WO1998043796A1

WO1998043796A1 - Method of making a block filter

Info

Publication number: WO1998043796A1
Application number: PCT/US1998/004556
Authority: WO
Inventors: David J. Emmons
Original assignee: Recovery Engineering, Inc.
Priority date: 1997-03-28
Filing date: 1998-03-09
Publication date: 1998-10-08
Also published as: AU6454198A

Abstract

A mold (12) is filled with a mixture (11) including carbon and a non-toxic thermo-plastic binder suitable for filtration. A pressure is applied to the mixture appropriate for producing a block (19) with sufficient porosity to be used as a filter. An electric current is passed through the mixture until there is sufficient electrothermal heating of the carbon to melt the binder.

Description

METHOD OF MAKING A BLOCK FILTER

Field of the Invention

This invention relates generally to manufacturing block filters, and more particularly to methods involved in making such filters in a mold.

Background of the Invention Block filters are well known in the art. They include a mixture of an active ingredient, such as carbon, and a binder material. The mixture is heated under pressure until the binder melts, and then cooled. In this way, a block filter of a desired shape is formed.

Two common methods are known for manufacturing block filters. The first is by extrusion, as shown for example in U.S. Patent No. 5,249,948. In this process, external heat is applied to a carbon and binder mixture prior to being passed through an extrusion die. The extruded shape is then cut to length to form the final block. The manufacturing cycle time of extrusion is limited by the speed at which the mixture can be passed through the extruder, which is also affected by the time that is required to heat and then cool the mixture to the necessary temperatures. The time required for cutting the extruded shape into the desired length also adds to the cycle time.

The second process is by molding, as shown for example in U.S. Patent No. 4,753,728. In this process, the mixture is placed in a mold of a desired shape. While pressure is being applied, the mixture is externally heated until the temperature is raised to the required level. An oven is typically used for this purpose. After heating, the mold is removed from the oven and must be cooled to below a certain temperature before it can be removed. This process also has long cycle time because heat transfer is accomplished through conduction. It takes substantial time to heat the entire mold and mixture (particularly the center of the mixture) and then cool the mold and mixture. Uneven heating of the mixture can also be a problem.

It is known to electrothermally heat a mixture of an electrically conductive material and a nonconductive material to make various articles. See for example U.S. Patent Nos. 4,193,956 and 4,783,288. Such methods, however, have not been employed in making block filters. Much lower pressures must be applied to ^' make a block which has sufficient porosity that it can be used as a filter. The toxic components often incorporated in prior art methods, such as thermoset resins employed as a binder, are also not appropriate for filtration applications where the purified air or water is intended for human consumption. What has been needed is a simple process for manufacturing block filters which is less expensive and more efficient than the processes previously employed.

Summary of the Invention

According to the present invention, various methods for making block filters are provided. The methods can be employed for making block filters appropriate for both water and air purification.

In one aspect of the invention, the method comprises filling a mold with a mixture including carbon and a non-toxic thermoplastic binder suitable for filtration. A pressure is applied to the mixture appropriate for producing a block filter of sufficient porosity to filter at least 100 milliliters of fluid per minute under 60 pounds per square inch of pressure. An electric current is passed through the mixture which causes sufficient electrothermal heating of the carbon to melt the binder. In another aspect of the invention, a pressure between 1 and 250 pounds per square inch is applied to the mixture.

In another aspect of the invention, a pressure is applied to the mixture appropriate for producing a block filter with at least 5 percent void space.

In certain preferred aspects of the invention, the mixture includes at least 40 percent carbon by weight, with carbon particles having an average diameter between 10 and 250 microns. The binder is preferably polyethylene comprising less than 50 percent of the mixture by weight.

Other preferred or alternative aspects of the invention include: passing the current through the mixture axially; controlling the total energy delivered to the mixture by monitoring the peak temperature to be between 200° and 600°F.; forcing gas through the mixture during electrothermal heating to more evenly distribute heat and/or after heating for cooling purposes; and precompressing the mixture to a pressure higher than that applied during electrothermal heating by compressing the mixture to a predetermined size. These and other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto. However, for a better understanding of the invention and its advantages, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter in which there is illustrated and described a preferred embodiment of the invention. Brief Description of the Drawings

Figure 1 is a cross-sectional view of an apparatus for practicing the method of the present invention; Figures 2A-D are schematic drawings of the apparatus of figure 1 in various stages of the method of the present invention; and

Figure 3 is a schematic diagram showing the process of controlling the total energy delivered in the method of the present invention.

