Stefan Schwank and Ulrich Schoepf discuss a modern way to tackle gold veins
Trench cutter technology, being a typical civil engineering method, has been used in recent times in various bulk sampling and trial mining applications. Nowadays, the cutter system technology has been further developed from applications in exploration and mining to a sustainable system for selective mining with one focus on near-vertical gold veins.
Trench cutters are widely used in civil engineering for the installation of diaphragm walls and cut-off walls. Diaphragm walls act as retaining walls to allow for open excavation pits, e.g. for the installation of underground car parks and subway lines, and foundations of buildings. Cut-off walls underneath and inside dams, also tailings dams, act as permanent water barriers to avoid seepage and collapse of the dam. To form the walls, trenches are excavated by the trench cutter in primary and overlapping secondary panels. The panels are supported by bentonite slurry during excavation, and backfilled with concrete, replacing the slurry, upon completion. During excavation, bentonite slurry in civil engineering and water in mining, is mixed with the crushed rock and pumped to the surface, where it is separated in a treatment plant. The cleaned slurry, or water, is pumped back to the cutter excavation in a closed loop to minimise consumption.
Bauer has been developing its trench cutter system for many years and is considered an expert in the sector, with more than 400 units working around the world. Key components of a trench cutter are two counter-rotating cutter wheels that can be fitted with different teeth arrangements for excavation of all different types of soils and rocks.
The cutter wheels and recovery pump are mounted inside a heavy steel frame that is equipped with hydraulically operated steering plates to control verticality or planned inclination in both directions. For better penetration in harder formations, especially in mining, the hard rock cutter system (HRCS) is mounted between the cutter frame and the cutter wheels. The locking system is designed for higher and constant load on bit compared to the free hanging system and transfers the loads applied into the sidewalls of the trench.
Both the mud delivery and the hydraulic hoses with electric cable are automatically fed into the trench and coiled up on drums positioned on the back of the rig. For moving the rig easily from one position to the next, the entire system is mounted on a crawler-based carrier that also provides sufficient engine power for the entire unit. As an alternative, electric power can also drive any of the Bauer cutter units.
The development of a potential mine comes along with big challenges during the technical and commercial planning stage. Therefore, a reliable and detailed bulk sampling programme is inevitable. In 2019, such a sampling programme was carried out with a Bauer trench cutter system on the Star Orion kimberlite fields for Rio Tinto in Saskatchewan, Canada.
High demands on depth capability, safety standards, and the integrity and quality of the samples required a new technology that was able to meet all expectations. The scope of the first phase of the programme comprised of 10 trenches with a maximum depth of 251m.
The objective of the work was to collect high-quality diamond bearing kimberlite samples. The kimberlite zone started from around 105m vertical depth and is overlain by so called overburden material from loose sands to sticky and hard clays with cobbles and boulders. The kimberlite chips with potential diamond content were washed, collected and stored for further processing.
This proven cutter technology will now be further developed to reach a 100 tonne per hour mining rate in up to 100MPa UCS rock. As an alternative to diesel, all drives for pumps and cutter wheels can be completely powered by electric only, with a total installed power of up to 2MW.
The cutter mining sequence will be designed to the dimensions and layout of each individual vein for maximum ore recovery and minimum dilution.
Overall Bauer recommends performing a four-month trial mining operation in the gold mine, first with HRCS on a standard Bauer cutter system and then to optimise the design to achieve the best performance for the future.
Gold veins, be it vertical or near-vertical, often constitute a challenge to miners. The small size of the ore body over great length and only little exposure to surface makes it difficult to mine economically. Open-pit mining of such ore bodies often comes to an end soon, due to the high stripping ratio and the enormous environmental impact.
Underground mining is expensive for the development and for mining due to the tight working conditions in the fissures. A combination of both – cutter mining to about 250m depth and underground mining below cutter mining – reduces development time and cost considerably and generates revenues at a very early stage of a new mining project.
Koidu, a BGS company, holds diamond mining leases in Sierra Leone for kimberlites, mostly kimberlite dykes. Jointly, the idea was conceived to use trench cutter equipment as a mining tool to excavate these dykes to greater depth. Despite the remote location of the leases in eastern Sierra Leone a test was carried out with a BC 33 cutter in 2010. The cutter width chosen was 800mm, based on Koidu’s assumption of a dyke width between 800 and 1,000mm.
The selective cutter mining system for gold veins will concentrate on the excavation of the vertical or near-vertical ore body only.
There are several key advantages of such a system, including the fact that mining of so far uneconomic small ore bodies and veins becomes viable. This approach allows operators to avoid open pit mining with a large environmental footprint and mine dumps and it doesn’t require any underground development – meaning there is a much lower installation cost. Minimum dilution of ore (almost zero strip ratio) is another advantage.
Additional benefits include increasing the mine lifespan after open pit mining ends without increasing the environmental footprint as well as increasing the mine life without any additional infrastructure. Finally, time to mine is extremely short and equipment can be used on several ore bodies or even mine sites.
Mining has always been at the centre of various interests. Economic viability, environmental impact and social acceptance are key factors that must be balanced to achieve a sustainable mining solution.
Cutter vein mining with its no, or very low, additional environmental impact and optimised economics by mining the ore body only, can achieve a much better acceptance from the authorities, and the public, than conventional mining. Cutter mining systems help to satisfy the demand for the much-needed commodities in our modern society that are increasingly focused on sustainability.
Stefan Schwank and Ulrich Schoepf are with Bauer Maschinen
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