Dolomitization of a limestone

In our example, we test the consequences of reacting an isotopically light (i.e., nonmarine) limestone at 60 °C with an isotopically heavier groundwater that is relatively rich in magnesium. We start by defining the composition of a hypothetical groundwater that is of known CO2 fugacity (we initially set /co2 to 1) and in equilibrium with dolomite  

We then pick up the fluid from the previous step as a reactant and define a system representing the limestone and its pore fluid. We specify that the rock contains 3000 cm3 of calcite, implying a porosity of about 25% since the extent of the system is 1 kg (about 1 liter) of fluid. The pore fluid is similar to the reactant fluid, except that it contains less magnesium. The procedure is 

The final commands define a reaction path in which 10 kg of reactant fluid gradually migrate through the system, displacing the existing (reacted) pore fluid. Typing go triggers the calculation. 

As expected, the fluid, as it migrates through the limestone, converts calcite into dolomite. For each kg of fluid reacted, about 0.65 cm3 (17.5 mmol) of calcite is consumed and 0.56 cm3 (8.7 mmol) of dolomite forms. 

To carry the calculation to high water-rock ratios, we enter the commands 

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Dolomite is a mineral that is obtained through dolomitization of a limestone deposit [7], i.e. through the progressive replacement of some Ca ions in the calcite structure by Mg " ions, which are brought into the limestone deposit under the form of salts (chlorides, sulphates...) dissolved in surface or ground waters. Dolomitization can be complete or partial. In the case of complete... 

The only fluid, common to oil field operations, that has a significant interaction with TKPP solutions was concentrated calcium chloride. Solutions of calcium chloride, spent acid, are generated during the acidization of a limestone or a dolomite formation. When solutions containing 10% calcium chloride were mixed in equal proportions with 14.5 ppg TKPP solutions, massive precipitation occurred. Similar precipitation was observed with oil field brines having calcium concentrations above 400 ppm. 

The passage of a limestone particle through a lime kiln can be divided into five stages. The following description refers to high-calcium limestone, but parallels can be drawn with magnesian/dolomitic limestones and dolomite. 

At Associated Electric Cooperative s Thomas Hill Unit 3, 12,000 to 14,000 ppm of magnesium in the limestone slurry was used to meet a 1.2 lb SfVmillion Btu onissions limit, but availability was unacceptable with this additive. The additive was switched to DBA and later to DBA and sodium formate (Moser and Owens, 1991 Rodoi, 1992). As of 1993, Public Service of Indiana s Gibson Unit 5 was the only plant known to be using a magnesium additive (dolomitic lime) to improve the c joation of a limestone system. Sulfur is used with the dolomitic Ume to inhibit oxidation. 

Abstract A new low density mineral material has been synthesized via a simple, flexible, cheap and easy to control process. This material is a synthetic carbonate produced by carbonation of a solid phase composed of a calcic part and a magnesian part. Typically, its production process includes the calcination of a raw dolomite (general formula CaC03.MgC03) into the oxide form, followed by an at least partial hydration of this oxide and a subsequent carbonation step. This process is thus close to the well-known process used for the production of Precipitated Calcium Carbonate (PCC), a common filler and pigment in plastic, paper and rubber, except that the raw material is a dolomite instead of a limestone. It has to be pointed out that flue gases from different industries can be used as a source of CO2 for the carbonation. 

Calcium. Soil minerals are a main source of calcium for plants, thus nutrient deficiency of this element in plants is rare. Calcium, in the form of pulverized limestone [1317-65-3] or dolomite [17069-72-6] frequendy is appHed to acidic soils to counteract the acidity and thus improve crop growth. Such liming incidentally ensures an adequate supply of available calcium for plant nutrition. Although pH correction is important for agriculture, and liming agents often are sold by fertilizer distributors, this function is not one of fertilizer manufacture. 

The neutralising power of lime and limestone and other alkaUes is compared in Table 2 (8). Of all these alkaUes, MgO is the strongest base, followed by CaO. Thus neutralization of a given acid requires less dolomitic limestone or lime than high calcium limestone or lime. 

