Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~)4G~4~3
This invention relates to methods for the treat-
ment of liquids to remove contaminants or pollutants from
them. More particularly, the invention is for the electro-
chemical treatment of spent processing solutions, rinse
t waters, e.g., plating rinses, waste water streams and
~ ef~luent or purge streams, as from cooling towers and the
.:., .
~ ~et scrubbing of contaminated gases, and the like. The
,i inyention also relates to electrolytic cell structures -~
utilizing iron, iron alloys or insoluble iron compounds
.. ,
for anodes thereof.
~ Prior art methods for the removal of wastes -
- from liquid or aqueous media have included chemical reactions,
drying and com~ustion, ion-exchange, sorption and electro-
lytic processes to convert the wastes to acceptable
different compounds or to concentrate them and make them
more readily disposable. The chemical processes often
require the steps of acidification, to obtain low pM;
~r. che~ical addition, to effect the reaction, such as reduction
: . . or oxidation; neutralization to precipitate out the re-
action products; and separation of the solid precipitate
from the liquid. ElectroIytic means have been employed
,. .
and iron and aluminum metals of electrodes have been convert-
~ ed to oxides or hydroxides, in the forms of flocculent pre-
- cipitates which physically entrapped or sorbed insolubles
- from the aqueous medium. However, such electrodes had not
`; been satisfactorily employed in electrochemical reactions
with contaminants to make insoluble electrode metal
28 salts or complexes of contaminants for removal
: .
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.. .. .
, . .
:; . - . . .
. .. . . ~ : .
1~64~3
", ~
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~ 1 from a liquid medium. ~ow, however, by following the method of, t~,
: "i
this inventionl with no pH adjustment of the influent or effluent
~i and no chemical additions, one can effectively remove ionic as
,',:
well as nonionic contaminants by converting them to insoluble
compoundsO The insoluble iron compounds or complexes of contami-
' nant ions formed with the electrode metal are readily removahle
::
~: from the aqueous medium in which they were orig.inally present,
r facilitating purification of the mediumO ~dditionally, in pre-
,..... .... ~ .
~t.-;' ferred embodiments the method converts the original ion to a less
toxic form by either oxidation or reduction, as in the case of
changing chromate ion containing hexavalent chromium to chromium
in trivalent formO Similarly, cyanide may he changed to ferro-
cyanide and ferrocyanate. Whereas it was previously considered
... .
: necessary to have a very high acidity in a solution to form
trivalent chromium from the hexavalent material according to -
the reaction:
Cr207 + 14~I~ + 6e - ~2Cr+3 + 7H20
. ~- = . .
in which E = 00902 + OoOl log Cr27 _ 0.~4 pH
Cr 3 ) 2
. ~:
(E being the Redox Potential), and whereas cyanide ion will be
. , ,;
: converted to HCN in highly acidic media, it is now found possible
.:. :.
''t',-'' to run the reactions of this invention at pH's hitherto con-
:-.
'"J,'` sidered antagonistic or unfavorable~
.. .-. :
- In accordance with the present invention a method for
. .
:~ electrochemically removing from an ioni~ing medium a contaminant
.~,.',,' '~
.~ ion which is capable of forming an insoluble iron compound, com-
ttt` plex or co-precipitate comprises passing an electric current
between an anode which has a surface or a portion of a surface
thereof of iron, iron alloy or insoluble iron compound, and a
~ 3 ~
. .
346~qll9
,j 1 cathode, through the ionizing medium containing the ion to be
removed, so as to produce anodically an insoluble iron compound,
' ~y~L~ or complex while cathodically reacting with the contami- :~
~a~t ion, to form insolublé products, and removing such insoluble
materials fr~m the aqueous mediumO In preferred embodiments of
the invention the contaminant, whi~h is initially toxic, is
converted to non-toxic or less tox.ic form, which is insoluble,
and is removed. ~or example, hexavalent chromium in chromate ~:
.: or dichromate salt form is changed to the less toxic trivalent
form which is capable of forming insoluble complexes and is
remoued as a complex of iron, which may also be thought of as
- a mixture of chromic hydroxide and iron o~ide or hydroxide. ~lso
. within the invention are apparatuses, such as electrolytic cells, .
- which include anodes having on their surfaces at least portion~ .
. of iron, iron alloy or insoluble iron compound and which are of -
structures to facilitate continuous flow through them of aqueous
. media to be detoxified, while avoidin~ blockages or covering of
the anodes with insulating insoluble reaction p~oductsO ;
~ . .
The present invention is applicable ko the treatment of
. 20 various liquids containing iron-precipitatable, co-precipitatable
... .
: or complex-forming impurities or toxicants,-such as those liquids ~.
resulting from cooliny tower operations~ chroma-te conversion
.. : : . . .
coatings, metal finishing industries, paper mill effluents,
: sanitary and municipal sewage, etc. Brac~is~ waters may also
be treated by this method but primarily it is useful for removal
of toxicants such as hexavalent chromium, cyanide and other such
materials from waste streams so as to avoid contamination of the
. waters into which such discharges ultimately flowO ~s a by-
- product of the process, the harmful constituents may often be
., , :
. ., : . , . ~ :
;. , , . ':,' , :. . . ~ .. , :
: ...... : . . . . . .
.. . . . . .
0~6~8
; 1 recovered in useful formO ~ven if it is decided that it is un-
economic to utilize them as recovered, possibly because of the
`.` inclusion with them of various impurities, the solid forms are
:
more readily disposed of than the dilute solutions of the toxi-
. cant initially present.
