Grinding Media — Its Effect on Pulp Chemistry and Flotation
Behaviour — Fact or Fiction?
C J Greet1, J Kinal2 and P Steinier2
ABSTRACT
A considerable volume of work exists in the literature examining the
electrochemical interactions between grinding media and sulfide
minerals. Broadly, most sulfide minerals are more noble than forged steel
grinding media used during comminution, therefore a galvanic couple
between the media and the sulfide mineral(s) exists, which increases the
corrosion rate of the 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.
This paper explores the science behind these corrosion reactions, and
provides an explanation of their impact on the pulp chemistry of sulfide
mineral slurries and subsequent flotation behaviour.
A number of laboratory and plant case studies are used to illustrate the
benefits of changing to a more inert grinding media.
INTRODUCTION
Total wear in grinding mills is made up of abrasion, corrosion
and impact. However, it is practically impossible to attribute
relative proportions for each towards the overall total wear under
wet grinding conditions. Work completed by Iwasaki et al (1985)
demonstrated that media wear rates were markedly higher for
wet grinding than dry grinding, however the particle size
reduction efficiency of dry grinding was considerably lower than
that observed for wet grinding. The difference in wear rates was
attributed to the effects of corrosion. They also noted that wear
attributable to corrosion increased significantly with increased
oxygen levels in the pulp, and/or in the presence of electroactive
minerals such as pyrrhotite and magnetite particularly for forged
steel grinding media.
The implications of media corrosion on sulfide mineral
flotation have been examined extensively within the literature
(for example, Iwasaki et al, 1983; Adam et al, 1984; Natarajan
and Iwasaki, 1984; Yelloji Rao and Natarajan, 1989a; Yelloji Rao
1.
2.
MAusIMM, Magotteaux Australia Pty Ltd, Suite 4, 83 Havelock
Street, West Perth WA 6005.
Magotteaux Australia Pty Ltd, Suite 4, 83 Havelock Street, West
Perth WA 6005.
and Natarajan, 1989b). Essentially, when sulfide minerals are
brought into contact with ferrous grinding media galvanic
interactions occur (Figure 1). Generally, the grinding media acts
as an anode, as it normally has the lowest rest potential of all the
components within the system (Table 1), and undergo oxidation,
while the sulfide minerals act as cathodes and undergo oxygen
reduction. Typical reactions are:
At the cathode (sulfide mineral): 12 O2 + H2O + 2e- → 2OH-
(1)
At the anode (grinding media): Fe → Fe2+ + 2e-
(2)
Invariably iron oxy-hydroxy species form on the surfaces of
the sulfide minerals, produced by the reaction of ferrous iron
(Fe2+) from Equation 2 with hydroxyl (OH-) ions derived from
the reduction of oxygen by Equation 1. It is these resultant
hydrophilic surface coatings which may affect the floatability of
the sulfide minerals (Kocabag and Smith, 1985; Johnson, 2002).
Therefore, preventing the formation of these iron hydroxide
species should have a positive impact on the flotation response of
sulfide minerals.
TABLE 1
Rest potentials of common sulfde minerals and of grinding media
in distilled water at near neutral pH (Cheng et al, 1993).
Mineral/media
Rest potential, V (SHE)
N2
Air
O2
Pyrite
0.389
0.392
0.395
Arsenopyrite
0.277
0.303
0.323
Pyrrhotite
0.276
0.285
0.318
Chalcopyrite
0.290
0.355
0.371
Cobaltite
0.200
0.275
0.303
Nickel arsenide
0.173
0.175
0.201
Galena
0.142
0.172
0.218
Mild steel
-0.515
-0.335
-0.175
Sulfide mineral/cathode
Oxygen reduction: 2e - + ½O2 + H2O <-> 2OH-aq
Fe2+
OH-
O2/H2O
e-
Grinding media corrosion: Fes -> Fe(1-x)s + xFe2+aq + x2e-
Forged grinding media/anode
FIG 1 - A schematic representation of the electrochemical cell when ferrous grinding media is in contact with sulfide minerals,
within an aqueous medium.
Centenary of Flotation Symposium
Brisbane, QLD, 6 - 9 June 2005
967
C J GREET, J KINAL and P STEINIER
Several studies have demonstrated that the composition of the
grinding media has a significant impact on its corrosion
resistance. Work completed by Isaacson (1989) clearly
demonstrated that in the presence of chalcopyrite the corrosion
rate of carbon steel markedly increased. Further, the corrosion
rate became more pronounced when the chalcopyrite was ground
in the presence of oxygen (Isaacson, 1989; Cheng et al, 1993).
