Download pdf - Column Flotation Circuits in Chilean Copper Concentrators

Pergamon MineraL~ Engineering, Vol. 7, No. 12, pp. 1473-1486, 1994

Elsevier Science Ltd Printed in Great Britain

0892--6875(94)00085-9 0892--6875/94 $7.00+0.00

COLUMN FLOTATION CIRCUITS IN CHILEAN COPPER CONCENTRATORS

G. SCtIENA§ and A. CASALIS"

University of Trieste, DINMA, Division of Georesources and Environment, 34127 Trieste, Italy t Department of Mining Engineering and Mineral Processing,

University of Chile, Santiago, Chile (Received 4 March 1994; accepted 17 July 1994)

ABSTRACT

This paper reviews the cohmm flotation practice in the largest Chilean copper-moly concentrators where this process unit has been increasingly chosen to replace multi stage cleaning circuits in new and existing operations. Process flowsheets incorporating columns are presented fi~r selected concentrators. Procedures for sealing up, attd process control are also illustrated.

Keywords Flotation columns, flotation circuits, processing costs

INTRODUCTION

The Chilean mineral industry has always been ready to accept novel and alternative process technology; it suffices to mention the large impact that semi-autogenous (SAG) milling has had since the early 80's and the on-going full scale testing of innovative hyperbaric-filtering technologies. In the last ten years colunm flotation has been increasingly accepted in the process circuits of the Chilean concentrators [1] and it may be expected that in the near future on going R&D will contribute to make this novel technology suitable to more applications. Certainly the low price of copper in 1993 (75 cents/lb, for high grade copper) has caused a short term general decline in re-investments and in the expenditure for pilot plant testing and R&D but has stressed the need to contain production costs. At the moment economy of scale and enhanced processing technology can contribute to better the economic performance of copper operations. A number of concentrators are considering capacity expansion (Andina & Disputada) and enhanced metallurgical recuperation projects have also been implemented (Andina, PARM) [2]. Column flotation can contribute both to cost saving and to increased revenues by improving the metallurgical performance of concentration circuits. Most of the column flotation experience in Chile is restricted to Cu-Mo bulk concentration and Cu/Mo separation. Table 1 summarizes colunm tlotation installations to date. In the copper industry the level of knowledge and experience is such that today columns can be considered in flotation flowsheets with increased confidence.

PROCESS FLOWSllEETS INCORPORATING COLUMNS

Column flotation has contributed to simplify copper flotation circuits, decreasing the number of units with the related benefits. Probably the most successful application has been in the flotation cleaning sections where the industrial column application began by replacing the third stage and evolved to substituting three stages of cleaning. Herewith selected tlotation circuits adopting colutlms are illustrated in some detail.

7:lz-s 1473

.-.d

4~

TA

BL

E 1

So

me

indu

stri

al a

nd p

ilot

plan

t ap

plic

atio

ns o

f fl

otat

ion

colu

mns

in

Chi

le.

Com

pany

Si

te o

f the

inst

alla

tion

Chi

lean

re

gion

&

m

ain

city

Min

eral

/s

>roc

esse

d (m

ain

spec

ies)

App

lica

tion

DIS

PU

TA

DM

L

as T

onol

as f

lota

tion

R

. Met

ropo

lita

na

Cha

lcop

yrit

e cl

eani

ng i

n th

e E

XX

ON

-CH

ILE

pl

ant (

800

m.a

.s.I

.)

-Col

ina

( 1.

2% C

u ca

.)

Cu-

Mo

bulk

fl

otat

ion

LOs B

mnc

es-

as a

bove

as

abo

ve

San

Fran

cisc

o pl

ant

as above

as above

AN

DIN

A/

CO

DE

LC

O

EL

TE

NIE

NT

E/

CO

DE

LC

O

Los

Bro

nces

- Sa

n Fr

anci

sco

plan

t

And

ina

grin

ding

- fl

otat

ion

plan

t (3

000m

.a.s

.I.)

