Improvements in and relating to the Treatment of Solutions of Carbonates of Soda resulting from Electrolysis of Chloride of Sodium, and in Apparatus applicable for use therein.
Year: 1897

page_text: Date of Application, fut Dept., 1997
Complete Specification Left, 15th June, 1898-Accepted, 15th Oct., 1898
PROVISIONAL SPECIFICATION.
Improvements in and relating to the Treatment of Solutions of
Carbonates of Soda resulting from Electrolysis of Chloride of
Sodium, and in Apparatus applicable for use therein.
I, JOHN HARGREAVES, of Widnes, in the County of Lancaster, Chemist, do hereby
declare the nature of this invention to be as follows:-
My invention relates to improvements in and relating to the treatment of solu-
tions of carbonate or sesqui-carbonate (hereinafter referred to as carbonate) of
soda resulting from electrolytic decomposition of chloride of sodium and in
apparatus applicable for use in such treatment. The principal objects of my
invention are to enable porous diaphragms, or colloid diaphragms which have
become porous by continued use, to be efficiently employed in electrolytic manu-
facture of soda; to separately obtain from solutions containing varying relative
proportions of undecomposed chloride of sodium, carbonate, or carbonate and bicar-
bonate of soda relatively free from chloride of sodium, and chloride of sodium
relatively free from carbonate of soda, same being rendered, if desired, suitable
for re-use in the electrolytic cell; and lastly to provide suitable apparatus for
carrying out my improved method of treatment. It will be evident that means
described for use under one method may be wholly or partially applicable under
other alternative methods hereinafter described.
$6
According to one mode of carrying out my invention, I employ two or more
troughs or conduits to separately convey solutions of carbonate of soda containing
relatively large, and solutions of carbonate of soda containing relatively small,
proportions of chloride of sodium-hereinafter referred to as "impure" and
pure" solutions-from a series of electrolytic cells. The solutions may, if
desired, be separately concentrated before treatment in the pans or vessels herein-
after more particularly described. On evaporating the impure solution of car-
bonate of soda, I obtain suitably sized crystals of monohydrated carbonate of soda,
the process of evaporation being carried out with the aid of suitable pans or vessels
arranged to operate in series with suitable pans or vessels containing pure solu-
tions of carbonate of soda. The impure solution of carbonate of soda preferably
enters the evaporating pan or vessel intermediate in series, and the pure solutions
enter the pan or vessel previous or first in series. The pure solutions are caused to
pass from the first pan or vessel in the direction of the last pan or vessel, whilst
crystals of monohydrated carbonate of soda containing undecomposed chloride
of sodium, obtained by evaporating impure solutions of carbonate of soda in the
last and intermediate pan or vessel, are passed through and washed or boiled with
impure, and finally with pure, solutions of carbonate of soda contained in the
intermediate and first pan or vessel; the chloride of sodium being thereby
eliminated from said crystals. Or each pan may be arranged to constitute in itself
a series of evaporating and washing vessels. To this end, the pans are furnished
with suitable partitions for keeping the impure solutions separated from the pure
solutions of carbonate of soda; conduits being provided for conveying the said

page_text: "
solutions to the compartment next in series. In working according to this arrange-
ment, the first compartment may be situated remote from and the last compartment
near the source of heat employed for effecting evaporation, or each compart-
ment may be separately heated. Or the evaporating and washing pans or vessels
may be arranged so that each pan or vessel becomes alternately first, intermediate
and last in the series, the crystals being passed through the series, while the
impure and pure solutions are evaporated without renewal of solution. Or, said
impure and pure solutions of carbonate of soda may be mixed and together suit-
ably evaporated and crystals of monohydrated carbonate of soda containing chloride
of sodium obtained in the well-known surface evaporating salting-down pan
employed in the Leblanc alkali process. Mixed solutions of carbonate of soda,
and chloride of sodium, obtained as mother liquor from said crystals.e., from
surface evaporated impure solutions of carbonate of soda-are then treated in the
manner hereinafter described; but the said crystals may be treated as herein-
before described, or they may be washed with pure solutions of carbonate of soda
or water, at suitable temperatures, or by means of steam from some economical
source, such as the exhaust from a steam-engine, in such a manner as to cause
relatively pure solution of carbonate of soda to pass from relatively pure crystals
of monohydrated carbonate of soda through crystals containing a relatively large
proportion of chloride of sodium.
