BACKGROUND OF THE INVENTION Mines use tailings storage facilities including above-grade structures and mined-out pits to contain solid and liquid residues from the mineral or metal extraction processes. Tailings storage facilities are designed to contain such by-products. Best practice management of these facilities sets out to minimise the volume of liquid (process water) residue in order to maximise the settlement and consolidation of solid (tailings) residues which accumulate in the bottom of the tailings storage facility.

To increase the rate of evaporation of process water residue from the tailings storage facility, one option is to increase the size of the storage facility.

However, this not always practical as the area which can accommodate an above grade tailings storage facility may be limited or the mined-out pit which is used for storage of mineral processing residues is fixed in size by the mining activity. Further, increasing the area of an above grade storage facility increases the area of land which is potentially contaminated by the residues of the mineral or metal extraction process. The rate of evaporation of process water (liquid residue) from a

tailings storage facility must match the rate of inflow (including rainfall) into the storage facility or the water level will rise. Physical constraints or regulatory controls and approvals may limit the height to which the process water (liquid residue) can be permitted to rise.

Finally, in the tropics, the net annual evaporation is the difference between evaporation and wet season precipitation. If this is small or zero, increasing the area of an above-grade storage facility may be an inefficient way to increase net evaporation. As a result, there have been efforts to increase the rate of evaporation of water from tailings storage facilities.

Previous attempts to improve evaporation have included the use of snow-making machines to pump water from tailings storage facilities and blow it into the air as a mist or spray. However, these techniques are not suitable for use in humid atmosphere where the air is already heavily saturated with water. Further, it is possible for contaminants in the water to be sprayed into the air with the water and spread over a wide area by the wind. This is a particular problem where the contaminants pose a potential health risk to humans or the environment.

For example, where the tailings storage facility is that of a uranium mine and the contaminants are radioactive.

Furthermore, in many tropical landscapes, where retention of wastewaters by mines or industrial operations is required by legislation so that downstream environments and ecosystems are protected from the potential for contamination by wastewaters.

Accordingly, there is a need for an alternative technique for enhancing water evaporation from a technique for enhancing evaporation from any body of water such as a tailings storage facility.

SUMMARY OF THE INVENTION Accordingly, the invention provides a method of enhancing water evaporation from a body of water comprising providing a plurality of evaporation enhancing structures at spaced apart locations on said body of water.

The method preferably involves anchoring the evaporation enhancing structures relative to the body of water so that they may move to align themselves with the prevailing wind. Typically, this comprises attaching each evaporation enhancing structure to a buoy anchored to floor of the body of water and providing each evaporation enhancing structure with wind alignment means-for example, in the form of a fin.

The method preferably comprises spacing the evaporation enhancing structures so they cannot interfere with one another when they move to align themselves with the prevailing wind.

Accordingly, the evaporation enhancing structures are preferably spaced from one another in a staggered array.

In one embodiment, each evaporation enhancing structure comprises a vortex generator for increasing the supply of dry air to the surface of the body of water, and floating support means for supporting the vortex generator when it is floating on the body of water.

The vortex generator is preferably a solid triangular wing with one side close and parallel to the water surface and one point elevated. The wing shape is approximately an equilateral triangle. The wing is preferably supported by the support means at an angle of

between 15 degrees and 35 to the surface of the body of water, most preferably at an angle of 25 degrees.

Where the evaporation enhancing structures include vortex generators it is preferred that the vortex generators are spaced so that the trailing vortices of the vortex generators do not interfere with the generation of vortices by other vortex generators. Typically, the spacing between the apex of each wing is at least 3 chord widths in both the streamwise and cross-stream directions.

In another embodiment, each evaporation enhancing structure comprises a wettable portion comprising at least one wettable surface, and floating support means for supporting the at least one wettable surface in a plane which is transverse to the surface of the body of water and transverse to the wind direction, when the evaporation enhancing structure is floating on the body of water.

The wettable portion may be provided by a rectangular frame covered by a wettable material. It is preferred that the wettable material is porous. In one embodiment, the wettable material is caused to come into contact with the body of water in order to draw water from the body of water by capillary action. In this embodiment, it is preferred that the wettable material has high capillarity. As an alternative to relying on capillary action, or in addition, a pump may be provided to draw water from the body of water in order to wet the wettable portion.

The height (H) to width ratio of the wettable portion is preferably in the range of 1: 10 to 1: 30 and most preferably about 1: 20.

The height (H) of the wettable portions is preferably in the range of 2m to 5m.

In this embodiment, it is preferred that the evaporation enhancing structures are arranged in a staggered array.

The method may also employ a mixture of different types of evaporation enhancing structures.

The method has particular application to bodies of water containing contaminants.

The invention also provides an evaporation enhancing structure comprising a wettable portion having at least one wettable surface, and floating support means for supporting the at least one wettable surface in a plane which is substantially transverse to the body of water when the evaporation enhancing structure is floating on the body of water.

