Process for modifying hydrophilic fibers with substantially water-insoluble inorganic substance

Abstract: A hydrophilic fibers modified with a substantially water-insoluble inorganic compound is produced by immersing hydrophilic fibers, for example, pulp fibers, in an aqueous solution of a water-soluble inorganic compound (a) which is precipiTated when brought into contact with a precipitant (b), adjusting the amount of the aqueous solution impregnated in the hydrophilic fibers to 60 to 40% based on the weight of the hydrophilic fibers, and bringing the hydrophilic fibers with the aqueous solution of the inorganic compound (a) into contact with the precipitant in the state of an aqueous solution to thereby cause the resultant substantially water-insoluble compound to be precipitated in and fixed to the hydrophilic fibers.

Description: BACKGROUND OF THE INVENTION 1) Field of the Invention The present invention relates to a process for producing hydrophilic fibers with a water-insoluble inorganic substance. More particularly, the present invention relates to a process for producing hydrophilic fibers modified with a substantially water-insoluble inorganic substance, for example, a substantially water-insoluble metal hydroxide, precipitated in and fixed to bores, pores and surfaces of the hydrophilic fibers, to provide modified hydrophilic fibers useful as a paper-forming material, a shaped article-forming material, and other functional materials in which the specific functions of the substantially water-insoluble inorganic substance carried by the hydrophilic fibers are utilized. 2) Description of the Related Art Various processes for producing hydrophilic fibers, for example, paper-forming pulp fibers, modified with a substantially water-insoluble hydroxide, for example, aluminum hydroxide, are known from Japanese Examined Patent Publication No. 56-18,712, which discloses a process wherein fine particles of aluminum hydroxide are adhered to surfaces of a paper sheet by a coating method; from Japanese Unexamined Patent Publication No. 57-144,754, which discloses a process wherein a paper sheet containing 70% by weight or more of aluminum hydroxide is formed from a pulp slurry containing an aluminum hydroxide powder; and from Japanese Unexamined Patent Publication No. 57-171,799, which discloses a process wherein a paper sheet containing 50 to 95% by weight of aluminum hydroxide is produced from a pulp slurry containing an aluminum hydroxide powder together with a sizing agent and binder. In these conventional processes, the substantially water-insoluble inorganic substance are adhered in the form of fine particles to a surface of a paper sheet, or mixed in the form of fine particles to provide a pulp slurry, but a water-soluble inorganic compound is not utilized to provide the substantially water-insoluble inorganic substance. As conventional methods wherein a water-soluble inorganic substance, for example sodium aluminate, is utilized for paper-making, Chizhov, G.I. et al., Mezhvuz, Sb. Mauch. Tr., Khimiya Tekhnol. Tsellyulozy, No. 8, 67-70 (1981) discloses a use of sodium aluminate mixed to a pulp, to enhance a mechanical strength of the resultant paper sheet; U.S.-A-3,706,629 discloses an addition of a polymeric electrolyte and an aluminate of an alkali metal to a pulp, to improve the dehydration property and retention of the pulp in the paper-forming step; and Canadian -A-964,808 teaches an addition of a water-soluble aluminum salt and sodium aluminate to a pulp. Nevertheless, these conventional methods do not teach a conversion of a water-soluble inorganic compound to a substantially water-insoluble inorganic substance on or within a hydrophilic fiber. Lagally, P. and Lagally, H., Tappi, 42 (11), 888 (1959) teach a method of precipitating a gel-like aluminum hydroxide on pulp fibers by immersing the pulp fibers in an aqueous solution of sodium aluminate and neutralizing the sodium aluminate aqueous solution with a mineral acid, but this method is disadvantageous in that, since the mineral acid is added to the pulp slurry containing sodium aluminate, a major portion of the resultant gel-like aluminum hydroxide remains in the pulp slurry but not in and on the pulp fibers, and thus the utilization efficiency of the resultant aluminum hydroxide for the pulp fibers is poor. This method is intended to increase the mechanical strength of the resultant paper sheet by the combination of the gel-like aluminum hydroxide with the cellulose pulp fibers, but the amount of the gel-like aluminum hydroxide picked up by the pulp fibers is relatively small, and thus the increase in the mechanical strength of the resultant paper sheet is unsatisfactory. A method similar to that mentioned above is disclosed by Hechler E., Wochenblatt für Papierfabrikation, 96 (23/24), 868 (1968). In this method, a beaten pulp slurry is supplemented with sodium aluminate in an amount of 5% based on the weight of the pulp, and then brought into contact with carbon dioxide, aluminum sulfate or calcium carbonate, to convert