IE46914B1 - Treatment of aqueous despersions - Google Patents

Description

This invention relates to a process for increasing the rate of zonal concentration and final density of solids in an aqueous dispersion of such solids and to apparatus for the practice of such process.

In many industrial processes, aqueous dispersions of solids are obtained as waste streams. For example, washing of ores, scrubbing of gas streams, precipitation of impurities, sewage disposal, etc., often produce waste streams referred to industrially as tailings, mud, etc. Such waste streams often cannot be conveniently disposed of due to materials handling difficulties or ecological or safety considerations. Therefore, it is a common practice to retain such Ig wastes in large ponds, holding tanks, or other containment means. To the extent the solids in the retained dispersions can be zonally concentrated by settling, flotation, etc., to provide a relatively solids-free aqueous phase which can be sewered or otherwise disposed of following any necessary filtration or purification procedures, space is provided in the containment means for retention of additional waste. However, in the case of many aqueous dispersions, zonal concentration of the solids by settling takes place at an extremely low rate, if at all. Thus, periodic construction of additional containment means at great difficulty and expense becomes necessary. Numerous attempts have been made at increasing the rate of zonal concentration of solids in such dispersions. However, the use of conventional means such as filtration, settling aids, or the like, is frequently physically impossible or prohibitively expensive. It has been found that laboratory techniques for effecting zonal concentration of solids in aqueous dispersions often cannot be successfully applied to large scale industrial waste retention systems. For example, it is known that applying an electrical potential between electrodes placed in an aqueous dispersion will cause migration of water towards the cathode to effect zonal concentration of solids via electroendosmosis. However, in large systems where the anode and cathode must, as a practical matter, be separated by relatively great distances, large and non-linear voltage drops occur which prevent effective application of the electroendosmotic technique.

It is thus well recognized that practical means for promoting zonal concentration of solids in aqueous dispersions have long been desired by those skilled in the art.

The process of the present invention is one for removing water 20 from an aqueous dispersion of solids which have an electrical charge in relation to ground zero via electroendosmosis, which process compri ses: mounting a first electrode means in the aqueous dispersion, mounting a third electrode means in the aqueous dispersion separated 25 from the first electrode means, mounting a second electrode means in the aqueous dispersion intermediate the first electrode means and the third electrode means β and closer to theSfirst electrode means than the third electrode means, establishing a direct current electrical potential between the first and second electrode means and the third electrode means wherein the water in the aqueous dispersion of solids is moved toward the first and second electrode means whereby the water may be removed and the solids remain.

The apparatus of the invention is one for increasing the rate of zonal concentration and the final density of solids having an electrical potential in relation to ground zero in an aqueous dispersion of solids, which apparatus comprises: a first electrode means having the same polarity as the dispersed solids, a third electrode means separated from the first electrode means ' and having the Opposite polarity from the dispersed soTids, < a second electrode means having the same polarity as the dispersed solids, intermediate the first electrode means and the third electrode means and closer to the first electrode means than the third electrode means, and means for applying a direct current electrical potential between the first and second electrode means and the third electrode means wherein water is moved toward the fii^st and second electrode means and may be removed from the aqueous dispersion, i There folflows a description of preferred embodiments of the invention.

I The process and apparatus or the present invention apply for use on aqueous dispersions contained in large (greater than 100 square meter area) impoundments, in which the dispersed particles are characterized by having an electrical charge in relation to ground zero. The greatest practical advantages of the invention are obtained with systems wherein zonal concentration of solids by natural settling does not rapidly occur to a high density.

In the practice of the present invention, electrodes are positioned within the aqueous dispersion as hereinafter described and a direct current electric potential is applied to effect zonal concentration of the solids via electroendosmosis, The dispersed particles within the hereinafter described aqueous dispersions may be characterized as- having a negative electrical charge in relation to ground zero.

The critical spacing of electrodes required in the present invention is described by reference to the drawings in which Figure 1 is a schematic top view of an electrode arrangement as employed in previously known electroendosmosis techniques in which a first cathode means 1 and an anode means 3 are disposed in an aqueous dispersion of solids 4. Figure 2 is a schematic top view of an electrode arrangement used in the practice of the present invention wherein a first cathode means 1, a second cathode means 2, and an anode means 3 are disposed in an aqueous dispersion of solids 4. Figure 3 is a schematic top view of a more preferred arrangement of electrodes according to the present invention wherein a first cathode means 1, a plurality of second cathode means 2, and a plurality of anode means 3 are disposed in an aqueous dispersion of solids 4.

