CA2121375A1 - In situ electrochemical bleaching of thermomechanical pulp - Google Patents

Abstract

An electrochemical process for the production of an in situ bleaching agent, sodium perborate has been developed. The bleaching agent is produced from an electrolyte containing borax, sodium carbonate, sodium bicarbonate and magnesium silicate using a platinum anode and stainless steel cathodes. If 1 % pulp consistency based on spruce /
balsam fir softwood is added to the electrolytic solution and the electrolysis is carried out at 10 to 40°C with an anode current density of 0.6 to 1.8 kA/ m2, the bleaching agent, sodium perborate that is generated in situ in the electrolysis cell simultaneously bleached the pulp.

Description

2121~7~
-- IN SITU ELECTEcOCHEMICAL BLEACHING OF THERMOMECHANICAL PULP

Background of the invention The present invention refers to a method for the bleaching of thermomechanical pulp from spruce / balsam fir softwood. More specifically it deals with an electrochemical method for bleaching pulp in which borax, sodium carbonate, sodium bicarbonate and magnesium silicate were employed to produce sodium perborate using a platinum anode and stainless steel cathodes.
In recent years, the pulp and paper industry is faced with a series of challenges presented by both environmental considerations and the pressure of economics. Since the past decade, there has been increased use of high yield pulps. The main interest of these pulps for the pulp and paper industry is due to the increasing pulp demand and to favor the conservation of wood resources. Given these reasons, a great concern for the bleaching of thermomechanical pulp ( TMP ) has increased considerably. Unfortunately, the level of brightness and the brightness stability of these pulps are blocking the possibility for new market, particularly, printing papers and therefore an expansion for TMP. In brightening high yield pulp, the objective is to decolorize or whiten the lignin and other components without dissolving them, thus maintaining the high yield. Aconsiderable amount of research has been done on the use of reducing and oxidizing agents such as sodium hydrosulfite, hydrogen peroxide and ozone for TMP bleaching.
Traditionally, bleached TMP is obtained by using sodium hydrosulfite, a reducing agent which preserved the high yield of TMP [ Liebergott, N., Joachimides, T., Pulp and Paper C~n~d~, 80, no 12, p 59, 1979]. The hydrosulfite process is basically simple: the chemical is mixed with the pulp under specified reaction conditions.
The optimum conditions for sodium hydrosulfite are 5 % consistency, 60 C, one hour retention and a pH of 5 - 6. Hydrosulfite is rapidly decomposed by air oxidation, thus it is especially important to exclude air during bleaching. Depending on wood species, the brightness increase varied from 4 - 8 points after treatment with 1 % Na2S2O4.
Hydrosulfite bleaching is a lower capital investment compared to hydrogen peroxide bleaching. However, the limitations of hydrosulfite bleaching are well known, and for 5 grades of paper requiring higher brightness levels, hydrogen peroxide bleaching is generally used.
Hydrogen peroxide has been used as brightening agent in the bleaching of mechanical pulps for many years [Richardson C.A., Tappi Journal 39 (6) 189, 1956, Dence, C.W., Omori, S., Tappi Journal, (10), pl20, 1986, Lachenal, D. de Choudens C., Bourson L., Tappi Journal, (3), pll9, 1987, Gagne C., Barbe M.C. and Daneault C., Tappi Journal, (11), p89, 1988, Carmichael D., Pulp and Paper Canada, 89 (10), 1988]. The response of a mechanical pulp to peroxide depends on a variety of factors, including wood species, condition of the wood, pulping process, bleaching mode, bleaching conditions. Hydrogen peroxide bleaching involves oxidation at ~lk~line pH, a 15 me~ m to high consistency and temperature between 60 - 70 C. The bleach liquor is a mixture of H2O2, NaOH, Na2SiO3, MgSO4 and trace of DTPA which deactivates free metals tending to catalyse peroxide decomposition. Many advantages are attributed to hydrogen peroxide bleaching of thermomechanical pulp; reached high brightness level ( 80 % ) and increased brightness stability. Peroxide bleaching of the TMP results in an 20 improved pulp as follows: the fiber to fiber bonding is increased, the flexibility of fibers is improved, the tensile strength and tear index are increased. The disadvantages with this oxidant are that required high capital investment, decreased the opacity and increased the biochemical oxygen ~lem~ncl ( BOD ).
Many studies have been undertaken on the effect of ozone on the properties of 25 TMP. In general, the effect of ozone on the strength properties of mechanical pulps is beneficial. The breaking length and tear index increased with ozone charge. This increase in paper properties is attributed to improvements in both fiber flexibility and bonding strength. The advantage with ozone treatment in gas phase results in a reduction of bleaching time and chemical consumption. Ozonization of mechanical pulps from softwoods leads to a brightness reduction, while the brightness of pulp from hardwood species shows a considerable increase in brightness [Soteland N., Pulp and PaperC~n~(la, 78 (7), T157-T160, 1977]. Treatment of mechanical pulps with ozone willdecrease the yield depending on the ozone charge, on the type of mechanical pulp and on the wood species. Ozone treatment, however, has a negative effect on optical properties such as brightness, brightness stability and opacity [Lindholm C.A., Paperi ja Puu, 4a, p217 1977].
