May 16, 1967 w. F. WALDECK 3,320,0 6 METHOD OF PREPARING CALCITE AND THE PRODUCT THEREOF Filed Feb. 20, 1964 5 Sheets-Sheet 1 PRIOR ART (.7, I____: ONE MICRON May 15, 1967 w. F. wALDEcK “ 3,320,026 METHOD OF PREPARING CALCITE vAND THE PRODUCT THEREOF Filed Feb. 20, 1964 5 Sheets-Sheet 2 5%, w M? l__:__.| ONE MICRON May 16, 1967 w. F. WALDECK 3,320,026 METHOD OF PREPARING CALCITE AND THE PRODUCT THEREOF Filed Feb. 20, 1964 5 Sheets-Sheet 5 O.l MICRON Fig.6 3,320,026 United States Patent nice Patented May 16, 1967 1 2 complete reaction to product substantially free of other 3,320,026 crystalline forms. METHDD 0F PREPARING CALCITE AND THE PRODUCT THEREOF Carbonatation may be conducted in standard gas-liquid William F. Waldeclr, Milford, N.J., assignor to Chas. contacting equipment. For example, an agitated tank P?zer & Co., Inc., New York, N.Y., a corporation of may be employed, carbon dioxide-containing gas being Delaware suitably injected beneath the agitator for thorough dis Filed Feb. 20, 1964, Ser. No. 346,349 persion throughout the reaction mixture. A typical plant 16 Claims. (Cl. 23-66) scale precipitation may require one or two hours, under which circumstances it will usually be appropriate to This invention is concerned with calcium carbonate, 10 maintain the temperature below 20° C. for about 10 or and more particularly with a calcite of novel crystal 15 minutes, by recirculating some of the slurry through habit, and processes for its preparation and use. a cooler. Cooling will usually be curtailed as crystalli Calcium carbonate can be precipitated in any of three zation proceeds, so that the temperature will begin to rise. isomorphons crystal groups, the calcite, aragonite and When carbonate formation is about half complete the vaterite groups. When obtained in calcite form, calcium cooling can usually be halted entirely. carbonate ordinarily assumes either a rhombohedral or a The carbon dioxide injected into the reaction mixture scalenohedral habit. Now, however, an especially valu may be pure, i.e. free of other diluent gases, but this is able calcite of novel habit has been obtained, differing by no means necessary. Flue gases containing about 12 from all known naturally occurring minerals or previ volume percent or more of carbon dioxide in admixture ously reported synthetic products. X-ray analysis estab 20 with non-reactive gaseous diluent are suitable, although lishes the fact that the new calcium carbonate is of the for more rapid absorption it will be preferable to employ calcite group, but the distinctive nature of its crystalline a somewhat richer source, such as the gases from lime habit will be clear from the accompanying illustrations, burning, which contain about 20-40% carbon dioxide. which are photographic reproductions of highly magni At the conclusion of the process, the calcite crystals ?ed electro micrographs. may be recovered from the aqueous suspension in dis FIG. 1 depicts prior art aragonite. crete form. Any coarse grit or un-reacted lime present FIG. 2 shows prior art vaterite. are best eliminated by an initial screening of the reaction In FIG. 3 is shown the prior art scalenohedral form slurry through a 325 mesh U.S. sieve screen, after which of calcite. the product is collected from the screened liquor by ?l FIG. 4 shows the prior art rhombohedral form of tration or other conventional procedure. The product calcite. may upon drying, be subjected to a single pass through FIG. 5 is an electron micrograph of the new stubby a hammer mill, to disintegrate any lumps and insure a prismatic calcite crystals of the present invention. uniform, free-?owing powder composed of discrete cal FIG. 6 represents a still further magni?ed view of these cite crystals. new crystals. ' It has been stated that at least 50 weight percent of The novel calcite is produced by the introduction of the calcium hydroxide particles employed in the process carbon dioxide into an aqueous calcium hydroxide sus of this invention must have a diameter above about 10 pension (“carbonatation”). Calcium carbonate has been microns. Electron micrographs suggest that calcium hy produced by such processes in the past, but the products droxide consists of aggregate clusters of fundamental obtained were of the conventional types mentioned. The 40 crystals about 0.01 micron in size, and the relatively critical process conditions required to produce the stubby large particles employed in the new process are believed prismatic calcite crystals are newly discovered and form to be such aggregate clusters. a part of the present invention. Since the lime particle diameter is a somewhat incon In the new process, carbon dioxide is introduced into venient value to measure in practice, a process criterion an aqueous suspension of coarse calcium hydroxide (at based on settling rate has been developed, In accordance least about 50 weight percent of the hydroxide consisting with this standard, the particle size should be such that of particles coarser than 10 