CA1047966A - Method of removing mercury from solution using particulate zinc cathode - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Removing heavy metals such as mercury, lead, cadmium, arsenic, and copper from contaminated aqueous solutions, such as waste waters, including steps of passing the solution through a bed of active metal or active metal-coated particles, amalgama-ting the heavy metal in the solution and the active metal on the particles or plating the heavy metal on the active metal in an electrochemical cell, and separating the heavy metal from the particles.

Description

~ L04~966 Back~round of the Invention There i~ considarable and growing concern over contamina-tion of the nation's waterway~ with mercury and other heavy metal~
Mexcury is known to be a neuro-poi~onJ beiny especially dangerou~
in the alkyl mercury ~orm o~ten found in water and aquatic li~e.
Cadmi~l in river water ha3 been identified as the cau e o~ a painful disea~e (itai itai)0 Lead, arsenic, and other heavy metal~
are suspected of being danger~u~ pollutants in our waters.
Many of these heavy metal~ e~ter.~our waters from industrial and mining scurcesO Recent step~s to limit the pollution ~rom the3e source~, eOg., in the case of mercury ~rom chlor-alkali plants, have been relatively effective. ~owever, some reduced but ~-significant quantities of the~e heavy metals will continue to come from industrie~ and mines unless treatment ~pecifically directed toward heavy-metal removal i~ given. -Heavy-~etal contamination may also arise from "natural"
sourcas, e.g., mercury at levels to cause concern has been ~ound in lakes where little, if anyJ human activity has occurred. To clean up these waters, the only course open is to treat specifical- ;
ly to remove the heavy-metal contaminants.
Prior ~rt Methods are reported to be available and being used for ~ ;
r~moval of heavy metal9 from water. For mercury, chemical treat- ;
ment (e.g., with FeC12 or Na2S~ to ~orm elemental mercury or an insoluble mercury compound has been used. A process invoIving ion exchange as one of it~ many step3 is claimed to be effective~
U.S. Patents 3,083,079 and 3,o8s,8sg ~how specific mercury removal proce~sesO However, with low mercury concentrations, the chemical methods require a large quantity of inert carrier ~olids for ~4~796~
efficient separation. This in ~urn requires the handling and dis-posal of larg~ volwn~3s of precipitate in order to remov~3 th~
mercury The ion-exchange proce~q generate~ a mercury-loaded resin which cannot be regenerated and mu3t be disposed o~. For other heavy metals, either the situation i~ similar or no satisfactory treatment method exi~t~0 Early u~e of zinc particles as a filtering medium for separating metals andpurifyiny water i seen in UOSo Patents 418,138 and 634,462. U.S. Patents 1,743,525 and 1,789,425 use a filter medium of metallic ~ilaments, mentioning zinc but no chemical action or release of zinc ions to the solution are mentioned. Karpiuk et al (U.S. ~atents 3,029,143 and 3,029,144) make use of sodium amal~am to remove mercury ~rom solutions using ~teel or non-metallic beds~ ~malgam and mercury metal accumulate below the bed where it i8 removed ~or further treabment. Town (UOS. 33361,559) shows a proce~s o precipitating elemental mercury from an aqueous solution of sodium ~ulfide-sodium hydroxide containing mercury by the addition of elemental antimony to tha solution~ U.S0 Patent 3,039,865 ~hows an ad~ition-~ al process of recovering mercury from a~ueous solutions.
Summary of the Invention The di~closed heavy-metal removal process for removing t~e heavy metal ~rom solutions containing low contaminative concen-trations thereof, u~es a DC electric curront cell comprising a ~, bed of active metal or active metal-coated particles which pre~
ferably are in a fluidized con~i~ion (or otherwi~e agitated con- ;-dition) bed cell~ The h0avy metal is deposited on the metallized particles which may be con~idered part of tha cathode in the call. After a heavy-metal depo~it is built up a~ an amalgam or coating with or on the active metal on the particles, the

- 2 -.

~ 7~66 particles are trans~erxad ~rQm ~he cell and regenerated~ u~ing either an electrochemical or chemical technique.
Da~ription of the,Drawings FigO 1 is a process flow diagram;
Fig. 2 is a cros~-sectional ~chematic viaw of a typical heavy-metal removal unit, and Fig. 3 iS a cro~s-~ectional schematic view of a regenera- ~-tion and ~.eavy-metal recovery unitO
Detailed Description .

