MASS-TRANSFER MEMBRANE AND PROCESSES USING SAME

WIPO Patent Application WO/1979/000040

A porous layer (22) is provided on one or both surfaces of a mass-transfer membrane (20) to enhance or modify the passage of ions. The membrane (20) may be a film of any of a variety of polymeric materials, and the porous layer (22) may be a surface layer on the membrane or a separate porous structure placed in contact therewith. The porous layer (22) or layers render certain normally ion-impermeable polymeric films capable of passing ions in an electrodialysis-type system.

Moeglich K.

PCT/US1978/000039

02/08/1979

07/11/1978

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INNOVA INC.

B01D69/12; C25B13/04; C25C7/04

WHAT IS CLAIMED IS:

1. In an iontransfer cell having at least two cham¬ bers, each containing a body of liquid, the chambers being separated by a masstransfer membrane assembly having an operative portion through which particles of mass pass in at least one direction from one body of liquid to the other, an improved membrane assembly which comprises a film and at least one porous layer on at least onesurface of the operative portion, said film and porous layer being, essentially chemically inert under the conditions existing in the cell.

2. A masstransfer cell as in claim 1, wherein the bodies of liquid are electrolytes, and wherein the film and the porous layer are made of electrically insulating material, said cell including at least one electrode in each body of liquid maintained at a different electrical potential than an electrode in the other body of liquid.

3. A masstransfer cell as in claim 1, wherein the material of the film is selected from the group consisting of dialysis membrane material, electrodialysis membrane material, reverse osmosis membrane material and synthetic polymeric material which is itself essentially ion impermeable.

4. A masstransfer cell as in claim 1 wherein said mem¬ brane assembly comprises a plurality of films alternating with porous layers.

5. In a masstransfer process carried out in a cell wherein particles in one body of liquid pass through an operative portion of a membrane assembly into another body of liquid the improvement which comprises using as the membrane assembly a film having on at least one surface of the operative portion thereof a porous layer, the film and layer being essentially inert under the conditions existing in the cell.

6. A masstransfer process as in claim 5, wherein ions pass through the membrane assembly under the influ¬ ence of an electrically charged electrode in each body of liquid, the film and porous layer being made of elec¬ trically insulating material.

7. A masstransfer process as in claim 5, wherein the material of the film is selected from the group consisting of dialysis membrane material, electrodialysis membrane material, reverse osmosis membrane material and synthetic polymeric material which is itself essentially ionimpermeable.

8. A masstransfer process as in claim 5 wherein the membrane assembly comprises a plurality of films alternating with porous layers.

9. A masstransfer membrane assembly capable of trans¬ ferring particles in at least one direction from one sur¬ face thereof to the other surface when said surfaces are in contact with separate bodies of liquid, said membrane assembly comprising a film having on at least one of its surfaces a porous layer.

10. A masstransfer membrane assembly as in claim 9 wherein the material of the film is selected from the group consisting of dialysis membrane material, electro¬ dialysis membrane material, reverse osmosis membrane material and synthetic polymeric material which is itself essentially ionimpermeable.

11. A masstransfer membrane assembly as in claim 10, wherein the film has a thickness in the range 1.0 micron to 1.0 millimeter and wherein the porous layer has a pore size in the range 0.1 micron to 5.0 millimeters and a thickness in the range 0.01 millimeter to 5.0 millimeters, the thickness being at least equal to the average pore size. ^'.

12. A masstransfer membrane assembly..as in claim 9 including a plurality of films alternating with porous layers.

13. In an electrochemical process carried out in a cell wherein ions in an electrolyte pass from one electro¬ lyte through a membrane into another electrolyte under the influence of an electrical potential applied between electrodes disposed in the electrolytes, the production of a low electrical resistance film material from a high electrical resistance film material by applying at least one porous layer on the operative portion of at least one surface? of said film, said porous layer being of electrically insulating material, and by applying ionic current through said film for aperiod of time.

