Mine backfill: a budgetary black hole or a savings opportunity?

Mine backfill is used on many underground mines and, although it can consume a big share of the operating budget, it is still a rarity to find a single person directly responsible for the process. With recent low commodity prices, costs and productivity are receiving laser-like attention. Backfill, which can account for up to 30 per cent of a mine’s operating expenses, warrants management focus.

Backfill is typically made from waste rock or dewatered tailings residues, and is often mixed with cement to achieve moderate strengths. It is then delivered to stopes either by truck, by pumping and/or gravity, or as dense slurry or paste through boreholes and pipeline

It fills open stope voids, maintaining stability of the adjacent working areas and reducing risk of local or regional ground failure. When cementitious binders are added, the blasting of adjacent pillars enables higher recovery of ore reserves by exposing the cured fill.

Backfill offers many environmental benefits too. Paste backfill – or tailings dewatered to a yoghurt or toothpaste-like consistency – can enable up to 50 per cent of the total tailings produced by an underground mine to be placed back underground. In some mines, acid-generating waste can be encapsulated in the backfill, sealing it into virtually impermeable cells. These activities reduce the environmental footprint of the mine and assist with final site rehabilitation.

The design, operation and management of backfill systems require a number of technical disciplines that often involve several management teams on a mine site. The processing department on surface is generally in charge of production and delivery of the backfill, and is responsible for the quality, operating costs and process monitoring. The underground geotechnical department specifies the strength of the fill, the cement dosing and the fill recipes. It also reviews the quality control (QC) data and analyzes fill performance to improve the fill recipes. The mine planning department develops the schedules and specifies where and how much fill is placed.

This convoluted process presents problems, though. The processing department carries all the production costs, but the geotechnical department is responsible for the cement dosing rates, and the mine planning and operating departments plan and place the fill. The net result tends to be a lack of focus on meeting the mining requirements and maximizing quality, which in turn produces excessive costs and low productivity.

To improve productivity and reduce costs in the backfill system, operations should assign one person to have overall responsibility. Most commonly, this is a superintendent level employee who is directly responsible for all underground backfill activities. Where management roles are split on a mine site by surface and underground, that person would also have a leadership role for assisting the other managers. The role is a coordinating function between processing, mine planning and operations, and geomechanics. Backfill is a technically complex system, and an engineer is required either directly or as support for this role.

Operations should also initiate a backfill system audit. The key performance indicators (KPIs) are both the quantity and quality of the fill placed underground. Ideally this should be a review of the current backfill system against the backfill management plan. However, many mines do not have such a document and the audit then serves to provide a systematic review that will initiate this plan.

The audit should start with a review of mining needs and work back through the system via placement, delivery and processing to the supply of the backfill components. Invariably, the backfill needs of the mine will have changed since the original design and commissioning of the backfill system. Increases in production rates, and changes in both mining area locations and mining methods will have occurred. An audit provides an opportunity to fine-tune each component to ensure that the backfill types and recipes are all relevant to the current operations.

Productivity improvements can emerge when focus is placed on activities that will shorten stope fill cycle times. Longer fill runs, higher slurry density, reduced drainage and improved barricade construction will all contribute to faster stope turnaround. Installation or recommissioning of line pressure sensors and video links will permit operators to identify and avoid impending line blockages, and the long and costly delays that accompany them.

Cost reductions can be found by addressing the fill quality and the components, particularly binders. Is the mine using the correct binder type or grade for your strength curing targets? When was the last review of the tailings or rock characteristics, and how do the QC test results compare with the design specifications? Have there been recent changes to recipes in response to either unexpected or excessive fill dilution that have increased binder or additive costs?

This article was originally published in the March/ April 2017 edition of CIM Magazine, click here to view.