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XLIV. On some Points in the Chemistry of Acid- manufacture. 

 By H. A. Smith, Junior Assistant in the Laboratory of Owens 

 College f Manchester^. 



Section I. On the presence of Arsenic in Alkali-manufacture, 



jyRESENCE of Arsenic. — The great drawback in the ma- 

 nufacture of sulphuric acid from pyrites is most undoubt- 

 edly the presence of arsenic. Its removal, even if it can be done 

 completely, is a work of difficulty and expense, as in our methods 

 for purification we must take into account the various uses to 

 which the acid is to be put. It is my intention in the present 

 part of my paper to trace this impurity (arsenic) from the original 

 pyrites, through the various operations with which the acid made 

 from it is connected, to the last stages of alkali-manufacture ; 

 and to show also that, not content with throwing the injurious 

 gases of sulphurous and hydrochloric acids into the air, alkali- 

 works must bear the blame of polluting the atmosphere with the 

 still more dangerous substance — arsenious acid. Although the 

 amount escaping from a single work is comparatively small, yet, 

 when we consider the number of works using pyrites in the for- 

 mation of sulphuric acid, we must confess that in the end it 

 mounts up to something very considerable. 



General amounts in various pyrites. — There are two things to 

 be looked to in choosing an ore for sulphuric acid-manufacture. 

 1st. Its breaking property, if I might so call it, — that is, its 

 power of breaking into small lumps without leaving what are 

 technically called " smalls."" 2ndly. Its freedom from arsenic. 



If we compare the amount of arsenic in published analyses of 

 various kinds of pyrites, we shall be astonished at the difference 

 between these and the amounts of arsenic found in the acid ma- 

 nufactured from the same ores. From ores containing from 

 0*21 to 0-31 per cent, arsenic we have acid containing from 1 to 

 1'5 per cent, arsenic, showing that some mistake has been made 

 in the analyses. 



Taking, for instance, the analyses of various pyrites given in 

 Richardson andWatts's 'Chemical Technology,^ I find that the 

 largest mean percentage of arsenic present varies from 0'31 to 

 0*33 per cent, arsenic, whilst some are mentioned as containing 

 merely a " trace,^'' and others as being perfectly pure. These 

 results, however, are never corroborated when these ores are 

 being worked on a manufacturing scale. 



Being obliged, in my capacity as chemist in an alkali-work, 

 to turn my attention to this subject, I determined to make an 

 extended series of analyses of such specimens of pyrites as were 



* Communicated by the Author. 



