CA1089797A - Fibrous electrolytic bed for the recovery of mercury and other heavy metal ions - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Small amounts of mercury are removed from an aqueous electrolyte solution containing sodium chloride, by recycling the solution through a chamber at a temperature of about 5° to about 98°C while subjecting the solution to the action of a direct current maintained between an anode and a cathode bed both located above the lower portion of the chamber, the cathode bed being below the anode and comprised of conductive fibers having a diameter of about 40 to about 1,000 microns and extending throughout a complete cross-section of the chamber.
Dissolved mercury is electrodeposited from the solution in the cathode bed and drops therefrom by gravitational force. The mercury is collected in the bottom of the chamber and is periodically removed therefrom. The invention also provides an electrolytic apparatus for carrying this process, which comprises a liquid containing means having liquid inlet means and liquid outlet means; an anode located at the outlet means; a cathode comprised of conductive fibers having a diameter of about 40 to about 1,000 microns, located at the inlet means; gas outlet means mercury metal outlet means: and electrical means connected to the anode and cathode for passage of a direct current between the anode and cathode. The invention is particularly suited for the removal of mercury contamination from liquid brine streams.

Description

'797 This invention relates to a process for removing small quantities of dissolved metal ions form an aqueous solution and the apparatus for employing such process.
The mercury type electrolytic cell for the production of chlorine has been used primarily because o~
the high grade caustic soda which is produced. However, recently the loss of mercury from the electrolytic cells into the waste streams has created ecological problems.
Thus, not only is the loss of mercury a costly expenditure in the chlorine producing field but it is also desirable to reduce the loss of mecury for ecological reasons. Accordingly, ; it is imperative that means be found for the removal of mercury and/or other heavy metal ions from liquid streams.
- U. S. patent No. 2,563,903 describes a process for the deposition of gold or silver employing charred excelsior as a cathode surface. U. S. patent No. 3,003,942 discloses a cell for the recovery of silver from spent photographic fixing baths employing stainless steel as a cathodic material.
U. S. patent No. 3,457,152 discloses the use of lead shot cathodes to remove trace quantities of metals from solutions.
It is also known in the prior art that a metallic coating can ~` be placed upon fibers to give the appearance of a uniform sheet of metal over theindividual fiber particles. These metal sheets can then be employed as electrodes for electrolytic devices. Nothing in the prior art, however, discloses the use of fibrous metals for the electrolytic recovery of small .~ .
quantities of metal ions from a liquid stream or, more specifically, the removal of small quantities of mercury ions.
In accordance with this invention there is provided a method for the removal of small quantities of metal ions from a liquid stream by electrolytic reduction comprising a process and apparatus employing a cathode which is comprised of ,5~, -2-... .

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conductive fibers.
The removal of small quantities of metal ions from a liquid stream by electrolytic reduction is complicated by the mass transport of the metal ions to the electrode surface where the actual reduction occurs. The quantities of trace metal which, it is contemplated, would be removed employing the apparatus of this invention can range from less than 1 part per million to 10,000 parts per million but on a practical basis the apparatus is particularly adaptable to a range of from about 5 to about 1,000 parts per million. In conventional cell design with planar electrodes, a long residence time in the cell and rigorous agitation of the liquid stream are necessary for efficient removal of the dissolved metal ions from the liquid stream.
In accordance with the present invention, there is provided a process for removing small amounts of mercury from an aqueous electrolyte solution containing sodium chloride, which comprises:
recycling the solution through a chamber at a tempera-ture of about 5 to about 98C while subjecting the solution to the action of a direct current maintained between an anode and a cathode bed both located above the lower portion of the chamber, the cathode bed being below the anode and comprised of conductive fibers having a diameter of about 40 to about 1,000 microns and extending throughout a complete cross-section of the chamber, whereby dissolved mercury is electrodeposited from ' said solution irl the cathode bed and drops therefrom by '~ gravitational force, collecting mercury in the bottom of the chamber, and periodically removing the mercury from the bottom of the chamber.

