US2253865A - Process for electrolyzing solutions containing tin - Google Patents

Description

Aug. 26, 1941. D. L. OGDEN min. I

PROCESS FOR ELECTROLYZING SOLUTIONS CONTAINING TIN 2 Sheets-Sheet 1 Filed Aug. 6, 193'? BEELE/N N kw N N M N wwwmvw W N. XEVR Qvfi Aug. 26, 1941. D. OGDEN EIAL PROCESS FOR ELECTROLYZING SOLUTIONS CONTAINING T Filed Aug. 6, 1937 2 Sheets-Sheet 2 INVENTORS DANIEL L- OGDEN MAX E W. HEBERLE/N 1% 01k ATTORZ ,6 EYS lyzing solutions ratus therefor.

,aqueous solutions containing Patented Aug. 26, 1941 PROCESS FOR ELECTROLYZING SOLUTIONS CONTAINING TIN Daniel Lattimer Ogden, Woodbridge, and Max F.

W. Heberlein, Rahway, N. American Metal Company,

1., assignors to The Limited, New York,

N. Y., a corporation of New York Application August '6, 1937, Serial No. 157,690

' 7 Claims.

' This invention relates to a process for electrocontaining tin and to an appa- It is more particularly directed to a process and apparatus for electrolyzing tin tetrachloride. It has heretofore been proposed to electrolyze solutions of tin tetrachloride, but one literature reference states that this could not be done satisfactorily. According to this reference (Mellor VII p. 439) the electrical conductivity of tin tetrachloride is .nil' or very small. According to this same source tin tetrachloride is not ionized and electrolysis of an aqueous solution yielded only a small deposit on the cathode due to secondary reactions, since it is improbable that the salt is revolved into Sn"" and 401' ions. other proposal made about thirty years ago was to electrolyze aqueous solutions of tin tetrachloride in a diaphragm cell for as long a period as a commercial current efi'lciency could be maintained. The basic anolyte and acid catholyte were then to be withdrawn from the electric system, combined, concentrated and returned for electrolysis. This process is not a practical one and could not be commercially practiced.

An object of the present invention is to provide a simple, practical process for depositing tin, which may be conducted without interruption if desired and without such reconditioning of the electrolyte. Another object is to provide a simple apparatus for carrying out such I process. Other objects will be in part obvious and in part pointed out hereinafter.

The invention accordingly comprises the novel processes and steps of processes, as well as the novel apparatus and combinations thereof, in

which such processes and steps -may be carried out, specific embodiments of which are described herein by way of example only and in accordance with the manner in which we now prefer to practice the invention.

Although the process of our invention may be carried out in other forms of apparatus, .we will now describe an apparatus forming part of our invention in which we prefer to operate the process. In the accompanying drawings, Fig. 1 represents a sectional elevation on the line |l of Fig. 2 of such apparatus, with portions shown diagrammatically; and Fig. 2 is a plan view of the apparatus shown in Fig. 1.

The following description of the apparatus shown in these drawings is a description of one electrolytic cell with its appurtenant apparatus. Although only one such unit is described it will be clear to those skilled in the art that two or any number of such units may be suitably connected to form a battery of cells. Such dupli-- cation of units and their connections will be well understood to those skilled in the art.

Referring now to the drawings, there is shown an electrolytic cell comprising a tank I, preferably of concrete, lined on'the interior 2 with a Harvel lining, which is prepared from suitably treated oil of cashew nuts. The Harvel lining composition is made by mixing oil of cashew nuts with para-formaldehyde. It is adapted to withstand the corrosive action of the solution 3 being electrolyzed. Other linings having a similar resistance may be employed. The electrolyte is supplied to the head tank 4 and passes therefrom by pipe 5 into the tank I. The head tank is part of a system for continuously circulating the electrolyte 2. This system comprises a sump tank 6 connected by an overflow pipe 1 to receive electrolyte overflowing through I from the tank I, such overflow electrolyte being delivered into the sump tank 6. The system also includes piping 8 leading back to the head tank 4, in which piping a rubber-lined pump 9 is inserted. A heat interchanger Ill may also be inserted in this line, if

desired. This system is made up of or lined with rubber or other material which will withstand the corrosive action of the circulating electrolyte.

