US1878235A - Electrodialysis - Google Patents

Description

sept? 2o, 1932.

R. A. GoR'rN'ER E'r AL ELEcTRoDILYsIs Filed Aug. 2o, '1928 Patented Sept. 20, 1932 UNITED STATES PATENT OFFICE ROSS AIKEN GORTNER, 0F ST. PAUL, AND WALTER F. HOFFMAN, 0F CLOQUET, MINNE- SOTA, ASSIGNORS T0 NORTHWEST PAPER COMPANY,- OF CLOQUET, MINNESOTA, A

CORPORATION 0F MINNESOTA unaccmonamrsrs Application 1ed August 20, 1928. Serial No. 300,662.

The present invention relates to electrodialysis and to method and means for increasing the time and current efficiency of the process. It has particular reference to the Ifemoval of inorganic matter and other substances capable of migration, or which can be caused to migrate, under the influence of the electric current from liquid or solid material, such as the impurities in lchemical wood pulp.

In a copending application Ser. No. 54,003, filed Sept. 2, 1925, by R. A. Gortner and W. F. Hoffman, of which this application is a continuation in part, there is described a process for treating wood pulp and like libers to remove coloring matter, mineral matter and other impurities by subjecting a slurry of the pulp to the action of an electric current in a diaphragm cell. The impurities are moved by the current from their former association with the pulp, into the liquid and from the liquid through'the diaphragms and away from the pulp. The electrical charge may ybe natural, or it may be induced in the impurities by the addition of certain materials.

In carrying out said process it haswbeen the usual practice to employ a slurry of low concentration of Tiber with the intent to limit the quantity of matter to be removed from a single batch, and hence to reduce the time required.

The present invention is based upon the observation that when a more concentrated slurry of pulp is employed, a unit quantity of liber can be purified in less time with less power consumption per unit of fiber treated. A study of the condition of operation has revealed that the ratio of fiber to suspending liquid is important, and that the time and current efficiencies increase with increase in concentration of slurry lwithin the workable ranges of the process. I v

Accordingly, one object of the present invention is to control the consistency of any stock which is subjected to electrodialysis so as lto obtain increased efficiency.

Another-object of the invention is the provision of a process for efficiently purifying may or may not be a source of the material to be so removed.

Still another object is to improve the dialysis processby other means which produce the same eect as the addition of liber or pulp.

In order to explaink theprinciples of the invention We have chosen to illustrate it by reference to wood pulp as it is dialyzed in the cell shown in the accompanying drawing. The process is not limited to such material nor apparatus as Will be clear from the following description and explanation of what is believed to be the underlying principle involved.

In the drawing the cell is illustrated in diagrammatic form. A suitable container l0 of box like structure may be constructed of wood and made watertight in any suitable manner, such as by coating withtar or pitch. Atypical cell which has been employed, but which'may be modifiedin its y dimensions without departing from the invention, is l0 inches long, 6 inches Wide, and

5 inches deep. Larger cellsemploying 2000 pounds of slurry have been employed., Partitions 11 and 12 are provided which are permeable to substances which it is desired to move out of the main chamber, and impermeable to substances desirably retained or animal tissue which permit ions to pass and which retains such materials as colloids.

The partitions divide the box into three compartments, the main compartment 13, the anode compartment. 14, and the cathode compartment 15. The size of the compartment is immaterial, but for efficiency, small electrode compartments are preferred. Suitable electrodes 1o and 17 of carbon may be used such as graphite plates. 'Carbon is the preferred material for the anode as it is sub- `Ject to corrosion, but the cathode may be iron, 'copper or 'other metal which is not attacked as cathode. Any cell involving the essential features of the exemplary cell above described may be used in the process of this invention.y

In use, a slurry of pulp is placed in the middle compartment 13. A current of electricity is passed through the cell by applying a voltage tothe electrodes. A voltage of about 200 has produced a current of about one ampere in the cell above described. The current, however, varies during operation, because the resistance of the cell increases as impurities are removed. When certa-in pulps high in electrolyte content are employed an initial current as great as ten'amperes may be observed, in which case the applied voltage may be reduced merely to prevent overheating. During the operation the impurities are caused to migrate into the electrode compartments, and in order to reduce the tendency of these impurities to fiow back through the partitions they are removed from the electrode compartment by providing intermittent or a constant replacement of water therein.

