Hydrometallurgical process for the recovery of silver as silver halide or silver pseudohalide from waste photographic paper and film and other sensitized materials

The invention provides a process for the recovery of transition and post-transition metals the halides and pseudohalides of which are hyperlinearly soluble in excess halide or pseudohalide, and especially of silver as halide or pseudohalide from waste photographic paper, photographic film, and other sensitized materials. The process comprises washing the material, removing wash, bringing the silver present to a form solubilizable in concentrated halide or pseudohalide solution, by oxidation if necessary, dissolving the silver salt or salts by means of a concentrated reagent solution of the halide or pseudohalide of an alkali or alkaline-earth metal, or ammonium, separating said complex solution from solid matter, washing the solid in turn with a dilute solution of the halide or pseudohalide and then with water, treating said separated complex solution with said dilute halide or pseudohalide wash and water-wash combined to form a precipitate and separating the precipitate of said silver halide or pseudohalide, reconcentrating dilute reagent, separating unwanted, accumulated salts from reagent and purifying silver salt.

The present invention relates to the application of the known property of 
certain halides and pseudohalides of silver, although insoluble or 
sparingly soluble in pure water, to dissolve to a lesser or greater extent 
in concentrated aqueous halide and pseudohalide solutions forming halo- or 
pseudohaloargentates, e.g., 
EQU mAgI+nKI.fwdarw.nK.sup.+ +Ag.sub.m I.sub.m+n.sup.(-n) ( 1) 
where, for each different combination of integers m and n which are 
applicable, there exists a separate species, and an equilibrium constant 
K.sub.m,n unique for that species. Different amounts of every existing 
species coexist in any complex solution, the relative amounts being 
governed by the Law of Mass Action 
EQU [Ag.sub.m I.sub.m+n.sup.(-n) ]=K.sub.m,n [I.sup.- ].sup.n, (2) 
where [I] is the free iodide-ion concentration (as opposed to total iodide, 
since some of the total iodide has been included in the iodoargentate 
complexes), it also being understood that correction for changes in 
activites of the various species, due to various factors, has been 
considered. 
So, it may be shown that the total silver concentration, i.e., the 
solubility, is simply the sum over m and n of all the species.times.m, 
EQU L.sub.AgI= sum(m.times.K.sub.m,n .times.[I.sup.- ].sup.n); (3) 
therefore, it is obvious that L.sub.AgI will increase non-linearly with 
iodide concentration; conversely, a relatively small dilution of a 
concentrated complex solution will precipitate most of the 
complexly-dissolved silver halide. 
All the above facilitates the separation and recovery of silver, by, and in 
the form of halides or pseudohalides, e.g., I.sup.-, Br.sup.-, or 
SCN.sup.-, from photographic and other wastes, where the metal or its 
compounds are dispersed. In all these applications, if the Ag appears in 
elemental form, it must be oxidized to a form which the above-mentioned 
complexing agent will solubilize and react with; this may be accomplished 
with acid treatment in most cases, as will be described. 
PREVIOUS ART 
The use of concentrated aqueous solutions of I.sup.-, Br.sup.-, or 
SCN.sup.- for the recovery of silver, or, for that matter, similar 
transition or post-transition metals, e.g., Pb, Cu, Hg, Bi, Au, and Tl, 
has not been found in the Art of extractive metallurgy. The use of 
concentrated brine, with or without CaCl.sub.2 for Ag, Cu, etc. leaching 
is very well known in the Art. In British Pat. No. 237,939 we find the 
oxidation of various Ag and Pb ores, followed by the lixiviation with 
concentrated NaCl, CaCl.sub.2, or other Cl.sup.-. However, the inventor 
specifically states in claim 5 that "ores . . . of Pb and Ag in too 
concentrated a form are mixed with inert substances . . . ," meaning that 
this method can not treat moderately concentrated materials. This is 
further verified in Iz. ANKAZSSR, Ser. Met. Ob. i Og. 1961, No. 2, 85-90 
(CA 56, 9493c), where mention is made that even hot, concentrated NH.sub.4 
Cl will dissolve no more than 3-4 g Ag/L. (See also JACS 33, 1937 (1911).) 
The use of complex formation for metal separation and recovery through 
solvent extraction is very well known in the Art now. Very recently 
(Research Disclosure No. 151, p. 49, Nov. 1976), Ag recovery as I.sup.- 
complex from green (unexposed) photographic wastes was proposed by B. C. 
