commissioning and operating an autogeneous mill at … · commissioning and operating an...

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Metallurgical Plant Design and Operating Strategies (MetPlant 2013) 15 - 17 July 2013, Perth WA

Commissioning and Operating an Autogeneous Mill at Karowe Diamond Mine

L M van Niekerk1, G N Ndlovu2 and N A Sikwa3

1. Senior Process Engineer, DRA Pacific. PO Box 1283, West Perth WA 6005. Email: [email protected]

2. General Manager, Karowe Diamond Mine, PO Box 44, Letlhakane, Boteti, Botswana. Email: [email protected]

3. Technical Superintendent, Karowe Diamond Mine, PO Box 44, Letlhakane, Boteti, Botswana. Email: [email protected]

ABSTRACT

In June 2012, Lucara Diamond Corporation officially commissioned its Karowe Diamond Mine in Letlhakane, Botswana. The processing plant circuit incorporates variable speed autogeneous milling, a first for African diamond processing – only previously practiced in Russia. The paper discusses the basis for the selection of milling against the more traditional circuits incorporating crushing and scrubbing. It covers early commissioning challenges of operating a mill in an ‘alien environment’ with no immediate reference plant to look up to for bench-marking.

The paper describes how these challenges were overcome and learnings quickly incorporated into the mill operating philosophy for maximum diamond liberation and minimum product damage. The plant was commissioned in three months and by August 2012 the production ramp up had reached name-plate capacity. The feed to the mill has varied from highly weathered material to extremely hard kimberlite and the operating philosophy had to be optimised for the full range of ore types. While more work is required to stretch the mill operating envelope coupled with other circuit additions to improve liberation of fine diamonds at minimum capital expenditure, the mill has, to date, performed to its billing.

KAROWE BACKGROUND INFORMATION

Karowe Mine is the mine developed from Lucara Company’s AK6 Project and is 100% owned by Boteti (Lucara Diamond Corporation, 2013). Lucara is a public company and member of the Lundin Group of Companies: listed on the TSX Exchange, NASDAQ OMX First North Exchange and the Botswana Stock Exchange and has a 100% indirect interest in Boteti. The Karowe Mine is located in north-central Botswana and is part of the Orapa/Letlhakane Kimberlite district, one of the world’s most prolific diamond producing areas. The kimberlite at the Karowe Mine (the “AK6 kimberlite”) comprises a single, tri-lobate kimberlite pipe, which is “pinched” at the surface, and its sub-outcrop consists of a core of kimberlite, covering an area of 4.2 ha, surrounded by an area where the kimberlite is capped by basalt or basalt breccia. Drilling has shown that the kimberlite bulges to a maximum area of 7 ha at a depth of 120 m.

The significant majority of the ore body consists of a competent ore whose main component, identified as Unit 13, is unusually hard for a kimberlite. It is also very abrasive and displays other abnormal properties including high (Phase 2) DMS yields, high crushing and low amenability to scrubbing. The Karowe diamond plant is designed to process 2.5 million tonnes of ROM kimberlite ore per annum (for Phase 1) with a single 200 mtph DMS module. The concentrate material from the DMS is subsequently treated through a 2.5 mtph wet X-ray Recovery for material reduction and diamond winning.

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AG MILLING VERSUS CONVENTIONAL CRUSHING, SCRUBBING AND SCREENING CIRCUITS

Historically, problems were experienced in the operation of diamondiferous ore crushing circuits in the Siberian sub-zero arctic conditions due to the choking of chutes with frozen ore. Milling was investigated as an alternative to crushing, with the critical investigative work being carried out on dry circuits at temperatures both above and below 0˚C. Wet milling was later found to be a superior process and has since been continually developed over a period of thirty years (Cambon and Shirley, 1994).

Autogeneous (AG) grinding mills have seen increased application in large high throughput mineral processing operations in recent years as used for example in dolerite, iron ore, gold, zinc/lead, platinum and vanadium slag processing. They are rotating/tumbling mills in which comminution (size reduction) takes place without steel or ceramic grinding media. The mills consist of a large diameter cylinder relative to their length (typically 2, 2.5 or 3 to 1) and use large lumps of ore as the grinding media. They use ore exclusively as their grinding media but if the hardness and abrasiveness of the ore does not lend itself well to full AG milling, then a small charge of steel balls (6 to 15 % by mass) is added to assist in size reduction. This is known as a Semi-Autogeneous (SAG) mill.

