Mill Ops Final Draft r2

Mill Operators’ Conference 2018
Paper Number: (place paper number here; save file as paper #_first author surname (eg 01_Smith)
M Bovell1, R Bose2, G Cheong3
Project Metallurgist, Donhad Pty Ltd, Perth, WA, Australia. Email: [email protected]
Mining Process Optimisation Manager, Donhad Pty Ltd, Perth, WA, Australia. Email:[email protected]
Group Metallurgy Manager, Westgold Resources Ltd, Perth, WA, Australia. Email:[email protected]
Grinding media is a major consumable in mineral processing and therefore decreasing costs associated with
its consumption is regularly considered a key goal for the minerals processing industry. Grinding media
manufacturers have historically developed steels with improved wear life for their products as a way of
delivering incremental media improvement. This has included increasing carbon levels from low-to-medium
carbon steels up to the eutectoid carbon composition of 0.77% where the relationship between higher carbon
and better wear performance is well understood. Producers today will regularly use grades in excess of 0.8%
to further improve wear performance particularly for rolled grinding media.
This paper first introduces the demonstrated relationship between this increased carbon content and improved
wear resistance along with metallurgical rationale behind the high carbon production process and limitations.
To test the performance improvement associated with the higher carbon steel grade, a series of Marked Ball
Wear Tests (MBWT) were carried out. This paper explores the relationship between ball property, test
methodology and wear performance. The paper concludes by correlating the increased carbon grade with a
reduced consumption rate, approaching 6%.
Grinding media developed for mineral processing are exposed to complex and variable wearing environments.
Various studies have assessed grinding media consumption (Aldrich, 2013; Azzaroni, 1987; Sepúlveda, 2004).
These have typically identified both milling operational factors such as energy and ore characteristics as well
as media factors such as chemical and mechanical properties can influence the overall consumption rate.
As shown in Figure 1, grinding media consumption is a major consumable cost associated with mineral
processing wherein a clear correlation between reducing wear rate and reducing operational costs is inherent.
Grinding media producers therefore attempt to maximise value to mill operators by improving the wear
Typical forged grinding media is manufactured with a low alloy steel. The general metallurgical material
properties of the steel will be defined by the carbon content, level of alloying additions and heat treatment to
produce a particular microstructure and hardness (Krauss, 1978).
Within the class of low alloy steel materials, correlation between hardness and resistance to wear is reasonably
well understood (Chandrasekaran, Natarajan and Kishore, 1991; Moore et al, 1988). Similarly, studies have
shown the relationship between carbon content and wear (Moore, 1974). Given that increased carbon content
is associated with increased hardness for an equivalent heat treatment (Jatczak, 1973; Krauss, 1978) it cannot
be clearly stated whether one or both of the aforementioned relationships are causal. Regardless, grinding
media types have historically been developed with steels providing greater resistance to wear by increasing
carbon content and hardness.
The relationships between carbon content, hardness and wear resistance are further complicated for alloy
steels where the level of carbon exceeds the eutectoid carbon composition of 0.77%. Beyond this limit, extra
consideration must be given in cases where carbon content is increased. The balance between lath and plate
martensite may change at these higher carbon levels (Krauss, 1978). Higher levels of carbon may be dissolved
in the austenite phase during heat treatment depending on the temperature thus affecting the resultant
martensite structure upon quenching (Owaku and Akasu, 1963). The maximum hardness attainable (ie that of
untempered 100% martensite) will no longer increase proportionally with increased carbon and may in fact
decrease (Jatczak, 1973). Alloy steel grades between 0.8% and 1.0% carbon content are typical today
particularly for ball mill environments (Sabih, Radziszewski and Mullany, 2017). There is therefore value to
grinding media users in understanding these factors that apply to hypereutectoid steels where the discussed
correlations between carbon, hardness and wear rates may no longer apply.
Developments where wear resistance is improved by increasing carbon content or hardness must also account
for the milling operational factors that contribute to consumption. Higher hardness materials might be more
resistive to abrasive wear, but might be more prone to wear by microspalling in high impact environments.
Increasing hardness for larger ball diameters may require higher alloy additions to maintain an appropriate
hardenability thus increasing production cost such that it outweighs the performance benefits. These and
multiple other factors may act as limits to media development for any particular milling environment.
