Chemical and Biological Engineering Publications Chemical and Biological Engineering 1960 Reductive Decomposition of Gypsum by Carbon Monoxide Thomas D. Wheelock Iowa State University, [email protected] D.R. Boylan Iowa State University Follow this and additional works at: http://lib.dr.iastate.edu/cbe_pubs Part of the Catalysis and Reaction Engineering Commons, Complex Fluids Commons, and the Other Chemical Engineering Commons The ompc lete bibliographic information for this item can be found at http://lib.dr.iastate.edu/ cbe_pubs/273. For information on how to cite this item, please visit http://lib.dr.iastate.edu/ howtocite.html. This Article is brought to you for free and open access by the Chemical and Biological Engineering at Iowa State University Digital Repository. It has been accepted for inclusion in Chemical and Biological Engineering Publications by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Reductive Decomposition of Gypsum by Carbon Monoxide Abstract Tremendous domestic reserves of gypsum and anhydrite constitute a potential source of raw material f i x sulfur-based chemicals. As in Europe today, calcium sulfate may become one of our principal raw materials for sulfuric acid. Several European acid plants are based on a process in which sulfur dioxide is freed from anhydrite by heating the latter with coke and shale to a sintering temperature (4). The uls fur dioxide is converted into acid and the clinker is used for portland cement. Disciplines Catalysis and Reaction Engineering | Complex Fluids | Other Chemical Engineering Comments Reprinted (adapted) with permission from Ind. Eng. Chem., 1960, 52 (3), pp 215–218. Copyright 1960 American Chemical Society. This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/cbe_pubs/273 T. D. WHEELOCK and D. R. BOYLAN I Department of Chemical Engineering, Iowa State University of Science and Technology, Ames, Iowa Reductive Decomposition of Gypsum by Carbon Monoxide Sulfur dioxide and lime can be produced from gypsum under newly determined conditions. Process provides a new route for manufacturing sulfuric acid from gypsum TREMExDOcS domestic reserves of gyp- carbon monoxide at elevated tempera- ditions at atmospheric pressure. calcium sum and anhydrite constitute a potential tures to produce sulfur dioxide and lime. sulfide cannot exist in the presence of source of raw material fix sulfur-based The solids do not sinter and the lime may calcium sulfate because of Reaction 3. chemicals. As in Europe today, calcium be a by-product of value. Because this Of course, the kinetics of this reaction sulfate may become one of our principal by-product can be disposed of in more may be unfavorable. raw materials for sulfuric acid. Several ways than portland cement, the process To show that a reducing agent is European acid plants are based on a should be more flexible than the Euro- needed for decomposing calcium sul- process in which sulfur dioxide is freed pean process. fate, equilbrium constants for Reaction from anhydrite by heating the latter with 4 are included. coke and shale to a sintering tempera- Thermodynamics ture (4). The sulfur dioxide is converted The principal reaction (Reaction 1) is Experimental into acid and the clinker is used for endothermic and therefore favored by portland cement. higher temperatures. Reaction 2, which For this study natural gypsum having In the work described here a simpler is undesirable, is exothermic and is the following composition was used. process for freeing sulfur dioxide \vas favored by lower temperatures and high The gypsum was crushed and then sep- investigated. Calcium sulfate reacts carbon monoxide partial pressures. arated by Tyler standard sieves into with a gaseous reducing agent such as Above 2100' F., under equilibrium con- narrow-size fractions. The -7f8-mesh fraction was used for most runs. Both commercial and chemically pure grades of carbon monoxide were used. Calculated Equilibrium Constants and Heats of Reaction for Reactions Involved in the Process The carbon dioxide was specified as 99.9791" pure, and the sulfur dioxide was AHR, refrigerant grade. The liquid nitrogen, Cal. ' Log,oK Mole which after vaporization served as a Reaction 1200' K. 1400' K. 1600' K. 1400' K. 1. CaS04 + CO = CaO + SO2 + CO? 0.31 1.48 2.28 43,400 ,~_~_- 2. CaSOa + 4CO = Cas + 4CO: 7.92 6.69 5.66 -48,400 - 3. 