Presently, the most effective and most widely used bleaching agents are hydrosulfite and hydrogen peroxide. These agents applied in one stage, are limited in their ability to bleach mechanical pulp from softwood to no more than 70 to 80 brightness. Therefore, technical development on bleaching could improve the situation.
The development of alternative bleaching methods is needed in order to obtain a pulp with higher brightness and brightness stability.
One interesting possibility is the utilization of electrical energy for in situ bleaching. Electrochemistry provides a good operating method. Depending on the potential applied, the electrode behaves as an oxidizing or a reducing agent. Its action can therefore be particularly selective. This technology will exploit our hydroelectric resources.
The generation of hydrogen peroxide in caustic soda via the cathodic reduction of oxygen has been known for long time [Traube, M. Chem. Ber., (1882),15, p. 2434;
Oloman, C., U.S. Patent # 3,969,201 (1976); Balej J., et al., Chem. Zvesti, (1976), 30, p. 611; Oloman, C., Watkinson, A.P., J. Appl. Electrochem., (1979),9, pll7;
Brown, G., et al., 1983 Pulping Conference, Tappi proceedings, p. 341]. Recent work has focused on new approach to the synthesis. H-D Tech has developed a new trickle bed cell design, which allows operation at near ambient temperature and pressure. This approach uses a packed bed of "composite chip" particulates as cathode m~tçri~l [Dong, D.F., Clifford, A.L., U.S. Patent # 4,891,107 (1990)]. This method is able to produce 1.5 - 2.0 % peroxide at 67 % current efficiency. The electrochemical process is favored to the chemical method for the preparation of peroxide, based on the catalytic process with anthraquinone, when the electricity cost is low and when the supplier is far from the site of consumption. It eliminates the dependance of pulp mills on the price anddisponibility of peroxide. The principle disadvantages for the electrochemical generation of hydrogen peroxide are the low stability of the graphite cathode ( around 1 year ), the inadequate ratio of sodium hydroxide / peroxide for bleaching mechanical pulps and low current densities which required large surface area electrode.
Ozone may be formed by the direct anodic oxidation of water. There are two main strategies for electrolytic generation of ozone. The first, the ABB-Membrel process use solid polymer electrolyte technology. This method is capable of producing a high level of ozone in water but the current efficiency is low ( generally 14 % ). Recent applications include sectors of the pharmaceutical and fine chemical industry. The principle disadvantages are hydrogen evolution and the requirement of ultra-pure water [Stucki, S., Ballm~nn, H., Christen, H.J., Kotz, R., J. Appl. Electrochem., (1987), 17, 773-778]. Alternatively, ozone may be evolved from glassy carbon anodes in high concentration of tetrafluoboric acid [Foller, Peter C., U.S. Patent # 4,541,989 (1985), Foller, Peter C., Goodwin, Mark L., Science and Eng., (1984), 6, 29-36, Foller, Peter C., Tobias, C.W., Goodwin, Mark L., U.S. Patent # 4,375,395 (1982)]. The use of an air cathode in this approach elimin~tes hydrogen management problems, lowers cell voltage, and increass current efficiency. Some applications for electrochemical ozone generation include sterilization of potable water, ~willlming pools, waste treatment is foreseen. The electrochemical process produces high concentrations of ozone ( 30 % ).
The advantages are in capital costs, ozone concentration and operating characteristics.
This technology is less expensive in terms of initial investment and operation cost than ~ 212~L37~
the conventional air phase corona discharge but power consumption is 50 % higher than traditional method. The high specific energy consumption tends to mitigate against their wider use. Therefore, it will be capital that electrochemical approach developed efforts to improve its energy efficiency.
We evaluate the possibility to modify one of these electrochemical technologies for in sitU bleaching of thermomechanical pulp. The advantages of in Situ bleaching versus competitive technologies are: the bleaching agent is formed in the electrolytic cell and reacts immediately with the pulp. It simplifies the process, in only one stage. It also makes it possible to solve problems associated with transporting and storing dangerous substances. It elimin~tes the dependance of pulp mills on bleaching agent producers. The ideal electrolysis method for in situ bleaching is: the generation of non chlorinated bleaching agent; to minimi7e waste effluent, to avoid pollution; to brighten the pulp; to improve the brightness stability; to reduce number of reaction steps; to use undivided electrolysis cell; to be in aqueous m~ m and in ambient temperature.
The use of in situ hydrogen peroxide generation does not appear to be a promizing method for bleaching mechanical pulp for multiples reasons: first, theelectrolysis needed high concentration of sodium hydroxide O.S-lM in the anodic compartment to achieve high current efficiency, therefore, such alkaline condition contributes to darkening of the pulp; second, the generation of hydrogen peroxide is in direct competition with water reduction, while aqueous medium is used for pulp bleaching.
The in situ ozone generation involved some technical problems: The ABB
Membrel process used an ultra pure water ~ 20 ~S cm-l which turned out to be very difficult to achieve as part of in sitU bleaching. Foller process used tetrafluoboric acid as electrolyte. The use of such an electrolyte seems difficultly justifiable for bleaching operations.

