Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to a process for recycllng
chromium recovered from a toxic sludge which results from
the removal o~ contaminating ions from waste water such as
cooling tower water or chromium plating solutions.
Wast~ liquid or aqueous media containing toxic
materials such as hexavalent chromium ions has presented
an acute disposal problem. However, in accordance with
the inventions described and claimed in U.S. Patent Nos.
3,926,754, 4,036,726 and 4,1231339, assigned to the assignee
of this invention, hexavalent chromium ions from cooling tower
waste water may be rapidly and efficiently removed electro-
chemically.
In the above patents, a process and apparatus were
described wherein waste water containing hexavalent chromium
ions is caused to flow between a plurality of electrodes.
It was discovered that when the anode has a suxface or a
portion of the surface of iron, an iron alloy or an insoluble
iron compound, an iron compound such as iron hydroxide will be
produced anodically. In turn, an insoluble trivalent
chromium compound, preferably as the hydroxide, will be
produced which will complex with or otherwise physically or
chemically combine with the inscluble iron compound to
thereby permit removal from solutionO Whereas it was pre-
viously considered necessary to reduce hexavalent chromium
to trivalent chromium in acidic solution, it was cliscovered
that the iron compound or complex formed will reduce hex-
avalent chromium and co-precipitate therewith in solution
ha~ving a pH from about 4 to about 11. Accordingly, the
lnvention described and claimed in said patents produces
an insoluble iron-chromium precipitate without pH adjustment
to thereby rapidly and efficiently remove toxic hexavalent
chromium from solution. The precipitate is then removed
from the aqueous medium utilizing conventional techniques
such as a clarifier, settling pond or the like and the
aqueous media thereby clarified is suitable for disposal.
In this process, hexavalent chromium undergoes
ca-thodic reduction to form trivalent chromium as insoluble
chromic hydroxide which complexes with iron which enters
solution at the anode. These products are not susceptible to
further electrolytic oxidation at the anode, back to
hexavalent chromium, apparently due to the difference in
ionization potential, at least in part because the production
of the hydroxide ion at the anode occurs at a much lower
potential than other electrode-oxidations. Thus, because
of the non-amphoteric state of the iron complex, the reaction
continues until the undesirable contaminating ions are
completely or substantially completely removed from solution
in the aqueous media.
Normally, the initial conta~inant concentration in
~0 water treated will be no less than about 0.03 parts per million,
and in most instances from l to 5,000 parts per million.
After treatment in the electrolytic cell as described above,
the water containing solids or floc normally flows lnto a
clarifier wherein the solids settle and collect at the
bottom thereof. The overflow to discharge is water contain-
ing less than 0.05 parts per million chromium, suitable for
disposal. The underflow from the clarifier then is normally
dewatered by centrifugation~ and the solids from the centrifuge,
filtered. Both the concentrate and filtrate are then returned
to the clarifier. The solids from the filter, iron-chromium
hydroxide at a concentration of a~out 50~ solids, are disposed
--2--
~' . ',:
of according to acceptable toxicsludge ~sposal techniques.
However, in the case of cooling tower water, fresh
cooling tower make up water must be added to the towex
continually~ and this make-up water requires the addition
of fresh hexavalent chromium salts. Accordingly, under
- prior techniques, the apparatus of-the above describ~d patents
is utilized to produce a sludge containing trivalent chromium
for disposal while new hexavalent chromium saltg are continually
~ added to fresh make-up water. Similarly, in the case of
plating solutions fresh make-up water must be provided with
chromium salts therein while spent solutions are purified
for disposal.
In accordance with this invention, it has been dis-
covered then that trivalent chromium may be rapidly and
efficiently separated from the sludge and oxidized to
hexavalent chromium for recycling in fresh make-up water -
for cooling towers and plating solutions. In addition, it
has been discovered that according to the process of this
invention, trivalent chromium precipitate may be oxidized
~0 to hexavalent chromium ions for recycling rapidly and eco-
nomically by utilizing a strong oxidi~ing agent in alkaline -~
media whereby the chromium is virtually completely removed
from the sludge to detoxify the sludge so that conventional
disposal techniques may be utilized therewithO
Accordingly, it is an object of this invention to
provide a process for reclaiming and recycling chromium from
toxic sludge.
It is another object to provide a process for reclaim
ing and recycling hexavalent chromium for reuse in cooling ~ -
tower and plating solution make-up water.
It is another object to provide a process for treating
3~
a chromium-iron hydroxide sludge to selecti~ely oxidize
chromium to soluble hexavalent chromium and separate the
soluble ions from the sludge.