The invention also extends to an evaporation enhancing structure comprising a vortex generator for increasing the supply of less humid air to the surface of the body of air, and floating support means for supporting the vortex generator when it is floating on the body of water.

Further features of the invention will become apparent from the following description of the preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows schematically how a plurality of evaporation enhancing structures can be deployed at spaced apart locations on a body of water ; Figure 2 shows one example of an evaporation

enhancing structure ; Figure 3 shows a"delta wing"vortex generator used in wind tunnel tests ; Figure 4 is a temperature pattern relating to the vortex generator of Figure 3 ; Figure 5 shows an array of"delta wing"vortex generators used in a wind tunnel test ; Figure 6 is a surface temperature pattern of the array of Figure 5 ; Figure 7 shows a staggered array of evaporation enhancing structures where the evaporation enhancing structures are porous mesh fences ; Figure 8 shows the surface temperature of the pattern for Figure 7 ; and Figure 9 shows an alternative evaporation enhancing structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the method of the first preferred embodiment of the invention a plurality of evaporation enhancing structures having vortex generators are located at spaced apart locations on a body of water in the form of a tailings storage facility. The tailings storage facility contains contaminants or by-products of a mineral or metal extraction process.

The arrangement of the evaporation enhancing structures incorporating vortex generators is illustrated schematically in Figure 1. The evaporation enhancing

structures 1 are anchored to respective anchor points 2 so that their movement is limited to being within circles 3.

The evaporation enhancing structures 1 are anchored in such a manner that they may align themselves with the direction of the prevailing wind 4. Each evaporation enhancing structure is attached to a buoy by a rope or chain and each evaporation enhancing structure is provided with wind alignment means-for example, in the form of a fin. The prevailing wind will then cause the evaporation enhancing structures to pivot around anchor point 2 in order to align themselves with the prevailing wind. As is shown in Figure 1, the circles which define the limit of movement of each of the evaporation enhancing structures are arranged so that they do not overlap so that the evaporation enhancing structures 1 cannot interfere with one another.

An evaporation enhancing structure 1 incorporating a vortex generator is shown in Figure 2.

The evaporation enhancing structure 1 includes a vortex generator in the form of a"delta wing"5. The"delta wing"5 is mounted on a floating support consisting of base member 7 which is formed from a plurality of hollow plastic tubing elements and struts 8 which support the wing 5 at the desired angle of attack. The preferred wing shape is an equilateral triangle with the point 6 of the wing 5 being elevated and oriented towards the prevailing wind. The wing is supported by struts 8 at an angle of 25 degrees to the surface of the water 10. This provides an ideal vortex generator. Persons skilled in the art will appreciate that less than perfect vortex generators can also be used. For example, a delta wing can be deployed at angles between 15 degrees and 35 degrees.

The evaporation enhancing structure 1 also includes additional float means 9 for improving the buoyancy of the structure, and rope 11 for anchoring the

vortex generator to a buoy.

As shown in Figure 1, the optimum arrangement of the vortex generators is a sparsely staggered array that ensures that the trailing vortices of one wing do not interfere with the development of vortices from the next row of wings downstream.

The vortices disturb a boundary layer of moist air which exists immediately above the surface of the water by drawing additional dry air down to the surface of water, thereby enhancing the rate of evaporation from the water. The ideal spacing for equilateral triangle vortex generators occurs when the apices 6 of the wings are arranged so that they are at least three cord widths apart in both the streamwise (ie. , the wind direction), and the cross-stream directions, successive rows displaced by one and a half cord widths relative to each other in the cross-stream direction (ie. , they are offset). More distant spacings are acceptable with only minor loss of effectiveness until the spacings are six cord widths in both the streamwise and cross-stream directions, in that successive rows displaced by one and a half cord widths.

The evaporation enhancement structure of the preferred embodiment were trialed in a wind tunnel. In a first test, an evaporation enhancement structure 1 having a"delta wing"was placed on an electrically heated black surface. Figure 4 shows a temperature pattern behind an isolated evaporation enhancement structure incorporating a vortex generator. As in Figure 1, arrow 4 indicates the prevailing direction of the wind in the wind tunnel.

Cooler region 16 directly behind the evaporation enhancing structure 1 show the development of strong trailing vortices caused by the evaporation enhancement structure.

Figure 5 shows an optimum array of evaporation

enhancing structures also prepared for a wind tunnel test.

The temperature pattern of Figure 6 shows that this arrangement leads to cooler regions 16a and 16b immediately behind vortex generators la and 1b as well as a cooler band 17 between regions 16a and 16b.

Figure 7 shows a test rig for a wind tunnel test of the method of a second preferred embodiment which uses an alternative evaporation enhancement structure. Each evaporation enhancement structure has a wettable portion which extends in a plane which is substantially transverse to the surface of the body of water. In the second preferred embodiment, the wettable portion is provided by a rectangular frame covered by wettable material. The wettable material would typically be porous and caused to come into contact with the body of water in order to draw water from the body of water by capillary reaction.

