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Case Study of Dry HPGR Grinding and Classification in Ore Processing

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Case Study of Dry HPGR Grinding and

Classification in Ore Processing

F.P. van der Meer

Humboldt Wedag GmbH, Cologne, Germany

Minerals Processing and HPGR Technology

Colonia-Allee 3

51067 Cologne, Germany

Tel: +49 221 6504 1473

Email: meer.fp@khd.de

Web: www.khd.com

S. Strasser

Humboldt Wedag GmbH, Cologne, Germany

Director, Process Design, Cement processing

Colonia-Allee 3

51067 Cologne, Germany

Tel: +49 221 6504 1473

Email: meer.fp@khd.de

Web: www.khd.com

ABSTRACT

Dry HPGR processing is gathering interest from industries that are operating in areas where

water scarcity or process conditions do require a minimizing of water addition and

consumption. High Pressure Grinding Roll (“HPGR”) technology could provide solutions in

operations where even a very fine grind is required, by applying closed circuit arrangements

with air classification.

Building on available processes and experiences from cement processing, tailored products can

be generated at high throughput rates.

This publication summarizes some of the features and experiences with dry HPGR processing.

A descriptive iron ore case study will be presented illustrating the process steps and equipment

design, and examples of potential applications in copper and gold ores. This includes a

comments on product handling, effects of moisture content, wear rates and desagglomeration

requirements.

KEYWORDS

HPGR, dry processing, autogenous lining, KHD, iron ore, heap leaching, roll surface wear

protection

INTRODUCTION

In the majority of present-day minerals processing operations, the ore handling is done by dry

crushing of generally moist ore, and subsequent wet grinding and classification. However,

especially case when the subsequent beneficiation process involves dry processing, such as dry

low or medium intensity magnetic separation, or in cases where wet handling of the finely

crushed ore is cumbersome (such as with earthy of clayey ores) dry grinding may provide a

more practical and economical route.

In addition, the number of minerals projects where moisture levels are low, or where the feed

material is nearly completely dry, and where water scarcity and cost plays a role (such as in the

high Andes, the Sahara, North West Australia or other desert locations) is increasing. As a

consequence, the design and cost of operation of an envisaged fine crushing and grinding stage

is influenced.

For these cases, a HPGR circuit can easily be designed to operate in closed circuit with dry

classification, in an integrated plant involving static and dynamic classification. In cement

processing, where comminution of limestone, slag, or clinker is done as feed for subsequent of

fine ball milling and kiln firing, dry HPGR grinding, or HPGR involving a drying stage, has

been successfully applied for many years.

Over the last decade HPGR has become a standard tool in cement preparation. Application of

dry air classification, either as a separate process stage after HPGR or as integrated part of the

grinding process [Strasser, 2010], has enabled HPGR application as a reliable pre-grinding

stages, but also as a single and final grinding facility without the need for ball milling. Product

quality in cement grinding averages around a specific Blaine surface area of up to 5,000 cm²/g

and a particle size of 80 % < 25-30 µm.

Product sizes for minerals processing are generally in a coarser range than those encountered

for cement processing, although ultrafine grinding (by various types of stirred mills) is used for

complex and finely intergrown ores. Based on the experiences of HPGR circuits operating in

cement applications, dry HPGR circuits are now being evaluated for minerals processing. In

this, several areas need to be considered, such as the effect of (low) moisture on operating

parameters, component wear life, product classification requirements, dust generation, and

circuit footprint.

UNIT PROCESS STAGES IN DRY GRINDING AND CLASSIFICATION

HPGR was initially developed in response to demand in the cement industry for a lower

energy, higher capacity grinding process to replace less efficient conventional crushing

processes ahead of a final ball milling stage. Following extensive studies (Schönert, Schwechten,

) on both single particle crushing phenomena and size reduction of samples in piston tests and

laboratory compression rolls, the HPGR technique (also called roller press) thus was developed

for this industry. Initial HPGRs, partially based on a re-design of smooth double roll crushers

and briquetting machines, did provide a solution but resulted in a modest throughput and a

significant maintenance effort as a consequence of the high wear from material slippage and

abrasion on the smooth roll surface.

