents,

avec des mesures jusqu'a 2 ~· Lors du broyage d'un

agr€gat mineral on obtient norrnalement deux maxima

sur la courbe de frequence des masses du produit

broye. D'une part un maximum du a la technique de

·broyage utilisee qui devient de plus en plus haut,

plus le broyage est fin, d'autre part un maximum du

a !'abrasion.

Dans une serie d'observations on a €tudi€ trois sys­

t~mes de broyage: concasseur a barre et a boulets,

broyeur a boulets et broyeur simple autogene. Pa­

rall~lement au prel~vement d'€chantillons a !'€chelle

industrielle, on a effectu€, en laboratoire, un

broyage dans un concasseur a barre et boulets et rendu

possible une comparison des diff€rents proc€d€s. Les

pararn~tres etudi€s sont: la consornrnation d'energie,

k 80 ainsi que la surface specifique. k80 est la

largeur suppos€e des mailles d'un grilla9e a travers

lesquelles 80% du poids total du materiau passent.

Une autre observation est basee sur deux proc€des

differents avec broyeur autog~ne-broyeur a pierres et

destinee a montrer comment un syst~me de broyage ouvert

avec broyeur autog~ne influe sur la courbe de freque nce

des masses du produit broye pour le produit et pour

la teneur des minerales.

Introduction

In a series of earlier lectures and articles /l-5/ I have

described studies of the particle size distributions,

specific surfaces and effects on subsequent mineral

dressing processes resulting from different methods

of grinding mineral raw materials. These studies have

shown how Rittinger's and Bond's laws have been found

to agree in the measurement of particle size distribu­

tion and specific surfaces. It has also been found

3

4

that this is due to the mechanical strength properties

of the crystalline structure of minerals. The normal

three-dimensional minerals, when ground, eventually

acquire a lattice structure in the 0.1-1.0 ~m particle

size range which is so strong that no further crushing

can be produced by the disintegrative forces available

in an ordinary mill. This particle size represents, so

to speak, a barrier against which the particle size

distribution is squeezed up by further grinding.

ACk 61og k

/' I I I I

: \ \

: ' .....

- TOlAL SCliO

/ \ ~ "' 1\ - 1-- / \

90

80

70

60

50

.:.o 30

/ ~,, J/ - -- GALEN.A

2'J

10

0

1)0

120

110

100

90

81)

70

60

50

4'J

)0

20

1'J

0

1/ .......... ,\ 'I

I ..... _

2 ) 4 5 6 s 10 20 30 40 6C 80100 ,-, I .fGALE1N~ : I \ ! I ~ TOTAL SOL 10

: I \ :f If \ 1 -\

/ ' ~ .\ // '"'\l \

fj ~ ~ .)1 \I\

\ \

\ \ \

\ ,_

200 400 6001ro k um

2 ) 4 5 6 8 10 200 3001.00 600 BOO k ~m

Fig. 1. Mass frequency curves for particle size

20 30 40 5060 8010C

distributions of total solid and galena after rod mill

grinding and after teritiary grinding with small balls.

Finely disseminated complex massive pyrite ore with

galena.

The procedure is illustrated in Fig. 1. /3/ Here we

see two stages in the grinding of a solid pyrite ore

from Canada which contains galena and other minerals.

The mass frequency diagrams show the particle distri­

bution of the total material and of the galena at diffe­

rent sizes after rod mill grinding (at the top) and

after the third grinding step and final classification

(at the bottom). Three peaks can be seen in the case

of this ore. The first and coarsest is a peak related

to the grinding process. The second, which in final

grinding coincides with the grinding peak, marks the

liberation _df galena crystals from their settings in

the ore matrix. The third peak, at about 3 pro, is an

abrasion peak that occurs with ores and some methods

of grinding. It is evident that progressively finer

grinding concentrates the particle size distribution

into an ever-narrower range.

In this lecture I propose to show the corresponding

particle size distributions for a number of ores of

different types ground by different methods.

