h. schubert-wet classification and wet screening of fine particles

7/23/2019 h. Schubert-wet Classification and Wet Screening of Fine Particles

http://slidepdf.com/reader/full/h-schubert-wet-classification-and-wet-screening-of-fine-particles 1/17

This article was downloaded by: [FU Berlin]On: 11 May 2015, At: 00:29Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer Hous37-41 Mortimer Street, London W1T 3JH, UK

Particulate Science and Technology: An InternationalJournal

Publication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/upst20

WET CLASSIFICATION AND WET SCREENING OF FINE

PARTICLESHEINRICH SCHUBERT

a

a Bergakademie Freiberg , East Germany

Published online: 24 Feb 2007.

To cite this article: HEINRICH SCHUBERT (1983) WET CLASSIFICATION AND WET SCREENING OF FINE PARTICLES, Particulate

Science and Technology: An International Journal, 1:4, 393-408, DOI: 10.1080/02726358308906384

To link to this article: http://dx.doi.org/10.1080/02726358308906384

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of Content. Any opinions and views expressed in this publication are the opinions and views of the authors, andare not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon ashould be independently verified with primary sources of information. Taylor and Francis shall not be liable foany losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoeveor howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use ofthe Content.

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

http://www.tandfonline.com/action/showCitFormats?doi=10.1080/02726358308906384

http://www.tandfonline.com/page/terms-and-conditions

http://www.tandfonline.com/page/terms-and-conditions

http://dx.doi.org/10.1080/02726358308906384

http://www.tandfonline.com/action/showCitFormats?doi=10.1080/02726358308906384

http://www.tandfonline.com/loi/upst20



WET CLASSIFICATION AND WET SCREENING

OF FINE PARTICLES

H N N R I C H S C HU E R T

Bergakademie Freibera

East Germany

I n t r o d u c t i o n

T h i s p ap er d e a l s w i t h t h e c l a s s i f i c a t i o n a nd s c r e e ni n g of f i n e p a r t i c l e s

w i t h cu t s i z e s d TS lmm.

n

i n d u s t r i a l - s c a l e p r o c es s e s , t h e c u t s i z e r a n g e

between and 0. 1

mm

i s

t h e t r a n s i t i o n r e gi o n f o r t he a p p l i c a t i o n o f s c re e ni n g

p r e v ai l i ng f a r d T > mm a nd t h e c l a s s i f i c a t i o n p r e v a i l i n g d T 0.lmm).

There a re , however, p ronounced t end enci es

t o

s h i f t t h e a p p l ic a t i on

l m t

of

s c r e e n i n g t o f i n e r cu t s i z e s . On t h e o t h e r h an d, p ro g re s s i s a l s o ob t ai n ed i f

h i gh e r c u t s i z e s

can be

u se d w i t h c l a s s i f i e r s . T h i s p ap er d e a l s e x c l u s i v e l y

w i t h t h e w et c l a s s i f i c a t i o n a nd we t s cr e e n i ng of f i n e p a r t i c l e s .

Impor tan t improvements i n the development o f wet c l as s i f i ca t i on and wet

s c r e e n in g h av e r e s u l t e d fr om h i g h er de ma nds i n g r i n d i n g c i r c u i t e f f i c i e n c y i n

mi n e ra l p ro ces s i n g . Fu r t h e r i mp o r t ant p ro g re s s i s due t o incr eas ed demands

c o nc e r ni n g p a r t i c l e s i z e a n a l y s i s o f s a nd s a nd g r a v e l s f o r s p e c i a l c o n c r e t e

c o n s t r u c t i o n s , f o un d ry s a n d s , a b r a s i v e s , k a o l i n ,

e t c .

Wet C l a s s i f i c a t i o n

B e f or e d e a l i n g w i t h r e c e n t de ve l op m en ts o f c l a s s i f i e r s , b o t h g r a v i t y a nd

ce n t r i f u g a l . t h e i r modes of o p e ra t i o n , and t h e co r r e s p o n d i n g s ep a ra t i o n mod e ls

a r e

p re s en t ed h e r e .

Modes of o pe ra t i on and sep ar at ion models

We

c a n

d i s t i n g u i s h t h e m o d e s o f o p e r a t i o n s h o w n i n F i g . 1 .

C h a r a c t e r i s t i c i s w h e t h e r t h e c o a r s e m a t e r i a l i n t h e c l a s s i f y i n g

z o n e m o v e s e s s e n t i a l l y a c c r o s s o r a g a i n s t t h e f l u i d f l o w o r f l o w

c om po ne nt d e c i s i v e f o r s e p a r a t i o n ) . T h e r e f o r e , w e c a n d i s t i n g u i s h

b e t w e e n c r o s s - f l o w a n d c o u n t e r - f l o w c l a s s i f i c a t i o n . I f

c l a s s i f i c a t i o n i s p e r f o r m e d i n t h e g r a v i t y f i e l d , t h e w e t

c l a s s i f i c a t i o n t e r m s , h o r i z o n t a l f l o w , a nd u p s t r e a m , a r e i n t r o d u c e d .

