April 1907. J 



225 



Timbers. 



and the caustic lime into carbonate of lime or chalk. The chalk forms a sludge 

 at the bottom of the vessel, from which the remainder of the liquor can be removed 

 by filter pressing. 



The sludge is pumped into a filter press to remove the liquor still remaining, 

 and water caused to percolate througii to remove the last traces. A clear caustic 

 liquor is ready to be used again in the process of boiling. The process of recovery 

 results in a certain amount of loss of the soda, amounting to about 15 per cent. This 

 has to be made good by the addition of a certain amount of caustic or carbonate of 

 soda. Soda wood pulp is generally of stuff of the nature of sulphite, and though Of 

 darker colour, is, as a rule, easier to bleach. Of recent yeras the soda process has 

 gone to a large extent out of: use and has been replaced by the sulphate process i 

 This process consists in treating wood chips in an iron digester with sulphate of soda 

 containing in the first instance a certain amount of caustic. The process is con- 

 ducted very much like the soda process. It is carried up to the stage of the recovery 

 process in a similar manner ; in the soda process, however, the recovered ash consists 

 of carbonate of soda, whereas in the sulphate process the recovered ash consists of 

 sodium sulphide and sulphate of soda. The liquor ready for use again consists of 

 caustic carbonate, sulphide, and sulphate of soda. The process is cheaper than the 

 soda process, because instead of making up for the loss of the soda by the addition of 

 caustic or carbonate it is made up with sulphate of soda, which is a much cheaper 

 chemical ; sulphate passing through the recovery process is reduced by the organic 

 matter to sulphide ; a considerable amount of this sulphide is decomposed through 

 the treatment of the wood, giving rise to sulphuretted hydrogen. The gases eman- 

 ating from a sulphate factory render it necessary to conduct the process in districts 

 where noxious factories are not interfered with. 



A. great deal of the wood pulp sold as soda pulp is, I am assured by a leading 

 expert in reality sulphate, and he tells me the proportion appears to be increasing 

 every year. On this subject I am unable to express an opinion, but I am quite 

 certain the British paper-maker secures delivery of chemical pulp capable of being 

 used for the purposes intended.— Journal of the Society of Arts. 



