Patent Description:
Safe treatment and storage of high volume industrial waste streams, such as Bauxite residue (red mud) from aluminum production, pose several waste management challenges. Land disposal of these materials often has negative environmental impacts such as contamination of soil and groundwater, and consumes vast areas of land, and, thus, result in substantial costs for the alumina producing industry. Since the production of red mud in the Bayer process is inevitable, the industry has focused on minimizing the volume of red mud to be disposed by extracting as much as possible liquid from the mud. Current processes particularly rely on decantation in large gravity settlers. However, such mud compacting processes require several hours to obtain a solid concentration of not more than about <NUM> and <NUM> %.

Other known slurry filtration apparatuses are not well adapted for red mud filtration applications and the like.

For example, the <CIT> describes a filtration apparatus in accordance with the preamble of claim <NUM>, that comprises a plurality of discs superimposed one upon another, the arrangement being such that fluid can percolate between the faces of discs, a passageway passing through the assembly of discs transversely with respect to planes thereof, an intermittently opening inlet at one part of said passageway for fluid to be filtered, a discharge for separated-out solids and/or semi-solids at another part of said passageway and means for feeding agglomerated solids and/or semi-solids to said discharge, the latter being arranged so that the solids and/ or semi-solids are compressed and discharged in compressed form.

In the above-mentioned filtration apparatus, the filter discs are maintained in a state of predetermined (but adjustable during operation) compression by a spring, in order to maintain a constant flow of filtrate. The spring-loading of the filter discs allows the forcing of the discs apart during the passage of some solids. The operation of the above-mentioned filtration apparatus therefore relies on the intermittently opening inlet for reversing the direction of the flow, by closing the inlet for clearing said solids, before resuming filtration by opening of the inlet again. This intermittent opening inlet and the resulting movements of the filter discs make the above-mentioned filtration apparatus more complex and more liable to blockage. Small particles, such as the one contained in red mud slurry, could remain stuck between filter discs, thereby creating preferential passages between filter discs.

In another example, <CIT> discloses a solid/liquid separation apparatus including an extruder press combined with a solid/fluid separation module for separating fluid from a mass of solids compressed by the extruder at elevated pressures. The solid/fluid separating apparatus includes a filter unit comprising filter plates that are sandwiched between two backer plates. During the separation process, liquid percolates through slots the filter plates
In another example, <CIT> discloses a modular screw device for processing fibre chip. The device includes at least one processor housing component or discharge housing component that can be altered to adjust the compression or drainage characteristics of the device, and a drainage housing comprising annular rings rigidly stacked together in which the material undergoes high compression while draining.

In another example, JPS59 <NUM> discloses screw press dewatering machine including filtration means comprising fixing plates that are separated by the sliding plates. The fixing plates and the sliding plates are provided eccentrically with respect to each other. This allows circular movement of the sliding plates 10a, 10b to ensure that fixing plates 9a, 9b and sliding plates 10a, 10b are in contact with each other at a single point. During filtration, the gap between the fixing plates and the sliding plates is filled with the deposited solid content and the sliding plates can slide to allow moisture to flow out while retaining solid matter between the filtration plates.

In another example, <CIT> discloses a filter comprising a stack of flexible metal sheets, each having a rough surface and a plurality of spaced apart projections and a housing and means to compress said sheets in said housing to provide a rigid pile so as to enable the filter to retain particles in a fluid flowing between said sheets. The above-mentioned filtration apparatus would not allow to reach an acceptable level of performance, for instance with respect of the solid concentration of the compacted slurry, and would not be adapted for a viable industrial filtration of red mud and the like, requiring the treatment of large quantity of slurry of solid in an acceptable time. In addition, the adjustable spring compression system would not be functional with the high pressures required for filtering slurries, such as red mud slurry.

Accordingly, there is a need to provide new solid-liquid separation equipment which addresses the above mentioned issues.

It is therefore an object to increase the solid fraction of compacted slurry, and to provide a reliable industrial filtration apparatus that can treat large quantity of slurry.

It is another object to provide an industrial filtration apparatus that can treat slurry having very fine particles, that is to say slurry having particles of less than <NUM> microns. More particularly, the industrial filtration apparatus can treat slurry having at least <NUM> %, preferably at least <NUM> %, more preferably at least <NUM> %, and up to <NUM> %, of particles of less than <NUM> microns, such as red mud slurry.