As a response, profiled rolls were developed to enhance the specific throughput of wet and dry

materials. Following experience of high wear rates of the roll surface, the stud-lined design was

invented by KHD Humboldt Wedag. This has developed to be the universal industry standard

for minerals applications and many cement plants, especially in the treatment of dry materials.

At present, hundreds of HPGRs are being applied worldwide in both cement and minerals

applications, with roll sizes of up to 3.0 m diameter and potential capacities per single unit

(depending on material performance) well over 3,500 t/h.

Dry classification was developed in conjunction with the HPGR technology, as a further

development from the classical dry gas cyclone facilities. As a first step, a cross-flow separator

(“V”-Classifier) was developed for a mid-range classification (80-1,500 µm), which could be

followed by a dynamic cage wheel separator (such as the SKS®-Classifier) for a finer range of

products (25-150 µm). Figure 1 does show an example of the V-classifier.

Figure 1 Operating Principle of V-Classifier (left)

and Classifier at installation (right) [Suesegger, Strasser]

The V-classifier, also called a V-Sifter, acts by passing an air flow across a stream of falling

particles in-between a cascading track of louver plates, thereby blowing the fines out of the

falling stream. These fines are then collected through a dust cyclone and bag house system, and

the coarse material, lean of fines, discharges through the bottom of the device. By variation of

air flow and angle of inclination, different separation cut sizes can be obtained, generally

varying between 80 and 1,500 µm.

The advantages of the VS-Classifier include:

- passage of coarse feed

- no moving parts

- reduction of wear in subsequent dynamic cage wheel classifier

- very low maintenance effort, both in frequency and wear parts

- low pressure difference of 4-8 mbar

- high material to air loading

- concurrent desagglomeration of compacted HPGR discharge

- concurrent drying by (hot) gas flow

Grinding plants world-wide are installed with KHD`s V-Separator and operate nearly

maintenance-free. In 2007 a redesign of the V-Separator into a compact version was introduced.

Designed for the same separating area, the new V-Separator-Compact has only 50% of the

height compared with the original design.

A cage wheel separator is generally applied as finishing stage in dry classifier arrangements

behind the V-classifier stage. A flow of air or gas, laden with the fine particles, is blown through

a vertically or horizontally spinning cage wheel (Figure 2). The coarser particles or middlings

particles in the bulk are thrown back by impact or friction with the vane blades and are

recovered from a bottom-discharge valve, whilst the fine dust passes through the slots, to be

collected in a gas cyclone and bag house dust collection system. By variation of air flow and

cage wheel rotational speed, different separation cut sizes can be obtained, generally varying

between 25 and 150 µm.

Figure 2 Cage Wheel (left) and HPGR with Static and Dynamic Classifier (right) [Strasser]

Presently, a large number of cement operations are applying the above unit processes to

generate either a finished product, or a pre-ground product for subsequent ball milling [Strasser

2008, 2010; Binner]. In these installations, a HPGR is combined with one or both of the above

classifiers.

The final separation is commonly done by a dynamic separator (“”Sepmaster”). This unit,

shown below on the left, has the advantage of compact design and excellent separating

efficiency. Another feature is the support of the rotor at both ends. Besides a vibration-free run,

the bearings are not subjected to process heat and dust and can be reached easily from outside.

The second main feature of the unit is the rotor drive at the bottom with separate support of

gear and motor instead of placing it on top of the separator housing. Dismantling of gear and

motor is not required for maintenance or if rotor bearings have to be replaced.

Figure 6 Final Product Separator – SEPMASTER and Static Type LS-Separator

In raw meal production for cement, where the product fineness is usually coarser than 88%

passing 90µm, a further static classifier (“LS-separator”) can be applied to scavenge the final

medium sized or residual coarse material from the rising fines fraction of the V-classifier. For

small to medium capacities, the static LS-Separator is an excellent choice, resulting in the lowest

energy consumption and maintenance (See above picture on the right). Since the V-Separator

takes already half of the separating job, the following static LS-Separator has a low feed to air

ratio and operates with excellent efficiency. COMFLEX references are available with both

separator configurations.

