_BLEACHING._
Bleaching Operation.--Sour Bleaching.--Bleaching with Chloride
of Lime.--Donkin's Bleach Mixer.--Bleaching with Chlorine
Gas (Glaser's Process).--Electrolytic Bleaching (C. Watt's
Process).--Hermite's Process.--Andreoli's Process.--Thompson's
Process.--Lunge's Process.--Zinc Bleach Liquor.--Alum Bleach
Liquor.--New Method of Bleaching.
=Bleaching Operation.=--The half-stuff treated in the
breaking-engine is run into the potcher, and the water it contains
is lifted out as far as practicable by the washer; the spent liquor
from the presses or drainers is then run in in lieu of water, and
as much fresh bleaching liquor as may be required is then measured
in, and in from two to six hours the pulp becomes perfectly
white. "However well managed a mill may be," says Mr. Arnot, "it
is scarcely possible to avoid having a small residue of unused
chlorine in the liquid which drains from the bleaching stuff." The
rule, therefore, is to use this liquor in the way above indicated,
by which the unexhausted chlorine, operating upon fresh half-stuff,
becomes available, and is, therefore, not wasted. "That as little
of this residual chlorine as possible may remain in the stuff,"
Mr. Arnot further observes, "when put into the beating-engine,
powerful hydraulic presses are employed to compress the stuff
and squeeze out the liquid. These presses should be large enough
to contain easily the whole contents of a poaching-engine, and
of unexceptional workmanship. The perforated lining especially
should be carefully prepared and properly secured. I have seen
much trouble from negligent workmanship in this respect. Recently
I examined a number of samples of press drainings, and found
the unexhausted chlorine to vary very much--from a few grains of
bleaching powder per gallon to about one ounce."
Sometimes it is the practice to partly fill the potcher with water,
and the engine being set in motion, the half-stuff is gradually
introduced until the full charge has been given, and the stuff is
then washed for some time, after which the drum-washer is raised,
and the bleaching liquor then run in, care being taken that the
necessary quantity is not exceeded, otherwise the fibre will
suffer injury from the chemical action of the bleaching agent.
When vitriol is employed to liberate the hypochlorous acid, the
vitriol, previously diluted with water, should be placed in a
small lead-lined tank in such a position that the acid liquor may
slowly trickle into the engine at the rate of 1 lb. of sulphuric
acid in twenty minutes. As soon as the bleaching is complete the
stuff is emptied into large stone chests, each of which will hold
the contents of two engines. On the bottom of these chests are
perforated zinc drainers, while a similar drainer runs up the back
of each chest. The bleached stuff is allowed to remain as long as
may be convenient in these chests, after which it is removed to
the beating or refining engines. In some mills the bleaching is
effected in the breaking-engine, while at other mills the operation
is performed in the beating-engine.
In bleaching it is considered to be more advantageous to employ
moderately strong liquors rather than weaker ones, inasmuch as
the object is effected in less time than when weaker liquors are
employed. An extreme in the opposite direction, however, must be
avoided, since a very strong bleach will inevitably cause injury to
the fibre. Sometimes the potchers are fitted with steam-pipes, in
order that the diluted bleaching liquor may be heated, if required,
to facilitate the operation. If the temperature be raised too high,
however, the effect upon the fibre will be at least as injurious
as if too strong a bleach were employed. It must also be borne in
mind that in either case, after the pulp has been bleached and the
liquor allowed to run off, the mass has to remain some time--even
if pressed to remove as much of the liquor as possible--in direct
contact with the products resulting from the decomposition, and
probably some undecomposed hypochlorite also, which will continue
their chemical action upon the fibre until removed by washing, or
neutralised by one or other of the agents employed for the purpose.
=Sour Bleaching.=--When the bleaching liquor, after acting upon
the half-stuff for some time, has become partially exhausted,
dilute sulphuric acid--about one part acid to fifteen parts of
water--is added, which, by liberating hypochlorous acid, hastens
the bleaching considerably, and when the chemical action resulting
from this treatment is nearly complete, the spent liquor is
allowed to drain away, and fresh bleaching liquor is introduced,
the strength being regulated by the progress made in the first
case, which will depend upon the character of the fibre treated.
