MENTS IN SEPARATING AND OBTAINING SILVER FROM METALLIC COMPOUNDS
Year: 1884

page_text: I, DESMOND GERALD FITZ-GERALD, of 6 Akerman Road, Brixton, in the county
of Surrey, Electrician, do hereby declare the nature of my invention for IMPROVE-
MENTS IN SEPARATING AND OBTAINING SILVER FROM METALLIC COMPOUNDS, to be
as follows:-
The objects of my invention are, firstly, to effect with greater convenience and
economy than heretofore the solution of silver contained in alloys of this metal,
such as the "quartation" alloy formed for the purpose of refining gold, or the
impure silver obtained in metallurgical operations; and, secondly, to recover the
silver in the metallic form by more perfect and advantageous means than have
hitherto been employed.
I effect the first of these objects by the use of certain substances to absorb the
nitric peroxide which is generated in large quantities when nitric acid is used for
dissolving silver. The escape of this gas ordinarily results in a great loss of acid,.
and necessitates the use of high shifts or flues for the purpose of dissipating the
noxious fumes. But by mixing the nitric acid with an oxidant such as the
binoxide of lead or of manganese I prevent the evolution of the fumes of nitric
peroxide, and subsequently recover nearly the whole of the nitric acid employed.
Or by causing the fumes to pass through an alkaline solution or through any
suitable fluid holding in suspension an oxide such as the protoxide of lead or red
lead, or an oxidising agent such as the peroxide of lead or the peroxide of
manganese, which may be in admixture with a salt such as baric carbonate, or
again through an oxidising fluid such as a solution containing chromic, manganic.
or permanganic acid, I completely absorb the fumes, and am thus enabled sub-
sequently to recover nearly the whole of the nitric acid originally employed to
effect the solution of the silver. The same means may be employed to prevent the
evolution of, or to absorb, the sulphurous acid gas ordinarily generated in large
quantities when sulphuric acid is used to dissolve the metal. But as sulphuric
acid is less convenient in use than nitric acid as a solvent for silver, being employed
only with a view to economy, and as the loss of nitric acid may be almost entirely
obviated by the present process, I prefer in all cases to use nitric acid for the
purpose in view.

page_text: The second of the above-mentioned objects I effect by precipitating the silver,
preferably in the form of oxide or of chloride, and reducing it to the metallic form
electrolytically, by means of a metal, such as zinc, which is not allowed to come
into contact with the compound to be reduced. When the silver is precipitated as
chloride by means of hydrochloric acid, the nitric acid previously combined with
the metal is at once liberated; and when calcic hydrate or haric carbonate is used
for precipitation the nitric acid may readily be recovered by decomposing the calcic
or baric nitrate with sulphuric acid. If due care be taken to obtain the insoluble
compound of silver in a condition of purity, the result of the electrolysis is pure
metallic silver, usually in a spongy form. But, in the contrary case, the reduced
metal may be purified by washing with diluted nitric acid until a trace of silver is
found in solution. A sheet of silver is preferably used as the negative element of
the voltaic couple formed in order to produce the electrolytic action. In the caso
of argentic chloride, the electrolyte may be dilute sulphuric acid; and in the case
of the oxide it may be constituted by a solution of caustic alkali. The zinc which
is preferably used as the positive element of the voltaic couple may advantageously
bo amalgamated, more especially when an acid solution is employed as the
electrolyte; and when the positive elements are arranged vertically their lower
edges may be inserted within a "shoo" of gutta-percha or other suitable material,
which serves the double purpose of containing a small quantity of mercury for
amalgamation and of obviating the liability of the reduced silver to become
contaminated by particles which may become detached from the zinc. When the
-zinc-elements-are-arranged horizontally, a diaphragm of parchment-paper may be
used for the purpose last-mentioned. The electrical energy generated-of-which-
the minimum value is ono horse-power-hour electrical for every 2 pounds
avoirdupois of zinc consumed, corresponding to 97 ounces troy of silver reduced-
may be utilised for any purpose which does not necessitate the introduction in the
circuit of a resistance such as would inconveniently augment the time requisite to
effect the reduction of a given weight of silver.
When by the use of peroxide of lead as an oxidant, in admixture with nitric
acid, a mixed solution of silver and lead is obtained, the silver, when precipitated
as chloride, should be freed from lead chloride by washing with hot water. Or the
silver may be directly reduced from the nitrate solution by precipitation with pure
lead; the reduced silver being in this case subjected to cupellation. The whole of
the nitric acid employed may then be recovered by precipitating with sulphuric
acid the lead in the resulting nitrate of that metal.
When the gold or silver to be refined contains other metals of value, such as
palladium nickel and copper, these metals may be accumulated in the nitric acid
(regenerated by the decomposition of the argentic nitrate) by using the same acid
over and over again for the treatment of the alloy. These metals may ultimately
be obtained by precipitating the silver with a chloride instead of by hydrochloric
acid, so as to obtain a neutral or nearly neutral solution of the chlorides of the
metals in question, from which solution these metals may readily be obtained by
well-known chemical means or by electrolysis. In precipitating silver as chloride
otherwise than by the use of hydrochloric acid, I employ, instead of the usual
solution of impure rodic chloride, the chloride of barium or that of calcium, or-
more especially when lead is already present in solution--the chloride of lead.
From the baric, calcic or plumbic nitrate thus formed, nitric acid may readily be
recovered by precipitating the base with sulphuric acid as aforesaid.
Dated this fifteenth day of April 1884

