processing of magnetite iron ores â€“ comparing - metso.com

Processing of Magnetite Iron Ores – Comparing GrindingOptions

B McNab1, A Jankovic2, D David3 and P Payne4

ABSTRACTAlthough the majority of current steel production is supported by iron oresourced from high-grade haemetite deposits, the long-term growingdemand for steel has led to higher raw material prices and opened theway for many new magnetite deposits to also be developed.

There is a rich and long history of magnetite ore processing in Westerncountries, including large operations such as Cleveland Cliffs in the USAand LKAB in Sweden, as well as smaller operations like Savage River inTasmania. The challenge for virtually all magnetite operations is tominimise operating costs, which is dominated by the cost of powerrequired to fine grind the ore in order to achieve acceptable concentrateiron grade together with low impurity content. This remains the casetoday and will be even more important in the future when a carbon tax isexpected to become a significant addition to the operating costs formagnetite deposits.

Historically, the lowest operating cost was achieved by multistage fullyautogenous grinding with integrated magnetic separation steps betweenthe stages. The major benefit of fully autogenous grinding is theelimination of steel grinding media costs and the need to discriminatebetween steel and magnetite in coarse magnetic separation. Theseparation step between grinding stages progressively reduces the amountof material to be ground.

Application of more efficient grinding technologies developed in thelast 20 years, including high pressure grinding rolls (HPGR) for finecrushing and stirred milling for fine grinding, has provided opportunitiesto further reduce the operating costs associated with comminution. Bothtechnologies are already implemented in some magnetite processingoperations, although in limited capacity.

The results of a theoretical option study for high capacity processing ofa hard, fine-grained silica-rich magnetite ore is presented in this paper,with the emphasis on comminution circuit options. Several circuit optionsare ranked based on a net present value analysis incorporating an estimateof carbon tax added in the operating cost. The study demonstrates thesignificant advantages of applying more efficient autogenous grindingtechnologies.

INTRODUCTION

World iron ore resources are estimated to exceed 800 billiontonnes and world iron ore production in 2006 was 1690 Mt. Newiron ore mining capacity taken into operation in 2007 reachedalmost 130 Mt globally (United Nations Conference on Tradeand Development, 2008). In Australia, magnetite is mined at theSavage River mine with reserves of 22 Mt of magnetite ore at52 per cent Fe and the Iron Magnet deposit which has 300 Mt ofmagnetite ore reserves grading 37 per cent Fe. There are atleast 4.5 billion tonnes of magnetite resources grading 33.5 -36.5 per cent Fe in Western Australia, 1.5 billion tonnes at 31 -50 per cent Fe in South Australia and 700 Mt at 25 - 52 per centFe in Tasmania and Queensland (Clout et al, 2004).

Compared to direct ship haemetite ores mined from the upperregolith, magnetite deposits require significant beneficiation,which typically involves grinding to a particle size wheremagnetite is liberated from its silicate matrix. Many banded ironformation deposits in the Pilbara and Yilgarn cratons withinWestern Australia and the Gawler craton within South Australiaare very fine grained, often requiring a final concentrate grindsize P80 (80 per cent passing size) of 25 - 35 µm. The amount ofenergy required to produce a magnetite product suitable for sale aspellet plant feed is substantially more than an equivalent directship lump (<32 mm >6 mm) and fines (<6 mm) haemetite project.

MAGNETITE ORE GRINDING OPTIONS

Various magnetite ore grinding flow sheets have beenimplemented in the past, including:

• conventional three (and four) stage crushing followed byprimary and secondary milling,

• primary crushing followed by wet semi-autogenous grinding(SAG) or autogenous (AG) milling and ball or pebblemilling, and

• air swept AG milling (for coarse grinding).

Historically, the lowest operating cost for fine-grained oreswas achieved by multi stage fully autogenous grinding(Koivistoinen et al, 1989) with integrated magnetic separationsteps between the stages. The major benefit of fully autogenousgrinding is the elimination of steel grinding media costs and theneed to discriminate between steel and magnetite in coarsemagnetic separation ahead of pebble crushing. The separationstep between grinding stages progressively reduces the amountof material to be ground and in many cases reduces the abrasiveproperties of the concentrate.

Four North American subsidiaries of Cleveland-Cliffs Incemploy autogenous milling. The original autogenous millingcircuit, consisting of an AG mill followed by cobber magneticseparation of pebbles, pebble milling of the magneticconcentrate, a finisher magnetic separation stage and silicaflotation, was installed at Empire Mines in 1963 and had sixindividual concentrating lines with a capacity of 1.6 Mt/a ofpellets (Weiss, 1985). There have been three expansions sinceand, in the 1990s, Empire Mines had a total of 24 individualconcentrating lines and a total plant capacity of 8 Mt/a of pellets.The target grind size of the circuit varies between the 90 -95 per cent minus 500 mesh (32 µm) depending on the ore andoperating conditions (Rajala, Suardini and Walqui, 2007).

