_RECOVERY OF SODA FROM SPENT LIQUORS._
Recovery of Soda.--Evaporating Apparatus.--Roeckner's
Evaporator.--Porion's Evaporator--American System of Soda
Recovery.--Yaryan Evaporator.
=Recovery of Soda.=--Probably one of the most important
improvements in modern paper-making, at least from an economical
point of view, is the process of recovering one of the most costly,
and at the same time most extensively used, materials employed in
the manufacture--soda. While not a great many years since (and
in some mills is still the case even now), it was customary to
allow the spent soda liquors resulting from the boiling of various
fibres to run into the nearest rivers, thus not only wasting a
valuable product, but also polluting the streams into which they
were allowed to flow, means are now adopted by which a considerable
proportion of the soda is recovered and rendered available for
further use. The means by which this is effected are various,
but all have for their object the expulsion of the water and the
destruction of the organic matters dissolved out of the fibrous
substances in the process of boiling with caustic soda solutions.
One of the main objects of the various methods of recovering the
soda from spent liquors is to utilise, as far as practicable, all
the heat that is generated from the fuel used, whereby the process
of evaporation may be effected in the most economical way possible.
The principle upon which the most successful methods are based is
that the flame and heat pass over and under a series of evaporating
pans, and through side flues, by which time the heat has become
thoroughly utilised and exhausted. When all the water has been
expelled, the resulting dry mass is ignited and allowed to burn
out, when the black ash that remains, which is carbonate of soda,
is afterwards dissolved out, and the alkaline liquor causticised
with lime in the usual manner. According to Dunbar, 8 cwt. of
recovered ash and 4½ cwt. of good lime will produce 900 gallons
of caustic ley at 11° Tw. The liquor is then pumped into settling
tanks, from which it is delivered to the boilers when required.
[Illustration: Fig. 64.]
=Evaporating Apparatus.=--An ordinary form of evaporator for the
recovery of the soda is shown in Fig. 64. It consists of a chamber
A, of the nature of a reverberatory furnace, lined with fire-brick,
the bottom of which is slightly hollowed. Above this is a tank
B containing the liquor, which is run down into the chamber as
required by means of a pipe C, provided with a tap. At one end of
the chamber is a furnace D, the flame of which passes through
the chamber and over the surface of the liquor lying upon the
floor, heating the chamber, evaporating, and at last incinerating,
its contents, and at the same time warming the liquor in the
tank above, and evaporating some of its water. The products of
the combustion in the furnace, and of evaporation, pass by the
flue into a chimney, and escape thence into the air. There is a
door E in the side of the furnace near the level of the floor of
the chamber, and this is opened from time to time to enable the
workmen to stir and move about the contents of the chamber, and
finally, when the process is sufficiently advanced, to draw out
the residue. The first effect produced is the reduction of the
liquor to the consistence of tar. Later on, a white crust, which
is the incinerated material, forms on the surface, and is drawn
on one side by the workmen, so as to allow of fresh crust being
formed. When all the charge has become solid it is drawn. The
charge is usually withdrawn before the conversion into carbonate is
completed; it is then raked out into barrows and placed in a heap,
generally in a shed or chamber, open on one side, but sometimes in
a closed brick-chamber or den, where the combustion continues for
several weeks. The result is the fusion of the material into a grey
rocky substance, which consists chiefly of carbonate and silicate
of soda.
Various modifications of the esparto evaporator and calciner have,
however, been introduced since the recovery of soda has become more
general, and are in use at various works, all having for their
main object the economising of fuel and the utilising of the waste
heat of the fire, which in the old-fashioned calciner goes up the
chimney and is lost. The leading principle, of all of them is to
use the waste heat in concentrating the liquor preparatory to its
being run into the part where the calcination is to be effected.
This is done by so extending and widening out the flue as to cause
the heated air and flame, after they have performed their function
in the calcination, to pass over or under their layers of liquor,
lying upon shelves or floors in such a way that the liquor shall
become more and more concentrated as it approaches the calciner by
successive steps or gradations.[32]--_Dr. Ballard._
[Illustration: Fig. 65.]
