1.INTRODUCTION

^{Milagres et al. (2011)} consider that the kraft pulps of Eucalyptus spp. are relatively easy to bleach because they present a higher quantity of hexenuronic acids. As a result, many studies have been done in order to decrease the number of stages of bleaching sequences and to reduce the costs of the process.

Setting up a modern large (3,000 tons/day) bleach plant with five stages (including the stage of oxygen delignification) for the pulp of Eucalyptus spp. requires investments of approximately 40 to 50 million dollars (^{Milanez et al., 2008}; ^{Milagres et al., 2011}). Due to this considerable cost, studies aiming to minimize the costs of implementing new bleaching sequences have been gaining recognition. One possible option in this case is reducing the number of bleaching stages, in other words, the preference for short sequences, taking into account that each stage incurred costs of approximately 10 million dollars (^{Milanez et al., 2008}).

Given the fact that the modern processes of pulping and washing are providing increasingly brighter pulps, there is a growing tendency for reducing the number of bleaching stages. ^{Colodette and Santos (2015)} argue that nowadays three-stage sequences provide effective results concerning brightness since they are considered sufficient to bleach the kraft pulp of the eucalyptus that was previously delignified with oxygen.

Thus, the objective of this study was to evaluate the physico-mechanical properties of two short bleaching sequences.

2.MATERIAL AND METHODS

In this study, the pre-delignified industrial pulp of eucalyptus was used with oxygen. The initial characteristics of the pulp were: 60.4% ISO brightness and kappa number 9.8.

Two reference bleaching sequence has been used, D₀(E+P)D₁P and A_HTD₀(E+P)D₁P, and other two bleaching sequences, A_HTDP and A_HTD(PO), were used as alternatives (short sequences). The Conditions are presented in Table 1.

Bleaching in non-pressurized stages was performed in polyethylene bags with representative samples of the pulp. Bleach liquor was added to the pulp at room temperature. Samples of the residual pulp liquor were extracted in the end of reaction, in order to conduct the pH analysis. The pulp was washed using a washing factor of 9 m³/t dry basis of clean water.

The pressurized hydrogen peroxide (PO) stage was performed in an autoclave system, with four individual stainless steel reactors, each with a capacity of 1.5 liters and temperature and pressure settings. For each capsule, 100 g dry basis of pulp was mixed with a bleach liquor, which is comprised by water, NaOH and H₂O₂. After the set temperature was reached, oxygen was applied to keep 500 kPa pressure. At the end of the stage, the pressure was relieved. Samples of the residual pulp liquor were extracted in order to conduct the pH analysis. The pulp was washed using a washing factor of 9 m³/t dry basis of clean water.

The bleached pulps were refined in a PFI mill at four levels: 0, 1000, 2000 and 3000 revolutions.

Compression resistance, smoothness, and tear resistance were performed on handsheets with grammage of 60 g.m^-2 acoording TAPPI 826 pm-92, TAPPI 479 cm-99 and TAPPI 414 om-98. The analyses of brightness and brightness reversion, according to the methods TAPPI 525 om 92 and TAPPI 525 om-92 (4 h, 105 ± 3 ºC, 0%UR).

The total costs of bleaching were calculated taking into account the quantity of reagents used in each stage (kg/odt) and the price of each reagent used (US$/ kg). For this calculation, were used chemicals prices (US$/kg of product) according ^{Favaro et al., (2014)}: chlorine dioxide = 1.0; hydrogen peroxide = 0.85; oxygen = 0.10; sodium hydroxide = 0.50; sulfuric acid= 0.08.

In order to evaluate the statistical difference between the average brightness and brightness reversion, the Test Method T was employed. In order to evaluate the differences between the values of compression indexes, smoothness, and tear indexes, the analysis of variance (ANOVA) was adopted. Tukey's multiple comparison tests were conducted among the bleaching sequences in order to find which of these sequences presented the most significant differences. The significance level adopted in the statistical analyses was 5%.

3.RESULTS

Physico-mechanical Test

In order to evaluate the impact of the different bleaching sequences, three tests were performed: compression resistance, smoothness, and tear resistance. Figure 1 describes the compression indexes found for each bleaching sequence.

Figure 1 Compression index for each bleaching sequence. 

Figura 1 Índices de compressão para cada sequência de branqueamento. 

Figure 2 describes the smoothness found for each bleaching sequence.

Figure 2 Smoothness for each bleaching sequence. 

Figura 2 Lisura para cada sequência de branqueamento. 

Figure 3 describes the tear indexes found for each bleaching sequence

Figure 3 Tear index for each bleaching sequence. 

Figura 3 Índice de rasgo para cada sequência de branqueamento. 

4.DISCUSSION

5.CONCLUSION

For a cellulose industry to adopt a new bleaching sequence, it is necessary to carry out many studies that focus on the quality of the final product and the costs of the process. This study permitted verifying that the short sequences, compared with long and reference sequences, presented lower brightness reversion, lower final cost of the bleaching reagents and higher smoothness. However, they presented worse performance regarding the tear index.