Given the complexities, comparisons of grinding media typically requires validation testing rather than relying
on theoretical results based on material properties. Validation testing may be in situ, such as with a marked
ball wear test (MBWT), or simulated, such as with a steel wheel abrasion test (Hosseini and Radziszewski,
2011) or ball mill abrasion test (Jankovic, Wills and Dikmen, 2016). Simulated tests obviously risk not directly
correlating with in situ results but are conducted with increasing sophistication to simulate actual milling
environments (Radziszewski, 2005). Conversely, in situ tests tend to be large scale and therefore cost more
and are less repeatable.
To make some assessment on the impact of increasing carbon content of low alloy steels in the hypereutectoid
range on wear resistance, two grinding media types were selected with equivalent specified hardness and
microstructure but different carbon contents. One type had a specified hardness of 62 HRc and specified
carbon content of 0.95% whereas the other had the same specified hardness but a specified carbon content
of 1.05%. The difference of 0.1% carbon was considered incremental so as to minimise any chance of major
differences in the steel microstructure brought about by the changed alloy balance between iron and carbon.
Further, by choosing both grinding media types with around 1% carbon it was considered that these would
definitively fit into the class of hypereutectoid steels. The media types were evaluated for various material
properties and a series of MBWTs were conducted to compare wear performance in actual milling
Methodology for Grinding Media Characterisation
The two grinding media types, henceforth referred to as Product A (specification of 0.95% carbon) and Product
B (specification of 1.05% carbon), were examined for chemical and mechanical properties to be assessed
against wear data. In conducting the experiment, sub-sets from the larger samples of each media type used
in the MBWTs were randomly selected for analysis and results averaged. The analysis included hardness,
chemical composition and microstructural evaluation.
Hardness testing was conducted by the Vickers HV30 method according to AS 1817.1 (2003) and converting
to the Rockwell C scale according to ASTM E140-12b (2012) for comparison with the specification for the
products, which was given on the same scale. A section was prepared from each sample such that risk of
damage to the material structure was minimised and tests were conducted near the surface and near the core
of the ball, as well as at a location approximating the mid-radius.
The same specimen was analysed for carbon content by spark atomic emission spectroanalysis. Triplicate
analysis of each specimen was conducted to improve the accuracy of the test method. The surface was
thoroughly cleaned prior to testing to remove any potential contaminants.
The traverse from the sample surface to the core of the ball was subsequently prepared for microstructural
analysis (Vander Voort, 1999). The surface was polished by a number of grinding and polishing steps to a 1
μm finish and etched with 10% nital solution to reveal the microstructure. The sample was observed using an
optical microscope at various magnifications between 50x and 1000x.
Methodology for In Situ Wear Testing
MBWTs were carried out in five different mineral processing plants as described in Table 1. The rationale for
choice of sites was to cover myriad of applications. Sites were chosen so that a mix of both primary and
secondary grinding stages were examined. This was due to the fact that a forged ball is a typical choice for
both primary and secondary grinding mills where impact is the predominant mode of breakage.
The purpose for conducting a range of MBWTs was that a particular mill environment would have particular
conditions benefiting certain specific properties associated with media. A discrepancy between performance
of Product A and Product B across a range of sites and operating conditions was considered important in
lending weight to any suggestion that the inherent media material properties may be beneficial in mill wear
The theoretical methodology for conducting an appropriate MBWT was an important consideration and was
applied to the MBWTs conducted. Before starting a MBWT, proper duration of the test [(Dt)MBWT, hrs], the
number of balls to be marked [NTOT], and the personnel required for the marked balls recovery crew [NCREW ],
once the test is completed, must be calculated.