It is yet another object to provide a process and
apparatus for treating by~product sludge from a water
purification process whereby hexavalent chromium is reduced
and precipitated as a hydroxide compound or complex with
iron, and ~he precipitate subsequently subjected to an
oxidizing agent for selective oxidation of the insoluble
chromic hydroxide to soluble hexavalent chromium whereby
chromium may be separated from the sludge for recycling or
reuse.
The present invention, therefore, resides in an
improvement in a process for recovering contaminating
hexavalent chromium ions electrochemically removed from an
aqueous medium by forming an insoluble iron compound or
complex therewith, which process comprises passing an
electric curren~ through the aqueous medium containing
the contaminating ions between an anode which has a surface
or a portion of a surface thereof of iron, iron alloy, or
a firs-t insoluble iron compound, and a cathode so as to
produce anodically a second insoluble iron compound or
complex in said medium while cathodically reacting said
contaminating ions with the medium to generate an
insoluble hydroxide of said ions wherefrom a third insoluble
iron compound or complex with the contaminating ion
hydroxide (comprising an iron-chromium hydroxide compound
or complex~ is produced, and whereby a substantlally
chromium free aqueous medium may be separated therefrom,
the improvement in question comprising:
?,S~.~
concentrating said third insoluble iron compound or
complex with the chromium hydroxide in said aqueous medium;
adjusting the alkalinity of said concentrate to a
pH of at least about 8;
selectively oxidizing the chromium constituent of
said third insoluhle compound or complex to soluble,
hexavalent chromium ions;
separating said soluble hexavalent chromium ions
from said third insoluble iron compound or complex contained
in said medium whereby said hexavalent chromium ions become
available for reuse; and
diluting said soluble hexavalent chromium ions
with water to form an aqueous medium for xecyc]ing thereof.
The foregoing and other objects will become
apparent with reference to the drawings and following
descriptions wherein:
Fig. 1 is a schematic representation of the
conventional sludge separation and dewatering process and
apparatus; and
Fig. 2 is a schematic representation of an apparatus
for carrying out the process of this invention.
In the patented process for electrochemical
contaminant removal as described in U.S. Patent NoO 3,926,754,
cooling tower blow down water or plating solutions
containing toxic hexavalent chromium are passed through a cell
having an anode of iron or an iron containing material such
as steel. Preferably, a plurality of electrodes are
provided as described in U.S. Patent Nos. 4,036,726 and
4,123,339 and the electrodes are steel.
As the aqueous solution passes by and between the
plurality of electrodes, erosion occurs and iron enters
- 4a -
solution, forms an h~y-~roxide, and reduces or reacts with
the hexavalent chromium to form chromic hydroxide. The
~0
- 4b -
~erric-chromic hydroxide is formed as a precipitate or
floc, and as noted above the reaction proceeds to completion
without reoxidation of the trivalent chromium to hexavalent
chromium primarily because of the non-amphoteric state
of the iron complex formed with the chromic hydroxide.
With attention to Fig. 1, typically in the prior art
water from the electrochemical unit must be clarified to separate
the solids therefrom for disposal. Water containing chromic
and ferric hydroxide solids is initially transferred to a
clarifier 10. The solids collect at the bottom 12 of clarifier
10, and the overflow water containing only acceptable maximum
concentrations of hexavalent chromium, such as a concentration
of less than 0.05 parts per million, is suitable for disposal.
The overflow water then proceeds to conventional disposal
(not shown) through overflow pipe 14, and the underflow is
dewatered for disposal.
The underflow from the clarifier 10 typically at a
concentration of about 1% solids flows through line 16 -to
centrifuge 18~ Solids are concentrated in the centrifuge
18 typically to a concentration of about 15% solids and are
then transferred to a holding tank 20~ Subsequently, the
concentrated solids are pumped through a sludge pump 22 to
filter 24.
In filter 24, the solids are dewatered to a con-
centration of about 50% solids, and then transferred to a
sludge receiver 26 for disposal as a toxic waste. The sludge
as noted above consists of ferric and chromic hydroxides.
The filtrate from filter ~4 and the concentrate from centrifuge
18 a~e then returned to the clarifier 10 for recycling.
