Dissolution limestone

Increases in permeability caused by limestone dissolution approximately doubled the injection index (the amount of waste that can be injected at a specified pressure). As of 1974, the effects of the pressure created by the injection were calculated to extend more than 40 miles radially from the injection site.167 An updip movement of the freshwater/saltwater interface in the injection-zone aquifer, which lies less than 32 km (20 miles) from the injection wells, was also observed. 

In wet scrubbing of SOp from boiler flue gas by limestone slurry, the concentration of dissolved species in the scrubbing liquor that can react with incoming SOp gas is very low, about one to two m-mole/1. This is far below the SOp make-per-pass in the scrubber, typically about 10 m-mole of SOp absorbed per liter of liquor for one pass through the scrubber. Therefore, the SOp absorption rate is largely dependent upon the slow rate of limestone dissolution into the liquor passing through the scrubber. 

Palmer AN (1996) Rates of limestone dissolution and calcite precipitation in cave streams of east-central New York state, northern section. Geol Soc Am 28 89 Parker BL, Cherry JA, Chapman SW, Guilbeault MA (2003) Review and analysis of chlorinated solvent dense nonaqueous phase liquid distribution in five sandy aquifers. Vadose Zone J 2 116-137... 

The pH profile is shown in Figure 3, A significant drop in the system pH was obtained when the total Mg2+ concentration was increased from 1060 to 2000 ppm. The scrubber bleed hold tank pH decreased from 6.3 to 5.6 and the forward feed hold tank pH also decreased from 6.7 to 6.2. However, the increase of pH across the reactors was observed even at 2000 ppm total Mg2+ concentration, indicating that the limestone dissolution was not completely stopped by increasing the Mg2+ concentration. 

In wet FGD processes, either DA or limestone slurry, the combined effects of calcium and magnesium actually determine the limestone dissolution rate. Sjoberg s results(fi) indicated that Ca2+ can inhibit the CaCO dissolution rate much more effectively than Mg2+ by the same surface adsorption phenomenon. The combined effects of Ca2+ and Mg2+ can be described as competitive adsorption, and the limestone surface will act as an ion-exchanger. The fraction of surface occupied by adsorbed Ca2+ and Mg2+ can be expressed as ... 

Equation (7) indicates that the relative effectiveness of Mg2+ and Ca2+ in inhibiting the limestone dissolution rate depends on the ratio of Mg2+ concentration to Ca2+ concentration. On the other hand, the sensitivity of limestone dissolution rate to the Mg2+ concentration is determined by the Ca2+ concentration. As indicated by Equation (7), when the minimum ratio ( Mg/0Ca) 0 5 is required... 

This effect is relatively small until the total magnesium ion concentrations reach about 1000 ppm. o The effect of Mg2+ concentration on limestone dissolution rate can be explained by a surface adsorption model. The adsorption of Mg2+ reduces the limestone dissolution rate because the surface is partially blinded by the adsorbed magnesium ions. The competitive adsorption of calcium and magnesium ions was described by a mathematical model based on the Langmuir adsorption isotherm. The model was used to explain the sensitivity of limestone dissolution rate to magnesium ion concentration under limestone DA operating conditions. 

Wentzler, T, H. and F. F. Aphan. 1972. Kinetics of limestone dissolution by acid waste waters. In C. Rampacek, Ed. Environmental Control. San Francisco, CA, pp. 513-523. 

AlkattanM., OelkersE. H., Dandurand J.L., and Schott J. (1998) An experimental study of calcite and limestone dissolution rates as a function of pH from -1 to 3 and temperature from 25 to80°C. Chem. Geol. 151(1-4), 199-214. 

Leak-off or loss of acid through the walls of worm holes often results in worm holes being too short to provide significant productivity increase. Therefore, effective stimulation often requires retardation of the mineral dissolution rate. The use of microemulsions is one method to accomplish this retardation. The hydrochloric acid is injected as an water-in-oil microemulsion. The diffusion rate of the dispersed aqueous acid to the rock surface is slower than molecular diffusion of acid from a totally aqueous system. Thus the rate of limestone dissolution is retarded with the microemulsion system. 

The solution composition varies significantly from pure CaC03 (e.g. pure dolomite aquifer). A contrary case arises when sulphuric acid from pyrite oxidation enhances limestone dissolution (e.g. Atkinson et al., 1983). 

The limestone dissolution rates at various pH values and partial pressures of carbon dioxide are shown in Fig. 3. 

Fig. 3 Limestone dissolution rates as a function of pH and carbon dioxide partial pressure, at a temperature of 25°C. (From RQfP )...

In the slurry scrubbing process, limestone dissolves at pH A to 6 and 55°C in both absorber and the hold tank/crystallizer. Because of HC1 accumulation from the flue gas, typical scrubbing solution contains 0.01 to 0.2 M CaCl2 C02 partial pressure can vary from near zero with forced oxidation to one atmosphere with CO2 evolution from the hold tank and is typically 0.1 atm in the absorber. Sulfite/bisulfite buffer can be present in concentrations up to 0.1 M. CaS03 and/or CaS04 crystallization must occur simultaneously with limestone dissolution. Buffer additives such as adipic acid should enhance both SO2 removal and CaC03 dissolution at concentrations of 3 to 10 mM (5). 

Limestone dissolution in throwaway scrubbing can be modeled by mass transfer. The mass transfer model accurately predicts effects of pH, Pcc>2> temperature, and buffers. For particles less than 10-20 pm, the mass transfer coefficient can be obtained by assuming a sphere in an infinite stagnant medium. This model underpredicts the absolute dissolution rate by a factor of 1.88, probably because it neglects agitation and actual particle shape. 

Toprac, A.J., and Rochelle, G.T., "Limestone Dissolution in Stack Gas Desulfurization Processes - Effect of Type and Grind", presented at AIChE Annual Meeting, New Orleans, November 8-12, 1981, in press, Env. Prog. 

Studies of the Major Factors Affecting Magnesium Limestone Dissolution... 

The dissolution of limestone is known to be controlled by both diffusion of ions in solution and surface reaction rates. The pH value influences which of these steps dominates in the limestone dissolution. For example, at pH values less than 5 the diffusion process dominates the dissolution with little dependence on surface reaction. On the other hand, at pH values greater than 7 the reaction at the limestone surface begins to dominate the dissolution process. In the pH range between 5 and 7 both dissolution steps can influence the overall rate. 

The bulk of the limestone dissolution in most SO2 scrubbers occurs in the 5-6 pH range. Thus, both solution mass transfer properties and the nature of the limestone must be considered at typical operating conditions. Therefore, the pH, solution composition, solution buffer capacity, and the nature of the limestone are important considerations when designing for maximum limestone utilization. This paper deals primarily with the measurement of the influence of limestone properties on the overall dissolution rate. 

Figure 4. Fredonia limestone dissolution rate vs. stirring rate, (50°C pH 5.8).

Takeda reported a series of experiments in which he determined the amount of S02 absorbed by limestone slurries(15). He observed significant differences in the rate of S02 absorption at different slurry concentrations. Here limestone dissolution probably plays an important role in the gas absorption process. 

---