Wear rate constant [kd] and energy specific wear rate constant [kdE] need to be calculated to assess the
outcome of the test using the Sepúlveda (2004) methodology.
kd is defined by the linear wear kinetics and can be expressed as
d = dR - kd t
Initial diameter of the marked balls
Diameter of the grinding ball after t hour of being charged in the mill
By analogy to mineral particle breakage kinetics, it is postulated that an even more representative and
scaleable quality indicator is the Energy Specific Wear Rate Constant [kdE, mm/(KWH/ton)], defined by the
= kdE (Pb/W b) / 1000
Power consumed by grinding media
Weight of the grinding media
Since kd is not a function of the ball size (Sepúlveda, 2004), for any given grinding media variety, the constant
kd becomes an indicator of its relative quality as compared to other alternative media types. A relatively large
value of this constant will be indicative of poor media quality. In general, the MBWT procedure has been
designed to measure kd in a full-scale operating environment. However, specific wear rate constant k dE
(µm/(kWh/ton)) is also utilised in the industry to evaluate the wear rate in the MBWT. On this basis and
considering any prior knowledge pertaining to the various types of media under evaluation, best estimates of
the anticipated wear rate constants applicable to each MBWT should be derived by the test designer.
The expected ball size at the end of the test [dF, mm] is determined by the original size of the marked balls at
the beginning of the test [dI, mm] and the average fractional weight loss [Dw, %] experienced by the marked
balls of any given type :
dF= dI (1 – Dw)1/3
Therefore, the MBWT duration [(Dt)MBWT, hrs] should be :
= (dI – dF) / kd = [ 1 – (1 – Dw)1/3 ] dI / kd
In preparation for the MBWT, the selected groups of balls must be properly marked to allow for recognition of
the recovered balls on completion of the test. There are two widely accepted means to identify the balls:
Method 1
The method of marking balls that allow each individual ball to be assessed is to drill each ball with an identical
radial hole (towards the centre of the ball), place an identification tag in the bottom of the hole and then plug
the hole with a low melting point metal alloy to secure the tag. At recovery, the plug is melted and the
identification tag removed.
Method 2
The second method does not require a tag and consists of using different combinations of hole diameters at
different relative orientations. Typically, two radial holes of equal or different sizes may be drilled at 90° or 180°
orientations. This method only allows for the identification of groups of balls and not each ball separately as is
the case in Method 1 above thus missing the possibility of assessing the variability in results as well.
In any case, drilling must be done carefully to avoid excessive localised heating in the drilling area, which could
alter the microstructure of some grinding media alloys which would reduce the impact resistance of the ball
and therefore disturb the reliability of the test results. In general, Electrical Discharge Machining (EDM) is the
recommended drilling technique.
The hole’s diameter may range from 6f mm to 9f mm, while the minimum penetration depth (mm) is specified
= (dI – dF)/2 + 10
so as to assure that the tag would not be removed and lost prior to the MBWT completion. After the balls have
been satisfactorily drilled and prior to the sealing of the tag, the marked balls must be carefully weighed to a
precision within a tolerance limit of ± 0.1%. The holes are then plugged to complete the preparation of the
On completion of the test, a marked balls recovery crew must enter the mill, for a normally limited number of
available hours, to inspect the exposed surface of the charge and recover as many as possible (ideally, no
less than 5) marked balls from each of the participating groups. A gradual turning of the mill (‘inching’) expands
the probabilities of recovering a larger proportion of the marked balls originally dumped in the mill significantly
and should always be practised whenever feasible. In very exceptional cases, the whole charge may be
emptied out of the mill for later, detailed inspection thus allowing for the recovery of virtually each marked ball
in every group. The required number of inspectors [NCREW ] to confirm the Marked Balls Recovery Crew may
be estimated from the simple relationship:
= estimate of the crew inspectors productivity, man-hours/m2
= area of recovery
= available operating hours for balls recovery
When using the recommended low melting point metal alloy for plugging the marking holes, ball tags may be
easily recovered by melting the plug in a regular drying oven or even with the aid of a welding torch. After
removing the plugs, recording the corresponding tags and thoroughly cleaning the recovered balls, these must
be carefully weighed within a precision tolerance limit of ± 0.1%. The marked balls were individually retrieved
and relevant measurements were taken by site personnel in collaboration bringing a sense of stewardship to
the process.
For each recovered marked ball (or group of balls) the following calculations must be performed:
Initial Ball Size:
dI = (6wI / πrb)1/3
Final Ball Size :
dF = (6wF / πrb)1/3
Wear Rate Constants: kd
kdE = 1000 kd / (Pb / W b)
= (dI – dF) / (Dt)MBWT
For the purpose of the experimental MBWTs undertaken, the second method of marking different ball types
was selected in order to reduce preparation time for the experiment given a reasonably large sample size for
the test. The requirement for this method was that a statistically significant sample must be trialled and
recovered so as to use mathematical analyses to approximate the consumption based on the population sets
before and after use.