LKAB’s Kiruna is the world’s largest, most modernunderground iron ore mine. Ore is mined using sublevel caving,with sublevels spaced at 28.5 m vertically. After primarycrushing and hoisting to surface, the ore is processed in Kiruna’scomplex consisting of a sorting plant, two concentrators and twopellet plants to give pellet and sinter fines products (seehttp://www.lkab.com/). Magnetite ore is ground using AG millsfollowed by cobber magnetic separation and pebble milling ofthe magnetic concentrate (Hahne, Palsson and Samskog, 2003).The target grind size is around 80 per cent minus 45 µmdepending on the ore and operating conditions (Tano et al, 2006).

Application of more efficient grinding technologies developedin the last 20 years, including high pressure grinding rolls

Iron Ore Conference Perth, WA, 27 - 29 July 2009 277

1. MAusIMM, GRD Minproc Limited, GPO Box Z5266, Perth WA6831. Email: [email protected]

2. MAusIMM, GRD Minproc Limited, The Precinct 2, Level 2,10 Browning Street, West End Qld 4101.Email: [email protected]

3. MAusIMM, GRD Minproc Limited, GPO Box Z5266, Perth WA6831. Email: [email protected]

4. GRD Minproc Limited, GPO Box Z5266, Perth WA 6831.Email: [email protected]

(HPGR) for fine crushing and stirred milling for fine grinding,has provided opportunities to further reduce the operating costsassociated with grinding. At Empire Mines a HPGR is installedfor processing crushed pebbles and its introduction has resultedin a primary AG mill throughput increase in the order of20 per cent (Dowling et al, 2001). Application of Vertimill® finegrinding technology at Hibbing Taconite Company enabledprocessing of lower grade ores and increased the concentrateproduction (Pforr, 2001).

The Whyalla magnetite plant in South Australia is specificbecause it utilises the HPGR technology for comminution of theprimary ore. The HPGR circuit is closed with a 3 mm aperturewet screen with the undersize reporting to the rougher magneticseparators (RMS). Banded iron formation (BIF) ores at Whyallaallow a relatively large silica rejection to the RMS tailings streamand so are well suited to the particle size that can be effectivelygenerated by a commercial size closed circuit HPGR system.Figure 1 shows the plant feed bin (left of photo) and the highpressure grinding rolls.

STUDY OPTIONS

An option study for a 10 Mt/a ore processing plant for aconsistently hard, fine-grained silica-rich magnetite ore wascarried out, with the emphasis on comminution circuit options. Acapacity of 10 Mt/a was selected to simplify the comparison bykeeping within the current single processing line limitations of theworld’s largest AG mill capabilities. In practice, GRD Minprochas undertaken studies of Australian concentrators from 10 Mt/aup to 80 Mt/a, whereby the level of design, layout and operationcomplexity significantly appreciates when multiple trains ofworld’s largest equipment need to be integrated.

For the purpose of the study, the concentrator was assumed tobe located in the Pilbara of Western Australia within 100 km of aport suitable for facilitating equipment delivery. It was assumedthat there were no restrictions on spatial layout and that theprocess facility would be built on ground of a sound geotechnicalcharacter. Any subsequent differences in tailings disposal, waterrecovery, operation and cost were not considered.

The approach taken in comparing various flow sheet optionswas in line with GRD Minproc’s typical conceptual or scopinglevel assessment methodology and delivers a plus or minus35 per cent capital and operating cost accuracy. Such an exerciseis recommended at the commencement or prior to theprefeasibility study phase of a magnetite project. To enable ameaningful study a minimum required level of comminution andbeneficiation test work should have been undertaken beforehand.

A minimum set of ore comminution properties are required fora conceptual design. An example is provided in Table 1 and isused as the basis for this theoretical study.

The magnetite concentrate weight recovery, specific gravity(SG), predicted Bond abrasion index (BAi), iron and silicacontent were based on the following relationships:

Concentrate weight recovery% = 10.737 ln(P80) - 3.0945

Concentrate iron content Fe% = -8.4667 ln(P80) + 98.455

Concentrate SG = 0.84( )% . % .Fe Fe×⎛

⎝⎜⎞⎠⎟

+ − ×⎛⎝⎜

⎞⎠⎟

⎡

⎣⎢

⎤

⎦⎥

518

7241

3 0

724

Concentrate Ai = 0.05(%Si020.4332)

Concentrate silica content Si02 % = 9.6966 ln(P80) - 29.571

The fine-grained nature of this hypothetical ore results in arelatively late release or liberation curve as illustrated in Figure 2.This fundamental property of a magnetite ore is generally one ofthe most salient drivers of flow sheet design and therefore flowsheet option generation. For example, it is not uncommon formagnetite banded iron formation ores to exhibit two distinctliberation zones, a coarse size consistent with the inherent silicabanding width and a finer size related to the unlocking ofsilicates within the magnetite bands.

