KINETICS OF PYRRHOTITE OXIDATION IN SEAWATER: IMPLICATIONS FOR MINING SEAFLOOR HOTSPRINGS

A rapid increase in the price of transition metals in recent years has piqued interest in deep sea in situ mining of seafloor massive sulfide (SMS) deposits. There are unique incentives to seafloor mining that make it more attractive than traditional land mining of sulfides, but these are accompanied by important unanswered questions about the potential environmental effects, particularly localized sulfuric acid generation. Currently there is a paucity of data on the oxidation kinetics of sulfide minerals in seawater. Pyrrhotite specifically is of interest because it is a non-economic mineral that will be disposed of on or above the seafloor during mining. Pyrrhotite oxidizes rapidly via an irreversible, acid-producing reaction. Knowledge of sulfide mineral oxidation rates will also provide constraints on metal and sulfur cycling in oceans by quantifying the natural, abiotic weathering rates of SMS deposits.

Laboratory experiments have been performed to evaluate the effects of pH, temperature, oxidant concentration, and mineral surface area on the rate of oxidation of pyrrhotite in seawater. Temperature controlled circulation baths, Teflon reaction vessels, synthetic seawater, and pure, hand sorted natural pyrrhotite crystals are used in experiments. Both batch and flow-though reactor methods are employed. Reaction products are analyzed using ICP-MS. The rate law is derived as follows:

R = -k (M_O2,aq)^a(M_H+)^b

where R is the specific oxidation rate of pyrrhotite, k is the rate constant (a function of temperature and surface area), and a and b are reaction orders for the molar aqueous species’ concentrations (M) to be determined experimentally. The initial rate method is used to isolate the reaction order of each variable.

Experiments to date indicate a rate of oxidation up to five times faster than chalcopyrite. This represents an upper limit to the anthropogenic and natural weathering rates of SMS deposits. The rate law can be used in computer models to predict the fastest local rate of oxidation and sulfuric acid generation specific to in situ mining, and to predict the natural weathering rates of SMS deposits and their contributions to the cycling of transition elements and sulfur to the oceans.