Media Characterisation
The metallurgical characterisation of media Product A and Product B was conducted by testing a sub-set of
balls taken from the greater sample set used for the MBWTs. Results of hardness testing were obtained by
testing on sectioned half-balls with a lightly ground surface. The half-balls were adequately supported for
mechanical testing and the results converted to the Rockwell C scale. The averages across the sub-sets from
each of the Product types at three locations within the ball sections are presented in Table 2. It was found that
the two media types had hardness approximately the same as each other and meeting the specifications for
each of 62 ± 3 HRc. There was some variation in the measured hardness, but it was considered that for the
purpose of in situ testing any results would not be significantly influenced by a correlation to these hardness
The samples taken for hardness testing were further prepared for analysis of the carbon content by sectioning
to a rectangular block that could be placed in the spectroanalyser. The specimens were cleaned and lightly
polished before analysis. The average carbon content by weight in Product A was found to be 0.95%. This
matched the specification for Product A which stated an aim of 0.95%. The average carbon content by weight
of Product B was higher at 1.07%. This was slightly higher than the specified aim of 1.05% but within the
standard tolerance limits. The test results showed a significantly higher carbon content for Product B in that
the difference exceeded the potential error of the testing method.
Finally, the samples were fine-polished and etched and the microstructure assessed throughout a traverse
from surface to core of the original ball sample. Each media type was found to have a predominantly martensitic
structure throughout the sample as shown in Figure 2 of bulk microstructures. Figures 2a and 2b showed the
Product A microstructure with Figures 2c and 2d showing the Product B microstructure. The structure indicated
an appropriate hardening and light tempering of the steel during manufacture. Combined with the hardness
and chemical composition results, the microstructure indicated a nearly full martensitic transformation with
some retained austenite. The manufacturing process was determined to be approximately equivalent for both
Products A and B.
The differences observed in the microstructures was attributed primarily to the increased carbon content. The
higher carbon in Product B would make it more likely to form plate martensite in preference to lath martensite
(Krauss, 1978). This could therefore be considered an inherent property of the carbon content and any effect
on wear properties would be attributable to the hypothesis of correlation with increasing carbon content.
In Situ Wear Testing
The results of the MBWTs are shown in Table 3 where wear rate measurements (kd and kdE) of Product B
forged grinding media were found to be lower than those of Product A. The results implied that the forged ball
with higher carbon content will be consumed at a lower rate in the milling environment.
Weight loss of the grinding ball per unit time was compared for a different mine site to assess Product A and
Product B in a different milling environment. As shown in Figure 3, weight loss per unit time is ranged from
1.07 g/ hr to 0.33 g/ hr and Product B was found better than Product A in all the trial conditions.
The wear rate constants of kd and kdE were calculated utilising Equation 9 and 10 and shown in Figures 4 and
Figure 5 respectively. As specified by Sepúlveda (2004), a lower value of kd and kdE is desirable for any grinding
media and it is used to compare different grinding media performance in the milling environment. A kd value
ranged from 7.1 to 15.3 and a k dE value ranged from 0.52 to 1.11 was recorded from the MBWT which is in
the lower range as per Sepúlveda (2004). This in general shows that both Product A and Product B are better
than other availble carbon steel forged grinding media. Also Figures 4 and 5 show that Product B was
performing better than Product A consistently across all sites. Hence, industry accepted wear rate constants
like g/hr, kd and kdE show a similar result, inferring that higher carbon levels in forged media correlate to better
wear resistance and thus will lower monthly consumption of the grinding media. Figures 3 to 5 also showed
that the grinding media wore less in the mine site 5 due to overall circuit arrangement, ore properties and
internal milling environment as per Table 1.
The characterisation of the media and subsequent MBWT results were considered to demonstrate an improved
resistance to wear by increasing the carbon content from just below 1% to just above 1%. This relationship
was observed whereby each MBWT’s saw in improved performance ranging between 3.1% and 6.1%.
