BACKGROUND TO THE INVENTION

THIS invention relates to a method of drying particulate material, and more particularly but not exclusively, to a method of drying a fine material using a belt filter.

Fine material required for drying can originate from various processes of mineral beneficiation and/or mineral extraction. Due to the nature of the fine material, water retention in these materials result in a number of undesirable issues. Of these, the loss of water into the surrounding environment is one of the major issues. Secondly the retention of water in coal ultra fines and fines results in a decrease in energy efficiency, and as such renders these fines undesirable. In order for the fine fractions to be saleable the moisture needs to be removed from the fine fractions to acceptable levels.

The mining industry, for one, has developed various methods for drying fine and ultra fine materials. These methods typically include, inter alia, drying beds, belt filters, belt filter presses and hydraulic filter presses. A brief description of the mentioned methods is provided below together with advantages and drawbacks associated with such methods.

Drying beds are typically provided by constructing lined, large surface area dams into which fines can be placed. The water can thereafter be removed from the drying beds with the use of, for example, penstock systems. Water is decanted from the surface of the fines, the so-called supernatant water, and once the drying beds are dry the fine material is mechanically excavated and sold in the case of coal fines. Making use of drying beds has the advantage that once the drying beds are in place, drying costs are minimal while the drying beds can handle large volumes of slurry. The drawbacks of making use of drying beds include a substantial capital outlay for constructing such drying beds, loss of water due to evaporation, loss of income due to delays during the period of drying, a potential for environmental spills due to over-topping and chemical breakdown of coal due to prolonged exposure to oxidation and various other complications.

A belt filter press typically comprises two belts between which slurry can be placed and thereafter pressed. A belt filter also makes use of a vacuum for sucking water from the filter. The advantages of using a belt filter press includes that continued drying can take place, a relative small operating area is required and water that is removed from slurry can be recovered immediately. Further advantages include that the dried product can be added to the product line and a relatively quick turnaround of revenue is possible. The drawbacks of using belt filter presses include high installation costs, high material and maintenance costs and consequential high running costs.

A further, and very significant, drawback of making use of a belt filter press is that by compressing hydraulically placed material, the permeability of such material is reduced with a consequential reduction in the ability of extracting all free water. Compressing the water containing material only serves to increase the pore pressure of the material with the consequential result that the largest volume of water is extracted prior to compaction. Energy which is therefore used with a view of removing free water is in fact wasted.

A hydraulic filter press comprises a bag in which fine material can be placed whereafter the material in the bag can be compressed hydraulically. The hydraulic filter press also makes use of a vacuum for sucking water from the bag. Upon completion of a drying cycle, batch material is blown from the bag into a bin and the cycle repeated. Advantages of making use of a hydraulic filter press include fast recovery of water, a minimal potential for detrimental environmental impact and fast realization of a saleable product. The drawbacks of hydraulic filter presses, however, include high installation costs, high running and maintenance costs as well as batch processing, i.e. there is no continuous flow of material and consequently large volumes of liquid should be stored. Making use of hydraulic filter presses also results in compressed materials with increased pore pressure and reduced permeabilities similar to the situation with belt filter presses.

Belt filters, comprising a permeable belt that rotates continuously and on which a layer of material to be filtered is placed, is well known in industry. The belt filter is generally exposed to a vacuum source to assist with the removal of the water from the particulate material. It has been proposed in the past to alternate the permeability of a particulate material by mechanical re-aligning the material, for example using cams, or by displacing the material from an elevated belt filter section to a lower belt filter section. This mechanical placement increases the permeability of the moist or partially wet material, but by at most one order of magnitude. The inventor believes that another methodology can be applied which will result in increased permeability. It is accordingly an object of the present invention to provide an alternative method for use in drying coal or other slurries which addresses the drawbacks of existing technology.

It is also an object of the invention to provide a continuous method of drying slurries which will be a useful alternative to existing methods.

SUMMARY OF THE INVENTION According to the invention there is provided a method of drying particulate material, the method including the steps of:

providing a body of substantially heterogeneously sized particulate material, the body having an operatively upper zone and an operatively lower zone;

continuously exposing the body from below the operatively lower zone of the body to a repetitive sequence of pressures above and below atmospheric pressure, in order for the pressure above atmospheric pressure to aid in the vertical alignment of particles of different sizes, and for the pressure below atmospheric pressure to aid in the removal of moisture from the material.

