August 2016

Design changes from the top down

Paste backfill has opened a new door for underground developers, and CEMI’s Doug Morrison wants to invite miners in

By Vince Morello

Underground mining has always been a costly endeavour. Most operations currently spend huge amounts of capital upfront by digging to the bottom of an orebody and mining up. However, Doug Morrison, CEO of the Centre for Excellence in Mining (CEMI), thinks this method of mine design, which he refers to as “bottom-up mining,” is inefficient. He believes there is an alternative way to design mines that could reduce the financial risk of projects by flipping current mine design convention: mining from the top of an orebody downwards. Before joining CEMI, Morrison worked at MIRARCO as interim CEO and as the global mining sector leader at Golder Associates. He has a master’s degree in rock mechanics and excavation engineering.

CIM: What caused you to start thinking about underground mine design?

Morrison: Basically it was out of our research project examining the opportunity of using paste fill in place of hydraulic fill. We did a project with Labrecque Technologies – they do discrete event simulations – where we looked at standard open stoping with a [hydraulic] backfill system and then we compared it to a similar stoping arrangement with paste backfill. [We] established that using paste is a better way to do things, [as] paste fill allows you to create a higher quality backfill product in place.

CIM: What kind of research is CEMI conducting now?

Morrison: What we really focus on is productivity, so that means improving the speed, reducing the cost and reducing the time in which we do things. All things [that] contribute to the productivity of operations on the ground and so one of the advantages that comes with using paste backfill as opposed to hydraulic backfill is you eliminate some tasks like building barricades, installing drainage towers and waiting for drainage.

CIM: What are the current problems with bottom-up mining?

Morrison: The biggest problem for us is that as we go deeper and deeper, the cost of mining goes up, and if we continue with the same mining methods as we go down then we’re becoming less profitable all the time, with the same [ore] grade in place. It’s becoming more expensive just because of the logistics of moving deeper and heat issues. It gets hotter as we go deeper and so we need to spend more money on ventilation, so it’s a greater energy and electricity demand for the ventilation systems.

CIM: What are the benefits of mining top-down?

Morrison: [In] bottom-up mining, you have to invest the capital to access all the way to the bottom of the orebody and then you have to also install the ventilation system to go all the way to the bottom of the orebody. You spend all [your] money before you get any production. Then you start producing ore at the bottom of the orebody [and that’s when] you begin to get a return on your investment.

If you do top-down mining, basically you’re only opening the top three levels, or maybe four levels, and then you start producing in the upper most levels so the total investment that you have to make is much less and you begin to get a return on investment much sooner than if you go all the way to the bottom of the orebody, and so you begin to move progressively down. The big advantage of this kind of approach [is that it’s] a low-risk approach because if there’s a big drop in the price of the metal you’re producing, you can stop production and wait for the price to return to an acceptable level. If you’re doing bottom-up mining, you’re putting all your capital upfront and so you can’t sustain mining when the price drops because you have no way to return or provide something to your investors. Normally bottom-up mining requires a projected stable price for the future, [such as the next] 15 or 20 years. If you don’t see a stable price then you won’t actually start the project because the risk of not getting a return is too high whereas with a top-down approach the risk is much less, and so you can afford to take the risk of moving forward with a less stable future price of metal.

CIM: What are the challenges with top-down mining?

Morrison: First of all you have to have the backfill to do it well, so that you have a stable backfill placement above you. Then you have to perfect the blasting techniques, so you can do up-hole blasting underneath the backfill, or if you are not comfortable with that you can actually go mining underneath the backfill and do conventional downhill drilling or blasting with open stoping. You have to look at the cost and benefits of both of those.

CIM: Are there any more or different safety risks involved with this method?

Morrison: We don’t see any more safety risks because the mining methods would [use] the same equipment and techniques as we have now so there’s no additional risks from those issues.

CIM: If the cost of bottom-up mining was so expensive, why did it take so long to innovate?

Morrison: It’s very expensive and difficult to innovate while operating mines. You have a lot of work that has to be done before you can introduce a new way of doing things into a mining operation. Mining operations are typically planned several years in advance and so usually your best opportunity to bring in a new innovation is with a new mine, or a new orebody within an existing mine.

CIM: What are the risks involved with innovating in this field?

Morrison: There are a lot of risks of doing something different but the risk of not changing anything is not zero. If you continue to go deeper and deeper without changing anything, your costs are increasing because of the logistical burden you have to carry, and because of a need for more electricity for the ventilation systems.

I would say the way to think of this is that the innovation is an opportunity cost rather than an expenditure risk so you are trying to gain the opportunity or benefit of doing it differently whereas continuing down the same path is going to take you to a place that is not productive and not profitable. It’s not like you can just keep on doing the same old, same old and have no effect.

CIM: How can this method of mining break into the mainstream of the mining industry?

Morrison: I think once miners become comfortable using paste [backfill], and have used paste for a long time, they’ll recognize that the quality of paste backfill is much higher than it was with hydraulic [backfill] and they’ll recognize that this becomes a much more cost effective way to do mining underground because it limits the amount of capital outlay you have at the beginning of a project.

CIM: Are a majority of mines still using hydraulic backfill?

Morrison: Most new mines will use paste fill. Many older mines that already ‎had an existing hydraulic backfill plant will likely not have changed over. In many cases they will calculate the savings from paste against the capital cost of a new plant and decide that there is not enough mine life to recover the investment. Few mines include the lost opportunity cost from productivity gains in this calculation – but this where the biggest benefit of paste fill is to be gained.

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