CATALYTIC PROCESS FOR ACCELERATED DECARBOXYLATION OF CALCIUM CARBONATE AND BLEND THEREFOR

Abstract: The method for accelerated decarboxylation of a calcium carbonate material comprising heating the calcium carbonate material to a temperature and for a time sufficient to decarboxylate the material to the degree desired while in the presence of a catalyst; said catalyst comprising at least one fused salt having the formula (MX) . (CaO) wherein M is an alkali metal and X is a halide, carbonate, or oxide, and a blend adapted to be heated to form CaO comprising a calcium carbonate material and at least one fused salt having the formula (MX) . (CaO) wherein M is an alkali metal and X is a halide, carbonate, or oxide.

Description: CATALYTIC PROCESS FOR ACCELERATED DECARBOXYLATION OF CALCIUM CARBONATE AND BLEND THEREFOR ' There are a number of procedures where it is desired to decarboxylate a calcium carbonate- containing material to calcium oxide. The most notable are in the production of lime and hydraulic cements. The American lime industry alone sells more than 21 million tons of lime a year with most of it being produced in energy-intensive rotary or shaft kilns. The lime manu acturing process is relatively simple in that particulate materials • are fed into refractory lined furnaces- mixed with suitable fuels, and subjected to intense heat caused by the combustion of the fuel. Provisions are made to optimize exposure of the feeds to the thermal environment and the system is an open one to per¬ mit escape of the products of calcination; lime and carbon dioxide and traces of moisture. The heating or calcination of limestone into lime requires break¬ ing an oxygen-to-carbon bond and liberating carbon dioxide. The calcination reaction is represented by the equilibrium equation: CaCO„-=«==-≥ > CaO + C0 3^ ^ 2 In commercial practice the actual energy employed is many times greater than the theoretical bond strength due to heat losses, heat transfer properties, and the impetus required to remove the liberated carbon dioxide. Large amounts of energy were used in such decarboxylation, when energy was inexpensive and abundant, to hasten calcination and promote the removal of CO2 from the reaction bed. Such usage, while not expensive, was wasteful, and, in some cases, when excessive temperatures were used, often resulted in overburning or hard burning and caused the reaction of lime with impurities producing hard grains in the lime. With escalation of energy costs due to the vast increases in the price of fossil fuels such as oil and coal, many efforts have been undertaken to con¬ serve energy and reduce costs in this energy in¬ tensive process. These have included changes in apparatus, the use of air purges, evacuation, or collection of the carbon dioxide, and the use of "dry feed" practices to eliminate the need for large amounts of energy to remove water from the calcined product. While effecting some reduction in fuel usage, they have not been entirely " successful in cutting costs. It is to be understood, of course, that the energy consumption in the production of lime by calcination of limestone is also applicable to the cement industry where the calcination of limestone represents a major portion of the energy-consuming process. 5 SUMMARY OF THE INVENTION The present invention overcomes the problems of the prior art and permits the rapid decarboxylation of calcium carbonate at faster rates and at lower kiln temperatures with up to about a 50 percent re- 10- duction in present fuel energy requirements. Briefly, the present invention comprises the method for accelerated decarboxylation of a calcium carbonate material comprising heating the calcium carbonate material to a temperature and for a time 15 sufficient to decarboxylate the material to the degree desired while in the presence of a catalyst; said catalyst comprising at least one fused salt having the formula (MX)• (CaO) wherein M is an alkali metal and X is a halide, carbonate, or oxide. 2.0 The invention also comprises a blend adapted to be heated to form CaO comprising a calcium car¬ bonate material and at least one fused salt having the formula (MX)• (CaO) wherein M is an alkali metal and X is a halide, carbonate, or oxide. 5 DETAILED DESCRIPTION While the instant invention broadly relates to any process or blend where a calcium carbonate material is to be decarboxylated by heat, it will be de¬ scribed in detail with respect to one of the most important industries where this occurs; namely, the manufacture of hydraulic cements. A major step in 5 that process is calcination of a calcium carbonate material. When referring herein to a "calcium carbonate material", it is intended to mean lime¬ stone, dolomite, or any other source of calcium carbonate conventionally used to make lime, cements, 10. and the like. Limestone is, of course, the most commonly used source of calcium carbonate for these purposes. The instant process requires the use of certain catalysts which are, in broadest outline, thermal 15 reaction products of calcium carbonate and an alkali metal halide or carbonate. These catalysts can be described as fused salts and have a vitreous nature. The calcium carbonate used to make the catalyst can be any calcium carbonate-containing material such as 20 the limestone, dolomite or calcium carbonate per se. With respect