The South A(~icall Institute o(Minillg alld Metallurgy Diamonds - Source to Use 2007 By J Charles Ntsele alld 2 Gerhard Sauermallll

THE HPGR TECHNOLOGY - THE HEART AND FUTURE OF THE DIAMOND LIBERATION PROCESS

ABSTRACT

The Polycom HPGR (High Pressure Grinding Roll technology), more commonly referred to in diamond applications as HPRC (High Pressure Rolls Crusher) or IPC (Inter - Particle Crusher) has been in use in this industry for over 21 years.

In this time, with the co-operative efforts of both operator and technology supplier it has evolved into being an indispensable part of the liberation and recovery process. New diamond winning challenges and commercial imperatives necessitate that the technology and user adapt to meet and exceed rising expectations.

This paper provides an overview of various flow sheet approaches employing the HPRC, showing how each has contributed to the "mature", state-of-the -art flow sheet employed in current designs. Major design and process control improvements that enabled / drove these developments are highlighted.

This will illustrate how the power of partnership between operators and technology suppliers ensures first, the successful implementation of a technology and then how this influences technology developments specific to the operator's challenges.

1 - Product Manager - Polys ius South Africa 2 - Marketing Manager, Minerals Polysius South Afi'ica

The South African Institute of Mining and Metallurgy Diamonds - Source to Use 2007 By J Charles Ntsele and 2Gerhard Sauermann

1. INTRODUCTION

The High Pressure Grinding Roll (HPGR) technology was first introduced in 1985 and was originally used in the cement industry treating relatively none abrasive materials. The benefits enjoyed by the cement industry through the use of this technology did not go unnoticed in industry in general. This saw some sectors trial the use of this technology in their comminution circuits. But due to the fact that ores in the mineral industry are between 20 to 50 times more abrasive than cement raw materials, higher than acceptable wear rates were experienced and this did make a good business case for the adoption of the technology in those comminution circuits. However for diamond ore comminution the business driver was somewhat different in that while liberation is key, diamond preservation during the process of liberation is equally important. This in addition to other benefits saw this sector take a leap of faith culminating in the introduction of the first HPGR in a kimberlitic application (1987) at the then Premier mine, now Cullinan mine. For diamond liberation, this is where the journey began for HPGRs.

2. THE HPGR PRINCIPLE

2.1 Brief Description of the HPGR

Figure 1 below is a representation of the HPGR with special reference to the rolls, frame, feed and hydraulic arrangement

Figure 1: HPGR representation

The HPGR Gonsists of two counter rotating rolls mounted in heavy duty bearings, enclosed in a strong frame. Pressure is applied to one of the rolls (floating) by means of a hydro-pnuematic spring system, while the other roll is held in a fixed position in the frame. The "free" or "floating" roll is allowed to slide (or float) on pads, reacting to the forces acting on the roll caused by the feed material and the hydro-pnuematic spring system. Feed to the rolls is provided by means of a hopper mounted above the rolls equipped with a level

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The South African Institute of Mining and Metallurgy Diamonds - Source to Use 2007 By lCharles Ntsele alld 2Gerhard Sauermann

controller to ensure that the rolls are continuously choke-fed. Normally, free flow of feed material within the hopper is sufficient to exert a separating force between the rolls. Special attention to the design of this hopper is required in applications where the feed material is fine and moist. The rolls are driven by separate motors connected to the roll shafts through gear reducers. The rolls can be operated at a fixed speed or variable speed depending on the demands of the process. A torque reaction system is included to prevent the gearboxes from turning and to divert any differential forces away from the frame. The HPGR rolls are of a solid make and their surface is protected against wear by wear resistant materials (more about this later).Cheek plates are typically used to contain material reporting to the edges of the rolls.

Roll diameters of HPGRs vary from 0.5 to 2.8 m. The forces applied range from 2,000 to 20,000 kN. Pressures between rolls range from 80 to 300 MPa. Most ores and minerals have compressive strength of between 50 and 280 MPa. Capacities range from 20 to up to 3,000 tph. Energy consumption is between 1 and 2.5 kWhlt. In certain application however, this can be higher due to low throughput rates associated with clay rich ores.

