Precipitation kinetics calcium carbonate

The second example is for the precipitation of calcium carbonate from Ca(HC03)2 solutions. The mechanistic model was derived by Plummer et al. [40] from a kinetic study of the dissolution of Iceland spar crystals. The approach has more recently been extended to the interpretation of crystal growth [41]. The model is based upon the reaction between a partially dehydrated [Ca-HC03]+ complex or ion-pair and possible anionic reaction... 

The dramatic inhibitory effect of even trace amounts of PMA on the precipitation of calcium carbonate has long been recognised [249] and it has thus found considerable commercial application as an inhibitor of the formation of scale in hard water systems [251]. However, the mechanism by which this species inhibits the precipitation reaction remains unclear. Furthermore, to our knowledge, there have been no kinetic or morphological studies on the effect of polymaleic acid on the dissolution process. 

In this chapter we will deal with the kinetics and equilibrium calculations of heterogeneous systems. Graphical and computational methods for solving equilibrium problems will be presented. The precipitation of calcium carbonate will be discussed in some detail, and the chemistry of phosphorus will be used as a detailed example of several heterogeneous equilibria that are of relevance to natural waters and treatment processes. 

A number of laboratory studies have been recorded recently aimed at characterizing the kinetics of both the chemical reaction and crystallization steps in a reaction crystallization process. Examples of liquid phase reactions studied for this purpose are the crystallization of salicylic acid from aqueous solutions of sodium salicylate using dilute sulphuric acid (Franck et al, 1988) and the crystallization of various calcium phosphates by reacting equimolar aqueous solutions of calcium nitrate and potassium phosphate (Tsuge, Yoshizawa and Tsuzuki, 1996). Several aspects of crystal size distribution control in semi-batch reaction crystallization have been considered by Aslund and Rasmuson (1990) who studied the crystallization of benzoic acid by reacting aqueous solutions of sodium benzoate with HCl. An example of crystallization arising from a gas-liquid reaction in an aqueous medium is the precipitation of calcium carbonate from the reaction between calcium hydroxide and CO2 (Wachi and Jones, 1995). 

The reactor has been successfully used in the case of forced precipitation of copper and calcium oxalates (Jongen etal., 1996 Vacassy etal., 1998 Donnet etal., 1999), calcium carbonate (Vacassy etal., 1998) and mixed yttrium-barium oxalates (Jongen etal., 1999). This process is also well adapted for studying the effects of the mixing conditions on the chemical selectivity in precipitation (Donnet etal., 2000). When using forced precipitation, the mixing step is of key importance (Schenk etal., 2001), since it affects the initial supersaturation level and hence the nucleation kinetics. A typical micromixer is shown in Figure 8.35. 

Hostomsky, J. and Jones, A.G., 1991. Calcium carbonate crystallization kinetics, agglomeration and fomi during continuous precipitation from solution. Journal of Physics D Applied Physics, 24, 165-170. 

Turner JV (1982) Kinetic fractionation of carbon-13 during calcium carbonate precipitation. Geochim... 

Although surfece waters are supersaturated with respect to calcium carbonate, abiogenic precipitation is imcommon, probably because of unfevorable kinetics. (The relatively rare formation of abiogenic calcite is discussed further in Chapter 18.) Marine organisms are able to overcome this kinetic barrier because they have enzymes that catalyze the precipitation reaction. Because fl declines with depth, organisms that deposit calcareous shells in deep waters, such as benthic foraminiferans, must expend more energy to create their hard parts as compared to surfece dwellers. 

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. 

The kinetics of calcium carbonate precipitation in simple solutions have received less attention than those of dissolution reactions. This perhaps reflects the fact that most sedimentary carbonates are initially formed biogenically and that the primary interest in carbonate precipitation reactions has been directed at reaction... 

Morse J.W. (1983) The kinetics of calcium carbonate dissolution and precipitation. In Reviews in Mineralogy Carbonates - Mineralogy and Chemistry (ed. R. J. Reeder), pp. 227-264. Mineralogical Society of America. Bookcrafters, Inc., Chelse, MI. 

Morse J. W. (1979) The kinetics of calcium carbonate dissolution and precipitation, 227. 

This is only an approximation because of the kinetic fractionation of isotopes during the calcium carbonate precipitation. An example of this fractionation is described, among the other scientific references, in Turner, 1982 or Emrich et al., 1970. 

To convert calciiun carbonate to dolomite, some of the calcium must have been replaced by magnesiiun, requiring the partial dissolution of the carbonate. This process is promoted by contact with acidic pore water, such as occurs in organic-rich sediments because remineralization produces carbon dioxide. This is probably why dolomites are presently forming in detrital algal mats buried beneath sabkhat. The restricted extent of these modern dolomites reflects a kinetic hindrance to precipitation. Apparently dolomite precipitation in this setting is too slow to form substantial deposits when sea level is rapidly fluctuating. 

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