It has Long been recognized that an electrode arrangement as shown in Figure I can be used to effect migration of water in an aqueous dispersion towards a cathode means 1 so long as the cathode means 1 and anode means 3 are separated by relatively small distance. However, when the distance between the cathode means 1 and anode means 3 is greater than a fewmeters, a large voltage drop occurs in close proximity to the cathode and anodes and a smaller drop •10 per linear meter more distant from the electrodes which preventseffective application of the electroendosmosis principle. In general, electroendosmosis techniques cannot be practically employed for increasing the rape of zonal concentrations in aqueous dis15 persions in systems wherein the voltage drop over any 5% of the linear distance between the cathode and -9 „ anode is greater than 30% of the total voltage drop between the cathode and anode. Where the 5% rule is violated,. large percentages of energy are wasted in the form of heat and the electroendosmosis system ceases to function properly.

In accordance with the present invention, it is unexpectedly found that proper positioning of the second cathode means 2 between the first cathode means 1 and anode means 3 as shown in Figure 2 will prevent the occurrence of unduly large voltage drops across email linear distances and will allow electroendosmosis techniques to be used in large impoundments In accordance with the present invention, the first cathode means 1 and the anode means 3 are separated by a distance sufficiently great that if the second cathode means 2 were relocated so that the first and second cathode means 1 and 2 were closely adjacent and the same distance from the anode means 3, and if a direct current electric potential sufficient to effect preferential migration of water in the dispersion toward the cathodes were imposed, the voltage drop over any 5% of the linear distance between the cathodes and anodes would be greater than 30% of the total voltage drop between the cathodes and anodes.

If the first cathode means 1 is located closer to the anode means 3, the advantage provided by the present invention is substantially reduced or eliminated. In the treatment of most aqueous dispersions, the maximum advantages of this invention are obtained wherein the first cathode means 1 is separated from the anode means 3 by a distance of at least about six meters.

In the practice of the present invention, the second cathode means 2 will be positioned closer to the first cathode means 1 than to the anode means 3 and will be disposed on a plane perpendicularly transverse to che plane between the first cathode means 1 and the anode means 3. The anode means 3 and second cathode means 2 each are comprised of a plurality of electrodes which are preferably positioned within the impoundment 4 in the form of two concentric circles about the first cathode means 1. The plurality of anodes comprising the anode means 3 are positioned such that each anode is equally spaced from its adjacent anodes. Similarly, the plurality of cathodes comprising the second cathode means 2 are positioned such that each cathode is equally spaced from its adjacent cathodes. The electrodes will be arranged such that a line drawn from the central cathode 1 through a cathode of the second cathode means 2 will bisect the angle formed by lines drawn between the central cathode 1 and alternate pairs of adjacent anodes of the anode means 3. Thus, in Figure 3 the angles 0 are all equal. Each electrode is anchored in its preferred position by a large weight which is attached by a nonconducting nylon rope. The relative spacing of the first cathode means 1, the second cathode means 2, and the anode means 3 will be such that when the potential chosen to effect electroendosmotic migration of water towards the first and second cathode means 1 and 2 is applied that no 5% of the linear distance between the first and second cathode means 1 and 2 and anode means 3 will exhibit a voltage drop greater than 30% of the total voltage drop between the first cathode means 1 and anode means 3. The optimum positioning of the electrodes, particularly the second cathode(means 2, for particular aqueous dispersions and applied electrical potentials can be readily determined by routine testing.

The maximum distance by which the anode means can be spaced from the first cathode means 1 is limited only,by the maximum distance at which required control of voltage drop can be obtained by proper positioning of' the second cathode means 2 and, as a more practical matter, the distance at which electroendosmosis can be induced by application of reasonable electric potentials. As a practical matter, spacings between the first cathode and anode greater than sixty meters will rarely be employed.

It will be recognized by those skilled in the art that the permissible and optimum spacings of electrodes will be dependent upon the characteristics of the aqueous dispersion of solids, electrode design, β and electric,power sources available. However,.as previously mentioned, for any given dispersion, electrode design, and electric power source permissible and optimum spacings within the limitations above set forth can be readily determined by routine tests.

The materials employed for the cathodes are not critical and any electrically conductive material can be employed. Preferably, the materials employed for cathode construction will be relatively resistant to chemical attack by the constituents of the aqueous dispersion. However, the cathode may be relatively light in weight as they, unlike the anodes, will not be decomposed electrically. The anodes are preferably largely iron. Iron is preferred as an inexpensive 6814 metal which is easily corroded to form iron-oxide.