For the ore considered, only 20 - 30 per cent of feed mass canbe rejected magnetically in the size range typical for closedcircuit HPGR operation. Conversely, significant mass up toapproximately 50 per cent can be rejected to tails at a P80 of150 µm which could be achievable with a closed circuit AG milloperation.

FLOW SHEET OPTIONSFour circuit options were considered for comparison with thefollowing acronyms used to identify the primary unit processwithin each:

• COS – coarse ore stockpile,

• SC – secondary crush,

• HPGR – high pressure grinding roll,

• AGC – autogenous mill in closed circuit with cyclones andpebble crusher,

• RMS – rougher magnetic separation,

• CMS – cleaner magnetic separation,

• CMS2 – second cleaner magnetic separation,

278 Perth, WA, 27 - 29 July 2009 Iron Ore Conference

B McNAB et al

FIG 1 - High pressure grinding rolls at Whyalla magnetiteconcentrator plant (Morgan, 2007).

Ore grade % FeT 32.2

Drop weight index (DWi) kWh/m3 11.1

Ore specific gravity 3.40

Concentrate specific gravity 4.30

Bulk density t/m3 2.01

Bond ball mill work index (BBWi) kWh/t 17.2

Bond abrasion index (BAi) 0.3

Bond rod mill work index (BRWi) kWh/t 17.7

Bond crushing work index (BCWi) kWh/t 20.6

Point load index (PLI) MPa 14.8

Unconfined compressive strength (UCS) MPa 355

Fibrous mineral content Nil

TABLE 1Ore design parameters.

• PM – pebble mill,

• PC – primary crusher,

• SM – stirred mill, and

• TSF – tailings storage facility.

Option 1 – PC/AGC/RMS/PM/CMS

Primary crushing – AG milling in closed circuit withhydrocyclones and pebble crushing – rougher magnetic separation– pebble milling – cleaner magnetic separation (see Figure 3).

Option 1 resembles the well known fully autogenous LKABand Cleveland Cliffs style operations. Absence of steel grindingmedia significantly reduces the operating cost. Pebble millcontrol and pebble transport and handling requirements addcomplexity to the design and operation. A P80 of 250 µm wasnominated for the RMS feed.

Option 2 – PC/AGC/RMS/BM/CMS/SM/CMS2Primary crushing – AG milling in closed circuit withhydrocyclones and pebble crushing – rougher magneticseparation – ball milling – cleaner magnetic separation – tertiarymilling using stirred mills – second cleaner magnetic separation.(see Figure 4)

Option 2 has an additional grinding and magnetic separationstage compared to Option 1 and is considered to be simple indesign and operation. In respect to the grinding flow sheet, it issimilar to the Savage River operation although the AG mill isclosed with hydrocyclones rather than screens, there is nohydroseparator and a third stage of comminution has been addedin respect of the finer liberation requirement. For reasons of costestimation and layout simplicity hydrocyclones have beenselected rather than screens to close the AG mill circuit for bothOptions 1 and 2.

A P80 of 250 µm was nominated for the RMS feed, 75 µm forthe CMS feed and 30 µm for the CMS2 feed.

Option 3 – PC/C SC/C HPGR/RMS/BM/CMS1/SM/CMS2Primary crushing – closed circuit secondary crushing – closedcircuit HPGR – rougher magnetic separation – ball milling – firstcleaner magnetic separation – tertiary milling using stirred mills– second cleaner magnetic separation (see Figure 5).

In Option 3 secondary crushing and HPGR effectively replaceAG milling with pebble crushing. Application of HPGR, stirredmilling and additional magnetic separation stage reduces thepower requirements compared to Options 1 and 2. The flow sheetis similar to that applied at Project Magnet south of Whyalla inSouth Australia and the proposed Gindalbie Project east ofGeraldton in Western Australia.

A P80 of 2300 µm was nominated for the RMS feed, 75 µm forthe CMS feed and 30 µm for the CMS2 feed. This wouldnecessitate wet screening to close the HPGR circuit.

