EP0685325A2 - Device for separating the liquid portion from the solid portion in two-phase systems - Google Patents

Abstract

The separating device has a two-stage structure with a first filter section and a second adjoining filter section. The first filter section produces a stationary filter cake layer on the filter assembly with a clearly higher solid content than the slime conveyed in the worm channel (5) which extends from the material supply (6) along the worm axis. The second filter section is built up of filters which have a higher abrasion strength than the filters fitted in the first section. The filters in the first section pref. comprise several layers of sintered-together fine mesh screen fabric or porous sinter metal whilst the filters in the second section are formed from porous ceramic such as silicon carbide or silicon nitride. <IMAGE>

Description

The invention relates to a device for separating the liquid fraction from the solid fraction of finely disperse mineral sludges, in particular a ceramic slurry, with a screw and with cylindrical filter means surrounding the screw with radial spacing and arranged along the screw, which with the screw limit a conveying channel for the slip , which is fed via a material feed at one end of the filter screw press and, after being conveyed through the channels of the rotating screw at the other end, is discharged as a solid discharge via a mouthpiece, the liquid fraction flowing through the filter medium being discharged via a filtrate discharge.

A device according to the preamble of claim 1 is already known (EP-B 0 138 920), in which the filter means surrounding the screw are arranged in a housing. The filter media surfaces facing the screw of the filter media accommodated in the housing are covered with an abrasion-resistant protective grille for protection against abrasion. A stationary filter cake forms in the mesh of this grid, which represents a significant flow resistance for the filtrate drain. This resistance becomes smaller, the smaller the thickness of the grating and the smaller the area portion of the filter medium surface covered by the grating bars. For reasons of strength and stability, but also to fulfill the protective function, the thickness of the grid and the proportion of the area covered by the grid cannot be reduced arbitrarily, so that there are limits to a reduction in the resistance for the filtrate outflow. Apart from the disadvantageous flow resistance for the filtrate outflow caused by the protective grid, the use of the protective grid also results a structural effort of such a known filter screw press.

The object of the invention is to eliminate the disadvantage of the prior art, in particular to provide a device for separating the liquid portion from the solid portion, which is structurally simple and does not require additional protective grids for the filter medium.

This object is achieved according to the invention by the features contained in the characterizing part of claim 1, expedient developments of the invention being characterized by the features contained in the subclaims.

The invention is based on the knowledge that from the beginning to the middle of the screw channel the two-phase system is still liquid to soft plastic, so that a filter cake resting on the filter medium is formed in the gap between the outside diameter of the screw and the inside diameter of the filter cylinder, the solids content of which is higher than that of the medium in the screw channel, so that the filter media are protected against friction due to the resting filter cake. For this reason, the use of filter media that do not have to be designed for high abrasion resistance is sufficient in this section, or additional protective measures of the filter media by connecting protective grids and the like can be omitted. The invention takes this into account by the two-stage structure of the device, filter media with lower abrasion resistance being able to be used in the first filter medium section, whereas filter media which are characterized by a higher abrasion resistance are used in the second filter medium section adjoining it. Porous silicon carbide or silicon nitride cylinders are particularly suitable for this. It is expedient here that the filter means are accommodated in filter elements with a limited axial length, which are advantageously constructed identically. This offers the possibility of assembling the filter elements in a modular manner, by arranging filter elements of the first filter medium section and filter elements of the second filter medium section next to one another along the screw axis. The filter elements are sealed against each other by conventional seals.

The filter media of the first filter section are distinguished by the finest pores close to the filter media surface, the pore system expanding into the filter media. As a result, fine particles are largely retained on the surface of the filter medium, whereas fine particles entering the pore system can easily pass through the filter medium. This largely prevents blockages.

In the case of finely disperse mineral sludges, in particular in the case of ceramic slurry, a stationary filter cake layer adheres firmly to the surface of the filter medium in the first filter section in such a way that abrasion and primary clogging are kept away from the filter medium. In zone 2, i.e. the second filter section, the two-phase system is already concentrated to a disperse plastic material. Here there is direct contact between the disperse solid particles in the entire screw channel. The solid particles are apparently so firmly fixed in a solid framework that the finest solid particles do not penetrate into the filter medium even if the protective stationary cake layer no longer exists here. Due to this fact, the protective grids on the filter media originally provided according to EP-B 0 138 920 can be dispensed with, which would greatly increase the resistance to the outflow of the filtrate and would thereby reduce the performance very much. At the same time, the construction is considerably cheaper. Relatively inexpensive filter elements that are sensitive to abrasion can be used for the first filter section, whereas only abrasion-resistant abrasion elements are required in the second filter section.

