_TREATMENT OF WOOD (continued)._
Sulphite Processes.--Francke's Process.--Ekman's Process.--Dr.
Mitscherlich's Process.--Ritter and Kellner's Boiler.--Partington's
Process.--Blitz's Process.--McDougall's Boiler for Acid
Processes.--Graham's Process.--Objections to the Acid or Sulphite
Processes.--Sulphite Fibre and Resin.--Adamson's Process.--Sulphide
Processes.--Mechanical Processes.--Voelter's Process.--Thune's
Process.
=Sulphite Processes.=--An important and successful method of
treating wood has been found in employing sulphurous acid,
combined in certain proportions with soda, lime, or magnesia,
whereby a bisulphite of the alkaline or earthy base is obtained.
One of the principal attributes of these agents is that in
boiling wood at high pressures oxidation and consequent browning
of the fibres is prevented. Of these sulphite, or more properly
bisulphite, processes, several of those referred to below have
been very extensively adopted, and vast quantities of so-called
"sulphite pulp" are imported into this country from Norway,
Germany, Scandinavia, &c., the product from the latter source
being considered specially suited for the English market. Some of
these processes are also being worked in this country, but more
particularly those of Partington, McDougall, and Ekman.
=Francke's Process.=--In this process, which is known as
the "bisulphite process," the active agent employed for the
disintegration of wood is an acid sulphite of an alkaline or earthy
base, as soda or potassa, lime, &c., but it is scarcely necessary
to say that the process has since been modified by others. The
invention is applicable to the treatment of wood, esparto, straw,
etc., and may be thus briefly described:--A solvent is first
prepared, which is an acid sulphite of an alkali or earth, that
is, a solution of such sulphite with an excess of sulphurous acid.
As the cheapest and most accessible base the inventor prefers
lime. It has long been known that a solution of sulphite of lime,
combined with free sulphurous acid, would, at a high temperature,
dissolve the intercellular portions of vegetable fibres, leaving
the fibres in a suitable condition for paper manufacture; but Mr.
Francke claims to have determined the conditions under which this
can be effected with rapidity, and in such a way as to preserve the
strength of the fibres, and to have obtained a practical method
of preparing pulp by his process. For his purpose he employs a
moderately strong solution of the solvent at a high temperature,
with gentle but constant agitation. The acid sulphite is produced
by this process at small cost and at a temperature nearly high
enough for use in the following way:--A tower or column is charged
with fragments of limestone, which are kept wetted with a shower
of water; fumes of sulphurous acid, produced by burning sulphur,
or by roasting pyrites, etc., are then passed through the tower.
The liquid which collects at the bottom of the tower is the
desired solvent, which should have a strength of 4° to 5° B. It
is not essential that the limestone should be pure, as magnesian
limestone, etc., will answer equally well. The soluble alkalies,
as soda and potassa, may also be used when their greater cost is
not an objection. But for these alkalies the treatment is modified,
as follows:--The tower is charged with inert porous material, such
as coke, bricks, etc., and these are kept wetted by a shower of
caustic alkali at 1° to 2° B., while the sulphurous acid fumes
are passed through the tower. In like manner carbonate of soda or
potassa may be used, but in this case the solution showered on the
porous material should be stronger than that of the caustic alkali,
so that it may contain approximately the same amount of real
alkali. Whichever alkaline base be employed, the liquid collected
at the bottom of the tower should have a strength of 4° to 5° B.;
this being the acid sulphite of the base is used as the solvent
employed for the manufacture of pulp. When wood is to be treated,
it is freed as much as possible from resinous knots by boring and
cutting them out, and is then cut--by preference obliquely--into
chips of a ¼ to ¾ of an inch thick. Esparto, straw, and analogous
fibres are cut into fragments. The fibrous material and solvent
are charged into a digester heated by steam at a pressure of four
or five atmospheres, and consequently capable of raising the
temperature of the contents to about 300° F. As agitation greatly
promotes the pulping of the materials, Mr. Francke employs a
revolving cylindrical boiler, which is allowed to revolve while the
charge is under treatment.