The present invention, in its most preferred fo~ns,
.. ~ chemically changes a toxic contaminant of a li~uid medium to a
less toxic form, eOg~ D hexavalent chromium to insoluble trivalent
chromium, preferably as the hydroxide, while producing an iron
~; 10 flocculent that aids in removing the less toxic.product by:form-
. ing a complex with it or otherwise physically:.or chemically .~.
combining with it to form a remo~able solid, gel or flocculent
.. material by the following synergistic reactions~ :
, ~ .
~ 1~ Cr(OH)3 is amphoteric and normally re~uires a speci-
..:
.. fic pH range for complete precipitation but in the presence of
. the iron complex of Cr~OH)3 shows less tendency to be so pH-
.. ~ sensitive;
....
~. 2) Complete precipitation is obtained over a much
wider pH range; and
..
,,: 20 3) Complete precipitation is obtained in a shorter
~A residence~timeO
'~''':
... Such reactions can be effected utilizing dilute solutions of the
~ contaminant to be treated and at reasonable p.~I~s, near neutralO
Prior art methods did not combine the use of ironO iron alloy
or insolu~ iron compound anodes with the reduction or other
.~. insolubilization reaction of the present invention and consequent-
ly, were unsuccessfulO For example~ although Belgian patent
.~ 739,684 oxidizes or reduces toxic ions in a semi-conductive bed
. of solid particles, additional chemical adjustments are required
. -5-
, .,
,:;
. ~ :
. , .
:
~ 1 to maintain the proper pH for the reaction and the solid particles
. have to be regenerated frequently~ Carbon or graphite electrodes
~ have been employed to oxidize cyanide and to reduce hexavalent
. chromium but very low pH's are re~uired to maintain use~ul reac-
- tion rates~ On the contrary, with the present invention pH
adjustment is essentially self-controlled and may be in a near
neutral range, e.g~, ~ to 11, preferably 6 to ll. In reactions
. in such ranges ef~luents containing even very small ~uantities
. of contaminants can be detoxified by treating them electrochemi-
cally in the present i~ron anode cells, making insoluble iron
.~ oxideD hydroxide~or other iron derivative, which rorms a floc-
: culent to remove suspended solid material, and at the same time
-`- causing hexavalent chromium to unaergo a rapid and complete
.. ~ reduction to trivalent chromiumO which forms chromic hydroxide
.. and separates out from the mediumO ~ complex of the iron floc-
; culent and chromic hydroxide forms and the combination is re- -
-. :
movable from the medium and entraps and sorbs other impurities :
.~:. present, both insoluble and soluble~ Unlike most reactions where- :.
. -~ ,~.
. in iron hydroxide is produced, in the present one no objection-
able rust color resultsO Also, the combination in the complex
. of both iron and chromium flocculents and/or precipitates makes :.
. - .
'. easier the separation of such solid materials from the treated
. streamO In a similar manner cyanide ion in the stream is converki-
ble to ferric ferrocyanide or other suitable d~rivativeO When
. . ~
.; either or both of cyanide ~d.hexavalent chromate and/or dichro-
mate are present in the aqueous medium to be treated the ferric
::~ ferrocyanide or other cyanide derivative and the complex of tri-
,,
. valent chromic hydroxide and iron hydroxide are readily removable
. by settling or filtration, centrifuging and other separation
.:: techniques.
-6-
... .
", ,,
0~6~4l~
.,;
The reaction does not require the presence of a
diaphragm or other separator between the anode and cathode
portions of the electrolytic cell, and because of the pH
self-controlling feature of the invention, in many cases
it does not require conductivity or pH adjustments. Sur-
prisingly, the hexavalent chromium undergoes cathodic reduction
to form trivalent chromium, insoluble choromic hydroxide
and the men~ioned complexes and these are not susceptible to
further electrolytic oxidation at the anode back to hexa-
.... .
~`- 10 yalent chromium, apparently due to the difference in ion-
, ,
ization potential, at least in part because the production
of the hydrox~de from the iron at the anode occurs at a
much lower potential than other electro-oxidations. Thus
~ecause of the non-amphoteric state of the iron complex,
the reaction continues until the undesirable contaminating
ion is completely or substantially completely removed from `
~ .
`1 the aqueous medium as a solid. Similarly, the reaction will
~~ be driven to completion when the product is a gas.
`;' Although the invention is directed primarily ~;
to the removal of hexavalent chromate contaminant from waste
,~ or process streams, it is also useful to remove phosphatej
~ cyanide, cyanate, arsen~te, pyroarsenate, antimonate, selenate
.~ .
and other such materials wherein the soluble form, upon
reduction, produces either an insoluble solid, preferably
complex-forming with iron hydroxides or oxides, or a gas.
O~ course, in cases where the gas is also poisonous, pro- -
yision will also be made for recover~ng it or chemically
converting it to non-toxic or disposable form.
In one particular aspect the present invention
.: .
provides a method for electrochemically removing from an
~- ionizing medium in an electrochemical treating zone a con-
taminant ion which is capable of forming an insoluble iron
cb/
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.. . .
,i,~, :
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104691~8
containing compound or complex, with said ion, which comprises
passing an electric current through the ionizing medium contain-
ing the ion to be removed between an anode which has a surface
or a portion of a surface thereof of iron, iron alloy or a
first insoluble iron compound, and a cathode so as to produce
anodically a second insoluble iron compound, derivative or
complex in said ionizing medium while cathodically reacting
said contaminant ion with the ionizing medium to generate an
insoluble hydroxide thereof and whereby an insoluble iron
~...................................................................... .~
..~ 10 compound or complex with the contaminant ion is produced in
'''! the treatment zone, and removing from the ionizing medium
in the treatment-zone said insoluble iron compound or complex
with tlle contaminant ion.
