Abstract:
A pressure-filter apparatus includes at least one filtration chamber coupled with a source of slurry. The filtration chamber is also coupled with a source of fluid at an elevated pressure, which may be arranged in series or in parallel with a source of wash fluid. The pressure-filter apparatus may also include a source of fluid at a conventional pressure. The source of fluid at elevated pressure has a limited volume for initially treating the slurry within the chamber. The source of fluid at conventional pressure has a substantially unlimited volume for completing the treating of slurry within the chamber. The relative volume capacity of the two fluid sources provides an improved efficiency in the treatment of the slurry and the overall efficiency of the apparatus by limiting the time and capacity of accumulation of elevated pressure fluid while using the conventional pressure for extended treatment of the slurry within the chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation in part of application Ser. No. 10/366,731, filed. Feb. 14, 2003, now abandoned, which is a Divisional of application Ser. No. 09/487,060, filed Jan. 19, 2000, now U.S. Pat. No. 6,521,135, and claims the benefit of U.S. Provisional Application No. 60/116,413 filed Jan. 19, 1999. 

   STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable 
   REFERENCE TO MICROFICHE APPENDIX 
   Not applicable 
   BACKGROUND OF THE INVENTION 
   This invention relates to a filter apparatus and method of its operation, particularly to a pressure filter and method for extracting liquids from slurry fluids and for producing a substantially-dry filter cake of the solid materials present in such slurry fluids. 
   In many industrial processes and other applications where a slurry of solids and liquids is produced, it becomes necessary to filter the slurry solids from the liquids so that these materials may be efficiently utilized or, alternatively, disposed in an environmentally safe manner. To separate the solids from the liquids, a filter apparatus, such as a pressure filter, is often employed. Known pressure filters may include one or more pairs of filter plates capable of relative motion. As an example, in a pressure filter having only a single pair of plates, the inlet plate is generally adapted for receiving the slurry, while the filtered liquids, called the filtrate, are collected by means of the outlet plate, which also provides the requisite support for a filter medium, positioned between the filter plates. This arrangement permits a filtration chamber to be defined by the inlet plate and the filter medium when the plates are pressed together. In the usual production cycle of such a filter, slurry is introduced into the filtration chamber under pressure through an inlet port, whereby it distributes itself throughout the chamber. After the filtration chamber is filled with slurry, the filter executes a series of programmed operations, namely, washing the contents of the chamber in a particular manner, as well as pressurizing the chamber, e.g., with compressed air, to force the liquids from the slurry through the filter medium, leaving the slurry solids, consolidated in a substantially dry filter cake, within the chamber. The slurry liquids are collected and are either used or disposed in an appropriate manner. The chamber is then opened, allowing the filter cake to be removed therefrom. 
   Known pressure filters are capable of successfully treating a wide variety of slurries. However, the use of such filters has shown to be impractical for filtering certain difficult-to-filter slurries, such as certain modified starch, pigment, and molybdenum sulfide slurries. These slurries have proven to be difficult to filter, either because they contain fine particles, e.g., fines on the order of 0.5–10 microns, or because they include compressible solids. In either case, the filter cake being formed In a filtration chamber of a conventional pressure filter may become virtually impermeable to liquids being forced through the slurry solids using gases under commercially available pressures on the order of 100 psi. As a result, the time required to produce a substantially dry filter cake for these types of difficult-to-filter slurries drastically increases, making it impractical to use known pressure filters for treating such slurries. One solution may be to increase the pressure of the gas(es) used to force the liquids from the forming filter cake. However, this alternative proves to be cost-prohibitive because it requires the use of additional equipment and a substantial amount of energy to continuously maintain large quantities of compressed gas at the requisite high pressure. 
   Thus, a need arises for a pressure-filter apparatus capable of efficiently treating difficult-to-filter slurries, such as the types of slurries described above. 
   It is also desirable to provide a pressure-filter apparatus that is energy-efficient and is capable of extracting the slurry liquids to produce a substantially dry filter cake in a minimum amount of time. 