C

olet

onas

co

ncen

trat

or

Mo

circ

uit

as a

bove

El

Salv

ador

co

ncen

trat

or

Chu

qui

conc

entr

ator

La

Esc

ondi

da

conc

entr

ator

EL

SA

LV

AD

OR

/ C

OD

EL

CO

V R

egio

n-

Los

And

es

VI

Reg

ion

- R

enca

gua

I11 R

egio

n

il R

egio

n -

Ant

ifag

-gta

A

ntof

agas

ta

CH

UQ

U1C

AM

AT

A/

CO

DE

LC

O

Cha

lcop

yrit

e (I

.25-

1.3

% C

u)

Cha

lcop

yrit

e cl

eani

ng u

n (I

.13%

Cu

ca.)

C

u/M

o se

p.

as a

bove

Cha

lcop

yrit

e (0

.9%

Cu

ca.)

Cha

lcop

yrit

e i (

1.3%

Cu

ca.)

C

alco

sina

an

d co

veli

na

(2.1

2% C

u ca

.)

ESC

ON

DID

A/

RT

Z - B

HP

bulk

cle

anin

g:

i I

3rd

and

sole

cl

eane

r cl

eani

ng i

n th

e 2

Cu-

Mo

bulk

fl

otat

ion

1

clea

ning

m

1

Cu/

Mo

sep.

cl

eani

ng i

n th

e 1

Cu-

Mo

bulk

fl

otat

ion

clea

ning

in

2 C

u/M

o se

p.

clea

ning

in

the

8 C

u-M

o bu

lk

flot

atio

n

Num

ber

of c

olum

n un

its

Col

umn

size

N

. B

uff.

T

otal

T

otal

In

du.s

hial

/ H

xLxW

or

I x w

vo

lum

e se

ctio

n pi

lot p

lant

H

xD [

m.l

[m

]x[m

] ~

unit

ar

ea

[m31

[m

2]

14.5

x2x

8 16

23

2 16

In

dtts

;, ia

l

13.5

x0.4

6 (1

8")

0.17

pi

lot

12x0

.91(

36")

0.

65

pilo

t

13.8

5x3.

71xl

.76

6.5

indu

stri

al

13.8

x2x6

.5

12

179.

4 13

in

d~hl

al

Ixl.

08

15x0

.9

0.71

pi

lot

13.3

x0.9

x0.9

0.

81

pilo

t

10x0

.76

(30"

) pi

lot

13.6

xl .8

3xl.

83

3.5

indu

stri

al

14.3

x4.0

x4.0

22

9 16

in

dust

rial

Typ

e of

T

ype

og

Bia

s Y

eas

bubb

le

cont

rol

+/-

of

star

t g

ene~

or

stra

tegy

up

and

en

d U

SB

M/C

II

h,+.

-ffe

ce

1992

- T

urbo

-Air

+

leve

l M

inno

vex

~;ir

a cl

oth

+ 19

85-

1987

filt

~r cloth

+ as

ab

ove

f.cl

oth

and

+ 19

87-

US

BM

/CII

19

91

USB

M/C

II

+ 19

91-

Tm

bo-A

ir

+ 19

89-

+ as

ab

ove

USBM/CII

+ 1992-

Turbo-Air

Com

inco

an

d M

inno

vex

+ 19

90-

+ 19

91-

.o

U'3 m

> ¢h

.>

Column flotation circuits 1475

Disputada de Las Condes - Las Tortolas concentrator.

