40 FOREST PRODUCTS 



Screening. 



The chips pass from this belt into a large revolving screen, or in some 

 cases, a flat jigger screen is used. As the chips pass along this screen 

 which has small openings at the head end, gradually increasing in size, 

 the fine slivers and dust are removed first. Next, the good chips them- 

 selves pass through the holes and the knots and large pieces drop out at 

 the lower end. These chips drop into a trough and are conveyed to a 

 storage bin, directly over the digesters and cooking room, while the waste 

 is conveyed to the boiler house and used for fuel. 



Acid Manufacture and Storage. 



In the sulphite process, the acid plant is one of the most important 

 parts of the mill. Acid making is a truly chemical process and in these 

 mills it is as much a part of the industry as the cooking or reduction of the 

 wood. The basis of this cooking liquor or acid is sulphurous acid and is 

 made by passing sulphur dioxide gas through water. 



In modern mills pure sulphur is burned either in caldrons or rotary 

 burners in the presence of an excess of oxygen. Part of the sulphur burns 

 to sulphur dioxide but a portion burns only to the oxide or monoxide. 

 In order to further oxidize this it is passed through a large oxidizer, which 

 is very similar to a Bunsen burner. Here oxygen is admitted and the 

 gas burns to the dioxide. The gas is then drawn through a series of three 

 water coolers where it passes through lead pipes surrounded with cold 

 running water. This cools the gas down to about 70° C. A set of fans 

 is arranged in this system and by their work they suck the gas this far, 

 furnishing the draft for the burner and oxidizer. The gas passes from 

 the last cooler directly into the fans and in the tower system of acid man- 

 ufacture is driven into large towers filled with limestone. The gas enters 

 these towers at the bottom and as it passes upward, it comes in contact 

 with many small streams of water which are trickling down over the 

 stone, the water being admitted at the top. The sulphur gas also unites 

 with a part of the limestone which has been dissolved by the water and 

 forms a solution of acid calcium sulphite which constitutes the cooking 

 liquor. As the liquor reaches the bottom of the system it is pumped into 

 large wooden storage tanks and there kept until needed. 



The acid, coming down through one tower is not sufficiently strong 

 to eat away the lignin, resins, etc., of the wood which must be reduced. 

 The proper strength is obtained by pumping the liquor from the bottom 

 of the first tower, up and into the top of a second, where it again passes 

 through a dense cloud of gas, as it runs down over the limestone. This 