This is very important for the economic and thus practical success of the device.

Fine-pored sintered sieve mesh and porous silicon carbide are examples of the filter media required in the respective sections. Of course, other suitable filter media can also be used.

The division according to the invention of the filtering screw cylinder into a number of self-supporting elements with a correspondingly smaller axial extension of the filter elements, in which the filter media are well fastened in undivided steel cylinders, is particularly advantageous, as a result of which the very high forces that occur during processing can be well absorbed. A closed housing, which accommodates the filter elements, is dispensable, rather a frame that defines the position of the filter elements and absorbs the torques that act on the individual filter elements is sufficient.

The filter elements are expediently designed to be self-supporting, the filter means either being welded into a solid steel cylinder or a steel cylinder being shrunk over the filter means, which is particularly the case when ceramic filter media are used. As a result of the self-supporting property of the filter elements, there is no need for an additional housing. After stringing together, the filter elements are only fixed by means of tie rods and otherwise secured against rotation on the outside by a frame. This results in an overall very simple, easy-to-assemble construction of a filter screw press.

Finally, a special rinsing process is also essential to clean both the screw channel and the filter elements after longer operating times. For this purpose, the snail channels through tap water are in a first work cycle rinsed, whereas in a second working cycle the filter elements are rinsed with changing flow direction, preferably individually with rinsing water under pressure. It is also possible to rinse the pore system of the filter elements with rinsing water intermittently against the flow direction of the filtrate without rinsing the screw channels beforehand.

• Figur 1 eine schematische Schnittansicht der Vorrichtung durch die Längsachse,
• Figuren 2 bis 5 Details eines Filterelements des ersten Abschnittes der Vorrichtung sowie
• Figuren 6 bis 9 entsprechende Detaildarstellungen eines Filterelements des zweiten Abschnittes der Vorrichtung.
• Figur 10 eine schematische Ansicht einer alternativen Ausführungsform der Vorrichtung.

A preferred embodiment of the invention is described below with reference to the drawing. Show in it

• FIG. 1 shows a schematic sectional view of the device through the longitudinal axis,
• Figures 2 to 5 details of a filter element of the first section of the device and
• Figures 6 to 9 corresponding detailed representations of a filter element of the second section of the device.
• Figure 10 is a schematic view of an alternative embodiment of the device.

The device according to FIG. 1 is formed by a filter screw press which has a conventional screw 1, which is arranged within a housing formed by filter elements 2 and 3 arranged next to one another and is rotatably mounted at 4. The screw 1, which is set in rotation by a drive (not shown), serves to convey ceramic slip which is transported through the screw channel formed between the screw 1 and the filter elements 2 and 3 for the purpose of separating the liquid portion from the solid portion. The slip is fed to the screw channel 5 via the material feed 6 at one end of the filter screw press and leaves the press via the mouthpiece 7 at the other end. At 8, a discharge opening is shown which serves as a bypass and is closer below is explained. The filter elements 2 of the first filter medium section extend over at least half of the part of the screw which is effective for the slurry conveyance. In the illustrated embodiment, four filter elements 2 are shown, to which three filter elements of the second filter medium section are connected. The cylindrical filter elements 2 and 3 arranged alongside one another along the screw axis are sealed next to one another by seals 9 of conventional design. The filter elements 2 and 3 are self-supporting, so that an additional housing for the filter screw press is not required.

Finally, the filter screw press comprises a flushing water supply line 10, via which, via a water pump 11 and a control valve 12, flushing water can be supplied to the individual filter elements with the interposition of valves V 1 to V _n . A further rinsing water circuit is formed by the water line 13 and the valve 14, which opens into the filter screw press at 15, and by the rinsing water line 15a and the control valve 16, the line 15a at the mouth end at 16a discharging the rinsing water from the screw channel. A sedimentation basin for the rinsing water is identified by 17, a line 18 also opening into the basin 17, which leads away from the bypass 8. Finally, in Figure 1 with 19 and 20 tie rods are shown schematically, which serve to fix the filter elements 2 and 3.