B ec au se o f t h e c r o s s - f l o v i n t h e c l a s s i f y i n g z o n e , t u r b u l e n c e c a n b e

p r o e e s s - d e t e r m i n i n g .

I t i s

r e a s o n a b l e t o s u b d i v i d e

i t

i n t o l a m i n a r

a nd t u r b u l e n t c r o s s - f l o v c l a s s i f i c a t i o n s . F i g . 2 g i v e s t u r b u l e n c e

p a r a m a t e r s o f c r o s s - f l o v c l a s s i f i e r s .

T ab l e sho ws t h e cor r e s p on d i n g s ep a ra t i o n mod e ls a cco rd i n g t o t h e p r e s en t

s t a t e o f k no wl ed ge . T he eq u a t i o n s fo r t h e cu t s i z e p r e s u p p o s es t h a t t h e

t e rm i na l s e t t l i n g r a t e of t h e p a r t i c l e s i s determined by th e Stokes fo rmula .

B u t t h i s

i s no

pr er eq u i s i t e f o r th e gene ra l model development 141 . These model s

h av e beco me more e f f e c t i v e s i n ce t h e p roces s -de t e rmi ni n g ro l e o f t u rb u l e n ce

f o r

t h e mast i mp o r t an t c ro s s - f l o w c l a s s i f i e r s h a s b een r ea l i z e d 14-71. I n t u rb u l en t

c ro ss -f lo w c l a s s i f i e r s , t h e p a r t i c l e s i n t h e c l a s s i f y i n g

zone

a r e

not on ly

Particulate Science and techno lop^

:393 407. 1983

9

Copyright 1983 by

Hemisphere

Publishing Corporation



subjected to the sedimentation caused by the force field,

but also to the eddy

diffusion transport flow. For modeling we have to proceed therefore, from the

fact that, near the product discharge, an equilibrium between the sedimentation

flow and the eddy diffusion flow has been achieved for the individual particle;

Fig. 1.

Modes of operation for wet classification 1 1 2 1 :

a)

laminar cross-flow;

b) turbulent cross-flow; c counter flow

Table 1. Separation models for wet classification

See 1-5) and 11)



etClassification and Screening

rorr Flow

Flow along a

wall

ler inlo fhe

Spiral Classifier

dgrratian

Fig.

2.

Turbulence parameters of cross-flow classifiers.

Rake Classif,er

Re-$

sizes range independently of each other and the concentration distribution in the

direction of the v-coordinate is valid: 11.21

Hydroseparalor

0

n

Up to now, however. due consideration in modeling has not yet been given to the

fact that, in cross-flow classification, suspensions of low solids concentration

(dilute suspensions, volume per cent of solids

< 5

to 10 . Fig. 3a),

s

well

as dense suspensions (Fig. 3b and 3c), must be h&dled. 181 Separaton in dense

suspensions are accomplished in some classifiers with satisfying separation

efficiency (e.g., hydrocyclones and mechanical classifiers in grinding circuits).

s is well know, the terminal settling rate vse of an irregularly shaped

particle in hindered settling [1 3] can be written as:

A g ~ f a f i o f l

d g i f a l i m

The hindered settling phenomenon is caused by the counter flow of the fluid due

to reasons of continity and also by an increased momentum transport within the

-1o?. .51oh

Re,,,,- 10'

n D 2

e

= 105

Re,,,, 10'

Y" -0p5...0,1

r n



~ i g . C ros s- fl ow c l a s s i f i c a t i o n i n d i l u t e a nd d e ns e s us pe ns io ns :

a )

d i l u t e f l o w s epa ; at i on , b ) d en s e f l o w s ep a ra t i o n ,

C i n t e r m e d i a t e f l o w s e p a r a t i o n ( co mb in ed s e p a r a t i o n )

s u s p e n s i o n s ( s o - c a l l e d s wa rm t u r b u l e n c e ) . R e c e n t l y .

