The 2023 feasibility study evaluates extending Karowe Diamond Mine life by mining underground following completion of open pit operations.
Report context
The 2023 Feasibility Study (FS) for Karowe Diamond Mine (KDM) was prepared by DRA Projects Pty Ltd. (DRA) on behalf of Lucara Diamond Corp. (Lucara), commissioned through JDS Energy & Mining Inc. (JDS). The study assesses extending the life of KDM by mining underground (UG) after the completion of open pit (OP) mining. The treatment plant evaluation drew on KDM operational data and personnel, including Assistant General Manager Lucas Ntsipe, Acting Process Manager Bailey Maila, Plant Metallurgist Glen Wright, Senior Process Engineer – Technical Tiroyaone Kesiilwe, and Acting Production Superintendent – Wet End Catherine Mrosso.
Processing route
Phase I (Greenfields) History
KDM was originally designed as a 2.5 million tonne per annum diamond bearing kimberlite processing plant with a single 200 t/h Dense Media Separation (DMS) module. Autogenous (AG) milling technology was utilised, a configuration previously seen predominantly in northern hemisphere diamond plants. AG mills accomplish size reduction that normally requires multiple stages of crushing, screening and grinding. Concentrate from the DMS was treated through a 2.5 t/h wet X-ray recovery circuit for diamond winning. The treatment plant and recovery sections were commissioned in April 2012.
Phase II (Brownfields) History
The Phase II expansion, completed in 2015, added secondary (gyratory) crushing, XRT sizing, and XRT diamond recovery circuits. XRT machines were employed to recognise and recover carbon-signature material (diamonds) in a large diamond recovery circuit. This technology increased the top cut-off size of the plant, allowing large stones to be recovered that previously would have been broken in the pebble crusher and mill. XRT also mitigated the impact of the high density of KDM kimberlite on DMS performance, as the DMS was limited to treating -8 mm material. A portion of the -20 +8 mm tails from the main XRT building was treated through a single 50 t/h capacity downstream sorter for both metallurgical accounting and scavenging purposes.
Mega Diamond Recovery and Phase III (Brownfields) History
The Mega Diamond Recovery (MDR) Project added XRT sorting technology ahead of the AG mill, with the aim of sterilising the feed of liberated mega diamonds above 50 mm by adding a recovery step that was top size limited only by available technology. Sorting was conducted on material passing 125 mm prior to AG mill comminution. Phase III provided XRT sorting technology to the 4 x 8 mm size fraction ahead of the DMS, to mitigate high-frequency near density content of KDM's Unit 13 (M/PK(S)) ore that could result in DMS yields exceeding approximately 25 per cent. This was the smallest fraction of XRT bulk sorting technology applied on a diamond mine at that time.
Current Plant Configuration
As documented in the 2023 feasibility study, ROM ore currently fed to the process treatment plant is Magmatic Pyroclastic Kimberlite (M/PK(S)) and Eastern Magmatic Pyroclastic Kimberlite (EM/PK(S)). Major equipment includes a primary jaw crusher (160 kW), secondary gyratory crusher (185 kW), AG mill (8.53 m diameter by 4 m long, 4,000 kW installed), pebble crusher (300 kW, cone crusher), tertiary wet flush cone crusher (220 kW), and multiple pumps. The block flow diagram (updated from 2019) shows equipment in black font denoting original Phase I kit, with green font denoting post-Greenfields changes.
Crushing Circuit
ROM material is delivered to the ROM tip by articulated dump truck and 777 rigid trucks. Primary jaw crushing reduces ore to an acceptable feed envelope. Depending on material treated, a proportion or the entire primary crushed stream is diverted through the secondary crusher circuit. Secondary crusher scalping is performed on the MDR screen, with oversize partially sent to the crusher depending on diverter setting. Secondary crusher product is reintroduced onto the mill stockpile feed conveyor.