It is another object to provide an industrial filtration apparatus that can treat large quantity slurry at high pressure.

In accordance with a general aspect of the present invention, there is provided a screw press in accordance with claim <NUM>.

In accordance with another general aspect of the present invention, there is provided a process for separating liquid from a solid-liquid mixture in accordance with the features of claim <NUM>.

Reference is now made to the accompanying figures, in which:.

<FIG> is representative of an application to which the principles of the present invention may be applied. More particularly, <FIG> illustrates a pressure filtration installation for removing liquid from a solid-liquid mixture. According to one application, the pressure filtration installation is particularly well adapted for dewatering red mud (the residue of Bayer process in the production of alumina from Bauxite ore). However, it is understood that the pressure filtration installation could be configured and use for dehydrating various types of slurry and is, thus, not strictly limited to red mud dewatering applications.

As can be appreciated from <FIG>, the installation generally comprises a reservoir <NUM> containing the red mud or slurry to be dehydrated, feeding means, such as a positive displacement pump <NUM>, operatively connected to the reservoir <NUM> for feeding the slurry under pressure to a screw press <NUM>, and a valve <NUM> for regulating the flow of dehydrated mud at a discharged end of the screw press <NUM>.

The positive displacement pump is usually combined with means for allowing the delivery of a substantially constant flow rate of slurry at a substantially constant inlet pressure. The input flow rate of slurry can be controlled by the stroke speed of a positive displacement pump. The inlet pressure and flow rate can be maintained during cycle/piston changeover of the positive displacement pump, by a non-return valve (or a check valve) to prevent reverse flow, and by a pressurized dampener, such as a reservoir, to supply slurry during the changeover.

As shown in <FIG>, the screw press <NUM> generally comprises a filter casing having a generally tubular body <NUM> for surrounding a screw <NUM> adapted to be rotatably mounted within the tubular body <NUM>. A motor (not shown) is mounted on a platform <NUM> adjacent to the filter casing for driving the screw <NUM> via a suitable transmission arrangement, such as a belt transmission or a direct drive (not shown). In operation, the screw <NUM> applies a longitudinal pressure gradient on the solid-liquid mixture to be dewatered. The pressure of the fed slurry or the slurry supply pressure, for instance at the outlet of the displacement pump <NUM>, causes the liquid to be squeezed out from the mixture and out of the screw press casing as schematically depicted in <FIG>. In addition to the slurry supply pressure, the action of the screw <NUM> on the solid-liquid mixture also causes the liquid to be squeezed out from the mixture and out of the screw press casing. The (size of the) opening of the outlet valve can be continuously altered to simultaneously maintain the required pressure within the apparatus and to control the outlet flow rate of the dewatered slurry.

As best shown in <FIG>, the screw <NUM> generally comprises a shaft <NUM> and a continuous flight <NUM> extending helically around a smooth outer surface of the shaft <NUM>. The screw flight <NUM> has a constant outer flight diameter, which is slightly less than an inner diameter of the tubular body of the filter casing by a predetermined flight clearance. According to one embodiment of the present invention, the outer diameter of the shaft <NUM> is constant along all the length of the screw <NUM>. Still according to this embodiment, the pitch (see P1 and P2 on <FIG>) of the flight <NUM> gradually decreases towards the discharge end of the screw press (i.e. in a downstream direction). As a result, the volume between adjacent turns of the screw flight <NUM> decreases progressively towards the discharge end of the screw press <NUM>, thereby gradually increasing the pressure on the solid-liquid mixture and promoting solid-liquid separation.

As shown in <FIG>, the tubular body <NUM> of the filter casing has axially opposed inlet and outlet sections <NUM>, <NUM>, and a filter section <NUM> between the inlet and outlet sections <NUM>, <NUM>. As schematically illustrated in <FIG>, the filter section <NUM> has fluid passages for allowing liquid to be evacuated out of the filter casing as the solid-liquid mixture is being conveyed from the inlet section <NUM> to the outlet section <NUM> by the screw <NUM>. The inlet section <NUM> is operatively connectable in flow communication to the output side of the positive displacement pump <NUM> for receiving a continuous feed of the slurry at a predetermined pressure. The inlet section <NUM> is preferably designed for maintaining continuous fluid communication with the tubular body <NUM>. Satisfactory results have been obtained by force-feeding the screw press <NUM> at a pressure preferably ranging from about <NUM> N/mm<NUM> (approximately <NUM> psi) to about <NUM> N/mm<NUM> (approximately <NUM> psi), and more preferably between about <NUM>-<NUM> N/mm<NUM> (approximately <NUM>-<NUM> psi). It is understood that the feeding pressure may change depending on the size of the screw press <NUM>. The outlet section <NUM> may have a conical passage section operatively connectable to the valve <NUM> to regulate the flow of dehydrated sludge coming out from the screw press and to maintain the desired filtering pressure inside the filter casing.

