Grinding the Primary Conditioner

8/4/2019 Grinding the Primary Conditioner

1/18

Grinding The Primary Conditioner

C J Greet1

and P Steinier2

1. MAusIMM, Magotteaux Australia Pty Ltd, Suite 4, 83 Havelock Street, West Perth WA

6005. E-mail: [email protected]

2. Magotteaux Australia Pty Ltd, Suite 4, 83 Havelock Street, West Perth WA 6005.E-mail: [email protected]

ABSTRACT

Most operations, when considering their comminution circuit, are primarily concerned with

achieving the desired particle size distribution to obtain adequate liberation for the separation

process, at minimum cost. Invariably, the effect the grinding media has on the subsequent

separation process (ie flotation, leaching) is not taken into account when selecting the media.

This choice is driven by cost rather than metallurgical outcomes.

Maximising valuable mineral recovery, with optimum selectivity against gangue minerals, is

about good particle preparation. And, particle preparation is strongly related to achieving

adequate mineral liberation with the correct pulp chemical conditions. However, the pulp

chemistry of a system is largely ignored and relies heavily on pH control and the addition of

appropriate reagents (collectors, frothers, activators, depressants). The changes effected by

the addition of the various reagents are obtained through conditioning the ground pulp prior

to or during the concentration stage, subsequent to grinding. Frequently, reagents are added

during grinding with good effect, but the impact of the grinding media on down stream

processing is usually overlooked.

A number of case studies examining the effect of grinding media on pulp chemistry and

subsequent flotation performance were completed on several different sulfide mineral

systems (for example, lead/zinc, copper, refractory gold). The evidence suggests that a

change in grinding media type from forged to high chrome (ie conditioning the pulp during

grinding) resulted in a shift in pulp Eh to less reducing conditions, an increase in the

dissolved oxygen content of the pulp and a reduction in the iron species present. The changes

have a positive impact on flotation behaviour. The results of the case studies are discussed.

INTRODUCTION

The key to a successful separation in mineral processing is the preparation of particles with

adequate liberation under the correct pulp chemical conditions.

While the importance of liberation on flotation separations is generally understood and well

documented in the literature (Johnson, 1987; Jackson et al, 1989; Young et al, 1997; Greet

and Freeman, 2000), the importance of pulp chemistry is more nebulous, particularly with

regard to the impact of grinding media. Extensive work examining the electrochemical

interactions between grinding media and sulfide minerals has been completed (for example,

Iwasaki et al, 1983; Natarajan and Iwasaki, 1984; Yelloji Rao and Natarajan, 1989a; YellojiRao and Natarajan, 1989b). Broadly, these studies indicate that most sulfide minerals are

more noble than the grinding media used during comminution, therefore a galvanic couplebetween the media and the sulfide mineral(s) exists, which increases the corrosion rate of the


2/18

grinding media. The corrosion products of the grinding media, iron oxy-hydroxide species,

invariably precipitate on to the surfaces of the sulfide minerals thereby affecting their

floatability (Johnson, 2002).

Cullinan et al (1999) completed laboratory experiments examining the effect of different

ferrous based grinding media on galena flotation performance in the lead circuit of the MountIsa Mines Lead/Zinc Concentrator. On a size-by-size basis, their work indicated that grinding

with high chrome grinding media increased the maximum galena recovery of the -3 micron

fraction from 63 per cent (for forged steel grinding media) to 79 per cent (Figure 1). This

improvement could not be attributed to entrainment, since slightly higher water recovery

values were obtained when grinding with forged steel grinding media. Improvements in

selectivity for galena against sphalerite and iron sulfides were also observed when high

chrome grinding media was employed.

Pulp chemical measurements recorded during these tests indicated that grinding with forged

steel resulted in significantly more reducing conditions than those observed when high

chrome grinding media was used (Table 1). The other feature of the pulp chemistry was theelevated levels of oxidised iron species (as measured using an EDTA extraction technique

(Rumball and Richmond, 1996; Cullinan et al, 1999; Greet and Smart, 2002)) recorded forthe forged steel case, which were 1.56 times greater than the values reported for the high

chrome alloy. The elevated levels of EDTA extractable iron for the forged steel grinding

media were the result of increased corrosion between this media type and the sulfide minerals

in the ore. It was postulated that the improvement in fine (-3 micron) galena flotation

response when the ore was ground with high chrome grinding media was due to lower levels

of oxidised iron species within that system.

