THE EVOLUTION IN EUCALYPTUS KRAFT MARKET PULP BLEACHING FROM FOUR TO TWO STAGES - A COMPARISON OF OPTIONS

ABSTRACT

State-of-the-art in kraft hardwood pulp bleaching is a four stage sequence. The alternating application of chlorine dioxide and hydrogen peroxide, D₀-Eop-D₁-P, has the lowest demand for bleaching chemical, a low effluent load, a high yield, top brightness and good brightness stability as well as good strength. Compared with this reference shorter sequences require less investment and less electrical energy for less equipment. Their disadvantage is a higher chemical demand.  

An option for fewer stages is the combination of acid hydrolysis of hexenuronic acids with chlorine dioxide delignification. The sequences D₀A_hot-Eop-D₁ or DA_hot-P represent this alternative. With very high charges of chlorine dioxide it is possible to reach 90 %ISO brightness; however, the resulting viscosity is low as result of the extended treatment at low pH and high temperature. A higher COD load indicates yield losses.

A very attractive option is the combination of an ozone stage with a directly added chlorine dioxide treatment. Removal of double bonds, lignin and hexenuronic acids, is very good and a high brightness stability results. The total electricity demand of the Z/D-P treatment is lower as is investment. One high consistency ozone reactor is followed by two medium consistency stages for D and P and two washers complete the equipment. Less electrical energy is needed driving machines. This compensates a higher requirement for chemical generation.

OBJECTIVE

Comparison of bleaching chemicals requirement in four, three and two stages bleaching to identical final brightness (>90 %ISO); comparison of electricity demand for chemicals generation and operation of bleach plant.

EXPERIMENTAL

All trials were run with commercially produced oxygen delignified eucalyptus kraft pulps. D and P stages were run in sealed plastic bags at 10% consistency with variation of time and temperature. All pulp samples were preheated to the bleaching temperature, and all the chemicals were added at once and mixed thoroughly with the pulp.

Pressurised peroxide stages (Eop) were conducted in a laboratory scale stainless steel high shear mixer at 10% consistency with variation of time and temperature, typically with an oxygen pressure of 0.3 MPa. Ozonation was conducted at high consistency (>30%) with fluffed pulp in a fluidised bed reactor¹⁾, pulp was diluted to 10% consistency directly after ozonation and immediately mixed with ClO₂.

After each bleaching stage pulps were repulped at 2% consistency and dewatered on a Buchner funnel without further washing. Samples of the effluent were used to measure the amount of COD. Any peroxide residual was decomposed with catalase before analysis of COD. COD was determined according to the Zellcheming Merkblatt X/2/76. Brightness and viscosity was measured according to the TAPPI standards. For ageing a hot and humid treatment, the former Tappi T260 standard was applied.

INTRODUCTION

The counting of the number of bleaching stages in this paper does not include the oxygen delignification step; this stage is seen as an integrated part of the pulping process. In addition only ECF bleaching is considered as a realistic option. Consequently only chlorine dioxide, hydrogen peroxide and ozone are applied in the experiments described.

Bleaching of pulp requires several stages because one bleaching chemical alone is not capable to oxidise all chemical structures remaining in pulp after the pulping process, at least without destroying the cellulose. Ozone would bleach pulp totally; however, degradation of the cellulose would be unavoidable at the amounts required.

Normally in bleaching acidic and alkaline stages are applied alternating, taking advantage of the improved solubility of oxidised lignin under alkaline conditions. Nowadays four bleaching stages represent the-state-of-the-art for most market pulp mills. Only integrated mills with a lower brightness requirement operate three stages bleach plants, e. g. a D₀EopD₁ sequence. With hardwood pulp they reach typically 87 to 88 %ISO brightness. However, it is not impossible to achieve a higher brightness with only three stages. The reason why most pulp mills prefer longer sequences first of all is the preference to run the process with a low chemicals consumption and secondly the significantly higher flexibility. The higher the number of stages, the easier difficulties, like an unusually high lignin residual or a malfunction of washers can be compensated. Off grade quality becomes much less likely. In consequence short sequence bleaching is the compromise between higher or lower investment and a very flexible operation or a very simple but riskier process. It is the task of this paper to shed some light on the options and to compare advantages and disadvantages. 