Option 4 – PC/SC/O HPGR/PM1/RMS/PM2/CMS1/SM/CMS2

Primary crushing – secondary crushing – screening – openHPGR – coarse pebble milling – rougher magnetic separation –fine pebble milling – first cleaner magnetic separation – tertiarymilling using autogenous stirred mills – second cleaner magneticseparation (see Figure 6).

Option 4 is an attempt to design a circuit with the lowestoperating cost through increased grinding energy efficiencyusing three stages of magnetic separation, traditionalautogenous milling, HPGR and stirred milling technology. Inthis conceptual flow sheet steel grinding media is eliminated.Circuit complexity is partially reduced by open circuitsecondary crushing, HPGR grinding and stirred millingoperation although recovery, storage and control of threeseparate sized media streams is introduced. It is assumed that amagnetic selection process is applied to collect each mediastream thereby maximising the power drawn thus capability ofthe downstream autogenous grinding unit.

A P80 of 500 µm was nominated for the RMS feed, 75 µm forthe CMS feed and 30 µm for the CMS2 feed.

ENERGY COMPARISON

With the exception of the primary crushing module, which isconsistent between options, estimates were developed for thetotal power drawn in the comminution, classification andmagnetic separation areas of each circuit. Energy consumed bymaterial transport machinery related to pumping between areaswas not considered at this level of study. A summary of theresultant unit circuit energy for each option is shown in Figure 7.


PROCESSING OF MAGNETITE IRON ORES – COMPARING GRINDING OPTIONS

y = 10.737ln(x) - 3.0945R² = 0.9705

30

40

50

60

70

80

90

10 100 1000 10000

P80 Size (µm)

Ma

ss

%o

ffe

ed

tom

ag

ne

tic

co

nce

ntr

ate

FIG 2 - Grind versus liberation.


B McNAB et al

FIG 3 - Option 1 flow sheet.


A significant circuit energy reduction is predicted withOptions 3 and 4, which include HPGR and stirred milling. Some33 per cent of additional energy separates the most energyefficient option (Option 4) from the least efficient, the twostage AGC Pebble circuit, Option 1. In today’s market and theprojected energy market during the life cycle of a typicalmagnetite concentrator the magnitude of this difference issignificant unless the cost of power is negligible (<$20/MWh),which is very rarely the case.

Interestingly if weighted averages are calculated for thecombined tailings from each circuit the outcomes for Options 3and 4 do not parallel the energy consumption comparison.Option 3 with the coarse feed to the RMS has an average tail P80of 736 µm and Option 4 284 µm. Some economic advantagesmay be realised by the coarser tails product although are beyondthe scope of this comparison.

According to Seidel et al (2006), the basic comminutionenergy requirement for the Boddington HPGR circuit option was14 per cent lower than the SAG option; however, the overallenergy requirement including conveying, screening, etc, wasonly five per cent lower. The Boddington copper gold ore is ofsimilar rock competency to that selected for this study and soprovides a good contrast between comminution processesdesigned to liberate minerals for flotation, in which the wholeore is ground to fine size, and comminution designed to rejectsilicates via magnetic processes. In the latter case energyefficiency between flow sheet options can be far more extreme.

PROCESS OPERATING COST

Albeit a high-level comparison, a fairly rigorous approach wastaken to the development of operating costs for each option.Consumption rates for power, wear and other consumables,labour and maintenance and materials were generatedconsidering each process flow sheet from the COS reclaimfeeders to either the final magnetic separator concentratedischarge or the magnetic separator tailings discharge. As such,no concentrate or tailings handling, filtration or storage costswere considered. For simplicity, some minor operating costs suchas metallurgical test work and analysis, which is consideredcommon to all options, have been omitted. Unit costs for power,grinding media, wear consumables and labour were referencedfrom average values within the GRD Minproc database forsimilar sized and located projects. A factoring approach fromdirect capital cost was used to develop cost estimates formaintenance materials. Key assumptions are listed in Table 2. Allcosts are estimated in Australian dollars and are presented as firstquarter 2009 costs.

The estimates as summarised below are judged to have anaccuracy of ±35 per cent. Unit cost breakdowns are presentedand shown graphically in Figure 8:

• Option 1 – $6.17,

• Option 2 – $6.42,

• Option 3 – $6.66, and

• Option 4 – $5.38.

The most significant operating cost variables between optionsare those relating to power, media and liner consumption. The twooptions including AG mill circuits have between 27 - 32 per centhigher power consumption cost relative to Option 4. Or lookingfrom the other perspective, application of new technologiesresulted in an average 25 per cent energy consumption reduction.