AG mills bring about size reduction by a combination of impact, attrition and abrasion forces. The ore is swept up one side of the mill and at a certain point it falls back to impact the toe of the charge at the bottom of the mill. Ore particles in the body of the charge slide when moving to different heights and are subjected to attrition and abrasion forces. The inside liner of AG mills consists of steel or polymer liners and are fitted with lifter bars.

When the grinding conditions are right, AG mill circuits (Figure 1) can accomplish the same size reduction work that normally takes multiple stages of crushing, screening and grinding methods (Figure 2), this accounts for its popularity for certain ore types. It also lends itself to high volume processing. Often the product can be finished size or ready for final grinding in a ball mill or pebble mill. They can grind run-of-mine rock (limited by top size) or primary crusher products with their feed size limited to what can practically be conveyed. In addition they can be less costly to operate with the reduction or elimination of expensive balls or rods. When treating materials of variable competency and degrees of weathering, a versatile pulping and size reduction process, like an AG mill, is often the most suitable and cost effective.

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OVERSIZE

SLIMES MIDDS

PRIMARY CRUSHING PEBBLE CRUSHING

COARSE/FINES

SCREEN

ING

AG MILLING

DMSDEGRIT/THICKENING

Fig 1 - AG mill diamond circuit

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O/S

MIDDS

OVERSIZE

MIDDS

MIDDS

SLIMES SLIMES

PRIMARY CRUSHING

HPGR

COARSE

SCREENING

FINES

SCR

EENING

SCRUBBING & SCREENING

DMS

DEGRIT/THICKENING

PEBBLE CRUSHING

Fig 2 - Conventional crushing/screening/scrubbing diamond circuit

Specific advantages for using an AG mill in a diamond plant flowsheet include the following:

smaller footprint (one AG mill vs. secondary, tertiary/HPGR crushing and scrubbing), smaller capital expenditure associated with smaller footprint, lower energy cost (one mill vs. secondary, tertiary/HPGR crushing and scrubbing load

requirements), achieving and maximising the desired diamond liberation through a single comminution step

(post primary crushing stage), removal of slimes (and clay material) much earlier in the process and improved control to optimise size reduction conditions.

KAROWE MILL DESIGN CRITERIA

Milling philosophy

The autogeneous high aspect ratio variable speed mill was required to treat various materials and kimberlite from the Karowe ore body, at a rate of 350 mtph (in closed-circuit with a cone crusher) to provide a minus 30 mm product with a waste fraction below 1.5 mm. The various minerals

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encountered during treatment ranged from soft sandstones and weathered kimberlites, to more competent kimberlites, mudstones and basalts. A large degree of variability was therefore expected, seeing as there is a fivefold variation in overall power requirements.

Milling design criteria

Material characterisation tests for plant design and diamond liberation involved rock mechanics and Drop Weight Tests (DWT). Rock mechanics data is typically used to communicate the scope of work and to obtain performance guarantees from crusher equipment suppliers (SGS South Africa, 2013). The DWT on the other hand are used to establish the rock’s relative amenability to comminution, by both impact and abrasion forces. The DWT allows for ores to be characterised independently of characteristics of comminution equipment and are primarily used in software models for the eventual sizing of comminution equipment. The initial ore dressing study (or ODS) test work was scrutinised and Table 1 lists the DWT values obtained.

Table 1: Drop weight test values

Unit DWi

(kWh/m3) SG UCS (MPa) A b A*b

1 1.30 2.01 12 68.6 2.26 154.9

2456 6.48 2.75 115 100.0 0.42 42.4

8 6.55 2.71 112 75.2 0.55 41.4

13 5.70 2.97 137 59.8 0.87 52.0

13 (JK Tech DWT, August 2010)

- 3.07 - 62.2 0.49 30.5

14 7.30 2.84 109 100.0 0.39 38.9

mdstn (mudstones) 7.39 2.47 117 45.9 0.73 33.4

sstn (sandstones) 0.67 1.98 37 64.6 4.60 297.0

The ore is characterised for impact breakage by the two parameters: A and b. The value of the multiplication of these parameters, the A*b value, has been found to have the best correlation with ore resistance to impact breakage. Lower values indicate harder ores. The ta value describes the particle size distribution of the product. As with the A*b value, a lower value of ta indicates a harder ore.