Located in the electrolytic tank is a series of alternate cathodes II, of soft steel or iron, and anode assemblies. The anode assemblies shown in Fig. 1 are each identical and the description of one will suffice for all. In each of the anode assemblies is an impregnated anode P2 of graphite. We prefer to use Atchison graphite. The graphite is impregnated with boiled linseed oil by immersing. the anodes in the oil at about 220 F. and allowing them to remain there for 60 hours. Instead of boiled linseed oil, impregnation may be made by similarly immersing the graphite anodes in melted paraffin. We, however, prefer the linseed oil-impregnated graphite anodes.

Each anode has a cross-bar l3, an end of each resting on bus bar It resting on the top of the tank 2 for conducting the current into the anode, the opposite end being suitably supported on the opposite wall of the tank. i

About each anode is a chamber made up of a wood cap I5 secured thereto from which depends a bag or envelope l6 which is preferably of glass cloth. The wood cap I5 may be made of hard rubber, if desired. The glass cloth is permeable to the electrolyte and to'the current.-

It also resists, as does the wood where it dips into the electrolyte solution, the corrosive actionof the electrolyte solution. The bags and wooden collars are designed so that they will be disturbed as little as possible if and when the anodes are changed. The edges of the glass fabric are preferably hemmed to prevent raveling where there is no selvage. It is also preferable to double the top of the bag which fits over the wooden collar. The solution creeps up into the cloth above the solution level of the electrolyte and makes the cloth gas-tight.

This bag IB extends somewhat below the anode (as shown in Fig. 1 and is open at the bottom. Instead of glass cloth we may employ asbestos or other material which will withstand the corrosive action of the solution being electrolyzed and at the same time is permeable to the solution and the current.

A pipe I! communicates with the chamber about the anode. In this chamber chlorine gas developed at the anode collects and passes then through the pipe I! to a manifold 18 and thence by pipes l9 and 20 to a liquid trap 2| where any liquid mechanically entrained with the chlorine gas is separated, the gas passing out through the top of the trap through a pipe 22 to a recovery device. The gas may be compressed to form liquid chlorine ready to be marketed or it may be converted into bleaching powder by known methods. The pipes l1, I8, I9, 20 and 22, as well as the liquid trap 2|, are all preferably made of rubber or lined with rubber or material capable of resisting chlorine.

The cathodes are composed of strips of soft steel l l as noted and these are supported by crossbars 23 which have one end resting on bus bars 24 supported by the top of the side wall of the tank 2 and the other end rests on the top of the opposite wall of the tank 2.

The top of the tank is closed by a. cover of hard vulcanized rubber or wood 25 vented at 26. The anodes l2 project therethrough and the cathodes II also project therethrough. Each cathode is provided with a wooden cap 21 which is attached thereto and which serves to cover the opening in the cover 25 through which the cathode passes into its position in the tank 24.

The sump tank 6 is provided with a valved opening 28 through which fresh solution of tin tetrachloride may be poured to replenish that used up by the electrolysis.

In operating the apparatus, tin tetrachloride to be electrolyzed is placed in the sump tank 6 and the pump 9 is started and the electrolyte is fed in until a suflicient quantity for electrolysis has been pumped. Anodes and cathodes being in place the current is turned on and electrolysis commences. Chlorine gas forming at the anode collects in the cap and bag and chamber, and passes out through the pipe l1, through the manifold I 8 and after being freed from liquid, passes out through the pipe 22 for recovery. The tin deposits on the cathode II. The electrolysis is continued until a sufiicient quantity of tin is deposited on the cathode which will occur in about 4 or days.

Although not restricted to such use, we may.

