This section is from "Scientific American Supplement Volumes 275, 286, 288, 299, 303, 312, 315, 324, 344 and 358". Also available from Amazon: Scientific American Reference Book.
We need for carrying out these manipulations, according to the size of the establishment, one or more reservoirs for precipitating the impurities of the water, and one pure water reservoir, to take up the purified water; from the latter reservoir the boilers are fed. The most practical idea would be to arrange the precipitating reservoir in such manner that the purified water can flow directly into the feeding reservoir.
The water in the precipitating reservoir is heated either by adding boiling water or letting in steam up to 60° C. at least. The precipitating reservoirs (square iron vessels or horizontal cylinders--old boilers) of no more than 4 or 4½ feet, having a faucet 6 inches above the bottom, through which the purified water is drawn off, and another one at the bottom of the vessel, to let the precipitate off and allow of a perfect cleaning. In a factory with six or seven boilers of the usual size, making together 400 square meters heating surface, two precipitating reservoirs, of ten cubic meters each, and one pure water reservoir of ten or fifteen cubic meter capacity, are used.
In twenty-four hours about 240 cubic meters of water are evaporated; we have, therefore, to purify twenty-four precipitating reservoirs at ten cubic meters each day, or ten cubic meters each hour.
It is profitable to surround the reservoirs with inferior conductors of heat, to avoid losses.
The contents of the precipitating reservoirs have to be stirred up very well, and for this purpose we can either arrange a mechanical stirrer or do it by hand, or the best would be a "Korting steam stirring and blowing apparatus." In using the latter we only have to open the valve, whereby in a very short time the air driven through the water stirs this up and mixes it thoroughly with the precipitating ingredients. In a factory where boilers of only 15 to 100 square meters heating surface are, one precipitating reservoir of two to ten cubic meters and one pure water reservoir of three to ten cubic meters capacity are required. For locomobiles, two wooden tubs or barrels are sufficient.
After the required quantity of lime and carbonate of soda which is necessary for a total precipitation has been figured out from the analysis of the water, respectively verified by practical experiments in the laboratory, the heated water in the reservoir is mixed with the lime, in form of thin milk of lime, and stirred up; we have to add so much lime, that slightly reddened litmus paper gives, after ¼ minute's contact with this mixture, an alkaline reaction, i.e., turns blue; now the solution of carbonate of soda is added and again stirred well.
After twenty or thirty minutes (the hotter the water, the quicker the precipitation) the precipitate has settled in large flocks at the bottom, and the clear water is drawn off into the pure water reservoir. The precipitating and settling of the impurities can also take place in cold water; it will require, however, a pretty long time.
In order to avoid the weighing and slaking of the lime, which is necessary for each precipitation, we use an open barrel, in which a known quantity of slaked lime is mixed with three and a half or four times its weight of water, and then diluted to a thin paste, so that one kilogramme slaked lime is diluted to twenty-five liters milk of lime.