Evaporation Table.
Showing the volume of liquor obtained by evaporating 1,000 gallons of weak black lye of density d to a higher density D.
| Lower Density d (at 100° F.). | Higher Density D (Twaddell) at 100° F. | ||||||||
| 20. | 25. | 30. | 35. | 40. | 45. | 50. | 55. | 60. | |
| 2 | 100 | 80 | 66·6 | 57·1 | 50 | 44·4 | 40 | 36·3 | 33·3 |
| 3 | 150 | 120 | 100 | 85·7 | 75 | 66·6 | 60 | 54·5 | 50 |
| 4 | 200 | 160 | 133·3 | 114·3 | 100 | 88·8 | 80 | 72·7 | 66·6 |
| 5 | 250 | 200 | 166·6 | 143 | 125 | 111·0 | 100 | 90·9 | 83·3 |
| 6 | 300 | 240 | 200 | 171·4 | 150 | 133·3 | 120 | 109 | 100 |
| 7 | 350 | 280 | 233·3 | 200 | 175 | 155·5 | 140 | 127 | 116·6 |
| 8 | 400 | 320 | 266·6 | 228·6 | 200 | 177·6 | 160 | 145·5 | 133·3 |
| 9 | 450 | 360 | 300 | 257 | 225 | 200 | 180 | 163·5 | 150 |
| 10 | 500 | 400 | 333·3 | 286 | 250 | 222 | 200 | 181·8 | 166·6 |
Example:—1,000 gallons of weak liquor at a density of 7° Twaddell are reduced to a volume of 200 gallons having a density of 35° Twaddell, or to a volume of 140 gallons with a density of 50° Twaddell, by evaporation.
Preparation of Caustic Soda.—The crude soda ash recovered from previous boiling operations is dissolved in large lixiviating tanks and extracted with hot water. The clear solution obtained after all impurities have been allowed to settle is pumped up into the causticising tanks, where it is converted into caustic soda, the loss due to the amount of soda not recovered being made up by the addition of ordinary soda ash. The causticising pans are large circular iron vessels usually 9 feet diameter and 8 or 9 feet deep, into which a known volume of the recovered carbonate of soda solution is placed.
A weighed quantity of ordinary quicklime is then put into a perforated iron cage which is fixed inside the causticising pan at such a level that the whole of the lime is immersed in the solution. The liquor is kept in constant circulation by means of an agitator and heated to boiling point, with the result that the chemical reaction sets in, the carbonate of soda being converted into caustic soda and the lime being thrown out as chalk. When the operation is completed, the steam is turned off and the chalk allowed to settle. The clear liquor is carefully strained off and pumped up into store tanks from which the required quantities are drawn off into the digesters as circumstances demand.
Washing.—The grass which has been partially washed in the digester is dug out by the workmen and discharged through a manhole fitted on one side of the digester near the bottom. It is then conveyed in any convenient manner to the breaking engine, in which the grass is more completely washed. This important machine has already been described on page [53]. The floor of the vessel slopes slightly upward towards the front of the roll and falls suddenly behind the roll, in order to promote a circulation of the contents of the engine round and round the vessel.
A definite weight of boiled grass is thrown into the engine together with a large quantity of fresh water. The circulation of the roll draws the mixture of pulp and water between the knives, breaking it up and at the same time discharging it behind the beater roll, and producing a continuous circulation of the mixture in the two sections of the vessels.
The dirty water is continuously removed from the vessel by means of a “drum-washer.” This is a large hollow drum, the outer surface of which consists of a fine wire cloth, the interior of the washer being fitted with specially curved scoops. The drum-washer is lowered until it is half immersed in the mixture of pulp and water, and as it rotates the dirty water finds its way through the wire cloth, being caught up by the internal scoops and discharged through a pipe to a drain outside the breaking engine. At the same time fresh water is run into the vessel at one end, and the continuous washing of the pulp thus effected.
Bleaching.—The clean boiled grass is bleached by means of a solution of chloride of lime.