3CaSO4 CaS = 4Ca0 4.502 -6.68 -0.77 3.44 222,200 + + - Silicon rubber tubing 4. CaSO, = CaO + SOZ + 1/2 02 -7.42 -4.51 -2.38 110,600 lnsulollng firebrick Crushed inrulotfng firebrick Leco zircon lube Alumina bolls -3+4 mesh Rotameter SO2 Gypsum particlar Crromel-olumel !hermOCOUPle I! Liqusd N2 The reactor (diameters, 0.75 to 1.13 Carbon monoxide and dioxide, sulfur dioxide, and nitrogen, either alone or in inches) was suspended inside the gas- combination, were continuously metered, mixed, and passed through the reactor fired muffle furnace from the triple- containing gypsum beam balance VOL. 52, NO. 3 MARCH 1960 215 Results Effect of Temperature. The effect Average Gypsum" Composition (1) of temperature on the desulfurization Calcium sulfate in the gypsum could be Constituent Weight To rate and on the formation of calcium sul- quantitatively decomposed by passing a fide was studied between 2100' and Hz0 19.6 stream of nitrogen over the gypsum CaO 30.9 2300 ' F. using various gas compositions. MgO 0.1 heated to about 2200' F. However, When the gas mixture fed to the reactor adding as little as 1% SOa 45.1 of the decomposi- contained 3y0 carbon monoxide, results c02 0.7 tion products, sulfur dioxide and oxygen, were obtained as shown in Figure 2. At R208 0 to the nitrogen prevented the decomposi- Si02 3.3 2110" F. the gypsum passed through an NaCl -0.3 tion. If several per cent of carbon mon- initial induction period where little or Total 100.0 oxide were also present, calcium sulfate no decomposition occurred. The reac- decomposed in the presence of as much as a Cnited States Gj-psum Co., Fort Dodge, tion rate soon increased, and a relatively Iowa. 7y0 sulfur dioxide. constant but rapid desulfurization rate Generally, calcium sulfate was con- was established. At the end the gypsum verted to calcium oxide. At times cal- was 8770 desulfurized, and the solids con- cium sulfide was produced. Conse- diluent for the gaseous reactants, con- tained 11yo calcium sulfide. Increasing tained less than 0.1% oxygen. quently the criteria chosen for comparing the temperature to 2200" F. increased The reactor, usually charged with a the effect of operating conditions were the the initial desulfurization rate: but the bed of gypsum 1 inch thick, was sus- rate of desulfurization and the concentra- maximum rate was unaffected. The tion of calcium sulfide in the residual pended inside the preheated furnace, and total desulfurization was increased to al- solids. The conversion to sulfide was ap- as its temperature rose, a mixture of sul- most looyo, and no calcium sulfide was proximately proportional to the percent- fur dioxide and air was passed through to found in the solids. When the tempera- age of calcium sulfide in the residue. prevent the gypsum from decomposing. ture was raised to 2310' F., the initial As operating temperature was ap- The total desulfurization or conversion rate was the same as for 2200' F.? but of calcium sulfate to calcium oxide was proached, nitrogen and carbon dioxide after 8 minutes the rate fell to a much calculated from the composition of the were added. When the reactor tempera- lower but constant value. final solids. By assuming that the ture had leveled out, the flow of air was i2'hen the gas fed contained 47, car- instantaneous conversion was propor- stopped and the flow of carbon monoxide bon monoxide, the initial and maximum tional to the weight lost by the gypsum was started. This marked the beginning rates (Figure 3) and the per cent calcium of a run. At regular intervals of 1 to 10 charge, desulfurization curves such as sulfide in the residue (Figure 4) varied minutes, depending on the rate of de- those in Figures 1 and 2 were plotted. ivith temperature. composition, the reactor weight was These curves usually had either one or two LVhen sulfur dioxide was excluded from noted. Operating conditions were kept constant rate periods for which the rates constant. After the reactor reached a could be reasonably correlated with the gas fed, the maximum desulfuriza- constant weight, it was slowly withdrawn reaction conditions. Generally the tion rate reached a peak value at about from the furnace, while nitrogen and a greater part of the desulfurization took 2150' F. with 4% carbon monoxide and small amount of carbon monoxide Lvere place at a constant rate which corre- 2250' F. with 2%. The variation in the passed through it. sponded to the maximum rate. rate kvith temperature was much greater Cndecomposed sulfate in the residual solids was determined gravimetrically (7). Sulfide and calcium were deter- mined iodometrically and by Venenate titration (2), respectively. loon90 'oo--m2400 IDewIfurization s F =- 60 2240 2 0 L c 0 2 50 2200 a E 1 E 40 2160 f D Gor Cornpoiition t -~ 2040 Figure 1. The desulfurization pro- Time, min. ceeded at a constant rate for much of the run when mixtures of carbon mon- Figure 2. When sulfur dioxide was present in feed gas containing less than oxide and nitrogen were fed 5y0 carbon monoxide, S-shaped desulfurization curves were obtained 2 16 INDUSTRIAL AND ENGINEERING CHEMISTRY GYPSUM DECOMPOSITION Figure 3. The maxi- mum desulfurization rate reached a peak value at about 2200' F. 0' 2100 220'3 2300 Temperature during maximum desulfurization rate period.