According to an alternative method of washing crystals of monohydrated car-
bonate of soda containing chloride of sodium obtained from impure solutions of
carbonate, water, or steam, say exhaust steam from an engine, may be employed;
the crystals being contained in an inclined trough provided with an Archimedian
or similar feed-screw. The crystals are conveyed in one direction while the steam
and the mother liquor resulting from the washed crystals flow in the opposite
direction.
Mixed solutions of carbonate of soda and chloride of sodium obtained as mother
liquor from crystals of monohydrated carbonate of soda may, if necessary, be
diluted and treated with carbonic acid gas in any suitable and well-known manner
for the purpose of rendering the carbonate of soda therein contained in the form
of bicarbonate; or the mother liquor may be evaporated whereby crystals of
chloride of sodium and suitably hydrated carbonate of soda are obtained, these
beng subsequently treated in a suitable vessel, or in a series of closed vessels, with
carbonic acid gas derived from any convenient source, such for instance as from
a lime-kiln, or with chimney gases, or the exhaust from a gas engine after being
suitably washed and purified may be employed. Or the said crystals of carbonate
of soda and chloride of sodium may be treated with carbonic acid gas in a suitable
vessel by mechanical means; the vessel being so arranged that the crystals are
stirred or moved about from time to time so as to present fresh surfaces to the
action of the carbonic acid gas and thereby form bicarbonate of soda.
to
Crystals of chloride of sodium and bicarbonate of soda are lixiviated
or washed, chloride of sodium dissolved and eliminated from bicarbonate
of soda, when contained in a series of vessels, and treated in manner
obtain strong and preferably concentrated solution of chloride of
sodium and bicarbonate of soda relatively free from chloride of sodium. After
suitably neutralising (if necessary) the carbonates of soda contained in the solu-
tion of chloride of sodium, said chloride may be re-used in electrolytic cells. By
suitably arranging the carbonating vessels so that the solutions of chloride may be
conveyed from vessel to vessel, these vessels may, without removing the crystals
of chloride of sodium and bicarbonate of soda therefrom, be employed alternately
as carbonating and washing vessels.
Apparatus adapted for employment according to my improved method may
comprise a series of evaporating and washing pans similar in construction to
the pans ordinarily used in obtaining common salt from brine, or the well-known
".boat pans may be used; the said pans being suitably arranged and provided
with conduits or connections for conveying the solutions from: the one pan to the
"

page_text: <<
next in the series, as well as means for fishing" and passing the crystals of
monohydrated carbonate of soda from one pan or vessel to the pan next in the
series. To form in one pan or vessel a series of evaporating pans and washing
vessels, partitions are provided in the pan for keeping the impure and pure solu-
tions of carbonate of soda separate, conduits being arranged for passing said solu-
tions from the one section to the next in series; the section first in series, which
contains pure solutions, being preferably placed furthest from the source of heat
while the last sections are placed nearest the said source. Or suitable means
may be provided for supplying the desired amount of heat to each compartment.
If preferred, the well-known surface evaporating salting-down pan operating in
conjunction with pans or vessels for washing crystals of monohydrated carbonate
of soda, obtained from said impure and pure solutions of carbonate of soda, may
be employed.
The vessel wherein the crystals of monohydrated carbonate of soda obtained,
for instance, from the surface evaporating pans are separately washed with rela-
tively pure solution of carbonate of soda may consist of a receptacle constructed
of metal, which may be provided with suitable arrangements for heating the con-
tents, and with means for conveying solutions of carbonate of soda from vessel to.
vessel, when arranged in series. Or vessels may be arranged to work in series in
which a column of said crystals are washed with water at a suitable temperature
or with steam; the said vessels consisting of a metal chamber, or a series of
lixiviating tanks, provided with means for passing water or steam therethrough
from the top of the chamber and for conveying therefrom mixed solution of
carbonate of soda and chloride of sodium, obtained therein as mother liquor, into
an evaporating pan or vessel, or into a series of evaporating pans or vessels, or
the said crystals may be washed in a covered trough-like vessel provided with an
Archimedean or like feed-screw.