Telford of Kodak. In his proposal, as given in an example, HI, KI, and 
acetonitrile--H.sub.2 O are used. Besides the waste of much HI by its 
combining with the gelatin present, and besides the use of unwieldly 
nitriles in his method, I present a method which bypasses the very need 
for solvent extraction, by direct aqueous recovery of the Ag as complex 
and its dilution and destruction (stripping) in purely aqueous form. It 
should be noted that solvent extraction is a much broader method than my 
proposed one, in that any complex which is preferentially partitioned to 
the non-aqueous solvent is usuable, whereas in my method only those 
particular types of complexes defined above are usable. 
It is interesting to note that the advantage of keeping the complexing 
agent in one phase has been noted (J. Gen. Chem. USSR 47, No. 3, pp. 
540-5, Mar. 1977), where the thiocyanate of an organic base in toluene 
carrier extracts various metals from aqueous solution. 
Caley used concentrated HI, and later, saturated NH.sub.4 I solution as a 
general quantitative reagent for determination of various metals, 
including Ag (Ind. Eng. Chem., Anal. Ed. 8, 63-7 (1936); "Analysis of 
Ancient Metals," p. 67, New York (1964). However, as mentioned above, HI 
would be wasted by side reactions. Also, he uses a great excess of each 
reagent, which would be uneconomical for recovery purposes. The use of 
Br.sup.- or SCN.sup.- are not mentioned there. In my method, the various 
factors influencing optimum reagent concentration will be treated below. 
Finally, U.S. Pat. No. 1,998,010 describes a process of purifying AgI 
obtained as an intermediate, in, inter alia, I.sub.2 recovery from 
oil-well brines by dissolving in conc. I.sup.-, filtering from impurities, 
diluting, and separating by filtration; however, there the case is one of 
an AgI precipitate, while here there is a dispersion of AgBr+AgCl, with 
perhaps a little AgI, the former which must be converted to AgI, in the 
emulsion gelatin. Also, here other reagents (Br.sup.-, SCN.sup.-) are also 
used. 
PROCEDURE 
(1) Solubility Influencing Factors 
The solubility of the complex silver halides or pseudohalides is a complex 
function of temperature, ionic strength, identity of "inert" salts 
present, especially the cation, which is the counterion of the complex 
anion (see Mironov, Russian J. Inorg. Chem. 7, 1366 (1962) and ibid., 5, 
138 (1960)), and, of course, the (free) ligand concentration. In general, 
it may be stated that, up to a point, the complex solubility increases 
exponentially with the increasing ligand concentration; increases linearly 
with the ionic crystal radius of the "inert" cation (e.g., KI should, up 
to a point, dissolve roughly 1.33/0.99=1.34 times the amount of AgI that a 
similar solution of NaI would); increases with total ionic strength. It 
almost always increases, sometimes greatly, with temperature more or less 
according to the known isochore relationship. After a certain point, 
however, various opposing effects can occur: 
(a) for very concentrated complex solutions double salts containing the 
desired metal may precipitate out of solution; 
(b) the rate of the exponential increase in solubility with ligand 
concentration decreases; 
(c) there is a reversal in the effect of the "inert" cations (see Mironov, 
ibid.), and the Li or Na salt of the ligand is a better/or as good a 
solvent for the metal halide or pseudohalide than the potassium or 
ammonium salt of the ligand would be (ammonium is usually a slightly 
better choice of cation for a given ligand). 
(d) according to equ. (3) above, the solubility is a function of ionic 
strength in that both the equilibrium constant for each species, and the 
activity factors for free iodide ion change with ionic strength, and this, 
differently for each concentration of different sets of salts of cations 
and ligands. Therefore, there can also be a reversal because of changing 
activity factors: in general, the relationship is complex. 
In this regard it should be noted that according to my proposed method as 
described, there will be an accumulation of certain amounts of salts 
caused by acid-base neutralization, when applicable, which, because of the 
above, will change the solubility, by changing the activity factors 
(changed total ionic strength); therefore, it is desireable to control and 
limit the accumulation of said salts as much as is possible, as will be 
described; 
(e) finally, the interesting effect of the enhanced solubility of some 
ligand-containing salts should be mentioned: in very concentrated complex 
solutions (e.g., KI+AgI), because of the withdrawal of iodie ion to the 
complex (since n in equ. (1) increases with iodide concentration; see 
Leden, Acta Chem. Scand., 10, 812 (1956)), so that the solubility product 
of KI is not exceeded, and the solution is "supersaturated" with the KI. A 
similar mutual solubility effect has already been noted with the system 
BeSO.sub.4 --BeO--H.sub.2 O. 
