CA1213561A - Chrome recovery from ferric chromic hydroxide by oxidation to dichromate - Google Patents
Chrome recovery from ferric chromic hydroxide by oxidation to dichromateInfo
- Publication number
- CA1213561A CA1213561A CA000384705A CA384705A CA1213561A CA 1213561 A CA1213561 A CA 1213561A CA 000384705 A CA000384705 A CA 000384705A CA 384705 A CA384705 A CA 384705A CA 1213561 A CA1213561 A CA 1213561A
- Authority
- CA
- Canada
- Prior art keywords
- chromium
- complex
- hydroxide
- compound
- insoluble
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F2103/00—Nature of the water, waste water, sewage or sludge to be treated
- C02F2103/02—Non-contaminated water, e.g. for industrial water supply
- C02F2103/023—Water in cooling circuits
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
Landscapes
- Treatment Of Water By Oxidation Or Reduction (AREA)
Abstract
CHROMATE RECOVERY PROCESS
ABSTRACT OF THE DISCLOSURE
A process for recovering and recycling chromium from chromium-iron hydroxide sludge for use in cooling tower make up water and plating solution is disclosed. The process comprises separating chromium from the sludge by electrolytically forming iron-chromium compound or complex and selectively oxidizing the trivalent chromium precipitate to soluble hexavalent chromium with a strong oxidizing agent such as chlorine gas, in alkaline medium. The hexavalent chromium ions then enter solution and are thereafter separated from the iron hydroxide precipitate as the sludge is dewatered.
There is also included an anode of iron, iron alloy or insoluble iron compound and a cathode, so that an insoluble iron compound or complex is produced in aqueous medium.
ABSTRACT OF THE DISCLOSURE
A process for recovering and recycling chromium from chromium-iron hydroxide sludge for use in cooling tower make up water and plating solution is disclosed. The process comprises separating chromium from the sludge by electrolytically forming iron-chromium compound or complex and selectively oxidizing the trivalent chromium precipitate to soluble hexavalent chromium with a strong oxidizing agent such as chlorine gas, in alkaline medium. The hexavalent chromium ions then enter solution and are thereafter separated from the iron hydroxide precipitate as the sludge is dewatered.
There is also included an anode of iron, iron alloy or insoluble iron compound and a cathode, so that an insoluble iron compound or complex is produced in aqueous medium.
Description
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 ~' . ',:
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 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 r ~,!, .. : . . ~' '........... . . ' ' ' :`
',~. '' .. ,_, , : . ' .
'' ' : ' 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.
- --11 .
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 ~' . ',:
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 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 r ~,!, .. : . . ~' '........... . . ' ' ' :`
',~. '' .. ,_, , : . ' .
'' ' : ' 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.
- --11 .
Claims (7)
1. 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 current 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 first 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 is produced, said third insoluble compound or complex comprising an iron-chromium hydroxide compound or complex; and whereby a substantially chromium free aqueous medium may be separated therefrom; the improvement comprising:
concentrating said third insoluble iron compound or complex with the previously formed 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 insoluble 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 recycling thereof.
concentrating said third insoluble iron compound or complex with the previously formed 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 insoluble 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 recycling thereof.
2. The process of claim 1 wherein the third insoluble iron-chromium hydroxide compound or complex comprises ferric hydroxide and chromic hydroxide.
3. The process of claim 2 wherein said hydroxides are present in a ratio of about 3:1 ferric hydroxide to chromic hydroxide, by weight.
4. The process of claim 1 wherein the pH of said concentrate is maintained at between about 8 and 10.
5. The process of claim 1 wherein the step of concentrating said third insoluble iron compound or complex comprises admitting the aqueous medium to a clarifier;
removing substantially chromium free aqueous medium from the upper portion thereof; and removing a mixture of said compound or complex in aqueous medium from the bottom portion thereof having a solids concentration of at least about 1% by weight.
removing substantially chromium free aqueous medium from the upper portion thereof; and removing a mixture of said compound or complex in aqueous medium from the bottom portion thereof having a solids concentration of at least about 1% by weight.
6. The process of claim 1 wherein the step of selectively oxidizing the chromium constituent of said third insoluble iron compound or complex comprises bubbling chlorine gas through said concentrate to oxidize said constituent to soluble hexavalent chromium.
7. The process of claim 6 wherein the step of separating said soluble hexavalent chromium ions from said third insoluble iron compound or complex comprises passing said oxidized concentrate through a centrifuge, collecting the concentrate, passing separated solids through a filter, and collecting filtrate, said concentrate and filtrate containing the hexavalent chromium ions in solution.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000384705A CA1213561A (en) | 1981-08-27 | 1981-08-27 | Chrome recovery from ferric chromic hydroxide by oxidation to dichromate |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000384705A CA1213561A (en) | 1981-08-27 | 1981-08-27 | Chrome recovery from ferric chromic hydroxide by oxidation to dichromate |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1213561A true CA1213561A (en) | 1986-11-04 |
Family
ID=4120812
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000384705A Expired CA1213561A (en) | 1981-08-27 | 1981-08-27 | Chrome recovery from ferric chromic hydroxide by oxidation to dichromate |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1213561A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102191390A (en) * | 2010-03-09 | 2011-09-21 | 刘杰 | Method for recovering hexavalent chromium resources from chromium slag |
-
1981
- 1981-08-27 CA CA000384705A patent/CA1213561A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102191390A (en) * | 2010-03-09 | 2011-09-21 | 刘杰 | Method for recovering hexavalent chromium resources from chromium slag |
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