The design of the individual electrodes must also be considered. It is generally preferred to use elongated electrodes of relatively small cross-sectional area. The electrodes could, for example, be formed from bar stock or I-beams, and railroad rails are particularly well suited. For longer operating times solid rods are preferred over hollow pipe because a greater mass is present for corrosion.

Preferred systems will employ a plurality of second cathodes spaced around a centrally located first cathode and a plurality of anodes spaced around the cathode system as shown in Figure 3. Such an arrangement permits optimization of electric fields in the system and effectively subjects large areas of the aqueous dispersion to the electroendosmotic effect.

For reasons of safety, it is desirable that the cathodes and anodes be totally submerged in the aqueous dispersion.

The arrangement of electrodes has been discussed relative to systems wherein the electrodes are laterally spaced. However, depending upon the characteristics of the aqueous dispersion and the containment means within which it is held, it may in some instances be desirable to employ vertically spaced electrode systems or combinations of vertically and horizontally spaced electrode systems. It is generally desirable that the anodes be disposed at a depth somewhat greater than the depth of the cathodes when water is removed from aqueous dispersions of negatively charged particles in order to promote concentration of the solids towards the bottom of the containment means in which they are confined. An example of vertical spacing has the shape of a pyramid wherein the anodes are positioned at the four corners of the base and the cathodes are located at the apex.

A direct current electrical potential is con9 6 914 nected between the anodes and cathodes. The current and voltage levels are chosen to remove the greatest amount of water from the aqueous dispersion of solids at the least power costs.

Theory dictates that an electrical field with a constant potential gradient should be optimum. However, in large impoundments the desired constant gradient is impossible to achieve and relatively large poten tial drops are suffered near the cathode and anodes.

To smooth the potential'gradient the second cathode means is added between the central cathode and the anodes. It is found that the measured field resistance of the aqueous dispersion is much more dependent upon the cathode area than the anode area when an anode rich system is used.

Several factors dictate the need for an anode rich system. One factor is that in the impoundments water moves toward the cathode. It is desirable to move the water toward a central location where it can escape relatively rapidly from the aqueous dispersion and be easily removed from the impoundment. Thus, a greater number of anodes are required surrounding a central cathode system. Additionally, the anodes are consumed electrochemically as predicated by Faraday's law. „ (anode) Fe —Fe+2 + 2e-’ (cathode) H20 + 26^0-2 + H2 Fe + HgO + 2e—^FeO + Hg + 2e ' Faraday's law states that one gram equivalent of metal will corrode for each 96,500 ampere-seconds, or, in other words, 56/2 = 28 grams of iron will corrode for each 96,500 ampere-seconds of electricity consumed.

Tt is, therefore, required that the mass of the anodes be large compared to that of the cathodes. The cathodes do not corrode because they are held at a high negative potential. It is the amperage, not potential, which consumes anodes and by adjusting the surface area of the cathodes che field resistance of the aqueous dispersion, and the corresponding amperagevoltage relationship, may be varied over large limits.

In the practice of the present invention various auxiliary techniques can be employed, if desired. For example, electrolyte can he added to the aqueous dispersion to increase the conductivity thereof or electrode cooling means can be provided to remove excess heat to prevent the complete drying of the material adjacent the anodes.

The drying of the aqueous dispersion surrounding the anodes, and the resulting buildup of solids on the anode, is one of the major problems encountered when using the techniques of this invention. To prevent the complete drying of the material surrounding the anode, a gas or liquid may be bubbled through the aqueous dispersion adjacent the anode. A more practical solution requires the periodic reversal of the electrical potential between the anodes and cathodes for a short period of time to cause migration of the liquid within the aqueous dispersion toward the anodes instead of toward the cathodes. However, reversal of the electrical potential causes rapid disintegration of cathodes which are relatively light in weight compared to the anodes. To prevent disintegration of the cathodes a second anode may advantageously he installed beneath the central cathode. When the polarity of the electrical potential is reversed, the cathodes are electrically removed from the circuit and the second anode is connected in their place so that the second anode disintegrates rather than the cathodes.

This invention may also be utilized when the dispersed particles within the aqueous dispersion are characterized as having a positive electrical charge in relation to ground zero. All parameters of the foregoing disquisition of aqueous dispersions containing negatively charged particles equally apply when Π the particles have a positive charge except that the positions of the anodes and cathodes must be reversed.

