IL23438A

IL23438A - Process and composition for tin plating

Info

Publication number: IL23438A
Application number: IL23438A
Authority: IL
Original assignee: M & T Chemicals Inc
Priority date: 1964-05-08
Filing date: 1965-04-28
Publication date: 1968-08-22
Also published as: US3346468A; FR1436922A; NL150170B; DE1592453C3; NL6505832A; GB1109676A; DE1592453A1; DE1592453B2; SE328269B

Description

V' as 3*!t warn T» nn Process and composition for tin plating H & T CHEMICALS INC. » C. 22553 NOVEL TECHNIQUE This invention relates to a novel process for electroplating of tin. More specifically it relates to a novel technique for replenishing the tin content of a tin-placing batho As is well known to those skilled in the art, tin may be electroplated onto various basis metal cathodes from electrolytic baths containing alkali metal stannate, preferably potassium stannate, and alkali metal hydroxide, preferably potassium hydroxide . As plating continues;, tin is removed from the bath. The tin content of the bath may be restored continuously by the use of a soluble tin anode » Use of a soluble anode system is disadvantageous in that it requires -operation within a rather limited range of anode current density. Operation outside of this limited range may yield either a rough dark plate or alternatively inactivation of the anode with resulting failure of the anode to replenish the tin depleted from the bath, this being accompanied by an undesirable increase in concentration of alkali metal hydroxide. Many practical plating operations may require operation outside the narrow limits of anode current density required by the use of soluble anodes f and in such systems,, it may not be advantageous to use a soluble tin anode.

Accordingly, it has been common to use inert anodes typically stainless steel anodes, and to attempt to replenish the depleted tin by the addition to the bath of tin compounds. Typically alkali metal stannate, e.g. potassium stannate, may be added^ but this is highly disadvantageous in that it adds alkali metal ion to the bath and this ultimately may build up the concentration thereof to a point at which no more alkali metal stannate will dissolve at which oint the bath must be discarded . Another disadvantage may be that the concentration of alkali metal hydroxide increases and this must be corrected by neutralization with acid, preferably acetic acid with the very real danger of over-neutralization and sludging.

It is an object of this invention to provide a process for electrodeposition of tin. It is a further object of this invention to provide a process for replenishing the tin content of an alkaline stannate tin-plating bath. It is a further object of this invention to provide a novel composition which may be added to alkaline tin-plating baths to replenish the tin content « Other objects will be apparent to those skilled in the art from inspection of the follow ing description.

In accordance with certain aspects of this invention an alkaline tin oxide sol characterized by its substantially complete convertibility to stannate when in contact with solutions containing 5-100 g/1 of potassium hydroxide at temperature of 50°Co-100°Co may be prepared by the process which comprises reacting at less than 75°C«, an alkali metal stannate in an aqueous solution with acid to a final pH of less than about 6 thereby precipitating hydrous stannic oxide, separating said hydrous stannic oxide from said aqueous medium, washing said hydrous stannic oxide thereby removing water-soluble ions, peptizing said hydrous stannic oxide with peptizing agent selected from the group consisting of potassium hydroxide and potassium stannate, maintaining the molar ratio of potassium to tin in the final solution at 0.1-1.5, and maintaining said hydrous stannic oxide at temperature below 75°C. prior to said peptizing.

The novel alkaline tin oxide sol of this invention may be prepared by the reaction of alkali metal stannate, preferably potassium stannate or sodium stannate, with acid typically as follows: Sn(0H)6= + 2H+ → Sn(0H)4 j+ 2H20 In practice of this process, a body of alkali metal stannate, sodium stannate or potassium stannate, typically sodium stannate may be employed containing 10-500 g/1, typically 50 g/1. To this solution at temperature below 75°C and preferably 35°C-70°C, preferably 50°C there may be added acid e.g. acetic acid, hydrochloric acid, nitric acid, sodium bicarbonate, etc. preferably dilute (e.g. 10%) sulfuric acid. Preferably the acid may be added slowly; when sulfuric acid is used, control may be effected to keep the temperature in the noted range. The acid may be added in amount of about two equivalents thereof per equivalent amount of tin; the pH during the addition may decrease fro a pH greater than 12 down to a final pH of less than about 6 and preferably 2.5-6, most preferably to about pH 4.6. As this occurs, the hydrous stannic oxide precipitates in the form of a flocculent white mass .