the sodium aluminate in the pulp slurry to aluminum hydroxide and thereby provide a filler-containing pulp usable for paper-formation. This method, however, is disadvantageous in that the effective utilization efficiency of the resultant aluminum hydroxide is unsatisfactory. Further, J.G., Soluble Silicates, ACS Monograph Series, Reinhold, New York, Vol. 2, 333 (1952) discloses a method in which sodium silicate (soluble glass) is added to a pulp slurry and the pH of the resultant sodium silicate-containing pulp slurry is lowered, to cause the resultant silicic acid gel to be precipitated. The purpose of this method is to size the resultant paper sheet with the silicic acid gel, and therefore, the amount of the silicic acid gel picked up by the pulp fibers must be relatively small. Cray, W.L., Pulp and Paper Magazine of Canada, August, 116 (1955) discloses a process in which a pulp slurry is supplemented with calcium chloride and then with sodium silicate to produce calcium silicate in the pulp slurry, and thereafter, aluminum sulfate is added to the pulp slurry to cause the resultant calcium sulfate to be precipitated in the pulp fibers in the slurry. This process, however, is not suitable for causing a large amount of a water-insoluble inorganic substance to be carried on the pulp fibers. Japanese Unexamined Patent Publication No. 62-144,901 discloses a process in which two different types of water-soluble inorganic compound aqueous solutions, which form a water-insoluble and flame-resistant inorganic compound when mixed together, for example, an aqueous solution of barium chloride and boric acid and an aqueous solution of hydrogen ammonium phosphate and boric acid, is proposed; a wood material is immersed in one of the above-mentioned aqueous solutions and then in the other aqueous solution, to cause the resultant water-insoluble and flame resistant inorganic compound to be dispersed and carried in the wood material. This method effectively produces a flame-resistant wood material, but is not suitable for modifying a hydrophilic fibrous material usable for paper. US-A-2 583 548 discloses a method of producing pigmented cellulosic pulp. Cellulosic fibers are immersed in a calcium chloride solution and subjected to a prolonged mechanical treatment, for example beating in the calcium chloride solution, so that the treated cellulosic fibers are swollen, hydrated and gelatinized in the calcium chloride solution; then a water soluble salt, which will react with calcium chloride to form a water insoluble calcium compound pigment, is added to the calcium chloride solution containing the cellulosic fibers, so that the water insoluble calcium compound pigment is precipitated in and on and around the gelatinized cellulosic fibers. The final product consisted of 80 % of calcium carbonate and 20 % of modified fibers on a dry basis. US-A-1,808,068 discloses a method of incorporating an inorganic alkali metal salt into paper filled with an alkaline filler. Fibrous material, for example pulp, is mixed with the alkaline filler and the alkali metal salt, for example sodium silicate, and a precipitant, for example aluminum sulfate, is added to the mixture so as to allow the resultant aluminum silicate to be deposited on or around the fibers material and then the resultant mixture is subjected to a paper forming procedure. DE-A-11 89 60 teaches the manufacture of paper filled with calcium carbonate. Said papers comprises about 80 % hydrophilic fibers and 20 % calcium carbonate. Accordingly, it is not as yet known how to impart and fix a large amount of a substantially water-insoluble inorganic substance to hydrophilic fibers, for example, paper-forming pulp fibers. SUMMARY OF THE INVENTION An object of the present invention is to provide a process for modifying hydrophilic fibers with a substantially water-insoluble inorganic substance, in a large amount and at a high efficiency. Another object of the present invention is to provide a process for modifying hydrophilic fibers with a substantially water-insoluble inorganic substance in a large amount, to thereby provide modified hydrophilic fibers having an enhanced flame resistance, dimensional stability, heat resistance, opacity, and/or hydroscopicity and useful for paper sheets, shaped articles and functional materials. The above-mentioned objects can be attained by the process of the present invention for modifying hydrophilic organic fibers with a substantially water-insoluble inorganic substance, which comprises the steps of, immersing hydrophilic fibers in an aqueous solution of a water-soluble inorganic compound (a) which is converted to a substantially water-insoluble inorganic compound when brought into contact with a precipitant (b); adjusting the amount of the water-soluble inorganic compound (a) aqueous solution impregnated in the hydrophilic fibers to a level of 60 to 400% based on the dry weight of the hydrophilic fibers; and bringing the impregnated hydrophilic fibers into contact with an aqueous solution of the precipitant (b) to cause