el efecto de las condiciones de molienda en la flotación de una mena de cobre sulfuro

8/12/2019 El efecto de las condiciones de molienda en la flotacin de una mena de cobre sulfuro

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Technical Note

The effect of grinding conditions on theflotation of a sulphide copper ore

K.L.C. Goncalves a, V.L.L. Andrade a, A.E.C. Peres b,*

a CVRD, Diretoria de Desenvolvimento de Projetos Minerais, Br 262 km 296, Santa Luzia, MG 33030-970, Brazilb UFMG, Department of Metallurgical and Material Engineering, Rua Espirito Santo, 35/206, Belo Horizonte, MG 30160-030, Brazil

Received 10 April 2003; accepted 23 May 2003

Abstract

The challenge of process development for the beneficiation of Salobos copper ore started in 1978 and the studies still go on.Copper is predominantly present as secondary minerals, such as chalcocite, bornite, and digenite, and liberation requires very fine

grinding. These minerals may undergo rapid oxidation at the alkaline pH range and under mildly oxidising conditions. The oxi-

dation products may adsorb onto the minerals altering their surface characteristics, flotation behaviour, and may also significantly

modify the mechanisms of interaction between the minerals and the collector. These facts impair the flotation process performance

and increase the reagents consumption, the required flotation cells volume, and the overall processing costs. This paper describes the

effect of grinding conditions on the flotation performance. Different media and mill construction materials were tested at bench scale

aiming at evaluating the effects of the pulp electrochemical potential and the availability of iron oxide and hydroxide compounds on

the flotation response. The results indicated that the grinding process affects significantly the flotation metallurgical performance of

Salobos ore. The conditions that yielded the highest levels of copper recovery and the fastest flotation kinetics were rubber lined

steel mill and stainless steel media.

2003 Elsevier Ltd. All rights reserved.

Keywords: Froth flotation; Grinding; Non-ferrous metallic ores; Sulphide ores

1. Introduction

Grinding precedes flotation in most concentrators

(outstanding exceptions are some iron ore flotation

plants in Brazil). The understanding of fundamental

aspects of the two operations is crucial for improving

the concentrator performance.

The floatability of ores and their separation selectiv-

ity are essentially determined by the surface properties.

The surface properties of sulphide minerals are mostly

controlled by the grinding processes and conditions(Xiang and Yen, 1998).

During grinding, a galvanic contact occurs among the

sulphide minerals themselves and also among the sul-

phide minerals and the grinding media, resulting in a

galvanic current due to rest potential differences. The

rest potentials for sulphide minerals are much higher

than that for iron (Rao et al., 1976), so the former act as

cathodes, while the iron grinding media act as anode,

being galvanically oxidised, while oxygen reduction oc-

curs on the sulphide mineral surface.

The oxygen consumption due to grinding media re-

sults in a reducing environment that affects the sulphide

and prevents xanthate oxidation and adsorption. At the

same time the sulphide particles surfaces are coated with

oxides layers. The sulphide minerals ground under these

conditions do not present self-induced floatability and

their flotation response in the presence of xanthate is

usually poor (Heyes and Trahar, 1979).When two sulphides presenting large differences be-

tween their rest potentials are in contact or are ground

in a porcelain mill, the sulphide with lower rest potential

will act as anode while the other will act as cathode. The

anodic mineral will exhibit enhanced flotation perfor-

mance due to the fact that a surface under oxidised state

favours xanthate oxidation and adsorption. On the

other hand, oxygen reduction occurs on the most noble

sulphide (usually pyrite). Its surface presents, then, very

low affinity towards xanthate due to its reduced condi-

tion (Xiang and Yen, 1998). This behaviour explains the

* Corresponding author. Tel.: +55-31-3238-1717; fax: +55-31-3238-

1815.

E-mail address: [email protected](A.E.C. Peres).

