Bleaching Powder.—This substance is prepared on a large scale by allowing chlorine gas to act upon dry slaked lime. The lime absorbs nearly one-half its weight of chlorine and forms a dry white powder, having a very pungent odour. The best bleaching powder contains about 37 per cent. of what is termed “available chlorine.” The substance, on being treated with water, gives a greenish-coloured solution known as bleach liquor, and when raw paper-making material, after having been digested with caustic soda, is treated with this solution, it is gradually bleached to a white colour. The composition of the powder may be represented approximately as follows:—
| Available chlorine (combined with lime) | 36·00 |
| Chlorine in the form of chloride | 0·32 |
| Chlorine in the form of chlorate | 0·26 |
| Lime | 44·66 |
| Magnesia | 0·43 |
| Silica, iron oxides, etc. | 1·33 |
| Insoluble matter | 17·00 |
| 100·00 | |
Since the amount of bleach used for wood pulps varies from 8 per cent. to 25 per cent. of powder on the dry wood pulp, the cost of bleaching in some cases is considerable. The economy of the process depends in some measure upon the care exercised in the purchase of bleaching powder of standard quality, the storage of same in a dark, cool place, and the efficient treatment or exhaustion of the powder when the bleach liquor is prepared.
The powder is usually agitated for about an hour with water sufficient to produce a liquor of 13°-15° Twaddell. The undissolved powder is allowed to settle and the clear solution siphoned off, after which the sediment is washed once or twice to remove all the soluble matter completely.
Bleach Liquor Table.
Showing for bleaching powder solutions of known density the quantity of powder necessary to produce 100 gallons of liquor and the number of gallons obtained from 1 cwt. of powder (adapted from Lunge and Beichofen).
| Twaddell. | Available Chlorine Pounds per 100 gallons. | Number of Gallons obtained from 112 lbs. of Powder. | Pounds of Powder per 100 gallons of Liquor. | ||
| 34 per cent. Powder. | 35 per cent. Powder. | 34 per cent. Powder. | 35 per cent. Powder. | ||
| 0·25 | 0·70 | 5,464 | 5,600 | 2·05 | 2·00 |
| 0·50 | 1·40 | 2,725 | 2,800 | 4·11 | 4·00 |
| 1 | 2·71 | 1,405 | 1,445 | 7·97 | 7·74 |
| 2 | 5·58 | 681 | 702 | 16·41 | 15·94 |
| 3 | 8·48 | 448 | 462 | 24·95 | 24·23 |
| 4 | 11·41 | 334 | 340 | 33·55 | 32·60 |
| 5 | 14·47 | 264 | 270 | 42·58 | 41·34 |
| 6 | 17·36 | 219·5 | 225 | 51·06 | 49·60 |
| 7 | 20·44 | 186 | 191 | 60·11 | 58·40 |
| 8 | 23·75 | 160 | 165 | 69·85 | 67·85 |
| 9 | 26·62 | 141 | 147 | 78·30 | 76·57 |
| 10 | 29·60 | 129 | 132·5 | 87·06 | 84·54 |
| 11 | 32·68 | 116·5 | 120 | 96·11 | 93·37 |
| 12 | 35·81 | 106·5 | 109·5 | 105·32 | 102·31 |
| 13 | 39·10 | 98 | 100 | 115·00 | 111·70 |
| 14 | 42·31 | 90 | 92·5 | 124·45 | 120·90 |
| 15 | 45·70 | 84 | 86 | 134·41 | 130·56 |
| 16 | 48·96 | 78 | 80 | 143·80 | 139·71 |
| 17 | 52·27 | 73·5 | 75 | 153·53 | 149·34 |
| 18 | 55·18 | 69 | 71 | 162·30 | 157·65 |
| 19 | 58·40 | 65·5 | 67 | 171·00 | 166·86 |
| 20 | 61·50 | 61·5 | 64 | 180·88 | 175·71 |
The best method for extracting powder is to agitate the material with water for a short period, and to stop the mixing process directly the maximum density has been obtained, which usually takes place in 15 minutes. Prolonged agitating prevents the powder from settling readily.
The maximum quantities of liquor which can be obtained from bleaching powder are shown on page [162]. The following table is useful as showing the amount of water required for diluting strong liquors, the figures being applicable to any solution independent of the nature of the dissolved substance.