US3423300A - Electrolytic regeneration of reduced chromium compounds - Google Patents

Description

Jan. 21, 1969 ob ETAL. 3,423,300

ELECTROLYTIC REGENERATION OF REDUCED- CHROMIUM COMPOUNDS Filed Oct. 25, 1967 IN V EN TOR. S

LOUIS A. J 06 FRANK E. SNODGRASS United States Patent 7 Claims ABSTRACT OF THE DISCLOSURE An electrolytic process for converting trivalent chromium values to the hexavalent form, which consists in subjecting at a temperature of about 90 C. an aqueous sulfuric acid solution to the action of direct current voltage applied in series to a water-cooled multi-unit filter press type cell assembled from frames coated with chromic acid resistant material lead plate electrodes and polytetrafiuoroethylene diaphragms having a thickness of about 0.01 inch, a porosity of about 50% and a pore diameter within the range of 50 to 150 microns. The sulfuric acid content of the electrolytic solution is preferably to by weight and the residence time of the solution in the cell is about one hour.

The process is characterized by high current density, low energy requirements and low anode plate requirements.

The invention is substantially an improvement over the process and equipment described in copending application, Ser. No. 493,995, filed on Oct. 8, 1965.

This invention relates to improvements in the electrolytic regeneration of trivalent chromium compounds to hexavalent chromium compounds. In another aspect, the invention pertains to modifications of a diaphragm type cell that allow these improvements to be carried out.

The prior art As is well known, hexavalent chromium solutions such as chromic acid in admixture with sulfuric acid, are powerful oxidizing agents. These solutions are particularly effectivein the oxidation of fused ring polynuclear hydrocarbons to quinones, the latter quinones being of course valuable starting materials and intermediates in the manufacture of dyes, drugs, and the like. Solutions containing available oxygen in the form of chromic acid are also commonly employed to effect the oxidation of relatively long chain unsaturated fatty acids and oils by producing a cleavage of the chain. The oxidation of fused ring hydrocarbons and the disruptive oxidation of double bonds are carried out by mixing the oxidizing solution with a batch of the hydrocarbon or fatty acid at an elevated temperature.

One of the advantages of chromic acid oxidation is that spent chromic sulfate solutions can be electrolytically regenerated to form C'r-O solutions. The regenerative electrochemical reactions are effected in an electrolytic cell of the bipolar electrode type. The present invention deals with chromic acid regenerative processes of this type, one of its objects being the improvement of the electrolytic cell.

In a typical cell used for the oxidation of trivalent chromium salts, a direct current (DC) voltage is applied to the electrodes to cause the electrolytic decomposition of water. The hydrogen atoms produced form molecular hydrogen, which is allowed to escape from the cell, while the oxygen is involved in the formation of lead dioxide which in turn is believed to oxidize the trivalent chromium to the hexavalent form that is present in chromic acid. Obviously, under such circumstances, means must be employed to prevent the migration within the cell of the freshly oxidized chromic acid to the cathode area where it would merely be reduced again. Porous ceramic diaphragms have therefore been used as barriers between the anode and cathode compartments of the cells.

The diaphragms constitute a chief source of wear and tear in the cells. While other potential difficulties such as electrode corrosion may be prevented by proper operation, ceramic diaphragms, although selected because of their chemical resistance, tend to pulverize on continued use. Furthermore, they do not lend themselves to large scale industrial operations.

A partial solution to these difiiculties has been provided by Joo et al., in co-pending application Ser. No. 493,995 mentioned earlier, by the replacement of ceramic diaphragms with porous polytetrahaloethylene membranes. Significantly, this successful substitution has opened the way to the use of filter press type bipolar cells and has facilitated the conversion of chromium salt regeneration into a continuous process.

Filter press type electrolytic cells are of course known in the art, but it is believed that they had not previously been employed for the oxidation of chromic acid solutions. Ceramic diaphragms are difiicult to use in this type of cell due to their rigidity and brittleness. Consequently, the diaphragms disclosed in the prior art for filter press type electrolytic cells are either fabric, such as canvas, or plastic, such as polyethylene. None of these can withstand the chemical attack of chromic acid.