The machines are compact and generate low levels of noise and vibration. Dust emanating from the process can be readily controlled.

2.2 Principle of Operation

Comminution in a HPGR takes place primarily through inter-particle comminution. In instances where the top size of the feed material is larger than the machine's working gap, single particle comminution precedes inter-particle comminution. In this case, these larger feed particles are nipped directly by the rolls and are pre-broken before entering the compression zone as illustrated by figure 2 below.

Figure 2: Comminution zones in a HPGR

Cracks. Preferentially Along Grain Boundaries

t

In the compression zone, direct contact of the ore and roll surfaces is minimised due to the fact that the forces that are applied are transmitted through the bed of particles in intimate contact with the surrounding particles. A combination of forces (typically in excess of the compressive strength of

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The South African Institute of Mining and Metallurgy Diamonds - Source to Use 2007 By I Charles Ntsele and 2Gerhard Sauermanll

most brittles ores) and high stresses present in the compression zone gives rise to breakage occurring preferentially along the weakest planes of individual progeny particles (typically along grain boundaries). This is the same mechanism that gives rise to the so called micro-cracking of intact material in the HPGR product. This weakened progeny structure is beneficial for down stream processing in applications that require further ball milling or leaching. The product of the HPGR is typically in the form of a cake (sometimes referred to as a flake). The resultant cake density typically ranges from 70 % (fine feed with high moisture) to 85 % (coarse feeds) of the material's real density.

Ideally, the most energy efficient method of comminution is the slow application of pressure to individual particles so as to cause structural failure, such that the energy lost as heat and noise is minimised. In industrial applications, the HPGR is the one device that is closest to this ideal and thus the most energy efficient industrial scale comminution device. This energy efficiency is essentially brought about by the fact that the energy transfer in the HPGR's compression zone is determinate and relatively uniform, whereas with other comminution devices the energy transfer is random and highly variable as depicted by table 1 and figure 3 respectively.

Figure 3: Forces imparted by various comminution devices

Table 1: Summary of differences in comminution machine characteristics

Device I Ball Mill HPGR i Roll Crusher Conventional Crushers icef'.',. !··V. : ',A';!) !!",,>\. "'(';,:~'1'>'>i)!!!) '"g:" ",'i,Iif)!)./' . , , , \:,·J',;t;,;'y:.: •• ,;, ' ,':jc':::~ ,::',.,

Comminution principle Surface I Particle Particle I Particle Surface I Particle Surface I Particle Gap setting Not applicable Self adjusting I Floating Preset! Fixed Preset I Fixed

Fructure force Low, attrition Controlled Uncontrolled Controlled, but difficult to

mani"ulate I' Choke feed Not applicable Yes No Yes

2.3 Key Operating Parameters

The performance of any comminution unit is mainly judged on achieving the prescribed duty and this essentially relates to throughput rate and product fineness. Product fineness relates to the liberation of valuables in a metallurgical processing plant while throughput rate addresses the economies of scale required to make an operation profitable. Both are equally important and in HPGRs, the applicable key operating parameters are the specific

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The South African Institute o(Mining alld Metallurgy Diamonds - Source to Use 2007 By lCharles Ntsele and 2Gerhard Sauermann

throughput rate and the required specific press force. These are explained in more detail below.

2.3.1 Specific Throughput Rate

The throughput (tons per hour) of an HPGR is given by the volumetric flow (L * s *um) through the machine's operating gap multiplied by the average density of the discharge material as shown by equation 1 below:

M = L * s * Urn * 8 * 3.6

Where,

M = throughput (t/h) s = working gap (mm) 8 = material density in the gap (tlm 3) L = roll width (m) Urn = material velocity in the gap (m/s)

[1]

The discharge of the HPGR consists of pressed, broken and bypassed material as depicted by figure 4, therefore the average density of the material passing the gap is the weighted average density of the various constituents.