the resultant substantially water-insoluble inorganic compound to be precipitated in and fixed to the hydrophilic fibers. DESCRIPTION OF THE PREFERRED EMBODIMENTS The hydrophilic fibers usable for the process of the present invention are not limited to a specific type of fibers, and can be selected from among known hydrophilic synthetic fibers; for example, polyvinylalcohol fibers and polyacrylamide fibers, and from hydrophilic natural fibers; for example, lignocellulosic fibers. The hydrophilic fibers are preferably lignocellulosic fibers, especially lignocellulosic pulp fibers for paper. The lignocellulosic fibers may be those derived from wood materials or herbaceous plants. The pulp fibers usable for the process of the present invention may be those produced by any type of pulping methods and treated by any type of procedures, for example, bleaching, beating, and dyeing, and by a chemical treatment. In the first step of the process of the present invention, the hydrophilic fibers are immersed in an aqueous solution of a water-soluble inorganic compound (a) capable of being converted to a substantially water-insoluble inorganic compound when brought into contact with a precipitant (b). In the immersing step, the water-soluble inorganic compound (a) is preferably present in a relatively high concentration of 6 to 60% by weight, more preferably 10 to 40% by weight, in the aqueous solution thereof, to cause a large amount of the inorganic compound (a) to be impregnated in the hydrophilic fibers. Some of the high concentration aqueous solution of the inorganic compound (a), for example, alkali aluminate, can swell the cellulosic fibers, but this effect is unsatisfactory when the concentration of the inorganic compound (a) is less than 6% by weight. The concentration of the inorganic compound (a) in the aqueous solution influences the amount of the substantially water-insoluble inorganic compound impregnated in the hydrophilic fibers. Namely, the higher the concentration of the inorganic compound (a), the larger the amount of the resultant substantially water-insoluble inorganic compound fixed to the hydrophilic fibers. The immersing step can be carried out at any temperature between the freezing point and the boiling point of the aqueous solution of the inorganic compound (a). Also, there is no restriction of the time for which the hydrophilic fibers are immersed in the aqueous solution of the inorganic compound (a). In the second step of the process of the present invention, the amount of the aqueous solution of the inorganic compound (a) impregnated in the hydrophilic fibers is adjusted to a level of 60 to 400% based on the dry weight of the hydrophilic fibers. In the adjusted hydrophilic organic fibers impregnated with the aqueous solution of the water-soluble inorganic compound (a), the dry content of the fibers is preferably 30% by weight or more. The amount of the inorganic compound (a) aqueous solution picked up by the hydrophilic fibers governs the amount of the resultant substantially water-insoluble inorganic compound fixed to the hydrophilic fibers. Namely, the larger the amount of the inorganic compound (a) aqueous solution picked up by the hydrophilic fibers, the larger the amount of the resultant substantially water-insoluble inorganic compound fixed to the hydrophilic fibers. The adjustment of the amount of the inorganic compound (a) aqueous solution picked up by the hydrophilic fibers can be effected by any one of the conventional procedures; for example, squeezing, gravitative dehydration on a net, suction dehydration, centrifugalizing, and pressing. By removing an excessive amount of the inorganic compound (a) aqueous solution from the immersed hydrophilic fibers, almost all of the aqueous solution located on the surfaces of the hydrophilic fibers can be removed, so that almost all of the picked up aqueous solution is located inside of the hydrophilic fibers (for paper-forming wood pulp fibers, on the cell walls in the fibers) and in the resultant modified hydrophilic fibers, the resultant substantially water-insoluble inorganic compound is located mainly inside of the fibers. When the amount of the inorganic compound (a) aqueous solution picked up by the hydrophilic fibers is less than 60%, the amount of the resultant substantially water-insoluble inorganic compound fixed to the hydrophilic fibers becomes too low, and the property of the resultant modified fibers becomes unsatisfactory. Also, if the amount of the inorganic compound (a) aqueous solution is more than 400%, a large amount of the substantially water-insoluble inorganic compound is formed on the surfaces of the hydrophilic fibers. The substantially water-insoluble inorganic compound on the fiber surfaces is easily removed, and therefore, the efficiency of modifying the imported substantially water-insoluble inorganic compound for the hydrophilic fibers becomes poor. In the third step of the process of the present invention, the impregnated hydrophilic fibers are brought into contact with the precipitant (b), to thereby cause the resultant substantially water-insoluble inorganic compound to be precipitated in and fixed to the hydrophilic fibers. In an embodiment of the process of the present invention, the water-soluble inorganic compound (a) is selected from water soluble alkaline earth metal salts, and the precipitant (b) comprises an aqueous solution containing at least one member selected from fluorine, phosphate, carbonate, sulfate, borate and chromate ions. In this embodiment, the alkaline earth metal salts are preferably selected from magnesium, calcium and barium. These salts can be converted to corresponding water-insoluble fluorides, phosphates, carbonates, sulfates, borates and chromates. In another embodiment of the process of the present invention, the water-soluble inorganic compound (a) is selected from aluminates, silicates and zincates of alkali metals, and the precipitant (b) comprises an aqueous solution containing a mineral acid, for example, hydrochloric acid and sulfuric acid, capable of converting the above-mentioned water-soluble compounds to substantially water-insoluble compounds, for example, hydroxides or oxides, of aluminum, silicon and zinc. In still another embodiment of the process of the present invention, the water-soluble inorganic compound (a) is selected from water-soluble salts of metallic elements other than alkali metals, and the precipitant (b) comprises an aqueous solution of ammonia. The above-mentioned water-soluble salts are preferably selected from nitrates, chlorides, and sulfates of zinc, aluminum, cobalt, zirconium, tin, titanium, iron, copper, lead, magnesium, cadmium, mercury and chromium. These specific water-soluble salts are converted to corresponding substantially water-insoluble hydroxides, upon reacting with ammonia. The aqueous solution of the precipitant (b) can be prepared by dissolving the precipitant (b) in water. Also, in the third step of the process of the present invention, the impregnated hydrophilic fibers with the water-soluble inorganic compound (a) aqueous solution are continuously introduced into the aqueous solution of the precipitant (b), while blowing the precipitant (b) in the state of a gas into the aqueous solution. There is no restriction of the temperature of the precipitant aqueous solution, as long as the temperature is in the range of from the freezing point and the boiling point of the aqueous solution. Also, there is no limitation on the time of contact of the impregnated hydrophilic fibers with the precipitant aqueous solution. The process of the present invention is useful for easily producing hydrophilic fibers carrying therein a large amount of water-insoluble inorganic compound, at a low cost and high efficiency. By utilizing the process of the present invention, a large amount of the water-insoluble inorganic compound can be precipitated not only on the surface but also inside of the hydrophilic fibers, and the resultant modified hydrophilic fibers exhibit a specific function, for example, an enhanced flame resistance, derived from the water-insoluble inorganic compound fixed to the fibers. EXAMPLES The present invention will be further illustrated by the following specific examples 1 and 2. Working Example 1 An unbeaten, bleached soft wood kraft pulp in an amount of 3.0 g was immersed in 200 ml of a solution of 30% by weight of sodium aluminate in an ion-exchanged water at room temperature for 3 hours. Thereafter, the immersed pulp was removed from the aqueous solution of sodium aluminate, sucked by a Buchner funnel, and then centrifugalized at an acceleration of gravity of 900 g for 3 minutes. The resultant pulp impregnated with the sodium aluminate aqueous solution had a weight of 15 g. The impregnated pulp was opened in the ambient air atmosphere by using a mixer, and the opened pulp was placed in a pressure container, and thereafter, the container was closed and filled with a carbon dioxide gas under a pressure of 1 kg/cm 2 G. The opened pulp was left to stand in the container under the above-mentioned condition for 10 minutes. After the carbon dioxide gas was discharged from the container, the resultant pulp was removed from the container, immersed in one liter of ion-exchanged water at room temperature for one hour, disintegrated in water by using a disintegrator, washed with water on a 150 mesh wire net, and then dried. The resultant modified pulp had the same appearance as non-modified pulp and was composed of individual pulp fibers which were separated from each other. The modified pulp was then incinerated at a temperature of 900°C, to determine the amount of aluminum hydroxide carried in and fixed to the pulp fibers, and as a result, it was confirmed that the amount of the fixed aluminum hydroxide was 43%, based on the dry weight of the pulp fibers. Working Example 2 and Comparative Example 1 In Working Example 2, a bleached hard wood kraft pulp beaten to a Canadian standard freeness of 350 ml in an amount of 3.0 g was immersed in 200 ml of a solution of 30 % by weight of sodium