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97~37 The invention also provides an electrolytic apparatus for carrying out the above process, which comprises:
(a) a liquid containing means having liquid inlet ... ~,.
means and liquid outlet means, (b) an anode located at the outlet means, (c) a cathode comprised of conductive fibers having a diameter of about 40 to about 1,000 microns, located at the ~; inlet means, (d) gas outlet means, ; 10 (e) mercury metal outlet means, and(f) electrical means connected to said anode and ` cathode for passage of a direct current between said anode and cathode.
The stream, containing the metal ions which it is desired to remove, is forced through the bed while a direct , current is passing through the apparatus. Metal ioD reduction occurs at the cathode and the metal is deposited on the fibers of the cathode.
The fibers may consist of any metal or alloy fiber but for greatest current efficiency a material with high hydrogen overvoltage is desirable. In a preferred embodimen~
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of this invention, lead fibers are used. Such a bed has a high internal surface area to volume ratio and the number of intersecting flow channels within it provide turbulent mixing within the electrode. The bed of fibrous material has several advantages over a bed of particles such as granules, spheres, and so forth. The latter depend on particle-to-particle contact for electrical continuity. The actual cross-sectional area of such a contact is generally very small and can lead to a high internal resistance through the bed.
In a bed of fibers the electrical path is along the fiber which is a path of much lower resistance and thus to a much lesser degree is dependent on fiber-to-fiber contact. High internal resistance in a cathode bed leads to poor current distribution and thus will decrease the efficiency of the bed. A particulate bed also has a tendency to settle with time, opening voids and permitting channeling through the bed and further increasing the internal resistance. A bed of entangled fibers has a far less tendency to settle and will be considerably more stable with time. The fiber length employed may range from about 0.01 times the length of the bed to about four times the length of the bed. This means the fiber may be of one continuous length which is folded back upon itself within the fiber bed. Generally, to be ~1 most effective, it is preferred that the fiber length range ;~ from 0.1 time the length of the bed to one that is equal to the length of the bed.
The accompanying drawing illustrates preferred embodiments of the invention which together with the description serve to explain the principles of the invention.
In this drawing :
FIGURE 1 is an elevational view partially in section of an electrolytic cell in accordance with the present invention , ~ ~V1~39~9~
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employing a single electrolyte stream ;
FIGURE 2 is an elevational view partially in section of an electrolytic cell in accordance with the present in-vention employing two separate electrolyte streams.
IN FIGURE 1, the apparatus comprises a cell body 10, having a top 12 connected at flanges 30 to body 10 by adjust-able spacer bolts 32 and nuts 34. A gasket 35 may be employed to ensure a gas tight seal. Such a gasket may be constructed of rubber or other inert pliable material. Cell body 10 may L) be constructed of glass, polypropylene, polyvinylchloride - / ' /

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s~ 7 and other inert materials. The bottom of cell body 10 is provided with metal plate ~2, outlet 24 and outlet valve 26. Metal p]ate ~2 may be constructed of a non-corroding metal such as titaniun. An electrical conductor 28 is connected to metal plate 22 to allow the imposition of a negative charge thereonto.
Cell body 10 is provided with both a liquid inlet pipe 20 and a liquid outlet pipe 16.
Cathode 18 is supported by screen 40. Screen 40 is supported by nubs 41 projecting from cell body 10.
Cathode 18 may be constructed of lead or other metallic fibers. Screen 40 may be constructed of conductive or non-conductive material such as lead or polypropylene fibers. Screen 43 is similar to screen 40 and rests on nubs 41. Screens 40 and 43 may be mounted on nubs 41 by ; any cc~nven~ion~l means. Anode 14 is located substantial ~y at the point of the liquid outlet pipe. The anode may be of DSA constructioll~ Cathode 18 and anode 14 are connected electrically through conductor 42 and conductor 38, resp~ctively, to battery 45 or another potential source.
Top 12 is provided with gas outlet 46.
Another embodiment of the invention is illustrated by FIGURE 2. The apparatus comprises a cell body 48 having a ~op 50 connected at f]an~es 82 to body 48 by adjustable spacer bolts 84 and nuts 86. A gasket 88 is provided to ensure a good seal. Gasket 88 may be rubber or other inert material. Cell body 48 may be constructed of glass or other inert materials. The bottom of cell body 48 is provided with metal plate 72, outlet 74, and outlet valve 76. Metal plate 72 may be constructed of a non-corroding metal such _6_ :