Bypassed

Figure 4: Product components of the HPGR

To calculate the specific throughput (tons seconds per cubic meter hour) it is assumed that for a given material type and operating conditions the working gap of the HPGR scales linearly with the roll diameter. Based on this a specific

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The South African Institute o(Minillg and Metallurgy Diamonds - Source to Use 2007 By 1 Charles Ntsele and 2Gerhard Sauermanll

throughput (m-dot) is derived from this proportional relationship as shown by equation 2 below.

m-dote

Where,

-dote cake oe D s

(s / D) * oe * 3.6 [2]

specific throughput (t*s/m3*h), calculated from the HPGR

density of pressed material (tlm3) roll diameter (m) working gap (m)

This equation can be re-written as follows:

M

Where,

M D u m-dot L =

m-dot * D * L * u

throughput [t/h] roll diameter (m) roll speed (m/s) specific throughput (t*s/m3*h) roll width (m)

[3]

The specific throughput is constant for a particular feed type and set of operating conditions and can be determined via test work and thus equation 4.

m-dotr

Where,

m-dotr feed

M I (D * L * u) [4]

specific throughput (t*s/m3*h), calculated from the HPGR

Salient to the two forms of specific throughput rates (me and mr) is that the ratio me I mr informs about the material's behaviour in the working gap. A ratio less than 1 indicates extrusion in the compression zone or Intemal aml or external bypass, while a ratio greater than 1 indicates that the whole width of the rolls may not be in use or slippage. Also, this ratio enables the calculation of the expected working gap given by equation 5 below.

s = (me * D) I (oc * 3.6) * (me Imr) = (mf * D) I (oe * 3.6) * c [5]

The factor c is calculated from the ratio me / mf. In most coarse feed applications this factor is between 0.85 and l.

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The South African Institute of Mining and Metallurgy Diamonds - Source to Use 2007 By J Charles Ntsele alld 2Gerhard SaUermal111

2.3.2 Grinding Pressure

In terms of product fineness, the major machine parameter that contributes to the degree of fineness is the grinding pressure applied. However, the grinding pressure can not be measured directly. Various parameters are therefore used to quantify the grinding pressure applied to the material bed. One such parameter is the specific grinding force. This is the grinding force divided by the projected area of the rolls as calculated by equation 6 below.

Where,

q> D F L

F I (1000 * L *D)

specific grinding force (N/mm2) roll diameter (m) grinding force (kN) roll width (mm)

[6]

The specific grinding pressure is particularly suitable for establishing correlations between the grinding pressure in the material bed and the achievable product fineness and for comparing grinding forces amongst HPGRs of different sizes.

Typically most ores reach their maximum product fineness when specific grinding forces of between 4 - 5 N/mm2 are reached, for kimberlites an even lower range of between 2 and 4 N/mm2 suffices to reach this point. Figure 5 below presents a typical relationship of product fineness as a function of specific grinding force.

Figure 5: Effect of specific grinding force on product fineness, general and kimberlite specific

2.3.2 Power

The motor power required to drive the rolls is proportional to the applied grinding force. The point at which the force is acting on each roll (figure 6 overleaf) is determined by the force angle p. The grinding force may be

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The South African Institute of Mining and Metallurgy Diamonds - Source to Use 2007 By J Charles Ntsele and 2Gerhard Sauermann

resolved into a radial and tangential component Ft. The tangential component gives rise to the torque which has to be provided by the main drive motors to turn the rolls. The motor power required is then calculated per roll for a given roll speed by equation 7 below.

\ , , \

\

'¥ Force acting angle ~

Figure 6: Action of grinding force on roll

Pr

Where,

Pr T F

P co n D

co*T = 2 * 11: * n I 60 * D I 2 sin ~ * F

motor power (kW) roll torque (m) grinding force (kN) force action angle (0) angular roll speed (1/s) roll speed (rpm) roll diameter (m)

The total motor power P is then

P = 2 * Pr = 11: * n I 30 * D * sin ~ * F

[7]

[8]

The specific energy absorbed by the feed material can then be calculated according to equation 9 below which is derived from equations 3, 4, and 8.

wsp P/M = 2000 * sin ~ I m-dot *

The South Afj'ican Institute of Milling alld Metallurgy Diamonds - Source to Use 2007 By J Charles Ntsele and 2Gerhard Sauerlllalln

2.3.3 Additional Parameters that Affect The HPGR Performance

This section briefly describes some of the other HPGR concepts and their impact (where applicable) on its performance.