An example of product size distributions for a fine (banded magnetite) ore material is given in

the Figure 4. In this example, a classifier product was achieved with a fineness of 80 % < 40 µm,

from a HPGR product of 80 % < 2 mm. By variation of air flow and rotor speed, any product cut

size can basically be achieved.

In operations with the traditional location of V-classifier above the HPGR is was found that the

discharge material from the HPGR, carrying a high proportion of fines, required specially

designed equipment to cope with the high dust load and spillage. For heated operations with

moist material, dust and vapor generated by the HPGR process resulted in blockages of chutes

and venting equipment. Furthermore, when grinding blast furnace slag, a highly reactive

product was generated at certain moisture levels, and a high maintenance effort was required

for the material handling equipment.

KHD, as one of the founders of HPGR and inventor of stud-lining for roll surface protection,

started development of an integrated grinding and classification system “COMFLEX” in 2007

[Strasser, 2010]. This system is a further development of the designs presented above, and

incorporates the placement of a V-classifier at the discharge of the HPGR. The advantage is that

the dust is swiftly taken out of the process, and only coarse, nearly dust-free material requires

conveying. Excessive dust and spillage is avoided. Where moisture is present, this can readily

be removed: moist material can be dried in the V-classifier by the air sweeping. The feed

distribution over the width of the classifier is ideal since the HPGR discharge and V-inlet are set

parallel orientation. Furthermore, deagglomeration of flake material potentially present in the

HPGR discharge can be taken care of by the cascading impact on the louver plates of the V-

classifier separation track.

A typical layout for such a grinding plant is shown in Figure 5a, with the VS classifier is placed

below the HPGR. Rejected coarse material from V-classifier and dynamic classifier is

transported back to be combined with the HPGR feed. A further improvement was found in

placing the dynamic classifier higher up. Above the HPGR (figure 5b), such that the fine rejects

could be fed to the HPGR by gravity, instead of having to be transported. Only the relatively

coarse, dust-free discharge of the V-classifier thus required transport. A newer design

V-classifier with a longer step grate and slightly flatter shape enables a lower overall height of

construction (about 10 m). In the COMFLEX system, the final cut for cement or slag is made by

the dynamic separator.

With the V-Separator and the Roller Press being on the right place, the final cut for cement or

slag meal is usually done by the dynamic separator SEPMASTER.

With large grinding plants the material circulation can be higher than the maximum capacity of

standard conveying equipment. To avoid a second bucket elevator, the layout has been

improved further:

The dynamic separator is placed on a higher level to convey its rejects directly to the Roller

Press by gravity chute. In this case the air required for the separating process is fully used for

pneumatically lifting of fine material from the V-Separator. The load to the bucket elevator is

reduced and can be designed 25 to 40% smaller. Investment cost is reduced.

The layout in above illustration compares the mass flow of Figure 3: Roller Press, Standard Sizes a

conventional system. Only one bucket elevator is required for COMFLEX®.

Below arrangement drawing shows a comparison between original the COMFLEX®-system and

COMFLEX-Compact, which includes all further developments of main equipments and layout.

Figure 5 - Lay-out Example of the Previous and Present Compact design for COMFLEX plant.

Where bucket elevators are common practice inmost cement operations, these may find less

acceptance in minerals operations where coarse and moist materials may lead to lower

availabilities. In these conditions, the application of conveyor belts may be more appropriate.

The dry processing of cement materials by HPGR (and associated classification) is well

established, also driven by the requirement of minimizing grinding costs in the energy

absorbing dry milling, and the obvious necessity to generate a dry product. The plant design is

different from the average minerals processing plant, with the requirement of rather

voluminous equipment (air classifiers) and internal transport facilities. Reducing the footprint

and building weight has been a focus of effort in the last few years. Certainly for Greenfield

operations in dry areas, the technology is available to be implemented in minerals processing.

CASE STUDY OF DRY HPGR GRINDING AND CLASSIFICATION

Magnetite Iron Ore Grinding. Case #1

An investigation was carried out to evaluate the feasibility of dry grinding and classification of

a North American magnetite iron ore from a feed size of about 50 mm to a dry product particle

size distribution of near 50 µm by using high pressure grinding and dry classification. The

operation was looking at a capacity 1100 t/h new feed, and did envisage to eliminate all tertiary

crushing, with using HPGRs to generate an as fine as possible product for downstream final

grinding and beneficiation. In this, it was considered that 1 mm screens are probably as small as

can practically be used at these tonnages.

