Chemical composition and physical properties of black liquors and their effects on liquor recovery operation in Brazilian pulp mills

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Chemical composition and physical properties of black liquors and their effects on liquor recovery operation in Brazilian pulp mills

Marcelo Cardoso

^a,^*

, Éder Domingos de Oliveira

^a

, Maria Laura Passos

^b

aDepartment of Chemical Engineering/School of Engineering, Federal University of Minas Gerais (UFMG), Rua Espírito Santo, 35–6°Andar, 30160-030 Belo Horizonte, Brazil

bDrying Center, Federal University of São Carlos (UFSCar), via Washington Luis, P.O. Box 676, 13565-905 São Carlos, Brazil

a r t i c l e i n f o

Article history:

Received 16 May 2007

Received in revised form 12 September 2008

Accepted 7 October 2008 Available online 6 November 2008

Keywords:

Eucalyptus and bamboo black liquors Chemical characterization

Physical properties Rheology Recovery unit

a b s t r a c t

Black liquor is the major by-product and fuel of pulp mills. In this work, effects of black liquor properties on its recovery unit operation are analyzed. Thus, an experimental methodology for characterizing the principal chemical and physical properties of eucalyptus Kraft and bamboo soda black liquors has been developed, including sample collections from six Brazilian mills. Based on results, eucalyptus and bamboo black liquors present higher contents of non-processing elements (NPEs), higher concentration and different molar mass of lignin than those reported by the pine Kraft black liquor. This leads to distinct rheological properties of these liquors. By comparing results obtained for the both liquors, the bamboo and the eucalyptus, the former has the lowest sulfur level, the highest silicon and lignin concentration and, consequently, the highest apparent viscosity.

1. Introduction

Kraft (or sulfate) and soda are the two major alkaline processes to produce chemical pulps, being the former the most important for pulp industries, while the latter is commonly applied to yield non-wood pulps, such as bagasse, straw, grass and bamboo. How- ever, in both processes, cellulose fibers are disassociated from lignin by chemical reactions. These reactions occur in a pressurized digester, where wood chips or fibers are heated and cooked with the cooking liquor, composed basically of NaOH (sodium hydroxide). Specifically in the Kraft process, the sodium sulfide (Na2S) is added to the digester for improving the disassociation of lignin from cellulose fibers, accelerating the wood cooking operation and increasing the mechanical resistance of the pulp[1]. Note that the products resulted from the digester reactions are the cellulose pulp and the black liquor.

In the Kraft recovery unit, the black liquor passes first through a set of multiple-effect evaporators, in which it is concentrated from 15% to about 70–75% of solids to become an adequate fuel. Before entering into the boiler, this liquor is generally mixed with the sodium sulfate to adjust the inorganic ion contents. The Kraft recovery boiler works as a chemical reactor (producing Na2S(l) and Na2CO3(l)), as a steam generator (using the heat of combustion of organic materials to produce vapor) and also as a residue inciner-

ator. The molten inorganic salts produced (Na2S(l)and Na2CO3(l)) flow from the furnace (lower boiler region) to the dissolving tank, in which they are mixed with the weak white liquor to generate the green liquor. This green liquor is clarified and causticized to re- cycle the calcium carbonate and to regenerate the white liquor, which returns to the process[1]. Although, in the soda recovery unit, the bamboo black liquor can be concentrated and burned, its high viscosity limits the solid concentration up to 50%, witch makes the bamboo black liquor recovery more difficult to perform [2].

The black liquor chemical composition depends on the type of the raw material processed, i.e. softwoods (such as pine), hardwoods (such as eucalyptus) or fibrous plants (such as bamboo), as well as, on the operational conditions of the pulping stage. How- ever, for all raw materials and pulping operational conditions used, the black liquor can be considered as a complex aqueous solution, comprising organic materials from wood or fibrous plants (lignin, polysaccharides and resinous compounds of a low molar mass) and inorganic compounds (mainly soluble salt ions). Hence, as summarized inFig. 1, the black liquor chemical composition affects its properties, basically those that govern its behavior in the recovery unit[3].

