IX, A, 4 Cox et al. : Water Supplies in the Philippines 375 



The preceding formula shows the total amount of solids (scale and 

 sludge) precipitated under ordinary conditions. 



The amount of matter deposited as a hard scale will be: 0.00833SiOi4- 

 0.0138Mg+ (0.016Cl-f 0.0118SO4-0.0246Na-0.0145K) . 



Though scale is a serious menace to boiler tubes and shell, a very thin 

 scale may sometimes be advantageous in a boiler, especially when a cor- 

 rosive water is being used. Kent " recommends the occasional addition of 

 lime water to water of this kind, so that the resulting thin scale may 

 protect the boiler. 



WATER SOFTENING AND SCALE PREVENTION 



For most technical and industrial work a water low in mineral content 

 is generally desired. In many localities such water is not available, and 

 the existing supply must be softened to make it suitable for use. The scale- 

 forming ingredients of water may easily be reduced to 50 parts per million 

 by careful treatment. When a boiler plant is supplied with impure water, 

 either the formation of hard scale must entirely be prevented and the soft 

 scale and sediment blown off or the plant must be in two parts, one the 

 boiler proper and the other the purifying apparatus which is so far as 

 possible independent of the boiler and in which chemical precipitation of 

 the scale-forming ingredients takes place where it will do no harm. As 

 hardness is generally due to salts of calcium and magnesium, the main 

 problem of water softening is to precipitate these salts as completely and 

 as cheaply as possible, without introducing any ingredient injurious to the 

 water. The cost of any process or method for the treatment of a boiler feed 

 supply depends to a large extent upon the chemistry of the water to be 

 treated. No one reagent, is known which will remove the scale-forming 

 ingredients from all waters, although almost everything from soda to catechu 

 has been proposed and tried. Calcium carbonate held in solution in water 

 by excess of carbonic acid may be precipitated by removing the carbon 

 dioxide by boiling or by the addition of lime. 



Calcium carbonate is practically insoluble, and therefore the reaction 

 proceeds practically to completion. Magnesium carbonate reacts similarly, 

 but owing to its slight solubility the reaction does not proceed to completion, 

 and in order to remove it completely a further quantity of milk of lime 

 must be added. 



The completeness of the precipitation of calcium carbonate is the same 

 whether water is boiled under atmospheric pressure or under a pressure 

 of several atmospheres. The completeness of the precipitation of mag- 

 nesium carbonate, however, is increased by increasing the pressure.^ 

 Freshly precipitated calcium carbonate settles very slowly, especially when 

 obtained by treating the bicarbonate with milk of lime in the cold, so that 

 the precipitation must be made in large tanks where the treated water 

 can remain a long time undisturbed. Heat hastens the settling. By 

 properly handling such a water in a boiler, a loose powdery sediment is 

 obtained which may be removed by blowing off, but if sulphates are present 

 it is deposited as hard scale. If sulphates are present, the chemical pre- 

 cipitation should be carried on outside of the boiler. 



The amount of the sulphate ion in boiler waters is important on account 



'- Loc. cit. 



"Knight, N., Eng. News (1905), 53, 311. 