Besides hydrogen peroxide and ozone, another oxidizing bleaching agent can also be produced electrochemically. Sodium perborate was manufactured industrially by chemical routes, but efforts were made to use electrochemical processes for the preparation of perborate. In the beginning of this century, the first method used a direct electrosynthesis by anodic oxidation of borate that proceeded with lln~ fartory current efficiency. A U.S. Patent No. 1,081,191 to Arndt discloses an addition of sodiumcarbonate to the electrolyte to improve the current efficiency for perborate formation. The electrochemical p~ e synthesis is made from an electrolyte containing borax, sodium carbonate, bicarbonate and water. A number of patents disclose electrolytic methods for the ~l~pa~ ~tion of sodium perborate, as e~emplifie~ by the following.
Bul. Pat. No. 1,444,399 to Kiselev et al. disclose that an addition of ammonium carbonate or bicarbonate or hydroxide to the electrolyte used in production of sodium p~ll~,al~, increases the efficiency of the process. Various addition agents like sodium or magnesium nitrates, sodium chloride, sodium tripoly-phosphate, carbamide, urea, diammonium hydrogen phosphate, ammonium cyanide, disodium salt of EDTA, ammonium sulfates are used to enhance the sodium perborate current yield and forreducing the voltage so that the energy requirements are reduced. For example, such as discussed in Bul. Pat. Nos.1,109,480 and 1,096,308 of Alekhin et al. issued Aug. 23 and June 7, 1984, Bul. Pat. No. 941,431 of Erkshov et al. issued July 7, 1982, Bul.
Pat. No. 920,080 of Ershov et al. issued April 16, 1982.
This electrochemical process to obtain sodium perborate is interesting because it generates an oxidative bleaching agent for wood pulps. It has now been surprisingly found that the wood pulp can be bleached in situ by sodium perborate produced in an electrochemical process.
In accordance with the present invention, an in situ electrochemical bleaching process has been developed which comprises producing sodium perborate and ~imllltaneously bleaching of pulp in the electrochemical cell. The perborate is produced ._ by optimizing a chemical composition containing borax, sodium carbonate, sodium bicarbonate, talc and water. The electrolysis is effected at specific temperature for a time and current density sufficient to cause bleaching of wood pulp.
The bleaching process provided by the present invention permits the use of a non-chlorinated bleaching agent to bleach pulp in a single-stage with minimllm waste effluent and by avoiding pollution.
It is, accordingly, an object of the present invention to provide an electrochemical process for bleaching pulp.This electrolytic process offers particular advantages for the pulp and paper industry where pelbwa~e mixtures can be used to bleach pulp. It is also object of the invention to provide optimized operating conditions for the electrolytic preparation of sodium perborate without pulp and finally, the optimi7~tion of pulp brightness during in situ bleaching of a thc~ ollRchanical pulp in electrolysis cell.