-MASS-TRANSFER MEMBRANE AND PROCESSES USING SAME

This invention relates to improvements in mass- transfer systems in which ions and/or molecules pass through a membrane assembly from one body of liquid to another.

Background of the Invention

Membrane materials used for chemical separation should be as thin as possible to increase the diffusion rate, mechanically strong to survive pressure and erosion conditions, chemically inert to the electrolytes used, have a very low hydraulic permeability to minimize unwan¬ ted diffusion, insensitive to temperature extremes likely to be encountered, immune from swelling or other dimen¬ sional changes, and of low cost. Further, membranes should be immune to organic fouling, and should, in many applica¬ tions, be highly ion-selective. In addition, where used in any electro-dialysis process, the membrane should have a low electrical resistance permitting a high current den¬ sity, and should have a high dielectric constant to elimi- nate problems of electrical breakdown.

Efforts in the past have attempted to find mate¬ rials which will satisfy the above requirements to the greatest extent possible, with optimization directed to¬ ward various specific applications. For example, reverse osmosis membranes must with¬ stand pressures up to 1000 psi. Such membranes are usu¬ ally permeable only to water, and accordingly are not used for separating other components of a solution, as is often required. Membranes used in the production of sodium hydrox¬ ide are hydraulically permeable, allowing considerable salt to pass through and thus contaminate the desired product. Membranes which will prevent such contamination and withstand the chemical conditions in the cell have not been available in the past.

Membranes used for ion selection, often called ion-exchange or permselective membranes, are expensive and are subject to organic fouling. Further, the selec-

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tivity, while necessary in a particular application, precludes the use of such membranes in applications where non-selectivity is desired.

Membranes used for osmotic (dialysis) processes are limited to those materials which exhibit high perme¬ ability to the constituents desired to be transferred through the membranes. Accordingly, the materials avail¬ able for such membranes are limited in variety and hence are limited in mechanical and chemical properties. In any membrane exposed to an electrolyte con¬ taining organic matter or other high molecular weight material, such material may be selectively absorbed by the membrane, causing clogging which reduces the desired permeability, and may cause the surface to become hydro- phobic, thereby preventing the transport of water across the membrane.

Most of the above-mentioned problems associated with dialysis, reverse osmosis, and electrodialysis are a result of the limited range of materials available for membranes which have permeabilities high enough for prac¬ tical purposes.

Summary of the Invention In accordance with the broad concept of the pre¬ sent invention, the passage of mass in the form of mole- cules and/or ions through a film of membrane material is altered by providing on one or both sides of the film a porous layer capable of allowing liquid from the adja¬ cent body of liquid to contact the film. Altering the passage of the particles of mass includes, for example, changing the ease or efficiency with which particles will pass through the film, changing the selectivity of the film with respect to certain particles and rendering certain ionrinpermeable films capable of passing ions. The film may be made of a known membrane material such as those conventionally used in making membranes for dialysis, electrodialysis and reverse osmosis, or it may be any one of several films which are by themselves

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practically impermeable to the passage of ions in solu¬ tion (and hence not heretofore used as ion-transfer members) , but which are rendered ion-permeable by the porous layer, for example, polyethylene, Saran (a poly- vinylidene chloride) and Mylar (a polyester) .

Therefore, it is a broad object of the invention to provide an improved mass-transfer membrane assembly which comprises a film which by itself may be either permeable or essentially impermeable to passage of par- tides and at least one porous layer, essentially inert to its operative environment, on the operative portion of at least one surface of the film.

The invention is particularly concerned with electrochemical membranes and their use in electrochemi- cal systems of the kind in which ion-transfer through the membrane takes place under the influence of an elec¬ trical potential applied between electrodes disposed in an electrolyte on either side of the membrane. It is therefore a further object of the invention to provide an electrochemical cell and an electrochemical process wherein at least two electrolyte-containing chambers are ^' provided with electrodes and are separated by a membrane assembly which comprises a film of electrically insulat¬ ing material and a porous electrically insulating layer on the operative portion of at least one surface of the film. It has been found that a number of low-cost syn¬ thetic polymeric films, such films of polyethylene, hylon, polyvinyl chloride, Tedlar (a polyvinyl fluoride) , Saran and Mylar are particularly useful. For example, a poly- ethylene film which will pass no electric current when used alone in an electrochemical cell, will pass up to one amp or more per square centimeter, the limit being film heating, when provided with a porous layer as des¬ cribed herein. The film may be comprised of permeable or of im¬ permeable material; the normal permeability of the film is not a factor in the invention.