   SUMMARY OF THE INVENTION 
   A pressure filter apparatus utilizing high-pressure fluid is disclosed. The filter apparatus includes at least one filtration chamber, a source of slurry coupled with the filtration chamber, a source of fluid at an elevated pressure coupled with the filtration chamber, and a source of fluid at a conventional pressure coupled with the filtration chamber. 
   The advantages of the invention and the effect on the efficiency of the apparatus will become apparent after consideration of the ensuing description and the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is Illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings, where: 
       FIG. 1  is a schematic view of a pressure-filter apparatus according to one embodiment of the present invention. 
       FIG. 2  is a schematic view of the pressure-filter apparatus according to another embodiment of the present invention. 
       FIG. 3  is a schematic view of the pressure-filter apparatus according to yet another embodiment of the present invention. 
       FIG. 4  is a schematic view of a multi-chamber pressure-filter apparatus. 
   

   For purposes of illustration, these figures are not necessarily drawn to scale. In all of the figures, like components are designated by like reference numerals. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
     FIG. 1  is a schematic view of a pressure-Filter apparatus according to one embodiment of the present invention. The apparatus comprises an inlet plate  100  and an outlet plate  102 , movable relative to each other, with a filter medium  104  capable of being disposed therebetween. Inlet plate  100  includes an inlet cavity  106  as well as an inlet port  108  in communication with cavity  106  and inlet piping  109 . Inlet port  108  may be vertical, as shown in  FIG. 1 , or may be horizontally disposed (not shown). Inlet cavity  106  is bounded by a continuous mating surface  110 . Outlet plate  102  incorporates a grid  112 , which provides a supporting surface for filter medium  104 . This supporting surface is bounded by and Is substantially coplanar with (e.g., within approximately 1/16 of an inch) a continuous mating surface  114 . Plate  102  further includes an outlet cavity  116  for collecting the filtrate liquids and an exit port  118  for discharging the filtrate, which may then undergo further processing or be properly disposed. The peripheral shape of plates  100  and  102  may take any form, but is usually rectangular or circular. 
   When plates  100  and  102  are pressed together, as illustrated in  FIG. 1 , a filtration chamber  120  is defined by inlet cavity  106  and filter medium  104 . It should be understood that the longitudinal and transverse dimensions of the filter medium inside chamber  120  exceed the corresponding dimensions of inlet cavity  106 . The depth of the filtration chamber may vary from about 0.25 of an inch to about 8 inches, depending on the particular application. An inlet distributor (not shown) may be disposed between inlet port  108  and cavity  106  to promote an optimal dispersion of slurry within the filtration chamber and to allow subsequent application of pressurized fluids without disturbing the uniform distribution of slurry inside the filtration chamber. 
     FIG. 1  provides only a schematic representation of the filter-plate configuration and certain elements of the apparatus either have not been shown or have been shown in simplified form to avoid unnecessarily obscuring the invention. For example, continuous mating surfaces  110  and  114  may contain recessed grooves having fluid connections for providing an effective seal between the mating surfaces of the filter plates to substantially reduce or completely eliminate leakage of slurry fluids between the filter plates. The specific details of such an arrangement are disclosed in U.S. Pat. No. 5,059,318 to the present inventor, which is hereby incorporated by reference. Similarly, grid  112  represents only one possible structure for providing the requisite supporting surface for filter medium  104 . Other alternatives for supporting the filter medium, as well as specific arrangements and construction of the filter plates, the filter medium, and other components of the filter apparatus have been disclosed in U.S. Pat. Nos. 5,292,424; 5,462,677; 5,477,891; 5,510,025; 5,573,667; and 5,615,713 to the present inventor, all of which are hereby incorporated by reference. Even though inlet cavity  106  has been schematically illustrated in  FIG. 1  as having square corners and vertical sidewalls, in practice it may be beneficial to provide an inlet cavity incorporating tapered sidewalls and radiused corners (not shown) to prevent the filter cake from adhering to the inside of inlet plate  100 . 