In the second application of the bleach no acid is used. When
sulphuric acid is added to the bleaching liquor, as above, the
process is termed _sour bleaching_. Sometimes hydrochloric acid is
used for this purpose, but in either case it is necessary to avoid
employing the acid in too concentrated a state, or in too great a
quantity, otherwise free chlorine will be liberated, which, besides
being injurious to the health of the workmen and the surrounding
machinery, also involves loss, while the colour and strength of the
fibre itself will also be impaired. In some mills the bleaching is
effected in the beating-engine, the bleaching liquor being pumped
in while the machine is in motion.
Respecting the time which the bleaching operation should occupy,
there appears to be some difference of opinion, or, at all events,
the practice seems to vary in different mills, but there is, no
doubt, an advantage, so far as ultimate yield is concerned, in
moderately slow bleaching at a moderate temperature, inasmuch as
there is less risk of chemical action upon the cellulose itself
than when strong liquors are used, at a higher temperature, with a
view to hasten the operation and economise the bleaching powder.
[Illustration: Fig. 20.]
=Bleaching with Chloride of Lime= (_Preparation of the Bleaching
Liquor_).--Chloride of lime, or hypochlorite of lime, commonly
called bleaching powder, when well prepared, contains from 32 to
35 per cent. of active chlorine. Being readily decomposed by the
air, and also by heat, this substance should always be stored in a
cool and dry place until required for use. A solution of bleaching
powder is generally prepared in large tanks lined with lead, which
are provided with agitators or stirrers, so that the powder, when
added to the water, may be freely diffused, and its active material
dissolved in the liquid. A machine, or "bleach-mixer," manufactured
by Messrs. Bryan Donkin and Co., of Bermondsey, is shown in Fig.
20, which is so constructed that the strong bleach liquor does
not destroy it. The device for agitating the contents of the tank
explains the principle of the machine. To prepare the bleaching
liquor, about ½ lb. of chloride of lime to each gallon of water
is used, which yields a liquor at about 6° T. When the required
quantity of bleaching powder and water have been introduced into
the mixer and sufficiently agitated, the vessel is allowed to
rest until the residue, which chiefly consists of free lime and
its carbonate, has subsided, when the clear liquor may be run off
for use. When all the clear liquor has been drawn off the residue
should be washed with water, and after again settling, the washing
water run off, and fresh water added, these washings being repeated
as often as necessary to remove the last traces of the "bleach," as
it is technically called. The washing waters may be used in lieu
of water in the preparation of fresh bleaching liquors. In some
mills the bleaching powder is mixed with from 2 to 3 times its
weight of water; the mixture is then well agitated and the residue
afterwards allowed to settle, the clear solution being afterwards
drawn off and the residue then washed as before. In either case the
residual matter is afterwards well drained and then cast aside. The
bleaching liquor is stored in large tanks ready for use, from which
it is withdrawn as required by means of a syphon or otherwise.
=Bleaching with Chlorine Gas= (_Glaser's Process_).--This method of
bleaching is not so much adopted in England as formerly, but has
found much favour in Germany; indeed, within the past few years,
namely, in March 3rd, 1880, a process was introduced by Mr. F. Carl
Glaser for treating straw, in which, after boiling with caustic
soda as usual, the pulp is bleached by the action of chlorine
gas. The straw, after being separated from weeds by a slight or
superficial picking, is cut into pieces of from ⅓ to ⅔ of an inch
in length. The cut straw is then placed in a rotary boiler for
about four hours, at a pressure of about 4 to 4½ atmospheres, in a
solution composed of 29 lbs. of caustic soda at 71°, and 48 lbs.
of calcined soda at 90°, rendered caustic, for every 220 lbs. of
straw. After boiling, the dirty ley is drawn off, and the boiled
straw subjected to two washings with water. It is then conveyed
to the washing-engine, where it is washed for an hour; the drum
of the machine should have a sieve or sifter, the apertures
of which are about 60 to the square inch. The washed straw is
next dried by centrifugal force in a hydro-extractor, until it
contains about 70 per cent. of water, which is necessary for the
action of the chlorine gas. To effect this, so as to obtain not
very solid or close cakes of straw, the holes of the wire of the
hydro-extractor should not be more than 50 to the square inch.