:Working Gap / Stop Gap

In HPGRs the stop gap refers to a pre-determined limit to movement of the floating roll towards the fixed roll, thus preventing the possibility of the rolls to be in contact. The working gap on the other hand is not pre-set, but rather adapts itself according to the nip-in characteristics of the feed material and has a linear relationship with the roll diameter. The working gap of the HPGR impacts on the coarse end of its product size distribution. Typically, the largest particle size that may be found in the product is either the size of the working gap (in cases where the feed top size is greater than the working gap itself) or the largest particle size in the feed (in instances where the largest feed particle is smaller than the working gap).

Roll Surface

The roll body of the HPGR rolls can either be cast or forged. F orgings require protection with either hard facing, hard metal tiles or studs. Hard or compound castings do not require additional protection, but the surface itself may be smooth, profiled or studded. Figure 7 below depicts these various surface types. Their development will be discussed elsewhere.

Figure 7: Types of roll surfaces protection

In addition to performing a protection role to the underlying materil, the profiles and studs also improve the nip in characteristics of the machine and increases the throughput rate of HPGRs. For instances instance, studded rolls have been found to have throughput rates that are between 50 to 100 % higher than smooth rolls while grooved (profiled) rolls are somewhere in between. Also, studded roll surfaces have also been found to be less sensitive to higher feed moisture than the other surface types.

Roll Aspect Ratio (WID)

The ratio of the roll width to the roll diameter is the so called aspect ratio of the rolls. There are different schools of thought that subscribe to either a high aspect ratio design or a low aspect ration design as indicated by figure 8 overleaf. .

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The South Afi'ican Institute of Mining and Metallurgy Diamonds - Source to Use 2007 By 1 Charles Ntsele and 2Gerhard Sal/ermann

Low Aspect (W/D) High Aspect (W/D)

Figure 8: Low and high aspect ratio roll designs

The pros and cons of each design philosophy include metallurgical, operating and capital aspects of the machine. In this section the metallurgical aspects will be discussed, while the others will be dealt with later in the paper. In terms of the metallurgical aspects, a low aspect ratio design, which is preferred by Polysius, has the advantage of being able to accept bigger feed top size due to the linear relationship (discussed earlier) of the roll diameter and top size. Also, the specific throughput rate is higher for coarse material in a high aspect ratio design mainly because the nip-in characteristics are improved in this configurati on.

Roll Speed

The relative roll speed which is defined as the roll velocity divided by its diameter has a moderate impact on the specific throughput of HPGRs. The relationship is such that as the relative roll speed increases, the absolute throughput (tph) increases linearly, while the specific throughput rate (t*s/ (m3*h)) decreases. Figure 9 below illustrates these relationships

300Tm~T~ 250

1 200

/ ~

V 150

100

1: 40

30

20

I ... Specific throughput AI. throughput ~ l

50 10

o o o 0.5 1 1.5 2

Relative roll speed u/O [1/5]

Figure 9: Throughput vs. relative roll speed

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The South African Institute of Mining alld Metallurgy Diamonds - Source to Use 2007 By 1 Charles Ntsele alld 2Gerhard Sauermallll

Of course, it is always a combination of both machine related parameters and ore characteristics that ultimately affect equipment performance. The influence of material characteristics is however beyond the scope of this paper.

3.0 Why the HPGR for Diamond Bearing Ores

Diamond winning comminution circuits unlike hard rock applications such as gold, copper and platinum more often than not do not require fine to ultra-fine (sub- 800 microns) grinding to liberate valuables. At most a final grind of approximately 1 to 1.7 mm is adequate for economic liberation. Also, diamond preservation during liberation is another significant aspect of the process. Therefore, any device used in the diamond liberation process should in addition to meeting capacity requirements, be energy efficient and preserve diamonds while liberating them using an appropriate amount of energy. All this should happen within acceptable operating costs.

The subsequent subsections will illustrate why the HPGR due to its ability to have both its gap and pressure controlled is a suitable unit process in the diamond liberation comminution circuit.

