Mineral tank floating cover and method of vapour recovery

Field of the Invention

The present invention relates to a cover for a mineral processing tank and relates particularly, though not exclusively, to a cover for a tank containing a gold ore and sodium cyanide slurry.

Background to the Invention

Most gold is currently separated from mined ore using the so-called "carbon in pulp" (CIP) process. The ore is crushed to a fine silt and mixed with water and sodium cyanide to form a slurry. Chemical reactions occur so that the cyanide attaches to minute particles of gold to form water- soluble gold-cyanide compounds from which gold can be recovered. The slurry is filtered to remove non-soluble compounds and then mixed with activated carbon pellets to which the dissolved gold-cyanide compounds attach. The carbon pellets are then removed and the gold is stripped from the pellets using a hot cyanide solution. The gold is then recovered from solution using electrolysis. Leftover cyanide is stored in tailing dams and may be re-used indefinitely.

Environmentalists are critical of the continued use by the mining industry of cyanide in mineral extraction because of its high toxicity. However the mining industry argues that the dilute cyanide concentrations employed and the rapid ultraviolet decomposition of cyanide into relatively non-toxic compounds make cyanide extraction safe. Nevertheless, plant workers in CIP gold processing facilities must follow strict health guidelines to prevent overexposure to noxious fumes from the cyanide solution tanks. The mineral processing tanks in which the cyanide and gold ore slurry are mixed are typically kept open to atmosphere. Atmospheric monitoring of cyanide fume concentration in the vicinity of the tanks is constantly maintained to ensure workers are not exposed to more than 10ppm of

cyanide in the air. Evaporation and ultraviolet decomposition of cyanide due to sunlight represent a substantial loss of cyanide during processing.

The present invention was developed with a view to providing a tank cover for reducing these losses to atmosphere of cyanide fumes. However it will be apparent that the cover may be used for other types of mineral processing tanks such as, for example, the caustic digestion tanks employed in the Bayer alumina process.

References to prior art in this specification are provided for illustrative purposes only and are not to be taken as an admission that such prior art is part of the common general knowledge in Australia or elsewhere.

Summary of the Invention

According to one aspect of the present invention there is provided a cover for a mineral processing tank, the cover comprising:

a plurality of segments made of lightweight material connected together in a generally circular configuration so as to be able to float on, and cover substantially the whole of, the surface in the tank.

Advantageously each of said segments is designed so that an upper surface of the cover slopes generally downwards in a radially inwards direction from a maximum height near an outer periphery to a minimum height near the centre.

Preferably said cover is inflatable. Preferably each of said segments is separately inflatable.

Preferably each of said segments is detachably connected to its adjacent segments wherein any one segment can be easily replaced should it be damaged. Preferably each of said segments is connected to an adjacent segment by means of a radially extending edge strip that connects to the radially extending edge strip of an adjacent segment. Typically said radially extending edge strip has a plurality of spaced apart eyelets that align with

the eyelets of an adjacent edge strip so that they can be fastened together with a suitable fastening means.

Alternatively, the segments may be connected in other ways. In a further form of the invention, each of said segments comprises attachment means on an upper wall of the segment to allow connection to an adjacent segment. The attachment means is preferably in the form of a series of loops extending from the upper wall of each segment and positioned radially at spaced apart intervals along an outer edge of the segment. Preferably the attachment means comprises two sets of loops, each set being positioned radially at spaced apart intervals along the outer edges of opposing sides of each segment. Typically the loops align with corresponding loops on an adjacent segment so that the segments may be fastened together with a suitable fastening means such as rope or cable ties.

Preferably each of said segments is substantially triangular in shape so that the plurality of connected segments forming the cover forms a generally circular configuration. Preferably the cover comprises six to sixteen segments. More preferably the cover comprises twelve segments. Preferably the segments are all of substantially similar dimension.

Each segment may further comprise a first tubular pocket extending radially from an edge of the segment. Tubular piping may be inserted into the first tubular pocket to assist in keeping the segment rigid. The tubular piping is typically in the form of polyvinylchloride (PVC) piping. The segment may further comprise a second tubular pocket extending from an opposing edge of the segment to that of the first tubular pocket. Likewise tubular piping may be inserted into the second tubular pocket.

The tank cover may further comprise an array of fins positioned between the upper wall and a lower wall of each of the segments. The fins are preferably in the form of baffles arranged radially in a substantially parallel orientation.

Preferably said cover is made from a flexible plastics material, for example, PVC. Preferably each segment has a handle fixed to an upper surface of the segment to facilitate easy handing of individual segments.

