THE EFFECTS OF ALKALINE FILTRATE RECIRCULATION TOWARDS THE PROPERTIES OF LONG FIBER PULPS WITH OD(EPO)DED BLEACHING SEQUENCE

Circuit closure can reduce water consumption, but negative effects on pulp quality and equipment wear may make it unfeasible. This study aimed to evaluate how the alkaline fi ltrate of the recirculation stage (EP) affects bleaching, pulp quality and characteristics of the fi ltrates produced. Pre-delignifi ed cellulose pulp from a mixture of three coniferous woods was used. Bleaching followed the D(EPO)DED sequence, with the addition of 5, 10 and 15 kg.odt-1 of the alkaline fi ltrate (EP) to the pre-O2 pulp. The physical and mechanical cellulosic pulp properties were evaluated in the control and with 10 kg.odt-1 of the alkaline fi ltrate. The inorganic compound accumulation in the system was evaluated in the control pulp and with 15 kg.odt-1 of the alkaline fi ltrate. The fi ltrate use increased the ClO2 consumption for bleaching and sulfuric acid and NaOH to adjust the pH of the stages. The pulp tensile index was higher and the tear index lower with the fi ltrate use in pulp without refi nement, however the properties of refi ned pulp were similar between treatments. The fi ltrate increased the calcium, chloride, sodium and sulfate levels in the D stage and that of sodium and sulfate after the (EPO) stage. Filtrate recirculation can reduce water use, but it increases bleaching costs and metal accumulation in the system. v.24 n.3 2018 THE EFFECTS OF ALKALINE FILTRATE RECIRCULATION TOWARDS THE PROPERTIES OF LONG FIBER PULPS WITH OD(EPO)DED BLEACHING SEQUENCE


INTRODUCTION
Circuit closure can recycle water and chemical reagents, reducing pollutant loads (Frigieriet et al., 2015;Frigieri et al., 2016).Bleaching is the industrial stage with the highest water consumption and reversing this scenario would result in economic and environmental gains (Kansalet al., 2008;Kamali and Khodaparast, 2015).The alkaline bleaching filtrate has characteristics similar to those of the black liquor normally sent to the recovery boiler and could therefore be recirculated (Kamali and Khodaparast, 2015).
The chloride and metal concentrations are lower in the alkaline than in the acid filtrate, making its reuse more feasible (Huberet al., 2014).In addition, sodium can be recovered from alkaline filtrates, which have high color concentrations and biochemical (BOD) and chemical (COD) oxygen demands (Blackwell, 1992;Martin et al., 1996;Kamali and Khodaparast, 2015).
The objective of this study was to evaluate how recirculation of part of the alkaline extraction filtrate (EPO) affected bleaching, final pulp quality and the characteristics of the filtrates generated.The results obtained could help in the implantation of the closed circuits in the bleaching of cellulose pulp mills.

Sample collection and preparation
Pre-delignified cellulose pulp and bleaching filtrates were collected in a Kraft pulp mill in western Canada that uses a softwood mixture (Picea spp., Pinus spp.and Douglas fir).The pulp sample was washed with excess water to remove the maximum of impregnated carryover material and maintain its characteristics constant during the experiment (Table 1).

Recirculation of the alkaline filtrate of the D(EPO) DED sequence
Organic loads of five, 10 and 15 kg .odt -1 of the alkaline filtrate (EP) were added to the pulp after the pre-O 2 stage to simulate filtrate recirculation.These values are commonly found in effluents from pulp mills.

Pulp bleaching
The pulp was bleached with the D(EPO)DED sequence (Table 2).The "D" represents stages with chlorine dioxide, "E" with sodium hydroxide, "P" with sodium peroxide with alkali and "O" oxygen with alkali.The "()" represents the absence of washing between the stages.Bleaching was done in a pilot bleaching plant developed by Paprican (Berry, 2000).
The first bleaching was adjusted to achieve brightness of 89.5 ± 0.5% ISO in the pulp without organic load.This fixed dosage was used in the pulp bleaching with organic loads of five, 10 and 15 kg .odt -1 of the alkaline filtrate (EP) (Table 2).The pH at the end of stage D, ClO 2 consumption, kappa number, brightness after (EPO) stage and final brightness were evaluated (Table 2).
The second bleaching was carried out until the pulps of all treatments reached 89.5 ± 0.5% ISO.The ClO 2 consumption in the bleaching and consumption of H 2 SO 4 and NaOH to adjust the final pH were evaluated.