B r a u e r

a n d h i s

c o - w o r k e r 6 p r o p o s e d m o d e l s w h i c h c o n s i d e r t h e s e p h e n o m e n a i n

s u s p e n s i o n s c o n t a i n i n g p o l y d i s p e r s e s o l i d s [ 9 , 1 0 1 . A c c o r d i n g t o

t h e m ,

i t

i s i m p o r t a n t

f o r

t h e c l a s s i f i c a t i o n i n d e n s e s u s p e o s i a o s

t h a t n e g a t i v e s e t t l i n g r a t e s o f f i n e p a r t i c l e s o c c u r ( F i g . 3 b

b e c a u s e o f t h e c o u n t e r - f l o v a n d u n d e r t h e e f f e c t o f t h e s w a r m

t u r b u l e n c e . F r o m t h i s , t h e s a t i s f y i n g s h s r p n e e s i n s e p a r a t i n g

c l a s s i f i c a t i o n p r o c e s s e s g o i n g o n i n d e n s e s u s p e n s i o n s m a y b e

e x p l a i n e d . I n c r o s s - f l o w c l a s s i f i e r s h a v i n g i n t e r m e d i a t e s o l i d s

c o n c e n t r a t i o n s i n t h e f e e d z o n e s r e f o r me d w h i c h a r e m o r e

o r

l e s s

d i s t i n c t l y s e p a r a t e d f r o m e a c h o t h e r ( F i g . 3 c . A t t h e b o t t o m o f

t h e c l a s s i f i e r , t h e r e i s

a

s e d i m e n t . O v e r l a y i n g i t t h e r e i s

a

z o n e

w i t h a r e l a t i v e l y h i g h s o l i d s c o n c e n t r a t i o n i n

a

f l u i d i z e d b e d - l i k e

s t a t e i n w h i c h t h e c o n d i t i o n s o f d e n s e f l o w s e p a r a t i o n p r e d o m i n a t e .

T h e u p p e r s u s p e n s i o n l a y e r e x h i b i t s l o w ~ o l i d s o n c e n t r a t i o n s , t h u s

p r o v i d i n g t h e c o n d i t i o n s o f a d i l u t e s e p a r a t i o n .

n

i n t e n s i v e

p a r t i c l e e x c h an g e t a k e s p l a c e b e t w e en t h e t w o

zones

When wet c l a s s i f i c a t io n i s c a r r i e d o u t w i t h s o l i d s o f i n c r e a s i n g l y f i n e

p a r t i c l e s a n d e s p ec i a l ly , a t a h i g h e r vo lu me p e r c en t o f s o l i d s , i t shou ld be

taken i n t o co ns i der a t ion tha t the suspe ns ions show an-Newton ian behav io r

1 13 ,3 71 . S t r u c t u r e b r eak i n g e f f ec t s c au s ed b y rh e t u rb u l e n ce p ro mo te t h e

s c p a r a c i o n

efficiency

G ra vi ty c l a s s i f i e r s

For wet c l a s s i f i c a t i o n i n th e g r a v i t y f i e l d , a l a r g e r number of c l a s s i f i e r s

i s a v a i l a b l e .

W it h r eg a rd t o t h e modes o f o p e ra t i o n an d t h e s ep a ra t i o n mo d el s ,

i t

i s ad v i s ab l e t o s u b d i v i d e them a s fo l l o w s :

a h o r i z o n t a l - f l o w c l a s s i f i e r s t o wh ic h t h e mo del o f

l ami n a r c ro s s - f l o w

i s

a p p l i c a b l e .

b ) m e c ha ni ca l c l a s s i f i e r s i n which t h e coarse m a c e r i a l i s di scharged by

means of a t ranspor t mechan i sm

( r a k e ,

s p i r a l ) , a nd a more

o r

l e s s

i n t e n s i v e a g i t a t i o n o f t h e p u l p s e f f e c t e d

s o

t h a t t h e mo de l of

t h c t u r b u l e n t c r o s s -f l o w c l a s s i f i c a t i o n must b e a p p l i e d , 141



Wet Classification and Screening 397

C upstream classifiers whose mode of operation corresponds to the model

of counter-flow classification.

Based on the separation model, the following requirements can be deduced

for the design and operation of a horizontal-flow classifier:

a laminar flow conditions should prevail in the classification zone.

b) the fines should be discharged with a maximum of liquid to the

overflow,

c

care should be taken to discharge the coarse material with a minimum

of liquid through the discharge opening at the bottom.

Several classification zones can

be

arranged in series to obtain products

of different finesness. To improve the sharpness of separation of horizontal-

flow classifiers, multi-stage arrangements are successfully used 11,121.

The most widely applied mechanical classifiers consist of a slightly

inclined trougb i n which a mechanism transports the coarse material along ?=he

bottom to the discharge

(rake

classifiers, spiral classifiers). Because the

turbule t diffusion coefficients of the rake classifiers

DT

about 0.004 to

0.0 1 m s-1)

are

essentially higher than those of spiral classifiers

D

about

between 0.0005 to 0.0025 d s - l ) the latter produce lower cut sizes. kh e wet

grinding circuits

are

the most important field of application for rake and

spiral classifiers. In recent times, rhey have been increasingly displaced by

hydrocyclones which have a simpler construction,

are

more flexible, and require

less space 1141.