The ferric hydroxide and chromic hydroxide sludge
resulting from the contaminant removal process above described
has a concentration of 3 parts ferric hydroxide to one part
chromic hydroxide, by weight~ The process is capable of
removing 24 pounds per day of hexavalent chromium from a
flow of~ for example, cooling tower blow down water of 200
-S gallons per minute having a concentration of about 10 parts
per million hexavalent chromium. The treated water after
clarification then will have an accepted chromi~n concentration,
as noted above, for disposal. The recovery process of the
instant invention then is intended to be capable of treating
the solids separated by the electrochemical process above
described, or in fact, any chromium containing sludge. The
instant process utilizes preferably the strong oxidizing
agent, chlorine gas to ~electively cause thP chromium con-
stituent to enter solution for separation. In cold, dilute
alkaline solution chlorine gas will react as fcllows to
form hypochlorite ion:
C12 + 20H~ OCl- ~ Cl- + H2O
The hypochlorite ion then reacts in turn with trivalent chromium
as follows:
~0 3Na OCl + 2Cr(OH~3 + 4NaOH ~ 2Na2CrO4 ~ 5H2O ~ 3NaCl
The overall reaction then for the oxidation of trivalent
chromium to hexavalent chromium is as follows:
3C12 + 2Cr(OH)3 ~ 10NaOH -- ~ 2NaCrO4 + 9H2O ~ 6NaCl
It has been discovered that the presence of ferric
hydroxide precipitate does not substantially interfere with
the above reaction, and therefore, the reaction proceeds very
rapidly, producing a bright yellow color solution as the
hexavalent chromium ion is formed.
I Preferably the reaction i5 maintained at a pH of about
¦ 30 8 or between 8 and 10, and at room temperatureO At eleva~ed
temperatures, hypochloxite lons will disproportionate and form
chlorate ion.
While the chlorate ion is also a good oxidizing agent,
excess may desirably have to be removed before the water is
reused. Hypochlorite ~ill readily dissociate at room
temperature to chlorine and oxygen, and preferably, the excess
will be removed merely by storage with agitation.
The process of the instant invention may be i~lplemented
with an apparatus as shown in Fig. 2 as will be subsequently
described. As will be obvious to those skilled in the art,
however, the instant invention is not in~ended to be limited
to the apparatus shown, and the following description is
merely illustrative of the process of this invention.
As noted abo~e, the underflow in line 16 from clari-
fier 10 normally contains solids in the concentration of about
1%. The solids are chromic hydroxide and ferric hydroxide.
In order to treat the solids, a first reactor tank 28 is
provided, and preferably a second reactor tank 30 is also
provided. As will be obvious to those skilled in the art,
the number of reactor tanks provided is a matter of choice.
The under~low from line 16 then is initially directed into the
first reactor tank 28. Typically, the flow into reactor
tank 28 will proceed at a rate of about 2 gallons per minute
until the tank is about half full, and contains around 200 -
250 gallons. At this point the flow will be diverted to
reactor tank 30 and the flow will begin collecting in reactor
tank 30 whlle the contents of tank 28 are treated as follows.
In order to raise the pH of the solutlon in tank 28 to
at least 8, a caustic solution is added.
Typically, a tank 32 contalniny a 25~35% sodium hydroxide
solution 34 will be utllized as a source of caustic. Caustic
will be added from tank 32 to reactor -tank 28 through a caustic
-7-
pump 36. The temperature and liquid level in tank 32 will ~e
continuously monitored by conventional temperature and liquid
level indic~tors 38 and 40 and the pH, temperature, and liquid
level of reactor tank 28 will similarily be monitored by
indicators 42, 44 and 46O Tank 28 preerably ~ontains a mixer
48 to ensure a continuous mixing of the solution therein. ~ike-
wise, reactor tank 30 will be provided with pH, temperature,
and liquid level indicators of conventional design, 50, 52
and 54. In addition, a mixer 56 will also be provided within
reactor tank 30. The caustic tank 32 then will be selectively
in communication with the interior of reactor tank 28~ or
reactor tank 30.
Utilizing a 2 gallon per minute flow rate from clari-
fier 10 and a volume of approximately 200 - 250 gallons to
~e treated according to the process of this invention, the
two tanks 28 and 30 may be utilized so that the contents
of one tank will be undergoing oxidation while the underflow
from the clarifier flows into the alternate tank. Typically~
the caustic will be added over about 15 minute period and
approximately 7.7 pounds of a 32~ sodium hydroxide solution
will be added.
As noted above, the preferred oxidizing agent is the
strong oxidizing agent chlorine gas. Chlorine gas is provided
in tanks 5B. After addition of the caustic to raise the p~l
to about 8, chlorine gas is bubbled through, pr ferably, a --
sparger pipe 60 in tank 28. A similar pipe 62 i5 provided in
tank 30 for alternate operation of the process of this invention
treating the contentsof that tankO About 4 pounds of chlorine
are bubbled through the sparger pipe at th~ bottom of reactor
tank 28 to treat from 200 - 250 gallons of the sol:ids contained ~-
in ~he liquid underflow from the clarifier 10. As soon as
--8-- -
r
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',~. '' .. ,_, , : . ' .
'' ' : '
chlorine is admitted, oxidation will begin and the trivalent
: - ,
chromium will be immediately converted to yellow hexavalent
chromium. Tests have indicated that the reaction proceeds
- . to completion in about on~ half hour.