Sunllll~y of the invention In relation to the present invention, an electrochemical process has now been provided to produce sodium perborate ( NaBO3 4 H2O ) in situ in an electrolytemixture containing pulp, thereby simultaneously bleaching the pulp. The electrolytic bleaching is effected at a low temperature and current density sufficient enough to produce minimllm sodium perborate required to bleach the pulp.
The improvement in brightness of pulp achieved by the present invention permits use of the said electrochemical process as a replacement for the peroxide bleaching of low consistency pulp, particularly for its advantages over peroxide bleaching process for indll~tri~l pollution control.
The said electrochemical process showed promise for an improvement in pulp bleaching ef~lciency by multiple stage bleaching of pulp by the residual perborate content in the electrolytic cell.

21~137~

The in situ bleaching of treated pulps carried out by sodium perborate produced from 10 - 50 g /1 of borax, 50 - 150 g /1 of sodium carbonate, 25 g /1 of sodiumbicarbonate and 1 g /1 of magnesium silicate within a temperature range 10 - 70 C, current density 0.6 - 1.8 kA / m2 and electrolysis time 30 - 120 min is within the scope 5 of the invention.

Detailed description of the invention The pulp used in the present invention was unbleached thermomechanical lo softwood pulp. The initial brightness of the pulp was 60 % ISO.
The process of the invention is an unique single-stage process for a generation of an oxidizing agent and ~imlllt~n~ously bleaching pulp. Whereby, the electrolytic cell has been designed. Figure 1 is a perspective view of an embodiment of an electrochemical cell of the invention. Refering to drawing, the embodiment of the invention shown, an 15 electrochemical cell 1 consists of a mesh pl~tini7e~1 tit~nillm anode 2 located in the center of the cell and two cathodes 3 and 4 were made of stainless steel screen ( 6 cm X 11 cm ). The cathodes 3 and 4 were fixed on either side of the anode 2. The spacing between anode 2 and cathode 3 or anode 2 and cathode 4 was 1 cm. The anode 2 and cathodes 3 and 4 are connected to a current rectifier 5. The current rectifier 5 comprises of three 20 devices: an ammeter 6 capable of producing a current up to 25 amperes, a voltmeter 7 capable of reading 0 - 12 volts range and a rheostat 8 for regulating the resistance for current production. The electrodes comprising anode 2 and cathodes 3 and 4 are submerged in a reactor 9 of 1 liter volume cont~ining pulps and electrolyte solution 10.
The pulp and electrolyte solution 10 are a~it;~te~l with a stirrer 11 and the speed of 25 agitation is controlled by a motor 12 fitted with the stirrer 11.
In a preferred embodiment of the invention, the chemical compositions used to generate the bleaching agent, sodium perborate is borax, sodium carbonate, sodium 2121~75 bicarbonate and talc or magnesium silicate. The balance was water to make up a constant volume in the cell. The glass cell is immersed in a temperature controlled water bath.
The presence of trace metals in the spuce / balsam fir softwood catalytically decompose the bleaching chemicals. TMP from softwood reveals itself to be effective only if it is pretreated with a chelating agent, DTPA ( diethylenetriamine pentaacetate ).
