Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ 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
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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 - ~
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~- : - - ,... . --. . . . ................. . . . .
. . - . . . : . . .
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
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~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
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:. :: . . . .. .. ..
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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
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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
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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
.:, ; - - ,
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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
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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,
EXAMPLE 1
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
:, : :
EXAMPLE 2
::
~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
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~ 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. " ~ :
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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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