case.
When a mixture of lime and carbonate of soda is used, a method much adopted on the Continent, the lime should be well screened from lumps before being mixed with the soda. The usual method of preparing this mixture is as follows:—A wooden tank, 15 feet long, 5 feet wide, and 4 feet deep is divided into three compartments, each of which has a false bottom perforated with ½-inch holes to keep back lumps, stones, pieces of coal, etc., which frequently abound in the lime. The fresh lime is put into the first compartment, where it is slaked with water in the usual way; the resulting powder is then put into the next compartment together with sufficient water, where it is agitated until converted into what is technically termed "milk of lime." In the partition which separates the second from the third division is a movable sluice, through which the milk of lime flows into the third compartment; in this is fitted a revolving drum, similar to the drum-washer of the breaking-engine, through which the milk of lime which flows from the sluice becomes strained, and is lifted in the same way as water is lifted by the drum-washer of the breaking-engine, and is thence discharged through a pipe into the rag boilers; an additional straining can be effected by placing a fine wire strainer over the mouth of this pipe leading to the boiler, which will prevent objectionable particles from entering the boiler. Each compartment is provided with a large waste pipe, through which, by the aid of a sufficient supply of water, all impurities which have been rejected by the drum are carried away. The soda solution is prepared by dissolving the required proportion in water, and the resulting liquor, after careful straining, is introduced into the boiler to which the charge of rags has been given; the head of the boiler is then fixed in its position and steam turned on, until a pressure of about 20 to 30 lbs. to the square inch is attained, and the boiling kept up for two to six hours, according to the quality of the rags. By the Continental system of boiling rags, for No. 1 stuffs, 216 lbs. of lime and 114 lbs., of 48 per cent., soda ash are used for every 4,000 lbs. of rags; for Nos. 3 and 5 stuffs, 324 lbs. of lime and 152 lbs. of soda ash are used; and for No. 4 stuff 378 lbs. of lime and 190 lbs. of soda ash, and the boiling in each case is kept up for twelve hours, under a pressure of 30 lbs., the operation being conducted in boilers which revolve horizontally.
In boiling the finest qualities of rags, it is considered preferable to boil with lime alone, which is believed to be less injurious to delicate fibres than caustic soda. Dunbar[14] gives the following proportions of 70 per cent. caustic soda per cwt. of rags:—
S. P. F. F. F. is boiled with lime alone, then washed in the boiler, and again boiled with 2 per cent. of soda ash.
| S. P. F. F. is | boiled | with | 12 | lbs. of | (70 per cent.) | caustic soda | per cwt. |
| S. P. F | " | " | 14 | " | " | " | " |
| Fines | " | " | 7 | " | " | " | " |
| Seconds | " | " | 6 | " | " | " | " |
| L. F. X. | " | " | 20 | " | " | " | " |
| C. L. F. X. | " | " | 27 | " | " | " | " |
| C. C. L. F. X. | " | " | 30 | " | " | " | " |
| F. F. | " | " | 15 | " | " | " | " |
These are all boiled at a pressure of from 20 to 25 lbs. for 10 hours, in stationary boilers without vomit, and also in boilers revolving horizontally. In some mills, where the best qualities of paper are made, iron boilers are objected to, as small particles of oxide of iron are apt to become dislodged from the interior of the boiler, and produce discolouration of the paper. In such cases wooden vats, with mechanical stirrers, are employed; sometimes a jacketed boiler is used.
Figs. 11 and 12.
Washing and Breaking.—The removal of the dirty water resulting from the boiling is effected in the washing and breaking engine, or "rag engine," as it is commonly called, which is constructed on the same principle as the beating engine, but is provided with an extra drum, called the drum-washer, which, being covered with wire gauze, allows the washing waters to escape without permitting the fibrous stuff to pass through. The rag engine, having been invented by a Dutchman, acquired, and still retains, the name of the Hollander, and although it has been considerably improved upon, its principle is still retained in the modern engines, of which there are many different forms. The ordinary rag engine, Figs. 11 and 12, consists of a cast-iron trough A, about 10 feet long, 4½ feet wide, and 2½ feet deep, and rounded at the ends, and is firmly bolted to a wooden foundation. It is provided with a partition termed the midfeather B, of such a length as to have the trough of uniform width round it. A cylinder, or roll, C, furnished with a series of steel knives, rotates in one of the divisions formed by the midfeather, and the floor of the trough in this division is inclined in such a manner as to cause the pulp, as it travels, to pass under the roll. Beneath the roll is the bed-plate, which is fitted with a series of steel knives c c similar to those on the exterior of the roll. The distance between the knives of the roll and the bed-plate is regulated by levelling screws, which are so adjusted that both ends of the roll are raised at the same time, which is a great improvement upon the older types of breaking engines in which only one end of the roll was raised, whereby the knives became unequally worn. By the present method of regulating the distance between the respective sets of knives, any required degree of fineness can be given to the fibrous substances treated. The roll is generally caused to rotate at a speed of about 230 revolutions per minute, causing the water and rags to circulate in the engine and to be constantly under the action of the knives. In the other division F F of the trough is the drum-washer H, which, being covered with fine gauze wire, allows the water to enter, but keeps back the fibrous material. The ends of the drum are formed of two discs of wood, generally mahogany, upon which the coarse gauze is fastened as a backing, and this is covered with the fine wire gauze. The interior of the drum is sometimes furnished with a series of buckets, which conduct the water to a trough in the axis of the drum, by which it is led away. This is also accomplished by dividing the interior of the cylinder into compartments by means of a partition. The drum-washer is so arranged that it can be wholly raised out of the trough, which is necessary in certain parts of the operation, when the removal of the liquid is not required; or it can be partially raised, or otherwise, according to requirement. The floor of the compartment containing the roll C is inclined at D, so as to cause the pulp to pass directly under the roll, and at D′ is the backfall, over which the pulp travels to the opposite side of the midfeather.