Figures 2 to 5 show the structure of the filter elements 2 of the first filter medium section. 2 shows the filter medium 21 of the first filter medium section, which is preferably formed from a plurality of layers of screen fabrics arranged one above the other, which are sintered together. This results in the porous system of the filter means 21, which is otherwise circular cylindrical. Instead of a screen mesh sintered together, the filter medium can also be formed from a porous sintered metal cylinder, in particular sintered steel.

The porous cylindrical filter media 22 of the second filter media section (FIG. 6) are formed from ceramic material, in particular silicon carbide or silicon nitride.

The sintered sieve fabrics or sintered steel cylinders for the first and the silicon carbide or silicon nitride cylinders for the second filter medium section are each received in a solid steel cylinder 23a or 23b, which is shown in FIGS. 3 and 7.

According to FIG. 3, channels 24 are arranged in the interior of the steel cylinder 23 at a distance of approximately 20 mm in the circumferential direction, which are connected to one another by one or more axially extending channels 25, which conduct the filtrate to outlet bores 26 in the steel cylinder 23a. Correspondingly, circumferential channels 24 are also provided in the steel cylinder 23b for receiving the filter medium 22 of the second filter medium section and are connected by longitudinal channels 25 in order to discharge the filtrate via the outlet bore 26.

The sintered sieve fabrics or sintered steel cylinders are welded into the steel cylinder 23a, as can be seen from FIG. 5. The silicon carbide or silicon nitride cylinders according to FIG. 6 are ground to the exact extent to form the filter element, so that the steel cylinder 23b can be thermally shrunk on. The composite filter element is shown in FIG. 9.

In the case of the filter element according to FIGS. 6 to 9, the tolerances of the dimensions are to be selected such that after the steel cylinder has cooled down and at a filtration pressure of 120 bar in the screw channel, the cylindrical filter medium (silicon carbide or silicon nitride) is on average free of stress in the circumferential direction. If the pressure in the screw channel or in Filter element assumes the value zero, the cylindrical filter medium gets circumferentially under high pressure through the shrunk-on steel jacket. The wall thickness of the cylindrical filter medium is chosen so that the silicon carbide or silicon nitride material is not overstressed. Since the materials mentioned can absorb high compressive stresses but only low tensile stresses, it is only possible to create filter elements from these materials that absorb high filtration pressures in the way shown. In this case, it is particularly also a question of the cylindrical design of the steel cylinder as a solid jacket, since when using half-shells it is not possible to ensure a sufficiently uniform pressure load on the circumference of the cylindrical filter element. This could break the filter elements.

The filter elements 2 and 3, as shown in Figures 2 to 5 and 6 to 9, are self-supporting and do not need to be supported from the outside by a special housing. They are only arranged on a corresponding frame, which is identified by 27 in FIGS. 4 and 8. This frame 27 has a recess 28 in which an extension of the steel cylinder engages, as a result of which the frame 27 absorbs the torque applied to the surface of the filter medium by the rotating screw on the inside.

The filter elements have a limited axial length, the length of the filter elements in the preferred embodiment being in the range from 100 to 200 mm. The division of the filter medium into individual filter elements not only results in a modular system, but the filter media made of screen mesh and sintered steel can also be welded to the surrounding steel jackets. This also ensures problem-free thermal shrinking of the steel jackets onto the filter media made of silicon carbide and silicon nitride. Even with the targeted ones described below This construction of the screw cylinder from individual filter elements 2 and 3 has proven itself in sections flushing the filter elements.