A flow diagram for the system using the novell!heavy-metal : ;~
removal method i8 shown in Fig. 1. The proc2ss shall be described ;;
with respect to mercury removal which involves four main teps (A) the collection o~ mercury from water in mercury removal electro- `
.. ,. ~.
chemical unit 10 containiny zinc-coated collector particl-s in- :
volving the deposition of metallic mercury and amalgamation of the mercury with the zinc; (B) electrolytic ~tripplng:.of.the zinc~
mercury amalgam from the collector particles by stripping unit 20;
.. ., ; .
(C) removal of mercury ~rom the stripping unit by line 21; and (D) electroplating zinc back onto the collector particles in a plating cell 30 and returning the collector particles back to the . ~.... .
remo~al unit lOo In the removal unit an electrical potential is applied between the fluidized balls and an anode so that (a) electrochemical rather than chemical deposition o~ Hg (or other metal) take~ place and (b) l~ss easily reduced foYms of combined mercury (or other me~al) may be depo~itedO Opsration~ (b) and (d) ;:~ :
may be carried out in a singl- cellO
The removal proces~ of Step A collects the mercury on the ' . :, zinc-coated ball3 by amalg~mationO For use in a fluidized bed electrochemical unit (Fig. 2), the balls preferably ~hould be of .
~ 3 - ~
' ',' `~ "

~- : - - ,... . --. . . . ................. . . . .
. . - . . . : . . .

1~479~
e~sentially uniform size and density. They al~o allow the collector particle~ to be conveniently reproce~ed repeatedly in the processing steps de~cribed. Agitation of the collector parti-cles in the fluidized bed is provided by action of the incoming waste wa~er or by rotatisn of the cathode (Fig~ 2)o Agitation will pro.ide effective transport of the mercury or other heavy-metal contaminants to the collector ~urface, allow suspended solids to move through the bed, prevent clogging or buildup of sludge in the bed, provide a burnishing action to densify the zinc coating on the particle~, thereby maintaining a reactive and adhering surface, and maintain a uniform di~tribution of mercury over the entire bed.
For optimum operation, round lightweight balls are pre-ferred. Metal-coated solid or hollow gla~s or plastic bead~
~erve this purpose. A primary metal layer such as silver, nickel, or copper i9 deposited by electroless plating or other means to impart electrical conductivity to tha balls~ A suitable electro-less process i9 seen in U,S~ Patents 2,532,283 and 2,53~,2840 The collector or active-metal coating i~ then applied by con-v~ntional electroplating, such as by deposition in a conventionalcyanide plating bath. The active-metal coating on the balls pre-~erably i9 zinc. While the detailed description herein refer3 to the use of 2inc, tin may be used in place o~ the zinc. Copper ~ ;
or nickel i8 preferred as the base or primary m~tal layer for reasons of cost. The preference of zinc a~ the active metal for the mercury removal process i~ ba~ed on the following: (1) it has a high diffu~ion rat~ in mercury (diffu~ion coefficient =
8.72 x 1o-2cm2/hr at 15C) thereby insuring that the surface concentration of marcury will always be low; (2) zinc has ~47966 favorable electrochemical properties for reprocessing; specifical-1YJ ~t can be easily separated from the metallic con~aminants and recycled; (3) zinc is considered to have low toxicity as an Lm- :
purity in water, thus any small Io5~ of zinc to the treated water will be relatively i~nocuous; and (4) it i~ a low co~t metal.
The water to be treated must have ~ome dissolved salt con-tent ~or electrolytic conductivity 90 that ~he electrolytic cell process may occurO This salt will normally be present in the effluent being prGce~sed but, if needed, addition of a small amount of an electrolyte~ eOgO, NaCl, to the contaminated heavy-metal-containing water may be made to enable it to be treated ~:
without serioa~ effect on ubsequent use of~the water. Strongly acid or strongly basic water~ should preferably be adjusted to a more nearly netutral pH before treatmentO This will not ~eriously affect the applicability of the method in that contaminated water .
of pH value suitable for legal di~charge is suitable for treatment by this method The conductivity of the solution 4hould be at least that of 100-200 ppm of dissolved salts (NaCl)O A conductivity :
of 200 ppm to 3.5% salt (NaCl) content is a preferred range ~he method is applicable to varlou~ form~ of mercury (for example, elemental and soluble and insoluble inorganic compounds and organic compounds, including methyl mercury) and compounds of the hcavy metals such as Pb, Au, Ag~ Cd, Cu, and ZnO Other . ~;~
heavy-metal compounds such as A~, Sb, Sn, Bi and Ni (in Cl-) may ~ . . .
be treatedO The parameters (size of cellg current, size of balls) for optimum removal will differ ~or the particular form of the particular heavy metal. Waters from a few ppb to over 100 ppm of `~
a heavy-metal LmpUrity may be treatedO It should be noted, though, :
that a given quantity of metal may be removed more easily and - 5 - ~:

, . ~ . - .. -~-.-. . .................... . ............ , .

:. :: . . . .. .. ..

~ 47~6~;
economieally the more concentrated it i~.
Fig. 2 shows a schematic of a metal removal unit 10, u~able ~o perform step A. The water (was~e water)containing the heavy-metal contaminant paqse~ upward through inlet 8 and through a fluidized bed 11 of metal-coated balls 12, which bed actc as the cathode in the electrolytic cellO A potential i5 applîed to the balls by their contact> as they move about, with each other and with a cathode. The contaminant metal is deposited by amalgamation (in the ca~e of Hg on Zn) with the metal coating the ball~ 120 The water leave~ the cell 10 as an effluent e~sentially ~ree of heavy metal contamin.ant through outlet 9. An anode 14 i~ provided which i8 kept separated from and out of contact with the balls 12 by a porous separator 150 The separator normally comprises a per-forated cylinder having a perforated bottom 16 which aids in holding ~he ball~ 120 At the anode 14, ~volution of a small amount of oxygen or chlorine or a mixture of the3e occurs simul-taneously with the metal removal proce~sO ~he cathode compri~es a central rod 17 having rods 18 spaced in a vertical plane along it~ length, the rod3 preferably being made of zinc-plated steelO
Th~ anode 14 i8 a right circular oylinder made of a Monel* ~creen in a carbon structureO Suitable terminals are provided on each of the anode and cathod~ connected to a direct current source (not shown). Voltages across the cell are in the preferr2d ranges of 2 to 12 volts. Flow rates of waste water are dependent on de~
sired removal, dwell times, and fluid velocity, and may range from small units processing 1 to 25 gpm up to large units handling from 1000 to 5000 gpm or larger~
Zinc is the preferred active-metal coating on the ball~
T~ade Mark 1~47~6~i when one i~ removlng mercury. Tin may be employedfo~ copper or lead removal wharein these heavy m~tal~ are electroplated out in the cell on the tin ~ubstrate~ The active-metal coating may be applied to balls of ~etal, glass, or pla~tic by ~uita~le known tr-atment~. Us~ of pla~tic balls in~tead o~ glas~ yields a lower overall dansity and permits lower ~low ra~e~ to achi~ve a given de-gree of fluidization. Smaller ball~ of either type would do like wise, A cylindrical cell is practical and effectiveO Some *aper ~-in the cylinder may improve uniformity of fluidization. Uniformity may al90 be improved by tangential entrance of the water, flow baffles, or u~e of air jet~
Upon the application of a potent~al to the collector or .: :
balls 12, mercury cations are deposited by the reaction:
~g+2 + 2e~ ~7Hg.

A similar reduction process i8 believed to occur for other -~
.''"' ' ' ' ' ' mercury compounds. With metal}ic mercury, the reaction is one of amalgamation: ~
Hg ~ Zn = Zn(Hg) amalgam. ~ ;
Zinc i8 not released~in these proce~ses and does not enter the water as a contaminantO
The application of the potential will reduce any corrosion of the zinc~ zinc i9 thermodynamically unstable in the presence o~ water and aqueous solutions and tends to dis301ve with the evolution of hydrogen, This reaction takes placo very slowly when ~-~
zinc is pure, due to the large hydrogen overpotential of zlnc. :
HoweverJ the reaction i~ more rapid if zinc i~ put in contact with a metal of low hydrogen overpotential, such a~ platinum In cases where tha water being treated contain~ metal impuritias other than mercury, other metal~ may be collected on tha zinc