According to another aspect of the present invention there is provided a method of recovering vapours from a solution or slurry in a mineral processing tank, the method comprising the steps of:

covering the tank with a lightweight, flexible cover that floats on the surface of the slurry or solution;

drawing any vapours that become trapped under the cover to a treatment means; and,

returning the treated vapours to the tank in liquid form.

Typically said treatment means comprises a gas scrubber.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Likewise the word "preferably" or variations such as "preferred", will be understood to imply that a stated integer or group of integers is desirable but not essential to the working of the invention.

Brief Description of the Drawings

The nature of the invention will be better understood from the following detailed description of several specific embodiments of the tank cover, given by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a plan view of a first embodiment of a tank cover in accordance with the present invention;

Figure 2 is a perspective view of a segment of the tank cover of Figure 1 ;

Figure 3 illustrates the manner in which the segments of the tank cover of Figure 1 are connected together to form the cover;

Figure 4 is a side view of the cover of Figure 1 floating on the slurry in a mineral processing tank;

Figure 5 is a side elevation of the segment of Figure 2;

Figure 6 is a perspective view of a second embodiment of a segment for a tank cover in accordance with the present invention; and,

Figure 7 is a perspective view of the segment of Figure 6 with the upper wall removed;

Figure 8 illustrates the manner in which the segments of the tank cover of Figure 6 are connected together to form the cover; and

Figure 9 illustrates a preferred method of recovering vapours from the mineral slurry tank in accordance with the present invention.

Detailed Description of Preferred Embodiments

A preferred embodiment of tank cover 10 in accordance with the invention, as illustrated in Figure 1, is used for covering a slurry or solution in a mineral processing tank 12. The tank cover 10 comprises a plurality of segments 14 made of lightweight, flexible material connected together in a generally circular configuration so as to be able to float on, and cover substantially the whole of, the surface in the tank 12. In the illustrated embodiment the tank cover 10 is placed over the surface of a gold and cyanide solution and comprises eight wedge shaped segments, with a hole 16 left at the centre to accommodate an agitator 18 located in the middle of the tank 12. If desired the central hole 16 can be closed with a flat disc of the same material.

In the preferred embodiment, each of the segments 14 is detachably connected to its adjacent segments wherein any one segment can be easily replaced should it become damaged. Figure 2 illustrates a single

segment 14 of the tank cover 10 in top perspective view. Each segment 14 is typically made by welding or sewing radial strips of the flexible material together to form an upper wall 20 and a lower wall 22 of the segment, which walls are sealed along their edges to form an airtight enclosure. The segment 14 is then preferably inflated with air, and for this purpose one or more non-return valves 24 are provided on the outer periphery as shown in Figure 2. Preferably each segment is also provided with a drain valve 26 for draining the enclosure of any fluid that may be contained therein. If desired the segments may also be partially filled with water or other liquid to weigh them down so that they do not blow off the top of the surface of the slurry.

Preferably each of the segments 14 is connected to an adjacent segment 14 by means of a radially extending edge strip 30 that connects to the radially extending edge strip 30 of an adjacent segment. Each radially extending edge strip 30 has a plurality of spaced apart eyelets 32 that align with the eyelets 32 of an adjacent edge strip 30 so that they can be fastened together with a suitable fastening means. Suitable fastening means include cable ties or a cable that is threaded through the eyelets like a shoe lace. Figure 3 illustrates the manner in which two adjacent segments 14a and 14b are connected together with cable ties 34 through the aligned eyelets 32 on their respective edge strips 30.

Advantageously each of the segments 14 is designed so that the upper wall 20 of the segment slopes generally downwards in a radially inwards direction from a maximum height near an outer periphery 36 of the cover 10 to a minimum height near the centre hole 16. Figure 5 is a side elevation of the segment 14 of Figure 2 illustrating the slope of the upper wall 20. Hence the upper surface of the cover 10 as a whole tends to direct any rainwater that may accumulate on the cover towards the centre hole 16. This inhibits excessive build-up of water on the upper surface of the cover which may cause it to sink.

Figure 4 illustrates a mineral processing tank 12 containing a slurry 40 with a tank cover 10 floating on the surface of the slurry 40. If desired one of more tethers 42 may be provided to secure the cover 10 to the tank. Because the cover 10 floats on the surface of the slurry it is self-supporting and requires no additional support structure to be built on or in the tank.

Preferably the tank cover 10 is made from a suitable plastics material, for example, PVC which has been treated to make it UV-resistant. It will be appreciated that any other suitable flexible, lightweight material may be used to make the segments 14, provided that it has the requisite chemical resistance to the compounds employed in the mineral processing tank. Preferably each segment 14 has a handle (not illustrated) fixed to the upper wall 20 of the segment to facilitate easy handing of individual segments.

