Improvements in the Manufacture of White Lead.
Year: 1891

page_text: Date of Application, 22nd Sept., 1891-Accepted, 31st Oct., 1891
COMPLETE SPECIFICATION.
Improvements in the Manufacture of White Lead.
A communication from abroad by CALVIN AMORY STEVENS, of No. 39, Broad
Street, New York City, New York, United States of America, Gentleman.
I, JOHN CORRY FELL, of the firm of Fell and Wilding, of 1, Queen Victoria
Street, in the City of London, Patent Agent, do hereby declare the nature of
this invention, and in what manner the same is to be performed, to be particularly
described and ascertained in and by the following statement :-
This invention relates to the manufacture of hydrated lead carbonate (known in
commerce as white, lead) by the aid of an electrical current. I am aware that
electricity has heretofore been employed for this purpose, as in U.S. Letters Patent
No. 292,119, dated January 15th 1884, issued to Johann K. Kessler, and U.S.
Letters Patent No. 414,935, dated November 12th 1889, issued to T. D. Bottome,
in both of which metallic lead is decomposed in an alkaline electrolyte into an
oxygen bearing salt of the metal, after which an attempt is made to convert the
said salt into hydrated lead carbonate by the introduction of carbon dioxide into
the electrolyte. But only a very small percentage of the hydroxide of lead formed
in the electrolyte under electrical action is precipitated upon the introduction of
the carbonic acid gas in either of the cases referred to for the reason that such salts
are soluble in the electrolytic solution and are retained thereby under the conditions
heretofore existing.
A leading feature of the invention consists in making the oxygen bearing salts of
lead insoluble in an acid electrolyte after their formation under the influence of an
electrical current by rendering the electrolyte neutral or nearly so, in which
condition practically all of the hydroxide of lead present will be converted into
hydrated lead carbonate upon the introduction of carbon dioxide. By thus
neutralizing the electrolyte at the proper stage in the process the manufacture of
white lead by the aid of electricity is rendered practical and profitable in a
commercial sense, a result not heretofore attained since while the use of electricity
in this connection has been experimentally successful on a small scale, the
precipitation of hydroxide of lead has been so slight as compared with the quantity
held in solution in the electrolyte as to render the process unprofitable and
impractical commercially.
The invention is designed for and adapted to the production of a higher grade
of hydrated lead carbonate, free from deleterious substances, in a more rapid and
economical manner than has heretofore been attained, and the invention consists
primarily in the following corelated steps, in the process of manufacture, to wit:
the electrical decomposition of metallic lead in an acid electrolyte into an oxygen
bearing salt of the metal, the neutralization of the electrolyte after it has become
charged with the oxygen bearing salts of lead, and the conversion of the said
oxygen bearing salts into hydrated lead carbonate and the precipitation thereof by
the introduction of carbon-dioxide either free or combined with the electrolyte.
In order to thus attain a pure high grade of white lead it is essential that all
traces of silver shall be eliminated from the lead during the process. In the manu-
facture of white lead by any of the processes which use metallic lead for this
purpose, if lead bullion or lead with only a very small percentage of silver be used,
the article produced is of a very inferior grade, owing to the presence of the salts
of silver, these salts under the influence of light and air turning black and giving
the whole mass a smudgy or dirty appearance. Hence the necessity under the old
methods, of using lead which has been submitted to some process of desilverization.
In this connection my invention includes, as a step in the process when metallic lead
contaminated with silver (as is usually the case) is used, the elimination of the

page_text: silver from the electrolyte, by means of electro-deposition, a current of electricity
of suitable intensity being passed through the electrolyte after it has become
charged with the lead and silver in solution and prior to the neutralization of the
electrolyte, the result being that the silver is thereby collected on a suitable
cathode.
By this new method of treatment the field of manufacture of white lead is thus
extended to include lead bullion or silver bearing lead, and the electrical elimination
of the silver directly from the solution of the decomposed bullion, and as an
essential step in the process when silver is present, is an important advance in the
manufacture of a high grade article of hydrated lead carbonate.
It will be seen that by this method an acid electrolyte is used, and distinguishing
features of this invention consist in the absence from the electrolyte of carbon
dioxide until after the electrolyte is charged or saturated with the oxygen bearing
salt, and in the neutralization of the electrolyte prior to the introduction of the
carbon dioxide in order to provide for and insure the complete precipitation of the
lead carbonate as formed.