Fig. 2is a schematic diagram of the black liquor structure, which comprises lignin and polysaccharide clusters, salt ions and water.

Note that, the lignin, the bonding agent of wood or fibrous plant fibers, is a polymer formed by phenyl-propane structures. During the pulping operation, the lignin is fragmented and the carbohy- drates are dissolved and converted into acids of low molar mass;

doi:10.1016/j.fuel.2008.10.016

*Corresponding author. Tel./fax: +55 31 34091789.

E-mail addresses: [email protected] (M. Cardoso), [email protected] (É.Domingos de Oliveira),[email protected](M.L. Passos).

Contents lists available atScienceDirect

Fuel

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / f u e l

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however, the fraction known as xylan (the main hemicellulose in hardwoods) cannot be degraded. Therefore, this polysaccharide survives from the pulping operation to compose the black liquor [3].

As well known in the literature [1], the concentration, molar mass and molecular conformation of lignin and polysaccharide, presented in the black liquor, affect strongly its rheological behavior. FromFig. 3a, it is seen that liquors with high lignin and polysaccharide concentrations tend to have a high viscosity, because these two compounds can cluster into amorphous and voluminous molecules of high molar mass. Conversely, as schematized in Fig. 3b, liquors with low lignin and polysaccharide concentrations

tend to present a lower viscosity, since these compounds can agglomerate in a more compact and spherical molecular structure [1]. However, as reported by Frederick[4], the lignin and polysaccharide macromolecule conformation is straightly related to the pH environment. For pH > 12.5, phenol groups are ionized and the lignin molecules become soluble, forming compact and spherical structures (Fig. 3b), which little affect the rheological liquor behavior. At an intermediate pH (12.56pH611.5), there is a par- tial dissolution of lignin that associates in shapeless and voluminous chains (Fig. 3a), influencing strongly the liquor viscosity. As a consequence, the analysis of the black liquor flow behavior requires the correct identification of its pH range.

In addition, the inert or non-processing elements (NPE) in black liquors (as potassium, chlorine, calcium, aluminum, silicon and iron ions) can also affect the liquor properties and, sometimes, haz- ard the continuous operation of the industrial black liquor recovery plant. As pointed out by Tran[5], the main operational problem caused by the presence of these NPE is their incrustation on equipment walls, corroding these surfaces. Note that, at low liquor temperatures and solid concentrations (Css< 75%), aluminum, calcium and silicon ions form, with the organic compounds, complexes that can avoid building up an insoluble crust on evaporator walls. How- ever, at high liquor temperatures and solid concentrations (Css> 75%), these complexes, when formed, become destabilized, releasing calcium, aluminum and silicon ions, which form calcium carbonate and/or aluminum silicate, incrusting on the wall-surfaces of heat exchange evaporators.

Furthermore, since fibrous plants contain a high silicon concentration[6–9], the bamboo liquor should present higher silicon ion content. Thus, because the tendency of this ion to form and build up insoluble complexes, the bamboo liquor recovery processing should be more difficult to perform.

In this work, the experimental methodology developed for characterizing the chemical composition and physical properties of eucalyptus Kraft liquor[1,10]has been brought to date and also ex- tended for characterizing the bamboo soda black liquor. Samples have been collected from six Brazilian mills to characterize these liquors. Results obtained have been analyzed to correlate the chemical composition and the main physical properties of these different types of black liquors, as well as, to identify the effects of these parameters on the recovery operational variables and predict the liquor behavior in the industrial plant.

2. Experimental methodology

As shown inTable 1, the experimental methodology developed consists in determining the elementary chemical composition of these liquors and other important parameters that should affect their physical properties. In parallel, this methodology evaluates Raw material (type of

wood or plant) and pulping conditions

Black liquor chemical composition Black liquor physical

properties

Black liquor behavior in the recovery unit (evaporators and boiler)

Fig. 1.Origin of the black liquor behavior in the industrial recovery unit (after Soderhjelm[3]).

