Calcium sulfite and gypsum

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. 

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. 

Waste materials such as calcium sulfite are difficult to dewater and cannot support a load. It is often desirable to stabilize such wastes before disposal. Methods for the stabilization of these materials, such as oxidation of calcium sulfite to gypsum, have been developed. In certain countries (such as W. Germany and Japan), waste materials like gypsum can be sold however, this is not generally true in the United States. 

An important technology for removal of S02 is Flue Gas Desulfurization (FGD), carried out in units known as scrubbers. Most scrubbers contact the flue gas with a slurry of lime or limestone to capture the sulfur oxides and produce a sludge containing calcium sulfite and calcium sulfate. However, disposal of sludge is another environmental problem, and some scrubbers include oxidation to convert all the calcium sulfite to sulfate (gypsum), which can be used for wallboard manufacture. Fluidized-bed combustion units add a sulfur... 

This is analogous to the limestone-gypsum process (section 12.5.2). Oxides of sulfur are absorbed by the milk of lime, forming crystallites of calcium sulfite and... 

Hydrated lime is also used to stabilize the calcium sulfite—sulfate sludge derived from thickeners at SO2 scmbbing plants that use limestone—lime. Hydrated lime (2—3%) is added to react with the gypsum sludge and flyash or other added siHceous material. Under ambient conditions the lime and siHca serve as a binder by reacting as calcium siHcates so that the material hardens into a safe, nonleaching, stable, sanitary landfill or embankment fill. 

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. 

This paper has described advantages of the sinq>le DDO ciystallizer for production of calcium sulfite in a bench-scale study and calcium sulfate dihydrate (gypsum) in an industrial process. 

The most common chemical used in scruhhers today is limestone (calcium carbonate CaCOj) because it reacts with sulfur dioxide readily to form calcium sulfite (CaSOj), which can be collected and sold as gypsum. In recent years, a new scrubber design that uses lime (calcium hydroxide Ca(OH)2) rather than limestone has been introduced in many plants ... 

This is easier than pretreatment of the fuel, but the waste is calcium sulfite which, unlike the more familiar sulfate (gypsum, Plaster of Paris), is quite toxic to plants and is therefore difficult to dispose of safely. 

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. 

For flue-gas desulfurization, lime and limestone processes are utilized in which the furnace gas is scrubbed with an aqueous liquid containing the necessary quantity of ealcium hydroxide or calcium carbonate for neutralizing the acidic furnace gas components. The end products are calcium sulfite/calcium sulfate sludges or on complete oxidation gypsum ... 

The solid product from each of these sets of reactions is primarily calcium sulfite hemihydrate (CaSOs-5 H2O), which has been confirmed by x-ray diffraction analysis of scrubber sludgesJ l A similar set of reactions collects sulfur trioxide (SO3) from the flue gases, forming gypsum (CaS04 2H2O) as the solid product, but under normal boiler conditions sulfur trioxide makes up only about 0.5% of the total sulfur oxides, and so its removal is less important than the removal of sulfur dioxide.l " ... 

There is a positive effect of the SCR unit on downstream equipment such as flue gas desulfurization, which produces gypsum. In the absence of a SCR unit, water from a FGD unit contains hydroxyl amine disulfonic acid which has been formed from NO2, water, and SO2. This compound acts probably as an inhibitor for the oxidation of calcium sulfite into calcium sulfate [129]. 

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