Summary of the invention The invention thus contemplates an electrolytic apparatus for the continuous regeneration of trivalent chromium salt solutions to form hexavalent compounds, which includes a plurality of electrode plates held by inert plastic coated hollow or tubular frames in a filter press type of arrangement forming a series of unit cells, the plates being placed in series with one side of each plate serving as anode and the other as cathode. A permeable polytetrahaloethylene diaphragm is interposed between adjacent electrode plates, dividing each unit cell into two compartments, an anolyte and a catholyte compartment. These compartments are connected by an orifice in the diaphragm. Means are provided for introducing a liquid feed stream into the catholyte compartment, for withdrawing regenerated chromium compound from the anolyte compartment, and for withdrawing hydrogen from the cat-holyte compartment. Means are also provided for cooling the cell during operation. Details of the apparatus are given in the drawing and in the text describing the figures therein.

The regeneration process of this invention is a surprisingly efiicient single pass process in which both high current density and high current efficiency are obtained. These desirable features are accomplished by using a polytetrahaloethylene diaphragm having a thickness in the vicinity of 0.01 inch, an average pore diameter of about 100 microns and an average porosity of about 50%. In addition to these structural features, the process requires preferably a net quantity of free sulfuric acid equal to about 15% to 20% of the total weight of the regeneration solution, a reaction temperature of about 92 C. and a retention time of about one hour, i.e. sufficient to achieve a 50% to conversion of the reduced chromium ions to the hexavalent state.

Description of the drawing The two types of cells that can be employed in the regeneration of chromic acid solution according to the present invention can be better visualized by reference to the accompanying drawing which is essentially similar to that found in Ser. No. 493,995, except that means for cooling the regeneration solution are now provided. It must also be pointed out that only equipment sufficient to illustrate the invention has been shown, pumps, valves and other secondary features being omitted for simplicity.

FIGURE 1 is a central vertical section of a tank type electrolytic cell.

FIGURE 2 gives a top view of an electrolytic cell comprising a filter press type assembly.

FIGURE 3 is a transverse sectional elevation of the cell of FIGURE 2.

Referring now to FIGURE 1, we see that the cell includes cylindrical anode 1 and cathode 3 concentrically arranged with porous diaphragm 2 therebetween supported on rods 5. Diaphragm 2 represented for ease of visualization as proportionally thicker than other cell elements than it actually is, is made of a polytetrahaloethylene rather than ceramic, with specifications that shall be described later.

Referring still to FIGURE 1, the unit is filled by means of conduit 6 leading into the cathode compartment. The solution then flows over the low or cutaway side of diaphragm 3 into the anode area A. Conduit 8 is provided in the anode compartment away from the cutaway portion for withdrawal of anolyte. During operation, the oxidation takes place in the anode compartment, and the molecular hydrogen formed at the cathode is removed through conduit 10 provided therefor. With such equipment, it has been found that the long life and durability benefits of the diaphragm of the invention are realized and that the system can be operated using either a batch or continuous approach.

During continuous operation, reduced chromium salts and metallic chromium tend to accumulate in the catholyte and cause unwanted deposits on the various compartment surfaces. However, a continuous chromium salt solution electrolytic regeneration process has been provided which can be operated much longer than any heretofore known continuous process. In addition a long lasting electrolytic cell has also been provided therefor. It has been found that the problems encountered in continuous operation in electrolytically regenerating trivalent chromium salt solutions to form hexavalent chromium compounds can be overcome by effecting circulation of both the catholyte and the anolyte. According to this practice, a feed solution of the trivalent chromium 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 permeable polytetrahaloethylene diaphragm. In this embodiment, shown in FIGURE 3, the feed stream is passed through the cathode compartment C then through at least one orifice in the diaphragm into the anode compartment A to provide a circulation pattern in the catholyte as well as in the anolyte compartment preventing salt deposition in said catholyte compartment.

Referring to FIGURE 2 and FIGURE 3, the continuous cell shown is composed of eight hollow frames F, forming four unit cells A-C between plate electrodes 14!; and 14a. In this particular cell the electrodes are so disposed that, with the exception of electrodes 14a and 14b, one side of each electrode 14 functions as an anode whereas the opposite side of the electrode serves as a cathode. This can be seen by referring to anode sections A and cathode sections C in the drawing.