. ~ , - .
: In a further aspect the present invention provides . -.. .
.~, . . .
a method for electrochemically removing from an ionizing medium
. in an electrochemical treating zone a contaminant ion which is
~j~ . I , . . :
capable of forming an insoluble iron containing compound or com- :
plex with said ion which comprises passing an electric current
~;~ throu~h an ionizing medium containing the ion to be removed ;
`~ 20 between an anode which has a surface or a portion of a surface
. - .
thereof of iron, iron alloy or insoluble iron compound and a
cat~ode to form ironhydroxide in said ionizing medium, said
hydroxide reducing the contaminant ion and forming an insoluble
hydroxide of the reduced contaminant ion, said hydroxide of `.
t~e reduced contaminant ion combining with said iron hydroxide
to produce an insoluble iron compound or complex in the treat-
ment zone and remoying from the ionizing medium in the treat-
'.s ? `'
;. ment zone said insoluble iron compound or complex.
:. The various methods, apparatuses, operations, con-
~ 30 structions, conditions, details, uses and advantages of the in-
; vention will be apparent from the following description,taken in
.~ conjunction with the illustrative drawing of preferred embodi-
. . ~.
. ments of the
i,, :.
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, ~ . . .
., ~ . , . , ' ~ : .
, .. . .
10~69~48
1 apparatuses used to practice the method, in which drawing:
FIGo l is a schematic diagram of an apparatus for removing
contaminants from liquids by the method of this invention;
FIG ~ 2 is a top plan view of an electrochemical cell of
the present invention for use in removal of contaminants from
:.
aqueous solutions:
.,.
FIG ~ 3 is a central vertical sectional view alony plane
. - 3~3 of FIGo 2;
FIGo 4 is a central vertical sectional view along ~ane
4-4 of FIG~ 2;
, .................................................................. .:~ FIGo 5 is a horizontal sectional view along plane 5~5
: of FIGo 3;
~ ..
FIG~ 6 is a plot trivalent chromium ion solubility in
."~, .
: water vsO pH; and
';
FIG~ 7 is an enlarged portion of the plot of FIGo 6
.,:
In FIGo 1~ electrolytic cell 11 includes anodes 13 of
:,
- iron, iron alloy, eOgO, steel, stainless steel or insoluble iron
compound, eOg., ferrous oxideD magnetite, connected to a source
15 Of direct electric current through an anode bus 17, and
:.,
. 20 cathodes 19, connected to a negative sink 21 through cathode bus
..:
23~ In the cell illustrated the anodes and cathodes are mo~opolar
. and a plurality of them is shownO However, different.numbers ~f
- anode-cathode combinations may be employed, both greater and less,
;. and bipolarD as well as unipolar arrangements can be used.
Pump 25 draws an aqueous liquid medium or s~htion 27 from
. .
tank 29 or other source through inlet line 31~ and discharges it
.. through line 33 to electrolytic cell llo Flow meter 37 measures
-'~- '
the flow of the solution containing contaminant to be removed so
.
~ that the flow may be controlled by adjusting pump speed~ valve 78 or
..
: _g_
.. . .
.
1046~4l51
;; 1 proportion through valve 67. The solution, now containing in-
soluble derivative of the contaminant, e.g., ferric hydroxide-
chromic hyaroxide complex, in flocculent precipitate form, flows
out of the cell through line 39 past valve 41, through line 43
into settling tank 45, where coagulant is added to coagulate the
flocculents. The coagulum 47 is removed from the flowing solu-
tion sbeamby sedimentationO ~ext, the clarified solution pass-
es through line 49 into storage tank 51 or other reservoir, or
- in some cases, through a disposal ]ine connecting to a s~, a
. ,
rec~cle line or other suitable stream. Instead of utilizing a
~ settling tank, in some instances it may be desirable to employ
,~ continuous filter means for removing the insoluble precipitate
from the treated liquidO
In FIG~ so 2-5 electrolytic cell 61 includes a cup-shaped
base 63, a cylindrical central section 65 and a flanged upper
portion 66, all joined together in li~uid-tight relationship
., .
to serve as a container for electrolyte and the positioned
electrodesO The container is principally made of polyvinyl
chloride or chlorinated polyvinyl chloride but other plastics and
,;, .
inert materials of construction may also be utilized. Inside
the cylindrical body is positioned a holder 71, grooved at a
plurality of locations 73 to support the electo~es 750 The
electrode holder 71 has an open bottom with a support 77 ex-tending
across it ~d supporting the angled electrodes, anodes 79 and
cathodes 810 Electricity is carried to the anodes 79 by lead
:
83, conn~ctor 85 and strap or bus 87 while the connection to
cathodes 81 is by means of lead 919 connector 93 and bus 950
Inlet 101 carries li~uid into the cell and outlet 103 is for
the e~fluent, includlng suspended insoluble materialO Top
g_
'",t '.
'`:'
.,`~, ,~ ~ .
.~. I ' ' :~
, . '
, . .
;; ~ 10~69~
.. 1 flange 105 is joined to the cell body and serves to hold various
~ other cell par-ts in position.
