WO2004035172A1

WO2004035172A1 - Method and apparatus for extracting liquid from a bed of granular solids

Info

Publication number: WO2004035172A1
Application number: PCT/ZA2003/000148
Authority: WO
Inventors: Marco Le Roux; Quentin Peter Campbell
Original assignee: The North West University
Priority date: 2002-10-14
Filing date: 2003-10-14
Publication date: 2004-04-29
Also published as: AU2003279297A1

Abstract

This invention relates to a method and apparatus for lowering the irreducible saturation point of a liquid - containing particulate bed. The apparatus includes pressure differential means for applying a pressure differential over the bed sufficiently to overcome the breakthrough pressure of the bed; and disturbing means for disturbing the structural coherency of at least part of the bed. The applicant found that the irreducible saturation point is lowered from a first level to a second level, wherein the first level is the baseline irreducible saturation point of the bed. The breakthrough pressure and lowered irreducible saturation point is also achieved in a relatively shorter time, obtaining a bed with a relatively lower moisture content than in the case where the structural coherency of the bed is not disturbed.

Description

METHOD AND APPARATUS FOR EXTRACTING LIQUID FROM A BED OF GRANULAR SOLIDS

INTRODUCTION This invention relates to a method and apparatus for lowering the irreducible saturation point of a liquid - containing particulate bed.

In this specification, the term "baseline irreducible saturation point" means the point where no more liquid is extracted from a differential liquid- containing particulate bed, when an uninterrupted pressure drop is applied over the bed.

Further in this specification, the term "breakthrough pressure" means the pressure drop over a differential liquid - containing particulate bed, required to initiate liquid extraction from the bed.

BACKGROUND TO THE INVENTION

The following types of water is found in particulate coal:

Surface or free moisture is the water on the surface of a coal particle. Hence, when a filter cake is formed, the free moisture will accumulate in the pores of the filter cake. Much of this moisture can be removed from the filter cake by mechanical means like filtration and centrifugation. Inherent or capillary moisture is the moisture allocated within the pores of each individual particle. It is also commonly known as intra-particle moisture, and can only be removed by using thermal methods.

Chemically bound moisture is found in the chemical structure of the ash fraction of the coal, as crystal water for example. This moisture cannot be removed except by pyrolysis, and was found to be in the order of 3-8 per cent for bituminous coals.

Studies done on fine coal beneficiation plants showed that a problem of conventional vacuum filtration methods is that it is not uncommon for the coal to have a final moisture percentage between 20-30 per cent after the baseline irreducible saturation point has been reached. This problem deserves attention for various reasons. It was previously shown that even a 1 per cent reduction in the moisture level of the three million tons coal produced annually in the USA, can lead to a saving of US$ 300,000. It was also estimated that the calorific value of the fuel during combustion will increase by 1.4 per cent per percent reduction in moisture.

Capillary pressure- or Dewatering curves

In conventional vacuum filtration methods, the dewatering process of any filter cake is described by a two phase flow of liquid and air through the cake. Capillary pressure curves are used to quantify and describe this process. These curves are obtained by performing filter experiments at specific pressure (or vacuum) differentials until no more filtrate flows from the cake (irreducible saturation point reached). The tests are then repeated at increasing pressure differentials, the resulting curve, as illustrated in figure 5, characterises the dewatering behaviour of the system.

The point at which the saturation is at 100% is the level where all the voids in the filter cake are completely filled with liquid, without an excess amount of supernatant liquid present. The first dewatering stage is the capillary stage, where the applied pressure differential is smaller than the capillary pressure of the pores. This results in no flow of liquid from the filter cake. The second, or funicular stage, commences when the applied pressure differential is greater than the breakthrough pressure (related to the capillary pressure of the largest pores). This is when most of the dewatering takes place at a fast rate. Liquid will flow rapidly from the filter cake until the third state, the pendular state (irreducible saturation point), is reached. No further dewatering will take place here, and the remaining free moisture will stay behind as small lenses on the surfaces of the particles, and in the micro pores of each individual particle as inherent moisture.

