_TREATMENT OF RAGS_ (_continued_).
Boiling Rags.--Bertrams' Rag Boiler.--Donkin's Rag Boiler.--
Washing and Breaking.--Bertrams' Rag Engine.--Bentley and
Jackson's Rag Engine.--Draining.--Torrance's Drainer.
=Boiling Rags.=--To remove greasy matters, and also to dissolve out
the cementing substances from the stems of flax and shell of the
cotton, the rags are next boiled in a solution of caustic soda,
caustic lime, or a mixture of carbonate of soda and lime. The
boiling has also the effect of loosening the dirt contained in the
rags, whereby the colour of the material is greatly improved, while
at the same time it is rendered more susceptible to the action of
the bleaching agent. Strong linen rags will sometimes lose from
one-third to one-fifth of their weight by the process of boiling.
The vessels for boiling rags are of various construction, and have
been the subject of numerous ingenious patents. These boilers
are either cylindrical or spherical, and are also stationary or
rotary--the latter form being devised for the purpose of keeping
the caustic alkali solution freely diffused throughout the mass of
fibre during the boiling.
[Illustration: Fig. 9.]
=Bertrams' Rag Boiler.=--An illustration of a spherical boiler, as
manufactured by Bertrams, Limited, of Edinburgh, is given in Fig.
9. The shell of this boiler is made from malleable iron, is 8 feet
in diameter and 9 feet deep. The boiler is constructed on what is
termed the "vomiting" principle, by which a free circulation of the
alkaline liquor is constantly maintained. These boilers are made
to withstand any pressure of steam, but the size given is usually
worked at from 35 to 45 lbs. pressure, and carries about 30 cwt. of
dry esparto.
[Illustration: Fig. 10.]
=Donkin's Rag Boiler.=--The spherical boiler of Messrs. Bryan
Donkin and Co. is shown in Fig. 10. Being of a spherical form, it
is twice as strong as a cylindrical boiler of the same diameter
and thickness. The plates used are, notwithstanding, of the usual
substance, thus rendering it perfectly safe, durable, and suitable
for high-pressure steam. The spherical shape also allows the rags
to fall out by themselves when the boiler is revolving with the
cover off. Within the boiler are strainers to carry off the dirt,
and lifters to agitate the rags during the process of either
boiling or washing. To avoid cement, or even lead joints, the
gudgeons and the boiler are turned true in the lathe to fit each
other, the joints being simply made with red lead. These boilers
are usually about 8 feet in diameter, and are capable of boiling
from 20 cwt. to 25 cwt. of rags. The idea of giving motion to the
boiler, so as to insure a perfect mixture of the rags and the
caustic liquor, is of American origin, and was first introduced
into this country by Messrs. Bryan Donkin and Co. It is usual to
fix the boiler so that it can be fed with rags through a trap in
the floor above, while the boiler is in a vertical position and the
lid removed. The trunnions are hollow, to admit the introduction
of steam, alkaline ley, or water, and its rotary motion, which is
about three times in two minutes, is given by the gearing on the
left of the illustration.
The alkalies used for boiling rags are either caustic soda, soda
ash, slaked lime, made into a cream and sifted, or a mixture of
slaked lime and carbonate of soda. A description of the preparation
of caustic soda ley will be found in another chapter. It has been
customary at most of the larger paper-mills to purchase their
caustic soda direct from the alkali manufacturers, who supply
it in a solid form enclosed in iron drums, hermetically closed,
which are broken and the contents removed and dissolved when
required for use. As to the strength of caustic soda liquor to be
used for boiling rags, this is regulated according to the nature
and condition of the material, and the quality of the paper it
is intended for (see p. 34). For the finest papers the caustic
soda should be perfectly pure, and as there are various grades of
this chemical substance sold by the alkali makers, only the purer
qualities are used for the better kinds of paper. The proportion
of caustic soda per cwt. of rags varies to the extent of from 5 to
10 per cent. of the former to each cwt. of the latter, the coarser
materials, of course, requiring more alkali than those of finer
quality. In cases where rags are boiled in an open boiler--as
was formerly the case--a much larger proportion of caustic soda
would be required than when the boiling is conducted under high
pressures, as is now very generally the custom. In boiling the
finer qualities of rags, less pressure of steam is required than
for the coarser qualities, and the heat being proportionately
lower, there is less destruction of the fibre. Some paper-makers
prefer to boil the rags with caustic lime only, in which case the
lime, after being slaked in the usual way, is mixed with water
until it attains a milky consistence, when it is passed through a
sieve to separate any solid particles which may be present. About
the same percentage of lime may be used as in the former case.
