CA1057234A

CA1057234A - Apparatus for the electrolytic regeneration of hexavalent chromium compounds and recovery of metals from waste bichromate solutions

Info

Publication number: CA1057234A
Application number: CA257,571A
Authority: CA
Inventors: Zenon Todorski
Original assignee: Noranda Inc
Current assignee: Noranda Inc
Priority date: 1976-07-22
Filing date: 1976-07-22
Publication date: 1979-06-26

Abstract

ABSTRACT OF THE DISCLOSURE:

An apparatus for the electrolytic regeneration of hexa-valent chromium compounds and recovery of metals from waste bichro-mate solutions comprises a tank, a plurality of semipermeable mem-branes subdividing such tank into a plurality of compartments, cathode and anode electrodes positioned in alternate compartments of the tank to form a series of cathodic and anodic compartments and means for circulating the waste bichromate solution through the anodic compartments to oxidize the chromium compound to its hexavalent form, and at least part of the waste bichromate solu-tion through the cathodic compartments for electrodeposition of metals on the cathode.

Description

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This invention relates to an apparatus for the electro- -lytic xegeneration of hexavalent chromium compounds and recovery of metals and sulphuricacid from waste bichromate solutions.
There have been several attempts to recover chromium and metals such as copper from bichromate solutions. Most of the investigators attempted to recover chromium and metals from waste bichromate solutions by chemical methods i.e. first reducing the hexavalent chromium to trivalent by means of a reducing agent such as sulphur dioxide and then precipitating chromium and metals from the solution in the form of a hydroxide by neutralizing the solu-, tion with a sodium or calcium base.
Canadian patent No. 219,176 granted May 30, 1922 disclo-~ sed and claimed what is believed to be the first electrolytic pro- ~ -,l cess for regenera~ing chromium acid from waste chromate solutions used for oxidizing organic compounds. The patentee of the above patent found that chromium sulphate in aqueous solutions may be economically transformed into chromic acid by a continuous elec-trolytic process in which approximately 80% of the chromium sul-~ phate content of the waste solution could be reconverted into chro-~ 20 mic acid for re-use in the oxidation process. The electrolytic process was later improved in Canadian patent No. 865,709 granted ~-March 9, 1971 wherein a feed solution of a waste trivalent chro-mlum~salt solution to be regenerated is continuously introduced into the catholyte compartment of an electrolytic cell having an anolyte compartment and a catholyte compartment separated by a ~ -i~ ~ semipermeable polytetrahaloethylene membrane. However, neither ~
. . , of the above patentees mention recovery of metals such as copper, zinc, etc. from the waste bichromate solutions. In fact, the -.: :
cells designed by these patentees are not suitable for the reco-very of copper, zinc, etc. from waste bichromate solutions.

Mitter e~ al, in an article entitled "Recovery of Pi-`~ chromate and Sulphurlc Acid Llquor from Waste Chrome Liquor", pu-:~, ` :, '.'.' -blished in the Journal o ~Scl5entlfic and Industrial Research tIn-dia) v. 2 1943, disclosed a batch electrolytic process using whi-te porous pots as semi-permeable diaphragms to recover both sodium bichromate and copper values. This process was labour consuming -owing to the fact that the ceramic pots had to be handled manual-ly. Also being made of fragile material, they had a short life span. Since this process could not be operated in an economical manner, it has not survived as a useful process.
It is therefore the object of the present invention to provide an apparatus for the electrolytic regeneration of hexava-lant chromium compounds and recovery of metalswhich can effective- -ly recover the above products in a more economical manner.
The apparatus, in acco~dance with the inv~ntion, compri-ses a tank, a plurality of semi-permeable membranes subdividing said tank into a plurality of compartments, cathode and anode electrodes positioned in alternate compartments in the tank to form a series of cathodic and anodic compartments and means for circulating the waste bichromate solution through the anodic com-partments to oxidize the chromium compound to its nexavalant form, ~ ~ 20 ~ and at least part of the waste bichromate solution through the ca-; ~thodic compartments for the electrodeposition of metals ~n the ~ athodes.
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~;~In cases where the metal content of the waste bichromate solution is high and no great load of regenerated hexavalant chro-mium is needed, all the waste bichromate solution may be past ; through the cathodic compartments to recover the metaI before fee-ding the solution through the anodic compartments to regenerate . .
,~the~chromium compound to its hexavalant form. To promote metal de-position on the cathode, the above apparatus may also be provided with means for produ¢ing internal recirculation of the catholyte through all or only part of the cathodic compartments.
~;In order to prevent corrosion, the tank and all other .:

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~57~34 components of the apparatus exposed to the bichromate solution should preferably be made of corro~ion proof material such as PVC
(polyvinylchloride).
Because the metallic deposit accumulates on the catho-des, thecathodic compartments have to be built so as to allow a sufficient space between the cathode and the membranes to permit their liftings from the compartments. This space has to be such as to accumulate enough metal deposit on each side of the cathode and thus make strippingof cathodes economical. Since on occa-sions, some of the cathode deposit breaks off and has to be fished out with a scoop, the cathodic compartment has to be sufficiently roomy to facilitate such an operation. The optimum spacing has been found to be between 1 and 4 inches. An alternative method to recover the metal falling off from the cathodes would be to place a PVC perforated basket having about 50~ free area within the cathodic compartments and suspend the cathode within the bas-ket.
The anodes of the apparatus must be made of insoluble material such as lead or antimonial lead as commonly used in the industry. Other materials are also suitable, such as platinum, ,' , but the high cost often precludes their use. The cathodes may be ~ ' . ,~ ~ ,.
made of any suitable ma~erial but are normally made of the same `~ matèrial as in the process to facilitate recovery of the metal by ' ' ',~, simply melting the cathode with the deposited metal.
The membranes must be made of a material which will -~ pass,cntions to the cathode but will not let through the anions. "~
A suitable material is Nafion (reinforced polytetrafluorethylene , ~, ;~ c1Oth~. Asbestos sheets have also been used but were found to be ' of limited strength. Ceramic plates could also be used but they , '~
have a high resistance~and have been found to develop cracks du~
~', ring operation. ~' ' The membranes have to be protected by a screen so as to ,~
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prevent damage thereof during mechanical handling of the cathodes which have to be removed from the cathodic compartments for reco-vering metal therefrom.
For mounting the membranes within the tank, internal flanges may be welded or otherwise secured on the bottom and the walls of the tank and additional flanges provided for securing the membranes and screens to such internal flanges. An additio-nal means of mounting the membranes would be to space the membra-nes and screens by means of U shaped holding channels extending the full thickness of the cathodic and anodic compartments.
The invention will now be disclosed, by way of example, with reference to the accompanying drawings in which:
Figure 1 illustrates a schematic diagram of anelectroly-tic cell which is commonly used for regeneratinghexavalent chromium compounds and recovering metals from waste bichromatesolutions;
Figure 2 illustrates a perspective view of an apparatus in accordance with the invention, Figure 3 illustrates a partial perspective view of a ¦ cathode used in the apparatus of Figure 2;
¦ 20 Figures 4 and 5 illustrate a method of mounting the .. ~ . .
membranes in the apparatus of Figure 2; and Figure 6 illustrates an alternative method of mounting the membranes of the apparatus of Figure 2.
Referrinq to Figure 1, there i~ shown a schematic dia-gram of a cell which has been used to ~recover both sodium bichro- -mate, sulphuric acid and metal by a batch process. The cell 10 . . , - ~ is made of any suitable electrically non-conductive and corrosion ~proof material and divided into two~compartments by a semipermea-~;~ ble membrane 12 which, when inserted into a waste bichromate solu-tion will~pass cations to the cathode but will not let through the anions. An anode electrode of insoluble material is placed in one compartment and a cathode electrode of a suitable material, - 4 - ~ ;