For the different Karowe material types, it is estimated that ta values range from 0.26 (JK Tech DWT, August 2010) - 0.36 to 1.56 from hard to soft material. It was also envisaged that during the initial stages of operation the mill will receive the bulk of weathered and softer materials (principally Unit 1) and will later treat the more competent materials like Unit 13, together with mudstones and sandstones making up the major diluents. The mill was conservatively sized to suit Phase 1 of the project, with a view to possibly doubling up for Phase 2. The variable speed capability is to cater for mill load stabilisation, and grate capacity. The mill is fed with jaw crusher product and pebble crusher recycle product. A 28’ Ø x 13’ EGL (8.53 m Ø x 3.96 m) 4MW variable speed (VSD) autogeneous mill operating in closed circuit with a double deck screen (30 mm top deck and 1.5 mm bottom deck) and cone crusher was proposed, operating at low percentage feed solids. Pebble ports of 70mm and 30mm grate slots were also initially proposed. Table 2 and Figure 3 below, highlight the most important design parameters used for the sizing and selection of the Karowe mill (Van Niekerk, 2010).

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Table 2: AG mill design criteria values

Criteria Units Phase 1 Phase 2

Operating work index kWh/t 4.6~5.0 7.4~8.6

Circuit feed size (fresh feed) F80 (mm) 40~71 40~159

Circuit product size P80 (mm)

%-1.5mm

3.4~4.8

68~73

4.8~19.0

45~67

Pinion power kW up to 3 500 up to 3 500

Installed power kW 4 000 4 000

Mill speed %TCs 65~70 75~80

Circulating load % 17~26 19~72

Circuit screen size mm 30 top

1.5 bottom

30 top

1.5 bottom

Product slurry

(-1.5mm)

Density (t/m3)

% (w/w)

1.17~1.20

24~27

1.15~1.18

20~24

0

10

20

30

40

50

60

70

80

90

100

0.01 0.10 1.00 10.00 100.00

Cumulative

% weight passing

Size [mm]

Jaw Fine South Lobe (Unit 1)

Jaw Coarse South Lobe (Unit 1)

Pebble Fine Unit 13

Pebble Coarse Unit 13

Pebble Extra Coarse Unit 13

Fig 3 - Simulated mill discharge product (minus 30 mm)

High level design mass balances regarding Unit 1 (weathered ore) and Unit 13 (competent ore) can be viewed in Figures 4 and 5.

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ORE = 350 tph

PEBBLE CRUSHER (+30mm) = 63 tph (18% of AG Mill Feed)

WATER = 350 m3/hr

DMS (‐30 +1.5mm) = 81 tph (23% of AG Mill Feed)

ORE : WATER (Ratio)

1 : 1

EFFLUENT (‐1.5mm) = 206 tph (59% of AG Mill Feed)

AG MILLING(Unit 1, Weathered)

Fig 4 - Unit 1 (weathered ore) AG milling design mass balance

ORE = 350 tph

PEBBLE CRUSHER (+30mm) = 55 tph (16% of AG Mill Feed)

WATER = 350 m3/hr

DMS (‐30 +1.5mm) = 147 tph (42% of AG Mill Feed)

ORE : WATER (Ratio)

1 : 1

EFFLUENT (‐1.5mm) = 148 tph (42% of AG Mill Feed)

AG MILLING(Unit 13, Competent)

Fig 5 - Unit 13 (competent ore) AG milling design mass balance

During commissioning of the Karowe mine, it was important to maintain the percentage solids in the mill at 50 % for all ore types. However, the loading as well as the percentage of critical speed also had to be adjusted for optimum performance (with varying ore blends). Table 3 indicates some of the required commissioning parameters and associated expected power draws during anticipated treatment of the different ore types.

Table 3 - AG mill commissioning control philosophy parameters

Description % Mill load % Critical speed RPM kW Fine Unit 1 South Lobe 20.0 65 9.5 1738.9 Coarse Unit 1 South Lobe 20.5 70 10.2 1906.9 Fine Unit 13 25.0 75 10.9 2385.3 Coarse Unit 13 28.0 80 11.6 2795.2 Extreme coarse Unit 13 28.0 80 11.6 2795.2

Also as part of the mill commissioning philosophy and upon each start-up of the mill, the Operator has to set, adjust (or confirm) the following input parameters:

A = feed tonnage (tph), B = % solids and C = mill speed (% of critical).

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AG MILL COMMISSIONING CHALLENGES AT KAROWE MINE

The Treatment Plant and Recovery section were successfully commissioned by April 2012 and the project was subsequently closed on 1 August 2012 with the sign-off of the final C5 document (i.e. “Final Hand-over and Project Completion Certificate”). During commissioning of the mill at Karowe with Unit 1 (weathered) material, the following challenges were experienced and successfully alleviated:

the gearbox “foamed” on a number of occasions and the reasons for it as explained by Harcliff Mining Services (sub-contracted by Outotec) in their report included the following (Aylott, 2012): defoamer removed by filter, oil cycles too high, feed pressure of pump too high, cross section of oil lines too small and intake of air.