The pretreatment process, i.e., a process according to which the pulp is treated with a chelating agent DTPA and consequently, a chelate complex with the free metal in pulp is formed. In practice of the invention, before in situ electrochemical bleaching, the pulp is pretreated with 0.4 % DTPA for 15 minutes at 60 C and 3 % consistency to elimin~te metal ions. Then, the pretreated pulps were dewatered to 20 % consistency and were bleached. The normal in situ bleaching procedure is as follows. The chemicals are mixed with a 10 g pulp sample, while the pulp consistency is 1 %. The pH of the pulp suspension is adjusted by the sodium carbonate and equalled 10. The duration of electrolysis varies from 30 to 120 minutes. Once the electrolysis is completed, the mi~lule was filtered and a filtrate sample was obtained. The filtrate was acidified with H2S04 and titrated for residual p~lb(J~t~ with 0.02 N KMnO4. The pulp is neutralized to pH = 5.5 and h~n~lcheets were made for brightness testing according to TAPPI T 452 om-87.
In electrochemical bleaching of pulp, the pretreated pulp of 1 % consistency is bleached in situ by the sodium perborate generation in the cell due to the electrochemical reaction of borax and sodium carbonate. During the electrolysis at least two intermediate products are formed: peroxocarbonate and hydrogen peroxide. The mechanism by which perborate forms by electrolysis was reported by Franchuk I.F., Brodskit A.I., [Doklady Akad. Nauk S.S.S.R.,l 18, 128-131 (1958)].
The temperature of electrolysis was varied over a range from 10 C to 70 C. Forpulp brightness, the temperature is an important factor and should be kept low. The extent of bleaching of pulp was good at near ambient temperature. For maximum - brightness a temperature preferably between 10 - 15 C is used. Perborate formation decreases on increasing temperature, an appreciable pelbold~e residual was obtained in the range of 10 - 25 C. The large perborate residual obtained for temperature range from 10 C to 25 C might be effectively used for a second batch of pulp bleaching process.
Then multiple batch bleaching per electrolytic solution seems feasible.
The borax concentration may range from about 20 g/l to 50 g/l of the electrolytic solution, although lower concentration down to about 10 g/l may be used with reduced beneficial effect. Similarly, improvements of the brightness of the pulp are not very significant with borax concentrations above 30 g/l of the electrolytic solution, while the additional cost is not justified by the small additional improvement. Generally, therefore, a borax concentration of between about 10 g/l to 50 g/l on the electrolytic solution, preferably around 30 g/l of electrolytic solution is used.
It is preferred to carry out bleaching with a sodium carbonate concentration varying from 50 g/l to about 150 g/l of the electrolytic solution preferably with lower concentration of sodium carbonate. Similarly, improvement in perborate formation are observed with sodium carbonate charges between 130 g/l to 150 g/l of the electrolytic solution, but substantial loss in brightness is observed.
In the present invention, the extent of electrolysis of the borax and sodium carbonate to form sodium perborate is influenced by the amount of current supplied.
Generally, the current density is varied between 0.6 to 1.8 kA / m2. A current density above 0.6 kA / m2 increased the perborate generation at the expense of additionnal cost.
No marked change in pulp brightness was observed for current density varied between 0.6 to 1.8 kA / m2. Therefore, a current density preferably around 0.6 kA / m2 is used.
The essential requirement for the process is that the electrolysis time must be sufficient to generate minimllm amount of pcll~l~lt; required to improve the bleaching of the pulp. The electrolysis time is varied between 30 to 120 min. Improvements of the pulp brightness are not very significant with electrolysis time above 30 minutes.

21213~
Therefore, a maximum reaction time corresponding to 120 min resulted to an increase of the operating cost.
The bleached pulp made from this invention may be used to a part or total replacement of peroxide bleached pulp with reduced environmental pollution.

