Paper making and fiber liberation – Processes and products – Plural fiber containing
Patent
1991-08-12
1993-11-16
Chin, Peter
Paper making and fiber liberation
Processes and products
Plural fiber containing
1621812, 1621813, 162183, 162189, 162191, D21H 1114, D21H 1770
Patent
active
052620069
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to reusing/recycling gypsum-containing cellulose fiber material in the manufacture of paper from a pulp stock of pH >6.5. The invention provides a technical solution so as to eliminate problems involved with the production of coated papers using gypsum pigments. It is applicable to the manufacture of coated and non-coated grades of paper, both wood-free and wood-containing, having a basis weight of 15 g/m.sup.2 or more and also comprising paperboard products. The invention provides a possibility of manufacturing gypsum-coated papers which have excellent optical properties (brightness, whiteness, opacity, and light scattering coefficient).
The cellulose fiber materials used in this process are primarily recycled, broke, and/or waste paper. The content of gypsum in the cellulose material, calculated as CaSO.sub.4 without water of crystallization, usually exceeds 0.5% (w/w) and may be for instance more than 1% or 2% (w/w). As a rule the gypsum content is less than 60% (w/w) although in some cases it may amount to up to 70% (w/w).
STATE OF ART
Within the field of papermaking systems, the reuse of cellulose fibers has been a time-honored classical expedient for minimizing the cost of raw materials. The procedure followed is to disintegrate either paper that has been used previously, in other words, paper, waste or paper that has been produced recently and been rejected as defective, that is broke paper, whereupon the resultant suspension is integrated into the stock employed for making the paper. The expression that "the suspension is integrated into the stock" means that its dry matter material wholly or partly forms the stock so as to totally or partially constitute the dry matter material of the stock "solids". Disintegration is normally performed in an aqueous medium Various procedures for processing broke and waste paper and problems inherent therein, have been described earlier in, for example, U.S. Pat. No. 3,865,684 and GB-A-9503. With respect to the reuse of gypsum-containing cellulose fiber materials, no good methods are available.
For a long time it has been known that gypsum may be used as a coating pigment in paper manufacturing techniques. See for example Eklund, D, Paperi ja Puu (1976) No. 9 pp. 559-70. Gypsum is a comparatively inexpensive material because it is obtained as a by-product in phosphate production processes and in systems for purifying SO.sub.2 -containing gases with lime.
For gypsum grades refined for paper manufacture see, for instance, EP-A-125,225, 125,224 and 112,317. It is believed that to obtain a high-quality coating on paper, a gypsum pigment may typically have a particle size of <10 microns preferably <3 microns. The best pigments in the market are recrystallized (reprecipitated) materials, and have an F content and a P.sub.2 O.sub.5 content of <0.3%. Calcium carbonate may be present in small amounts as an impurity. For further information see, inter alia, EP-A-112,317.
Calcium carbonate (CaCO.sub.3) is frequently used as a filler. In nature, it occurs in the form of, for instance chalk and calcite and, after being subjected to grinding, it has been used in paper making processes. However, the form of calcium carbonate with which the best results have been obtained has been a synthetically produced, precipitated calcium carbonate (PCC); this is obtained with a very homogeneous particle size distribution and in the form of uniform crystals. The usual way of producing PCC is either to react milk of lime with carbon dioxide or to react an aqueous solution of calcium chloride with sodium carbonate. In both of these processes, controlled and well-defined conditions are required in order to obtain a PCC of suitable physical properties. But PCC may be an expensive material as compared to other fillers; consequently such other fillers have often been chosen instead. For a survey see Gill, R. and Scot, W., Tappi Journal, Jan. 1987, pp. 93-99.
Problems involved with the reuse of gypsum-containing cellulose fiber
material
Pape
REFERENCES:
patent: 4470877 (1984-09-01), Johnstone et al.
patent: 4853085 (1989-08-01), Johnstone et al.
Eklund, D., "The Use of Gypsum in Pigment Coating", Paperi ja Puu, No. 9, pp. 558-570 (1976).
Andersson Kjell R.
Kuni Stefan O.
stensson Per O. L.
Chin Peter
Mo Och Domsjo Aktibolag
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