1992 Volume 42 Issue 234 Pages 241-247
The dissolution of limestone collected from Akiyoshidai, Yamaguchi Pref., Japan, in dilute hydrochloric acid solution has been followed by measurements of calcium concentration and pH as a function of time, under various conditions of initial pH, temperature and stirring rate. The initial dissolution rate increases as the stirring rate increasing. The Arrhenius plot shows that the apparent activation energy at 700rpm of stirring rate is 15kJ/mole for initial pH=4 and 25kJ/ mole for initial pH=5.
The viscosity of water depends on temperature and differs about two times within the range from 5°Cto 4°C in this experimental conditions, so stirring rate cannot express the real conditions of bulk fluid. In spite of stirring rate, Stirring Reynolds Number ReS, which is one of non-dimension numbers possible to express fluid condition, defined as ρd2n/μ where ρ is the density of water, d characteristic length of stirring rate, n the revolutions per second, μ the viscosity of water, is proposed.
The initial dissolution rate at initial pH=4 is almost proportional to ReS and is independent from temperature. These facts suggest that the dissolution reaction process of calcite is much faster than the mass transport process of species. In the case of initial pH=5, the initial dissolution rate increases as ReS increasing and as temperature rasing. The apparent activation energy of initial dissolution rate at ReS =7×104 for initial pH=4is 1kJ/mole and for initial pH=5 is 12kJ/ mole. These values are smaller than those obtained from stirring rate, because the previous studies have not considered fluid condition in estimation of the activation energy.
The facts mentioned above suggest that dissolution of calcite in weak acid region is mass transport controlled reaction.