The cakes of straw thus formed are then exposed to the action
of chlorine in leaden chambers of the ordinary kind, in which
they are placed in layers upon hurdles, or upon shelves. If the
chlorine is produced by hydrochloric acid, for every 220 lbs. of
unboiled straw, 51½ lbs. of the acid at 20° B., and a corresponding
quantity of 70 per cent. peroxide of manganese are used. After
the bleaching operation, the acid formed is removed by washing in
a washing-engine. If a complete reduction of the fibres has not
been effected by the bleaching, this may be completed by the aid
of well-known machines, and either before or subsequent to the
after-bleaching there is used for 220 lbs. of straw about 4½ lbs.
of chloride of lime, at 35° [per cent.?] The patentee then gives
the following explanation:--"As pine wood or fir is chemically
freed from its colouring principle and transformed into fibres as
well as cellulose, the object of the intense action of the chlorine
is to destroy the mucilage of the straw, as well as the incrusting
matters which have not been destroyed by the boiling with caustic
soda, and consequently to strip or expose and open the fibres." It
will be readily seen that this process bears a close resemblance to
Mr. C. Watt's wood-pulp process.
=Electrolytic Bleaching= (_C. Watt, jun.'s, Process_).--At the
present time, when the means of obtaining the electric current for
practical purposes in the arts have so far exceeded that which
would have been deemed probable some forty years since, we find
that many ingenious processes, which were found to be unpractical
at that time from the want of cheap electrical power, have since
reappeared in the form of patented inventions, which would seem to
possess every merit--but originality.
So long ago as September 25th, 1851, the author's brother, Mr.
Charles Watt, obtained a patent for, amongst other claims,
decomposing chlorides of sodium and potassium, and of the metals of
the alkaline earths into hypochlorites by electricity. It may be
well to make a few extracts here from his specification in order
that some of the subsequent patents, to which we shall refer, may
be traced to what may, perhaps, be considered their true origin.
In the specification in question, the inventor says:--"The third
part of my invention consists of a mode of converting chlorides of
potassium and sodium, and of the metals of the alkaline earths,
into hypochlorites and chlorates, by means of a succession of
decompositions in the solution of the salt operated upon, when
induced by the agency of electricity.... Electricity first
decomposes the chloride, the chlorine being eliminated at one of
the electrodes, and the alkaline or earthy metallic base at the
other electrode.... The liberated chlorine will, when it is set
free, combine with a portion of alkali or alkaline earth in the
solution, and a hypochlorite will be formed. The hypochlorite thus
formed will, by the continued action of heat, be resolved partly
into a chlorate of the alkali or alkaline earth, and partly into
a chloride of the metallic base, and the chloride will again be
subjected to decomposition, and a hypochlorite formed.... If I
desire to produce a hypochlorite of the alkali or earth, I merely
keep the vessel warm ... and continue the process until as much
of the saline matter has been converted into a hypochlorite as
may be required for the purpose to which the solution is to be
applied. This mode of forming a hypochlorite of the alkalies and
alkaline earths may be used for preparing a bath for the purpose of
bleaching various kinds of goods, and the bath may be strengthened
[recuperated] from time to time by the action of the electric
current."
Thus it will be seen that this specification clearly described a
process by which the chlorides of sodium and potassium, and of the
metals of the alkaline earths (chloride of magnesium, for example),
may be converted into hypochlorites by electrolysis, and the
hypochlorite solution obtained used for the purposes of bleaching.
It would appear difficult to conceive how any subsequent patent for
accomplishing the same thing, and using essentially the same means,
can claim originality in the face of such "prior publication" as
was effected by the usual "Blue-book," which any person can buy for
eightpence.
