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202
the increased viscosity of the emulsions. This reduced mobility combined with the orienting effect of the polar interface appears to be responsible for the rapid polymerization. It is probable that emulsions containing chloroprene particles of the order of 2 p in diameter will polymerize slowly enough so that a-polymer can be isolated. It would be desirable to study chloroprene dispersed through a continuous phase which would eliminate water from consideration. h~~other mater,al suitable for preparing such has been found, the exception Of glycol which, because of the hydroxyl groups, is quite analo-
Vol. 23, Yo. 2
gous to water. Chloroprene dispersions made in ethylene glycol behave in a manner similar to water emulsions. LITERATURE CITED (1) Carothers, Tyilliams, Collins, and Kirby, J. Am. Chem. 53, 4203 (1931). (2) Ibid., 53, 4221 (1931). (3) ~ ~ i f fE,i L,, ~ , Ibid., 45, 1651 (1923).
,Sot
,
RECEIVEDAugust 29, 1932. Presented before the Division 01 Rubber Chemistry a t the 84th Meeting of the American Chemical Society, Den.,er, Colo., August 22 t o 26, 1932. Thie paper is Contribution 27 from Jackson Laboratory, E. I. du Pont de gemours & Company.
Electrolysis of Silver-Bearing Thiosulfate Solutions K. HICKMIAX, W, WEYERTS,AIVD 0. E. GOEHLER,E a s t m a n Kodak Company, Rochester, N. Y.
A
T LEAST 100 tons of s i l v e r halides are dissolved annually in America alone in photographic thiosulfate baths. The s i l v e r has been recovered in a number of ways ( 7 ) ,perhaps the favorite being the precipitation as sulfide ( I S ) , followed by smelting. The separation of silver sulfide is an unpleasant operation lvhich may well be replaced by electrolysis (l6),l if the latter can be done conveniently in thiosulfate solution. The procedure might be extended to the treatment of silver ores to the exclusion of the more dangerous and expensive cyanide. It is the purpose of this paper to describe recent developments in the electrolysis of silver-bearing hypo (thiosulfate) solutions on an industrial scale. CHEMICBL CONSIDERATIONS
simple thiosulfate solution may contain or present to a suitable chemical host all or any of the above, together with liydrosulfurous and sulfoxylic acid.. If we consider these in p a r t i a l i o n i c dissociation, adding the water molecule, hydrogen and h y d r o x y l ions, and diswlved oxygen and hydrogen a t T arying oxidation-reduction potentials, we comprehend in some measure the number of events which may occur to the d v e r ion in free solution or 111 the neighborhood of the electrodes. T h e p r a c t i c a l evidence of this complex situation i- the immediate formation of colloidal s i l v e r sulfide when a silverbearing thiosulfate solution i. e l e c t r ol y z e d under ordinary l a b o r a t o r y c o n d i t i o n s . -45 soon as the current is applied, brown s t r e a m s of c o l l o i d a l silver sulfide fall a w a y f r o m the cathode, yielding a dirty unfilterable liquid. Crabtree and Ross (S), in their comprehensive review of fixinn-bath recovery methods, dismiss the electrolysis of hypo as impracticable for this reason. When the simple stationary electrode is replaced by a rotating cathode, and electrolysis is performed under varying pH conditions, a wide region is found where satisfactory separation of silver occurs. I n general, the proper solutions contain acid, which thiosulfate will not tolerate except in the pre.sence of sulfite, and, accordingly, all the solutions described in this paper contain sulfite. The equilibrium
-Much of the siltier accumulating in motion picture fixing baths, which used to be reclaimed as silver sulfide, is now recovered as metallic silver by electrolysis, and Ihe bath which used to be thrown io waste is replenished and recirculated. The electrolytic regenerat ion incokes the use of large cells containing 100 square ,feet of cathode surface through which a current of 300 amperes is passed at 1 to 1.5 colts. At the anode, thiosulfate is oxidized io tetrathionate and trithionate sulfate; at the cathode, silver is deposited with small quantities of silver sulfide and gelatin; some of the tetrathionate is reduced to thiosulfate. The adjustment of the bath to secure good plating must be made within certain critical limits: cigorous agitation, together with the presence of acid, sulfite, and certain promoting agents is essential. The electrical plating eficiency caries between 65 and 80 per cent in large installations, and the yield per million feet of Jilm is about 1200 ounces ( 1 kg. per 10,000 meters). The consumption of fixing baths is reduced to 35 per cent of the quantity previously used.