Grinding media and wear lining costs range between $0.41/tand $1.82/t. Option 3 has the highest media and wear lining costas two ball mills of 8.8 MW installed power are required to grind8 Mt/a of RMS concentrate from P80 2.3 mm to P80 75 µm. Thisis the largest ball milling duty of all options being some 2.8times larger than the Option 2 requirement.

A carbon tax is expected to be introduced in the near futureand would add a significant cost to the operation. For thisexercise a simplified estimate of the effect of carbon tax isconsidered. It was assumed that the carbon tax would be appliedto total circuit energy and steel consumption relating to mediaand comminution equipment wear liners. The following criteriawere applied for the carbon tax estimate:

• CO2 emission, 5 t per 1 t of steel media (Price et al, 2002); and

• CO2 emission, 1.0 kg per kWh of electricity, CO2 tax, $23 pertonne of CO2 (Australian Government, 2008).

Table 3 shows a summary of calculations related to carbonemission and carbon tax effect on OPEX. It can be observed thatthe introduction of carbon tax at $23/t would increase OPEX inthe order of nine to 11 per cent. The majority of carbon emission


B McNAB et al

Power $/MWh 120

Ball mill steel media $/t delivered 1501

Stirred mill steel media $/t delivered 1814

Labour on-cost % 50

Total HPGR cost $/t of HPGR feed 0.35

TABLE 2Key operating cost inputs.

33.0

31.6

25.0 24.7

0

5

10

15

20

25

30

35

Option 1 Option 2 Option 3 Option 4

Cir

cuit

En

erg

y(k

Wh

/fe

ed

ton

ne

)

FIG 7 - Circuit energy comparison.

is from electrical energy consumption while indirect contributionfrom steel consumption (dominated by grinding media) is inthe order of five to 16 per cent for Options 2 and 3 that utiliseball milling.

PROCESS CAPITAL COST

The scope of the estimates follows the work breakdown structuredeveloped specifically for the study and considers each flowsheet from the COS reclaim feeders to either the final magneticseparator concentrate discharge or the magnetic separator tailingsdischarge. As such, no concentrate or tailings handling, filtrationor storage was considered. For simplicity some equipment orcosts considered common to all options have been omitted.

The estimate is developed based on the premise that the processis located inland in north-west Western Australia. All costs areestimated in Australian dollars and are presented as first quarter2009 costs. They are judged to have an accuracy of ±35 per cent,which is commensurate with the accuracy requirements for a highlevel options study of this nature.

The capital cost estimates have been structured into threemajor categories, detailed below.

Direct costs

Direct costs are those expenditures that include supply ofequipment and materials, freight to site and construction labourrelevant to the particular option.

Indirect costs

Indirect costs are those expenditures covering engineering,procurement and construction management (EPCM) servicestogether with the supervision of the commissioning of the works.Contract works and goods in transit insurance have also beenincluded. Temporary construction facilities have been included inIndirects.

Contingencies (growth allowance)

Contingencies have been assigned as an overall percentage to thetotal estimate. Contingency has been applied to the estimate tomake allowance for the following risks:

• minimal design input,

• preliminary scope definition,

• quantity survey errors and omissions,

• material and labour rate accuracy,

• equipment budget costing, and

• incorrect ‘bulks’ factor application.

Estimation methodology

An industry standard methodology for a conceptual levelestimate has been applied for the cost comparison and includes,‘bulk’ quantities, equipment and platework, freight, capitalspares, temporary facilities, indirect costs, temporary facilitiesand EPCM and contingencies (growth allowance).

A detailed equipment list has been prepared and imported intothe estimate. For each item of equipment costs have been enteredas per the basis outlined above. Budget quotes have been receivedfor certain items of major equipment. GRD Minproc’s databaseinformation (or allowances) has been used for other equipmentitems appropriately factored and escalated where necessary.

The estimates are summarised and tabulated in Table 4:

• Option 1 – $346.6 M,

• Option 2 – $356.9 M,

• Option 3 – $321.3 M, and

• Option 4 – $312.6 M.




Power CO2 t/a 329 503 315 768 248 757 238 328

Steel CO2 t/a 5804 18 256 37 300 8306

Total CO2 t/a 335 307 334 023 286 057 246 634

CO2 tax $/t 0.77 0.77 0.66 0.57

OPEX $/t (no CO2 tax) 6.17 6.42 6.66 5.38

CO2 tax % OPEX 11.1 10.7 9.0 9.3

TABLE 3Carbon emissions and carbon tax summary.

Option 1

Option 2

Option 3

Option 4


MISCELLANEOUS 0.17 0.18 0.17 0.17

LABOUR 0.80 0.85 0.87 0.90

MAINTENANCE MATERIALS 0.83 0.86 0.81 0.79

GRINDING MEDIA & WEAR LINERS 0.41 0.74 1.82 0.66

POWER 3.95 3.79 2.99 2.86

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

Op

era

tin

gC

osts

($/f

ee

dt)

FIG 8 - Operating cost comparison.