=Hermite's Process.=--The following description of this process
has been furnished by the engineers engaged in connection with
the process to the _Paper Trade Review_:--"Briefly described,
the Hermite process consists in manufacturing a solution of high
bleaching power by electrolysing an aqueous solution of magnesium
chloride. The salt is decomposed by the current at the same time
as the water. The nascent chlorine, liberated from the magnesium
chloride, and the nascent oxygen, liberated from the water, unite
at the positive pole, and produce an unstable oxygen compound of
chlorine of very high bleaching power. The hydrogen and magnesium
go to the negative pole; this last decomposes the water and forms
magnesium oxide, whilst the hydrogen is disengaged. If in this
liquid coloured vegetable fibre is introduced, the oxygen compound
acts on the colouring matter, oxidising it. Chlorine combines with
the hydrogen to form hydrochloric acid, which finding itself in the
presence of magnesium in the liquid combines with it, and forms the
initial chloride of magnesium."
=Andreoli's Process.=--This process consists, avowedly, in
bleaching pulps "by means of hypochlorite of sodium, produced
by electrolytical decomposition of a solution of chloride of
sodium." In carrying out his process, M. Andreoli uses as an
electrolyte "concentrated or non-concentrated sea-water, or a
solution of chloride of sodium, the specific gravity of which
varies according to the quality and nature of the materials to be
treated. Generally the solution to be electrolysed works better
with a density of 8° to 12° B., but although salt is cheap, and the
solution when exhausted may be regenerated by passing an electric
current, I always endeavour to have when possible (_sic_) a weak
solution, and with some kinds of pulp an electrolyte having the
density of sea-water (3° B.) is sufficiently strong to bleach."
The foregoing are the only electrolytic processes for bleaching
fibres that need recording, and we fancy there will be little
difficulty in tracing the resemblance between the two latter and
the process of Mr. C. Watt.
=Thompson's Process.=--This process, for which a patent was
obtained on February 3rd, 1883, may be thus briefly described:--In
bleaching linen fabrics the material is boiled for about three
hours in a solution of cyanide of potassium or sodium--about
half an ounce of the salt to each gallon of water--to remove the
resinous matter from the fibre, so that the cellulose may be
exposed to the action of the bleach. The fabric is then washed,
and again boiled for three hours more in a similar solution, and
after being again washed is ready for bleaching. With cotton the
preliminary boiling is not necessary, unless the material is
greasy, in which case a solution of half the strength and two
hours' boiling is sufficient. In ordinary cases cotton is not
boiled at all, but is simply washed in cold water and squeezed.
In bleaching, all vegetable fibres are treated in the same way,
the only difference being in point of time. The cotton or linen,
after being treated as described, is then piled somewhat loosely
in an air-tight vessel, 9 lbs. of cloth to the cubic foot of space
being considered sufficient. The vessel is then filled with a weak
solution of bleaching liquor, consisting of about one ounce of dry
bleaching powder to each gallon of water. "After the vessel has
been filled, the liquor is immediately run out, and is replaced
by an atmosphere of carbonic acid, which quickly liberates the
chlorine on the fibre, and thus decomposes the water, uniting with
the hydrogen and liberating the oxygen, the result of which, is
to bleach the fibre or fabric. In about an hour the whole of the
bleaching liquor in the fibre will have been thus decomposed, and
this operation must be repeated until the material is of the proper
whiteness to be withdrawn from the action of the chlorine. The
material is then washed and squeezed. Chlorine, however, always
leaves these materials of a yellowish white." To remove this tint,
the material is passed through a solution of oxalic acid--about 2
oz. to the gallon--squeezed as it passes out of this solution, and
then passed through another solution made by dissolving ¼ grain
of triethyl rose aniline to the gallon of water, or 20 grains
of indigo, as may be preferred. To this solution oxalic acid is
added until it becomes of an opaque but bright turquoise blue. The
material, after washing, is then white.
The patent describes and illustrates the apparatus to be used
in conjunction with certain parts of existing apparatus used in
bleaching.
