Calcium sulfite solid

Thickener The lime reactor products flow by gravity to a thickener where the calcium solids settle. The thickener overflow, which is a clear liquor, is separated and is piunped to a clarifier to remove any dissolved calcium sulfate. The thickener underflow is pumped to a rotary vacuiun filter to further concentrate the calcium sulfite solids. 

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. 

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. 

Lime slurry and limestone scrubbing are suitable for relatively low SO2 concentrations and moderate SO2 removal efficiencies ( 95%). Each process generates calcium sulfite solid waste that can be placed in a nonhazardous landfill. The waste solids are actually a mixture of mostly calcium sulfite with a small amount of calcium sulfate. The mixture is often represented as CaSO solids. 

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 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. 

Alvarez-Dalama, A., "Calcium Sulfite Hemihydrate Crystallization in Liquors with High Total Dissolved Solids," M.S. Thesis, University of Arizona, Tucson (1986). 

Rotary Drum Vacuum Filter The underflow from the thickener is the feed to this filter. The filter concentrates the solids (predominately calcium sulfite), which then leave the filter in a cake form and are transferred to a cake repulp tank where they can be reslurried. The liquid generated from the filtering process (filtrate) is collected and pumped to the lime reactor vessel. 

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 LCM was accurate within 20 percent error in SO- vapor pressure for the 50°C data, as well as data at 35, 70, and 90°C. The solid line shows the general trend of the LCM predictions. Figure 4 plots the apparent equilibrium constant as a function of ionic strength for a calcium sulfite/bisulfite system at 25, 50, and 60 C... 

Calcium sulfite (CaS03) is a solid that can be removed from the inside of the smokestack. 

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. 

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). 

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 " ... 

When the solid sludge is removed from the scrubber as unoxidized calcium sulfite, as is done in many older... 

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). 

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 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 ... 

At the start of each run, various materials were added to the hold tank. These materials included deionized water, adipic acid, calcium sulfite seed crystals, NaCl, MnS( H20, Fe2(S( )3, fly ash, etc. depending on the purpose of the test. The resulting thin slurry was circulated through the scrubber where it was contacted with flue gas. This procedure was continued for several hours, allowing time for the mass of solids in the hold tank to increase and the concentration of ionic species (particularly S03) to reach a constant level. At this point, a quantitative amount of slurry was withdrawn from the hold tank. 

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. 

At the Shawnee Test Facility, major emphasis has been placed on the use of adipic acid in conjunction with forced oxidation of calcium sulfite to calcium sulfate, since this system results in better sludge dewatering properties and reduced waste solids disposal costs. Furthermore, the more tightly closed liquor loop,... 

In any within-scrubber-loop forced oxidation system, irrespective of whether it is additive promoted or not, the possibility exists for calcium sulfite blinding of limestone because the recirculated slurry lacks the solid CaSC>3 crystal seeds. 

Bleed stream oxidation of unenhanced lime or limestone slurry is usually not feasible because the pH rise caused by the residual alkali in the oxidation tank makes it difficult to redissolve the solid calcium sulfite. With adipic acid-enhanced limestone scrubbing, however, this constraint is removed because of the low operating pH and low residual alkali in the bleed slurry. Thus, the oxidation tank can be maintained at a low pH for good sulfite oxidation, while achieving high SO2 removal efficiency with a sufficiently high concentration of adipic acid in the scrubber liquor. 

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 sulfate co-precipitates with the calcium sulfite, resulting in a mixed crystal (or solid solution) of calcium-sulfur salts. Gypsum is not formed. The relatively high sulfite concentrations in the solution prevent soluble calcium concentrations from reaching the levels required to exceed the gypsum solubility product, and the system operates unsaturated with respect to calcium sulfate. 

There was a slow buildup of a layer of scale on the walls of the first reactor. Insufficient agitation may have contributed to the deposition of this scale which was primarily made up of calcium sulfite/sulfate solids. The formation of this scale was also evident in the overflow pipe connecting the first and second reactors. This pipe had to be replaced in early March as the 6-inch line had been reduced to a 3-1/2 inch line due to scale buildup. This overflow line was rather long—6 feet—and had an angle of incline of only 3° because of limitations in the existing plant layout. A greater angle of incline may have reduced this scale buildup. 

Another type of solids deposition in the form of round/oval beads was also observed in the first reactor. These beads—1/8 to 3/8 inch in size—were also made up of calcium sulfite/sulfate which built up in layers around a seed particle much like the growth of pearls. Scale buildup in the other reactors was minimal. Based on the time of operation at Scholz, it appears that a semiannual cleaning operation as part of a regular maintenance program might be adequate to control reactor scaling. 

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