To

improve the sharpness of separation, mechanical multi-stage

classifiers have been developed (Bathos-classifie r of Chemie and Metal1 GmbH.

Rheax) 1151.

Recently, upstream classifiers have gained in importance. In some of them

a higher solids concentration in the separation zone is avoided due to an

adequate design of the classifier (dilute flow separation). whereas in others

the fluidized bed conditions

are

realized in this

zone

(dense flow separation).

or combination of dilute and dense flow separation is applied. Many upstream

classifiers have an automatically controlled discharge for the coarse material.

The classifier shown in Fig. 4a, type Pheax, avoids an enrichment of the

solids in the classification

zone

by its design and realizes a mostly laminar

flow and a high sharpness of separation 2.2) with cut sizes between 0.4

and 2.5 mm. The rotationally symmetrical upstream classifier, a Sagreah

(Lavoflux) type (Fig. 4b), resulted from the further development of gravity

concentration equipment and combines dilute flow separation in the upper part of

the classification zone. Here, the

coarse

material rapidly forms

a

sediment

on

the

cone

and

a

downward sliding dune, with its control classification in the

lower ring space at higher volume per cent of solids [16.171. For cut sizes

between 0.1 and 1 mm, u-v alu es 1.5

are

said to be obtainable. Also in the

so

called pleated classifier (Fig. 4c), the coarse particles are rapidly removed

from the upward suspension flow and underlie a control classification

(cleaning). The main field of application are cutsizes 0.5 mm. -values of

from 1.4 to 1.6 being possible. rotationally symmetrical pleated classifier

is shown in Fig. 4d. The upstream classifier type, Larox represented in Fig.

4c , was developed in order to improve the separation efficiency of grinding-

circuit classification [18,191. It consists mainly of an open cylinderical

upper section, a conical lower section, and a vertical mechanism which rotates

at a low speed. The classification in

a

radially-laminated upwardly-directed

pulp column is combined with the cleaning of the sands in the conical seccion.

This classifier is especially recommended for the reclassification of the

underfllow of hydrocyclones in grinding circuits [19]. Multi-compartment

classifiers have been in use for a long time. Several separation zones are

connected in a s ri s so that different cuts can be realized.



Fig. 4. Upstream classifiers.

a) Rheax, b) Sogreah Lavoflux) c

Rheax

Pleated classifer)

d) Hydrofors, e

Larox

Centrifugal classifiers

We can distinguish two groups of centrifugal classifiers; the

hydrocyclones and the decanter centrifuges.

Theoretical and experimental investigations on the role of turbulence in

hydrocyclone flow have

een

under consideration for a long time 120,211. Only

recently however. the conclusion was drawn that

an

adequate model of turbulent

crass-flow classification

can

better reflect the nature of the hydro-cyclone

separation than the conventional models

11,3-6.22-251.

The model development w s based on the ideas shown in Fig. 5 so that from

respective equation for the cut siz Table 1, 11.21 it follows that:



Wer Classification and Screening

99

C o n s t a n t f o r t h e a d a p t i o n t o t h e h y d r o c y c lo n e g e om e tr y

t h e o r

F i g .

5

T u r b u le n t c ro s s -f l o w c l a s s i f i c a t i o n i n h y d r o cy c l o nf s .

A c c o r d i n g t o t h e s i m p l i f y i n g a s s u m p t i o n s e s p e c i a l l y w i t h r e g a r d

t o

p a r t i c l e s h a p e i n f l u e n c e

o f

h i n d e r e d s e t t l i n g

a n d

t h e p a r a m e t e r s

o t h e t u r b u l e n t t w o - p h a s e f l o w

v

o b t a i n e d b y

s u s t i t i o n

a n d

a c c o r d i n g t o t h e r e s p e c t i v e c o r r e c t i o n s :

I t s w e l l known t h a t t h e p a r t i c l e s i z e d i s t r i b u t i o n of t h e h y d r o cy c l o ne f e e d

ha s

a n

i m p o r ta n t i n f l u e n c e

o n

t h e s e p a r a t i o n p r o c e s s . T h i s may b e e x p l a i n e d by

t h e t u r b u l e n c e d am pi ng e f f e c t

of

t h e s o l i d s w hi ch i n c r e a s e s w i rh t h e

f i m n e s s [ Z 7 1 . T h i s s t a k e n u nd e r c o n s i d e r a t i o n

L28

by t h e e m p i r i c a l

c o r r e c t i o n f a c t o r k ~d .

