Magnesium limestone dissolution

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

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 ... 

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. 

The impact of water hardness due to calcium or magnesium ions on detergents was explained in Section 7.3.1 The source of most Ca2+ and Mg2+ in hard water is the dissolution of limestone (CaCOs) or dolomite [CaMg(COs)2]. Magnesium carbonate is fairly soluble (1.26 mmol L 1 at ambient temperature), but CaCOs is much less so (0.153 mmol L 1). However, if the water contains dissolved CO2 (as indeed it will if it is exposed to the air see Exercise 14.9), the relatively freely soluble Ca(HCOs)2 forms, and the limestone slowly dissolves away ... 

Congruent and incongruent dissolution and precipitation reactions, other than for halite, which probably control the major cation compositions of formation waters include dolomitization of limestone, resulting in a major increase of calcium and a major decrease of magnesium, as in reaction (2) ... 

Stoichiometry and Solids Dissolution. One thousand cu ft of gas at standard conditions contains 3.16 g moles of sulfur dioxide when the sulfur dioxide content of the gas is 2500 ppm. At a liquid-to-gas ratio of 50 gal/standard 1000 cu ft, 16.7 mmoles of basic species/1. are required to react with this amount of sulfur dioxide. Very few of the simulated solutions in Tables III and IV attained this basicity except under extreme conditions of the variables, conditions unlikely to be controlled consistently in lime or limestone scrubbing. Consequently, under most conditions, additional basic species must enter the liquid phase in the scrubber to neutralize the dissolving gas. These species come from the dissolution of calcium carbonate or calcium sulfite in the scrubbing tower. The amount of solids dissolution required to achieve stoichiometry is reduced greatly by the presence of large amounts of magnesium in solution. 

The dissolution increases alkalinity (HCOa ) with the added benefit of increasing an equivalent amount ofhardness (Ca ). Limestone is a sedimentary rock primarily composed of calcite, aragonite and vaterite. Calcite can be pure in composition (CaCOa) or can contain low concentrations of magnesium-forming magnesium calcite. Natural limestone can contain minerals or impurities such as dolomite [CaMg(C03)2] and quartz (Si02). 

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