From the entry of the slip at 6 into the screw channel 5 to the middle of the screw channel (up to approx. 60% by weight dry matter), the two-phase system in the screw channel is still liquid to soft plastic. The filter elements 2 are arranged in this area. The screw is not yet effective (slightly falling pressure in this zone). A filter cake resting on the filter cylinder is formed in the gap between the outer diameter of the screw and the inner diameter of the filter cylinder and has a significantly higher solids content than the medium in the screw channel. This filter cake also has a significantly higher mechanical strength than the material in the screw channel. This resting filter cake layer is very important in the first section, since it keeps the friction away from the filter medium. This applies if a certain layer thickness is not undershot, which is approximately 0.5 to 1 mm based on test results. The resistance for the filtrate outflow in this first filter medium section lies essentially in this resting filter cake layer, because in the screw channel there is almost no direct contact between the solid particles (solid framework pressure 0) for reasons that cannot be explained here in detail. The resistance to the filtrate outflow cannot be reduced arbitrarily, since the minimum thickness of the resting filter cake layer cannot be undercut. But without the use of a protective grille that is unnecessary in the first filter medium section, it is in any case smaller than with a protective grille. In this zone, a few layers of fine-meshed sieve fabrics that have been sintered together or porous sintered steel cylinders are sufficient as filter media, since there is no risk of the filter media due to abrasion.

Appropriately, the filter media are made from sintered screen fabrics or sintered steel in known pore structures, the finest pores being located close to the filter media surface where the filter cake is formed. From there, the pore system expands into the respective filter medium. Fine particles are largely retained on the surface of the filter medium. The finest particles that penetrate the filter medium are very likely to completely pass through the widening channels of the filter medium and come off with the filtrate. This significantly reduces the possibility of constipation.

In the second immediately following filter medium section, the solids concentration in the screw channel has risen to over 60 percent by weight to such an extent that a clear distinction between the medium in the screw channel and the filter cake layer on the filter medium is no longer possible. The snail is clearly effective. Dewatering takes place here essentially by compression of the disperse plastic medium in the screw channel, the solid particles already touching one another. To avoid abrasion, a filter medium made of very abrasion-resistant silicon carbide or silicon nitride must be used, although a protective grille is not required. Due to the very rough surface structure of the silicon carbide, wall sliding is sometimes prevented, but wall sliding processes can no longer be completely excluded at the highest solid concentrations.

Over a longer period of operation, as occurs in practice, the filtrate flow gradually decreases, which is due to clogging processes. The fine pores of the filter medium and especially the stationary cake layer on the filter medium are gradually added over a longer period of operation by the finest solid particles also present in the screw channel in the two-phase system, so that the flow resistance for the filtrate flow increases. However, this blockage can be reversed by rinsing. The rinsing takes place in two steps, namely at the end of a long stationary operation of the filter screw press.

For the purpose of flushing, the mouthpiece 7 or the bypass 8 for the outflow of the plastic material at the mouth end of the screw channel is opened in order to greatly reduce the flow resistance for the outflow of the plastic material from the screw channel. The screw continues to rotate and the slip flow via the material feed 6 is also maintained. The rotating screw is thus capable of largely transporting the plastic material out of the screw channel through its conveying action, which takes place until slip slips through from the material feed 6 to the outlet at the mouthpiece 7 or bypass 8. Then the slip supply 6 is shut off and the water supply tap 14 is opened so that water is fed into the filter screw press. The water now flows through the screw channels and now, instead of slip, leaves the housing of the filter screw press via the mouthpiece 7 or the bypass 8. This effectively rinses the screw channel, which is largely cleaned of slip and plastic material. The rotation of the screw supports the rinsing process. The rinsing process is finished when slurry and residues of plastic material have been rinsed out of the screw channel.

It is then expedient to rinse the filter elements. For this purpose, the connection to the water supply network at 14 and the outflow openings 7 and 8 are closed. Then rinse water is passed under higher pressure via automatically controlled valves 12 and 16 in alternating flow directions through the pore system of the filter medium. The change of direction is controlled in a predetermined time cycle by the solenoid valves 12 and 16. The Flushing water flow for the filter medium is conveyed by a piston pump 11, for example hydraulically driven. The discharge pressure can then be easily controlled hydraulically (10 to 30 bar). The flushing water flow would decrease after a short time in flushes with a constant flow direction, since the fine particles which clog the pore system and are first released from their positions by the flushing water flow, after a short distance, settle again at any angle of the pore system and lead to new blockages . This is avoided by a repeated reversal of the direction of the flushing water flow, which transports the fine particles out of very angled pore systems, which is especially true for filter media made of porous ceramic.

Expediently, only a limited section of the filtering screw cylinder is also always flushed in order to avoid that the flushing water flow flows over sections of the screw cylinder that have already been flushed out, while other sections which are still blocked remain unflushed. The individual filter elements 2 and 3 are therefore rinsed individually. This is accomplished in that only one of the valves V 1 to V _{n is} opened one after the other.

