Calcium sulfite saturation

Liquor saturated with both calcium sulfite and gypsum... 

Figure 3. Equilibrium SOi partial pressure as a function of pH and magnesium concentration for liquors saturated with calcium sulfite and gypsum at 50°C with...

For liquors saturated with both calcium sulfite and gypsum, the concentration of dissolved sulfite species increases linearly with dissolved magnesium concentration, decreases linearly with dissolved chloride concentration, and is only weakly affected by pH. 

Calcium Sulfite. (CAS 10257-55-31 CaSOj- 2H2O. white precipitate, pK = 7.9, formed by reaction of soluble calcium salt solution and sodium sulfite solution, or by boiling calcium hydrogen sulfite solution calcium hydrogen sullite. Ca(HSOj) . formed in solution by saturating calcium hydroxide nr carbonate suspension with sulfurous aeid. 

Clear Solution Kinetics. Without buffer, clear solutions of calcium sulfite have such low saturation concentrations in the pH range of interest that the resulting oxidation is quite slow, subject to low-level extraneous influences, and hard to work with. There are few reports in the literature which contain experiments at the needed conditions (11,12). Of the... 

Gladkii(16) at the State Scientific Research Institute of Industrial and Sanitary Gas Cleaning at Moscow did work on the three-phase calcium sulfite slurry oxidation system, finding that the liquid phase oxidation (pH 3.6-6) is first order with respect to the sulfite species. He pointed out, on the basis of pH versus time data from his semi-batch reaction, that the slurry oxidation had different periods in which either reaction kinetics or solid-liquid mass transfer controlled the oxidation rate. He also presented an omnibus empirical correlation between pH, temperature, and the liquid phase saturation concentration of calcium sulfite solution for predicting the slurry oxidation rate. The catalytic effect of manganese... 

The first case is calcium sulfite dissolution without chemical reaction. Using a film model will allow the calculation of the surface concentrations of all the species and the rate of dissolution. With a knowledge of the particle population and solution concentrations, most of the variables are known when the experiment starts. To specify all the starting values of the variables, the conditions at the particle surface are required. From the consideration of saturation concentration In the previous section during dissolution, the bulk liquid must obey ... 

The results in this preliminary analysis give important information about the driving force, (C - C ), which governs the calcium sulfite dissolution that used in the model of the oxidation. The constant surface pH 8,18, which is naturally the pH of the saturated solution, can be used in the transient state analysis of the dissolution. 

Precipitation of Carbonate Salts. In the Appendix, equations are given for the calculated degrees of saturation of calcite, S3, and of dolomite, S4, based on regression of these properties at the conditions shown in Table III. Figure 7 shows the effects of pH and PCo2 for solutions that are saturated with calcite or dolomite. In each case the solutions are saturated with calcium sulfite and gypsum, and the content of sodium chloride is 200 mmoles/kg H20. 

Degree of saturation of calcium sulfite c Magnesium content in mmoles/kg H20... 

An altemadve approadi for die prevention of calcium sulfate scale dqiosition is to operate with a solution less than saturated with respect to CaS04. Hiis can be accomplished by continuously removing CaS04 from solution as a co-precipitate with calcium sulfite at a rate which is equal to that at vdiich it is formed by oxidation. Up to about IS mol % sulfrite can be incorporated into die calcium sulfite crystals as a solid solution (Radian Corp., 1976). Therefore, if oxidation can be maintained at a rate less than 15% of the rate of formation of new sulfite, absorber operation free of gypsum depositirm is possible. A convenient technique to assure this mode of operation is oxidation inhibition. 

Dissolved Concentrations of Calcium and SO2 Species. The equilibrium dissolved concentrations of total calcium and SO2 (sulfite plus bisulfite) species are important because comparison of these equilibrium concentrations with actual measured values determines the degree of gypsum saturation, and hence the potential for gypsum scale formation in the scrubber. As a first approximation, the fraction gypsum saturation of a scrubber liquor, having specified pH and specified concentrations of magnesium and chloride, is proportional to the measured calcium concentration, and inversely proportional to the measured S02 concentration. 

For a liquor of known pH and magnesium and chloride concentrations, the degree of gypsum saturation can be determined by measurement of either the total dissolved calcium or the total dissolved SO2 (sulfite plus bisulfite). The chemical model has been used to obtain correlations for gypsum saturation, presented below. The correlations, Equations 7 and 9, are valid for a typical scrubbing temperature of 50 °C, and for the same ranges of pH, magnesium, and chloride as for Equations 1-4. 

The degree of gypsum saturation of a 1imestone/magnesia scrubbing liquor can be calculated from either dissolved calcium or dissolved S02 (sulfite plus bisulfite) analyses. 

The common feature of the thermolysis of saturated five-membered cyclic sulfites is the elimination of sulfur dioxide (Scheme 7). 4,5-Dimethyl-l,3,2-dioxathiolane 2-oxide (38) decomposes at 275 °C on calcium oxide to a mixture of 2,3-dimethyloxirane (39) and... 

Heterogeneous crystallization is minimized by providing seed crystals for homogeneous crystallization (crystallization on the seed crystals) and by designing the reaction tanks so that the liquor leaving them is close to saturation and not highly supersaturated. This requires the knowledge of precipitation kinetics of calcium sulfate and sulfite. 

As the concentration of sulfate increases relative to sulfite, the amount of sulfate precipitation increases. Thus, as the rate of oxidation increases, the ratio of sulfate to sulfite in solution will increase until the rate of calcium sulfate precipitation is sufficient to keep up with the rate of sulfate formation by oxidation. This self-adjustment by the system may, however, be limited by the need to maintain a high active sodium concentration which will limit sulfate concentrations (and consequently the sulfate/ sulfite ratio) simply by solution saturation considerations. Furthermore, the sulfate/sulfite ratio may also be limited by the need to ensure high limestone utilizations and good solids properties. 

Scale-free operation. Calcium sulfate and sulfite concentrations are well below saturation levels. 

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