The inlet and outlet sections <NUM>, <NUM> each include upper and lower half-shell members 24a, 24b; 26a, 26b adapted to be bolted to one another to form a complete cylindrical casing section. Bushings or the like (not shown) may be provided in the inlet and outlet sections <NUM>, <NUM> on the inner surfaces of the half-shell members 24a, 24b; 26a, 26b to rotatably support the axially opposed ends of the screw <NUM>.

Referring to <FIG> and <FIG>, it can be appreciated that the filter section <NUM> generally comprises a plurality of stationary filtration plates <NUM> axially clamped to be continuously maintained in intimate face-to-face contact by a clamping assembly <NUM> operable for applying a predetermined axially clamping pressure substantially uniformly about an inner diameter of the plates <NUM>. The predetermined axial clamping pressure is maintained constant. Each filtration plate <NUM> may be provided in the form of a flat disc defining a central hole <NUM>. Once assembled, the central holes <NUM> of the plates <NUM> are axially aligned to jointly form an axially extending core passage for receiving the screw <NUM>. Since the filtration plates <NUM> are continuously maintained in intimate face-to-face contact, there is no risk that some of the discs be forced apart, which would create preferential passages and results in intermittent decrease of the slurry pressure inside the core passage below the pressure at the outlet of the displacement pump <NUM>. Consequently, there is no risk that small particles, such as the one contained in red mud slurry, could remain stuck between filter discs. It is a significant advantage over the filtration apparatus of the prior art to be able to maintain the slurry pressure inside the screw press of the invention at a relatively constant value. The screw press of the present invention is consequently preferably operated in steady state most of the time.

As will be seen hereinafter, the clamping pressure and the surface roughness of the plates <NUM> are selected to provide for the formation of a predetermined "micro" inter-plate gap <NUM> (<FIG>) between each pair of adjacent plates <NUM>. The inter-plate gap <NUM> is selected to be sufficiently large to allow the liquid, which has been squeezed out by the screw <NUM>, to percolate between the plates <NUM>, while being sufficiently small to prevent the passage of the solid particles, thereby allowing for the formation of a cake of dehydrated mud on the inner diameter of the filter section <NUM>. Once formed, the solid particle cake contribute to maintain the pressure inside the filter section <NUM> despite the presence of the inter-plate gaps <NUM> (i.e. it limits pressure escape through the inter-plate gaps <NUM>). The thickness of the solid particle cake is maintained by the screw, which also acts to trim said cake. Depending on the solid-liquid mixture to be dehydrated, the inter-plate gaps <NUM> may range from about <NUM> to about <NUM>, and preferably from about <NUM> microns to about <NUM> microns. For red mud dewatering applications, the inter-plate gap <NUM> is preferably from about <NUM> microns to about <NUM> microns and more preferably from about <NUM> microns to about <NUM> microns. It can be generally said that the inter-plate gaps <NUM> are selected to be smaller or in the same order of magnitude than a medium size value of the solid particles contained in the solid-liquid mixture to be processed and sufficiently large to allow liquid percolation.

As mentioned herein above and as schematically illustrated in <FIG>, each inter-plate gap <NUM> is function of the surface roughness of the plates <NUM>. The surface roughness (R) of the filtration plates <NUM> is defined as the average peak height of the asperities at the surface of the filtration plates <NUM>. When the plates are clamped together, the peaks extending from the opposing faces of the plates <NUM> prevent the plates from mating in perfect face-to-face sealing engagement, thereby resulting in the formation of micro-passages extending from the inner diameter of the plates <NUM> to the outer peripheral edge thereof. Depending on the solid-liquid mixture to be dehydrated, filtration plates having a surface roughness ranging from about <NUM> micron to about <NUM> microns are being used. Tests have shown that the optimum range of surface roughness for red mud filtration applications is between about <NUM> microns and about <NUM> microns. However, satisfactory results may also be obtained with surface roughness ranging from about <NUM> microns to about <NUM> microns.

