Process of producing calcium
US RE23794 E
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Description (OCR text may contain errors)
Reissued Mar. 2, 1954 PROCESS OF PRODUCING CALCIUM CARBONATE Milton 0. Schur, Asheville, and Robert M. Levy,
Brevard, N. 0., assignors to Ecusta Paper 001'- poi-ation, a corporation of Delaware No Drawing. Original No. 2,631,922, dated March 17, 1953, Serial No. 108,416, August 3, 1949. Application for reissue 4 Claims.
This invention relates to the manufacture of calcium carbonate and especially in a form suited for use as a filler in cigarette paper for imparting thereto the desired opacity, porosity, and thickness characteristics.
In our copending application Serial No. 745,728, filed May 3, 1947, now Patent No. 2,538,802 issued January 23, 1951, we have disclosed and claimed a process of producing calicum carbonate in certain desired particle sizes for use in cigarette paper in connection with the porosity and opacity characteristics'of the paper, the process comprising a first step of producing calcium carbonate particle sizes finer than desired for use in the paper and a second step of increasing the particle size of these fine particles so as to obtain the optimum size particles. Our two-step process is a decided improvement in the control of particle size in the finished product over" a single-stage process.
Nevertheless, there are characteristics of the calcium carbonate filler, other than particle size, that must be controlled to give the optimum properties in the final high-quality cigarette paper, and while these characteristics are difficult to identify specifically, we believe that they include the varying tendencies of the particles to aggregate, the retention of the fillerinth'e paper furnish during the manufacture thereof,
the softness of the filler, and the bulking'properties of the filler. These are apparently important characteristics, but we do not wish our in vention to be limited to'them or to any theory of why our new'process produces the type of calcium carbonate filler that gives the desired results in the finished cigarette paper.
In our research which led to the present invention, we found that one of the main problems was to produce and reproduce consistently batches of the desired type of calcium carbonate. Despite strict control of the overall process conditions, variations would occur in the quality of the final precipitated calcium carbonate, and the end result in any one batch could not be definitely anticipated. We finally discovered that the carbonating conditions at the very start of carbonation are exceedingly important in determining the quality of the final filler, probably because the particles of calcium carbonate formed at the outset constitute nuclei, the physical-chemical characteristics of which determine the general characteristics of the batch. In short, in the-consistent production and reproduction of the desired endprod'uct, the most important conditions during carbonation are April 29, 1953, Serial Matter enclosed in heavy brackets [jappears in the original patent but forms no part of this reissue specification; matter. printed in italics indicates the additions made by reissue.
those at the outset of carbonation. We discovered that thej'essential condition is to have the slaked lime, calcium hydroxide suspension, initially at a temperature of about 15 C. to 25- C. and preferably at about 20 C. When this initial condition has been established and carbonating of the lime with CO2 begun, the temperature of the mixture may be allowed to rise. Usually the temperatures reach an equilibrium level of about C.-. C. Oncethe proper initial relatively low temperature has been established, it is not necessary to maintain that temperature or any fixed low temperature throughout the carbonating reaction. This is contrary to prior beliefs of other workers in this field, as illustrated by the'prior art patents that recommend maintainingthe temperature during carbonation at a fixed low value or within some specified narrow range of temperatures. Our new discovery in this regard is of substantial commercial value because it avoids the necessity of maintaining a low temperature during the carbonating reaction, which is exothermic, and which would require considerable cooling to absorb the heat of reaction.
We recognize that prior patents disclose processes for obtaining certain characteristics of calcium carbonate by control of the tempera ture of carbonation" and other operating conditions. A typical prior art process of this type involves the maintaining during carbonation of a relatively low temperature of the lime being carbonated, so as to produce a colloidal calcium carbonate. Our present invention fundamentally distinguishes over these prior art processes in that we have found it is not necessary to maintain the low temperature throughout the carbonation reaction, provided the initial temperature is adjusted properly and, also, by permitting the temperature to rise-weavoid the production of colloidal calicum carbonate. We have determined from tests that colloidalcalcium carbonate is much tco fine in particle size to give the above-described desired properties in the ciga such tra tor exam l'e b iexte al 'oqolingbf the vessel containingthe lime slurry or by direct cooling of a thickened, i. e., concentrated, lime slurry with cold water used for dilution to bring it to the proper concentration for carbonating.
