Screw press with filter plate

Abstract

A screw press for separating a liquid from a solid-liquid mixture includes a housing having an inlet section and an outlet section and a filter section between the inlet and outlet sections. The filter section includes a set of coplanar filter plates defining an axially extending core passage for receiving a screw configured to convey the solid-liquid mixture from the inlet section to the outlet section while compressing and dewatering the solid-liquid mixture by forcing at least a portion of the liquid component of the mixture through an inter-plate gap defined between each pair of adjacent filter plates to the exterior of the housing. The filter plates are secured relative to one another and clamped in direct intimate face-to-face contact by a clamping assembly operable to apply a predetermined axial clamping pressure substantially uniformly about the core passage. The inter-plate gap is controlled by the surface roughness of the plates and the clamping pressure.

Description

[0001] This application is a divisional application of the invention patent application with international application date of April 9, 2015, national application number 201580020994.6, and title "Screw press with filter plates". TECHNICAL FIELD

[0002] The present invention relates generally to solid-liquid separation equipment, and more particularly, to a screw press for dewatering a slurry, such as red mud generated during the production of alumina from bauxite. The present invention also relates to a method for separating liquid from a slurry, such as red mud generated during the production of alumina from bauxite, by using a screw press. BACKGROUND

[0003] The safe handling and storage of large volume industrial waste streams, such as bauxite residue (red mud) from aluminium production, presents many waste management challenges. Land disposal of these materials often has negative environmental impacts, such as soil and groundwater contamination, and consumes large areas of land, thus resulting in significant costs to the alumina production industry. As the production of red mud in the Bayer process is unavoidable, the industry has focused on minimising the volume of red mud to be disposed of by extracting as much liquid as possible from the red mud.

[0004] Current methods rely particularly on decanting in large gravity settlers. However, such mud compression methods require several hours to achieve a solids concentration of no more than about 45% and 50%.

[0005] Other known slurry filtration apparatuses are not well suited for red mud filtration applications and the like.

[0006] For example, UK Patent No. 1,037,384 describes a filtration apparatus comprising a plurality of discs stacked one above the other, the arrangement being such that fluid can percolate between the faces of the discs; a passageway transverse to the assembly of discs relative to the plane of the assembly of discs; an intermittently open inlet at a portion of the passageway for fluid to be filtered; a discharge for separated solids and / or semi-solids at another portion of the passageway; and means for feeding coagulated solids and / or semi-solids to the discharge, the means being arranged such that the solids and / or semi-solids are compressed and discharged in compressed form.

[0007] In the above-mentioned filter apparatus, the filter discs are maintained in a predetermined (but adjustable during operation) compressed state by a spring, thereby maintaining a constant flow of filtrate. The spring loading of the filter discs allows the discs to be forced apart during passage of some solids. Operation of the above-mentioned filter apparatus thus relies on the intermittently open inlet for reversing the flow direction by closing the inlet to clear the solids and then resuming filtration by opening the inlet again. This intermittently open inlet and the resulting movement of the filter discs makes the above-mentioned filter apparatus more complex and more prone to clogging. Small particles, such as those contained in red mud slurry, can become stuck between the filter discs, thereby creating preferential pathways between the filter discs.

[0008] The above-mentioned filter apparatus would not allow to reach an acceptable level of performance (e.g. in terms of solids concentration of the compressed slurry) and would not be suitable for a viable industrial filtration of red mud and the like, which requires to treat large quantities of solid slurry in an acceptable time. Moreover, in case of high pressure required to filter the slurry (such as red mud slurry), the adjustable spring compression system would not work.

[0009] Therefore, there is a need to provide a new solid-liquid separation apparatus that solves the above-mentioned problems. SUMMARY

[0010] Therefore, one object is to increase the solid fraction of the compacted slurry and to provide a reliable industrial filter apparatus that can treat large quantities of slurry.

[0011] Another object is to provide an industrial filter apparatus that can treat a slurry having very fine particles (in other words, a slurry having particles smaller than 40 microns). More specifically, the industrial filter apparatus can treat a slurry (such as a red mud slurry) having at least 10%, preferably at least 40%, more preferably at least 60% and up to 100% of particles smaller than 40 microns.

[0012] Another object is to provide an industrial filter apparatus that can treat large quantities of slurry at high pressure.

[0013] According to one general aspect of the present application, there is provided a screw press for separating liquid from a solid-liquid mixture, the screw press comprising: a general tubular body having an axially spaced-apart inlet section and outlet section and a filter section therebetween; the filter section comprising an axially extending set of coplanar filter plates defining a core passage; and a rotatable screw mounted in the tubular body and axially extending through the core passage for conveying the solid-liquid mixture from the inlet section to the outlet section while compressing the solid-liquid mixture and dewatering the solid-liquid mixture by forcing at least a portion of the liquid component of the mixture through liquid passages of the filter section surrounding the core passage to the exterior of the tubular body, the liquid passages being formed by inter-plate gaps defined between each pair of adjacent filter plates; characterized in that the filter plates are continuously maintained fixed relative to each other and clamped in direct intimate face-to-face contact by a clamping assembly operable to apply a predetermined axial clamping pressure substantially uniformly around the core passage, and in that the filter plates have a predetermined surface roughness (R), the inter-plate gaps being a function of the predetermined surface roughness (R) and the axial clamping pressure.

