Alexander Watt

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.

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.

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.

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[211]
[212] 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.

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 B¹ B² B³ B⁴, 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, A¹ A² A³ A⁴, of enlarged diameter, and at the supply ends of the coils with the coils with return bend ends, C¹ C² C³ C⁴. From each separating chamber, A¹, A², valve pipe D¹ D² D³ leads into the shell of the next heating cylinder, as B², B³, B⁴, and vapour pipe D⁴ leads from the last separator A⁴ to the condenser H, and the vacuum pump H¹. A cylindrical catch-all chamber E¹, E², E³, E⁴, 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, G¹, G², G³, G⁴, 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, t¹, having valves h, h¹, as best shown in Figs. 66, 68, and 72, lead respectively from chambers G¹, G², of the first and second effect to the manifold feed pipes leading into the cylinders B³, B⁴, 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 f¹, Figs. 66, 67, and 68, connects with the heating cylinder B¹ 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 C¹ C², 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.

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 C¹ C², 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 B¹ B², 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 K¹, 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 K¹, 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 K¹. The suction pipe I of pump I extends into box K¹, 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 E¹ E², 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 D¹, 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 G³ of the third effect to the separating chamber A⁴ of the fourth effect, the latent heat being carried off in the vapours drawn by the vacuum pump H¹ 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 B¹ B², 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 B⁴ 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.