GB2151939A - Apparatus and process for the dewatering of fine granular materials - Google Patents

Abstract

An apparatus and process for the dewatering of fine granular material is disclosed. The apparatus (1) comprises a base member (2), a compressing member (3), and a housing member (4) together defining an enclosed compression chamber (6). During compression of the material (7), fluid (12) is expressed and a small percentage (13) of the material is intentionally permitted to escape through a self-cleaning opening (9) or (16) in the apparatus. By maintaining the smallest dimension of the opening (9) in the range of one to five times the diameter of the largest particle in the material (7), the opening remains essentially clear of any material which could cause clogging. <IMAGE>

Description

SPECIFICATION Apparatus and process for the dewatering of fine granular materials This invention relates to an apparatus and process for the dewatering of fine granular materials, which are present in the underflow of solid-liquid separating devices, such as thickeners, or in filter cakes produced, for example, by filtration of centrifugal means.

Wet granular materials, such as those produced by sedimentation, filtration, or centrifuging from slurries, generally require some dewatering, partial or complete, in orderto improve their handling and storage qualities. For example, the underflow from thickeners used to collect the water employed in coal cleaning processes contains from 25 percent to 35 percent coal and 65 percent to 75 percent water. The use of vacuum filters can reduce the amount of water to the range of 25 percent to an excess of 30 percent depending upon the relative coarseness of the solid particles. Centrifuges, which are costlier than vacuum filters in terms of capital, energy and maintenance expenses, may be utilized to reduce the moisture content to about 18 percent. However, the filter cakes produced by either centrifuges or vacuum filters are sloppy and subject to freezing in cold weather.Additionally, these filters cakes usually contain more moisture than is desirable, even after mixing with drier, coarser fractions, for different end uses such as pulverized coal combustion or as feed to coke-making ovens.

In order to facilitate handling and storage and to reduce the moisture content of the total washed material to an acceptable level of approximately 6 percent of final use, thermal drying is practiced. Thermal drying is very energy intensive and also necessitates the extensive use of pollution control means. In the thermal drying of moist coal, for example, a fire hazard may exist during the drying process due to overdrying caused by the variability of the moisture content of the coal being dried.

Another means of dewatering materials involves the use of presses. Typicaily, presses are utilized with fibrous materials which, when compressed, produce a matter mass that prevents the loss of solids and the occlusion of any drain openings. Press dewatering of granular materials, while being less costly than any of the above mentioned methods, also has a variety of attendant difficulties. The major problem is that the drains or openings in the devices used in such a process tend to become clogged with the material being compressed. This clogging inhibits the escape of the expressed fluid from the materials. Various means have been developed to circumvent this problem including the use of self-cleaning tapered openings or mechanical or fluid cleaning of the openings.Examples of such prior art are disclosed in United States patent nos. 1,488,744,2,398,135, 3,520,411,4,043,832 and and 4,159,947.

In U.S. patent No. 4,208,188 a dewatering apparatus is disclosed which utilizes a porous drainage member having a quasi-triangular porous structure similar to a woven screen having a mesh size in the range of about 50 to 100 microns. During compression, water drains through the drainage screen ad the solid particles consolidate and tend to agglomerate and bridge the interstices of the screen rather than following the tortuous path through the screen. Use of the above described means leads to either increased construction costs or increased complexity in the structure of the dewatering apparatus.

The present invention in one aspect provides apparatus for the dewatering of fine granular material and compressing the material into a puck, comprising: a base member, a compressing member, and a housing member, mounted on the base member, having a passage therethrough for receiving the material to be compressed and the compressing member, the passage being in communication with the base member, the members forming an enclosed compression chamber and defining a drainage opening in connumication with the compression chamber, the smallest dimension of the drainage opening being one to five times the diameter of the largest particle in the material, whereby the material received is compressed between the base and compressing members causing fluid to be expressed from the material via the drainage opening and producing the puck.

The invention in another aspect provides a process for the dewatering of fine granular material and producing a coarse puck product, comprising loading the fine granular material into an enclosed compression chamber having a drainage opening therein, wherein the said granular material has a particle size one to five times smaller than the width of the drainage opening; and applying a compressive force against the material thereby initially expressing fluid and material from the compression chamber via the drainage opening until the intergranularfriction of the particles of the material is increased so that material fluidization ceases allowing agglomeration of the material to occur and bridging of the drainage opening to take place thereby forming a coarse puck product.