   Referring to  FIG. 1 , the pressure-filter apparatus further includes a slurry source  122  coupled with inlet piping  109  via a shut-off valve  124 . A fluid source  126 , containing fluid at an elevated pressure, is coupled with inlet piping  109  by means of a shut-off valve  128 . A fluid source  130  contains fluid at a conventional pressure and is coupled with the inlet piping via shut-off valve  132 . Source  134  contains fluid at a wash-input pressure and is coupled with the inlet piping through shut-off valve  136 . Inlet piping  109  also contains a purge valve  138 . All sources discussed above, including sources  126  and  130 , are arranged in parallel. 
   Sources  126  and  130  comprise holding tanks, the pressure in each of which is maintained by, e.g., at least one compressor of conventional design (not shown). Source  122  comprises a slurry-holding tank having at least one pressure-supply apparatus (not shown, such as a conventional hydraulic pump (not shown). A wash-fluid source  134  may have a configuration similar to that of source  122  if it is designed to hold a liquid. Alternatively, it may be configured in a manner similar to sources  126  and  130  if its purpose is to hold a fluid such as pressurized steam. 
   Depending on the specific application, different combinations of fluids and pressures in the tanks are possible. For example, the slurry supplied from source  122  may be delivered to the filtration chamber at a slurry-input pressure from about 15 to about 125 psi. Similarly, source  126  may contain fluids at an elevated pressure from about 100 to about 400 psi and source  130  may contain fluids at a conventional pressure from about 30 to about 150 psi. Wash fluid in source  134  may be at a wash-input pressure from about 30 to about 200 psi. It should be noted that even though for many applications the relationship between the pressures is such that the slurry-input pressure will be the lowest, the elevated pressure the highest, and the conventional pressure will be higher than the wash-input pressure, this need not be true in all cases. In any particular situation, the only relationship between the above-mentioned pressures that must always be satisfied is that the elevated pressure of the fluid in source  126  must always be higher than the conventional pressure of the fluid in source  130 . It is possible, for example, that in a specific application the slurry-input pressure of source  122  may exceed the elevated pressure of source  126 . Similarly, the wash-input pressure of source  134  may be greater than the conventional pressure of source  130 . 
   When the plates  100  and  102  are closed to define the chamber  120  having a volume capacity and slurry from source  122  is introduced through inlet port  108 , the pressure within the chamber becomes the pressure of the input slurry. When slurry input is terminated, wash fluid from source  134  is introduced and the pressure within the chamber becomes the pressure of the wash fluid. At that time in the operation of the filter the wash fluid pressure will be higher than the slurry pressure; e.g., if the slurry pressure is at a low range of 15 psi, the wash fluid pressure will be at least 30 psi. Likewise, as fluid from elevated pressure source  126  is introduced that pressure will be at least higher than the wash fluid pressure; e.g., if wash fluid is at 30 psi the elevated pressure fluid will be substantially higher in the example ranges of 100 psi to 400 psi. After the fluid at elevated pressue from souorce  126  is applied the fluid at conventional pressure from source  130  is applied to continue the filtering operation. 
   It should be apparent that the specific wash fluid pressure, while in the suggested range shown, will always be higher than the slurry input pressure. Also, the fluid at the elevated pressure in the suggested range shown will always be higher than the selected wash fluid pressure and the fluid at conventional pressure in the suggested range will be higher than the slurry input pressure but lower than the elevated pressure fluid. Each of the inputs of slurry, wash fluid, fluid at elevated pressure and fluid at conventional pressure are applied in sequence so that the selected pressure of each input is coordinated with the preceding fluid input to accomplish the desired filtration separation result. 
   As mentioned above, sources  126  and  130  may contain a variety of different fluids, such as compressed air, nitrogen, CO2, or steam. Source  134  may hold wash liquids such as water or may contain a fluid such as steam. Depending on the requirements for a particular application sources  126  and  130  may incorporate the same or different fluids. 