[0014] According to another general aspect of the present application, there is provided a method for separating liquid from a solid-liquid mixture by using a screw press comprising a general tubular body having an axially spaced-apart inlet section and outlet section and a filter section therebetween, the filter section comprising an axially extending set of coplanar filter plates and inter-plate gaps between each pair of adjacent filter plates, the method being characterized in that it comprises:

[0015] - providing filter plates having a predetermined surface roughness;

[0016] - applying a predetermined axial clamping pressure substantially uniformly around the core passage using a clamping assembly to continuously maintain the filter plates fixed relative to each other and clamped in direct intimate face-to-face contact by a clamping assembly, the inter-plate gaps being a function of the predetermined surface roughness and the axial clamping pressure;

[0017] - introducing a solid-liquid mixture through the inlet section;

[0018] - using a rotatable screw mounted in said tubular body and extending axially through said core passage to convey the solid-liquid mixture from the inlet section to the outlet section while compressing the solid-liquid mixture and dewatering the solid-liquid mixture by forcing at least a portion of the liquid component of the mixture through a liquid passage of said filter section surrounding the core passage to the exterior of the tubular body, said liquid passage being formed by said inter-plate gap; and

[0019] - producing a dewatered mixture at the outlet section.

[0020] The predetermined axial clamping pressure is preferably maintained constant.

[0021] Generally, the invention discloses the following technical solutions of 1 and 17, and the following 2-16 and 18-21 are preferred technical solutions:

[0022] 1. A screw press (14) for separating liquid from a solid-liquid mixture, the screw press (14) comprising:

[0023] a generally tubular body (18) having axially spaced apart inlet (24) and outlet (26) sections and a filter section (28) between said inlet (24) and outlet (26) sections; said filter section (28) including an axially extending set of coplanar filter plates (36) defining a core passage; and

[0024] a rotatable screw (20) mounted in said tubular body (18) and extending axially through said core passage for conveying said solid-liquid mixture from the inlet (24) to the outlet (26) section while compressing the solid-liquid mixture and dewatering the solid-liquid mixture by forcing at least a portion of the liquid component of the mixture through a liquid passage of said filter section surrounding the core passage to the exterior of the tubular body (18), said liquid passage being formed by an inter-plate gap (40) defined between each pair of adjacent filter plates (36);

[0025] characterized in that said filter plates (36) are continuously maintained fixed relative to each other and clamped in direct intimate face-to-face contact by a clamping assembly (38) operable to apply a predetermined axial clamping pressure substantially uniformly about said core passage, and said filter plates (36) have a predetermined surface roughness (R), said inter-plate gap (40) being a function of said predetermined surface roughness (R) and said axial clamping pressure.

[0026] 2. The screw press (14) according to the preceding paragraph 1, characterized in that the inlet section (24) allows maintaining continuous fluid communication with the overall tubular body (18).

[0027] 3. The screw press (14) according to the preceding paragraph 1 or 2, characterized in that a pressurized feeder, preferably a positive displacement pump (12), is operatively connected to a source of solid-liquid mixture to forcibly feed the tubular body (18) of the screw press at a pressure in the range of about 3.4 N / mm 2 to about 14 N / mm 2 and more preferably between about 4 N / mm 2 and about 10 N / mm 2 .

[0028] 4. The screw press (14) according to any one of the preceding paragraphs 1 to 3, characterized in that the surface roughness (R) of the filter plates (36) substantially corresponds to the average of the highest and lowest points on the mutually engaging surfaces of the filter plates (36), the surface roughness (R) being between about 1 micron and about 30 microns, preferably between about 2 microns and about 10 microns, more preferably between 1.4 microns and 3.5 microns.

[0029] 5. The screw press (14) according to any one of the preceding paragraphs 1 to 4, characterized in that the inter-plate gap (40) is between about 1-60 microns, more preferably between about 2-20 microns.

[0030] 6. The screw press (14) according to any one of the preceding paragraphs 1 to 5, characterized in that the clamping pressure exerted on each filter plate (36) is between about 1.4 N / mm 2 and about 3.5 N / mm 2 , preferably between about 2 N / mm 2 and about 2.8 N / mm 2 .

[0031] 7. The screw press (14) according to any of the preceding solutions 1 to 6, characterized in that the clamping assembly (38) comprises first and second clamping plates (42, 44) disposed at opposite ends of the filter section (28), said filter plates (36) being arranged between said first and second clamping plates (42, 44), each of said first and second clamping plates (42, 44) having a load distribution portion (42a, 44a) in uniform bearing contact with an adjacent one of said filter plates (36), said first and second clamping plates (42, 44) being thicker than each of said filter plates (36).

[0032] 8. The screw press (14) according to the preceding solution 7, characterized in that the clamping assembly (38) further comprises at least four, preferably six, axially extending stay bolts (48) uniformly distributed circumferentially around the core passage, said at least four stay bolts (48) extending through aligned mounting holes defined in said filter plates (36) and said first and second clamping plates (42, 44), the clamping assembly (38) further comprising nuts (50) threadably engaged at opposite distal ends of said at least four stay bolts (48) and all tightened with the same predetermined torque.

[0033] 9. The screw press (14) according to the preceding solution 7 or 8, characterized in that said load distribution portions (42a, 44a) are provided in the form of a ring projecting from an inner face of each of said first and second clamping plates (42, 44), said ring encircling said core passage.

[0034] 10. The screw press (14) according to any of the preceding solutions 1 to 9, characterized in that at least one intermediate support plate (52) is interposed between two of said filter plates (36), said intermediate support plate (52) being thicker than each of said filter plates (36).

[0035] 11. The screw press (14) according to the preceding solution 8, characterized in that a calibrated tightening torque of about 56 to about 560 is applied to each of said nuts (50).

[0036] 12. The screw press (14) according to any of the preceding solutions 1 to 11, characterized in that said filter plates (36) have a filter height extending radially from an inner diameter of said filter plates (36) to a peripheral edge of said filter plates (36), and said filter plates (36) are in close face-to-face contact along the entire extent of said filter height.

[0037] 13. The screw press (14) according to any one of the preceding solutions 1 to 12, characterized in that said inter-plate gap (40) is smaller than or of the same order of magnitude as a median size value of solid particles contained in the solid-liquid mixture to be treated.