3 surface. Although none of the~e will have hydrogen overpotentials ~ 7 -~ L~4796~i as low as platinumJ the hydrogen overpotentials of iron~ copper, and nickel, for example, are .~mall enough to increase the cor-ro.~ion rate of zinc. PrelLminary values for the corrosion rate of 0.6 mm diameter zinc particles which had collected Hg, Cd, and As were found to be about 20 ~a/cm2 at pH 605. This correRponds to ~ 1% of the rate of heavy~metal pickup by the particle~ in a bedO Raising the pH to 705 will cut this corro~ion rate by about an order of magnitudeO Imposition of a cathodic potential ~hould make it negligible. ~:

A chemical mode of ac~ion may al~o occur as follows.
Mercury or other heavy metal~ are displaced from many of their compounds by metals which are above it in the electromotive ~oree seriesO Thus, for example, mercury in an oxidized state may be di~placed by the e metalæ> zinc, for example, according to the reaction:
: . . . .
~g ~ Zn = ZN + Hg . .
This reaction takes place under a wide variety o~ conditions in-cluding variations in pH and the pre~ence of other cations and anions at high concentrations. The reaction causes the active metal to go into solution as a cation~ leaving a deposit of mercury which amalgamates with the unreacted metal, ~ince mercury amalgamatas with almost all metals (a notable exception i~ iron)0 ~.
While zinc or another active metal working alone (iOe., without applied potential) will remove mercury and it~ compounds (see UOSD Patent 3,039,865), the electrochemical process offers sever~l clear advantagesO The application of a negative potential to the zinc collector will speed up 9 luggish reaction3 and promote ~:
the deposition of mercury from relatively stable mercury eompounds, thus making the method useful for an even wider range of conditionsO

.:, ; - - , ~047~
The proce~s also becomes u~eful ~or other metals le~ noble than mercury. In the electrochemical proce~s, zinc i~ not relea~ed to the waterO In fact, any zinc which may tend to be released in a chemical mode would be removed by redeposition under the ;~
applied potentialO Further, a cost saving i~ lik~ly to be re- `~
alized using the electrochemical rather than tha chemical proce~s.
Step3 B, C, and D may be carried out together a~ ~hown -: . .
in FigO 3. The mercury-loadadJ zinc-coat~d balls 26 containing the amalgamated materials are fed into the anode compartment 21 of a fluidized or rotating particle bed cell 2~. ~ere, the zinc is stripped off, zinc ions thereby goiny into ~olution in the electro~
lyte l9o Mercury i5 left behind as the zinc is stripped away from the collector particlesO The mercury will be in the form of droplets which will coalesce and fall to the bottom of the con~
tainerO The mercury can then be removed through outlet 23 and r~covered for u~eO When fully stripped, the ball3 27 are trans~
ferred to the cell cathode compar~ment 24 where they are zinc platedO The plated balls are then recycled to the removal unit shown in Fig. 2. A separator 25 divides the compartmented cellO

~he ~eparator is porous~ allowing access of the zinc-ion-containing electrolyte, such a8 ZnO2~~ in a KOH 901ution, f~om the anode side to the cathode side of the cell. The rotating particle bed cell may comprise that shown in UOSo Patent~3~663,2980 ~ith the fluidized bed design, wa3te waters with ~uspended solids will be amenable to treatmentO For ~xample, while raw ~^
sewage would likely foul the system, it is feasible to treat primary effluent for heavy-metal removal. This would prevent such heavy-metal contaminant from interfering with biological ~-(secondary) treatment. The constant agitation of the fluidized g_ - - , - ~4~79~6 bed will tend to minimize any fouling due to organic growth on the particles and other cell componentsO Slurries consisting, for exampleg o~ mud drawn from benthic layers in lakes and streams may be treatedO Thus, mercury or other heavy metal~ which have accumulated i~ lakes and estuaries may be removed using this method, Test aqueous solutions containing heavy-me~al compounds wexe passed downward through column~ packed with granular xinc.
10 DC potential was applied between the bed (cathode) and a stainless st~el screen (anode), separated from the b~d by a porou~ separator~
The resulte are summarized in Table Io TABLE I

: COLUM~ REMOVAL OF HEAVY ~ETALS WrTH DC POTENTIAL APPLIED
. : . ; - ; _ . _, Form: Re3i- ~ Dis- ~: Per- ~.
Aqueous Applied dence Input charge cent Solution Column Potential Time ConcO ConcO Removal : ~. Motal of Siz(Volts) ~ ~ (PP~) _ :

: Cd Cd(:N03)2 2.2cm dia~J6 f~l100 ~2 98 ~ hi2h cm ~. .

