It is often neces sary, as in the case of paper having a water mark, that the sheet should be cut with great exactness, so that the device shall come exactly in the centre. The ordinary cutter cannot be relied on for this purpose, and, in its place, a machine called a "single-sheet cutter" is used (Fig. 1065). It consists essentially of a large wooden drum A, fixed on a horizontal axis, over which the paper is led by a pair of drawing-in rolls B. The paper is held against the drum by a clamp worked by the arm C. The paper is cut by the knife E moving against the stationary knife D. After the cut, the drum describes part of a circle, the paper being still held, so that it cannot go back with the drum. As soon as it has gone far enough, the clamp is removed, and the drum returns, bringing the paper with it. The length of the aro through which the drum moves, and therefore the size of sheet, is regulated by the length of the crank-arm F. If, from any cause, the cut should not take place at the right time, the man in charge can, by press ing against the clamp, retard the motion of the paper, and thus bring back the cut to the right place. The small roller G is for the purpose of keeping the paper always tight.
Process. — In former years, the liquors in which rags, esparto, and other paper material had been boiled, was run into a river or stream near ; but now, owing partly to the fact that it is insisted on by the land owners, but chiefly because it can be made remunerative, all such liquors are preserved, and the soda in them is utilized. The method adopted is to evaporate to dryness, and ignite the residue, which then contains the soda, originally used as caustic soda, chiefly in the form of carbonate, mixed with a quantity of silicate and other salts.
The ash so obtained is dis solved in water, sometimes fil tered, and boiled with a quan tity of lime sufficient to reconvert it into caustic soda, the lime at the same time being changed into carbonate. The latter is allowed to separate out by settling, and the clear liquor is run ofE The oarbonate of lime is washed once or twice with water, the liquor, if very weak, being used to dissolve fresh ash, and then, in the best conducted mills, it is allowed to drain on filters, beneath which a vacuum is produced by an air-pump, similar to the arrangement used in alkali-works. Of course the whole of the soda cannot be recovered in this way : lose by leakage, in addition to that left in the fibre, must inevitably occur ; this is generally replaced by fresh caustic soda, or good soda ash.
The apparatus for accomplishing the evaporation varies with almost every mill. In some, it is very primitive and crude, consisting perhaps of only a furnace for incinerating the residue, and over it a pan containing the liquor, the latter being heated and evaporated by the heat from the furnace. It is obvious that, with such an arrangement, a large quantity of beat must be wasted. To economise as much as possible of this waste beat, various plans have been sug gested. That of Roeckner, of Newcastle, appea s to be to a great extent efficacious. It consists practically of a series of shallow trays B (Fig. 1066) placed in a brick chamber, alternated so as to
allow the heated air from the furnace below to play upon the surface of each in succession, on its way to the chimney, with which the whole system is in connection. Above the chamber containing these trays, is a large tank C containing a store of the liquor to be evaporated, placed there so as still further to economise the heat, and from which the liquor runs on to the trays. The fur nace A below is of the ordinary rever beratory kind ; below it, and connected with it by a kind of damper, is a large chamber J where the calcined residue from the furnace is put to cool, thus preventing any nuisance from the smell of the burning mass. The chamber is provided with a pipe L, through which the vapours pass into the furnace. Several pipes E from the furnace pass through the tank, to assist in warming the liquor. The residue, when cold, is drawn through doors from the chamber below the furnace. Roeckner has devised an apparatus (Fig. 1067), consisting of a small chamber containing a series of pipes A, through which, a stream of cold water constantly runs, in connection with the flue from his evaporator, for the purpose of condensing volatile bodies, and thus preventing, to a certain extent, contamination of the surrounding air.
An economical evaporator, theoretically considered, is that invented by Porion, a French distiller, and named after him. It is largely used on the Continent, and at several mills in Scotland and England, where it has given great satisfaction. In connection with it, a " smell - consumer " has been invented by Menzies and Davis. It con sists essentially of a large chamber, the floor of which is in clined slightly from the chimney. The li quor to be evaporated is run in at the end nearest the chimney, from a tank. Fan ners, dipping about in. into the liquor, revolve with great rapidity, and produce a very fine spray, thus presenting a very large evaporating surface. The evaporation is caused by the heat from the furnace on its way to the chimney. The " smell-consumer " consists of fire-brick chamber, having courses of walls, built in such a way as to retard the draught somewhat, and so give time for the products of combustion from the furnace to be completely burnt. Time is all that is necessary, as the heat is quite enough, and there is always sufficient oxygen present. The doors are used for cleaning out this chamber. The tank is placed over this chamber, in order to warm the liquor, and thus still further economise the heat. The chamber is for the purpose of retaining any solid particles carried forward. The evaporated liquid is run from one or more of the doors along a spout to the pan, from which it is run into the furnace. The furnace is of the ordinary kind, but with two beds. Here the liquor is still further evaporated, and the residue is incinerated. If the latter operation is properly conducted, no nuisance from smells need arise, and the combustion in the smell-consumer is perfect. The draught is regulated by a damper. (Continued ou p. 1508.)