Specific description of the invention In order to disclose more clearly the nature of the present invention, the following examples illustrating the invention are given. It should be understood, 10 however, that this is done solely by way of example, and is intended neither to cleline~te the scope of the invention nor limit the ambit of the appended claims.

This example illustrates the effect of electrolysis temperature on pulp properties.
It can be seen from the results of table lA that the ten~ature is an important factor and should be kept low. The electrochemical bleaching of TMP at 1 % consistency can obtain a gain in brightness of 4.5 points. In table lA, b* is a coefficient that 20 measures the yellow color of the pulp. For unbleached pulp, b~ equals 10.8, the yellowness increased drastically with temperature above 40 C. Perborate consumption decreases on increasing temperature, an appreciable perborate residual was obtained in the temperature range of 10 - 25 C. An increase of temperature favors the decomposition of perborate to perhydroxyl ions. But it also increases the rate of oxidation of water 2s against the oxidation of carbonate ions, the latter being one of the steps in the formation of perborate. Perborate consumption in table lA was obtained by substracting pell,ol~te residual from perborate formed by same electrolysis conditions without pulp. To calculate the perborate consumption we made a supposition that the presence of pulp in the electrolysis does not interfere the generation of perborate. The total energy consumption is slightly decreased with an increase of temperature.

This example illustrates the effect of borax on pulp brightness and perborate formation.
It can be seen from the results of Table lB that the increase of borax concentration brings about an increase in pulp brightness. The addition of borax slightly increases the perborate consumption. It is seen that the brightness of pulp is increased to about 64.5 % ISO at 30 g /1 of borax and a further increase of borax concentration has only marginal effect on brightness increase at 10C. A concentration of borax above 30 g /1 does not give higher pulp brightness. On the other hand, an increase of boraxconcentration above 30 g /1 only increased the amount of p~ e consumed. The total energy consumed remains constant.

This example shows the effect of current on pulp brightness and perborate generation.
It can be seen from the results in Table lC that the increase in current production 25 resulted in an increase in the production of perborate thereby, leaving a high concentration of residual perborate. For current range of 3 to 9 A, the gain in brightness is minim~l even if the energy supplied to the system is increased. An increase in supply 21213~5 current has a positive influence on pulp yellowness and a current of 9 A reducesyellowness by 21 %. This result is very advantageous for bleached thermomechanical pulps, because it can produce a Ueached pulp with very low b* value.
The total energy consumption is increased dramatically with increasing current.
Therefore, a complolllise has to be made between the level of yellowness reduction obtained and the energy con~llm~

This example illustrates the effect of sodium carbonate on pulp brightness and perborate consumption.
It can be seen from the results of Table lD that the pulp brightness is higher with lower concentration of sodium carbonate. Thus, an increase of sodium carbonate concentration has a negative influence on pulp brightness. On the other hand, the addition of sodium carbonate increased the perborate residual concentration indicating a good potential for mlllti~t~ge bleaching. The yellowing remains constant in the range of 50 to 150 g /1. Table lD shows that an increase in the concentration of sodium carbonate decreases the concentration of perborate con~llmed The total energy consumption remains constant within the range of sodium carbonate between 50 to 150 g /1.

This example illustrates the effect of electrolysis time on pulp brightness and perborate fnrm~tion 2121~75 It can be seen from the results of Table lE that the maximum electrolysis time to yield beneficial results is about 30 min at 10 C, with longer times being used no substantial improvement in pulp brightness was observed. The increase in bleaching time increases the energy consumption. Thus, a good brightness can be achieved with 5 relatively less amount of energy, if the electrolysis time is reduced at about 30 min.
Similarly, a low bleaching time reduced the energy requirement to 0.7 kWh / kg of dry pulp without sacrifice the brightness of pulp.