The vessels wherein the carbonating is effected may consist of a chamber, or a
series of chambers, preferably of iron, having a perforated grid or false bottom to
furnish a free passage for, and to equalise the distribution of, the carbonic acid gas
into and amongst the mixed crystals of chloride of sodium and carbonate of soda.
When employed in series, said chambers are provided with conduits for the pas-
sage of carbonic acid gas from one chamber to another.
When carbonating vessels are so arranged as to serve alternately the purpose
of carbonating and washing vessels they are provided with suitable conduits for
the passage of the solution of chloride of sodium from vessel to vessel in the series,
and may constitute a double interchangeable series of carbonating and washing
vessels; suitable means being provided for discharging the washed bicarbonate of
soda.
A carbonating vessel may consist of iron or other suitable metal, wherein are
disposed a series of shelves placed one above another for the reception of the
crystals of chloride of sodium and carbonate of soda, and the said crystals are moved
by mechanical means, thereby causing fresh surfaces to be exposed to the action
of the carbonic acid gas; the said crystals passing downwards from shelf to shelf.
The vessel is provided with inlets or passages for the carbonating gas and for
charging the crystals and with an outlet or discharge for the sodium chloride
and bicarbonate.

page_text: Improvements in and relating to the Treatment of Solutions of
Carbonates of Soda resulting from Electrolysis of Chloride of
Sodium, and in Apparatus applicable for use therein.
I, JOHN HARGREAVES, of Widnes, in the County of Lancaster, Chemist, do hereby
declare the nature of this invention and in what manner the same is to be per-
formed to be particularly described and ascertained in and by the following state-
ment:-
My invention relates to improvements in and relating to the treatment of solu-
tions of carbonate or sesqui-carbonate (hereinafter referred to as carbonate) of
soda resulting from electrolytic decomposition of chloride of sodium and in
apparatus applicable for use in such treatment. The principal objects of my
invention are, first, to enable porous diaphragms.e.,. those which permit unde-
composed chloride of sodium to pass from the anode compartment to the cathode
compartment in electrolytic cells or, colloid diaphragms which have become
porous by continued use, to be efficiently employed in electrolytic manufacture of
soda; secondly, to separately obtain from solutions containing varying relative
proportions of undecomposed chloride of sodium, carbonate, or carbonate and
bicarbonate, of soda relatively free from chloride of sodium, and chloride of sodium
relatively free from carbonate of soda, same being rendered, if desired, suitable
for re-use in electrolytic cells; and, thirdly, to provide suitable apparatus for
carrying out my improved process. It will be evident that the means hereinafter
described admit of considerable variation and that the treatment which is described
for use under one method may be wholly or partially applicable under other alter-
native methods such as hereinafter specified.
Diaphragms in electrolytic cells are either separated from the cathode, in juxta-
position thereto, or they constitute a combined diaphragm and cathode. Caustic
soda formed at the cathode, may be carbonated, by injection of carbonic acid gas
directly into the cathode compartment of the electrolytic cell, or indirectly after
passing from the said cathode compartment. Porosity of diaphragms usually
increases with continued use, the resultant solution being more or less impure.
The accompanying drawings illustrate diagrammatically the means employed
in carrying out my invention; the various figs. being hereinafter described as
reference is made thereto in defining the several operations.
According to one mode of carrying out my invention, I separately collect solution
of carbonate of soda containing relatively large, and solution of carbonate of soda
containing relatively small proportions of undecomposed chloride of sodium,
hereinafter referred to as impure" and "pure" solutions, yielded from a series
of electrolytic cells, by means of two or more troughs or conduits; the said conduits
being also applicable for use in separately obtaining from electrolytic cells "im-
pure solutions for employment in the manufacture of soda cystals, and of "pure"
solutions for use in obtaining either relatively pure monohydrated carbonate of
soda, or caustic soda.
