Una comparación de la flotación del mineral de la Merensky Reef después de la molienda en húmedo...

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.Int. J. Miner. Process. 60 2000 115129

www.elsevier.nlrlocaterijminpro

A comparison of the flotation of ore from theMerensky Reef after wet and dry grinding

D. Feng, C. Aldrich)

Department of Chemical Engineering, Uniersity of Stellenbosch, Priate Bag X1, Matieland 7602,Stellenbosch, South Africa

Received 22 January 1999; received in revised form 3 January 2000; accepted 3 January 2000

Abstract

The effect of dry and wet grinding on the flotation of complex sulfide ores from the Merensky

Reef in South Africa was investigated. Topographical examination of the ground particle surfaces

by scanning electron and atomic force microscopy showed that the dry ground samples hadrelatively rough particle surfaces with a high concentration of microstructural defects, while the

wet ground samples had smoother, cleaner surfaces. As a result, the activated particle surfaces

from the dry ground ore accelerated the dissolution of the particles, as well as the adsorption of

reagents onto the particle surfaces. The dry ground samples exhibited more stable, higher loaded

froths and faster flotation kinetics, owing to the activated particle surfaces. High intensity

conditioning of the dry ground ores prior to flotation could improve flotation by cleaning the

particle surfaces through high shear force fields in the pulp. Moreover, by combining dry and wet

grinding, the kinetics, as well as the final grades and recoveries of the sulfides, could be improved.

q 2000 Elsevier Science B.V. All rights reserved.

Keywords:grinding; mechanochemistry; froth flotation; high intensity conditioning

1. Introduction

Although the primary objective of grinding is to reduce particle size, several

phenomena can accompany comminution. These mechanochemical transformations in-

)

Corresponding author. Fax: q27-21-808-2059. .E-mail address: [email protected] C. Aldrich .

0301-7516r00r$ - see front matter q2000 Elsevier Science B.V. All rights reserved. .P I I : S 0 3 0 1 - 7 5 1 6 0 0 0 0 0 1 0 - 7


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( )D. Feng, C. AldrichrInt. J. Miner. Process. 60 2000 115129116

clude formation of amorphous phases, surface activation, polymorphic transformation,

chemical reaction, etc. The formation of highly active surface areas, as well as changes

in the physical behaviour of the solids can be directly attributed to permanent rearrange-

ment of the crystal lattice, and structural alteration of the grains during grinding. These

effects have been investigated, along with comminution processes, by Boldyrev and

. . .Avvakumov 1971 , Butyagin 1971 and Katchalova 1978 among others. From theseearly studies, it was clear that the grinding environment has a major influence on the

mechanochemistry of the solids, and hence the behaviour of the solids during subsequent

processing, such as flotation.

Industrially, wet grinding is preferred to dry grinding, among others owing to

downstream processing requirements, as well as the higher energy efficiency associated

with wet grinding. Wet grinding proceeds with the preferential formation of new

surfaces and little bulk deformation in the particles. Since the contribution to the

non-equilibrium defects to the integral excess enthalpy content is considerably higher

than the contribution resulting from the surface energy, the expected magnitude of .excess enthalpy in wet grinding is lower than in dry grinding Tkacova, 1989 .

The liquid side of the solidliquid interface has historically been the focal point of

flotation research, while the solid phase has received much less attention. The submicro-

scopic and microscopic structure of the particle surface can significantly influence the

surface properties, collector adsorption and flotation of a mineral. Aplan and Fuerstenau .1962 have summarized the effect structural differences in minerals may have on their

flotation. These differences need not be large, but can arise from subtle defects in the . .solid, as postulated by Welch 1953 . Subsequently, Plaksin and Shafeev 1960 and

.Simkovich 1963 , working independently, have demonstrated the profound influencethat solid state point defects may have on flotation. These observations are supported by

.more recent research by Ducker et al. 1989 , who have shown that surface roughness

can have a marked effect on flotation recoveries.