If the body of alkali metal stannate is a crude liquor, as obtained from a detinning operation, and particularly if it contains organic impurities such as degraded lacquers, etc., it may be preferred to use sodium bicarbonate as the acid and to achieve partial neutralization to pH of about 8. The sodium bicarbonate may be added to the alkali metal stannate solution or may be generated in situ, e.g. by bubbling C02 into a solution of sodium stannate. Other equivalent acids A- At this pH, the hydrous stannic oxide may precipitate and after separation from supernatant liquor, may be readily-washed free of impurities . The so-washed precipitate may then be reslurried in an aqueous medium and treated with acid, preferably acetic acid to the final pH of less than about 6 and preferably 2.5-6, most preferably to about pH 4.6.

The so-precipitated hydrous tin oxide may be separated from supernatant aqueous medium as by decantation, filtration, centrifuging, etc. - preferably by decantation.

The precipitate may be washed by mixing with water, and again separating. Preferably, washing may be done 4-6 times. At the conclusion of washing the precipitate may be substantially free of (a) sodium ions which may have been present e.g. if the charge material was sodium stannate; and (b) anions including e.g. sulfate which may have been introduced as from the precipitating acid. The total content of water-soluble ions in the' precipitate may normally be less than about 0.2%. In the preferred embodiment, five decantation steps may be employed followed by filtration. The precipitate may be found tq have a tin content of 10%-50% on a wet basis. It is preferred to filter to a tin content of 28%- 0% by weight on a wet basis. At this point, it may be preferred to adjust the tin content to a predetermined level, depending on the amount of tin desired in the final sol. Typically, it may be desirable to adjust the tin content of the precipitate to about 28%.

To this slurry, there may then be added a peptizing agent selected from the group consisting of potassium hydroxide and potassium stannate, in amount sufficient to form a colloidal solution of the precipitate and to effect peptization. The peptizing agent may be added in amount sufficient to permit attainment typically of about 30% tin in the final solution and of a ratio of potassium to tin of 0.1-1.5. Preferably, the peptizing agent may be added as a solid, with agitation, during which time the temperature may be below about 75°C. and preferably 20°G-30°C, typically room temperature.

The colloidal solution so prepared may be characterized by its tin content of 10%-50%, preferably 30%, and by its ratio of potassium ion to tin ion of 0.1-1.5, preferably 0.3. It is a particular property of this solution that it may be stable for an indefinite period of time at temperature of 50°C or less, typically at room temperature. It is a further characteristic that the novel alkaline tin oxide sol may readily be dispersed to give a clear, stable dispersion when contacted with solutions containing 5-100 g/1 of potassium hydroxide at a temperature of 50-l00°C. The resultant dispersion may be maintained in the noted temperature range, whereupon the tin oxide in the sol may be converted to stannate , Typically, 100% conversion to stannate may be realized in a short time, typically four hours or less. It is a particular advantage of this process that the novel alkaline tin oxide sol will retain its ready convertibility to stannate after prolonged storage, in contrast with stannic oxide hydrate paste wh.ich is not completely convertible to stannate after storing.

It is a further feature of the novel solution of this invention that, when viewed under an electron microscope, it appears to contain a plurality of highly crystalline-crystallite stannic oxide particles each of size less than 10 Angstrom units., When the solution is permitted to dry in air, the dry particles appear to form an interlocking network of particle chains each having a thickness of about -8 particles and a length of about 10-50 particles.

According to certain of its aspects the novel tin plating process of this invention may comprise electrodepositing tin from an alkali metal stannate bath containing a insoluble anode Cor a low efficiency soluble anode) and a cathode thereby depleting the tin content of said bath, and replenishing the tin content of said bath by adding an alkaline tin oxide sol containing crystalline-crystallite stannic oxide.

Electroplating of tin in practice of this invention may be effected from an aqueous bath containing alkali metal stannate, preferably potassium stannate in amount of 15 g/1-450 g/1, preferable 100 g/1 and alkali metal hydroxide, preferably potassium hydroxide in amount of 7,5 g/1-35 g/1, preferably 22.5 g/l„ The anode may preferably be an insoluble anode e.g. stainless steel, etc. The cathode may be any metal on which an electrolytic tin plate is desired e.g. steel, brass, copper, etc.