55
כי
The apparatus is illustrated in Figs. 1, 2 and 3, whereof Fig. 1 is an end eleva-
tion and Figs. 2 and 3 side elevations, and may comprise a series of cells a whereof
the earthenware portion 6 rests in a trough c, the solutions resulting from the
cells being delivered by the pipes d d into the conduits e e. According to the con-
dition of the diaphragms impure or pure solutions are rendered in the trough c;
the latter being divided into compartments by means of the part b. In the
arrangement shown impure and pure solutions may be collected from either side
of the cell; the pipes & being severally turned so as to serve alternatively for

page_text: collecting the impure and the pure solutions from the cells. By inverting the
pipes the flow of solution may be cut off. The pipes d may be provided with
stop-cocks.
I find that upon evaporating a solution of chloride of sodium containing a rela-
tively small proportion of carbonate of soda in solution, the relative amount of
carbonate of soda increases, a mother liquor containing chloride of sodium and
carbonate of soda in the constant relative proportions of about two equivalents of
the former to one of the latter resulting; and that when crystals of chloride of
sodium, or mixed crystals of monohydrated carbonate of soda and chloride of
sodium, are heated or boiled in a sufficiently large excess of a concentrated solution
of carbonate of soda, the chloride of sodium is dissolved and replaced, from the
solution of soda, by monohydrated carbonate of soda.
Upon the cooling and crystallisation of carbonate of soda from mother liquor
containing undecomposed chloride of sodium, resulting from the separation of
monohydrated carbonate of soda from the solution of carbonate of soda from elec-
trolytic cells, a final mother liquor may be obtained, largely impregnated with
chloride of sodium, suitable for treatment either with carbonic acid gas, or for
direct neutralisation as hereinafter described for the recovery and re-use of the
undecomposed chloride of sodium contained therein.
The "impure" and "pure" solutions may, if desired, be separately concentrated
before treatment.
The evaporating and treating pans or vessels may be the ordinary "boat"
or the well known evaporating pans, such as illustrated in Figs. 4 and 5, which
pan,
represent respectively a transverse section and a plan of a series of such pans, or
be similar in construction to the pans ordinarily employed in obtaining common
salt from brine. The solutions are convoyed to the pans f, f', f by the con-
duits e e.
The first and second pans f f of the series are each provided with a
partition g, which descends nearly to the floor, and ducts h h connect the pans,
the solution near the bottom of one pan being thereby conveyed to the surface
of the solution in the pan next in series. Any suitable means may be provided
for "fishing" and passing the crystals of monohydrated carbonate of soda from
the one pan to the other. is a drainer for. receiving the fishings from the pans.
k is a draining aperture which may be closed by a plug kl. A fire or other source
of heat is preferably placed beneath each pan. The pure solution is conveyed to
the pan f and the impure solution to the pan f'; the product being discharged at k.
On evaporating the "impure" solution of carbonate of soda, I obtain crystals
of monohydrated carbonate of soda of a size suitable to facilitate treatment by the
methods hereinafter described; the process of evaporation being carried out with
the aid of pans or vessels, arranged to operate in series" with pans or vessels
containing "pure" solution of carbonate of soda. The "impure" solution of car-
bonate of soda preferably enters the evaporating pan or vessel intermediate in
series, and the "
pure" solution enters the pan or vessel previous or first in series.
Pure solutions are caused to pass from the pan or vessel first in series towards
the pan or vessel last in series; while crystals of monohydrated carbonate of soda,
containing, or impregnated with, undecomposed chloride of sodium, obtained by
evaporation of "impure solutions of carbonate of soda in pans or vessels last
and intermediate in series, are passed through and treated in a hot, and preferably
boiling, "pure" solution of carbonate of soda contained in the first, or in the inter-
mediate and first pan or vessel in series, whereby undecomposed chloride of
sodium is eliminated from said crystals.