Detailed Description The thickness of the films used in the membrane assemblies of the invention is not critical and may be in range of conventional mass-transfer membranes. ^' Typi- cally the thickness may vary from one micron up, with a preferred range being 5 to 50 microns. As described above, the films themselves may be known ion-transfer films or films of various synthetic polymeric materials not heretofore used for ion-transfer membranes. The porous layer which alters the ion-transfer properties of the film may be provided on one or both surfaces of the film. It is intended that the identifi¬ cation of the porous layer as being "on" the film be interpreted broadly, to include separate porous struc- tures which are in tight contact with or loose contact with the film as well as coatings adhered to the film and surface treatments of the film which render the sur¬ face porous. Separate porous structures include pre¬ formed sheets of filter paper> asbestos paper, woven and non-woven fabrics of natural and/or synthetic fibers, glass fiber, sintered particulate refractory material, open-cell polymeric foams and gelatin. Coating-type porous layers may be deposited on the film by chemical or vacuum deposition, by electrostatic flocking or by allowing suspended solid matter to settle on the film when placed horizontally in a liquid bath. Certain chem cal precipitates may also be used as the porous layer. Surface treatments to form a porous layer include chemi¬ cal etching or leaching of embedded soluble materials. The physical characteristics of the porous layer

^' may vary widely. The pore size must, be sufficient to permit a film of liquid from the body of liquid adjacent the membrane assembly to contact the ion-transfer film but insufficient to permit free flow of liquid between the ion-transfer film and the body of liquid. Generally pore size may vary from 0.1 micron to 5 millimeters, with a preferred range of 20 to 100 microns. The pore

size is dictated in part by practical considerations of permeability, mechanical strength and stability, and chemical inertness.

The thickness of the porous layer may vary from the thickness of a few molecules of water up to several millimeters, a preferred range being 0.01 mm to 1.0 m. In an electrochemical environment the thickness is die- tated by the voltage drop which is tolerable for practi¬ cal purposes. The thickness should be greater than the average pore size.

In;an electrochemical environment both the film and the porous material are normally electrically insula- tive., that is, they do not act as electrodes in the elec¬ trolyte. The film material, if made from the plastics given as examples, is normally impermeable to water.

That is, hydrostatic pressure alone will not cause water to pass through the film to any practical extent. It has been found unexpectedly that a membrane assembly as described above will pass an electric current when the assembly is used in an electrolytic cell, even though the film component alone may be one which will not pass a current under the same conditions. For example, a polyethylene film which will pass no electric current when used alone in an electrochemical cell, will pass up to one amp or more per square centimeter, the limit being film heating, when provided with a porous layer as des¬ cribed above.

When the membrane assembly is to be used in an electrolytic cell it is generally desirable to "condition" the membrane assembly to increase the permeability.

This is accomplished by operating the membrane assembly at a higher voltage initially in the electrolyte. The current gradually increases to a steady state value indi¬ cating an increased permeability. This "conditioning" is beneficial, but not essential to the invention. The conditioning can be done as a separate process if desired to provide pre-conditioned membrane assemblies.

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Some film materials exhibit selective ionic per¬ meability when used according to the invention; therefor it may be desirable to test several types of materials for each application and select the most suitable. 5 The membrane assembly includes at least one film and at least one porous layer but may include a pluralit of films and a plurality of layers. In one particularly useful embodiment a film is sandwiched between two porous layers to form an assembly which will pass ions in both

10 directions through the assembly. Alternatively a one- film, one-layer assembly can be used if desired; however the permeability of the assembly will generally be less than if both sides of the film are treated or in contact with a porous layer. Several layers can be utilized as

15 desired, for example, two or more layers of paper or two or more layers of film or both. Also, combinations of materials may be employed in any one membrane assembly such as two or more types of plastic film and two or more types of paper, if paper is used as the material contact-

20. ing the film. Further, multiple alternating layers may be used as desired.