F u r t he r i m pr ovem e n ts o f t h i s m ode l

a r e

m a in ly p o s s i b l e i n c o n n e c t i o n w i t h

a dv an ce s i n t h e d e s c r i p t i o n of t wo -p ha se f l o w s e s p e c i a l l y t u r b u l e n t t wo - ph as e

f l o w s a nd f o r t h e c o r r e l a t i o n s b et we en d e s i g n a nd t u r b u l e n c e p a r a me t e rs .

F u r t he r m or e

t

s n e c es s a ry t o a d e q ua t e ly t a k e i n t o c o n s i d e r a t i o n t h e i n f l u e n c e

of

h i gh e r vol um e o f C onc e n t r a t i on s t h e s o l i d s see F i g .

3 c .



Various results of other authors can be regarded as

an

indirect

confirmation of this model. Lynch et a1 1291 wrote: The results of the present

study confirm the earlier conclusions (Lynch and Raa. 1975 Caaliari) that the

critical design variables for a hydrocyclone operation are the inlet and outlet

diameters. The hydrocyclone operation body is merely

a

housing required to

carry out these constituent parts.

Based an the model of turbulent cross-flow classification, the suspension

discharge ratio determines the sharpness of classification

see

Table 1). The

sharpness of classification increases with decreasing proportion of the pulp volume

flow of the coarse product. Although only few investigations on the relationship

between sharpness of classification and flow or design parameters are known. the

sy~t emat ic esearch work by Lynch et sl. 114.29-311 and others 135,361 speaks well

for the principal validity of this model. further confirming result was submitted

by Plitt et el 1221. Furthermore, the model is in agreement with recent results by

Trawinsk. according to which hydrocyclones with larger cone angles or even full-

length cylindrical cyclones yield not only coarser cut sizes (Dt increases with the

cone ngle 151) but also a satisfactory sharpness of classification does not

depend on the cone

angle

132,341.

Also, recently further summarizing contributions were published on the

hydrocyclone classification 134.38.511 Lynch et al 114.29-311 presented

extensive experiment l

test results with larger hydrocyclones. Their model

consists of a series of regression equations which describe pressure-throughput

relationship, reduced efficiency curve. waterflaw ratio. and cur sire.

Though the hydrocyclone was introduced into processing mare than 30 years

ago, ncw fields of application are accessible, and further technical development

is ongoing. For normal hydrocyclones, the range of obtainable cut sizes is

between about 5 and 200 um. Special designs achieve up to 500LIm (dense-pulp

cyclones, full-length cylindrical cyclones

or

cyclones with large

cone

angles)

ond up to 2,ym for fine sizes. The hydrocyclones used have a diameter up to

about 1400 mm. However, even with higher throughputs. diameters of 750 mm are

only seldom exceeded.

The following areas of further development should be particularly stressed:

a ) Manufacturers increasingly apply spiral inlet nozzles instead of

tangential ones. In this way the turbulence intensity is reduced

and thus lower cut sizes can be obtained) and higher suspensions

throughputs are achieved.

b) Smell and medium-sized hydrocyclones (up to about 250 in diameter)

arc increasingly made from plastics and polyurethanes. Medium-size

to big hydrocyclones of cast steel or sheet steel

are

lined with

wenr resistant material (rubber, polyurethane. other plastics,

Ni-hard, hard porcelain).

C) By using suitable precautions (preseparation of clogging materials

and devices for the removal of blockages). the operational

reliability of smaller cyclones was essentially improved

so

that

they are increasingly considered for the separation of fine particles

up to d about 2 Um.

d) Increased attention was paid to the stabilization of the hydrocyclone

operation by the application of underflow control (pneumatically

r hydraulically adjustable

nozzles

and throttle valves 132.33.381.

C

Moreover, technologically profitable effects have been attempted by

developing special designs which more

or

less deviate from the

standard design

e . g . ,

Tagawa-swirl cyclone 139,401 double-cone

cyclone 1411, and others).



Wet C lassification and Screening 401

In spite of the progress made with regard to the application of the hydro-

cyclones for cut sizes d t 10 um, the increasing application of decanter

centriguges, for example, can be observed for the classification of kaolin,

phosphate slimes, and similar materials

4 2 1 .

Wet Screening

In wet screening. the fines are essentially transported through sieve

apertures by the fluid flow. Movements of the sieve decks if used) serve to

counteract the blinding of the sieve area

as

well as promote coarse material

transport along the sieve deck. The separation efficiency of screening

processes is to

a

large sxtent influenced by the amount of oversize present.

because

cake-like sediments in the oversize highly impair the screening action

of the fluid flow.

The most important screens far wet screening can be subdivided

as

follows:

a

stationary screens

b) sieve drums

C

vibrating screens

In

the following, new developments will be discussed according to this

classification.