~s illustrated, the electrolytic cell is of the ~onopolar
.. type, with alternating electrodes of different sign~ How~ver,
: this may be modified to a bipolar arrangement, in known
''',
~-~. mannerO
:~ '
.` The curves of FIG~s 6 and 7 show pH ranges wherein tri-
valent chromium is insoluble. The 'linner" curves of both figures
.;, :,
-~ represent trivalent chromium solubility in the absence of other
.:
... 10 materials such as ironO The outer curves show the extensions of
'
. the pH range in which trivalent chromium may be precipitated when
~ iron is presentO Thus, ferric hydroxide-chromic hydroxide mix-
. .
tures and complexes are producible to xemove chromium in trivalent
form over a wider pH range from a contaminated liquid medium
which originally contained hexavalent chromiumO
-.; The contaminant to be insolubiliæd by the present re-
~: action may be in various liquid media in whlch ionic reactions
-:.
may occur but in almost all cases the medium will be aqueous and
usually it will be primarily, over 50% water. Generally, the
csntaminant will be substantially completely or entirely dis
: ;
-: solved in the medium~ The water content of the medium will
~ normally be over 90%, usually over 95% and o~ten over 99%O
::; . .
~ However, various non-interfering solvents, solutes and suspe~ded
`.',:. :
. materials may also be presentO For example, an agueous alcoholic
~ medium may be utilized and additional dissolved solids such as
. .
'~! are present in brac~ish waters, sea waters, chromate conversion
s:` coating rinses, metal~inishing treatment rinses, metal plating
.. rinses, cooling to~er purge waters, foundry wastes and agricultu-
: .
.; ral run-offs are treatable by the presentmethod, along with the
,,
10--
'.
:.
:'. ' : ' '
48
~ 1 particular mentioned contaminants to be removed from themO
. In addition to removing the toxic contaminants pre-
viously mentioned, the present processes andapparatuses are
. capable of treating other materials which may accompany tho~e
mentionedO Included in this group are ferrocyanides, borates,
tungstates, phosphites, sulfates, sulfites, persulfates, carbo-
nates, silicates and phosphates and various forms of such materi-
als of higher and lower acidities and higher and lower oxidation
levels, eOgO, perborates, bisulfatesF sesquisilicates, etcq If
the iron oxide or hydroxide complex of such material or an iron
.:
salt of it is less soluble than the concentration of the material
in the medium to be treated, it will usually be removable by the
process. Even in soma cases where such materials are ~uite
,
soluble, they tend to be sorbed by the flocculent reaction
. products and their concentrations are diminishedO
.
The various contaminants are normally presant in or were
added to the medium being treated as soluble salts or as acidsO
; Generally, when they are present as salts the salts will be
.` ammonium or alkali metal salts such as sodium and potassium
. 20 salts but because the reactions are essentially ionic the parti-
.; cular salt-forming cations are not of highest importance~ Mix-
tures of such salt-forming materials may be present and of course, ~.
mixtures of the contaminants may be trea-tedO
:
The anodes will have the active sur~aces theraof at least
partially of a material which releases iron in the agueous
.:~ medium due to the ac~ion of the electric current. It is not
,. .,~
;,
.; essantial that the iron donating compound or material should
.: cover the entire surface of the anode but usually it will be at
least 50% and preferably over 90~ thereofand in most preEerred
. ,' ''. .
~,: ' . ' , . .
.. . . .
`- ~046~9~8
l cases the entire electrode will be of such material. Mixtures
of iron, iron alloys and insoluble iron compounds may be employed,
.~ The cathodes are preferably also of iron, to facilitate their
;,. ::
~. use as anodes when it is desirable to reverse current flow for
.. - cleaning purposes, to increase cell life, etcO, but they may
also be of other suitable electrode materials previously men~
~. tionedO Because the cathode is relatively inert with respect
.~ to removal of the contaminant ions from the aqueous medium bein~
treated the nature of th~ cathode material is not critical but
it should be compatible with the anode and the electr~yte and
of cours~, should not interfere with the insolubilization of
the soluble contaminant to be removedO
The electrolytic cell employed may be of any structure
in which the electrodes are located in any suitable configuration
and the electrodes may be either monopolar or bipolarO ~oweverf
;~ vertical stacks of monopolar iron electrodes in which each
, ..
.. ~ electrode is monolithic are preferredO E~uivalent forms, such
. . .
.. - as screens, perforated curved sheets, mixtures of screens and
,i:
~ perforated sheets, solid sheets, bars and shot may be used to
.
~- . 20 promote the best flowsO Also, electrodes of the different
~ materials already mentioned may be employed and the anodes and
.~. cathodes may be the same or differentO In some instances, it may
.:i be desired to utilize different materials for individual anodes
or a ~rality of such materia~ may be employed in an anode or
`~ cathode. The assembled electrolytic cell, with electrodes,
. .
:~ frame, conductorsO piping and seals in place, is usually sized
.` to fit the particular application contemplated and sizes,
: clearances and designs may be varied, as is suitableO
' ':` '
. .
. -12- :
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:':, " . " '
- `` 10~6441~
; 1 The method of this invention may be applied to various
concentrations of the anionic contaminant in ionizing medium.