However, by changing the grinding media to a high chrome
white iron, the corrosion rate under the same test conditions was
dramatically reduced. Presumably a reduction in the corrosion
rate, by a change in grinding media alloy, results in a reduction in
corrosion products, which should have a positive impact on
sulfide mineral flotation.
Forssberg’s group, at the Technical University of Luleå, have
investigated the influence of grinding method on sulfide mineral
flotation at both the laboratory and pilot plant scale. These
studies (Forssberg et al, 1988; Forssberg et al, 1993; Yuan et al,
1996) have concentrated on the differences in pulp chemistry and
flotation response of complex sulfide ores prepared by grinding
in either an iron free environment (ie autogenous grinding), or
with steel grinding media. Their data indicated that the
autogenous grinding environment, in both the laboratory and
pilot plant resulted in significant differences in the pulp
chemistry of the system. That is, the autogenous grinding
produced pulps that had less reducing Eh values, and contained
higher dissolved oxygen concentrations (Table 2). Further, the
flotation performance was enhanced. In the laboratory study
completed on Langsele ore the copper concentrate grade was
improved as the selectivity for chalcopyrite against pyrite was
enhanced when grinding was completed in an iron free
environment (Table 2). The pilot plant study completed on
Viscaria ore demonstrated that autogenous grinding produced a
significant improvement in copper recovery compared to
grinding with steel (Table 2).
Similar behaviour was noted for the pilot plant testing of
complex copper/lead/zinc ore from Renstrom (Forssberg et al,
1993). It should be noted that Forssberg’s work is not an isolated
occurrence. Work completed by Iwasaki et al (1985) on a
copper-nickel sulfide ore demonstrated that improved
metallurgical response was possible if autogenous grinding was
employed compared to grinding with steel. And, similar
behaviour was observed at the industrial level by Griffin et al
(1993) in the copper concentrator at Mount Isa Mines. The
improved flotation performance was attributed to changes in the
pulp chemistry of the system.
Autogenous grinding in many instances is not an attractive
proposition in terms of particle breakage, therefore it is
necessary to use ferrous based grinding media. However, as
Isaacson (1989) indicated, high chrome white iron alloys provide
improved wear resistance, and are a viable alternative to forged
steel grinding media. In terms of metallurgical response it is
expected that these alloys will provide similar results to those
observed for autogenous grinding.
LABORATORY STUDY
A laboratory study was completed on a copper/gold ore
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® (Greet et al, 2004) using SAG
mill feed. In this case it should be noted that the plant was
already operating with high chrome grinding media, so the
laboratory mill was calibrated with the 21 per cent chrome alloy.
The target parameters and the results achieved using the
laboratory mill are listed in Table 3.
TABLE 3
Magotteaux Mill® calibration data for a copper ore: targets and
results.
Parameter
Plant
Range
Magotteaux
Mill®
Match
P80
190
±5
190
Yes
% -38 microns
40
±2
38
Yes
pH
10.3
±0.2
10.5
Yes
Eh, mV (SHE)
160
±20
150
Yes
Size distribution
Pulp chemistry
DO (ppm)
0.5
±0.5
0.0
Yes
% EDTA Fe
2.4
±0.1
1.4
No
With the laboratory mill calibrated, a series of tests were
performed using forged, 15, 21, and 30 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 4. An Eh-pH diagram comparing the effect of
grinding media on pulp chemistry changes during laboratory
flotation tests is given in Figure 2.
Table 4 and Figure 2 indicate that changing the grinding media
from forged steel to high chrome resulted in an increase in Eh to
more oxidising potentials, and the pH remained approximately
the same in all instances. The dissolved oxygen content of the
pulp was zero in the mill discharge for all cases. The oxygen
demand (MagO2®) rate constant (Greet et al, 2004) decreased in
the flotation feed as the chrome content of the grinding media
increased (Table 4), which corresponded with 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 was increased.
†
Ethylenediaminetetraacetic acid disodium salt (EDTA) is a chelating
agent used to leach oxidation products from mineral slurries.
TABLE 2
Variation in pulp chemistry and metallurgy for Langsele and Viscaria ores ground in autogenous and steel mills (Forssberg et al, 1988).
