The Impact of Ore Body Variability on Mine Waste Tailings Disposal Systems

Ore body variability can have a significant effect on the various elements of mine waste tailings disposal systems, including:

• thickening systems,
• tailings pump and pipeline systems, and
• tailings storage facilities.

Ore bodies that are exploited using open-pit mining methods often consist of softer, weathered, clay-containing material near to the surface (oxides), transitioning to harder rock ores (sulphides) with increasing pit depth. A typical example is that of oxide and sulphide ores common to the Central African Copper Belt and some open-pit gold mining operations in West Africa.

The extractive metallurgical processes used for optimum refining of such oxide and sulphide ores are well understood and covered extensively in the literature—e.g. the roasting of copper sulphides ores prior to leaching. However, management of the tailings resulting from such processes appears to be less well understood by industry, particularly in terms of the impact of variable mineralization of the ore body on tailings thickening and pumping systems—potentially resulting in inappropriate sizing and selection of tailings thickeners and tailings pump and pipeline systems that could influence the deposition properties of the material.

This short article focuses on typical open-pit copper and gold mining operations that often treat a range of ore types that result in tailings with distinct rheological (viscosity and yield stress) properties. These would typically include:

• Tailings from the weathered oxide ores with significant clay content occurring at surface and relatively shallow pit depths
• Tailings from the hard-rock sulphides ores containing negligible clays which are located in deeper zones of the pit, and
• Tailings from the transition zone between these two ore types.

A compounding factor is the often arid climate and scarcity of water, and so consistent water recovery from the tailings thickeners and deposition site is a key priority:

• This requires that the tailings thickeners must produce the required underflow densities irrespective of the changes in mineralogy of the feed material
• Variations in feed due to differing clay components have a significant impact on thickener performance and require different conditioning and flocculation to achieve similar underflow densities, and the rheology will vary considerably at equivalent densities.

Historical vs Modern Processes

Historically, tailings management typically either involved unthickened tailings or conventional tailings thickening with targeted underflow densities in the region of 50% by mass (i.e. one metric tonne of water to transport one metric tonne of solids).

This relatively low slurry density, in combination with coarser grind sizes typical of older operations (i.e. reduced liberation of fines and clays if present) meant that rheological properties were usually insignificant; and, that tailings behaviour did not change significantly as a function of the properties of the ore being treated.

However, more modern processes with increasingly fine grind sizes, advancing thickener technology and environmental pressures to reduce water consumption often target tailings underflow densities well in excess of 50% concentration by mass—often 60% and above.

Typical Rheological Data

The figure below shows yield stress vs solids concentration by mass for the three tailings types from a deposit with typical oxide to sulphides transition. The three curves, from left to right, are for the oxide, transition and hard rock (sulphide) tailings, respectively.

The highly variable rheological characteristics of the three tailings types need to be considered carefully in the design of the tailings management system, particularly if thickening to higher underflow densities.

From the data it is evident that mass concentrations in the 50% range have insignificant yield stress differences, while concentrations in the 60% to 70% range exhibit dramatic differences.

For example, at a required thickener underflow density of 68% by mass, the sheared yield stress of the three tailings types is as follows:

• Oxide (clay containing) material—approximately 156 Pa
• Transition material—approximately 55 Pa
• Sulphide (hard rock) material—approximately 8 Pa.

The difference is presented visually in the following photos, which show Boger cylinder slump tests for oxide and sulphide thickened slurry samples at the same solids concentration.

• The difference in slump (and corresponding slurry yield stress) is clearly seen and is due to the fine, weathered clay fraction present in the oxide ore and absent in the hard rock ore
• As mining progresses through the upper zone it is clear that the requirements for thickening, pumping and deposition will vary considerably over the life of the mine.

Thickening Considerations

To achieve a consistent underflow density to satisfy the water requirements requires a thickening strategy capable of producing the same underflow densities for materials with different settling characteristics and viscous properties.

The oxide ore would require modern paste or high rate thickeners with enough surface area and bed compression height to achieve the required densities, compared to the hard rock ore body that could be thickened with smaller diameter high rate or conventional thickeners.

In addition, the thickener feed conditioning and flocculation requirements are likely to vary considerably. The oxide ore would usually require higher flocculant dosing rates than the hard rock ore and, in addition, may also require feed conditioning using coagulants.

Pumping Considerations

To reach the furthest corner of the TSF from the tailings pump station would also require different pump and pipeline combinations:

• High pressure positive displacement pumps and high-pressure steel piping and valves would need to be considered for the oxide ore,
• Centrifugal pumps and steel piping could be considered for the transition ore, and
• Centrifugal pumps and HDPE piping are an option for the sulphide ore.

Deposition Considerations

The behaviour of these different materials at the tailings storage facility will clearly also be very different. The high yield stress oxide materials will likely form a more uniform and less segregated deposit than the lower yield stress material, and as mining progresses and the nature of the ore body changes, the tailings facility must be able to accommodate these variations.

Summary

For these operations with a highly variable mineralogy it is not always practical to design a dewatering and pumping solution that consistently delivers the required underflow density for a wide range in ore type and subsequent changes in rheology. It is often necessary to select a thickening and pumping system based on slurry rheology that will result in a moderate thickened underflow pumping density for the more viscous ore zones and will be able to achieve higher underflow densities for the less viscous materials.

To quantify the differences in a variable ore body it is necessary to conduct slurry test work to properly understand the material behaviour so that appropriate dewatering, pumping and deposition options are considered.

Failure to understand the variations in the material and the consequences on the operation can lead to significant equipment and piping selection errors and create a bottleneck for plant performance. This can have serious consequences in terms of CAPEX, OPEX and lost production.

Paterson & Cooke has the experience and capabilities to design optimum tailings thickening and pumping solutions, including:

• Mobile benchtop scale test equipment to conduct on-site thickening and rheology test work for correct characterisation of slurries
• An extensive database of slurries that have been tested
• A selection of slurry flow models which ensure accurate modelling of slurry pipeline flow for all slurry types
• Extensive experience in the design of both positive displacement and centrifugal pumping systems.