CHAPTER XV.

SITE FOR PAPER-MILL, WATER-SUPPLY, WATER PURIFICATION, ETC.

In choosing a spot on which to build a paper-mill, the manufacturer has to take into consideration several very important circumstances. Chief of these is the necessity for having a large supply of water at command. Not only is a large quantity needful, but it should be free from impurities, such as suspended matter and iron. The former, it is true, can be removed by settling and filtration; the latter cannot, and is liable to injuriously affect the colour of the paper. Again, as a question of economy in working, it is advantageous to have convenient water-power; therefore for this, as well as for the former reason, paper-mills are usually situated on the bank of a stream. In choosing such a site, paper-makers are probably also influenced by the fact that it affords a ready means for the removal of impurities. In properly conducted mills, where suitable apparatus is employed for evaporating the liquors in which the raw material has been boiled, the stream should not be polluted to any very great extent. Generally speaking, the greater the pollution, the more are valuable materials being lost to the manufacturer. It is obvious that the site for a mill should also be chosen with reference to its proximity to means of transit for the raw and manufactured materials.

The disposition of the different divisions of a paper-mill depends of course upon the nature of the paper it is intended to make and to a large extent upon local circumstances. Fig. 79 will give some idea of the general arrangement of a mill for manufacturing esparto paper. {211}

_Water Purification._—Arnot in his Cantor Lectures on the Technology of the Paper Trade states that from 30,000 to 40,000 gallons of water are required for the manufacture of each ton of paper. The importance of a pure and copious supply is therefore very evident.

The impurities of water consist of two classes, insoluble and soluble. The former can be readily removed by processes of settling and filtration. For this purpose most paper-mills are provided with large ponds capable of holding several days’ supply. These are sometimes supplemented by filtering beds. Insoluble impurities can also be removed by passing the water through filter-presses.

Fig. 80 is an illustration of Rœckner’s patent clarifier, which may also be applied to this purpose.

The cylinders C dip below the surface of the water to be purified, which is contained in the reservoir A. They are open at their lower ends, but are closed at the top by the domes D. These are connected with the pump I by means of the pipes H. On starting the pump the water rises slowly in the cylinders. As soon as it reaches the level of the top, the action of the pump is stopped and the cocks K opened, when the water commences to flow down the pipes G, which together {212} with the cylinders form a kind of siphon. If the reservoir A be kept full, the siphons can be made to act continuously. The flow of water being slow, insoluble impurities have time to subside.

The impurities may from time to time be removed by means of the small pump L connected with the bottom of the reservoir.

The clarifier can also be used for the purification of the effluent water from paper-mills.

Insoluble impurities can be removed from water by means of the Stanhope purifier, shown in Fig. 81.

The soluble impurities of water consist mainly of carbonate and sulphate of calcium and iron, and occasionally organic matter. It is a moot point among paper-makers {213} whether or not the presence of the two former in a water is objectionable. For boiling and bleaching purposes it undoubtedly is, as when mixed with caustic soda or bleaching liquor they form a precipitate of carbonate of calcium, which lines the insides of boilers, breakers, and potchers as a hard scale, which is always liable to become detached, and to find its way into the finished paper. Moreover, carbonate of calcium is precipitated in the fibre, and carries with it a certain quantity of colouring matter, the subsequent removal of which is difficult.

Carbonate of calcium, though practically insoluble in pure water, is soluble in water containing carbonic acid. When this carbonic acid is neutralised by lime or soda, the carbonate is precipitated. The sulphate of calcium is unacted upon by lime, but by the action of caustic soda is converted into free lime and sodium sulphate. The lime then neutralises the free carbonic acid in the water, and forms carbonate of calcium, which is of course precipitated.

These reactions may be represented by the following equations:—

Calcium Carbonic Lime. Calcium carbonate. acid. carbonate. 1. CaCO_{3} + CO_{2} + CaO = 2 CaCO_{3}.

Calcium Caustic Calcium Sodium sulphate. soda. hydrate. sulphate. 2. CaSO_{4} + 2 NaOH = CaH_{2}O_{2} + Na_{2}SO_{4}.

Calcium Carbonic Calcium Water. hydrate. acid. carbonate. CaH_{2}O + CO_{2} = CaCO_{3} + H_{2}O.

If sodium carbonate be used instead of caustic soda, the decomposition will take place thus:—

Calcium Sodium Calcium Sodium sulphate. carbonate. carbonate. sulphate. CaSO_{4} + Na_{2}CO_{3} = CaCO_{3} + Na_{2}SO_{4}.

It will be seen from the above equation that in the boiling processes the lime salts are removed from the water at the expense of an equivalent quantity of caustic soda. The amount thus decomposed is not sufficiently large to make it advisable {214} on that account to purify the water. It is indeed so small that processes of purification, based upon the use of lime and caustic soda, are now largely used, the cost for chemicals rarely exceeding 1_d._ per 1000 gallons. But for the reasons we have stated above, and also from the fact that it serves to remove dissolved iron and organic matter from a water, such a purification process is in certain cases advisable.

The processes now in use consist, as we have indicated, in the addition to the water of lime and caustic soda or carbonate of soda, the quantities being regulated according to the hardness of the water, and the relative proportions of carbonate and sulphate of calcium.

The method of removing carbonate of calcium from water by the addition of lime is due to the late Dr. Clark, of Aberdeen, and the process is still called after him. It is in use in its original form in many places. The plant necessary consists simply of a tank for mixing the water and the lime, and of large settling tanks in which the carbonate of calcium subsides.

Various modifications of this process have been proposed, chiefly in the direction of improved plant.

In the Porter-Clark process the carbonate of calcium is removed by passing the water after the addition of lime (in the form of lime-water) through a filter-press.

In Fig. 81 is shown the Stanhope purifier, which is largely used for the purpose of softening water. The following details will render its action clear: A is a store tank containing caustic soda solution. B is a tank into which the water to be treated flows, and which is maintained at a constant level by means of a valve and float. C and C are two tanks which are used alternately, and in which lime water is prepared. A definite quantity of caustic soda solution is added from B; this mixture forms the reagent by which the softening of the water is accomplished. D is a small tank for the purpose of maintaining a constant head of reagent. The reagent and the water are mixed together in the pipe F, the quantities being regulated by the cocks {216} shown at H. The pipe F leads into the vessel E, which is fitted with a number of V-shaped shelves, placed at an angle of 45°, and riveted alternately to opposite sides of the vessel. This arrangement causes the water to take a serpentine course. The position of the shelves is indicated by the dotted lines.

As soon as the mixture of water and reagent flows down the pipe F, and enters the vessel E, the calcium carbonate commences to precipitate and settles upon the V-shaped shelves. At the top of the tank a layer of wood shavings, inclosed in wire-netting, is placed as shown by the dotted lines. This acts as a filter, and intercepts any particles that may not have subsided. The clear purified water passes away by the pipe I.

The precipitate that collects on the V-shaped plates is from time to time drawn off by the cocks shown at G; this may be done without interfering with the working of the apparatus. While there can be no doubt that the use of soft water is advantageous in the boiling, washing, and bleaching processes, and also of course for supplying steam boilers, it is probably an advantage to use a hard water for diluting the pulp before running it into the machine.

In the case of papers which are loaded with any of the forms of calcium sulphate (pearl-hardening, crystal-hardening, &c.) the use of very soft water is objectionable from the fact that a certain quantity of calcium sulphate is dissolved. This would not take place to the same extent with water which is already charged with sulphate and carbonate of calcium.

The removal of soluble iron from a water is effected by the softening processes described.

{217}