"
Or, each pan may be arranged to constitute in itself a series of evaporating and
treating vessels, as shown for example in longitudinal section in Fig. 6 and in
plan in Fig. 7. To this end, the pans f, etc., are each furnished with dividing
walls for separating the "impure" from the "pure" solutions of carbonate of
soda; ducts being provided for conveying the said solutions from the lower
part of the solution in one compartment to the surface of the solution contained in
the compartment next in series. In working with apparatus arranged in this

page_text: manner, the first compartment f may be situated remote from, and the last com-
partment near the source of heat employed for effecting evaporation. Or each
compartment may be provided with means for heating. Or a series of evaporating
and treating pans may be arranged so that each pan or vessel becomes alternately
first and last, or first, intermediate and last, according to the number of pans or
vessels employed; the crystals being passed through the series, while the solu
tions are evaporated without being removed from the pans.
"
According to another mode of procedure, impure" and "pure" solutions of
carbonate of soda may be mixed together and evaporated in salting down pans,
such as employed in the Leblanc alkali process; inonohydrated crystals of car-
bonate of soda containing or impregnated with undecomposed chloride of sodium
being obtained.
The crystals of monohydrated carbonate of soda above referred to, obtained for
instance, by surface evaporation are separately treated with relatively pure solu-
tion of carbonate of soda from electrolytic cells (said solution being rendered hot
and preferably concentrated) in a series of lixiviating tanks, such, for example,
as the well known Leblanc black ash lixiviating tanks, side openings being pro-
vided therein for the discharge of the treated crystals.
Mixed solutions of carbonate of soda and undecomposed chloride of sodium
obtained as mother liquor from said crystals, i.e., from surface evaporated
"impure" solutions of carbonate of soda, are then treated in manner hereinafter
described; but the said crystals may be treated as herein before described, or they
may be treated with hot or boiling "pure" solutions of carbonate of soda, or by
means of exhaust steam from an engine, in such a manner as to cause hot or
boiling relatively pure solution of carbonate of soda to pass from relatively pure
crystals of monohydrated carbonate of soda through said crystals containing or
impregnated with relatively large proportions of undecomposed chloride of sodium.
According to an alternative method of treating the said crystals of monohydrated
carbonate of soda, illustrated in Fig. 8, which represents a longitudinal section of
the apparatus employed, hot water, hot solution of carbonate of soda, or steam,
say the exhaust from an engine, may be employed, the crystals meanwhile being
contained in an inclined trough or cylinder 7 provided with an Archimedian or
similar feed screw . The crystals are conveyed through the trough or cylinder
in one direction while the steam or mother liquor resulting from treated crystals
flow in the opposite direction. When steam is used, the hoppers m m³ are sealed.
n is the outlet for the product.
Mixed solutions of carbonate of soda containing undecomposed chloride of sodium
obtained as mother liquor from monohydrated carbonate of soda may be cooled,
crystals of carbonate of soda further separated, carbonate of soda in mother liquor
therefrom neutralised, and chloride of sodium recovered and re-used in electrolytic
cells as hereinafter described; or mother liquor from monohydrated, or from
soda crystals, may, if necessary, be diluted and treated with carbonic acid gas
preferably by means of carbonic acid gas obtained as products of combustion from
a gas engine, hereinafter described; the pressure of the exhaust being utilised for
forcing the products of combustion through the said mixed solutions; or carbonic
acid gas may be employed in any other suitable and well known manner for the
purpose of rendering the carbonate of soda contained in said mixed solutions in
the form of bicarbonate. The mother liquor may moreover be evaporated and the
resulting magma allowed to cool, whereby mixed crystals of undecomposed chloride
of sodium and suitably hydrated carbonate of soda, say containing about four
equivalents of water, are obtained, these being subsequently treated with carbonic
acid gas in a suitable vessel forming one of a series of vessels employed for wash-
ing the bicarbonate of soda formed. The carbonic acid gas may be derived from
any convenient source, such as from a lime kiln, or chimney gases, but, by pre-
ference, the exhaust from a gas engine, after being cooled and suitably washed
and purified, is employed. Figs. 9 and 10 represent a transverse section and a
plan respectively of a series of vessels wherein the bicarbonating and washing are

page_text: effected, same comprising vessels o 01 02 0³ constructed in a similar manner to the
well known Leblanc black ash lixiviating tanks; each vessel being furnished with
a down pipe p for the passage of the carbonic acid gas. The gas supply pipe p¹
is interchangeable from one down pipe to another as may be required. Each
vessel is provided with a grid or perforated false bottom q whereby a free passage
is afforded for the gas and its equal distribution into and amongst the crystals
ensured. By means of the pipe 21 the vessels are rendered interchangeable; con-
stituting in one series a carbonating vessel and a series of washing vessels. The
solution of chloride of sodium obtained by washing and dissolving the crystals
are conveyed from vessel to vessel by the means ordinarily employed in the black
ash lixiviating tanks; the vessels being capable of employment alternately as
carbonating and washing vessels without removing the crystals therefrom.