Detailed Description of the Preferred Embodiment

Referring now to the drawings wherein like numerals designate like parts, an apparatus for practicing the method of the present invention is shown in figures 1 and 2.

Referring to figures 1 and 2A, mixture 1 1 is gravity fed into mold 12 from bin 10. Mixture 1 1 preferably includes at least 40% carbon by weight and less than 50% thermoplastic binder by weight, most preferably 69% carbon and 17% binder. The carbon is particles preferably having an average diameter between 1 and 2000 microns and more preferably between 10 and 250 microns. The preferred carbon is 80 x 325 granular activated carbon, which has an average diameter between .0015 and .0070 inches (between 38 and 178 microns). The size of the carbon particles chosen will depend on the level of filtration desired and the expected pressure drop across the block. The preferred thermoplastic binder is a polyethylene binder available under the trade name MICROTHENE FN510. Other non-toxic thermoplastic binders suitable for filtration, such as a different polyethylene, polypropylene or ethyl vinyl acetate ("EVA"), could also be used. Additional components can also be added to the mixture, such as an adsorbent material (14% of the preferred mixture by weight) for removing impurities.

The moisture level of the mixture is preferably less than 6%. High moisture levels require more input energy and longer cooling time. After mold 12 has been filled with mixture 11, shuttle 14 is moved to enclose mixture 1 1 in mold 12 between electrodes 13, as shown in figure 2B. Mixture 1 1 is then compressed by clamp 16 of hydraulic press 15 as current is passed through mixture 11 by electrodes 13. In this way, the carbon in the mixture is electrothermally heated to a point where the binder melts so as to bind the particles of the mixture together.

Precompressing the mixture to a pressure higher than that applied during electrothermal heating may reduce the amount of time necessary for heating and result in more even heat distribution. This can be done by precompressing the mixture to a predetermined size reduction, approximately a 20% reduction in volume in the preferred method.

The clamp pressure which must be applied is relatively low because of the porosity required in block filters, in the range of 1 to 250 psi. The pressure must be high enough, however, to effect adequate electrical conductivity so that cycle time is acceptably low. The clamp pressure range is preferably between 70 and 80 psi. Applying pressure to the mixture can also be accomplished by compressing the mixture to a predetermined size. This can be done, for example, by having stops in the mold and applying a clamp pressure sufficient to cause the stops to come into contact.

Current is passed through mixture 11 axially by electrodes 13. This is preferable over passing the current radially (i.e., by placing an electrode in the center of mixture 1 1) because it results in more even heating. In the radial arrangement, the decreasing circumference toward the center can result in overheating there and underheating near the periphery.

The amount of energy that is delivered to the mixture can be controlled as shown in figure 3. A potential of approximately 100 volts is applied between the electrodes. The resistance of the mixture decreases over time, and therefore the current passed through the block will gradually increase. Voltage and current are incrementally integrated until a predetermined energy level is reached. In the preferred method, this in the range of 10-30 KJ, preferably 20 KJ. In order to optimize the energy applied, the peak temperature can be measured, and then the set energy level adjusted accordingly as the block is formed. Instead of providing a temperature feedback loop during the manufacture of the same block (as shown in figure 3), peak temperature can also be used to automatically or manually adjust the energy applied to the next block. The peak temperature can be between 200° and 600°F within the principles of the invention, preferably between 375° and 425°F.

