Calcium sulfite and

Calcium thiosulfate has been prepared from calcium sulfite and sulfur at 30—40°C, or from boiling lime and sulfur in the presence of sulfur dioxide until a colorless solution is obtained. Alternatively, a concentrated solution of sodium thiosulfate is treated with calcium chloride the crystalline sodium chloride is removed at low temperature. Concentrated solutions of calcium thiosulfate are prepared from ammonium thiosulfate and lime the Hberated ammonium ion is recycled to the ammonium thiosulfate process (85). 

Applicability Self-cementing processes require large amounts of calcium sulfate and calcium sulfite and are appropriate for immobilizing heavy metals. 

Limestone (calcium carbonate) is also used as a dry sorbent, forming calcium sulfite and carbon dioxide gas ... 

The lime and limestone processes, as indicated in Figure 3, produce a sludge consisting mainly of calcium sulfite and calcium sulfate by the following reactions (limestone) ... 

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... 

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. 

Lime Reactor The efflnent from the EDV system is pumped to an agitated reactor vessel where the EDV efflnent is reacted with lime to form calcium sulfite and active sodium species via the following reactions ... 

When heated in dry air or oxygen, the compound is oxidized to calcium sulfite and then to the sulfate, CaS04 ... 

Lime (portlandite) spray dryers are often used to remove sulfur dioxide from flue gases at coal combustion facilities. Calcium in the spray and flyash may also simultaneously remove arsenic. The resulting arsenic-bearing flyash and calcium sulfites and sulfates are then collected in baghouses or with electrostatic... 

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. 

Spray towers can be used to absorb gaseous reactants. The most widely used spray tower is for flue gas desulfurization. SO2 in a combustion gas is passed upward through an alkaline solution that usually contains calcium oxide. The SO2 is absorbed into the liquid, which then reacts to calcium sulfite and continues on to calcium sulfate. 

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... 

The calcium sulfite and any remaining unreacted sulfur dioxide are removed by injecting an aqueous suspension of lime into the combustion chamber and the stack, producing a slurry (a thick suspension), as shown in Fig. 5.31. 

The controlling chemical reactions for the lime/limestone wet scrubbing SO2 removal systems have been established. In both the lime and limestone systems, the principal absorption reaction is calcium sulfite plus sulfur dioxide to form calcium bisulfite. Methods of preventing both calcium sulfite and calcium sulfate scaling are presented. 

Calcium sulfite and calcium sulfate scaling in the system can be a problem for the lime/limestone wet scrubber systems. Scaling occurs when the solutions are supersaturated to a point where heterogeneous crystallization (crystallization on foreign surfaces such as the scrubber walls, overfiow pots, marbles) takes place, resulting from nucleation. The ratios of the products of the activities (A) of Ca and S04 " or to their solubility product constants Kgp) as a measure of the degree of supersaturation are ... 

The Air Quality Control Systems (AQCS) using lime/limestone wet scrubbing have three basic types of chemical process equipment (1) scrubbers, (2) reaction tanks, and (3) solid-liquid separators, in addition to several auxiliary pieces of equipment such as pumps, demisters, and reheaters. The SO2 in the flue gas is transferred into the liquid in the scrubber, the sulfur in the liquid is converted to solid calcium sulfite, and calcium sulfate in the reaction tanks and solid calcium sulfite and sulfate are separated from the liquid and disposed from the solid-liquid separators such as clarifiers, vacuum filters, and ponds. 

The alkaline lime, in a gas-solid phase reaction, reacts with sulfur dioxide in the combustion gases to form solid particles of calcium sulfite and calcium sulfate which are captured in electrostatic precipitators (Eqs. 3.39 and 3.40). 

Write balanced chemical equations for each of the following processes (a) Calcium phosphate reacts with sulfuric acid to produce calcium sulfite and phosphoric acid, (b) Calcium phosphate reacts with water contaming dissolved carbon dioxide to produce calcium hydrogen carbonate and calcium hydrogen phosphate. 

Representative results are shown in Table VIII for T = 50°C pH = 5.0. The manganese concentration was varied from 0.1 to 1.0 ppm, values which are representative of those found in scrubber liquors (12). The concentration of manganese is the total value, l.e., the sum of that contributed by the calcium sulfite and that added to the solution. These results were also fit to an expression of the form of equation (2) with the maximum likelihood estimates being ... 

Solids from the batch precipitation tests were also examined by scanning electron microscopy. In tests where no adipic acid was added, the calcium sulfite solids formed a single platelet crystal. However, upon addition of 3,000 ppm adipic acid prior to solids precipitation, the calcium sulfite crystals formed as platelet clusters or rosettes. As the concentration of adipic acid was increased the crystals became smaller and less plate-like until at 10,000 ppm adipic acid in the slurry solution the crystals were submicron in size and resembled popcorn shaped spheres (5). These results suggest that adipic acid effects the nuclea-tion rate of calcium sulfite and certainly can drastically change the particle size distribution and crystal morphology of precipitated solids. 

The spent scrubbing solution is regenerated by reaction with limestone. This reaction precipitates mixed calcium sulfite and sulfate solids, resulting in a slurry containing up to 5 wt. % insoluble solids. The regeneration process involves basically the following overall reaction ... 

The quicklime reacts with SO2 to form calcium sulfite and some calcium sulfate ... 

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. 

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