Chemistry of pulp and paper making . type of burners was con-siderably more difhcult to control than the burning of sulphurand it could be worked advantageously only where the burnerscould be grouped together in sufficient numbers to insure gas ofeven composition. Many of these difficulties have been over-come by modem mechanical furnaces of which the Herreshoff fur-nace is a type. These burners (see Fig. 25) usually have fiveshelves over which the pyrites is raked in succession by mechan-ically operated rakes. The shafts and arms are hollow and are PREPARATION OF SULPHUR DIOXIDE 167 cooled by
Chemistry of pulp and paper making . type of burners was con-siderably more difhcult to control than the burning of sulphurand it could be worked advantageously only where the burnerscould be grouped together in sufficient numbers to insure gas ofeven composition. Many of these difficulties have been over-come by modem mechanical furnaces of which the Herreshoff fur-nace is a type. These burners (see Fig. 25) usually have fiveshelves over which the pyrites is raked in succession by mechan-ically operated rakes. The shafts and arms are hollow and are PREPARATION OF SULPHUR DIOXIDE 167 cooled by a current of air supplied by a fan; part of the hot airthus produced is used in the lower parts of the chamber andmaterially assists combustion. The heat produced by the oxida-tion of the sulphur and iron is sufficient for carrying on the opera-tion and once the furnace is in good working condition no fuel isrequired. The spent pyrites or cinders leaves the furnace withI per cent to 4 per cent of sulphur. The separation of dust is. Fig. 25. Herreshoff Pyrites Burner particularly important where pyrites is used. This was formerlydone by passing the gas through long chambers of large area sothat the velocity of the gas should be slight. In modem practiceit is more successfully accompHshed by passing the gas throughtowers into which water is sprayed; this not only removes dust 168 THE SULPHITE PROCESS but also takes out SO3 and cools the gas. Provided the washwater is discharged at 175° F. (80° C.) the loss from dissolvedSO2 is very slight. The gas from sulphur burners operating under satisfactoryconditions generally contains 14 to 18 per cent of SO2 by volume;the maximum which it can possibly contain is 21 per cent. Inthe case of pyrites the theoretical maximum is per cent ofSO2 and it generally runs about 10 to 14 per cent. As a rule whenburning sulphur about 2 to 3 per cent is converted into SO3 whilewith pyrites as much as 13 per cent may be lost in that way. After lea
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