Mine backfill: one size fits all?

Jon Lawson

Martijn-K Mannot-Russell & Zlatko Martic explore the relationship between backfill and production

In the past, at many mining sites utilising backfill methods there may have been a tendency to focus more on the mining production than the backfill element.

For cut & fill operations, the success of the mining operation also relies heavily on the success of the backfilling operations. In fact the mining, mineral processing and backfilling methods are inextricably interlinked, with an immediate effect should one of the processes be disrupted. The total cost of most mining operations depends on the all processes being well connected and part of the total net present value (NPV) of the mine. It means that backfill optimisation needs to be evaluated as a part of the total mining cycle.

Backfill cost and benefits are related to the cost of the binder but also essentially, to the efficiency, production rates, energy costs, total tailings and water management and maintenance costs (wear and tear).

There a several types of mine backfill and to avoid any misunderstanding, it is worth clarifying what each type is.

Cemented rock fill (CRF) is rock, aggregates from external sources or waste rock from footwall/development drives placed in the stopes, fine tailings with binder pumped in to fill any voids and bind the backfill or sprayed onto the rocks that are on the trucks dumping close to the stope and placed by LHDs.

Cemented hydraulic fill (CHF) is backfill material consisting of a fluid mix of classified tailings or sand and binder pumped into stopes fitted with barriers to allow the drainage of excess water to pass through. Tailings or sand are classified to enhance dewatering process (drainage) and excess amount of water is needed to permit flowability/ transportability of CHF. During the drainage certain amount of fines and binder are drawn with the water, causing inhomogeneity of the fill and loss of cement with the water. The solid faction separates out with the excess water being pumped out to surface or re-used elsewhere in the mine.

‘Grout pack’ fill is a typical South African development from the narrow reef stopes of the gold and platinum mines as an alternative to the traditional support methods using timber. These bags or pillows are filled with a CHF.

Cemented paste fill (CPF) is the latest backfill system, using full tailings stream from run of mine or dry excavated tailings. This method uses a mixture with a considerably higher solids content, with the minimum content of 15% passing 20 microns, which per definition doesn’t set and bleed. In practice there is none or considerably reduced amount of excess water that has to be disposed, as in the case of CHF.

Backfill binders are a blend that can consist of the various ratios of fly ash or slag blended with cement. Typical blends are either 90:10 or 70:30 fly ash/slag: cement ratios respectively. Note that cement properties will also vary from region to region and country to country, just to make life interesting.

Are binder costs the key driver?

Binder cost in a backfill operation can represent up to 70% of a total backfill cost and backfill as an operation up to 25-30 % of total mining cost. The mine or project location will have an effect on the binder costs. Reduction in binder consumption volume without affecting the final backfill strength is a key focus for most mines. This is more so in the case of remote mines that have to transport binders over vast distances. In some cases, the cost of the binder is the main driving factor with some remote mines having binder costs as much as US$200 per ton or even higher.

Backfill plants and systems rely on the preparation of the fill, transportation and placing of it. In essence there is a limit to the capabilities of such plant that use only water, binder and tailings as they are limited by the properties of the materials themsevles (PSD, density, mineralogy, binder type, water chemistry and conditions as temperature) and limitations of the plants and pipes. Certain chemicals can increase the solids content and flowability, at the same time reducing binder content maintaining same flow and UCS. In certain operation, some can also decrease wear of the system (paddles of the mixer and pipe wear). In addition it is possible to reduce the amount of the water needed for the paste preparation and also increase amount of the tailings in the paste, while decreasing tailings send to the pond (surface disposal).

Off-the-shelf or customer-specific solution?

Is there a standard product and or dosage best suited for your mine backfill? The answer is, unfortunately, no. Each mine by its very nature is unique. The mineralogy of the orebody, the processing method(s), the tailings management method(s), and the water properties are all unique and all have influences and affect the properties of mine backfill. As the mine develops the mineral processing systems may be adjusted to improve recovery rates and hence there may be some variation in chemical properties of the tailings from case to case. This will need a study for any projects to find out a best-suited chemistry. It also needs to be stated that besides material properties, conditions and requirements from the mine can play a major role in selection of most suitable technology or blend of the technologies currently available.

The only way to obtain an effective understanding of how the mine backfill can be optimised is by small-scale laboratory testing followed by site testing and trials. A regular testing regime is recommended to ensure consistent production of mine backfill with the desired properties such as flow ability (viscosity), solids content, and final compressive strength for the lowest price and maximum benefit.

The search for the holy grail of cost optimisation

Cost optimisation is a continuous process linked to the cost and availability of the required binders, which in some cases is linked to the amount of available tailings left over from the mineral extraction process.

In the search for cost optimisation there has to be certain element of trust between the mine operator and the chemical supplier. The mine operator needs to divulge the costs per cubic meter and the key elements to be optimised. Note that in some cases further optimisation on an economic scale is simply unfeasible.

Chemicals have the ability to increase/extending the operating range of backfill plants so should be viewed as a complementary element, though the cost benefit, if any, is the ultimate deciding factor.

So be wary of anybody claiming that they have the single ingredient to solve all your mine backfill challenges with no testing. If this was the case, then our job would be replaced by a web page and you would order the required product online.

For more information, visit, www.engineerlive.com/ime

Martijn-K Mannot-Russell & Zlatko Martic are with BASF Construction Chemical Europe.

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