[0038] 14. The screw press (14) according to any one of the preceding solutions 1 to 13, characterized in that said filter plates (36) are made of a material selected from the group consisting of stainless steel, black steel, steel with baked enamel finish, and ceramic.

[0039] 15. The screw press (14) according to any one of the preceding solutions 1 to 14, characterized in that said filter plates (36) have a bead-blasted surface finish.

[0040] 16. The screw press (14) according to any one of the preceding solutions 1 to 15, characterized in that said filter plates (36) are coated with a mixture of paint and particles.

[0041] 17. A method for separating a liquid from a solid-liquid mixture by using a screw press (14) comprising a general tubular body (18) having an axially spaced apart inlet section (24) and an outlet section (26) and a filter section (28) between said inlet section (24) and said outlet section (26), the filter section comprising an axially extending set of coplanar filter plates (36) defining a core passage and an inter-plate gap (40) between each pair of adjacent filter plates (36), said method being characterized in that it comprises:

[0042] - providing filter plates (36) having a predetermined surface roughness (R);

[0043] - applying a predetermined axial clamping pressure substantially uniformly around the core passage using a clamping assembly (38) to continuously maintain said filter plates (36) fixed relative to each other and clamped in direct close face-to-face contact by a clamping assembly (38), said inter-plate gap (40) being a function of said predetermined surface roughness (R) and said axial clamping pressure;

[0044] - introducing a solid-liquid mixture through the inlet section (24);

[0045] - using a rotatable screw (20) mounted in said tubular body (18) and axially extending through the core passage to convey the solid-liquid mixture from the inlet section (24) to the outlet section (26) while compressing the solid-liquid mixture and dewatering the solid-liquid mixture by forcing at least a portion of the liquid component of the mixture through a liquid passage of said filter section surrounding the core passage to the outside of the tubular body (18), said liquid passage being formed by said inter-plate gap (40); and

[0046] - producing a dewatered mixture at the outlet section (26).

[0047] 18. The method according to claim 17, characterized in that the solid-liquid mixture is continuously introduced through the inlet section (24).

[0048] 19. The method according to the preceding solution 18, characterized in that the clamping pressure exerted on each filter plate is between about 1.4 N / mm 2 (200 psi) and about 3.5 N / mm 2 (500 psi).

[0049] 20. The method according to the preceding solution 17 or 19, characterized in that said filter plates (36) have a predetermined surface roughness (R) on the entire surface of the two lateral faces.

[0050] 21. The method according to any one of the preceding solutions 17 to 20, characterized in that the solid-liquid mixture is a residue produced by the Bayer process during the production of alumina from bauxite.

[0051] Further details of these and other aspects of the present application will become apparent from the detailed description and drawings included hereinafter. BRIEF DESCRIPTION OF DRAWINGS

[0052] Reference will now be made to the drawings wherein:

[0053] Figure 1 is a schematic view of a pressure filter device according to an embodiment of the present application;

[0054] Figure 2 is an exploded perspective view illustrating construction details of a screw press forming part of the pressure filter device shown in Figure 1

[0055] Figure 3 is a partially exploded perspective view of a filter section of the screw press;

[0056] Figure 4 is​Figure 3 a longitudinal cross-sectional view of a filter segment shown in

[0057] Figure 5 is a perspective view illustrating details of an external coupling between two screw segments of the screw press. Figure 3 and Figure 4 is an enlarged view of an inter-plate gap between two adjacent filter plates of a filter segment shown in

[0058] Figure 6 is a perspective view illustrating details of an external coupling between two screw segments of the screw press. DETAILED DESCRIPTION

[0059] Figure 1 represent applications in which the principles of the present invention can be applied. More specifically, Figure 1 A pressure filtration apparatus for removing liquid from a solid-liquid mixture is illustrated. According to one application, the pressure filtration apparatus is particularly suitable for dewatering red mud, a residue of the Bayer process during the production of alumina from bauxite. However, it will be appreciated that the pressure filtration apparatus can be configured and used for dewatering a variety of types of slurries and is therefore not strictly limited to red mud dewatering applications.

[0060] As can be understood from Figure 1 the apparatus generally comprises a reservoir 10 containing the red mud or slurry to be dewatered, a feed device, such as a positive displacement pump 12, operatively connected to the reservoir 10 for feeding the slurry under pressure to a screw press 14, and a valve 16 for regulating the flow of dewatered mud at the discharge end of the screw press 14.

[0061] The positive displacement pump is typically combined with a device for allowing the delivery flow rate of the slurry to be substantially constant at a substantially constant inlet pressure. The inlet flow rate of the slurry can be controlled by the stroke speed of the positive displacement pump. The inlet pressure and flow rate can be maintained by a check valve (or non-return valve) to prevent reverse flow during the cycle / piston transition of the positive displacement pump, and the slurry is supplied by a pressurised damper (such as a reservoir) during the transition.

[0062] As Figure 2As shown, the screw press 14 typically includes a casing having an overall tubular body 18 for surrounding a screw 20, which is adapted to be rotatably mounted within the tubular body 18. A motor (not shown) is mounted on a platform 22 adjacent to the casing for driving the screw 20 via a suitable drive arrangement (such as belt drive) or a direct drive (not shown). In operation, the screw 20 applies a longitudinal pressure gradient over the solid-liquid mixture to be dehydrated. For example, the pressure of the slurry supplied at the outlet of the displacement pump 12 or the slurry supply pressure causes the liquid to be squeezed out of the mixture and extruded outside the screw press casing, as... Figure 1 The diagram is schematically depicted. In addition to the slurry supply pressure, the action of the screw 20 on the solid-liquid mixture also causes the liquid to be extruded from the mixture and forced outside the screw press housing. The opening (size) of the outlet valve can be continuously changed to simultaneously maintain the required pressure within the equipment and control the outlet flow rate of the dewatered slurry.