This rinsing process of the filter elements is interrupted a few times by rinsing the screw channel with tap water, as described above, in order to convey residues of the disperse plastic material which have still detached from the surface of the filter medium out of the screw channel.

In special cases, a simplified flushing method also proves its worth, which can be used during the ongoing operation of the screw press without first flushing the screw channels. Flushing water is passed through the pore system of the filter elements at a higher pressure than is present in the screw channel in the opposite direction to the filtrate flow. The flushing water flow must be switched on repeatedly for a limited time interval. As soon as the rinsing water flow is switched off for the subsequent time interval, a part of the rinsing water entered in the screw channel flows back through the pore system of the filter medium in the direction of the filtrate flow due to the high pressure in the screw channel, so that there is also a rinsing with changing flow direction. Again, it makes sense to rinse the filter elements one after the other.

It is particularly advantageous that the cycle times for the rinsing processes are selected to be of different lengths in the different directions. The result of this is that a particle in the direction of the longer cycle time statistically covers a larger distance than in the opposite direction and thus has a chance of being removed from the interior of the filter medium. With this procedure it is possible to remove the gradual clogging of the filter elements caused by the finest particles, which occurs during longer operating times.

The formation of individual filter elements together with the division of the filtering screw cylinder into two sections with different filter media are particularly important when it comes to designing the device for greater performance. The increase in the transport speed of the multiphase system in the screw channel and the flow rate of the liquid phase in the pore system, which is formed by the disperse solid phase, are subject to strict limits for fluidic reasons. An increase in performance of the device by extending the axial extent of the screw and by increasing the screw channel depth is therefore only possible to a very limited extent. Essentially, performance can only be achieved by increasing the diameter. Filter elements with screen mesh for the first filter section can be produced without major production problems and with reasonable Effort can be made with a larger diameter. In contrast, filter elements with porous ceramics can only be increased in diameter to a limited extent. Even with diameters of 400 mm, there are hardly any manageable manufacturing problems when processing by grinding and correspondingly high costs. To increase performance, the use of a plurality of filter screws in parallel with one another is proposed for this section. Such an embodiment is shown in FIG. 10. Here, the first filter section 29 forms a filter screw with a large diameter. In contrast, the second filter section 30 is formed by a plurality of filter screws with a smaller diameter working in parallel with respect to the first filter section 29. In the embodiment according to FIG. 10, four filter screws 31 working in parallel are provided. The diameter of the filter screws 31 of the second filter section 30 is expediently a quarter of the diameter of the filter screw 32 of the first filter section 29. The filter screws 31 feed the extrudate into a common mouthpiece 33, from which then only one strand 34 of plastic mass emerges. The opening cross section of the mouthpiece 33 is controlled in a known manner so that a constant predetermined pressure prevails in front of the mouthpiece. The common rotational frequency of the screws of the second filter section 30 is so matched to the rotational frequency of the screw 32 of the first section that in the transition from section 29 to section 30 the concentration is established in the two-phase system, which necessitates the change of the filter medium. Finally, in FIG. 10, 35 drive units of the individual filter screws 31 are designated.

Claims (20)