In this process of manufacture I am enabled to use as, or in, the electrolyte all
the acids, as for instance nitric acid (HNO,) acetic acid (HC,H,O,) sulphuric
acid (H,SO) hydrochloric acid (HCI) aqueous carbonic dioxide (H.CO₂) or
carbonic acid (CO2) oxalic acid (H,C,O) tartaric acid (H,C,H,O) citric
acid (H,CH,O).
2
By investigation and experiment I have discovered that while metallic lead may
be decomposed in solutions of all the substances named, by aid of the electric
current, some of them are better adapted to the work of decomposing the lead and
allowing its conversion to the state of hydrated lead carbonate with greater facility
than others; this being due to the peculiarities of chemical affinity existing in the
elements entering into the various substances; that when metallic lead is converted
into an oxygen bearing salt of the metal, by aid of the electric current with any
of the above substances the nature of the solution can be so changed by diluting
it and bringing it to or near the point of neutrality that the hydrated lead carbonate
will remain within it when formed by the addition of carbonic acid gas (either
with or without pressure) in an insoluble state.
This electrical decomposition of metallic lead is produced with the greatest
facility in an acid electrolyte, especially one containing nitric acid (HNO₂). Also,
that the compound of lead which is formed when the metallic lead is electrically
decomposed in the solutions of the substances just mentioned is most readily
converted to the state of hydrated lead carbonate when the solution of the substance
in which it has been decomposed is rendered neutral, owing to the fact that
hydrated lead carbonate is soluble in solutions which are highly acid and insoluble
in neutral solutions.
In utilizing nitric acid to effect the decomposition of the lead into oxygen
bearing salts &c., the electrolyte should be prepared by diluting 30:) cubic
centimetres of chemically pure nitric acid, or its equivalent in the commercial acid,
with 2000 cubic centimetres of water. This is placed in a suitable glass, porcelain
or wooden vessel. The electrodes are metallic lead in any form either lead bullion
or pure, and for this amount of the electrolyte solution, I prefer a plate inch
thick and 34 inches square for the anode.
The cathode may be of lead and of the same dimensions, or of carbon or other
suitable material of sufficient surface to be the equivalent of the anode in con-
ductivity. The electrodes are immersed in the electrolyte and connected with a
source of electricity and a current of say 10 amperes per square foot of anode
surface passed through the electrolyte. The lead anode is rapidly decomposed
into a compound of lead which remains in solution, until the electrolyte is
thoroughly saturated, which occurs after from ten to twenty minutes passing of
such current. When the point of saturation is reached, fine crystals of metallic
lead will be seen commencing to attach themselves to the cathode.
When this occurs the current of electricity should be stopped and the electrodes

page_text: removed. If lead containing silver has been dissolved, the silver should then be
collected and removed from the electrolyte by electro-deposition as hereinbefore set
forth. In the absence of silver in the electrolyte, or after its elimination therefrom,
sufficient caustic soda or potash sodium or potassium hydroxide, or preferably a
mixture of the two should now be added to render the solution neutral, care being
taken to add only enough to bring the electrolyte to neutrality, as in highly alkaline
solutions the hydrated lead carbonate cannot be precipitated, and that which is
precipitated in only slightly alkaline solutions has such a decided alkaline reaction
as to unfit it for the purposes for which white lead is used. As the electrolytic
solution approaches the neutral state its color becomes milky white from the
formation of plumbic hydroxide, and if any carbonic acid gas be present as is
frequently the case to a greater or less extent in water which has not been
subjected to some process of purification, a small curdy precipitate of hydrated
lead carbonate will be seen falling to the bottom of the vessel. But this carbonic
acid gas is not naturally present in sufficient quantity to convert any great amount
of the plumbic hydroxide to the state of hydrated lead carbonate and for this
purpose it must be added to the electrolyte which may be done either with
slight pressure or without. When the electrolyte is saturated with free carbonic
acid gas, the conversion of the plumbic hydroxide to the state of hydrated lead
carbonate is very rapid, to the extent of about two thirds of the weight of the
plumbic hydroxide. The precipitate may now be removed from the electrolyte and
prepared for market.