. employ the apparatus above described for the carrying out of the process of our invention.

We have found in accordance with our invention that aqueous solutions containing tin tetrachloride may be electrolyzed with practical commercial results. We have found in accordance with our invention that the tin deposit should be dense and it is important that after the tin has been deposited that conditions be maintained such that there is substantially no re-solution of the tin by the solution being electrolyzed. At the same time it is also important that a satisfactory current efilciency shall be maintained.

In carrying out the process of our invention in order to produce a high grade product and to conduct the process efficiently, we have found it important to distill the tin tetrachloride in order to obtain it in a purified form. We have found that the presence of impurities has a strong infiuence on the electrolysis, and if present to any considerable extent will interfere with the operation of electrolysis for an extended period. Various impurities may be found in tin tetrachloride such as lead arsenic, antimony, selenium, tellurium, sulphur, etc. These impurities if not removed tend to deposit on the cathode at the beginning of the run and form a dark and loosely adhering powder or may promote the formation of trees. Sulphur is a very undesirable impurity because of its destructive effect on the graphite anodes. If not removed it may be necessary to shut down the process and filter the electrolyte to get rid of elementary sulphur. We have found that attempts to remove impurities by cementation, that is by the addition of metallic tin to an aqueous solution of tin tetrachloride, is not satisfactory. We prefer to employ a redistillation of tin tetrachloride at a temperature of -120" C., and in this way a tin tetrachloride suitable for electrolysis can be obtained.

In conducting the process an electrolytic tank has been employed like that described hereinabove, having a length of approximately 12 feet, and a width of about 30 inches and a depth of electrolyte in the tank of about 45 inches. The ranges of concentrations of hydrochloric acid and chlorine and tin content in the solution being electrolyzed, as well as the temperature employed referred to specifically below, have been ascertained by using this apparatus and satisfactory results have been obtained thereby. It is possible that the limits mentioned may vary somewhat in case of the use of other apparatus of larger or,

smaller size or differently constructed, but such variations or changes would be clear to those skilled in the art upon operation of such other apparatus. In preparing to use the apparatus herein described of the dimensions set forth, tin tetrachloride preferably purified as above described is dissolved in water so that there is present at least 30 to 40 grams, but preferably 50-60 grams, of tin per liter. We have found that this solution should contain about 45 grams per liter of hydrochloric acid and this is preferably added to it prior to commencing electrolysis so that a dense tin deposit shall be obtained. The expression dense tin deposit as used herein and in the appended claims, is intended to designate a bright, dense and smooth form of tin deposited on the cathode when the process is carried out in accordance with our invention. The character of this deposit will naturally vary somewhat as operating conditions vary. It should not contain too large a portion of loose and spongy metal to interfere with the efiicient operation of the process,

Prior to starting the electrolysis, and thereafter if desired, cresylic acid or similar addition agent is preferably added to the solution. The cresylic acid is added in the amount of approximately 27 pounds per ton of tin deposited at the cathode. The cresylic acid is added by mixing it with a,

the electrolysis is cathode and chlorine gas forms, and is collected about the anode in a suitable chamber from which it is sucked out. During the course of the electrolysis both chlorine and hydrochloric acid are present in the solution being electrolyzed, and we. have found that it is important to maintain a concentration of at least about 45 to about 120 grams per liter of hydrochloric acid in order to obtain a dense deposit. We have also found that it is important to regulate the content of chlorine in the solution and to maintain it at about 1.0 gram per liter or less. Although it is important to have suillcient hydrochloric acid present in order to produce a "dense deposit of tin, if the content of hydrochloric acid rises much above 120 grams per liter or the content of the chlorine rises much above 1 gram per liter there is apt to be re-solution of the tin. deposited. It is also important to maintain a suitable temperature in the solution and we have found that with a circulation of about 3 gallons per minute of electrolyte, or at such a rate as to prevent the electrolyte leaving the deposition tanks from falling below 30gms. per liter of tin, and with a current density of approximately 30- amperes per square foot cathode area, and with the electrolytecontent of tin, hydrochloric acid, chlorine and cresylic acid as indicated above, that a temperature of about 130-100 F. will produce an efficient electrolysis. This temperature under the conditions given above may be maintained without adding any heat to the system, but if it is desired to heat or cool the electrolyte this may be done by applying or removing heat byheatinterchanger ill, for example, as hereinabove mentioned. I f