(2) Description of Process of Invention 
Except for the case of a mixture of the silver salt with some other 
material, as in green photographic materials, the metal will usually be in 
the form of the element, e.g., silver dispersed on exposed photographic 
paper or film, specifically, in the gelatin layer. 
According to this process, the silver must be brought into a form which 
will be soluble as a complex in the solution finally used to recover it. 
To this end, the volume of the original mixture is first reduced as much 
as possible, e.g., by burning photographic paper or film, or sensitized 
copying papers, or any mixture thereof. Also, the gelatin containing Ag 
may be stripped, e.g., by conc. NaOH solution, e.g., in Fr. Pat. No. 
633685, then dried, washed, and burned, as above. Then the Ag is oxidized, 
e.g. by "acid curing," with the minimum amount of a concentrated acid, 
e.g., HNO.sub.3 or H.sub.2 SO.sub.4 ; if it is possible to use dilute 
acid, this is preferable, so as to minimize extraneous salt content, as 
mentioned. (To this end, also, the abovementioned ash should be washed and 
drained of liquid before the acid treatment. The use of suction will help 
removal of excess water.) Heat is often needed in acid treatment. The 
excess acid is neutralized with minimum base having the same cation as the 
complexing salt to be used. 
Knowing the approximate concentration of the silver, and other salts 
present, e.g., from the above-mentioned neutralization, and the 
temperature of the mixture, it is possible to calculate the minimum volume 
of the highest concentration of ligand usable, considering also mixing 
facility. As may be shown, using the highest possible original reagent 
concentration (on the condition that the solubility of the metal complex 
is hyperlinear, i.e., exponential, in this concentration range of reagent) 
will result in the lowest final volume when the complex-carrying reagent 
is later diluted, as will be explained below. This is important 
economically, as the reconcentration of the diluted reagent, after removal 
of the precipitated metal salt, is by evaporation, and it is desired to 
evaporate the smallest volume possible. To the above amount of ligand must 
be added the stoichiometric equivalent of the silver, to convert the 
solubilized silver to the iodide; a 5-10% excess of the above volume 
should be added, because of uncertainties in silver content, to ensure 
complete solubility. The mixture is mixed by means known in the Art, and 
the liquid is separated by vacuum filtration, centrifugation, or any other 
means known. It may be necessary to filter the filtrate/centrifugate 
through activated charcoal. 
On the basis of the amount of complex solution still retained by the 
material remaining after filtration, centrifugation or other means, the 
minimum volume of the reagent, at the lowest concentration which will just 
keep the complex remaining in the material in solution, is added to wash 
the remaining complex, and other salts, out of said material. It is 
possible to break this wash up into small portions, applying vacuum, or 
other means, after each portion, for more efficient washing. A second, 
final wash of pure water is added, the volume of which is determined by 
the remainder of salts in the material, or the desired final concentration 
of the reagent, to which the two above-mentioned washes will be added, 
whichever determined volume is greater. 
Both washes, or the first and part of the second, which had also been 
separated by filtration, etc., and filtered, if necessary via activated 
charcoal, are added to the solution of Ag complex, whereupon most of the 
Ag precipitates out as the insoluble, or sparingly soluble salt of the 
ligand, which is separated by known means. Some Ag always remains in 
complex solution. 
The desired final concentration of the reagent will be determined by 
considering the economic trade-off between the greater energy which will 
be needed to reconcentrate the reagent by evaporation, in the case of 
greater dilutions, v.s. the incomplete stripping of the Ag salt from the 
reagent by the lesser dilution, which will lower its solvent power for the 
following material treatment, so that a unit volume will treat less 
material, with the same time, equipment, and work. 
It is also possible to pass the reagent having low Ag-complex content, 
i.e., after dilution, through an ion-exchange column to strip it of said 
complex. 
