WO2014191634A1

WO2014191634A1 - Filter element and a method for manufacturing a filter element

Info

Publication number: WO2014191634A1
Application number: PCT/FI2014/050435
Authority: WO
Inventors: Bjarne Ekberg
Original assignee: Outotec (Finland) Oy
Priority date: 2013-05-31
Filing date: 2014-05-30
Publication date: 2014-12-04
Also published as: CN105307747A; AR096509A1; FI20135608A; BR112015029547A2; FI125576B; EA201592087A1; CN105307747B

Abstract

A microporous ceramic capillary-action filter plate (22) made of recrystallized silicon carbide, i.e. pure silicon carbide. Silicon carbide (SiC), also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. Grains of silicon carbide can be bonded together at elevated temperatures to form very hard ceramics. Unlike other types of silicon carbide materials, which uses additives (binders) to help bond the silicon carbide particles together, re-crystallized silicon carbide ceramics are produced from pure silicon carbide powder having bimodal grain size distribution, e.g. extremely fine and coarse grains. A filter plate based on pure crystallized silicon carbide is extremely corrosion resistant material which enables significantly longer lifetime of the filter plate also in filtration application wherein fluorine is present.

Description

FILTER ELEMENT AND A METHOD FOR MANUFACTURING A FILTER ELEMENT

FIELD OF THE INVENTION

The present invention relates generally to ceramic filter elements.

BACKGROUND OF THE INVENTION

Filtration is a widely used process whereby a slurry or solid liquid mixture is forced through a media, with the solids retained on the media and the liquid phase passing through. This process is generally well understood in the industry. Examples of filtration types include depth filtration, pressure and vacuum filtration, and magnetic, gravity and centrifugal filtration.

Both pressure and vacuum filters are used in the dewatering of mineral concentrates. The principal difference between pressure and vacuum filters is the way the driving force for filtration is generated. In pressure filtration, overpressure within the filtration chamber is generated with the help of e.g. a diaphragm, a piston, or external devices, e.g. a feed pump. Consequently, solids are deposited onto the filter medium and filtrate flows through into the filtrate channels. Pressure filters often operate in batch mode because continuous cake discharge is more difficult to achieve.

The cake formation in vacuum filtration is based on generating suction within the filtrate channels. Several types of vacuum filters exist, ranging from belt filters to rotary vacuum drum filters and rotary vacuum disc filters.

Rotary vacuum disc filters are used for the filtration of suspensions in large scale, such as the dewatering of mineral concentrates. The dewatering of mineral concentrates requires large capacity in addition to producing a cake with low moisture content. Such large processes are commonly energy intensive and means to lower the specific energy consumption are needed. The vacuum disc filter may comprise a plurality of filter discs arranged in line co-axially around a central pipe or shaft. Each filter disc may be formed of a number of individual filter sectors, called filter plates, that are mounted circumferentially in a radial plane around the central pipe or shaft to form the filter disc, and as the shaft is fitted so as to revolve, each filter plate or sector is, in its turn, displaced into a slurry basin and further, as the shaft of rotation revolves, rises out of the basin. When the filter medium is submerged in the slurry basin where, under the influence of the vacuum, the cake forms onto the medium. Once the filter sector or plate comes out of the basin, the pores are emptied as the cake is deliquored for a predetermined time which is essentially limited by the rotation speed of the disc. The cake can be discharged by a back-pulse of air or by scraping, after which the cycle begins again.

In a rotary vacuum drum filter, filter elements, e.g. filter plates, are arranged to form an essentially continuous cylindrical shell or envelope surface, i.e a filter drum. The drum rotates through a slurry basin and the vacuum sucks liquid and solids onto the drum surface, the liquid portion is "sucked" by the vacuum through the filter media to the internal portion of the drum, and the filtrate is pumped away. The solids adhere to the outside of the drum and form a cake. As the drum rotates, the filter elements with the filter cakes rise out of the basin, the cakes are dried and removed from the surface of the drum.

The most commonly used filter media for vacuum filters are filter cloths and coated media, e.g. the ceramic filter medium. The use of a cloth filter medium requires heavy duty vacuum pumps, due to vacuum losses through the cloth during cake deliquoring. Capillary action filters utilize the capillary action in microporous hydrophilic ceramic materials to prevent air flow through the filter material. In other words, when wetted, it does not allow air to pass through, which further decreases the necessary vacuum level, enables the use of smaller vacuum pumps and, consequently, yields significant energy savings. The principle of capillary action filtration is described in US4357758. The ceramic filter plates have been based on alumina (e.g. AI3O2) and made by casting or pressing. Examples of such materials are described in US4863656 and US4981589.