Stationary

screens

Typical o these screens is that the sieve

area

is relatively steeply

inclined. This results in a reduced amount of rinsing water. In addition, the

cut size is essentially smaller than the sieve apertures. The latter is

favorable to counteract blinding. With all of them, whatever sizing is going to

be used, separation is completed within

a

relatively short distance from the

feed point. Long lengths are, therefore, not used

nd

the capacities are rated

in terms of width and sieve aperture rather than

area.

They all produce optimum

results over a fairly narrow range of pulp consistency, between approximately

and 20 percent volume of solids. They require reasonably constant feed rates

for

efficient operation and

a

uniform distribution of the feed pulp over the

width of the screen.

The first screen of the stationary type was the sieve bend Fig. 6a) 1 4 3 1

It consisted of fairly rugged stationary surface comprising parallel wedge bars

running at right angles to the pulp flow. The layers of slurry adjacent to the

surface are successively peeled off at each slot, the size of particles passing

through are smaller than approximately half the gap width. Sieve bends have

mostly been installed with gap widths between 0.3 and 1.0

mm

whereby cut sizes

between 100 and 500 um

can

be realized. There is a minimum pulp velocity for

each gap width to counteract blinding. This is characterized by

a

Reynolds

number

of

about 300

I441

1.e.. the smaller the gap width. the higher must be the

fluid velocity. To overcome the abrasion problem, the Rapifine-screen Fig.

7 a

1 4 4 4 6 1

was designed

in

the 1960s. It is

a

steeply inclined, slightly curved

sieve bend equipped with

a

pneumatic rapping device working in adjustable

intervals to prevent blinding. Consequently, the pulp velocity can be reduced

compared with the conventional sieve bends. A uniform pulp distribution is

ensured by a saw-tooth design of the overflow edge i n the feed box. Two-stage

screens Fig. 7b) improve the separation efficiency.

The gap width

of

Rapifine



H chubert

6

Mode of operation

of

s tat ionary

screens: a s i ev e bend, b

wedge-wire screen.

Fig .

7

Stationary v t screens: a Rapif ine,

b

Rapifine, two-srage, c

KHD,

two-stage. d Bartles

CTS,

two-stage.



Wet

Classification and Screening

4 3

screens used in industry corresponds to that of conventional sieve bends

[44,461. Even a apertures of 100 um, the throughputs with single-stage screens

are 8 to 10 t/m h processing of iron ores [46l.

The development of the Rapifine screen suggested the conclusion that

a

curved design of the sieve

re

is o prerequisite for the separation effect.

Cansequently, at the beginning of the 1970s, flat wedge-wire screens (Figs 6b

and 7c) were designed 147,481. In the type shown in Pig. 7 c the sieve deck is

linked to the base frame by smooth rubber blocks. At intervals, it is excited

to cleaning vibrations. The inclination of the screen area is adjustable.

Additional rinsing water can be fed from adjustable boxes arranged above the

sieve areas.

Another development proceeding from the conventional sieve band is to be

found in the processing of Cornwall tin ores. The type shown in Fig. 7d uses a

woven wire cloth as the sieve area L44.451. Early testing proved that an

improvement in efficiency was obtained using a short radius of cloth, but the

inherent difficulties

of

screen blinding remained. Observations indicated that

the major problem was hydrodynamic. That is, while separation occurred on the

face of the screen,

a

thin layer of pulp, having Passed through the apertures,

continued down the back side of the cloth, being held on by a wall effect. To

overcome this problem, the pulp is peeled off from the back of the cloth by

a

series of wedges.

Sieve drums

Horizontal sieve drums, whose sieve re is externally rinsed with water.

are

being used in Scandinavian mineral processing plants for screening pulps

with cut sizes d t l m m . This washing prevents blinding and additionally

increases the capacity.

The type shown in Fig.

8

is a combination of

a

vertical sieve drum and an

upstream classifier. It was designed for the reclassification

of

the

hydrocyclone underflow in grinding circuits. 119,491. This apparatus is similar

Fig. 8 Hydraulic trommel

screen

(Hukki screen cell).



4 4

H Schuberf

t o t h e u p st r ea m c l a s s i f i e r shown i n F i g . 4 e .T h e ma in d i f f e r e n c e i s t h e d e s i g n o f

t h e c y c l i n d r i c a l t o p s e c t i o n.

I t

i n c l u d e s

a

p e r i p h e r a l w ed ge w i r e s c r e en . T he

f e e d i s i n t ro d u c ed c e n t r a l l y a t t h e t o p and i s d i s t r i b u t e d a l o ng t h e i n s i d e o f

t h e trommel by means of

a

low-speed ro ta t i ng mechanism compr is ing

a

p l a t e o f

d i s c s a nd f i t t e d w i t h r a d i a l s we ep er b l a de s . T he se s e r v e t o d i r e c t t h e p u l p

t ow ar ds t h e p e r i p h e r y i n t h e f or m o f a p u l p r i n g t r a v e l l i n g a t c o n s t a n t a n g u l a r

s pe e d w i t h c e n t r i f u g a l a c c e l e r a t i o n o f

a b ou t g a g a i n s t t h e

screen

s u r f a c e .