Nomenclature

C_ss solids concentration () C_lig lignin concentration () D shear strain (s¹) HV heating value (kJ/kg) M_w mass molar (Da) MMlig lignin mass molar (Da)

O/I ratio of organic compound mass per inorganic compound mass ()

T temperature (°C)

g

apparent viscosity (cP)

q

_liquor liquor density (kg/m³)

q

_liq liquid phase density (kg/m³)

q

_ss solid phase density (kg/m³)

s

shear tension (mPa)

ABNT Brazilian association of technical norms HPLC high-pressure liquid chromatography NPE non-processing elements

GPC gel permeation chromatography

TAPPI technical association of the pulp and paper industry

Fig. 2.Simplified schematic representation of the black liquor structure.

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the liquor properties listed inTable 1to identify the influence of chemical composition on them.

2.1. Sampling and storage

Five of the six Brazilian industries, selected in this work, use Kraft process for producing cellulose pulp fromEucalyptus grandis (hardwood basically). One used soda process fromBambasa vulga- riswoody grasses. To characterize their black liquors, samples have been taken from each industrial plant, from April 2000 to July 2005. In Kraft process mills, samples of weak (12%6Css617%), intermediate (38%6C_ss644%) and strong (63%6C_ss672%) liquors have been taken and stored at 4°C. In the soda process mill, only samples of weak liquors (14%6Css616%) have been taken and stored at 4°C.

Prior to experiments, these samples have been equilibrated to the room temperature. When necessary, two or three different

concentrated eucalyptus liquor samples have been mixed together at known proportions to achieve the solid concentration range required for performing the viscosity and density tests. Bamboo weak liquor samples have been mixed, when necessary, with the dried liquor powder (obtained from drying tests in the laboratory) to reach the required solid concentration range.

2.2. Chemical characterization of black liquors

Methods employed for the liquor chemical characterization are shown inTable 2. Results of these analyses supply data for predict- ing the liquor physical properties, especially its density and heating value. The black liquor chemical composition is determined using the elementary analysis technique. In this work, only the compositions of the most important elements are presented, as shown inTable 2. Such analyses have been performed according to Technical Association of the Pulp and Paper Industry (TAPPI) and Brazilian Association of Technical Norms (ABNT) standards.

Carbon, nitrogen and hydrogen (CHN) compositions are determined by injecting samples into a CHN elementary analyzer (Per- kin–Elmer, model 2400).

To analyze the organic to inorganic mass ratio (organic/inorganic ratio), the technical standard procedure T625 cm-85 is used (Table 2). This consists in determining the amount of the sul- fated-ash through the following steps: (a) heat liquor in a muffle for drying; (b) add drops of concentrated sulfuric acid to the dry li- Fig. 3.Schematic representation of lignin and polysaccharide conglomerates presented in black liquor: (a) voluminous and shapeless and (b) compact and spherical (after Cardoso et al.[1]).

Table 1

Main parameters to be analyzed for black liquor characterization[1,10].

Chemical characterization Physical characterization Chemical composition

(elementary analysis)

Density (q_liquor)

Organic/inorganic ratio (O/I) Calorific heating value (heating value, HV) Lignin concentration (Clig) Rheological behavior (apparent viscosity (g) as

function of temperature (T) solids concentration (Css) and shear rateD)

Lignin molar mass (MMlig) Boiling point rise (BPR)

Table 2

Techniques used in chemical characterization of eucalyptus and bamboo black liquors from Brazilian mills.

Analysis Measured parameters Technique and equipment used References

Elemental analysis Carbon (C) Hydrogen (H)

Nitrogen (N) Combustion of dried liquor in oxygen (>1000°C) Elementary Analyzer Perkin–Elmer, model CHN

2400 (based on thermal conductivity measurements) [1,10]

Sodium (S) Atomic absorption spectroscopy with addition of HCl TAPPI Test T266

om-94

Potassium (K) Atomic absorption spectroscopy TAPPI Test T266

om-94 Sulfur (S) Combustion of dried in oxygen under pressure following gravimetric sulfate determination by

BaSO4

ABNT Test MB 106/65 Chlorine (Cl) Liquor oxidation under pressure following potentiometric titration with AgNO3 TAPPI Test T699

om-87

Silicon (Si) Colorimetry [2]

Lignin Lignin concentration (Clig) Lignin precipitation methods [1,10,11]

Molar mass (MMlig) High pressure liquid chromatography with gel permeation columns using tetrahydrofurane as a movable phase – Shimadzu (GPC-802; GPC-803; GPC-802C)

[1,10]

Organic/inorganic

materials Organic to inorganic ratio

(O/I) Liquor combustion followed by dust analysis [1]

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quor [1] and (c) burn the organic matter by heating the liquor excessively (T> 700°C).

