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AbstractThis study compares the energy consumption of highpressure grinding rolls with the laboratory rod milling tests and the

pilot plant. Three types of minerals were used as test specimensnamely: pyroxenite (I), pyroxenite (2) and pyroxenite (3). These testsamples were ground using jaw and cone crushers to provide feed for

laboratory milling test deposit and to evaluate a circuit comprised ofhigh pressure grinding rolls comminution followed by laboratorymilling. Attempts at improving the comminution machines generallyhave been directed towards increasing the performance efficiency,

particularly increasing throughput rate and decreasing energy

consumption.To evaluate the potential energy benefits of this novelcircuit arrangement, energy consumption related to comminution wascalculated for the circuit using power draw readings off the mainmotor and the throughput recorded during testing. To provide a basis

for comparison, the energy requirements for laboratory rod millingtests, the high pressure grinding rolls tests and the piloting campaignwere determined through high pressure grinding rolls pilot-scaletesting.

KeywordsComminution, Efficiency, Hpgrs, Rod Milling, PilotMilling Test.

I. INTRODUCTIONHE mining industry all over the world has been facing thechallenge of ore size reduction as some minerals are soft;

hard, clay or may exhibit distinct cleavage whichcauseshigh energy consumption in mining practices such as

comminution [1, 2, 3]. Comminution is required in mineral

processing to provide the liberation between useful and

gangue minerals [4].

High pressure grinding rolls (HPGRs) technology is rapidly

gaining a wide acceptance within the mineral processingindustry [5]. This technology is based on operation principle

that applying enough pressure to a bed of particles might bring

gains regarding energy efficiency. One of its attractivefeatures is that, it could lead to better liberation of the precious

mineral species. The use of the HPGRs could also have a

Rudzani Sigwadi is with the Chemical Engineering Department, Universityof South Africa, P/Bag X6, Florida 1710, Johannesburg, South Africa.

Molebogeng Nkobane is with the Chemical Engineering Department,

University of South Africa, P/Bag X6, Florida 1710, Johannesburg, South

Africa

Touhami Mokrani is with the Chemical Engineering Department,University of South Africa, P/Bag X6, Florida 1710, Johannesburg, South

Africa.

Ayo Afolabi is with the Chemical Engineering Department, University of

South Africa, P/Bag X6, Florida 1710, Johannesburg, South Africa, Tel:0027114713617; Fax: 0027114713054; e-mail; [email protected].

favourable impact on overall energy consumption [1, 6, 7, 8].

The slow load application to the particles causes the grainsstructural collapse and minimizes energy losses as heat and

noise. Among the comminution equipments, HPGRs are the

most efficient from the energy viewpoint [4]. In mineral

processing, comminution accounts for a substantial portion of

the energy consumed in the plant almost 80% [9]. Efforts toincrease comminution efficiency come not only from the need

for high production rates but also because of high energy costs

associated with the inherent low-efficiency of conventionalcomminution systems and the low grade ores that have to be

milled.

This study focuses on the development of a comminution

circuit design to address these issues. One option is to replace

the tertiary crusher and primary rod mill with single HPGRs[10]. The HPGRs product would then serve as feed to a single

stage rod mill in closed circuit with a vibrating screen to

achieve the target grind for flotation (80 per cent passing size

of 265 m). Interest in the application of these mills to thegrinding of minerals and ores is also increasing and has led to

attempts at modeling the high pressure rolls size reduction

performance with particular regard to minerals and ores so that

the effects of introducing high pressure rolls into typicalmineral processing circuits may be predicted.

II. MATERIALS AND EXPERIMENTAL PROCEDUREThree mineral samples namely:pyroxenite (1), pyroxenite

(2) and pyroxenite (3) were used individually for testwork and

in admixtures for comminution in the HPGRs, laboratory

milling test (Bond rod mill test) and pilot milling test.