=Ekman's Process.=--In this process, which in some respects bears
a resemblance to the preceding, native carbonate of magnesia
(magnesite) is first calcined to convert it into magnesia; it is
then placed in towers lined with lead, and sulphurous acid gas,
obtained by the burning of sulphur in suitable furnaces, is passed
through the mass, a stream of water being allowed to trickle down
from the top of the towers. The supply of gas is so regulated that
a continual formation of a solution of bisulphite of magnesium, of
an uniform strength, is obtained; great care, however, is necessary
to avoid excess and consequent loss of sulphurous acid by its
conversion into sulphuric acid. In boiling, the fragments of wood,
previously crushed by heavy rollers, are placed in a jacketed,
lead-lined, cylindrical boiler, suspended on trunnions, so that it
can be inverted to remove the charge. The pressure in the outer
jacket is 70 lbs. per square inch, and that within the boiler is
90 lbs. per square inch. The boiling occupies twelve hours. This
process has been extensively worked by the Bergvik and Ala Company,
of Sweden, for many years with great success, and we understand
that the company has been turned over to an English company--the
Bergvik Company, Limited. The Ilford Mill and Northfleet Works have
been largely supplied with sulphite pulp from the Swedish works.
One great drawback to the bisulphite processes is that the boiling
cannot be effected in iron boilers unless these be lined with some
material which will protect the iron from the destructive action of
the bisulphite, which, being an acid salt, would exert more action
upon the iron than upon the fibre itself, and the solution of iron
thus formed would inevitably prove injurious to the colour of the
fibre. In several of the systems adopted iron boilers lined with
lead have been used, but the heavy cost of this material and its
liability to expand unequally with the iron, especially at the high
temperatures which the solvent necessarily attains under pressure,
causes the lead to separate from the iron, while it is apt to bulge
out in places, and thus becomes liable to crack and allow the acid
liquor to find its way to the interior of the iron boiler which it
was destined to protect. To overcome this objection to the simple
lead lining, Dr. Mitscherlich patented a process which has been
extensively adopted in Germany, and is now being carried out by
several companies in different parts of America. This process is
briefly described below.
=Dr. Mitscherlich's Process.=--The digester employed in this
process is lined with thin sheet lead, which is cemented to the
inner surface of the boiler by a cement composed of common tar and
pitch, and the lead lining is then faced with glazed porcelain
bricks. In this process a weaker bisulphite of lime is used than
in Francke's, and the time of boiling is consequently considerably
prolonged.
=Ritter and Kellner= have proposed to unite the inner surface of
the boiler to its lead lining by interposing a soft metal alloy,
fusible at a temperature lower than that of either metal, and it
is claimed that the iron and lead are thus securely united, while
the alloy being fusible under the normal working temperature of the
digester, the lead lining can slide freely on a boiler shell.
=Partington's Process.=--This process, which has been for some
time at work at Barrow, and for the further development of which
a private company, entitled the Hull Chemical Wood Pulp Company,
Limited, has been formed, consists in the employment of sulphite
of lime as the disintegrating agent. The process consists in
passing gaseous sulphurous acid--formed by burning sulphur in a
retort, into which is forced a current of air at a pressure of
5 lbs. to the square inch--through a series of three vessels,
connected by pipes, the vessels being charged with milk of lime.
The first two of these vessels are closed air-tight, and the gas
is then introduced, while the third vessel remains open; from this
latter a continuous stream of nitrogen escapes, due to the removal
of the oxygen by the burning sulphur from the air passed into the
retort. This process is said to be a very economical one, so far as
relates to the cost of materials used.
=Blitz's Process.=--This process consists of employing a mixture
composed of bisulphite of soda 2 parts, caustic soda 1 part; and
vanadate of ammonia 1 gramme, in hydrochloric acid 4 grammes to
every 6 kilogrammes of the bisulphite. The wood, after being cut
up in the ordinary way, is submitted to the action of the above
mixture, under a pressure of three or four atmospheres, for from
four to eight hours, and the pulp is then ground; it is said to
possess some of the qualities of rag pulp and to look much like it.