FIG 1 - Maximum galena recovery versus particle size for Mount Isa lead/zinc ore

ground with different grinding media types (Cullinan et al, 1999).

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

1.0 10.0 100.0

Size, microns

Rmax,%

Forged steel high chrome


3/18

Cullinan (1999) completed more fundamental work examining the effect of the grinding

environment on galena flotation. In these tests, 100 grams of Rapid Bay galena was ground in

different mills: mild steel mill using forged steel balls; a stainless steel mill using high

chrome balls; and a ceramic mill using ceramic balls. The resultant recovery versus time

curves are provided in Figure 2, and illustrate that the galena recovery increased markedly as

the media type changed from forged to high chrome, to ceramic. The pulp chemistrymeasurements taken for these experiments are listed in Table 2. While the variations in pH

and Eh were comparatively minor, the amount of EDTA extractable iron decreased

dramatically as the grinding media type was changed to more inert materials. This

corresponded to an increase in the galena flotation response, again suggesting that the

corrosion products on the grinding media impact on flotation response.

TABLE 1Pulp chemical measurements for Mount Isa lead/zinc ore ground with different grinding

media types (Cullinan et al, 1999).

Media type pH Eh, mV (SHE) Per cent EDTA extractable Pb Zn Fe

Forged steel

High chrome

7.8

8.2

72

275

1.7

3.1

0.053

0.071

1.11

0.71

FIG 2 - Mass recovery versus flotation time curves for rougher flotation tests completed

on 100 grams of Rapid Bay galena ground to a P50 of nine microns using forged steel,

high chrome, and ceramic media. The tests were completed in demineralised water

(pH 7), and employed 1000 grams per tonne of sodium ethyl xanthate collector.

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

Flotation time, minutes

Massrecovery,

%

Forged steel High chrome Ceramic


4/18

Samples of galena were collected after grinding in the three different milling environments,

and their surfaces examined using x-ray photoelectron spectroscopy (XPS). The atomic

concentrations of oxygen, lead, iron and sulfur for galena particles ground using the three

different grinding media are listed in Table 3. There is a marked difference in the surface

concentration of iron. That is, it decreased from 16.6 per cent for the forged steel case tobelow the detection limit for ceramic grinding media. The XPS spectral data (Cullinan, 1999)

suggested that the iron present on the surface occurred as oxy-hydroxide species (ie Fe(OH) 3,

FeOOH, Fe2O3, and Fe3O4). When the sample was ion etched, to determine the thickness of

these oxidised iron surface layers, there was no appreciable alteration in the Fe2p spectra or

the atomic concentration, which suggested that these layers were relatively thick. The surface

concentration of these species decreased as the grinding media became less reactive, and

resulted in increasing exposure of the lead sulfide surface. These findings were consistent

with the EDTA extraction data provided in Table 2.

TABLE 2

Pulp chemical measurements for Rapid Bay galena ground with differentgrinding media types (Cullinan, 1999).

Media type pH Eh, mV (SHE) Per cent EDTA extractable Fe

Forged steel

High chrome

Ceramic

5.5

5.5

5.0

300

300

370

1.24

0.16

0.02

TABLE 3Composition, determined via XPS, of the un-etched surfaces of Rapid Bay galena ground

with forged steel, high chrome and ceramic grinding media (Cullinan, 1999).

(Note: The data was normalised to remove the percentage of surface carbon.)

Media type Atomic composition, %

O Pb Fe S

Forged steel

High chrome

Ceramic

53.1

50.0

33.6

15.6

20.6

32.0

16.6

10.2


5/18

METHODOLOGY

In the past, many laboratory studies have not used plant-operating conditions during testing.