During electroplating, the bath may typically be maintained at 60°C=95°C, preferably 80®C. The cathode current density may be up to 10 amperes per square decimeter (asd) and typically a out 6 asd over a plating time of 5-30 minutes , typically 20 minutes. The anodic current density may be several times higher than the cathodle;. ■*> ' current densities. Typically the anodic current density may be as ¾¾igh as 50 asd. This unexpected feature of the invention facilitates plating on the inside of pipes, couplings, rings where the geometry of the article is such that the anode musts because of the geometry,, be considerably smaller than the cathode.

As the bath is used over a period of time, for every 118» 7 grams of tin plated out, 112 grams of potassium hydroxide (in the case of the potassium bath) or 80 grams of sodium hydroxide (in the case of the sodium bath) is generated as by the following equation? K2Sn(0H)6 → Sn + 2K0H + 02 + 2H20 If no corrective action be taken, the bath becomes unbalanced, the tin content drops, and in due course no more tin will plate out. Th content of free hydroxide rises and the cathode current efficiency drops due to this factor alone.

In practice of this invention, there may be added to the electroplating bath the alkaline tin oxide sol hereinbefore disclosed. The addition of the colloidal solution may be determined by the ampere hours for which the bath is used or by measuring the tin content of the bath or e.g. the potassium hydroxide content of the bath.

Preferably the colloidal solution may be added to the bath at a rate of 1 ,107 grams of tin per ampere hour of plating. In the preferred embodiment wherein the colloidal solution contains 30% tin, 3 .69 grams of solution may be added per ampere hour .

If control be effected by measuring the tin content of the bath, then for example 3 ,3 grams of 307o tin oxide sol may be added for each one g/1 of tin lost, thus compensating for the tin plated out , It will be apparent that the addition may be continuous or incremental. When incremental, it preferably may be done at intervals sufficient to prevent the free e.g., potassium hydroxide from rising above or preferably closely approaching 30 g/1 In a typ al industrial operation using currents of the order of 0 .25 -0 «50 amperes per liter, then the free potassium hydroxide content may rise by 0 =28-0 ,56 g/1 per hour; and this may be corrected by the addition of 0 ,9 -1 ,8 grams of the preferred 30% tin sol per liter per hour .

Use of this novel composition permits tin plating to be carried on indefinitely from a tin bath with no deterioration of the quality of the plate.

It is a particular feature of the process of this invention that addition of the colloidal solution permits maintenance of the bath with no undesirable excessive build-up of potassium ion.

If replenishment of tin were made by addition of potassium stannate, the excess potassium hydroxide may have to be neutralized by acid eg, acetic acid (as is commonly done), then the potassium content of the bath may rise to such an extent that after about 375 ampere hours per liter have passed through the bath, no more potassium stannate can be dissolved in the bath because of the presence of the excess of potassium ions§ the bath may have to be discarded as unuseablec It is also a particular feature of the novel product of this invention that when added to tin plating baths as herein noted;, it goes into solution as the desired stannate ion.

Practice of this invention may be observed from the following examples? EXAMPLE 1 A solution of 1000 grams of potassium stannate dissolved in 2 liters of water was heated to 9®C and 700 grams of sodium bicarbonate was added thereto in small increments o During the addition, the solution was stirred and the temperature was maintained at ®C + 3®C« The precipitate which formed was filtered and washed with 500 ml of cold water . After washing;, the precipitate was suspended in 1.5 liters of water and 120 ml. of glacial acetic acid was added to bring the pH to about 5. The precipitate was again filtered and washed with water. It was then mixed with 200 grams of solid potassium stannate, whereupon a fluid clear, slightly amber alkaline tin oxide sol was obtained. The product sol weighed 1465 grams, had a specific gravity of 1.63, and contained 28.6% by weight tin.

