Dissolution Kinetics of Dolomite in Water at Elevated Temperatures

Abstract

Kinetic experiments of dolomite dissolution in water over a temperature range from 25 to 250°C were performed using a flow through packed bed reactor. Authors chose three different size fractions of dolomite samples: 18–35 mesh, 35–60 mesh, and 60–80 mesh. The dissolution rates of the three particle size samples of dolomite were measured. The dissolution rate values are changed with the variation of grain size of the sample. For the sample through 20–40 mesh, both the release rate of Ca and the release rate of Mg increase with increasing temperature until 200°C, then decrease with continued increasing temperature. Its maximum dissolution rate occurs at 200°C. The maximum dissolution rates for the sample through 40–60 mesh and 60–80 mesh happen at 100°C. Experimental results indicate that the dissolution of dolomite is incongruent in most cases. Dissolution of fresh dolomite was non-stoichiometric, the Ca/Mg ratio released to solution was greater than in the bulk solid, and the ratio increases with rising temperatures from 25 to 250°C. Observations on dolomite dissolution in water are presented as three parallel reactions, and each reaction occurs in consecutive steps as

$$ \hbox{CaMg}({{\rm CO}_{3}})_{2} ({\rm s})={{\rm MgCO}_{3}}({\rm s})+{{\rm Ca}^{2+}}+{\rm CO}_{3}^{2-} $$

$${\rm MgCO_{3}}({\rm s})={{\rm Mg}^{2+}}+{{\rm CO}_{3}}^{2-} $$

where the second part is a slow reaction, and also the reaction could occur as follows:

$$ {{\rm CaMg}({{\rm CO}_{3}})_{2}}({\rm s})+{{\rm Mg}^{2+}}={{\rm Ca}^{2+}}+{2{\rm MgCO}_{3}}({\rm s}) $$

The following rate equation was used to describe dolomite dissolution kinetics

$$ {\rm Rate}= \Upsigma r_{ij}=\Upsigma k_{ij}(a_{i})^{n} $$

where $\Upsigma{r}_{ij}$ refers to one of each reaction among the above reactions; k _ij is the rate constant for ith species in the jth reaction, a _i stands for activity of ith aqueous species, n is the stoichimetric coefficience of ith species in the jth reaction, and define $n=n_{ij}$. The experiments prove that dissolved Ca is a strong inhibitor for dolomite dissolution (release of Ca) in most cases. Dissolved Mg was found to be an inhibitor for dolomite dissolution at low temperatures. But dissolution rates of dolomite increase with increasing the concentration of dissolved Mg in the temperature range of 200–250°C for 20–40 mesh sample, and in the temperature range of 100–250°C for 40–80 mesh sample, whereas the Mg²⁺ ion adsorption on dolomite surface becomes progressively the step controlling reaction. The following rate equation is suitable to dolomite dissolutions at high temperatures from 200 to 250°C.

$$ {-r_{\rm Ca^{2+}}}= k (m_{\rm Ca^{2+}})^{n}+k_{\rm ad} ({{K}_{\rm Mg^{2+}}} m_{\rm Mg^{2+}}) /(1+ {K}_{\rm Mg^{2+}} m_{\rm Mg^{2+}} ) $$

where $-{r}_{\rm Ca^{2+}}$ refers to dissolution rate (release of Ca), $m_{\rm Ca^{2+}}$ and $m_{\rm Mg^{2+}}$ are molar concentrations of dissolved Ca and Mg, k _ad stands for adsorption reaction rate constant, K _Mg refers to adsorption equilibrium constant.

At 200°C for 40–60 mesh sample, the release rate of Ca can be described as:

$$ {-r ({\rm mol}\,{{\rm m}^{-2}}\,{{\rm s}^{-1}})}={0.55\times10^{-4}}(m_{\rm Ca})^{-0.36}+0.135\times10^{-4}(m_{\rm Mg})/(1+ 0.14\,m_{\rm Mg}) $$

Appendix 1: Saturation Index

The dolomite saturation index was defined as

$$ \hbox{sid} = 1/2\,{\rm log}\,({\rm Activity\,product\,of\,output\,aqueous\,species\,of\,Ca,\,Mg,\,CO}_{3})/ K_{s} $$

(A1)

where K _s, the thermodynamic solubility product of dolomite was also calculated by using Shvarov software HCh (Shvarov 1989).

Concerning the bulk concentration of the solution in which calcite dissolves, the difference between the solubility product K _s, and the ion activity product Q i.e.,

$$a_{{\rm CO}_3^{2-} } a_{{\rm Ca}^{2+}} $$

The (${K}_{\rm s}-Q$) could be also described for the distance of the system from equilibrium as well as saturation index (Morse 1983; Lasaga 1981; Sjöberg and Rickard 1983).

In general, to determine the correct rate law to use, experiments would be carried out at far from equilibrium. Thus, for calcite dissolution processes, the simplified form of the rate law can be described as

$$ -r=k(T)Sa(a_{{\rm Ca}^{2+}}a_{{\rm CO}_{3}^{2-}}-K_{S})^{n} $$

(A2)