Chromate recovery process

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 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.

This invention relates to a process for recycling chromium recovered from a 
toxic sludge which results from the removal of contaminating ions from 
waste water such as cooling tower water or chromium plating solutions. 
Waste 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. Pat. Nos. 
3,926,754 4,036,726 and 4,123,339, assigned to the assignee of this 
invention, hexavalent chromium ions from cooling tower waste water may be 
rapidly and efficiently removed electrochemically. Accordingly, the 
disclosures of these patents are hereby incorporated by reference. 
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 
surface 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 insoluble iron compound to 
thereby permit removal from solution. Whereas it was previously considered 
necessary to reduce hexavalent chromium to trivalent chromium in acidic 
solution, it was discovered that the iron compound or complex formed will 
reduce hexavalent chromium and co-precipitate therewith in solution having 
a pH from about 4 to about 11. Accordingly, the invention 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 media 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 cathodic 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 contaminant concentration in water treated will be no 
less than about 0.03 parts per million, and in most instances from 1 to 
5,000 parts per million. After treatment in the electrolytic cell as 
described above, the water containing solids or flocculent normally flows 
into a clarifier wherein the solids settle and collect at the bottom 
thereof. The overflow to discharge is water containing 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 centrate and filtrate are then 
returned to the clarifier. The solids from the filter, iron-chromium 
hydroxide at a concentration of about 50% solids, are disposed of 
according to acceptable toxic sludge disposal techniques. 
However, in the case of cooling tower water, fresh cooling tower make-up 
water must be added to the tower continually, and this make-up water 
requires the addition of fresh hexavalent chromium salts. Accordingly, 
under prior techniques, the apparatus of the above described patents is 
utilized to produce a sludge containing trivalent chromium for disposal 
while new hexavalent chromium salts 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 discovered 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 to hexavalent chromium ions for 
recycling rapidly and economically by utilizing a strong oxidizing 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 therewith. 
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 reclaiming 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 a chromium-iron 
hydroxide sludge to selectively oxidizing 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 the 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.

In the patented process for electrochemical contaminant removal as 
described in U.S. Pat. No. 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. Pat. 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 solution, forms an hydroxide, and reduces 
or reacts with the hexavalent chromium to form chromic hydroxide. The 
ferric-chromic hydroxide is formed as a precipitate or flocculant, 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 concentration 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 24 and the centrate from centrifuge 
18 are then returned to the clarifier 10 for recycling. 
The ferric hydroxide and chromic hydroxide sludge resulting from the 
contaminate 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 gallons per minute 
having a concentration of about 10 parts per million hexavalent chromium. 
The treated water after clarification then will have an accepted chromium 
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 selectively cause the chromium 
constituent to enter solution for separation. In cold, dilute alkaline 
solution chlorine gas will react as follows to form hypochlorite ion: 
EQU Cl.sub.2 +20H.sup.- .fwdarw.OCl.sup.- +Cl.sup.- +H.sub.2 O 
The hypochlorite ion then reacts in turn with trivalent chromium as 
follows: 
EQU 3Na OCl+2Cr(OH).sub.3 +4NaOH.fwdarw.2Na.sub.2 CrO.sub.4 +5H.sub.2 O+3NaCl 
The overall reaction then for the oxidation of trivalent chromium to 
hexavalent chromium is as follows: 
EQU 3Cl.sub.2 +2Cr(OH).sub.3 +10NaOH.fwdarw.2NaCrO.sub.4 +9H.sub.2 O+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. 
Preferably the reaction is maintained at a pH of about 8 or between 8 and 
10, and at room temperature. At elevated temperatures, hypochlorite ions 
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 will readily 
disassociate 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 implimented 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 
intended to be limited to the apparatus shown, and the following 
description is merely illustrative of the process of this invention. 
As noted above, the underflow in line 16 from clarifier 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 underflow 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 while the contents of 
tank 28 are treated as follows. 
In order to raise the pH of the solution in tank 28 to at least 8, a 
caustic solution is added. 
Typically, a tank 32 containing a 25-35% sodium hydroxide solution 34 will 
be utilized 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 be continuously monitored by conventional 
temperature and liquid level indicators 38 and 40 and the pH, temperature, 
and liquid level of reactor tank 28 will similarily be monitored by 
indicators 42, 44 and 46. Tank 28 preferably contains a mixer 48 to ensure 
a continuous mixing of the solution therein. Likewise, 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 clarifier 10 and a volume of 
approximately 200-250 gallons to be 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 58. After addition of the 
caustic to raise the pH to about 8, chlorine gas is bubbled through, 
preferably, a sparger pipe 60 in tank 28. A similar pipe 62 is provided in 
tank 30 for alternate operation of the process of this invention treating 
the contents of that tank. About 4 pounds of chlorine are bubbled through 
the sparger pipe at the bottom of reactor tank 28 to treat from 200-250 
gallons of the solids contained in the liquid underflow from the clarifier 
10. As soon as 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 one half hour. 
Upon completion of the reaction, the contents of reactor 28 are drained 
through line 64 to a conventional centrifuge 18. The centrate from the 
centrifuge 18 then is collected in a product tank 66. Solids from the 
centrifuge 18 are then transferred to a conventional filter 24. The 
filtrate from filter 24 is also transferred to product tank 66. 
Once the material has passed through filter 24, wash water is added thereto 
to wash the filter cake. The sludge from the filter 24 is then deposited 
in a sludge receiver 26 for disposal. The sludge will consist of only 
ferric hydroxide with a very neglible amount of chromic hydroxide or 
hexavalent chromium 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 measures. Accordingly, both reactor tanks 28 
and 30 and product tank 66 are vented to a chlorine analyzer (not shown) 
to eliminate inadvertent release of chlorine into the atmosphere. 
When reactor tank 28 is empty, the underflow from clarifier 10 previously 
routed to reactor tank 30 will be diverted to reactor tank 28. Reactor 
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 the contents 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 centrifuge 18, the solids will be filtered in 
filter 24, and ultimately, conveyed to sludge receiver 26. The liquid 
separated in centrifuge 18 and filter 24 including the wash water, will 
then be conveyed to the product tank 66 as described above with relation 
to the contents of reactor tank 28. 
In this fashion, the solids from the clarifier may be continuously treated 
at alternate tanks 28 and 30 to selectively oxidize trivalent chromium to 
hexavalent chromium. If desired, the hexavalent chromium collected in 
product 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 treatment of water 
containing about 10 parts per million hexavalent chromium initially to 
separate the chromium therefrom and subsequently recover hexavalent 
chromium for reuse in make-up water. The water is initially treated, 
preferably, according 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 contain about 1% 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 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 
pounds of chlorine in less than one half 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 FIG. 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 filter, but not limited thereto. 
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 embraced therein.