At the
side of each vessel is a door or opening for discharging the washed bicarbonate.
Or said mixed crystals of carbonate of soda and chloride of sodium may be treated
with carbonic acid gas in a carbonating vessel of iron or other suitable metal
wherein are disposed a series of shelves placed one above another for the reception
of the said crystals which are stirred or moved about from time to time by
mechanical means so as to cause fresh surfaces to be exposed to the action of the
carbonic acid gas and thereby form bicarbonate of soda, the said crystals passing
downwards from shelf to shelf; provision being made for admitting and passing
the carbonating gas to and through the said vessel, for charging the vessel with
crystals and for discharging the sodium chloride and bicarbonate therefrom.
Crystals of chloride of sodium and bicarbonate of soda are lixiviated or washed,
chloride of sodium dissolved and eliminated from bicarbonate of soda when con-
tained in a series of vessels to obtain strong, and preferably concentrated, solution
of chloride of sodium, and bicarbonate of soda relatively free from chloride of
sodium. In order to diminish the back pressure on the gas engine when the.
exhaust is employed as the source of carbonic acid for bicarbonating under the
several alternative methods herein described, a gas reservoir may be used for
receiving the under pressure, and an outlet valve provided on the engine to allow
the last part of exhaust to pass into the air. After suitably neutralising the car-
bonates of soda contained in the solution of chloride of sodium, said chloride may
be re-used in electrolytic cells.
gras
Having separated the carbonate or the carbonate and bicarbonate of soda from
the solutions of carbonate of soda containing undecomposed chloride of sodium
obtained from electrolytic cells, by any one of the several alternative methods
herein described, when desired to re-use recovered undecomposed chloride of
sodium in electrolytic cells, I prefer to react upon the carbonate of soda con-
tained in the resulting mixed solutions of chloride of sodium and carbonate of
soda with salts of the alkaline earths, such as salts of magnesium and calcium
contained in common salt, rock salt or brine, previous to the employment of said
solutions, as source of chloride of sodium, in the electrolytic cell; thereby jointly
and economically neutralising the said carbonate of soda, recovering the chloride
of sodium in suitable condition for re-use in the process of electrolysis and, from
the said impure chloride of sodium, eliminating in the form of carbonate, the said
alkaline earths sesqui and bicarbonates of soda contained in the recovered solu-
tion of undecomposed chloride of sodium are reduced to monocarbonate of soda,
before reacting upon said earthy salt-contained in salt or brine. The quantity
of carbonate of soda allowed to remain in the said resulting mixed solutions of
chloride of sodium and carbonate of soda, should be in proportion to the amount
required to separate the desired quantity of the said alkaline earths from impure
chloride of sodium to be employed in the process of electrolysis. The said result-
ing mixed solutions of chloride of sodium and carbonate of soda may be
neutralised by filtering through, or otherwise bringing in direct contact with,
common salt, or rock salt; or by mixing the said solutions with brine containing
salts of the alkaline earths.

page_text: To facilitate the separation of precipitated carbonates of lime and magnesia
from brine, the said precipitates are allowed to remain in the brine until they
form crystals. These are then separated from the purified brine and are subse-
quently mixed with impure brine from which said earthy carbonates are to be
precipitated and for which the said crystals serve as nucleus for crystallization.