Lime is among a family of chemicals which are alkaline in nature and contain principally calcium, oxygen and, in some cases, magnesium. In this grouping are included quicklime, dolomitic lime, hydrated lime, dolomitic hydrated lime, limestone, and dolomite. The most commonly used additives are quicklime and hydrated lime, but the dolomitic counterparts of these chemicals (i.e., the high-magnesium forms) are also widely used in wastewater treatment and are generally similar in physical requirements. 

Over the past decades, advances have been made that reduce environmental impacts of coal burning in large plants. Some arc standard and others experimental. Limestone (mainly calcium carbonate) scrubber smokestacks react with the emitted sulfates from the combustion and contain the chemical products, thereby reducing the release of SO., into the atmosphere by a large factor (of ten or more). Pulverization of coal can also allow for the mechanical separation of some sulfur impurities, notably those in the form of pyrites, prior to combustion. Currently deployed—with more advanced versions in the development stage—are various t yies of fluidized bed reactors, which use coal fuel in a pulverized form, mixed with pulverized limestone or dolomite in a high temperature furnace. This technique reduces sulfate release considerably. There are... 

If a limestone or dolomite sorbant is added to the hot sand bed, the calcium oxide (CaO) produced reacts and combines with S02, thus reducing the emission of the pollutant. 

Marble. The word marble is used as the common name for two types of monomineral rocks one derived from limestone and therefore composed of calcium carbonate, the other derived from dolomite and composed of calcium magnesium carbonate. Extremely high pressures and heat during past geological times modified the structure of both limestone and dolomite, compacting them into a characteristic crystal structure. Most marble is white however, minor and trace amounts of metallic impurities cause the formation of stains in a variety of colors, hues, and patterns, or of colored marble. 

Fig. 19.2. Isotopic composition (bold lines) of dolomite formed by reaction between a limestone and migrating groundwater, assuming that minerals maintain isotopic equilibrium over the simulation. Fine lines show results of simulation holding minerals segregated from isotopic exchange, as already presented (Fig. 19.1).

For WGS, commercial catalysts are only operated up to 550 °C and no catalysts are available for higher temperatures, because adverse equilibrium conversion makes the process impractical in the absence of a CO2 sorbent. Han and Harrison [38] have shown that, at 550 °C, dolomite and limestone have a sufficiently high WGS activity. For SMR a conventional Ni SMR catalyst is used in a 1 1 ratio with CaO [30]. Meyer et al. [32] have also used a Ni-based catalyst in combination with limestone and dolomite, and achieved CH4 conversions of 95% at 675 °C while the CH4 conversion at equilibrium was 75%. 

Sulfur Emissicms Sulfur present in a fuel is released as SO2, a known contributor to acid rain deposition. By adding limestone or dolomite to a fluidized bed, much of this can be captured as calcium sulfate, a dry nonhazardous solid. As limestone usually contains over 40 percent calcium, compared to only 20 percent in dolomite, it is the preferred sorbent, resulting in lower transportation costs for the raw mineral and the resulting ash product. Moreover, the high magnesium content of the dolomite makes the ash unsuitable for some building applications and so reduces its potential for utilization. Whatever sorbent is selected, for economic reasons it is usually from a source local to the FBC plant. If more than one sorbent is available, plant trials are needed to determine the one most suitable, as results from laboratory-scale reactivity assessments are unreliable. 

In addition to Ni catalysts, Lee and Park explored some unconventional catalysts, such as limestone, dolomite, and iron ore, in a fluidized bed reactor to carry out SR of kerosene and bunker oil. H2 yields from SR of bunker oil over various catalysts (temperature = 800°C, bed height = 10 cm, superficial gas velocity = 20 cm/sec, and S/C = 1.6) were sand (20%), iron ore (29%), commercial Ni catalyst (89%), limestone (93%), and dolomite (76%). Limestone as a SR catalyst looked very promising, but H2 yields over a limestone catalyst decreased over time due to elutriation of fines during the reaction. A fluidized-bed reactor was advantageous for reforming of higher hydrocarbons, due to its ability to replace coked catalyst with fresh catalyst during operation. 

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