By surface roughness of filtration plates, it is generally meant the surface roughness on the entire surface of both faces of each plate.

The liquid passages of the filter section <NUM> are formed by inter-plate gaps <NUM> defined between each pair of adjacent filtration plates <NUM>. The liquid passages extend from the inner diameter of the plates <NUM> to the outer peripheral edge thereof. The liquid passages surround the core passage defined by the axially extending stack of coplanar filtration plates <NUM>. The filtration plates <NUM> being maintained clamped continuously in direct intimate face-to-face contact, the resulting liquid passages, which extend from the inner diameter of the plates <NUM> to the outer peripheral edge thereof, are uniformly distributed around the core passage, thereby preventing the creation of preferential passages.

In other words, the filtration plates <NUM> are maintained clamped continuously in direct intimate face-to-face contact, on a surface that extends from the inner diameter of the plates <NUM> to the outer peripheral edge thereof, so that the resulting liquid passages are uniformly distributed around the core passage, thereby preventing the creation of preferential passages.

Tactile or optical roughness depth measuring equipment is used to ensure that the plates <NUM> have the desired surface roughness. Preferably, the plate surface roughness is measured using a contact-type instrument having a stylus adapted to be placed in direct contact with the surface of each of the filtration plates <NUM>. As the stylus traces across a plate, it rises and falls together with the roughness on the plate surface. This movement in the stylus is picked up and used to measure surface roughness.

The filtration plates <NUM> may be made out of a wide variety of materials, including, for instance: stainless steel, black steel, steel with a baked paint finish, and ceramic. It has been observed that a baked paint finish allows improving the permeability of the filter section <NUM> while offering a good protection against abrasion and corrosion. The selected material must be able to sustain corrosive environments, stable at the operating temperatures (e.g. <NUM> <NUM>C), and strong enough not to collapse or be subject to compression/deformation over the entire range of clamping pressures applied by the clamping assembly <NUM>. The plate material is also selected so that the fluid flow resistance through the inter-plate gaps <NUM> is inferior to the resistance of the solid particle cake formed on the inner diameter of the plates <NUM>. In other words, the fluid flow resistance of the filtration plates <NUM> is selected so that it is less limitative than that of the solid particle cake. It is noted that different materials with different surface roughness may be used to obtain similar liquid flow resistances between the filtration plates <NUM>. For instance, it has been found, while conducting red mud dewatering experimentations, that stainless steel plates with a <NUM> surface roughness and steel plates with a baked paint finish and a surface roughness of <NUM> offer similar liquid flow resistances.

The liquid flow resistance through the inter-plate gaps <NUM> is also function of the filtration height which corresponds to the distance along which the plates <NUM> are urged in intimate face-to-face contact between their inner diameter and their outer peripheral edge. The greater the filtration height, the greater the flow resistance through the plates will be. The filtration plates <NUM> being maintained clamped, at all time, or continuously, in direct intimate face-to-face contact, the resulting liquid passages extend over the whole filtration height, thereby preventing the creation of preferential passages.

The filtration plates <NUM> may be subject to various surface treatments to obtain the desired surface roughness and liquid flow resistance. For instance, the plates <NUM> may be subjected to a bead blasting surface treatment. Glass beads surface treatment is preferred over sand blasting surface treatment. Sand blasting is more abrasive and results in greater surface roughness values than those obtained with glass bead blasting.

Various coatings may be applied to the filtration plates <NUM> to protect them against corrosion, to change their hydrophobic or hydrophilic properties and/or to alter their surface roughness. For instance, a mixture of paint and particles could be applied over the plates <NUM> to protect them against corrosion and to obtain a desired surface roughness.