3.1 Diamond Liberation and Preservation

The HPGR efficiently liberates diamonds from the host rock (kimberlites) due to its ability to expose ore particles to extremely high pressures in a material bed between the two rolls, resulting in efficient inter-particle breakage. Figure 10 below illustrates this principle of grinding. It is this efficient inter-particle breakage that causes the host rock to break preferentially along grain boundaries whereby the typically softer material (host rock) yields to the pressure while the diamonds are liberated undamaged. Furthermore, the movement of ore particles within the material bed further cleans the diamonds and improves liberation from the waste material. Conventional cone crushers on the other hand, exposes the diamond bearing particle to undefined forces between the surfaces of the crushing chamber resulting in insufficient liberation or even diamond damage.

HPGR

Undamaged Diamond F ."' .. fter Material Bed ComminuHon

In a Large Working Gap

Crusher

Broken Diamond After Single Particle Comminution

In a Small Working Gap

Figure 10: Principles of comminution - HPGR vs. cone crushers

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F

The South African Institute of Mining and Metallurgy Diamonds - Source to Use 2007 By 'Charles Ntsele and 2 Gerhard Sauermann

The ability of the HPGR to produce a finer particle size distribution while operating with a bigger gap size (20 to 40 mm) minimises the likelihood of diamond damage. In cone crushers and standard rolls crushers, the gap setting has a direct impact on the product particle size distribution and therefore a compromise between product fineness, possible diamond damage, throughput and diamond liberation has to be negotiated continuously. Whereas with HPGRs an even finer product size distribution can be achieved at a larger operating gap without compromising diamond liberation, while decreasing the likelihood of large diamond breakage. Also, a larger working gap means a bigger volumetric flow through the unit, thus increased throughput. Figure 11 below depicts typical product distributions of HPGRs vs. conventional cone crushers for the same duty requirements.

~-~~.~~-~~ ... ~~---~---~~-.--~-.-~--- ... ~~~~-~~--,

100.0

90.0

~ 80.0

.[ 70.0 60_0 ~ 50.0 0

40.0

.....

=~=R9···········-····· d} ,+ "d~ - - ----';it. i f V T

- .~----- / :i~·-7--HPGR products

//. /'/ I ! , .--c-.

.--

The South African Institute Q(Mining and Metallurgy Diamonds - Source to Use 2007 By 1 Charles Ntsele and 2Gerhard Sauermallll

3.3 Throu.ghput / Particle Size Distribu.tion

Often in comminution a balance is required between throughput rate and the degree of product fineness. The finer the product requirements, the more the throughput rate has to be compromised. This relationship needs to be optimised and monitored at all times as different ore types will affect it differently. Cone crushers inherently have limitations in this area especially if the ore gets progressively harder because to maintain the same degree of fineness, the crusher's capacity will have to be reduced and if it is not economic to reduce throughput then the crusher gap will have to increased so that the throughput rate can be maintained, in which case product finesses will be compromised. Both solutions may not be ideal for the economics of a diamond processing plant. In instances where it can be illustrated that there is a possibility to enhance revenue by crushing finer, unlike cone crushers, the HPGR can achieve this without compromising on the rate of production of the target grind. This is due to the ability to readily control the grinding pressure and higher machine capacities enabled by a relatively larger working gap, thus volumetric flow.

3.0 Application of HPGRs in Diamond Bearing Ore Comminution Circuits

The flow-sheets of diamond ore treatment plants may vary greatly to an extent that a standard flow-sheet does not exist. Typically though, the HPGR can be employed up-stream or down-stream of a Dense Medium Separation (DMS) plant and this impacts on the selection and design process of the unit.