This has led to a suspicion of laboratory results. To avoid this complication a new tool, the

Magotteaux Mill, has been developed (Greet, et al; in press).

The Magotteaux Mill (Figure 3) allows the researcher to generate a product in the laboratory

that has nominally the same physical properties (particle size distribution) and pulp chemical

properties (Eh, pH, dissolved oxygen, oxygen demand and EDTA extractable iron) as an

equivalent sample taken from the plant. This is achieved by grinding an appropriate sample to

achieve the particle size distribution of the flotation feed, and manipulating the pulp

chemistry, by purging the system with gas, so that it matches the grinding mill discharge.

FIG 3 - Schematic representation of the Magotteaux Mill.

The experimental strategy adopted to achieve the desired outcomes is completed in three

phases:

Phase 1 - Plant data collection: The collection of plant data is vital to the success of

the test program, for this data forms the basis of the calibration process by definingthe target parameters. This initial step involves:

o The completion of a pulp chemical and EDTA survey of the grinding andadjoining flotation circuit;

o Determination of the oxygen demand at strategic points within the circuit;o The completion of a metallurgical survey;o The collection of conditioned flotation feed for laboratory testing; ando The collection of a bulk sample of the grinding circuit feed for further testing.

It is important to note that the metallurgical survey must include both a down-the-

bank survey of the flotation stage immediately following grinding, and a block surveyof the plant to determine overall metallurgical performance.


6/18

Phase 2 Magotteaux Mill calibration: The data collected in Phase 1 essentiallydescribes the circuit under consideration, and provides targets for the Magotteaux

Mill calibration. The calibration process uses the same grinding media as the

operating plant. The objective of the calibration process is to produce a laboratory

mill discharge which has the same particle size distribution as the conditionedflotation feed, and the pulp chemistry of the plant grinding mill discharge. To achieve

this match involves careful manipulation of the dissolved oxygen, Eh and grinding

time, such that all the measured parameters line up when grinding the bulk sample

collected during the metallurgical survey. This task is not trivial.

Once the Magotteaux Mill is calibrated, oxygen demand and flotation tests are

completed on the ground ore. These data are compared with the results of tests

conducted on the conditioned flotation feed. If they are the same, a match is achieved.

Phase 3 Media testing: With the Magotteaux Mill calibrated, alternative grindingmedia are substituted into the mill for testing. The procedure determined during thecalibration process for the current grinding media is then applied while grinding the

bulk sample employing the alternative grinding media. In this way, it is possible to

measure changes in pulp chemistry and flotation response. The changes observed are

attributed to the variations in grinding media composition, as the only intentional

parameter being changed in the test is the grinding media.

When the flotation results of tests completed on the bulk sample prepared in the Magotteaux

Mill using the current grinding media are correlated with the metallurgical survey of the

plant, it is possible to determine the scale-up between the laboratory and the plant. Knowing

this relationship (scale-up), and the laboratory flotation response of the bulk sample whenground with high chrome grinding media, a prediction of the plant performance can be made

if the alternative grinding media were to be used. Thus, this approach provides a robust

laboratory methodology that firstly provides a link between the laboratory and the plant using

the existing grinding media and secondly investigates changes to the pulp chemistry and

metallurgical outcome when high chrome grinding media is substituted into the grinding

circuit. Further, it provides a means of estimating plant performance should the best high

chrome grinding media be installed in the plant.

If used correctly, this strategy will provide an excellent screening procedure that will yield

valuable information about the best grinding media for the operation. This should focus

pilot plant and plant trials on only the most promising of grinding media types.

CASE STUDIES

A considerable amount of work has been completed in recent times employing the above

approach. Three case studies have been selected to illustrate the success of this methodology.

Lead/zinc

Work was completed at a lead/zinc mine examining the effect of high chrome grinding

media, employed during lead regrinding, on pulp chemistry and flotation performance. Plant

data was collected, and subsequently used to calibrate the Magotteaux Mill using fresh lead


7/18

regrind circuit feed. The target parameters and the results achieved using the laboratory mill

are listed in Table 4.