page_text: *6
I DESMOND GERALD FITZ GERALD, of 6 Akerman Road, Brixton, in the County
of Surrey, Electrician do hereby declare the nature of the said invention for
IMPROVEMENTS IN SEPARATING AND OBTAINING SILVER FROM METALLIC COM-
POUNDS and in what manner the same is to be performed, to be particularly
described and ascertained in and by the following statement:-
"
The objects of my Invention are, firstly, to effect with greater convenience and
economy than heretofore the solution of silver contained in alloys of this metal,
such as the "quartation" alloy formed for the purpose of refining gold, or the
impure silver obtained in metallurgical operations; and, secondly, to recover the
silver in the metallic form by more perfect and advantageous means than have
hitherto been employed which permit of the recovery of the greater portion of the
nitric acid originally employed.
I effect the first of these objects by the use of certain substances to oxidise the
nitric peroxide which is generated in large quantities when nitric acid is used for
dissolving silver. The escape of this gas, ordinarily, results in a great loss of acid,
and necessitates the use of high shafts or flucs for the purpose of dissipating the
noxious fumes; but by mixing the nitric acid with an oxidant such as the binoxide
of lead, or of manganese, I prevent the evolution of the fumes of nitric peroxide
and subsequently recover nearly the whole of the nitric acid employed. The
proportion of the oxidant used must be sufficient to oxidise into nitric acid the
lower oxides of nitrogen generated by the action of the nitric acid upon the
oxidisable metal.
The second of the above mentioned objects I effect by precipitating the silver,
preferably in the form of chloride or of oxide, and reducing it to the metallic form
electrolytically by means of a metal such as zinc, or iron, which is not allowed to
come into contact with the compound to be reduced.
When the silver is precipitated as chloride by means of hydrochloric acid, the
nitric acid previously combined with the metal is at once liberated. In order that
the nitric acid liberated in the production of argentic-chloride should sufficiently
strong to be immediately employed in the solution of more silver, it is important
that hydrochloric acid in the gaseous condition (i.e. free from water) should be
employed in preference to the hydrochloric acid of commerce. If the nitrate of
silver from which the chloride of silver is precipitated contain nitrate of lead
(derived from peroxide of lead employed in reoxidising the lower oxides of nitrogen
in the earlier operation of dissolving the silver as herein before described) chloride
of lead will be precipitate with the chloride of silver. The latter salt may be
purified from chloride of lead by washing with hot water. If this washing be not
performed the silver reduced as hereinafter described, may be contaminated with
metallic lead and in this case it should be purified by washing with dilute nitric
acid or the silver after fusion should be subjected to cupellation. The nitric acid
may be recovered from the nitrate of lead in solution by the addition of sulphuric
acid.
I, sometimes use calcic or baric hydrate to precipitate the silver as oxide and in
this case the nitric acid may readily be recovered by decomposing the calcic or baric
nitrate with sulphuric acid.
In the case of chloride of silver if zinc is employed as the positive element of the
voltaic couple (which I form in order to produce the electrolytic action which
results in the reduction of the chloride of silver to the metallic form), an oxychloride
of zinc, which is white and insoluble in water, accompanies the reduced metal, and