A. Laboratory Milling TestTwo types of laboratory milling tests were conducted. The

first one involved basic batch tests that effectively simulated

open circuit milling with plug flow material transport. The

other test simulated closed circuit milling, also with plug flowmaterial transport of the pulp. The latter procedure was similarto the locked cycle procedure used in the standard Bond rod

mill test, except that it involved slurry, rather than dry

material, and the milling energy was expressed directly interms of specific energy input, rather than mill revolutions.

Both procedures included the identification of model

parameters that characterized the breakage kinetics inside the

mill and can be used in computer simulation studies to predict

the performance of complete milling circuits. The new feedand the screen undersize and oversize from the last cycle of

Effect of High Pressure Grinding Rolls on

Comminution Circuit Designs

Rudzani Sigwadi, Molebogeng Nkobane, Touhami Mokrani and Ayo Afolabi

T

International Conference on Chemical, Ecology and Environmental Sciences (ICCEES'2011) Pattaya Dec. 2011

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each test were sized down to 38 m using standard sieves

conforming to a root-two geometric progression of mesh sizes.

B. High pressure grinding rolls (HPGRs)The testwork was operated under an open circuit tests with

the pressure settings ranges from 1.5 to 4.5 N/mm2. Theoptimum pressure setting for each ore type was picked based

on energy considerations, particle size distribution (PSD)

analysis, and observed behaviour. The nitrogen pressure was

kept at a constant ratio of 0.8:1 of the hydraulic pressure.Initial no-load power consumption, operating pressures and

gaps were recorded using the data recorder. The throughput

was determined by taking timed sample using a specially

designed sampler that split the HPGR product into a centreproduct and two edged products. This centre and edge product

was weighed and sub-sampled for PSD analysis.

C. Pilot milling testThe commissioning tests showed that the maximum new

feed rate that the mill could handle was about 0.25 t/h due to

constraints imposed by the capacity of the feed inletarrangement for injecting new feed and recycled pulp into the

mill. It was necessary to operate the mill at very low powerlevels because of the very low specific energy requirements,determined by the rod load. During the commissioning run,

the rod load was set at 15% of the mill load. Even at this load,

the grind size proved to be significantly finer than the target

grind (a P80of 0.265 mm). Accordingly, the main tests were

conducted at a rod charge of only 10% of the mill volume. An

estimate of the net power draw of the mill was obtained from

the known rod charge mass, the fractional milling of the mill

with rods, and the fixed mill speed.

III. RESULTS AND DISCUSSIONTable I shows the measured energy size relationship for

three different pyroxenite ore samples that were subjected toopen and close milling circuits. The given net energy

requirements are based on the new feed rate in both cases. It

can be seen from this table that the pyroxenite 2 appears to be

an order of magnitude softer than the other two ore samples interms of the net specific energy required to achieve the target

grind.TABLEI

SPECIFIC ENERGY REQUIREMENT FOR OPEN AND CLOSED

CIRCUIT ROD MILLING

Ore type Open circuit

[kWh/t]

Closed circuit

[kWh/t]

Pyroxenite 1 8.5 3.8



To determine the transfer size between the closed circuit

milling, three different top sizes (265, 300 and 425m) were

evaluated. These particle sizes tested the limits for closed

circuit milling and an evaluation was based on the specific

energy requirements for comminution and whether the closedcircuit milling could operate and grind effectively. Table II

shows that apart from the pyroxenite 1, the measured P80s for

closing screen sizes of 300 and 425 m straddled the target

grind of 265 m. Although complete accuracy was not

achieved in reconciling the measured and predicted specific

energy requirements, the trends are clear with regard to therelative grindability of the different types of ores. The specific

energy requirement for achieving the target grind for the

pyroxenite 2 is less than a third of that required for the

pyroxenite 1 and the pyroxenite 3. The specific energyrequirement for the pyroxenite 3 appears to be marginally

higher than that required for the pyroxenite 2.