=Lunge's Process.=--In this process acetic acid is used in place
of hydrochloric or sulphuric acids, etc., to set free the chlorine
or hypochlorous acid, in the ordinary method of bleaching with
hypochlorite of lime, or bleaching powder, which, the inventor
says, "combines all the advantages of the materials formerly
employed, without any of their drawbacks.... The price is no
impediment, for a minimal quantity is sufficient, the same being
regenerated over and over again. At first acetic acid and chloride
of lime decompose into calcium acetate and free hypochlorous
acid. In the bleaching process the latter yields its oxygen,
hydrochloric acid being formed. The latter instantly acts upon the
calcium acetate; calcium chloride is formed and acetic acid is
regenerated, which decomposes a fresh quantity of chloride of lime,
and so forth. Consequently the smallest quantity of acetic acid
suffices for splitting up any amount of chloride of lime.... The
hydrochloric acid formed is never present in the free state, as it
instantly acts upon the calcium acetate. This is very important,
since hydrochloric acid weakens the fibre by prolonged contact,
whilst acetic acid is quite harmless. Since there are no insoluble
calcium salts present, the operation of 'souring' after bleaching
is quite unnecessary; this not merely saves the expense of acid,
and of the subsequent washing of the fabrics, but it also avoids
the danger, especially present in the case of stout fabrics, of
leaving some of the acid in the stuff, which concentrates on drying
and weakens the fibre; it may also prove injurious in subsequent
dyeing operations. But in the new process no free acid is present
except acetic acid, which has no action upon fibre, even in its
concentrated state and at a high temperature."
The acetic acid may be employed in various ways, including the
following:--A small quantity of the acid may be added from the
first to the bleaching liquor; or the fabric, after being treated
in the ordinary way with a solution of the bleaching powder, may
be steeped, without previous washing, in water containing a little
acetic acid; or the fabric may be steeped in water acidulated with
acetic acid, and bleaching liquor afterwards run in slowly and
gradually, with continuous agitation in the usual way. In the case
of hard water, or of impure bleaching liquors, a good deal of the
acetic acid would be consumed in neutralising the lime; in this
case, some hydrochloric or sulphuric acid may be added, but only
sufficient for the purpose, so that no acid but hypochlorous or
acetic acid exists in the free state. The process is applicable to
the bleaching of vegetable fibres, whether spun or in the unspun
state, and for bleaching paper pulp made from rags, wood, straw,
esparto, etc. Besides acetic acid, any other weak organic acid of
an analogous nature may be used.
=Zinc Bleach Liquor.=--Strong acids are often objectionable for
liberating chlorine from bleaching powder, and especially in
bleaching some classes of paper pulp. If a solution of sulphate
of zinc be added to one of bleaching powder, sulphate of lime is
precipitated, and the zinc hypochlorite formed at once splits up
into zinc oxide and a solution of free hypochlorous acid. Chloride
of zinc acts similarly; for a saturated solution of zinc in
hydrochloric acid decomposes as much bleaching powder as half its
weight of concentrated oil of vitriol.--_Varrentrapp._ Consequently
zinc salts can be employed in place of sulphuric acid, and thus
bleach the paper pulp very quickly. When this mixture is employed
in bleaching pulp, the precipitated sulphate of lime resulting from
the reaction and also the oxide of zinc formed, remain in the pulp,
and serve as loading materials.
=Alum Bleach Liquor.=--Orioli[19] recommended for use, in
paper-mills especially, a bleach liquor made by decomposing
equivalent quantities of a solution of chloride of lime and
sulphate of alumina, formerly known as _Wilson's Bleach Liquor_.
Sulphate of lime is precipitated, and hypochlorite of aluminium
remains in solution; this being a very unstable salt can be applied
for bleaching without the addition of an acid, splitting up into
aluminium chloride and active oxygen. Consequently the liquid
always remains neutral, and the difficulty caused by the obstinate
retention of free acid in the fibre, by which it is strongly acted
upon in drying, in this case does not exist. The aluminium chloride
also acts as an antiseptic, so that the paper stock may be kept for
many months without undergoing fermentation or other decomposition.
The solution is allowed to act for about ten minutes in the
engine.--_Lunge._
=New Method of Bleaching.=--Young's Paraffin Oil Company have
recently introduced what they term an "intermediate oil for
paper-making," to be used with alkali in the boiling of rags and
esparto, for the purpose of increasing the bleaching power of
the powder, and producing a softer pulp, at the same time having
no smell. Several well-known paper-makers have tried, and speak
favourably of it. The quantity of oil to be added to the caustic
varies for different stock, but may be said to average about 1½
gallon per ton.[20]