REFERENCE SEQUENCES

The conventional sequence for bleaching eucalyptus kraft pulp has four stages. Equipment suppliers recommend a D₀EopD₁D₂ sequence. Their concession towards lower investment costs is the elimination of the washer between the final stages. The alternative is a D₀EopD₁P sequence, which is definitively more economical from a chemicals consumption point of view^{2, 3)}.

Table 1 has the typical amounts consumed for the different chemicals. The possibility to wash the pulp several times and remove the oxidised lignin stepwise does not require a high charge of chemicals in either one of the stages. There is no need to push lignin oxidation and extraction. The Kappa factor for the amount of ClO₂ applied in the D₀ stage can be low with a value as low as 0.17. The consequence is a moderate oxidation of the lignin and a low yield loss. At the same time viscosity and strength are protected from degradation.

Table 1: Amounts of chemicals consumed in D₀EopD₁D₂ and D₀EopD₁P bleaching of oxygen delignified eucalyptus kraft pulp to reach 90⁺ %ISO brightness. All amounts in kg per ton

sequence	ClO2	NaOH	O2	H2O2	COD
D0EopD1D2	34	12	5	5	23.1
D0EopD1P	27	15	5	7	23.3

The COD reaches just 23 kg/t, with understandably the high amounts resulting from the D₀ (6.6 kg/t) and the Eop stage (10.6 kg/t). The final stages contribute only marginally to the COD load; they are indeed just "cleaning" stages. The brightness achieved with this input of chemicals is >90 %ISO. Brightness stability is very good, in moist accelerated heat ageing the pulp loses around four points with a final D stage and three points after a final P stage. End viscosity is between 15 and 20 mPa^.s.

ELECTRICITY DEMAND in BLEACHING

Caustic soda, chlorine dioxide and ozone are generated using electrical energy. Caustic soda is produced in modern membrane cells from sodium chloride with an input of about 2.500 kWh/t⁴⁾. Because of the cogeneration of chlorine and hydrogen the amount of energy attributed to caustic soda was set to only 1.250 kWh/t (1.25 kWh/kg).

Sodium chlorate is generated from 565 kg NaCl and 4.535 kWh⁵⁾. In order to generate 1 ton of chlorine dioxide approximately 1.7 tons of chlorate are required. The amount changes a bit depending on the process for the generation. A value of 3 kWh/kg for active chlorine was used in the calculation.

Ozone is generated from oxygen in modern medium frequency generators with an input of only about 10 kWh/kg⁶⁾. This is an average value; the demand for energy depends on the concentration of ozone in the oxygen. If the required concentration is very high, the energy demand increases. Oxygen is made by compression and expansion using either pressure swing generators or cryogenic processes. Because the excess of oxygen in the ozone containing gas can be used in the oxygen stage or in other process steps in the mill, the energy required for O₂ generation is not included in the comparison.

Hydrogen peroxide is generated normally using hydrogen from natural gas. The production certainly requires some electricity; however, the demand is rather low compared to the electrochemical processes for caustic soda or chlorate.

In addition electricity is required for pumping and mixing of the pulp and to operate the washers. In modern pulp mills the demand can be as low as 30 kWh/t for each bleaching stage. For a four stages bleach plant therefore in first approximation a demand for about 120 kWh/t can be assumed. A three stages bleach plant will need 90 kWh/t and a two stages bleach plant 60 kWh/t. This assumption ignores the differences between stages, however, it is good enough to start a comparison. These values are on the low side; smaller or older mills do have a significantly higher specific electrical energy demand for their bleaching steps. From the data in table 2 the high impact of the electrochemical generation of chlorine dioxide and for the electrical drives becomes visible.

Table 2: Electricity requirement for bleaching in four stages, demand in kWh/t of pulp

sequence	NaOH	ClO2 (act. Cl)	electricity for drives, pumps	total electricity
D0EopD1D2	15	100	120	235
D0EopD1P	19	80	120	220

• In a four-stage sequence more electricity is required for motors than for the generation of bleaching chemical.

THREE STAGE BLEACHING

In three stages bleaching the number of options for the stages in reality does not change. In ECF bleaching there is no alternative to a start of the sequence with either chlorine dioxide or ozone. In principle the sequence remains similar to the four stages option, just the final stage is cut off. For the sequence without ozone this means a D₀EopD₁ treatment. Eiras and Colodette⁷⁾ combined a hot D₀ stage with Eop and a final D stage and needed in total an amount of 3.9 % active chlorine to reach 90 ± 0.5 %ISO. The consequence of the elimination of the final D or P stage is an increased demand for active chlorine in the D₀ and in the D₁ stage. The Kappa factor increases from a value below 0.2 in the four-stage sequence to 0.3.