The precipitated metal salt may be purified any number of times by 
redissolving in concentrated ligand solution, and reprecipitation. For 
example, AgI dissolves to an extent of 5.68 M/original L 8.46 M HI 
(25.degree. C.), which latter may be recovered as the azeotrope after 
dilution and precipitation of the AgI; 4.27 M/7.95 M LiI (25.degree.); 
4.63 M/8.55 M NaI (30.degree.); 4.14 M/6.26 M KI (30.degree.). The pure 
AgI, for example, may be used for weather modification, in 
temperature-indicating pigments, or in the form of compounds MAg.sub.4 
I.sub.5, where M=metal or NH.sub.4.sup.+, in batteries, or reduced to the 
metal, by known means. 
When the silver is originally present as the salt, e.g., in green 
photographic materials, there may not be a need to burn the supporting 
substance, e.g., paper or film, but to directly contact the material with 
the concentrated reagent. Burning may be used, however, when applicable, 
if it is not desired to recover the paper or film base. 
When possible, the material should be washed to remove soluble salts, but 
not the desired metal, and the wash removed as thoroughly as possible, as 
mentioned above. 
Factors such as diffusion must be taken into consideration: photographic 
paper and the gelatin layer absorb almost immediately, whereas the 
cellulose acetate base of film takes a few hours to completely absorb the 
reagent, so that the film should be washed immediately after being 
contacted with the reagent, while there is no difference for the paper. 
In the case of some voluminous materials, such as some low-silver content 
green photographic wastes, there is a volume problem, in that the 
complexing power of a given volume of reagent may be adequate to treat all 
the silver salt, but the large volume of the material makes it necessary 
to divide the material into batches, when the materal volume (area x 
thickness, for photographic paper, for example) exceeds a certain % of the 
reagent volume which could treat it. Alternatively, it is possible to use 
a larger volume of less-concentrated reagent; here, there is a trade-off 
between the final volume, and the number of batches per weight (silver 
content) of material. The material may be contacted with the reagent by 
dipping each batch into it when the batch is suspended in a net; or by 
having the reagent flow from container to container, possibly by 
gravitational force, etc. Agitation is used in all cases, including the 
abovementioned ash treatment. Elemental Ag separating from green 
sensitized materials may be separated by filtration. As with treatment of 
materials after the "acid cure," here too an excess reagent should be used 
when the amount of metal and/or ionic strength is not known for sure. Then 
too, with green materials, carryout of solution slightly lowers the 
reagent concentration for the last batches. Although the silver complexes 
are light-stable, as are their complex solutions, many of the original 
salts are characterized by their decomposition in light, so that 
safe-light conditions should be used when necessary. 
After the reagent has been reconcentrated by evaporation, which should be 
done by a combination of heat and vacuum, to reduce the corrosion of the 
evaporating vessels, a stoichiometric amount of ligand is added for the 
conversion of the next cycle of material. 
When the reagent solution has become contaminated with an excess of salts, 
from acid neutralization, conversion of silver to ligand salt, etc., which 
interfere with the process outlined above, it is necessary to remove the 
unwanted salts by fractional crystallization. The complex silver remaining 
in the solution may be removed first, e.g., by passing through an 
ion-exchange column. 
As is obvious, the recovered reagent (ligand salt) need not be 100% pure. 
It is possible to burn off organic material from some reagents used in 
this method, if necessary. In further purifications of the silver salt, 
the degree of purity of the reagent will depend on the purity desired. 
(3) Choice of Ligand, Cation, and Acid 
This proposed method presents a very wide variety of possibilities for 
silver recovery as regards form of recovered salt; concentration of silver 
in material treated; purification of salt recovered. According to equ. 
(2), for different ligands at the same concentration of free ligand (and 
the same activity), the concentration of each species is a function of the 
equilibrium constant's numerical value; also, for each ligand, the set of 
values m and n that are applicable to that ligand are different than that 
set applicable to other ligands. From this, and equ. (3), it is seen that 
the solubilities of the ligand salts of the same metal in excess ligand 
solution will differ from ligand to ligand. In general, the more stable 
the compound (e.g., AgSCN), the more soluble as complex. 
Therefore, at moderate concentrations, the solubility rises from Br.sup.- 
to I.sup.- to SCN.sup.-. (The effect of the cation has been mentioned 
above.) 6.44 M NH.sub.4 Br dissolve 0.427 M AgBr/L; 3.39 M KI dissolve 
1.11 M AgI/L; while 3.26 M KSCN dissolve 1.33 M AgSCN/L. At higher 
SCN.sup.- concentrations, however, double salts precipitate out, so for 
very high Ag concentrations, the iodides are the best solvents. At these 
high concentrations, the cation effect is diminished, and HI is almost as 
good a solvent as KI. One case where the weaker solvent Br.sup.- might be 
favored over the other two ligands is where a larger volume is desired 
because of the small quantity of metal in a large volume (as mentioned 
above.) Silver bromide may be converted to the iodide with ease. AgBr may 
be, inter alia, recycled for further photographic use (see, e.g., U.S. 
