Changing nature of mine fill

Water-borne sand has been used as an underground fill for more than 60 years. These early applications were generally for unusual circumstances where it was required to mine pillars or to stabilize ground that was honeycombed with earlier workings.

Soon after the Second World War, large-scale, systematic hydraulic filling became commonplace in Canada. It was an essential part of the cyclical cut-and-fill methods then in vogue and cut-and-fill mining was a necessity for the production of clean, undiluted mill feed.

The application of fill was to permit the use of selective mining methods. The fill was deslimed mill tailings; the tailings were sometimes supplemented with natural sands when the mill supply was insufficient.

Ground support was obviously improved by the use of sand fill but at the same time, the support was strictly local. It was confined to the area in the immediate vicinity of the stope.

Overall support for the mine as a whole was not greatly benefited as the sand fill yielded under presssure. It compacted to about 90% of the volume it originally occupied and the rock stresses induced by mining were only partly allayed as a consequence. The sand fill did not have the requisite stiffness to give effective support.

None of this was of serious concern at shallow mining depths, if in fact it was recognized, but as mining went deeper the picture changed. Fill was needed to permit clean mining methods but it was also becoming a necessity for maintaining the overall stability of the rock mass in which the mine operated.

At this point, cement came into use and it is now a virtually universal additive to water-borne fills. (Interestingly, cyclical cut-and-fill mining has almost disappeared from the Canadian mining scene and has been superseded by bulk-mining methods. Cemented sand fill has also been superseded but only in part. Thus, water-borne cemented sand fill is as frequently used today as a binder for coarse rock fill as it is as a single, all-in-one fill material.) In just two generations, the industry has witnessed a change in the basic function of fill. Initially, it was the means for promoting selective mining. Now it is the essential component of a system designed to ensure the overall stability of the mine.

What is next for fill? According to a recent paper presented by Golder Associates, well known consultants in the rock mechanics field, the newest development is “paste fill.” This paper was presented at the Ninth International Conference on the Hydraulic Transportation of Solids in Pipes. It was written by R. Brummer and A. Moss of Golder and titled “The fill of the future.” (A summary of the paper will form part of the forthcoming issues of the Canadian Mining Journal, a Southam publication).

Essentially, the entire mill tailings fill is thickened to about 80% solids, cement added and the thick paste delivered underground to wherever it may be needed.

There are several differences between this paste and conventional hydraulic fill. Only deslimed tailings can be used in the latter and provision must be made for the long-term impoundment of the unused slime fraction. This entails a major expense particularly today with the proliferation of environmental regulations. Paste fill will reduce the volume to be impounded on surface by about a half, according to the authors.

In its underground application, paste fill will produce a higher strength fill with less cement, it will eliminate the need for elaborate stope bulkheads and the reduced water content will lead to a reduction in mine pumping costs. Already, four mines in South Africa have adopted paste fill and a number of others are investigating its possibilities.