The fact that the benefit was observed to vary across the five MBWTs was thought to relate to the variety of
operational conditions for each mill. Media can respond differently in different environments (Massola, Chaves
and Albertin, 2016), and therefore the beneficial properties of Product B may be enhanced or negated
depending on the milling operational conditions present. Given that in each test conducted, some improvement
in resistance to material loss was observed for Product B, it was considered reasonable to conclude that some
beneficial properties were inherent for the media with higher carbon content.
In characterising the materials, comparing the hardness results was critical because of the known correlation
between hardness and wear resistance (Moore, 1974). The results showed very similar hardness values
between the two media types, and Product B was on average very slightly lower than Product A. Given that
actual MBWT results were contrary to the theory of higher hardness giving better wear resistance, it appeared
that increased carbon could actually improve performance regardless of the hardness of the steel.
The fact that the higher carbon Product B had similar or slightly lower hardness than Product A was possibly
related to the alloy composition itself (Jatczak, 1973). Up to the eutectoid composition of 0.77% carbon in steel,
increased carbon content is associated with increased hardness. Beyond the eutectoid, maximum attainable
hardness will reduce with increased carbon. Factors such as the amount of carbon in the austenite phase prior
to quenching to form martensite during heat treatment will further complicate the attainable hardness even
before any tempering processes are considered.
The most likely cause for increased wear resistance of Product B was considered to be the effect of the carbon
content on the final microstructure. The most likely mechanisms for wear of the grinding media during the
MBWTs were some combination of abrasive, impact, attrition, and high stress abrasion wear mechanisms
(Aldrich, 2013; Gates et al, 2007; Varenberg, 2013). The ability of the media material to resist these
mechanisms would depend not only on their bulk material properties but also its microstructural properties.
It is postulated that Product B was likely to have been more resistant to abrasive wear due to the coarse plate
martensite within the structure. The hard martensite plates may have acted to resist gouging abrasion by hard
ore particles. Martensite plate cracks may be present in such materials (Solano-Alvarez and Bhadeshia, 2014)
which would counteract the benefit but were not observed in the samples analysed.
Materials resistant to abrasive wear might typically be more prone to mass loss by microspalling under impact.
Given the microstructures of the two media types, it was possible that this did not occur in the case of Product
B due to the strength and ductility of the interplate material. Spalling is known to be initiated by first plastically
deforming a small grain or piece before subsequent fracture of the deformed microregion (Yang et al, 1994).
If the strength of the material resists the initial deformation, the spalling effect can be mitigated. Given Product
B outperformed Product A in all MBWTs conducted, where a variety of impact energies would have been
present in the mill, this or some other explanation must have applied to the higher carbon media’s performance.
The full wear mechanism in each given mill is not fully understood and therefore the precise nature of each
media’s performance could only be hypothesised. Ultimately the wear performance of the higher carbon
grinding media was demonstrated by the conducted MBWTs to be superior to the other tested media type.
This could not be explained by hardness or manufacturing processes which were thought to be largely similar
to each other. The increased carbon level and subsequent inherent effects on the alloy structure were thus
considered to be the primary factors influencing wear.
While this paper has explored the relationship between wear resistance and carbon content, it was noted that
the grinding media may not have been exactly equivalent in all other properties. Different levels of alloying or
trace elements in the steel chemistry, the presence of carbides or other non-metallic phases, the residual
stresses in the ball and many other grinding media metallurgical and mechanical properties could influence
the wear experienced. The MBWT is limited in its ability to assess a wide range of variables due to the scale
of the test. Laboratory testing, where it can adequately simulate the real milling environment, may allow for a
broader experimental design to assess these various properties and the relationships between them.
Ultimately, the results obtained showed a strong correlation between increased carbon content and improved
wear resistance of grinding media. Despite the inherent limitations of the MBWT in assessing a wide range of
properties, this was considered to demonstrate a clear benefit in increasing carbon content of grinding media
for otherwise similar hypereutectoid steels. Higher hardness was found to not correlate to improved wear
performance. The conclusion was strongly supported by the fact that the result was found across 5 mine sites
with a variety of milling environments.