Regarding the carbon dioxide gas, which is introduced into the calcium hydroxide suspension for efiecting carbonation, as described below, the CO2 concentration will vary with the type of gas used. For example, if flue gas is used, it will contain approximately 8% tol2% CO2, whereas if kiln gas is used, the percentage will vary within the range of about 10% to 30%. The percentage concentration of CO2 in the gas will. determine the concentration of the lime, slurry to be used. For example, when flue gas is used, the lime concentration, calculated as CaCOe, should be approximately 220 to 280 grams per liter.
The process or" our invention will be more clearly understood from the following description of a typical laboratory preparation, and a typical pilot plant production.
A good grade or 1" rotary kiln, high calcium, quicklime was slaked by addition to boiling water in the ratio of 54 lbs. of lime to 33 gal. of water. The lime suspension was slowly agitated for one hour to insure complete hydration of the lime.
Sixteen liters of the above lime slurry were adjusted in concentration to contain 250 grams/ liter expressed as CaCOs, then were cooled to C.
and were transferred to a small laboratory turbo carbonator, 12" in diameter. A 10% (302-90 70 air mixture was introduced under the surface of the liquid in the turbo-carbonator at the rate of 0.6 C. F. M. The 4 diameter turbo-mixer in the carbonator was rotated at about "500 R. P. M. (The equipment used is sold by the Turbo-Mixer Corporation as 4" laboratory turbo-gas absorber.) ,5
The temperature of the batch was increased-by means of a heating coil at. the uniform rate of C. increase in temperature in the time required to carbonate one-half of the lime present. This rate of increase in temperature corresponds approximately to the increase obtained in large scale apparatus as a balance between the heat of reaction, on the one hand, and radiation and other heat losses, on the other hand, The temperature was then held at C. for the remainder of the time of carbonation. From time to time, a small amount of water was added to replace that lost by evaporation. About 12 hours was required to convert all of the lime initially present to calcium carbonate.
Handsheets of cigarette paper made with numerous batches of calcium carbonate prepared according to the above procedure and from limes from different localities, consistently gave sheets having opacity, porosity and thickness values satisfactory in commercial made cigarette paper.
Pilot plant production A good commercial grade of 1"v high calcium quicklime burned in a rotary .kiln was slaked by the gradual addition to boiling water of 2340 lbs. of lime to 880 gals. of water. The lime suspension was gently agitated with compressed air for several hours to insure complete hydration.
Of the above lime slurry, 7750 gallons were adjusted in concentration so as to contain 250 grams per liter expressed as CaCOa and were pumped into a 10,000-gal'. commercial turbo-carbonator. The. lime, slurry .was then cooled wi h coolin water sprayed onitl'iej'outside' ,ortheiank'to about 20 c. Boiler flue easnc n ainine a ut 9%; CO2 was then adde'd'atthe rateor'about-800 4 C. F. M. under the surface of the liquid near the blades of the rotating turbine. The turboagitator was rotated at about 50 R. P. M. The temperature gradually rose at a relatively uniform rate until a level of about 50 C. was attained. This corresponded approximately to the time when of the lime initially present had been converted to calcium carbonate. The temperature then remained practically constant during remainder of the carbonation period, the head of reaction evidently being balanced by radiation and other heat losses. The time required to convert all the lime to carbonate was about 18 hours.
Batches of this pilot plant filler were first evaluated with the aid of the laboratory handsheets of cigarette paper. The results obtained were identical with the corresponding filler similarly made in the laboratory as described previously. In contrast with'these results, similar batches of calcium carbonate made either in the laboratory or in the pilot plant at a temperature initially above 30 C. gave variable and frequently poor results, when tested in the cigarette paper.
Other methods than those described above for initially cooling the lime slurry prior to carbonation may be employed. They include the use of a continuous centrifuge associated with a rotating discharge screw and operating so as to produce from the hot lime slurry a relatively thick, heavy mass of the hydrated lime, the bulk of the initial water content of the hydrated lime being thrown out by the centrifuge and being recycled, if desired, for use in the slaking of another batch of lime. The discharge screw will slowly feed the dewatered thick mass of hydrated lime away from the centrifuge and into a suitablevessel for dilution of this heavy slu y with process water at suificiently low temperature to convert the heavy sludge or mass of slaked lime into a watery suspension having a temperature of about 20 C. This procedure, which involves first concentrating the lime slurry by centrifuging, and then diluting the concentrated sludge thus produced with cold water to effect a lowering of the temperature of the entire mass to 20 0., has a definite practical advantage from the heat trans fer standpoint. It is difiicult to obtain quickly a lowering of the temperature of the center portion of the mass by external cooling. This difiiculty is eliminated, however, when cold water is introduced directly into the concentrated mass. This water serves the dual function of reducing the temperature of the resulting suspension to the desired low point and of bringing the lime slurry to the desired concentration.