Physical and mechanical pulp characterization
After bleaching, the physical and mechanical pulp properties without carryover and with the addition of 10 kg .odt -1 were evaluated.The number of rotations for refinement was chosen to facilitate the development of pulp properties.Each test was replicated five times (Table 3).
The tear and tensile indexes, as well as pulp viscosity without refinement were compared using the t test at 5% significance.The statistical analysis of the identity model was performed to evaluate if the refinement level and pulp properties without carryover and with 10 kg .odt -1 could be adjusted in a model at 5% of significance.

Characterization of inorganic compounds in the system
The acetate, carbonate, chloride, chlorate, formate and sulfate content in the effluent were verified according to TAPPI 669 and the metal and total oxalate content according to TAPPI -266.

RESULTS AND DISCUSSION
Potassium permanganate consumption pulp with the presence of carryover from (EPO) filtrate Addition of the alkaline filtrate increased the pulp kappa number.The filtrate has large quantities of lignin and extractives (Huber et al., 2014), which increases chemical reagent consumption such as chlorine dioxide and ozone (Brogdon et al., 2015, Sarto et al., 2015) (Figure 1).
The carryover presence from the (EPO) stage increased the potassium permanganate consumption linearly by 17% with an organic load at 15 kg .odt -1 .Similar behavior was reported for Eucalyptus globulus pulp (Barroca et al., 2002), but to a lesser extent.This happened because the alkaline filtrate compounds of the bleaching sequence from long fiber pulp consumed more potassium permanganate than the resultant of the bleaching sequence from hardwoods (Brogdonet al., 2015).

Carryover recirculation and consumption of chemical reagents
The carryover uses increased pH values, kappa number and reduced pulp brightness after the (EPO) stage and after complete bleaching sequence (Table 4).
Recirculation of 15 kg .odt -1 of carryover increased the kappa number by 29.9% and decreased brightness by 8.23% and 1.56% after (EPO) stage and after the Final pH 2.8-3.210-10.5 3,0-3,5 9.5-10 3.5-4.5  entire bleaching sequence, respectively.Therefore, the kappa number increase was more significant, indicating the presence of permanganate oxidizable filtrate compounds, which do not affect pulp brightness (Sezgiet al., 2016).In addition, the greater reduction in brightness after the (EPO) stage indicates that those which followed partially corrected the negative impact of the carryover.
The addition of the alkaline filtrate to the pre-O 2 pulp, mainly with 15 kg .odt -1 required pH adjustment in the D stage.Increasing the pH reduces the performance during this stage, which can be evidenced by the increased of chlorine dioxide consumption (Sevastyanova CERNE SOUZA et al. et al., 2012;Sezgi et al., 2016).Above pH 4.0, chlorite formation occurs (ClO 2 -), reducing the chlorine dioxide delignifi cation rate.The chemical reagent loss in D stage is lower in pH from 3.0 to 4.0.However, formation of chlorate (ClO 3 -) at pH below 2.0 is also detrimental to the dioxidation stage (Dence e Reeve, 1996).
The increase in the chlorine dioxide (ClO 2 ), sulfuric acid (H 2 SO 4 ) and sodium hydroxide (NaOH) load was required to reach 89.5 ± 0.5% ISO in pulps with carryover (Table 5).The increase in the chlorine dioxide (ClO 2 ) consumption is due to the organic load in the fi ltrate (Brogdon., 2015;Wilke et al., 2015).The higher pH of the D stage with 10 and 15 kg .odt -1 of carryover required the use of H 2 SO 4 for its reduction (table 5).The NaOH consumption did not increase with 5 kg .odt -1 of carryover, however, the use of 10 and 15 kg .odt -1 of carryover increased consumption by 18 and 19%, respectively.The ClO 2 consumption generates organic acids, consuming the NaOH and, therefore, the load of the latter must be increased in the subsequent stages (Biermann, 1996;Ribeiro et al., 2014;Wilke et al., 2015).
Increasing reagent consumption in bleaching is detrimental to the economic viability of the sector, because from softwoods depends on the tracheid quality, the tear index is better related to tracheid strength and tensile index with the number of connections between these structures (Biermann, 1996;Gharehkhaniaet al., 2015).Therefore, changes during bleaching with carryover addition might affect the interactions between them.
The use of carryover had damaged the tracheids connections, thereby reducing the tensile index.The this is the most expensive industrial step in the pulp and paper manufacturing process (Perng and Wang, 2016).

