The new underground mining concept put forward by Torex Gold’s President and CEO Fred Stanford is gaining some traction at the company’s early-stage Media Luna project in Mexico, with the new technology potentially able to cut upfront capital requirements, reduce operating costs and decrease the time to commercial production.

IM reported on the highlights of the latest preliminary economic assessment in an earlier story, which showed the after-tax IRR going from 27% to 46% using the Muckahi Mining System (MMS) concept. But, the filing of the latest technical report has brought out some more details.

The report states on MMS: “The system challenges the status quo in many ways with the goal of establishing more efficient and cost effective alternatives to established mining processes.”

The MMS requires the use of a one-boom jumbo, service platform, mucking machine and tramming conveyor to create a more continuous mining process that can accelerate return on investment. It also significantly reduces the ventilation needs in underground mines by using conveyors as the main transport solution, playing into the mine electrification theme that is gaining traction.

Stanford, who is credited as the originator of the technology, explains the design rationale in the report:
“The production system in a mine is effectively a serial set of processes, with the ultimate objective of delivering rock, at specification, to the processing facilities. Each process step will have a primary design objective of either transformation, transport, or storage. In some processes there will also be inadvertent, non-design, transformation. This inadvertent transformation is generally not a desired outcome (ore pass slough, oxidation, etc).

“It is quite common for the ‘rates’ or ‘availability’ of processes in a serial set of processes to be out of alignment/coordination with each other. When this is the case, the productive capability of the entire system is reduced.

“To increase the productive capability of the system, designers frequently insert storage processes between transformation and/or transport (T&T) processes. These storage processes serve to reduce the inter-dependence between T&T processes and thereby increase throughput. This can be an effective design feature to maximise output, but it is expensive.

“In an underground mine these storage facilities, whether they are for rock or supplies, must be excavated and equipped, which consumes capital. They frequently also require re-handling, which consumes operating dollars. A design objective for Muckahi was to eliminate the need for storage processes by finding ways to bring into alignment the rates and availability of the entire set of T&T processes.”

He continues: “If the quality (size) of the rock product from the primary blast is not adequate for downstream processes, then a secondary sizing process will need to be added to the ‘set of processes’. Having ore-passes in the mine design will also force a requirement for a secondary sizing process. This is due to the uncontrolled size of the wall rock that, over time, will slough into, and dilute, the ore product.

“Secondary sizing processes, particularly underground crushers, are expensive and time consuming to build and expensive to operate. A design objective for Muckahi is to eliminate large size secondary size reduction processes and just deal with minor oversize management with mobile rocker breakers or ‘chunk’ blasting.”

To materially reduce the capital, operating cost, and mine build schedule, the MMS design approach sought ways to reduce the number of process steps and make the remaining process steps more efficient.

This involved eliminating secondary sizing processes that required ‘constructed’ facilities such as a crusher station – thereby eradicating the need for ore passes – cutting out all storage facilities, and replacing the current logistics model of one-way traffic in large tunnels, with two-way traffic in tunnels half of the size.

Stanford said the MMS has been able to achieve all of these requirements on a conceptual level by using five solutions:

• Blasting rock down to a smaller size – if the rock is to go directly onto a conveyor, then the product of the primary blast must be in the range of 95% passing -400 mm. Achieving this specification is not a challenge for ‘short hole’ primary blasts, such as used in development or cut and fill production mining methods. For ‘long hole’ production methods, it will require much tighter control of drilling procedures, explosives placement, and detonator timing;
• Twin roof (back) mounted monorails in all tunnels – this technology from the European coal industry solves several of the design challenges. It provides a stable platform for ‘long and skinny’ loads, allows climbs up steep 30° ramps and two-way traffic (one rail for inbound traffic and the other for outbound). SMT Scharf Group and Becker Mining Systems are two companies currently supplying these systems to the mining industry;
• A new transport concept named a ‘Tramming Conveyor’ (pictured) – this machine deals with the ‘first mile’ from the face/drawpoint, when straight lines for conventional conveyors are not an option. The conveyor is end loaded at the drawpoint until the belt is fully loaded. The belt then stops ‘turning’ and the whole unit drives away on the outbound rail to the discharge point. At the discharge point, the belts starts turning again and discharges its load (conveyor-to-conveyor transfer). The unit then switches to the inbound rail and returns to the drawpoint. While it was away from the drawpoint, other units have been loaded – hence, one of the benefits of two-way traffic;
• Ramps at 30° instead of the conventional 7.5° – the rubber tyres on conventional equipment lose traction on gradients that are much steeper than 7.5°. The back-mounted monorails remove the need for rubber tyres, hence the ability to steepen the ramps to the 30° gradient that can be handled by the cog drive system;
• Twin tunnels in waste – the tunnels in a Muckahi mine are half the volume of the tunnels required for a 50 t truck in a conventional mine. Half the volume means less rock to remove, less ground support, fewer holes to drill and load in the face, etc. This means they can be driven much more quickly. In a Muckahi mine, there are also no muck bays to be driven, which reduces metres by approximately 20%. The net effect is that excavation rates in a 4 m x 4 m tunnel should be two to three times faster than in conventional tunnel of 5.5 m x 5.5 m.

Torex said the concept is now shifting to the underground testing phase, with manufacturing of the first of the prototype machines underway in partnership with Medatech Engineering Services out of Canada. This could see the first trials underground at the company’s ELG mine in the March quarter.

In summary, the key expected benefits of Muckahi are:

• Continuous muck handling system and the elimination of re-handle and storage;
• All-electric operation and significant reduction in ventilation requirements;
• Ability to travel on ±30° (58%) slope and major reduction in both permanent and operating development;
• Ability for bi-direction travel in 4m x 4m tunnel.