South Africa has large coal reserves, and is one of the world's leading producers and exporters of coal. The country currently produces approximately 290 million tons of ROM coal per annum. Of this, about 14 per cent is thought to be finer than 500 μm (defined as fine coal) and 2-3 per cent finer than 100 microns (ultra-fine coal). The fines content in the ROM had been increasing in recent years with the introduction of mechanised mining methods. Coal producers have a few options as to what to do with the fines. Initially, fines and ultra-fines were dumped in discard streams, but since the value of these fines had been recognised, it was either added to the product streams as is, or alternatively, upgraded and sold.

The economic potential of coal fines has led to developments in fine coal beneficiation processes, like spirals and froth flotation. The major problem of coal fines and reason for it not being fully exploited as an additional energy source is the high moisture levels found in the fine fractions of coal.

Owing to a lack of an economical dewatering technique in the industry for lowering the moisture content of fine coal products, the utilisation thereof has been hampered causing a negative impact on the environment, and limiting the economical commercialisation thereof.

OBJECT OF THE INVENTION

It is accordingly an object of the present invention to provide a method and apparatus for lowering the irreducible saturation point of a liquid - containing particulate bed with which the aforesaid problems may be overcome or at least minimised.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a method for lowering the irreducible saturation point of a liquid - containing particulate bed including the steps of:

- applying a pressure differential over the bed sufficiently to overcome the breakthrough pressure of the bed; and

disturbing the structural coherency of at least part of the bed.

The step of at least partially disturbing the structural coherency of the bed may include the step of mechanically deforming at least part of the bed.

The step of mechanically deforming at least part of the bed may include the step of forming at least one aperture in the bed.

The step of forming at least one aperture in the bed may include the further steps of punching spaced apart apertures into the bed. Alternatively, the step of forming at least one aperture in the bed may include the step of cutting a plurality of spaced apart elongate groove formations in the bed.

Further alternatively, the step of mechanically deforming the bed may include the step of deforming at least part of the bed out of its plane.

Yet further alternatively, the step of disturbing the structural coherency of at least part of the bed may include the steps of applying the pressure differential over the bed for a first period of time; interrupting the pressure differential over the bed for a second period of time; and reapplying the pressure differential for a third period of time.

Further according to the invention the irreducible saturation point is lowered from a first level to a second level, the first level being the baseline irreducible saturation point of the bed.

According to a second aspect of the invention there is provided apparatus for lowering the irreducible saturation point of a liquid - containing particulate bed including :

pressure differential means for applying a pressure differential over the bed sufficiently to overcome the breakthrough pressure of the bed; and

disturbing means for disturbing the structural coherency of at least part of the bed.

The disturbing means may comprise a disturbing member for mechanically deforming at least part of the bed.

The disturbing member for mechanically deforming the bed may be elongate and adapted to form at least one aperture in the bed.

The elongate disturbing member may be a first member and the apparatus may include further similar members adapted for punching additional spaced apart apertures into the bed.

The bed may be elongate and may be conveyed along a conveyor and the elongate disturbing members may extend radially outwardly from an axle rotatably mounted in close proximity to the conveyor for rolling over an upper surface of the bed while the bed passes beneath the axle, the arrangement being such that the elongate members punch apertures into the bed.

Alternatively, the disturbing member may be in the form of at least one disc mounted on an axle for cutting an elongate groove formation in the bed while the bed passes beneath the axle.

Further alternatively, the disturbing member may be adapted to deform at least part of the bed out of its plane.

The disturbing member may be in the form of an undulation in the conveyor for deforming the bed out of its plane when the bed passes over the undulation.

Yet further alternatively, the disturbing means may comprise a member for interrupting the pressure differential over the bed, before reapplying the pressure differential over the bed.

The member may be in the form of a valve for releasing the pressure differential.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail without limiting the scope of the invention, inter alia at the hand of the enclosed drawings wherein:

figure 1 is a perspective view of an apparatus according to a first embodiment of the invention for lowering the irreducible saturation point of a liquid - containing particulate bed;

figure 2 is a perspective view of an apparatus according to a second embodiment of the invention for lowering the irreducible saturation point of a liquid - containing particulate bed;

figure 3 is a perspective view of an apparatus according to a third embodiment of the invention for lowering the irreducible saturation point of a liquid - containing particulate bed;

