Calcium sulfite

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. 

A variation of the n on regen erabi e absorption is the spray dry process. Time slurry is sprayed through an atomizing nozzle into a tower where it countercurtendy contacts the flue gas. The sulfur dioxide is absorbed and water in the slurry evaporated as calcium sulfite-sulfate collects as a powder at the bottom of the tower. The process requires less capital investment, but is less efficient than regular scmbbing operations. 

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). 

Calcium sulfite [10257-55-3] and acid sulfite may be prepared by reaction of SO2 and hydrated lime or limestone. Calcium acid sulfite [13780-03-5] Ca(HS02)2, has been used to remove lignin (qv) from wood pulp in paper manufacture (6) (see Paper Pulp). 

Compare the solubilities in water of calcium carbonate, calcium sulfite, calcium sulfate, magnesium sulfate, and dolomite. 

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

A number of chemical reactions occur in the absorber beginning with the reaction of limestone (CaCO,) with the SO, to form calcium sulfite (CaSO,). The calcium sulfite oxidizes to calcium sul-... 

Sulfur dioxide can be removed from power plant exhaust gas by a scrubber s tem. One common method involves the reaction of SO2 with calcium oxide (lime) to form calcium sulfite S02(g) + CaO( ) CaS03 ( ) Unfortunately, scrubber systems are expensive to operate, and the solid CaS03 is generated in large enough quantities to create significant disposal problems. 

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

Scale formation in the scrubber can lead to sodium carbonate as an additional dry sorbent in the scrubber. Alternatively, limestone is also introduced into combustion chambers to treat sulfur dioxide emissions. Decomposition of CaC03 into CaO and CO2 occurs in the combustion chamber, and the resulting CaO combines with S02 to produce calcium sulfite. Notice that this process produced another potentially environmentally harmful pollutant (CO2) as it gets rid of a definite environmentally harmful pollutant (SO2). 

Bischoff A flue-gas desulfurization process. A slurried mixture of lime and limestone is sprayed into the gas in a spray tower. The calcium sulfite in the product is oxidized by air to calcium sulfate. Used in Europe in the 1980 s. Lurgi Bishoff is a part of the Lurgi group. The process is offered by Lentjes, Germany, a subsidiary of Lurgi. 

Desox A flue-gas desulfurization process in which limestone slurry absorbs the sulfur dioxide, forming calcium sulfite. This is then oxidized to saleable gypsum ... 

Fersona A process for stabilizing the calcium sulfite/sulfate waste from FGD processes, so that it may be used for landfill. The waste is mixed with ferric sulfate waste from another process (e.g., metallurgical leaching) to form sparingly soluble basic sodium ferric sulfates. Developed in the 1970s at the Battelle Columbus Laboratories, OH, under contract with Industrial Resources. See also Sintema. 

Harloff A process for purifying sugar syrup by the addition of lime and sulfur dioxide. The precipitated calcium sulfite carries down many of the impurities in the syrup. 

Terra-Crete A process for stabilizing the calcium sulfate/sulfite waste from flue-gas desulfurization, so that it may be used for landfill. Calcination converts the calcium sulfite to cementitious material to which proprietary additions are made. Developed by SFT Corporation, York, PA. See also Terra-Tite. 

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) ... 

The calcium sulfite or sulfate solids are allowed to settle from the solution. The regenerated solution is returned to the absorber. The solids are concentrated to around 70%. Because these solids are not a mixture of the sulfite and sulfate, their properties are far superior to lime or limestone process sludge (unless oxidation is used) and disposal should be easier. 

The throw-away processes with aqueous slurries of lime or limestone as the scrubbing media are the most extensively installed processes. These processes create a waste sludge containing calcium sulfite, calcium sulfate, fly ash, unreacted alkali, and other minor dissolved species in the free water contained in the sludge. Since flue gas contains oxygen, some of the dissolved sulfur dioxide is oxidized, and calcium sulfate is formed. 

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... 

The scaling tendency of the lime or limestone processes for flue gas desulfurization is highly dependent upon the supersaturation ratios of calcium sulfate and calcium sulfite, particularly calcium sulfate. The supersaturation ratios cannot be measured directly. They are determined by measuring experimentally the molalities of dissolved sulfur dioxide, sulfate, carbon dioxide, chloride, sodium and potassium, calcium, magnesium, and pH. Then by calculation, the appropriate activities are determined, and the supersaturation ratio is determined. Using the method outlined in Section IV, the concentrations of all ions and ion-pairs can be readily determined. The search variables are the molalities of bisulfite, bicarbonate, calcium, magnesium, and sulfate ions. The objective function is defined from the mass balance expressions for dissolved sulfur dioxide, sulfate, carbon dioxide, calcium, and magnesium. This equation is... 

These values are based upon a pH of 5.5, temperature of 50°C, carbon dioxide partial pressure of 0.12 atm, calcium sulfite supersaturation ratio of 1.0, and calcium sulfate supersaturation ratio of 1.25. 

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. 

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