Table 3 compares the demand of chemicals required to reach a final brightness of 90⁺ %ISO with the stages D₀EopD₁. Without hydrolysing (hot) conditions in the D₀ stage the complete removal of hexenuronic acids is difficult. As long as hexenuronic acids are present in the final pulp, yellowing of the pulp in heat ageing takes place more easily. Thus a high input of chlorine dioxide is needed to make sure all hexenuronic acids are degraded. Another effect also causes a high demand for chlorine dioxide: Provided temperature and retention time is high enough, nearly all amounts of chlorine dioxide applied will be consumed. Chlorine dioxide reacts not only with the lignin in the fibre but also with already dissolved lignin and oxidises it further. This effect is one of the limitations restricting the development of very high brightness by simply increasing the amount of ClO₂ applied in D₁ or D₂⁸⁾.

Table 3: Chemicals required to reach 90⁺ %ISO with three stages: D₀EopD₁. Kappa factor in D₀ 0.3; amounts in kg/t. D₀ at 70°C, 1h, pH <3; Eop 90°C, 1.5h; D₁ 75°C, 2h    

ClO2	NaOH	O2	H2O2	total COD
45	12	5	6	25

The COD load increases slightly because of the more drastic conditions required in all stages. Viscosity decreases to 16.5 mPa^.s and brightness loss in accelerated heat ageing increases dramatically. It now reaches 8 points. This high reversion is not tolerable.

Regarding the total demand for electrical energy, the cut of one bleaching stage down to three nearly balances the higher requirement for chlorine dioxide. The addition of all values gives a total demand of 238 kWh/t (Table 4).

Table 4: Demand for chemicals in bleaching eucalyptus kraft pulp in 4 and 3 stages to a brightness >90 %ISO. The amount of oxygen applied is constant at 5kg/t in all sequences

sequence	demand for chemicals (kg/t)				total electricity (kWh/t)	viscosity (mPa.s)	ageing loss (T260) (points)
	ClO2	H2O2	O3	NaOH
D0EopD1P	27	7	--	15	220	19.5	3.5
D0EopD1	45	6	--	12	238	16.5	8

An option to improve the results is the application of ozone in combination with the first chlorine dioxide treatment. This does not necessarily mean the addition of an extra stage. Ozone reacts very fast with pulp and requires acidic conditions as chlorine dioxide. Therefore it is possible to add ClO₂ directly after ozone. Washing is not required.

Figure 1 shows the impact of increasing amounts of ozone on the Kappa number with a Z-E treatment. The analysis of the Kappa number does not allow differentiating between lignin and hexenuronic acids. This becomes possible after an additional treatment. Hot acid hydrolysis removes hexenuronic acids and leaves the lignin in the pulp. After oxygen delignification the pulp had a Kappa number of 11.2.

Fig. 1: Decrease of the Kappa number with application of increasing amounts of ozone or with acid hydrolysis. Treatment of oxygen delignified eucalyptus kraft pulp, Kappa 11.2, ozonation at high consistency followed by extraction with 0.5% NaOH, 70°C, 0.5h, 10% cons., subsequent acid hydrolysis at pH <3, 95°C, 2h

A hot acid treatment decreases this Kappa number to 5.5. The amount of permanganate consuming compounds, which can be removed by hydrolysis, is very high. This demonstrates on one hand the effectiveness of the oxygen stage; it has obviously removed more than 50 % of the real lignin. After the oxygen stage only 5.5 Kappa units can be attributed to lignin. On the other hand this shows clearly how much of the Kappa number is hexenuronic acid, more than half of the Kappa number is not lignin. The application of increasing amounts of ozone followed by an extraction decreases the Kappa number. If this Z-E treatment is in addition followed by a hydrolysis step, it becomes possible to estimate how much lignin and how much hexenuronic acid are removed with ozone. The ozone application decreases the part of the Kappa number which can be attributed to hexenuronic acid faster than the residual, which has to be lignin. Consequently, the application of ozone is a good tool to remove hexenuronic acids from pulp.