Data of lignin concentration (Clig) and its molar mass (MMlig) are essential to describe changes in the rheological liquor behavior during its evaporation. Before measuring these parameters, lignin needs to precipitate from the black liquor. For this, a modified ver- sion of the method proposed by Kim et al.[11]is used in this work [10].

The high-pressure liquid chromatography technique (HPLC) with gel permeation chromatography columns (GPC) is used to determine MMlig(lignin molar mass). Tetrahydrofurane is selected as the movable phase in this technique.

Since there are random oscillations in the operational variables of the liquor recovery unit due to usual perturbations in the industrial plant, one of the six mills has been chosen arbitrarily to estimate the effect of these time oscillations on the liquor chemical characterization. In this mill, the chemical composition of the black liquor has been monitored during one year and three months of operation and 56 liquor samples have been collected and analyzed.

Based on these data, the standard deviation of each chemical composition parameters in relation to its mean value (averaged over one year and three months) have been calculated to analyze the ac- tual data dispersion due to the usual oscillations and perturbations occurred in the industrial process.

2.3. Physical characterization of black liquor

Methods used in the physical characterization of black liquor are shown inTable 3. The solid concentration (Css) is obtained by using the oven drying method at controlled temperature and the density (

q

_liquor) is determined by the pycnometric technique[2].

Black liquor calorific, i.e. the heating value (HV) is evaluated using

TAPPI standard test – T684 om-90. Following this method, a calorimetric bomb (Shimadzu model C-03) has been used to determine the heat of combustion of these black liquors atC_ssP45%.

Tests for describing the rheological liquor behavior have been carried out in a rotary rheometer (COLE-PARMER, models 98936- 00/20). Following methodology proposed by Costa et al. [12],

s

(shear stress) vs.D(shear rate) curves are determined as a function ofC_ssandT(liquor temperature). Each experimental point on

s

vs.

Dcurves is averaged over two or three replications. These curves are statistically analyzed to determine the apparent viscosity (

g

) of eucalyptus and bamboo black liquors as the function ofTand Css[2,12,13]. Based on this

s

vs.Dcurves, the range ofCss, at which the liquor changes its behavior from Newtonian to pseudoplastic fluid, can be identified.

3. Results and discussion

Results of the elementary analysis for the eucalyptus and bamboo black liquors are shown inTable 4. The composition of each element (carbon, hydrogen, nitrogen, sodium, potassium, sulfur, silicon and chlorine) expressed as the mass percentage of this element to dry solids has been obtained from a series of (at least) three tests, with a standard deviation lower than 1.5%. Data of the organic/inorganic ratio, O/I, of the lignin concentration and molar mass,C_ligand MMlig, and of the Kappa number are shown inTa- ble 5for these liquors. For comparison,Tables 4 and 5include also data obtained from the literature[7,14,15].

Mill E is the one, in which the chemical composition of its liquor has been monitored during one year and three months. As shown inTables 4 and 5, the chemical composition data dispersion of this liquor (identified by the standard deviation of each chemical composition parameter) is lower than one of the chemical composition

Table 3

Techniques used in physical characterization of the eucalyptus and bamboo black liquors from Brazilian mills.

