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Dissolution Kinetics of Dolomite in Water at Elevated Temperatures

Content Provider	Springer Nature Link
Author	Zhang, Ronghua Hu, Shumin Zhang, Xuetong Yu, Wenbin
Copyright Year	2007
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 $$ {\rm Rate}= Σ r_{ij}=Σ k_{ij}(a_{i})^{n} $$ where $$Σ{r}_{ij}$$ 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: $$ {-r ({\rm mol}\,{{\rm m}^{-2}}\,{{\rm s}^{-1}})}={0.55\times10^{-4}}(m_{\rm Ca})^{-0.36}+0.135\times10^{-4}(m_{\rm Mg})/(1+ 0.14\,m_{\rm Mg}) $$
Starting Page	309
Ending Page	338
Page Count	30
File Format	PDF
ISSN	13806165
Journal	Aquatic Geochemistry
Volume Number	13
Issue Number	4
e-ISSN	15731421
Language	English
Publisher	Springer Netherlands
Publisher Date	2007-11-01
Publisher Place	Dordrecht
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Dolomite High temperature Dissolution kinetics Release rate Adsorption reaction Hydrogeology Geochemistry
Content Type	Text
Resource Type	Article
Subject	Geophysics Geochemistry and Petrology

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