Metal sorption onto the surface of sulfides in flotation pulps is a common phe-

nomenon during the processing of complex sulfide ores. As a result, the surface

properties of sulfide mineral grains are changed, leading to changes in flotation responsewhich can be a major contributor to a loss of processing efficiency Ralson and Healy,

1980; Allison, 1982; Wang, 1984; Senior and Trahar, 1991; Basilio et al., 1996; Trahar

.et al., 1997 . The metals available in solution in the pulp are derived by oxidation ofsulfide minerals, and in polymetallic sulfide ores, Pb, Zn and Fe are the most common

.metals present Senior and Trahar, 1991 . Wet grinding can favour the chemical surface

reaction products. Dry fine grinding can result in the oxidation of sulfides.

As can be expected, changes in structure and enthalpy of solids during grinding in air

and in aqueous environments are different, and as a result, subsequent flotation of the

ore greatly depends on the grinding environment. Nonetheless, most flotation experi-

ments have been conducted on solids reduced in wet grinding environments, so that

relatively little is known with regard to the effect of dry grinding on specific flotation

systems. In this paper, the effects of wet and dry grinding on the flotation of a complexsulfide ore are consequently compared, and exploitation of the characteristics of each

process in order to enhance flotation of the ore is considered.


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( )D. Feng, C. AldrichrInt. J. Miner. Process. 60 2000 115129 117

2. Experimental work

The platinum-bearing sulfide ore from the Merensky Reef in South Africas Bushveld

Igneous Complex had a complex mineralogy, consisting mainly of silicates, oxides and

base metal sulfides. The main base metal sulfides were pyrrhotite, pentlandite, chalcopy- .rite, pyrite and several other sulfides Vermaak and Hendricks, 1976 . The sample was

taken from the feed to the milling plant in Rustenburg, and its mineralogical and

chemical analyses are shown in Table 1. The sample was crushed in a jaw crusher and a

rotary cone crusher to 100% passing 4 mm. For comparative purposes, an oxide ore .sand with a particle size of 3 mm was also investigated.

Table 1

Mineralogical and chemical analyses of Merensky ore

( )Mineralogical analysis %

Sulfides Fe-sulfides 0.45

Pentlandite 0.33

Chalcopyrite 0.19

Other-sulfides 0.02

Silicates Feldspar 46.52

Orthopyroxene 37.04Clinopyroxene 7.47

Olivine 0.43

Mica 0.61

Quartz 1.48

Other-silicates 1.08

Altered silicates Serpentine 0.06

Chlorite 0.60

Talc 0.16

Oxides Chromite 3.11

Oxides 0.26

Others Carbonates 0.19Others 0.00

PGM 5.3 grt

Total 100.00

( )Chemical analysis %

S 0.32

Al 8.47

Ca 6.38

Cr 0.76

Cu 0.06

Mg 7.24Ni 0.14

Si 23.00

PGM 5.3 grt


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Particle size was determined via sieve analysis, while the specific surface area of the

particles was determined by the BET method. The reagent content in solutions was

determined by ultraviolet spectral analysis. The topology of the particle surfaces was . .studied by scanning electronic microscopy SEM and atomic force microscopy AFM .

A rubber-lined cylindrical laboratory rod mill with a diameter of 200 mm and a

length of 300 mm was used for dry and wet grinding in all the experiments. Fifteenstainless steel rods with a dimension of 22 mm in diameter and 270 mm in length were

used as milling media.

The apparatus used for the input of energy to the slurry consisted of a baffled agitated

vessel with a 90 W stirrer with a six-bladed standard Rushton impeller. The concentra- .tion of the slurry within this high intensity conditioning HIC cell was maintained at

approximately 50% solids, while the impeller speed was kept constant at approximately

2000 rpm.

In each case, a 1-kg sample was milled for a specified period. Once the sample had

been milled, it was removed from the mill and placed directly into the Leeds flotationcell. The pulp in the flotation cell was diluted to 30% solids.