Some particle size distributions

By way of additional background to our view of diffe­

rent particle size distributions and their implications,

I will show you in Fig. 2 /5/ the mass frequency curves

for the feed to three large European flotation plants

for complex sulphide ore. The grinding layout in these

three plants is of standard type, with a rod mill and

one or two ball mills in series plus hydrocyclone

classification. The finenesses differ, as can be seen

5

6

~ Ck ~ logk

150

~

I 7 I

I I

\ c k80 39 ~m

\ \

' v-1\. B k80 1.1 ~m

/ ~ \ k.

140

130

120

110

100

90

eo 70

&) I ~v \ 1\. ~ ~ \A k60 135 um

50

40

30

20

10

~

~ ~

/ v f-.... //

v ./ v v

J / i""'oo.,

r--... It 7_. ........ r-- t>C r- l,.;' ~

~ v \ \ ~ ~ ~ \

\ \ \ i\

\ \ \ ' 1\

1\ 1\ \ 0 1 s 10 50 100 200 ~m(k)

Fig. 2. Mass frequency curves. Feed to flotation at

three European flotation mills. Estimated quantity of

particles per m3 .

A:5 X 1015 , B:20 X 1015 , C:50 X 1015

from the values of k 80 quoted for each end product.

k80 is defined as the imaginary screen aperture through

which 80% by weight of the material will pass.

As we see, each curve has two peaks: a grinding peak

that grows higher the farther the grinding is carried,

and an abrasion peak. The particle size distribution

is determined down to about 2 ~ and tends to stop at

about 1 pro. Determination of the exact distribution

in this range calls for special methods and apparatus

which were not available in the present case.

An interesting point is that these particle size de­

terminations can be used to compute ' the number of

particles produced by each method of grinding. The

result works out at something of the order of five

to fifty times ten-to-the-fifteenth particles per

cubic metre of solid material.

The central mineral processing research organisation

in Sweden has been engaged since 1969 on a programme

of sampling and measurement in full-scale operational

plants to investigate how different grinding systems

CONE CRUSHER

fU)

t.41LL

SYSTEM I

n---,...--c~::BALL ILL

SYSTEM ][

RAKE CLASSIFIER

SYSTEM m

FIG 3

GRQJNO PROD.

GROUND PROO.JCT

Fig. 3. Three simplified principle grinding systems

compared by individual laboratory tests.

7

8

work with reference to the properties of the ground

product as well as operating costs and energy consump­

tion. Let us look at some results from three principle

systems.

System I is a standard layout comprising fine crushing,

rod mill and ball mill in a closed circuit with a

hydrocyclone. The ore is a gangue-dominated complex

sulphide ore.

System II is an older layout, with closed fine crushing

circuit followed by ball mill in a closed circuit with

a scraper classifier. The ore is sulphide-bearing,

gangue-dominated skarn iron ore.

System III is an autogenous grinding system with run­

of-the-mine material fed straight to the mill, which

works in a closed circuit with a hydrocyclone. The

ore is a gangue-dominated magnetite ore of skarn type.

To obtain a comparison between the systems, ore samples

were taken as well as samples of products from the

circuits. The former were subjected to standardised

grinding on a laboratory scale in Bond rod and ball

mills. The net energy input was determined, and the

products were analysed for particle size distribution

and specific surface.

System I showed much the same results on both full

and laboratory scale as regards energy input, particle

size distribution (Fig. 4) and specific surface. The

energy input to k 80= 108 ~m was about 8.8 kilowatt­

hours per ton in the laboratory and 9.9 kWh/ton in

full-scale operation.

Fig. 4. Mass frequency curves of ground products.

Grinding system I.

Specific surface at k 80=108 ~ wa s 10 000 square

centimetres per cubic centimetre in the lab and

9 500 cm2/cm3 i n the f ul l -scale plant.

Figure 4 shows how the pyrite content of the ore

distorts the particle size distribution to some extent

in full-scale operation due to the effect of the closed

circuit through the hydrocyclone; in the laboratory

the material was ground in an open circuit.

System II showed less favourable figures for energy

input and specific surface in full-scale operation.

Energy input to k 80= 75 ~m was 8.8 kWh/ton in the lab

9

10

and 17.0 kWh/ton in the full-scale plant.