A conventional wet ball mill grinding system could achieve the requirements, which would

involve wetting the material before grinding, wet grinding, and subsequent sedimentation,

thickening and filtration the product. The anticipated costs and complexity of this route for the

very fine product was reason to investigate a dry grinding system. A dry grinding ball mill

system however was expected to result in a high operating cost, and it was considered that a

dry HPGR and air classification system might provide a good alternative.

The work was carried out including several series of closed circuit grinding tests, involving

HPGR grinding, screening and air classification, at various operating parameters and moisture

levels. In evaluating the test work, a two-stage circuit arrangement was considered as a basis for

the design of a grinding plant, and two systems were compared: a two-stage HPGR system,

each in closed circuit with screening (dry 7 mm and wet 1 mm), and a subsequent fine ball

milling on one hand, and a two stage HPGR system with a first stage HPGR in closed circuit

with dry 7 mm screening, and a second stage in closed circuit finish grinding with dry air

classification as alternative. Figures 6 and 7 do illustrate these flow sheets.

Figure 6 – Three Stage Flow Sheet with Two Stage HPGR and Ball Mill Fine Grinding.

Figure 7 – Two Stage Flow Sheet with the Second HPGR and Air Classification for Finishing

Fine Grinding.

The first stage HPGR, for a feed top size of 50 mm in closed circuit with 7 mm screening, was

indicated to generate a product of 70-80 % passing 7 mm, at a proportion of about 15 % 90 µm

fines. The general performance of the HPGR was very satisfactory, and well comparable to the

performance of HPGR’s in high density magnetite iron ore in Chile or North America. Specific

throughput in closed circuit was high, and the net specific energy consumption for the first

grinding stage averaged 1.3 kWh/t. Depending on the screening efficiency, a circulating load of

about 40 % (recycle in proportion to fresh feed) was calculated, and the overall energy input to

this grinding stage thus was calculated to be 1.8 kWh/t. For a duty of 1,100 tph, a single HPGR

could process this at a net power consumption of 2,300 kW

The second stage HPGR, for a feed top size of 7 mm in closed circuit with 1 mm wet screening,

did generate a product of 55 % passing 1 mm. The screen undersize product was about 80 %

passing 550 µm, with 44 % passing 90 µm. The general performance of the HPGR again was

very satisfactory. Whereas the closed circuit tests with dry screening did generate little flakes,

the second stage tests, at a moisture content of 4 %, did generate a proportion of about 25 % by

volume in the centre discharge. The finely crushed ore adequately filled-in the space in-

between the studs on the rolls’ surface, thus providing a competent autogenous wear layer.

Specific throughput in this case was equivalent to that of the first stage, and the net specific

energy consumption averaged 1.2 kWh/t. Depending on the screening efficiency, a circulating

load of about 110 % was calculated, and the overall energy input to this grinding stage thus was

near 2.6 kWh/t. A single HPGR could perform the required duty at a net power consumption of

2,900 kW.

For the final grind, a calculation was made of the power input to the ball mills required for

grinding a feed of 80 % < 550 µm to a product of 80 % < 90 µm. Assuming a reverse circuit with

classification ahead of the ball milling, a mass proportion of about 30 % of finished product

included in the HPGR screen undersize could bypass the mills. Given a measured Bond work

index of about 12 kWh/t (a reduction by 8 % as compared to non-HPGR feed), and assuming 4

ball mills of a size 4.4 m (15 feet) diameter by 7.2 m (24 feet) length, a power input per mill of

about 1,925 kW would be required for an 704 t/h capacity. Thus the total power consumption

would be 4 x 1,925 =7,700 kW. In addition, some power consumption needs to be attributed to

the pumps for the materials handling system and the hydrocyclones.