=Roeckner's Evaporator.=--This apparatus, an illustration of which
is shown in Fig. 65, is thus described by Dr. Ballard, medical
officer of the Local Government Board, who was specially appointed
by the board to investigate the effluvium nuisances which arise
in connection with certain manufacturing industries. "In this
apparatus there is above the calcining floor a series of shelves
or shallow pans, alternating in such a manner that the liquor
flowing from the tank above into the uppermost of them, flows,
after a partial evaporation, over the edge of the shelf into the
shelf or shallow pan next below, and in this way from shelf to
shelf, still becoming more and more concentrated until it reaches
the final floor, over which the flame from the actual fire plays,
and where the first part of the calcination is effected. The
heated air, in passing to the chimney, passes over each of these
shelves in succession, heating them and concentrating the liquor
upon them. There is between the lower shelves an arrangement for
causing the liquor to pass from the upper to the lower by means of
a pipe, instead of its running over the edge. At the top of all
is a covered tank, where the temperature of the liquor is raised
before it is run into the evaporator. In order to promote the
heating of the liquor in this tank, the lower part of the tank is
made to communicate by side pipes with tubes passing across the
evaporator near the fire, as, for instance, at the bridge and at
the further end of the calcining floor. In this way a circulation
of liquor is set up which serves to heat the liquor in the tank
more effectually. A pipe from the top of the tank leads to the
chimney-shaft, conducting any vapours into it. As the incinerated
crust forms it is raked on one side, and when sufficient of it has
accumulated it is drawn to an opening (provided with a damper) at
the side or end of the floor, and discharged down this opening
into a brick chamber below, which is inclosed by iron doors, and
from which a flue conducts the vapours that arise during the final
fusion through the fire in such a way as to consume them." By
recent improvements Mr. Roeckner has constructed an apparatus for
condensing and rendering inoffensive the vapours eliminated from
the liquor during its evaporation on the successive shelves of his
evaporator.
=Porion's Evaporator.=--This evaporator and incinerating furnace
much resembles in principle an ordinary reverberatory furnace,
except that it is provided with paddle agitators, which project the
liquid upwards, causing it to descend in a spray, thus increasing
the surface of the liquid coming in contact with the hot air and
current of smoke traversing the furnace. By this method the expense
of fuel is greatly reduced. The residue is in a state of ignition
when it is withdrawn from the furnace, and is piled in heaps so
that it may burn slowly. When the combustion is complete, the
resulting calcined mass is treated with water, and the carbonate
of soda formed is afterwards causticised in the usual way. About
two-thirds of the soda is thus recovered.
=The Yaryan Evaporator.=--Mr. Homer T. Yaryan, of Toledo, Ohio,
U.S.A., has introduced some important improvements in evaporating
apparatus, which have been fully recognised in America, and appear
to have been attended with success. The principle involved is that
of multiple effects, in which the evaporation takes place while
the liquid is flowing through heated coils of pipe or conduits,
and in which the vapour is separated from the liquid in a chamber,
at the discharge end of the coils, and is conducted to the heating
cylinder surrounding the evaporating coils of the next effect,
from the first to the last effect. The objects of the invention
are: (1) to provide extended vaporising coils or conduits and
increased heating surface for each liquid feed supply in the
heating cylinders, and provide improved means for feeding the
liquid, whereby each set or coil of vaporising tubes will receive
a positive and uniform supply of liquid without danger of the
feed ducts being clogged by extraneous matter; (2) to positively
control the amount of liquid fed by the pump to the evaporating
coils, and make it more uniform than heretofore, regardless of the
speed of the pump; (3) to provide improved separating chambers at
the discharge ends of the vaporising coils so as to better free
liquid and solid particles from the vapours; (4) to provide for
the successful treatment of the most frothy liquids by causing
the vapours carrying solid and liquid particles to pass through
catch-all chambers, where they are arrested and precipitated and
then returned to the evaporating coils; (5) to secure a more
positive flow and circulation of liquid from the evaporating
cylinder of one effect to another, under the influence of a better
vacuum than heretofore in multiple-effect vacuum evaporating
apparatus; (6) to provide for transferring a better concentrated
liquid into the separating chamber containing cooler concentrated
liquid in direct connection with the condenser and vacuum pump, so
as to equalise the temperature of the two liquids, and then draw
off both by one tail pump.