Generally, the proportion of such contaminant will be bss than
1% in the medium although as much as 15% may sometimes be presentO
Normally, because of the limiting effects of the solubilities of
the complexes and other insoluble derivatives of the contaminants
the initial contaminant concentration will be noless than 0O03
parts per million~ In most instances it will be from l to 5,000
pp~mO and usually is from 2 to 500 pOp.m. The current density
. 10 range, in amperes per square foot (~SF~ will be within the 0O05
. . ~
-: to 500 range, preferably l to 10 and more pre~erably 2 to 3 for
~ both bipo~r and unipolar arrangementsO The voltage is usually
; from Ool to 50 volts, preferably 0O5 to 20 volts and most prefer-
.` ably from 5 to 15 volts in unipolar arrangementO The current is :
in the r~ge o~ 0O5 to 20,000 amperes~ p x ~erably from 2 to l,000
am~eres per cell module and most preferably from 50 to 500 amperes
for unipolar~ For bipolar cells tha voltage range is 500 to
`.~ 50000 when the current is from 0O5 to 50 amperes. For unipolar
. configurations current will be high and voltage low whereas for
: 20 bipolar cells the reverse will usually be trueO
The pH is maintained within the range of about 4 to ll,
,}
:~ more preferably from 6.)to lOo It is recognized that it should
~ be such that the insoluble derivative or complex of the contaminant
:~ fo~n will have a solubiIity sufficiently low so that the treatment
: decreases its concentration in the aqueous medium and such decrease
~i
:,-
is usually to less than half the quantity initially presentO
~? Normally the concentration will be lowered to less than 5 pOp-m-
and preferably to less than l pOpOmO with decreases to less than
;' O~l pOpomo~ OoOl pOp~m~ and even to 0O00 pOp~mO not being uncommonO
/. --13--
.
.
... . .
~;
* Of course, the pH, as well as the electrical conditions,
can usually be adjusted or controlled to produce the most
. . ~
desirable removal of contaminant and such adjustments will
- depend on the nature of the contaminants. The temperature
~; of the contaminated solutions to be treated will normally
;
- be within the 10 to 50C. range, preferably about 15 to
: 30C., and often will initially be ambient and will be
raised by the electrolytic reaction about 2 to 20C. above
ambient.
Dwell times in the various apparatuses for the
^~ contaminated medium to be treated may vary widely, being
as little as 0.001 minute to as long as an hour but normally
., .~
`'~ at least five seconds or 0.1 minute is required. The
-~ dwell time is determined by the flow of electricity, with. ~;. .
six coulombs being the theoretical amount and being found
; in practice to be about the charge required to convert one
. i. ~, .~ milligram of hexavalent chromium to trivalent chromium.
~,;, .
When utilizing single pass continuous processes and electro-
lytic cells it is found that an excess of from 50 to 200
of electricity m~y often be required, generally about 100%. ;
` After the insoluble contaminant compound or complex of iron
: i^
~` has been made it may be filtered out or settled out from
: . .
- the effluent liquid from the cell by conventio~al means.
:~ .
- To help speed the separation of the insoluble
Y complex from the liquid and to help remove other insoluble
materials, after removal of the treated liquid from the ~-
;' :
electrochemical cell it may be further treated with a suit-
~~ able polyelectrolyte or coagulant. Among the most useful
!s of such materials are the polyfunctional polyelectrolytes.
.~ 30 The polyelectrolyte or o~her coagulant or
,.- ,,
.. ....................................................................... .
,,~,, `,.: ~
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flocculent treatment is preferably effected before any separ-
ation of flocculant or other insolu~les ~rom the effluent from
the treatment c~ll or afterward. Such treatments may be
repeated but usually only one is enough. The treated and
separated effluent may be recycled to another or the same
electrochemical cell for retreatment or may be cascaded
:. ~
~` to other such cells for subsequent treatments. In each
case, polyelectrolyte or equivalent treatment may be
employed to assist in removing the insolubles but usually
.. .
it will be effected only after the final treatment or
~- after the initial and final treatments.
The following examples serve to illustrate the
` lnYention but do not limit it. All parts are by weight
- ~ and all temperatures are in C. unless otherwise mentioned.
,, .
' The examples given are mostly with respect to the removal
of low concentrations of hexavalent chromium in the form
v ~ :
~ of chromate ion from waste waters because this is an
f~ important and difficult problem which has been solved by
the present invention. Nevertheless, it is evident that
~any other impurities are also removable by the same or
' slightly modified methods.
;,, .~ EXAMPLE 1
~-;
:~,i,.,~i,
~ To an apparatus of the type illustrated in Figs.
.,.~.;. .
2-3~ having a volume of twenty gallons, equipped with
multiplate iron anodes and cathodes and spacers of poly-
~i vinyl chloride, 0.03 inch thick, there is fed an aqueous
solution containing chromate ion corresponding to 150
'~ p.p.m. of hexavalent chromium. The flow of such solution
~ 29 is maintained at a rate of five gallons per minute.
. - :
:~'
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- 15 -
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....
",:. ~ - . - . . . ..
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0~6~L4~3
,:
; The pH of the incoming solution is 6.5 and the reaction is
carried out without making any adjustments of pH by addition
of acid, base or buffer, othex than by the production of
hydroxyl ions at the cathode. A DC voltage of ten ~olts
is applied and the current flow is 50 amperes. The current
density is 2 to 10 ASF and is preferably held at 2.S ASF.
The electrochemical reaction for the operation of this
example takes 6 to 12 coulombs to reduce 1 mg. of hexa-
;
valent chromium to trivalent chromium. For oxidations
" 10 or reductions of other materials proportional quantities
o~ electrlcity are used, according to Faraday's law.
The reaction is monitored by periodic withdrawals of
samples of the aqueous medium and by analyses using s-tand-
ard photoelectric means, as well as iodometric techniques.
The treated medium is removed and to it are
. ... .
added about 3.5 p.p.m. of polyfunction~l polyelectrolyte ;
' obtained ~y the reaction of 77.7 parts of acrylamide mono-
. . ~ . .- mer, 29 parts of maleic anhydride, 5 parts of 37~ aqueous
~ormaldeh~de, 0.1 part potassium persulfate and 400 parts
o aluminum sulfate octadecahydrate. The polymer has a
Yiscosity of about 9,000 centipoises at 25C.