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as titallium. ~n el.~ctrical conductor 78 is connected to metal plate 72 to allow the imposition of a negative charge thereontor Cell body 48 is provided with both a liquid inlet pipe 70 and liquid outlet pipe 62. Additionally, liquid inlet pipe 54 and liquid outlet pipe 56 are provided for ~ the anode 52 portion of the cell.
.. ~ Cathode 66 is supported by screen 64. Screen 64 is supported by nubs 65 projecting from cell body 48.
Screen 64 may be constructed of conductive or non-conductive ~.-.; material such as lead or polypropylene fibers. Screen 67 simil.ar to screen 64, rests on nubs 65 and may be mounted thereon by any conventional means. Cell separator 60 is located just above the liquid cutlet pipe 62 and rests on nubs 61. Cell separator 60 rests on nubs 61 which project from cel.l body 48 and may be mounted by any conventional , means. Separator 60 may ~e composed of porous glass, porous .l ceramic, porous polymeric membranes, or ion exchange membra.nes.
Anode 52 may be of DSA construction. It is located in such a position that it is in the path of the . liquid flow from inlet pipe 54 to outlet pipe 56.
.l Cathode 66 and anode 52 are connected electrically through conductor 68 and conductor 90, respectively, to . battery 69 or another potential source. Top 50 is provided with gas outlet 58.
The apparatus of FIGURE 1 may be employed as a single pass system or the stream may be recycled until the desired amount of impurities has been removed. Aqueous salt :30 solutions contaminated with mercury may be purified flowing .
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3~7 thro~1g11 th. i~>e1^ bcd past the DSA type anode formin~ a continuous electrica] path. D~A type anodes are well known to those skilled in the art and do not re~uire any further explanation. When a NaCl solution is being purified, ch~orine is evolved at the surface of the anode and leaves the cell through the gas outlet while the sodium which forms at the cathode reacts with water to form sodium hydroxide The lower section of the cell acts as a collection point for any metallic mercury which might leave 0 the fiber bed under the force of gravity. The metal plate - is malnt:ained at a negative potential to prevent re-oxidation of the mercury. As the amount of mercury collects at the bottom of the cell it may be drawn off as desired. Other impurities which may be removed from aqueous streams include soluble salts of cadmium, zinc, antimony and tin. These other metaLs would, of course, remain on the cathode.
Ihe apparatus of FIGURE 2 may be used advanta-geously to recycle the contaminated elec~rolyte stream until the desired level of metal concentration is achieved. This cell is operated with a separator such as a diaphragm or membrane. The contaminated aqueous salt solution flows through the cathodic fiber bed. A separate flow of elect:ro-lyte is maintained past the anode. As both the electrolyte flowing through th~ fiber becl and that flowing past the anode are in contact with the separator,a continuous electrical path is formed. When a NaCl solution is employed at the anode~ chlorine would be evolved. This particular embodiment may be used for removal of small ~o quantities of metals from those solutions wherein the 39~97 reduction products at the cathodic fiber bed were soluble or the anodic products were not gaseous and readily separable. This apparatus can, however, also be used with a gas evolving anode. Provisions are also made at the lower section of the cell for removing metallic mercury, - if this is the trace metal which is the contaminant.
` The cell currents, which are employed in the apparatus of either FIGURE 1 or FIGURE 2, are dependent upon the concentration of the metal it is desired to remove from the solution, the flow rates employed, and the metal oxidation state. For example, in a single pass application at a 5 mg/liter concentration of divalent mercury and a 5 ml/min/in2 area of cathode bed flow rate, a minimum 24 ma/in2 of bed current would be required. At the same concentration of mercury and a 200 ml/min/in2 area of cathode bed flow rate, a minimum current of 960 ma/in2 of bed would be required. If the concentration of mercury is ., 1,000 mg/liter and a flow rate of 5 ml/min/in2 area of cathode bed is employed, theminimum current requirement would be 4.8 amp/in2 of bed. At a 200 ml/min/in2 area of cathode bed, a minimum current of 192 amp/in2 of bed would be needed.
As mentioned earller, the diameter of the fibers employed as cathode generally ranges from about 40 to about 1,000 microns. The optimum range would be from about 100 to about 1,000 microns. The fibers should then be packed in the bed so that the void volume of the bed ranges from about 30 to about 90 percent with an optimum range of about 50 to about 80 percent.

The ~eTr~perature ranOes which may be employed are fro~l about 5" ~. to about 9~ C. The optimum is from about 20 C. to abou~ 80 C.
The elec~rolyte concentrations can range from very dilute to saturated solutions. The minimum concen-tration is one whicll would be sufficient to reduce the resistance of the solution~ For solutions of sodium chloride, the NaCl concentration can range from about 6 to about 30 welght percent.
The following examples are illustrative of the present invention and, therefore, are not intended in any way as a limitation thereof. Parts and percents are by weight unless otherwise indicated. These examples illustrate the utility of both the apparatus and the process for the removal of small quantities of mercury.