aluminate in an ion-exchanged water at room temperature for 3 hours. The immersed pulp was removed from the sodium aluminate aqueous solution, sucked by a Buchner funnel, and then centrifugalized under an acceleration of gravity of 3000 g for 15 minutes. The centrifugalized pulp had a weight of 10 g. The pulp was opened in the ambient air atmosphere by using a mixer, and the opened pulp was placed in a treatment vessel. A carbon dioxide gas was then flowed through the treatment vessel at a flow rate of 500 ml/min under the ambient air atmospheric pressure, and this treatment was continued for 10 minutes. Thereafter, the carbon dioxide gas was discharged from the vessel, and the resultant modified pulp was removed from the vessel, immersed in one liter of ion-exchanged water at room temperature for one hour, opened in water by using a fiber opener, washed with water on a 150 mesh wire net, and then dried. The resultant modified pulp was then incinerated at a temperature of 900°C, and as a result, it was confirmed that the amount of aluminum hydroxide carried in the pulp fibers was 32%, based on the dry weight of the pulp. Also, the pulp fibers were observed by a microscope, and as a result, it was confirmed that almost all of the aluminum hydroxide imparted to the pulp fibers was located inside of the pulp fibers, and that substantially no aluminum hydroxide was located on the surface and in the lumen of the pulp fibers. The modified pulp (A) was converted to a paper sheet having a basis weight of 100 g/m 2 , by a customary paper forming process. In Comparative Example 1, a paper sheet (B) with a basis weight of 100 g/m 2 was produced from an aqueous slurry of a mixture of the same non-modified pulp as used in Working Example 2, with aluminum hydroxide dispersed therein. The amount of the aluminum hydroxide contained in the paper sheet (B) is the same as that contained in the paper sheet (A). The flame resistances of the paper sheets (A) and (B) were measured in accordance with JIS A 1322. The results are shown in Table 1. Table 1 clearly shows that the modified pulp paper sheet of Working Example 2 exhibited a satisfactory flame resistance, whereas the non-modified pulp paper sheet of Comparative Example 1, in which aluminum hydroxide in the same amount as in Working Example 2 was mixed with the non-modified pulp fibers, exhibited substantially no flame resistance. Working Example 3 The same procedures as in Working Example 1 were carried out, with the following exceptions. In the immersing step, the aqueous solution contained sodium silicate in a Baumé degree of 40. After the centrifugalizing step, the amount of the aqueous solution of sodium silicate picked up by the pulp was 250% based on the dry weight of the pulp. After the drying step, the resultant modified pulp had the same appearance as the non-modified pulp. By the incineration test, it was confirmed that the modified pulp fibers carried therein a silicic acid gel in an amount of 49% based on the dry weight of the pulp. Working Example 4 and Comparative Example 2 In Working Example 4, the same procedure as in Working Example 1 were carried out, with the following exceptions. In the immersing step, 200 ml of an aqueous solution of 23% by weight of aluminum sulfate in an ion-exchanged water was used. After the centrifugalizing step, the pulp was impregnated with the aluminum sulfate aqueous solution in an amount of 300% based on the dry weight of the pulp. The impregnated pulp was placed in a pressure container and the container was closed and filled with an ammonia gas under a pressure of 0.5 kg/cm 2 G. The impregnated pulp was treated with the ammonia for 60 minutes. After the ammonia gas was discharged, the resultant modified pulp was removed from the container, and treated and dried in the same manner as in Example. The resultant modified pulp had the same appearance as the non-modified pulp, and was composed of individual pulp fibers which were separated from each other. As a result of the incineration test at 900°C, it was confirmed that the resultant modified pulp carried therein aluminum hydroxide in an amount of 40.4% based on the dry weight of the pulp. The modified pulp was converted to a paper sheet (C) having a basis weight of 100 g/m 2 . In Comparative Example 2, a paper sheet (D) having the same basis weight as in Working Example 4 was produced from an aqueous slurry of a mixture of the non-modified pulp and aluminum hydroxide dispersed altogether in water. The paper sheets (C) and (D) were subjected to the same burning test (JIS A 1322) as in Working Example 2. The results are shown in Table 2. Working Example 5 The same procedures as in Working Example 2 were carried out with the following exceptions. In the immersing step, the aqueous solution contained 20% by weight of magnesium nitrate. The centrifugalizing step was carried out at an acceleration of gravity of 1500 g for 15 minutes. The resultant impregnated pulp contained the magnesium nitrate aqueous solution in an amount of 230% based on the dry weight of