[0063] like Figure 1 As best shown, the screw 20 generally includes a shaft 32 and a continuous thread 34 extending screw-shaped around the smooth outer surface of the shaft 32. The screw thread 34 has a constant external thread diameter, which is slightly smaller than the inner diameter of the tubular body of the filter housing by a predetermined thread clearance. According to one embodiment of the invention, the outer diameter of the shaft 32 is constant along the entire length of the screw 20. Still according to this embodiment, the pitch of the thread 34 (see...) Figure 1 The volume between adjacent turns of the screw thread 34 gradually decreases toward the discharge end of the screw press 14 (i.e., in the downstream direction). As a result, the volume between adjacent turns of the screw thread 34 gradually decreases toward the discharge end of the screw press 14, thereby gradually increasing the pressure on the solid-liquid mixture and promoting solid-liquid separation.

[0064] like Figure 2 As shown, the tubular body 18 of the filter cover has an axially opposed inlet section 24 and an outlet section 26, and a filter section 28 between the inlet section 24 and the outlet section 26. Figure 1 As schematically illustrated, filter section 28 has a fluid passage for allowing liquid to be discharged outside the filter housing as the solid-liquid mixture is conveyed from inlet section 24 to outlet section 26 via screw 20. Inlet section 24 can be operatively connected in fluid communication to the output side of positive displacement pump 12 to receive a continuously supplied slurry at a predetermined pressure. Inlet section 24 is preferably designed to maintain continuous fluid communication with tubular body 18. This is achieved, preferably at approximately 2 N / mm². 2 (Approximately 300 psi) to approximately 14 N / mm 2in the range of approximately 2000 psi and more preferably between about 4-10 N / mm 2 Satisfactory results have been obtained with the screw press 14 being forced fed at a pressure in the range of approximately 600-1500 psi. It will be appreciated that the feed pressure can be varied depending on the size of the screw press 14. The outlet section 26 can have a conical passage section operatively connected to the valve 16 to regulate the flow of dewatered slurry from the screw press and to maintain the desired filter pressure inside the filter housing.

[0065] The inlet section 24 and the outlet section 26 each comprise upper and lower half shell members 24a, 24b, 26a, 26b adapted to be bolted to one another to form a complete cylindrical housing section. Bushings or the like (not shown) can be provided within the inlet section 24 and the outlet section 26 on the inner surfaces of the half shell members 24a, 24b, 26a, 26b to rotatably support the axially opposite ends of the screw 20.

[0066] With reference to Figure 3 and Figure 4 It will be appreciated that the filter section 28 generally comprises a plurality of stationary filter plates 36 which are axially clamped in close face-to-face contact by a clamping assembly 38 operable to apply a generally uniform predetermined axial clamping pressure around the inner diameter of the plates 36. The predetermined axial clamping pressure is preferably maintained constant. Each filter plate 36 can be provided in the form of a flat disc defining a central aperture 37. Once assembled, the central apertures 37 of the plates 36 are axially aligned to jointly form an axially extending core passage for receiving the screw 20.

[0067] Because the filter plates 36 are continuously maintained in close face-to-face contact, there is no risk that some of the discs will be forced apart, which would create preferential passages and cause the slurry pressure inside the core passage to intermittently decrease below the pressure at the outlet of the displacement pump 12. Therefore, there is no risk that small particles, such as the particles contained in the red mud slurry, will get stuck between the filter discs. One significant advantage over prior art filter apparatuses is that the slurry pressure inside the screw press of the present application can be maintained at a relatively constant value. Therefore, preferably the screw press of the present application operates in steady state most of the time.

[0068] As will be seen hereinafter, the clamping pressure and the surface roughness of the discs 36 are selected to form a predetermined "micro" inter-plate gap 40 (typically in the range of 0.1-0.5 mm) between each pair of adjacent plates 36. Figure 5The plate gap 40 is chosen to be large enough to allow the liquid squeezed out by the screw 20 to seep between the plates 36, while being small enough to prevent solid particles from passing through, thereby allowing the formation of clumps of dewatered mud on the inner diameter of the filter section 28. Once formed, the clumps of solid particles help maintain pressure inside the filter section 28 despite the presence of the plate gap 40 (i.e., it restricts pressure from escaping through the plate gap 40). The screw maintains the thickness of the clumps of solid particles and also acts to trim the clumps. Depending on the solid-liquid mixture to be dewatered, the plate gap 40 can be in the range of about 1 micrometer to about 60 micrometers, and preferably in the range of about 2 micrometers to about 20 micrometers. For red mud dewatering applications, the plate gap 40 is preferably about 4 micrometers to about 6 micrometers, and more preferably about 5 micrometers to about 6 micrometers. Generally, it can be assumed that the plate gap 40 is selected to be smaller than or on the same order of magnitude as the medium size of the solid particles contained in the solid-liquid mixture to be processed, and that the plate gap 40 is large enough to allow liquid to seep out.

[0069] As mentioned above, and as... Figure 5 As schematically illustrated, the gap 40 between each plate is a function of the surface roughness of the plate 36. The surface roughness (R) of the filter plate 36 can be defined as the average peak height of the asperity at the surface of the filter plate 36. When the plates are clamped together, the peaks extending from the opposite faces of the plates 36 prevent the plates from fitting in a perfectly face-to-face seal, resulting in the formation of minute pathways extending from the inner diameter of the plate 36 to its outer peripheral edge. Depending on the solid-liquid mixture to be dehydrated, filter plates with surface roughness ranging from about 1 micrometer to about 30 micrometers can be used. Tests have shown that for red mud filtration applications, the optimal range of surface roughness is between about 1.4 micrometers and about 3.5 micrometers. However, satisfactory results can also be obtained using surface roughness in the range of about 2 micrometers to about 10 micrometers.

[0070] The surface roughness of a filter plate generally refers to the surface roughness of the entire surface of both sides of each plate.