Media
Pulp chemistry
pH
Cu metallurgy
Eh, mV (SHE)
DO (ppm)
Grade (%)
Recovery (%)
Langsele
Autogenous
7.8
250
2.0
5.07
75.3
Steel
8.2
-210
0.0
1.72
76.9
Autogenous
9.6
155
1.4
10.1
90.8
Steel
9.7
26
0.3
11.3
79.5
Viscaria
968
Brisbane, QLD, 6 - 9 June 2005
Centenary of Flotation Symposium
GRINDING MEDIA — ITS EFFECT ON PULP CHEMISTRY AND FLOTATION BEHAVIOUR
TABLE 4
Pulp chemical data for Magotteaux Mill® discharge and flotation feed for laboratory tests conducted on SAG mill feed samples ground with
forged, 15, 21 and 30 per cent chrome grinding media.
Magotteaux Mill® discharge
Media
Flotation feed
pH
Eh, mV (SHE)
DO (ppm)
pH
Eh, mV (SHE)
MagO2 k (min-1)
EDTA Fe
Forged
10.4
-225
0.0
11.5
210
0.151
2.6
15% Cr
10.5
25
0.0
11.5
220
0.035
1.1
21% Cr
10.5
150
0.0
11.5
225
0.037
1.1
30% Cr
10.7
160
0.0
11.5
220
0.034
1.1
300
3
Eh, mV )SHE)
200
2
1
100
0
1
-100
1. Mill discharge;
1
-200 2. Flotation feed; and
3. Rougher tailing.
-300
9
9.5
10
10.5
11
11.5
12
pH
Forged
15% Cr
21% Cr
30% Cr
media (as evident by the higher percentage EDTA extractable
iron value), and the oxidation of sulfide minerals. During
flotation (ie point two to three) the Eh-pH curve was parallel to
the water-oxygen line suggesting that any changes in Eh and pH
were related to the maintenance of water equilibria. The Eh-pH
curves for the three high chrome alloys tested become more
parallel to the water-oxygen line as the chrome content increases,
suggesting that these systems are comparatively inert with fewer
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 30 per cent chrome grinding media. The
copper grade/recovery curves for theses tests are provided in
Figure 3. The copper grade and diluent recoveries, at 80 per cent
copper recovery, are given in Table 5.
FIG 2 - Eh-pH curves for laboratory grinding and flotation tests
conducted on SAG mill feed ground with forged, 15, 21 and
30 per cent chrome grinding media.
23.0
21.0
The Eh-pH curves (Figure 2) and provide an excellent
indication of where reactions are occurring. From the Nernst
Equation 3 there is a dependence of redox potential on pH:
E = Eo +
a

0.059
log 10  Reactants 
 aProducts 
n
(3)
(4)
This can be simplified further (Johnson, 1988; Natarajan and
Iwasaki, 1973) for an oxygenated aqueous solution with no well
defined redox couples to (Equation 5):
E O 2 = +0.9 − 0.059 pH
(5)
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 5. 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.
The forged steel grinding media Eh-pH curve (Figure 2) is
perpendicular to the water-oxygen line from point one to point
two (ie from the mill discharge to the flotation feed), suggesting
that this system is very reactive. It is assumed that the dominant
reactions occurring are the corrosion of the forged grinding
Centenary of Flotation Symposium
17.0
15.0
13.0
11.0
9.0
7.0
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 4), above which water decomposes and oxygen is
evolved, and below which water is stable:
E O 2 = +1.23 + 0.015 log 10 PO 2 − 0.059 pH
Cu grade, %
19.0
5.0
60.0
65.0
70.0
75.0
80.0
85.0
90.0
Cu recovery, %
Forged
15% Cr
21% Cr
30% Cr
FIG 3 - Copper grade/recovery curves for laboratory flotation tests
conducted on SAG mill feed samples ground with forged, 15, 21
and 30 per cent grinding media.
TABLE 5
Copper grade and diluent recoveries, at 80 per cent copper
recovery, for laboratory flotation tests completed on SAG mill feed
samples ground with forged, 15, 21 and 30 per cent grinding
media.
Media
Cu grade
(%)
Diluent recovery (%)
Au
IS
Forged
12.5
54.0
15.0
1.8
15% Cr
20.0
53.0
8.0
1.8
21% Cr
18.0
55.0
8.0
1.8
30% Cr
17.5
57.0
8.0
1.8
NSG
The pulp chemical changes observed had a positive impact on
both copper and gold flotation response. That is, at 80 per cent
copper recovery, there is an increase of 7.5 per cent copper grade
between forged and 15 per cent chrome media. The increased
copper concentrate grade can be attributed to improved
Brisbane, QLD, 6 - 9 June 2005
969
C J GREET, J KINAL and P STEINIER
200
0.20
150
0.18
100
0.16
0.14
50
0.12
0
0.10
-50
0.08
-100
0.06
-150
0.04
-200
0.02
-250
0.0
0.5
1.0
1.5
2.0
2.5
Oxygen demand k, min-1
Eh, mV (SHE)
selectivity for chalcopyrite against iron sulfides. The change to
high chrome grinding media also had a subtle positive influence
on gold recovery to copper concentrate (Table 5).