the pulp. The carbon dioxide gas was replaced by an ammonia gas. By the incineration test at 900°C, it was conformed that the resultant modified pulp carried therein magnesium hydroxide in an amount of 32% based on the dry weight of the pulp. Example 1 The same procedures as in Working Example 4 were carried out with the following exceptions. The pulp impregnated with 300% by weight of the aluminum sulfate aqueous solution was opened in the ambient air atmosphere by using a mixer, and immersed in a 10% ammonia aqueous solution for 60 minutes. Thereafter, the resultant modified pulp was removed from the ammonia aqueous solution, immersed in one liter of ion-exchanged water at room temperature for one hour, disintegrated in water by using a disintegrator, washed with water on a 150 mesh wire net, and dried. The resultant washed pulp had the same appearance as the non-modified pulp and was composed of individual pulp fibers which were separated from each other. As a result of the incineration test at 900°C, it was confirmed that the resultant modified pulp carried therein aluminum hydroxide in an amount of 11%, based on the dry weight of the pulp. The modified pulp could be converted to a paper sheet by a customary paper-forming process, without difficulty, and the resultant paper sheet exhibited an excellent flame resistance. Example 2 The same procedures as in Working Example 5 were carried out with the following exceptions. The magnesium nitrate was replaced by calcium chloride. The pulp impregnated with the calcium chloride aqueous solution, in an amount of 230% based on the dry weight of the pulp, was immersed in an aqueous solution of 20% by weight of sodium carbonate at room temperature for 60 minutes, removed from the solution, immersed in one liter of an ion-exchanged water for one hour, disintegrated in water by using a disintegrator, washed with water on a 150 mesh wire net, and dried. As a result of a incineration test at 900°C, it was confirmed that the modified pulp carried therein calcium carbonate in an amount of 32%, based on the dry weight of the pulp. The modified pulp exhibited an enhanced light-scattering coefficiency compared with the non-modified pulp, and was useful for forming a paper sheet having a high opacity.

Claims: A process for modifying hydrophilic fibers with a substantially water-insoluble inorganic compound comprising the steps of: A) immersing hydrophilic fibers in an aqueous solution comprising 6 to 60% by weight of a water-soluble inorganic compound (a) which is converted to a substantially water-insoluble inorganic compound when brought into contact with a precipitant (b) to impregnate the hydrophilic fibers with the aqueous solution of the water-soluble inorganic compound (a); B) bringing the impregnated hydrophilic fibers into contact with an aqueous solution of the precipitant (b) to cause the resultant substantially water-insoluble inorganic compound to be precipitated in and fixed to the hydrophilic fibers, characterized in that before bringing the impregnated hydrophilic fibers into contact with the precipitant (b), the amount of the water-soluble inorganic compound (a) aqueous solution impregnated in the hydrophilic fibers is adjusted to a level of 60 to 400% based on the dry weight of the hydrophilic fibers by removing almost all of the water-soluble inorganic compound (a) aqueous solution from the surface and from the surrounding of the hydrophilic fibers. The process as claimed in claim 1, wherein the hydrophilic fibers are selected from polyvinylalcohol fibers, polyacrylamide fibers and lignocellulosic fibers. The process as claimed in claim 2, wherein the cellulosic fibers are pulp fibers. The process as claimed in claim 1, wherein the water-soluble inorganic compound (a) is selected from water soluble alkaline earth metal salts, and the precipitant (b) comprises an aqueous solution containing at least one member selected from fluorine, phosphate, carbonate, sulfate, borate and chromate ions. The process as claimed in claim 1, wherein the water-soluble inorganic compound (a) is selected from aluminates, silicates and zincates of alkali metals, and the precipitant (b) comprises an aqueous solution containing a mineral acid capable of converting the above-mentioned water-soluble compounds to corresponding substantially water-insoluble compounds. The process as claimed in claim 1, wherein the water-soluble inorganic compound (a) is selected from water-soluble salts of metallic elements other than alkali metals, and the precipitant (b) comprises an aqueous solution of ammonia. The process as claimed in claim 6, wherein the water soluble salts of metallic elements other than alkali metals are selected from nitrates, chlorides, and sulfates, of zinc, aluminum, cobalt, zirconium, tin, titanium, iron, copper, lead, magnesium, cadmium, mercury and chromium.

Inventors: NAKAJIMA KEIHACHIRO, NAKAJIMA, KEIHACHIRO
Applicants: OJI PAPER CO, OJI PAPER CO., LTD.
Application year: 1991
Main classification: D21H17
IPC codes: D06M11, D21H13, D21H11, D21H17