The greater part of sulphuric acid plants in Japan roasts pyrite or pyrrhotite in multiple hearth mechanical roaster such as Herreshoff furnace. As the demand of sulphuric acid is remarkably increased at present, much more quantities of sulphide are are inclined to be fed to roasters than their capacity; the increment of residual sulphur content of cinder is due to this fact. However, increment of residual sulphur means unprofitable utilization efficiency of sulphur in manufacturing sulphuric acid, and moreover, pyrite cinder of high residual sulphur content is met with some obstacles when it is used as iron ore. The authors carried out some experinients of industrial scale using multiple hearth mechanical roaster, such as Herreshoff furnace or Wedge furnace. The results are summarized as follows:
1. The residual sulphur content of pyrite cinder increases when the charging rate of roaster deviates from its normal capacity.
2. The residual sulphur decreases when the revolving velocity of arm increases.
3. Usually, mechanical roasters are supplied the air to their lowest hearth. But when the greater part of air is supplied to upper hearth, instead of the lowest hearth, the residual sulphur decreases from 3% to 1.5%, and it is possible to roast 26 t/d of pyrite in the Herreshoff furnace of 20 t/d capacity without increasing residual sulphur.
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