=McDougall's Boiler for Acid Processes.=--This invention is
intended to obviate the difficulties which arise in using
lead-lined boilers, owing to the unequal expansion and contraction
of the lead and the iron on their being alternately heated by steam
and cooled, on the discharge of each successive batch of pulp. This
invention consists in constructing the boilers with an intermediate
packing of felt, or other compressible and elastic material,
so that when the interior leaden vessel is heated, and thereby
enlarged and pressed outwards by the steam, the compressible and
elastic packing yields to the pressure and expansion. Also in the
cooling of the vessels the packing responds to the contraction,
and approximates to its original bulk and pressure between the two
vessels, and so prevents the rupture or tearing of the lead and
consequent leakage and other inconveniences. Another part of this
invention consists in the construction of the outer iron or steel
vessel in flanged sections, which are fitted to incase the interior
leaden vessel with a space between the two vessels, into which the
compressible and elastic materials are packed. In the construction
of these vessels the iron or steel flanged sections are placed on
to the leaden vessel and packed with the compressible and elastic
lining in succession. As each section is packed it is screwed
close up to the adjoining section by the screw bolts, fitted into
corresponding holes in the flanges of the contiguous section until
completed. This method of construction secures economy by the
retention of the heat, which is effected by the packing between the
two vessels. The materials used for the packing are caoutchouc,
felt, flocks, asbestos, etc., and a space of about two inches
between the vessels is preferred, into which the packing is filled.
=Graham's Process.=--This process consists in boiling fibrous
substances in a solution of sulphurous acid, or a sulphite or
bisulphite of soda, potash, magnesia, or lime, or other suitable
base and water. The boiling is preferable conducted in a closed
boiler, lined with lead, to protect it from the action of the
chemical substances used, and is fitted with a valve which can be
opened to allow the gases and volatile hydrocarbons contained in
and around the fibres to escape. The method of carrying out the
process has been thus described:--"In carrying out the process
there is a constant loss of sulphurous acid gas going on, and
consequently a continual weakening of the solution employed, to
avoid which it is preferable to employ monosulphite of potash,
soda, magnesia, lime, or other suitable base, and water. Either
of these substances, or a suitable combination of them, and
water are placed in the boiler with the fibrous substances to be
treated, and the temperature raised to the boiling point. After
the hydrocarbons, air, and gases natural to the fibrous substances
have been driven out by the heat and allowed to escape, sulphurous
acid, in its gaseous or liquid state, or in combination with
either of the bases referred to, is pumped or injected into the
boiler. There is thus forming in the closed boiler a solution
containing an excess of sulphurous acid above that required to
form, in combination with the base, a monosulphite. The operation
of injecting sulphurous acids, or the sulphites, may be repeated
from time to time during the boiling, so as to fully maintain,
and if necessary increase, the strength and efficiency of the
chemical solution. It is said that by this process a saving of the
chemicals employed is effected, as little or no sulphurous acid gas
is lost during the time the gaseous hydrocarbons, air, and other
gaseous matters are being expelled from the fibrous materials.
If an open vessel is used instead of a closed boiler, it will be
necessary to keep the solution at a fairly uniform strength, and
if necessary to increase the strength, but the result will be
substantially the same; but as it is evident that, when using an
open boiler, the excess of sulphurous acid supplied during the
boiling will be constantly driven off as gas, it must be replaced
by further injections, while the acid fumes may be conveyed away
and condensed, so as to be available for further use. When the
fibrous substances are boiled as above, with the addition of
potash, soda, etc., during the boiling, the result will be equally
beneficial. The inventor prefers to inject the sulphurous acid
or its combinations into the boiler at the bottom, and to cause
it to come in contact with the solution therein before reaching
the fibrous materials. For this purpose there is formed a kind of
chamber beneath the boiler, but separated from it by a perforated
disc or diaphragm of lead or other suitable material not acted upon
by the solution, so as to allow the latter to fill the chamber, to
which is connected a pipe, through which the sulphurous acid or
solutions of the sulphites is forced by any suitable apparatus.
