WOOD PULP AND PAPER 41 



liquor is then pumped from the bottom of this second tower, up and into a 

 third and the process is repeated. On reaching the bottom of this third 

 tower the acid is strong enough to do its work and is pumped away. At 

 this stage it is strong enough to "eat" metals and is particularly harmful 

 to iron and steel. This makes it necessary to handle the substance in 

 lead pipes and containers or wooden tanks, etc. 



Several other systems of sulphite acid manufacture are in use, tanks 

 and flat vats fiUed with milk of lime being charged with sulphur dioxide 

 to form acid calcium sulphite, but the tower system is believed to be the 

 most efficient and economical. 



In handling the gas, great care must be taken to keep all copper and 

 brass out of the wa}- as the gas wOl unite with the copper to form copper 

 sulphide. In fan blades and all parts which must be hardened, a hard- 

 ened lead is used. This consists of a mixture of lead and antimony. 



The limestone which is used in these towers is not pure lime car- 

 bonate and as the lime is dissolved away a large amount of refuse in the 

 form of sand, and other minerals is left. This must be cleaned out at 

 intervals of from three to five days. As it is necessary to shut down a 

 system entirely while it is being cleaned, an extra system must be main- 

 tained and run while any other system is closed for cleaning and refilling 

 with stone. 



A test of the acid is made ever}- hour and record is sent into the office 

 each day. 



Statistics show that it requires a three-tower system, with two towers 

 making acid twenty-four hours a day for six days of the week and twelve 

 hours on one day to suppty this cooking liquor for a loo-ton mill. This 

 varies in \^-inter and siunmer as more acid and stronger add can be made 

 in the winter with cold water than in the summer when the water is 

 warm. This requires the burning of 8000 lb. of sulphur and the use of 

 about 25 tons of limestone per day. 



Cooking. 



The cooking, which is the chemical process which reduces the wood 

 elements to soluble compounds leaving only the cellulose, is carried on in 

 large steel retorts, which taper to a neck at each end and vary in size 

 according to the desired capacity. The outside measurements of a 3-ton 

 digester are 32 ft., neck to neck, and it is 10 ft. in diameter. A 5-ton 

 retort is 45 ft., neck to neck, and is 10 ft. 6 in. in diameter. Retorts vary 

 in size from 3 to 20 tons. Modern mills use the largest size because of 

 the econom\- in operation. These retorts are lined with two layers of 