[0071] The liquid passages of filter section 28 are formed by interplate gaps 40 defined between each pair of adjacent filter plates 36. The liquid passages extend from the inner diameter of the plate 36 to its outer peripheral edge. The liquid passages surround a core passage defined by a stack of axially extending, coplanar filter plates 36. The filter plates 36 are continuously held in a direct, tight, face-to-face contact, and the resulting fluid passages—extending from the inner diameter of the plate 36 to its outer peripheral edge—are evenly distributed around the core passage, thereby preventing the formation of preferential passages.

[0072] In other words, the filter plates 36 are consistently maintained clamped in direct intimate face-to-face contact across the surfaces extending from the inner diameter of the plates 36 to the outer peripheral edges of the plates 36 such that the resulting fluid passageways are evenly distributed around the core passageway, thereby preventing the formation of preferential passageways.

[0073] Tactile or visual roughness depth measuring equipment is used to ensure that the plates 36 have the desired surface roughness. Preferably, the plate surface roughness is measured using a contact instrument having a stylus adapted to be placed in direct contact with the surface of each of the filter plates 36. As the stylus traverses the plate, it rises and falls with the roughness on the surface of the plate. This movement of the stylus is captured and used to measure the surface roughness.

[0074] The filter plates 36 can be made from a wide variety of materials including, for example, stainless steel, black steel, steel with a baked finish, ceramic. It has been observed that a baked finish allows for increased permeability of the filter section 28 while providing good protection against wear and corrosion. The material selected must be able to withstand a corrosive environment, be stable at operating temperatures (e.g., 100°C), and be sufficiently robust so as not to collapse or suffer compression / deformation across the range of clamping pressures applied by the clamping assembly 38. The plate material is also selected such that the fluid flow resistance through the inter-plate gaps 40 is lower than the resistance of the solid particle mass formed on the inner diameter of the plates 36. In other words, the fluid flow resistance of the filter plates 36 is selected such that it is limited compared to the resistance of the solid particle mass. It is noted that different materials having different surface roughnesses can be used to achieve similar liquid flow resistances between the filter plates 36. For example, it has been found that stainless steel plates having a surface roughness of 1.4 and steel plates having a baked finish and a surface roughness of 3.5 provide similar fluid flow resistances when subjected to a red mud dewatering experiment.

[0075] The liquid flow resistance through the inter-plate gaps 40 is also a function of the filter height, which corresponds to the distance that the plates 36 are forced into intimate face-to-face contact between the inner diameter of the plates 36 and the outer peripheral edges of the plates 36. The greater the filter height, the greater the flow resistance through the plates will be. The filter plates 36 are always or consistently maintained clamped in direct intimate face-to-face contact, with the resulting liquid passageways extending across the filter height, thereby preventing the formation of preferential passageways.

[0076] The filter plates 36 can be subjected to a variety of surface treatments to achieve the desired surface roughness and liquid flow resistance. For example, the plates 36 can be subjected to a bead blasting surface treatment. Glass bead surface treatment is preferred compared to sand blasting surface treatment. Sand blasting is more aggressive and results in surface roughness values that are greater than those obtained with glass bead blasting.

[0077] A variety of coatings can be applied to the filter plates 36 to protect the filter plates from corrosion, to alter the hydrophobic or hydrophilic properties and / or to modify the surface roughness of the filter plates. For example, a mixture of paint and particles can be applied to the plates 36 to protect the filter plates from corrosion and to achieve a desired surface roughness.

[0078] As mentioned above, the inter-plate gap 40 is also a function of the clamping pressure applied to the filter plates 36. This clamping pressure must be applied as uniformly as possible around the core passage defined by the filter plates 36 to avoid leaks. With reference to Figure 3 And Figure 4 It will be appreciated that the clamping assembly 38 generally includes first and second clamping plates 42, 44 disposed at opposite ends of the filter section 28, with the filter plates 36 arranged between the first and second clamping plates 42, 44. Each of the first and second clamping plates 42, 44 has a load distribution portion which can take the form of a ring or cylindrical projection 42a, 44a extending from one face thereof for uniformly bearing contact concentrically around the central aperture 37 of the filter plates 36 with an adjacent one of the filter plates 36. Uniformly circumferentially distributed ear segments or eyelet projections 46 extend radially outwardly from the cylindrical projections 42a, 44a of the filter plates 36 and clamping plates 42, 44 for engagement with axially extending stay bolts 48. It will be appreciated that the mounting holes defined by the eyelet projections can be provided in other ways. For example, the mounting holes can be defined directly within the filter ring surface of the plates 36. At least four, and preferably six, sets of eyelets and stay bolts are circumferentially distributed around the core passage defined by the filter plates 36. Nuts 50 are threadably engaged at opposite distal ends of the stay bolts 48 to axially clamp a set of filter plates 36 between the clamping plates 42, 44. The nuts 50 are tightened with the same predetermined torque. A suitable tool, such as a torque wrench, is used to ensure that the exact same torque is applied to each of the nuts 50. According to one application of the present invention, a calibrated tightening torque of between about 56 (approximately 500 lbf-in) and about 560 (approximately 5000 lbf-in) is applied to each of the nuts 50. The torque required increases with the size (length and diameter) of the screw press 14. For example, for a screw having a diameter of about 0.1 m, the clamping torque can be about 56 (approximately 500 lbf-in); however, for a diameter of 0.3 m, the clamping torque can be between 225 Approximately 2000 lbf-in. The thickness of the clamping plates 42, 44—including the cylindrical protrusions 42a, 44a and the eyelet 46 or ear section—is chosen to avoid any deformation under such tightening conditions. This is why the clamping plates 42, 44 are much thicker than the filter plate 36. This allows for a uniform pressure distribution on the plates between adjacent nuts 50, and therefore allows for a uniform pressure distribution around the circumference of the central hole 37 of the filter plate 36. It should be understood that the value of this torque will vary depending on the size / geometry of the filter plate 36. This torque is chosen to generally correspond to approximately 1.4 N / mm on each filter plate 36. 2 (Approximately 200 psi) to approximately 3.5 N / mm 2 (Approximately 500 psi) and preferably around 2 N / mm 2 (Approximately 300 psi) to approximately 2.8 N / mm 2 The clamping pressure is approximately 400 psi.