Accordingly, it is preferred that the wettable material has high capillarity. Thus, the evaporation enhancing structures of this embodiment work by a combination of drawing water from the body of water so as to wet a vertical surface that is exposed to the wind, hence increasing the area undergoing evaporation, and reducing the aerodynamic resistance of the water surface.

The evaporation enhancement structures are arranged in parallel rows. A row consists of a series of evaporation enhancement structures aligned normally to the long axis of the row like rungs on a ladder. The long axis of each row is aligned with the prevailing wind. In any one row the distance between evaporation enhancement structures in the direction of the wind may be greater but not less than 10H with an optimum spacing of 12H. Adjacent rows of devices are staggered by half the spacing between devices in any one row. The distance between the centrelines of adjacent rows should be approximately 10% less than the cross-wind width of the devices so that

devices in adjacent rows overlap when viewed in the direction of the wind. This produces an overall arrangement of devices consisting of a staggered array so that the projected area of fence normal to the wind divided by the plan area is 1 divided by 20.

For such a staggered array, the change in aerodynamic resistance is indicated in Figure 8, where there are cooler patches 21 proximate the edges of the evaporation enhancing structures 20.

The aerodynamic resistance is reduced by 18% by these evaporation enhancing structures compared to a reduction of 29% by the evaporation enhancing structures 1 incorporating vortex generators. However, there is the additional effect of the increased area for evaporation due to water being drawn from the surface by capillary reaction.

The height to width ratio of the wettable portion is 1: 20, although other height to width ratios can also be used. For example, height to width ratios in the range of 1: 10 to 1: 30.

The height (H) of the wettable portion is typically in the range of 1-5 metres.

Table 1 summarises the predicted changes in evaporation for various different wind speeds.

Table 1. Dry Season Results. Evaporation in mm/day Windspeed Water Stability Water + Ratio of Vortex Ratio of m/s only Z/L Fences Water + Generators Vortex Fences to + water Generators + water only water to water only 1. 010% 2.7826 0. 1 3. 272 1. 176 3. 37264 1. 212 1. 7510% 3.78693 0. 1 4. 32826 1. 142 4. 45037 1. 175 2. 510% 4.49668 0. 1 5. 10394 1. 133 5. 23401 1. 162 5. 010% 6. 16767 0. 1 7. 07465 1. 145 7. 27848 1. 179 7. 0+10% 7.27849 0.1 8. 46145 1. 162 8.73168 1.199 Stability Z/L Water Wind Water + Ratio of Vortex Ratio of only speed Fences Water + Generators Vortex m/s Fences to + Generators + water only water water to water only -1 5. 36293 2. 5 6. 13286 1. 143 6. 26663 1. 169 0. 1 4. 49209 2. 5 5. 10352 1. 135 5. 23401 1. 164 1 3. 21128 2. 5 3. 69397 1. 149 3. 83208 1. 193 Table 1 Wet Season Results. Evaporation in mm/day Windspeed m/s Water StabilityZ/L Water + Ratio of Vortex Ratio of only Fences Water + Generators Vortex Fences to + Generators + water only water water to water only 1. 010% 3.04811 10 3. 54421 1. 162 3. 6434 1. 194 1. 7510% 4.0396 10 4. 53122 1. 121 4. 62824 1. 146 2. 510% 4.66321 10 5. 15031 1. 104 5. 24798 1. 125 5. 010% 5.90161 10 6. 48182 1. 098 6. 60749 1. 120 7. 010% 6.6074 10 7.31413 1.095 7.47157 1. 119 StabilityZ/L Water Wind speed Water + Ratio of Vortex Ratio of only m/s Fences Water + Generators Vortex Fences to + Generators + water only water water to water only -1 5. 34291 2. 5 5. 89104 1. 102 5. 96699 1. 117 0. 1 4. 66321 2. 5 5. 15031 1. 133 5. 24798 1. 171 1 3. 48392 2. 5 3. 9525 1. 104 4. 0816 1. 125 Table 1

In Table 1, the evaporation enhancing structures 1 which include vortex generators are referred to as vortex generators, whereas the evaporation enhancing structures which include a wettable first portion are referred to as fences. It will be seen that in both cases the fences and the vortex generators provide improved evaporation compared to water alone, with the vortex generators providing the greater improvement in evaporation.

It will be apparent to persons skilled in the art that various modifications may be made to the invention without departing from the scope and spirit of the invention. For example, where the evaporation enhancing structure is a wettable portion (ie. , a floating fence), the wettable portion 25 need not be rectangular, as shown in Figure 9 which utilises a triangular wettable portion 25. Again, the frame 26 of the structure can be formed by tubes of plastic piping and a fin 27 can be provided in order to allow the structure to align with wind direction 4.

In Figure 9, Figure 9a is a plan view, Figure 9b is a side view, and Figure 9c is a front view, with Figure 9d being a perspective view.

These and other modifications will be apparent to persons skilled in the art.