Upon completion of the reaction, the contents of
. reactor 28 are drained through line 64 to a conventional
- centrifuge 18. The concentrate from the centrifuge 18 then is
collected in a product tank 66. Solids from thecentrifuge 18
are then transferred to a conventional filter 24. The filtrate
from filter 24 i5 also tra~sferred to product tank 66.
Once the material has passed through filter 24, wash
water is added thereto to wash the fil~er cake. The sludge
from the filter 24 is then deposited in a sludge receiver
. ~6 for disposal. The sludge will consist of only ferric
lS hydroxide with a very negligible ~nt of chromlc hydroxide
or hexavalent chromi~m therein. Accordingly, conventional
disposal techniques may be utilized with the sludge from the
receiver 26. The wash water from filter 24 will also be
conveyed to the product tank 66. As will be obvious to those
skilled in the art, the presence in the system of chlorine
gas will require certain safety measùresO Accordingly, both
reactor tanks 28 and 30 and product tank 66 are vented to a
chlorinP analyzer (not shown) to eliminate inadvertent release
of chlorine into the atmo~phere~ -
When reactor tank 28 is empty, the under10w from
clarifier 10 previously routed to reactor tank 30 will be ~.
diverted to reactor tank 28. R~actor tank 30 will then be
similarly treated with caustic from the caustic tank 32 through
pump 36. When the pH has been established at the preferred
level, chlorine from tanks 58 will be bubbled through th contents
- ~9
~ . . .
.~ . . .
.-... .
, j - -
of tank 30 through sparger pipe 62. The contents of tank 30 `
will then be diverted through line 68 to centrifuge 18 for
separation of the liquid therein. After treatment, in the
centrifuye 18, the solids will be filtered in filter 24,
and ultimately, conveyed to sludge receiver 260 The liquid
separated in cen~rifuge 18 and fllter 24 including the wash
water~ will then be conveyed to the product ta~k 66 as described
- above with relation to the contents o~ reactor tank 28.
In this fashion~ the solids from the clarifier may be
L0 continuouslytreated at alternate tanks 28 and 30 to selectively
oxidize trivalent chromium to hexavalent chromium. If desired,
the hexa~alent chromium collected in proauct tank 66 may be
acidified or otherwise treated, and is available for reuse in
cooling tower make up water or plating solutions, as desired.
The above described process is described for the treat-
ment of water containing about 10 parts per million hexavalent
chromium initially to separate the chromium therefrom and
subsequently recover hexavalent chromium *or reuse in make-up
water. The water is initially treated, preferably, according
~0 to the process described in the above identified patents and
clarified to separate a chromic- ferric hydroxide precipitate~
The underflow from the clarifier then will con-tain about 1%
,,, ~ . j, ~
solids in the form of 3:1 iron to chromium hydroxide. The~
underflow at a rate of slightly less than 2 gallons per minute
., ~
is then treated as described above according to the instant~
recovery process to continuously separate the chromium from
the insoluble ferric hydroxide whereby the chromium is oxidized
to hexavalent chromium for reuse in for example cooling tower
or plating solution make-up water.
The reaction described above relative to the use of `~
__ ._ _.. __,_ _ .. , _ _ ., ~ . _ ___ _ . ___~ .. , . .. _ ... _ _ _ . _ __ _ . _ ... .
`
.5
- `
chlorine as a oxidizing agent has been found to proceed very
rapidly whereby at least 200 gallons of the material to be
treated may be subjected to oxidation with four po~nds of
chlorine in less than one hal~ hour. It will be obvious to
those skilled in the art that it is not intended to limit
this process to a particular flow rate, or to the quantities
of materials treated. The above description is intended to
be illustrative only of a preferred embodiment of this invention.
The above process as described may be characterized
as a batch or batch-continuous process. However~ this invention
is not intended to exclude continuous operation. For example,
chlorine gas and caustic could be continuously supplied to
line 16 to oxidize trivalent chromium in the line thereby by-
passing the need for reaction tanks.
While the apparatus shown in Figure 2 and described
herein includes both centrifuge 18 and filter 24, as will be
obvious to those skilled in the art,it is technically possible
to achieve the desired results with a filter only. Furthermore,
in certain centrifuges it is possible to collect sludge and
wash the sludge therein~ Accordingly, this invention contemplates
dewatering with any desired apparatus including a centrifuge
and/or a ilter, but not limited theretoO
The invention may be embodied in other specific forms
without departing from the spirit or essential characteristics
thereof. The present embodiment is therefore to be considered
! ~ in all respects as illustrative and not restrictive, the
scope of the invention being indicated by the appended claims
rather than by the foregoing description, and all changes
which come within the meaning and range of equivalency of
the claims are therefore intended to be er~raced therein.
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