[0079] The clamping pressure applied to the filter plate 36 is such that the filter plate is always or continuously clamped in a direct and tight face-to-face contact manner.

[0080] like Figure 3 and Figure 4 As shown, at least one intermediate support plate 52 (three in the illustrated embodiment) is inserted between two adjacent filter plates 36. The number of support plates 52 will vary depending on the axial length of the filter section 28. The support plates 52 are inserted at predetermined intervals along the axial length of the filter section 28 to provide uniform support and prevent the set of filter plates 36 from deforming under the clamping force applied thereto by the clamping plates 42, 44. The support plates 52 help to stabilize the plate assembly while providing a mounting interface or base 54 for securing the filter section 28 to the bottom mounting interface or base 54 of the frame structure 55 below. Figure 1 Furthermore, the intermediate support plate 52 may have a pair of lugs 56 at one of its upper ends to aid in the handling and transport of the assembled filter section 28. Mounting holes are also defined in the intermediate support plate for engagement with the support bolts 48. The intermediate support plate 52 is thicker than the filter plate 36. It provides a stable and uniform support surface for adjacent filter plates 36, and thus helps maintain uniform clamping pressure across the entire filter plate assembly. Similar to the filter plate 36, each intermediate support plate 52 has a central hole 58 defining a portion of the core passageway of the filter section 28. The intermediate support plate 52 typically has the same surface roughness as the filter plate 36. Therefore, the filtration gap on opposite sides of each intermediate support plate 52 is similar to the inter-plate gap 40 between adjacent filter plates 36.

[0081] The embodiments of the pressure filtration apparatus described above allow for increased expression of the solid-liquid mixture. In other words, more liquid can be extracted from the mixture. For red mud dewatering applications, testing has shown that the dewatered mud can be 70% to 75% solids by weight and sometimes up to 77% solids by weight as it exits the outlet of the outlet section 26 of the screw press 14. For calcium fluoride (CaF2) dewatering applications, testing has shown that the dewatered mud can be up to 80% solids by weight as it exits the outlet of the outlet section 26 of the screw press 14. For iron tailings dewatering applications, testing has shown that the dewatered mud can be up to 89% solids by weight as it exits the outlet of the outlet section 26 of the screw press 14. This is an increase of approximately 20% over conventional red mud weight decanting processes. It can generally be said that the pressure filtration apparatus allows for an increase in the solid phase fraction of the compressed slurry discharged from the outlet section of the screw press while maximizing the solid-liquid separation rate.

[0082] As can be understood from Figure 2 the tubular body 18 and screw 20 can be of modular construction. According to the illustrated embodiment, the tubular body 18 has a first filter section 28a and a second filter section 28b which are connectable in series to one another, and the screw 20 has corresponding first and second screw sections 20a, 20b which are connectable in series to one another adapted for mounting in the first and second filter sections 28a, 28b respectively for joint rotation as an integral component. However, it will be appreciated that the tubular body 18 and screw 20 can comprise more than two sections.

[0083] The first and second screw sections 20a, 20b are joined together so as to have a continuous screw thread between the sections 20a, 20b without an interruption point, and to ensure that the coupling 62 does not reduce the volume between adjacent turns of the thread 34 at the junction of the two screw sections 20a, 20b. As Figure 6 shown in the drawings, the screw sections 20a, 20b are detachably coupled to one another by an external coupling 62 provided at the outer diameter of the thread 34. Typically, the screw sections are coupled via their shafts. Such a shaft coupling arrangement can in some cases require that the shafts be reinforcements at their point of junction, resulting in a reduction in slurry compression volume between adjacent thread turns at the transition point from one shaft section to the next. Therefore, to not interfere, it is proposed herein to couple the shafts externally from the volume defined between adjacent thread turns, thus maintaining the cross-sectional area through which the slurry is passed, which minimizes flow restriction and reduces the likelihood of plugging.

[0084] The coupling 62 generally comprises a first coupling member 62a mounted to the first screw thread section 34a at one distal end of the first screw section 20a and a second coupling member 62b mounted to the second screw thread section 34b at one adjacent end of the second screw section 20b. The first and second coupling members 62a, 62b are detachably fastenable to each other, such as by bolting.

[0085] The first and second coupling members 62a, 62b can comprise semi-cylindrical plates or annular segments mounted to the outer diameter surface of the screw thread sections 34a, 34b respectively. Each of the screw thread sections 34a, 34b can be provided in the form of a half-thread segment. The inner side end of the semi-cylindrical plates can be integrally provided with a frusto-conical section 63a, 63b adapted to interconnect with the associated screw shaft section 32a, 32b via a strut-like member 65. When interconnected, the semi-cylindrical plates form a complete support ring around the first and second screw thread sections 34a, 34b having an inner diameter corresponding to the outer diameter of the threads 34. Thus, the coupling 62 does not reduce the volume between the thread sections 34a, 34b. The semi-cylindrical plates can be welded to the outer diameter surface of the first and second screw thread sections 34a, 34b on their inner surface. The coupling member 62a, the screw thread section 34a and the associated strut 65 are preferably mounted to the screw shaft section 32a as a pre-assembled unit. Likewise, the coupling member 62b, the screw thread section 34b and the associated strut 65 are preferably mounted to the screw shaft section 32b as a pre-assembled unit. According to one embodiment of the present application, the first and second thread sections 34a, 34b are welded to the inner surface of the coupling members 62a, 62b and the pre-assembled coupling and screw thread assembly is then subjected to a heat treatment process to improve the mechanical properties of the coupling assembly. Thereafter, the heat treated coupling and screw thread assembly is mounted to the respective screw shaft section 32a, 32b by welding the screw thread sections 34a, 34b to the outer surface of the shaft and to the end of the thread present on the respective shaft section 32a, 32b. The screw thread sections 34a, 34b are welded to extend in a continuous manner to the thread already present on the shaft section 32a, 32b. The strut 65 is also welded to the screw shaft section 32a, 32b.