Examining FIGURE 2, it can be seen that the electrolytic cell shown therein has the advantage that it overcomes the need to step down the voltage. Unit cell voltage is usually between 4 and 8 volts D-C, requiring a transformer and a large number of electrical connections. These are eliminated by the series arrangement of FIG- URE 2. The current is supplied at the two ends of the series of plates, and the cell voltage is determined by the number of cells connected in the series. Each electrode 14 preferably is lead. Nickel plated anodes and iron cathodes have been used, but due to corrosion, these metals are not recommended.

As indicated, the electrolytic cell of FIGURES 2 and 3 is particularly suited to continuous operation. To effect circulation of electrolyte, the chromium salt feed solution is introduced into catholyte compartment C through inlet conduits 24 connected to header 20. Circulation of catholyte is accomplished by this introduction coupled with flow through the diaphragm and residence time in the anolyte compartment. To achieve flow through the diaphragm, the polytetrafiuo-roethylene membrane 16 is provided with one or several smaller orifices 15 allowing unrestricted flow at that point. Although it was first believed that an orifice in the diaphragm would render the system conductive by short circuiting the unit, actual operation of the unit has demonstrated that the effect of the orifice is negligible. The orifice is positioned near the bottom of the catholyte compartment so that the feed must pass through that compartment. Any compound reduced in the catholyte compartment will be oxidized in the anolyte compartment. The orifice area is such that passage from the catholyte compartment to the anolyte compartment is no more rapid than the rate of anolyte withdrawal. Residence time in the anolyte is one unit cell volume per 0.5 to 1.5 hours. The regenerated chromium compound solution may be withdrawn through manifold 18 controlled by vacuum, or alternatively and preferably, it may be collected through overflow means provided for each anode compartment. Hydrogen formed at cathode 14 is withdrawn through manifold 12. During the continuous operation, an electric current applied at 21 and 22 passes continuously through the catholyte, the diaphragm and the anolyte to bring about the anodic oxidation of ionic chromium. Cooling is provided to maintain the desired reaction temperature by allowing water to circulate in the hollow interior of the frames 23.

Detailed description of the invention A better understanding of the critical improvements which constitute the essence of this invention is afforded by the following examples. These examples are provided for illustrative purposes and are therefore not intended to limit the invention to a scope narrower than that covered by the claims appended to this disclosure.

EXAMPLE 1 An eight unit filter press type electrolytic cell was set up substantially according to the drawing already described and was used to regenerate trivalent chromium sulfate solution. The frames selected were made of aluminum and were coated with a film of polychlorotrifluoroethylene, a material commercially available under the trade name of Kel-F. The assembly was operated at average cell temperatures exceeding C. for approximately 95 hours, being exposed in the process to solutions containing better than 60% chromium in the hexavalent form. No corrosion was suffered by the frames under such drastic conditions. These frames had been previously used for numerous shorter runs and have been used since, still with no detrimental effect observed.

From the standpoint of corrosion resistance, particularly at elevated temperature, the only construction materials tested found to be unaffected by the sodium dichromatesulfuric acid mixtures were lead, polytetrafiuoroethylene (Teflon) and polychlorotrifluoroethylene, with the latter preferred for coating frames since it is more easily applied as a pin-hole free layer on said frames.

EXAMPLE 2 The regeneration of a spent chromium liquor from a naphthalene oxidation process was attempted in a cell having a polytetrafiuoroethylene diaphragms of 0.125 inch thickness and of 40% porosity. The chromium liquor was composed of sodium sulfate, chromium sulfate, water and a very small percentage of sulfuric acid, and it contained EXAMPLES 5 TO 13 a net amount of 7 to 8 g. chromium ions per 100 g. solution.

On electrolysis, the current densities noted for the solution were extremely low, in the order of 0.0095 amp./cm. of electrode, and regeneration was negligible. Dilution of 5 the solution with water increased the density only slightly,

These runs were carried out in a filter press type oell substantially similar to that of the drawing except that Teflon polytetrafluoroethylene membranes of different porosity were used. The voltage and the residence time were varied. Table 2 summarizes the variations and their results.

TABLE 2.EFFECT OF DIAPHRAGM POROSITY ON ELECTROLYTIC OXIDATION OF CR(III) Unit Cell Residence Anode Plate Energy Ex. Diaphragm Type l Potential Time Current (ttfi/lb. (kWh/lb. Percent (volts) (hrs.) Density N 82C12O NazCrzO Conversion (amps/cmfl) hr.) hr.)