While the process of our invention has been described hereinabove with particular reference to use of the calcium carbonate, produced by this process, in cigarette paper, in which it has a special value, it is to be understood that the process may be utilized for producing carbonates for other end uses within the scope of our invention. In fact, in any case where it is'desired to reproduce consistently a predetermined type of precipitated calcium carbonate, this process may be employed. Once the desired particle size and other characteristics of the calcium carbonate are determined, this product can be reproduced much more consistently by means of the process of our present invention than by any other known prQcess and as described above our processis particularly characterized by the use of the'initial cooling of the lime slurry to a temperature within the a proximate range of '1 5 C. to 25 C., and
1. The process of controlling the characteristics of precipitated calcium carbonate so as to render it especially suitable for use as a filler in cigarette paper comprising first, slaking quicklime, next cooling the slalred lime slurry to a temperature of approximately C. to C., then adjusting the concentration of the cooled slurry to approximately 220 to 280 grams per liter, expressed as calcium carbonate, introducing gas, containing approximately 8% to 12% carbon dioxide, into this slurry to effect an exothermic carbonation reaction between the slurr and the carbon dioxide, continuing this exothermic reaction on a gradually increasing temperature scale to an equilibrium value of about 50 C. to 60 C. until substantially all of the lime has been converted into calcium carbonate of a non-colloidal form.
2. The process of controlling the characteristics of precipitated calcium carbonate so as to render it especially suitable for use as a filler in cigarette paper comprising first, slaking quicklime, next cooling the slaked lime slurry to a temperature of approximately 20 C., then adjusting the concentration of cooled slurry to approximately 220 to 280 grams per liter, expressed as calcium carbonate, introducing gas, containing approximately 8% to 12% carbon dioxide, into this slurry to effect an exothermic carbonation reaction between the slurry and the carbon dioxide, continuing this exothermic reaction on a gradually increasing temperature scale to an equilibrium value of about 50 C. to 60 C, until substantially all of the lime has been converted into calcium carbonate of a non-colloidal form.
3. The process of controlling the characteristics of precipitated calcium carbonate so as to render it espec ally suitable for use as a filler in cigarette paper to provide the desired porosity and opacity characteristics, comprising first preparing a lime slurry having at least 220 grams 5 the prec pitation of calcium carbonate, permitting the temperature to increase gradually from the start of the carbonation reaction at a substantiallg uniform rate until the carbonation reaction mass has attained an equilibrium temperature of about C. to 0., at which stage in the process a substantial portion of the l me in the lime slurry has been converted to calcium carbonate, and then completing the carbonation of the remaining lime at this equilibrium tempera ture to produce non-colloidal calcium carbonate suitable for use as a filler in cigarette paper.
4. The process of controlling the characteristics of precipitated calcium carbonate so as to render it especially suitable for use as a filler in cigarette paper to provide the cleisred poros ty and opacity characteristics, comprising first preparing a lime slurry having at least 220 grams per liter of lime, calculated as 06603, cooling the lime slurry to an nitial temperature of approarimately 20 C., introducing into this lime slurry a gas containing carbon dioxide to effect the procipitation of calcium carbonate, permitt ng the temperature to increase gradually from the start of the carbonation reaction at a substantially uniform rate until the carbonation reaction mass has attained an equilibrium temperature of about 50 C. to 60 0., at which stage in the process a substantial portion of the lime in the lime slurry has been converted to calcium carbonate, and then completing the carbonation of the remaining lime at this equilibrium temperature to produce non-colloidal calcium carbonate suitable for use as a filler in cigarette paper.
MILTON O. SCHUR. ROBERT M. LEVY.
References Cited in the file of this patent or the original patent UNITED STATES PATENTS Number Name Date 1,870,259 McKee Aug. 9, 1932 1,909,924 Schweitzer 1 May 16, 933 2,058,593 Rafton et a1 Oct. 27, 1936 2,081,112 Statham et a1 May 18, 1937 2,140,375 Allen et al Dec. 13, 1938 2,386,389 Elkington et a1 Oct, 9, 1945 2,442,525 Wrege et al. June 1, 19 8 FOREIGN PATENTS Number Country Date 563,111 Great Britain July 31, 1944 OTHER REFERENCES Paper Trade Journal, April 11, 1929, page 54.
Mellor: Inorganic and Theoretical Chemistry," vol. 3, pp. 817, 818. Longmans, Green and Co., New York, 1923 ed.