Hilden, M.

Powell, M.

University of Queensland eSpace

Multiple-pass High Pressure Grinding Rolls Circuits 
M. Hilden and M. Powell 
University of Queensland, JKMRC, QLD
Introduction, Background and Objectives 
High Pressure Grinding Rolls (HPGR) are widely 
considered to be more energy efficient devices for 
comminuting rock than tumbling mills such as 
SAG mills. Recently, HPGR have been installed 
and commissioned at Cerro Verde (Peru), 
Mogolokwane (South Africa), and Freeport 
(Indonesia) with reported energy savings of 
around 19% over more conventional grinding 
routes [1]. 
To date, HPGR have been installed as single unit, 
either in open circuit or closed circuit with a 
screen. A single HPGR unit is typically limited to 
around 3 kWh/t, compared with SAG mills which 
may have energy inputs exceeding 10 kWh/t, and 
therefore despite being less efficient can generate 
a finer product.  
This project aims to determine whether a series of 
HPGR units could be used to perform most of the 
grinding duty in a process plant, increasing the 
amount of breakage performed in these highly 
efficient devices. Such a circuit could 
substantially reduce the energy and grinding 
media used in milling circuits and therefore 
reduce the carbon footprint of new processing 
plants. 
Methodology / Experimental Technique 
Three ore samples have been tested to date. The 
samples were a hard copper-gold ore, a softer 
lead-zinc sulphide ore, and the third was a 
porphyry copper ore. The samples were crushed to 
-32 mm top-size and processed in a three-pass 
HPGR circuit (Figure 1). 
HPGR
HPGR
HPGR
Figure 1:  Three HPGR in series. 
Test work was carried out on two large pilot-scale 
Köppern HPGRs at AMMTEC, Perth. The first 
two samples were tested using 0.22 m (W) x 0.75 
m (D) rolls in July 2007. The third sample was 
tested using a newer 0.25 m (W) x 1.0 m (D) 
HPGR (Figure 2) in March 2008. 
Figure 2:  Köppern’s Andrew Gardula and JKMRC’s 
Malcolm Powell discussing the pilot-scale HPGR at 
AMMTEC Perth. 
The third sample was also tested in a wide range 
of circuit configurations that included screening 
between passes. Test work was also carried out 
using one or two passes with the Köppern HPRG 
with subsequent passes carried out at CSIRO 
Pinjarra Hills using a laboratory-scale HPGR 0.10 
m (w) x 0.25 m (D). In each case, up to three 
passes of the HPGR were used. 
The energy consumption and product size was 
measured for each stage in the flow sheets to 
allow energy efficiency of each circuit to be 
determined. 
Key Results / Findings 
The feed and product sizes from the samples 
tested to data show that the three-HPGR circuit 
produces a broad product size distribution, but 
also that the magnitude of size reduction reduces 
with subsequent passes. Two passes were found to 
be effective; however the third pass continues to 
generate fines without substantially reducing the 
top size. The result is a flatter or broader product 
size distribution as shown in Figure 3. 
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0.01 0.1 1 10 100
Size, mm
%
 P
a
s
s
in
g
Feed
Pass 1
Pass 2
Pass 3
Figure 3:  Product sizing for the three-stage circuit. 
A number of three-stage circuit HPGR 
configurations were compared with the above 
three-pass HPGR circuit. Two approaches were 
found to be effective in generating a steeper 
product size distribution:  
1. Removing excess fines from the feed to the 
HPGR (Figure 4a), and 
2. Using a smaller diameter rolls in the second 
and/or third pass (Figure 4b). 
A B
HPGR
HPGR
HPGR
2.0 mm Screen
48% 
52% 
1.0 m HPGR
1.0 m HPGR
0.25 m HPGR
HPGR
8 mm Screen
HPGR
HPGR
1.0 m HPGR
22% 
78% 
t
0.25 m HPGR
0.25 m HPGR
Figure 4:  Two of the alternative flow sheets tested.  
As shown in Table 1, these circuits produce a 
similar-sized product using less energy compared 
with three passes in the 1.0 m rolls. In the case of 
flow sheet 4b, just two HPGR passes are needed 
to grind to an equivalent product fineness 
(measured using the 80% passing size, P80, and 
the quantity of -150 microns fines generated), and 
uses 33% less energy. 
Table 1:  Comparison of various three-pass HPGR circuits. 
Pass 1 Pass 2 Pass 3
P80, um 9391 3788 2414
% -150um 24% 38% 47%
Energy, kWh/t 2.77 5.23 7.53
P80, um 8804 2824 1425
% -150um 22% 35% 45%
Energy, kWh/t 2.83 4.13 5.48
P80, um 4710 1764 1195
% -150um 27% 46% 53%
Energy, kWh/t 3.23 5.03 5.83
Triple-pass circuit Fig 1.
Modified Circuit Fig 4a.
Modified Circuit Fig 4b.
Highlights / Benefits 
• Test work confirms that the HPGR can be 
used in series to efficiently generate a very 
fine product. After three passes, around half 
of the product can be made into final flotation 
feed size.  
• Two flow sheet modifications further improve 
the performance of the three-stage HPGR 
circuit and use less energy, albeit using a 
potentially more complex flow sheet. 
Conclusions and Future Direction 
This work has shown that a multi-pass HPGR 
circuit can generate a high proportion of fines, but 
only two passes appear to be beneficial without 
either a classification stage or a reduction in roll 
diameter. One or two additional samples will be 
tested in coming months.  
Acknowledgements 
This work was carried out under the auspice of the 
Centre for Sustainable Resource Processing, 
which is established and supported under the 
Australian Government’s Cooperative Research 
Centres Program. The large-scale test work was 
carried out with the assistance of Köppern 
Machinery Australia and the small-scale HPGR 
test work was carried out by Steve Suthers of 
CSIRO, Pinjarra Hills. 
References 
1. C. M. Rule, I. Smit, A. J. Cope, and G. A. Humphries, 
"Commissioning of the Polycom 2.2/1.6 5.6MW HPGR 
at Anglo Platinum's new Mogalakwena North 
Concentrator," presented at Comminution 08, Falmouth, 
2008. 


Multiple-pass high pressure grinding rolls circuits

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Multiple-pass high pressure grinding rolls circuits

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