A preferred embodiment for general testing pur¬ poses consists of one plastic film of thickness from 10 to 50 microns sandwiched between layers of paper such as

25 filter paper or asbestos paper, with the several layers being held together in contact by non-conductive backing plates such as plastic sheets with a multiplicity of small holes, 1 to 3 mm diameter, formed or drilled in the plates providing fluid access to the membrane while

30 retaining mechanical rigidity of the plate. An open area of at least 50% of the total plate area is preferre To maximize the open area while maintaining strength, honeycomb type plates, plates with reinforceing bars or rods, layers of plates with special pores., holes, and

35 rods, and other similar methods may be used. It is desirable to minimize the thickness of the reinforcing plates especially when using liquids not having high con

ductivity, since the electrode spacing is thus minimized, the cell voltage is reduced, and the power requirement reduced. For high current density use, the backing plates may be used to keep the temperature of the me - brane materials within desirable limits by using the plate as a heat exchanger with the circulation of a coolant fluid through closed channels formed in the plate or plates.

The use of backing plates may also be dispensed with entirely where the membrane layers are formed into an integral assembly, and then formed into a bag or pouch which contains- one electrode and an electrolyte, with the bag and electrode then being immersed into another cell containing the other electrode and the other elec- trolyte.

No critical pressure between the film and porous layer is required, although the film and layer must be very closely spaced or in contact throughout the entire film area which is to be utilized. When the porous layer is a separate sheet, the film and sheet may be held together mechanically in any of a variety of ways. They may be held together by mechanical supports such as porous or perforated plates usually of an inert sub¬ stance such as plastic. Electrodes may be used as the support membmers if properly formed to allow fluid access to the membrane. The membrane assembly may also be held together by bonding methods such as adhesives or heat seals; by mechanical methods such as pins, rivets, screws, clamps, or plugs; by manufacturing methods which would result in an assembly of the elements in a single package for example as in the use of a plastic film having a roughened, coated, fuzzy or hairy surface to perform the same function as a porous paper layer; or by hydaulic methods which utilize the dynamic or static pressures of fluids to maintain the layers in contact. Where the con¬ figuration allows, gravity alone may be sufficient to hold one layer to another, for example where the layers

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are horizontal.

Backing plates may be omitted if the membrane and the porous layer or layers are sufficiently rigid to maintain the layers in contact and to withstand the flow conditions in the cell.

Selective permeability of the membrane assembly to various ion species may be achieved by several means, for example, by selection of materials used for the film or porous layer, by selection of the thickness of the film or porous layer, by using a multiplicity of films and porous layers, and by adjustment of the voltages use between electrodes.

Voltages between 0.15 volt and 1000 volts may be employed, but more typical voltages used are 2 volts to 10 volts per membrane assembly.

Current densities at the membrane may vary from

.001 amp per cm 2 to 2.0 amps per cm2 with the preferred range from .15A/cm 2 to .5A/cm2.

The improvements of the invention find utility in recovering uranium, other heavy metals, and nobel metals from waste waters, acids, or caustic solutions. The improvements of the invention may also be used in metal recovery from industrial processes, and in the purification of water to remove undesirable contaminants such as in the purification of sea water to produce pot¬ able water, and generally in the purification of waste waters containing valuable or undesirable ionic constitu ents. Waste waters containing from a few parts per billion up to twenty percent or more of contaminants may be purified to undetectable limits.

Acid or caustic solutions may be concentrated or ^" purified. For example, salt in sodium hydroxide can be removed by transferring the chlorine out of the solution or by transferring the sodium out of the solution and reconcentratirig it as a catholyte. In the latter case, the sodium hydroxide is also purified o.f organic materia and the concentration level of the NaOH can be increased

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to at least 50% .