To cause migration of water toward a central location, the anodes, rather than cathodes as discussed above, must be located at the center of the electrode system. Referring to Figure 3, in an aqueous dispersion, a first anode means 1 is surrounded by concentric circles of the plurality of anodes forming a second anode means and the plurality of cathodes forming the cathode means 3.

By the use of this invention, water is separated from the solids and brought to the upper surface of an aqueous dispersion where it may be removed and the solids are more rapidly compacted than by natural settlement. Thus, the volume of aqueous dispersion of solids which can be accepted by the impoundment within any time span is increased over that volume which can be accepted by the impoundment if settling is accomplished solely by natural means. Similarly, the more rapid settling of the solids caused by this invention more rapidly restores the impoundment to an acceptable habitat for aquatic life.

Working Example An electroendosmosis system as described herein is tested in a large 120 hectare pond. The system consists of 20 stations, each station comprising one central cathode, 4 secondary cathodes and 8 anodes arranged as in Figure III. Each secondary cathode is placed 3 meters from the central cathode and each anode is placed 36 meters from the central cathode. Each cathode is an iron pipe about 5 meters long, submerged such that the tops thereof are about 1 meter below the mudline. Each anode is a railroad rail about 10 meters long, submerged such that the tops thereof are about 3 meters below the mudline.

The pond is a storage pond for aqueous illite clay slime from the benefication of phosphate ore. At start-up of the electroendosmosis system the mudline in the pond is at an elevation of 225 meters above sea level and rising with the addition of further slime at a rate of about 15 centimeters per month.

About 25,000 watts of D.C. electrical power are applied across each station. After 8 months of operation the mudline rises only 30 centimeters. Without electroendosmosis the rise would be about 120 centimeters. During operation the temperature of the slime is measured frequently at various locations between the central cathode and the anodes as a measurement of the uniformity of the voltage drop between the cathodes and the anodes. Temperature variations within +5°C. between locations at any given time shows that the voltage drop is substantially linear.

This invention can also be used, for example, in separating water from muds, sludges or slimes resulting from mining and/or processing of various native, nonorganic or fossilized organic ores such as bauxite, alumina, fluorspar, feldspar, barite, pyrophyllite, talc, ilmenite, andalusite and cyanite; coal; peat; micas; diatomaceous earths; clays such as kaolin, bentonite, fullers earth, ball clay, fire clay and crushed stone. The invention has further utility in, for example, separating water from sewage and various other muds, sludges or slimes resulting from river dredging, paper manufacture, beet and cane sugar processing or .phosphate sludges.

Claims (26)

1. A process for removing water from an aqueous dispersion of solids which have an electrical charge in relation to ground zero via electroendosmosis, which process comprises: 5 mounting a first electrode means in the aqueous dispersion, mounting a third electrode means in the aqueous dispersion separated from the first electrode means, mounting a second electrode means in the aqueous dispersion intermediate the first electrode means and the third electrode means and closer to 10 the first electrode means than the third electrode means, establishing a direct current electrical potential between the first and second electrode means and the third electrode means wherein the water in the aqueous dispersion of solids is moved toward the first and second electrode means whereby the water may be removed and the solids 15 remain.

2. A process according to Claim 1, in which the said dispersed solids have a negative electrical charge in relation to ground zero, the first electrode means is a first cathode means, the third electrode means is an anode means, and the second electrode means is a second cathode 20 means.

3. A process according to Claim 1, in which the said dispersed solids have a positive electrical charge in relation to ground zero, the first electrode means is a first anode means, the third electrode means is a cathode means, and the second electrode means is a second anode 25 means.

4. A process according to any of the preceding claims, which comprises mounting the second electrode means intermediate the first electrode means and the third electrode means wherein no five percent (5%) of the linear distance between the first electrode means and the third electrode means exhibits a voltage drop greater than thirty percent (30½) of the total voltage drop between the first electrode means and the third electrode means.

5. A process according to Claims 2 and 4 in combination, in which the second cathode means is disposed in a plane perpendicularly transverse to the plane between the first cathode means and the anode means, and separating the first cathode means and the anode means by a distance such that if the first cathode means and the second cathode means were located at the same distance from the anode means and closely adjacent, some 5½ of the linear distance between the combined first and second cathode means and the anode means would exhibit a voltage drop greater than 30% of the total voltage drop between the first cathode means and the anode means.

6. A process according to Claim 5, which comprises separating the first cathode means and the anode means by a distance of at least six meters.

7. A process according to Claim 6, which comprises disposing a plurality of second cathode means circumferentially around the first cathode means, and disposing a plurality of anode means circumferentially around the first and second cathode means.