Fig. 2: Kappa number after extraction (with Eop) following a D₀Eop or a Z/D₀Eop treatment; ozonation at >30%cons., 40°C, D₀ at 10% cons., 60°C, 1h; Eop with 1.2% NaOH, 0.6% H₂O₂, 0.3 MPa O₂ pressure, 85°C, 10% cons.

The combined application of only 0.4% ozone plus 1.8% chlorine dioxide have a much high impact on the decrease of the Kappa number than twice the amount of chlorine dioxide applied alone. The combination of both chemicals in one stage allows a much better performance of a three stages process. This becomes visible in figure 2, which shows the Kappa number after the Eop stage following a Z/D or a D treatment. The Z/D₀EopD₁ sequence needs fewer chemicals and gives a more stable pulp. This becomes obvious in the comparison in Table 5, which adds the ozone pre-treatment ahead of the chlorine dioxide. The value for reversion is still higher compared to the reference; however, it is much better with the ozone stage than without.

The three-stage process compared to the four-stage reference produces a pulp with a lower viscosity and a higher sensitivity towards ageing. If only chlorine dioxide and peroxide are applied, the total demand for chlorine dioxide becomes significantly higher. The application of ozone allows keeping this increase very moderate. It decreases on the other hand the final viscosity.

Table 5: Demand for chemicals in bleaching eucalyptus kraft pulp in 4 and 3 stages to a brightness >90 %ISO. The amount of oxygen applied is constant at 5kg/t in all sequences

sequence	demand for chemicals (kg/t)				electricity for chemicals	total electricity (kWh/t)	viscosity (mPa.s)	ageing loss (T260) (points)
	ClO2	H2O2	O3	NaOH	(kWh/t)
D0EopD1P	27	7	--	15	100	220	19.5	3.5
D0EopD1	45	6	--	12	150	240	16.5	8
Z/D0EopD1	26	5	4	12	133	223	13.2	5.5

• Conventional conditions in a three-stage sequence (D₀EopD₁) are not attractive. Brightness stability is poor and energy demand about 10% higher.
• The inclusion of ozone in the sequence (Z/D₀EopD₁) improves the result moderately. Energy demand becomes identical at slightly lower brightness stability and viscosity compared to the reference.

TWO STAGE BLEACHING

The brightness achieved with the first two stages in the three-stage version was already very high. Even in the four stages version Eop brightness was between 81 and 82 %ISO. With three stages and a high input of ClO₂ values up to 84 %ISO are achieved. This is not very far away from the target of 90⁺ %ISO. Thus it is an only consequent question, whether not bleaching in even less, namely two stages would be possible. Would not a very high charge of ClO₂ in the D₀ stage and a very high input of H₂O₂ in a hot final P stage allow a top brightness?

The problem of very high charges of chlorine dioxide in a D₀ stage is its limited consumption under the normally moderate conditions. At 50°C and with only one hour retention time a high residual of ClO₂ remains. This problem was overcome easily in the three stages experiments by a temperature increase to 70°C. Doubling of the chlorine dioxide amount from the "normal" Kappa factor level now becomes possible. Figure 3 shows the impact of the application of very high charges of chlorine dioxide at 70°C on final brightness with the short sequence D-P. The application of very high amounts of peroxide did not lift the brightness high enough at lower ClO₂ inputs (Kappa factor 0.25). However, with a very high amount of ClO₂ (Kappa factor 0.35) an input of 3% H₂O₂ was sufficient to reach 90 %ISO.

The demand for hydrogen peroxide is rather high. To increase the brightness from 89.3 to 90 .1 requires the doubling of the peroxide input. This indicates how close together in such a sequence either success or failure is. Small changes will have no impact at all and even bigger changes not necessarily allow predicting the success. On the other hand, between the application of Kappa factor 0.25 and 0.35 in the D₀ stage something of importance seems to change. Figure 4 shows the impact of more ClO₂ on the Kappa number after a subsequent extraction.

There is obviously a minimum Kappa number required to allow the brightness to increase above the 90% level. The Kappa number obviously needs to be below 2 to allow a top brightness.