Total estimated capital costs for each circuit are within 14 percent, which does not infer any one option is a standout from acapital cost perspective. Due to the higher power drawn byOptions 1 and 2, which include AG mill circuits, a lower capitalcost to power consumption ratio results. Option 1 offers thelowest ratio with $8.4 k/kW consumed, Option 2 has $9.0 k/kWconsumed and Options 3 and 4 with $10.3 k/kW $10.1 k/kWrespectively (refer Table 4).

FINANCIAL COMPARISON

Applying a ten per cent discount rate over 12 years of operation,high level, pre-tax, net present value (NPV) determinations werecalculated for Options 1 to 3 relative to the base case, Option 4,which returned the lowest capital and operating cost. Figure 9compares these outcomes.

Options 1 and 3 have a similar NPV outcome ranging betweennegative $94 - 95 M relative to Option 4. A $1 M variance isviewed as being immaterial relative to the accuracy of the study.Option 2 shows the least favourable outcome with a $118 M NPV

deficit relative to Option 4. This option is disadvantaged by bothhigh capital and operating cost.

The all autogenous Option 4 flow sheet is $94 M lower thanthe next best option, Option 1. The conclusion drawn from thisfinancial evaluation is that highly energy efficient autogenousprocessing routes can offer significant benefits for competentmagnetite ores requiring fine grinding. Were this hypotheticalprocess design to be advanced, piloting test work would be welljustified to explore the validity of the key autogenous unitprocesses proposed within Option 4.

Having established a relative NPV comparison a variabilityanalysis was undertaken to understand each flow sheet option’ssensitivity to two key operating cost inputs namely power andball mill media. The outcomes are presented graphically inFigures 10 and 11.

Not surprisingly, Options 1 and 2, which are the most energyintensive circuits, are found to be highly sensitive to power costwhen compared relative to Option 4. Only in the case that powercould be supplied at a zero cost could Option 1 (AGC/RMS/PM/CMS) approach the NPV value of Option 4.


B McNAB et al

Area number Area description Option 1 Option 2 Option 3 Option 4

Direct costs $ $ $ $

001 Coarse ore feed 7 237 040 7 237 040 7 151 313 6 241 940

002 AG mill grinding 80 068 915 88 965 461

002 Secondary crushing 18 809 132 13 923 099

002A Tertiary crushing 11 969 442

003 Pebble recycle crushing 17 704 056 14 972 636

003 HPGR circuit 34 215 535 20 430 750

003A Pebble mill grinding and pebble storage 36 389 765

004 Rougher magnetic separation 2 272 362 2 272 362 3 282 300 2 524 846

005 Ball mill grinding 18 264 249 48 448 884

005 Pebble mill grinding 90 872 360 27 801 316

006 Concentrate separation and fine milling 76 680 928 76 680 928 66 959 736

006 Concentrate separation 2 758 989

Total – process plant 200 913 722 208 392 675 188 588 092 186 240 895

Site preparation and improvements 2 098 103 2 083 927 1 885 881 1 862 409

Control aystems 4 552 915 4 586 582 3 668 577 3 463 224

Total – plant infrastructure 6 651 018 6 670 509 5 554 458 5 325 633

First fill reagents and consumables (allowance) 0 2 646 705 3 413 705 0

Ocean freight 10 180 855 10 095 453 7 373 659 7 405 066

Spares 5 630 444 5 612 860 4 325 727 4 219 437

Mobilisation/demobilisation and preliminaries 6 447 027 6 287 927 6 554 941 6 759 735

Commissioning assistance 2 098 103 2 083 927 1 885 881 1 862 409

Total – miscellaneous 24 356 429 26 726 872 23 553 914 20 246 647

Total – direct cost 231 921 169 241 790 056 217 696 463 211 813 176

Indirect costs

Temporary facilities 19 265 417 19 343 204 17 415 717 16 945 054

EPCM 36 122 657 36 268 508 32 654 469 31 771 976

Total – indirect costs 55 388 074 55 611 713 50 070 186 48 717 031

Total costs (net) 287 309 243 297 401 769 267 766 649 260 530 207

Growth, contingency, risk 59 241 158 59 480 354 53 553 330 52 106 041

Total costs (overall) 346 550 401 356 882 123 321 319 979 312 636 248

Delta total cost relative to Option 4 33 914 153 44 245 875 8 683 731 0

Total cost/kW drawn 8414 9041 10 289 10 108

TABLE 4Capital cost comparison.