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~L~1)57Z34 such as copper when copper is to be recovered, is positioned in the other compartment. The membrane 12 thus subdivides the cell into an anodic and a cathodic compartment. When a copper contai-ning waste bichromate solution is placed in the cell and a suita-ble potential E applied across the electrodes, copper is deposited at the cathode while the trivalent ions of the waste bichromate solution are reoxidized to their hexavalent form in the anolyte compartment. The overall reaction can be represented as follows:
At the Anode Cr2(S04)3 + 8H20 - 6e = 2H2CrO~ + 3H2S04 ~ 6H

Waste bichromate Regenerated chromium compound or .
if only the ions taking part in the reaction are considered Cr3+ - 3e ~ Cr6+
At the Cathode . . ~
Cu++ + 2e ~Cu From Copper Deposited Sulphate Metallic Copper In the normal course of operation of a pickling bath uslng bichromate solutions, the waste bichromate solution is dum-p~d when its specific gravity reaches about 1.25 and its active bichromate solution is at the lower range of operating concen~ra-tion usually between 4 to 6%. From a solution containing even a small amount of active bichromate solution, it is difficult to deposit copper at the cathode since copper is being continuously redissolved by the active chromic solution. Therefore, before ` copper can be deposited on the cathode, the hexavalent chromium which remains in the waste pickling solution has to be reduced to a trivalent form. It has been found that the percentage of hexa-valent chromium in the solution should be ~1.5%. In the batch ;
operation, as described by Mitter et al, it wasnecessary to first '~: '' ;' , .....
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3~5~ 34 place the waste bichromate solution in the catholyte compartment to decopperise the solution. At the same time, the active chro-mic solution was presumably reduc~d to a trivalent form thus per-mitting deposition of copper on the cathode. The content of the catholyte compartment was then placed into the anolyte compartment to oxidize the solution back to its hexavalent form. This batch process is cumbersome and uneconomical since it requires a lot of manual handling as each compartment had to be emptied individual-ly. Furthermore, the membranes 12 have to be made of strong ma-terial or reinforced so as to withstand the hydrostatic pressure from the solution left in the other half of the cell.
The apparatus, in accordance with the present invention, provides for continuous or semi-continuous operation by circula-ting the waste bichromate solution through a series of anodic com-partments where the bichromate is regenerated, and at least part of the bichromate solution through a series of cathodic compart-ments wherein metal is recovered. Sulphuric acid is regenerated in both the cathodic and anodic compartments.
Referring to Figure 2, the apparatus in accordance with ; 20 the invention consists of a tank 20 divided into a plurality of ~- compartments 1 to 8 by semipermeable membranes 22 which will pass cations to the cathode but will not let through the anions The membranes must be made of strong material and a sui-. .
table material has been found to be Nafion (reinforced polyte-trafluore`thylene cloth). Asbestos sheets have also been tried but were not found to be sufficlently strong to withstand handling o the cathodes within the cathodic compartments. Ceramic mate-~ ~ rials have also been used but they were found to have a high re-- ~ sistance and have also developed crac~s during operation.
A plate electrode is inserted in each compartment and , these electrodes are connected to a source of D.C. potential E
:
so as to form alternate cathodes 24 and anodes 26 which are sup-.