Preferably, the body is in the form of a layer of particulate material to be filtered or segregated, and which is located on a moving belt which is at least partially permeable, substantially an entire surface of which is in constant flow communication with a plurality of alternating blower and vacuum source sequences, in order the material transported by the belt to be continuously exposed to the blower and the vacuum source sequences.

There is provided for the body of material to be exposed to a plurality of alternating pressure sequences which act continuously on the body in order for the body to become at least partially segregated, , with an operatively upward fining particle size distribution.

The body may be a layer of material to be filtered or segregated in order to improve the qualities of that body.

The layer of material may be located on a moving belt which is at least partially permeable, and which is in constant flow communication with a plurality of sequential and alternating blowers and vacuum sources in order the material transported on the belt to be sequentially exposed to the blowers and the vacuum sources.

The blowers and vacuum sources are arranged immediately adjacent one another in order to ensure that a particular zone of the moving layer is at all times exposed to either a blower or a vacuum source.

Substantially the entire surface of the moving belt is arranged to be in direct flow communication with the blowers and vacuum sources so as to prevent the formation of dead zones.

There is provided for the constant pressure above atmospheric pressure to be less than 20 kPa (gauge), more preferably less than 15 kPa (gauge), and most preferably less than 10 kPa (gauge) at any of the blowing ports to which the belt passing over is exposed. There is provided for the constant pressure below atmospheric pressure to be a vacuum of less than 20 kPa (absolute), more preferably less than 15 kPa (absolute) and most preferably less than 10 kPa (absolute)within the chamber to which the belt passing over is exposed.

DETAIL DESCRIPTION OF THE INVENTION

The methodology of the invention is described with reference to Figure 1 , which is a schematic representation of the particle distribution along a layer of material being dried using the method as set out above. The methodology utilized in this method of drying is based on a uniquely different principal to conventional horizontal belt filters in that it is designed to ensure the pathways between the solids particles are maintained through pneumatic agitation. The method is applied to separate the solids and liquor contained in fines, and the gist of the method comprises the material to be dried 20 being continuously and repetitively exposed to an alternating cycle of downward vacuum and upward blowing.

When applying a continuous low upward air pressure through a moving layer or cake (particles) (indicated in the Figure as "Blowing"), the particles tend to align vertically as opposed to horizontal alignment which is experienced during normal mechanical or hydrological placement.

In addition to the vertical alignment, it has been found that the particles tend to segregate to give a medium or cake which is upward fining. This means that larger particles are located towards the bottom of the layer, and smaller particles towards the top of the layer. This happens due to the smaller particles being displaced further than the larger particles, and a natural redistribution of particles ensues. The blowing must be sufficient to partially fluidize the bed, but low enough to prevent the particles from being displaced from the belt. In addition, the layer has a homogeneous horizontal particle size distribution (PSD), as can be seen in the Figure where the initial PSD is heterogeneous, and where segregation increases the further the particles are displaced along the belt from the starting point. It has been found that, when applying a low vacuum to the material being deposited, the carrying medium or liquor 33's capillary force can be easily overcome and the liquor separated from the solids (30, 31 and 32).

In use, the particulate material is located on an endless belt 10 which is in the form of a filter cloth having a predetermined permeability. The filtrate (liquor) 33 drains through the filter cloth 10 and is re-introduced into a plant's water circuit. The product, which is the solids (30, 31 and 32) component of the feed (for example a slurry), discharges as a dried unconsolidated cake ready for further processing. The constant pneumatic agitation is facilitated by providing blowers having a delivery pressure of about 10kPa (g). The low pressure is in turn facilitated by a vacuum arrangement having a suction pressure of about 10kPa (a). The speed of the belt filter, as well as the feed rate of material, can be adjusted in accordance with the actual operational conditions in a given environment.