' A~ter mixing of the polyfunctional polyelectro-
i Iyte the liquid is allowed to settle for two minutes, after
;-~ which time a clear effluent is obtainable. Upon analysis -
` it is found that the chromate content has been reduced to ~
, ~ .
' 0.001 p.p.m. Because of the excellent reduction in
i, chromate content the liquid medium is not cascaded to -
- other cells nor is it recycled. Instead, it is ~
:. , :
~; 28 directly dischargeable into waste streams. When the
,'' ~``. '
;' ` . ,
: : '
: , . - ,:
.
," c~,,' ~:
~''' .
~ ~ .. , . . , . . , . . ~
: - ` 10~6~L~8
.
experiment is repeated, using as the contaminant liquid a rinse
obtained from the chromium plating of metals, similar excellent
results are obtainedO The followlng is a comparison of initial
and final concentrations of components of such a rinseO
:~ Initial Concentration F I ~c~ntr~s~
:-~ (p.pOmO~ (pOpOmO)
~ . Chromium ~total) 205 0.001
,
Cr~6 200 0.001
3 30 o.00
- Nickel 0O7 oOoo
. 10
Iron 0O4 0O00
; S04 120
:,-- = ..
.... Fe(C~)6 ~ Fe (C~)4 80 20
:;~
SiF4 ~ SiF6 80 20 :;
.::
i.~ pH 605 9.0
:.; In another run, using a charge of rinse water effluent :.
.*: .
~- from a phosphate metal treatment process, a 100 pOp.mO concentra-
tion of chromate is reduced to OoOl pOpomo ~ under the same
conditionsO
From the results of the experiments it is apparent that
., ,~ .
the method is economically sound, and technically correctO The
:~ good rates of reaction and extents of chromate removal are shown
.
c~ to be obtained even at very low concentrations, apparently be-
:~,
cau~e they depend primarily on the electrical energy applied,
~: within the 6 to 12 coulomb/mgO of hexavalent chromium rangeO
~ hen, instead of using the chromate rinses from metal
plating solutionsO similar rinses from conversion coatings or
related metal treatments are employed or when purges from cooling
, .. .
; -17-
~. ~
:' ,
. :: ............ . . .
~ ~ .
10~6~4~
.
` tower waters are treated, corresponding results are obtain-
ed as long as the above number of coulombs is applied.
This also applies, in cases of removal of other contamina-
tions, over the ranges of pH, currents, voltages, current
. :j
densities and other factors mentioned previously in the
-
~ specification for chromate removal.
.. :
The use of-my polyelectrolyte or coagulant addi-
tionally aids in separating out the insoluble complex and/or
other products by the described reactions but good reductions
~ 10 in the quantities of toxic ions present in the solution
- ~ .
to be ~reated are also obtainable without the use of the
polyelectrolyte or similar material. The polyelectrolyte,
at 3 p.p.m., when used after the electrochemical reaction
of this invention to treat plater's waste containing
CrO4 and CN in usual concentrations, is especially
effective in reducing such ions to less than l or often,
:, .
; D.l p.p.m.
- EX~MPLE 2
i, Following t~e method of Example 1, the effluent
2~ from a chromate conversion coating system, used to conversion
~; coat aluminum parts of automobile heat exchangers in a
s~ CQmmercial manufacturing plant, is passed through the electro-
~- chemical cell and is subsequently treated with polyelectrolyte.
The current density utilized is controlled to between 2.Q
ana 2.5 ASF and the electrical energy requirements are set
to apply about 6 to 12 coulombs/mg. of hexavalent chromium.
' ., .:
2~ The following results are obtained.
.. i,, .. ,. ~ .
".''. '':
.,~
'
~, .
- 18 -
.,.', .
~ cb/
,.. ~ ~ :
.'. ' .
, . . .. .. . . . . . .
1~4~
1 Initial Final E~PoA ~*
Concentration Concentration Standard
'' ~P Op Om ~ ) (E~,Om ~ ) (p-p Om O )
. . _
`~ Hexavalent chromium 70 0002 Ool
~ Suspended solids80 102 20
- Fluorine 20 5.0 15
Copper 6 Ool
Iron 5 OoOl
Aluminum 60 0.01
..
Zinc 50 Ool 5
, ~
Turbidity 80 106 2.0
; pH 5.8 8.6 805
.:
., -
;- *Environmental Protection Admin~tration
:',,,' :
EX~MPLE 3
. . . ~
~ In a twenty gallon volume cell like that of Example 1,
.,~
;~ and also using the apparatus of FIG. lo 15 gallons of a chromium
plating rinse having a hexavalent chromium content of 50 pOpOmO
and o an initial pH of 602 are treated at room temperature and
... .
at atmo~pheric pressureO over the twenty pairs of anodes and
cathodes there is applied a voltage of about four volts, corres- -
ponding to a current density of 305 ~SF over the total 1.7 sq.
~` ft. area of each electrode. The applicatlon of the mentioned
voltage is continued ~or ten minutes, during which time the
electrolyte is circulated through the cell by the pumpO ~t the
,
completion of the operation the pH has risen to 905, the product,
, .
i~ of a dark brown color, is removed and th~ hexavalent chromium
content is reduced to 00000 pOp .mO The precipitate is coagulated
and filtered off in the same manner as described for Example 1.
~.' ' .
EX~MPLE 4
At a throughput rate of about ten gallons per minute,
19--' '
,'~,'
, ..................................................................... .