As shown in the first embodiment and as illustrated in Figure 1, the segments 14 are all of substantially similar size. Having a "standard" size means that manufacturing costs are reduced, and that segments can be readily replaced, or interchanged as required. The shape of the segments being substantially triangular means that the segments may be readily joined together to form the overall circular shape of the tank cover.

Figure 6 illustrates a segment 44 of a second embodiment of the tank cover 50 according to the present invention. In this second embodiment the segment 44 comprises an upper wall 46 and a lower wall 66, and resembles in many respects the segment 14 of the tank cover 10 of the first embodiment. The segment 44 also includes a non-return valve 62 positioned on the upper wall 46 for inflating the segment 44, as well as a drain valve 64 for draining purposes.

The segment 44 is provided with two sets of spaced apart loops 52 extending radially from the upper wall 46 as shown in Figure 6. The loops 52 of one segment align with corresponding loops on an adjacent segment

so that the segments may be readily fastened together. Using cable ties 60, adjacent segments 44 are connected as shown in Figure 8.

Each segment 44 further includes two tubular pockets, a first tubular pocket 54a and a second tubular pocket 54b, extending radially from opposing edges of the segment 44 as can be seen in Figure 6. Tubular piping 56 is inserted into the two tubular pockets 54a and 54b to assist in keeping the segment rigid. The tubular piping 56 is typically in the form of polyvinylchloride (PVC) piping.

Figure 7 shows the inside of the segment 44 with the upper wall 46 having been removed. The inside of the segment 44 comprises an array of fins in the form of baffles 58 arranged radially in a substantially parallel orientation. The baffles 58 assist in keeping the upper wall 46 in a spaced apart relationship from the lower wall 66, and assist in maintaining the overall shape of the segment 44.

It will be appreciated that the tank cover 10 (or 50) provides an effective means of partially or wholly sealing the contents of a slurry in a mineral processing tank. In the case of the CIP gold recovery process, it can therefore help to reduce the incidence of noxious cyanide fumes and vapours in the vicinity of the tank, as well as reducing the loss of cyanide due to evaporation and ultraviolet degradation. The reduction in the noxious cyanide fumes is important for the health and safety of persons working near the tank. There is often legislation in force which requires certain safety standards regarding the allowable concentration of noxious fumes and the tank cover can assist in meeting these requirements.

A preferred method of recovering vapours from slurry in a mineral processing tank according to the present invention will now be described with reference to Figure 9. The method preferably comprises a first step of covering the tank with the lightweight, flexible cover 50 that floats on the surface of the slurry 40. Any vapours that may become trapped under the cover 50 are drawn off to a treatment means 54. In the illustrated

embodiment the treatment means is a gas scrubber 54, which has a fan 56 that sucks any vapours or fumes trapped below the lower wall 66 of the cover 50 into the scrubber 54 for treatment. Cyanide fumes or vapours agglomerate in the scrubber 54 and are returned to the slurry 40 in the tank in liquid form via a return pipe 58. Clearly other forms of treatment may be employed to recover chemical reagents or to purify gases or vapours prior to release to the atmosphere.

Now that preferred embodiments of the tank cover and the method of recovering vapours have been described in detail, it will be apparent that it provides a number of advantages over the prior art, including the following:

(i) The tank cover provides a cost-effective means of partially or wholly sealing off the tank to minimise losses due to evaporation or UV degradation.

(ii) A principal advantage of the tank cover is that it is self-supporting and requires no additional support structure to be built on the tank.

(iii) As the cover is made in segments it is easily deployed on, or removed from, the surface of the slurry, or individual segments can be easily removed for repair or replacement.

(iv) It can be manufactured from inexpensive materials and therefore does not involve significant additional expenditure on plant.

(v) It can be used to recover fumes or vapours from the slurry to further reduce losses of valuable reagents.

(vi) The tank cover assists in minimising evaporation and losses of toxic substances such as cyanide, thereby providing a safe and healthier environment for persons working near the tank.

It will be readily apparent to persons skilled in the relevant arts that various modifications and improvements may be made to the foregoing embodiments, in addition to those already described, without departing from the basic inventive concepts of the present invention. For example,

whilst the segments of both the described embodiments are inflatable, it is possible to make the segments of any other suitable lightweight material such as, for example, a blown plastics material. Therefore, it will be appreciated that the scope of the invention is not limited to the specific embodiments described.