- ~ 15qo aqueous solution of sodium chloride containing 1~50 ppm ~f mercury in the form of salts was passed through the apparatus of the embodiment shown in FIC.URl~ 1. This apparatus had an annular diameter in the fibrous bed of 3.8 cm. and a bed length of 20.3 cm. The lead fiber in the bed had diameters of 0.388 mm. + o.o6 mm.
The void volume of the bed was 77.6~. The lead fiber length ranged from 5 to 15 cm. The apparatus was operated at a curren~ of 300 ma, a temperature range of 22-30 C.
at essentially atmospheric pressure. The data of Table I
shows the percentage of mercury removal at various flow rates.

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, El.lw Rate ~,I, Hg ml/mi.nute Removal . 51.l~ 9 ; 1~7-5 93.5 v A 15U,~ aqueous solution of sodium chloride con-O taining 4lO ppm of mercury was electrolyzed under conditions similar to Example 1 employing the apparatus of Example 1 at a curren~ of 300 ma. The temperature rangecl from 24-27" C. The results obtained are shown in Table II
below .

Table II
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F`low ~oncentration Conce+tration R~ate of Hg+~ of Hg Z
ml/ Entering Bed, Leaving Bed, ~ Hg minute ppm ppm Removal ~0 lOO 35 5 86 ;::
.: 138 90 15 83 930 2~ 15 40 . 1040 15 10 33 :
:~ 1476 4~ 25 44 `
A 15'~o aqueous solution of sodi.um chloride con-taining 565 ppm mercury was electrolyzed using the apparatus of Example l. The flow rate was maintained at a constant 200 ml/minute ancl the stream was recycled -- ~.t~~ 3~ ~

throu~,h the app.l~atus at essentially atmospheric pressure.
The entire system has a 6-Li~er capacity. The current employed was 200 ma and the temperature ranged from - 22-31 C. The results obtained are shown in Table III.
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Table III
Time, Mercury, ~lo Hg minutes ppm Removal 0 565 o 58 95 8~
0 100 1~ 97 165 ~ 99-5 210 2 99.6 .' .
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Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrolytic apparatus comprising:
(a) a liquid containing means having liquid inlet means and liquid outlet means, (b) an anode located at the outlet means, (c) a cathode comprised of conductive fibers having a diameter of about 40 to about 1,000 microns, located at the inlet means, (d) gas outlet means, (e) mercury metal outlet means, and (f) electrical means connected to said anode and cathode for passage of a direct current between said anode and cathode.

2. An electrolytic apparatus comprising:
(a) a liquid containing means having two sets of liquid inlet means and liquid outlet means, each inlet and outlet separated by means of a separator, (b) an anode located on one side of the separator, (c) a cathode comprised of conductive fibers having a diameter of about 40 to about 1,000 microns, located on the opposite side of the separator, (d) gas outlet means located at the anode side of the apparatus, (e) mercury metal outlet located at the cathode side of the separator, and (f) electrical means connected to said anode and cathode for passage of a direct current between said anode and cathode.

3. The apparatus of claims 1 or 2, wherein said fibers are composed of lead.

4. A process for removing small amounts of mercury from an aqueous electrolyte solution containing sodium chloride, which comprises:
recycling said solution through a chamber at a tempe-rature of about 5° to about 98°C while subjecting the solution to the action of a direct current maintained between an anode and a cathode bed both located above the lower portion of the chamber, said cathode bed being below the anode and comprised of conductive fibers having a diameter of about 40 to about 1,000 microns and extending throughout a complete cross-section of the chamber, whereby dissolved mercury is electrodeposited from said solution in the cathode bed and drops therefrom by gravitational force, collecting mercury in the bottom of said chamber, and periodically removing the mercury from the bottom of the chamber.

5. The process of claim 4, wherein the mercury is collected on an auxiliary cathode at the bottom of the chamber whereby reoxidation of the metallic mercury is prevented.

6. The process of claim 4, wherein said conductive fibers are composed of lead.

7. The process of claim 4, wherein a separator is provided between the anode and the cathode bed.

8. The process of claim 4, wherein said electrolyte solution contains sodium chloride.