With the laboratory mill calibrated a series of tests were performed using forged, 15, 21, and

32 per cent chrome grinding media. The pulp chemical data of the Magotteaux Mill

discharge and flotation feed for each grinding media type are listed in Table 5. An Eh-pHdiagram comparing the effect of grinding media on pulp chemistry changes during laboratory

flotation tests is given in Figure 4.

TABLE 4Magotteaux Mill calibration data for a lead/zinc ore: targets and results.

Parameter Plant Range Magotteaux Mill

Match

Size distribution

P80

%-38 microns

Pulp chemistrypH

Eh, mV (SHE)

DO, ppm

% EDTA Fe

NA

94.1

7.85

-141

0.61

0.85

NA

1.5

0.50

30

0.20

0.15

NA

94.9

8.30

-145

0.00

0.87

Yes

Yes

Yes

No

Yes

FIG 4 - Eh-pH curves for laboratory grinding and flotation tests conducted on fresh lead

regrind circuit feed ground with forged, 15, 21 and 32 per cent chrome grinding media.

Table 5 and Figure 4 indicate that changing the grinding media from forged steel to high

chrome resulted in an increase in Eh to more oxidising potentials, and a shift in pH to more

basic levels. The dissolved oxygen content of the pulp also increased, which corresponded to

3

21

32

1

-300

-200

-100

0

100

200

300

400

7.0 7.5 8.0 8.5 9.0 9.5 10.0

pH

Eh,mV(SHE)

Forged Average 15% Cr Average 21% Cr Average 32% Cr Average

1. Mill Discharge

2. Pb Clr Feed

3. Pb Clr Tail


8/18

a decrease in the percentage EDTA extractable iron. There were subtle differences in the pulp

chemistry for each of the high chrome alloys tested, with the pulp becoming more oxidising

as the chrome content is increased. It is postulated that these changes in pulp chemistry are

strongly related to the corrosion mechanisms encountered for each grinding media type.

The Eh-pH curves profiling the grinding and flotation process of the laboratory tests areshown in Figure 4, and provide an excellent indication of where reactions are occurring.

From the Nernst Equation 1 there is a dependence of redox potential on pH:

+=

oducts

tsaco

a

a

nEE

Pr

tanRe10log

059.0(1)

TABLE 5Pulp chemical data for Magotteaux Mill discharge and flotation feed for laboratory tests

conducted on fresh lead regrind circuit feed samples ground with forged, 15, 21 and 32 per

cent chrome grinding media.

Media Magotteaux Mill discharge Flotation feed

pH Eh, mV (SHE) DO, ppm pH Eh, mV (SHE) DO, ppm EDTA Fe

Forged

15% Cr

21% Cr

32% Cr

8.30

8.89

8.82

8.65

-145

189

187

194

0.00

3.41

4.22

1.60

7.92

8.38

8.33

8.14

-182

192

191

190

0.00

2.91

3.38

0.55

0.87

0.44

0.44

0.40

Applying the Nernst equation to water results in a Pourbaix diagram that describes three

domains, separated by lines of equilibria. The upper most of these is the water-oxygen line

(Equation 2), above which water decomposes and oxygen is evolved, and below which wateris stable:

pHpE OO 059.0log015.023.1 22 10 ++= (2)

This can be simplified further (Johnson, 1988; Natarajan and Iwasaki, 1973) for an

oxygenated aqueous solution with no well defined redox couples to (Equation 3):

pHEO 059.09.02 += (3)

What does this mean in terms of chemical reactions that occur in dilute aqueous solutions? In

broad terms, if the changes in Eh and pH result in a line parallel to the water-oxygen line this

means that water equilibria is being maintained. That is, any change in Eh is directly

proportional to a change in pH with a similar relationship to that expressed in Equation 3. If

the changes in Eh and pH result in a line that is perpendicular to the water-oxygen line then

the evidence suggests that oxidative reactions are occurring.