0892-6875/$ - see front matter 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.mineng.2003.05.006

Minerals Engineering 16 (2003) 12131216This article is also available online at:

www.elsevier.com/locate/mineng
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enhanced flotation selectivity in the separation between

chalcopyrite, galena, and sphalerite from pyrite, when

the ore is ground in porcelain or stainless steel mills as

compared with iron or carbon steel mills.

Salobos deposit, located in the Carajaas area, is the

largest Brazilian copper reserve (geological reserves of

approximately 1 billion tonnes of ore, copper grade

0.86%, and open pit mineable reserve of 255 million

tonnes of ore averaging 1.11% Cu). Results of extensive

on site pilot plant scale testwork, performed in the 80s,

were reported by Pereira et al. (1991). This stage in-

cluded a demonstration run lasting 210 h and treating

380 tonnes of ore. The major conclusions were that

Salobos copper ore is hard to treat mainly due to its

high Bond work index, its liberation in a fine size range,

its complex mineralogy and uncommon mineralogical

associations, and its need of high flotation retention

times and high collector consumption. The importance

of controlling the electrochemical conditions of the pulp

was stressed in the report, despite the fact that thiscontrol was not performed due to technical difficulties

related to time and geographical constraints.

CVRD decided to concentrate the efforts on process

development of smaller deposits presenting higher grade

and easier concentration conditions. Serra do Sossego

project start up is predicted for 2004. In the mean

time, Salobos ore should be submitted to further

investigation, especially concerning the correlation

between grinding conditions and flotation performance

(Goncalves, 2002).

The effects of sodium sulphide additions and of using

nitrogen as gas phase in flotation will be presented inother publications.

2. Materials and methods

The ROM copper ore sample was crushed in a roll

crusher, in closed circuit with screening, to produce

100% passing 1 mm with minimal ultrafines production.

This sample was homogenised and then cone and

quartered to produce 1100 g fractions for the flotation

experiments.

Grinding was performed in the absence of reagents,

at 60% solids:

(i) rubber lined jar mill, stainless steel rods;

(ii) rubber lined jar mill, carbon steel balls;

(iii) unlined jar mill, carbon steel rods;

(iv) ceramic mill, ceramic balls.

The grinding time was determined based on the re-

quirement of achieving 90% < 100# (150 lm).

Rougher flotation experiments were performed with

1100 g ore samples, at 35% solids, in a 2.5 L Denver

laboratory machine. Potassium amyl xanthate (100 g/t)

and sodium dithiophosphate (25 g/t) were utilised as

collectors and a polyglycol alcohol (60 g/t) was em-

ployed as frother.

Flotation kinetics was evaluated by means of froth

collection at the following time conditions: 1.5, 3, 5, 10,

15 and 20 min. Froths collected in the first three flota-

tion stages were designated as rougher #1 and the other

three froths constituted rougher #2, #3, and #4. Re-

agents were dosed in all four steps, prior to each flota-

tion stage, 40% of each reagent being dosed prior to

rougher #1 and 20% prior to each other stage, namely

rougher #2, #3, and #4.

A combined platinum electrode was utilised for rest

potential determinations. An increment of 197 mV was

added to the figure read in the equipment in order to

convert it to the normal hydrogen electrode standard.

Slurry potentials were recorded after grinding and also

after each reagents addition stage and rougher flotation

stage.

3. Results

The chemical analysis showed 1.35% Cu, 0.55% S,

and 0.99 g/t Au. The copper sulphides present in the ore

are bornite (4%), chalcopyrite, covellite, and chalcocite/

digenite (1% each). Pyrite content is 0.5%. The pre-

dominance of bornite is confirmed by the Cu/S ratio

approximately 2.5. The sulphide particles are present in

the fine size range. Associations with magnetite and with

silicates, as inclusions, are common.

Fig. 1 illustrates the size distribution of the sampleafter grinding under different conditions. The coinci-

dence of the curves was achieved by using different

grinding times for each grinding condition:

(i) rubber lined jar mill, stainless steel rods: 16 min;

(ii) rubber lined jar mill, carbon steel balls: 15 min;

(iii) unlined jar mill, carbon steel rods: 09 min;

(iv) ceramic mill, ceramic balls: 29 min.