It will be clear from the foregoing description of the apparatus that too concentrated a slurry will form a mechanical ba-rrier to the migration of the impurities through the solution and it will also minimize the water channels through which migration may take w` place. We have found that an 8% slurry of a commercial sulphite spruce pulp is closel to the upper limit of concentration. When a more dilute slurry is employed, such as a 3% slurry greater current and a longer time will be required than when an 8% slurry is used, and this in spite of the fact that there i's only three-eighthsas much impurity to be removed. Accordingly, we believe that an 8% slurry lies between the upper and lower limits of concentration for chemical wood pulps.

We are unable to set by exact figures the lower limit, but believe the limit to be determined by the manner in which current is proportioned between liquid and pulp. In a very thin slurry current is carried largely by the suspending liquid. In a 3% slurry the mass is mobile and moves about to a disadvantage. An 8% slurry is thick and but slightly mobile. The condition approximates a close packing of the wet fibers. Such a condition is believed to establish a continuous fibrous path across the cell for conducting the current, and the fibrous path to be of greater conductivity than the water paths.

This belief is supported by the observation that ashless fibers, such as those prepared by this process, when added to distilled water, or to water containing electrolytes, have the property of increasing the conductivity generally in proportion to the fiber concentration up to a certain limit. This limit of course depends upon the particular conditions and materials involved. The added fibers are by nature insulating in character when dry, but when in combination as above described they appear to have a property of conductance. Whereas, one would expect a less conductivity by adding an insulating solid non-ionizing material to a conductive liquid, it is nevertheless a fact that a greater conductivity is found when pulp is added to water. Accordingly, in the cell with the high concentrations of fiber current appears to be carried principally by the liquid associated with the fibers. The potential diHerence causing current to flow tears the impurities apart, or away from their fixed condition with the fibers, so that they may readily enter the liquid current-carrying channels for free approach to the separating diaphragms.

It is not known whether the path of increased conductivity at the fibers is through the fiber or at the surface of the liber, or whether it is at-both, or elsewhere. Inasmuch as distilled water with pulp free from electrolyte has only ions of water present to carry electric charges, it is possible that a selective separation of the ions has been made by the addition of pulp. This could be explained by the double layer theory characterizing many adsorptive and surface phenomena. According to this theory positive and negative charges which normally neutralize each other are separated into two layers in such a manner that each layer assumes a positive or negative charge indicating a disturbance of the normal neutralization. In accordance with this theory it is possible to suppose that with a proper concentration of pulp in water, the mixture subjected to electrodialysis contains positive and negative channels through the mass into which channels various charged impurities are drawn and in which they migrate with rapidity and with high current efficiency.

The time and electrical efficiency of the process of this invention as applied to pulp is greater than when the dialyzing is carried out with concentrations under the lower limit. For example, a 3% slurry operating ona given applied voltage has taken over a week to reach a given stage of completion, while an 8% slurry under the same conditions of applied voltage has attained the same stage in two days. Although the 8% slurry carries a higher current, the power consumed per unit of pulp, is much less than required for the 3% slurry, and the time is much shorter.

With too low a concentration of pulp we have determined that when the ash content of pulp is reduced to 0.1% the ionic concentration resulting from such ash is insufficient to give a current capable of removing the ash without anundue expenditure of time and electric energy. But when there is a so-calledsurface conductance across the cell caused by the presence of sufficient pulp an increased current will flow. Such increased current causes rapid and eicient removal of the ash ,to much lower concentrations and even permits rsubstantially complete removal. In other words, ionic conductance by the impurities to be removed is not relied upon, but rather the so-called surface conductance along the fibers. It appears difficult to define the limits of concentration, but generally it is believed a sufiicient concentration is one at which there is an appreciable conductance through the mass after the electrolyte content has been decreased to a point where there is no appreciable ionic conductance through the body of the liquid which lies in the'interstices of the fiber mass.

Although the invention has been described and explained with reference to wood pulp it is not to be considered as limited to wood.

`pulp. It-has been carried out with other materials such as cinchona bark, from which quinine has been removed'by the process described in a copending. application of Anton Hogstad, J r. Ser. No. 95,203, filed March 17,

1926, nux vomica and other alkaloid-contain ing materials, and other processes of electro-` dialysis of distinctive character. The invention does not necessarily require that pulp or fibrous material present during the dialysis be the source o f material to be recovered, for

pulp will function according to this invention independently of impurities therein. In.

consequence, the invention contemplates that fiber, such as wood pulp, be added to any electrodialysis cell for ,the purpose of producing an effective concentration of solid material therein to give all the advantages herein set forth.