EXAMPLE 2 50 ml. of an aqueous potassium hydroxide solution containing 240 grams KOH per liter was transferred to a 100 ml. volumetric flask . An amount of the alkaline tin oxide sol of Example 1 equivalent to 11.5 grams of tin (41 grams of the sol) was added thereto and the volume adjusted to 100 ml. with water. Throughout the addition and mixing steps, the materials were maintained at room temperature « After mixing, a clear, stable solution, free from undissolved material, was formed.. From this example, it may readily be seen that the novel alkaline tin oxide sols produced by the process of this invention are characterized by their unexpected ready solubility in aqueous potassium hydroxide solutions .

EXAMPLE 3 A tin plating solution was made up by dissolving potassium stannate and potassium hydroxide in water to give 283 grams per liter of K2Sn(0H)6 and 20 grams per liter of free KOH. The bath was heated to about 77°G and electro-lyzed at a cathode current density of 6.5 amperes per square decimeter using stainless steel anodes and steel cathodes.

The alkaline tin oxide sol of Example 1 was added to the bath at the rate of 3.9 grams of sol for each ampere-hour of operation to replenish the tin depleted by plating.

The bath was operated and maintained in this manner for 123 ampere-hours per liter. Throughout the test, the tin electroplate obtained was highly satisfactory and had a light grey, satin, smooth appearance „ At the end of the test, the bath was analyzed and found to contain 350 grams per liter of K2Sn(0H)fe and 2 grams per liter of free KOH. From this example9 it may be seen that the novel alkaline tin oxide sols of this invention are outstanding sources of tin for the replenishment of tin plating baths that the tin plate obtained was highly satisfactory? and that the use of the novel products of this invention substantially eliminates the problem of build-up of free KOH in the plating bath.

EXAMPLE For purposes of compariso 9 the procedure of Example 1 was repeated9 except that the temperature was not maintained within the limits of this invention as hereinbefore set forth.

Specif cally,, a solution of 1000 grams of potassium stannate dissolved in 2 liters of water was heated to 82®G and 700 grams of sodium bicarbonate was added thereto in small increments. During the addition,, the solution was stirred and the temperature was maintained at 82®C + 3®C„ The precipitate which formed was filtered and washed with 500 ml. of cold water. After washing;, the precipitate was sus ended in 1.5 liters of water and 120 ml. of glacial acetic acid was added t© bring the pH to about 5. The precipitate was again filtered and washed with water. It was than mixed with 200 grams of potassturn staraaat®9 whereupon there was obtained a viscous 9 murky product containing numerous undissolved particles which were not disscslved. after 2h hours.

When the product of this example was subjected to the solubility test described in Example 29 it gave a dense whit® precipitate which was not dissolved even after prolonged agitation-EXAMPLE 5 IpOOO liters of a solution obtained from a de nning operation, was analyzed and found to contain 49.5 grams per liter of tin and 3 grams per liter of free sodium hydroxideβ Due to the presence of decomposition products from lacquers contained on the scrap9 the solution was black and contaminated with organic residues., The solution was heated to 51®C and 90.7 kilograms of sodium bicarbonate was added incrementally with stirring while the temperature was maintained at about 50®C The precipitate which formed was allowed t© settle overnight and the supernatant liquor was decanted. The precipitate wa®' transferred to a filter and washed with water^ until white. The washed precipitate was suspended in 100 liters of water and 10 liters of glacial acetic acid was added thereto t© bring the pH to 4.6. The precipitate was again filtered and washed with water to a neutral filtrate. The residue after washing weighed 119„3 kilograms and contained 42.89% tin.

To this residue was added 63 liters of. water and the mixture was stirred. ¼5<A kilograms ©f potassium stannate was added with stirring at room temperature^ whereupon there was obtained 227.7 kilograms of a clea 0 fluid slightly greenish;, alkaline tin oxide sol having a specific gravity of 1.66 and a tin content of 30% by weight. This sol was treated with carbon t© remove the coloro When this product was subjected to the solubility test described in Example 2, it produced a clear, stable solution with no evidence of insoluble material.

When it was employed as the tin-replenishing agent in a plating operation as described in Example 3, it was found that the bath was still producing highly satisfactory tin plate after 430 ampere-hours per liter of operation and that the free potassium hydroxide content of the bath remained essentially constant throughout the test .

