Chemistry of inorganic compounds – Carbon or compound thereof – Oxygen containing
Reexamination Certificate
1999-07-21
2001-06-26
Hendrickson, Stuart (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Oxygen containing
C423S430000
Reexamination Certificate
active
06251356
ABSTRACT:
A portion of the disclosure of this patent document contains material that is subject to copyright protection. The owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
FIELD OF THE INVENTION
This invention is related to a process for the production of calcium carbonate via precipitation from solubilized calcium ions and carbonate ions, and to the products of the process, and to paper products produced using the products of the process.
BACKGROUND
The manufacture and supply of high quality calcium carbonate for paper filler and for paper coatings is now widely practiced around the world. Relatively recently, particularly as alkaline papermaking has become popular, the on-site manufacture of precipitated calcium carbonate (“PCC”) from aqueous solutions in atmospheric tanks has also been developed and implemented at a variety of locations. These on-site plants have developed because transportation costs of either a dry powder or of a liquid slurry of calcium carbonate was generally prohibitive. However, the variability of product quality from the heretofore-employed on-site PCC plants has been problematic at times. Such problems are especially acute in those locales where relatively impure sources of carbon dioxide have been employed, such as from boilers burning a variety of solid or liquid waste fuels. Also, the particle size distribution of the PCC obtained from various prior art processes has been less than optimum, and consequently, it would be advantageous to provide a process in which the particle size distribution could be more effectively controlled.
In processes employed for the manufacture of precipitated calcium carbonate, several fundamental chemical reaction steps are normally employed, which steps can be generally summarized as follows:
(1) Calcination—heating limestone (calcium carbonate) and driving the carbon dioxide out, resulting in the formation of lime (calcium oxide).
(2) Slaking—reacting lime with water to form a lime slurry (calcium hydroxide; Ca(OH)
2
); this reaction is accompanied by the evolution of heat.
(3) Carbonation—reacting the lime slurry with carbon dioxide so that the solubilized calcium from the calcium hyroxide is reacted with the carbonate produced by bubbling the carbon dioxide in water, to form the desired calcium carbonate; this reaction is also exothermic.
Various prior art techniques disclose methods of preparing different PCC crystal morphologies, shapes, sizes, and size distribution of for the precipitated calcium carbonate. Although the prior art known to me teaches the use of process variables such as carbon dioxide concentration, calcium hydroxide concentration, temperature, and the use of chemical additives, none of such prior art processes known to me utilizes the step of carbonation under pressure, either alone or in combination with other heretofore utilized variables, as a technique for increasing the reaction rate, carbonation efficiency, or for making finer PCC particles. The prior art has also not employed pressurization of the carbonation reaction as a method for increasing the rate of formation of carbonate and calcium ions, the formation of which (and especially the latter) are the primary limitation in increasing the rate of carbonation reaction.
Moreover, the various prior art methods utilized for production of precipitated calcium carbonate in papermaking operations can be characterized in that the carbonation reaction has been carried out in an atmospheric pressure vented or open vessel. This means that the partial pressure of carbon dioxide available in the carbonation reactor has been limited based on the concentration of carbon dioxide available in an incoming gas stream.
It is in the carbonation reaction that the soluble calcium from the calcium hydroxide is converted to calcium carbonate. Then, more solubilization of the calcium ion takes place as the calcium hydroxide (lime slurry) is dissolved, and this proceeds until all of the available calcium hydroxide is converted into calcium carbonate. In this reaction, the reaction rate of calcium ions combining with carbonate ions is almost instantaneous. Consequently, the slow kinetic step which controls the overall reaction rate is believed to be the rate of dissolution of calcium hydroxide in the lime slurry, so that calcium ions are available for reaction. In conventional industrial processes for the manufacture of calcium carbonate, a slurry of approximately 200 gm/L of calcium hydroxide placed in an atmospheric reactor, and a gas containing from about 15% to about 20% by volume of carbon dioxide is bubbled through the slurry. In general, such prior art processes have a reaction rate such that calcium carbonate is formed at the rate of from about 0.5 grams per liter of slurry per minute to about 1.5 grams per liter of slurry per minute. Thus, for a batch charge of 200 grams per liter of calcium hydroxide, about 200 minutes is required to complete the reaction, per liter of slurry.
In general, the currently utilized manufacturing processes are slow, with low carbonation efficiencies. Thus, manufacturing plants utilizing such prior art processes require large equipment, resulting in high capital costs per unit of calcium carbonate production.
Relatively recently, approximately eighty percent (80%) of the world paper production has been converted to an alkaline papermaking process. In that process, precipitated calcium carbonate (“PCC”) is employed as the primary filler. An average papermill may require from about 20,000 to about 100,000 tons per year of PCC. To meet such demands, the production of PCC has shifted from off-site to on-site. One important advantage of on-site PCC production has been the saving of transportation costs. Also, a primary raw material for PCC production, namely carbon dioxide, is available free at many mills, as a waste product from lime kiln flue gas. Such gas normally contains from about twelve percent to about twenty five percent (12%-25%) of carbon dioxide. However, one limitation encountered was that variability and fluctuation in the carbon dioxide concentration in the flue gas produced variability in the resulting PCC. Moreover, some mills do not have lime kilns, and free on-site sources of carbon dioxide are limited to flue gas from gas fired boilers, which only have seven to ten percent (7-10%) carbon dioxide concentration. In such situations, it has not heretofore been economical to place an “on-site” PCC plant at the mill location.
Thus, in order to manufacture large quantities calcium carbonate as required in papermaking operations, it has heretofore been necessary to provide very large reactors (for example, reactors in the 18,000 gallons to 20,000 gallons range are common). Thus it is evident that it would be desirable to provide a process in which the overall production rate of calcium carbonate is increased, thereby reducing the reactor size for a desired PCC production rate. It would also be advantageous to develop a process which (a) can utilize low CO2 containing gas, and (b) in which the effects of fluctuation in CO2 concentration on particle size distribution of PCC can be minimized.
Several prior art processes are known which superficially resemble portions of my process to some limited extent. In U.S. Pat. No. 3,304,154 issued on Feb. 14, 1967 to Dimitrios Kiouzes-Pezas for a Process for Producing Spheroidal Alkaline Earth Metal Carbonates, carbon dioxide gas is bubbled through a cylindrical autoclave reactor having a calcium hydroxide suspension therein. Pressure in the reactor was accumulated until a pressure from about 4 to 6 atmospheres gauge, and preferably about 5 atmospheres gage, was built up. Then, the reactor was rotated, while keeping the temperature between 60° to 90° Centigrade. However, that process has some practical limitations and thus is not well suited to the on-site production of PCC. First, it is difficult to
G. R. International, Inc.
Goodloe, Jr. R. Reams
Hendrickson Stuart
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