PRODUCTION OF SULPHITE PULP PROM SPRUCE. 9 



fed in through a U-tube, the receiving end (which was about 5 feet 

 higher than the top of the cylinder) having a large lead dish attached, 

 into which the sulphuric acid was poured. As the sulphuric acid 

 mixed with the sodium bisulphite, pure sulphur dioxide was liber- 

 ated. This was passed through a Wbulf bottle, containing a little 

 water as a trap to catch any sulphuric acid which might boil over. 

 From here the gas entered the absorption tank. As the gas pro- 

 duced was pure, sulphur dioxide solutions of any strength desired 

 could be produced. 



Making the cooking liquor was not so quick or easy, nor was it so 

 simple and convenient with the gas as with the sulphur dioxide in 

 liquid form, so that the liquid was again used as soon as a supply 

 was once more available. 



METHOD OF COOKING. 



The method of cooking received a great deal of consideration, as 

 one variable, the pressure, was hard to control. The ordinary method 

 of cooking by direct steam and relieving sulphur dioxide at the top 

 is unsuitable for experimental cooking because, first, direct heating 

 causes too much condensation of steam in the digester with conse- 

 quent dilution of the cooking liquor and, second, relieving S0 2 at 

 the top would have a tendency to destroy the effect which we were 

 trying to study. For example, in studying different strengths of 

 cooking liquor, there would be no object in starting with a strong 

 liquor, only to blow all the sulphur dioxide out of the digester in a 

 short while. Therefore it was decided to heat the digester by means 

 of indirect steam, that is, by means of a lead coil placed in the bot- 

 tom of the digester; and no gas was allowed to escape until the cook 

 was finished. While this method of cooking differs from that used 

 in commercial practice, it will bring out more than any other the 

 factors under investigation. 



The method of procedure in making a cook was as follows: The 

 chips, the amount of which had previously been determined by 

 means of a bone-dry sample, and which remained the same for every 

 cook, were put into the digester and 63 gallons of cooking liquor in 

 each case were then run in and the digester tightly closed. The 

 steam to the coil was then turned on and the cook started. The 

 cooking curve for temperature was the same in each case, taking 

 two hours to reach 100° C, and three hours to reach the maximum 

 temperature, where it was held until it was finished, the pressure 

 being allowed to go where it would. Keeping the full strength of 

 the liquor to the finishing point in this way gave a hard pulp, which 

 had the characteristics resulting from beating for a long time. 



The curves for all cooks are shown in figures 3 to 7. In each 

 figure, curve 1 shows the way the temperature was increased and 

 14646°— 18— Bull. 620 2 