In an up-stream application, (Argyle and Cullinan diamond mines), two HPGRs (2.2 and 2.4 m diameter respectively) processes in the order of 300 to 700 tph . These units treat material prepared by two stages of open circuit crushing resulting in a feed top size to the HPGR of approximately 75 mm, occasionally lumps of up 250 mm are found to be present in the feed to the HPGR due to the lack of control of the crusher circuit product. To facilitate the handling of such a coarse top size (the intended 75 mm), the units are equipped with smooth segmented liners as a studded roll surface will limit the top size that can be fed into the unit. In this mode the HPGR is said to be operating in a tertiary (traditional nomenclature) comminution stage. Material being treated in this mode still contains all the large diamonds. This then requires a large '.Xlorking gap generally in the order of 30 mm depending on the operation's requirements in terms of their diamond size frequency distribution. In spite of such a big gap size, typically in the order of 45 to 55 % minus 1 mm product is generate by the HPGR in open circuit. Once the product is scrubbed and the minus 1 mm material removed from the HPGR product, the net effect is a reduction in the amount of tons that reports to the DMS plant. Figure 12 overleaf presents an example of this impact on the DMS plant whereby lower capital and operating costs can be expected from the DMS plant.

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The South Atrican Institute of Mining and Metallurgy Diamonds - Source to Use 2007 By 1 Charles Ntsele and 2Gerhard Sauermann

I Up to 44 % smaller DMS plant by applying HPGR in circuit I ~ Crushing without HPGR I I Crushing with HPGR I

1000 tph 1000 tph 20%

The South Afi-ican Institute o[Millillg alld Metallurgy Diamonds - Source to Use 2007 By J Charles Ntsele alld 2Gerhard Sauermanll

The net result of this decision was that the plant capacity was doubled at 5 % of what it originally cost to build the plant.

The continued decline in the grade of the ore-body necessitated further increases in plant capacity so that the annual carat targets can be maintained. Computer simulations were then employed to seek the best option to increase plant capacity. This work showed that the most effective way of achieving this was to install a second HPGR.

Later on, a review of the operation indicated that within 4 years the operation would close. This stemmed from the fact that while there was sufficient ore in the deposit, it was just getting difficult to extract the diamonds profitably with the cost structure that was employed by the operation. The simple option of increasing production in order to gain the benefits of scale so as to off-set the cost structure was made difficult by the fact that access to the ore within the mine was getting progressively difficult to a point where mining declared that it has reached the limit and can not deliver more. Several initiatives were then embarked on to avoid mine closure and this mainly involved cost cutting so as to maintain the required carats production per annum. When no further costs could be cut some fairly major changes to the plant were proposed that could potentially solve the problem, but these were found to carry significant technical risks.

The option that was finally chosen was to enhance the revenue for the same annual tons treated. A number of investigations showed that there were un-liberated diamonds in the coarse tailings that could be recovered if the crusher in the re-crush section was to be changed from cone to HPGR. This would enable the top cut size to be changed from 8 mm to 6mm without necessarily impacting negatively on throughput and increasing the possibility of diamond damage. In addition to finding a solution, the mine took this opportunity to compare all aspects of operation between the HPGR and cone crusher in the re-crush mode. This was achieved by configuring the circuit to accommodate both crushers and on instances when the HPGR was not operating, the cone crusher was used instead and performance data captured. Below are some salient outcomes of this comparative work published by Argyle.

);> For the same duty requirements the product distributions of the ~NO crushers were compared as illustrated by figure 13 overleaf.

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The South Atrican Institute of Mining alld Metallurgy Diamonds - Source to Use 2007 By 1 Charles Ntsele and 2 Gerhard Sauermalln

100.0 t~~II~R~eCfr~US~h~H~P~R~c~p~rO~je~c~t 1~r=1II--~ I Nominal Crusher Product Size Distributions .............. _ ............. _ ..................... _ ............ ··"4.' .. ·•· .. Cl --c 'iii f--- I--

,/' ID V '" 10.0 a.

~ :; 1.0 j E :::I t--.

0 --r- .

0.1 I1 - f----0.01 0.10 1.00 10.00 100.00

Product Size (mm)

r= Recr~sh F~;d _ HPRC 100bar -Cone Crusher I

Figure 13: Product size distribution of HPGR and cone crushing (Argyle mine)

The above diagram is based on real operating data and it aptly demonstrates the ability of the HPGR to produce a finer product than the cone crusher albeit using a bigger working gap.

