Calcium carbonate models

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

Figure 8.27 Comparing Monte Carlo model predictions with MSMPR experimental data for calcium carbonate due to Hostomsky and Jones, 1991 (Faiope etal., 2001)...

Hostomsky, J. and Jones, A.G., 1993c. Ibid., Modelling of calcium carbonate precipitation in the reaction between gaseous carbon dioxide and aqueous solution of calcium hydroxide. Indem. pp. 2055-2059. 

In the North American market, water heaters are almost always made with the cold water inlet and hot water outlet lines coming out of the top of the tank. The hot water outlet opens right into the top of the tank and so draws off the hottest water. The hot water has risen to the top of the tank because of its lower density. The cold water on the inlet side is directed to the bottom of the tank by a plastic dip-tube. In some models the dip-tube is curved or bent at the end to increase the turbulence at the bottom of the tank. This is to keep any sediment from settling on the bottom of the tank. As sediment— usually calcium carbonate or lime—precipitated out of the water by the increased temperature builds up, it will increase the thermal stress on the bottom of a gas-fired water heater and increase the likelihood of tank failure. On electric water heaters the sediment builds up on the surface of the elements, especially if the elements are high-density elements. Low-density elements spread the same amount of power over a larger surface of the element so the temperatures are not as high and lime doesn t build up as quickly. If the lower elements get completely buried in the sediment, the element will likely overheat and burn out. 

Examples of reversible breakdown of structure have been reported for procaine penicillin dispersions (7), for model systems of calcium carbonate in polybutene ( ), and for numerous other systems. During shear the particles are forced into contact with each other with sufficient kinetic energy to overcome any natural barrier against their displacement of a lyosphere around each individual particle. A dispersion which is inherently stable can thus be forced by shear into a condition of instability. 

Fricke M, Volkmer D (2007) Crystallization of Calcium Carbonate Beneath Insoluble Mono-layers Suitable Models of Mineral-Matrix Interactions in Biomineralization 270 1-41 Fujimoto D, see Tamura R (2007) 269 53-82... 

On the other hand, the alkoxide system presented several problems in formulation. The system first chosen as a model consisted of a trimethoxymethyl silane crosslinker, 8000 centistoke HEB siloxane, and a catalyst. A number of catalysts were used and each exhibited different cure rates and electrical properties. DuPont tetraalkoxytitante-Tyzor appears to he one of the better catalysts used in this type of curing system. Fillers are usually incorporated into the silicone formulation to improve mechanical properties, promote adhesion, and to serve as light screening and pigment agents. Cab-o-sil, a form of fumed silica, carbon-black, titanium dioxide and calcium carbonate are then used as RTV fillers. 

The spring waters of the Sierra Nevada result from the attack of high C02 soil waters on typical igneous rocks and hence can be regarded as nearly ideal samples of a major water type. Their compositions are consistent with a model in which the primary rock-forming silicates are altered in a closed system to soil minerals plus a solution in steady-state equilibrium with these minerals. Isolation of Sierra waters from the solid alteration products followed by isothermal evaporation in equilibrium with the eartKs atmosphere should produce a highly alkaline Na-HCO.rCOA water a soda lake with calcium carbonate, magnesium hydroxy-silicate, and amorphous silica as precipitates. 

We may now examine specific information from chemical analyses of the Great Lakes (7, 8, 13) to determine to what degree the variations of the proposed model fit the actual data. Rather than consider all of the variables at once, it is simpler to consider smaller portions to get a better idea of what actually is happening. We shall look at calcium carbonate equilibria, dolomite equilibria, phosphate equilibria, and silicate equilibria. 

Over the last few years, as powerful but relatively low-cost personal computers have become widely available, new indices and models have been proposed that use complex computer programs and often a different starting point than that of LSI to more accurately predict the formation of calcium carbonate and many other common types of scale, as well as calcium carbonate in combination with these scale types (as tends to happen in real life). 

L. N. Plummer and E. Busenberg, The Solubilities of Calcite, Aragonite, and Valerite in Carbon Dioxide-Water Solutions Between 0 and 90 °C, and an Evaluation of the Aqueous Model for the System Calcium Carbonate-Carbon Dioxide-Water , Geochim. Cosmochim. Acta., 46, 1011-1040 (1982). 

Carbonate minerals are among the most chemically reactive common minerals under Earth surface conditions. Many important features of carbonate mineral behavior in sediments and during diagenesis are a result of their unique kinetics of dissolution and precipitation. Although the reaction kinetics of several carbonate minerals have been investigated, the vast majority of studies have focused on calcite and aragonite. Before examining data and models for calcium carbonate dissolution and precipitation reactions in aqueous solutions, a brief summary of the major concepts involved will be presented. Here we will not deal with the details of proposed reaction mechanisms and the associated complex rate equations. These have been examined in extensive review articles (e.g., Plummer et al., 1979 Morse, 1983) and where appropriate will be developed in later chapters. 

Numerous models have been proposed for the processes occurring near the sediment-water interface in deep sea sediments that lead to a balance between dissolution and retention of calcium carbonate in these sediments. Investigation of these processes is currently one of the most active areas of research in the study of calcium carbonate behavior in the oceans. A major difficulty in studying and modeling these processes is that many of the most important changes take place over distances of only a few millimeters in a highly dynamic environment. 

Early models for the process of calcium carbonate dissolution from deep sea sediments (e.g., Takahashi and Broecker, 1977) were based on simple diagenetic models in which calcium carbonate dissolved into the pore waters of the sediments. 

Figure 6.7. A model for the interaction of organic matter (OM) with calcium carbonate. Variables are total organic carbon (TOC), dissolved oxygen, and composition of organic matter. (After Mitterer et al 1985.)...

Both authors calculations also indicated that it is possible for solutions of reasonable compositions for natural waters to produce mixtures of freshwater and seawater that were undersaturated with respect to calcite but supersaturated with respect to dolomite. This observation is a cornerstone for some dolomitization models that are discussed later in this chapter. It is also important to note that the extent of undersaturation which results from mixing is strongly dependent on the initial Pco2 °f the dilute water when it is in equilibrium with calcite. Waters high in CO2 can cause more extensive dissolution. If these waters enter a vadose zone where CO2 can be degassed, they will become supersaturated and calcium carbonate can precipitate. This process provides an excellent mechanism for cementation near the water table. Because the water table can oscillate vertically, a considerable zone of cementation can result. 

Morse J.W. and Berner R.A. (1979) The chemistry of calcium carbonate in the deep oceans. In Chemical Modelling—Speciation, Sorption, Solubility, and Kinetics in Aqueous Systems (ed. E. Jenne), pp. 499-535. Amer. Chem. Soc., Washington, D.C. 

Schink D.R. and Guinasso N.L., Jr. (1977) Modelling the influence of bioturbation and other processes on calcium carbonate at the sea floor. In The Fate of Fossil Fuel CO2 in the Oceans (eds. N.R. Anderson and A. Malahoff), pp. 375-400. Plenum Press, New York. 

The goal of this research was to improve activity coefficient prediction, and hence, equilibrium calculations in flue gas desulfurization (FGD) processes of both low and high ionic strength. A data base and methods were developed to use the local composition model by Chen et al. (MIT/Aspen Technology). The model was used to predict solubilities in various multicomponent systems for gypsum, magnesium sulfite, calcium sulfite, calcium carbonate, and magnesium carbonate SCU vapor pressure over sulfite/ bisulfite solutions and, C02 vapor pressure over car-bonate/bicarbonate solutions. 

---