Paterson & Cooke’s Maureen McGuinness chatted to CIM Magazine to discuss the evolution of backfill and the key role it has to play in mine planning.

What role does backfill play in the mining cycle, and where are the opportunities for mining companies to optimize this?

The role of backfill is to help manage the stress in the mine and aid in local regional ground stability; it doesn’t hold up the mine, but it helps with that whole process. The increased stability allows mining companies to get more out of their mine, for example they can work safer, faster, extract more or have better cycle times, all with the goal of getting more ore out of every deposit.

While the main role of backfill is stress management, the resulting benefit is that we can recover more ore and it helps provide different mining options to a mine site. Backfill is at the end of the mining cycle, so it is often thought of as a necessary evil, but there is an opportunity to see it as an extra tool in the mine design toolkit that can push the limits of the mine being developed or designed.

There are also opportunities to look at the waste management of the site as a whole, to see what materials from the surface can be used as backfill and how that can help in other ways. Using backfill allows a mine to maximize how much of a mine’s waste products can be put back underground, minimizing surface impact.

The other part is safety; having a fill that you can rely on allows the designers to be more confident when they’re looking at taking out different stopes. Backfill should be a planned, engineered process with results that can be foreseen.

What key aspects do mines need to consider when designing and operating mine backfill systems?

One of the first things is to look at what material is available for the fill and to get a good sense of what the mining process will produce in terms of fill potential—it could be waste rock or tailings, or depending on the situation of the mine, there may not be tailings or backfill components close by. If the mine is sending the ore off site, then a key consideration will be what can be used as backfill.

It is also important to size the backfill system appropriately, and make sure that it will meet the needs of the mine in the short and long term. The backfill system can help a mine to catch up if there have been delays throughout the different processes, and having it sized appropriately and robustly allows you to bring that mine cycle back on track if it can fill fast.

Understanding the strength requirements of the fill is important, including short-, medium- and long-term strength requirements. You may need to understand how strong it needs to be very early on, even after one or two days, to meet the requirements of the filling process. For a cemented backfill, it could be how quickly, or how far in the future, you want to expose the adjacent stope, depending on the mining sequence.

Sometimes these are components that we figure out later, but they should be done upfront with mine design. By understanding what the requirements need to be, then we can dial in the recipes and make sure that we can meet a complex profile of strengths.

Then of course the other key aspect, which makes backfilling unique, is that we’ve got plants on surface and then the whole distribution system underground. We need to make sure that those two systems are working in sync, and that one is not developed in a silo compared to the other. For a successful backfill system, it is very important that the plant matches the underground distribution system, and that the operation is configured or managed with that whole entity in mind.

Is this an area that is sometimes undervalued in mine design?

Definitely. Backfill is the last part of the mining cycle and tends to get less attention than other parts of the mine design, especially in earlier stages of studies. Often, these studies will have quite a detailed look at how the ore will be extracted and processed, and then there’s one line that says, “and it will be backfilled.”

But if we plan backfill at an early stage, then we can identify opportunities to match the backfill with tailings storage, which can help alleviate some of the pressures of the surface disposal. It also helps if we can have fewer surprises going forward. If everybody knows what it means to backfill, and its requirements in terms of costs, footprint and effort, then they won’t try to squeeze it into a predetermined design.

I would like to show people that we need to plan for backfill so that it can work more efficiently. Our end goal with backfill would be—if you don’t hear about it, it’s going well. But if that doesn’t happen and you’ve let things go, then everything will be in an emergency state. Reacting to a problem usually works out more expensive in the long run, versus planning backfill properly.

We find that backfill is a little undervalued in operations as well. Often, mine sites have no dedicated people working on backfill, so you see people managing the backfill as a secondary part of their job.

Would the Canadian mining industry be considered to be a leader in backfill?

Canada is definitely a leader in pastefill, which had its origins right here in Sudbury, and Canadian mines were early adopters of paste backfill. We also have extensive backfill research being done in Canadian universities, including some very new studies about how backfill behaves in situ. While the industry has been backfilling for a long time, actually understanding how it works and how the strength develops is relatively new, and Canada has been a great provider of that information. There is a hub of engineering companies, too, in Canada that are really specialized in backfill, which I think indicates where we are in the field.

And then of course, there are a lot of Canadian mining companies that have this background and expectation of backfill, and as they are going out worldwide and starting up different mining operations, they are bringing that expertise with them.

What changes have you seen in this area of the mining industry over the course of your career?

There has been a shift, especially recently with the new guidelines on how to manage tailings storage, including the benefits of filtered tailings disposal as opposed to wet tailings. There are a lot of synergies because if you’re going to filter these tailings, then that sets you up for synergies with tailings-based backfill. We are seeing a lot of companies diverting filtered tailings as much as possible to pastefill, and disposing of the rest on the tailings stack. That’s probably one of the most recent changes that we’ve seen.

I started working in backfill in the early 2000s, and I was starting up one of the new paste systems that were coming in. At that time, the industry was focused on the fundamentals of backfill and how to distribute it, in terms of pastefill in particular, as that was a newer technology that was becoming more mainstream. We did not have that full understanding yet of the theory behind how the hydraulics work or how the rheology or flow properties of the material affect how we place it underground. This meant that startups were difficult, and there were a lot of places where it didn’t run smoothly. But once they got running properly, then sites immediately saw the benefits of it. As an industry, we have learned about the best methods of delivering the paste underground, and looking at what did and didn’t work in the plant as it became more popular in Canada.

For example, the mining industry used to just take a mixer from the concrete industry, but now it has adjusted these and made them specific for pastefill. It is the same with binders—general use cement used to be the main ingredient for the binder, but now slags and other additions are being used in mining. What works as a specialized admixture in the concrete industry isn’t really feasible at the mining scale, so the mining industry is now developing admixtures specifically for backfill.

There has been a refinement of understanding, too, of the material components that can be used in backfill. Instead of taking what’s available, people are refining it in terms of particle size or mineralogy, or choosing to split up types of tailings so that more problematic materials are either selectively removed from the backfill, or put into the backfill because it is an opportunity to get it in an isolated condition underground. I think these have all been really interesting developments over the last 25 years.

What do you think is the future of mine backfill?

Pastefill is coming into maturity now. We have largely figured out the mechanics, although there is always more to be done on that side. We are seeing a lot more clients wanting to know about the processes they need in place to make sure their backfill system is sustainable, reliable and high quality all the time. Once the fill becomes reliable, the mine can react accordingly and reap even more benefits.

I think we’re going to see a lot more instrumentation being used. The digitalization of mining has benefitted backfill because before, there wasn’t really a way to get a lot of real-time information. We are seeing a lot more equipment like cameras, automatic valves and widespread use of pressure transmitters; now there is more connectivity between the systems and on-demand information. Even simple things like measuring the height of the backfill in the stope as you’re pouring, which sometimes is very difficult—those are the types of challenges that we’re going to see get solved.

We are seeing a rethink of binders too. One of the main costs of backfill is the cementitious materials used as binders, and as a global industry, we’re looking for ways to use less cement, or to further bring out every ounce of strength we can get from the materials that we put in.

Backfill already has a key role in mine waste disposal, and I think we’ll see that expanding as mines go deeper, and as more mines are opened in remote areas like northern Canada. Anything you can do to lower the amount of material that you have to ship onto site is important; or at least if you do use it, to make sure that you’re using it to its maximum capacity.

Read the full original article published by CIM Magazine.

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