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 oxygen
demand of the pulp decreased and the levels of EDTA extractable
iron decreased significantly. In fact, there appears to be a
reasonable correlation between these three parameters (Figure 4).
That is, a high EDTA extractable iron value corresponds to a
higher oxygen demand and lower Eh reading. This circumstance
usually exists when forged steel is used as the grinding media,
and suggests that media corrosion is a dominant feature of the
chemistry of the system. Changing to a corrosion resistant alloy
significantly reduced the EDTA extractable iron (a measure of
media corrosion), the oxygen demand and increased the Eh to
less reducing pulp potentials. These changes had a positive
impact on copper and gold flotation behaviour, with a positive
shift in the copper grade/recovery curve, particularly for the
15 per cent chrome alloy. The relationship between pulp
chemistry and metallurgy is illustrated in Figure 5, where a
change to a more inert media saw an increase in copper
concentrate grade, at 80 per cent copper recovery, as media
corrosion decreased (ie a decrease in EDTA extractable iron).
Similar observations have been made on numerous ores at the
laboratory scale (Greet and Steinier, 2004), with the same trends
apparent in the majority of cases.
0.00
3.0
EDTA extractable Fe, %
Eh
PLANT STUDY
Based on the results of a marked ball test, which indicated that
media consumption cold be halved by changing to a suitable high
chrome alloy, the operation decided to conduct a plant trial in its
primary ball mills. While the plant trial produced positive results
based on media wear, the impact on metallurgical response was
examined much later in the piece. The operation provided their
shift data from January 1997 to March 2004 so that a statistical
analysis of the metallurgical performance could be completed.
Before giving details of the analysis it is worth noting that the
plant trial was completed in stages. Further, changing from forged
steel to high chrome was achieved by ‘topping up the mill’ with
the new alloy. Therefore, the lag time between commencing to
charge high chrome and obtaining fully converted seasoned charge
took upwards of nine months. Thus, the time required to complete
a trial of this nature was considerable, and brought with it
significant challenges when analysing the data. However, as the
plant operates two parallel lines some of these issues were
alleviated because a circuit configuration of this type made it
possible to compare the performance of one line against the other,
on nominally the same feed stock.
The trial commenced with high chrome being loaded in to one
ball mill in one line. Once the performance of the media could be
assessed, in terms of wear, the second mill in this line was
converted to high chrome. This process took approximately two
years. Having successfully converted one line, the second line
was changed, which took a further year to complete. So, of the
seven years worth of data three years have been discarded as
these years represent transition periods when the mills were
being converted to high chrome grinding media.
Hence, caution is urged when interpreting the data, as during
the trial period many other unrelated trials took place, and
conditions (feed, equipment, operating practice, etc) would
almost certainly have changed in ways which would be difficult
and probably impossible to bring into the analysis. Also, the fact
that a particular condition was correlated with performance
improvements does not necessarily imply a causal relationship.
The conclusions should therefore be seen with these important
caveats in mind.
The following conclusions are based on statistical tests of
significance:
1.
Oxygen demand
FIG 4 - The correlation between Eh, oxygen demand and EDTA
extractable iron for a copper/gold ore ground with forged, 15, 21,
and 30 per cent chrome grinding media.
There was no difference in metallurgical performance
between the two process lines when operated with forged
steel grinding media (Table 6).
TABLE 6
Statistical comparison between Lines 1 and 2 when operated with
forged steel grinding media.
Cu grade at 80% Cu recovery, %
21.0
Quantity
19.0
Line 2
STD
Mean
STD
17.0
Cu feed grade (%)
0.816
0.189
0.861
0.242
15.0
Au feed grade (ppm)
0.715
0.169
0.760
0.247
13.0
Cu recovery (%)
84.62
4.46
84.51
5.00
11.0
Au recovery (%)
71.83
5.13
71.97
5.96
9.0
Cu concentrate grade (%)
34.74
3.60
34.43
2.98
7.0
Au concentrate grade (ppm)
26.57
6.99
26.64
7.26
5.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
EDTA extractable Fe, %
FIG 5 - The relationship between copper concentrate grade, at
80 per cent copper recovery, and EDTA extractable iron for a
copper/gold ore ground with forged, 15, 21, and 30 per cent
chrome grinding media.
970
Line 1
Mean
2.