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ported within the thus formed catholyte and anolyte compartments by any suitable means (not shown). The cathodes are removably mounted since, as mentioned previo,usly, metal is deposited there-on and, therefore, must be stripped therefrom. A suitable design is shown in Figure 3 and includes a hanging loop 28 into which is inserted a rod 30 for suspending the electrode. A deposit sup-porting flange 32 is provided at the lower end of the cathode 24 for preventing the cathode deposits from falling off to the bot-tom of the cathodic compartment. Sheet copper is normally used as the cathode material where copper is to be recovered from the waste solution as mentioned previously. The anodes must be made of a material which is insoluble in the bichromate solution. A suitable anode material is lead or antimonial lead (5% antimony). ;,;~
As illustrated more clearly in Figures 4 and 5, the tank 20 is provided with a plurality of internal flanges 34 whioh are welded or otherwise secured to the walls and the bottom of ;
the tank. Each membrane 22 is covered with a supporting screen 36 and both the membrane and the supporting screen are secured to flanges 34 by means o holding flanges 38 made of PVC or other suitable non-metallic material. The protective screens 36 are used to prevent damage to the membranes during raising and lowe- -ring of the cathodes 24. In order to seal the compartments so that there is no leaking o the anolyte into the catholyte or vi-~ . . .
ce-versaj`a rubber seal 40 is placed between the internal flanges 34 and the membranes 22. The top edge of each membrane and sup- ,' porting screen is covered with a U shaped clamp 42.
~: :
` ~ Another method of mounting the membranes is illustrated ~ ~-in Figure 6 where the internal flanges of the tank 20 have been completely eliminated and the membranes 22 and supporting screens 36 are held in place in between two sets of holding channels 44. ~' The cell units are thus stacked against each other like a pack of , ~' ; ~ ' - 7 -.:
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cards in a box.
The opexation of the apparatus so far disclosed will now be described with reference to Figure 2 of the drawings. In the continuous or semi-continuous operation, at least part of the waste bichromate solution is fed into the bottom of cathodic com-partment No. 1 through inlet pipe 45 and overflows, through an externally connected pipe 46 into the bottom of cathodic compart-ment No. 3. The solution then overflows from the top of compart-ment No. 3 into the bottom of cathodic compartment No. 5 through an externally connected pipe 48 (opposite side of the tank). The solution subsequently overflows from the top of cathodic compart-ment No. 5 into the bottom of cathodic compartment No. 7 through pipe 50. The solution has then completed its circulation through the cathodic compartments and copper is substantially removed therefrom. From the top of cathodic compartment No. 7~ the solu-- tion overflows into the top of anodic compartmentNo. 8by pipe 52.
The waste bichromate solution is also fed to the top of anodic compartment No. 8 through pipe 53 and the total solution flows from the bottom of anodic compartment No. 8 into the top of anodlc compartment No. 6 through external pipe 54. From the bot-tom of anodic compartment No. 6, the solution flows into the top of anodic compartment No. 4 through external pipe 56 (opposite si~
de of tank~. From the bottom of anodic compartment No. 4, the so-lution flows into the top of anodic compartment No. 2 through ex- - ;~
~; ternal pipe 58. The so}ution finally flows out of the bottom of anodic compartment No. 2 through an external pipe 60. The solu-tion has then completed a full pass through the anodic compart-ments whPre the chromium compound is regenerated into its hexava-lent form. The size of the tank, the number of cathodic and ano-.:
dic oompartments as well as the flow rate of the solution is de-termined by the available retention time for depositing copper from the solution and regenerating the bichromate to its hexavalent ~; , ~
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form. When operating at a cell voltage of about 5.8 volts D.C., a current of about 1500 amp., and a current density of about 30 ASF, it has been found that with each compartment being ~0 in-ches wide 30 inches high and 6 inches thick and the flowrate about 15 litres per minute, the solution will completely go through all the anodic compartments in about 20 minutes. Of course, a compa-rable retention time would prevail in the cathodic compartments.
In the above described apparatus, part of the bichroma-te solution needs to be circulated through the cathodic compart-, 10 ments for recovering metal from the solution to prevent a built s up of metal in the solution and also to prevent back up of the ;
s hexavalent chromium through the membranes as no ionic membrane is perfectly non-permeable. It is to be understood however that all the bichromate solution could be passed through the cathodic s ~ compartments before being fed to the anodic compartments when the `
, metal content in the solution is high and no great load of regene-rated hexavalent chromium is needed.
The above apparatus may also be provided with means for , -. ~
providing internal recirculation of the catholyte throug~ all oron-1~ a number ofthe cathodiccompartments topromote metaldepositionon the cathodes. Such a means is illustrated by external pipe 62 and pump 64 intexconnecting cathodic compartments No. 7 and No. 2 in Figure 2. ~he rate of addition of fresh untreated bichromate solution must be adjusted such that the concentratipn of the hexa-¦~ valent chromlum in the bulk of the solution is sufficiently low to permit the deposition of metal ions on the cathode.
:
Becaase the metallic deposit accumulates on the catho-des, the cells have to be built so as to allow a sufficient space :
;~ between the cathode and the membranes to permit their liftings from the cell. This space has to be such as to accumulate enough copper deposit on each side of the cathode and thus make stripping -of the cathodes economical. Since on occasions, some of the ca-.~
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thode deposit breaks off and has to be fishedout with a scoop, the cathodic compartments have to be sufficiently roomy to fa-cilitate such an operation and are generally thicker than the ano-dic compartments as illustrated in Figures 5 and 6. However, from the design point of view, larger spacing increases the cost of the cell for a giving capacity and also lowers the efficiency of the process by increasing the residence time of the catholyte in the cell. The optimum spacing has been found to be between 1 and 4 inches.
In order to facilitate removal of the metal deposits from the cathodic compartments, a PVC perforated basket 66 having about 50% free area may be placed in these compartments as illus-trated in Figure 6~ The cathodes would be suspended in the cen-ter of the basket to collect any metal deposit falling off.
The bichromate solution is fed to the above disclosed apparatus at room temperature. There is a certain amount of heat generated during the process and the solution should be kept be-low 180F, preferably less than 100F to prevent damage to the PVC tank and other PVC components of the apparatus. The circula-tion rate of the solution should therefore be adjusted so as tofeed in enough fresh untreated solution to maintain the tempera-ture below lOOOF.
The acid content of the bichromate solution is normally ;~
about 4.5~ and is normally reduced during circulation through the ~athodic~compartments. The addition of fresh bichromate solution must be controlled in such a way as to maintain the acid content ~bo~e 1.5%.
Although the invention has been disclosed with referen-ce to a preferred embodiment, it is to be understood that various modifications may be made to such embodiment within the scope of . .