[Illustration: Fig. 66.]
[Illustration: Fig. 67.]
[Illustration: Fig. 68.]
[Illustration: Fig. 69. Fig. 70. Fig. 71.]
[Illustration: Fig. 72. Fig. 73. Fig. 74.]
The present invention comprises a series of important improvements
on an apparatus described by Mr. Yaryan in a former English patent,
No. 14,162 (1886), and covers a number of important modifications
in construction, whereby improved results are secured. It is only
necessary, therefore, to give the details of the new patent,
No. 213 (1888), since it embodies the latest improvements which
practical working of the apparatus has suggested. In reference to
the accompanying illustrations the following details are given:
Fig. 66 represents a side elevation of the apparatus; Fig. 67,
the front elevation; Fig. 68, a top plan view; Fig. 69, a vertical
section of a cylinder showing the evaporating coils and separating
chamber; Fig. 70 is a horizontal section; and Fig. 71, a vertical
section of the separating chamber shown in Fig. 69, both on
reduced scale; Fig. 72 is a broken section of the cylinders for
showing the connections of the liquid pipe from the first to the
third effect evaporator; Fig. 73 is a rear end view of a cylinder
with manifold, the feed pump and a sectional view of the feed
box and supply devices; Fig. 74 represents a sectional view, on
enlarged scale, of the manifold and a feed duct; Fig. 75 is an
inside view of a return bend-head; Fig. 76 an inside view of a
section of the head; Fig. 77, a vertical cross section thereof
on enlarged scale, and showing the partitions forming cells
for connecting the ends of the evaporating tubes; Fig. 78 is a
vertical longitudinal section of a catch-all chamber; Fig. 79, a
cross section thereof; Fig. 80 is a vertical longitudinal section
of new form of separating chamber; and Fig. 81 represents a side
view and Fig. 82 an end view of the cylinders for showing the pipe
connection between the separating chambers of the third and fourth
effect evaporators.
[Illustration: Fig. 75. Fig. 76. Fig. 77.]
[Illustration: Fig. 78. Fig. 79. Fig. 80.]
The evaporating cylinders are mounted upon a framework Y, supported
upon columns X X, or other suitable supports. The apparatus is
shown arranged as quadruple effect, with four connected cylinders,
but multiple effect apparatus may be constructed with an increased
number of cylinders up to ten or twelve. The heating cylinders
B^1 B^2 B^3 B^4, containing the evaporating tubes or coils, are
preferably arranged in the same horizontal plane, and are provided
at the discharge ends of the evaporating coils with separating
chambers, A^1 A^2 A^3 A^4, of enlarged diameter, and at the supply
ends of the coils with the coils with return bend ends, C^1 C^2
C^3 C^4. From each separating chamber, A^1, A^2, valve pipe D^1
D^2 D^3 leads into the shell of the next heating cylinder, as
B^2, B^3, B^4, and vapour pipe D^4 leads from the last separator
A^4 to the condenser H, and the vacuum pump H^1. A cylindrical
catch-all chamber E^1, E^2, E^3, E^4, is connected in each vapour
pipe between each separator and each successive heating cylinder,
as shown in Figs. 66, 67, and 68, and in detail in Fig. 75. Gauge
glass and liquid receiving chambers, G^1, G^2, G^3, G^4, connect
with the bottom of each separating chamber for receiving the
liquid as it is separated from the vapour, and a gauge glass _g_
is applied to each of such chambers. Liquid discharge and transfer
pipes _t_, _t^1_, having valves _h_, _h^1_, as best shown in Figs.