As mentioned herein before, the inter-plate gaps <NUM> are also function of the clamping pressure applied on the filtration plates <NUM>. The clamping pressure must be applied as uniformly as possible about the core passage defined by the filtration plates <NUM> in order to avoid leakage. Referring to <FIG> and <FIG>, it can be appreciated that the clamping assembly <NUM> generally comprises first and second clamping plates <NUM>, <NUM> respectively provided at opposed ends of the filter section <NUM> with the filtration plates <NUM> disposed therebetween. Each of the first and second clamping plates <NUM>, <NUM> has a load distribution portion, which may take the form of a ring or cylindrical projection 42a, 44a extending from one face thereof, for entering in uniform bearing contact with an adjacent one of the filtration plates <NUM> concentrically about the central hole <NUM> thereof. Uniformly circumferentially distributed ear sections or eyelet projection <NUM> extend radially outwardly from the filtration plates <NUM> and the cylindrical projections 42a, 44a of the clamping plates <NUM>, <NUM> for engagement with axially extending stay bolts <NUM>. It is understood that the mounting holes defined by the eyelet projections could be otherwise provided. For instance, mounting holes could be defined directly in the filtering ring surface of the plates <NUM>. At least four, preferably six, sets of eyelets and stay bolts are circumferentially distributed about the core passage defined by the filtration plates <NUM>. Nuts <NUM> are threadably engaged at opposed distal ends of the stay bolts <NUM> to axially clamp the stack of filtration plates <NUM> between the clamping plates <NUM>, <NUM>. The nuts <NUM> are tighten at a same predetermined torque. A suitable tool, such as a torque wrench, is used to ensure that the exact same torque is applied at each nut <NUM>. According to one application of the present invention, a calibrated tightening torque comprised between about <NUM> N-m (approximately <NUM> Ibf-in) and about <NUM> N-m (approximately <NUM> Ibf-in) is applied on each of the nuts <NUM>. The required torque increases with the size (length and diameter) of the screw press <NUM>. For instance, for a screw having a diameter of about <NUM>, the clamping torque could be about <NUM> N-m (approximately <NUM> Ibf-in); whereas for a diameter of <NUM>, the clamping torque could be in the vicinity of <NUM> N-m (approximately <NUM> Ibf-in). The thickness of the clamping plates <NUM>, <NUM>, including the cylindrical projections 42a, 44a and the eyelets <NUM> or ear sections, is selected to avoid any deformation under such tightening conditions. This is why the clamping plates <NUM>, <NUM> are much thicker than the filtration plates <NUM>. This allows to ensure uniform pressure distribution on the plates between adjacent nuts <NUM> and, thus, about the circumference of the central hole <NUM> of the filtration plates <NUM>. It is understood that the value of the torque will vary depending on the size/geometry of the filtration plates <NUM>. The torque is selected to generally correspond to a clamping pressure of between about <NUM> N/mm<NUM> (approximately <NUM> psi) and about <NUM> N/mm<NUM> (approximately <NUM> psi), and preferably between about <NUM> N/mm<NUM> (approximately <NUM> psi) and about <NUM> N/mm<NUM> (approximately <NUM> psi) on each of the filtration plates <NUM>.

The clamping pressure applied on the filtration plates <NUM> is such that the filtration plates are maintained clamped, at all time, or continuously, in direct intimate face-to-face contact.

As shown in <FIG> and <FIG>, at least one intermediate support plate <NUM> (three in the illustrated example) is interposed between two adjacent filtration plates <NUM>. The number of support plates <NUM> will vary depending on the axial length of the filter section <NUM>. The supports plates <NUM> are inserted at predetermined intervals along the axial length of the filter section <NUM> to provide uniform support and prevent deformation of the stack of filtration plates <NUM> under the clamping forces applied thereon by the clamping plates <NUM>, <NUM>. The support plate <NUM> contributes to solidify the plate assembly while providing a bottom mounting interface or foot <NUM> for fastening the filter section <NUM> to an underlying frame structure <NUM> (<FIG>). Also, the intermediate support plate <NUM> may be provided at an upper end thereof with a pair of ear projections <NUM> for facilitating handling and transportation of the assembled filter section <NUM>. Mounting holes are also defined in the intermediate support plate for engagement on the stay bolts <NUM>. The intermediate support plate <NUM> is thicker than the filtration plates <NUM>. It offers a stable and uniform bearing surface for the adjacent filtration plates <NUM> and, thus, contributes to maintain a uniform clamping pressure across the whole filtration plate assembly. Like the filtration plates <NUM>, each intermediate support plate <NUM> has a central hole <NUM> defining a portion of the core passage of the filter section <NUM>. The intermediate support plate <NUM> typically has the same surface roughness as the filtration plates <NUM>. Accordingly, the filtration gaps on opposed sides of each intermediate support plate <NUM> are similar to inter-plate gaps <NUM> between adjacent filtration plates <NUM>.