Specific surface at k 80=75 ~was 7 000 cm2;cm3 in the

lab and 8 700 cm2/cm3 in actual operation.

A Ck A log k tSO

100

so

0 1

~ v

, II

5 10

""'"' LA~ v TOTAL

v 'Jr-..

lli ~

~ PLANT TOTAL

\

50 100 500 1000 &Jm

Fig. 5. Mass frequency curves of ground products.

Grinding system II.

Figure 5 shows the particle size distributions for

both laboratory-scale and full-scale grinding. The

full-scale curve is somewhat flatter.

System III showed a higher energy input and specific

surface for grinding to the same value of k 80 as well

as a much flatter particle size distribution curve.

Energy input to k 80=120 ~was 10.1 kWh/ton in the lab

and 17.5 kWh/ton in the full-scale plant.

Specific surface at k~ 0=120 pro was 7 90.0 cm2 /cm3 in

the lab and 12 200 em /cm3 in the full-scale plant.

fl Ck fllog k 150

'

5

!/ I

10

/"' 1'\

v !"o"

~I

50

~As YoiAL

" ~PLANT OTAL

\ \ 100 500 1000 ~m

Fig. 6. Mass frequency curves of ground products.

Grinding system III.

Figure 6 shows the particle size distribution curves

for both laboratory-s cale and full - scale grinding .

At first sigh t one might assume that this type of

grinding gives unfavourable results. However, the

magnetite concentrate produced here goes to pelletisa­

tion, a process which works better with a high specific

surface and a flat particle size distribution. In

actual fact it is much more economically advantageous

to generate the surface by this type of closed-circuit

autogenous grindin~ than by grinding systems of more

traditional type.

11

12

The investigations of these three grinding systems

show that for normal grinding purposes, where the

object is to obtain a good particle collection with the

lowest possible specific surface and the lowest possible

energy input, the value of k 80 required for the mill

product can be reached most economically if the process

is divided into several stages, that is to say fine

crushing, road mill grinding and ball mill grinding in

one or more stages in a closed circuit.

Multistage Autogenous Grinding

Experienc e from many parts of t he world i ndicates that

for purposes of flotation, singel-stage autogenous

grinding in a closed circuit is not the bes t choice

from the point of view of grinding costs and flotation

results. Aut9genous grinding is an abrasion process

capable of giving optimum results in grinding to a

particle size distribution which match~s the natural

disintegration structure of the ore. But if, as is

normally the case, the grinding has to be much finer

than that in order to liberate the minerals, this can

be accomplished much more economically by normal fine

grinding of the autogenous mill product in a ball or

pebble mill in a closed circuit with a classifier or,

hydrocyclone.

Let us study two examples of such grinding. One refers

to a low-grade lead ore in quartzitic sandstone, and

the other to a low-grade copper ore in the form of mica

schist impregnations with some pyrite. Figure 7 shows

the flowsheets in simplified form. The energy input per

ton of ore is probably somewhat higher than for multi­

stage grinding after fine crushing. The lead ore is

ground to k 80 = 125 pro and the copper or to k 80 = 185 ~·

Figures 8 and 9 show the resulting particle size distri­

butions.

If these distributions are compared with those in

Fig. 2, we find that the products from primary auto­

genous grinding followed by pebble mill grinding show

a more concentrated range of sizes with less abrasion

slime than the products from rod and ball mills. This,

however, is probably also influenced by the age of the

geological formations from which the respective ores

come. There is, however, no abrasion peak in the curves

for the mill products of the Swedish ores in Figures

8 and 9.

It is of some interest to note the slime content of the

lead ore. The quartzitic gangue here contains a few per

cent of clay, part of which is slurried in the fractions

smaller than 1 urn. This is because clay minerals have a

two-dimensional lattice structure of low mechanical

strength. But the galena too seems to produce some fine

slime, despite an otherwise well-grouped. particle size

range peaking at about 30 urn.