loEXAMPLE 6 This example compares the effect of electrochemical bleaching and the conventional hydrogen peroxide bleaching, for a given pulp consistency.
It can be seen from the results of Table 2 that the maximum brightness with 2 %
15hydrogen peroxide at 1 % consistency and 70 C is 64.6 % ISO which is about the same as we observed for in situ bleaching ( Table lA ). Table 2 indicates that we obtained a brightness of only 63.5 % ISO at 10 C with the conventional hydrogen peroxide bleaching compared to 64.5 % ISO for electrochemical bleaching ( Table 1 A ).
Therefore, Table 2 shows that the hydrogen peroxide based conventional bleaching is 20 less effective compare to the perborate based electrochemical bleaching for low pulp consistency as well as for bleaching at near ambient temperature.
Notably, an electrochemically bleached pulp needs only 30 min bleaching to achieve a brightness of 64.8 % ISO at 1 % consistency ( table lE ), whereas the same pulp bleached with conventional peroxide needs 120 min to obtain similar brightness.
25 This is a significant advantage. Table 2 indicates that conventional peroxide bleaching could achieve 11 points gain in brightness only with high pulp consistency ( lO % ).
However, the increase in pulp consistency for conventional peroxide bleaching 212~37~
deteriorated the pulp yellowness by 8 % compare to the electrochemically bleached pulp.
This is a clear disadvantage of conventional high con~i~tency peroxide bleaching process over the low consistency in situ electrochemical bleaching. Moreover, these examples although illustrated electrochemical bleaching at 1 % pulp consistency only, there is a 5 promise to use this method even at high pulp consistency as well as for multistage bleaching.

21213 l 5 TABLE lA
ELECTROLYSIS CONDITIONS:
30 g/l Borax, 100 g/l Carbonate, 25 g/l Bicarbonate, 3 A, 2 hours, 1 g/l talc.

Temperature, C
Q
ISO Brightness, % 64.5 63.2 62.3 60.6 57.0 b* 10.1 10.3 10.5 11.5 14.0 Yellownessreduction, % -6 -5 -3 6 30 Perborate residual, g/l 1.2 1.0 0.6 0.3 0.1 Perborate consumption, g/l 1.6 0.8 0.2 0 0 Perborateconsumed, % 57 44 25 0 0 Voltage, V 4.5 4.5 3.9 3.7 3.4 Energy consumed, kWh/kg dry pulp 2.7 2.7 2.3 2.2 2.0 TABLE lB
ELECTROLYSIS CONDITIONS:
100 g/l Carbonate, 25 g/l Bicarbonate, 3 A, 2 hours, 10 C, 1 g/l talc.

Borax, g/l ISO Brightness, % 63.4 63.8 64.5 64.7 64.8 b* 10.0 10.0 10.1 10.4 10.5 Yellowness reduction, % -7 -7 -6 -4 -3 Perborate residual, g/l 1.4 1.2 1.2 1.1 1.0 Perborate consumption, g/l 0.8 1 1.6 1.6 1.7 Perborateconsumed, % 36 45 57 59 63 Voltage, V 4.3 4.4 4.5 4.2 4.0 Energy consumed, kWh/kg dry pulp 2.6 2.6 2.7 2.5 2.4 Unbleached TMP brightness and b*: 60.0 % and 10.8.

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~ TABLE lC
ELECTROLYSIS CONDITIONS:
30 g/l Borax, 100 g/l Carbonate, 25 g/l Bicarbonate, 2 hours, 10 C, 1 g/l talc.

Current, A

ISOBrightness, % 64.5 64.7 64.9 b* 10.1 9.6 8.5 Yellownessreduction, % -6 -11 -21 P~ll,ol~Le residual, g/l 1.2 2.5 3.2 Perborate consumption, g/l 1.6 1.1 0.8 Perborateconsumed, % 57 30 20 Voltage, V 4.5 6.5 7.2 Energy consumed, kWh/kg dry pulp 2.7 7.8 13.0 TABLE lD
ELECTROLYSIS CONDITIONS:
30 g/l Borax, 25 g/l Bicarbonate, 2 hours, 3 A, 10 C, 1 g/l talc.