figure 4 is a perspective view of laboratory apparatus according to a fourth embodiment of the invention emulating industrial apparatus for lowering the irreducible saturation point of a liquid - containing particulate bed;

figure 5 is a graph illustrating the dewatering process of a filter cake, obtained by performing filter experiments at specific pressure (or vacuum) differentials until no more filtrate flows from the cake, and repeating the tests at increasing pressure differentials;

figure 6 is a graph illustrating the effect of the releasing of vacuum to atmospheric level during filtration before reapplying the vacuum on the breakthrough pressure and irreducible saturation point;

figure 7 is a graph illustrating the experimental results for the tests done with a vacuum break duration of 30 seconds, and initial break times of 15, 30 and 45 seconds respectively;

figure 8 is a graph illustrating the results for experiments where the initial breaking time was kept constant at 30 seconds, while the different break duration times were used;

figure 9 is a graph illustrating the final moisture level of the three different break duration times compared to the baseline irreducible saturation point; and

figure 10 is a graph illustrating the dewatering curves of a bed where the structural coherency of the bed has been deliberately disturbed.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The apparatus and method in accordance with the invention are particularly suitable for the extraction of liquid from a bed of liquid containing fine

particles (particle diameter less than 500μm), such as the dewatering of fine

particulate coal. However, the application of the method and apparatus of this invention is not limited to the dewatering of coal, but could also be utilised practically in any industry where liquid is to be extracted from a bed of particulate solids through a process of filtration. For example, it could be applied in the washing and extraction of valuable compositions and elements in solution from the bed and in a vast array of other industries such as the food and medical industries. Specific non-limiting embodiments of the apparatus and method are described below with reference to the drawings.

In the drawings, like components are allocated the same or like numerals. In this description, the terms filter cake and particulate bed are used alternately to describe the same thing namely a liquid - containing body of solid particles. The particles typically have different diameters less than 10 mm.

Referring to figure 1 , apparatus according to a first embodiment of the invention for lowering the irreducible saturation point of a liquid - containing particulate bed 12 from a baseline irreducible saturation point is generally designated by reference numeral 10. The apparatus 10 comprises pressure differential means for applying a pressure differential over the bed 12 in the form of a vacuum pump assembly 14; and disturbing means for disturbing the structural coherency of at least part of the bed in the form of a roller assembly 16.

The bed 12 is in the form of an elongate layer of liquid-containing particulate solids disposed on a porous endless conveyer filter belt 18 for transporting the bed over a suction plate 20 connected to the vacuum pump assembly 14. In use, the vacuum pump assembly 14 applies a pressure differential over the bed 12, sufficient to overcome the breakthrough pressure of the bed 12.

The roller assembly 16 includes an axle 16.1 rotatably mounted in close proximity over the bed 12 and a plurality of elongate members 16.2 extending radially outwardly from the axle 16.1. In use, the elongate members 16.2 punch a plurality of spaced apart apertures 22 (Detail A) into the bed 12 while passing beneath the roller 16, thus to disturb the structural coherency of the bed 12. It will be appreciated that the elongate members 16.2 could be replaced with discs (not shown) for cutting parallel extending elongate groove formations (also not shown) into the bed 12.

The applicant has found that by disturbing the structural coherency of the bed 12 by punching the aperture 22 or by cutting the grooves into the bed 12, the airflow through the bed is increased and a irreducible saturation point is lowered relative to a baseline irreducible saturation point where the structural coherency of the bed 12 has not been disturbed.

Referring to figure 2, apparatus according to a second embodiment of the invention for lowering the irreducible saturation point of a liquid - containing particulate bed from a baseline irreducible saturation point is generally designated by reference numeral 10A.

The apparatus 10A is similar to the apparatus 10 described above, with the exception that the disturbing member for disturbing the structural coherency of the bed 12 is in the form of an undulation 20.2 in the suction plate 20, for bending the conveyor belt 18 and thus the bed 12 when passing over the undulation 20.2. The arrangement is such that when the bed 12 is bent in this fashion, the structural coherency of the bed 12 is disturbed, causing an increased airflow through the bed 12 and thus lowering the irreducible saturation point.