Thus the present invention provides an apparatus and a process for the dewatering of fine granular materials. The apparatus comprises a base member, a compressing member, and a housing member for holding the material. Either the base member or the compressing member or both may contain drainage openings made in accordance with the present invention. In addition, these drainage openings may also be defined by the junction of base member and housing member or the interface between compressing member and housing member. The drainage openings may be slotted, circular, annular or arcuate. The housing member has a passage therethrough which is in communication with the base member. The base, compressing and housing members, when positioned for the dewatering operation, define an enclosed compression chamber having drainage openings as disclosed herein.

In one embodiment of the invention the passage in the housing member receives both the material and the compressing member. In an alternative embodiment of the invention the housing member has two openings i.e. a passage therethrough which is in communication with the base member and is used to receive the compressing member and an opening in the side of the housing member which is in communication with the passage and is used to conduct the material into the housing member.

The preferred orientation of the drainage openings is in a direction perpendicular to a plane formed by or parallel to the member or members defining the drainage openings.

Upon the application of compressive force in the range of 10,000 to 20,000 psi (7 > c 106 to 14 x 106 kg/m2) for approximately 10 seconds, fluid is expressed from the material via the drainage openings. At the same time, a small portion of the material is intentionally permitted to escape with the expressed fluid. Material continues to escape until the intergranularfriction of the particles in the material prevents furtherfluidization of the material. At this point the particles agglomerate and form bridges over the drainage openings. These bridges prevent the further escape of material. They appear as projections on the surface of the puck which is formed as a result of the compression. When the puck is withdrawn from the apparatus, the bridges, being a part of the puck, are also removed, thus keeping the drainage openings clear.While a small percentage of the material is expressed, the net effect of these interactions is that the bulk of the material is retained inside the housing and is compressed into a puck having a lowered moisture content in the range of 10 percent.

Thus there may be provided a process and an apparatus with self-cleaning openings for the dewatering of fine granular materials which reduce the moisture content of wet, fine granular materials in an energy efficient manner while eliminating pollution and fire hazards. There may be obtained a coarse, free-flowing, puck-like product that is easily handled, transported, and stored.

The invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a vertical cross-sectional view of one embodiment of a dewatering apparatus according to the present invention; Figure 2 is a sectional view taken along line ll-ll of Figure 1; Figure 3 is a vertical cross-sectional view of another embodiment of a dewatering apparatus of the invention, shown in an open position to illustrate detachable mounting of the housing; Figure 4 is a vertical cross-sectional view of a further embodiment of a dewatering apparatus of the invention, utilizing separate openings in the housing member for receiving the compression member and the material to be compressed; and Figure 5is a schematic representation of a dewatering apparatus utilizing a means for recirculating the fluid expressed.

Referring to the drawings, an apparatus 1 for dewatering fine granular material comprises a base member 2, a compressing member 3 and a housing member 4 having a passage 5 extending therethrough which is in communication with the base member 2. The base member 2, compressing member 3 and housing member 4 define an enclosed compression chamber 6. The compressing member 3 and the material 7 to be compressed are received into the housing member via the passage 5. Except as noted below, the passage 5 is made to closely receive the compressing member 3 in order to prevent the escape of material 7 between the edge of the compressing member 3 and the inner wall 8 of the housing member 4. The base member 2 is shown having slotted drainage openings 9 extending therethrough. These slotted drainage openings 9 may be located in the base member 2, the compressing member 3, or both.While slotted drainage openings are shown in the base member 2, other shapes of drainage openings may be used. As an alternative, drainage openings 9 may be located only at the junction 10 of the base member 2 and the housing member 4, or only at the interface 11 of the inner wall 8 of the housing member 4 and the compressing member 3, or both, or in combination with drainage openings 9 in either the compressing member 3 or base member 2 or both. As the fine granular material 7 is pressed between the compressing member 3 and the base member 2, the material 7 begins to dewater with fluid 12 being expressed through the slotted drainage openings 9, each having length L which exceeds its width W.

During the initial stages of dewatering, a small percentage of the fine granular material 7 is intentionally allowed to pass with the fluid 12 through the slotted drainage openings 9 and is defined as escaped material 13. As dewatering continues, the intergranularfriction of the particles increases causing the particles to agglomerate and bridge the slotted drainage openings 9. These bridges 15 (see Figure 3) prevent the continued exit of the material 7. As a result, the bulk of the material 7 is retained in the housing member 4 and is compacted into a hard coarse puck 14 having a thickness T and bridges 15 (see Figure 3).

