Nucleation kinetics in the reactions of nickel basic carbonates with hydrogen sulfide: The carbonate precipitation reactions of divalent nickel

Basic carbonates of nickel (Ni(CO3)(x)(OH)(y)center dot zH(2)O, abbreviated hereafter as NBC) are uniquely reactive in absorption with reaction of dilute hydrogen sulfide (H2S) in nitrogen gas (N-2). The rate is fast at ambient temperature and the very high conversion is unusual, by comparison with other oxide and carbonate absorbents. Precipitated basic carbonates are amorphous but can be prepared in quite narrow composition ranges via partial precipitation in a narrow pH range. At the lowest pH (typically 5.4) and in a system saturated by CO2 at 1 atm, the CO32-:Ni(II) ratio is x approximate to 0.75 (LpH-NBC chosen for detailed study). At pH similar to 11, with ambient CO2 level, the ratio is x approximate to 0.33. These solids are unstable in the atmosphere and undergo hydrolysis or carbonation, respectively. The sulfiding reactions have variable induction periods, after which the rate accelerates to a maximum (nucleation kinetics) at a Ni(II) conversion that is frequently in the range of 0.4-0.5. The maximum rate, its crest time and the conversion (X-m,X-Ni) were used as the principal observables in the kinetic study and in modeling. Apparently, the nickel sulfide product is the catalyst for the nucleation process, while water vapor is also a promoter. For freshly precipitated LpH-NBC material, the nucleation rate increases as the H2S concentration increases but not as the water vapor concentration increases, whereas initiation is promoted by water. In ambient air, deactivation of LpH-NBC material is progressive and probably is associated with hydrolysis as a shrinking-core process; the crushing of aged NBC effects substantial reactivation. Two-term initiation-nucleation power laws were used to model the reaction in a packed-bed reactor (pseudo-homogeneous tanks in series). The nucleation (or autocatalytic) kinetics provides a useful probe for flow characteristics in such a reactor.