The filter plate is affected by slurry particles and extraneous compounds, especially in the field of dewatering of mineral concentrates. In many filtration applications the chemical conditions are to very corrosive which may shorten the lifetime of the filter plates. As the replacement of a plate can be expensive, it is important to increase the time-in-operation of an individual filter plate.

BRIEF DESCRIPTION OF THE INVENTION

An aspect of the present invention is to increase a life-time of ceramic filter element used in removal of liquid from solids containing material to be dried in a capillary suction dryer. Aspects of the invention are a filter ele- ment, a filter apparatus and a method according to the independent claims. Embodiments of the invention are disclosed in the dependent claims.

An aspect of the invention is a filter element to be used in removal of liquid from solids containing material to be dried in a capillary suction dryer, the filter element comprising a porous ceramic plate which is microporous at least at a top surface zone to provide capillary action during filtration, and filtrate channels within the porous ceramic plate for draining the filtrate sucked through the porous ceramic plate from at least the top surface of the plate.

The filter element is characterized in that the porous ceramic plate comprises two recrystallized silicon carbide half-plates glued together with glue strings, said glue strings being dimensioned and patterned to define said filtrate channels between opposing flat surfaces of said recrystallized silicon carbide half-plates.

Another aspect of the invention is a filter apparatus comprising one or more filter elements according to the invention.

A further aspect of the invention is a method for manufacturing a filter element to be used in removal of liquid from solids containing material to be dried in a capillary suction dryer, wherein the method comprises the steps of:

providing a first porous half-plate made of recrystallized silicon carbide, at least one side of the first half-plate being substantially flat,

providing a second porous half-plate made of recrystallized silicon carbide, at least one side of the second porous half-plate being substantially flat,

applying glue in spaced strings to form a glue string pattern on the flat side of the first porous half-plate,

placing the flat side of the second porous half-plate on top of the glue string pattern while keeping the flat sides of the first and second half- plates at a predetermined distance from each other,

hardening the glue strings pattern so that the glue pattern fixes the first and second porous half-plates together to form a united filter plate, the spaces in the glue string pattern providing said filtrate channels,

sealing the peripheral edge of the united filter plate.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail by means of example embodiments with reference to the accompanying draw- ings, in which Figure 1 is a perspective top view illustrating an exemplary disc filter apparatus, wherein embodiments of the invention may be applied;

Figure 2 is a perspective top view of an exemplary sector-shaped ceramic filter plate;

FIGS. 3A, 3B and 3C illustrate a schematic example of a functional structure of a ceramic capillary action filter plate and different phases of a filtering process;

Figure 4 shows a graph illustrating a comparison between one filter material made of recrystallized silicon carbide and one standard filter material based on glass bonded alumina;

Figure 5A illustrates a cross-sectional top view of a ceramic substrate (e.g. a bottom half-plate) provided with a glue string pattern according to exemplary embodiment of the invention;

Figure 5B is an enlarged illustrates cross-sectional top view of a portion of the ceramic substrate shown in Figure 5A;

Figure 5C is an enlarged cross-sectional side view of a final filter plate taken along line A-A shown in Figure 5B;

Figure 5D is a cross-sectional side view of a final filter plate having a microporous membrane layer taken along line A-A shown in Figure 5B. DESCRIPTION OF EXEMPLARY EMBODIMENTS

Principles of the invention can be applied for drying or de-watering fluid materials in any industrial processes, particularly in mineral and mining industries. In embodiments described herein, a material to be filtered is referred to as slurry, but embodiments of the invention are not intended to be restricted to this type of fluid material. The slurry may have high solids concentration, e.g. base metal concentrates, iron ore, chromite, ferrochrome, copper, gold, cobalt, nickel, zinc, lead and pyrite. In the following, example embodiments of filter plates for rotary vacuum disc filters are illustrated but the principles of the invention can be applied also for filter media of other types of vacu- urn filters, such as rotary vacuum drum filters.