When thiosulfates are electrolyzed in aqueous solution with moderate current densities applied through inert electrodes, no gases are evolved, both the oxygen and hydrogen being absorbed by the solution. In general, tetrathionates (IO) are generated a t the anode, and sulfides (21) or derivatives of sulfoxylic acid a t the cathode. The substances mix by natural convection to reform thiosulfate. The quantity of salt lost by irreversible decomposition to sulfur or sulfate (6) is small. The great number of substances existing in a solution Tvhich purports to contain thiosulfate, and the variety and ease of their mutual transformations is remarkable. A study of the literature (20) concerning the thionic acids ( 2 ) indicates that a solution of sulfide or colloidal sulfur exposed to the air may soon contain hydrogen sulfide, sulfur dioxide, thiosulfate, the four thionic acids, and sulfate. Conversely, a 1 Many so-called electrolytic processes have been used in which metal strips or bimetallic packs are immersed in the exhausted fixing bath. The silver is removed but another metal takes its place.
HA03 eH2SOa
+S
apparently (3) involves the intermediate formation of trithionate and hydrogen sulfide, thus: 2HiS Oa
F-'
H2S
+ HA08
and the reformation of thiosulfate:
I N D U S T R I ,4 L A N D E N G I S E E R I S G C H E 21 I S T 1%T
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203
PIATIKG MEDIUM Two types of solution are generally useful for study. They K e may assume, with Bassett and Durrant, that Le Chatelier's theorem applies usefully to the reactions of the sulfur acids. Addition of an ion or other chemical entity will qhift the equilibrium of a reaction in the direction diminishing the concentration o f that ion or entity. Sulfite is neces-ary to prevent the spontaneous formation of' sulfur or hydrogen sulfide from the hypo solution in acid condition.? On casual consideration, it is puzzling to know how a n acid condition, which appears to promote the formation of hydrogen sulfide, can combat the sulfiding of the silver a t the cathode. There is, however, besides the sulfur presented by the free solution, some sulfide produced a t the cathode by rrduction of the thiosulfate:
-+
H~S203 2H +Hi3
contain, besides the materials listed below, certain additions or accumulations which will be described later.
+ Hi301
.4lthougli thiosulfate is best formed under alkaline condition. from sulfur and sulfite, the reaction between sulfite and d f i d e ion occurs most readily in slightly acid solution (4): 2NaHS
+
4SaHS03
+ 3SapS203+
3&0
CONDITIOXS FOR P L ~ T I S G The fundamental chemical condition for the deposition of kilver instead of d v e r wlfide is the presence of sulfurous acid. The more obvious physical conditions offering opportunities for control are agitation, temperature, and current density. -4gitation is, perhaps, the most important, owing l o the kind of ions involved. Silver bromide requires 4.5 times its weight, and silver chloride 3.9 times its weight of Sa2S20s.5H20in 1 per cent qolution to effect solution. The corresponding figures for 20 per cent hypo are 5.4 and 4.9. The silver thiosulfate available from double decomposition, forms, in turn, silver sodium thiosulfate (1) and the heavier complex Ka5.4g3&Os).,, which remains soluble in water. The silver-bearing ions in solution are reputed to be &3(SnOs)r- and Ag&Os-, which, by slight secondary dissociation, yield free silver ions: SaAgS2Os
Sa+
+ AgS203- * A g + + S203--
The constant for the final silver dissociation is stated by so that most of the silver Bodldnder (6) to be less than is held in electrically inert molecules or in negatively charged ions. The silver is wandering aJvay from the pole a t which it is to be deposited. -4 remedy for this adverse migration is stirring, and the current which can be tolerated should show some relation to the velocity of stirring. The criterion for toleration i5 the deposition of bright metallic silver, leaving the solution uncolored by sulfide. When once the tolerated maximum is passed, the plating surface blackens and the solution becomes cloudy. The toleration is found to increaqe with the temperature, with the concentration of silver, and also, within limits, with increase in acidity and SOs-- content. Manifestly it would be possible to construct families of curve. shoning the composite relation of all the principal factors, from which prediction could be made for plating equipment to handle any chosen solution of silver halide in hypo. The composition of the solution is, however, generally governed by considerationi which are quite irrelevant, from an electrical point of view, so that the work is not ~vorthwhile for the present purpose. Instead, a few curves will be given d-hich will point to the more important relations. 2 That hydrogen sulfide is actually liberated during the acidification of hypo, although sulfur dioxide is Boon the only gas detectal)le b y amell, readily g h o a n b y heating a 10 per cent thiosulfate solution u i t h boric acid and euspendin:: lead a c e t a t e paper in the vapoi
