Application prospect of the hottest chlorine dioxi

Application prospect of chlorine dioxide in paper industry

in recent years, great progress and gratifying results have been made in the research of element free chlorine bleaching (ECF) and complete chlorine free bleaching (TCF). From the point of view of completely eliminating organic chlorides, it is undoubtedly necessary to take the TCF bleaching route. Judging from the actual situation in China, it is difficult for most small and medium-sized paper mills to adopt TCF bleaching in all factories. ECF bleaching has its unique advantages in many aspects, and it is suggested that qualified paper mills should consider adopting it

this paper discusses the application prospect of chlorine dioxide in paper industry

1 the gain and loss of TCF bleaching

Sweden and Finland have conducted in-depth research on TCF bleaching and have put it into practice in production. For example, the newly-built enocell factory in Finland generally produces coniferous wood pulp with a kappa number of 15. TCF bleaching can obtain higher whiteness. However, compared with the pulp with the kappa number 20 and bleached with the ECF, pulp with the low kappa number and bleached with the TCF has lower yield. When the kappa number is reduced from 30.8 to 14, the yield of bleached pulp will be reduced from 45.5% to 43.7%. With the reduction of kappa number, the yield of bleached pine pulp will decrease, which will increase the wood consumption required for the production of bleached pulp by 5% - 10%

when super batch cooking method is adopted, the average yield of unbleached pulp with kappa number of 30 is 1.5% higher than that of sulfate pulp. In order to adapt the pulp to TCF bleaching, the super batch cooking method loses the advantage of high yield when the cooking is extended to the pulp kappa number below 20

if modified continuous cooking (MCC) is used, 1% more wood is consumed per ton of pulp than ordinary sulfate cooking at a given kappa number. When the kappa number decreases from 30 to 20, the wood consumption of bleached pulp is 2.2% higher than that of kraft pulp. Therefore, this method of cooking to low kappa number for TCF bleaching may increase the pressure on wood resources

tcf bleaching loss of yield is also greater than ECF bleaching loss, eucalyptus pulp with d-eo-d (ECF) sequence bleaching, pulp yield is 97.5%, if using a-zq-eop (TCF) sequence bleaching, yield is 96.5%. TCF bleaching with ozone is used to obtain the material of pulp δ and ψ The splicing rate of the sample after experimental fracture can be 0.5% - 1% lower than that when bleached with ECF

in addition, several articles have reported the strength loss of Scandinavian softwood TCF pulp. For example, when the tensile strength is fixed, the tear strength is 10% - 20% lower than the specified value

when making TCF pulp, it not only increases wood consumption and loses pulp strength, but also increases the load of the chemical recovery system of the pulp mill. Therefore, weighing the gains and losses of ECF and TCF bleaching, it can be considered that the ECF bleaching sequence will still have broad prospects in the near future or in the medium and long term, which means that the prospect of chlorine dioxide in pulp bleaching is very considerable

2 chlorine dioxide can be used as a separate bleaching agent

chlorine dioxide can be used as an oxidant for bleaching pulp alone. Unbleached pulp is treated with chlorine dioxide at low concentration and low temperature, and then extracted with hot alkali, and then treated with chlorine dioxide at higher temperature and concentration for the second time. The three-stage bleaching of ded in this way can bleach the kraft pulp to 72 ~ 82G E. Whiteness, bleach sulfite pulp to 85 ~ 89g E. Whiteness

if the first stage chlorine dioxide is bleached at 40 ℃, the kraft pulp can also be bleached to a higher whiteness. For example, the brightness of the pulp after D25 ℃ ed bleaching is 78.7g E. , while with D40 ℃, ED can drift to 81.4g E. Whiteness. Under the condition of 40 ℃ and 12% slurry concentration, the added chlorine dioxide (2.35% for the absolute dry slurry weight) can be consumed in 2H

for sulfite pulp, removing resin is an important problem. The ED bleaching sequence can be stored and printed with the CED experimental results. The resin content in the bleached pulp is 1.50%, while the resin content in the bleached pulp is only 0.77% with the deded bleaching sequence