Two grinding media types were compared with largely similar properties except for a difference in carbon level
where one type had just below 1% carbon and the other had just above 1% carbon. The microstructure was
found to be slightly different in the two media types largely in line with the increased carbon level. Both were
hypereutectoid alloy steels where relationships between hardness, carbon content and abrasive resistance
are less well understood.
A number of in situ MBWTs were conducted to compare the two grinding media types. The milling
environments were selected to represent a variety of typical conditions. MBWTs were selected to assess the
products as they did not rely upon simulation of actual milling environments. Improved understanding of wear
mechanisms in situ can lead to further developments in laboratory scale wear testing which can then overcome
some limitations with multivariable analysis of grinding media in MBWTs where the scale of the trial render this
expensive and impractical.
In each MBWT the performance of the higher carbon media type was superior, with a reduced mass loss of
between 3.1% and 6.1%. Further, the energy specific wear rate constant (k dE) of the higher carbon media type
was between 2.2% and 5.4% better. The use of kdE as a metric further goes to support the results in that it
removes expostulations relating to the ambiguity around the mine-specific milling conditions.
In conclusion, it is theorised that the improved performance of the higher carbon grinding media could be
explained by the effect of increased carbon on the microstructure of the material. While the mechanisms of
wear in a mill remain complex and understandings limited, comparative testing can be a valuable technique to
forecast grinding media consumption. Ultimately the authors found that by comparing two hypereutectoid
steels, the experiment demonstrated a better wear performance of higher carbon steel media albeit with
equivalent hardness.
The authors would like to thank Donhad Pty Ltd for allowing the presentation of the paper and facilitating the
test work conducted. The authors would like to particularly acknowledge Mr Alan Faulkner and Mr Luke Turner
for work related to the conducted experiments and analysis, and Dr Maruf Hasan for guidance in the
interpretation and presentation of the results obtained.
The test work undertaken was also supported by undisclosed companies who wished to understand the results
as they relate to their operations. The results presented could not have been obtained if the trials had not been
actively facilitated by these companies.
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2a: Product A structure at low magnification
2b: Product A structure at higher magnification
2c: Product B structure at low magnification
2d: Product B structure at higher magnification
Product A
Product B
Mine Site 1
Mine Site 2
Mine Site 3
Mine Site 4
Mine Site 5
Product A
Product B
kd (µm/hr)
Mine Site 1
Mine Site 2
Mine Site 3
Mine Site 4
Mine Site 5
Product A
Product B
kdE (µm/(kWh/ton))
Mine Site 1
Mine Site 2
Mine Site 3
Mine Site 4
Mine Site 5
FIG 1 – The operating cost of grinding media relative to liners and energy (Image courtesy Metso Minerals)
FIG 2 – Microstructures of Product A and Product B (etched with 10% nital solution)
FIG 3 – Weight loss as a function of time (g/hr) for Product A and Product B in different mine site
FIG 4 – Wear rate kinetics (kd) for Product A and Product B in different mine site
FIG 5 – Specific wear rate kinetics (k dE) for Product A and Product B in different mine site
Mine Site 1
and Cyclone
Mine Site 3
Mine Site 4
Mine Site 5
Primary Ball
Primary Ball
SABC and
Mill and
Mill and
Copper -
Bond Wi (kWh/t)
Primary Ball Mill
Mine Site 2
Product A
Product B
Near Surface Hardness (HRc)
Mid-Radius Hardness (HRc)
Near Core Hardness (HRc)
Site 1
Duration of Trial
Possible Nominal
Loss of weight (g)
Initial diameter of
marked ball (mm)
Nominal Loss of
Diameter (mm)
Site 2
Site 3
Site 4
Site 5
Differential Loss
in media diameter
Differential Loss
in media diameter
- 4.1%
- 5.4%
- 2.2%
- 5.0%
- 3.7%
wrt Product A (%)
kd (µm/hr)
Differential kdE wrt
Product A
Loss of initial
weight (%)
- 4.1%
- 5.4%
- 2.2%
- 3.8%
- 3.7%
Weight loss as a
function of time
Weight loss wrt
Product A (%)
- 4.8%
Features of the MBWT mine sites
Hardness testing result of Products A and B
Detailed result of MBWT for five mine sites
- 6.1%
- 3.1%
- 3.3%
- 3.2%