The surprisingly well-grouped mill product from the

fairly soft-grained copper ore contains very little

slime. The hard pyrite gives no slime at all , whereas

the chalcopyrite is rather softer. We can also see how

cocrystallisation between pyrite and chalcopyrite has

influenced the particle size distribution of the

chalcopyrite too. The bulk concentrate of pyrite and

chalcopyrite is liberated in a regrinding stage just

ahead of the selective final flotation.

The distributions in Figs. 8 and 9, then, show that

primary autogenous grinding of suitable ores can produce

favourable particle size groupings, provided that final

grinding to liberation is done in a normal ball or

pebble mill.

13

14

Autogtneous mill

Ore ebbles

Ptbblt mills

MILL SYSTEM FOR LEAN LEAD ORE GRINDING

Feed ort Autogt>n. ------------r---et mill

Ore pebbles

Hydro­cyclone

Ground product

Hydro­cyclont

Ground product

MILL SYSTEM FOR LEAN COPPER ORE GRINDING

Fig. 7. Open autogeneous grinding systems with

pebble milling circuits.

t. Ck t.log k 150

100

50

0 1

--~

[,I~

~ ~ ~~

~

5

GALE NV 1\

v 1 I j

v ~:v ~ ~

10

V' TOTAL v,..~ SOLID

~ ~ I \ ~ ~ ~

" 50 100 500 1000 um

Fig . 8. Mass frequency curves of ground products

from lean lead ore grinding.

15

16

A Ck A log k 150

100

50

0 1

~ :::: F;;;.

. 5 10

~HAL CO-

~~v~ ~ J J

;v~

~ ~~AL SQID

50

"' · II

~~ ~ l! \

~ ~ riTE '~

100 500 1000 um

Fig. 9. Mass frequency curves of ground products

from lean copper ore grinding.

Milling in practice

We shall now see what this means in practice:

In rod milling, the material is well classified in­

side the mill. In the Loesche mill with built-in wind

screen and the Aerofall mill, a composite unit with

autogeneous mill and wind screens, excellent results

are obtained in the right grinding ranges because

the material is so quickly classified after each

crushing. Figure 10 shows how well the parcticle sizes

are grouped, measured as the value of S \lk80 , when

Malmberg ore is milled in an Aerofall mill. The com­

parative figures for equivalent dry and wet rod

milling confirm the differences previously pointed

out between these systems. In terms of energy, dry

·milling in Aerofall and Loesche mills is not necessa-

rily bette·r than other milling methods, owing to the

energy consumed by the blower equipment. The crushing

operation itself, however, beats all records for low

energy consumption.

Excellent results are of course obtained if we proceed

f ar ther on the multistage breakdown principle illustra­

ted in Fig. 11 but introduce classification steps

wherever appr opr iate . Figure 1 2 s hows s u c h a well­

designed system . It gives an optimum mill product for

selective flotation, with low energy consumption and

a very well grouped particle size distribution.

Figure 13 represents a milling system of the type

that was common during the thirties, with crushing

in a closed circuit and one-stage milling with coarse

balls in a closed circuit. Where this technique is

adopted, great importance must be attached to the

working of the classification equipment, because of

the small number of stages and the inherently poor

17

18

classifying capability of the grinding equipment.

This system, which was called ·'one easy step' , in­

volved a much higher energy consumption and greater

milling costs on account of the poor particle size

grouping and increased slime generation.

In the extreme autogeneous milling process in Fig. 14, .

simplification has been carried to the limit with the

entire breakdown taking place in one step - apart

from coarse crushing - in a closed circuit with a

hydrocyclone. This results in a higher energy consump­

tion , as autogeneous mil l ing scrubs the r e ject material

instead of milling it, producing more slime and a

flatter particle size distribution curve . In this

case the material is an iron ore destined for pelle­

tisation, and extra milling is applied to obtain a

large specific exterior surface in the magnetite

concentrate, so the milling is very favourable.

If, however, it is desired to obtain a more optimum

milling from the point of view of energy input and

particle size grouping, the autogeneous milling cir­

cuit should be made open and should be followed by

one or two ball or pebble mills as shown in Fig. 15.