Referring to figure 3, apparatus according to a third embodiment of the invention for lowering the irreducible saturation point of a liquid - containing particulate bed from a baseline irreducible saturation point is generally designated by reference numeral 10B. The apparatus 10B is similar to the apparatus 10 described above, with the exception that the disturbing member for disturbing the structural coherency of the bed 12 is in the form of a vacuum release valve 14.1 for interrupting the pressure differential over the bed 12. In use, the pressure differential is applied over the bed for a first period of time; and then interrupted by releasing the vacuum via valve 14.1 to atmosphere for a second period of time; and then reapplied by closing the valve 14.1 for a third period of time.

The applicant has found that the interruption of the applied vacuum disturbs the structural coherency of the bed 12, thus causing an increase in the airflow through the filter bed, with a concomitant lowering of the irreducible saturation point, so that a lower final moisture content of the bed 12 is achieved relative to the case where the structural coherence of the bed is not disturbed.

The applicant has further found that by compromising or disturbing the structural coherency as set out above, the breakthrough and irreducible saturation points are achieved relatively faster than in the case where the structural coherency is not disturbed.

Referring to figure 4, apparatus according to a fourth embodiment of the invention for lowering the irreducible saturation point of a liquid - containing particulate bed from a baseline irreducible saturation point is generally designated by reference numeral 10C.

The apparatus 10C emulates the operating conditions of the apparatus 10B or an industrial vacuum belt filter plant (not shown) and includes a bench scale Buchner vacuum filter assembly 11C, mounted on a glass bell jar 24 and a filter head 26 fitted onto the top of the bell jar 24. A glass beaker 28 is disposed inside the bell jar 24 for collecting the filtrate. The beaker 28 was placed on a load cell 30 to measure the filtrate mass continuously. Data logging was done by a computer (not shown).

A series of dewatering tests were done with the apparatus 10C, where the applied vacuum was interrupted at different instances in the dewatering cycle, and for different time durations, to disturb the structural coherency of the bed 12C during filtration. It yielded a product with a final moisture content that had an irreducible saturation point that was 7 per cent lower than the baseline irreducible saturation point that could be achieved using the same set-up, with an uninterrupted vacuum. There was also an increase in the dewatering rate, as well as a lower breakthrough pressure.

Further investigation showed that if the structural coherency of the filter cake/bed 12C was disturbed by intentionally damaging the filter cake 12C (causing for an increased air flow rate, and a decrease in applied vacuum), a product with lower irreducible saturation point and thus lower final moisture was also obtained. An increase in the dewatering rate was also observed in this case. This led to the conclusion that increased air flow through a filter cake 12C was more advantageous than an increase in the applied vacuum and that a relatively lower irreducible saturation point could be reached in a relatively shorter period of time by disturbing the structural coherency of the bed 12C.

While determining desaturation curves, the applicant further discovered that, instead of a continuous application of incrementally higher vacuum levels, the release of vacuum to atmospheric level (referred to as a vacuum break in the subsequent text) before the next increment yielded much better results. These results, shown in figure 6, showed a significantly lower breakthrough pressure, resulting in a lower final moisture percentage coupled with a higher rate of dewatering. It was found that the vacuum break causes a disturbance in the structural coherency of the filter cake.

EXAMPLE 1

Experimental setup

A fine coal sample was obtained. The sample was dewatered using pressure filters (not shown). It was then air dried for some time, before the final moisture was removed in an oven at 60°C. Table 1 shows the proximate

analysis of the coal, while a particle size analysis indicated the coal to be 100

per cent -600μm and 50 per cent -212μm. The coal samples were then

repulped to make up the feed to the filtration experiments.

The experiments were conducted on the apparatus 10C illustrated in figure 4. The apparatus 10C was designed to emulate commercial belt filters, taking into account the use of the same type of coal, having the same filter cake thickness and residual time on the filter, and finally using the same filter clothThe same applied vacuum level of 45 kPa was also used.

Each experiment was done under an applied vacuum of 45 kPa. The total filtration time was taken to be approximately 300 seconds after the 100 per cent saturation point was reached, while the cake thickness was kept at approximately 15 mm. This related to about 40g of dry coal per experiment. After each test the moisture level in the cake was determined using the SABS 924 standard.