Carbonate, g/l ISO Brightness, % 64.6 64.5 64.2 64.1 b* 10.0 10.1 10.1 10.0 Yellowness reduction, % -7 -6 -6 -7 Perborate residual, g/l 1.0 1.2 1.8 2.2 Perborate consumption, g/l 1.3 1.6 1.8 2.0 Perborateconsumed, % 57 57 50 48 Voltage, V 4.3 4.5 4.3 4.4 Energy consumed, kWh/kg dry pulp 2.6 2.7 2.6 2.6 Unbleached TMP brightness and b*: 60.0 % and 10.8.

TABLE lE
ELECTROLYSIS CONDITIONS:
50 g/l Borax, 100 g/l Carbonate, 25 g/l Bicarbonate, 3 A, 10 C, 1 g/l talc.

Electrolysis time, min ISO Brightness, % 64.6 64.8 64.8 b* 10.4 10.4 10.5 Yellowness reduction, % -4 -4 -3 Perborate residual, g/l 0.8 1.0 1.0 Perborate consumption, g/l 1.3 1.6 1.7 Perborateconsumed, % 60 62 63 Voltage, V 4.4 4.2 4.0 Energy consumed, kWh/kg dry pulp 0.7 1.6 2.4 Unbleached TMP brightness and b*: 60.0 % and 10.8.

HYDROGEN PEROXIDE BLEACHING CONDITIONS:
1.65 % NaOH, 2 % H202, 3 % Na2SiO3, 0.05 % MgS04, 0.4 % DTPA.

Temperature, C 70 70 70 70 10 70 Consistency, % 1 1 1 1 1 10 Bleaching time, min 15 30 90 120 120 120 ISO Brightness, % 63.0 63.8 64.6 63.9 63.5 71.3 b* 11.0 11.5 12.0 12.3 10.9 12.2 Yellownessreduction, % -3 2 6 9 -4 8 Peroxide residual, g/l 1.8 1.5 1.4 1.4 1.6 0.4 Peroxide consumption, g/l 0.2 0.5 0.6 0.6 0.4 1.6 Peroxide consumed, % 10 25 30 30 20 80 Unbleached TMP brightness and b*: 59.8 % ISO and 11.3.

Claims (11)

1. A method for bleaching thermomechanical pulp in an electrolytic cell while sodium perborate is generated in situ and bleaches simultaneously the pulp therewith, which method comprises a 1 liter electrochemical reactor having a mesh platinized titanium anode and two stainless steel screen cathodes disposed on either side of the anode, an electrolyte containing sodium borate, sodium carbonate, sodium bicarbonate, magnesium silicate, pulp and balance water, said electrolytic solution having a pH of about 10 and pulp in said electrolytic solution is of 1 % consistency, maintaining a constant current between the anode and the cathode, the sodium perborate that is generated in situ in the said cell simultaneously bleached the pulp without reducing the pulp brightness below about 62 % ISO.

2. An electrochemical bleaching method for pulp according to claim 1, wherein the amount of sodium borate employed is between 10 to 50 g/l of the electrolytic solution.

3. An electrochemical bleaching method for pulp according to claim 1, wherein the amount of sodium carbonate employed is between 50 to 150 g/l of the electrolytic solution.

4. An electrochemical bleaching method for pulp according to claim 1, wherein the amount of sodium bicarbonate and magnesium silicate were 25 g/l and 1 g/l of electrolytic solution respectively.

5 . An electrochemical bleaching method for pulp according to claim 1, wherein the bleaching temperature is between 10 - 40 °C.

6. An electrochemical bleaching method for pulp according to claim 1, wherein the current density used for electrolysis is between about 0.6 kA/ m2 and 1.8 kA/ m2.

7 . An electrochemical bleaching method for pulp according to claim 1, wherein the electrolysis time is between about 30 to 120 minutes.

8 . An electrochemical bleaching method for pulp according to claim 1, wherein the pulp material is softwood thermomechanical pulp.

9. An electrochemical bleaching method for pulp according to claim 1, wherein the pulp material is pretreated with 0.4 % DTPA.

10. An electrochemical bleaching method for pulp according to claim 1, wherein the said electrolysis is perform at a pH = 10.

11. An electrochemical bleaching method for pulp according to claim 1, wherein the consistency of the said pulp is about 1 %.