EXAMPLE 6 To a solution of 725.7 kilograms of potassium stannate in ,542.4 liters of water was added a solution of 18 parts by volume concentrated sulfuric acid and 82 parts by volume water until a pH of 3.0 was reached. The temperature during the addition was maintained below 35°C| the precipitate was washed with water to a neutral filtrate. The washed precipitate was mixed with 215 kilograms of potassium stannate at room temperature, whereupon a clear, fluid alkaline tin oxide sol containing 130 grams per liter of tin was formed.

When this sol was subjected to the solubility test described in Example 2, it produced a clear, stable solution with no evidence of insoluble material.

EXAMPLE 7 To a solution of 500 grams potassium stannate in 1200 ml. of water was added 410 ml. of a solution of one part concentrated sulfuric acid in 4 parts of water to give a pH of 3.85. During the addition, the solution was stirred and maintained at 32-38°C. An additional 1000 ml, of water was added and the precipitate formed during the addition was filtered and washed to a neutral filtrate „ The washed precipitate was mixed with 50 grams of potassium hydroxide whereu on there was formed 1800 ml, of fluid9 clear alkaline tin oxide sol containing 108 grams of tin per liter» When subjected to th@ solubility test of Example 2P the sol of this example produced a clea 9 stable solution with no evidence of insoluble material.

Although this invention has been illustrated by reference to specific examples,, numerous changes and modifications thereof which clearly fall within the scope of the invention will be apparent to those skilled in the art.

Claims

23438/3

oxalate:

1> A process for preparing alkalis© tin oxide sol

characterized by its substantially complete convertibility to stannate when in contact with solutions containing 5-100 g/1 of potassium hydroxide at temperature of 50-100°G> which comprises reacting at eea than 75°C. an alkali metal

stannate in aqueous solution with acid to a final pH of less than about 6 thereb precipitating hydrous stannic oxide,

separating the hydrous stannic ©aside from the aqueous solution, washing the hydrous stannic oxide thereby removing water*

soluble ions, peptizing the hydrous stannic oxide with a peptizing agent selected from the group consisting of potassium hydroxide and potassium stannate, maintaining the molar ratio of

potassium to tin in the final solution at 0*1-1.5, and maintaining the hydrous stannic oxide at temperature below 75°C. prior to the peptizing operation.
2. A process according to Claim 1, wherein the alkali metal stannate in aqueous solution comprises 10-500 g/1 of alkali metal stannate.
3. Λ process according to Claim 1, wherein the alkali metal stannate in aqueous solution is reacted with acid at a temperature of 35-70°C. and the hydrous stannic oxide is maintained at temperature of 35-70°C. prior to the peptizing operation.
4. A process according to claim 1, wherein the final pH is 2.5-6.

23438/5
5· A process according to Claim 1, wherein the reaction of the alkali metal stannate in aqueous solution with the acidic agent is conducted in two steps, in the first of which the solution is reacted with acid to a pH of about 8 and the precipitated hydrous stannic oxide is separated from the

aqueous solution, washed and re-slurried, and in the second step the slurry is reacted with acid to a final pH of less than about 6.
6. A process according to any one of Claims 1 to 5» wherein the tin content of the hydrous stannic oxide obtained after precipitation and washing is adjusted to 28-40 by weight, prior to the peptizing operation.
?, A process according to any one of Claims 1 to 6, wherein the acidic agent is sodium bicarbonate.
9. A process according to Claim 1, substantially as described.
10. The alkaline tin oxide sol characterized by its substantially complete convertibility to stannate when in contact with solutions containing 5*100 g/1 of potassium

hydroxide at temperatures of 5O-100°C when prepared by a

process according to any of Claims 1 to 9*
11. A tin plating process which comprises electro-depositing tin from an alkali metal stannate bath containing an insoluble anode and cathode* thereby depleting the tin content of the bath, by adding thereto an alkaline tin

oxide sol prepared according to Claim 10*
12. A process according to Claim 11, wherein the sol is prepared according to Claim 7 and the hath ia maintained at 60-95°C.
13. A process acoording to Claim 11» wherein the alkaline tin oxide sol is added to the hath at a rate of about 1.107 g of tin per ampere hotir of plating.

1 o A tin plating process according to Claim 11 substantially as described.

For

DR.