The onset time and duration of the interruption in the applied vacuum was varied. Three different vacuum break durations were used, namely 15, 30 and 60 seconds. The initial break instance was chosen to start at 15 seconds after the 100 per cent saturation level, and every 15 seconds thereafter, for the subsequent tests. Some tests were done using a number of breaks during one test.

Results and Discussion

Figure 7 shows the experimental results for the tests done with a break duration of 30 seconds, and initial break times of 15, 30 and 45 seconds respectively. The fourth line indicates the results of the test where no break in applied vacuum occurred, that is, the way it is currently operated in practice, i.e. the baseline irreducible saturation point for this specific particulate coal.

There was a significant increase in the dewatering rate for the experiments where the applied vacuum was interrupted. Another aspect that shows a remarkable difference is the final moisture level of each experiment. A much lower final moisture level is shown for the experiments with interrupted vacuum application. These trends were also observed for tests with other initial breaking times.

EXAMPLE 2

Figure 8 shows the results for experiments where the initial breaking time was kept constant at 30 seconds, while the different break duration times were used. Again similar trends were visible, with the only difference being the final moisture level at each condition. These results suggested that there should be an optimum break instance in terms of both the initial breaking time and the break duration.

EXAMPLE 3

Figure 9 summarises the previous results by plotting the final moisture level of the three different break duration times compared to the final moisture of the current dewatering method. It indicates an optimum interruption time for an initial breaking time of 30 seconds and a break duration time of 30 seconds. This suggests that the optimal break occurs during the funicular stage of dewatering. A final moisture of 25,2 per cent could be achieved, as compared to the standard filtration final moisture of 32,6 per cent.

The effect of repeated breaks were also studied, and the results showed that even though better rates of dewatering could be achieved, lower final moisture levels were not obtained. This suggested that the phenomenon occurred owing to once-off structural changes in the filter cake, i.e. a once - off disturbance of the structural coherency of the filter cake.

EXAMPLE 4

The airflow through the filter cake increased owing to the formation of cracks at the bottom of the filter cake during the dewatering cycle, caused by the interruption in the applied vacuum. To test this result, it was decided to deliberately damage the filter cake after 30 seconds into the dewatering cycle, using two parallel cuts.

The result of this test is compared with that of the optimum condition of the first set of tests, as well as with the normal filtration (figure 10). A final moisture level of 25,6 per cent was obtained - a level similar to that obtained before. The cuts in the filter cake also caused a decrease in applied vacuum from 45 kPa to 29 kPa, and despite this, there was an increase in the rate of dewatering in comparison to the optimum experiment, reaching the pendular stage in less than a third of the normal time. These results show that it is more advantageous to increase the air flow through the cake rather than increasing the pressure differential. The increased air flow aids in the withdrawal of moisture that would under normal circumstances be trapped as inherent moisture.

Conclusions

A clear advantage was observed in using this technique where the air flow rate through the cake was increased at the expense of applied vacuum. It was possible to lower the irreducible saturation point from a baseline saturation point, thus to decrease the final moisture percentage in a coal filter cake from 32 per cent down to 25 percent. Also, the rate of dewatering increased drastically, giving a drier final product in a much shorter time. These results showed that the drying time could effectively be reduced by approximately 60 per cent.

It was shown that kinetics are involved, due to the time dependence of the effects. The results showed that it is more advantageous to apply the vacuum break during the funicular stage of dewatering. An optimum vacuum interruption point was determined as 30 seconds into the dewatering cycle, lasting for 30 seconds* The flow of air through a filter cake plays an integral part in the dewatering. Increasing the airflow through a filter cake by deliberate damage to the cake results in a much drier final product. This means that dewatering can take place at a low pressure differential across the filter cake.

As discussed in the background, efficient dewatering of fine coal (-600μm) is problematic, but avoiding the use of water in coal washing plants is not possible. By using conventional mechanical dewatering methods, such as vacuum filters, the final moisture content of fine coal is usually found to be in the order of 30% by weight. These high moisture percentages in fine coal create various problems in transportation costs, quality and handling. Consequently fine coal dewatering is a necessity and it is important to identify cost-effective techniques that can be conveniently retrofitted onto existing unit processes to improve dewatering and reduce fine coal product moisture. The applicant found that a deliberate break of a fine coal filter cake during dewatering thereof, resulted in an increase of the filtration rate as well as a decrease in the final moisture content of the cake.