Fig. 3: Effect of increasing the active chlorine input in the D₀ stage on brightness after a subsequent P stage. D₀ at 70°C, 1h, P stage at 90°C, 2h, with 1.5% NaOH activation

Fig. 4: Effect of a sharp increase of the chlorine dioxide amount in D₀ on Kappa after subsequent extraction or peroxide bleaching. D₀ at 70°C, 1h, pH <3; E at 75°C with 1.5% NaOH, 1.5h; P with 1.5% NaOH at 90°C, 2h

Fig. 5: Brightness stability and viscosity resulting with four stages or two stages bleaching. Very high input of ClO₂ and H₂O₂ in two stages bleaching, D₀ with factor 0.35 at 70°C, 1h; P with 2% H₂O₂, 90°C, 2h

The pulp viscosity suffers visibly from the very high input of chemical. The very high temperature obviously causes a lot of radical side reactions. Very high ClO₂ and H₂O₂ input obviously are negative. The brightness is not very stable; the losses are by far higher compared with our reference. Thus, the cellulose not only gets degraded; it also becomes oxidised by the drastic reaction conditions. Fig. 5 compares the viscosity and ageing data with the reference pulp.

As mentioned, Eiras and Colodette achieved a brightness ± 90% ISO in only three stages using hot acid hydrolysis in the D₀ stage. In a four stages sequence hot acid hydrolysis would be applied to lower the demand for ClO₂ in the D₀ stage. It could be a tool in two stages bleaching to reach a low Kappa number without the need for an extreme ClO₂ input. Figure 6 shows the resulting Kappa number using different amounts of ClO₂ and retention time in a hot D0 stage followed by a P stage. The input of NaOH and H₂O₂ were held constant.

Fig. 6: Decrease of the Kappa number with different amounts of ClO₂ applied at 90°C followed by prolonged time for hexenuronic acids hydrolysis. Kappa determined after P stage. Sequence D₀A-P, all at 10% cons. pH in D₀ <2.6; P with 1.5% NaOH, 3% H₂O₂ at 90°C

The longer the time at low pH, the lower the Kappa number becomes. Hot acid hydrolysis clearly removes the hexenuronic acids. Ageing values for the pulp are very good. If the retention time is too short, the Kappa number stays high even at a high input of chlorine dioxide.

Table 6: Effect of increasing ClO₂ addition ahead of hexenuronic acids hydrolysis on final brightness and viscosity. D₀/A stage at 90°C, 3h, pH 2.5; P stage with 3% H₂O₂, 90°C, 2h

active chlorine factor	Kappa after P	brightness (%ISO)	viscosity (mPa.s)	aging loss (points)
0.2	1.6	87.2	8.9	4.5
0.25	1.3	87.9	8.7	3.8
0.3	1.0	88.6	8.3	3.1

Thus hot hydrolysis seems to be the right tool to lower the "lignin" residual and allow easy bleaching. Unfortunately there are other negative side effects. First of all the brightness does not increase as high as expected. All peroxide stages show a residual, so decomposition could not be the problem. On top of this viscosity drops to very low levels. Table 6 shows the results for increasing input of ClO₂ with 3 hours time for hydrolysis.

The prolonged hot acid treatment has a negative impact also on the COD load. Fig. 7 compares the COD load with our reference sequence. The damage done to the pulp in the acid stage results in a sharp increase of the COD in the D₀ stage and a further rise in the alkaline stage. There the shortened cellulose chains are solubilized. The increase in COD is a sign for yield losses. The impossibility to reach the target brightness further shows that a low Kappa number per se does not mean all chromophores are easily oxidized. Thus the combination of an application of high amounts of ClO₂ with a hot acid treatment cannot be recommended as the right tool to bleach with only two stages. Hot acid hydrolysis can only be used for moderate amounts of ClO₂ in four or three stage sequences.

Fig. 7: COD generated in four stages bleaching or in two stages with acid hydrolysis combined with high charges of ClO₂

Like in the three-stage alternative already described, this leaves the option to combine an ozone treatment with a chlorine dioxide stage. The effect of the ozone application is a visible decrease of the required amounts for chlorine dioxide and hydrogen peroxide to reach full brightness. Figures 1 and 2 already explain the impact of the combination of ozone with chlorine dioxide on lignin removal. The Kappa number decreases to very low levels with moderate amounts of ozone and additional chlorine dioxide. The only difference to the three stages sequence is a higher charge of hydrogen peroxide applied in the Ep stage. This makes the extraction stage more a P stage than an Ep stage.