Flowsheet

The concentrator is located at 800 m. above see level near Colina and one hour by car from Santiago. It is fed through a 56 km. long pipeline with the product of the Los Bronces grinding plant at 3600 m. above sea level (m.a.s.1.). The feed (37000 tpd; 80% -65 mesh or (212 am), 1.2% Cu) is mainly chalcopyrite (Cu-34.6 %) but chalcocite (Cu-79.8%) is sometimes present. It is treated in two parallel flotation lines with a common 3.8 x 8.2 m. (12.5 x 27 ft.) regrinding ball mill for the combined rougher concentrates (Figure 1). The rougher stage consists of two parallel sections of nine 85 rrr a (3000 cu. ft.) Wemco cells. The rougher concentrate is ground to 80% -325 mesh (45/zm) in a ball n'fill circuit closed by ten 0.50 m. (20 in.) diameter Vulco cyclones. The overflow joined to the scavenger concentrate is fed to two parallel flotation colunms with cross section 16 m 2 and 14.5 m. high. The colunm Cu-Mo concentrate is de-watered and filtered in two 48 m 2 hyperbaric Andritz filters that produce a cake humidity of 8%. The scavenger stage consists of two parallel sections of seven 42.5 m 3 (1500 cu. ft.) Wemco cells each.

_ = - = _ - = ~ ~ %ter~,~, I [LasT°rt° las 'regr indingandl '~ ~ ', z~-~ ' " t ~ s ~ " I flotation plant [

~ o , , . t ~ ~ / . . ' - I L ~ ~OU,~,..OT,,~O~,~L, L _ _ _ _ J

, F W:- LOO REAGENTS I • I

i ~ WATER WATER ~ I ~ l

Fig. 1 Las Tortolas Cu-Mo concentrator flowsheet (1992-)

Cohtmn flotation practice

Each column treats 200 tph of mineral at 20% solids. Copper recovery in the column is 60% . The overall copper recovery of the circuit is 86 %. The grade of the column concentrate varies between 28- 30%. but may be as high as 38% when the mineral is mainly chalcocite.

The 14.5 m. column is fed 2.5 m. from the top. The pulp-froth interface is kept between 0.6 m. and 1.5 m. from the colurnn top by the interface level control system. The sprays tbr an eventual washing water addition are above the froth surface. However wash water is rarely needed to upgrade the concentrate and the column operates prevalently with negative bias. The baffles divide the 2 x 8 m. section (16 m 2) in 16 sub-columns of 1 m 2 area each. The baffling work starts 0.1 m above the bubble generator.

1476 G. SCHENA and A. CASALI

The two columns are equipped with two different external bubble generation systems. One unit uses the USBM/CII Turbo-Air device. The other unit uses a bubble generator marketed by Minnovex-Canada. The two bubble generation systems allow comparable metallurgical results. They are tested in parallel to compare their wearing performances. The Minnovex bubble generator device made from steel lasted a few days, a carbon-tungsten steel device has worked for several weeks without need of maintenance. Water addition to sparger air for bubble generation is kept at minimum values and frother is not required.

The colunm feed and products flow rates are measured on-line. Wash water and air flows are also measured. Pulp-froth interface level is calculated from multiple pressure measurements. The column control system is used to keep the interface level within suitable limits by adjusting the tailing discharge valve. However, on line measurements can be used to adjust the washing water addition as well. A display in the control room allows visual control of the top of the column.

The new 36000 tpd Las Tortolas flotation circuit was conceived and designed on the basis of the industrial and colunm pilot plant experiences in the previous Los Bronces-San Francisco concentrator (3600 m. above sea level). The earlier San Francisco conventional circuit (Figure 2) was successively modified to a configuration similar to that of Las Tortolas. The initial treatment capacity of San Francisco was 4800 tpd (1978), rising to 8400 tpd in 1981 and was 12000 tpd in 1988. The plant was put out of service and finally substituted by Las Tortolas in 1991. One 0.45 m. (18") and one 0.90 m. (36") flotation columns have been tested at San Francisco since 1985. Based on the finding from pilot testing in 1987 a full scale 3.91 x 1.76x 13.85 m. colunm was put in service as third cleaner (Figure 3). Later the cleaning section was fully converted to columns - that is replacing three stages of cleaning with one unit (Figure 4). The experiences are thoroughly reported by Villanueva and Vergara [3]. The flowsheet of Figure 1 used today at Las Tortolas and sketched in Figure 5 is indeed a further evolution of the Los Bronces flowsheet in Figure 4.