The sulfide ore sample was conditioned in the flotation cell for 5 min in the presence .of CuSO at a concentration of 50 grt. The collector SIBX sodium isobutyl xanthate4

with a concentration of 80 grt was added together with 40 grt of Dow 200 frother, and

allowed to condition for another 5 min. A Guar type depressant, at a concentration of

100 grt, was added for a further 1 min. After this total conditioning time of 11 min the

air was switched on. The aeration rate was 4 lrmin. The froth was allowed to build up

for 30 s before the individual concentrates were taken. The froth in the cell was

controlled at a height of 5 mm and removed at 10-s intervals with a scraper. The totalfloat time was 15 min. Four concentrates were taken after 2, 5, 10 and 15 min. These

concentrates were dried and weighed and analysed for their sulfur content using a Leco

sulfur analyser.

The sand sample was conditioned in the flotation cell for 5 min in the presence of

collector and frother hexadecyl trimethylammonium bromide at a concentration of 700

grt. Other flotation parameters were the same as those for the above-mentioned sulfide

flotation.

The froth structure was monitored with an ELMO colour charge coupled device

.CCD . The CCD was mounted on a bracket to insulate it from undue cell vibration andwas connected to a computer equipped with a frame grabber which was used to digitize

and process captured images. Lighting was provided by a single 100-W bulb next to the

CCD. Images were captured at intervals of 30 s, after every third scrape of the froth. .Only three parameters were captured, i.e. the so-called small number emphasis SNE ,

.average grey level AGL and the froth instability, as described previously by Aldrich et . al. 1997 . The SNE parameter had an inverse relationship with the bubble size adjusted

.multiple squared correction coefficient of approximately 0.60 , while the AGL could beseen as an indication of the loading of the froth the higher the AGL, the lower the

. loading . The instability was a measure of the rate of bubble collapse in the froth the.higher the instability, the less stable the froth .


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3. Results and discussion

3.1. Merensky sulfide ore

3.1.1. Effect of grinding enironment on the properties of the particlesThe rod mill was run at a constant speed of 80 rpm. Three samples were ground to

approximately 60% passing 75 mm under the conditions of dry milling, as well as wet

milling with a pulp concentrations of 66% and 90% solids. During dry milling, nitrogen

was fed to the mill to drive off oxygen before milling started. The grinding results are

shown in Table 2.

From the point of newly formed surface, the energy consumption in wet grinding is

lower by about 20% than in dry grinding. The effect of water on grinding used to be

interpreted in accordance with two different theories. According to the first, adsorption

.changes the fracture mechanism adsorption-induced decrease in strength , while accord-ing to the second, adsorption influences the properties of fragments created by breakage .Tkacova, 1989 . Based on empirical observation, the conditions of energy and mass

transport in wet grinding mill are improved significantly. Microplastic deformation of

the near-surface layers of solid particles under cyclic stress causes an increase of the

adhesive forces between their external surfaces. These adhesive forces grow with

decreasing particle size, and hence, the conditions of mutual interaction between

surfaces of the newly created particles improve. In contrast, cyclic stressing during dry

batch grinding leads to a recombination of newly created particles into unstable

aggregates as a result of partial fusion of the surfaces of particles.From an energy equilibrium viewpoint, some of the adsorbed energy during dry

milling is dissipated in distortion of the particle structure. Consequently, in wet grinding,

preferential formation of new surfaces and little bulk deformation in the particles can be

expected. The contribution of these non-equilibrium defects to the internal excess

enthalpy content in dry grinding is much higher than that in wet grinding. This was

verified from the topographies of the particle surfaces, as discussed in more detail

below.

3.1.2. Topographical study of particle surfaceThe above-mentioned samples of sulfide ores dry grinding, and wet grinding with

.66% and 90% solids content were examined microscopically. Typical SEM and AFM

images are shown in Figs. 1 and 2.

Table 2

Grinding results of Merensky ore

2 . . .Milling method Milling duration min Particle size % 75 mm Surface area m rg

Dry milling 40 58.0 1.23 .Wet milling 66% solids 30 57.5 1.29 .Wet milling 90% solids 36 58.5 1.25


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. .Fig. 1. Topographies of the sulfide particle surfaces by SEM, with a wet grinding with 66% solids, b wet .grinding with 90% solids, and c dry grinding.

