This novel approach to rapidly minimizing the volume of tailings slimes is based on the realization that the pore spaces existing between larger particles of stratified deposits are both filled with fluid, and at the same time potential reservoirs for smaller particles. Simply mixing together separate seams of uniformly sized particles will result in the volume reduction of the saturated solids, while the liberated pore water becomes supernatant fluid available for futher mine use. The application of vibration will cause further densification. The Strata Mixer is designed to perform both actions concurrent throught the full depth of slimes within a tailings pond.
The natural processes of fluid transport of particles (tailings is discharged into storage containment as a fluid) results in particles settling out of suspension strictly according to their size and the lateral velocity of the transporting fluid. Consequently, the resulting deposit of the solid phase is inherently a layering of uniformly sized particles. Furthermore, because the discharge from the mill is variable and episodic (rather than the relative constant flow of a river) the particles come out of suspension in strata of uniform size which have limited lateral extent. Therefore, the typical tailings deposit consists of seams of uniformly sized grains wherein the particle packing is inherently very loose, with large pore volumes filled with fluid.
These seams/lenses are interlayered to an extent which depends on the degree of mill discharge temporal consistency/inconsistency as well as the location at which the mine staff choose to position the discharge pipe around the perimeter of the pond.
The result is that the volume occupied by tailings is unnecessarily high because of being made up of seams of uniformly sized particles. The solution advocated here is to intentionally disturb these naturally formed seams, so that the particle sizes are mixed together in order that the volume occupied by the solid phase is reduced by the pore space between larger particles becoming infilled with smaller particles. This procedure will result in making the overall volume previously needed to store the separate seams of uniformly sized particles much reduced. And in consequence, the fluid volume previously occupying the pore spaces will be liberated as a supernatant liquid which can be pumped back to the mill or for further treatment.
Apart from the primary benefits associated with the Strata Mixer of reducing the volume occupied by mine waste, and the liberation of slimes-entrapped water for reuse, there is the following:
Mixing of grain sizes ensures that the resulting aggregation cannot liquefy. An important implication of this is that treated tailings should thereafter allow the option for safe use of "upstream" construction above them. It is anticipated that the placement of a layer of earthfill (for an increase in tailings dam crest elevation) could commence within a week or so of treatment by the Strata Mixer.
The mixing function is achieved by slowly rotating paddles – much the same as in a kitchen blender – around the a vertical axis of the poker.
The deployment of the Strata Mixer is the same as for the other Phoenix™ hardware, and this tool could be used either as a single machine, or deployed in the Trident configuration. Because of the nature of slimes and their deposition in tailings ponds, it is likely that, rather than moving the equipment to the treatment site by tractor, the machine would be operated from a barge.
Once the nose cone of the poker has reached its assigned depth within the tailing mass the paddles are extended out into the surrounding tailings and made to rotate.
Our proprietary "spiral coupling" is incorporated in the drive mechanism so that verical vibrations of the paddles can be alternated with their rotation; this should contibute further to volume reduction of the slimes.
To cover the plan area selected for volume reduction, the columnar treatment process would be reinacted at multiple interconnected positions across that area. It would be for the mining/design engineer to lay out an array of treatment axes suitable to optimize their operation.
Although what is written here is expressed in terms of mine slimes, this Strata Mixer could also be used in the natural soils where, as for instance in the case of deltaic deposits, they are similarly stratified.
This idea of Hodge has been granted proprietary status by virtue of US Patent applications 61/882,621 and 62/004,402. The sketch to the left shows the main operating mechanism of the 62/004,402 tool.
Subsequently, a new tool has been designed specifically for the problem COSIA is experiencing in the Alberta Oil Sands with that fraction of the tailings which apparently has the consistency of yoghurt, and which is not amenable to consolidation within a reasonable timeframe.
As we understand it, the concern is with the space the yoghurt-like MFT currently occupies, rather than any noxious properties inherent in this material. If this is so, we suggest it be taken from its present storage areas and be placed instead in the huge volume of "empty" space available within the coarser tailings.
Masses of tailings commonly consist of about equal parts of solid particles and pore fluid; consequently, each cubic metre of tailings contains about 500 litres of free water/fluid which is replaceable and reusable. What we propose here is to store the yoghurt in these pore spaces by injecting it under pressure (much like a grout) and at the same time facilitate the escape of the pore water/fluid by means of a suction pump activated close above the injection site. In this manner, we feel it is reasonable to expect to create subsurface storage, amounting to about 35% of the coarser tailings volume, for the MFT – with an equivalent volume of fluid being recovered for recycling.
Essentially, what we propose doing is simply the opposite/inverse process of oil extraction by means of oil well drilling. Here, instead of pumping oil out of a porous formation, it is a matter of pumping a waste material into a porous formation. And in this case, to environmental advantage.
The GI hardware we would probably use, at least for testing purposes, would be a modified version our existing proprietary "strata mixer". Figs 8 & 9 from our new US/Canada patent applications include these features:
Items 56 and 58 on Fig 8 show the filtered drain element through which expelled pore fluid/water would be sucked out of the coarser tailings above the working zone – for clarity, the submersible pump is not shown inside casing (30). On Fig 9, the MFT is pumped under pressure through a central conduit (37) and out into the coarse tailings through ejection points 54. For simplicity of presentation holes 54 are shown only through the upper part of the nose cone (39), however, depending on the viscosity of the yoghurt it may prove advantageous to also provide 54 openings in the paddle arms themselves.