=Objections to the Acid or Bisulphite Processes.=--While
the various methods of boiling wood in caustic soda at high
temperatures are well known to be open to serious objections, the
acid treatment of wood also presents many disadvantages, which
it is to be hoped may be yet overcome. In reference to this,
Davis makes the following observations:--"In the acid treatment
of wood for the purpose of converting the fibres into pulp for
use in paper manufacture, the general practice has been to use
alkaline solutions of soda, combined in various proportions with
certain acids, such, for instance, as sulphurous acid, hydrochloric
acid, etc. These solutions have been heated in digesting vessels,
and the high temperature resulting from this process of heating
developing a pressure of from six to seven atmospheres, the wood
being disintegrated by the action of the boiling solutions, the
gum, resinous constituents, and other incrustating or cementing
substances that bind the fibres together are decomposed, destroyed,
or dissolved, while pure cellulose, which constitutes the essential
element of the ligneous fibres, is separated therefrom. To this end
high temperatures had to be employed, otherwise the disintegration
was found to be only partial, the wood remaining in a condition
unfit for further treatment. The high temperature not unfrequently
converts a large proportion of the resinous and gummy constituents
of the wood into tar and pitch--that is to say, carbonaceous bodies
that penetrate into the fibre and render its bleaching difficult,
laborious, and costly, while the frequent washing and lixiviation
necessary to bleach such products seriously affect the strength
of the fibre and its whiteness, and also materially reduce the
percentage of the product, in some instances to the extent of 18
per cent. These difficulties and detrimental results materially
enhance the cost of production, while the fibre itself suffers
considerably in strength from the repeated action of the chloride
of lime.... The difficulties are chiefly due to the carbonisation
of certain constituent parts of the fibres under temperatures
exceeding 212° F., such carbonised matters being insoluble and
incapable of being bleached, and as they permeate the fibre, cannot
be entirely removed.
"To overcome these difficulties, the wood should be chemically
treated at a temperature sufficiently low to ensure that the
decomposition of the connecting substances of the fibres will
remain chemically combined with the other elements, such as
hydrogen, oxygen, and nitrogen, in order to obtain an increased
product of superior quality and render the process more economical."
=Sulphite Fibre and Resin.=--A German manufacturer sent the
following communication to the _Papier Zeitung_, which may
be interesting to the users of sulphite pulp:--"In making
[disintegrating] cellulose by the soda or sulphite process, the
object in boiling is to loosen the incrusting particles in the
wood, resin included, and to liberate the fibres. The resin is
dissolved both in the soda and sulphite processes, but in the
former it is at the same time saponified, and is consequently
very easily washed out. In the case of sulphite fibre, however,
the resin attaches itself by its own adhesiveness to the fibres,
but can also be removed by as hot washing as possible, and adding
a little hydrochloric acid, which produces a very great effect.
At the same time, however, sulphite fibre loses in whiteness
by thorough washing, and assumes a reddish-grey shade. As the
paper manufacturer insists upon white fibre, the manufacturer of
sulphite fibre not only often omits washing, but adds some sulphite
solution (bisulphite of lime). This not only enables him to give
his customers white fibre, but he also sells a quantity of the
incrusting particles and sulphite residuum as cellulose.
"So long as the manufacturer looks more to white than to
well-washed cellulose, or does not wash it well before working up
the fibre, these annoyances cannot be avoided. Not only this, but
other disadvantages will be added in the course of time, as the
action of the sulphurous acid in the pulp will have very injurious
consequences on metals--[and on the fibre itself?] especially
iron--coming in contact with it. This should be the more avoided,
as the whiteness of the unwashed cellulose is of very short
duration. The paper made from it soon turns yellow and becomes
brittle. Well-washed sulphite fibre, on the other hand--provided
no mistakes have been made in the boiling process--makes a strong,
grippy paper, which can withstand both air and sunlight. I have
made no special studies as to resin, but believe that pine and fir
act differently, especially with solvents."
=Adamson's Process.=--Mr. W. Adamson, of Philadelphia, obtained
a patent in 1871 for the use of hydrocarbons in the treatment of
wood. His process consisted in treating the wood with benzine in
closed vessels, under a pressure of 5 to 10 lbs., according to the
nature of the wood. His digester consisted of an upright cylinder,
in which the wood-shavings were placed between two perforated
diaphragms. The mass was heated beneath the lower diaphragm by
a coil through which steam was passed. The vapours which were
given off were allowed to escape through a pipe on the top of the
digester, to which was connected a coil immersed in a vessel of
cold water, and the condensed liquid then returned to the lower
part of the digester. The remaining portion of the benzine in the
digester, which was still liquid but saturated with the extracted
matters, was drawn off through a faucet at the bottom. Benzine
being a very cheap article in America, a similar process was
recommended in another patent by the same author for extraction of
pitch and tar from rags [tarpaulin, ropes, &c.?], and for removing
oil from rags and cotton waste.
