Improvements in the Purification of Caustic Alkalies.
Year: 1898

page_text: Date of Application, 23rd May, 1898
Complete Specification Left, 16th Feb., 1899-Accepted, 23rd May, 1899
Improvements in the Purification of Caustic Alkalies.
I, GEORGE WILLIAM JOHNSON, of 47, Lincoln's Inn Fields, in the County of
London, Patent Agent, do hereby declare the nature of this invention (which has
been communicated to me from abroad by Chemische Fabrik Griesheim, of Frank-
fort-on-Main, in the Empire of Germany) to be as follows:-
Although dialysis affords convenient means whereby the separation of crystalline
from uncrystallisable substances can be effected, this agency has been seldom applied
to the practical separation of crystalloids from one another, because the diffusion
constants of crystalloids, which have been as yet determined, and are in general
found to be considerably greater than those of colloids do not usually differ from one
another in a ratio greater than 2:1. These constants are (Ninkelmann, Handbuch
der Physik, 1891, Vol. 1, p. 638), for caustic potash, 214, potassium chloride, 149,
caustic soda, 162, sodium chloride, 128.
A closer investigation of the diffusion of a mixture of caustic potash with
chlorides, sulphates, sulphides, and the like, revealed the unexpected fact that, under
certain conditions, caustic potash may be almost completely separated from salts.
It was found, for example, that the velocity ratio of diffusion of alkali and chloride,
which in dilute solutions is about 2:1 very greatly increases with increased concen-
tration, becoming about 16:1 in highly concentrated solutions if care be taken to
prevent the concentration of the lye falling below a certain limit.
The concentration of the pure lye obtained by diffusion in water is without
influence, and inay be allowed to approach that of the crude lye.
The following examples serve to illustrate the foregoing :-
1. By dialysis of a dilute potash lye containing

page_text: that in the case of high concentrations, such as 750 g. per litre, the centents-ratio
unexpectedly drops from 100: 1.60 to 100: 0.11.
The behaviour of concentrated solutions of sodium hydroxide is similar, as is
shewn by the following example.
3. By dialysis of a deep red, crude soda lye, containing silica, iron, alumina, and
lime, obtained by the Leblanc Soda. Process, and having a gravity of 50° Beaumé,
containing
ratio 100 NaOH : 0.03 Na,S: 0.04 NaCl, 0.00 Na₂SO
By evaporation of the dialysed lyes, and subsequent dialysis, the amount of
other constituents may be reduced to unrecognizable traces.
Since the dialysis of caustic alkalies by means of membranes such as parchment,
proceeds with extraordinary rapidity, and the evaporation of the water added during
the process costs but little, the process according to this invention, affords a new and
practical process, of far-reaching technical application, for the preparation on a large
scale and in a simple manner of caustic alkalies in a state of great purity, a result
which has hitherto been only obtained by crystallisation from alcohol, which has
considerably increased the cost of the product.
Dated this 23rd day of May 1898.
JOHNSONS & WILLCOX,
47, Lincoln's Inn Fields, London, W.C., Agents.
COMPLETE SPECIFICATION.
Improvements in the Purification of Caustic Alkalies.
I, GEORGE WILLIAM JOHNSON, of 47, Lincoln's Inn Fields, in the County of London,
Patent Agent, do hereby declare the nature of this invention (which has been
communicated to me from abroad by Chemische Fabrik Griesheim, of Frankfort-on-
Main, in the Empire of Germany) and in what manner the same is to be
performed, to be particularly described and ascertained in and by the following
statement
Although dialysis, affords convenient means whereby the separation of crystallizable
froin uncrystallizable, bodies can be effected it is not practically suited to the separa-
tion of crystalloids from one another, because the diffusion constants of crystalloids,
which have been as yet determined, and are, in general, found to be considerably
greater than those of colloids, do not usually differ from one another in a ratio greater
than 2:1. These constants are for example, (Ninkelmann, Handbuck der Physik
1891 Vol. 1, page 638), for caustic potash 214, potassium chloride 149, caustic soda 162,
sodium chloride, 128.
A closer investigation of the diffusion of a mixture of caustic potash with chlorides,
sulphates, sulphides, and the like, revealed the unexpected fact that, under certain
conditions, caustic potash can be almost completely separated from salts. It was
tound, for example, that the velocity ratio of diffusion of alkali and chloride which

