RECOVERY OF THE NITROGEN COMPOUNDS.—The fact that nitrogen compounds are absorbed by sulphuric acid of a certain atrength, furnishes us with a means of recovering a portion escaping unused in the exit gases, and which may be re-ueed in the process. For one method, and that by far the moat generally used, of applying thin fact we are indebted to the celebrated Gay Lussac.
Fig. 50 shows a vertical section, and Fig. 51 a ground-plan, of the Gay-Lussac " absorbing " tower. It consists principally of a leaden tower K, 25 ft. high and 5 ft. 6 in. in diameter, placed in a wooden frame in the same way as the chambers. The cover or roof is not, how ever, burnt to the sides, but is made movable. it is composed of a board frame covered with sheet lead, and having a border burnt on all round. This border fita into a gutter which is made in the topmost joist of the framing of the tower, and over which the side lead is drawn. This gutter is filled with acid and the joint thus luted. At the bottom of the tower a aort of grating of fire bricks is erected. Theae are arranged in parallel rows, and form channels about 1 ft. 6 in. high. They are crossed by others at right anglea, leaving spaces of about 2 in. broad through which the ascending gas and deacending acid can freely paas. The tower ia packed with pieces of hard coke, resting upon the grating and reaching cloao up to the inlet pipe N, these pieoea being largeat at the bottom and gradually decreasing in size towards the top. Three manholes are made in one aide of the tower, in order that the coke may be more easily introduced. These are closed with wooden doora covered inside with sheet lead and made tight with putty. The gases paas from the last chamber through the pipe J into the tower, traversing the damper box L, which is provided for the purpose of deflecting the gases through the pipe M, so that when repairing or refilling the tower the work need not be stopped. In that case the damper b ia opened while c is shut. On the other hand, when b is shut and c is opened, the gases pass through the ahort pipe d under one side of the tower, and apreading themselves among the channels in the grating rise up through the coke in the tower, while at the same time acid of about 149°-150° Tw. flows down and absorbs the nitric acid from the gas, and flows out at the bottom of the tower, having the same composition as a solution of chamber crystals. The denitrated gaaes escape at N and M, passing another damper box which is only intended to he used while the damper e ie cloaed to cut off communication between the tower and the pipe M, when the gases are to he conveyed directly into the air and not through the tower. The nitro-sulphuric acid flows from the tower through the pipe f, seen in the ground-plan, into a cistern whence it is taken to the so-called "denitrating" tower, when the nitric acid is re-eliminated, as we shall presently describe.
In some works the absorbing tower is placed quite close to the denitrator, which stand° near the first leaden chamber into which the nitric acid is introduced. This necessitates the gases from the last ohamber being taken through a very long pipe to the tower. The check thus created to the draught is probably the reason why it has been found necessary with this arrange ment to have the pipe M in connection with a chimney. We have already stated that such an arrangement is sometimes adopted, and is, in fact, almost universal in this country. In the other ease, it is better to put the absorb ing tower in the immediate vicinity of the last chamber, so that the gases have only to pass a short pipo. With this arrangement it is not necessary to take the gases into a chimney, they may be conveyed away simply by the pipo which is seen at P.
In order to observe the colour of the gases as they enter and leave the tower, glass windows are placed opposite each other in the two damper boxes L and 0, or a part of each of the two pipes J and N is fitted with a glass cylinder. Before entering the tower the gases should appear orange-coloured, afterwards colourless. It is of importance in the foregoing process, that the sup ply of acid for absorbing the nitro gen compounds shall be precisely regulated so that it may be distri buted evenly over the ooke, other wise with even an excessive supply of acid, the gases may still escape without eurrendering their nitrous acids. A special apparatus there fore is required. This apparatus consists of two leaden cisterns S and T, and a delivery vessel g at the top of the tower. The acid is generally forced into the cis tern S by an air force-pump. The acid is collected below in an air-tight iron cistern lined with lead, and upon it air is forced until the acid rises up an escape pipe from the bottom. A part of the ascending pipe is seen at h. The acid enters the cistern S through a leaden rose, which detains all solid bodies which may have accidentally got into the acid, and eau be removed for cleaning. The acid passes from S through a leaden pipe into the smaller cistern T. The inflow to this second cistern corresponds with the out flow by means of an automatic arrangement, consisting of a leaden float hanging from one arm of the balance k, by the rising of which the other arm is depressed and closes the exit pipe. The pipe 1 is only to prevent any chance of the acid overflowing in consequence of an accident to the balance. The acid runs from T to the coke-packed tower B, through the intervention of the delivery apparatus.