_Vacuum Multiple Effect Evaporation._--Advantage is taken of the fact that water boils at a lower temperature in a vacuum than at the ordinary pressure of the atmosphere. There are many forms of apparatus based on this principle, amongst which the most recent is Scott's evaporator. The black liquor from the boilers is pumped through tubes heated externally by high-pressure steam. The liquor is passed into a chamber in which a slight vacuum is maintained, so that immediately on entering, the liquor parts with a good deal of water in the shape of steam. The steam liberated is utilised in producing further evaporation of the partially concentrated liquor, and this operation is repeated several times until the concentration is effected to the desired point. In most cases the actual incineration of the thick liquor is carried out in a rotary furnace when such an apparatus as this is used. 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 | Higher Density D (Twaddell) at 100° F. Density _d_ +----------------------------------------------------- (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.