| CaCO3 | - | + | Ca(OH)2 | = | 2CaCO3 | + | 2OH2. | (3) | |
| H2CO3 |
Hydric magnesium carbonate is also precipitated by lime, but the reaction is somewhat different, the magnesia being removed as hydrate as follows:—
| - | MgCO3 | + | 2Ca(OH)2 | = | 2CaCO3 | + | 2OH2 | + | Mg(OH)2. | (4) | |
| H2CO3 |
It will be noted that 2 equivalents of lime are required to precipitate 1 of magnesia. Two molecules of sodium hydrate (NaOH) or potassium hydrate (KOH) may be substituted for 1 of Ca(OH)2 with similar results, and in some cases it is practically advantageous to use the former, as the sodium carbonate formed in precipitating the temporary hardness reacts again on the permanent, throwing down the lime and magnesia as carbonates. (See [p. 101].)
Fig. 19.—Plan of Archbutt and Deeley’s Apparatus.
[Larger plan] (110 kB)
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The use of lime for softening temporary hard waters was originally proposed by Thomas Henry, F.R.S., of Manchester, but was first applied as a practical process by Clark, who, after adding the requisite quantity of lime to the water in a mixing vat, allowed it to stand in a large tank to clear by subsidence, the precipitated carbonate of lime taking from 6 to 12 hours to settle. The process in its original form is a perfectly satisfactory one, except for the capacious settling tanks which are required, which in some cases are inconvenient and expensive. Messrs. Archbutt and Deeley[62] have patented a modification of the Clark process, by which the time of subsidence is much shortened, and according to which the precipitated carbonate of lime of previous operations is allowed to remain in the tank, and the fresh charge of water and lime is mixed up with it by means of steam-injectors, which blow in a current of air through perforated pipes at the bottom of the tank, and at the same time very slightly warm the water. The action goes on much more rapidly at a slightly raised temperature than in the cold; and rather curiously, the stirred up precipitate, instead of increasing the time of clearing, settles rapidly and carries down with it that formed in the new operation. It is particularly suitable for treating waters containing magnesia, from which a compound of lime and magnesia is apt to be precipitated in a colloid form which chokes filter-cloths and will not readily settle. After softening, the water is usually “carbonated” by passing the gases produced by burning coke into the floating exit-pipe through which it falls, in order to retain any remaining traces of carbonates of lime and magnesia in a soluble form, and prevent their subsequent precipitation in the pipes. The apparatus is made by Messrs. Mather and Platt, of Manchester, and its arrangement is shown in Figs. 19 and 20.
[62] Journ. Soc. Chem. Ind., 1891, p. 511.