-$140,000,000

-$120,000,000

-$100,000,000

-$80,000,000

-$60,000,000

-$40,000,000

-$20,000,000

$0

700 800 900 1000 1100 1200 1300 1400 1500 1600 1700

Ball Mill Media Cost ($/t)

Option 1

Option 2

Option 3

De

lta

NP

VR

ela

tive

toO

pti

on

4($

)

FIG 11 - Net present value sensitivity to ball mill media cost.

-$160,000,000

-$140,000,000

-$120,000,000

-$100,000,000

-$80,000,000

-$60,000,000

-$40,000,000

-$20,000,000

$0

0 20 40 60 80 100 120 140 160 180 200

De

lta

NP

VR

ela

t ive

t oO

pt i

on

4( $

)

Power Cost ($/MWh)

Option 1

Option 2

Option 3

FIG 10 - Net present value sensitivity to power cost.

-94

-118

-95

-140

-120

-100

-80

-60

-40

-20

0

Option 1 Option 2 Option 3

De

lta

NP

VR

ela

tive

toO

pti

on

4($

M)

FIG 9 - Net present value (NPV) (comparison).

Ball mill media cost is only relevant to Options 2 and 3 as theother circuits include autogenous grinding. Figure 11 highlightsthe high NPV sensitivity to ball mill media cost for Option 3,which considers the largest ball milling duty. Although the NPVrelationship shown is steep, Option 3 would not becomeeconomically equivalent to Option 4 even at a zero ball millmedia cost.

DISCUSSION

Some clear understanding of the economic merits of processcircuit options can be gleaned by developing a comparative studyas illustrated by this paper. In this case it was found that highlyenergy efficient autogenous processing routes can offersignificant benefits for fine-grained competent magnetite ores.The traditional AG mill and pebble mill style comminutioncircuit or those requiring significant steel grinding media tooperate have been found to be suboptimal from a pure economicperspective. Circuit options favouring multistage magneticseparation and with energy efficient autogenous comminutionequipment are more likely to add project value.

The approach taken in this paper to compare flow sheetoptions was purely economic and to a large degree simplified byassumptions. In practice there are many other flow sheetselection drivers that can become relevant or even exclusivelydominant. Some examples of these from recent GRD Minprocexperience include:

• the identification of fibrous minerals within the ore;

• the availability/cost of water;

• the capability of the project owner to accept risk;

• spatial layout constraints, ground slope and geotechnicalcharacteristics;

• a prescribed study and development schedule that does notallow sufficient time or budget for comparative test workprograms;

• variability in rock competency or magnetite liberation;

• social and environmental risks relating to dust and the costsassociated in dust collection and control;

• the effect of moisture on HPGR performance and cakingproperties of HPGR product;

• long-term predictions for the cost of power and grindingmedia; and

• the cost impacts of tailings including capital, operating costand environmental risks.

To expand further on the final point other capital and operatingcosts components related to different options for tails disposalwould need evaluation in the next phase of study. Options whichallow coarse RMS tailings, such as Options 3 and 4 in this paper,offer the potential to be inexpensively dewatered and eitherconveyed or transported by dump truck to be either comingledwith mining waste or dumped in a separate tailings storagefacility (TSF). Cost savings in process water consumption andTSF capital may be realised with this approach.

The justification and value of this type of evaluation process isreflected by the magnitude of the NPV delta produced. In thiscase a $118 M delta resulted between the options considered. Itis therefore not unreasonable to expect that flow sheet selectionhas the potential to vary project value by up to one billion dollarsfor the world’s largest scale magnetite projects being considered.In comparison, the total cost of sample collection, metallurgicalbench and pilot scale test work and engineering studies becomesinconsequential for such projects.

GRD Minproc places a high importance on developing andunderstanding geometallurgical relationships during the projectevaluation phase and as such would recommend that the

requisite test work is completed prior to undertaking comparativeconceptual or prefeasibility studies. This approach infers testwork and study planning need to be suitably timed to ensure anadequate basis is available at the commencement of a studyperiod. The rapid pace of project evaluation and developmentexperienced during the 2006 to 2008 mining boom did notalways permit this strategy and in many cases has led to higherproject risk, inefficient engineering practices and slowcommissioning and ramp up. Further information describingGRD Minproc’s approach to geometallurgy and analysis can bereferenced in David (2007).

CONCLUSION

The primary conclusion drawn from this financial evaluation isthat highly energy efficient autogenous processing routes withmultiple separation stages offer significant financial benefits forfine-grained hard magnetite ores. For the ore type evaluated, theapplication of HPGR and stirred mill technology is indicated toreduce energy consumption by up to 25 per cent compared toconventional flow sheets with wet tumbling mills.