The present invention provides for the presence of sufiicient solid material, such as the fiber of the wood pulp, which has the property of imparting to the suspending liquid an electrical conductance distinct from and in addition to. that which is due to conducting ions in the liquid in the absence of such solid material. When it is attempted to remove the last portions of electrolyte from a solution (solid being absent) by the migratory influence of current flowing through the 4 solution, itis clearly obvious to those who are informed on such processes, that the current decreases as the electrolyte decreases, for thel reason that the strength of current depends in part upon the amount of electrolyte pres-i ent. But when an additional conductance is provided by the presence of solid as described p in this invention the removal of the last portions does not operate to reduce the current to a substantially ineffective value. It has been found that the additional current is fully as effective, (if n ot more effective) in removing the electrolytes, as the current due directly to such electrolytes.

We are aware of the fact that wood pulp has`a`high degree of dispersion, that is, that it provides a very large surface, upon which y surface we believe there is a special conductance phenomenon. We are aware that other substances having a high degree of dispersion exhibit a simllary property of increasing conductance of liquids in which they are sus,

pended. For example, asbestos fibers, silica gel, precipitated and dried aluminum oxide, clay, carbon, etc. Other substances exhibit the property but nevertheless in a less pronounced degree. Powdered glass has a slight effect, and powdered quartz has an even less effect. Providing our theory is correct as to the so-called surface conductance along such materials, any of the foregoing substances may be added to material being electrodia-l'- lyzed in order to' increase the speed of migration and to decrease the power consumption.

Another theory which can be offered to explainthe effect produced bythe addition of pulp is `that the increased current which is shunted across the liquid channel through the fibers creates an increased magnetic field in which the ions and charged particles in the liquid or the solution are more readily moved by the applied potential at the elecand not inconsistent with a theory of operation herein presented, and furthermore in terms calculated todefinethe invention in its broadest scope including the range of equivalents which can be ascertained upon establishing the nature of the forces involved in the process.

We claim:

1. The method of' removing impurities from chemical wood pulp which comprises subjecting an aqueous suspension of said pulp in an electrodialysis apparatus to the action of an electric potential to form a current through thefmass, the pulp being present in sufficient quantity to provide towards the completion of dialysis a major portion of thel the vicinity of said confining means after pascurrent along a fiber channel and a minor porsage therethrough.

tion in a water channel, permitting impuril In witness whereof we have hereunto afties to migrate away from the mass, refixed our signatures.

straining movement of the pulp, and restraining movement of the liquid around electrodes from entry into the suspending liquid about the pulp. i

2. The method of removing f impurities from fibrous material which comprises subjecting an aqueous suspension of fiber in an electrodialysis apparatus, to the action of an electric potential to form a current through the mass, the fiber being present in suiiicient quantity to proyide towards completion of dialysis amajor portion of current along a fiber channel and a minor portion in a water channel, permitting impurities to migrate away from the mass, restraining movement of the fiber, and restraining flow of liquid across a semi-permeable partition separating theisuspension from the electrodes.

3. In an electrodialysis process the method of increasing the speed and decreasing the consumption of power necessary to move the last portions of material which is caused to migrate in a liquid by the electric current which comprises, adding fibrous material in suiiicient quantity to provide a current channel along the ber' having a conductance greater than the conductance of the liquid in the absencel of said fibers at a time near the end of the process.

4; In an electrodialysis process the method of increasing the speed and decreasing the consumption of power necessary to move the last portions of material which is caused to migrate in a liquid by the electric current which comprises, adding material having a.

high degree of dispersion in, suiicient quantity to provide a current channel along such material having a conductance greater than the conductance of the liquid in the absence of such material at a time near the end of the process.

5. In an electrodialysis process the method of increasing the speed and decreasing the consumption of rpower necessary to move the' last portions of material whichis caused to migrate in a liquid by the electric current' which comprises, adding material capable of producing a conductance along the surface of said material when immersed in the liquid.

6. The method of electrodialysis which comprises subjecting a confined suspension of fibrous material which provides a continuous ibrous path toan electrical potential whereby a current ows along said' continuous fibrous path, confining the mass by means permeable to passage of the resulting migrating units in the mass and impermeable to `fiow of liquid therethrough, whereby said units are removed by the current from the mass, and removing some or sald units from

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