page_text: requires to be separated therefrom by washing with a dilute acid, such as sulphuric
acid, until the washings when tested by a suitable reagent are found to be free
from zinc. The silver may then be washed with water, dried, and fused. When
iron is employed in lieu of zinc as the positive element of the couple (in which case
the reduction of the silver occupies a longer time than when zine is used) careful
washing, or the employment of a suitable flux in fusing the silver, is necessary in
order to obtain a clean ingot of the metal.
In
In the case of argentic-chloride the electrolyte may be dilute sulphuric acid; and
in the case of the oxide it may be constituted by a solution of caustic alkali.
either case a sheet of silver (with which the insoluble compound of silver is in
contact) is preferably used as the negative element of the voltaic couple.
"
If zinc be used as the positive element of the voltaic couple it should preferably
be amalgamated, more especially when an acid solution is employed as the
électrolyte; and when the positive elements are arranged vertically their lower
edges may be inserted within a "shoe of gutta-percha or other suitable material,
which serves the double purpose of containing a small quantity of mercury for
amalgamation and of obvinting the liability of the reduced silver to become con-
When the zine
taminated by particles which may become detached from the zinc.
elements are arranged horizontally, a diaphragm of parchment-paper may be used
for the purpose last mentioned. The saine diaphragms would also be used when
iron is employed as the positive element of the cell.
The electrical energy generated (of which the minimum value is one horse-power-
hour electrical for every two pounds avoirdupois of zinc consumed, corresponding to
97 ounces troy of silver reduced) may be utilised for any purpose which does not
necessitate the introduction-in-the-circuit of a resistance such as would incon-
veniently augment the time requisite to effect the reduction of a given weight of
silver.
When the gold or silver to be refined contains other metals of value, such as
palladium, nickel and copper, these metals may be accumulated in the nitric acid
(regenerated by the decomposition of the argentic nitrate) by using the same acid
over and over again for the treatment of the alloy. These metals may ultimately
be obtained by precipitating the silver with a chloride instead of by hydrochloric
acid, so as to obtain a neutral or nearly neutral solution of the chlorides of the
metals in question, from which solution these metals may readily be obtained by
well known chemical means or by electrolysis. In precipitating silver, as chloride,
otherwise than by the use of hydrochloric acid, I employ, instead of the usual
solution of impure sodic chloride, the chloride of barium or that of calcium, or-more
especially when lead is already present in the solution-the chloride of lead. From
the baric, calcic, or plumbic nitrate thus formed, nitric acid may readily be recovered
by precipitating the base with sulphuric acid as aforesaid.
When iron is used as the positive element of the cell in which the insoluble
compound of silver is reduced, I sometimes employ the current from a dynamo-
machine with a view to reduce the time necessary for the reduction of the silver,
the positive pole of the dynamo-machine being connected to the iron and the
negative pole to the silver e'ements of the cell.
Having now particularly described and ascertained the nature of the said
Invention and in what manner the same is to be performed I declare that what
I claim is
1st In the process of dissolving silver alloys by means of nitric acid the method
of preventing the evolution of nitric peroxide which consists in mixing with the said
solvent an oxidant such as the binoxide of lead or of manganese, as specified.
2nd The process of obtaining silver from solutions containing this metal, which
consists in first precipitating the silver as chloride by means of hydrochloric acid
and washing the chloride, and then reducing it to the metallic form in an electrolytic
cell having a positive element of zinc or iron which is not allowed to come into
contact with the chloride, and finally washing the reduced motal with a dilute acid
and with water, as hereinabove described.

page_text: 3rd The process of obtaining silver from its solution as nitrate, which consists in
first precipitating the silver as oxide by means of calcic or baric hydrate and
subsequently reducing the silver to the metallic form in an electrolytic cell having
a negative element (preferably of silver) with which the silver oxide is in contact,
an electrolyte of potassic or sodic hydrate in sclution, and a positive element of
zinc, as herein described.
4th The process of obtaining silver from solutions containing this metal, which
consists in first precipitating the silver as chloride by means of the chloride of
barium, of calcium, or of lead (with the production of nitrate of barium, of calcium,
or of lend from which nitric acid may be recovered by precipitating the earthy base
or the lead with sulphuric acid) washing the chloride, and then reducing it to the
metallic form in an electrolytic cell, as hereinabove described.