page_text: in dilute solutions is about 2:1 very greatly increases with increased concentration,
becoming about 16:1 in highly concentrated solutions if care be taken (say by stirring,
rapid circulation or the like) to prevent the concentration of the lye falling below a
certain limit. As to the lye obtained by the diffusion the result is satisfactory even
if the strength of it comes up to 42° Beaumé.
The following examples serve to illustrate the foregoing :-
1. By dialysis of a dilute potash lye containing
ratio 100 NaOH: 0·03 Na,S: (0·04 NaC), 0·00 Na̸„SO̟
By evaporation of the dialysed lyes, and subsequent renewed dialysis, the percentage
5 of secondary constituents was reduced to imperceptible traces.
But even where dialysis is performed but once, the impurities in the osmotic lye
may readily be reduced to a minimum, provided that care is taken, that, in the
dialysing chamber, the water is, at all stages of diffusion, confronted with a concen-
trated lye, and that, at the partition between the water and the lye, a dilute lye does

page_text: not form from time to time, as that would tend to mix the water again by osmose.
This end can be attained, for example, by stirring the caustic potash lye, or by
causing a rapid circulation, or movement, thereof.
The following tables shew the results attained by this method:-
(a) A crude caustic potash lye of 50° Baumé was taken, containing per litre
In Example V, whenever the specific gravity of the potash lye in use fell to 45°
Beaumé, fresh 50° Beaumé lye was added. The resulting lye, at 30.5° Beaumé, cou-
tained, per litre, 365 grammes of KOH and 16 grammes of KCl.
(b). A crude caustic soda lye at 49° Beaumé was taken, containing, per litre,
707 grammes of NaOH; 51 grammes of Na,S; 12-2 grammes of NaCl; 1.3 grammes
of Na₂SO. The concentration of this lye, which, in the course of the experiment
was repeatedly renewed, dropped to 46-5° Beaumé when dialysis was applied, and
there was obtained a diffused lye of 19° Beaumé, containing, per litre, 155, ONaOH;
016 Na,S; 042 NaCl; and traces of Na, SO4. Ratio per 100 NaOH: 01 Na,S;
027 NaCl; and .000 Na₂SO,
Diffusion without a membrane leads to the same results in regard to quality, while
quantitatively the results of such diffusion are not quite so favourable.
Diffusion on this system may be conducted by either of the following methods.
(1). Sheets, (or layers), of crude lye and water, of a large horizontal area, are care-
fully superposed, and the diffused lye is, sometime afterwards, drawn off, nearly down
to the surface of the crude lye. The layer of diffused lye in immediate contact with
It is therefore
the crude lye is strongest, though its degree of purity is lowest.
expedient to take off such lye a few millimètres above the line of contact. Or: (2)
operations may be conducted in a system of grooves, or channels, by causing water
to flow slowly, but continually, over the crude lye, and taking off the diffused lye
through a horizontal slot-the level of which is adjustable-at a suitable height
above the plane of contact of the two layers.
Inasmuch as the crude lye becomes inore dilute towards its upper surface, and as
the diluted part thereof collects at the top, if it be desired that the operations should
be perfectly continuous, the following alternative course may be adopted :-

page_text: (3). The crude lye, also, may be caused to flow, but in the direction opposite to
that of the water, the level of the contact line being kept unchanged.
The following examples will serve as an illustration of what has been stated
above:--
The results of the tests 4 to 9 were unsatisfactory, evidently because, in
consequence of the increased friction of the oppositely flowing liquid layers, besides
diffusion, an actual mixing of the liquids was in progress.
In the osmose of caustic
soda lye, generally speaking, the results are more fluctuating, being influenced by
5 various circumstances over which there is no absolute control.

page_text: Since the dialysis of caustic alkalis proceeds with extraordinary rapidity, and the
evaporation of the water added during the process costs but little, the process accord-
ing to this invention, affords a new and practical process of far-reaching technical
application for the preparation, on a large scale, and in a simple manner, of caustic
alkalies in a state of great purity, a result which has hitherto been only obtained by
crystallization from alcohol, which has considerably increased the cost of the
product.
Having now particularly described and ascertained the nature of this inven-
tion and in what manner the same is to be performed, I declare that what I
claim is :--
The separation of caustic alkalies from dissolved secondary constituents, such as
inorganic, or organic, salts, oxides, hydroxides, and colouring matters, by causing the
highly concentrated lyes to diffuse in water, with, or without, the employment of a
membrane of organic, or inorganic, material.