One might say that this follows the system in Fig. 12

but replaces the crusher and rod mill with an auto­

geneous mill.

An estimate of results and costs according to different

systems gives the figures shown in the table.

Here we must bear in mind that total optimation must

also include the results of subsequent processes, i e

the yields and grades obtained in concentrates and

the balling and sintering steps in pelletisation.

Circuit stability, liberation, compaction of the

concentrate and so on must be considered here.

It is evident, however, that systems b and e are

suitable for concentration by flotation, while system

d is eminently suitable for pelletisation.

System kWh/m 3 Skr/m 3 kWh/m3 Skr/m 3

for for for for milling same milling same to k 80= to spec. 80 Jim surf.

1520003 em /em

a. Crusher, rod mill, ball mil.l, cylpebs mill without classification 60 14.00 85 20.00

b. Crusher, rod mill, ball mill, eylpebs mill with · classification 50 13.50 85 . 23.00

c. Crusher, ball mill with closed circuit classification 80 16.00 100 20.00

d. Autogeneous mill wi th closed circuit classification 70 11.00 70 11.00

e. Autogeneous mi l l ~n

open circuit, ball mill, cylpebs mill with classification 55 12.00 80 17.00

The operating costs of crushing and milling depend

almost entirely on the size of the equipment used, and

only to a very small degree on the amount of material

passing through the plant. Figure 16 shows a plant

where milling has been divided into a large number of

parallel sections. Despite the large processing capacity

of the plant, the operational cost per ton of material

19

20

is no lower than if it had consisted of a single sec­

tion of the same size. On the contrary, inaccessibility

for purposes of repair and maintenance means added

expense. Lower costs can be obtained if the mill size

is increased, and this is a line of development that

has grown more and more pronounced in recent years.

Figure 17 shows how great savings in the required

building volume can be made in principle if a few

large mills are chosen in preference to a larger num­

ber of small ones. In the case illustrated here, the

necessary floor area was reduced by 65%.

The t rend t owards l a r ge mill s has been s pec i a lly

linked to the development of autogeneous milling. In

Fig. 18 a s tandard mill size of 1 000 horsepower has

been assumed, with a basic cost put at 100%. The basic

cost here pr·esumably refers to investment and installa­

tion costs per unit weight of material. The figure

shows how the basic cost can be significantly reduced

by the choice of larger mills. Al ·though this figure

naturally only shows one calculation example based on

certain given assumptions, the result is valid in

general terms.

In mill dimensioning studies it is usual to express

the milling requirement for a mineral or ore in units

of kilowatt-hours per ton. In project planning and

preliminary costing, the cost of crushing and milling

is stated in kronor or other monetary units per kWh

per ton. These costs may be of the order of magnitude

given in the table below (cost levels as of about

1970).

Cost of various items

Energy consumption

Lining and repairs

Grinding media

Inspection and supervision of operations

Capital cost of equipment

Capital cost of buildings

Total cost in Skr/kWh-ton

Cost in Skr/kWh-ton

0.05

0.03

0.11

0.01

0.08

0.02

0.30

The cost' of milling a copper ore for flotation, then,

works out at roughly 3 kronor per ton if the milling

requirement is 10 kWh/ton. Fine milling of a coarse

magnetite concentrate to pelletising fineness, which

requires an energy input of 20 kWh/ton, would thus by

the same reasoning cost about 6 kronor per ton.

21

22

Literature references

/1/ P G Kihlstedt: The Relationship between Particle

Size Distribution and Specific

Surface in Comminution. Symposium

Zerkleinern. Verlag Chemie-VDI­

Verlag (1962) 205-216

/2/ P G Kihlstedt: Assessment of Comminution by means

of Particle Size and Specific Sur­

face. VII International Mineral

Processing Congress, New York.

Volume 1. Gordon and Breach (1965)

11-17

/3/ P G Kihlstedt: Particle Size Distribution and

Separation Results of Selective

Flotation of Complex Sulphide Ores.

VIII International ~ineral Pro­

cessing Congress, Leningrad (1968)

/4/ P G Kihlstedt: The Influence of Materials and

Methods on Particle Size Distri­

bution and Specific Surface in

Comminution Processes for Minerals.