With Line 2 converted to high chrome grinding media it
was found that the mean copper and gold recoveries were
1.7 and 2.8 per cent, respectively, better than those reported
for Line 1 (Table 7). Subtle increases in copper and gold
concentrate grades were also observed. Assuming that no
other effects were in play, this could be taken as evidence
that the high chrome media produces an improvement in
metallurgical performance.
Brisbane, QLD, 6 - 9 June 2005
Centenary of Flotation Symposium
GRINDING MEDIA — ITS EFFECT ON PULP CHEMISTRY AND FLOTATION BEHAVIOUR
TABLE 7
Statistical comparison between Lines 1 (forged steel) and
2 (high chrome).
Quantity
Line 1
Line 2
Mean
STD
Mean
STD
Cu feed grade (%)
0.931
0.272
0.936
0.289
Au feed grade (ppm)
0.915
0.336
0.917
0.329
Cu recovery (%)
76.15
9.93
77.87
9.09
Au recovery (%)
63.29
9.14
66.11
8.33
Cu concentrate grade (%)
29.46
2.89
29.76
2.90
Au concentrate grade (ppm)
24.08
6.17
25.00
6.32
Media wear:
Coefficient of superiority
3.
1.0
2.0
Once both lines were converted to high chrome it was hoped
that the analysis would show that there was no difference in
metallurgical performance between the two process lines, as
was observed when the plant was operating with forged
steel. Unfortunately, differences were observed, making it
difficult to draw any firm conclusions. However, such a
conclusion does highlight the inherent dangers of conducting
plant trials over such long time frames.
During the conversion from forged steel to high chrome
grinding media there is anecdotal evidence to suggest that the
collector consumption was also reduced by as much as 30 per
cent. Nevertheless it is again difficult to attribute this reduction
solely to the use of high chrome grinding media. It should be
noted however that it is not uncommon to see a reduction in
collector consumption when comparing forged steel and high
chrome grinding media in the laboratory.
A laboratory study on a copper/gold ore demonstrated in the
Magotteaux Mill® that changing from forged steel to high
chrome white iron grinding media resulted in a change in the
pulp chemistry of the system. That is, the Eh of the system
shifted to less reducing potentials, the oxygen demand of the
pulp decreased, and the amount of EDTA extractable iron
decreased. These pulp chemical changes were accompanied by a
positive improvement in the copper grade/recovery curve.
A plant trial completed at the same mine provided data that
suggested high chrome grinding media had a positive impact on
copper and gold metallurgy. If one third of the metallurgical
improvement was attributed to high chrome grinding media it is
estimated that the increase in revenue through improved copper
and gold recoveries is worth US$ 7 580 000 per annum. This is
significantly greater than the cost savings ascribed to a reduction
in media costs through reduced consumption.
THE FUTURE
While good technical work is being completed at laboratory,
pilot plant and plant scale to demonstrate the advantages of using
high chrome grinding media on metallurgical performance a
valid methodology of scaling the laboratory results to the plant is
required. However, this represents only a small part of the
challenge that lies ahead. We also need to educate by providing
technically valid information about the potential benefits to the
flotation process of employing high chrome grinding media
during comminution.
ACKNOWLEDGEMENTS
The authors wish to thank Magotteaux for giving permission to
publish this paper.
REFERENCES
ECONOMIC EVALUATION
In this case the decision to charge the primary ball mills with
high chrome grinding media was based solely on reduced media
consumption. However, it is believed that the change has also
delivered significant metallurgical benefits and a reduction in
collector consumption. When dollar values were assigned to each
of these operational cost savings and revenue improvements the
following summary was generated:
• wear component (ie 50 per cent reduction in media
consumption (or a coefficient of superiority of 2.0, see Table
7): US$ 450 000 per annum;
• metallurgy component (if one third of the copper and gold
recovery increases were attributed to installation of high
chrome grinding media): US$ 7 580 000 per annum;
• reagent component (a dollar value was not assigned because
the data set is incomplete); and
• total cost saving/revenue increase of: US$ 8 030 000 per
annum.
It is abundantly clear that the improvement in metallurgical
performance dominates the financial equation.
CONCLUSIONS
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effect of grinding environment on sulfide flotation performance.
It is generally agreed that grinding with forged steel has a
deleterious affect on the flotation of sulfide minerals through the
deposition of media corrosion products (iron hydroxides) on to
their surfaces. These hydrophilic species can depress the
flotation properties of valuable sulfide minerals.
Centenary of Flotation Symposium
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Brisbane, QLD, 6 - 9 June 2005
Centenary of Flotation Symposium