TABLEII

PREDICTED AND MEASURED ENERGY REQUIREMENTS FOR

CLOSED CIRCUIT MILLING

Ore type Predicted at

265 m P80

Measured at 300

m closing screen

Measured at 425

m closing screen

Pyroxenite 1 3.8 kWh/t 3.2kWh/t(80% -270 m)

2.4 kWh/t(80% -311 m)

Pyroxenite 2 0.4 kWh/t 1.1 kWh/t

(80% -234 m)

1.2 kWh/t

(80% -303 m)

Pyroxenite 3 4.0 kWh/t 5.2 kWh/t

(80%-219 m)

2.5 kWh/t

(80% -305 m)

A detailed investigation of the effects of HPGRs pressingforce was conducted recently on three different samples.

HPGR press tests ranging from 1.5 to 4.5 N/mm2press force

were compared. Fig. 1 illustrates the particle size distribution

curves for each specific pressing force. The total upper limit

for a press force is more than 4.5 N/mm2. The PDS values for

1.5 N/mm2, 2.0 N/mm2, 3.0 N/mm2PSD are very similar.

Fig. 1 PSD of Pyroxenite 1 sample at different specific pressing

forces

Fig. 2 shows the particle size distribution curves of

pyroxenite 2 and the feed size distributions. The results

were obtained from measuring specific pressing force. The

figure shows that the specific pressing forces follow almostsame trend but the HPGRs feed is shown to be a bit lower.

However, the feed also produced the cumulative passing of

100%, so only the shape of the curve could be compared.

Although these size distributions resemble a similar trend,


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the size distributions of different specific pressing forces

could be considerably different.

Fig. 2 PSD of Pyroxenite 2 sample at different specific pressingforces

Fig. 3 shows the summarized results of pyroxenite 3 and

the feed size distributions. These results were also obtainedfrom measuring specific pressing force.

Fig. 1 PSD of Pyroxenite 3 sample at different specific pressingforces

Fig. 4 shows the measured as reconciled size distributionfor the low and high feed rates. The results appear to be

anomalous because the size distributions and circulating load

ratio seem to be insensitive to the feed. It should also be noted

that the measured solids flowrate for the screen undersize is

much lower than the measured new feed rate. Thisdiscrepancy could be due to either circuit instabilities or

inherent inaccuracies associated with trying to estimate

flowrates from timed grab sample cuts taken over very shorttime periods. The screen over and under sizes can be up to 10

mm and mass percentage sizes of 100% was achieved. This

pilot milling test provides a steeper over size screening

distribution as compared to a feed, milling discharge andscreening under size product.

Fig. 4 Measured and reconciled size distributions at low feed rates

IV. CONCLUSIONSA comprehensive testworks including laboratory rodmilling

tests, HPGRs tests and piloting campaign were conducted on

three pyroxenite ore samples to explore a more efficient and

easier method of operating milling circuit to achieve a targetgrind of 80% passing 265 m. The new circuit explored

consists of replacing the tertiary crusher and primary rod mill

in the actual configuration by a single stage HPGRs followedby a single stage rodmill in closed circuit with a screen to

achieve this set target grind. HPGRs tests showed that increase

in the specific grinding force increased the specific energy

consumption. These conditions were applied to the lockedcycle tests for each ore type. The specific energy inputs (kWh

per ton new feed to the HPGRs) for the pyroxinite 1,2&3samples averaged about 1.4 kWh/t at new feed rates around 40

t/h and circulating load ratios around 20 to 30 per cent. These

results indicated that all three ore types were amenable toHPGRs treatment with the HPGRs used as a tertiary. These

ore samples are therefore amenable to HPGRs treatment as a

replacement of a tertiary crusher and primary mill with

HPGRs. It was established that the target grind of 80% passing

265 m can be achieved in a single-stage using a rod millclosed by a vibrating screen.

ACKNOWLEDGMENTThe authors gratefully acknowledge the financial supports

of the National Research Foundation (NRF) and University of

South Africa.

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