V i b r a t i n g screens

A l s o v i b r a t i n g s c r e e n s a r e u se d f o r we t s c r e e n i n g. T he m ai n p r ob l em s

a r e

the same s e x p e ri e n c ed w i t h o t h e r wet s c r e e n s . I n a d d i t i o n , t h e v i b r a t i o n mu s t

n o t h i n de r t h e f l u i d f l ow .

A t y p e d e s i g n e d p a r t i c u l a r l y f o r w et s c r e e n i n g

i s

t h e v i b r a t i o n s c r ee n

s hown i n F i g . 9

1441

To t a k e a dv a n t ag e o f t h e r a p i d s c r ee n i A g a c t i o n , t h e

s c r e e n i s made a s a m u l t i p l e f e e d u n i t . The s i e v e d e ck v i b r a t e s w i t h h i g h

speed ( 30 00 -3 60 00 v i b r a t i a n s / r n i n u t e ) an d l ow am p l i t u d e s t r an s m i t t e d b y

a

v i b r a t i n g m o to r m ou nt ed ab ov e t h e cen t e r of t h e s c r een . Fo r m aki ng s ep a r a t i o n s

w i th c u t s i z e s < 3 5 ~ on d i l u t e f e e d s , t h e d e ck c o n s i s t s of two s u pe r im p os e d

F i g . 9 V i b r a t i o n s c r e e n .

and

e q u a l l y t e n si o n e d s t a i n l e s s s t e e l w i r e c l o t h s s u p p or t e d i n ho op t e n s i o n

t o

o b t a i n u n if o rm a c t i o n o v er t h e e n t i r e s u r f a c e a nd t o a v oi d f l u t t e r a nd

f a t i g u e .

T h i s s an dw ic h c o n s t r u c t i o n r e s u l t s i n

a

s l i g h t r e l a t i v e movement o f t h e t o p

and t h e b o tt o m c l o t h s . t h e r eb y av o i d i n g b l i n d i n g and m ak in g p o s s i b l e t h e u s e o f

h c a v i e r g au ge w i r e t h a n i n c o n v e n t i o n a l l y u s ed i n t h e m a n uf a c t ur e of c l o t h o f

t h e s ame ap e r t u r e . T he t h r o u g h p u t i s s a i d t o be 1 3 . 3 t1 h. m f o r a c u t s i z e d ~

-15Cvm,

2 0 p e r c e n t w e i g ht o f s o l i d s i n t h e f e e d p u l p ,

a

s o l i d d e n s i t y o f 2 7 00

kg/m and 15 %

o

o v e r s i z e f e e d .

T he c l o t h s h a ve

a

mesh s i z e o f 250 u

[ I .



Wet lassirication and Screening

NOMENCLATURE

Clas~ ifyln g rea

Acceleration; centrifugal acceleration

Diameter

Eddy diffusion or transport coefficient

Particle Size

Cut size

Volume diameter

Particle size for P3 d) 0.25 or

0 7 5

respectively

Force Field

Fluid

Partlcle size mass distribution

Gravity acceleration

Height of the classifying zone

Feed. oversize

or

undersize

o

a classifier

Hindered settling factor

Particle shape factor

Number of revolutions

Particle number concentration of the i-th

size

range at

y y respectively

Rapping device

Grade efficiency

urve

separation curve, separation

function)

Relative intensity of turbulence

Pulp volume flow rate of f eed, oversize or undersize

respectively

Terminal settling rate

o

particles of diameter dV

Fluid velocity

Dynamic viscosity

Kinematic viscosity

Fluid density

Particle density

Volume per cent of particles

median

value

of the particle size distribution



406 H Schubert

REFERENCES

1. R. Schubert.Aufbereitunn fester mineralischer Rohstoffe. Vol. 1, 3rd Ed.

Leipzing, Grundstoffverlag 1975.

2. H. Schubert. et al, Mechanische Verfahrenstechnik 11. Leipzig. Grundstoff-

verlag 1980.

3. H. Schubert. et al, Mechanische Verfahrenstechnik I, Leipzing, Grundstaffver-

log 1977.

4. Th. Neesse and H. Schubert, Chem. Tech. 7 (1975) 529, 8 (1976) 80, 273,

29 (1977) 10.

5

Rcesse, Th.: Thesis Bergakademic

F r e i b e r g

1978.