`'':
.
. .
`~ 6~918
.I. 1 a cooling tower water purge containing 20 pOpOm. chromate, 96
`:
pOp.m. sulfate, 65 pOpOmO zinc ion, 95 pOpOm. phosphate, 150
pOp~mO chloride, 40 pOpOm. calcium ion and about 100 pOp.mO of
.~ suspended material is subjected to the electrolytic chemical
treatments described in Example 2~ The solution has an initial
-. pH o~ 608 and after electrolysis at 6 amperes it is clarified
and is found to contain no chromate, no zinc and no calcium ions
: and greatly diminished quantities o~ the anions, less than 20%
. of the proportions originally presentO ~t the end of the reaction
the solution p~ is 8.50 ~ith the solution 50 pOpOmO of the poly- :~
; electrolyte of Example 1 are admixed at room temperature to
.; coagulate out the s~lid produced and those solids originally
. presentO The polyelectrolyte addition results in the pH being
. reduced to about 7.
'., '~
. EX~MPLE 5
A foundry effluent containing 2% of colloidal clay, 30
.; . .
- pOp.mO of hexavalent chromium, 10 pOpOmO of CN , 500 pOpOm~ of
`~ Ca++, 300 pOpOm. of Fe+++, 10 pOpOm. of phenol, 5 p~pOmO of
Cu~ and 3 pOpOmO Ni~ and having a pH of 7.8, is treated by the
.-- 20 method described in Example 4O Electrolytic treatment is effect-
ed without prior filtration. ~fter five minutes of treatment~
with the solution being treated flowing through the cell, the .:~
colloidal suspension is clarified and insoluble metal com~xes
and apparently coagulated colloidal material are separated out
. from the a~ueous mediumO Analysis of the clear effluent shows
: that it contains 0 pOp.m. chromate, 0 pOpOmO of the mentioned
metal ions and significantly reduced (to less than 20% of the
original quantities) amounts of otner listed components of the
original s~htion being treated.
:. -20-
:,.
;'
:, . , . '. . ,.; . :~ :
' ' 1_X~MPIIE 6
The following tables describe the decrease in chromate
concentration as treatments of rinses from chrome plating opera-
tions are continuedO It is noted that the time for reduction to
'' O pOpcm. chromate concentration i~ about the same in the experi-
,`' -ments reported in Tables 1 and 2 although the conce~tration of
.
the chromate,in the material, the treatment of which is described
,~, in Table 2; is s-ubsta,ntially greater initially.
Ti~BLE 1
10Time ~,fter Start p~ Potential Current Chromate ConcO kwO-hrs.*
~`'(minutes) _ ~volts) ~amperes) (POPom~
?~
`-' O 7~1 600 7.0 4807 0
. 1 9ol 5.0 600 902 0.15 '
2 9.1 5~0 600 201 0030
3 9ol 5.0 600 Ool 0045
.,,~ .
4 902 5.0 600 0 0~,60
, . .
~' * kilowatt hours ~,
'.. ` T;I~BLE 2
'~.'
Time After Start pEI Potential Current Chromate ConcO kwO-hrsO
(minuteS2 ~ _ (volts~ (amperes) (pOpOmO)
` 20 0 5.2 7~0 700 86~oO O
,
~ 1 7~,5 700 7.0 5002 0025
. .:.- .
2 9ol 7 00 7 ~0 205 0050
3 9~1 700 7~.0 0 0.75
E~MPLE 7 '~
Table 3 describes additional experiments run utilizing an
electrolytic cell of the type shown in FIGS's 2-50 As is noted,
~,, the chromate concentrations are reduced to less than one p.p.mO
.. ...................... .
from concentrations as great as 300 pOp~mo The reaction ta'kes
.......
place in all cases in a single pass of room temperature liS~uid
, .....
" --21--
.. '~ . .
,:
o, ~ 46A~8
1 through the cell~ For higher concen~rations a multipass, recycle
; or cascade arrangement of cells is used and found to be addition-
ally successfulO Tandem arrangeme.nts are also useful for per-
. iodic clean-outs and maintenance of the cellsO
. ' .,
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o o o o o o
C.)--- O o o O o
: .
~I LnLn O Ln ~ a~~') Nl-1 Ln n
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,
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.~ ~ . O ~ ~9 ~ ~ CO CO ~ Ln Ln ' '
~ ~ NO ~D Ln L-) ~ CO ~ S) Ln
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,
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:. ~ .,~ O O . O O O ~ ~n ~ O Ln
.i ~ n ~ I` I` co co O~
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.,. _ ...
U~
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o Ln ~ o O " ~o O o Ln.
j p~ - ~ :
046~
1EX~M.PLE 8
In a continuous process, utilizing the electrochemical
cell-o~ FIGsSo 2-5, sixty different samples o~ chromate-contain-
. ing aqueous solutions are treated to remove the chromate~ The
- electrodes are hot rolled iron and the current passed through
~'
them is 50 amperes at 5 volts, with the current density being
~ about 2.5 ~SF and the flow rate of the aqueous medium treated
.~: being about five gallons per minute. Results o~ the experiments
:.
are summarized in Table 4, followingO
.
~ 10
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,
~ . ,:
~ .
,~,,
. .;
:, .
.:- 20
,.,
...
. :'
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...
. ~ ., .
-25_
:"'
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.
:.,
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10~6~48
OO O O
n ,1 o o
--I 0 V~-`1 NLl') ~5)
,
~ .