[0086] By welding the threaded sections 34a and 34b of the screw to the connecting components 62a and 62b before the heat treatment process, and then connecting the connecting components 62a and 62b to the screw shaft sections 32a and 32b, the structural integrity of the connecting components 62a and 62b can be preserved. In fact, directly welding the threads of the connecting components 62a and 62b to the shaft sections 32a and 32b could potentially adversely affect the mechanical properties of the connecting components 62a and 62b.

[0087] Or as Figure 6 As shown, wear plates 64a and 64b are removably mounted to the outer surface of each of the semi-cylindrical connecting members 62a and 62b for use with a screw support section 66 arranged between the first filter section 28a and the second filter section 28b (see Figure 1). Figure 2 The corresponding segmented wear-resistant annular structures (not shown) in the screw 20 engage with each other. Therefore, the coupling 62 can also be used to provide intermediate support for the screw 20 approximately at the midpoint between opposite ends. The screw support section 66 may include an upper shell member 66a and a lower shell member 66b adapted to be detachably bolted to each other. This ensures rapid engagement of the coupling 62. The segmented wear-resistant annular structures (not shown) disposed within the screw support section 66 are configured to wear before the wear plates 64a, 64b on the outer surfaces of the semi-cylindrical plates of the coupling members 62a, 62b. Bolt flanges 68a, 68b are provided at their opposite ends of the upper shell member 66a and lower shell member 66b for attachment to corresponding bolted flanges 70 provided on the clamping plates 42, 44 of each filter section 28a, 28b. In this way, each screw and associated filter segments 20a, 28a, 20b, 28b can be easily removed as a unit or cartridge and replaced with a similar screw and filter "cartridge" by simply removing flange 70 from flanges 68a, 68b, removing upper shell member 66a, removing screw connector members 62a and 62b, and removing flange 70 at the other end of the screw and filter segment to be replaced. All bolts—including those used to fasten the first screw connector member 62a and the second screw connector member 62b—are easily accessible.

[0088] The above description is merely exemplary, and those skilled in the art will recognize that changes can be made to the described embodiments without departing from the scope of the disclosed invention. Based on a review of this disclosure, modifications falling within the scope of the invention will be apparent to those skilled in the art, and such modifications are intended to fall within the appended claims.

Claims

1. A screw press (14) for separating liquid from a solid-liquid mixture, the screw press (14) comprising: A general tubular body (18) having axially spaced inlet section (24) and outlet section (26) and a filter section (28) between said inlet section (24) and said outlet section (26); said filter section (28) includes an axially extending set of coplanar filter plates (36) defining a core passage; and A rotatable screw (20), which is mounted in the overall tubular body (18) and extends axially through the core passage, is used to convey the solid-liquid mixture from the inlet section (24) to the outlet section (26) while compressing and dehydrating the solid-liquid mixture by forcing at least a portion of the liquid component of the mixture through a liquid passage of the filter section surrounding the core passage to the outside of the overall tubular body (18), the liquid passage being formed by an interplate gap (40) defined between each pair of adjacent filter plates (36), wherein the pitch of the thread of the rotatable screw (20) gradually decreases toward the discharge end of the screw press (14); The filter plates (36) are characterized by being continuously held in a direct, tight face-to-face contact relative to each other by a clamping assembly (38) operable to apply a predetermined axial clamping pressure substantially uniformly around the core passage, wherein the filter plates (36) have a predetermined surface roughness (R), the inter-plate gap (40) being a function of the predetermined surface roughness (R) and the axial clamping pressure, wherein an intermediate support plate (52) is inserted between two filter plates in the filter plates (36), wherein the intermediate support plate (52) is inserted at predetermined intervals along the axial length of the filter section (28), thereby providing uniform support and preventing a set of filter plates (36) from deforming under the clamping pressure applied thereon. The rotatable screw (20) includes at least two screw sections (20a, 20b) connected to each other by a coupling (62) at the outer diameter of the thread (34) of the rotatable screw (20) such that at the connection of the at least two screw sections (20a, 20b), the volume between adjacent turns of the thread (34) is not reduced due to the coupling (62).

2. The screw press (14) according to claim 1, characterized in that, The inlet section (24) allows for continuous fluid communication with the overall tubular body (18).

3. The screw press (14) according to claim 1, characterized in that, A pressurized feeder is operatively connected to a solid-liquid mixture source to supply power at 3.4 N / mm. 2 Up to 14 N / mm 2 The overall tubular body (18) of the screw press is forcibly supplied with pressure within a certain range.

4. The screw press (14) according to claim 1, characterized in that, The surface roughness (R) of the filter plate (36) generally corresponds to the average of the highest and lowest points on the mating surfaces of the filter plate (36), and the surface roughness (R) is between 1 micrometer and 30 micrometers.

5. The screw press (14) according to claim 1, characterized in that, The interplate gap (40) is between 1 and 60 micrometers.

6. The screw press (14) according to claim 1, characterized in that, The clamping pressure applied to each filter plate (36) is 1.4 N / mm. 2 Up to 3.5 N / mm 2 between.

7. The screw press (14) according to claim 1, characterized in that, The clamping assembly (38) includes a first clamping plate (42) and a second clamping plate (44) respectively disposed at opposite ends of the filter section (28), the filter plate (36) being arranged between the first clamping plate (42) and the second clamping plate (44), each of the first clamping plate (42) and the second clamping plate (44) having a load distribution portion (42a, 44a) that is in uniform support contact with one of the adjacent filter plates (36), and the first clamping plate (42) and the second clamping plate (44) being thicker than each of the filter plates (36).