When a mixture of lime and carbonate of soda is used, a method
much adopted on the Continent, the lime should be well screened
from lumps before being mixed with the soda. The usual method
of preparing this mixture is as follows:--A wooden tank, 15
feet long, 5 feet wide, and 4 feet deep is divided into three
compartments, each of which has a false bottom perforated with
½-inch holes to keep back lumps, stones, pieces of coal, etc.,
which frequently abound in the lime. The fresh lime is put into the
first compartment, where it is slaked with water in the usual way;
the resulting powder is then put into the next compartment together
with sufficient water, where it is agitated until converted into
what is technically termed "milk of lime." In the partition which
separates the second from the third division is a movable sluice,
through which the milk of lime flows into the third compartment; in
this is fitted a revolving drum, similar to the drum-washer of the
breaking-engine, through which the milk of lime which flows from
the sluice becomes strained, and is lifted in the same way as water
is lifted by the drum-washer of the breaking-engine, and is thence
discharged through a pipe into the rag boilers; an additional
straining can be effected by placing a fine wire strainer over
the mouth of this pipe leading to the boiler, which will prevent
objectionable particles from entering the boiler. Each compartment
is provided with a large waste pipe, through which, by the aid of a
sufficient supply of water, all impurities which have been rejected
by the drum are carried away. The soda solution is prepared by
dissolving the required proportion in water, and the resulting
liquor, after careful straining, is introduced into the boiler to
which the charge of rags has been given; the head of the boiler is
then fixed in its position and steam turned on, until a pressure of
about 20 to 30 lbs. to the square inch is attained, and the boiling
kept up for two to six hours, according to the quality of the rags.
By the Continental system of boiling rags, for No. 1 stuffs, 216
lbs. of lime and 114 lbs., of 48 per cent., soda ash are used for
every 4,000 lbs. of rags; for Nos. 3 and 5 stuffs, 324 lbs. of lime
and 152 lbs. of soda ash are used; and for No. 4 stuff 378 lbs. of
lime and 190 lbs. of soda ash, and the boiling in each case is kept
up for twelve hours, under a pressure of 30 lbs., the operation
being conducted in boilers which revolve horizontally.
In boiling the finest qualities of rags, it is considered
preferable to boil with lime alone, which is believed to be less
injurious to delicate fibres than caustic soda. Dunbar[14] gives
the following proportions of 70 per cent. caustic soda per cwt. of
rags:--
S. P. F. F. F. is boiled with lime alone, then washed in the
boiler, and again boiled with 2 per cent. of soda ash.
S. P. F. F. is boiled with 12 lbs. of (70%) caustic soda per cwt.
S. P. F " " 14 " " " "
Fines " " 7 " " " "
Seconds " " 6 " " " "
L. F. X. " " 20 " " " "
C. L. F. X. " " 27 " " " "
C. C. L. F. X. " " 30 " " " "
F. F. " " 15 " " " "
These are all boiled at a pressure of from 20 to 25 lbs. for 10
hours, in stationary boilers without vomit, and also in boilers
revolving horizontally. In some mills, where the best qualities of
paper are made, iron boilers are objected to, as small particles
of oxide of iron are apt to become dislodged from the interior of
the boiler, and produce discolouration of the paper. In such cases
wooden vats, with mechanical stirrers, are employed; sometimes a
jacketed boiler is used.
[Illustration: Figs. 11 and 12.]
=Washing and Breaking.=--The removal of the dirty water resulting
from the boiling is effected in the washing and breaking engine, or
"rag engine," as it is commonly called, which is constructed on the
same principle as the beating engine, but is provided with an extra
drum, called the _drum-washer_, which, being covered with wire
gauze, allows the washing waters to escape without permitting the
fibrous stuff to pass through. The rag engine, having been invented
by a Dutchman, acquired, and still retains, the name of the
_Hollander_, and although it has been considerably improved upon,
its principle is still retained in the modern engines, of which
there are many different forms. The ordinary rag engine, Figs. 11
and 12, consists of a cast-iron trough A, about 10 feet long, 4½
feet wide, and 2½ feet deep, and rounded at the ends, and is firmly
bolted to a wooden foundation. It is provided with a partition
termed the _midfeather_ B, of such a length as to have the trough
of uniform width round it. A cylinder, or _roll_, C, furnished with
a series of steel knives, rotates in one of the divisions formed
by the midfeather, and the floor of the trough in this division is
inclined in such a manner as to cause the pulp, as it travels, to
pass under the roll. Beneath the roll is the _bed-plate_, which
is fitted with a series of steel knives _c c_ similar to those on
the exterior of the roll. The distance between the knives of the
roll and the bed-plate is regulated by levelling screws, which
are so adjusted that both ends of the roll are raised at the same
time, which is a great improvement upon the older types of breaking
engines in which only one end of the roll was raised, whereby the
knives became unequally worn. By the present method of regulating
the distance between the respective sets of knives, any required
degree of fineness can be given to the fibrous substances treated.
The roll is generally caused to rotate at a speed of about 230
revolutions per minute, causing the water and rags to circulate in
the engine and to be constantly under the action of the knives. In
the other division F F of the trough is the drum-washer H, which,
being covered with fine gauze wire, allows the water to enter, but
keeps back the fibrous material. The ends of the drum are formed
of two discs of wood, generally mahogany, upon which the coarse
gauze is fastened as a backing, and this is covered with the fine
wire gauze. The interior of the drum is sometimes furnished with a
series of buckets, which conduct the water to a trough in the axis
of the drum, by which it is led away. This is also accomplished by
dividing the interior of the cylinder into compartments by means of
a partition. The drum-washer is so arranged that it can be wholly
raised out of the trough, which is necessary in certain parts of
the operation, when the removal of the liquid is not required; or
it can be partially raised, or otherwise, according to requirement.
