The dyeing of textile fabrics . -Clarks Apparatusfor Softening Water. Chap. IX.] PURIFICATION OF WATER. 133 settling of the precipitate. Mechanical impurities, alsoiron, aluminium, and earthy phosphates are completelythrown down. Permanent as well as temporary hardness is removedby the use of caustic soda, since the calcium andmagnesium sul-phates present 1A Jl ^are decomposedby the sodiumcarbonate pro-duced in theabove reaction. Should thewater correctedin this mannerbe required forpurposes wherealkalinity is tobe avoided, itcan be readilyneutralised be-fore use. 87. Porter-Clark Process.—The
The dyeing of textile fabrics . -Clarks Apparatusfor Softening Water. Chap. IX.] PURIFICATION OF WATER. 133 settling of the precipitate. Mechanical impurities, alsoiron, aluminium, and earthy phosphates are completelythrown down. Permanent as well as temporary hardness is removedby the use of caustic soda, since the calcium andmagnesium sul-phates present 1A Jl ^are decomposedby the sodiumcarbonate pro-duced in theabove reaction. Should thewater correctedin this mannerbe required forpurposes wherealkalinity is tobe avoided, itcan be readilyneutralised be-fore use. 87. Porter-Clark Process.—The essentialimprovementeffected by thisprocess is asaving of space,time, and la-bour, through the application of machinery to the ordi-nary Clarks process. Figs. 40 and 41 give plan and elevation of an arrange-ment for supplying over 6,000 litres of softened water perhour, but there is practically no limit to the amountwhich may be supplied if the apparatus is made largeenough. The lime-water is prepared in the small horizontal. Fis 41.—Pcrter-Clarks Apparatus for SofteningWater (elevation). 134 DYEING OP TEXTILE FABRICS. [Chap. IX. cvlinder A, by constantly chuming up slaked lime withwat^r admitted under pressure direct from the reservoiror main. By a pipe midway in the height of the chum,the more or less saturated lime-water (a saturated solutioncontains about 1-4 gi-ams of lime per litre), and withsome lime in suspension, is led into the large cylindricalvessel B, where the lime and water are kept in slightagitation, to assist in completing the saturation. Asthe lime-water ascends, the particles of lime in suspension^adually settle out, and tolerably clear lime-water passesout at the top into the cylinder c, where it is continuouslymixed with the water to be purified, in accurately deter-mined proportions. The supply of each is regulatedby valves furnished with dial plate and index. A brisVAgitation is maintained in the mixing cylinder c, in orderto facilitate the chemical
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