The smallest dimension of each of the drainage openings relative to the material particle size is important to the self-cleaning feature of the apparatus as well as to the aforementioned bridge effect. The width of the openings is selected to be one to five times the diameter of the largest particle in the material 7. Such openings remain clear while only about 0.2 to 0.6 percent of the material escapes with the expressed fluid.

Overall, it has been found that the percentage of escaped solid material in the expressed fluid is in the range of only about 1 to 3 percent. If the width of the openings is greater than five times the diameter of the largest particle in the material 7, a large portion of the material 7 escapes the dewatering apparatus 1 without adequate dewatering. If the smallest dimension chosen is less than the diameter of the largest particle in the material, the percentage of escaped material found in the fluid expressed decreases; however, the drainage openings tend to clog and thus they require frequent cleaning. Accordingly, when the width W of the slotted drainage openings is selected to be one to five times, preferably two times, the diameter of the largest particle in the material, both self-cleaning and effective dewatering are achieved.

It is possible to utilize the apparatus of this invention for the dewatering of any of a variety of fine grained materials. Examples of such materials are beneficiated iron ore, red mud, phosphate slime and coal filtercake. Table I below illustrates the results of the dewatering of coal filtercake obtained from a coal washing plant. The coal filtercake having passed through a screen having 0.6 mm to 0.3 mm openings therein, contained 25 to 30 percent water prior to dewatering. Each test series represents the average results of three to five tests. A cylindrical housing member 4 having a passage 5 with a diameter of 90 mm and a compressing member 3 having a diameter of slightly less than 90 mm were used for the test series. The slotted openings 9 were 180 mm in length and were located in the base member 12.

In test series 1 through 11, the slotted drainage openings 9 remained clear or essentially clear of material 7.

Reducing the ratio of slotted opening width W so that the width W was smaller than the particle size resulted in increased water content in the puck 14 and clogging of the slotted drainage openings 9. This is seen in the results of the test series 12 through 15.

The puck produced using the invention was firm and maintained its compacted shape, strength, and moisture content even when submerged in water for periods exceeding three days. The puck also withstood three freeze/thaw cycles before crumbling. During testing, it was also observed that the puck 14 increased in thickness T, as much as 11 percent after being removed from the dewatering apparatus. This increase in thickness is termed relaxation and it is believed that this relaxation causes the puck to have lower density than those produced by dewatering apparatuses, utilizing lower compressive forces and longer compression times.

As indicated above, any of a variety of fine grained materials can be dewatered utilizing the apparatus and process of this invention. The following fine grained materials are presented as examples only and are not to be construed a the only fine grained materials susceptible to treatment in this dewatering apparatus.

Example 1 Beneficiated iron ore is a resultant product from a process in which taconite ore with approximately 33 percent iron is first groundto a very fine mesh (325 to 500 mesh) and is then separated from the gangue by a flotation process. It is then typically dewatered by a vacuum filtration process prior to the pelletizing operation that agglomerates the fine grained beneficiated iron ore to a suitable size for blast furnace feed.

However, the vacuum filtering operation does not always provide sufficient dewatering and as a result thermal drying, which is both expensive and pollution-prone, has been utilized. Although the beneficiated iron ore is of a very fine mesh with a material particle maximum size of about 0.04 mm, an effective slot width of 0.19 mm in an apparatus according to this invention resulted in the successful dewatering of the material. Although the slot width is nearly five times the material particle maximum size, this fine grained material did not tend to escape from the apparatus with the expressed fluid. The particular embodiment of the dewatering apparatus utilized for the beneficiated iron ore had a plain platen and the slot through which the fluid was expressed was defined by the annular clearance at the interface 11 of the inner wall 8 of the housing 4 and the compressing member 3.

Example II Red mud is an extremely fine residue from the Bayer Process which converts bauxite ore to alumina for subsequent aluminium production. The red mud consists primarily of iron oxides and has silica and titanium materials as minor constituents. This material has always presented a difficult disposal problem for the aluminium industry. Again, filtration techniques are inadequate and thermal techniques are too expensive.

Dewatering of the extremely fine red mud, often with as small a particle size as 0.015 mm, was accomplished using a platen with slots 0.6 mm in width. The slotted platen was covered with approximately a 3 mm layer of 28 mesh coal with the largest particle size being about 0.6 mm. The red mud was loaded on top of this sealer/filter layer to a depth of approximately 40 mm. A second 3 mm layer of 28 mesh coal was placed on top of the red mud to form a sealer/filter between the compressing member (a piston) and the red mud. The red mud water content of about 39 percent was reduced by compression to about 17.6 percent when loaded as described above.