The new method enables the liquor to be removed more efficiently and with much lower input power requirement. If thickener underflow densities are low, de-watering cyclones may be installed prior to the inlet feed box of a belt filter that incorporates the new methodology, in order to increase the relative density of the slurry to between 1.25 - 1.35. Cyclone underflow is then placed, via the feed box, onto the filter belt and cyclone overflow is returned to the thickeners for further settling. Additional process requirements such as cake washing, heat drying etc, can easily be incorporated as part of the method. Cake thickness is controlled by adjusting the speed of the belt and cake discharge, is on most applications, spontaneous. When handling very thin or sticky cakes a polymer scraper may be used in combination with the new drying method to assist the removal of product.

A variety of filter cloths 10 can be used with the new method, ranging from monofilament cloths to dimensionally stable needle felts. The filter cloth is continuously cleaned which results in maintained throughput as cloth blinding is reduced.

Significantly, the pneumatic agitation caused by the blowers and vacuum enables the courser particles to settle first forming a pre-coat with finer particles on top, thereby improving filtration efficiencies. More particularly, as the slurry is continuously exposed to alternating sequences of positive pressure and vacuum, an upward fining filter cake is obtained, in which larger particles 30 settle on top of the belt filter, whereas smaller particles 32 moves to the top of the filter cake, and with a gradual decrease in particle size being present there between. This is an important advantage of the new method, in that the very particle distribution that is created (upward fining) aids the filtering process, with larger particles towards the bottom acting as a porous filter rather than a densely compacted retaining layer of low permeability, which is typically formed in belt filter presses.

In one example suitable operating parameters were found to be as follows, although it would be appreciated that most of the parameters are variable, provided that the basic alternating vacuum and pneumatic agitation operating principle is applied:

Maximum volume of slurry 80m ³ per hour

Required feed SG: 1.1 - 1.3

Filtrate SG: 1.01 - 1.02

Particle size distribution: <1500μηη

Vacuum: <10 kPa (a)

Blower pressure: <10 kPa (g)

The above operating parameters have provided very good results, and resulted in a higher degree of water removal at lower energy input levels when compared to prior art techniques. The selection of the positive blower pressure and the negative vacuum pressure has been found to be very important. In particular, a sufficient amount of blowing is required to assist in the rearrangement of particles, but without completely displacing particles from the belt.

In order to achieve the continuous exposure to vacuum and pressure, the belt filter used in this method does not include a rotating carrier mat, and is in direct flow communication with the pressure and vacuum sources. This ensures that no dead zones are formed in the layer of material on the belt, as the vacuum / pressure are not exerted through discreet channels formed in the carrier mat. It has been found that the carrier mat is no longer required due to the upward force exerted by the blowers onto the belt, thus providing sufficient lift of the belt to allow smooth operation. This requires the blowers to be relatively close to one another, and for the frequency of the alternating pressure / vacuum sources to be relatively high. Preferably, the distance between adjacent blowers is less than three times the height of the layer of particulate material, more preferably less than twice the height. Also, the width of a blowing zone is similar to the width of a vacuum zone, thus resulting in an optimally balanced configuration. Preferably these widths are equal. It would be appreciated that the provision of blowers and vacuum sources at short intervals are not merely a simple design choice, but that the optimal frequency is important to achieve sufficient support of the belt filter (so as to do away with the carrier mat) whilst also providing optimal fluid interaction (blowing and sucking) with the particulate material. If pressure and vacuum are applied at intervals that are too long, there would be a negative impact on the structural support of the belt filter, as well as on the hydrodynamic interaction with the material on top of the belt filter. From a support perspective, the pressure will not be able to overcome the load carried by an extended length of belt under vacuum, in addition to the downward force exerted on the belt due to the application of the vacuum, which will result in high friction in the vacuum areas, and potentially the need for a separate carrier mat. The proposed remedy for this is the introduction of blowers at shorter intervals, which will then exert a counteracting upward force on the belt. In addition, from a hydrodynamic perspective the optimal water removal would have occurred at an early part of a long interval vacuum cycle, and the removal of water during a latter part of such long interval vacuum cycle would not be efficient prior to the bed having been exposed to the blowers again - which would again be resolved by using shorter vacuum / pressure intervals.

It will be appreciated that the above is only one embodiment of the invention and that there may be many variations without departing from the spirit and/or the scope of the invention.