The floor of the compartment containing the roll C is inclined at
D, so as to cause the pulp to pass directly under the roll, and at
D′ is the _backfall_, over which the pulp travels to the opposite
side of the midfeather.
In working the rag engine, it is first partly filled with water,
and then set in motion; the boiled stuff is then gradually put
in, and a constant supply of clean water is run in from a cistern
provided with means of preventing sand or other impurities from
finding their way into the engine. It is of the utmost importance
that the water should be abundant and of good quality, more
especially as the material (rags) is mostly required for making
the finer qualities of paper. In this respect the county of Kent
and a few other localities on the chalk formation are considered
specially suitable for this particular manufacture.
With respect to the driving of the engines, this was formerly
effected by what is called _toothed gearing_, but cog-wheels were
afterwards replaced by iron spur-wheel gearing, which enabled
manufacturers to drive four or more engines from one source of
power, by continuing the line of shafting and spur-wheels; but even
with small rolls the wear and tear on this system was considerable,
while it was quite inadequate to the driving of a number of
large rolls of 30 inches in diameter, such as are now used. The
introduction of belt-gearing, by Messrs. G. and W. Bertram, proved
to be a great improvement on the older system, and it is found that
the rags are broken not only more uniformly, but in less time, as
the rolls work more steadily on the plates than with any system of
wheel-gearing, while the various working parts of the engine last
longer than when subjected to the vibrating action of wheel-gearing.
[Illustration: Fig. 13.]
=Bertrams' Rag Engine.=--This engine, of which a drawing is shown
in Fig. 13, may be used either as a washing and breaking engine,
potcher, or beater. It is provided with double lifting gear, and
has "all sweeps, curves, and angles" of the most improved design
to save lodgments and ensure steady and thorough travelling of
the pulp. The drum-washer is shown lifted by rack and pinion and
worm gear, and empties down the midfeather direct to mouthpiece.
The emptying can be done by spout and pipe, or by a chamber cast
on the engine, down back or front side, as well as through the
midfeather; but it is not advisable that it should be emptied down
the midfeather if the rag engine is to be used as a beater.
[Illustration: Fig. 14.]
=Bentley and Jackson's Rag Engine.=--This form of engine is shown
in Fig. 14. The trough is of cast-iron, and made whole, and the
engine can be obtained of any required dimensions. The trough is
provided with a sand-well, cast-iron grate, and cock in front of
the roll, and a sand-well, cast-iron grate, and brass valve on
the back of the midfeather, a brass let-off valve and a brass
waste-water valve. The bottom of the trough is "dished," to prevent
the stuff from lodging. There are two movable bridge trees, fitted
with pedestals and brass steps, and wrought-iron lifting links
and screws, worm-wheels, worms, cross-shaft and hand-wheel for
simultaneously lifting the roll on both sides. The roll is covered
by a polished pitch-pine cover. The drum-washer may have either
iron or wooden ends, has strong copper brackets, and is covered
with brass backing and covering wires, mounted on a wrought-iron
shaft, and carried by cast-iron stands, fitted with improved
lifting gear, driving-wheels, and pulley.
When the engine is set in motion by the revolving shaft or spindle,
the combined action of the knives of the roll and bed-plate causes
the rags, which circulate in the water, to be gradually cut into
small fragments, and the operation is kept up until the rags are
converted into what is technically termed _half-stuff_. While
this process is going on, fresh water is constantly supplied by a
pipe at the end of the washing-engine; and when it is found that
nothing but clear water escapes from the drum-washer, this is
raised, and the spindle bearing the roll is lowered, so as to bring
the respective knives closer together, to enable them to cut the
reduced material still finer.
=Draining.=--When the material is sufficiently _broken_, as it is
termed, the engine is then emptied by means of its valves, and
the contents run into large vats or _drainers_, furnished with
perforated zinc floors, in which it is allowed to drain thoroughly;
and in order to remove the water more effectually, the pulp is
afterwards pressed, either by an extractor or a centrifugal
drainer, which dries it sufficiently for gas-bleaching, or for
treatment in the _potcher_ or _poacher_. This is a larger engine
than the washer, and instead of the cylinder and bars, has a
hollow drum which carries on its periphery a number of cast-iron
paddles, which thoroughly agitate the pulp, and thus render it
more susceptible of being freely and uniformly acted upon by the
bleaching agent. The drum-washer of this engine should have a finer
wire than is used for the breaker.
=Torrance's Drainer.=--This machine, which has been extensively
used, is manufactured by Messrs. J. Bertram and Son, of Edinburgh.
It consists of a perforated cylindrical box, enclosed in a fixed
case, which revolves at about two hundred and fifty revolutions per
minute. The machine is capable of treating about 4 cwt. of pulp per
hour.