Pat. No. 3,600,175 and Brit. Pat. No. 1205395). The factors of temperature 
and cation identity have been mentioned above, and are additional 
variables which may be used to enhance the versatility of the method. The 
effect of ionic strength on the solubility of the complex in the diluted 
reagent must be considered, because, at a reagent concentration of about 1 
M, increased ionic strength, because of additional salts, etc., as 
mentioned, will always raise the solubility of the complex. Although 
CN.sup.- salts, classed as "pseudohalides," form very strong complexes 
with silver, they are "linear" solvents in that, as opposed to Br.sup.-, 
I.sup.-, and SCN.sup.-, the CN.sup.- salts form strong ionic complexes of 
the type M(CN).sub.2.sup.-, which, according to the Law of Mass Action, 
has a concentration proportional to the first power of the CN.sup.- 
concentration; S.sub.2 O.sub.3.sup.= also dissolves these metal 
thiosulfates in an almost linear manner, for, although there is formation 
of the series of polynuclear complexes of the general formula 
EQU Ag.sub.m (S.sub.2 O.sub.3).sub.m+2.sup.(m+4).sbsp.-, 
there is a large amount of the "linear" complex Ag(S.sub.2 
O.sub.3).sub.2.sup.-3 (Arkiv Kemi, 12, 229.) 
Therefore, reasonable dilution would not precipitate enough of the 
dissolved salt. 
The neutral ligand NH.sub.3 in water solution dissolves many Ag salts, and 
in a non-linear way. NH.sub.3 is used in leaching Cu from ores. It also 
figures in a Hungarian Patent (by J. and J. Kovacs) (CA 78, P60867h) for 
the recovery of Ag from photographic wastes. However, its solvent 
properties for these metals salts is in the cold (0.degree. C.), at high 
NH.sub.3 concentrations. There is no immediately usable product; it is 
difficult to handle concentrated NH.sub.3 solutions; there is the danger 
of the formation of explosive compounds with Ag. Therefore, the ligands 
mentioned in my process are preferred. 
The combination of ligand and cation should give a salt which is stable at 
all concentrations, in slightly alkaline media, upon heating, if this is 
employed (e.g., with bromides), and with the components of the material 
from which the metal is being recovered. It is useful that there be a 
relatively big difference between the solubility of this salt, and that 
formed by neutralization of the acid used for "acid curing," to facilitate 
fractional crystallization. E.g., there is a big difference in solubility 
between KI and K.sub.2 SO.sub.4, the latter dissolving to a small extent 
in concentrated solutions of the former. 
If the same volume (minimum covering volume) of H.sub.2 SO.sub.4 is used as 
that of HNO.sub.3, the amount of neutralizing base must be double, and the 
relative addition of the ionic strength will be 6 times as much, since 
SO.sub.4.sup.= is doubly charged. 
If HNO.sub.3 is used, the fumes of NO.sub.2 may be absorbed in urea. The 
heat of neutralization may of course be taken advantage of to warm the 
added reagent (e.g., a bromide), to increase the solubility. 
It should be noted that, in the acid treatment of some wastes, the presence 
of some halide, along with the acid, while increasing the corrosiveness of 
the mixture to the reaction vessel, will have a rate-increasing effect on 
the reaction. 
Following are embodiments of the proposed process as given in Working 
Examples; they should not limit the scope of this process either as to 
concentrations; possible combinations of ligand, cation; form and source 
of silver; acid used; etc. E.g., a column extraction of the metal by 
reagent may be used. A third wash may be introduced. Etc.