FEED

TAIL

iI t

I REGRINDING

FINAL CONCENTRATE

Fig.2 San Francisco flotation plant - earlier conventional circuit (1978-87)

Future developments

A flotation plant for the treatment of the bulk concentrate for copper/naolybdenuna separation is under construction and will be operating early in 1994. The new flowsheet incorporates column flotation and is shown in Figure 6.

Currently the expansion of the treatment capacity from 36000 to 50000 tpd is also being investigated.

FEED

II I

RF.GRINDING

Fig.3

FINAL CONCENTRATE

TAIL


Evolution of San Francisco flotation plant - column as third cleaner in the conventional cells circuit (1987-88)

FEED

I I

REGRINDING

TAIL

FINAL CONCENTRATE

Fig.4 Evolution of San Francisco flotation plant - column cleaning circuit (1988-92)


FEED

l

REGRINDING

' rAIL

FINAL C O N C E N T R A T E

Fig.5 Further hypothetical evolution from San Francisco flotation plant to that in use at Las Tortolas.

u~Mo

CONC. Cu

Fig.6 Las Tortolas Cu/Mo separation tlowsheet (1994-)

CONC. Mo

Andina concen[rator

Flowsheet

The operation is located in Los Andes 120 Km from Santiago leading north. The grinding-flotation plant is in a cave at 3000 m. a.s.1.. The concentrator can be reached by tbur-wheel drive in spring and summer but in winter with severe climate conditions the access is possible only by snow-mobiles. The mill treats


33500 tpd of ore mineralised by chalcopyrite. Average feed grades vary between 1.25-1.3 % Cu. The feed is treated in three parallel grinding circuits. Each grinding line has one 3.7 x 4.9 m. (12 x 16 ft.) Marcy rod mill and the discharge is split in three secondary 3.8 x 4.9 m. (12.5 x 16 ft.) ball mills in close circuit. The rod mill discharge feeds three batteries of cyclones each consisting of three 0.4 m. cyclones. The underflow of a battery of three cyclones feeds one ball mill. The 80% - 270/am (60 mesh) overflow of the nine cyclones goes to five rougher flotation lines each with nine cells. There are 4 parallel lines with 90utokumpu-36 (1350 of.) cells and 1 line and with 9 -42.5 m 3 (1500 cf.) Wemco cells. The combined rougher concentrate is reground in closed circuit with one 2.75 x 3.96 m. (9 x 13 ft.) and seven 0.4 m (15 in.) cyclones. The overflow, along with the re-ground cleaner-scavenger concentrate, feeds the two parallel rectangular 6.5 x 2 m. columns 13.8 m. high. (Figure 7). The scavenger is a bank of 13 Outokumpu-36 cells. The re-grinding of the scavenger concentrate is in open circuit. The bulk concentrate is treated in a copper moly separation circuit. Copper is depressed and molybdenite floated. The Me concentrate has 49 % Me and less than 3 % Cu. The separation is carried out by roughing and six cleaning stage in counter-current sections.

FEED

TAlL

1 REGRINDING

I

FINAL CONCENTRATE

Fig.7 Andina Cu-Mo bulk flotation flowsheet.

Column flotation practice

The column feed is 80% - 63 #m (230 mesh) and 20% solids. In Table 2 a tentative mass balance is reported for the flotation - regrinding plant of Figure 7. Each cohmm is divided by baMes in 12 sections. The device is operated with positive bias and tile washing water kept to 10%. of the feed flowrate. The USBM/CII Turbo air system is in use. In the past (1986-87) pilot plant tests have been carried out in a 1.0 m. diameter, 13 m. high column. Different flowsheet configurations were tried with the column in parallel with other conventional flotation banks. This circuit has been operating since the recent completion early in 1992 of the marginal project termed PARM (Proyecto Aumento de Recuperacion Metalurgica) that succeeded in bringing the copper concentrate grade to 30 % from the average values of 28 % and increasing the overall copper recovery by 3 %.

