US2116138A - Apparatus for the electrolytic production of chromic acid and caustic alkali - Google Patents

Description

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May 3, 1938.

Patented May 3, l938 PATENT' OFFICE APPARATUS FOR THE ELECTROLYTIC PRO- DUCTION OF CHROMIC ACID AND CAUSTIC ALKALI John W. Boss, Livingston, Mont., assignor to Chromium Products Corporation, Livingston, Mont., a corporation of Montana Original application June 15, 1936, Serial No.

Divided and this application October 12, 1936, Serial No. 105,321

3 Claims.

The present invention relates to an improved apparatus for the electrolytic production of chromic acid and caustic alkali. This invention relates to an improvement in the apparatus de- Iscribed in my Patent No. 2,055,962, and the preslent application is a division of my application Serial No. 85,405, flledJune 15, 1936.

`An object of the .present invention is to produce in a single cell pure chromic acid from a soluble chromate salt.

A further object is to overcome the defect. caused by accumulation of sludge between the` feed solution and the caustic alkali' solution in v cells of this type as heretofore known.

l5 A further object is to revivify used plating solutions.

Another object is to produce ina single cell both crude and finished chromic acid.

Still another object is to provide means for effecting more complete removal of basic sludge from the cell than has been possible by other means.

With the foregoing and other objects in view, the invention will be more fully described herechromium 25 inafter, and will be more particularly pointed out in the claims appended hereto. In the drawing, wherein like symbols refer to -like or corresponding parts throughout the several views.

Figure 1 is a central vertical `section of a cell in operating condition, and

Figure 2 is a top plan view of a modiiication of the cell in which the feed pipes do not` lie in the same vertical plane.

35 This invention relates to a process and appa-- ratus for the electrolytic` production of chromic acid and of caustic alkali. In the prior art it is customary to produce chromic acid (CrOs) bythe action of anacid such as sulphuric on an 4.0 alkali chromate or dichromate such as sodium chromate or dichromate. Separation of the chromicl acid is accomplished by crystallization and draining off the mother liquor containing in solution other products of the reaction.

45 Some of the disadvantages of such a process are: That diillculty is encountered in removing other acid radicals from the chromic acid; and

that the alkali originally combined as chromate or dichromate appears as a salt such as sodium 50 sulphate. This salt is not of much value and is not suitable for use in preparing new quantities of sodium chromate.

When a used plating solution is reviviiied according to the procedure taught in the applica- 55 tions above mentioned some difculty arises because of the precipitation of bases at the plane separating the caustic soda solution from the n impure acid solution. As the acid solution underlies the caustic solution. these bases or sludges tend to settle lin the acid, -redissolve and reprecipitate at the plane of separation.

This cyclic behavior lowers the efliciency of the cell and requires an excessive ow of caustic soda solution to carry out the precipitates; it also requires closer supervision of the cellto regulate the removal of these precipitates. In the present invention as applied to revivifying used plating acid there is no contact between it and the caustic solution. Instead the caustic alkali solution oats on one part of an alkali chromate feed solution while an impure chromic acid solution iioats on another part of the feed solution. The used plating acid is then mixed with pure chromic acid solution and floated on top of the impure chromic acid solution. The alkali chromate feed solution can be maintained neutral or nearly so and bases precipitated at its contact surface with the crude acid solution do not tend to redissolve but settle out towards the bottom of the feed solution.

In the drawingy Ill designates the body of a cell which may be formedl of glass, stoneware or other suitable material resistant vto the corrosive action of the solutions employed. 'I'he cell I0 is divided longitudinally into two equal compartments by a partition II which extends down into the cell a substantial distance but stops short of the bottom of the cell. As shown the partition II extends about three-quarters of the `way down into the cell. There are, therefore, formed two side compartments I2 and I3 communicating at their lower ends to form a common chamber or compartment I4.

The cell II) is provided near the upper part of the compartment I2 with an overflow pipe Iii.

A movable .sleeve I6 connects the pipe I5 to the take off I 1. The distance between the take o I'l` and the overow pipe I5 may be regulated by means of the sleeve I 6 to alter the rate of overflow from the compartment I2. Also lower down in the compartment I2 but yet a substantial distance above the bottom of the partition II the cell I0 has a second overflow pipe I8 similarly provided with a sleeve I9 and take ol! 2U. 1n the compartment I3 approximately opposite to the overflow pipe I8 in the other compartment, is placed an overflow pipe 2 I with sleeve 22 and take ofi? 23. At one side of the common compartment I4 and below the lower edge ,of the partition II is placed another overflow tube 24 with sleeve 25 and take oil 26. An anode 21 is suspended in the compartment I2 and a cathode 28 is suspended in the compartment I3. The anode 21 may be made of lead and the cathode 28 of iron, steel orcopper. It will be noted that the. anode 21 is positioned in the upper part of the compartment I2 at a region approximately opposite the overflow pipe I5 while the cathode 28 is placed lower in the compartment I3 approximately opposite the overflow pipe 2|.

