Page:EB1911 - Volume 20.djvu/786

Rh handling till it is made into finished paper at the end of the machine; this effects a considerable saving in cost of labour and reduces the waste of material incidental to a series of disconnected operations.

Masson, Scott and Co., Ltd. . 5.—Esparto Bleaching and Beating Plant.

From the potcher or breaking engine the esparto or wood pulp is discharged, by means of a patent circulatory or pump, into the first of a series of upright bleaching towers. These towers (fig. 5) are built up of wrought-iron rods and a special kind of cement. They are usually about 16 ft. high in the parallel by 8$1⁄2$ ft. in diameter; the bottom of the tower is conical and connected to a powerful circulator or pump, which discharges the pulp into the top of the tower and causes thereby a continuous circulation and a thorough mixing of the pulp and bleach. A special form of concentrator is fixed on the top of the first tower, which reduces the water in the pulp as it leaves the potcher to the minimum quantity necessary for perfect circulation in the tower; by this means a considerable saving is effected in the quantity of bleach required. After the necessary concentration of the pulp in No. tower, the bleaching liquor is added and the circulator at the foot of the tower put in motion. A two-way valve in the discharge pipe allows the pulp to pass on to tower No. 2, and so on through the series. The circulator in each tower is only put in working for a short time once in every hour and there is never more than one circulator working in the series at one time. There is no manual labour in working the process, perfect cleanliness, and a great saving in power over the old process. Each tower will hold about two tons of dry pulp. When the pulp is fully bleached in the last tower of the series, fresh water is run into it, and a second concentrator, similar to the one on the first tower, is put in motion and washes out all traces of the bleach in about 25 to 30 minutes. These concentrators effect also another purpose, taking to some extent the place of the presse-pâte machine for removing roots, weeds and other impurities.

From the last tower and concentrator the bleached pulp is pumped through a line of pipes to the beaters, valves being fixed in the line of pipes to discharge into whichever beater is desired. These beaters are constructed in tower-form like the bleachers, the roll and plate being fixed on the top of the tower and the circulation effected in the same way as in the bleachers. Fig. 5 shows plan and elevation of such an arrangement of beaters and bleachers arranged in series. The beaters are made to hold each about 500 ℔ of dry paper and a series of four of these can make from 55 to 60 tons of paper per week.

Fibres like jute, hemp, manila, &c., are chiefly used for the manufacture of coarse papers where strength is of more importance than appearance, such as wrapping-papers, paper for telegraph forms, &c. The boiling processes for them are similar to those used for esparto and straw.

The alkaline liquors in which rags, esparto and other paper-making materials had been boiled were formerly run into the nearest water-course; but now, partly because it is insisted upon in England by the Rivers Pollution Acts, and partly because the recovery of the soda can be made remunerative, all these liquors are preserved and the soda they contain utilized. One of the best and most economical of the simple recovery plants is that invented by Porion, a French distiller, and named after him. This consists of an evaporating chamber A, on the floor of which a few inches of the liquid to be evaporated rest. By the action of fanners B, B revolving at a high speed and dipping into the liquid, it is thrown up in a fine spray through which the heated gases pass to the chimney. After being concentrated in the evaporating chamber the liquid flows into the incinerating furnaces C, C, where the remaining water is driven off by the heat of the fire D, and the mass afterwards ignited to drive off the carbonaceous matter. A considerable feature in this evaporator is Menzies and Davis's patent smell chamber E, a chamber filled with masonry in which the strongly-smelling gases from the incinerating furnace are allowed to remain at a red heat for a short time. After being recovered, the soda, in the form of crude carbonate, is lixiviated and re-causticized by boiling with milk of lime.

. 6.—Porion Evaporator.

Porion's method is open, however, to the objection that the whole of the sulphur in the coal employed for the furnaces finds its way into the recovered soda, and forms sulphur compounds, thus reducing the value of the ash for boiling purposes; in addition, a considerable amount of soda is volatilized during the evaporation. By the application of the system of multiple-effect evaporation to the recovery of waste liquors these drawbacks disappear, and an important change has been made in the soda-recovery plant of the paper-mill. This system of multiple-effect evaporation, originally introduced by M. Rillieux, was perfected by the invention of Flomer T. Varyan, of Toledo, Ohio, U.S.A. This type may here be taken for description, though other types of evaporator are now also employed, notably the ordinary vertical tube multiple effect evaporator as used for concentrating sugar liquors. The Yaryan evaporator was originally applied in the United States to the concentration of the waste alkaline liquors of paper-mills; it then came into extensive use for the manufacture and refining of sugar, the production of glucose and a variety of other purposes. The principle of multiple-effect evaporation is to utilize the latent heat of a vapour given off from a liquid under a certain pressure to vaporize a further quantity of the liquid under a pressure maintained by mechanical means below that of the first. The essential feature which distinguishes the Yaryan evaporator consists in the boiling of the liquor to be treated while it is passing through a series of tubes, which constitute a coil and are heated externally by steam or vapour. The quantity of liquor entering the coil is so controlled that it is only permitted partially to fill the tubes, and thus leaves room for the instantaneous liberation of the vapour and its free escape. As the liquor descends from tube to tube it becomes concentrated and reduced in volume until it ultimately passes into a "separator," where it impinges on a plate or disk, which causes a complete separation of the vapour and liquid; each then passes on to the next "effect," the liquid through the second coil of tubes and the vapour to the chamber enclosing them. This combination of a series of tubes, or coil, and separator constitutes a vessel or "effect," and the evaporator consists of a series, usually three or more, of these vessels, one above the other (fig. 7). The vital feature, it will be understood, is therefore that the latent heat of the original steam, after performing its function in the first effect, is passed on to the second and then to the third or more effects, in each of which an equal amount of work is done before passing to the final condenser, where a vacuum is maintained. Thus, if the total temperature be divided three times, the result is a triple-effect, if four times, a quadruple-effect. Taking an evaporation of 10 ft of water per pound of coal, a single-effect apparatus will evaporate 10 ℔ of water, a