8. The screw press (14) according to claim 7, characterized in that, The clamping assembly (38) also includes at least four axially extending support bolts (48) evenly distributed circumferentially around the core passage, the at least four support bolts (48) extending through registration mounting holes defined in the filter plate (36), the first clamping plate (42), and the second clamping plate (44), the clamping assembly (38) also includes nuts (50) threaded into the opposite distal ends of the at least four support bolts (48) and all tightened with the same predetermined torque.

9. The screw press (14) according to claim 7, characterized in that, The load distribution portion (42a, 44a) is configured as a ring protruding from an inner surface of each of the first clamping plate (42) and the second clamping plate (44), the ring surrounding the core passage.

10. The screw press (14) according to claim 1, characterized in that, The intermediate support plate (52) is thicker than each of the filter plates (36).

11. The screw press (14) according to claim 8, characterized in that, In 56 Up to 560 A calibrated tightening torque is applied to each of the nuts (50).

12. The screw press (14) according to claim 1, characterized in that, The filter plate (36) has a filtration height that extends radially from the inner diameter of the filter plate (36) to the peripheral edge of the filter plate (36), and the filter plate (36) is in close face-to-face contact along the entire range of the filtration height.

13. The screw press (14) according to claim 1, characterized in that, The interplate gap (40) is on the same order of magnitude as the average size of the solid particles contained in the solid-liquid mixture to be processed.

14. The screw press (14) according to claim 1, characterized in that, The filter plate (36) is made of a material selected from the group consisting of: stainless steel, black steel, steel with painted finish, and ceramic.

15. The screw press (14) according to claim 1, characterized in that, The filter plate (36) has a bead-sprayed surface treatment.

16. The screw press (14) according to claim 1, characterized in that, The filter plate (36) is coated with a mixture of paint and particles.

17. The screw press (14) according to claim 1, characterized in that, A positive displacement pump (12) is operatively connected to a solid-liquid mixture source to operate at 4 N / mm. 2 Up to 10 N / mm 2 The pressure between them is forcibly supplied to the overall tubular body (18) of the screw press.

18. The screw press (14) according to claim 1, characterized in that, The surface roughness (R) of the filter plate (36) generally corresponds to the average of the highest and lowest points on the mating surfaces of the filter plate (36), and the surface roughness (R) is between 2 micrometers and 10 micrometers.

19. The screw press (14) according to claim 1, characterized in that, The surface roughness (R) of the filter plate (36) generally corresponds to the average of the highest and lowest points on the mating surfaces of the filter plate (36), and the surface roughness (R) is between 1.4 micrometers and 3.5 micrometers.

20. The screw press (14) according to claim 1, characterized in that, The interplate gap (40) is between 2 and 20 micrometers.

21. The screw press (14) according to claim 1, characterized in that, The clamping pressure applied to each filter plate (36) is 2 N / mm. 2 Up to 2.8 N / mm 2 between.

22. The screw press (14) according to claim 7, characterized in that, The clamping assembly (38) also includes six axially extending support bolts (48) evenly distributed circumferentially around the core passage, the support bolts (48) extending through registration mounting holes defined in the filter plate (36), the first clamping plate (42), and the second clamping plate (44), the clamping assembly (38) also includes nuts (50) threaded into the opposite distal ends of the support bolts (48) and all tightened with the same predetermined torque.

23. The screw press (14) according to claim 1, characterized in that, The gap (40) between the plates is smaller than the average size of the solid particles contained in the solid-liquid mixture to be processed.

24. A method for separating a liquid from a solid-liquid mixture using a screw press (14) according to any one of claims 1-23, said screw press (14) comprising an overall tubular body (18) having axially spaced inlet section (24) and outlet section (26) and a filter section (28) between said inlet section (24) and said outlet section (26), said filter section comprising an axially extending set of coplanar filter plates (36) defining a core passage and an interplate gap (40) between each pair of adjacent filter plates (36), the method characterized in that it comprises: A filter plate (36) with a predetermined surface roughness (R) and an intermediate support plate (52) are inserted between two filter plates in the filter plate (36), wherein the intermediate support plate (52) is inserted at predetermined intervals along the axial length of the filter section (28), thereby providing uniform support and preventing a set of filter plates (36) from deforming under the clamping pressure applied thereon. A predetermined axial clamping pressure is applied substantially uniformly around the core passage using a clamping assembly (38) to continuously maintain the filter plates (36) fixed relative to each other and clamped in a direct and tight face-to-face contact manner by a clamping assembly (38), the gap (40) between the plates being a function of the predetermined surface roughness (R) and the axial clamping pressure. A solid-liquid mixture is introduced through this inlet section (24); The solid-liquid mixture is conveyed from the inlet section (24) to the outlet section (26) using a rotatable screw (20) mounted in the overall tubular body (18) and extending axially through the core passage, while simultaneously compressing and dehydrating the solid-liquid mixture by forcing at least a portion of the liquid component of the mixture through a liquid passage formed by the interplate gap (40), wherein the thread pitch of the rotatable screw (20) gradually decreases toward the discharge end of the screw press (14); and A dehydrated mixture is produced at the outlet section (26).

25. The method according to claim 24, characterized in that, The solid-liquid mixture is continuously introduced through the inlet section (24).

26. The method according to claim 25, characterized in that, The clamping pressure applied to each filter plate is 1.4 N / mm. 2 Up to 3.5 N / mm 2 between.

27. The method according to claim 24, characterized in that, The filter plate (36) has a predetermined surface roughness (R) on the entire surface of both sides.

28. The method according to claim 24, characterized in that, This solid-liquid mixture is a residue produced by the Bayer process during the production of alumina from bauxite.

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