S~ ,
O ~ ~q
~r F
~'~
~ ~ co
o ~d' ~ ~ N
~ O O
.. ~ ,_1u)In o o
a) ~. ~~ ~ u~ o
00 0 1` ~9 `
~' 0 5~` .,
'; ~ E-~' : '
,' P:3.`
tf) ~~ ~
Id, D O O
~ a~ c~o a) ~
~j. . ...
.~ ~
. ~ . 1 ~o ~o ~ ~ ~
.~., .
,', H
., ~
~' o
0. ~ .`
~,, P O O O, o , `
~'
0~
~' ~ o ~ ~ ~ ~
o O O O O
rl O O O ~n
H t~
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a) ; `-
~,: O
, ~Z; U~
;: .. ,
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",'' ' ' : ` ' , ` '' ; ', ~. , '` . .:
"' '' . ` . ` ` . ~ ` .'. '.
L6qL~8
During continuous operations, such as those of this
~ example, it is preferable that bothlanode and cathode be made
.. of the same material, such as hot rolled i.ron or steel, so
; that periodic current reversals may be effected, causing equal
:. ;
::` consumptions of both electrodes, thereby prolonging the useful
~ life of the electrochemical cell. Normally, the direction
~ .
`~ of the flow of the current is reversed every eight hours.
EXAMPLE 9
u~aS
The apparatus of FIG'5 2-5/modified so that the
monopolar electrodes are alternately iron and lead, like those
described in Canadian patent application S.N. 163,376 which
was filed on 9 February, 1973. Setting the electric current.
flow so that the lead electrodes are anodes, the hexavalent
; chromium of an aqueous solution containing about 30 parts per
:~ million chromate ion is converted to lead chromate, according to
.~
.. ~ the method of above identified Canadian patent application
163,376 after which the current direction~is reversed, making
the iron electrode the anode. The~iron hydr~oxide, chromic
` hydroxide (from hexavalent chromium not reduced or trivalent
, 20 chromium not previously convert.ed to the hydroxide, now insol-
~ ubilized to chromic hydroxide) and the complex or mixture of
ii-' the hydroxides and/or oxides, insoluble in the aqueous medium
~,.
at the pH developed, about 8.5 are removed by settling and/or
~, centrifuging. Also removed with the flocculent precipitate is
the small amount of lead which was previously soluble in the
system. By following this method, the addition of phosphate
. to the water being discharged is avoided (phosphate is employed
. ~ ~, . .
., in the method of above mentioned Canadian Patent application
163,376 to remove any lead in excess of that required to
:. 30 combine with the hexavalent chromium to form lead chromate).
': '
, .
:
.. ` an/sv - 27 -
:':;'
,.~
": ,~r~
. '. ~ c~
~,.. .
'
': .
~ 1 In the foregoing experiments the reactions can be speeded
by application o~ additional electriclty above the theoretical
amountD especiall~ in continuous single-pass cells. A broad
range is from 100 to 500O/o of the t:heoretical charge~ although
150 to 300% is more usual, and 200% is most preferred for the
continuous processes.
EX~MPLE 10
Effluents from two different sources, one containing hexa-
valent chromium and the other containing cyanide are blended
together in about 1:1 proportion of the mentioned contaminants
: and are then treated by passing the mix through the treating
cell of Example 1 in the manner therein described~ In the ratio
.~ of mixing employed the p~ of the mixture is about 5, although
.... p~',s in the range of 4 to 7 oP 8 are also o~ten acceptable. ~t .
the completion of treatment the pH has risen to about 7 but final
:. ph"s in the range of 6~5 to 9 are also acceptable an~ often
,` result in excellent decontaminations of treated svlutionsO The
s initial and final concentrations of vaxious components of th~
. solution are given below.
, , .
.-.... 20 Ions Prese~t Ini-tial p.~mO 3~J~ Ç~
CrO3 i CrO4 (about 1:2) 100 o
. Zn+~~ 50 lol
C t~ 3~ 0O3
Ni~+ 4
+ . .
~1 50 0.1
. Ca 60
. . .
Cl 100
CO3 50
pO4 60
.: -28-
j:
:; ~ . . . ,, . .. ~
;:., ~ i .
~0~64~8
. _
. 1 C~ 105 OoOl
Na~ 80
. . +
K 30
;``' Sa~ 100 _ ,,
~ s is seen from the foregoing results9 excellent removals
of hexavalent chromium and cyanide ions are obtained. Wh~n the
:. ratios of CrO3 to CrO4 are varied over the range of 1:20 to
s .
. 20:1 or when the hexavalent chromium containing contamlnant is
.
solely CrO4 or Cr2o7 initially, equally good removals are
~ 10 obtainable in the presence of the cyanide ion, which is also
; removed to a similar extentO ~hen the ratios of effluents are
.... .
. varied so that the proportion of chromate or other he~avalent `-
.: ~
., chromium-containing ion(s) to cyanide ion is in the range of
~;'
~ I00 to 100:1, preferably 1:50 to 50:1, 1:5 to 551, such good
."........ .
,~. removals also result. The blends of hexavalent chromium and
;.~ .
~: cyanide-containing effluents may be obtained from various commer- -
cial sources, including different plating solutionsa cooling
tower water purge streams, phosphatizing ~lutions, anodizing
......... solutions and plating rinses~
The invention has been described with respect to illustra- :
:
tions and examples thereor but is not to be limited to them
because itwill be obvious to one o~ ordinary skill in the artO
:,.
with the present speclfication before him, thdt equlvalents
;` ana sub.stitutes may be utilized without departing from the
`; scope of the inventionO
.
.: .
.
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....
: .
, . . .
~ -29-
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