Future developments

A major expansion, almost doubling the actual treatment capacity, has just received approval (November 1993); treatment capacity will increase from 32000 to 64000 tpd. Column flotation has been inclt, ded in

1480 G. SCH]ENA and A. CASALI

the new Cu-Mo bulk flotation plant and also in the Cu/Mo separation plant. Here roughing and cleaning stages will use columns, and mechanical cells will be used in the scavenger section. A probable flowsheet of the integrated process is shown in Figure 8.

TABLE 2 Tentative mass balance around the columns operating at Andina

F . e d ~ I ~ R.t .ill h= T.il. solid tph 1400.00 Iv ~ ~ 1084 • I v 1349,60 C u % 1.28 ~ R concentrates O. 19013283 T 0,19

Me % 0,02 ~ 346,00 0,004 I S 0,01 solid % 036 4.60 39,87 28,43 pulp inch 2947.20 O,O9 1979,96 .tails 3890.00 solid s g 2,80 28,98 2.7 | 295,60 2,70

= watar 967,24 l O, 17

~ . td % - 2,90 0,02 I c ..... ,,,,e ~ ~ , , . . 3 I 14.o8

o,4o ,t' .,.d .... 1 3°,5° ~ 0 - ~ ' q ' 164.80 ~ . 0,44 COLUMN I 4,96 5,70

29.23 CLEANER I 0,55 1,61 134,03 t 19,42 26,33

4,20 2279,55 507,19

I ~ S C 3.20 3 , 6 0 F " '

avenger feed b,~ [SCAVENGER =did % - solid tph 460,50 I~ ~ lY Cu % 2.15 Me % 0.59 solid % O. 19 pulp inch 2145.52 solid a g 3,00

solid tph

Cu % Me % solid % pulp inch

solid s g

rougher column scavenger overall

Yield 24,71 9,86 35.81 3.60 Cu recover'/ 88.82 60.92 95.01 86,06 Me recovery 87.20 7,54 97.60 67,33

FEEl

l, l BULK Cu-Mo ]

TAIL >

Fig.8 Probable Andina flowsheet after expansion. Cu-Mo bulk flotation and Cu/Mo separation plants (1996-)

CONC Cu

CONC. Mo


El Teniente - Col6n concentrator

Cu/Mo separation by column flotation

At E1 Teniente colunms have been evaluated since from for application in the Cu/Mo separation circuit where copper is depressed and Mo is floated. The objective is a radical simplification of the counter current cleaning molybdenum circuit that is in use today. Two pilot plant column were used for the evaluation : one circular unit 15 m. high and 0.9 m. diameter and one square unit 13.3 m. high and 0.9 m. wide.

The conventional circuit has a rougher stage and 6 cleaning stages. The rougher stage consists of 5 parallel banks of 13 - 2.83 m 3 (100 ft 3) Wemco cells. The first cleaning section has 4 banks of 12 - 1.7 m 3 (60 ft 3) Wemco cells each. The banks operate in counter-current mode with a 4-4-4 arrangement. The second has 5 banks of 6 - 1.1 m 3 (40 ft 3) Denver 21 arranged 4-2. The third cleaning section has 3 banks of 6 - 0.68 m 3 (24 ft 3) ceils. The 4th, 5th and 6th cleaning section each consist of 2 parallel banks of 6 - 0.68 m 3 (24 ft 3) Denver 18 cells in counter-current. In these sections the cells are counter-current arranged according 4-2, 4-2-3, 2-1 respectively.

Tile Mo grade in the Cu-Mo bulk concentrate feeding tile conventional Mo rougher-cleaning circuit is in the range 0.45-0.5 % and in the final concentrate 42-45 %. Since molybdenite has 60 % Mo the mineral grade of tile concentrate is 70-75 %. The Cu content in the Mo concentrate is kept below 4%. Concentrate production is in tile range 10-15 tpd. The rougher tail that is tile Cu concentrate has 30% Cu. The grade of the rougher concentrate that is the cleaning circuit feed is 0.8-1.0% Mo.