Figure 1 is a perspective top view illustrating an exemplary disc filter apparatus in which filter plates according to embodiments of the invention may be applied. The exemplary disc filter apparatus 10 comprises a cylindrical- shaped drum 20 that is supported by bearings on a frame 8 and rotatable about the longitudinal axis of the drum 20 such that the lower portion of the drum is submerged in a slurry basin 9 located below the drum 20. A drum drive 12 (such as an electric motor, a gear box) is provided for rotating the drum 20. The drum 20 comprises a plurality of ceramic filter discs 21 arranged in line co-axially around the central axis of the drum 20. For example, the number of the ceramic filter discs may range from 2 to 20. The diameter of each disc 21 may be large, ranging from 1 ,5 m to 4 m, for example. Examples of commercially available disc filters in which embodiments of the invention may be applied, include Outotec Larox CC filters, models CC-6, CC-15, CC- 30, CC-45, CC-60, CC-96 and CC-144 manufactured by Outotec Oyj.

Each filter disc 21 may be formed of a number of individual sector- shaped ceramic filter elements, called filter plates, mounted in a radial planar array around the central axis of the drum to form an essentially continuous and planar disc surface. The number of the filter plates may be 12 or 15, for example. Figure 2 is a perspective top view of an exemplary sector-shaped ceramic filter plate. The filter plate 22 may be provided with mounting parts, such as fastening hubs 26, 27 and 28 which function as means for attaching the plate 22 to mounting means in the drum. FIGS. 3A, 3B and 3C illustrate a functional structure of a ceramic capillary action filter plate and different phases of a filtering. A microporous filter plate 22 may comprise a layered suction structure which may include a ceramic porous substrate and a microporous membrane 31 on one or both sides of the substrate 32. Alternatively, a microporous substrate 32 may be provided, in which case no separate microporous membrane 31 may be needed. Filtrate channels (not shown) are created within the microporous filter plate 22. The filter plate 22 may also be provided with connect- ing part 29, such as a filtrate tube or a filtrate nozzle, for drainage of fluids. The flow channels will have a flow connection with collecting piping in the drum 20, e.g. by means of a tube connector 29. When the collecting pipe is connected to a vacuum pump, the interior of the filter plate 22 is maintained at a negative pressure, i.e. a pressure difference is maintained over the suction wall. The microporous filter plate (e.g. the membrane 31 or the substrate 32) contains micropores that create strong capillary action in contact with water. The pore size of the micropores may preferably be in the range of 0.2 to 5 micrometer and that will make possible that only liquid is flowed through the microporous layer. The edges of the plate may be sealed by means of painting or glazing or another suitable means to seal, thus preventing flow through the edges. In exemplary embodiments the filter plates 22 of the consecutive discs are disposed in rows, each row establishing a sector or zone of the disc 21 . As the row of the filter discs 21 rotate, the plates 22 of the each disc 22 move into and through the basin 9. Thus, each filter plate 22 goes through four different process phases or sectors during one rotation of the disc 21 . In a cake forming phase (illustrated in Figure 3A), a partial vacuum is transmitted to the filter plates 22 and filtrate is drawn through the ceramic plate 22 as it is immersed into the slurry basin 9, and a cake 35 forms on the surface of the plate 22. The liquid or filtrate in the internal filtrate channels is then transferred into the collecting pipe and further out of the drum 20. The plate 22 enters the cake drying phase (illustrated in Figure 3B) after it leaves the basin 9. A partial vacuum or over-pressure is maintained in the filter plates 22 also during the drying phase so as to draw more filtrate from the cake 35 and to keep the cake 35 on the surface of the filter plate 22. If cake washing is required, it is done in the beginning of the drying phase. In the cake discharge phase illustrated in Figure 3C, the cake 35 is scraped off by ceramic scrapers so that a thin cake is left on the plate 22 (gap between the scraper and the plate 22). After the cake discharge, in a cleaning phase (commonly called a backwash or backflush phase) of sector of each rotation, water or filtrate is pumped with overpressure in a reverse direction through the plate 22 to wash off the residual cake and clean the pores of the filter plate.

The inventor has observed that in many filtration applications the chemical conditions are too corrosive to standard ceramic filter materials based on alumina. For example, in some applications the corrosion problems have resulted in a lifetime as short as 0.5 - 1 year while a normal expected lifetime is approximately 3-4 years. Serious corrosion problems have been faced especially in filtration applications in which fluorine is present in the water used in the process. Even fluorine in low concentrations of some few ppm's (e.g. 1 - 10 ppm) in acidic conditions will attack the filter material and shorten its lifetime significantly. The attack of fluorine on the filter materials takes place during the cleaning with acid. High end pH applications (e.g. from pH9 to pH14) such as alumina processes are also challenging to alumina based materials especially in high temperatures. This fact prevents feasible implementation of filtration applications with high pH.