In the case of the forged steel grinding media the Eh-pH curve (Figure 4) is perpendicular to

the water-oxygen line, and indicating that this regime is very reactive. It is assumed that the

dominant reactions occurring are the corrosion of the forged grinding media (as evident by

the higher percentage EDTA extractable iron value), and the oxidation of sulfide minerals,probably pyrite (due to the reduction in pH to more acid levels). The Eh-pH curves for the


9/18

three high chrome alloys tested are approximately parallel to the water-oxygen line,

suggesting that these systems are comparatively inert with few oxidative reactions occurring.

Standard laboratory rougher, rate flotation tests were completed on fresh lead regrind circuit

feed ground in the Magotteaux Mill with forged, 15, 21, and 32 per cent chrome grinding

media. The lead grade/recovery curves for theses tests are provided in Figure 5. The lead anddiluent recovery, at 85 per cent lead recovery, are given in Table 6.

The pulp chemical changes noted above had a positive impact on galena flotation response.

That is, at 85 per cent lead recovery, there is an increase of 1.5 per cent lead grade between

forged and 21 per cent chrome media. The increased concentrate grade can be attributed to

improved selectivity for galena against chalcopyrite and non-sulfide gangue (Table 6). It is also

possible to realise an improvement in lead recovery through the use of high chrome grinding

media. That is, at 50 per cent lead grade, there is an increase in lead recovery of over four per

cent when using 21 per cent chrome grinding media compared to forged steel (Figure 5).

FIG 5 - Lead grade/recovery curves for laboratory flotation tests conducted on fresh

lead regrind circuit feed samples ground with forged, 15, 21 and 32 per cent grinding

media.

TABLE 6Lead grade and diluent recovery, at 85 per cent lead recovery, for laboratory flotation tests

completed on fresh lead regrind circuit feed samples ground with forged, 15, 21 and 32 per

cent grinding media.

Media Pb grade, % Diluent recovery, %

Ag Cu Zn IS NSG

Forged

15% Cr

21% Cr32% Cr

51.55

48.95

53.0751.63

83.20

84.17

85.3685.88

74.28

81.32

74.3282.89

41.44

48.27

43.9243.04

32.54

53.25

32.4535.52

28.25

25.54

27.6214.66

30.0

35.0

40.0

45.0

50.0

55.0

60.0

50.0 60.0 70.0 80.0 90.0 100.0

Recovery - Pb(%)

Grade-Pb(%)

Forged 3

15% Cr 1

21% Cr 2

32% Cr 2


10/18

These data suggest that a change from forged to high chrome grinding media changed the

pulp chemistry of the system such that the Eh was shifted to more oxidising potential, the

dissolved oxygen content of the pulp increased and the levels of EDTA extractable iron were

decreased significantly. These changes had a positive impact on galena flotation behaviour,

with a positive shift in the lead grade/recovery curve, particularly for the 21 per cent chrome

alloy.

Copper

A similar study was completed at a copper/gold mine examining the effect of high chrome

grinding media, employed during primary grinding, on pulp chemistry and flotation

performance. Plant data was collected, and subsequently used to calibrate the Magotteaux

Mill using SAG mill feed. The target parameters and the results achieved using the

laboratory mill are listed in Table 7.

TABLE 7

Magotteaux Mill calibration data for a copper ore: targets and results.


Match

Size distribution

P80

%-38 microns

Pulp chemistry

pH

Eh, mV (SHE)

DO, ppm

% EDTA Fe

195

33

9.5

-75

0.5

1.2

5

2

0.2

20

0.5

0.1

195

31

9.3

-65

0.3

1.3

Yes

Yes

Yes

Yes

Yes

Yes

With the laboratory mill calibrated, a series of tests were performed using forged, 15, 21, and

32 per cent chrome grinding media. The pulp chemical data of the Magotteaux Mill

discharge and flotation feed for each grinding media type are listed in Table 8. An Eh-pH

diagram comparing the effect of grinding media on pulp chemistry changes during laboratory



conducted on SAG mill feed samples ground with forged, 15, 21 and 32 per cent chrome

grinding media.