Analysis Measured parameters Technique and equipment used References

Solids contents Solids concentration (Css) Dry a known mass of liquor in oven at controlled temperature until mass constant – Digital Metler

balance AB204 (±10⁴g) TAPPI Test T650

om-89[2]

Density Liquor density atCss

(q_liquor)

Pycnometry (mass and volume measurements) – Digital Metler balance AB204 (±10⁴g), heater plate and standard 25 ml volumetric bottles

[2,12]

Calorific heating value

Heating value (HV) Heating by a complete oxidizing liquor in an adiabatic calorimetric bomb (Shimadzu model C-03) TAPPI Test T684 om-90[2]

Viscosity Liquor apparent viscosity

atCssandT(g) Viscosity measurements at different shear stresses and temperature – Calibrated rotational Cole–

Parmer viscometers, 98936-00 and -20 model with cylindrical spindles, adapters, unit of controlled temperature

[2,12]

Table 4

Results from the elementary analysis of different black liquors.

Liquor (type of processed wood) Mill location Elementary composition (% mass per dry solids)

C H N Na K S Cl Si

Hardwood/Eucalyptus grandis^a Brazil/mill A (Kraft process) 30.8 3.6 0.01 21.8 1.8 3.7 4.5 0.1

Hardwood/E. grandis^a Brazil/mill B (Kraft process) 35.2 3.7 – 21.2 2.1 3.0 4.3 –

Hardwood/E. grandis^a Brazil/mill C (Kraft process) 29.6 3.6 – 18.7 2.2 4.4 2.6 –

Hardwood/E. grandis^a Brazil/mill D (Kraft process) 34.8 3.4 0.04 18.3 2.1 3.6 3.2 –

Hardwood/E. grandis^a Brazil/mill E (Kraft process) 32.3 ± 0.3 3.1 ± 0.1 0.04 ± 0.01 23.5 ± 3.2 1.8 ± 0.3 4.9 ± 0.5 2.2 ± 0.3 –

Fibrous plant/Bambasa vulgaris^a Brazil/mill F (soda process) 35.4 3.6 0.30 19.3 3.3 0.2 1.3 3.8

Softwood/Pinus sylvestrisand Pinus caribaea^b

Scandinavia and North America (Kraft Process)

33.9 to 35.8 3.3 to 3.6 0.06 to 0.07 17.2 to 19.8 1.4 to 2.2 4.6 to 5.7 0.3 to 0.9 –

Hardwood^b Scandinavia (Kraft process) 33.2 3.3 0.08 20.8 2.6 5.2 0.3 –

Hardwood/Eucalyptus^b North America (Kraft process) 37.3 3.6 0.09 17.3 1.8 3.4 1.6 –

Fibrous plant/Straw^c South America (soda process) 39.1 4.5 1.0 8.8 4.1 0.8 3.5 0.23

a In this work.

b In Ref.[14].

c In Ref.[7].

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among the five eucalyptus liquors analyzed. This means that the usual oscillations occurred in the industrial process operation affect less significantly the liquor chemical composition. Therefore, considering, as the experimental error, two times the standard deviation of each parameter presented inTables 4 and 5, one can conclude that the five eucalyptus liquors analyzed here have their own chemical composition.

As shown inTable 4, the bamboo black liquor contains the lowest sulfur content and highest silicon level. Both results are expected since there is no addition of Na2S in the soda process (hydroxide ion is the only agent responsible for the lignin degradation) and the bamboo itself has high silicon content in its composition.

Data fromTable 4show also a high concentration of chlorine, a non-processing element (NPE), in the Kraft eucalyptus liquor composition. Therefore, serious operating problems should occur in the recovery boiler because chloride and potassium ions tend to com- bine with the sodium ion to form a salt, which can build up on the wall of recovery boiler tubes, plugging and corroding them (especially in super-heaters). In the boiler super-heater region, values of Cl/(Na⁺+ K⁺) and K⁺/(Na⁺+ K⁺) ratios establish the adhesive temperature range of this salt incrustation[5]. Therefore, each specific Kraft mill must monitor and control these two ratio parameters to avoid problems with scaling.

Mean values of O/I andC_ligobtained fromTable 5for the eucalyptus black liquor are, respectively 1.94 and 41.5%, higher than those reported for the pine black liquor. This high organic matter and lignin concentration in these eucalyptus black liquors can be explained by the pulping process, expressed by a low Kappa number, and the internal wood structure. As known in the literature [16], a low Kappa number is one of the hardwood pulp character-

istics, since these pulps have a small amount of residual lignin.

