Calcium sulfite reaction

Following the initial steps of hydration (Reaction 1) and formation of calcium sulfite (Reaction 2), removal of SO2 in the lime or calcium hydroxide system depends on the formation of calcium bisulfite by reaction of suspended calcium sulfite with sulfur dioxide and water (Reaction 3). 

The reactions in which sulfite is oxidized to sulfate (Reaction 8) and soluble bisulfite is converted to a insoluble calcium sulfite (Reaction 9) account for the waste products as well as the regeneration of the solid calcium sulfite reactant that is recirculated to the scrubber. The ratio of calcium sulfite to calcium sulfate found in the air pollution control system solid waste depends on the extent to which these reactions go to completion. 

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). 

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. 

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) ... 

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 ... 

Calcium Sulfite. (CAS 10257-55-31 CaSOj- 2H2O. white precipitate, pK = 7.9, formed by reaction of soluble calcium salt solution and sodium sulfite solution, or by boiling calcium hydrogen sulfite solution calcium hydrogen sullite. Ca(HSOj) . formed in solution by saturating calcium hydroxide nr carbonate suspension with sulfurous aeid. 

There arc two main processes for the industrial production of phosphoric add, H3PO4. from phosphate rock (1) the wet process which involves tlie reaction of phosphate rock with H2SO4 to yield phosphoric acid and insoluble calcium sulfites, Several of the impurities present in the rock dissolve and remain with the product add. These are not important when the add is used for fertilizer manufacture. However, the impurities are deleterious to the manufacture of phosphorus chemicals. For a purer product, (2) the furnace process is used, wherein the phosphate rock is combined with coke and silica, producing elemental phosphorus as previously described. Oxidation of the phosphorus produces P2O5 which, when combined with H2O, yields H3PO4. 

These processes could contribute a damaging amount of SO, to the atmosphere if precautions were not taken to remove it. Limestone and sand, which are added to the mixture, form a molten slag that removes many of the impurities as well as the S02. For example, calcium oxide (a basic oxide) from the limestone reacts with the SOz (an acidic oxide) to produce calcium sulfite in a Lewis acid-base reaction ... 

In the sodium-based dual alkali process, the acid gases are absorbed by a solution of sodium salts at a pH range of 5-8. The solution is regenerated outside the scrubber with lime or limestone to produce a solid waste containing calcium sulfate and calcium sulfite. Some sodium salts are lost with the waste and must be made up by the addition of NaOH or Na2C03. The principal chemical reactions are as follows ... 

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. 

The control of sulfite scaling requires that a minimum amount of free hydroxide ion be recirculated to the scrubber therefore, fresh additive (lime or slaked lime) is added in the reaction tank external to the scrubber where calcium sulfite is formed (Reaction 4). An amount of calcium sulfite equivalent to the SO2 removed (or the fresh Ca(OH)2 added) is conveyed from the system to a pond or vacuum filter, and the remainder is recycled to continue the removal process. 

Experimental work with lime scrubbing has shown that sulfite scaling occurs in the scrubber bed when free hydroxide is introduced. By proper control of the pH of the spray slurry (less than 10) entering the scrubber, calcium sulfite scaling will be prevented within the scrubber. In the calcium carbonate system, the buffering action of the carbonate-bicarbonate couple (Reaction 5) maintains a system pH between 5 and 6 thus sulfite scaling is not encountered. 

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. 

Laboratory studies are in progress to determine the calcium sulfite precipitation kinetics and the oxidation kinetics of sulfite to sulfate. Until these reaction rate expressions are developed, the experimental data obtained from the pilot plant, prototype, and field units will be used to design the reaction tanks and scrubbers to eliminate calcium sulfite scaling. 

The endothermic, reversible dissociations of sulfites exhibit a superficial resemblance to the reactions of carbonates (MSO3 MO + SO2). However, the possibility of anion oxidation to the thermally more stable sulfate makes investigations of the simple reactions of these salts difficult. Some of the problems are exemplified by the many studies of the reactivity of CaS03. Calcium sulfite is a probable intermediate in the reactions of CaC03 used to desulphurize the flue... 

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). 

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). 

We have measured the rate of oxidation of sodium and calcium sulfite to sulfate in a solution containing an organic buffer and the catalysts manganese and iron. The work was carried out in order to develop kinetic rate expressions rather than to explore the fundamentals of the reaction scheme. 

A model has been developed for oxidation of calcium sulfite in a three-phase, semibatch reactor, The overall rate of conversion to sulfate depends on the rates of solid dissolution and liquid phase chemical reaction. In this first treatment of the problem, gas-liquid mass transfer resistance did not affect the overall rate of oxidation. 