The separation results obtained by operating tile pilot cohmms in parallel to different sections of the complex existing conventional circuit have demonstrated that 4 stages of cleaning will produce a concentrate with a Mo grade averaging 50 % whereas the conventional circuit gave grades of 45 %. The final column circuit that will probably enter operation in 1995 is shown in Figure 9. Basically the carrying capacity was the variable used to scale up the full-scale columns and superficial velocities were kept close to optimum values found during pilot plant testing.

Walhwater

From rougher

Wash water

Wa#hwatcr

To rougher

Air

WaJh water

A i r

Mo Cone,

Fig.9 El Teniente Cu/Mo probable counter-current columnar circuit (1996-) ?:IZ-C


Chuquicamata - Cu/Mo circuit.

At Chuquicamata columns are used in the molybdenum circuit. The bulk copper- moly concentrate is thickened and fed to the Cu/Mo separation circuit that is schematically shown in Figure 10. The final moly concentrate averages 54 % Mo and less than 0.2 % Cu. The copper concentrate averages 35 % Cu.

Tile rougher stage consists of two parallel lines with 8 Denver (500 cf.) cells each. The first cleaner stage has 12 Agitair (48 cf.) followed by 8 Denver (300 cf.). The second and third cleaner stage are carried out in two square section (1.83xl.83 m.) columns operating in parallel; that is one column flotation stage instead of two conventional cleaning stages. The fourth cleaner consists of two benches of 12 Agitair (48 cO. Regrinding is in two stages: 4 x 100 mm Krebs cyclones are followed by 2 Marcy (4x8 ft.) ball mills. The second re-grinding stage is closed with 8 x 100 mm Krebs cyclones.

The fifth and sixth cleaning stage are two parallel benches of Agitair (48 cf.) in a 8-3 counter-current arrangement.

The final molybdenum concentrate is leached to reduce copper content, thickened and filtered and dried.

Bulk Cu-Mo

Canc. Cu

Fig. 10 Chuquicamata Cu/Mo separation circuit.

Escondida concentrator

Tile processing plant was design for a nominal capacity of 35000 tpd. The initial reserve exploited should have an average copper grade 2.12 % and total reserves 1.6 %. The projected life is 52 years. However the average feed grade for the first years is 2.85 % Cu. Concentrate production is 760000 tpy with 320000 tpy of copper (42% Cu). Moly plant is not in operation and its activation is not expected in the short run due to low Mo grade of the concentrate currently produced. Grinding is carried out in two identical lines. Each line includes a 8.5 x 4.3 m (28 x 14 fi) SAG mill and a battery of 10 Krebs D-26 cyclones. Tile underflow is re-ground in a 5.5 x 7.5 m (18 x 24 ft) ball mill and re-classified. The cyclone overflow is pumped to a rougher flotation bank. Flotation llowsheet is shown ill Figure 11; the two parallel rougher tlotatiort banks include 40 (2 x 20) 44 m 3 (1500 cf.) Dorr Oliver cells and are designed to provide 22 minutes retention time with a pull~ solid content of 30%. The rougher COllcentrate of each


line is classified in two parallel batteries of ten Krebs D-15 cyclones and the underflow re-ground in 4.3 x 8.1 (14 x 26.5 ft) ball mills. The overflow is fed to the cleaning columns. A total of eight 4 x 4 x 14.3 columns provide a cleaning retention time of 20 minutes. The column tails are re-treated in a scavenger bank of 20 44 m 3 Dorr Oliver flotation cells; the tailings are de-watered and finally rejected and the concentrate re-classified in the D-15 cycloning batteries. Column concentrates are de-watered in two 52 m diameter thickeners to 65 % solids and delivered by pipeline to the Coloso filtering-storage buildings. The expansion to a nominal capacity of 55000 tpd by installing a third processing line is under evaluation.

F E E D

1

~ G R ~ D ~ G

Fig. 11 Escondida copper flotation circuit.

