Calcium Carbonate Crystallization from Hydrocarbonate Solutions

Abstract

The kinetics of CaCO3 precipitation from model solutions formed by preliminary saturating deionized water with CO2 and by adding NaHCO3 and CaCl2 has been considered. The characteristic feature of the method is the almost simultaneous measurement of the activities of major components of the aqueuos calcium hydrocarbonate system (ACHCS): Ca 2+ , CO2, HCO − 3 , as well as pH and the temperature. CaCO3 crystallization was provided with the help of the CO2 degassing by air. The processes running in the ACHCS during the degassing can be divided into four stages: dissociation of CaHCO + 3 complexes; formation of crystal nuclei in the solution; a transitive stage, which includes the final phase of crystal nucleation and the initial growth of newly formed crystals; and intensive growth of crystal nuclei, which gives rise to the mass CaCO3 precipitation. The product (Ca 2+)•(CO 2− 3) is a reaction coordinate for the second stage of CaCO3 precipitation, whereas (Ca 2+)•(HCO − 3) for the third and fourth stages. The kinetics of growth of crystal nuclei and their concentration are calculated for the second stage, by using the concept of CaCO3 dissolution product that depends on the crystal nucleus size. During the process of crystal nucleation, the crystal size remains practically stable (≈8 × 10 −8 m), and the concentration reaches 1.5 × 10 −15 m −3. The mass CaCO3 precipitation (the fourth stage) starts when the crystal nuclei reach dimensions of about 3 × 10 −7 m. K e y w o r d s: calcium carbonate crystallization, reaction coordinate, crystal nuclei, CaCO3 precipitation stages.

SOLID CaCO3 FORMATION IN WATERS OF CIRCULATING COOLING SYSTEMS OF POWER PLANTS UNDER THE CONDITIONS OF ELECTRIC LOAD CHANGE

“…This in turn reduces water exchange in CCS and, consequently, salts supply for CCS. Note that conclusions from expressions (13) and (14) qualitatively coincide with data [15], however, obtained here formulas (13) and (14) give the opportunity for agreed calculation of acids and inhibitors doses, as well as the degree of feed water softening with electric load of units. Their uses allow carrying out technical and economic optimization of the process of CaCO3 formation minimization and CCS operation regime.…”

“…3 curve 1. Besides, units operation with maximum load favors the increase of solid CaCO 3 formation rate This growth is of the same nature both for stability index, see expression (13) and solid CaCO 3 formation rate, see (14). It is also evident that feed water softening and CW acidification reduce Dve.…”

“…As we can see from (13) the similar I st behavior corresponds to φ, V and C HCO3oo changes. Dependence of I st on gross efficiency of the plant is shown in fig 2. The reduction of I st at the efficiency growth is due to A(η) dependence, see (3) and (13). In practice it is connected with the reduction of the degree of salt concentration due to reduction of consumption for evaporation, see (7) and (12).…”

“…Note that the degree of CW stability at the selected law of solid CaCO3 formation does not depend on calcium ions concentration. Actually it is not so, since C HCO3oo and C Caoo concentrations are linked with each other, see [13].…”

“…Let us use the expressions obtained to analyze the operation of a specific heat power plant [14] where partial softening of feed water and CCS with cooling towers are used. Based on this data we obtain from (8) and (13) the dependence of I st on Ne, see fig. 1 for different meanings of kinetic K constant.…”