The chlorides and sulphates of calcium and magnesium are readily soluble in water, but the carbonate of lime or magnesium (marble or ordinary limestone) is soluble onlin water which contains carbonic acid gas The greatest difficulty with the magnesium salts is that they prevent or delay the removal of the calcium salts and by their presence raise the point of solubility for calcium carbonate. An increased addition of lime in this case results in the formation of magnesium hydroxide which forms very slowly and is very likely to assume a colloidal form, absorb carbonic oxide from the air and return to its first form. Calcium carbonate will remain in solution in ordinary samples of water to the extent of 2.04 grains per gallon. This amount, how ever, is not large enough to rate the water as If all the carbon dioxide is re moved from the water the limit of solubility will be only 1.08 grains per gallon. And this remnant cannot be dislodged by the addition of caustic lime. Magnesium carbonate is much more soluble where carbon dioxide is present and while it rarely exceeds five or six grains per gallon, it is sometimes in much larger quantity. Even though all the carbon dioxide be removed the content of magnesium car bonate may remain at 12 grains per gallon. The higher the pressure in a boiler the more of this salt will be held in solution. Magnesium chloride, although highly soluble, is very ob jectionable in boiler water because of its tend ency to decompose and produce hydrochloric acid, which is extremely corrosive. Calcium chloride is more stable, but under high pres sure is believed also to break down into hydro chloric acid. It is a simple matter to soften water which contains only the bicarbonate of calcium, because it is necessary only to steal away the extra molecule of CO, by adding a little fresh-slaked lime which has a great af finity for carbonic acid gas and combines with it to form a molecule of mono-carbonate of calcium, which is insoluble, and. like the remain ing molecule of mono-carbonate, settles to the bottom as precipitate. The chemical reaction is stated thus: Calcium Slaked Cakium mono bicarbonate lime carbonate Water CaC0.(CO3) Ca(OH). 2CaCOs + and thus there is precipitated to the bottom not only the original dissolved limestone but also the slaked lime which has become limestone by uniting with the molecule of carbonic acid gas which held the original limestone in solution. This discovery or invention was made in Eng land by Dr. Clark about 1840 and no better or cheaper method has since been found for re moving carbonate of lime from water. Cal cium carbonate alone does not form a hard scale in boilers but produces a soft, slimy silt. In combination with other salts, however, it makes the hardest of scale.
In the case of the sulphate of calcium heavy crystals are deposited whenever the boiler pressure reaches or exceeds 50 pounds. The sulphate of calcium scale is regarded as worst of all. The solubility of calcium sulphate is increased by thepresence of magnesium chloride and decreased by the presence of cal cium chloride. At high boiler pressure very much less of calcium sulphate is held in solu tion, the excess being precipitated. At ordi nary temperatures water will bold in solution 150 grains per gallon. At a temperature of F., equivalent to a pressure of 132 pounds per square inch, the solubility per gallon is but 16 grains, the remaining 134 grains per gal lon having been deposited as scale. The ordinary method of taking out sulphate or chloride of calcium is to add to the water the proper amount of sodium carbonate (soda ash or washing soda), when the sodium and calcium exchange acids as follows: Calcium Sodium Calcium Sodium sulphate carbonate carbonate sulphate CaSO. -f- NasCO, = CaCO.
The calcium carbonate settles as a while the sodium sulphate remains dissolved in the water, and, for boiler or washing purposes, is harmless in any ordinary amount. In water softening operations the treatment for the mag nesium salts is practically identical with that for cakium salts. Mud in suspension is taken out of soft water by introducing a coagulant, as alum, or the combination of two substances which will form a precipitate — such as sulphate of iron and caustic soda, so that the precipitate in settling will carry down with it the fine particles of mud. The oil taken up by steam
in engine cylinders is extracted from the con densed water in the same way, so that the water may be used again in the boilers. For boiler purposes it is most important that the softened water should be freed from precipitate and made perfectly clear, because the ,presence of particles of solid matter suspended in the water is the most potent cause of boiler foaming. When the reactions take place and the young and fine precipitates are formed, in the presence of old precipitate, an agglomerating action re suits which gathers the small particles together in balls so that they settle readily. The best machines in use for carrying out these proc esses are automatic in their action and require only to be supplied with chemicals and to have the settled precipitates discharged by openi a valve at the bottom of the settling tank, each once in 24 hours.
The most recent and apparently the most successful of all water softening processes is the use of :collies, natural or artificial. These substances are silicates containing water, and which, in contradistinction to other silicates, decompose and are soluble in dilute acids. The artificial form is produced by fusing together feldspar. kaolin. pearlash and soda in certain fixed proportions ; and goes by the commercial name of •permunt.' Two forms are in use sodium permutit and manganese permutit.
When water is made to flow through a layer of sodium perreutit, the latter exchanges its sods for the calcium and magnesium salts of the water and the degree of hardness in the latter is reduced to zero. The perm:tit has to be regenerated as it grows lax in action. This is easily accomplished by washing it for a few bones with a solution of •camped' salt. Man ganese permutit in connection with marble dust removes all iron and acids from waters thus alerted.
The practice of softening the entire water supply of a municipality has grown steadily and in not a few large manufacturing cities the municipal softening plant is saving the citi zens hundreds of thousands of tons of coal per annum and adding years of life to the community's boilers. The principal require ments are (I) mixing chambers, in which the softening chemicals are thoroughly mixed with the water; (2) capacious settling basins sigh to give the tmessary u o tune tor the slower tractions when the temperature is low, as in sinter; (3) a device fur adding a coagulant (alum r ferrous sulphate) at both the en trance poi t and the exit port of the water into and from the settling basins; (4) substantial mechanical filters. In addition, as one of the =s the ' • " he wise, reduce Ili Via • pguyca swui '.
and feed it in proper quantity automatically into the At nie plant at Cleveland. Ohio, 150.000,000 gall• f water per day quire the daily application 40 tons of cone :r. This is slaked with I, t water, taken to tanks where it and to the proper consistency and finally pumped at a uniform rate into the flowing water.
In a recent report (1916) upon a water sup ply for the city of Sacramento, Cal., the rel ative hardness of the available waters was con sidered in estimating the cost of the project. The engineers showed that while a supply of eater from ground wells would be cheapest in initial outlay, that the water of the Sacra mento River would be more economical for the consumers because of its lower degree of hardness; the ultimate figures being $.40 per 1.01:0,000 gallons for the river w•: ter as against $42.40 per 1,000,000 gallons for well water. On the basis of the city's daily consumption of .)3,000.000 gallons the saving in soap alone to the consumers amounted to $120 per day. Consult Booth. W. H., 'Water Softening and Treatment' (London 1906); Christie, W. W.. 'Water: Its Purification and Use in the In dustries' (New York 1912) ; Whipple, G. C.. 'The Value of Pure Water' (New York 19(Y).
C. iir1esom. KorL, Consulting Engineer, New York.