5 High Porosity (-70%). 5. 0. 73 0. 26 2. 25 2. 18 36. 6 d0 5. 5 0. 74 0. 29 2.07 2. 47 40. 4 6. 0 0. 84 0.31 2. 05 2. 86 46. 4 5. 0 1. 22 0. 11 3.98 1. 96 34. 5 5. 5 1. 28 0. 14 3.02 2. 47. 6 6. 0 v 1. 44 0. 12 3. 72 2. 54 47. 2 5. 0 0. 53 0.23 1. 84 1. 94 38. 7 5. 5 0.50 0. 27 1. 65 2. 23 41. 0 6.0 0. 57 0.30 1. 49 2. 43 52. 4

1 The porosity or void content measured for these difierent membranes is the sum of closed pores and through pores determined by density measurement methods. The measure has some value in that there exist, for each porosity level, a certain proportion of open or through pores through which diffusion of electrons takes place. The 50% porous material contains the preferred concentration of functional or through pores.

i.e. to 0.0148 amp/cm. and achieved a Cr to- Cr The best results are evidently obtained with membranes conversion rate of less than 5% in one hour. The addition 25 of porosity of about 50%. With greater porosities, even at of sulfuric acid however, at a ratio of about 3 g. 9 8% higher anolyte retention time and slightly higher current H 80 per 100 g. solution, caused a sensible increase of densities, results were generally inferior, indicating possicurrent density to 0.052 amp/cm. thus permitting oxidably some diifusion of Cr ions back into catholyte with tion at an acceptable rate to attain conversion levels of subsequent reduction at the cathode. over 20% in one hour. In the case of low porosity diaphragms, much lower Further runs with the improved cell of this invention current densities were obtained along with a greater plate have demonstrated the essential need of sulfuric acid for requirement resulting in a rather slow process as evithe obtention of good regeneration results. While the denced by a comparison of the conversion-retention time actual sulfuric acid concentration needed to maximize currelationships of the low porosity examples to those of rent density may vary with alterations in other factors, it 5 Examples 11 to 13.

has been found that the optimum concentration is in the vicinity of 15% to 20% by weight. A gradual lowering EXAMPLES 14 To of this concentration will result obviously in a gradual A two-unit iilter press type assembly was set up substanlowering of current density while higher proportions of tlally as described in the drawing with the following dith id ill b tt d d b Solubility robl Higher 4 mensional characteristics: electrode separation, 5.08 cm.;

concentrations will tend to cause sulfates to precipitate. anode area, 2:714 0111-2 P unit cell; Cathode area, same as Limits of 3% and 23% are imposed on the process by anode area; dlaphfagm maleflal, t'hlck Teflon P 3- th cgnsideratigns tetrafluoroethylene sheet of 50% porosity. The cell was EXAMPLES 3 AND 4 run at various temperatures at a unit cell potential of 5.0

volts cooling Was provided to maintain the operating Cells were assembled essentially as per the drawing of temperatures desired. The results obtained are summarized the invention except that in one case, Example 3, a 0.125 in Table 3.

TABLE 13.-EFFECT OF TEMPERATURE ON ELECTROLYTIC OXIDATION OF CR(III) SOLUTIONS IN A TWO-CELL FILTER PRESS UNIT Cell Anolyte Plate Energy Ex, Temp. A.O.D. Retention (it. Pb/lb. (kwh./lb. Percent C.) (amps/cm?) Time NazCr2O1/ N azClzO Conversion (hlS.) hr.) hr.)

inch thick Teflon polytetrafiuoroethylene membrane of For a given cell potential, the current density (arnps/ porosity was used for diaphragm while in the other, cm increased with the temperature while the plate re- Example 4, a thinner membrane of the same porosity was quirement decreased resulting in much greater Cr(III) employed. Results obtained with each diaphragm are given to Cr (VI) conversion at a given retention or residence in Table 1. time. Energy consumption was essentially comparable.