The +80 mm mill screen product and 32 x 80 mm LDR XRT tailings are processed through the pebble crusher. Pebble crusher product is sized at 32 mm, with +32 mm material reporting to mill feed. A proportion of -32 mm material bypasses the mill via the bleed screen to reduce mill loading. The 20 x 32 mm tailings from XRT bulk sorters are processed through a wet flush tertiary crusher circuit, with product reintroduced via the bulk sorter sizing screen.
Comminution – Milling, Bleed Screening and Pebble Crushing
Fresh mill feed enters the AG mill from the feed stockpile along with a variable portion of pebble crusher product. The bleed screen on the pebble crusher product stream allows a proportion of -32 mm material to be bled out of mill feed to alleviate mill loading. The AG mill discharge grate incorporates Turbo Pulp Lifter technology for improved discharge efficiency.
XRT Circuits
The mill screen product (1.25 x 80 mm) is sized on the bulk sorter sizing screen and XRT sizing screen. The 32 x 80 mm oversize fraction reports to the Large Diamond Recovery (LDR) XRT section to recover large diamonds before the stream enters the pebble crusher circuit, reducing potential diamond breakage.
Recent plant upgrades include wet dust scrubbing at primary crushing (commissioned December 2018) and pebble crushing sections, a secondary gyratory crushing feed bin installed in December 2018, XRT replacement/refurbishment in 2020, and an XRT sorthouse upgrade completed in December 2018 with holding bins, feeders, washer driers and sort boxes.
Key reported parameters
| Parameter | Units | Value | Basis |
|---|---|---|---|
| Design annual tonnage | dry mt/a | 2.5 – 3.5 | Client specified |
| Overall utilisation | % | 81.0 | DRA design estimate |
| "On ore" hours per year | hrs pa | 7,095 | DRA design estimate |
| Design throughput | t/h | 350 – 500 | Client specified |
| Top cut off size | mm | 60.0 | DRA design estimate |
| Bottom cut off size | mm | 1.5 | Client specified |
| ROM moisture content | wt % | 8.0 | Assumed |
| Clay mineral content | % | 3.0 | Assumed |
| Pre-crusher feed F100 | mm | 300 | Vendor information |
| Secondary crusher closed side setting | mm | 60 – 75 | DRA design requirement |
| AG mill circuit feed F80 | mm | ~125.0 | DRA from testwork |
| AG mill circuit product P80 | mm | ~37.5 – 50.0 | DRA from testwork |
| AG mill pinion power | kW | ~3,045 – 3,783 | DRA design requirement |
| AG mill installed power | kW | 4,000 | DRA design requirement |
| AG mill speed | % Nc | ~80 – 82 | DRA design estimate |
| AG mill circulating load | % of fresh feed | ~5.5 – 12.5 | DRA design estimate |
| Pebble crusher closed side setting | mm | 25.0 | DRA design estimate |
| XRT diamond recovery (Large, Coarse & Middles) | % | ≥ 98 | Client specified |
| DMS feed size | mm | 1.5 – 8 | DRA design requirement |
| DMS de-rated throughput | t/h | 150 – 200 | DRA design requirement |
| DMS average expected yield | % | 7.40 | DRA design estimate |
| DMS 75th percentile expected yield | % | 11.1 | DRA design estimate |
Project website: https://lucaradiamond.com/operations/karowe-mine/
*Source codes: D1 = DRA design requirement, D2 = DRA from testwork, D3 = DRA from other inputs, A = Assumed, C = Client specified, V = Vendor/third party.*
Technical qualifications
The Process Design Criteria presented in the study is a high-level summary from predominantly Phase I and II design and built, using source codes to reference the origin of each item. A desktop evaluation conducted by DRA in 2018 considered front end plant modifications for UG mining, including anticipated challenges such as water retention in broken ground from sub level cave, steel rebar entering the crusher, high carbon-containing materials potentially affecting XRT and thickening, and high waste contents diluting grade. The study notes that water management and potential impacts on the macro water balance when finding water at depth remain aspects requiring consideration for UG mining.
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*Source: Karowe Diamond Mine | 2023 Feasibility Study, Section 17 , Process Description / Recovery Methods, Sections 17.1–17.4*
