Oxygen mass transfer into flotation slurries - Part 1: effect of mild steel corrosion during grinding of quartz

Under ordinary conditions of milling the consumption of oxygen by the grinding media relative to its consumption by a massive sulphide ore is not known. To help to distinguish between the two oxygen mass transfer into flotation slurries has been investigated under various conditions. Aeration studies were conducted on a quartz slurry following its grinding in a mild-steel mill in the absence and presence of sodium chromate as a corrosion inhibitor. During these studies such variables as the oxygen concentration, pH and pulp potential were monitored as a function of time by means of a customized data-acquisition system. A systematic increase in the amount of iron-bearing deposition on quartz particles was observed with increasing grinding time, which was accompanied by a decrease in pulp potential. The oxygen mass-transfer coefficient showed an exponential decrease from a value of 7.5 s(-1) (in tap water) to about 0.11 s(-1) (in slurry) with increasing fineness of quartz ground in the absence of sodium chromate. This trend appears to be completely reversible through the use of sodium chromate in grinding, the coefficient attaining a value that can be even greater than its original value prior to any grinding. The variations of pH and pulp potentials are discussed in relation to certain electrochemical and hydrolysis reactions relevant to the current system. An Eh-pH diagram constructed using specific experimental conditions indicates formation of an iron chromate as the main product of corrosion inhibition.