New diamond-processing facility for Letšeng Mine, Lesotho
A second diamond-treatment plant has been established by Bateman Engineering at the Letšeng-la-Terae mine in the Mokhotlong District of northern Lesotho, about 100 km from the town of Buthe Buthe. The first ore was treated through the plant on 23 March 2008 and, with final commissioning in progress, full production from the gravity fed DMS plant is expected during the second quarter of 2008.
Letšeng Diamonds Proprietary Limited is owned 70 % by Gem Diamonds and 30 % by the Government of the Kingdom of Lesotho and was re-opened in 2004 after lying dormant for years. The second plant, which commenced construction in late 2006, will double Letšeng Mine’s hard rock processing capacity from 2.6 Mtpa to 5.2 Mtpa, making it the world’s seventh largest diamond mine by throughput.
The kimberlite at the mine is known for its high proportion of large and high-value diamonds, with 76,873 carats being sold from Letšeng Mine in 2007 and the mine expected to produce approximately 100,000 carats in 2008. It is estimated that nearly 15 % of the diamonds in the ore are larger than 10 carats in size and in the past 2 years alone the mine has recovered two of the world’s top 20 diamonds. This includes the 15th largest, called the Lesotho Promise, of 603 carats, unearthed in August 2006 and sold for US$12.4 million, and the 18th largest, the 493 carat Letšeng Legacy, unearthed in September 2007 and sold for US$10.4 million.
The processing plant at the mine processes run-of-mine ore from current mining operations as well as stockpiled ore produced by mining operations at Letšeng during the 1980s. The new plant, known as Plant No. 2, augments the production from the existing plant, Plant No. 1. Plant No. 2 has a nominal processing capacity of 350 t/h with a planned production of about 50,000 carats/yr during a 20-year life.
Both plants are fed by material from the existing primary crusher, but a new crushed-ore stockpile was provided to feed both processing plants.
The crushed ore from the stockpile is processed through a jaw crusher and scrubber. The oversize from the scrubber discharge screen reports to secondary crushing via a vertical conveyor. The fine material is screened out and the diamond-containing fraction conveyed to the dense-media separation (DMS) circuit comprising two 200 t/h (one fine and one coarse) custom-designed Bateman DMS plants. The coarse floats are recirculated to a re-crush crusher. The concentrate from the DMS circuit proceeds to the existing diamond-recovery facility.
A new 20 m-diameter Bateman Thickener is used to recycle a large portion of the process water and the thickened waste is sent to the existing tailings dam.
The Letšeng operation is the highest diamond mine in the world, located at an altitude of approximately 3,200 m in the Maluti Mountains. With regular snowfalls, particularly in winter, the ambient temperature ranges from about -10 ˚C to 25 ˚C. Because of the exposed nature of the site, however, the chill factor of the strong winds often reduces this to -20 ˚C and below.
To provide adequate protection for man and machine in these conditions, the whole plant is enclosed. The footprint of the building is minimised by the extensive use of vertical conveyors.
Most of the equipment and materials for the plant were sourced from neighbouring South Africa, where as much as possible of the fabrication and painting of the components of the plant were completed at the workshops. This ensured that a minimal amount of assembly and painting had to be done on site under such harsh conditions.
The transport to the remote site had to negotiate winding mountain passes. Although the roads were mostly tarred, the icy conditions required careful handling and resulted in a lot of maintenance to keep the roads in a passable condition.
Work on the project commenced in August 2006 and was completed during May 2008, with mechanical completion being achieved at the end of February 2008.
Bateman Engineering provided the engineering design, procurement, construction and commissioning of the new plant under a ZAR58 million (approx US$8 million) EPCM contract. Bateman Engineering tendered for the project, which had a total value of ZAR305 million (approx US$42 million), after having conducted the original feasibility study and value engineering on the project (between May 2005 and June 2006).
Some 94,970 man hours were required to complete the project. This was completed with one fatality and two lost-time injuries. The on-site labour peaked at 535 (Dec 2007) and the lost-time injury-frequency rate (LTIFR) for the project was 0.62 per 200,000 man hours.
Additional information on the Letšeng project may be obtained from Vincent Diesel, Project Manager on +27-11-899-9111 or email Diamonds@BatemanEngineering.com.
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