For each 1 pound of tin deposited at the cathode approximately 1.2 pounds of chlorine are liberated at the anode,and it isthereforenecessary (1) that an anode is used which will withstand the effect of this highly corrosive gas, and (2) that provisionsare made to withdraw the chlorine from the systemeflectively so that it maybe recovered. Since it isimportanttd maintain a low content of chlorine and .since' chlorine is beingv generated at the anode, the chlorine gas shouldbe removed and this is preferably done by collecting the chlorine in a bag such as shown by l6 in Fig. 1 above mentioned and suckme it through the system such as shown in Fig. l of the drawings for recovering chlorine. Most of the chlorine will be removed in this way. Some of it necessarily will evaporate from the surface of the circulating liquid and hence in the apparatus used it is desirable to vent this chlorine as described. It is important that the anodes It be of a material which will resist chlorine. The anodes described above in connection with the description of the apparatus are made of a high grade graphite, preferably impregnated with linseed oil or similar corrosion-resistant.material.

When a sufilcient deposition of tin is collected on the cathode which will occur in about 4 to 5 days, the cathodes are removed and placed in a cathode rack and run through a steam-sweating process which may be like that described in Patent No. 1,989,852.

The following is an example oi. an embodiment of the process as we now prefer to practice it. It will be understood that this example is purely illustrative and that the invention is not to be considered as limited thereto exceptas indicated in the appended claims.

Tin tetrachloride is distilled in a simple vessel constructed of soft steel, this vessel being submerged in an oil bath at a temperature of 115-120 C. The tin tetrachloride fumes were condensed in a water-cooled glass coil and collected in a glass vessel to secure a tin tetrachloride containing only traces of impurities. In making up the electrolyte, in starting, the purifled tin tetrachloride was dissolved in water acidified with hydrochloric acid in proportions to give 50-60 grams of tin per liter and approximately 45-50 grams of free hydrochloric acid per liter. Sufllcient crude cresylic acid and glue are then added to the bath of tin'tetrachloride to give a content of 0.5 gram of cresylic acid per liter and 0.1 gram of animal glue per liter.

The solution is now ready for electrolysis and is poured into the sump tank and the pump started and the electrolytic cell -filled to proper depth, and the current being turned on the electrolysis is started. The tin deposits and chlorine is withdrawn from the solution. After the electrolysis has been started regular further additions, preferably continuous, of the conditioning agents are made at the rate of 27 pounds of cresylic acid and 5.5 pounds of glue for a ton of tin deposited at the cathode. Additions of tin chloride are made from time to time in order to keep'th'e tin content at 50-60 grams of tin per liter. Such tin tetrachloride solution should 'be redistilled to free it from sulphur, etc. but if it contains precipitated sulphur should be filtered before addition. I

While the electrolysis proceeded care was taken that the temperature did not drop below F. Such a decrease of temperature increases the solubility of chlorine gas and, will cause a lower efficiency and a higher rate of corrosion of the anode. A high chlorine content of the electrolyte has its destructive effect on the tank lining. These difficulties were avoided by maintaining a temperature of the electrolyte at about 80- F. The electrolysis was continued for about 220 hours with a cathode current density of 30 amperes persquare foot, and a cell voltage of 2.3 to 2.9, averaging 2.45. Free' hydrochloric acid 'content variedfrom day to day between a 'maxi- ,mum of ,3 grains perliter'and a'minimum of 72.7 gramsper liter. The over-all current efliciency noted was'95.3%. The chlorinecontent was less than 1 gram per liter. The tin deposit was then removed by steaming by the process of Patent No. 1,989,852. The dross produced while remelting amounted to approximately 1%. The tin recovered upon spectographic investigation had a purity of 99.94%, containing the following small proportions of metals indicated:

their deposits of tin and replace them by others,

which may-be accomplished in a few minutes.

