Self-Diffusion In Minerals, Particularly Copper Sulphides

IN a study1of the action of amyl xanthate on chalcocite in water suspension, it was found that if the xanthate is added in relatively large quantity the xanthate group can be accounted for in three forms: (1) as xanthate ion remaining in water, (2) as benzene-soluble cuprous xanthate on the chalcocite, (3) as a benzene-insoluble coating on the chalcocite. Reduction in the amount of xanthate added leads to disappearance of form 1 then of form 2. With minute quantities of added xanthate, substantially all the reagent is of the "unleachable" kind. Yet this unleachable xanthate can be dissolved by pyridine to form a soluble cuprous xanthate complex, and it is sufficient in effectiveness to make chalcocite flotable even if insufficient in quantity to form a complete monomolecular film on the mineral. In that study it was also found that chalcocite abstracts cuprous xanthate from benzene solution, in what may (or may not) be a case of molecular adsorption. It seemed desirable, therefore, to study further the "unleachable" xanthate on the chalcocite, and particularly to ascertain whether it is formed as a result of the benzene leach or is actually formed during the conditioning of chalcocite in aqueous xanthate solution. A splendid tool to make such studies has recently been perfected; that is, the use of radioactive spies or tracer atoms. With this tool, it was thought possible to answer the problem stated above, but, unfortunately, this did not prove to be true. Answer to the flotation question was clouded by the existence of rapid and selective diffusion of copper atoms in the solid mineral-a phenomenon that was totally unexpected yet remains interesting and perhaps important. METHOD OF RADIOACTIVE SPIES The method of radioactive spies is discussed in many places. Two convenient references are the article by Evans2 and the book by Pollard and Davidson.3 The method was first developed by Von Hevesy and Paneth in 1913, when only natural radioactive isotopes were available. These isotopes are few in number, and so the method remained limited in scope. Recently the synthesis of over 350 active isotopes of all the known stable chemical elements4 has opened widely expanded possibilities for the method. A fundamental tenet of the method is that a radioactive atom and a nonradioactive atom are exactly alike until disintegration of the radioactive atom. Then, by its sudden death, the spy reveals its location. The relative abundance of spies at various locations makes it possible to ascertain the relative abundance of the corresponding nonradioactive atoms at the same times and locations. A second fundamental tenet of the method of radioactive tracers is that the radioactivity lies in the nucleus of the atom: transfer of radioactivity from one