Kinetics of Calcite Dissolution in pH-region of Acid Rain.

本文研究了日本山口明代石灰石在稀盐酸溶液中的溶解过程，测定了不同初始pH、温度和搅拌速度下钙离子浓度和pH随时间的变化关系。初始溶出速率随搅拌速度的增大而增大。阿累尼乌斯曲线表明，在搅拌速度为70orpm时，初始pH为4时的表观活化能为15kJ/ﾟ，初始pH为5时的表观活化能为25kJ/ﾟ。在本实验条件下，水的粘度与温度有关，在5~40ﾟC的范围内，粘度相差约2倍，因此搅拌速度不能反映大块流体的真实状态。不考虑搅拌速度，定义了表征流体状态的无量纲数之一的搅拌雷诺数Re_S，定义为Pd^2n/u，其中Rho为水的密度，d为搅拌速度的特征长度，n为每秒转数，u为水的粘度，初始pH为4时的初始溶解速率几乎与Re_S成正比，与温度无关。这些事实表明，方解石的溶解反应过程比物质的传质过程快得多。在初始PH5的情况下，初始溶出速率随着Re_S的增大和温度的升高而增大。在Re_S=7×10~(-4)时，初始pH_4的表观溶解活化能为1kJ/m ol，初始p H_5时的表观溶解活化能为12kJ/m o l。由于以前的研究在估算活化能时没有考虑流体条件，这些数值比搅拌速度得到的数值要小。上述事实表明，方解石在弱酸性区域的溶解是传质控制的反应。

The dissolution of limestone collected from Akiyoshidai, Yamaguchi Pref., Japan, In dilute hydrochloric acid solution has been followed by measurement both 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 apparent activation energy at 7OOrpm of stirring rate is 15kJ/mol for Initial pH4 and 25kJ/mol for Initial pH5.The viscosity of water depends on temperature and differs about two times within the range from 5ﾟC to 40ﾟC in this experimental conditions, so stirring rate cannot express the real conditions of bulk fluid. In spite of stirring rate, Stirring Reynolds Number Re_s, which is one of non-dimension numbers possible to express fluid condition, defined as pd^2n/mu, where rho is the density of water, d characteristic length of stirring rate, n the revolutions per second, mu the viscosity of water, is proposed.The initial dissolution rate at initial pH4 is almost proportional to Re_s 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 pH5, the initial dissolution rate increases as Re_s Increasing and as temperature raising. The apparent activation energy of initial dissolution rate at Re_s = 7X10^4 for initial pH4 is lkj/mol and for initial pH5 is 12kJ/mol. 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.