1s
FIGURE1. EXPERIMENTAL CATHODEAND AUODECARBONSA V D DISKS
A. PHOTOGR~PHIC FIXING BATHS. S e w baths generally hold about 20 per cent hypo; sodium sulfite and acid (equivalent to about 1 per cent sodium bisulfite); and a hardening agent for gelatin, such as potash or chrome alum, a1.o in approximately 1 per cent concentration. After use, the bath holds quantities up to 1 per cent of silver halides, up to 0.5 per cent of gelatin degradation products, and varying quantities of stale developer. The acidity is considerably lowered but usually remains above the isoelectric point of gelatin. T e may adopt the following solution as a n experimental standard: Grams
Commercial hypo (Na&203 5H2O) Sodium bisulfite (NaHSOa) Chrome alum ICrg(SOd3 KzSOa 24HzOl Silver (as AgBb) Water t o
0.5 100
€3. DILUTETHIOSULFATE BATHS. Sometimes quantities of dilute thiosulfate solutions are available which contain relatively large proportions of silver salt', and are consequently worthy of electrical recoi*ery. A typical solutic)n would be: Grams HYPO
SO? gas or NaHSO3
2
0.5
Silrer halide Kater to
Grnms 0.4 100
AGITATIOS X a n y ways are known of renewing the liquid in the sluggish surface layer on the cathode. The means available when accurate measurements of velocity are required are limited, and it seems preferable to use tv-o concentric cylindrical
INDUSTRIAL AND ENGINEERING CHEMISTRY
204
electrodes moving a t ascertainable relative velocities at a specified distance from each other. For the outer cylinder there is employed a nest of arc lamp carbons secured to a metal band held above the beaker in which the experiment is carried out. The cathode is a cylinder of silver 2 inches (5 cm.) in diameter, rotating so that there is a channel 1 inch (2.5 cm.) wide between its surface and the carbons. The complete dimensions are shown in Figure 1. I
~
8
IB
32.
G 4
FIGURE 2
At any given separation and surface velocity, the size and shape of the electrode can introduce variations of their own. Small unperforated cathodes can accommodate higher current densities than large smooth ones because on the latter a sluggish surface stream of liquid can travel for a longer time without disturbance. Cathodes 2 feet (60 cm.) square in commercial plating sets have been found to accommodate 2 amperes per square foot (0.2 amp. per sq. dm.) in solutions of type A, whereas a small electrode 2 inches (5 cm.) in diameter moving with the same surface speed would take 6 amperes per square foot (0.6 amp. per sq. dm.). The ratio of current density to agitation cannot be discussed without reference to the composition of the solution. The curves in Figure 2 show the change in toleration as we pass from alkaline through neutral, to an acid condition of type A baths. Since chrome alum is precipitated in alkali, this substance and potash alum are omitted in all three experiments, but are included in baths 4 and 5 (Figure 3), which is otherwise similar to bath 3, Figure 2. Commercial plating solutions which are recirculated accumulate quantities of halogen salts, and bath 6, with extra potassium bromide, is included in the series (Figure 4). Evidently, baths containing acid sulfite permit so much greater current densities than the others that it becomes unnecessary to cover the alkaline and neutral variants for baths of type B. These are, accordingly, represented by the single curves of Figure 5. SILVERCONTENT : CURRENT DENSITYRATIO Two general processes occur in the neighborhood of the cathode-reduction of ions with respect to silver, and reduction with respect to sulfur. There are from 100 to 500 sulfurbearing ions for each silver complex, even in solutions considered fairly rich in silver, and the deposition of metal instead of sulfide is possible only because the silver ions are discharged a t a lower potential. The competing reactions may be summarized:
++
cathode to carry the current. The permissible current density a t constant agitation varies directly with the silver content until only one part of metal in a thousand of 20 per cent hypo is reached. Thereafter, the current may be maintained a t this level without darkening the electrode until the last trace of silver has been removed, a happening of vital importance to the industrial application of the process. The relation for solutions of type A, with stirring at 1 foot (30 cm.) per second, is shown in Figure 6.