From these figures, it can be seen that the particle surfaces resulting from wet

grinding were much smoother than those resulting from dry grinding. Two- and

three-dimensional defects appeared in the particle surfaces by dry grinding. Higher .stresses were induced in the particles in the denser slurry 90% solids , and as a

consequence, some defects also appeared on these particle surfaces.

During grinding, the overall enthalpy change of the macroscopic state is attributed to

changes in the free surface area, lattice strain, reduction of crystallite size and formation

of amorphous phases. In contrast, in wet grinding the free surface area change dominates

and the other three factors are negligible. When the same new surface formed, drygrinding consumed more energy than wet grinding. This meant that the particles in dry

grinding conserved more energy, some of which existed in the form of defects. These

defects played an important role in subsequent particle dissolution and reagent adsorp-

tion.

3.1.3. Particle dissolution

The ground samples were stirred in a vessel with a concentration of 28% solids. The

ion concentration change with the dissolution time is shown in Table 3.

The dissolution rate of particles after dry grinding was higher, as indicated by the

concentrations of Cu, Ni and Pd in Table 3, for example. In contrast, the wet ground

sample was more difficult to dissolve.

3.1.4. Collector adsorption kinetics

An 80-grt SIBX was added to the pulp for adsorption tests. The initial SIBX

concentration was 30 ppm. The collector adsorption with time is shown in Fig. 3.

The collector adsorption rate increased with an increase in the particle surface energy.

The activated surface easily adsorbed the surfactant. The collector could decrease theparticle surface energy making the system achieving a thermodynamic equilibrium. The

dry ground sample had faster collector adsorption kinetics than the wet sample, as

shown in Fig. 3.


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. . Fig. 2. Topographies of the sulfide particle surfaces by AFM. a Wet grinding with 66% solids b Wet gr


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Table 3

Variation of ion concentration in pulp after wet and dry grinding

. .Sample Time min Ion concentration ppm

Ag Au Cu Fe Ir Ni Pb Pd Pt

Dry ground sample after 0 0 0.05 0.01 0 0.04 0.04 0.01 0.06 0.04

40 min milling

15 0.01 0.06 0.02 0 0.05 0.04 0.01 0.07 0.06

45 0.01 0.06 0.04 0 0.06 0.06 0.02 0.08 0.08

120 0.01 0.06 0.05 0 0.08 0.08 0.04 0.10 0.09

Wet ground sample with 0 0.01 0.06 0.04 0 0.05 0.08 0.03 0.09 0.07

66% solids after

30 min milling

15 0.01 0.06 0.04 0 0.05 0.08 0.03 0.09 0.07

45 0.01 0.06 0.04 0 0.05 0.08 0.03 0.09 0.07

120 0.01 0.06 0.04 0 0.06 0.08 0.03 0.09 0.08

3.1.5. Effect on flotation

Figs. 4 and 5 represent the flotation performance of samples ground under different

milling conditions. All the samples had similar particle sizes and surface areas. In Figs. .4 and 5, the following conditions applied: dry milling, 40 min; wet milling 66% solids ,

.30 min; wet milling 90% solids , 36 min; dryqwet milling implied dry milling 30 min .followed by 10 min wet milling 66% solids .

The dry ground sample exhibited much faster flotation kinetics than the wet ground

samples, while the wet ground samples showed higher concentrate grades and slightlyhigher recoveries. The samples obtained with dry milling conserved more energy in the

form of defects, and displayed activated particle surfaces, onto which reagents easily

adsorbed. In the samples obtained from dry milling, some very fine particles also

adsorbed onto the activated surfaces, which resulted in lower flotation selectivity. The

wet ground samples had smoother particle surfaces and lower surface energy, so that

Fig. 3. Adsorption kinetics of SIBX collector on sulfide particles.


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Fig. 4. Variation of sulfur recovery with flotation time.

both the reagent adsorption and flotation rates were lower. In wet grinding, the particle

surfaces were cleaner, resulting in higher flotation selectivity. In the denser wet grinding .media 90% solids , some of the energy was also converted into particle surface energy.