The invention will be further understood from the following more detailed description taken with the drawings in which: FIGURE 1 is a schematic vertical cross-sectional view of an electrochemical reaction cell utilizing two membrane assemblies according to the present invention;

FIGURE 2 is a schematic cross-sectional view, on an enlarged scale, of one of the membrane assemblies of FIGURE 1;

FIGURE 3 is a schematic vertical cross-sectional view of a second embodiment of an electrochemical reac¬ tion cell;

FIGURE 4 is a schematic vertical cross section of a third embodiment in which the membrane is disposed hori¬ zontally;

FIGURE 5 is a plot of electrical resistance ver¬ sus current density for a polyethylene film; and

FIGURE 6 is a plot of current efficiency versus hydroxide content for a polyethylene film and for a Nafion film.

Referring to Figure 1, there is shown schemati¬ cally an electrochemical cell including a tank (10) , two spaced-apart membrane assemblies (12), a cathode (14), and an anode (16) . The membrane assemblies (12) divide the tank (10) into three compartments, each of which con¬ tains an electrolyte (18) .

Figure 2 schematically illustrates the construc¬ tion of one of the membrane assemblies (12) ; the thick- nesses of the membrane components are greatly enlarged for explanatory purposes.. As shown, the assembly (12) includes a film (20) of electrically insulating material such as polyethylene. The film (20) is sandwiched be-- tween two layers (22) of porous electrically insulating material such as filter paper. The film (20) and layers (22) are held in contact over the entire front and back surfaces of the film (22) by backing plates (24) such as

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rigid sheets of polyethylene. The backing plates (24) are provided with, a large number of relatively large holes (26) which allow contact of the electrolyte (18) with the layers (22). The assembly (12) is held togethe in any suitable way such as by clips, tape, bolts or othe fasteners, none of which is shown. The fasteners are constructed of materials which are inert under the reac¬ tion conditions in the cell. Means (not shown) is pro¬ vided for applying a direct current potential difference to the electrodes (14) and (16) .

Figure 3 illustrates an electrochemical cell having a membrane assembly (12) of the kind described above disposed between a perforated cathode (26') and a perforated anode (28). Each chamber of the cell has an electrolyte inlet (30), an electrolyte outlet (32) and a gas outlet hood (34) disposed above the electrolyte leve (36).

Figure■4 illustrates an electrochemical cell in which a membrane assembly (12) as described above, is disposed horizontally between horizontal perforated elec trodes (26*) and (28). Each of the upper and lower cham bers has an electrolyte inlet (30) and an electrolyte outlet (32) . A gas collection hood (38) is located abov the electrolyte level .(36) in the upper chamber. The removal of evolved gas through an outlet (40) from the lower surface of the membrane assembly (12) is essential to prevent the gas from screening the membrane from the electrolyte. Such collection may be achieved by several methods such as a gas screen, fluid flow, cell inversion, or a curved or slanted membrane surface. A gas screen is a fine mesh screen or fabric which will not pass gas bubbles, but will allow liquid to pass. Most fine mesh screens will accomplish this function to a sufficient extent to allow the gas bubbles to coalesce and be re- moved by other means such as tilting the screen and collecting the gas at the upper end. Fluid flow can be used to sweep the gases to one side for separation by

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conventional means such as settling or centrifugation. The cell can be periodically inverted for gas collection from alternate ends. The membrane assembly can be sian- ted or made concave or convex to guide the gas bubbles to a convenient collection point. Other such methods are well known to those familiar with the art and are not essential to the inventive concept here described.

It is to be understood that in any of the embodi¬ ments more than one membrane assembly may be used, and that one or more electrodes may be used between the mem¬ brane assemblies as desired for specific ionic transfer control and pH control.