6. Th. Neesse. Chem. Tech. 3 (1971) 146.

7. Schubert

Th

eesse, Proceedings Tenth Int. Miner. Process. Cang.

London 1973. 213.

R . ~ e e s s e , ~ h . n d s c h u b e r t . H.: P a v e r ~ e r f a h r e n s t e c h ~

Jahresta ung Dres den, 1978.

9.

A

A

~asias. nelsls Teehn. Univers. Wesr-Berlin 1970.

10. H. Brauer and H. Thiele. Chemie-I~K.-Techn. 5 (1973) 909.

11. 0. Molerus and H. Hoffman, Chemie-I~K echn.

4

(1969) 340.

12. H. Trawinski, Ullmanns Encykl. techn. Chemie.. 4th Ed.. Vol. 2. 70.

13. H. Kirchberg, et al, Proceedings Eleventh Int. Miner. Process Cannress,

Cagliari 1975, 219.

A. J Lynch. Minerals Crushing and Grinding Circuits, Elsevier 1977.

N.N.. Aufbereitungs-Tech . 7 (1976) 360.

B. Gaucherad and

J.

Crassart. Rev. Ind. Minerale (1971) 531.

L. Abraham and

P.

Piso. Aufbereitunus-tech . 1977) 121.

R. T. Hukki. EnKnR. Min. J

78

(1977) 66.

K. Hciskanen, World Mining 32 (1979) 44.

M. G. Driessen, Rev. Ind. Minerale 3 (1951) 482.

K Rietema. Proceedings Symposium Interaction between Fluids and Particles,

Paper C44. London 1962.

22.

L.

R. Plitt and S. K. Kawatra. Int. J Miner. Process. (1979) 369.

23. L. R. Plitt, Canad. Min. Metallurg. Bull.

64

(1971) 42.

24. E A. Nepornnjascij et al, Garnij Z (Sverdlovsk), 1974. 164.

25. P. I Pilov. Theisis Mining College Dnjepropetrovsk 1976.



Wet Classification and Screening 407

P.

H. Fahlstrom, Proceedings Sixth Int. Miner. Process. Canaress, Cannes.

1963, 87.

Th.

Neesse

et al, Chem. Techn.

9

1977, 544.

Mueller,

8.

et al.: Freibe rger

Forth.-H.

(1975) 357.

T. C.

Rao

et al. Int.

J

Miner. Process.

3

(1976) 357.

A. J Lynch, et al. Inter

J.

Miner. Process i(1974) 173.

A.

J.

Lynch, et al. Proceedings Eleventh Int. Miner. Process Conaress,

Cagliari 1975. 245.

H. Trawinski. Verfahrenstechn.

2

(1978) 710.

H. Trawinskl, SolidILiquid Separation Equipment Scale-up, 241. (Hydra-

cyclomes). Croydonl ngl.. Uplands Press Ltd. 1977.

H. Trawinski, Chemie-Ing. Techn.

5

(1978) 789.

Ohnet.

Verfahrenstechn. (1969) 376.

D. Ocepek. Papers Rittinger-Symp., Leoben 1972. 153.

K.

Heiskanen and H. Laapas. Preprints XIII. Int. Miner.

Process

Congress,

Warsaw 1979. Vol. I. 189.

A. I. Povarov. Gidrociklony

na

obogatitel'nych fabrikach, Moscow, Nedra

1978.

R.

E. Zimmermann. Min. Engng. 3 (1978) 189.

H. Grond, Aufbereitungs-Techn.

18

(1977) 107.

L. Simek, Verfahrenstechn (1977) 18.

H. Kellervessel and P. Zahr. Erzmetall

3

(1979) 227.

P.

3

Pontein, Glue cka uf 9 1 (1955) 781 .

S.

K.

De Kok,

J

South Afr. Inst. Min. Metall. (1975) Oct. 83.

R.

0. Burt, Mining Magazine 128 (1973) 463.

Jenn er, F.W. and Loesel.

W :

Erzmetall

26

3 7 6

H. Bauer et

al.

Aufbereitungs-Tech .

12

(1972) 428.

R. P. Pearsall. US-Patent 3 483 974 (1969).

R.

T. Hukki, Dechema-Monographien

Nr.

1549-1575,

79

(1976) 319.

S. Bednarski. Pirst European Conf. on Mixing and Centrifuaal Separation

Cambridge 1974. Paper E4.

51. P. Schmidt. Aufbereitungs-Techn.

14

(1973) 411.



4M H

chuberr

52. S e h u b e r t A .

a n d N e e s s e

T.:

p r e p r i n t s I nt . C o n f .

o n

Hydrocyclones Cambridge. U R 1980 P a p e r

3.

h. schubert-wet classification and wet screening of fine particles

Documents