A method and device for dewatering and drying a wet mineral concentrate

The invention relates to a method and device for the dewatering and drying of fine grained valuable materials

(concentrates or slurries) produced in mineral processing plants up to the inherent moisture by the use of geotextile structures.

There are a number of different ways to dewater and dry a valuable concentrate from a processing plant before it can be sold or further processed. The processes of dewatering and drying are often separated from each other with each process requiring individual pieces of equipment or processes. The most commonly used method of dewatering is to add chemicals (such as flocculants) to thicken the pulp in large thickeners and then filter the thickened pulp using a variety of different mechanical dewatering devices. Some of these devices include mechanical vacuum filter belts, pressure filter belts and vacuum filter drums . Another method of dewatering the concentrates instead of thickeners is the use of hydrocyclones. Hydrocyclones are classifying devices that use centrifugal force to accelerate the settling rate of particles. Under the correct operating parameters, hydrocyclones can be used as pre-dewatering devices rather than classifying devices. Centrifuges are also found in the industry in dewatering applications. Most commonly the solid bowl screen centrifuge (decanter) having the widest usage due to its ability to discharge solid products continuously.

Once a slurry has been dewatered, it is normally required to be dried. Some of the devices and processes used for

drying are, but not limited to, rotary dryers, fluidised bed dryers, flash drying, spray drying and steam injection systems. Rotary dryers are all based on the concept of a drum or vessel rotating on a set of riders or trunnions. The drying medium (generally hot air) is fed from one end and the exhaust gases, together with the vapour, are discharged from the opposite end. The product may be fed from either end. If the flow of product and hot gas is the same, it is termed co-current. If the flow of product opposes the gas flow, then it is counter- current. Flash drying systems comprise of a feed arrangement which may include a back mixing recycle system (to counter stickiness), a hot gas generator, drying duct, and primary and secondary air-dust separation systems, from which the product is discharged. Fluid beds operate under the principle of passing air through a product bed. The product is then fluidised and simulates "boiling". The bed is expanded and the heat carrier passing through the product creates intimate contact with the expanded and fluidised particles, promoting drying. A spray dryer consists of a vessel through which the heat carrier, normally air, passes either in a counter, co-current or fountain mode. Spray drying is a technique that uses the method of converting a liquid into a free-flowing powder. The product, which must be in a pumpable slurry form, is sprayed or atomised into a fine mist, in either counter, co- or fountain mode. The fine particles make intimate contact with the hot gas stream resulting in a dry powder. The product is then discharged from the base of the vessel. The moist gases are exhausted from the top or cone of the vessel where they pass through a dust-air separation system.

Another form of dewatering and drying a mineral slurry concentrate is to use large open air ponds where the mineral slurry is open to the elements and water is naturally evaporated and surface water decanted and returned to the processing plant.

One of the disadvantages of using the thickening and filtration method is the cost associated with the chemicals required for flocculation and for filtration equipment capital and operating costs. Another disadvantage is that the filtering equipment available can only produce dewatered concentrates at a moisture level which is still well above that of the naturally occurring surface moisture of the minerals let alone the natural internal moisture. The use of hydrocyclones are attractive because they are cheap and simple however they are only able to produce a thickened slurry that would still require some form of filtering or drying. Centrifuges, by comparison are complex and more expensive, but have more flexibility than hydrocyclones and can produce greater solids concentrations than that achieved with cyclones however they are known to lose ultrafine material .

One of the disadvantages of rotary dryers is that they are very large pieces of equipment and are capital intensive. The energy requirements for drying the minerals are also very high resulting in expensive operating costs. A disadvantage of flash drying systems is that the feed to the drier must be strictly controlled because the feed material must be friable and non-sticky. Because of the high velocity that particles are passed through the system, the wear rate of critical components has to be very carefully monitored, and if incorrect

materials are used, this results in high maintenance costs and extended downtime for repairs. Fluidised bed dryers need to be individually designed for each application resulting in high capital and design costs. Spray dryers, if not operated correctly can be dangerous when working with combustible particles resulting in explosions. All of the mentioned processes and pieces of equipment require that additional hot air is produced resulting in high operating costs.

