CHEMISTRY OF OXIDATION OF PYRITE 275 



lime, or manganese. That the mineral coating thus formed on the jig- 

 concentrates is clearly chalcocite can not be doubted from a mere inspec- 

 tion of the samples and comparison with ore from the mines. 



Chemistry of Oxidation of Pyrite 



The chemical changes involved in the oxidation of pyrite liave been 

 discussed by S. H. Emmons* and by Penrose,t and more recently by 

 Van Hise, Weed, Emmons, and others, and the equations given indicate 

 the formation of sulphur and sulphurous anhydride as a product of such 

 oxidation. 



The real agency of this SO2 and its probable role as an active reducing 

 agent, capable, for example, of reducing cupric sulphate to cuprous sul- 

 phate preparatory to its deposition as cuprous sulphide, seems to have 

 been overlooked. The equations given by Emmons are as follows : 



" 1. FeS, + O3 + HjO = FeS + H2SO4. 



2. FeS + H2SO4 = FeSO, + H2S. 



3. FeSa + O + 2H2S = FeS + 2H2O -f 3S. 



4. S + O2 + H2O = H2SO4. 



That is to say, the oxygen of the atmosphere and the moisture of the ground 

 and air convert part of the sulphur into sulphuric acid and leave a residue of iron 

 mcnosulphide, which is then attacked by the sulphuric acid with formation of 

 ferrous sulphate and evolution of sulphuretted hydrogen. This latter reacts with 

 the sulphurous anhydride formed (together with sulphuric acid) by the oxidation 

 of the sulphur in the marcasite, and produces water and free sulphur, the latter of 

 which is in its turn oxidized and produces a further quantity of sulphuric acid, and 

 so on. Hence, as the result of the first attack on the ores we should expect to find 

 ferrous sulphate, sulphuretted hydrogen, free sulphur, and surphuric acid. 



Now, a solution of ferrous sulphate eagerly absorbs atmospheric oxygen and sul- 

 phuric acid to form ferric sulphate, thus, 



2FeS04 + O + H2SO4 = FeaCSOJs + H2O, 

 and therefore, although I have spoken of ferrous sulphate and free sulphuric acid 

 as amongst the first results of the gossan-forming action, they are rapidly converted 

 into a solution of ferric sulphate ; and it is in this latter form that they are usually 

 found in mine-waters and the like. In some cases, however, where local circum- 

 stances impede peroxidation, ferrous sulphate remains in considerable quantity, 

 as, for example, in the manufacture of copper by exposing large heaps of pyrite to 

 the action of the atmosphere and moisture. 



Let us next consider what will be the action of the ferric sulphate upon the 

 remaining ferrous sulphide. This is shown by the following equation : 



FeS + Fe2(SO,>3 = 3 FeSO^ f S ; 

 or, in other words, one molecule of ferric sulphate will abstract one molecule of 

 iron from ferrous sulphide, forming 3 molecules of ferrous sulphate and setting 

 free the sulphur." 



* Engineering and Mining Journal, Dec. 17, 1892, p. 582. 

 t Journal of Geology, 1894, pp. 288-317, 