8. A process according to Claims 3 and 4 in combination, in which the second anode means is disposed in a plane perpendicularly transverse to the plane between the first anode means and the cathode means, and separating the first anode means and the cathode means by a distance such that if the first anode means and the second anode means were located at that same distance from the cathode means and closely adjacent, some 5% of the linear distance between the combined first and second anode means and the cathode means would exhibit a voltage drop greater than 30% of the total voltage drop between the first anode means and the cathode means.

9. A process according to Claim 8, which comprises separating the first anode means and the cathode means by a distance of at least six meters.

10. A process according to Claim 9, which comprises disposing a plurality of second anode means circumferentially around the first anode means, and disposing a plurality of cathode means circumferentially around the first and second anode means.

11. An apparatus for increasing the rate of zonal concentration and the final density of solids having an electrical potential in relation to ground zero in an aqueous dispsersion of solids, which apparatus comprises: a first electrode means having the same polarity as the dispersed solids, a third electrode means separated from the first electrode means and having the opposite polarity from the dispersed solids, a second electrode means having the same polarity as the dispersed solids, intermediate the first electrode means and the third electrode means and closer to the first electrode means than the third electrode means, and means for applying a direct current electrical potential, between the first and second electrode means and the third electrode means wherein water is moved toward the first and second electrode means and may be removed from the aqueous dispersion.

12. An apparatus according to Claim 11, in which the said dispersed solids have a negative electrical potential in relation to ground zero, the first and second electrode means are first and second cathode means and the third electrode means is an anode means.

13. An apparatus according to Claim 11, in which the said dispersed solids have a positive electrical potential in relation to ground zero, the first and second electrode means are a first and second anode means and the third electrode means is a cathode means.

14. An apparatus according to Claim 12, in which the second cathode means is positioned intermediate the first cathode means and the anode means such that no five percent (5%) of the linear distance between the first cathode means and the anode means has a voltage drop greater than thirty percent (30%) of the total voltage drop between the first cathode means and the anode means.

15. An apparatus according to either Claim 12 or Claim 14, in which the second cathode means comprises a plurality of cathodes arranged in a concentric circle about the first cathode means, each of the plurality of cathodes being equally spaced from its adjacent cathodes.

16. An apparatus according to Claim 15, in which the anode means comprises a plurality of anodes arranged in a concentric circle about the first and second cathode means, each of the plurality of anodes being equally spaced from its adjacent anodes.

17. An apparatus according to Claim 16, in which the plurality of cathodes of the second cathode means and the plurality of anodes of the anode means are arranged such that a line drawn from the first cathode means through one of the plurality of cathodes will bisect the angle formed by lines drawn between the first cathode means and alternate pairs of adjacent anodes of the plurality of anodes.

18. An apparatus according to either Claim 16 or Claim 17, in which each cathode and anode of the first and second cathode means and the anode means comprises an elongated metal rod of a relatively small cross-sectional area. 5

19. An apparatus according to Claim 13, in which the second anode means is positioned intermediate the first anode means and the cathode means such that no five percent (5%) of the linear distance between the fi rst anode means and the cathode means has a voltage drop greater than thirty percent (30%) of the total voltage drop between the first 10 anode means and the cathode means.

20. An apparatus according to either Claim 13 or Claim 19 in which the second anode means comprises a plurality of anodes arranged in a concentric circle about the first anode means, each of the plurality of anodes being equally spaced from its adjacent anodes. 15

21. An apparatus according to Claim 20, in which the cathode means comprises a plurality of cathodes arranged in a concentric circle about the first and second anode means, each of the plurality of cathodes being equally spaced from its adjacent cathodes.

22. An apparatus according to Claim 21, in which the plurality 20 of anodes of the second anode means and the plurality of cathodes of the cathode means are arranged such that a line drawn from the first anode means through one of the plurality of anodes will bisect the angle formed by lines drawn between the first anode means and alternate pairs of adjacent cathodes of the plurality of cathodes. 25

23. An apparatus according to either Claim 21 or 22, in which each cathode and anode of the first and second anode means and the cathode means comprises an elongated metal rod of a relatively small cross-sectional area.

24. A process according to Claim 1 for removing water from an aqueous dispersion substantially as described in the Example. 5

25. An apparatus according to Claim 11 substantially as hereinbefore described with reference to, and as illustrated in the accompanying drawing.

26. An apparatus according to Claim 11 substantially as described in the Example.