using chlorine dioxide as pulp bleaching agent not only bleaches the pulp with high strength, but also has good whiteness stability. Using the DED bleaching sequence, a dissolved pulp with high whiteness and good whiteness stability can be produced from sulfite birch pulp. After being treated with sodium borohydride, the pulp was heated in an oven at 105 ℃ for 18h. After thermal aging, the whiteness of the dissolved pulp was only 96.1g E. Reduced to 95.7g E. This may be one of the whitest and most stable pulps that can be made

what is more valuable is that larch, as the main papermaking material in Northeast China, has poor bleachability. When CEH three-stage bleaching is used, the chlorine content is more than 15%, and the whiteness of the pulp can only reach about 60%. When the effective chlorine content is 8%, the bleached Larch pulp can be bleached to 84% whiteness by using deded bleaching sequence, and the whiteness stability is also very good

3 bleaching with chlorine dioxide can reduce the generation of AOX

under the same amount of effective chlorine, the AOX produced by chlorine dioxide is 1/5 of that produced by elemental chlorine. Various measures have been proposed to further reduce AOX generated during chlorine dioxide bleaching. For example, the chlorine dioxide section adopts low slurry concentration; Cancel the washing section between chlorine dioxide and alkali extraction section; Strengthen the alkali extraction section by oxidation; In the chlorine dioxide section, the method of changing pH is adopted (low pH2 ~ 4 at the beginning, and high pH8 ~ 9 at the end); Adding chemical additives (such as dimethyl sulfoxide, sulfamic acid or hydrogen peroxide) can reduce the generation of organic chlorides during chlorine dioxide bleaching

however, in some industrial chlorine dioxide solutions, the elemental chlorine accounts for 3% - 5% of the total effective chlorine. According to the empirical formula, it is estimated that about 21% of the AOX generated by the chlorine dioxide section is generated by the organic chloride generated by the chlorine in it. Therefore, completely removing chlorine from chlorine dioxide solution can reduce AOX by at least 20%. Foreign literature has introduced that adding a small amount of hydrogen peroxide can eliminate chlorine in industrial chlorine dioxide solution

in fact, the chemical cost of adding H2O2 to reduce the generation of AOX is quite small, because the ordinary sulfate pulp (kappa number 30) is bleached with industrial chlorine dioxide solution containing 5% chlorine, and the H2O2 required to eliminate chlorine is only 0.14% (for pulp). Therefore, this method is technically feasible and economically reasonable

4 there are new methods for preparing chlorine dioxide

there are many methods for preparing chlorine dioxide, such as R2, R3, R4, R5, R6, etc. The chlorine dioxide manufacturing equipment purchased by Guangxi Hexian pulp mill and Liujiang paper mill in China uses the R8 method of Canadian albright&wilson company; Zhalantun pulp mill in Inner Mongolia uses the R8 method of cellchem company in Sweden. When preparing chlorine dioxide by R8 method, naclo3+ch3oh (methanol) is used. The price of methanol is high, so the cost is slightly high, and the by-products need to be treated

Canada has studied the R10 method of preparing chlorine dioxide, which is an improved method of preparing chlorine dioxide by R8 method. R10 method can save 0.25t acid per ton of chlorine dioxide without neutralizing the sesquidic acid produced by R8 method, and it can also save 0.2T sodium hydroxide per ton of ClO2

5 catalytic activation of vanadium pentoxide

when bleaching pulp with chlorine dioxide, chlorine dioxide may decompose into chlorate, reducing the bleaching capacity of chlorine dioxide

Rapson, a famous bleaching expert, proposed that careful control of the pH of chlorine dioxide bleaching section to 4 can reduce the formation of chlorate and improve the whiteness effect during chlorine dioxide bleaching. He also proposed that adding vanadium pentoxide, under the condition of 70 ℃ and ph=4, the maximum whiteness effect can be obtained, because vanadium pentoxide can play a catalytic activation role under this condition, so that the chlorate generated by the decomposition of chlorine dioxide in the bleaching process can be regenerated into chlorine dioxide, so that chlorine dioxide can play a full bleaching effect

naclo3+2h++2cl- → nacl+clo2+1/2cl2+h2o

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