As ~a AsO 0.5 cm dia 6 004 10 ~ 7 ~3o 3 3 x l9cm .
. high ~0~ ,2 cl dl~ I ~ 6 ~ I ¦ 10 ¦ ~ 2 ¦ ~ ôO

:, : :

::
~Test~ wers run with dilute solution-s of s~dium ethyl- ~ :

mercury thiosalicylate (merthiolate)* in waterO Mixing the solution with 30-me~h zinc for one hour produced a lightening * Trade Mark - 10- , ~ . ., ~ 9 6 6 in color of the ~olution, indicating a r~action wa3 occurring A ~imilar test was carried out where a 2-volt DC pot~ntial was applied between the zinc and a platinum anode. The ~olution colour was lightened ~ignificantly mors in the te~t with applied potential. A imilar t~st in which tho mercury compound, in a water ~olution with NaCl9 deposited in a~hiny ~ilm on a copper ~trip in 15-20 minutes with a potential of 2-3 volts applied, was also completed. Tests were conducted u~ing ~IgC12 (~ 100 ppm) ~ .. j ~ .
~i~solved in H20 with 8-hydroxyquinoline and NaOH base added, with an electrical potential o~ 2-3 volts pa~ed to ~inc and ;~
copper elactrode~. Shiny depos its were apparent on the electrode ` ~:
substrat~s. Without a charge on the copper strip no shiny surface appeared.
EXAMPLE 3 ;~
A laboratory scale 3yRtem comprising a feed tank, pump~
flowme~er, and from one to five fluidized bed electrochemical celLs was constructed. The cells contained a carbon rod anode, plastic screen separator, and cathode par~icles made of zinc- `;
plated, silvered, glass ~pheres. Copper ion in a 3.5~ NaCl brine wa9 pumped through each cell in ~equence, A DC potential was applied acro~s the carbon rod and the cathode~particles (uslng a screen cathode collector). The results bbtained are sununarized in Table II. " ~ :

- :

~::J
7~,6 TABLE I I

COPPER REMOVAL DATA
===cs~ _ ~ - -~ ~ r; ~ __=__~c _ Re~ i~ In it ia l Avg O F ina 1 Appl ied dence Copper Copper Te~tNo. of Voltage Time concO concO Percent NoOCells (~T) (sec) (ppm) (ppm) Removal ~ _. .-. _ ........ . .... . _ _ 1 1 6 ~ 6 o.375 00260 31 2 4 12 ~ 24 Oo 220 Oo 067 7o `:
3 4 12 ~ 24 o 560 Oo 125 78 ~ .

4 4 12 5 ~ 2L~ o 120 0 o 028 77 . .

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Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE
DEFINED AS FOLLOWS:

1. A method of removing mercury from an aqueous solution containing the same comprising:
passing an electrically conductive aqueous solution containing mercury through an electrochemical cell containing an anode and a cathode and comprising a cathodic bed of particles having zinc surfaces, passing a direct current from said anode through said solution to said particles whereby the mercury is deposited upon and amalgamated with the zinc surfaces of said particles, and discharging said solution of reduced mercury content.

2. The method of claim 1 wherein said particles are particulate zinc.

3. The method of claim 1 wherein said particles are zinc-coated nonmetallic substrates.

4. The method of claim 1, 2 or 3 wherein an applied voltage of from about 2 to 12 volts is maintained between said anode and said particles.

5. The method of claim 1, 2 or 3 wherein the mercury is recovered from said particles.

6. A method of removing mercury from an aqueous solution containing the same comprising passing an electrically conductive aqueous solution containing mercury through an electrochemical cell containing an anode and a cathode, and comprising a cathodic bed of particles having zinc surfaces, passing a direct current from said anode through said solution to said particles whereby the mercury is deposited upon and amalgamated with the zinc surfaces of said particles, discharging said solution of reduced mercury content, and recovering said mercury by electrolytically stripping the zinc from the amalgam whereby the mercury forms droplets which coalesce and are recovered.

7. The method of claim 6 wherein said particles are particulate zinc.

8. The method of claim 6 wherein said particles are zinc-coated nonmetallic substrates.

9. The method of claim 6, 7 or 8 wherein an applied voltage of from about 2 to 12 volts is maintained between said anode and said particles.