The chemical changes which have taken place under the above manipulation are
explained in the following equations:
=
1st. On passing the electric current through the nitric acid electrolyte with
lead electrodes we have 6 Pb + 12 HNO,- 6 Pb(NO), +12 H the hydrogen
being set free from the decomposed bath, and lead nitrate remaining in solution
within it.
The removal of the salts of silver from the electrolyte if present therein by
electrolysis is here effected in the manner as hereinbefore set forth.
2nd. On the addition of the caustic solution we have 6 Pb(NO,), + 12 KOH
3 Pb,O (OH), + 12 KNO, +3 H,O, the lead nitrate being converted into lead
hydroxide, with the formation of the potassium nitrate and water.
=
3rd. On the addition of the carbonic acid gas we have 3 Pb,O (OH), +4 CO,
= H₂O + 4 PbCO, + 2 Pb(OH)2 =2 (2 PCO, Pb(OH)2, the lead hydroxide
being converted into hydrated lead carbonate.
Carbonates of the alkalies (sodium or potassium preferably) may be used to
neutralize the acid thus introducing at the same time carbonic acid in a combined
form. In this case the reaction is between the oxygen bearing salt of lead which
has been produced by electrical action, the acid of the bath and the carbonate salt.
But while these methods of conducting the process are operative, in practice it is
preferable to introduce the carbonic acid gas in a free state, for which purpose it
may be made in any of the well known manners, -as by the burning of lime stone,
the destruction of a carbonate with an acid, &c. It should be introduced into the
electrolyte immediately after it has been rendered into a condition in which the
lead carbonate may exist within it in an insoluble state, by neutralization.
When the electrolyte becomes charge with the oxygen bearing salts of lead if
an electrode has been used which contains silver the silver is also decomposed, and
is in solution providing that the electrolyte is a solvent for the silver, in which case
the silver must be eliminated from the electrolytic bath, by passing a suitable
current of electricity through it, with a carbou anode and silver cathode, the silver
being deposited in a metallic state on the cathode. In cases where the silver is not
soluble in the electrolyte it falls to the bottom of the vessel and is removed as an
impurity from the bath. This separation is necessary where it is desired to produce
a high grade article of white lead.
The removal of the silver by electro-deposition as above set forth is effected

page_text: prior to the neutralization of the electrolyte and the introduction of the carbon
dioxide.
It is to be noted that this process is made up of a series of independent successive
steps in each of which the complete chemical reaction desired takes place unimpeded
by the presence of substances necessary in effecting other reactions in the process, as
in the case of the Patents to Bottome and Kessler hereinbefore mentioned in which
all the ingredients are present together in the electrolyte and act and react on each
other until the solution becomes clogged and inoperative when it has to be discarded,
thus entailing considerable and unnecessary waste of material and rendering the
process unprofitable.
By this method of treating each re-action separately practically all the material
is utilized, and a purer and cheaper product is obtained than has heretofore
been possible.
It is to be understood that I do not confine myself strictly to the relative
proportions of substances, or to the exact manner of manipulation herein set forth
by way of illustrating the practical utilization of my improvements since it is
obvious that various modifications may be introduced into the process without
deviating materially from the essential features of my invention.
I am aware that in what is known as the Riffault process of manufacturing white
lead from peroxide of lead &c., the solution of oxide of lead is neutralized prior to
the introduction of the carbon dioxide, and I do not herein seek to cover the
feature of neutralization, excepting only as an essential step in the successful
manufacture of white lead by electrolysis.
Having now particularly described and ascertained the nature of this said inven-
tion and in what manner the same is to be performed, I declare that what I
claim is :
First The process of manufacturing hydrated lead carbonate substantially as set
forth, consisting in decomposing a metallic lead anode by a current of electricity in
an acid electrolyte to form oxygen bearing salts of lead, then rendering the
electrolytic solution neutral or nearly so, and treating the electrolyte with carbon
dioxide for the purpose described.
Secondly The process of manufacturing hydrated lead carbonate from lead
bullion consisting in dissolving the same by a current of electricity in an electrolytic
solution to form oxygen bearing salts, removing the silver from the electrolytic
solution by electro-deposition, rendering the remaining solution neutral, or nearly
so, and treating the electrolyte with carbon dioxide for the purpose and substantially
in the manner set forth.
1
נו.on
1001
FELL & WILDING,
1, Queen Victoria Street, London, E.C., Agents for the Applicant.