A microporous ceramic capillary-action filter plate can be made of recrystallized silicon carbide, i.e. pure silicon carbide. Silicon carbide (SiC), also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. Grains of silicon carbide can be bonded together at elevated temperatures to form very hard ceramics. Unlike other types of silicon carbide materials, which uses additives (binders) to help bond the silicon carbide parti- cles together, recrystallized silicon carbide ceramics are produced from pure silicon carbide powder having bimodal grain size distribution, e.g. extremely fine and coarse grains. The manufacturing process may include shaping of a green body by slip casting or press moulding, for example. The green body is fired at a very high temperature, such as 2300-2500 °C in a protective atmos- phere (e.g. in argon gas), which causes the evaporation and condensation of silicon carbide. The result is a porous (typically 30-50 volume-% of porosity) self-bonded silicon carbide ceramic material. A filter plate based on pure crystallized silicon carbide is extremely corrosion resistant material which enables significantly longer lifetime of the filter plate also in filtration application wherein fluorine is present. Figure 4 shows graph illustrating a comparison between one filter material made of recrystallized silicon carbide and one standard filter material based on glass bonded alumina. The tests were carried out in a laboratory test where corrosion rate was accelerated in low pH and 5 ppm concentration of fluorine. The corrosion rate is expressed as weight loss of materi- al during the test. The corrosion rate of recrystallized silicon carbide is a fraction of the corrosion rate of the alumina based material. There are other types of silicon carbide materials manufactured in a different way but recrystallized silicon carbide shows the best chemical stability. Other advantages of recrystallized silicon carbide filter plates in addition to high corrosion resistance in- elude: material is very hard and it is extremely resistant to abrasion; material has a very high mechanical strength (double compared to standard glass bonded alumina); and the specific gravity is significantly lower compared to alumina based materials and a filter element with less weight can be produced (20-30%).

There are also reaction sintered silicon carbide materials resulting from the reaction between carbon and liquid silicon in protective gas atmosphere, but their corrosion resistance is not as good for filtration applications due to residual silicon after the reaction. Silicon carbide is often added to ceramic materials to give the material certain properties. However, if the material is then sintered in atmosphere containing oxygen, most of the silicon carbide is oxidized to silica and no corrosion resistant features will be obtained. Thus, recrystallized silicon carbide material is superior filter plate material also among silicon carbide materials generally.

Example embodiments of microporous ceramic capillary-action filter plates made of recrystallized silicon carbide will now be illustrated without in- tention to limit the invention these embodiments.

A filter plate 22 comprises two half-plates 32A and 32B glued together to a filter plate with a given thickness, as illustrated in Figures 5A-5D. The half-plates 32A and 32B may be flat on both sides without any recessed areas. The half-plates 32A and 32B may consist of one or two material layer. In an embodiment illustrated in Figure 5D the half-plates 32A and 32B are each made of two layers, the top layer (outmost layer) being a thin microporous recrystallized silicon carbide membrane 31 made on the top of the respective porous recrystallized silicon carbide substrate half-plate 32A and 32B. The microporous membrane 31 provides the capillary action during filtra- tion. The pore size of the microporous membrane 31 may be in the range of 1 - 5 micron, for example. In an embodiment illustrated in Figure 5C the half- plates are made of one layer such that the substrate half-plates 32A and 32B made of recrystallized silicon carbide substrate are microporous all through the material so as to provide the capillary action during filtration. The pore size of the substrate half-plates may be in the range of 1 -5 micron, for example. An advantage of the one-layer half-plates is that the steps of making and especially firing the separate membrane are avoided.

The recrystallized silicon carbide half-plates 32A and 32B are glued together with glue strings 51 . The glue strings 51 are dimensioned and pat- terned so as to define said filtrate channels 33 between opposing flat surfaces of said recrystallized silicon carbide half-plates 32A and 32B. In other words the spaces between the glue strings 51 form the filtrate channels 33 within the porous ceramic plate for draining the filtrate sucked through the porous ceramic plate from the top surface of the plate. The gluing may be done in a jig in such a way that there is a distance (e.g. 5-10 mm) between the two half-plates 32A and 32B. The glue may be applied in spaced strings 51 to form a glue string pattern on the flat side of the bottom half-plate 32A, and the flat side of the second half-plate 32B may be placed on top of the glue string pattern while keeping the flat sides of the first and second half-plates at a predetermined distance from each other by stoppers in the jig. The jig is arranged to press the plates while assuring the total plate thickness is the desired and constant along the final filter plate. The glue strings 51 will be in contact with both half-plates 32A and 32B binding them together. The glue may be any suitable glue, such as a two-component type glue that hardens in a short period of time, e.g. in a few hours' time. Example of a commercially available glue is DP490 manufac- tured by 3M. The peripheral edge of the plate may be sealed in a suitable manner, for example with the same type of glue as the glue strings 51 , to achieve a leak free plate structure, as illustrated by a glue seam 52 in Figures 5C and 5D. Moreover, the inactive parts of the plate surface may be coated with an impermeable coating, e.g. painted with impermeable paint. Such inac- tive parts may include the peripheral edge of the plate and/or the surface areas surrounding the mounting parts 26-28 and the nozzle 29. Finally, the plate may be equipped with mounting parts 26, 27 and 28 (Figure 2) and filtrate nozzle 29.

