Calcium sulfite oxidation

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-... 

Erwin(13) investigated the calcium sulfite oxidation at high manganese concentrations in a flow calorimeter under the following conditions ... 

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. 

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 ... 

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. 

Cake Repulp Tank The cake repulp tank is an agitated vessel whose function is to reslurry the calcium sulfite solids so that they can be oxidized. The solids are mixed with recycled filtrate and fresh make-up water to produce a slurry that is then oxidized. 

Oxidizer Column The oxidizer column is where the calcium sulfite slurry is reacted with air to form the by-product calcium sulfate. Air compressors are used to supply the oxidation air. In addition, the proper pH is maintained to sustain the... 

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. 

With sulfur dioxide, calcium sulfite is the product which slowly oxidizes to calcium sulfate ... 

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

Both lime and slaked limes are use to reduce sulfur emissions, which contribute to acid precipitation, from power plants, particularly coal-fired plants. By using lime, more than 95% of the sulfur can be eliminated from the emissions. Calcium oxide reacts with sulfur dioxide to produce calcium sulfite CaOfe) + S02( —> CaS03(). Sulfur dioxide is also removed by spraying limewater in the flue gas. Limewater, also called milk of lime, is a fine suspension of calcium hydroxide in water. Other pollutants removed with lime include sulfur trioxide, hydrofluoric acid, and hydrochloric acid. 

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 ... 

Waste materials such as calcium sulfite are difficult to dewater and cannot support a load. It is often desirable to stabilize such wastes before disposal. Methods for the stabilization of these materials, such as oxidation of calcium sulfite to gypsum, have been developed. In certain countries (such as W. Germany and Japan), waste materials like gypsum can be sold however, this is not generally true in the United States. 

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... 

In the future, it seems likely that most processes that currently entail the discharge of SO2 will have to be adapted with the introduction of scrubbers . Two main methods can be employed for this either the reduction of SO2 to snllur using the Claus process or, alternatively, neutralization nsing milk of lime (equation 18) to give either calcium sulfite or, with complete oxidation, calcium sulfate (gypsum). 

Sulfates are widely known in the environment and a few such as those of calcium, strontium, barium, and lead are insoluble. The sulfates of magnesium and the alkali metals are soluble and so are typically found in brines and salt deposits. Sulfites are rarer because of the ease of oxidation, but calcium sulfite, perhaps as the hemihydrate, can be detected on the surface of building stone in urban atmospheres containing traces of sulfur dioxide (Gobbi et al, 1998). 

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. 

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. 

For flue-gas desulfurization, lime and limestone processes are utilized in which the furnace gas is scrubbed with an aqueous liquid containing the necessary quantity of ealcium hydroxide or calcium carbonate for neutralizing the acidic furnace gas components. The end products are calcium sulfite/calcium sulfate sludges or on complete oxidation gypsum ... 

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 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 " ... 

There is a positive effect of the SCR unit on downstream equipment such as flue gas desulfurization, which produces gypsum. In the absence of a SCR unit, water from a FGD unit contains hydroxyl amine disulfonic acid which has been formed from NO2, water, and SO2. This compound acts probably as an inhibitor for the oxidation of calcium sulfite into calcium sulfate [129]. 

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 rates of the catalytic oxidation of sodium and calcium sulfite in several organic acid buffers have been determined. In the paper particular attention is given to oxidation in succinic acid. 

The experiments with calcium sulfite were done with succinic acid concentrations from 0 to 0.2 m/1 and it is seen that the organic acid is a substantial inhibitor for the oxidation. 

Forced oxidation in flue gas desulfurization (FGD) systems converts calcium sulfite (CaSO. H O) to calcium sulfate, or... 

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