Chemistry of inorganic compounds – Carbon or compound thereof – Oxygen containing
Reexamination Certificate
1999-06-01
2003-09-30
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Oxygen containing
C423S430000
Reexamination Certificate
active
06627170
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to processes for preparing calcium carbonate giving useful performances as a paper filler in the causticization step of the sulfate or soda pulp process, and more specifically processes for preparing calcium carbonate giving useful performances as a paper filler by defining the quick lime used and slaking and causticization conditions or other factors.
PRIOR ART
Normally, a filler is internally added to printing or writing papers for the purpose of improving brightness, opacity, smoothness, writing suitability, touch, printability or other properties. Such papers are made by the so-called acid process at about pH 4.5 or the so-called neutral process at a neutral to weakly alkaline pH of 7 to 8.5 using talc, clay, titanium oxide or the like as a filler. In the neutral process, domestic calcium carbonate can be used in place of expensive imported talc or clay. In recent years, neutral papers obtained by the neutral process have attracted interest because of the papers' storability, and the number has been growing and will become increasingly widespread because of many advantages including paper quality, costs, environmental influences.
A feature of recent demands for paper is a significant growth in the field of leaflets, catalogs, pamphlets, direct mails or the like in commercial printing and in the field of books related to computer, multimedia and family computer to meet the development of the information age, magazines, collections of photographs, mooks, comics or the like in publishing. Thus, paper users increasingly desire to lower the cost and seek downgraded and lighter paper.
As the demands for inexpensive and light neutral paper increase as described above, the role of calcium carbonate as a filler will become very important. Calcium carbonate used as a filler in neutral papers includes heavy calcium carbonate obtained by mechanically dry- or wet-grinding a natural limestone and precipitated calcium carbonate chemically synthesized (synthetic calcium carbonate).
However, in heavy calcium carbonate obtained by grinding a natural limestone by a mill such as ball mill it is difficult to control the shape and it severely abrades plastic wires during paper making processes when it is used as an internal filler. Moreover, normal fine-quality papers or coated papers prepared with such a filler are insufficient in bulk, brightness, opacity, smoothness, writing suitability, touch, printability and other properties.
Recent advances in weight saving make the above problem graver. High-specific surface fillers (e.g. pulverized silica, white carbon), highly refractive fillers (e.g. titanium dioxide) or precipitated calcium carbonate (synthetic calcium carbonate) have been so far used as common means for improving the opacity of light printing papers.
The precipitated calcium carbonate is known to be prepared by (1) a reaction between carbon dioxide gas and a milk of lime obtained from a calciner of lime or the like, (2) a reaction between ammonium carbonate and calcium chloride in the ammonia soda process, (3) a reaction between a milk of lime and sodium carbonate in the causticization of sodium carbonate, etc. Among these processes, (2) and (3) have been less examined with respect to controlling the shape of the resulting calcium carbonate because both reactions produce it as a by-product and are now being replaced by novel formats for obtaining the main products. On the other hand, (1) has been widely studied with respect to techniques for preparing calcium carbonate in various shapes and have actually created some examples prepared on-site in paper factories, partly because the reaction system is relatively simple (water, slaked lime, carbon dioxide gas). However, the production costs of this process are very high because calcium carbonate is the sole product and so it can not satisfy users' demands for cost-saving and can not be used, or at most in a greatly limited amount, for inexpensive types of papers.
A possible alternative is to use calcium carbonate by-produced during the causticization step for recovering and regenerating a digesting agent in a kraft pulp process, as a paper making material.
In the sulfate or soda pulp process, wood is digested with a mixed chemical solution of sodium hydroxide and sodium sulfide at high temperature under high pressure to isolate cellulose. Cellulose is separated as a solid phase and purified into pulp, while the chemical solution and other elements than cellulose eluted from the wood are recovered as a pulp waste liquor (black liquor) and concentrated and burned. During that time, the eluted elements from the wood are recovered as a heat source while inorganic matters based on sodium carbonate and sodium sulfide in the chemical solution are recovered and dissolved in water or a dilute chemical solution called weak liquor in which are dissolved a part of white liquor components generated when calcium carbonate sludge formed by the reaction shown below is washed to give a green liquor. This green liquor is mixed with a quick lime to produce calcium carbonate by the reactions (1) and (2):
CaO+H
2
O→Ca(OH)
2
(1)
Ca(OH)
2
+Na
2
CO
3
→CaCO
3
+2NaOH (2)
This calcium carbonate can be prepared at very low cost because it is a by-product of the preparation of the main product white liquor. Moreover, it can be expected to improve the reactivity of the above reactions (1) and (2) and the clarity of the white liquor and to reduce waste, because the inside of the system can be clarified and the circulating lime can be highly purified by extracting calcium carbonate from the calcium circulating cycle (calcium carbonate, quick lime, slaked lime) in the causticization step which is conventionally a closed system.
However, it was difficult to control the shape of the thus obtained conventional calcium carbonate and it was always amorphous or massive including various shapes such as cube or hexahedron with large particle diameters similar to conventional heavy calcium carbonate and normal fine-quality paper or coated papers prepared with such a filler were insufficient in bulk, brightness, opacity, smoothness, writing suitability, sense of touch, printability or other properties. With recent large-scale paper-making machines producing paper at high speed, serious problems in plastic wire abrasion resistance and wet end yield also occurred.
Thus, it was difficult to efficiently and inexpensively prepare calcium carbonate, which is useful as a filler or pigment to give a good plastic wire abrasion resistance during paper making processes; it was difficult also to produce highly opaque fine-quality paper or coated paper while maintaining printing quality.
Considering the above situation, an object of the present invention is to provide inexpensive and light calcium carbonate with a controlled shape self-generated in the causticization step, which gives a good wire abrasion resistance during paper making processes and can be used to produce fine-quality paper or coated paper having high opacity and excellent printability or other properties.
SUMMARY OF THE INVENTION
As a result of careful studies to overcome the above problems, we found that they can be solved by slaking a quick lime containing calcium carbonate at a specific level or less with a white liquor to prepare a milk of lime and taking advantage of the causticization step of the sulfate or soda pulp process to continuously add a green liquor generated in the causticization step of the sulfate or soda pulp process in an amount comparable to that required for preparing a white liquor in conventional procedures to said milk of lime at a controlled loading rate and reaction temperature, and accomplished the present invention on the basis of this finding. According to the process of the present invention, the shape of calcium carbonate can be controlled without significant change in the conventional causticization step to prepare calcium carbonate in the form of spindle- or rice-like particl
Kanai Kiyoshi
Nanri Yasunori
Okamoto Yasuhiro
Takahashi Kazuto
Hendrickson Stuart L.
Nippon Paper Industries Co. Ltd.
Roylance, Abrams, Berdo & Goodman LLP.
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