0

10

20

30

40

50

60

70

80

90

100

1 10 100 1000

particle size (m)

cumulativepercentagepassing

lined; stainless steel rods

unlined; carbon steel rods

ceramic; ceramic balls

lined; carbon steel balls

Fig. 1. Size distributions after grinding under different conditions.

1214 K.L.C. Goncalves et al. / Minerals Engineering 16 (2003) 12131216


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The condition of eliminating the flotation feed size

distribution as a variable was attained, as illustrated in

Fig. 1.

Fig. 2 illustrates the slurry potential variation along

rougher flotation, after grinding under different condi-

tions. Table 1 and Fig. 3 show copper flotation recovery

for grinding under different conditions.

4. Discussion

The similarity among the curves plotted in Fig. 1

shows that the different grinding conditions utilised didnot produce samples with significantly different size

distributions.

The results of rest potential variation along rougher

flotation are discussed at the light of Fig. 2.

Grinding in ceramic mill (ceramic balls) and lined

steel mill (stainless steel rods) yielded positive values of

the pulp rest potential, approximately +200 mV, the

oxidising environment being caused dissolved oxygen.

This condition is ideal to promote the oxidation of

xanthate to dixanthogen as well as to oxidise moderately

the surface of copper sulphides present in the slurry

(bornite, chalcocite/digenite, and chalcopyrite). A layer

of metal sulphide deficient in the metal is formed at the

surface of the minerals, favouring the adsorption of

xanthate oxidised species and also enhancing the self-

induced floatability of the copper minerals.

Grinding in lined steel mill (carbon steel balls) and

unlined steel mill (carbon steel rods) yielded negative

values of the pulp rest potential, approximately )50 and

)150 mV, respectively, the reducing environment being

caused by the availability of ferrous ions in the system.

The co-existence of ferrous ions and sulphide minerals in

the system causes a galvanic current that induces the

oxidation of species presenting lower redox potential

(ferrous ions) and oxygen reduction on the surface of the

sulphide minerals, resulting in a strongly reducing en-

vironment. Iron oxides and hydroxides tend to precipi-tate on the surface of the sulphide particles. Sulphide

minerals ground under this condition do not present

self-induced floatability and their flotation response with

xanthate is normally poor.

The high copper recovery in the first minute of flo-

tation after grinding in a lined mill and stainless steel

grinding media, shown in Table 1, confirms the hy-

pothesis that the existence of an oxidising environment

during grinding enhances the floatability of copper

minerals. Grinding conditions that generated a reducing

environment yielded low copper recoveries in the first

-300

-200

-100

0

100

200

300

1 2 3 4 5 6 7 8 9stages

Eh(mV)





1 - after grinding 4 - after reagents 7 - after rougher 3

2 - after reagents 5 - after rougher 2 8 - after reagents

3 - after rougher 1 6 - after reagents 9 - after rougher 4 Fig. 2. Slurry potential variation along rougher flotation of the copper

ore.

Table 1

Copper flotation recovery after grinding under different conditions

Time (min) Lined-stainless steel rods Ceramicceramic balls Lined-carbon steel balls Unlined-carbon steel rods

Cu (%) Cu recovery

(%)

Cu (%) Cu recovery

(%)

Cu (%) Cu recovery

(%)

Cu (%) Cu recovery

(%)

1.5 26.4 58.8 34.4 41.0 34.5 36.5 38.4 21.1

3 18.4 73.7 21.9 64.0 26.0 52.5 26.3 42.1

5 15.4 79.9 17.6 73.0 21.5 61.8 20.9 53.2

10 10.0 89.5 10.5 87.5 9.6 87.7 8.8 86.8

15 8.5 92.1 8.6 91.0 8.0 91.8 7.2 91.9

20 7.6 93.5 7.8 92.5 7.1 93.8 6.5 93.7

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30 35 40 45

copper grade (%)

co

pperrecovery%





Fig. 3. Copper recovery as a function of copper grade for different

grinding conditions.