In the claims the expression oily material referring to the material employed for impregnating the carbon anodes is intended to designate linseed oil as well as parafiin or similar material.

During the operation of the process employing the insoluble anodes, impurities will usually build up in the electrolyte or the amount of electrolyte mayhave to be depleted and in that event one of the cells of the system should be cut out from the general circulation system and the electrolytic deposition continued as before, but without addition of tin tetrachloride. Cresylic acid and glue are added to the cell and the latter should be agitated slightly by hand or some mechanical means. The tin begins to deposit as usual and is gradually removed from the solution. With a tin content of below grams per liter the cathode deposit starts to become coarse and rough, this roughness increasing with the decrease in tin content. The chemical purity of the tin deposit is about equal to that of the tin produced during regular operations. It appears inadvisable to continue the electrolytic depletion of the electrolyte after about 15 grams of tin per liter has been reached. After this point the depleted electrolyte can be placed in a precipitation tank where the tin value can be recovered by neutralizing the tin tetrachloride solution with soda ash or ground limestone to precipitate the tin as stannic acid which is then resmelted.

While the invention has'been described in detail with respect to a particular preferred example, it will be understood by those skilled in the art after understanding the invention, that various changes and further modifications may be made without departing from the spirit and scope of the invention, and it is intended therefore in the appended claims to cover all such changes and modifications.

What is claimed as new and desired to be secured by Letters Patent of the United States solution at a temperature of about F. to F., by adding tin chloride from time to time during the course of the electrolysis and by removing chlorine from the solution, whereby a dense deposit of tin is effected and resolution of the deposited tin is negligible.

2. A process according to claim 1 in which cresylic acid is present in the solution being electrolyzed.

3. A process according to claim 1 in which cresylic acid and glue are present in the solution being electrolyzed.

4. A process as defined in claim 1, in which the electrolysis is carried out by electrolyzing the solution with a current density of about 30 amperes per square foot.

5. A process as defined in claim 1, in which the generated chlorine is removed from the electrolyte solution'in such amount that the solution will not contain more than about 1 gram of chlorine per liter of solution.

6. A process of the kind described which comprises, taking for electrolysis an aqueous solution of tin tetrachloride, substantially free from impurities, adding thereto sumcient hydrochloric acid to give a contentof said acid of at least about 45 grams per liter and not more than about grams per liter upon the commencement of the electrolysis, electrolyzing the solution in a cell having a chlorine-resistant anode surrounded by a chamberfor collecting chlorine gas, circulating the electrolyte through the cell, adding tin tetrachloride to the electrolyte as electrolysis proceeds and removing chlorine from the electrolyte so that it contains no more than about 1 gram of chlorine per liter and maintaining the electrolyte solution at a temperature of about 80 F. to about 100 F. as well as the other aforesaid conditions of circulation, addition of tin tetrachloride and removal of chlorine, so that the hydrochloric acid content of the electrolyte solution is held within said range of about 45 grams to about 120 grams per liter.

7. An electrolytic process for depositing tincomprising electrolyzing an acidified aqueous solution of tin tetrachloride containing at least approximately 30 grams of tin per liter of said solution and approximately 45 to 120 grams of hydrochloric acid per liter of solution without adding substantially further quantities of tin, to efiect deposition of the tin and to deplete it below 30 grams per liter but not below about 15 grams per liter while forming chlorine and hydrochloric acid, the said concentration of hydrochloric acid in the electrolytic solution being maintained by maintaining the solution at a temperature of about 80 F. to 100 F., and by re- Y moving chlorine from the solution.

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