SU933848A1 - Wood pulp bleaching method - Google Patents

Description

I

The invention relates to methods for bleaching wood pulp with reducing agents and can be used in pulp and paper industry in the manufacture of bleached wood pulp intended for various types of paper and cardboard.

There is a method of bleaching wood pulp by treating it with a reducing agent — sodium hydrosulfite in an amount of 0.5% by weight of absolute dry fiber. The pH of the medium is maintained by trisodium phosphate, the consumption of which is 0.3% by weight of absolutely dry fiber l

One of the disadvantages of this method of bleaching wood pulp is the low stability of the bleaching solution used. In the presence of atmospheric oxygen, the decomposition of hydrosulfite proceeds very quickly, and this is the main cause of hydrosulfite loss during bleaching. In addition, the products are unstable; reversing, therefore, with aging, the wood mass is again prone to yellowing.

There is also known a method of bleaching wood pulp by treating it with borohydride at a consumption rate of the last 1% of the absolutely dry mass, at 2035 C and a pH of 10.5. Sodium borohydride is a very reactive chemical that reduces all types of carbonyls, i.e. quinones, aldehydes and ketones, with the exception of some flavones, in the corresponding alcohols. In this case, a small amount of intensely colored secondary stilbene-type chromophores can develop. The whiteness achieved with bleaching with borohydride is more stable than with bleaching with sodium hydrosulfite .2.

The disadvantages of this method are the high cost of the bleaching agent, as well as the possibility of forming secondary chromophores, which reduce the effect of bleaching. The closest to the invention. According to the technical essence and the achieved effect is the method of bleaching wood pulp by treating it with an aqueous solution of alkaline or alkaline earth metal hypophosphite at elevated temperatures. This method is carried out by sequential processing of wood pulp with an aqueous solution of SOj, and then with a solution of hypophosphite in an amount of 1-5 by weight of absolutely dry fiber with a pH of 5 6.0 for 1 hour. At the same time, hypophosphite acts rather selectively on wood chromophores, . providing them only partially f3j. Achieved whiteness is unstable, and wood pulp is prone to yellowing when using bleached wood pulp in the composition of writing and printing types of paper. The purpose of the invention is to increase the degree of whiteness and light resistance of wood pulp. The goal is achieved by the 4TQ method of bleaching wood pulp by treating it with an aqueous solution of alkaline or alkaline earth metal hypophosphite at elevated temperatures, according to the invention, before treating the wood pulp of alkali or alkaline earth metal with wood solution treated with sodium boron hydride. The wood pulp is treated with borohydride at a flow rate of 0, 0% by weight of absolutely dry fiber, pH 10-11 and a temperature of 20-35 s, and an aqueous solution of alkaline or alkaline earth metal hypophosphite at its consumption of 1-1.5%, pH 2-6 and temperature yo-eo with. Due to the combination of sequential treatments with borohydride and hypophosphite, increased rates of light fastness and whiteness are achieved with respect to weight. When processing with borohydride in an alkaline medium, intensely colored secondary stilbene-type chromophores are formed, which contribute to a partial decrease in the bleaching effect. It is possible that subsequent treatment with an aqueous solution of SOoH with hypophosphite contributes to the creation of conditions under which stabilization and blocking of the formation of secondary chromophores occurs.

Comparisons are carried out in a single stage. The proposed method is carried out as follows. Wood pulp is loaded into a separate unit equipped with a stirrer, and borohydride in the amount of 0.5-2.0% by weight of absolutely dry fiber is added to the same place. The bleaching is carried out at 20-35 ° C, the pH of the medium is 10-11 for 0.5-1.0 hours. After the end of bleaching, the aqueous solution of SO / g is metered into the mass. until the pH of the medium reaches 2.0-6.0. Then an aqueous solution of alkaline or alkaline earth metal hypophosphite is added in an amount of 5% and heated to 70-3ffC for 0.5-1 h. After HdTe bleaching time, the mass is discharged, drained, and then the whiteness of wood pulp and the degree of yellowing are determined (light fastness). Example 1. 10 g of absolutely dry wood pulp, previously crushed in a disintegrator, is loaded into a laboratory bleaching unit equipped with a mixer. 8 mass is added with sodium borohydride in an amount of 2.0% of absolutely dry wood pulp, mass concentration 6%, pH 11. The bleaching is carried out at 35 ° C for 1 hour. Then liquid 50 is metered into the mass to pH, add hypophosphite aqueous solution sodium in the amount of 1.0% of the absolutely dry mass, heated to and kept for 1 hour. The mass is discharged, the whiteness index is determined and tested for light fastness in the Light 3 apparatus. Example 2, The pulp bleaching process is carried out similarly to Example T. Borohydride consumption 0.5% by weight of absolutely dry fiber, pH 1 0.0, bleaching time 1.0 h, 30 is dosed into the mass to pH 2. Example 3. The mass bleaching process is carried out as in Example 1. The borohydride is bleached and the treatment is carried out with calcium hypophosphite at 80 C. Example. The mass bleaching process is carried out analogously to example 1. The borohydride consumption is 1.0% by weight of absolutely dry fiber, pH 10.5, temperature, treatment with sodium hypophosphite at pH 2.5, and temperature for 0.5 m. Example 5 (control) . For bleaching with borohydride at a flow rate of 1.0 from absolutely dry mass, pH 10.0, temperature 35 С, bleaching time 1 h. Example 6 (by a known method), Bleaching with hypophosphite is carried out at a flow rate U to an absolutely dry mass, pH 6.0, temperature, bleaching time 1 h. The test data are given in the table.

In comparison with the known method, the proposed method allows to increase the whiteness index and the light resistance of the mass by 10-20%. In addition, the combined use of bleaching agents at the indicated ratios leads to an effect exceeding the efficiency of bleaching by individual components.

Claims (3)

1. The method of bleaching wood pulp by treating it with an aqueous solution of alkali or alkali hypophosphite an earth metal at an elevated temperature, characterized in that, in order to increase the degree of whiteness and light resistance of the pulp, before treatment with an aqueous solution of alkaline or alkaline earth metal hypophosphite, the pulp is treated with sodium borohydride. 2. The method according to claim 1, about t l and h and yush and the fact that the processing of wood pulp with sodium borohydride is carried out at a rate of 0.5-2%, pH 10-11 and a temperature of 20-35 C, and water rast 79338 .88 thief of an alkaline or alkali hypophosphorite. 2. Lebedeva I, A. and others. Bleaching no earth metal at its consumption of wood mass. M, Lesna about 1-1, 5%, pH 2-6 and a temperature of 70-90 0. intensity, 1973, p. 88-90. Sources of information taken into account in the examination of $ 3. USSR author's certificate |. Author's certificate of the USSR № 58621, cl. D 21 C 9/10, 1375 H- 326273, cl. D 21 N 3/00, 1967 .. (prototype).