IMPROVEMENTS IN BLEACHING WOOD PULP
Year: 1879

page_text: LETTERS PATENT to Frank Wirth, of the Firm of Wirth and Company,
Patent Solicitors, at Frankfort on the Main, in the Empire of Germany, for
the Invention of "IMPROVEMENTS IN BLEACHING WOOD PULP." A communica-
tion from the Society for the Manufacture of Wood Pulp, at Grellingen, in the
Republic of Switzerland.
Sealed the 20th January 1880, and dated the 13th August 1879.
PROVISIONAL SPECIFICATION left by the said Frank Wirth at the Office
of the Commissioners of Patents on the 13th August 1879.
+
FRANK WIRTH, of the Firm of Wirth and Company, Patent Solicitors, at
Frankfort on the Main, in the Empire of Germany. "IMPROVEMENTS IN BLEACHING
WOOD PULP." [A communication from the Society for the Manufacture of Wood
Pulp, at Grellingen, in the Republic of Switzerland.]
This Invention relates to the bleaching of wood pulp which has been mechanically
produced by the use of sulphurous acid.
In practising the said Invention I expose the pulp while moist to the action of
sulphurous acid gases produced by burning sulphur. The action is more quick or
short, according to the kind and sort of wood used, the difference in time taken by
the process, however, can be counted by seconds. The sulphurous acid is best used
In the
in the form of vapour. The pulp can be treated either as stuff or mash.
first method the stuff is passed through a chamber or box containing the gases. In
the latter case the finished mash is allowed to fall through a chamber or box con-
taining the gas, to the action of which it is thus exposed and arrives at the bottom
bleached. The vapour can be introduced and drawn off by means of ordinary
valves.
Wood pulp can also be bleached by a watery solution of sulphurous acid from a
sulphate or hydrosulphate.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Frank Wirth in the Great Seal Patent Office on the 13th February
1880.
FRANK WIRTH, of the Firm of Wirth and Company, Patent Solicitors, at
Frankfort on the Main, in the Empire of Germany. " IMPROVEMENTS IN BLEACHING
WOOD PULP." A communication from the Society for the Manufacture of Wood
Pulp, at Grellingen, in the Republic of Switzerland.
The problem of bleaching wood pulp made by mechanical means in a thoroughly
satisfactory manner, although of the greatest importance, has not heretofore been
solved.
Among the many substances recommended for bleaching this material sulphurous
acid has been named; experiments, however, which have been made have not
produced any practical result, as demonstrated in Muspratt's work, 5 vol., page 820.
Moreover, Kick and Gintl in their Technical Dictionary only mention sulphurous
acid as a bleaching material for animal substances, other agents being named for
vegetable substances. In fact it is now well known that sulphurous acid is not
considered as one of the bleaching agents for vegetable substances, and in the present
state of the manufacture of wood pulp it is unknown as such.
. The use of sulphurous acid for bleaching wood pulp is stated to be impracticable
by Müller (in his work "Die Fabrikation des Papieres," published by I. Springer,
Berlin, 1877, page 56), by Muspratt and by Kick and Gintl.
In the most recent work on the manufacture of wood pulp, by B. Dropisch
(Holzstoff und Holzcellulose published by F. Voigt, Weimar, 1879), the use of this
acid is passed over unnoticed.
By this Invention, however, the practical application of this substance is accom-
plished in such a manner that although it was discarded both in theory and practice
as a bleaching agent I apply it with the greatest success, so that henceforth it will
occupy the most important place among bleaching agents used in the manufacture
of wood pulp.
This result is obtained by a special method in which the acid in a particular
form is brought in contact with the wood pulp, which is also in a special condition.
This Invention is based upon the discovery that wood pulp is practically indif-
ferent to the action of sulphurous acid in any form, if, as has always been the case,
this acid is used on the material when the latter is dry, or even when the same has
been dried and then wetted again. And on the further discovery that wood pulp is
easily acted upon by the bleaching properties of sulphurous acid as vapour if
exposed to the same without having been first dried, that is to say, in the damp
state in which it is left after its reduction to pulp.
The method employed, according to this Invention, is as follows, that is to
say,-
-
The finished wood stuff is either passed through a chamber containing sulphurous
acid vapour or gas while it is still damp, or the stuff in a semi liquid state is passed
as it comes from the machine directly through a chamber containing sulphurous
acid gas or vapour, this vapour can be either produced in the chamber or introduced
into the same by any suitable means.
It is important in practising this Invention that the wood pulp be treated in the
damp state in which it is first obtained after production; and, second, that the
sulphurous acid be used in the form of gas.
The wood stuff in its first damp condition, that is to say, when first produced
and not dried, and then damped or wetted again, is in a very finely separated state,
and so disintegrated that each molecule is surrounded by water, the latter serves
as a vehicle for the sulphurous acid which follows, and brings the same into close
connection with the colouring particles of the stuff in such a manner that they are
easily acted upon by the bleaching power of the acid.

page_text: Wood pulp which has been dried after its manufacture can, when damped again,
never be so thoroughly disintegrated that each molecule will be exposed to the
direct action of the acid.
The use of sulphurous acid in the state of vapour is important, as only when
thus used can the necessary concentrated strength of such acid be obtained by reason
of the quantity of water contained in the wood pulp.
The wood stuff thus treated with sulphurous acid is very white, but it is then
banked up and left for 15 to 20 days; during this time the sulphurous acid which
is most thoroughly mixed with the wood-pulp continues its bleaching action, and at
the end of the above specified time the pulp will be brought to a degree of
whiteness practically unattainable by the use of oxalic acid or chlorine, or any
other agent proposed till now for the purpose in question.
I am well aware that De Angelis Rocco in the Specification of his Letters Patent,
No. 1155, A.D. 1875, has described a process for manufacturing paper pulp by
which the pulp not yet refined is with the addition of a great quantity of water
subjected to the action of gaseous sulphurous acids. But by these means, as experience
has proved, paper pulp cannot be bleached in a satisfactory manner; although
Rocco in his said Specification states that "the action of this acid and of the
continued exposure of the air of the substances under operation converts them into
colourless pulp," it might be difficult to prove that this result is in fact obtained.
A good result is obtained only by the present process, by which the finished wood pulp
in its original damp condition, without any essential addition of water, is exposed to
the action of gaseous sulphurous acid, and then banked up for 15 to 20 days, during
which time the sulphurous acid continues its bleaching effect without access of the
outer air, which cannot enter into the wood pulp properly banked up.
Having thus fully described the said Invention, and the manner of performing
the same, I wish it understood that I do not claim broadly or generally the use of
sulphurous acid for bleaching wood pulp, but I claim,-
In the bleaching of wood pulp subjecting the pulp in its original damp, moist, or
wet condition to the action of sulphurous acid in the form of vapour or gas, and
then banking it up, all substantially as and for the purposes set forth.
In witness whereof, I, the said Frank Wirth, have hereunto set my hand and
seal, this Tenth day of February, in the year of our Lord One thousand
eight hundred and eighty.
FRANK WIRTH. (L.S.)