66, 68, and 72, lead respectively from chambers G^1, G^2, of the
first and second effect to the manifold feed pipes leading into the
cylinders B^3, B^4, of the third and fourth effect for the purpose
hereafter described. The main steam supply pipe F, having a safety
valve _f_ and stop valve _f^1_, Figs. 66, 67, and 68, connects with
the heating cylinder B^1 of the first effect. The evaporating tubes
1, 2, 3, 4, 5, are expanded or otherwise secured in the tube sheets
_d_ and _e″_ at opposite ends of the cylinders, and are properly
connected at the ends in sets of five to form coils. The outer rear
return-bend head C^1 C^2, etc., are provided on their insides with
numerous short intersecting partition plates _c_, forming single
and double cells, properly arranged for connecting the evaporating
tubes in sets of five, as shown in Figs. 75, 76, 77.
[Illustration: Fig. 81. Fig. 82.]
The heads are pierced with holes _c′_ for connecting the liquid
supply pipes M of the manifolds L. The inner return-bend head T in
the separating chambers are formed like heads C^1 C^2, etc., with
intersecting partition plates _x_, and are provided with discharge
openings _t″_ for every fifth tube, as shown in Fig. 69. Tube sheet
_d_ is made of considerably larger diameter than cylinders B^1
B^2, etc., and acts as a vibrating diaphragm, to accommodate the
expansion and contraction of the tubes. The separating chambers may
be constructed with dash plates _b_ _b_, two or more in number,
having openings _g′_ _g′_ alternately upon opposite sides for the
passage of vapour, and opening _a′_ at the bottom for the passage
of liquid, as shown in Fig. 80. Here a tube sheet _z_ is provided
near the openings of the evaporating tubes, and in such sheet are
set numerous small horizontal tubes _n_, which discharge against a
vertical arresting plate _b′_ set near their open ends. Water and
solid matter are impelled against the plate and thereby arrested
and caused to flow down to the bottom of the chamber. The liquid
feed apparatus consists of a supply tank K, stand-pipe J, feed box
K^1, double pump I, manifold L, and connecting pipes and valves.
The liquid to be evaporated flows from tank K, through pipe _k_, to
stand-pipe J and box K^1, the flow being constant and uniform, and
of the desired quantity, by means of a valve _k′_ having a lever
handle _r′_ which is connected by a cord or chain passing over a
pulley _j_ with float _q_ in stand-pipe J. The valve opening in
pipe _k_ being properly adjusted by means of the float, etc., the
liquid is admitted to the stand-pipe J while the column of liquid
is automatically maintained at any desired height and pressure
regardless of the quantity in the supply tank, by means of the
float _q_, which, as it rises, tends to close valve _k′_, and as
it falls, to open the valve. From the bottom of the stand-pipe J,
nozzle _j′_ discharges a constant and uniform stream of liquid into
feed box K^1. The suction pipe I″ of pump I extends into box K^1,
where it terminates in a turned-down nozzle provided with valve _i_
having a lever handle and float _z_. As a given amount of liquid is
constantly running into the box, should the pump run too fast the
float lowers, partially closing the valve and lessening the amount
of liquid drawn at each stroke of the pump, and preventing air from
being drawn in, since the end of the suction pipe is always sealed
by the liquid. The liquid is forced by pump I into the manifolds
L, from which it flows through the contracted ducts _l_ into the
enlarged feed pipes _m_, as shown in Figs. 73 and 74. Ducts _l_ are
of about one-half inch diameter, and the upper and lower sections
thereof are connected by a union coupling, one portion of which
_l′_ has a reducer with opening one-quarter inch diameter, more or
less, according to the amount of liquid it is desired to feed.
The catch-all chambers E^1 E^2, etc., Figs. 66, 78, and 79, are
provided each at its inlet end _e_, with tube sheet _o_ extending
across its diameter a short distance in front of the opening of
vapour pipe D^1, and in such sheet are fixed numerous longitudinal
tubes _p_ extending to near the opposite head _e′_, so that vapours
carrying watery or solid particles are impelled against the head
and arrested. Liquid and solid matter, arrested in the catch-all
chambers, flow through pipes _v v′_ _v″_ down into the fluid
transfer pipe _t t′_ (Figs. 67, 68, and 72), and thence into the
evaporating coils and through pipe _v‴_ directly to the tail pump
W, Fig. 67. By use of the catch-all chambers the most frothy
liquids can he readily and economically managed. A liquid transfer
pipe _s_, having a valve _h″_, leads directly from receiving
chamber G^3 of the third effect to the separating chamber A^4 of
the fourth effect, the latent heat being carried off in the vapours
drawn by the vacuum pump H^1 into the chamber H, and the finished
liquid of both effects is drawn off through pipe _w_ by one and
the same tail pipe pump W. The water of condensation accumulating
in the heating cylinders B^1 B^2, etc., is transferred from one
to the other through connecting pipes _u u′ u″_ having valves
_y_, shown in Figs. 66, 67, and 68; and finally from cylinder B^4
through pipe _u‴_ directly into condenser H. The specification of
the patent, which those interested will do well to consult, next
describes the operation of the apparatus.