Or, if the soda to be neutralised, and contained in solution of chloride of sodium,
resulting from the treatment of solution of carbonate of soda from electrolytic
cells, exists as sesqui or bicarbonate of soda, I mix the said solution with brine
containing said earthy salts, adding thereto sufficient milk of lime to reduce the
sesqui or bicarbonate to monocarbonate of soda; the carbonated particles of lime
carry down and facilitate the rapid separation of the said precipitated carbonates
of the alkaline earths.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed, I claim :-
L
1. In the treatment of solutions of carbonates of soda containing undecomposed
chloride of sodium, with a view to the production of carbonate of soda, separately
obtaining "impure" and "pure" solutions of carbonate of soda, evaporating the
'impure," and treating the resulting salts of monohydrated carbonate of soda
with the "pure" solution, substantially as herein described.
#6
2. The herein described process for the production of carbonates of soda from
solution of carbonate of soda containing undecomposed chloride of sodium from
electrolytic cells; such process consisting in separating from the solution mono-
hydrated carbonate of soda relatively free from chloride of sodium, cooling the
mother liquor, and further separating therefrom carbonate of soda in the form
of crystals, treating the resultant mother liquor with carbonic acid gas to obtain
bicarbonate of soda, and then neutralising the carbonates of soda contained in
the resultant solution of chloride of sodium, and thus rendering the latter capable
of re-use in electrolytic cells.
3. In the production of carbonate of soda, the herein described method of treat-
ing a solution of carbonate of soda containing undecomposed chloride of sodium
obtained from electrolytic cells and separating therefrom carbonate of soda rela-
tively free from chloride of sodium, neutralising the carbonate of soda contained
-in the resultant solution of chloride of sodium, and re-using the latter in electrolytic
cells.
:
4. The herein described means for treating monohydrated carbonate of soda
impregnated with undecomposed chloride of sodium obtained from solutions
yielded from electrolytic cells with exhaust steam from an engine, substantially
as and for the purpose set forth.
5. Treating mixed solutions or mixed crystals of carbonate of soda and chloride
of sodium, yielded after the separation of carbonate of soda from a solution thereof
obtained from electrolytic cells, with the exhaust from a gas engine for the pur-
pose of separately obtaining bicarbonate of soda and chloride of sodium, sub-
stantially as set forth.
6. The improved means operating substantially as herein described for separately
obtaining from mixed solutions of the character herein referred to a carbonate of
soda relatively free from chloride of sodium and undecomposed chloride of sodium,
whereby porous diaphragms are rendered capable of efficient use in electrolytic
cells.
7. The combination with a series of electrolytic cells, of two or more conduits
for separately conveying and collecting "impure" and " pure" solutions of
carbonate of soda yielded in said cells, substantially as set forth.
8. A series of evaporating and treating pans provided with conduits for the
passage of the solution from the lower part of the solution in one pan to the sur-
face of the solution in the next pan in the series, substantially as herein described.
9. The employment of a series of evaporating and treating pans wherein the
solution remains in the pan until the evaporation is completed, whilst the salts

page_text: of monohydrated carbonate of soda are passed to, and are treated in, the pan next
in the series, substantially as and for the purposes herein described.
10. In the herein described treatment of monohydrated carbonate of soda, the
employment of a trough or similar receptacle provided with a feed screw whereby
the crystals are fed and conveyed in one direction while the hot solution or exhaust
steam traverses the said receptacle in the opposite direction, substantially as and
for the purpose specified.
11. In the bicarbonating and washing of mixed crystals of chloride of sodium
and carbonate of soda, the employment of a series of vessels wherein one of said
vessels is capable of being employed alternately as a bicarbonating vessel while
the remainder of the series are used as washing vessels, substantially as herein
described.
Dated this 15th day of June 1898.
For the Applicant,
G. G. M. HARDINGHAM,
Chartered Patent Agent, 191, Fleet Street, London, E.C.