   One salient feature of the present invention is that the volume of source  126 , which contains fluid at elevated pressure, is considerably smaller than that of source  130 , which contains fluid at conventional pressure. Thus, the volume of source  126  may be from about 0.04 cu. ft. to about 2 cu. ft. per square foot of filter area within chamber  120 . By comparison, the volume of source  130  may be from about 2 cu. ft.3 to about 10 cu. ft. per square foot of filter area within chamber  120 . The relatively small size of source  126  when compared to source  130  helps improve the energy efficiency of the filter apparatus, since less energy and equipment is required to generate high pressure in a small tank versus a large tank. Energy is further conserved because source  126  has to be pressurized only once per filtration cycle and useful work can be performed by the entire quantity of fluid contained therein. 
   As a measure of energy consumption, about 1 horsepower of energy is required to provide 4.25 cubic feet per minute (CFM) of fluid at 100 psi whereas only ½ horsepower of energy is required to provide 4.25 CFM of fluid at 40 psi. Thus supplying a fluid source volume at elevated pressure can require more energy than the same volume at a reduced (conventional) pressure. In a filtration operation as described herein, the elevated pressure fluid is applied for a short period of time from a source of lower volume than the conventional pressure fluid applied for a longer period of time from a source of larger or unlimited volume. The capacity the source of elevated pressure fluid can be from 0.25 to 25 times the chamber volume which can depend upon the solids content of the slurry being filtered. The capacity of the source of conventional pressure fluid can be 1 to 100 times the chamber volume, or substantially unlimited in capacity. 
   In other words, due to its high initial pressure, the fluid originating from source  126  need not be continuously maintained at the elevated pressure to be effective for the purpose of separating the slurry liquids from the solids. Conversely, the fluid in source  130  should be continuously maintained at conventional pressure to provide peak operating efficiency of the filter. However, because the fluid pressure in source  130  is much lower than that in source  126 , it is relatively inexpensive to maintain pressure therein. Moreover, just as with fluid at elevated pressure, once valve  132  is closed after the fluid at conventional pressure is supplied to chamber  120  from source  130 , the entire quantity of released fluid is available for producing useful work of separating slurry liquids and solids. 
   Slurry source  122  may have a volume from about 0.01 cu. ft. to about 100 cu. ft. per square foot of filter area, depending on several factors such as solids content and filterability of solids. Wash-fluid source  134  may have a volume from about 0.01 cu. ft. to about 5 cu. ft. per square foot of filter area, depending on wash or leaching requirements. 
   The filter apparatus described above may contain additional hardware and peripheral devices to enhance its operational capabilities. For example, sources  122 ,  126 ,  130 , and  134  and their associated piping may include flow meters (not shown). Pressure sensors may be placed inside the filtration chamber and/or sources  122 ,  126 ,  130 , and  134  to determine the pressure therein. A load cell (not shown) may be integrated into the assembly that incorporates plates  100  and  102  so that the weight of the contents of filtration chamber  120  may be ascertained. Conventional actuators (not shown) may be used to operate valves  124 ,  128 ,  132 ,  136 , and  138 , whose operation may be controlled, for example, in accordance with various timers (not shown). All of these devices may be electrically coupled with and controlled by a conventional electronic control unit (also not shown). 
   The method of operation of the above-described embodiment of the pressure-filter apparatus is discussed below with reference to  FIG. 1 . Initially, valve  124  is opened and a quantity of slurry at the slurry-input pressure is directed into filtration chamber  120  to be uniformly distributed therein. The inlet flow of slurry may be turned off based on elapsed time. Alternatively, the slurry supply may be shut off when the back pressure inside the chamber, measured by a pressure sensor (not shown), approaches the slurry-input pressure. Other conventional devices that may be used to ascertain when the flow of slurry into the chamber is to be terminated are a flow meter (not shown) and a load cell (also not shown). Thus, the flow of slurry into the chamber may be shut off when a flow meter, which measures the flow rate of slurry from source  122 , indicates that the flow rate has decreased to a specified value. Similarly, where a load cell is utilized, slurry will cease to be supplied into the filtration chamber when the contents of the chamber approach a specified weight. 
