ASX-listed Magnum Mining & Exploration has concluded Phase 1 of the trail mining campaign at its 74%-owned Gravelotte emerald project in South Africa, where the company is targeting the re-establishment of commercial mining operations.
South Africa – In late February, Magnum Mining & Exploration commenced Phase 1 of a trial mining programme, which was aimed at mining and crushing 2 112 t of material sourced from four historic low grade and waste rock dumps onsite.
The overall objective of the trial mining programme is to provide critical information to assist with the re-establishment of commercial mining operations at Gravelotte – which was a major producer of emeralds over several decades up until the early 1980s.
The programme is designed to confirm historical emerald grades, optimise mining and processing techniques and recoveries, and allow the company to establish the value of the emeralds produced in the open market.
Phase 1 successfully recovered 11 774.8 carats of emeralds from the treatment of 256 t of dump material at an average recovered grade of 46 carats per tonne.
Based on the results of the Phase 1 programme, the company has made significant progress on assessing the Phase 1 key objectives, and to allow a final costing and timetable for the construction of a trial mining processing plant to commence.
Phase 1 at a glance
Magnum Mining & Exploration mined 52.2 t from Dump 25 and to date has treated 46.9 t of ore from this dump for a recovery of 9 135.8 carats. This is an average recovery of 194.9 carats per ton. This is considered to be an abnormally high-grade dump and approximately half of this small dump was mined in the Phase 1 trial mining programme.
A further 536.1 t were mined from Dump 001, with 38 t treated to date and 132 carats recovered. This is an average recovery of 3.5 carats per ton.
At Dump 100, the company mined 612 t, treated 34.1 t and recovered 170.3 carats. This is an average recovery of 5 carats per ton.
Lastly, at Dump 86, the company mined 667.4 t, treated 137.6 t and recovered 2 336.8 carats. This is an average recovery of 17 carats per ton.
The grade variability between dumps, highlights that a detailed sampling programme will need to be undertaken as a pre-cursor to the commercial exploitation of these dumps. The trial mining plant will be available onsite to undertake this sampling programme.
Phase 1 of the trial mining programme tested both jaw and SAG crushing to determine the optimum method to maximise liberation of the emeralds, minimise damage to the emeralds, and provide a uniform ore fraction size for efficient recovery of the emeralds.
The ore material from the dumps was stockpiled before crushing using a mobile jaw crushing plant. The crusher’s sizing gap was operated at different settings (25mm and 50mm) to test which aperture would produce the better particle size distribution for sorting and recovery.
Both crush sizes reported oversize material and tests have been undertaken to determine if the volume of oversize material can be easily reduced without increasing emerald breakages.
Two studies were consequently completed onsite to simulate a SAG mill process to assess its suitability in achieving this aim. The results are currently being evaluated.
In addition, the company has also commenced an onsite small-scale crushing operation to evaluate different crush sizes and methods to re-crush the oversize material. This test work will look at oversize material that has been through the sorting process previously, but which requires a re-crush to see if additional emeralds can be recovered.
The data received from this ongoing test work will be used to finalise the crushing circuit for the trial plant.
The company has trained eight employees to recover emeralds by hand washing and sorting the crushed material. The company originally sorted over sorting tables with 1 mm, 2 mm and 3 mm screens. All sorting tables have now been changed to 3 mm screens.
The change to larger screens on all tables and natural improvement in methodology has led to a steady but slow increase in current daily throughput.
A review of operating performance has shown that hand sorting is significantly slower than anticipated, and the company’s external consultants have recommended the evaluation of an optical sorter for emerald concentration.
Optical sorters have a history of use in the emerald industry and it is likely that the use of an optical sorter will significantly increase the efficiency of future operations at Gravelotte.
In this regard Magnum Mining & Exploration’s external consultants have highlighted the potential for optical sorting to significantly increase the processing rate, security and recovery rate of the recovery circuit, whilst reducing operating costs.
Phase 1 of the trial mining programme has allowed the company to provide freshly crushed and processed ore to optical sorter manufacturers for further detailed assessment. The work has highlighted the need for additional testing to clarify issues around uniformity of particle size, moisture content and washing of material in order to maximise the recovery of both liberated and host rock-attached emeralds.
The optical sorting trials being undertaken will focus on the customisation of the sorter’s various parameters to suit the Gravelotte project requirements.
This work is planned for late July, and once completed, the company will be able to assess the merits of the various alternative optical sorting alternatives available.
Processing plant design
The current treatment methodology employed on site is for the ore to be washed over a 3 mm screen to remove the minus 3 mm material and clean up the ore for hand sorting and recovery.
In a positive implication for the potential commercial operation the testing to date indicates that a significant percentage of the crushed ore reports to the minus 3 mm fraction which, even when emerald bearing, has little to no commercial value.
This has highlighted the importance of a Trommel to wash the ore to remove the fine material and hence the volume of ore to be sorted which in turn will maximise the utilisation and efficiency of an optical sorter.
The Phase 2 trial mining plant ([rocessing plant) will be designed to recover and re-use all water used in the Trommel washing operation.
The processing plant will also require sizing of various ore fractions to accommodate maximum efficiency parameters of the optical sorter.
Assuming a single shift operation on a five day week, the processing plant is being designed to be able to treat 2 000 tpm of ROM.
This processing plant has now been conceptually designed and plant specifications have been completed. The company is currently assessing the design and specifications to ensure they are appropriate for a trial mining plant