Rittingersyrnposium, Leoben, June

(1972)

/5/ P G Kihlstedt: Grinding of Minerals. Gruvforsk­

ningen Serie B. Swedish Miner's

Association Stockholm. No . 194

(1974)

SfKso 2000

500

2.3

1'\

L

1 ~ ' "' -.-1-

r--

7 ~ .-I

0,01 0,1 1,0 10,0 SIZE OF GROUND PRODUCTS K(8Q) CM

SVKgQ AS A FUNCTION OF Kgo

MEASURED VALUES TAKEN FROM B. FAGERBERG, H ORNSTEIN: GRINDING TESTS ON MAGNETITE ORES WITH A~ROFALL MILL AND WITH DRY AND WET ROD MILLS. VOLUME OF PROCEEDINGS OF THE INTERNATIONAL MINERAL PROCESSING CONGRESS, LONDON 1960 CURVE 1 - DRY ROD MILL CURVE 2 - WET ROD -MILL CURVE 3 AEROFALL MILL

Figure 10

BIN

~ FEEDER CO 0")

JMI CRUSHER DJ ~#IIIII~

~~~ GYRATORY V~ ~~ CRUSHER

~ ~ BIN

SCREEN

~ ¢) O)

FEEDER BALL ~1 ILL

CYLPEBS MILL

FLOWSHEET FOR OPEN CIRCUIT CRUSHING AND GRINDI NG IN SEVERAL STEPS.

BIN

~ co o) FEEDER ~ GYRATORY~ {' ~~ CRUSHERS •

SCREEN

~ © o)

FE EDER

HYDROCYCLONES

BALL MILL

FLOWSHEET OF CRUSHING AND

SE VERAL STEP GRINDING IN SEPARATE

CLOSED Cl RCUI TS.

CYLPEPS MILL

1\.)

V1

FEEDER

JAW

~ (o o)j CRUSHER 0!}

SCREEN

SCREEN

CO o) FE EDER

BALL MIll

FLOWSHEET FOR CLOSED CIRCUIT CRUSHING

AND GRINDING.

FROM MINE <250mm

CYCLON

r

AUTOGENOUS MILL

4. 5 x 4. 8 m

i PUMP

0 ORE 75 - 80°/o --.a• <200 mesh

I MAGNET ITE

• SEPARATION

TAILI NGS

INPUT 1450- 2000mmel GRATE 7 mm

AUTOGENOUS GR IND ING .

MILL OF CONVENTIONAL DIMENS IONS .

ONE CLOSED CIRCU I T CLASSIFICATION.

28

RU N- Of - NINt OR PRINARY •:RUSHER PRODUCt

l - IN PE BBLE SIZE HOLE IN '~ATE NILL

~

HARD ING£ CASCAOf WILL

TROWWEL WIT H l - IN

PEBBLE Off LECTOR

'

CLEU PfBBLES 10 BIN -}

1 HF. SMALl . hnlo:' in, idc tho: Ca~cado: Mill help prevent an oversupply of pebble' that would cause a major conveying problem.

Figure 15

Figure 16 .

Figure 17

120

~ 110

~

~

:i ~ 100 "'

0\ 0 0

~

.... 90 ~

~ .... .. 80 c ... 0 .... z 10 .... u

"' .... ~

60

SIX- 1500 HP GRINDING MILLS FLOOR SPACE • 29,900 SQ. fT

JLOOR SPACE SAYING • 65 PER.CENT

H O~E I I I I

•INCLUDES OAIVE AND IIOTOA-

/ BAS E POINT i I .. -

l\ _J_ -- -- -r- -

\ I

I !

"' =r-1-- -- --

0

_J_ -~ --I

50o 05 10 15 2.0 25 30 35 40 4.5 5.0 55 60 u 7.0 u 1.0 AVERAGE IIILL CON~ECTEO • OASlPOWEA · IN THOJ!1!.!!.Q!.

. Mill co~ts versus connected horsepower in wet autO/!.('nou: grinding.

Figure 18

29