Fig. 8: Impact of the combination of ozone and chlorine dioxide on Kappa number decrease in Z/D-P (E) or D-P (E) treatment. Eucalyptus kraft pulp Kappa 11.8 after oxygen delignification. Ozone charge 0.4%, act. Cl amount corresponds to Kappa factor 0.25 or 0.35, D stage at 70°C, 1h, E stage at 75°C, 1.5h, 1.5% NaOH; P with 2% H₂O₂ at 90°C, 2h

Figure 8 shows the delignification achieved with O₃ and increasing amounts of ClO₂. Because ozone is added ahead of the chlorine dioxide addition, it would not be correct to give a Kappa factor for the ClO₂ amount. The impact of the ozone treatment on the "lignin" removal is extremely pronounced. While the treatment with chlorine dioxide only even at a high active chlorine addition leaves a lot of "double" bonds in the pulp, the ozonated pulp has very little residual. The impact of hydrogen peroxide – the difference between E and P treatment - on this residual is visible but not very high.

Brightness increase follows the decrease of the double bonds. The impact of the ozone plus chlorine dioxide treatment results in a very low Kappa pulp with an improved bleachability. The bleachability is much lower if only ClO₂ is applied. In this case the target brightness is only reached with a very high input of ClO₂ and H₂O₂. The synergy between ozone and chlorine dioxide is obvious.

Fig. 9: Increase of brightness using the sequences D-P or Z/D-P for two different acts. Chlorine amounts, effect of different peroxide charges (1.5% and 2% H₂O₂), other conditions see Fig. 8

Brightness increase follows the decrease of the double bonds. The impact of the ozone plus chlorine dioxide treatment results in a very low Kappa pulp with an improved bleachability. The bleachability is much lower if only ClO₂ is applied. In this case the target brightness is only reached with a very high input of ClO₂ and H₂O₂. The synergy between ozone and chlorine dioxide is obvious.

It is now possible to calculate the chemical demand for such a short bleaching sequence. The ozone and the chlorine dioxide charge have to be high enough to remove hexenuronic acids and lignin. With the information from figures 1 and 2 and in addition 8 and 9 the demand for ozone can be estimated to be between 0.3% and 0.5%. The chlorine dioxide demand similarly can be estimated to be between 2.5% and 3% as active chlorine. These data are used for the calculation of the electricity demand.

Fig. 10: Impact of the ClO₂ addition in the Z/D stage on brightness increase with hydrogen peroxide. Ozone charge constant at 0.44%, P stage at 90°C, 2h

Figure 10 in addition allows estimating the demand for hydrogen peroxide. The 90 %ISO brightness level is passed clearly with an input between 1% to 1.5% H₂O₂. Table 7 compares all data with the reference and the three and two stage alternatives.

Not unexpected, the demand for bleaching chemical is higher in the shorter sequences. However, the differences are not very big. There is more ClO₂ required, in addition ozone and more hydrogen peroxide. This results in a higher electricity demand for chemicals. In total this is more than compensated by the savings in the electricity requirement for pumps and other drives. In part this is also the result of the independence of the hydrogen peroxide production from electricity. This raises the question why the longer sequences still are the standard? One topic becomes obvious by a comparison of the viscosity values. The application of ozone and higher amounts of hydrogen peroxide results in a lower final viscosity. This is a drawback for the very short sequences. On the other hand brightness stability is not bad even for the very short 2½ stages sequence. As long as the lignin and the hexenuronic acids level in the final pulp are low enough, reversion seems not to represent a problem.

Table 7: Comparison of bleaching with four, three and two stages to reach 90⁺ %ISO brightness, chemical demand in kg/t, electricity in kWh/t. The chemical amounts represent the optimized input for the four stages sequence and the "safe side" for the three and two stages sequence.

sequence	ClO2	H2O2	O3	NaOH	electricity for drives	total electricity	viscosity (mPa.s)	ageing loss (points)
D0EopD1P	27	7	--	15	120	220	19.5	3.5
Z/D0EopD1	26	5	4	12	90	223	13.2	5.5
Z/D0P	30	15	4.5	12	60	210	10.2	3.5

• The conditions required in a two-stage sequence for the combination of chlorine dioxide and hot acid hydrolysis are too drastic to be acceptable. Yield and viscosity losses become too high.
• Despite a high demand for bleaching chemical the very short Z/DP sequence has a lower demand for electrical energy and good brightness stability. The drawback is a lower viscosity.