WORKING EXAMPLES 
EXAMPLE 1 
500 g of ash, with a 10% Ag content, from the burning of a contrasty 
single-weight paper, medium speed B&W film, waste emulsion, and a small 
amount of metallic Ag leached from exposed film by means known in the Art 
were soaked with H.sub.2 O to remove salts (soluble), and the wash removed 
by vacuum filtration. The waste was wetted by the minimum amount of 
H.sub.2 SO.sub.4 (conc., tech.), heated to 90.degree. C. for 10 min., and 
cooled. KOH was added to neutralize excess acid, and 1.34 L sat. KBr at 
60.degree. C.+0.74 M solid KBr were added, the mixture agitated, and 
vacuum filtered. The remaining material was washed with 250 ml 3.33 M KBr, 
and the latter filtered also. A final H.sub.2 O wash of 1.62 L was used, 
and this also filtered. The filtrates were combined, whereupon AgBr 
precipitated, and was separated by filtration. 
Yield as Ag=49.5 g, or 99%, including 5.25 g remaining in solution. 
EXAMPLE 2 
166 g ash from processed industrial x-ray film, with an Ag content of 30% 
was treated as in Ex. 1, except that the acid was HNO.sub.3 (conc., 
tech.), the neutralizing base was KOH, the halide solution was 76 ml sat. 
KI at 30.degree. C.+0.76 M solid KI; the first wash was 55 ml 0.40 M KI; 
and the final wash was 0.93 L H.sub.2 O. 
Yield as Ag=49.4 g, or 98.8%, including Ag in solution. 
EXAMPLE 3 
200 g of a 50% photographic ash was treated as in Ex. 2 except that the 
reagent was 188 ml sat. NH.sub.4 SCN at 25.degree. C.+0.69 M NH.sub.4 SCN 
(solid); the first wash was 50 ml 0.32 M NH.sub.4 SCN; and the final 
H.sub.2 O wash was 3.97 L. (The neutralizing base was NH.sub.4 OH.) 
Yield as Ag=99.4 g, or 99.4%, including Ag in solution. 
EXAMPLE 4 
1 kg of a voluminous ash with a Ag content of 5%, which was obtained by the 
burning of a mixture of film framed in cardboard, and of copying paper was 
treated as above, except that the acid used was H.sub.2 SO.sub.4 (conc., 
tech.); the base was CaO, the reagent 0.6 L of sat. CaBr.sub.2 at 
60.degree. C.; the first wash 0.73 L 1.52 M CaBr.sub.2 ; and the final 
water wash 1.29 L. The treatment was as above. The solid which remained 
after the filtration of the washes was saved to ascertain whether its 
CaSO.sub.4 content might endow it with useful properties. 
Yield as Ag=49 g, or 98%, including Ag in solution. 
EXAMPLE 5 
8.8 m.sup.2 of unexposed waste industrial x-ray film with a thickness of 
0.1 mm, containing 73.4 g Ag were shredded and the shreds rinsed with 
water in a container, and the water removed as thoroughly as possible by 
vacuum. 1 L 3.59 M NH.sub.4 I was added to the container, and the mixture 
was stirred for 10 min. The solution was removed as completely as possible 
with the aid of vacuum. 1 L wash of 0.57 M NH.sub.4 I was added, removed 
as completely as possible with the aid of vacuum, and a second H.sub.2 O 
wash of 3.27 L added and removed as above. The filtrates were combined, 
when AgI precipitated and was separated by filtering. 
Yield as Ag=73.1 g, or 99.6%, including Ag in solution. 
EXAMPLE 6 
11.3 m.sup.2 unexposed waste color negative film with a thickness of 0.1 mm 
and containing 24.8 g Ag were shredded and washed as in Ex. 5, and divided 
into two equal batches. Each batch was treated in turn, as above, with 705 
ml 3.05 M NaI; in turn with a first wash of 0.5 M NaI of the same volume; 
and then, in turn, with a H.sub.2 O wash of 750 ml. 
Yield as Ag=24.6 g, or 99%, including Ag in solution. 
EXAMPLE 7 22.5 m.sup.2 double weight unexposed waste low-contrast 
photographic paper with a thickness of 0.28 mm was divided into 9 equal 
batches consisting of 1.times.1 dm.sup.2 cards. The total Ag content was 
24.8 g. The batches were treated as in Exs. 5-6, except that reagent was 
876 ml NH.sub.4 Br, sat. at 25.degree. C.; the first wash the same volume 
of 2.0 M NH.sub.4 Br; and the H.sub.2 O wash 860 ml. 
Yield as Ag=24.3 g, or 98.2%, (including Ag in solution. 
Notes: (a) the photographic film (exposed) was burned as completely as 
possible to avoid reaction of cellulose compounds with HNO.sub.3 to form 
nitrocellulose; (b) safe-light conditions were used where applicable.