Calcium sulfite-bisulfite solutions

Figure 4 (right). LCM Prediction of Apparent Equilibrium Constant in Calcium Sulfite/Bisulfite Solutions. Data from Kuzminykh and Babushkina (31). 

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. 

Several power plants have been equipped with dual alkali processes. These are throw-away processes with two liquid loops. In one common process, the scrubbing liquid is a clear solution of sodium sulfite. The absorption of sulfur dioxide converts the sodium sulfite to sodium bisulfite. In the regeneration loop, an alkali such as lime slurry is added the sodium sulfite solution is regenerated and a mixture of calcium sulfite and calcium sulfate is precipitated. The slurry is... 

It combines with sulfur dioxide to form calcium sulfite hemihydrate, CaSOs MiH20 which can oxidize in air in the presence of moisture to give calcium sulfate dihydrate, CaS04 2H2O. However, when SO2 is passed through a solution of calcium hydroxide, calcium bisulfite, Ca(HS03)2 is obtained. The solution is yellowish when it contains bisulfite in aqueous SO2. 

The LCM accurately predicted the vapor pressure of SO and CO2 over sulfite/bisulfite and carbonate/bicarbonate solutions, respectively, for salts of sodium, calcium, and magnesium. Figure 3 plots the LCM predictions of SO2 vapor pressure as a function of theQtotal SO to sodium ratio for a sodium sulfite/bisulfite system at 50 C... 

Calcium Sulfite Deposition. Calcium sulfite (CaS03 %H20) is formed in the scrubber under those conditions that favor sulfite formation. These conditions are apparent when one considers the sulfite-bisulfite equilibrium and compares the relative solubilities of the corresponding calcium salts. As seen in Figure 2, extremely soluble bisulfite in solution changes to relatively insoluble sulfite when the solution pH shifts from 4 to 10. When SO2 is absorbed, the scrubber solution is usually between pH 4 and 6 and, therefore, the predominant species is... 

Since calcium sulfite is not very soluble in water, the last stage(s) of liquor preparation involve absorption of sulfur dioxide into the cold liquor under pressure. This eventually gives a ratio of about 1 mole of calcium hydrogen sulfite (calcium bisulfite) to a total of 4-5 mole of (sulfur dioxide + sulfurous acid). At this stage the pulping liquor is an aqueous solution, which contains 6.5-9% dissolved sulfur dioxide in all forms. 

A considerable amount of work has been done on the oxidation of sulfite and bisulfite anions in aqueous solutions (25). In this paper the discussion is limited to oxidation of calcium sulfite (9), which has received much less attention than oxidation of sodium salts. The attention here is on the oxidation of calcium sulfite, catalyzed by metal ions in the presence of organic acid buffers, occuring in solid-liquid-gas slurry reactors. The organic acid buffers not only moderate pH changes during the reaction, but also inhibit the rate of chemical reaction (10). 

Calcium Bisulfite, Solution. CafHSOj), is known only in soln. Prepn from sulfite liquor Arend, Chem. Produce 10, S3 (1947) Lougheed, Pulp Paper Mag. Can. 49(3), 215(1948) Sehoeffel, U.S. pat. 2,696,424 (1954 to Sterling Drag). The product here described is substantially a soln of okium sulfite in an aq sulfur dioxide soln. 

Lime is added to a clarified side stream of the solution. This results in the precipitation of insoluble calcium sulfite to form a slurry, which is added to the main solution-stream leaving the gas washer. The resulting thin slurry is passed into a clarifier. The calcium sulfite and any fly ash which may have been picked up are then removed as slurry. This slurry is acidified by contacting it with a portion of the product sulfur dioxide. Acidification results in conversion of the calcium sulfite to the more soluble bisulfite form and reaction of the dissolved calcium ions with any sulfate present in the solution to form calcium sulfate, which is relatively insoluble under these conditions. Precipitated calcium sulfate and undissolved ash are removed together in a small filter. The resulting desulfated solution containing dissolved calcium bisulfite is then treated with lime to form the slurry, which is recycled to the process. 

The basis of the sulfite pulping process is the discovery by Tilghman in 1866 that white cellulose fibers are obtained by cooking wood under pressure in an aqueous solution of calcium bisulfite Ca(HS03)2. In chemical terms, the process is founded on the fact that lignin, the solid binder of wood, reacts with sulfite to form water-soluble sulfonic acids. In this fash-... 

Aqueous solutions of ligninsulfonates do not form micelles, although the surface tensions are lowered to levels of 40 mN/m. In paper making, wood chips are cooked in a sulfite digestor with calcium bisulfite... 

Metal Chelate Addition. Metal chelates, such as ferrous ethylene-diamine-tetra-acetate (Fe(II) EDTA), enhance the absorption of NO, into aqueous solutions by reacting quickly with dissolved NO to form the complex Fe(II) EDTA NO. The coordinated NO can react with sulfite and bisulfite ions, forming hydroxylamine-N-disulfonates (HADS) and releasing the ferrous chelate to react with additional NO. When the aqueous solution is a sodium or calcium-based SO2 absorbent, the addition of ferrous chelate results in a combined NO,/SO, control process. 

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