COMPARISON of QUALITY and COST

The conventional approach with four bleaching stages requires the highest investment. With the assumption of cost of about 5 to 7 million US$ for a washer and a tower, the four stages sequences requires 20 to 28 million $ of investment. In contrast, a two and a half stages sequence would need only 10 to 15 million $ in direct equipment; two towers and two washers plus the high consistency ozone reactor. It would on the other hand require a slightly bigger electrolysis for chlorate and the generator for ozone. Still investment and capital cost would be significantly lower.

On the electrical energy side the differences are not very pronounced at all. The combination of ozone and chlorine dioxide for delignification and hydrogen peroxide for brightness increase is rather effective. The slightly higher demand for chemical is more than compensated by the elimination of two washers and two towers and the required motors and pumps.

The higher demand for bleaching chemicals is difficult to translate into cost. Depending on the location mills run their own electrolysis for caustic soda and sodium chlorate generation. An optimised operation might in addition produce an excess of electricity, which might or might not be sold to a public net, where it might or might not generate good revenue. Credits for the on site generated by-product salt cake are as well different from mill to mill. Thus it would be of limited use to put in some regional costs for ClO₂ or H₂O₂. With the amounts given, anybody can do simple math and calculate the correct numbers with the locally valid prices.

Table 8 summarises the advantages and disadvantages for "normal" and "very short" sequences. On the pulp quality side the lower viscosity is the only negative parameter. It might be a parameter with limited relevance in some markets. The most negative thing to say against the very short sequence is its inflexibility. If there is a problem in one of the stages, no chance for compensation or a bypass exists. Consequently such a short sequence requires a perfect maintenance of all important parts and in addition a perfect team in the control room. The short sequence will give limited options for small adjustments in the chemical dosage. It will be important to stay all the time on the safe side and this means to stay on the high side of the chemical addition. Anything else would mean to take chances and end with off grade product.

Table 8: Comparison of advantages and disadvantages of long and short bleaching sequences

	long sequence	short sequence
high brightness	yes	yes
low reversion	yes	yes
high viscosity	yes	no
low electricity demand	yes	yes
low investment	no	yes
low maintenance cost	no	yes
flexibility during operational problems	yes	no

CONCLUSIONS

Ozone is a very good tool to remove hexenuronic acids and improve brightness stability.

The combination of a treatment with ozone and chlorine dioxide followed by a hydrogen peroxide stage allows bleaching of eucalyptus kraft pulp in only two stages (Z/D-P) to >90 %ISO brightness at very good stability.

The following was found to be valid for such a very short sequence:

• Bleaching chemical demand is higher, there is slightly more ClO₂ required, in addition O₃ and more H₂O₂,
• Total demand for electrical energy is lower despite the higher demand for the generation of chlorate and ozone,
• Investment and maintenance costs become significantly lower,
• Bleaching basically becomes much simpler.
• This should offset the higher chemical cost.

LITERATURE cited

1. Nimmerfroh, N., Süss, H.U., Hafner, V., Überlegungen zum großtechnischen Einsatz von Ozon zur Zellstoffbleiche, Wochenbl. f. Papierfab. 120 (21) 860(1992)

2. dos Santos, C. A., Süss, H. U., Mambrim Filho, O., Flexibilização da sequência de branqueamento ECF da Bahia Sul Celulose s. a.; ABTCP 28th Annual Congress, (1995)

3. Henrique, P. M., Costa, M. M., Correia, F. M., Fonseca, M. J., Santos, J. R., Landim, A. B., Leporini; C., Hydrogen peroxide in an ECF bleach plant: Cenibra's industrial experience; Tappi Pulping Conference 2001

4. Minz, F.-R.; Production of sodium hydroxide solution; in: Ullmann's encycl. of industrial chemistry, Vol A24, 348, (1993)

5. Elektrizitätsbedarf zur Herstellung von Natriumchlorat, in: Greenwood, N. N., Earnshaw, A., Chemie der Elemente, 1116, ISBN 3-527-26169-9, VCH, (1988)

6. Ozone generation, Energy requirements and efficiency, in: Kirk-Othmer, Encyclopedia of Chemical Technology, Vol 17, 975 (1996)

7. K. M. Eiras, J. L. Colodette, Optimization of the high temperature chlorine dioxide stage (D_HT) for hardwood kraft pulp; Tappi Pulping Conference, 2001

8. Suess, H. U., Leporini Filho, C., Schmidt, K., Bleaching of eucalyptus kraft pulp to very high brightness; ABTCP annual meeting, (2000)

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