Forged

15% Cr

21% Cr

32% Cr

9.3

9.0

9.1

9.4

-65

174

205

240

0.2

1.2

1.2

1.7

9.5

9.7

9.5

9.4

138

195

225

260

5.0

6.6

6.5

6.1

1.3

0.2

0.1

0.1


chrome resulted in an increase in Eh to more oxidising potentials, and the pH was

comparatively stable. The dissolved oxygen content of the pulp also increased, whichcorresponded to a decrease in the percentage EDTA extractable iron. There were subtle


11/18

differences in the pulp chemistry for each of the high chrome alloys tested, with the pulp

becoming more oxidising as the chrome content is increased.

Again, the forged steel grinding media Eh-pH curve (Figure 6) is perpendicular to the water-

oxygen line, suggesting that this system is very reactive. It is assumed that the dominant

reactions occurring are the corrosion of the forged grinding media (as evident by the higherpercentage EDTA extractable iron value), and the oxidation of sulfide minerals. The Eh-pH

curves for the three high chrome alloys tested are approximately parallel to the water-oxygen

line, suggesting that these systems are comparatively inert with few oxidative reactions

occurring.

FIG 6 - Eh-pH curves for laboratory grinding and flotation tests conducted on SAG mill

feed ground with forged, 15, 21 and 32 per cent chrome grinding media.

Standard laboratory rougher, rate flotation tests were completed on SAG mill feed ground in

the Magotteaux Mill with forged, 15, 21, and 32 per cent chrome grinding media. The

copper grade/recovery curves for theses tests are provided in Figure 7. The copper and gold

grades and gold recovery, at 90 per cent copper recovery, are given in Table 9.

The pulp chemical changes observed had a positive impact on both copper and gold flotation

response. That is, at 90 per cent copper recovery, there is an increase of 1.5 per cent copper

grade between forged and 21 per cent chrome media. The increased copper concentrate grade

can be attributed to improved selectivity for chalcopyrite against iron sulfides and non-sulfide

gangue. The change to high chrome grinding media also had a marked positive influence on

gold recovery to copper concentrate (Table 9).

-100

-50

0

50

100

150

200

250

300

7.0 7.5 8.0 8.5 9.0 9.5 10.0

pH

Eh,mV(SHE)

Forged 15% Cr 21% Cr 32% Cr

1

2

3

1

1

1

2

2

2

3

3

1. Mill discharge

2. Cu rougher feed

3. Cu rougher tailing


12/18

TABLE 9Copper and gold grades, and gold recovery, at 90 per cent copper recovery, for laboratory

flotation tests completed on SAG mill feed samples ground with forged, 15, 21 and 32 per


Media Grade Au recovery, %Cu, % Au, ppm

Forged

15% Cr

21% Cr

32% Cr

23.1

24.5

24.6

15.7

41.5

42.6

41.5

29.4

80.5

87.0

88.6

90.0

However, it is worth noting that the flotation performance of the 32 per cent chrome alloy

was inferior to the other grinding media types tested. It is postulated that the poor

performance of this alloy may be related to the system being either over oxidised (ie too high

an Eh), or over reagentised. It is important to realise that increasing the chrome content of the

media further than the optimum grade does not automatically translate into better metallurgy.

Such a course of action may in fact result in increased levels of oxygen within the pulp,which may oxidise the sulfide minerals and retard their flotation. The choice of alloy is very

much driven by the mineralogy of the system under investigation. Alternatively, it must be

recognised that by using high chrome grinding media, the surface chemistry of the particles

of interest have been dramatically changed. Therefore, the reagent regime employed when

grinding with forged media may no longer be adequate. At the very least, this means a

reduction in collector addition. However, it is more likely that another collector may be more

appropriate.

FIG 7 - Copper grade/recovery curves for laboratory flotation tests conducted on

SAG mill feed samples ground with forged, 15, 21 and 32 per cent grinding media.

0.00

5.00

10.00

15.00

20.00

25.00

30.00

40.00 50.00 60.00 70.00 80.00 90.00 100.00

Recovery - Cu(%)

Grade-Cu(%)

Forged pH 9.5 - Average 15% Cr pH 9.5 - Average 21% Cr pH 9.5 - Average 32% Cr pH 9.5 - Average


13/18

Again, these data suggest that a change from forged to high chrome grinding media changed

the pulp chemistry of the system such that the Eh was shifted to less reducing potential, the

dissolved oxygen content of the pulp increased and the levels of EDTA extractable iron were

decreased significantly. These changes had a positive impact on both copper and gold

flotation behaviour, with a positive shift in the copper grade/recovery curve, particularly for

the 21 per cent chrome alloy.