T A I L

F I N A L C O N C E N T R A T E ),

COLUMN FLOTATION PRACTICE

Scaling up

Scaling up procedures are largely based on experiences in pilot plants. Full scale equipment is scaled-up by the use of carrying capacities measured in pilot plants and matching ranges of superficial velocities. However the height of all the columns operating in Chile is between 12-14 m.

Table 3 reports typical values used in copper cleaning. The data were collected from bibliograplly [4,5,6] and enquiries during plant visits.

Bubble generalors and energy requirement

The Minnovex, the Cominco and the Turbo-Air USBM/CI[ external bubble generators are in use in Chile.

As an example, in a typical copper cleaning operation two 16 m 2 cohmms consume 1200-1500 m3/h of air at standard - i.e. atmospheric- conditions. The compressed air at 480 kPa (70 psi) of the air network is ted to the cohmm at 290 kPa (45 psi). Each column has 16 inlet pipes. As a rule of thumb 50 m3/h of air in standard conditions per square meter of colunm section are necessary. Power requirement for the application is in the order of 0.1 kWh/m 3 of air in standard conditions, i.e. 1 bar. No applications of the Microcel bubble generation systems are reported in Chile. With such a system the energy requirement is higher because about 0.3 kWh per m 3 of feed pulp are probably needed by the re-circulating puml~.


Power consumption is essentially application specific: increasing with decrease in the mineral size and depending on the recovery. There are experimental indications that air flow rate permits movements along the recovery-grade curve of the column.

TABLE 3 Typical values of key operating variables and geometry for tlotation columns.

min.- max. Notes: values for Cu-Mo bulk flotation

Carrying capacity 1.5-2.5 (tph. m-Z solid in conc.)

Carrying capacity 0.5-1.2 (tph .m-2 Cu in conc.) Treatment capacity 5-10 (tph. m-2 solid feed )

Bias : (T-F)/F .10-.20 Bias ratio : T/F 1.1-1.2 (T-F)/F =T/F-1

Superficial velocities flowrate/section area (cm.s-l) :

washing water 0.2-0.5 air or gas 2.0-3.0 volume of air in standard conditions bias 0-0.3 pulp 1.0-1.5

Pulp retention time 7-9 collection zone volume / tailing flowrate in col. zone (minutes)

Froth height (m.) 0.6-1.5

Gas hold-up (%) 14-18 estimated figures

Pulp weight % solid 19-22

Geometrical ratios: Overall height / overall width o r 2-3.5 Typically a column is 12-14 m. high diameter ratio height / width ratio 10-15 the Typically the baffling work divides

column in sets 1 m. wide

Flotation column control in Chile

The lowest level control strategy presupposes the automatic regulation of tile pulp-froth interface position. It is showed in the sketch 'a' of Figure 12. A constant interface level is achieved by operating the tailing discharge bottom valve. This simple strategy is conceived to allow a stable operation and not directly meant to improve metallurgical performance. Washing water and gas tlowrates are kept constant or manually regulated. A multi-point pressure measurement method is used to eliminate the need of bulk


density in the pulp and froth zones for the calculation of the interface depth. Floating bobs or bubbling- pressure measurement systems are also used. Sonar technology is in use at Escondida,

In many cases positive bias assures better metallurgical performance. The higher levels of control are based on the bias control. This is achieved either by maintaining a constant positive difference between tailing flowrate and feed flowrate or by ensuring a constant ratio ( > 1) between the two flowrates.

WHh water

SET LEVEL

F , , d k m l Co.~

Tail

Air

a) b) c) d)

Fig. 12 Control strategies : a) interface level with discharge flowrate - fix washing water; b) bias with flowrate difference and level with washing water addition; c) bias with flowrate ratio and level with

washing water; d) bias with water addition and level with discharge flowrate Legend : FC: flow controller; FDC: flow difference controller; FRC: flow ratio controller; FI: flow

indicator; FT: flow transmitter; LT: level transmitter; LC: level controller; BC: bias controller

In this class of strategies different possibilities exist. One method is to set a bias value as the difference in the measured feed and tail flowrates and maintain the (positive) setting adjusting the bottom tailing valve. An independent control loop operates the washing water addition valve in order to keep a constant interface level ( Figure 12 - b). The method can be improved by setting the bias as a ratio greater than one between the measured tailing and feed flowrates (Figure 12 - c). This second strategy is more suitable for variable feed flowrates.

