Timbers- 



224 



[April 1907. 



digester, which is closely packed with chips of the wood until the liquor just covers 

 over the wood. The lid is put on, steam is introduced until the temperature slowly 

 rises to about 100° 0. This causes all the air from the pores of the wood to escape 

 and the solution to take its place, and takes a few hours. The temperature is then 

 increased by the introduction of further steam until it slowly rises to, say, 117°C. • 

 115° is about the temperature at which chemical action begins to take place ; 120° is 

 the maximum temperature above which it is unsafe to go. The temperature there- 

 fore must be maintained within these limits during the process of boiling. The 

 progress is judged by withdrawing samples of the liquor and examining their colour, 

 sedimentation, and by other means. When the process is complete, the digester is 

 blown off, the pulp washed with hot water, after which it is put into potchers, where 

 it is further washed, and then it is passed through screens for separating out any 

 untreated particles, and collected in the machine in the form of sheets containing 50 

 per cent moisture, packed into bales for shipment. 



If required in the bleached state, when in the potcher, it is mixed with 

 solution of bleaching powder from 10-20 per cent of the weight of the material, 

 emptied into " steeping" tanks. When the chlorine is exhausted, the liquor is 

 allowed to drain away, and the bleached product restored to the potchers and treated 

 in the same way as the unbleached product. 



Chemical wood pulps now enter into the manufacture of the highest class 

 papers, and such a degree of excellence has been achieved in this, that only au expert 

 could tell the difference between a chemical wood fibre paper and an expensive all 

 rag paper. 



SODA PROCESS. 



The heating is effected either by means of coils or live steam. When the 

 latter, allowance must be made for the amount of condensation. Little makes the 

 statement that the temperature can be raised quickly. I have, however, reason to 

 know that with soft soda aspen the temperature has to be raised slowly and with 

 the utmost care, and also lowered again. The filling of the boiler is similar to 

 that of the sulphite ; the full pressure is, however, readied as quickly as 

 possible, and maintained until the end of the treatment, the pressure formerly 

 adopted being from 60 to 75 lbs. per square inch, but latterly it vas employed 

 at about 100 lbs. per square inch, and sometimes 110. The time of boiling is from 

 eight to ten hours ; as the pressure is increased the strength of the liquor can be 

 somewhat diminished. Unlike the sulphite pulp that obtained by means of the 

 soda process is of a greyish brown colour, whilst the liquor is a darkish brown and 

 of a peculiar odour. This liquor contains the incrusting and resinous matters in 

 combination with the soda as a soluble soap. 



Caustic soda, being an expensive chemical, has to be recovered. This is 

 effected by evaporating the liquor down to a thick syrup, after which they are 

 make to flow into a revolving furnace, where they catch on fire, their own organic 

 matters supplying a large amount of heat necessary for the incineration as well as 

 for the evaporation of the weaker liquors. The evaporation is much economised 

 by the adoption of what is known as the tiiple or quadruple effect evaporator, by 

 means of which the water is removed at the least possible expenditure for fuel t 

 The incinerated ashes as discharged from the furnace appear in greyish and 

 blackish masses in the form of a sort of clinker. This mass, consisting of carbonate 

 of soda mixed with carbon, is " lixiviated " or treated in hot water, whereby the 

 soluble carbonate of soda goes into solution, leaving a black mud of charred and 

 useless matter, from which the liquor is freed by sand filtration. The clear liquor 

 standing at from 10—20 Tw., is heated in iron coppers, and causticised by treatment 

 with caustic lime, whereby the carbonate of soda is converted into caustic soda, 