   After the closing of valve  124 , inlet piping  109  may optionally be drained of slurry using purge valve  138 . Next, valve  128  is opened and fluid (e.g., compressed air) at elevated pressure is introduced into the filtration chamber from source  126  to force the liquids from the cake forming in the filtration chamber. Once the liquids begin to clear the solids, valve  128  is closed to allow the falling residual pressure in the inlet piping and filtration chamber  120  to continue driving the liquids through the filtered solids. The shut-off point of valve  128  may be determined, e.g., by a pressure sensor located inside the filtration chamber or in the inlet piping. More specifically, when the pressure in the chamber begins to drop as the liquids start to clear the solids, the sensor provides an appropriate signal to the control unit, which in turn proceeds to close valve  128 . Alternatively, the shut-off point of valve  128  may be based on elapsed time. The above sequence of operations results in a substantially-dry filter cake being produced in the filtration chamber and slurry liquids being collected in outlet cavity  116 . 
   To produce a filter cake having an even lower liquid content, valve  132  may be opened for a specified time so that a fluid, such as compressed air, may enter the filtration chamber from source  132  at conventional pressure when the residual pressure in the filtration chamber drops sufficiently to be substantially equal to that conventional pressure. 
   Alternatively, if the washing of the contents of the filtration chamber is required, a cake-washing operation may be performed after the slurry has been distributed throughout the filtration chamber. In this instance, after the closing of valve  124 , valve  136  is opened so that wash fluid is introduced into the filtration chamber at the wash-input pressure. To end the washing operation (the duration of which may be based, e.g., on elapsed time), valve  136  is closed and the filtering process resumes with the opening of valve  128 , whereby fluid at elevated pressure is introduced into the filtration chamber, as has been previously described. 
   As evident from the method discussed above, the apparatus as illustrated in  FIG. 1  is capable of a variety of operating sequences, based on the requirements of a particular application. Alternatively, if the apparatus of  FIG. 1  is to be used to perform only a specific task not requiring all the above-recited capabilities, its configuration may be simplified, as desired, by eliminating structural elements not necessary to perform a particular function. For example, if no need exists to produce a filter cake which is virtually liquid-free, the operation involving the application of fluid (e.g., compressed air) at conventional pressure to the contents of the filtration chamber may be omitted and the corresponding hardware (i.e., fluid source  130 ) may be eliminated. 
   The above-described method of using the filtration apparatus of  FIG. 1  may be illustrated with a specific example of a modified starch slurry. The slurry is introduced into the filtration chamber at the slurry-input pressure of approximately 85 psi. The slurry flow is terminated when the back pressure in the chamber approaches 85 psi. Alternatively, the slurry flow may be shut off after about 14 seconds. Water is pumped into the filtration chamber as wash fluid at about 125 psi and the wash cycle continues for about 20 seconds. After the wash cycle has been completed, compressed air at the elevated pressure of about 200 psi is supplied into the filtration chamber from source  128  to force the wash liquid and the slurry liquids through the slurry solids. As the liquids begin to clear the slurry solids and the pressure inside the chamber begins to drop, valve  128  is closed, allowing the falling residual pressure in the inlet piping and the filtration chamber to continue forcing the liquids through the slurry solids. When the residual pressure drops to about 100 psi, the filtration chamber is pressurized with compressed air at the conventional pressure of about 100 psi for about 30 seconds to dry the resulting filter cake. 
   Another embodiment of the pressure-filter apparatus according to the present invention is described with reference to  FIG. 2 . In this configuration, fluid source  126  is arranged in series with a wash-fluid source  140  so that source  140  is located between source  126  and filtration chamber  120 . A shut-off valve  142  is disposed between source  140  and inlet piping  109 . The volume of the wash-fluid source  140  is from about 0.01 cu. ft. to about 5 cu. ft. per square foot of filter area. Thus, wash-fluid source  140  has a smaller volume than the corresponding wash-fluid source  134  illustrated in  FIG. 1 . Moreover, source  140  does not include a pump, but instead relies on the elevated pressure of the fluid in source  126  to push the wash fluid into the filtration chamber. Wash-fluid source  140  may contain liquids such as water, acid, caustic, or solvent. 
