Reactions of acid with limestone and dolomite

The following factors affect the spending rate of acid in carbonate formations  

Temperature. Acid reaction rate increases with temperature. At about 150°F, the reaction rate of HCl and limestone is about twice that at SOT. The reaction rate of HCl in limestone is faster than in dolomite up to about 250°F (or somewhat less). At higher temperatures, the rates of reaction in limestone and dolomite are equally fast. 

Pressure. Pressure greater than 500 psi has little effect on reaction rates. Below 500 psi, increased pressure accelerates reaction rate. 

Acid type. Acid strength varies with acid type. Add strength is defined by ionization strength, or the degree to which acid ionizes to hydrogen ion 

Acid concentration. The rate of HCl spending reaches a maximum at a concentration of about 20%. At higher concentrations, the large volumes of CaClj and CO generated in solution have the effect of retarding the reaction. Acetic and formic adds are naturally retarded by their reaction products, caldum acetate and calcium formate, respectively. The benefit from increased concentration diminishes. 

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Sulfur dioxide emissions may affect building stone and ferrous and nonferrous metals. Sulfurous acid, formed from the reaction of sulfur dioxide with moisture, accelerates the corrosion of iron, steel, and zinc. Sulfur oxides react with copper to produce the green patina of copper sulfate on the surface of the copper. Acids in the form of gases, aerosols, or precipitation may chemically erode building materials such as marble, limestone, and dolomite. Of particular concern is the chemical erosion of historical monuments and works of art. Sulfurous and sulfuric acids formed from sulfur dioxide and sulfur trioxide when they react with moisture may also damage paper and leather. 

One of the processes for extracting zirconium from the mineral zircon (ZrSi04) is to fuse the mineral with limestone or dolomite. The reaction product disintegrates on cooling into powdered calcium silicate and coarse crystals of calcium zirconate (equation 12.7). The zirconate is dissolved in acid and converted into zirconium salts or zirconium oxide, much of which are converted into the corrosion-resistant zirconium metal [12.38]. 

Formation con osition. The chemical and physical compositions of the formation are very important in defining the acid spending time, and, subsequently, the acid penetration distance. Acid spends very rapidly in highly reactive (>95%) carbonates. Acid spending time can be much slower in formations with lower HCl reactivity (65%-85%). As mentioned previously, the reaction rate of acid in limestone is about twice that in dolomites (at lower temperatures). Therefore, live acid penetration can be deep in low-solubility, lower-temperature dolomites. 

Reaction with aqueous acids. In general, limestones react readily with adds and are used for acid neutralisation. High calcium limestones react readily with dilute hydrochloric and nitric acids at ambient temperatures, whereas dolomite and dolomitic limestones only react readily when the dilute acid is heated. The reaction of limestone with sulfurous acid (formed by the dissolution of sulfur dioxide in water) is the basis of a flue gas desulfurisation process (see section 12.5.2). The reactions with acids which form insoluble or sparingly soluble calcium salts (e.g., sulfurous, sulfuric, oxalic, hydrofluoric and phosphoric acids) are inhibited by the reaction product. 

Acidizing with HF finds no application in carbonates, as it forms solid calcium fluoride (CaF ) in limestone and both calcium fluoride and magnesium fluoride (MgF ) in dolomite. In any case, HF reaction in sandstones cannot be considered analogous to HCl reaction in carbonates. Whereas HF reaction in sandstones is controlled by the surface area of siliceous minerals—that is, by the surface reaction kinetics—HCl reaction in carbonates is controlled by the mass transport of acid to the mineral surfaces. In sandstones, the acid transport rate is high compared to surface reaction rates, and in carbonates, surface reaction rates are high compared to the acid transport rate. The slower rate step (acid transport or surface reaction) will control the reaction kinetics. Overall, HCl reactions in carbonates are much fester than HF reactions in sandstones. 

Distilled water is fairly nonreactive and practically does not interact with carbonates or silicates, such as limestone dolomite and marl or granite, basalt, or sandstone. However, when enriched with C02, water turns into carbonic acid and can dissolve rocks by means of the reaction... 

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