3738
Effects of the Discharge Design on
Wet Grinding Mill Performance
Sami Makni, Robert McIvor
Corem
ABSTRACT: The grate discharge was historically favored over the overflow discharge for wet ball milling.
Additionally, the presence of a large slurry pool in autogenous or semi-autogenous grinding ((S)AG) mills
has been shown to have a negative effect on grinding performance. Industrial experiences comparing the
performances of both types of these mills operating at higher versus lower slurry level were reviewed, in addition
to pilot plant and laboratory scale test work. The impact of slurry level on both ball and (S)AG mill grinding
performances is clearly demonstrated. In this paper, we focus on the effects of the discharge design on wet
grinding ball mill performance.
INTRODUCTION
For wet ball mills, there is no strong reason given, beyond
personal preference, for the choice of an overflow vs. a grate
discharge. Taggart (1945) provides tables of operating data
on many ball mills, about one half the number (28) of over-
flow type versus (58) grate discharges in that era. Several
plants that have discharge grate mills continue to operate
today, or at least recently (e.g., at Morenci, Copper Range,
LTV Steel). The simpler overflow mills will be slightly less
expensive. Since today’s designers are presented with no
clear options, overflow discharge ball mills are exclusively
chosen for new plants, essentially by default, without any
discussion, over ball mills with grate discharges.
Compared to an overflow ball mill, Michaelson (1944)
at Allis-Chalmers said that an intermediate-level diaphragm
(grate) discharge ball mill draws approximately 11% more
power per ton of grinding media, and provides 15% greater
grinding capacity a low-level discharge mill draws approxi-
mately 20% greater power per ton of media, and approxi-
mately 25% greater grinding capacity. These differences
between power and capacity equate to approximately 4 to
5% higher milling efficiency. Allis-Chalmers continued to
use this 4–5% energy efficiency advantage for a grate dis-
charge compared to overflow when sizing mills for many
decades. However, during his late writings, Fred C. Bond
(1985) put the relative efficiency advantage of grate versus
overflow ball mills at 10%.
When sizing a new mill, the length can always be
adjusted to achieve desired power draw. If a grate is installed
into an existing overflow mill, a portion of the grinding
length will be used for the discharge grate arrangement.
However, it is evident that the substantially higher power
draw per ton of grinding media, besides a higher charge
level, can make up for that.
REPORTS AND OBSERVATIONS ON
PLANT TESTING
In the section of his book discussing “Height of Discharges”
on ball mills, Taggart (1945) references the reports from
Hollinger, Sylvanite and Lake Shore, among others. He states
that Hollinger (1937) demonstrated increased production
per HP-h by 5% to 8%, depending on the reference mesh
Effects of the Discharge Design on
Wet Grinding Mill Performance
Sami Makni, Robert McIvor
Corem
ABSTRACT: The grate discharge was historically favored over the overflow discharge for wet ball milling.
Additionally, the presence of a large slurry pool in autogenous or semi-autogenous grinding ((S)AG) mills
has been shown to have a negative effect on grinding performance. Industrial experiences comparing the
performances of both types of these mills operating at higher versus lower slurry level were reviewed, in addition
to pilot plant and laboratory scale test work. The impact of slurry level on both ball and (S)AG mill grinding
performances is clearly demonstrated. In this paper, we focus on the effects of the discharge design on wet
grinding ball mill performance.
INTRODUCTION
For wet ball mills, there is no strong reason given, beyond
personal preference, for the choice of an overflow vs. a grate
discharge. Taggart (1945) provides tables of operating data
on many ball mills, about one half the number (28) of over-
flow type versus (58) grate discharges in that era. Several
plants that have discharge grate mills continue to operate
today, or at least recently (e.g., at Morenci, Copper Range,
LTV Steel). The simpler overflow mills will be slightly less
expensive. Since today’s designers are presented with no
clear options, overflow discharge ball mills are exclusively
chosen for new plants, essentially by default, without any
discussion, over ball mills with grate discharges.
Compared to an overflow ball mill, Michaelson (1944)
at Allis-Chalmers said that an intermediate-level diaphragm
(grate) discharge ball mill draws approximately 11% more
power per ton of grinding media, and provides 15% greater
grinding capacity a low-level discharge mill draws approxi-
mately 20% greater power per ton of media, and approxi-
mately 25% greater grinding capacity. These differences
between power and capacity equate to approximately 4 to
5% higher milling efficiency. Allis-Chalmers continued to
use this 4–5% energy efficiency advantage for a grate dis-
charge compared to overflow when sizing mills for many
decades. However, during his late writings, Fred C. Bond
(1985) put the relative efficiency advantage of grate versus
overflow ball mills at 10%.
When sizing a new mill, the length can always be
adjusted to achieve desired power draw. If a grate is installed
into an existing overflow mill, a portion of the grinding
length will be used for the discharge grate arrangement.
However, it is evident that the substantially higher power
draw per ton of grinding media, besides a higher charge
level, can make up for that.
REPORTS AND OBSERVATIONS ON
PLANT TESTING
In the section of his book discussing “Height of Discharges”
on ball mills, Taggart (1945) references the reports from
Hollinger, Sylvanite and Lake Shore, among others. He states
that Hollinger (1937) demonstrated increased production
per HP-h by 5% to 8%, depending on the reference mesh