-1
PCUtPMCUAL SPECD--FEET PER S E C O N D IZ L
Vol. 25, No. 2
+
(Ag-thionic complex) H + +Acid hypo complex Ag metal (Na-thionic complex) H + -+- Hydrosulfide complex &. (Ag-thionic complex) Silver sulfide
+
The smaller the silver concentration, the greater must be the agitation in order that enough silver complex shall reach the
THE DEPOSIT At low current densities the first deposit is a brilliant matte white. As the current increases, the color turns creamy, then yellowish, until a t the limit of tolerance it is a deep brown. Beyond the limit, streamers of colloidal silver sulfide wander into the solution, and it is not then easy to obtain good plating a t reduced current densities, even with a new electrode. No matter how safe the current, the deposit becomes less uniform and brilliant as it grows in thickness. With 0.013 inch (0.5 mm.) deposited, the surface becomes crystalline, passing to a rough nodular structure on continued plating. The tolerated current maximum falls off considerably, owing to the dead liquid which remains lodged in the crevices between the crystals. Early systematic experiments were carried out with two kinds of solutions which may be called synthetic and natural. The former were made with weighed quantities of silver; the latter, by fixing waste film until the bath contained approximately the required silver. It was soon found that data acquired from the synthetic solutions were almost entirely inapplicable to the natural baths. Gelatin dissolving from the film completely altered the course of electrolysis.
L\Z 4
8
lG
32.
6 4
I
FIGURE 3
When the gelatin content was kept low by using, for instance, cooled solutions charged with chrome alum, plating could be conducted faster than usual. The silver presented a new, quite characteristic bluish appearance, the surface rivaling a mirror in brilliancy. Plating could be carried on to any reasonable thickness because conditions favoring entrainment of liquid did not arise and there was, consequently, freedom from sulfide degradation. It was found that the protein was being deposited with the silver, necessitating frequent additions of gelatin to prevent the beneficial effects from wearing off. The blue metal was hygroscopic, and would shrink when withdrawn from the bath, becoming very brittle when dry. On heating, separation into laminas occurred, followed by charring, and the emission of a strong smell of burnt gelatin. The roasted metal was firm and very pure silver. The trace of gelatin so beneficial to plating was, perhaps, one-hundredth that occurring in a heavily worked fixing bath. Higher concentrations, far from assisting, actually
February, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
prevented plating. The cathode, even a t low current densities, became blackened and caused fouling of the bath. Sometimes a solution, set aside as worthless and reexamined a day later, would be found to plate brilliantly. It was believed that some activating material was generated during the decay of gelatin. Alternatively, it was thought that gelatin exerted two functions-that of restrainer and that of promoter. I n high concentration the latter would be overpowered by the former, which was considered dependent on the protective properties of the colloid. In stale solutions, the gelatin would be degraded to glue, spoiling the colloid but perhaps leaving unharmed the promoter principle ( I ? ) . A search was therefore begun for likely substances to promote the deposition. These experiments, arising a t a time when Sheppard's work on photographic sensitizers (25) was earning recognition, seemed best furthered by trying the effect of labile sulfur compounds. While small quantities of the alkali thionates and of thiocyanates produced no improvement, immediate success was secured with the true photosensitizers. Thiourea, the isothiocyanates, certain thio acids, and conjugated disulfides were effective in concentrations as low as 20 p. p. m. In general, activation was produced by substances having the groupings:
s=c
s----c=N-;
203
culation of the solution is impeded (resulting in a local scarcity of silver), and the fatal sulfiding results. The function of thiourea or other promoter is probably twofold. As a polar molecule it can migrate to the cathode where it competes for position with the gelatin and, as a substance having a great affinity for silver halide, it can act as a carrier for silver, forming a complex which migrates in the correct way for silver deposition instead of in the opposite direction which is taken by the silver thiosulfate ions. On reach-
s
145 4
-a
1332
-
I
g
a
"-