Moreover, the particle surfaces were comparatively clean, leading to an improvement in

both the flotation kinetics and concentrate grade.

Since dry milling promoted high surface energy and faster flotation kinetics, and wet

grinding favoured clean particle surfaces and higher flotation grades, a combination of

wet and dry milling was subsequently investigated in order to improve both the flotationkinetics and grade. Dry milling induced high stresses in the particles, resulting in an

activated layer on the particle surfaces, while the presence of an aqueous environment

enlarged the specific surface area of the samples. The aggregation of particles could be

suppressed in samples that had first been subjected to dry grinding, followed by brief

regrinding in an aqueous environment. The results portrayed in Figs. 4 and 5 indicate

Fig. 5. Variation of sulfur content with flotation time.


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Fig. 6. Variation of recovery with flotation time after high intensity conditioning.

that the combination of dry and wet milling could improve both the flotation kinetics

and grades.

From the experiments described above, it could be expected that the flotation kinetics

could be further improved by increased energy input, while flotation selectivity could be

enhanced by cleaning the particle surfaces through high shear force. High intensity

conditioning of the flotation pulp was therefore conducted in an agitated vessel. Dry

milling took 40 min, and wet milling with a pulp concentration of 66% solids took 30

min. The high intensity conditioning time was 10 min for the dry ground sample and 35min for the wet ground sample. The flotation results are shown in Figs. 6 and 7.

For the dry ground sample, the flotation kinetics were more or less the same, while

the flotation selectivity improved appreciably after 10 min of high intensity condition-

ing. The high shear forces involved in the HIC process removed the adherent fines from

the particle surfaces, which resulted in better selectivity.

Fig. 7. Variation of concentrate content with flotation time after high intensity conditioning.


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For the wet ground sample, the flotation kinetics greatly improved and the flotation

recovery and concentrate grade improved to some extent as well after 35 min of high

intensity conditioning. The energy input through the HIC process accelerated the reagent

adsorption kinetics and subsequent particle-bubble attachment. Consequently, suitable

energy input into the flotation system could enhance flotation.

For the dry ground sample, only the particle surfaces needed to be cleaned and for thewet ground sample, more energy was required to improve the kinetics. The required

high intensity conditioning time for the dry ground sample was much shorter than that

for the wet ground sample for similar flotation kinetics.

Since the adsorbed energy in dry milling was stored in the form of permanent defects,

the effects were essentially permanent. In contrast, the energy adsorbed by the particles

during high intensity conditioning was stored in the form of elastoplastic deformation of

the particle surfaces. This deformation was temporary, owing to the subsequent relax-

ation of the crystals. In order to exploit the effects induced by high intensity condition-

ing, flotation should follow conditioning without delay. This has been verified by .Bunkell et al. 1996 . When viewed as a whole, the higher energy consumption during

milling can be compensated for by the lower energy consumption in flotation with

improved kinetics.

3.1.6. Effect of the grinding enironment on the flotation froth structure

The most obvious result of the flotation test was the effect that the grinding

environment had on the flotation froth structure. The froth on the floats associated with

the wet ground samples was visibly unstable and the shallow froth layer had a low

loading. In contrast, the froth associated with the dry ground samples was comparativelystable, the froth layer was thicker and the bubble loading was higher. This can probably

be attributed to the increased reagent adsorption rate and the increase in the attachment

of particles and bubbles, which tended to stabilize the froth. The image analysis results

are shown in Figs. 8 and 9.

Fig. 8. Variation of AGL with flotation time.


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Fig. 9. Variation of SNE and stability with flotation time.

From Figs. 8 and 9, it can be seen that the bubble loading in the dry milled ore floats .was higher than the loading in the wet milled ore floats lower AGL values in Fig. 8 .

The average bubble size distributions of the two floats appeared to be very similar SNE.values in Fig. 9 . Also, as indicated in Fig. 9, the froth from the dry ground samples

float was more stable than that of the wet sample.