Device for separating the liquid portion from the solid portion of finely disperse mineral sludges, in particular a ceramic slip, with a screw (1) and with cylindrical filter means surrounding the screw and arranged along the screw axis, which with the screw (1) provide a delivery channel (5) for limit the slip which is fed via a material feed (6) at one end of the device to the screw (1) and, after being conveyed by the rotating screw at the other end, is discharged as solid discharge via a mouthpiece (7), the liquid portion flowing through the filter medium is discharged via channels (24, 25) leading to a filtrate discharge, characterized by a two-stage construction of the device with a first filter medium section for the construction of a resting filter cake layer on the filter medium (21) with a significantly higher solids content than that conveyed in the screw channel (5) Schlicker, the sic h extends from the material feed (6) along the screw axis, and with a second adjoining filter medium section, which is constructed from filter medium (22), which have a higher abrasion resistance than the filter medium (21) of the first filter medium section. Apparatus according to claim 1, characterized in that the filter means, preferably those of the first filter medium section (21), have very fine pores close to the filter medium surface facing the screw (1), from where the pore channels widen into the filter medium (21). Device according to claim 1 or 2, characterized in that the filter means (21) of the first filter means section from several layers of preferably sintered, fine-mesh sieve fabrics or from porous sintered metal and the filter media (22) of the second filter media section are formed from porous ceramic material, in particular from silicon carbide or silicon nitride. Device according to one of the preceding claims, characterized in that the first filter medium section extends from the slip inlet to at least the middle of the screw (1), preferably over a range in which the solids content of the slip is 42 to 60 percent by weight, preferably up to a maximum of 60 Weight percent, which is then followed by the second filter medium section. Device according to one of the preceding claims, characterized in that the two filter medium sections are each constructed from self-supporting filter elements (2, 3) arranged next to one another, each of which filter element consists of a solid steel cylinder (23) with a cylindrical filter medium (21) or (22) is formed. Apparatus according to claim 5, characterized in that the filter means (21) of the first filter medium section are welded in each case in the steel cylinder (23), and that the filter elements (3) of the second filter medium section by thermally shrinking the corresponding steel cylinder (23) onto the filter medium (22 ) are formed. Apparatus according to claim 6, characterized in that the tolerances of the dimensions are selected in the filter elements (3) of the second filter medium section so that after the cooling of the steel cylinder (23) and at a filtration pressure of about 120 bar in Screw channel (5), the cylindrical filter means (22) is stress-free in the circumferential direction on average. Device according to one of claims 5 to 7, characterized in that the filter elements (2, 3) have an axial extent in the range from 100 to 200 mm. Device according to one of claims 5 to 8, characterized in that channels are formed within each filter element (2, 3) between the steel cylinder (23) and the filter means (21, 22) accommodated therein. Apparatus according to claim 9, characterized in that the channels of the filter element (2, 3) are formed from channels (24) which run preferably at a distance of about 20 mm in the circumferential direction and which are connected by one or more axially running channels (25) that lead to one or more outlet bores (26) in the steel cylinder (23). Device according to one of claims 5 to 10, characterized in that the filter elements (2, 3) are arranged on a frame (27) arranged along the screw axis so as to be secured against rotation. Device according to one of claims 5 to 11, characterized in that circumferential seals (9) are arranged between adjacent cylindrical filter elements (2, 3). Device according to one of the preceding claims, characterized by a flushing system in which the screw channels are flushed in a first working cycle and the filter elements (2, 3) are flushed in a second working cycle. Apparatus according to claim 13, characterized in that the screw channels are rinsed in that the mouthpiece (7) or a bypass (8) arranged at the end of the screw (1) on the mouthpiece preferably with a rotating screw (1) and continuous slip input via the material feed (6) is opened until slip passes through from the inlet to the outlet at the mouthpiece (7) or bypass (8), then the slip supply is prevented and water is fed in at the material feed end of the device. Apparatus according to claim 13 or 14, characterized in that the filter elements (2, 3) are rinsed in that, after rinsing the screw channel with the mouthpiece (7) closed, bypass (8) and slip supply (6), pressurized rinsing water is valve-controlled in changing flow direction is passed through the pore system of the filter elements (2, 3). Device according to one of claims 1 to 12 with a flushing system, characterized in that during operation of the filter screw press without flushing the screw channel, flushing water at a higher pressure than is present in the screw channel from the flushing water pump (11) via the valve (12) in The opposite direction to the filtrate flow is passed through the pore system of the filter elements, the valve (12) being controlled after a predeterminable time cycle in such a way that during a flushing process, a time interval in which the flushing water flow is pumped through the filter elements in the opposite direction to the filtrate flow Time interval follows in which a free filtrate outflow from the filter elements is set, so that a liquid flow through the filter elements in the direction of the filtrate flow takes place through the filter elements due to the high internal pressure in the two-phase system. Device according to one of claims 13 to 16, characterized in that the rinsing process is supported by rotation of the screw (1). Device according to one of claims 13 to 17, characterized in that the individual filter elements (2, 3) are rinsed one after the other. Device according to one of claims 13 to 18, characterized in that the rinsing of the filter elements (2, 3) is interrupted a few times by rinsing the screw channel. Device according to one of the preceding claims, characterized in that a plurality of filter screws (31) working in parallel are provided in the second filter section (30).

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