The use of open air drying ponds is disadvantageous as they are exposed to elements such as rain and wind. Fine valuable minerals can be blown away and lost when the surface starts to dry and rain results in the mineral slurry being made wet again resulting in the pond flooding and longer periods of drying time being required. The water recovery from open air ponds is also lower than average due to the evaporation and is a labour intensive operation.

Accordingly the present invention is directed to a method and device for dewatering valuable concentrates using geotextile structures and an appropriate product recovery process without the use of pre-dewatering thickening.

The geotextile structures have a cylindrical shape and have a predetermined number of filling ports for adding concentrate slurry. The structures are manufactured from geotextile fabrics such as, but not limited to, polyester or polypropylene and different materials can be used to match the desired price and concentrate that is to be dewatered. The structures vary in size from as small as 10m ³ to as large as, but not limited to, 10,000m ³ .

A large ground area would be prepared with drainage for the water that is removed from the concentrate to be pumped back into the processing plants re-circulating water circuit. A feeding system would be setup so that the feed can be distributed to different geotextile structure bags as the initial bags fill up. Starting at one end of the structure farm, the concentrate will be pumped into the first structure. Once the structure is filled, the feed will be sent to the next structure in the farm. This process will continue for a predetermined time period at which the first structure that was filled will have reached the desired moisture content. This bag will then be cut open and the concentrate removed via front end loaders or other appropriate means. Once the structure has been emptied, a new structure will be installed to replace it and the remains of the first structure appropriately disposed of or recycled. The second structure in the farm will then be cut open whilst other structures are filled. This process of filling, cutting, removing and replacing will continue as a continuous operation after the initial predetermined time period.

This provides the advantage that the use of this method will results in cheaper dewatering costs by removing the need for expensive capital cost for mechanical dewatering and drying systems. Furthermore the reduction in flocculation reagent cost will result in lower operating costs .

Advantageously the geotextile structures only allow water to flow from the inside to the outside hence no roof or covering will be reguired in outdoor applications.

Advantageously the water recovered can be returned to the processing plant without further need for clarifying.

Examples of devices made in accordance with the present invention will now be described in relation the accompanying drawings, in which:

Figure 1 shows a side view of a geotextile structure filled with mineral slurry; Figure 2 shows a cross-section of the geotextile structure shown in Figure 1 taken along the line I-I shown therein;

Figure 3 shows a preferred embodiment of a geotextile structure farm for producing dewatered and dried mineral concentrate.

Figure 1 shows a geotextile structure 12 attached to a feed pipe 10 by a number of feed ports 11 on the structure for the dewatering and drying of a mineral concentrate. The feed pipe 10 can be connected to the feed ports 11 in a variety of ways but a standard flange and hold configuration is shown.

Figure 2 shows a cross-section of the geotextile structure 12 along the line I-I. Under the effects of gravity the water in the concentrate drains out of the structure 12 through the fabric of which it is- composed.

Drainage channels 13 are provided in the ground which are covered with grates 14 and are present on either side of the geotextile structure 12 for the efficient recovery of water.

Figure 3 shows an embodiment of the method with a structure farm consisting of in this case five geotextile

structures 12 on a prepared area of ground with drainage channels 13 covered by grates 14. A feed distribution system 15 is shown that can be moved from one geotextile structure 12 to another. A water return pump 16 is positioned to collect the water from that drainage channels 13 and return it to the mineral processing plants water system.

Numerous variations and modifications to the present process may occur to the reader familiar with the mineral processing field without departing from the scope of the present invention. For example the size of the geotextile structures may vary as well as the number of structures used in each structure farm. Also the method of removing and recovering the dewatered and dried material can vary.