the following claims.
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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An apparatus for the electrolytic regeneration of hexavalent chromium compounds and the recovery of metals from waste bichromate solutions comprising:
a) a tank at least covered with non-corrosive material;
b) a plurality of semipermeable membranes subdividing said tank into a plurality of compartments;
c) cathode and anode electrodes positioned in alterna-te compartments of said tank to form a series of cathodic and anodic compartments; and d) means for circulating the bichromate solution through the anodic compartments to oxidize the chromium compound to its hexavalent form, and at least part of the bichromate solu-tion through the cathodic compartments for the electrodeposition of metals on the cathodes.

2. An apparatus as defined in claim 1, wherein the waste bichromate solution is completely circulated through the cathodic compartments before being fed to the anodic compartments.

3. An apparatus as defined in claim 1, further compri-sing means for recirculation of the waste bichromate solution in-ternally through at least some of the cathodic compartments.

4. An apparatus as defined in claim 1, wherein the tank is made of electrically non-conductive and corrosion proof material.

5. An apparatus as defined in claim 1, wherein the anodes are made of lead or antimonial lead.

6. An apparatus as defined in claim 1, wherein the ca-thodes are spaced about 1 to 4 inches from the membranes.

7. An apparatus as defined in claim 1, wherein the ca-thodes are made of the same metal as the main metal in the bichro-mate solution.

8. An apparatus as defined in claim 1, wherein the membranes are made of reinforced polytetrafluoroethylene.

9. An apparatus as defined in claim 1, wherein screens are positioned on the membranes for protection against mechanical damage during removal of the cathodes for stripping metal there-from.

10. An apparatus as defined in claim 9, wherein inter-nal flanges are provided on the bottom and the walls of the tank and wherein means are provided for securing said membranes and screens to said flanges.

11. An apparatus as defined in claim 10, wherein said means are holding flanges.

12. An apparatus as defined in claim 9, wherein said membranes and screens are spaced by means of U shaped holding channels extending the full thickness of the cathodic and anodic compartments.