IMPROVEMENTS IN THE MANUFACTURE OF POTASH AND SODA
Year: 1882

page_text: PROVISIONAL SPECIFICATION left by Alexander Melville Clark at the
Office of the Commissioners of Patents on the 17th November 1882. A
communication to him from abroad by Jean Baptiste Marie Prosper Closson,
of Paris, France
ALEXANDER MELVILLE CLARK of the firm of A. M and W Clark of 53, Chancery
Lane, in the County of Middlesex, Patent Agents. IMPROVEMENTS IN THE
MANUFACTURE OF POTASH AND SODA A communication to him from abroad by
Jean Baptiste Marie Prosper Closson, of Paris, France
This process consists in the caustification of the alkaline sulphates. This
caustification has been attempted under the most diverse conditions without
yielding commercial results. I obtain it in a very simple manner by means of lead
oxide.
To perform the operation I dissolve the alkaline sulphate in hot water and treat
it with a milk of line and of lead oxide. The operation may be performed even
in the cold, but it is found that a temperature approaching ebullition is the most
suitable. I obtain finally the caustification of two-thirds of the sulphate if I
operate in a solution which corresponds to about one half of the maximum of
solubility. This proportion of sulphate caustified is reduced to about 45 per cent
when operating on liquors at their maximum of concentration. For 1000 kilos
of sulphate of potash for example, I employ 40 kilos of protoxide of lead and
obtain about 325 kilos of potash which is found in the liquor with about the
same weight of sulphate of potash. It is advisable to perform the operation three
times with the same oxide of lead, adding fresh lime each time. A methodical
washing is then effected, and for this purpose the apparatus at present used in the
manufacture of carbonate of soda by the Leblanc process answers perfectly. I
may also use filter presses, hydro-extractors, or decanting tanks.
The residue consists of sulphate of lime, of lime in excess, and of a small
quantity of oxide of lead or of plumbite of lime-In the liquor resulting from the
washing, there remains also some oxide of lead in the form of alkaline plumbite.
It may be removed in several ways:-
(1) By the employment of the alkaline carbonate corresponding to the sulphate
used, in which case there still remains a little carbonate of lead in solution.
(2) By hydro-sulphuric acid or an alkaline sulphide; this method is radical.
(3) By electrolysis; by this means the last trace of lead is removed.

page_text: I may also combine methods 1 and 2. In this manner I obtain a solution of
alkali and of the corresponding alkaline sulphate, from which by evaporations and
crystallizations, the alkali itself is obtained."
The quantity of lead oxide which remains in the residue is about 10 per cent of
the total quantity of lead oxide employed. This proportion may even be reduced
to less than 5 per cent by adding water to the residue after the washing and
boiling. In short, there are but 8 to 10 kilos of lead oxide in the residue for a ton
of potash. This lead oxide is removed very easily and completely by electrolysis.
may also treat this residue with hydrochloric acid, separate the chloride, and
precipitate the lead by lime. In this case it is desirable to limit as much as
possible the excess of lime to be employed if the hydrochloric acid is to be
economised.
By electrolysis, if the operation be performed at a low tension, nearly the whole
of the lead oxide is regenerated in the form of protoxide; there is a little metallic
lead at the negative pole. In certain cases it is desirable to regenerate the lead in
the metallic state, and for this it is only necessary to operate at a higher tension.
If the lead has been otherwise regenerated, for example in the form of carbonate,
I treat with hydrochloric acid and afterwards with boiling water and with lime,
and again have protoxide of lead.
The caustification may also be effected when, instead of precipitated. protoxide
of lead, I employ litharge or another oxide of lead. In this case more is used,
about double the amount in the case of litharge. This leads to a mechanical
separation by washing for the recuperation of part of the lead oxide.
The yield is considerably reduced, hardly more than 36 per cent of the alkaline
sulphate being caustified.
Instead of evaporating and crystallising, I may also employ carbonic acid to form
a relatively insoluble bicarbonate from which I obtain the alkaline carbonate
corresponding to the sulphate employed.