In the following description sodium chromate will be used as an example but it is to be understood that any alkali salt of chromic acid may be used. In filling the cell the compartment I4 is filled with a solution of sodium chromate of specific gravity, for example, 1.450. This solution is introduced through the feed pipe 29 and the'solution is added until it fills the lower ends of the side compartments I2 and I3. In the compartment I3 on top of the sodium chromate solution is floated a caustic soda solution of, for example, specific gravity 1.200. This solution is added through the supply pipe 30 to substantially fill the compartment I3. In the other side com'- partment I2 is floated flrst a crude chromic acid solution of specific gravity, for example, 1.325. This solution is added through the feed pipe 3|. The amount of this solution must be such as to extend from the top of the sodium chromate solution to some point below the anode 21.. Fi-

nally, on top of this crude chromicl acid solution is floated a pure chromic acid solution of specific gravity, for example, 1.200. This v,last solution is added through its feed pipe 32. It will be necessary, of course, in adding these various solutions to balance the heads of solutions in the opposite chambers so as to prevent mixing beneath the partition II. It is also necessary, as soon as the solutions are added, that current be passed between the electrodes so as to prevent diffusion of the various solutions.

When the electrodes are connected to a. suitable source of direct current the pure chromic acid solution loses basic ions to the impure solution on which it floats while the crude solution loses chromate ions to the solutions above it. The pure solution thus tends to increase in specific gravity while the impure solution loses in specific gravity. In view of this fact, it is necessary to regulate the specific gravity of the crude solution in order to prevent mixing with the pure solution as they tend to attain the same specific gravities due to the operation of the cell. The crude solution receives from the sodium chromate solution underlying it, chromate ions and loses to the underlying solution alkali or basic ions. However, the crude solution loses chromate ions to the pure solution above it at a rate greater than the sodium chromate solution supplies these ions to it. It is, therefore, necessary to replenish the crude chromic acid solution from outside sources to maintain the supply of chromate ions. Such a source of the crude acid may be a simple cell such as described in my above identified applications. The caustic soda solution takes from the sodium chromate in the lower compartment basic ions and the solution in the compartment I3 increases both in volume and in density requiringfdrawing off and regulating of the density in order to maintain the plane of separation between it and the sodium chromate solution underlying it.

` 'I'he sodium chromate feed solution decreases in volume due to loss of its ions as described and the plane of separation between it and the solutions floating above it moves downward toward the partition II. It is, therefore, necessary to replenish this solution to prevent mixing of the solutions in the other compartments The sodium chromate solution also tends to accumulate some caustic soda. Some basic substances such as aluminum, forming soluble compounds with the caustic soda, may also accumulate in this feed solution; In addition to this there is a sludge of insoluble basic materials which may appear at the separation plane between the sodium chromate and the crude chromic acid. This sludge is insoluble in the sodium chromate and eventually settles to the bottom of the cell where it lies until such time as the cell may be cleaned.

In operation of this cell it is of the utmost importance to maintain the planes of separation between the various solutions. Thus, if the sodium chromate feed solution be permitted to drop below the lower edge of the partition II the solu-l tions in the side compartments will mingle and the contents of the cell destroyed. If, on the other hand, the sodium chromate solution be allowed to rise as high as the anode 28 evolution of gas will cause breaking of the plane of separation between it and the caustic soda solution. In the chamber I2 the sodium chromate solution must not rise higher than the feed pipe 3| and the overflow opening I8, which are both situated at the mid point between the lower edge of the partition I I and the anode 21. A rise above that point would cause mixing of both feed and overflow with the sodium chromate solution. Similarly the plane of separation between the plain and crude chromic acid solutions must not 'drop as low as the opening of the feed pipe 3| or the opening of the overflow pipe I8 as this would cause mixing of the two chromic acid solutions. Also the plane between these acid solutions must not rise as high as the anode 21 for here gas evolution would destroy the plane of separation between the solutions.

In view of the requirement for regulating these planes of separation it will be seen to be necesi sary that some means of observing the planes be provided. 'I'hus the cells themselves may be made of transparent material, for the various chromate solutions are readily distinguishableby color while the caustic soda is similarly distinguishable by its lack of color. An alternative means of observing the planes is` to lower a glass tube into the cell, close the .top and carefully withdraw it from the cell. The solutions carried in the glass tube will indicate by their color and planes of separation the relation existing in the cell between these solutions.

The most convenient way to operate this cell and the way for which the cell illustrated is intended is a continuous operation. It will be noted that the lower ends of the tubes 30, 3| and 32 are turned so as to direct the streams of in# coming liquid away from the nearest plane of separation, the object being to avoid disturbing this plane. The feed pipes and overflow pipes are adjusted and connected with sources of solution to promote flow of these solutions through the cell to prevent a rise in specific gravity which would tend to destroy the planes of separation. The rate of overflow is adjusted by sliding the take oil pipes in the sleeves so that more or less pressure will be required to force solution out of the cell.