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). 

Gladkii(16) at the State Scientific Research Institute of Industrial and Sanitary Gas Cleaning at Moscow did work on the three-phase calcium sulfite slurry oxidation system, finding that the liquid phase oxidation (pH 3.6-6) is first order with respect to the sulfite species. He pointed out, on the basis of pH versus time data from his semi-batch reaction, that the slurry oxidation had different periods in which either reaction kinetics or solid-liquid mass transfer controlled the oxidation rate. He also presented an omnibus empirical correlation between pH, temperature, and the liquid phase saturation concentration of calcium sulfite solution for predicting the slurry oxidation rate. The catalytic effect of manganese... 

The first case is calcium sulfite dissolution without chemical reaction. Using a film model will allow the calculation of the surface concentrations of all the species and the rate of dissolution. With a knowledge of the particle population and solution concentrations, most of the variables are known when the experiment starts. To specify all the starting values of the variables, the conditions at the particle surface are required. From the consideration of saturation concentration In the previous section during dissolution, the bulk liquid must obey ... 

The liquid phase concentration of sulfite in the slurry is governed by two competing mechanisms. Sulfite in the liquid is lost due to the oxidation reaction, but is replenished by the dissolution of the solid calcium sulfite. The sulfite concentration is represented by a differential equation of the form ... 

Highly Catalyzed Experiments. In conjunction with experiments in which the manganese concentration was 2000 ppm, the model was first used to determine the mass transfer coefficient of calcium sulfite (T=40°C, pH=5.0). A reaction order of 1.5 was obtained from previous liquid phase kinetic studies with a rate constant of 85 1 ° 5 mol 0,5 sec-1 (25). Computer curves were generated using a series of mass transfer coefficients and plotted along with the experimental kinetic results on Figure 7. A mass transfer coefficient of 0.015 cm/sec most closely fits the data. The bulk pH drops quickly during the initial several seconds and then stays around 2.9, but the surface pH remains almost constant at 5.15 which implies that the oxidation only has small influence on the surface conditions. 

The buffering activity of adipic acid limits the drop in pH that normally occurs at the gas-liquid interface during SO2 absorption, and the resultant higher concentration of SO2 at the interface significantly accelerates the liquid-phase mass transfer. The capacity of the bulk liquor for reaction with SO2 is also increased by the presence of calcium adipate in solution. Thus, the SO2 absorption becomes less dependent on the dissolution rate of limestone or calcium sulfite in the absorber to provide the necessary alkalinity. 

Buffer Reaction Mechanism. The mechanism by which adipic acid buffers the pH is simple. It reacts with lime or limestone in the effluent hold tank to form calcium adipate. In the absorber, calcium adipate reacts with absorbed S02(H2S03) to form CaS03 and simultaneously regenerates adipic acid (the buffer reaction). The regenerated adipic acid is returned to the effluent hold tank for further reaction with lime or limestone. With a sufficiently high concentration of calcium adipate in solution, usually on the order of 10 m-moles/liter to react with the absorbed S02, the overall reaction rate is no longer controlled by the dissolution rate of limestone or calcium sulfite. 

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 ... 

Reaction of calcium ions from calcium oxide, calcium hydroxide, or calcium carbonate with sulfite ions from sulfuric acid to form and precipitate calcium sulfite. 

There is data indicating that the hydration of sulfur dioxide to sulfuric acid, H+, and HS03, the dissociation of HS03" to S032", the reaction of lime and limestone in an acidic medium to form Ca2+ ions, and the final reaction of Ca2+ and S032- ions to precipitate calcium sulfite are rapid reactions (JO). The rate controlling mechanisms are, therefore, either... 

Reaction with acidic gases. Limestones react readily with gaseous hydrogen chloride and hydrogen fluoride, forming calcium chloride and fluoride respectively. Dry sulfur dioxide reacts with limestone at 95 °C and above to produce calcium sulfite. Sulfur trioxide also reacts with limestone to produce the sulfate. 

The rates of absorption of sulfur oxides in milk of lime and of solution of the slaked lime are considerably faster than the analogous reactions with limestone. For a given removal efficiency, therefore, the absorber is considerably smaller than that for limestone. This helps to reduce capital costs and the power consumed in recirculating the slurry. However, because of the rapid precipitation of calcium sulfite hemi-hydrate with some gypsum, the particles tend to be finely divided and form a thixotropic sludge which is difficult to de-water for disposal [12.1]. 

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