microns) while maintaining 1000 ml. of homogeneous water suspension at 65° C. the temperature below about 20° C. at least until calcite containing 150 grams of said particles exhibits a settled crystallization is in progress. Carbon dioxide introduc particle volume below about 960 ml. in a standard 1000 tion is then continued, and the temperature may be per~ - ml. graduated cylinder after 10 minutes’ standing. That mitted to rise, while calcite crystalliaztion continues to is to say, the interface between the clear supernatant wa substantial completion. ter and the cloudy suspension should lie below this limit. The initial temperature during calcite crystallization is Such a slurry is indeed a coarse suspension, in contrast critical. If carbon dioxide is introduced to the lime sus to so-called “milk of lime,” which typically exhibits a pension at a temperature substantially above 20° C. there settled volume of 990 ml. or more in the described test. is a strong tendency to precipitate scalenohedral calcite, The ?nely divided milks have been employed almost uni~ and at still higher temperatures, aragonite is obtained. versally in the past in general industrial practice, being Any temperature below 20° will be satisfactory, and ex favored because of their resemblance to true solutions in cellent results are obtained at 15-16". Once prismatic handling properties. calcite crystallization is in progress, however, the tem 60 Ordinarily, lime milks are prepared by slaking reac perature may be permitted to rise while crystallization tive lime with hot water, or with limited amounts of of the novel product continues. Apparently the initial water (with subsequent dilution if necessary), so that the crystallization insures massive inoculation of the slurry exothermic reaction produces a rather high temperature, with the desired crystal form, whereby this type continues ften 90-100“ C. The resulting products are entirely to appear even at the higher temperatures where scaleno 65 unsatisfactory in the process of the present invention. hedrons might otherwise be expected. Permitting the In order to produce a calcium hydroxide suspension of temperature to rise during crystallization is, in fact, often the requisite coarseness for the new process, the tempera desirable, to insure complete conversion of even the slow ture during slaking must never be permitted to exceed reacting, coarser calcium hydroxide particles. A maxi 70 65° C. It is therefore, of course, prudent to provide mum crystallization temperature in the range between ef?cient agitation during slaking, to avoid the possibility about 30 and 45° C. is particularly preferred, to insure of local overheating. ’ 3,820,026 A; The maximum temperature during slaking will natural the best commercial chalks, i.e., the rhombohedral cal ly vary with the initial water temperature, the quicklime: cites. This dramatic advantage is accompanied by a slake water ratio ‘and the reactivity of the quicklime. very considerable increase in printability, achieved at low (It can also be controlled, of course, by external cooling er calender pressure or with fewer than customary passes if desired.) Experience has shown that a slaking tem through the calender rolls. “Calgon T Demand,” a meas perature below 65° C. can be assured, and particularly ure of the quantity of this polyphosphate dispersant which good results obtained, by providing water at an initial is required to achieve minimum viscosity of a high solids temperature of no higher than 35° C. for the slaking chalk slurry, is also desirably low. Even in combination reaction; in addition, the quicklimezslake water ratio with conventional forms of calcite, the stubby prismatic should preferably not exceed that which will produce 10 product affords signi?cant advantages, provided at least about 20 grams calcium hydroxide per 100 ml. suspen about 50% by weight, and preferably 75—80% or more sion. Particularly desirable is a quicklime proportion to of the new product is present. produce a suspension containing from about 10 to 15 To reduce cost, paper-coating pigments are usually grams calcium hydroxide per 100 ml. suspension. clay-‘based, with chalk added primarily for whiteness. Very low quicklime proportions, i.e., those which pro Most commercial chalks are, in fact, incapable of provid duce a suspension containing less than about 10 grams ing the desired gloss and printability when employed calcium hydroxide per 100 ml., tend to be uneconomic. alone. While the new calcite overcomes these technical This disadvantage may be overcome by subsequently in disadvantages, the economic factor will often dictate .its troducing additional lime, at least 50 weight percent of use in combination with clay for many paper-coating ap which is coarser than 10 microns, so as to raise the total plications. Inorganic clay-based pigments containing at calcium content to a level equivalent to about 10-20 least 20% by weight of the stubby-prismatic calcite pro grams of calcium hydroxide per 100 ml.