Such characteristic is corroborated by Kappa number data obtained for eucalyptus liquors belonging to Brazilian mills, which are, in average, lower than those reported for pine liquors (seeTable 5).

According to MacDonald and Franklin[17] and Britt [18], hardwoods delignify easier during pulping operation because its internal structure presents a larger number of open vessels, which enhances the penetration and flow of the cooking liquor into the wood chip. In softwoods, these vessels are resinous and obstructed, hindering this mechanism of lignin removal. Besides, hardwoods have a higher percentage of lignin in the medium lamella, promot- ing its easy removal due to this outer location.

As shown inFig. 4, for determining MMligof eucalyptus liquors, the permeation time of lignin, obtained in each liquor sample, has to be compared to permeation time of each one of seven standard polystyrenes with known molar masses (Mw = 2500, 5000, 9000, 17500, 30000, 50000 and 382000 Da). Using this procedure, MMlig

for each eucalyptus black liquor analyzed has been calculated and its mean value is presented inTable 5. To verify the efficacy of the method developed for precipitating lignin from liquors, the absorption spectrum in the infrared region for each precipitated lignin obtained here has been compared to one for the lignin ‘‘in situ” ofE.

grandisspecies, reported by Morais[19]. The similarity between them corroborates that the precipitated material, obtained from these liquors, isin factlignin[10,20].

FromTable 5, it can be inferred that values of O/I, Cligand MMlig

are higher for the bamboo liquor, meaning that this liquor contains more lignin with larger structure molecular than does the eucalyptus liquor. The soda black liquor tend to have a high lignin molar mass because of a lower degree of lignin dissociation in these liquors since, as mentioned earlier, the hydroxide ion is the only Table 5

Organic/inorganic ratio, concentration and lignin molar mass for black liquors coming from Kraft and soda processes.

Liquor (type/material processed) Mill location Kappa number O/I () C_lig(%) (mass per mass dry solids) MMlig(Da)

Hardwood/Eucalyptus grandis^a Brazil/mill A (Kraft process) 17 1.81 42.3 820

Hardwood/E. grandis^a Brazil/mill B (Kraft process) 17 2.2 40.2 1641

Hardwood/E. grandis^a Brazil/mill C (Kraft process) 17 1.94 41.8 1401

Hardwood/E. grandis^a Brazil/Mill D (Kraft process) 17 2.1 42.3 1050

Hardwood/E. grandis^a Brazil/mill E (Kraft process) 17 1.86 ± 0.09 39.7 ± 2.2 1871 ± 221

Fibrous plant/Bambosa vulgaris^a Brazil/mill F (soda process) – 2.30 45.3 3282

Softwood/Pinus Caribaea^b Scandinavia (Kraft process) 17–125 1.33 39.0 2728

aIn this work.

b In Ref.[15].

0 10 20 30 40 50 60

Time

Polystyrene standards Lignin (Black liquor) M.w 382000 M.w 50000

M.w 30000 M.w 17500

M.w 9000 M.w 2500

Toluen

Fig. 4.Typical chromatogram of the molar mass of lignin present in the eucalyptus black liquor of the sample coming from mill A with 73% of dry solids.

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chemical responsible for it. Conversely, in the Kraft wood pulping, hydroxide and hydrosulfide ions act together to accelerate the lignin degradation and dissociation, reducing significantly its molar mass.

Based on data inTable 5, it is possible to conclude that, on average, MMligis lower for eucalyptus liquors than it is for pine liquors.

This is explained by the faster delignification mechanism, characteristic of hardwoods, and also by the high sodium content in the eucalyptus liquor. Zaman and Fricke[16] have pointed out the alkalis present in white liquor aqueous solutions are responsible for breaking macromolecules of lignin and, consequently, for reducing MMlig of the liquor. Higher alkali contents are usually indicated by the presence of higher tenors of sodium ions. Due to these reasons, the Kraft eucalyptus liquor contains the lowest molar mass of lignin molecules among all black liquors.