A ceramic filter plate may be microporous on only one side of the fil- ter plate, so that the filtering operation is carried out only through that side. Only the first half-plate made of recrystallized silicon carbide may be microporous all through the material so as to provide the capillary action during filtration, while the other half-plate made of recrystallized silicon carbide may not be microporous. One half-plate may be made of two layers, the top layer (outmost layer) being a thin microporous recrystallized silicon carbide membrane made on the top of the respective porous recrystallized silicon carbide substrate half- plate, while no microporous recrystallized silicon carbide membrane may be provided on the outer surface of the other half-plate. These approaches may be particularly suitable for filter elements of drum filters. In the case of drum filter plates, the microporous surface of the half-plate may be a curved surface.

The microporous membrane layer 31 may be produced on the half- plates 32A and 32B by a dip coating process or spraying, for example, and subsequent firing. The microporous membrane layer 31 may be produced on the half-plates 32A and 32B prior to or after attaching the half-plates into an united filter plate 22.

Upon reading the present application, it will be obvious to a person skilled in the art that the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

1 . A filter element to be used in removal of liquid from solids containing material to be dried in a capillary suction dryer, the filter element comprising a porous ceramic plate (22) which is microporous at least at a top sur- face zone to provide capillary action during filtration, and filtrate channels (33) within the porous ceramic plate (22) for draining the filtrate sucked through the porous ceramic plate (22) from at least the top surface of the plate (22), characterized in that the porous ceramic plate (22) comprises two recrystallized silicon carbide half-plates (32A, 32B) glued together with glue strings (51 ), said glue strings (51 ) being dimensioned and patterned to define said filtrate channels (33) between opposing flat surfaces of said recrystallized silicon carbide half-plates (32A, 32B).

2. A filter element according to claim 1 , wherein each of the two recrystallized silicon carbide half-plates (32A, 32B) consists of one material layer being a microporous recrystallized silicon carbide substrate that provides the capillary action during filtration.

3. A filter element according to claim 1 , wherein each of the two recrystallized silicon carbide half-plates (32A, 32B) consists of two material layers, the first layer being a porous recrystallized silicon carbide substrate and the second layer being a microporous recrystallized silicon carbide membrane (31 ), said second layer forming the outermost layer on each of the two recrystallized silicon carbide half-plates (32A, 32B).

4. A filter element according to any one of claims 1 to 3, wherein the peripheral edge of the ceramic plate (22) at the point where the two half-plates (32A, 32B) are connected is sealed with a glue seam (52).

5. A filter apparatus, characterized in that the filter apparatus comprises one or more filter elements according to any one of claims 1 -4.

6. A method for manufacturing a filter element to be used in removal of liquid from solids containing material to be dried in a capillary suction dryer, characterized in that the method comprises the steps of:

providing a first porous half-plate (32A) made of recrystallized silicon carbide, at least one side of the first half-plate (32A) being substantially flat,

providing a second porous half-plate (32B) made of recrystallized silicon carbide, at least one side of the second porous half-plate (32B) being substantially flat,

applying glue in spaced strings (51 ) to form a glue string pattern on the flat side of the first porous half-plate (32A),

placing the flat side of the second porous half-plate (32B) on top of the glue string pattern (51 ) while keeping the flat sides of the first and second half-plates (32A, 32B) at a predetermined distance from each other,

hardening the glue strings pattern (51 ) so that the glue pattern fixes the first and second porous half-plates (32A, 32B) together to form a united filter plate (22), the spaces in the glue string pattern (51 ) providing said filtrate channels (33),

sealing the peripheral edge of the united filter plate (22).