=Sulphide Processes.=--Many attempts were made about thirty years
ago, and in subsequent years, to employ the soluble sulphides as a
substitute for caustic soda in boiling wood and other fibres, but
these processes do not appear to have been very successful. Later
improvements in the construction of boilers or digesters, however,
seem to have induced further experiments in this direction, and we
understand that several sulphide processes are being worked on the
Continent, the processes of MM. Dahl and Blitz being amongst them.
One of the supposed advantages of these sulphides over caustic soda
is that by evaporation and calcination of the liquors, or leys, by
which the organic matters become destroyed, the original product
would be recovered, which merely requires to be dissolved out for
further use. There are, however, several important objections to
the use of sulphides in this way, amongst which may be mentioned
the deleterious vapours which they emit; and this alone would
doubtless prevent their employment--at all events in this country.
=II. Mechanical Processes.=--Besides the various chemical methods
of separating cellulose from woody fibres, before described,
certain processes have been devised for reducing wood to the
condition of pulp directly by mechanical means without the aid
of any chemical substance whatsoever. In this direction Heinrich
Voelter, of Wurtemburg, appears to have been the first to introduce
a really practical process for the conversion of wood into pulp
for paper-making, although, as far back as 1756, Dr. Schaeffer,
of Bavaria, proposed to make paper from sawdust and shavings
mechanically formed into pulp: the process was not successful,
however, with the machinery then at his command.
=Voelter's Process for Preparing Mechanical Wood Pulp.=--In 1860-65
and 1873 Voelter obtained patents in this country for his methods
of treating wood mechanically, and the process may be thus briefly
described:--Blocks of wood, after the knots have been cut out by
suitable tools, are pressed against a revolving grindstone, which
reduces the material to a more or less fine condition, but not in
a powdery form, and the disintegrated fibre is caused to press
against a wire screen, which allows the finer particles to pass
through, retaining the coarser particles for further treatment.
[Illustration: Fig. 17A.--Voelter's Wood-pulping Machine.
[_To face page 78._]
The apparatus employed, which is shown in Fig. 17A, consists of a
pulping apparatus A, with vat K, in which the revolving stone S is
placed; the blocks of wood are held against the stone at _p p_,
and water is introduced at G, and the revolving stone carries the
pulp against the screen E, which admits the passage of the finer
particles of the wood, while the coarser particles are led by the
trough F to the first refining cylinder B, after passing through an
oscillating basket, which retains the coarser particles. From
thence it is led through a distributing apparatus and hopper C, to
be uniformly supplied to the refining cylinder D, these cylinders
being of the ordinary construction, and, as usual, covered with
fine gauze wire sieves. The ground material which fails to pass
through the sieves is transferred by an elevator to the millstones
E, which are of ordinary construction, and after leaving these
unites with the finer fibres which pass through E, the whole now
entering a mixing reservoir F, whence it is thrown on to the
cylinder G, and the pulp which passes into this is distributed
on to a similar cylinder H, the contents of which then passes
through the last cylinder I, which is differently constructed
to the others, inasmuch as its lower part is surrounded by an
impervious leather jacket, so that the pulp ascends in order to
enter it. The disintegrated fibres that are retained by the wires
of the cylinders pass into the refiners, which consist of a pair
of horizontal cylinders of sandstone, one of which (the upper one)
only revolves, and by the action of these the coarser fibres become
further reduced, the finer particles, as before, passing through
the wire gauze of the cylinders, the operation being repeated in
the same order until the whole of the fibres have passed through
the sieves.
=Thune's Process.=--Mr. A. L. Thune, of Christiana, U.S.A., has
recently patented an apparatus for disintegrating wood, which
consists of a grinding apparatus connected to a turbine. In this
arrangement the grindstone, fixed on a shaft, is worked by a
turbine, and the wood, which is used in small blocks, is pressed
against the stone by means of a series of hydraulic presses. The
fine pulp is afterwards made into thick sheets by means of a
board-machine, the pulp, mixed with water, passing down a shoot
into a vat beneath, in which is a revolving cylinder covered with
wire-cloth, which in its revolution carries with it a certain
quantity of pulp in a continuous sheet; this is taken on to an
endless travelling belt by means of a small couch-roll, and passes
on to a pair of rolls, round the upper one of which the sheet
becomes wound, and is removed when sufficiently thick.