Refractory gold

A study examining the effect of high chrome grinding media used in primary grinding on pulp

chemistry and flotation performance was completed at a refractory gold operation. Plant data

was collected, and subsequently used to calibrate the Magotteaux Mill using SAG mill feed.

The target parameters and the results achieved using the laboratory mill are listed in Table 10.

TABLE 10Magotteaux Mill calibration data for a refractory gold ore: targets and results.


MatchSize distribution

P80

%-38 microns

Pulp chemistry

pH

Eh, mV (SHE)

DO, ppm

% EDTA Fe

204

36

7.0

-215

0.0

1.3

5

2

0.2

20

0.5

0.1

201

35

7.1

-235

0.0

2.3

Yes

Yes

Yes

Yes

Yes

No

Once the Magotteaux Mill was calibrated a series of tests were performed using forged, 15,

21, and 32 per cent chrome grinding media. The pulp chemical data of the Magotteaux Mill

discharge and flotation feed for each grinding media type are listed in Table 11. An Eh-pH

diagram comparing the effect of grinding media on pulp chemistry changes during laboratory



conducted on SAG mill feed samples ground with forged, 15, 21 and 32 per cent chrome

grinding media.



Forged

15% Cr

21% Cr

32% Cr

7.1

7.0

7.0

7.0

-235

70

120

105

0.0

0.5

0.5

0.2

6.8

6.6

6.6

6.5

-28

145

140

145

0.1

0.7

0.8

1.3

2.3

1.3

1.2

1.2


chrome resulted in an increase in Eh to more oxidising potentials, and a marginal shift in pH

to more acidic levels. The dissolved oxygen content of the pulp also increased, which

corresponded to a decrease in the percentage EDTA extractable iron. There were subtle

differences in the pulp chemistry for each of the high chrome alloys tested, with the pulp

becoming more oxidising as the chrome content is increased.


14/18

In the case of the forged steel grinding media the Eh-pH curve (Figure 8) was again

perpendicular to the water-oxygen line, and indicating that this regime is very reactive. It is

assumed that the dominant reactions occurring are the corrosion of the forged grinding media

(as evident by the higher percentage EDTA extractable iron value), and the oxidation of

sulfide minerals, probably pyrite (due to the reduction in pH to more acid levels). The Eh-pH

curves for the three high chrome alloys tested are approximately parallel to the water-oxygenline, suggesting that these systems are comparatively inert with few oxidative reactions

occurring.

Standard laboratory rougher, rate flotation tests were completed on SAG mill feed ground in

the Magotteaux Mill with forged, 15, 21, and 32 per cent chrome grinding media. The gold

grade/recovery curves for theses tests are displayed in Figure 9. The gold and sulfur grades,

and diluent recovery, at 70 per cent gold recovery, are given in Table 12.

FIG 8 - Eh-pH curves for laboratory grinding and flotation tests conducted on SAG mill

feed ground with forged, 15, 21 and 32 per cent chrome grinding media.

The pulp chemical changes noted above had a positive impact on gold flotation response.

That is, at 70 per cent gold recovery, there is an increase of 29 grams per tonne gold grade

between forged and 15 per cent chrome media. The increased gold concentrate grade can be

attributed to improved recovery of sulfur, and better selectivity for pyrite against non-sulfide

gangue (Table 12). It is also possible to realise an improvement in gold recovery through the

use of high chrome grinding media. That is, at 200 grams per tonne gold grade, there is an

increase in gold recovery of over 11 per cent when using 15 per cent chrome grinding media

compared to forged steel (Figure 9).