EXAMPLES Example 1. No current using plain plastic film. An electrolytic cell, equipped with two electrodes, was divided into two chambers by a membrane consisting of a film of polyehtylene 0.0013 cm thick (1/2 mil). The anolyte was deionized water; the catholyte was dilute phosphoric and sulfuric acid with pH = 1.5. A potential of 150 volts D.C. was applied to the electrodes. During and after 8 hours of voltage applied, no current was measured, indicating a membrane resistance in excess of several megohms. The pH of the deionized water did not change. This test shows that under normal conditions, a film of polyethylene plastic will not serve as a mem¬ brane.

Example 2. Current flow using film plus paper.

The same cell as in Example 1 was used, with the same film material, except that the film was covered on both sides with filter paper. The electrolytes were the same as in Example 1. With a potential between the electrodes of 30 volts D.C. the current increased to 0.5 amp within 8 hours. After 350 amp-minutes, the deionized water in the anode compartment changed in pH from 7 to 0, while the acid in the cathode compartment changed in pH from

1.3 to 7.5, showing a transfer of the negative phosphate

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and sulfate ions through the membrane. Each chamber held 200 ml of electrolyte. The anode was platinum; the

2 cathode, graphite. The active membrane area was 13 cm .

Example 3. Effects of Non-Wettable Porous Layers. The same cell as in Example 2 was used, except that the film was covered on both sides with a polyester felt material having a negative angle of wetting; that is, the surface of the material repelled water, however wate did flow through the felt. With a potential of 150 volt applied, no current was noted over a period of 21 hours. The same experiment was repeated using a teflon-coated glass fiber mat. With 40 volts applied, the current started at 47 microamps and increased to 530 microamps after 14 hours. The pH of the anolyte decreased from 7. to 6.9. This' xample shows that the use of non-wetting porous materials does not decrease the membrane resis¬ tance below 75,000 ohms whereas the use of wettable porous materials, -for the same cell and electrode geome¬ try, as shown in Example 2, decreases the membrane resis tance to well under 50 ohms.

Example 4. Membrane resistance.

The membrane resistance decreases with temperature and ' with an increase in current density, up to about 1 amp/i 2 (.15A/cm ). Using a 1/2 mil polyethylene film between filter papers, 400 cm 3 electrolyte, 28 cm2 membrane area graphite electrodes 1 cm diameter, anolyte H ₃ P0. + H^SO. with pH » 0; catholyte H ₃ P0 ₄ + H ₂ S0 ₄ with pH = 0.1; at

18°C the membrane resistance was a minimum of 10.8 ohms at a current density of .68 amp/in 2 (.105 amps/cm2). The current was 3.0 amps at a voltage of 32.5 volts.

Figure 5 shows the membrane resistance vs. current den¬ sity.. When the electrolyte temperature was increased to 40°C, the membrane resistance decreased further as shown in Figure 5. Other electrolytes and other cell conditio will result in other values for the optimum current, den¬ sity.

Example 5. Current efficiency.

The.actual current efficiency was measured using tap water as the anolyte and a mixture of H-PO., H ₂ SO., and HF as the catholyte, with a pH of 1.3. The attained current efficiency was 80 to 90% of the theoretical maxi¬ mum as given by Faraday's Law. This data was obtained from the cell of Example 4, with the electrolytes as given above.

Example 6. Current efficiency. Example 5 showed a high current efficiency for an acidic electrolyte. This example deals with the current effici¬ ency in a basic electrolyte. Polyethylene film 1/2 mil thick was used between layers of filter paper as the mem¬ brane. The anolyte was 400 ml of 5% OH; the catholyte ^' was 400 ml of deionized water. The potassium ions were transferred across the membrane into the catholyte where purified KOH was built up to a concentration of 25% before the test was discontinued. The initial low cur¬ rent efficiency shown in Figure 6 is due to "conditioning" process of the new membrane. For comparison purposes, in this same figure is shown the current efficiency of a new ion exchange membrane "NAFION" by duPont, designed for sodium hydroxide use. It may be noted from this figure that even without cell optimization, the membrane of this invention is superior to "NAFION" at the higher levels of hydroxide concentration.

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