Alkaline fi ltrate recirculation and cellulosic pulp quality
The tensile index of the control sequence was 23% higher than that with 10 kg/odt of carryover in the unrefi ned pulp.Increasing the chlorine dioxide consumption during bleaching in the sequence with carryover may cause greater fi ber degradation (Feria et al., 2013;Ribeiro et al., 2014).This was not recorded in this research, because the pulp tear index was 16% higher and viscosity was similar in the sequence with 10 kg .odt -1 of carryover (Table 6).The paper resistance refi nement of the cellulosic pulp can reduce this problem by increasing the connection numbers between tracheids (Biermann, 1996;Bhardwajet al., 2007;Gharehkhania et al., 2015).Thus, in all evaluated parameters, the regression models generated for the two pulp types (with and without carryover) can be adjusted to one model (Figure 2).Refining increased the pulp tensile index, the parameter that depends mainly on the connection numbers between the tracheids.This also occurred with drainage, because the greater connection numbers hamper the water passing through the pulp (Gharehkhaniaet al., 2015).This behavior was reported for cellulose pulp from grasses (Andrade e Colodette, 2016), softwoods (Chen et al., 2016;Chen et al., 2017) and hardwoods (Zanuncio et al., 2016).
Refinement reduced the tear index, a parameter mainly dependent on the tracheid quality, which are degraded during refinement.The softwood tear index was reduced, even at low levels of refinement, as reported for Pinus massonianae and China Fir (Chen et al., 2016).In hardwoods, the tear index increased at the initial refinement levels due to the increase of connections and decreased at more intense refinement levels due to fiber degradation (Gharehkhania et al., 2015), as reported for Corymbia citriodora (Severo et al., 2013) and Eucalyptus grandis × Eucalyptus urophylla (Zanuncio et al., 2016).

Inorganic compound accumulation in the system
Calcium, magnesium and sodium concentrations increased after the D stage with 15 kg .odt -1 of the filtrate (EPO) recirculation (Table 7).The increase of calcium and sodium ions concentrations generated precipitated salts, causing incrustation and increasing the carbonate, oxalate and sulfate concentrations (Frigieri et al., 2016).
The carbonate, chlorate, chloride and sulfate anion concentrations in filtrate after the D stage increased with the addition of the alkaline filtrate to the pre-O 2 pulp at 134; 5.7 and 15 and 267%, respectively.The higher chloride concentration may favor organochlorine (AOX) formation in the filtrates, impairing the treatability of the effluent and the aquatic biota, when released into rivers (Huber et al., 2014).
The ions and salt concentrations varied depending on carryover use after (EPO) stage (Table 8).Calcium and magnesium content was lower and sodium content increased 21% in the filtrate from the (EPO) stage due to the increase in NaOH consumption by 19% in the extraction alkaline stage (Frigieri et al., 2016).Partial circuit closure of the alkaline filtrate did not change the calcium or carbonate concentrations in the filtrate (EPO).Incrustation due to calcium carbonate causes more problems during the bleaching stages (Hermosilla et al., 2015).The potassium concentration was lower in the (EPO) filtrate, but that of the chloride increased by 2% with the addition of 15 kg .odt -1 of carryover.The sulfate content in the (EPO) filtrate increased 10 times with the recirculation of 15 kg .odt -1 of carryover, favoring the recovery cycle (Huber et al., 2014).The oxalate content of the (EPO) filtrate was similar and that of the formate increased.

CONCLUSIONS
The alkaline filtrate concentration in the pre-O 2 was found to hamper bleaching and increasing the ClO 2 consumption by 0.08 kg .odt -1 per unit of carryover.In addition, the filtrate carryover (EPO) increased the sulfuric acid and NaOH consumption for pH adjustment of the stages.The tensile index decreased and the tear index increased with filtrate use in the unrefined pulps.

CERNE
SOUZA et al.
However, the physical and mechanical pulp properties were similar after refinement.The calcium, sodium, sulfate and chloride contents were higher after "D" stage with the recirculation of 15 kg .odt -1 of the alkali filtrate and the sodium and sulfate contents were higher in the filtrate (EPO).Recirculation of the alkaline reduce the effluent generation and the usage of water in bleaching.

FIGURE 2
FIGURE 2 Tensile index, Drainability (freeness) and tear index behavior in refi ned bleached pulps and in the control with 10 kg .odt -1 of carryovers.

TABLE 2 D
(EPO)DED bleaching sequence using fixed dosage of the reagents in all treatments.

TABLE 3
Methodology for physical and mechanical properties evaluation of pulps after bleaching.Relation between the carryover charge of the filtrate and the kappa number of the pre-O 2 pulp from softwoods.

TABLE 6
Tensile index, tear index and viscosity (Visc.) in unrefi ned pulps without the fi ltrate and with 10 kg .odt -1 of carryover.

TABLE 7
Concentration (Conc.kg .odt -1 ) of ions and salts in the pulp bleaching effluent after D stage.

TABLE 8
Ion and salt concentration (Conc.) in the bleaching effluent after (EPO) stage.