As alternative to the second stage HPGR, a closed circuit grinding was run for a feed top size of 7

mm in closed circuit with dry air classification, for which the moisture of the system feed was

below 1 %. In the classification, variation process conditions such as air flow and cage wheel rotor

speed do provide a means to establish a cut size at virually any selction point. Obviously,

depending of the fineness of the HPGR discharge product, this does reflect in the achieved

circulating load of the rejected coarse fraction from the classifier.

Figure 4 - Examples of Air-Classifier Product Size Distributions

In the present test work, the classifier could be set to generate a product of 80 % passing 90 µm at a

recycle load of about 380 %. The general performance of the HPGR again was very acceptable.

Considering the dry status of the feed, an acceptable autogenous wear layer was generated. A

minimal proportion of flakes was present in the centre discharge. These disintegrated almost

immediately when handled or upon entering the air classifier. Specific throughput in closed circuit

with air classification did indicate to be near 320 ts/hm³, and the net specific energy consumption

for this grinding stage averaged 0.88 kWh/t. Depending on the classification mass split and

efficiency, a circulating load of about 380 % was calculated, and the overall energy input to this

grinding stage thus was near 3.1 kWh/t. For a duty of 1,100 tph, two HPGR’s operating in parallel

could process this at a net power consumption of 2,350 kW per unit, or 4,700 kW in total. In

addition, some power consumption needs to be attributed to the materials handling system

(conveying) and the Fan and Motor of the air classifiers.

Overall, the comparison for the energy input to the grinding system, to grind from a feed of top

size 40 mm to a product of 80 % y< 90 µm can be summarized as follows:

HPGR and Ball

Milling

HPGR and Air

Classification

1st Stage HPGR from 50 mm to 7 mm 2,300 kW 2,300 kW

2n Stage HPGR from 7 mm to 1 mm 2,900 kW

Fine Grind by ball milling from 1 mm to 90

µm

7,700 kW

Fine Grind by HPGR 7 mm to 90 µm 4,700 kW

Total Grinding Power 12,900 kW 7,000 kW

Pumps, Fans, Materials Handling 15 % 1,935 kW 1,050 kW

Total 14,835 kW 8,050 kW

Grinding Power Savings 6,785 kW = 45 %

Comparison at progressing grinding fineness…

Magnetite Iron Ore Grinding. Case #2

An investigation was carried out to evaluate the amenability of dry HPGR grinding and

classification of a magnetite iron ore from a feed size of about 40 mm to a dry product particle

size distribution of between 100 % < 125 µm and about 50 % < 45 µm by using high pressure

grinding and dry classification.

Given that a liberation of a significant proportion (25 %) of the gange occurs as a 3 mm size,

magnetic cobbing was proposed for this size. Therefore the application of HPGR was

considered in a flow sheet of tertiary crushing in closed circuit with 3 mm screening,

subsequent magnetic cobbing to reject coarse gangue, and a final HPGR grinding stage in

closed circuit with dry air classification to produce a – 125 µm iron ore pre-concentrate for final

magnetic separation beneficiation.

As an alternative, a direct grinding of the 50 mm feed in a Comflex plant to produce a – 125 µm

iron ore pre-concentrate for final magnetic separation beneficiation was calculated. In this set-

up, an intermediate product could be bled from the classifications circuit to be directed to a low

intensity magnetic separation stage.

For this purpose, a series of HPGR comminution tests were carried out using a pilot HPGR and

Comflex unit at the KHD Humboldt Wedag test facilities in Cologne, Germany.

The work was carried out including several series of closed circuit grinding tests, involving

HPGR grinding and air classification, at various operating parameters and moisture levels. On

the basis of the test work, two circuit arrangement options were indicated as a basis for the

design of a grinding plant.

The first selected system did involve a pre-grinding stage of HPGR in closed circuit with dry

screening to generate a feed to an intermediate stage of dry low intensity magnetic separation.

This beneficiation would reject a 20-25 mass % low iron content tailings mass, and produce a

pre-concentrate for further grinding. The subsequent fine grinding stage would HPGR in closed

circuit with dry air classification.

Figure 6 – Two-Stage Flow Sheet

Figure 7 – Full Comflex Flow Sheet.