~ A statement of efficiency was fonnulated which compared the energy input required to generate what was considered to be the key size for the operation (- 2.3 mm) by both the cone crusher and the HPGR, based on the plant trails. The HPGR was found to have applied 1.355 kWhlt, while the cone crusher applied 0.289 kWhlt. The energy ratio to produce the minus 2.3 mm material was thus 1.335 10.289 = 4.69 : 1 (HPGR: Cone)

However the HPGR was found to have generated 60,888 tones of the -2.3 mm material as opposed to the cone crusher that produced 5,172 tones of the same size material. Therefore the tonnage ratio was 60,888 15,172 = 11. 71: 1 in favour of the HPGR

The Relative Comminution Efficiency (R.E.C.) of the HPGR compared to the cone crusher was thus 11.72 /4.69 = 2.51: 1 implying that the IIPGR is 2.51 times more energy efficient than cone crusher in producing the nominated minus 2.3 mm size material.

As the grinding tends towards single particle breakage as is the case with the coarser components of the feed, the magnitude of the REC is less pronounced, in this case at 8 mm was found to be 1.54 times more than that of cone crushing, which is still significant.

);;> In tenns of the impact of the HPGR on the plant mass balance it was found that the unit enabled an increase in the re-crush new feed of approximately 10 % (of Headfeed) while allowing a 20 % drop (of

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The South Africall Institute o(Mining and Metallurgy Diamollds - Source to Use 2007 By 1 Charles Ntsele and 2Gerhard Sauermallll

Headfeed) in the amount of tons crushed in the re-crush itself. This was significant in that the implication was an equivalent of 1 million tons per annum that would otherwise not enter the -re-crush circuit could now be treated other than reporting to the coarse tailings, implying an increased e carat liberation. The overall plant throughput remained virtually unchanged even though the re-crush circuit was the plant's bottle-neck prior to the changes. The implication was that the feed to the DMS plant as a percentage of Headfeed increased to an extent that this part of the circuit now became the plants bottle neck.

6.0 Wear and Protection

The rolls of an HPGR are either of a solid design or are equipped with replaceable tyres or segmented liners.

The factors affecting wear rates in HPGR can be grouped into three categories, namely, the wear properties of the surface protection material, the physical properties of the ore and machine settings. In terms of the surface protection material, pertinent factors that affect wear rates are as follows:

~ The wear resistance characteristics of the material used, ~ Ability to build-up and retain an autogenous wear protection layer.

Ore characteristics that affect wear rates are:

~ Feed size ~ Moisture ~ Hardness ~ Mineralogical composition and ~ Grain size

Machine parameters that affect wear include,

~ Grinding force and ~ Roll speed

How each element contributes to wear will not be discussed here as it is beyond the scope of the paper, but of importance is how through partnerships with operators the surface protection has evolved.

Initially the HPGR rolls had a smooth surface as depicted in figure 14a below.

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The South African Institute of Mining and Metallurgy Diamonds - Source to Use 2007 By 1 Charles Ntsele and 2Gerhard Sauermann

Figure 14a: Smooth HPGR roll surface Figure 14b: Profiled HPGR roll surface

While this kind of roll surface enables the machine to accept a bigger feed top size, its life is sh0l1er than that of other surface types. In co-operation with users, to increase the service life of rolls, the roll surfaces were hard faced with profiles as depicted in figure 14b. This surface type is also tolerates bigger feed top sizes as it is limited to about 5 % of the roll diameter, for instance a 2.5 m diameter rolls machine can accept a feed top size of up to 125 mm.

This profiling enhanced the life of wearing surface due to less slip and extrusion as compared to smooth rolls. The profiles however require frequent renewal when treating abrasive ores. To overcome this, studded rolls (tungsten carbide pins) were then developed and introduced to operations. Figure 14c overleaf.

This kind of protection surface is characterised by allowing the formation of an autogenous layer between the studs, thus protecting the underlying base material. In most applications, this protection system offers the longest roll life. The limitation of such a protection system is that when treating very coarse and hard feed material pin breakage may occur due to localised pressure peaks on the roll surface. Therefore to optimise performance, pin quality, ore hardness, feed top size and the expected working gap should be well considered prior design. F

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The South Afhcan Institute oflvJining and Metallurgy Diamonds - Source to Use 2007 By 1 Charles Ntsele and 2Gerhard Sauermann

Figure 14c: Studded HPGR roll with an autogenous layer between the studs

This development has increased the operational availability of the HPGR to beyond 96 %.