The feed pipe 29 for the common chamber I4 communicates with a source of sodium chromate solution of the desired specific gravity and the supply is regulated to compensate for the rate of consumption of this -solution in the cell. Also if the operation of the cell is being impaired by accumulation of insoluble impurities and by accumulating caustic soda in this chamber the volume of solution fed may be increased and the excess carried off through the overflow pipe 24, this having the effect of flushing the chamber I4 of these impurities.

In operating this cell to revivify fouled plated solutions the cell is filled as described and the fouled solution introduced through the pipe 32 into the purified chromic acid layer. As pointed out above thechromic acid content of this puri- 'ied layer will be increased by operation of the i cell and the basic content will be decreased.

'I'hese actions then purify fouled plating acid when it is introduced into the finished chromic acid layer. Since the cell is most conveniently operated by maintaining continuous flow of solution, a portion of the overflow from the pipe |5 is adjusted in specific gravity and returned to the cell through the feed pipe 32 along with a portion of the fouled acid to be purified. The portion of overflow which is not adjusted and returned is taken as product solution to be used in a chromium plating bath or for the recovery of chromic acid. In a similar manner a portion of the caustic soda solution removed through the overflow pipe 2| may be adjusted and returned through the feed pipe 30 while the remaining portion of the overflow maybe withdrawn as caustic soda product solution.

Basic substances removed from .the floating purified chromic acid layer in chamber I2 will be found as in insoluble mud in the feed solution in chamber Il of the cell.

By a selection of the proper sizes of cells and current values, an acid making cell as herein described can be operated to treat solution from chromium plating cells so that the efficiency of the plating operation is maintained and the chromium content of the plating bath held constant.

In operating the cell it has been found that a current density of about 30 amperes per square foot of area of anode and of cathode chamber section works well.

Referring to Figure 2 a modification in the location of overflow pipes is seen.v In this form the upper overflow pipes i5' and 2|' are located near one end of the cell while the lower overflow pipes I8' and 24' are located near the opposite end of the cell. In other respects the construction is the same. The location of these overflow pipes, as well as the location of the feed pipes is subject to variation in a horizontal plane, the only requirement being that the vertical position of their outlets conform with the requirements of solution position.

It Will also be apparent that the cell is not limited to the structure shown with regard to the number of anode and cathode chambers but that any desired number of these upper chambers may be provided communicating with a lower feed chamber. In such a case each anode and cathode chamber is provided with anode or cathode and with the various feed and overflow pipes.

It is obvious that various changes and modifications may be made in the details of construction and design of the above specifically described embodiment of this invention without departing from the spirit thereof, such changes and modi flcations being restricted only by the scope of the following claims:

What is claimed isz- 1. A cell for electrolytic production of chromic acid and caustic alkali consisting in a cell body, a vertical partition extending partly down into the body dividing said body into an anode and a cathode chamber communicating at their lower ends with a common chamber, the cathode chamber adaptedto receive a caustic alkali product solution, said anode chamber adapted to receive a chromic acid product solution in the upper part floating on an impure chromic acid solution in the lower `part of said anode chamber, said cathode chamber solution and said impure chromic acid solution both floating on an alkali chromate feed solution in said common chamber, an anode in the upper part of the anode chamber in the region of the product solution, .a cathode in the cathode chamber, supply pipes in the region of each of the solutions, and removal pipes in the region of each of the solutions.

2. A cell for electrolytic production of chromic acid and caustic alkali consisting in a cell body, a vertical partition extending partly down into the body dividing said body into an anode and a cathode chamber communicating at their lower ends with a common chamber, the cathode chamber adapted to receive a caustic alkali solution, the anode chamber adapted to receive a chromic acid solution, said cathode chamber solution and said anode chamber solution floating on an alkali chromate feed solution in said common chamber, an anode in the anode chamber, a cathode in the cathode chamber, supply pipes in the region of each of the solutions, removal pipes in the region of each of the solutions, and means for varying the rate of flow in the said removal pipes.

3. A cell for electrolytic production of chromic acid and caustic alkali consisting in a cell body, a vertical partition extending partly down into the body dividing said body into an anode and a cathode chamber communicating at their lower ends with a common chamber, the cathode chamber adapted to receive a caustic alkali product solution, said anode chamber adapted to receive a chromic acid product solution in the upper part floating on an impure chromic acid solution in the lower part of said anode chamber, said cathode chamber solution and said impure chromic acid solution both floating on an alkali chromate feed solution in said common chamber, an anode in the upper part of the anode chamber in the region of the product solution, a cathode in the cathode chamber, separate supply means for each of said solutions, and separate removal means for each of said solutions.

JOHN W. Boss.

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