The global trend of decreasing metalliferous ore grades and increasingly complex mineralogies is leading to finer grind sizes to achieve the necessary liberation for ore beneficiation. The impact of this is expressed throughout the ore processing cycle, including mine backfill; by leaving lower fractions of coarse material suitable for hydraulic fill and, in particular, influencing modern paste backfill which traditionally has sought to utilize the entire tailings stream.
The impact of a finer grind size typically leads to increasing the binder consumption and therefore the unit operating cost. The rise in binder consumption can be attributed to changes in particle surface area, as well as an increase in rheology, resulting in lower solids concentration backfills for the same flow characteristics. Consequently, opportunities exist to adjust the particle size to drive down on binder consumption and unit cost.
Adopting rheology modifiers or admixtures to improve flow characteristics of finer particle size backfills can offer a technical solution, but inevitably requires trading a portion of the cement saving to cover the cost of purchasing the rheology modifier. Admixtures have become commonplace in the industry and their adoption will continue to increase, not least because the performance of the available products continues to improve.
An alternative to this supplementary ingredient is to physically modify the particle size distribution of the tailings to achieve more favorable characteristics. Such modification may require the addition of material, such as a sand or aggregate to the backfill, or the removal of ultra-fines by cycloning, or other applicable separation processess. Recent test work programs undertaken by Paterson & Cooke have demonstrated the performance improvement associated with this particle size adjustment. The first example (Graph 1) illustrates the difference in unconfined compressive strength and rheology of a paste backfill when prepared with full tailings and cycloned tailings.
Looking at the data for a given water to cement ratio and for these particular tailings, the cycloned tailings would return a 50% higher strength than the equivalent full tailings. Additionally, the increase in solids concentration for a given yield stress (Graph 2) demonstrates the second benefit offered by the cycloned tailings. The ability to operate at a higher solids concentration (due to lower yield stress values) allows for a further reduction in the relative water to cement ratio and consequently a lower binder dose for the same target strength. Using the above data, and assuming a target strength 750 kPa with a backfill reporting a 150 Pa yield stress, the operation might expect to see a 30% binder reduction.
Another option available to backfill operations is the addition of material to the backfill in the form of sand or aggregate. This can be applicable in instances where the grind size of the tailings precludes fines removal, when the removal of material might lead to a deficit in the available backfill tonnes, and / or a suitable supply of additional material is available.
Data presented in Graph 3 are derived from a tailings material with 62% Passing 20 microns and an aggregate blend of < 16mm aggregate. In this instance the option to utilize 100% tailings proved to be technically unviable because of the lower strengths achieved and consequently the addition of the aggregate provided viability to the backfill solution.
In summary, opportunities may exist to optimise a backfill product by investigating adjustment to the particle size, leveraging significant binder reductions to achieve unit cost savings to the operation. In this way, the industry can begin to treat the tailings feed for backfill as a product, designed and manipulated to meet a consistent specification.
About The Author
BEng (Hons), ACSM, CEng, MIMMM
Stephen is a Mining Engineer with more than 12 years’ experience of providing consultancy services to the minerals industry, with particular specialisation in tailings dewatering and mine backfill. He has a Mining Engineering degree from Camborne School of Mines in Cornwall, and is a chartered engineer with the Institute of Materials, Minerals and Mining. In 2013 Stephen established the Paterson & Cooke Cornwall Office.