Example III Phosphate slimes are suspensions of very fine (150 mesh) particles of waste products resulting from phosphate mining and beneficiating operations. A concentrate of dilute slime with a water content of about 30 percent could be dewatered to form agglomerates with a water content of about 11.5 percent. Such a dewatered phosphate slime could be used as a stabilized landfill. Here again it is advantageous to utilize a sealer/filter layer of appropriate material, preferably 28 mesh particles of phosphate.

Table II below presents the results of the dewatering of the fine grained materials discussed above. As will be readily understood, the apparatus and process of this invention for dewatering extremely fine materials may be successfully applied to other fine grained materials not specifically mentioned herein.

A modified form of apparatus of the present invention is illustrated in Figure 3. In this embodiment the housing member 4, which is detachably mounted on the base 2, is shown in the open position. During compression of the material 7, the operation is identical to that previously described. After compression, the housing member 4 is separated from the base 2, thus permitting ready removal of the puck 14.

A further modification involves the use of drainage passages 16 in the housing 4 as shown in Figure 1 to permit fluid 12 to be expressed from the material 7, the diameter of these drainage passages 16 being one to five times, preferably two times, the diameter of the largest particle in the material 7. The operation of the dewatering apparatus 1 is the same as that previously described.

Another modification of the apparatus 1 is presented in Figure 4. Here an opening 17 is provided which extends through the side of the housing member 4 and into the passage 5. This opening 17 would be used to receive the material 7 while passage 5 would be used to receive the compressing member 3.

In Figure 5 the apparatus of the invention is shown in schematic form in cooperation with a recycling means 18. The recycling means 18 illustrated has a solid-liquid separating means 19, a means 20 for collecting and transporting the expressed fluid 12 and escaped material 13 to the separating means 19, and a means 21 for combining the underflow 22 of the separating means 19 with the material 7 being fed to the apparatus 1.

Various modifications may be made within the scope of the invention. While the detailed description given illustrates an apparatus utilizing slotted drainage openings, other designs have drainage openings made in accordance with the ratios of the smallest dimension of the drainage opening to particle diameter size disclosed will also perform in the same manner as those described. Such openings may be arcuate, circular, or annular, each having its smallest dimension determined by the size ratios given herein. Furthermore, if annular drainage openings between the housing member and compressing member and/or between the housing member and the base member are incorporated in the design of the apparatus, these annular drainage openings may be employed in lieu of or inconjunction with the use of drainage openings in the base or compressing member.

TABLE 1 Test Compression Compression Material Slot Material Material Water Expressed Fluid Colgged Series Pressure Time Compacted Width Particle Content after Solids Content of Slotted Thickness W Top Size Compression Openings T (psi) (sec) (mm) (mm) (mm) (%) (% Solids) 1 10,000 10 52 0.6 0.6 11.8 2 No 2 10,000 10 40 0.6 0.6 11.3 2 No 3 10,000 10 22 0.6 0.6 10.2 2.1 No 4 10,000 10 38 0.6 0.3 10.8 2.9 No 5 10,000 10 21 0.6 0.3 9.6 2.9 No 6 20,000 10 62 0.6 0.6 9.9 1.9 No 7 20,000 10 39 0.6 0.6 8.6 2.1 No 8 20,000 10 19 0.6 0.6 8.2 2.1 No 9 20,000 10 60 0.6 0.3 9.8 2.8 No 10 20,000 10 40 0.6 0.3 8.4 3.0 No 11 20,000 10 21 0.6 0.3 8.2 3.1 No 12 20,000 11 24 0.1 0.3 10.8 0.3-0.8 Yes 13 20,000 11 41 0.1 0.3 11.9 0.3-0.8 Yes 14 20,000 11 25 0.05 0.3 13.0 0.3-0.8 Yes 15 20,000 11 39 0.05 0.3 13.9 0.3-0.8 Yes TABLE II Compression Compression Effective Material Sealer-Filter Material Water Material Water Pressure Time Slot Particle Material Content Before Content After Material (psi) (sec) Width Top Size Particle Compression Compression (mm) (mm) Top Size (%) (%) (mm) Beneficiated Iron Ore 20,000 10 0.19 0.04 --- 16.7 8.4 Red Mud 20,000 12 0.6 0.15 0.6 39.0 17.6 Phosphate Slime 20,000 10 0.6 0.11 0.6 30.0 11.5

Claims (23)

1. Apparatus for the dewatering of fine granular material and compressing the material into a puck, comprising: a base member, a compressing member, and a housing member, mounted on the base member, having a passage therethrough for receiving the material to be compressed and the compressing member, the passage being in communication with the base member, the members forming an enclosed compression chamber and defining a drainage opening in communication with the compression chamber, the smallest dimension of the drainage opening being one to five times the diameter of the largest particle in the material, whereby the material received is compressed between the base and compressing members causing fluid to be expressed from the material via the drainage opening and producing the puck.