Indicative Energy Comparison for Dry Processing Options

An indicative comparison could be derived for dry grinding energy consumption between a

system of HPGR and dry ball milling (Figure 16), and a system involving HPGR and VSK®-

classification (Figure 17). As an example base case, a material with a Bond Work Index of 14

kWh/t was considered, and a HPGR specific energy consumption of 1.8 kWh/t (depending on

pressure applied), to be ground from a top size of 50mm to a product size of 125 µm.

HPGR pre-grinding ahead of ball milling in closed circuit with 6 mm screening could be done at

a circulating load of about 40 % (as % of fresh feed supplied), followed by ball milling from a

P80 of about 3.5 mm to a product of P80 of 125 µm. Overall grinding energy for this circuit

could be calculated as near 16 kWh/t of feed.

HPGR pre-grinding in closed circuit with VSK®-classification indicated that a circulating load of

about 400 % (as % of fresh feed supplied) would be achieved for VSK®- product at a P80 of 125

µm. Overall grinding energy for this circuit was calculated as near 10 kWh/t of feed, and thus

implies a reduction in overall comminution specific energy input of near 35 %.

Figure 17 – Comflex Plant Lay-Out, Front View

Figure 18 – Comflex Plant Lay-Out, Side View

CONCLUSIONS

HPGR is worth evaluating for dry processing and dry classification in minerals processing. Dry

processing HPGR circuits and equipment can be considered as proven and available

technology, and are widely applied in cement industry. Similar systems can provide a feasible

solution for minarals processing projects where there is a strong need to minimize water usage,

such as from the increasing tendency world-wide to reduce water consumption in populated

areas, or in arid areas with limited availability of water. and also when processing of a dry or

low moisture feed, or where dry processing ahead or after grinding is required.

The application of HPGR with dry classification by static V-classifier or a combined static and

dynamic classifier (VSK®-Classifier) can generate a fine product as desired for subsequent

beneficiation, or pelletization. The grinding by HPGR and dry classification alone, without the

requirement for a final ball mill grinding stage downstream, may provide a means to further

reduce operating cost and energy consumption.

Dry processing can also provide a process arrangement to dressing a HPGR product suitable for

heap leaching, in cases where excessive fines may hinder the percolation process and heap

stability, especially in areas where water is scarce.

Wear rate on the roll surface when treating dry materials can effectively be managed by

applying a newly designed RollSpray® facility for building-up a strong autogenous surface

coating on stud-lined rolls, requiring only about 0.1 % water.

REFERENCES

Binner, J. (2006). Grinding and Classification Technology for Slag. Global Slag. November 2006,

pp. 12-17.

Strasser, S., and Seemann, S. (2008). A New Look at Slag Grinding. 4th Global Slag Conference. 10-

11 November 2008, Strasbourg, France.

Strasser, S. (2001) V-Separator & VSK-Separator. A Development by KHD Humboldt Wedag.

.Internal Presentation, 03-10-2001

Strasser, S., and Seemann, S. (2010). COMFLEX® – Highly efficient and flexible comminution

system. 16th Arab-International Cement Conference and Exhibition 6-8 December 2010 Ras Al

Khaimah, UAE

Van Der Meer, F.P., Matthies, E., Westermeier, C.P., Gallardo, V.H., Negroni, P. (2009). Success

and Reliability of HPGR Crushing at Compania Minera Huasco in Chile Procemin 2009, Sixth

International Minerals Processing Seminar. 02-04 December 2009, Santiago, Chile.

Van Der Meer, F.P., and Dicke, R. (2008). High Pressure Grinding; How high can you go?

Procemin 2008, Fifth International Minerals Processing Seminar. 22–24 October 2008, Santiago,

Chile.

Suesegger, A. (1996). V for Victory. Practical experience with the V-Separator. International

Cement review No, 12, Decvember 1996

Van Der Meer, KHD Humboldt Wedag. (2007) German Patent Application DE102007030896.7

“Verfahren und zugehörige Guttbettwalzenmühle zur kontinuierlichen Zerkleinerung spröden

Mahlgutes. 03-07-2007

Van Der Meer, F.P. (2011) Feasibility of Dry High Pressure Grinding and Classification.

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