=American System of Soda Recovery.=--Mr. Congdon gives an
exhaustive description[33] of the method of recovering soda
in the United States, from whose interesting paper we extract
the following:--The spent liquors are delivered to the Yaryan
evaporator from the pans at a density of 6° to 7° B. at 130°
F. Here they are concentrated to 34° to 42° at 140° F. At this
density they are fed into furnaces of a reverberatory type, where
they are burnt to a cherry-red heat; and the ash then raked out.
This ash, which averages 50 per cent. of soda, is weighed in iron
barrows on suitable scales, and wheeled into the leaching-room
for lixiviation. The system of leaching, as it is termed in the
States, is conducted as follows:--Iron tanks are used, with
suitable piping, that allows pumping from one tank to another, and
also to pump from any one of them up to the causticising tanks in
the alkali-room. There is also a water-line by which water may be
pumped into any of the tanks, and there is a spout used in washing
away the black ash sludge. The leaching-tanks have false bottoms
of 2in. by 2in. stuff, placed crosswise, over which is a layer of
gravel, on which lies a layer of straw, by which the liquor is
filtered. The gravel is removed every few days, and the straw
with every charge. When one of the tanks is filled with black ash,
it is "wet down" with the stored liquor (the strongest of the
stored weak liquors), and also with the strongest weak liquors
from the tanks, and with weak liquors obtained from these tanks by
pumping water upon them and keeping them full. This is all pumped
up to the causticising-tank until the strength is reduced to 2°
or 1½° B. The remaining liquor is then drained into a tank known
as the "clear-liquor" tank, owing to there being no black ash in
it. The liquor from the next weakest pan is then pumped upon the
pan containing the black ash, and the next weakest liquor pumped
upon this. The weaker pans are then in succession pumped upon
the stronger, and the water pumped upon these, and thus a very
perfect washing is obtained. The sludge left behind is nothing but
charcoal, with a slight trace of carbonate of soda. Mr. Congdon
illustrates the above system thus. The tanks stand as follows:--
No. 1. Clear liquor, 1° to 2° B. (strongest).
No. 2. Black ash sludge (weaker than No. 3).
No. 3. Black ash, after sending up to causticising-tank (strongest
sludge).
No. 4. Fresh black ash.
No. 5. Weaker than No. 2 (sludge only).
No. 6. Weaker than No. 5 (sludge and weakest liquor).
The method of procedure is as follows:--
Liquor from No. 3 drained into No. 1 (now full).
No. 6 pumped on to No. 2 (No. 6 sludge thrown away).
Liquor from No. 2 drained upon No. 3.
Water put on No. 5.
No. 5 pumped upon No. 2 (No. 5 sludge thrown away).
The black ash is treated thus:--
No. 4, full of black ash, is wet down with Nos. 1, 2, and 3, and
pumped up to the causticising-tank.
Water is pumped out to Nos. 2 and 3, and then drained upon No.
4, the liquor still being pumped up from No. 4 while the water
is being pumped upon Nos. 2 and 3, which are kept full. This is
continued until the liquor tests only 2° to 1° B.
No. 4 is now drained upon No. 1.
No. 3 pumped upon No. 4, and this drained into No. 1 (now full).
No. 3 pumped upon No. 5.
Water pumped upon No. 2 (No. 2 the next to be thrown away).
No. 5 is by this time full of fresh black ash, and the same process
is carried out with No. 4.