2. Apparatus as claimed in claim 1,wherein the drainage opening is formed at the interface of the compressing member and the inner wall of the housing member.

3. Apparatus as claimed in claim 1, wherein the drainage opening is located at the junction of the housing and base members.

4. Apparatus as claimed in claim 1, wherein the drainage opening is provided in the compressing member.

5. Apparatus as claimed in claim 1, wherein the drainage opening is provided in the base member.

6. Apparatus as claimed in claim 1, wherein the drainage opening is annular.

7. Apparatus as claimed in claim 1, wherein the drainage opening is arcuate.

8. Apparatus as claimed in claim 1, wherein the drainage opening is circular.

9. Apparatus as claimed in any of claims 1 to 8, wherein the smallest dimension of the drainage opening is two times the diameter of the largest particle in the material to be compressed.

10. Apparatus as claimed in claim 1,4 or 5, wherein the drainage opening is slotted having a length greater than its width.

11. Apparatus as claimed in claim 10, wherein the drainage opening extends through the member defining the drainage opening in a direction perpendicular to a plane formed by the member.

12. Apparatus as claimed in claim 10 or 11, wherein the width of the drainage opening is two times the diameter of the largest particle in the material to be compressed.

13. Apparatus as claimed in claim 1, wherein the housing member has a plurality of circular drainage passages distributed about the periphery of the housing member with the drainage passages having a diameter one to five times the diameter of the largest particle in the material to be compressed.

14. Apparatus as claimed in claim 13, wherein the diameter of the drainage passages is two times the diameter of the largest particle in the material to be compressed.

15. Apparatus as claimed in any of claims 1 to 14,further comprising means for recycling the fluid and escaped material, the said recycling means including a solid-liquid separating means, a means for collecting and transporting the expressed fluid and escaped material to the separating means, and a means to combine the underflow produced in the separating means with the material being fed to the apparatus.

16. Apparatus as claimed in any of claims 1 to 15, wherein the housing member is detachably mounted on the base member allowing the puck formed therein to be readily removed from the apparatus.

17. Apparatus as claimed in any of claims 1 to 15, wherein the housing member is provided with an opening passing through the side thereof and in communication with the said passage whereby the passage is used to receive the compressing member and the side opening is used to receive the material to be dewatered.

18. Apparatus according to claim 1 for the dewatering of fine granular material, substantially as herein described with reference to, and as shown in, Figures 1 and 2, Figure 3, Figure 4, or Figure 5 of the accompanying draings.

19. A process for the dewatering of fine granular material and producing a coarse puck product, comprising loading the fine granular material into an enclosed compression chamber having a drainage opening therein, wherein the said granular material has a particle size one to five times smaller than the width of the drainage opening; and applying a compressive force against the material thereby initially expressing fluid and material from the compression chamber via the drainage opening until the intergranularfriction of the particles of the material is increased so that material fluidization ceases allowing agglomeration of the material to occur and bridging of the drainage opening to take place thereby forming a coarse puck product.

20. A process as claimed in claim 19, including the step of loading a second, fine granular material having a particle size more than five times smaller than the width ofthe drainage opening wherein the fine granular material having a particle size one to five times smaller than the drainage opening is between the drainage opening and the said second fine granular material.

21. A process as claimed in claim 20, including the step of loading additional fine granular material having a particle size one to five times smaller than the drainage opening into the compression chamber wherein the second fine granular material is between two layers of the said granular material having a particle size one to five times smaller than the drainage opening.

22. A process as claimed in any of claims 19 to 21, wherein the material is su bjected to a compressive force in the range of about 10,000 psi to 20,000 psi (7 x 106 to 14 x 106 kg/m2) for a time period of less than 1 minute.

23. A process according to claim 19 for the dewatering of fine granular material, substantially as herein described with reference to Figures 1 and 2, Figure 3, Figure 4, or Figure 5 of the accompanying drawings.