Flow sheet design and option selection should be tailored tothe magnetite ore comminution and liberation characteristics. Forfine-grained ores the addition of a third grinding stage whichutilises energy efficient stirred milling benefits the economics intwo ways: reducing the amount of material that need to be fineground and grinding at high energy efficiency. The application ofHPGR technology significantly reduces the energy consumptioncompared to AG milling. A ‘synergy’ of HPGR and primarypebble milling, as proposed for Option 4, can result in a veryeffective circuit from a capital and operating point of view withHPGR working in open circuit feeding the primary pebble millwhich in addition to grinding, generates grinding media(pebbles) for the secondary pebble milling circuit. The productfrom the primary pebble mill is much finer than what can bepractically obtained from a closed HPGR circuit and thus the tailrejection at the RMS is higher which significantly reduces theduty of the following pebble milling stage.

The magnetite market in general and the required scale ofmagnetite concentrator capacity is rapidly expanding and as aresult will amplify the importance and value of soundmetallurgical investigation and process design. To this end a wellstructured and scheduled study and evaluation period is ofconsiderable importance if the objective is to maximise life cycleproject value and mitigate financial and stakeholder risk.

An introduction of a carbon tax at levels indicated by theAustralian government is predicted to increase the operating costfor this type of operation significantly, in the order of nine to11 per cent or $0.57 - $0.77/t of ore.

ACKNOWLEDGEMENT

The authors acknowledge the permission of GRD Minproc topublish this paper and the assistance of James Higgie incompiling flow sheets and operating costs during his studentvacation work period.

REFERENCESAustralian Government, 2008. Carbon Pollution Reduction Scheme:

Australia’s Low Pollution Future – White Paper, volume 1,December.

Clout, J M F, Trudu, A, Zhu, D, Holmes, R J and Young, J, 2004.Australian magnetite resources and pellet plants, in Proceedings2004 Pelletizing Conference, Dalian, China, 19 - 22 August.

David, D, 2007. The importance of geometallurgical analysis in plantstudy, design and operational phases, in Proceedings Ninth MillOperators’ Conference 2007, pp 241-248 (The Australasian Instituteof Mining and Metallurgy: Melbourne).


B McNAB et al

Dowling, E C, Corpi, P A, McIvor, R E and Rose, D J, 2001. Applicationof high pressure grinding rolls in an autogenous – Pebble millingcircuit, in Proceedings SAG 2001 Conference, Vancouver, vol III,pp 194-201.

Government of India, 2007. Comprehensive industry document on ironore mining, Ministry of Environment and Forest, Government ofIndia [online]. Available from: <http://www.cpcb.nic.in> [Accessed:2007-08].

Hahne, R, Palsson, B I and Samskog, P O, 2003. Ore characterization for– and simulation of – primary autogenous grinding, MineralsEngineering, 16:13-19.

Koivistoinen, P, Virtanen, M, Eerola, P and Kalapudas, R, 1989. Acomminution cost comparison of traditional metallic grinding,semiautogenous grinding (SAG) and two stage autogenous grinding,in Proceedings SAG 1989 Conference, Vancouver.

Morgan, A, 2007. Project Magnet HPGR’s [online]. Available from:<http://www.amped.net.au/2007_06_01_archive.html>.

Pforr, B, 2001. Fine screen oversize grinding at Hibbing TaconiteCompany, SME Annual Meeting, Denver, February.

Price, L, Sinton, J, Worrell, E, Phylipsen, D, Huc, X and Li, J, 2002.Energy use and carbon dioxide emissions from steel production inChina, Energy, 27(2002):429-446.

Rajala, G, Suardini, G and Walqui, H, 2007. Improving secondarygrinding capacity at Empire Concentrator, SME Annual Meeting,Denver, 25 - 28 February.

Seidel, J, Logan, T C, LeVier, K M and Veillette, G, 2006. Case study –Investigation of HPGR suitability for two gold/copper prospects, inProceedings SAG 2006 Conference, Vancouver, vol IV, pp 140-153.

Tano, K T, Pålsson, B I, Alatalo, J and Lindquist, L, 2006. The use ofprocess simulation technology in process design when time andperformance is critical, SME Annual Meeting, Denver, February.

United Nations Conference on Trade and Development, 2008. The IronOre Market 2007 - 2009 [online]. Available from:<http://www.unctad.org/infocomm/Iron/covmar08.htm#abstract>.

Weiss, N L (ed), 1985. SME Minerals Processing Handbook, pp 20-27(Society for Mining, Metallurgy and Exploration, Inc: Littleton).



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