3.2. Flotation of sand

Two 1-kg sand samples were ground under dry and wet milling conditions. Dry

milling took 50 min to ensure that by mass 75% of the sample passed 75 mm. The dry

ground sample had a specific surface area of 1.68 m2rg. Wet milling at a pulp

concentration of 66% solids took 37 min to ensure that by mass 75% of the sample

. .Fig. 10. Topographies of ground sand by SEM. a Wet milling b Dry milling.


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Fig. 11. Flotation result of sand samples under different milling environments.

passed 75 mm. The wet ground sample had a specific surface of 1.76 m 2rg. Dry milling

consumed approximately 25% more energy than wet milling to achieve more or less

similar surface areas. Some of the energy adsorbed in the dry milling process was owed

to particle deformation, which was stored in the form of surface defects on the particles.

In addition, the particle surface was activated in the dry milling process. The topogra-

phies in Fig. 10 give some idea of the particle surface structures generated by dry and

wet milling.

As can be seen from Fig. 10, the wet ground sample had a smooth particle surface

and few loose fine particles adsorbed onto the surface, while the dry ground sample had

a rough particle surface and many small particles attached onto the surface. Some

defects appeared on the particle surface in the dry ground sample, and these surface

defects served as the active centres in during reagent adsorption.

The stored energy could be exploited by improved flotation kinetics of the sand. The

results are shown in Fig. 11.

The dry ground sand showed significantly faster flotation kinetics and a slightly

higher flotation yield than the wet ground sand. This demonstrates that dry milling could

improve flotation kinetics, owing to the higher energy adsorbed in the particles.

In practice, dry milling of sulfide ores is complicated by the tendency of fine sulfides

to oxidize in air. This is not the case with oxide ores, but the higher energy consumption

has to be weighed against the enhanced flotation kinetics.

4. Conclusions

From this investigation, the following can be concluded.v Based on the newly formed surface area, the energy consumption with dry rod

milling was approximately 25% higher than with wet rod milling at pulp concentrations

of 66% solids. Wet milling rapidly enlarged the surface area, while dry milling resulted


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in the activation of the ground samples. A study of the topographies of the particle

surfaces indicated that the dry ground samples had comparatively rough particle

surfaces, as opposed to wet ground samples, which had smooth surfaces.v In dry milling, some of the energy is stored in the form of crystal defects. These

defects serve as active centres for speeding up particle dissolution and reagent adsorp-

tion.v The dry ground samples showed more stable, highly loaded froths, faster flotation

kinetics and lower flotation selectivity. The fast flotation kinetics could be attributed to

the activation of particle surfaces in dry milling, while the low flotation selectivity could

be attributed to non-selective adsorption of fines onto the activated particle surfaces. In

contrast, the wet ground sample exhibited higher flotation selectivity and slower

flotation kinetics.v .In the dense wet milling process 90% solids in the pulp phase , the particle

surfaces had some defects, and the ground sample appeared to have faster flotation

.kinetics than the less dense dilute milling process 66% solids in the pulp phase . Thecombination of wet and dry milling could improve flotation. Dry milling process was

used to increase the enthalpy of the ground sample, and wet milling was used to depress

the non-selective aggregates resulting from dry milling.v By use of high intensity conditioning prior to flotation, the selectivity of the dry

ground sample could be improved further. The shear force fields generated by high

intensity conditioning in an agitated vessel removed non-selectively adsorbed fines from

the activated particle surfaces, resulting in an increase in flotation selectivity. In the case

of the wet ground ore, the energy input through high intensity conditioning activated the

particle surfaces, resulting in improvement of the flotation kinetics. In the case of thewet ground ores, this effect tended to dissipate with time, since the energy was stored in

the form of particle surface stress concentrations, which were prone to stress relaxation.v Although dry milling can greatly improve flotation kinetics, it is difficult to apply to

practical sulfide flotation systems, owing to the tendency of fine sulfides to oxidize in

air. This is obviously not a problem as far as oxide ores are concerned.

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