to the alkali metal halide, it can be sodium chloride, potassium chloride, lithium chloride, lithium iodide, lithium fluoride, sodium iodide, potassium iodide, cesium iodide, rubidium chloride, 25 and the like. As to the carbonates, illustrative examples are sodium carbonate, potassium carbonate, lithium carbonate, cesium carbonate, and the like. OMPI Of the alkali materials used, it is preferred to use sodium or potassium carbonate. The fused salts have the formula (MX)* (CaO) in which M is the alkali metal and X is the halide, carbonate, or oxide. The catalysts are formed by admixing; preferably in equal molar ratios, a calcium carbonate material with any one or a mixture of the alkali metal com¬ pounds. Molar ratios of carbonate material to alkali metal compound ranging from 50 to 1 to 1 to 5Q can be used. The mixture is preferably ground to a homogenous blend and heated to a temperature sufficient and for a time sufficient to assure fusion to a vitreous-like mass; usually a temperature of about 900° to 1000° C. (+ 100° C. ) . The fused salt is then cooled and ground to a particle size which is preferably either above or below that of the calcium carbonate material to be decarboxylated or calcined for the reason set forth below. Grinding to a mesh size of about -100 to +200 is preferred. s to the process of decarboxylation or cal¬ cination, from about 0.1 to about 20 parts by weight of the granular catalyst; preferably about 1 to 10 parts by weight, are then added for each 100 parts by weight of the calcium carbonate material, the mixture then fed into a furnace, and then heated to a temperature and for a time sufficient to get the g τ_iEAir O PI desired degree of decarboxylation or calcination. While the precise calcining temperature and treat¬ ment time will vary depending upon the efficiency of the furnace, the particular catalyst, and the uniformity of the heating, it has been found that temperatures as low as 450° C. , but preferably 800° to 900° C., for about thirty to forty-five minutes is sufficient for proper calcination. This is to be contrasted with calcination temperatures of about 1200° to 1350° C. required for longer periods of time in the absence of the fused salt. When the heat treatment is finished, the treated mixture will comprise free lime and magnesia (from dolomitic limestone) with the unaltered fused salt catalyst. By providing for a difference in the particle size between the catalyst and the untreated limestone, it is possible to easily separate the catalyst for reuse by using any conventional separation procedure, such as screening or air classification. In instances where the fused salt is one formed by reacting a sodium or potassium carbonate with calcium carbonate, such catalyst may be allowed to remain as part of the final product since it will have a very low sodium or potassium content and will not be deleterious for many of the uses for which the cement is required. For most effective results the catalyst and calcium carbonate material should be finely ground to about -100 to -200 mesh powder. This optimizes the activity of the catalyst and assures a more com- plete burn and economy of f el. Thus, for present cement operations using coarse feeds, a pre-grinding of the feed to the noted mesh is suggested. The invention will be further described in con¬ nection with the following examples which are set forth for purposes of illustration only. EXAMPLE 1 Equal molar ratios of limestone and sodium carbonate are blended and ground to an homogenous blend. The mixture is then heated to about 1000° C. for 6Q minutes to assure fusion to a vitreous mass, cooled and ground to about -100 + 200 mesh. Ten parts by weight of the granular catalyst is then added to 100 parts by weight of ground lime¬ stone (-200 mesh) , fed into a furnace, and heated at 800° - 850° C. for sufficient time to assure a uniform degree of heating; at least 30 minutes but not necessarily greater than 45 minutes. The emerging mixture will comprise free lime (and magnesia) with the unaltered fused salt catalyst. The catalyst may be lightly coated with calcination product but gentle abrasion will restore its original surface and allow its reuse. In 15 minutes the catalyzed mixture will achieve over 60% conversion compared to 20 - 30% when un- catalyzed. In 30 minutes at 850° C. the catalyzed mixture will be approximately 99% calcined compared to less than 50% when uncatalyzed. EXAMPLE 2 The same procedure as Example 1 is followed except that cesium chloride is substituted in like amount for the sodium carbonate. The rate and degree of calcination at 15 minutes is reduced from that obtained in Example 1 from 60% to 45 - 50% with the halide. EXAMPLE- 3 The procedure of Example 2 is followed, except that separately, and in turn, the same amounts of Lil, NaCl, K Cl, LiF, Nal, and Cel are substituted for the CeCl. Equivalent results are obtained. EXAMPLE 4 To determine the potential contribution of the fused salt catalysts of the present invention in a typical Portland cement kiln-feed system, 12 samples were fired at 850° C, 95Q° C, and 1232° C. for 30 minutes and 6Q minutes, weighing before and after the firing to monitor rate and degree of decarbox- ylation. Samples (6) containing a fused salt of potassium carbonate and calcium carbonate prepared as set forth in Example 1 were found to