Once electrothermal heating is completed, the mixture must be cooled. This can be done for example by surrounding the mold 12 with a cooling jacket through which water is passed. An alternate cooling method is shown in figure 1. A cooling gas 18, preferably an inert gas such as nitrogen, is forced through mixture 11 at a pressure of about 30 psi, which results in a flow rate of about 6 cfm through the mixture. Upper electrode 13 has a bottom face 21 made of sintered porous metal to allow the gas to enter mixture 11. Passages 17 are provided in clamp 16 to permit gas^" to exit mixture 11. Cooling gas could also be employed during electrothermal heating in order to more evenly distribute heat in the mixture, thereby reducing heating time. This would preferably be done at a lower pressure of 10 psi, which results in a flow rate of about 2 cfm through the mixture. After mixture 11 is sufficiently cooled (below a temperature of about 400°F in the preferred method), shuttle 14 is moved and block 19 is ejected by press 15, as shown in figure 2C. As shown in figure 2D, shuttle 14 is then moved back to its original position, pushing the finished block 19 away from mold 12, and now in position for filling mold 12 with a mixture for making the next block.

The block filter that results from the preferred method is intended for use in water purification applications where household water pressure is the source. It is a cylindrical filter where water is purified as it passes radially inward to a central core 20 (formed by pin 22 in mold 12) from which it exits. For this particular application, the preferred parameters described herein are intended for a 2.0 inch diameter 100 gram filter and result in approximately 30% to 40% void space. The cycle time for making each block is on the order of one minute or less.

It will be understood, however, that the methods of the present invention could be employed to produce a variety of other water, as well as air, filters. This would include block filters which have a void space of 5% or higher, or which have sufficient porosity to filter at least 100 milliliters of water per minute under 60 pounds per square inch of pressure. The particular parameters chosen for the method will depend on the filtration application for which the block filter is intended.

Claims

WE CLAIM:

1. A method of making a block filter, comprising:

(a) filling a mold with a mixture including carbon and a non-toxic thermoplastic binder suitable for filtration; (b) applying a pressure to the mixture appropriate for producing a block filter of sufficient porosity to filter at least 100 milliliters of water per minute under 60 pounds per square inch of pressure; and

(c) passing an electric current through the mixture to cause sufficient electrothermal heating of the carbon to melt the binder.

2. A method of making a block filter according to claim 1, wherein the mixture includes at least 40 percent carbon by weight.

3. A method of making a block filter according to claim 1 , wherein the mixture includes less than 50 percent thermoplastic binder by weight.

4. A method of making a block filter according to claim 1, wherein the thermoplastic binder is polyethylene.

5. A method of making a block filter according to claim 1 , wherein the pressure applied is between 1 and 250 pounds per square inch.

6. A method of making a block filter according to claim 1 , wherein the pressure applied is appropriate for producing a block filter with at least 5 percent void space.

7. A method of making a block filter according to claim 1, wherein the carbon in the mixture is carbon particles having an average diameter between 1 and 2000 microns.

8. A method of making a block filter according to claim 7, wherein the carbon in the mixture is carbon particles having an average diameter between 10 and 250 microns.

9. A method of making a block filter according to claim 1 , wherein the electric current is passed through the mixture axially.

10. A method of making a block filter according to claim 1 , further including the step of controlling the total energy delivered to the mixture.

11. A method of making a block filter according to claim 10, wherein the step of controlling the total energy delivered includes monitoring the peak temperature of the mixture.

12. A method of making a block filter according to claim 1 1, wherein the peak temperature is between 200 and 600┬░F.

13. A method of making a block filter according to claim 1 , further including, during the electrothermal heating, the step of forcing gas through the mixture so as to more evenly distribute heat in the mixture.

14. A method of making a block filter according to claim 1, further including the step of cooling the heated mixture by forcing a gas therethrough.

15. A method of making a block filter according to claim 1 , further including, prior to the step of passing an electric current through the mixture, precompressing the mixture to a pressure higher than that applied during electrothermal heating.

16. A method of making a block filter according to claim 1, wherein the step of applying a pressure to the mixture includes compressing the mixture to a predetermined size.

17. A method of making a block filter, comprising: (a) filling a mold with a mixture including carbon and a non-toxic thermoplastic binder suitable for filtration;

(b) applying a pressure to the mixture between 1 and 250 pounds per square inch; and

18. A method of making a block filter, comprising:

(a) filling a mold with a mixture including carbon and a non-toxic - - thermoplastic binder suitable for filtration; (b) applying a pressure to the mixture appropriate for producing a block filter with at least 5 percent void space; and