The above described embodiment of the pressure filtration installation allows improving the compaction of the solid-liquid mixture. That is more liquid can be extracted from the mixture. For red mud dewatering applications, tests have shown that the dehydrated mud may be <NUM>% to <NUM>%, and sometime up to <NUM>% solid in terms of weight at its exit from the outlet section <NUM> of the screw press <NUM>. For calcium fluoride (CaF2) dewatering applications, tests have shown that the dehydrated mud may be up to <NUM>% solid in terms of weight at its exit from the outlet section <NUM> of the screw press <NUM>. For iron tailing dewatering applications, tests have shown that the dehydrated mud may be up to <NUM>% solid in terms of weight at its exit from the outlet section <NUM> of the screw press <NUM>. This is an improvement of about <NUM>% over conventional red mud gravity decanting processes. It can generally be said that the pressure filtration apparatus allows to increase the solid fraction of compacted slurry discharged from the outlet section of a screw press, while maximizing the solid-liquid separation rate.

As can be appreciated from <FIG>, the tubular body <NUM> and the screw <NUM> can be of modular construction. According to the illustrated example, the tubular body <NUM> has first and second serially interconnectable filter sections 28a; 28b and the screw <NUM> has corresponding first and second serially interconnectable screw sections 20a, 20b adapted to be respectively mounted in the first and second filter sections 28a, 28b for joint rotation as a unitary component. However, it is understood that the tubular body <NUM> and the screw <NUM> could comprise more than two sections.

The first and second screw sections 20a, 20b are joined together so as to have a continuous screw flight with no discontinuities between the sections 20a, 20b and to ensure that the volume between adjacent turns of the flight <NUM> at the junction of the two screw sections 20a, 20b is not reduced by the coupling <NUM>. As shown in <FIG>, the screw sections 20a, 20b are detachably coupled to one another by an external coupling <NUM> provided at the outside diameter of the flight <NUM>. Typically, screw sections are coupled via their shafts. Such shaft coupling arrangements may in some instances require that the shafts be reinforcement at their junction, thereby resulting in a reduction of the slurry compression volume between adjacent flight turns at the transition from one shaft section to the next. Accordingly, in order not to be intrusive, it is herein proposed to couple the shaft exteriorly from the volume defined between adjacent turns of the flight, thus maintaining the cross sectional area the slurry passes through, which minimises flow restrictions and reduces the likelihood of blockage.

The coupling <NUM> generally comprises a first coupling member 62a mounted to a first screw flight section 34a at a distal end of the first screw section 20a, and a second coupling member 62b mounted to a second screw flight section 34b at an adjacent end of the second screw section 20b. The first and second coupling members 62a, 62b are detachably fasteneable to one another, such as by bolting.

The first and second coupling members 62a, 62b may comprise semi-cylindrical plates or ring segments mounted to the outside diameter surface of the screw flight sections 34a, 34b, respectively. Each of the screw flight sections 34a, 34b may be provided in the form of a half-flight segment. The inboard end of the semi-cylindrical plates may be integrally provided with a frusto-conical section 63a, 63b adapted to be interconnected to the associated screw shaft sections 32a, 32b via strut-like members <NUM>. When interconnected, the semi-cylindrical plates form a complete support ring about the first and second screw flight sections 34a, 34b, the support ring having an inside diameter corresponding to the outside diameter of the flight <NUM>. Therefore, the coupling <NUM> does not reduce the volume between flight sections 34a, 34b. The semi-cylindrical plates may be welded on an inner surface thereof to the outside diameter surface of the first and second screw flight sections 34a, 34b. The coupling member 62a, the screw flight section 34a and associated struts <NUM> are preferably mounted as a pre-assembled unit to screw shaft section 32a. Likewise, the coupling member 62b, the screw flight section 34b and associated struts <NUM> are preferably mounted as a pre-assembled unit to screw shaft section 32b. According to an embodiment of the present invention, the first and second flight sections 34a, 34b are welded to the inner surface of the coupling members 62a, 62b and then the pre-assembled coupling and flight assemblies are subject to a heat treatment process in order to improve the mechanical properties of the coupling assembly. Thereafter, the heat treated coupling and screw flight united pieces are mounted to respective screw shaft sections 32a, 32b by welding the screw flight sections 34a, 34b to the outer surface of the shaft and to the end of the existing flight on respective shaft sections 32a, and 32b. The screw flight sections 34a, 34b are welded to extend in continuity to the flight already present on the shaft sections 32a, 32b. The struts <NUM> are also welded to the screw shaft sections 32a, 32b.