   Alternatively, wash-fluid source  140  and fluid source  126  of  FIG. 2  may be integrated into a single unit such as a fluid source  144 , illustrated in  FIG. 3 . Such a fluid source would include at least one pressure-supply apparatus (not shown), such as a compressor of a conventional type to produce the requisite elevated pressure. The volume of source  144  ( FIG. 3 ) should be approximately the same as the combined volume of sources  126  and  140  ( FIG. 2 ). 
   The method of operation of the above-described embodiment of the pressure-filter apparatus is discussed below with reference to  FIG. 2 . Initially, valve  124  is opened and a quantity of slurry at the slurry-input pressure is directed into filtration chamber  120  to be uniformly distributed therein. The inlet flow of slurry may be terminated based on elapsed time, back pressure inside the chamber, flow rate of slurry from source  122 , or the weight of the contents of the filtration chamber, as previously discussed with reference to  FIG. 1 . 
   After the closing of valve  124 , inlet piping  109  may optionally be drained of slurry using purge valve  138 . Next, valve  142  is opened so that the entire volume of wash fluid (e.g., water) contained in fluid source  140  is introduced into the filtration chamber, propelled by the elevated pressure of the fluid (e.g., compressed air) in source  126 . The elevated pressure of the fluid in source  126  proceeds to force the slurry liquids and the wash liquid from the cake forming in the filtration chamber. Once the liquids begin to clear the solids, valve  142  is closed to allow the falling residual pressure in the inlet piping and filtration chamber  120  to continue driving the liquids through the slurry solids. The appropriate time to close valve  142  may be determined as discussed previously with reference to  FIG. 1 . The above sequence of operations results in a substantially-dry filter cake being produced in the filtration chamber and slurry liquids being collected in outlet cavity  116 . 
   To produce a filter cake having an even lower liquid content, valve  132  may be opened for a specified time so that a fluid, such as compressed air, may enter the filtration chamber from source  132  at conventional pressure when the residual pressure in the filtration chamber drops sufficiently to be substantially equal to that conventional pressure. 
   The above-described method of using the filtration apparatus of  FIG. 2  may be illustrated with a specific example which involves a molybdenum sulfide slurry with impurities dissolved in a cyanide slurry mother liquor. The slurry is introduced into the filtration chamber at the slurry-input pressure of approximately 90 psi. The slurry flow is terminated when the back pressure in the chamber approaches 90 psi. Alternatively, the slurry flow may be shut off after about 18 seconds. Next, valve  142  is opened so that a quantity of approximately 0.5 gallons of wash fluid (e.g., water) per square foot of filter area is introduced into the filtration chamber propelled by the elevated pressure (about 200 psi) of the fluid (e.g., compressed air) in fluid source  126 . The elevated pressure of the fluid in source  126  acts to force the slurry liquids and the wash liquid from the cake forming in the filtration chamber. Once the liquids begin to clear the solids, valve  142  is closed to allow the falling residual pressure in the inlet piping and filtration chamber  120  to continue driving the liquids through the filtered solids. When the residual pressure drops to about 100 psi, the filtration chamber is pressurized with compressed air at the conventional pressure of about 100 psi for about 45 seconds to dry the resulting filter cake, if so desired. 
   The previously-described embodiments of the present invention may be implemented not only in a filter apparatus having a single filtration chamber, but also in an apparatus having a plurality of stacked shallow filtration chambers, as schematically represented in  FIG. 4 , each individual chamber being constructed substantially as has been described above. The necessary details regarding the basic configuration of such a multi-chamber filter apparatus are disclosed in U.S. Pat. Nos. 5,510,025 and 5,573,667 to the present inventor. 
   The above configurations of pressure-filter apparatus are given only as examples. Therefore, the scope of the invention should be determined not by the illustrations given, but by the appended claims and their equivalents.