General causes included:

gear oil mixed with oil of another brand, gear oil contaminated by bearing grease, gear oil contaminated by dust and gear oil contaminated by water.

The following corrective actions were taken to mitigate the challenge experienced with the mill gearbox “foaming” (Styger, 2012):

An o-ring was installed on the trunnion bearing housing to stop the oil leaks. It was later found that leaks on the trunnion bearing seals were caused by an incorrect oil delivery pulse rate interval set-point (1.5 minutes instead of 15 minutes),

the gearbox was also inspected with regards to the “foaming” and the nozzle angles were adjusted.

the AG Mill port grate liners were also investigated with regards to percentage open area and found to be 5.5 % with the initial installed configuration (60 x 70mm grates). Based on a site visit and subsequent observations captured in a DRA specialist report (Bester, 2012) with back-up trajectory simulations by Weir Minerals (Kokoroyanis, 2012) it is believed that with the 5.5 % open area, excess fines generation was created with minimal oversize reporting to the pebble crushing circuit,

Weir Minerals (Kokoroyanis, 2012) has recommended three options using the standard port grate sizes to increase percentage open area by changing the port grates installation configuration (i.e. adding more 60 x 70 mm grates) or replacing the standard port grate sizes with 90 x 150 mm port grates and

the final remaining work proposed at the time of post-commissioning ramping-up regarding Karowe’s AG Mill, included the stripping and repair of the girth gear guard as well as the leaking trunnion housings (Meadway, 2012).

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Fig 6 - View of the Karowe AG mill

After commissioning and formal hand over of the Plant by the EPCM contractor to the operations team (Figure 6), Karowe operations started pushing throughput. On weathered ore, milling rates in excess of 500 mtph were achieved. This was sustained for extended periods of time until water shortage became a constraint in the early months of operation. At Karowe Diamond Mine, there can be no mention of the AG Mill without reference to water supply.

PROCESS WATER REQUIREMENTS

The design for the water supply system was based on a lower fines content of mill discharge as per the feasibility study. There was, however, an unexpected high generation of fines from the highly weathered kimberlite from the upper part of the crater. This resulted in high water consumption. In addition to this, water supply from pit dewatering boreholes was significantly lower than that predicted from modelling studies. It therefore became quite clear early into the operation of the plant that water conservation would be vital to sustainable operation. Several studies were initiated to reduce water consumption per tonne of ore treated to sustainable levels. These initiatives included:

filtration test work, modified slurry rheology, thickener control philosophy and accelerated water reclaim from the slimes dam.

After initial trials with a test unit, filtration was not considered further due to the long lead time to implement and high capital cost requirements. Other tests and studies were completed but were found to be impractical or would yield insignificant benefit.

In the end, modifications to the thickener control philosophy and steady operations resulted in high underflow pulp densities being pumped to the slimes dam leading to increased overall water recovery. This contributed the most to reducing water usage. At commissioning, water consumption was as high as 2 m3/tonne against a target of 1.0 m3/tonne. This was gradually reduced to 1.6 m3/tonne treated.

HARD ORE

The first encounter with hard/competent ore happened in July 2012 just when name-plate throughput had been reached on weathered ore. The milling rate on this hard material dropped to less than half of

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that on weathered ore. During this period, the mill loading increased rapidly to the point of tripping the mill. This was countered by having to stop feed from time to time and flushing the mill with water. This worked successfully and is now standard operating procedure on processing hard ore. This initial encounter with hard ore was for a short period and feed supply returned to weathered ore.

Sustained delivery of hard/competent kimberlite to the plant came through in October 2012. Plant throughput was adversely affected as previous mill optimisation and operating philosophy had not anticipated the extent of this level of hardness. The hard fragmental kimberlite had been reached a bench earlier than was anticipated in the geology model. Milling rates deteriorated to as low as 200 mtph (Figure 7a) while pebble recycle increased (Figure 7b). For several days, daily throughput remained below 5 000 tonnes.