Another strategy in use controls the interface level that is kept constant by operating the tailing valve and the bias by adjusting the washing water addition (Figure 12 - d). Therefore the measures of feed and tail flow rates are used to modulate the water addition in order to keep a given bias setting; the tailing valve operates to maintain the interface level. The tailing discharge system is regulated by a pinch valve.

All the control strategies reported are used in Chile; however in a number of plants the strategies based on bias originally implemented were later modified and the simpler control of level is now adopted. It seelns that to date available instrumentation does not offer the precision sufficient for successful colunm control. Optimal control is not yet implemented, but is planned.

Research at tile Department of Chemical Engineering of the Technical University of Valparaiso is devoted to column flotation variable identification and control. Alternative methods for the precise measurement of the level interface position based on temperature and electrical conductivity profiles obtained along the colunm are being investigated. The research group works close association with eolunm operators at El Teniente. These novel methods will permit the estimation ofinterface levels and bias with a precision superior to that allowed currently by tile use of available instrumentation. This will permit better


performance of the existing control systems and a step forward for the development of optimal control strategy algorithms that require an improved knowledge of the relationships between variables and metallurgical results.

CONCLUSIONS

Colunm flotation teclmology has significantly contributed to the simplification of the copper flotation circuits, allowing a decrease in the number of units and the number of stages, while maintaining or bettering the overall circuit metallurgical performance. They have also improved the economics of the concentrators, column technology being less costly than conventional flotation. However, to date colunms have yet to demonstrate their capability to fully substitute for conventional cells in all applications. Indeed in modern copper circuits cells must still be used in the rougher section where coarse material is treated and in tl'te scavenging section of the cleaning flowsheets. There are also important indirect benefits related to the use of columns such as those related to the automatic control of the circuit. However, the lack of fundamental knowledge of this novel technology does not allow scaling up of plants with confidence on the basis of laboratory tests and it is wise to undertake pilot plant tests. Although Chile has much expertise with sulphide copper ores there is very little experience with other minerals. However experiences continuously arising from new industrial applications and R&D will contribute to a better understanding of the technology.

ACKNOWLEDGEMENTS

This research was financially supported by CEC-IMPEXFLOTCOL Brite-Euram project.

The authors are grateful for the assistance to Mr. B. Soto and Mr. A. Cristiansen (Las Tortolas), Mr. P. Molinet and Mr. G.Von Borries (Andina), Mr. D. Riveros (El Teniente), Mr. C. Barahona (C.M. Disputada de Las Condes), Mr. J. Valdivieso (Chuquicamata), Prof. J. Yianatos (Universidad T6cnica Federico Santa Maria).

CODELCO USBM CII

ACRONYMS

Corporacion del Cobre de Chile United States Bureau of Mines Control International Inc.

I.

2. 3.

4.

5.

6.

R E F E R E N C E S

Toro, 1t., Flotacion colmnn.'lr en Chile VI S3,mposium ARMCO, Vifia del Mar, Chile, (1990). Anon, P.A.R.M. Proyecto aumento de recuperacion met.'durgica division And ina - Codelco Chile Vill.'mueva, F. & Vergara, M.A., Desarrollo de la tlotacion columnar en el concentrador de los Bronces - CMD. Minerales, 54, 191. Yianatos, J.B. & Finch, J.A., Condiciones tipicas de disedno y operaciones de colunmas de flotacion industrial. Mineria Chilena, Informe expecial, No. 74, (May 1987). Yianatos, J.B. & Murdock, D.J., Nuevos avances en la tecnolo~ia de columnas de llotacion. Mineria Chilena, N. 125, (Nov. 1991). Bergh, L.G. & Yianatos, J.B., Control alternatives for llotation columns Minerals Engineering, 6, 6, (1993)