By welding screw flight sections 34a, 34b to the coupling members 62a, 62b prior to the heat treatment process and by then connecting the coupling members 62a, 62b to the screw shaft sections 32a, 32b, the structural integrity of the coupling members 62a, 62b can be preserved. Indeed, welding the coupling members 62a, 62b directly to flights on the shaft sections 32a, 32b could potentially negatively affect the mechanical properties of the coupling members 62a, 62b.

Also as shown in <FIG>, wear plates 64a, 64b are removably mounted to the outer surface of each of the semi-cylindrical coupling members 62a, 62b for engagement with a corresponding segmented wear ring structure (not shown) mounted in a screw support section <NUM> (see <FIG>) disposed between the first and second filter sections 28a, 28b. Accordingly, the coupling <NUM> may also be used to provide an intermediate support to the screw <NUM> generally mid-way between the opposed ends thereof. The screw support section <NUM> may comprise upper and lower half-shell members 66a, 66b adapted to be detachably bolted to each other. This ensures ready access to the coupling <NUM>. The segmented wear ring structure (not shown) provided inside the screw support section <NUM> is configured to wear out prior to the wear plates 64a, 64b on the outer surface of the semi-cylindrical plates of the coupling members 62a, 62b. The upper and lower half-shell members 66a, 66b are provided at opposed end thereof with bolting flanges 68a, 68b for attachment with corresponding bolting flanges <NUM> provided on the clamping plates <NUM>, <NUM> of each filter sections 28a, 28b. In this way, each screw and associated filter section 20a, 28a; 20b; 28b can be readily removed as a unit or cartridge and replaced by a similar screw and filtration "cartridge" by simply unbolting flange <NUM> from flanges 68a, 68b, unbolting the top half-shell member 66a, unbolting the screw coupling members 62a and 62b and unbolting the flange <NUM> at the other end of the screw and filter section to be replaced. All the bolts, including the bolts used to secure the first and second screw coupling members 62a and 62b, are easily accessible.

Claim 1:
A screw press (<NUM>) for separating liquid from a slurry mixture, the screw press (<NUM>) comprising:
a generally tubular body (<NUM>) with an axially spaced-apart inlet section (<NUM>) and outlet section (<NUM>), and a filter section (<NUM>) between said inlet section (<NUM>) and outlet section (<NUM>); said filter section (<NUM>) including an axially extending stack of stationary coplanar filtration plates (<NUM>) defining a core passage; and
a rotatable screw (<NUM>) mounted in said tubular body (<NUM>) and extending axially through said core passage for conveying the slurry mixture from the inlet section (<NUM>) to the outlet section (<NUM>) while compressing and dewatering the slurry mixture by forcing at least part of the liquid of the slurry mixture to be expelled out of the tubular body (<NUM>) through liquid passages of said filter section surrounding the core passage, said liquid passages being formed by inter-plate gaps (<NUM>) defined between each pair of adjacent filtration plates (<NUM>);
characterized in that the stationary filtration plates (<NUM>) are continuously maintained fixed relative to one another and clamped in direct intimate face-to-face contact by a clamping assembly (<NUM>) operable for applying a constant axial clamping pressure substantially uniformly about said core passage, and in that the filtration plates (<NUM>) have a predetermined surface roughness (R), the inter-plate gaps (<NUM>) being function of said predetermined surface roughness (R) and said axially clamping pressure such that some of the discs cannot be forced apart to create preferential passages and result in intermittent decrease of the slurry pressure inside the core passage,
wherein the surface roughness of the filtration plates corresponds to an average of highest
and lowest points on mutually engaging surfaces of the filtration plates, said surface roughness being between <NUM> to <NUM> microns, and wherein the clamping assembly further includes first and second clamping plates (<NUM>, <NUM>) at opposite ends of the stack of filtration plates (<NUM>) and a plurality of stay bolts (<NUM>) threadably engaged to a nut (<NUM>) at opposed distal ends of the stay bolts to axially clamp the stack of filtration plates between_the first and second clamping plates.