The 24 m high boom of the ROM stacker
Lětseng Diamond mine located high in the Maluti Mountains in the Kingdom of Lesotho, Southern Africa is owned by Gem Diamonds limited with a 70 percent stake, while the government of Lesotho owns the remaining 30 percent.
Lětseng processes ore from two kimberlite pipes, Main and Satellite, both bearing extremely low grade ore (averaging under two carats per hundred tonnes), as well as from existing stockpiles. The mine currently processes around 7 million tonnes of ore, producing about 100 000 carats per annum.
Lětseng is famous for its large, top quality diamonds with the highest percentage of large (+10.8 carat) diamonds of any kimberlite mine, making it the highest dollar value per carat kimberlite diamond mine in the world, but are also from the highest diamond mine in the world, at an altitude of 3 100 metres.
The two kimberlite pipes are mined using conventional open-pit mining techniques on a split shell design. The split shell concept was adopted to optimize waste stripping and enhance cash-flow. Major mining activities except blasting are outsourced to contracting companies. Drilling is done with Atlas Copco Rock L8 & L6 machines. The mine employs a mixed loading and hauling fleet comprising CAT 385C and CAT 390D excavators, together with CAT 773 rigid dump trucks for hauling waste and CAT 740 articulated dump trucks hauling ore. The mixed fleet results from different ore and waste profiles during different stages of mine development.
Approximately 7 million tonnes of ore and 20 million tonnes of waste are mined per annum.
Lětseng operates two main kimberlite treatment plants, termed Plants No. 1 and No.2, which together treat 5.7 million per annum, and thus treated by contractor Alluvial Ventures through their two pan plants. Production through Plant No. 1 commenced in March 2004, and through Plant No.2 in March 2008. Full production commissioning was achieved in record time at the end of the Q2 2008.
The flow-sheets of the two plants are similar, though small improvements were made to Plant No.2 based on the learning from Plant No.1. They are fed by a Primary Crushing unit (the PCA) that crushes raw kimberlite ore to boulders smaller than 200 millimeters. Each comprises primary, secondary and tertiary crushing, scrubbing, sizing and concentration of the different size fractions in DMS (dense media separation) units. The DMS concentrate is fed to the recovery facility.
Technological Operational Improvements
Lětseng resource is unique, as it combines a relatively low grade with very high quality large diamonds. This uniqueness necessitates that special care be taken during the diamond recovery process to ensure that diamond damage is avoided and that the optimal diamond liberation steps are included in the process plant.
At Lětseng , a year of solid operational performance saw an improvement over the prior year’s production results, with costs well controlled, says Gem Diamonds’ CEO Clifford Elphick.
“Both the implementation of the Plant 2 Phase 1 upgrade and the new Coarse Recovery Plant projects remain on track for commissioning in Q1 and Q2 of 2015 respectively – on time and budget.”
Letseng Mine continues to perform strongly despite drop in carat output
According to Gem Diamonds report of the fourth of 2014 (October 1 to December 31), saw an “encouraging end to a very positive year”. It further states that the Letseng mine in Lesotho, of which it owns more than 70 percent share, has continued to perform strongly, despite a drop in the mount of diamonds being recovered from the mine- 25,525 carats during the quarter compared to 28, 365 carats in the third quarter.
The December tender of goods from Letseng achieved an average of US$2, 799 per carat (p/c), resulting in an average value of US$2, 140 p/c being achieved compared to US$2,603 p/c in the third quarter of the year. This result brought the average for 2014 to US$2, 540 p/c compared to US$2, 043 per carat in 2013.
Thirteen rough diamonds exceeded US$1 million each during the quarter, including a 112.6 carat white diamond and a 90.4 carat white diamond that sold for US$5.8 million and US$4.2 million, respectively.
A 299.3 carat yellow diamond was recovered and extracted at rough valuation during the quarter and was sold into a partnership arrangement this month (January 2015) with Letseng to share in 50 percent of the polished uplift.
“Based on the positive results achieved in 2014, Gem Diamonds remains on track to declare a maiden dividend to shareholders following the 2014 full year results announcement in March 2015”, states Elphick.
Lětseng’s Project Kholo
The overland conveyors designed by DemcoTECH in 2008 for the taillings disposal system
DemcoTECH supports Lětseng’s expansion focus
Having had a working relationship with Lětseng Diamond Mine that dates back to 2008, bulk materials handling specialist, DemcoTECH Engineering continues to service the mine’s expansion initiatives, with its most recent work focused on upgrading part of the mine tailings materials handling capability.
In 2008, DemcoTECH designed, engineered and supplied the original tailings disposal system for the second diamond treatment plant at the mine, located in the Maluti Mountain, in Lesotho, and has participated ever since in a number of studies for the mine. Current work is focused on upgrading the tailings system to handle higher capacities resulting from Lětseng’s Project Kholo, which, amongst other objectives, is aimed at increasing ore throughput.
“Following a design audit on the run-of-mine (ROM) stockpile system which we carried out for Lětseng, we are busy upgrading the ROM stacker as a turnkey contract. This involves relocating the drive of the 24 m high boom down to ground level for ease of maintenance, as well as redesigning the head arrangement to ensure that the material is distributed evenly above the stockpile reclaimer feeders below,” says Paul van de Vyver, General Manager, DemcoTECH Engineering.
In addition, a new WEBA headchute has been installed to ensure that material particle sizes are distributed evenly over the dump.
To minimize any disruption to production, the improvements to the ROM stacker were implemented during a shut down period of 10 days during February 2015.
“We have also recently completed the conveyor design and expansion layout to be able to increase the tailings dam to handle the expanded throughput,” explains van de Vyver.
Given the location of Lětseng and the fact that it is the highest diamond mine in the world, at an altitude of some 3 200 m, the system DemcoTECH originally supplied for the mine presented a number of challenges. This included the fact that the route of the overland conveyors had to accommodate Lesotho’s mountainous terrain, with steep inclines and declines en-route to the tailings dump.
“This required us to employ special engineering solutions, such as the inclusion of a regenerative braking system on the tail pulley of the extendable conveyor to prevent the conveyor from running away,” notes van de Vyver.
“The system was also required to operate at ambient temperatures ranging from +35°C to – 20°C in wind speeds higher than 100km/h on a very exposed site.”
DemcoTECH’s contract to supply the system was part of the establishment of a second diamond treatment plant at Lětseng Mine (Plant No 2), which doubled the mine’s hard rock processing capacity from 2.6 million t/yr to 5.2 million t/yr, making it the worlds’ seventh largest diamond mine by throughput.
The project included a conveyor with fixed tripper and multiple discharge points, a 1.6 km overland conveyor and a 1 km long tail-driven downhill extendable conveyor with a rail -mounted tripper and boom spreader as well as an emergency dump system .
DemcoTECH services are offered though contracting mechanisms from EPCM to Lumpsum Turnkey including studies and from concept design through to detailed feasibility studies. After-sales services