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 The charge is usually withdrawn before the conversion into carbonate is completed; it is then raked out into barrows and placed in a heap, generally in a shed or chamber, open on one side,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.
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.