TABLE 1.EFFECT OF DIAPHRAGM THICKNESS ON THE ELECTROLYTIC OXIDATION OF CHROMIUM SOLUTIONS Diaphragm Unit Cell A.C.D. Plate Energy Percent Material Potential (amps/cm!) (ftfl/lb. (kwh./lb. Conversion Thickness (volts) Nags/15201] Nalrgrfml The generally superior results obtained with the thinner In addition to the polymer of tetrafluoroethylene used membrane become evident on noting the higher anode curin the examples, there may be employed, as diaphragms, rent densities (A.C.-D.) that it permits and the lower elecmembranes of polytrifiuorochloroethylene. These known trode plate areas that it requires even at lower cell popolymers are described broadly in U.S. Patents 2,393,-

tential. 967 and 2,600,202, the ones usable here being permeable and having a sufiiciently high molecular weight to be solids. A permeable sheet means a porous sheet, with a porosity such that electron flow is permitted while the fiow of ions and hydrogen is inhibited. For this invention, the pore size should be in the range of l to 300 microns at its largest. Thus the reduction of regenerated chromic acid at the cathode will be minimized and the energy and anode area requirements will be at their lowest due to inhibited ionic flow. The preferred pore sizes are in the range of 50 to 150 for a permeable membrane. It should be noted also that successful use has been made of non-porous Tefin polytetrafluoroethylene fibers woven or matted into a cloth. In such instances, the only voids present in the resulting structure were the spaces between the filaments. Yet, the cloth has been made to the proper specifications so that its resistance to current is minimal and its barrier properties are satisfactory. Fibrous membranes of this nature are available to industry under the trade name of Zitex.

The foregoing examples and descriptions illustrate a durable and efficient filter press type electrolytic cell. It is superior to ceramic diaphragm cells which crack under their own weight in such use and it can be used in circumstances where other diaphragms fail. The cell permits long continuous operation at current densities and efficiencies heretofore unattainable. While the favored operating parameters have been described, various adaptations of the cell and process in terms of size, voltage, chromium compound, electrolytic composition and the like are contemplated and fall within the scope of the invention. Moreover, it is further realized that the electrolytic process and equipment are applicable with minor modifications to the production of chromium (VI) compounds from chromite ores, to the regeneration of chromic acid baths and to other similar tasks. Such ramifications and variations are deemed to be within the scope of the invention.

What is claimed is:

1. A continuous process for the electrolytic oxidation of reduced chromium compounds to the hexavalent state,

which consists in submitting to the action of direct current voltage in a cell equipped with a polytetrahaloethylene membrane having a thickness not greater than 0.2 inch and a pore diameter of 1 to 300 microns,

an aqueous solution of the compounds containing from 3 to 23 by weight sulfuric acid,

at a temperature within the range of to 100 C.,

for a net residence time suflicient to achieve the desired degree of conversion of chromium to the hexavalent state.

2. The process of claim 1 wherein the reduced chromium compound solution contains the Cr(III) species obtained from the oxidation of an organic compound by hexavalent chromium.

3. The process of claim 1 wherein the reduced chromium compound solution is a chromite ore sulfuric acid extract.

4. The process of claim 1 wherein the cell is a multiunit type in filter press arrangement.

5. The process of claim 1 wherein the cell is comprised of a single unit.

6. The process of claim 1 wherein the membrane used has a thickness of about 0.01 inch, a porosity of about 50% and an average pore size of about 50 to about 100 microns.

7. The process of claim 6 wherein the sulfuric acid content is within about 15% to 20% by weight, the temperature is about C. and the net residence time about one hour.

References Cited UNITED STATES PATENTS 630,612 8/1899 Le Blanc et al. 20497 1,878,918 9/1932 Udy 20497 FOREIGN PATENTS 9,636 8/ 1907 Great Britain. 715,075 9/ 1954 Great Britain. 136,338 7/ 1960 Russia.

OTHER REFERENCES McKee et al., The Journal of Industrial and Engineering Chemistry, vol. 12, No. 1, pp. 16 to 26. Copy in 20497.

JOHN H. MACK, Primary Examiner.

D. R. JORDAN, Assistant Examiner.

US. Cl. X.R. 20497, 78

wy UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3,423,300 Dated January 21, 1969 I Inventor) Louis A. Joo and Frank E. Snodgrass It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

I' In the heading of this printed patent, delete "Continuation in-part of application Ser. No. 493,995, Oct. 8, 1965. This application Oct. 25, 1967, Ser. No. 677,952" and substitute -Fi1ed Oct. 25, 1967, Ser. No. 677,952.

SIGNED AND SEMEU MAR 101970 Men:

M M. Fletcher, Ir.

. IHILIMI E. SGHUYIER, JR.

Atteaung Officer Oomissioner of Patents

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