-300

-200

-100

0

100

200

300

5.0 5.5 6.0 6.5 7.0 7.5 8.0

pH

Eh,mV(SHE)


1

2

3

1

1

1

2

3

1. Mill discharge

2. Rougher feed

3. Rougher tailing


15/18

The 32 per cent chrome alloy again showed inferior flotation performance compared to the 15

and 21 per cent chrome grinding media. The arguments detailed in the copper ore case study

apply equally to this ore type.

Once more the data suggest that a change from forged to high chrome grinding media

changed the pulp chemistry of the system such that the Eh was shifted to less reducing potential, the dissolved oxygen content of the pulp increased and the levels of EDTA

extractable iron were decreased significantly. These changes had a positive impact on gold

flotation behaviour, with a positive shift in the gold grade/recovery curve, particularly for the

15 per cent chrome alloy.

TABLE 12Gold and sulfur grades, and diluent recoveries, at 70 per cent gold recovery, for laboratory

flotation tests completed on SAG mill feed samples ground with forged, 15, 21 and 32 per


Media Grade Diluent recovery, %Au, ppm S, % S NSG

Forged

15% Cr

21% Cr

32% Cr

210

239

213

144

22.46

27.82

26.15

16.07

50.33

55.03

53.53

69.97

0.89

0.65

0.77

1.97

FIG 9 - Gold grade/recovery curves for laboratory flotation tests conducted on SAG mill

feed samples ground with forged, 15, 21 and 32 per cent grinding media.

Other sulfide minerals

Similar work has been completed on ores containing platinum group metals and nickel. All

demonstrate similar trends to the data presented above.

0.00

50.00

100.00

150.00

200.00

250.00

300.00

20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00

Recovery - Au(ppm)

Grade-Au(ppm)



16/18

DISCUSSION

The data clearly demonstrate that for the same ore, treated in the same way, changing the

grinding media had a significant impact on the pulp chemistry of the system. That is, the Eh

shifted to more oxidising potentials, the oxygen content of the pulp increased and the level of

iron species in the pulp decreased. These changes in pulp chemistry had a positive impact onthe flotation response of the valuable mineral in each of the case studies.

The reason for this positive outcome is directly related to the reactivity of the grinding media,

and how the corrosion products of the grinding media interact with the sulfide minerals

present in the ore. Forged steel grinding media, which has a rest potential considerably lower

than the high chrome grinding media and sulfide minerals, behaves as the anode and

corrodes, releasing iron oxy-hydroxy species into the pulp. As discussed by Johnson (2002),

these species do alter the flotation response of sulfide minerals. Changing the grinding media

to something more electrochemically inert minimises media corrosion, and subsequently

reduces the levels of iron oxy-hydroxy contaminating the system. This reduction in

contamination improves flotation behaviour as illustrated in the case studies above.

Thus, by changing the grinding media it is possible to condition the pulp during grinding to

achieve the best pulp chemistry for flotation. Importantly, this work remains applicable

should high pressure grinding rolls (HPGR) supersede SAG milling, as the foremost

candidate for this approach is the primary ball mill following either of these unit processes.

CONCLUSIONS

In all instances, changing from forged steel to high chrome grinding media had an impact on

the pulp chemistry by:

Shifting the Eh to more oxidising potentials,

Increasing the level of oxygen in the pulp, and

Reducing the amount of EDTA extractable iron in the pulp.

These phenomena had a positive impact on the flotation behaviour of the valuable minerals,

although it must be noted that the highest chrome content of the grinding media does not

always give rise to the best metallurgy. The optimum alloy for an ore is dependant on the

mineralogy of the system.

FURTHER WORK

Based on results of this nature, a number of plant trials investigating the effect of high

chrome grinding media on pulp chemistry and flotation response are currently underway. It is

Magotteauxs intention to monitor the progress of these trials both in terms of pulp chemistry

and metallurgical performance. With these data, it is anticipated that a more thorough

evaluation of the economic performance of high chrome grinding media can be obtained, that

is, the impact on grinding media wear, metallurgical performance and reagent consumption.


17/18

ACKNOWLEDGEMENT

The authors wish to thank Magotteaux for granting permission to publish this paper.

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Documents

Grinding the Primary Conditioner