Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With chemical or physical modification of liberated fiber
Reexamination Certificate
2000-08-09
2002-05-14
Nguyen, Dean T. (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With chemical or physical modification of liberated fiber
C162S004000, C162S181200, C162S181400
Reexamination Certificate
active
06387212
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for synthesizing calcium carbonate in contact with fibers and to the novel product obtained. It relates more particularly to a process for obtaining fibers integral with calcium carbonate particles, in which the fibers to be treated are contacted with carbon dioxide generator means and at least one composition comprising Ca
++
ions capable of reacting with the carbon dioxide so as to give “in fine” a precipitation of calcium carbonate “in situ” on the fibers.
The invention likewise relates to a process for removing calcium carbonate from the other insoluble compounds present in various aqueous media, originating in particular from papers for recycling and from deinking sludges.
2. Description of the Related Art
In the various fields concerned with fiber-based products, including the field of paper pulp, it is well known that it is appropriate to provide said products with fillers, generally mineral fillers, in order to impart certain physical properties to said products. Moreover, in view of the high cost of producing and converting said fibers, the fillers act as a less expensive substitute, making it possible to reduce the manufacturing cost of the products containing them.
In particular, in the papermaking sector, in addition to the economies they bring about, the fillers impart numerous qualities to the paper, said qualities including opacity and whiteness, density and porosity, and printability and handle.
The opacity is an essential quality for the paper, especially for papers intended for printing and writing, where it is desirable for the ink to show through as little as possible on the reverse of the sheet. For printing, and other applications, qualities of whiteness are also sought, which are not always exhibited by the fibers alone. In this case, the papermaker adds fillers.
The fillers are, in general, mineral powders; they are added to the fibers before the paper sheet is formed; in this case the reference is to fillers added to the pulp during the manufacture of the fiber suspension which feeds the paper machine (when they are added after the sheet has dried, the reference is to pigments added by coating on the dried paper sheet emerging from the paper machine; the operation is called “coating”).
Generally, the fillers are mixed with the fibers during the manufacture of the fiber suspension. Synthetic or natural, they are prepared “ex situ”, i.e., they are precipitated or ground, and are sieved before being employed in the paper mill.
The principal natural fillers are kaolin (aluminum silicate), talc (magnesium silicate) and calcium carbonate; the principal synthetic fillers are titanium dioxide, aluminum hydroxide, the mixture of aluminum sulfate plus lime which is called “satin white”, and precipitated calcium carbonate.
Recent developments in sizing the paper in an alkaline medium have promoted the use, among these various fillers, of calcium carbonates, both natural and precipitated, the precipitates taking on an increasingly important role by virtue not only of their greater whiteness but also of their morphological characteristics.
The fillers are introduced in variable amount depending on the paper type, on average at between 5 and 35% by weight. There is an economic interest in increasing the filler content when the paper is sold on the basis of its weight, or per sheet: the high cost of the fibers is partially substituted by the low cost of the fillers. However, too high a filler content weakens the mechanical performance of the paper, which leads the manufacturers to use binders and retention aids; other chemical additives are also used, including sizing agents to reduce the sensitivity of the sheet to water and drainage aids to facilitate flow during the forming of the sheet.
Optimizing the proportion of fillers will rest essentially on the form and distribution of the mineral crystals in the fibers. Their purity and their crystallographic characteristics will influence the qualities of the paper.
Paper is therefore a composite material whose manufacture requires a sequence of steps employing a number of technologies for the mixing of raw materials having very different chemical and physical properties, the formation of a wet sheet by removal of water, the drying of the wet sheet, the possible treatment of the surface of the sheet, and the recycling of the various liquors resulting from the process, which are termed “white waters”.
Although the principal raw materials are the plant fibers and the fillers, the chemical additives, the majority of which are expensive products, are necessary to the good progress of each step; one objective for the papermaker is therefore to reduce the quantities of said additives.
Incorporating the fillers into the fibers is an essential step of the papermaking process. In the face of the numerous difficulties which are encountered in the realization of this step, a variety of processes and products have been proposed with a view to ameliorating the impact of the fillers both on the papermaking process and on the qualities of the finished product. One of the recommended routes consists in preparing the fillers “in situ”, i.e., in the presence of fibers, so as to retain and distribute said fillers more effectively within the fiber mat.
U.S. Pat. No. 2,583,548 thus discloses a process which consists in impregnating cellulose fibers with a solution containing calcium chloride, then in reacting this salt with sodium carbonate in accordance with the double decomposition reaction of two salts:
CaCl2+Na
2
CO
3
→CaCO
3
+2 NaCl.
The impregnation of the calcium chloride in the fibers then makes it possible to precipitate the calcium carbonate in the fibers or around the fibers. The sodium chloride, which is a by-product of the reaction, must be removed by washing, which complicates the industrial implementation.
In addition, U.S. Pat. No. 5,096,539 discloses a process which describes, in accordance with this same principle, an “in situ” precipitation of calcium carbonate from calcium chloride, this process introducing a step of washing the fibers before adding the sodium carbonate, in order to remove the calcium chloride situated on the outside of the fibers and to precipitate more specifically the calcium carbonate in the hollow part of the fibers, the lumen. This process, although it enhances the retention and the maintenance of the mechanical properties, by promoting contact between the fibers and therefore the fiber/fiber bonds (since the filler is inside the latter), employs successive washing operations, which limit considerably the industrial scope of the invention.
Moreover, Japanese Application J60-297382 discloses a process for carbonating calcium hydroxide in accordance with the following reaction:
Ca(OH)
2
+CO
2
→CaCO
3
+H
2
O
In this process, calcium hydroxide is placed in the presence of fibers. The calcium hydroxide is added in solid form. The fibers are present in the form of a suspension; they are required to exhibit fibrils at their surface in order to permit subsequent retention of the calcium carbonate. The whole batch is mixed with stirring for a period of the order of around ten minutes. The carbon dioxide is then blown in with stirring in order to carbonate the lime. This stirring phase is indispensable; it ensures the uniformity of the reaction and the production of uniform particles of calcium carbonate. The reaction time depends on the proportion of lime added and on the concentration of carbon dioxide; it is generally of the order of 30 minutes. This process, although it has the advantage of not requiring a washing step, remains complex to implement continuously. In particular, during the first step, it is necessary to prepare the milk of lime in contact with the fibers, and especially fibrils, in a relatively concentrated medium, and generally a greater amount by weight of quicklime than of fibers is added. In the second step, which is very intricate to conduct,
Burns Doane Swecker & Mathis L.L.P.
L'Air Liquide Societe Anonyme a Directoire et Conseil de Su
Nguyen Dean T.
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