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Initial rates were measured for the reduction of single pellets of nickel oxide with carbon monoxide a t atmospheric pressure and temperatures from 566 to 796°C. The nickel oxide pellets were porous (; l o = 0.032 -0.35) and intrapellet diffusion retarded the rate so thot the shrinking core model was not applicable. Effective diffusivities for the lower porosity pellets were very small, corresponding to tortuosity factors of about 180 for 50 = 0.032 and 92 for GI = 0.062.The reaction appeared to be first order in carbon monoxide. Rate constants, bosed upon the intraparticle area, indicated an activation energy of 47 kcol/g.-mole over the temperature range 566 to 682°C. A t higher temperatures the rate was constont. These results, while not conclusive, are in agreement with a reaction sequence consisting of formation of nuclei of nickel atoms in the nickel oxide surface, rapid adsorption of carbon monoxide a t the nickelnickel oxide interface, migration of the adsorbed carbon monoxide to the adjacent lattice, and a slow reaction to extract oxygen from the lattice.The reduction of ores is a major industry throughout the world, yet the design of reduction equipment has not advanced to the level of many other kinds of reaction apparatus, for example, fixed-bed catalytic reactors. An appropriate starting point for improving design concepts is a study of the intrinsic kinetics of the gas-solid, noncatalytio reactions that occur during reduction. Metallic oxides are seldom nonporous. Hence, observed rates are not a measure of intrinsic kinetics due to coupling with transport processes and by uncertainty about the true reaction area.To avoid these problems data have heretofore been analyzed (3, 10, 12, 1 7 ) by the shrinking-core model (19).Except for those few cases where a very sharp boundary exists between unreacted oxide and metal product, this model gives unrealistic rate constants and activation energies. The present work was undertaken in order to obtain realistic rate constants (intrinsic kinetics) by measuring intraparticle areas and accounting for intraparticle diffusion for a practical system, Intrinsic kinetics were investigated for the reduction of nickel oxide pellets with carbon monoxide in the presence of carbon dioxide. Initial rates were measured gravimetrically for single, sintered pellets with porosities of 0.35 to 0.032. Pore-volume distributions were used to evaluate reaction areas and, with the rate data, to obtain effective diffusivities for the pellets of lower porosity.The reduction reaction is essentially irreversible up to temperatures above 796 "C.Also the operating conditions were chosen (temperatures of 566 to 796°C. and carbon monoxide partial pressures of 0.07 to 0.33 atm.) such that no deposition of carbon by the reaction 2CO -+ C + COe was predicted from thermodynamic information. X-Ray analysis of some of the pellets after reduction verified the absence of carbon and of Nig C. Both substances might be produced at lower temperatures and higher carbon monoxide concentrations. The system ...

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