A decision was taken to increase mill speed from 69 % to 82 % of critical speed (10 to 12 rpm) in October. This was 85 % of the VSD output. Power draw immediately doubled from a nominal 1.6 MW to 3.2 MW. Milling rate stepped up significantly by over 20 %. The positive side to this was on water consumption which dropped in line with modelled figures to below 1.0 m3/tonne due to the reduction in slimes generation from > 65 % to less than 55 % (Figure 8). The DMS feed split increased from 8 to 30 % of mill feed (Figure 9a). Mill recycle increased from < 2.5 to about 8 %. The DMS feed was noticeably coarser (Figure 9b) with possible negative implications on diamond liberation. The new mill operating conditions were accompanied by an increase in gear oil temperature. This was mitigated, somewhat, by erecting a shade over the motor to shield it from direct sunlight.

Fig 7a - Effect of ore hardness on mill throughput Fig 7b - Effect of ore hardness on pebble recycle (Sept 2012 (soft) and Nov 2012 (hard) feed)

Fig 8 - Comparison of mill effluent at 69 and 82 % of critical speed

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Fig 9a - Mill product to DMS from soft and hard ore Fig 9b - Mill product to DMS from hard ore at 69 and 82 % Nc

Fig 10 - Effect of ore hardness on yield to DMS

MODIFIED MILL DISCHARGE PORTS

Following successful operation of the mill at higher mill speed, the next stage of capacity enhancement on hard ore was to install modified discharge grates in line with previous recommendations. In addition to the intended increase in milling rate, the bigger discharge opening was expected to reduce fines generation on weathered ore.

Now the total open area of the new ports is a nominal 10% (Figure 10) compared to that of the original area of 5%. The modified ports were successfully installed at Karowe during a planned mill full reline over the period 21 to 25 January 2013. While mill performance following this change is being evaluated, preliminary results show that milling rates on hard ore have improved by at least 10%. The mill effluent PSD from soft and hard feed remains unchanged. Port size does not seem to have an influence on effluent PSD. The grind of the mill product to DMS appears to have remained virtually the same. Key performance factors resulting from these changes are shown in Figures 11a through to Figure 12c.

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Fig 10 - AG mill internal showing discharge grating

Fig 11a - Mill product to DMS from hard ore Fig 11b - Comparison of mill product at 10% opening

Fig 12a - Mill effluent on soft ore at 5 and 10% opening Fig 12b - Mill effluent at 10% discharge opening

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Fig 12c - Mill effluent on hard ore at 5 and 10% opening

CONCLUSION

Karowe Diamond Mine has successfully installed, commissioned and operated autogeneous milling technology in a diamond recovery application. While there is more work to be done towards optimising the various operating parameters, to date, the advantages of this technology are being realised. Diamond breakage, though not fully evaluated, is significantly better (estimated at <10 %) than that achieved through the conventional circuit.

ACKNOWLEDGEMENTS

The authors would like to thank the following individuals for their assistance and/or contribution to this paper:

William Lamb and Tony George, Lucara Diamond Company, Ace Sikwa and Gerry Ndlovu, Boteti Mining (Karowe Plant), Paul Morgan, DRA Mineral Projects, Paul Bester, DRA Mineral Projects, Johan de Villiers, DRA Mineral Projects, DRA AK6 Project and Commissioning teams, Demitri Kokoroyanis, Weir Minerals Africa and Wouter Styger, Outotec: Sub-Saharan Africa.

REFERENCES

2010. Drop Weight Test Report on a single sample from DRA Mineral Projects, JKTech Job No. 10002/P9.

Aylott, P, 2012. Re. AK06 Gearbox site visit report, Harcliff Mining Services.

Bester, P, 2012. Visit to Karowe diamond mine 20/07/2012 to 24/07/2012, DRA Mineral Projects.

Cambon, J and Shirley, J M, 1994. Diamond Processing, Russian vs. Western Diamond Recovery Plants: A Technical and Financial Comparison in Sixteenth CIM District 6 Meeting 1994, pp 1-8.

Kokoroyanis, D, 2012. Personal communication. September.

Kokoroyanis, D, 2012. Weir Mill Liners: Outside Ball Trajectory simulations. Weir Minerals Africa.

Lucara Diamond Corporation, 2013. Corporate profile, [online]. Available from: <http://www.lucaradiamond.com> [Accessed: 10 February 2013].

Meadway, C, 2012. Personal communication. October.

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SGS South Africa, 2013. Media, global documents, flyers, [online]. Available from: <http://www.sgs.co.za/~/media/Global/Documents/Flyers%20and%20Leaflets/SGS-MIN-WA232-JK-Drop-Weight-Test-EN-11.pdf> [Accessed: 12 February 2013].

Styger, W, 2012. Personal communication. August.

Van Niekerk, L M, 2010. Boteti AK06, Process Design Criteria, Rev 3, DRA Mineral Projects.