Calcium sulfite analysis

The other type of salt precipitating in the column is calcium based. Elimination or reduction of calcium ions in the liquor is critical if the temperature in the system drops. A larger amount of precipitate was observed in the pilot plant when overnight temperature dropped to about 60°F. Most of these salts returned to solution after the system was reheated to operating temperatures. This relationship between the temperature and precipitation must be taken into consideration in the design and operation of a full-scale plant. As indicated by the analysis shown in Table V, these salts are believed to be primarily calcium sulfite and sulfate. 

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 " ... 

Solid Calcium Sulfite. Recent thermodynamic studies of calcium sulfite by mass spectroscopy indicated that calcium sulfite dissociates into calcium oxide and sulfur dioxide (12). Under atmospheric pressure, this dissociation reaction is slow in the range below 250°C. We find under these conditions substantial decomposition of sulfite, yielding sulfate, elemental sulfur as well as thiosulfate. These observations are consistent with experiments by Brewer (13), and confirm old observatons made by wet-analysis of these complex solids (14). Our work confirms seventy year old literature reports which suggested evidence for thiosulfate, trithionate and dithionate in old pulping sulfite liquor which yellows when kept in air-free, sealed ampules (15-18). 

The chemicals used in the experiments were all of reagent grade except the calcium sulfite. This material was a hemi-hydrate of commercial quality ( 47 wt% anhydrous CaS03) product containing about 0.006 wt% Mn and 0.024 wt% Fe. [A complete analysis appears in reference (12)]. 

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. 

The batch precipitation tests show dramatic effects of adipic acid slurry concentration and solid phase oxidation fraction on coprecipitation of adipic acid in scrubber solids. Real world scrubbers would probably never operate at adipic acid concentrations as high as those tested and would also not likely ever produce pure phase calcium sulfite hemihydrate. Therefore, the magnitude of the results observed is somewhat a product of the laboratory test conditions. The results do, however, establish the potential importance of adipic acid coprecipitation and, hence, the need for analysis of scrubber solids for adipic acid when determining adipic acid chemical degradation rates by a mass balance calculation approach. 

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