The applicant further found, using -600μm coal, that a deliberate damage of

a fine coal filter cake during vacuum dewatering, increased the desaturation rate. This yielded a final moisture content of 4-6% lower than when using conventional vacuum filtration methods. There is an optimum time for damaging of a filter cake; this time was found to be before the point of 100% saturation is reached. The optimum configuration of the break was obtained by using objects, e.g. glass rods, that promotes natural cracking of the cake after 100% saturation is reached. This resulted in unique coal-to-airflow liberation ratio's, giving its own indefinite results. By optimising this technique, it was lastly shown that it is a more than capable replacement for the highly cost intensive flocculant addition currently in operation.

It will be appreciated that variations in detail are possible with a method and apparatus according to the invention, for lowering the irreducible saturation point of a liquid - containing particulate bed without departing from the scope of the appended claims.

Claims

1. A method for lowering the irreducible saturation point of a liquid - containing particulate bed including the steps of:

disturbing the structural coherency of at least part of the bed.

2. A method according to claim 1 wherein the step of at least partially disturbing the structural coherency of the bed includes the step of mechanically deforming at least part of the bed.

3. A method according to claim 2 wherein the step of mechanically deforming at least part of the bed includes the step of forming at least one aperture in the bed.

4. A method according to claim 3 wherein the step of forming at least one aperture in the bed includes the further steps of punching spaced apart apertures into the bed.

A method according to claim 3 wherein the step of forming at least one aperture in the bed includes the step of cutting a plurality of spaced apart elongate groove formations in the bed.

6. A method according to claim 2 wherein the step of mechanically deforming the bed includes the step of deforming at least part of the bed out of its plane.

7. A method according to claim 1 wherein the step of disturbing the structural coherency of at least part of the bed includes the steps of applying the pressure differential over the bed for a first period of time; interrupting the pressure differential over the bed for a second period of time; and reapplying the pressure differential for a third period of time.

8. A method according to any one of claims 1 to 7 wherein the irreducible saturation point is lowered from a first level to a second level and wherein the first level is the baseline irreducible saturation point of the bed.

9. Apparatus for lowering the irreducible saturation point of a liquid - containing particulate bed including :

- pressure differential means for applying a pressure differential over the bed sufficiently to overcome the breakthrough pressure of the bed; and

10. Apparatus according to claim 9 wherein the disturbing means comprises a disturbing member for mechanically deforming at least part of the bed.

11. Apparatus according to claim 10 wherein disturbing member for mechanically deforming the bed is elongate and adapted to form at least one aperture in the bed.

12. Apparatus according to claim 11 wherein the elongate disturbing member is a first member and the apparatus includes further similar members adapted for punching additional spaced apart apertures into the bed.

13. Apparatus according to claim 12 wherein the bed is elongate and is conveyed along a conveyor and wherein the elongate disturbing members extend radially outwardly from an axle rotatably mounted in close proximity to the conveyor for rolling over an upper surface of the bed while the bed passes beneath the axle, the arrangement being such that the elongate members punch apertures into the bed.

14. Apparatus according to claim 10 wherein the bed is elongate and is conveyed along a conveyor and wherein the disturbing member is in the form of at least one disc mounted on an axle for cutting an elongate groove formation in the bed while the bed passes beneath the axle.

15. Apparatus according to claim 10 wherein the disturbing member is adapted to deform at least part of the bed out of its plane.

16. Apparatus according to claim 15 wherein the bed is elongate and is conveyed along a conveyor and wherein the disturbing member is in the form of an undulation in the conveyor for deforming the bed out of its plane when the bed passes over the undulation.

17. Apparatus according to claim 10 wherein the disturbing means comprises a member for interrupting the pressure differential over the bed, before reapplying the pressure differential over the bed.

18. Apparatus according to claim 17 wherein the member is in the form of a valve for releasing the pressure differential.

19. A method for lowering the irreducible saturation point of a liquid - containing particulate bed from a baseline irreducible saturation point substantially as herein described with reference to the accompanying drawings.

20. Apparatus for lowering the irreducible saturation point of a liquid - containing particulate bed from a baseline irreducible saturation point substantially as herein described and as illustrated in the accompanying drawings.