7.0 The Future

Although not a trivial task, understanding the liberation characteristics of an ore body is the key towards unlocking the full potential of HPGRs. Liberation for diamond processing plants can be defined as the amount of in-situ carats, above a given size that are released by comminution processes. While this definition sounds straight forward, other financial drivers often come into play and these result in liberation being based rather on the equitable amount of stones that can be liberated. Also, carat liberation is intrinsically tied to revenue liberated. The revenue is however a function of the diamond size frequency distribution of a deposit. This is an underlying relationship that should be considered during plant design to establish the exact point where further grinding has little or no returns in terms of carats / revenue liberation. Of course, tied to all of this is the use of energy to achieve particular grinds. All operations should be aiming for the most efficient use of energy during comminution. .

Comminution devices typically use a combination of mainly impact and abrasion energy to achieve size reduction. The prevalence of a type on energy input i~ dt:pt:mlcnt on the unit type. For instance, scrubbers are typified by predominantly low fracture energy to effect abrasion breakage, while AG mill and HPGRs are predominantly characterised by the high fracture energy to effect breakage.

The characteristics of an ore body will reveal how much 'fines' can be generated by these predominant modes of fracture. An understanding of the amount of carats / revenue that is released is then very invaluable in deciding on how or where to position the HPGR in the circuit, since this is the most energy efficient comminution device.

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The South African institute ofMinillg alld Metallurgy Diamonds - Source to Use 2007 By 1 Charles Ntsele alld 2 Gerhard Sauermallll

High energy fracture is best suited to liberating fine and ultra-fine diamonds. Apart from being a high energy fracture device, the HPGR has the added dimension of utilising this energy efficiently (inter-particle crushing), while having the ability to preserve diamonds. For these significant reasons, there is no reason why a carefully considered comminution plant design should not be configured to primarily prepare material in the best way possible for the HPGR to do what it does best, especially for instances where the frequency distribution of the big stones is low and the split of carats / revenue release is more biased toward high energy fracture.

1 rLarge Diam and concentration ; & Recovery Circuit L""".,,c._.~. __ ......• _.~c __ .... '

Material requiring re-

crushing

Smaller DMS plant

Figure 15: Conceptual Flow Sheet of the Future - Combined Grinding

Main Liberation Circuit I perhaps

multi - stream

Figure 15 above is a conceptual example of a circuit geared towards liberating the majority of the carats / revenue through the HPGR. In instances where there are large stones, then a dedicated large diamond facility is incorporated, while in their absence, everything will report to the HPGR-Scrubbing circuit. In the absence of large diamonds, the front end of the circuit is dedicated to prepare feed adequately for the HPGR.

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The South African Institute ofMinillg and Metallurgy Diamonds - Source to Use 2007 By 1 Charles Ntsele and 2 Gerhard Sauermallll

8.0 Conclusions

The unique aspects of the HPGR have been discussed, illustrating how and why this comminution unit is indispensable in the diamond recovery flowsheet. Also the basic concepts of HPGR comminution have been explained. It is thus concluded that the HPGR technology is state of the art in diamond ore processing. The HPGR enhance revenue by improving liberation and minimising diamond breakage. This impacts directly on the capital cost as the rates of return are improved and on the operating costs with the higher availability and enhanced dollar per ton treated. This compelling value proposition has even overcome the myth that HPGRs are for major operators to the exclusion of junior minors as more of the juniors are embracing the technology and are implementing it in their comminution circuits.

The optimum comminution circuit configuration utilising an HPGR remains an exciting prospect and will differ from operation to operation as it is informed by the highest achievable diamond recovery counter balanced by the lowest capital and operating costs.

9.0 References

I. Various Polysius Internal publications 2. D. Maxton, C. Morley and Dr R Bearman, Re-crush HPRC Project- The

Benefits of High Pressure Rolls Crushing 3. E. Burchardt, HPGR, A Metallurgical Tool for the Diamond Industry

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