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:—
1.
CaCO3
Calcium
carbonate.
+
CO2
Carbonic
acid.