K.L.C. Goncalves et al. / Minerals Engineering 16 (2003) 12131216 1215


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minute of flotation. A gradual decrease in the float-

ability of the sulphide minerals is observed for pulp rest

potentials reaching less positive and then negative val-

ues. More positive potential values represent a larger

availability of iron ions in the system, and consequently

a larger amount of oxidised iron species (oxides and

hydroxides) are present on the surface of the sulphide

minerals.

The expectation that grinding in a ceramic mill would

yield flotation results similar to those produced by

grinding in a lined mill (with stainless steel rods) was not

confirmed, despite the fact that the pulp rest potential

was practically identical for both conditions (see Fig. 1).

The impaired flotation performance of the ore ground in

the ceramic mill may be attributed to the morphology of

the particles, submitted to longer abrasion action (29

min in the ceramic mill 16 min in the lined mill with

high chromium rods). Another explanation could be the

precipitation of oxidised copper and iron species on the

sulphides surfaces. The extension of this phenomenonwas larger for grinding in the ceramic mill due to the

longer residence time.

Data presented in Table 1 and Fig. 3 show that the

highest selectivity (higher grade for the same copper

recovery) was achieved for grinding in a lined mill with

stainless steel rods, following ceramic mill with ceramic

balls, then lined mill with carbon steel balls, and finally

unlined mill with carbon steel rods. The direct contact

between the carbon steel rods and the mill shell causes

enhanced wear of the shell and grinding media, liber-

ating ultrafine iron particles into the pulp. The rate of

the oxidation of iron particles reaction is accelerated,with consequent precipitation of larger amounts of iron

oxides and hydroxides on the surface of the sulphide

minerals, explaining the impaired selectivity and the

slower flotation rate up to the 10th minute of the test.

The high dosage of reagents necessary for the flota-

tion of Salobos ore is responsible for the high final

copper recovery even for grinding conditions less ade-

quate for flotation. Very high collector dosages are

necessary to provide the adhesion of the oxidised copper

sulphide particles to air bubbles. Nevertheless, improved

flotation selectivity is achieved only for grinding condi-

tions providing an oxidising environment.

5. Conclusions

The grinding conditions affect significantly the sub-

sequent flotation stage of this sulphide copper ore. The

presence of iron ions in the slurry is deleterious to the

flotation of copper minerals and may be avoided by

the utilisation of lined mills and non-ferrous or corro-

sion resistant grinding media, such as stainless steel or

pebbles.

Monitoring the slurry rest potential provides an in-

dication of the flotation performance. Mild oxidising

potentials (positive rest potentials) are adequate for

enhanced copper recoveries for favouring xanthate oxi-dation and adsorption onto the minerals surface.

References

Goncalves, K.L.C., 2002. Effect of surface oxidation on the flotation of

Salobos copper and gold ore, M.Sc. thesis, CPGEM-UFMG,

p. 138 (in Portuguese).

Heyes, G.W., Trahar, W.J., 1979. Oxidationreduction effects in the

flotation of chalcocite and cuprite. International Journal of

Mineral Processing 6, 229252.

Pereira, C.E., Peres, A.E.C., Bandeira, R.L., 1991. Salobo copper ore

process development. In: Proceedings of COPPER 91, Ottawa,

Canada, pp. 133144.Rao, S.R., Moon, K.S., Leja, J., 1976. Effect of grinding media on the

surface reactions and flotation of heavy metal sulphides. In:

Fuerstenau, M.C. (Ed.), Flotation A.M. Gaudin Memorial Vol-

ume, vol. 1. AIME, New York, pp. 509527.

Xiang, H.W., Yen, X., 1998. The effect of grinding media and

environment on the surface properties and flotation behaviour

of sulfide minerals. International Journal of Mineral Processing 7,

4979.

1216 K.L.C. Goncalves et al. / Minerals Engineering 16 (2003) 12131216

el efecto de las condiciones de molienda en la flotación de una mena de cobre sulfuro

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