lose weight ^ ^ E OMPI _ fa WIPO uch faster than the untreated controls (6) at all temperatures with the catalyzed samples going to completion in 30 minutes at 1232° C. whereas the control was no further calcined at 60 minutes/1232° C. than in 60 minutes at 950° C. and only slightly better than at 30 minutes/950° C. The specimens were then analyzed for free lime by titration with ammonium acetate. The catalyzed samples were found to contain significantly less free lime than the controls and when compared to the titration of con¬ ventionally produced, fresh Portland cement, the low free lime content of the catalyzed feed was even more impressive. Free Lime Concentration Specimen Calcination Uncatalyzed Catalyzed Time and Temperature Specimens % Specimens % Free Lime Free Lime 30 minutes at 85Q°C. 14.85 13.60 60 minutes at 850°C. 13.55 12.15 30 minutes at 95Q°C. 13.05 11.05 60 minutes at 950°C. 12.95 10.90 30 minutes at 1232°C. 2.85 .75 6Q minutes at 1232°C. 2.75 .60 From the above it can be seen that it is possible to reduce the kiln temperatures required to econom- ically produce the quantities of calcium oxide or cement currently generated without reducing production rates. The reduced operating temperatures permit a longer service life and reduced maintenance and repair of calcining equipment, extended service life of refractories, reduced heat losses through the insulators, and delivery of a product from the calcining zone at lower temperatures thereby re¬ ducing the amount of energy input and subsequent energy loss during cooling and packaging. Also, the calcium oxide (lime) which emerges from the catalyzed lower temperature system is of improved quality. A soft-burned-more porous lime results which is very chemically active and readily useful in the production of countless calcium salts including the perchlorates essential to manufacture of bleach and chlorinating agents. While the halides are operative, they are not as suitable as the carbonate catalysts by virtue of the fact that the halides present can have a corrosive effect on the equipment used " to heat the mixture of calcium carbonate material and catalyst. Further, to make the catalytic action even more effective, the surface of the refractory lining of the kilns can be coated with the fused salt. In addition to the fused salt catalysts described in detail above, it has also been found that certain alkali metal molybdates also function to catalyze the carboxylation of calcium carbonates. However, these materials; such as sodium silicomolybdate, sodium phosphomolybdate, and the equivalent potassium salts are not commercially acceptable since they are expensive and not readily available materials. While the invention has been described in con¬ nection with a preferred embodiment, it is not in¬ tended to limit the scope of the invention to the particular form set forth, but, on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A method for the accelerated decarbox¬ ylation of a calcium carbonate material comprising heating the calcium carbonate material to a tem- perature and for a time sufficient to decarbox¬ ylate said material to the degree desired while in the presence of a catalyst; said catalyst com¬ prising at least one fused salt having the formula (MX)• (CaO) wherein M is an alkali metal and X is a halide or oxide. 2. The method of Claim 1 wherein the calcium carbonate material is limestone or dolomite and the catalyst is admixed therewith. 3. The method of Claims 1 or 2 wherein the accelerated decarboxylation is the calcining step in the manufacture of a hydraulic cement. 4. The method of Claim 1 wherein said catalyst is used in a ratio of 0.1 to 10 parts by weight for each.100 parts by weight of calcium carbonate material and the temperature is from about 560° to 9.0.0° C. 5. The method of Claims 1, 2, or 4 including the step of separating the catalyst from the de- carboxylated material for reuse. 6. The method of Claims 1, 2, or 4 wherein the catalyst is selected from Na 2 0«CaO or K 2 0*CaO. 7. A method of making a cement comprising calcining a calcium carbonate material in the presence of a catalyst; said catalyst having the formula (MX)• (CaO) wherein M is an alkali metal and X is a halide or oxide and catalyst is used in the ratio of about 1 to 10 parts by weight for each 100 parts by weight of the calcium carbonate material. 8. The method of"CΪaim 7 wherein the calcium carbonate material is selected from limestone or dolomite and the catalyst is selected from Na^O* CaO or K 0 CaO. SL. A blend adapted to be heated to form CaO comprising a calcium carbonate material and a catalyst; said catalyst comprising at least one fused salt having the formula (MX)« (CaO) wherein M is an alkali metal and X is a halide or carbonate. OMPI 10. The blend of Claim 9 wherein the calcium carbonate material is selected from limestone or dolomite, the fused salt is selected from Na_,0* CaO or K O'CaO, and the ratio is from about 0.1 to 2 10 parts by weight of catalyst for each 100 parts by weight of calcium carbonate material. O PI _.

Inventors: DZUIK JEROME J JR, FUNNELL JOHN E, MALLOW WILLIAM A, DZUIK, JEROME, J., JR., FUNNELL, JOHN, E. (DECEASED), MALLOW, WILLIAM, A.
Applicants: SOUTHWEST RES INST, SOUTHWEST RESEARCH INSTITUTE
Application year: 1984
Main classification: C01F11
IPC codes: B01J23, B01J27, C01F11, C04B2