The chemical composition of black liquor, basically the amount of polymeric organic matter (lignin and polysaccharides) and of inorganic compounds, as well as, their specific concentration, influences directly its physical properties, such as density (

q

liquor) and viscosity (

g

). Results from earlier works[2,12]indicate that

q

_liquor

varies withC_ssaccording to the following equation:

q

_liquor¼ C_ss

q

_ss^þ

ð1C_ssÞ

q

_liq

" #1

ð1Þ

where

q

_ssis the density of the solid phase and

q

_liqis the density of the liquid phase, both at the operating temperature. These densities,

q

ssand

q

liq, have been fit to experimental data and their values are presented inTable 6for eucalyptus liquors (mills A and C) and for bamboo liquor (mill F).Fig. 5shows the experimental data for these three liquors fitted by their curves obtained from Eq.(1).

As pointed out by Frederick[4], the variation of

q

_liquorwithC_ssis strongly influenced by the presence and concentration of inorganic

compounds encountered in black liquor. Such statement is corroborated by data inFig. 5for eucalyptus black liquors from mills A and C. Note that these two liquors present similar values of density since their inorganic compositions are quite the same, as seen in Tables 4 and 5. Furthermore, specifically for Kraft liquors (basically pine liquors), Frederick[4]have suggested a linear relationship between

q

_liquorandC_ssup toC_ss= 0.65. For 0.65 <C_ss< 0.80, changes in

q

_liquorwithC_ssare more pronounced due to the liquor transition from water-continuous phase to polymer-continuous phase. As shown in the

q

_liquorvs.C_sscurves ofFig. 5, there is, for these liquors, a transition region after which

q

_liquorrises faster with an increase inCss. This occurs atCssﬃ0.50 for both eucalyptus liquor and at C_ssﬃ0.40 for the bamboo liquor. Such result also corroborates the effect of the amount of inorganic matter on the liquor density since these bamboo and eucalyptus liquors have a quite distinct inorganic composition (seeTable 4).

The heating value (HV) of eucalyptus (mills A and E) and bamboo (mill F) liquors are presented inTable 7. Since HV of the lignin extracted from hardwood is 25110 kJ/kg and lower than HV (26900 kJ/kg) of the lignin extracted from softwood[4], one would expect similar behavior for the HV of hardwood and softwood liquors. However, HV data inTable 7are within the typical range of the pine liquor HV, which is from 13400 to 15500 kJ/kg [4].

Returning toTable 5, one can see thatC_ligin the eucalyptus black liquor is 39.7 to 42.3%, little higher thanClig(39%) in the pine liquor. This higherC_ligcounterbalances the lower HV of hardwood lignin, resulting in the similar HV range for both, eucalyptus and pine, liquors. This explanation also justifies HV data obtained for the bamboo liquor, since HV of the lignin extracted from bamboo is 24500 kJ/kg[9], lower than HV of the lignin extracted from both, soft and hard, woods. Furthermore,C_lig in the bamboo liquor is 45.3% (Table 5), higher than C_lig in both, eucalyptus and pine, liquors.

Zaman and Fricke[21]have reported that HV ofslashpine liquor varies strongly with C_lig, but insignificantly with O/I. Their data are compared to HV data of the eucalyptus black liquor from Table 6

Adjustable parameters of Eq.(1)to estimate the liquor density in the range of 26–

30°C.

Mill/liquor qss(kg/m³) qliq(kg/m³)

Mill A/eucalyptus 1926 (±258) 1005 (±20)

Mill C/eucalyptus 1936 1012

Mill F/bamboo 2098 947

900 1100 1300 1500 1700

0 20 40 80

C_ss (%) liquor (kg/m3)

Mill A Mill C Mill F - bamboo Equation 1- mill A Equation 1 - mill C Equation 1 - mill F

60

ρ

Fig. 5.Liquor density (q_liquor) as a function of the solids concentration (Css) for eucalyptus black liquors from mill A and C and for bamboo black liquor from mill F at temperature between 26 to 30°C. (density error between experimental data and correlation from Eq.(1): ±12 kg/m³for eucalyptus liquor from mill A, ±5 kg/m³for eucalyptus liquor from mill C and ±42 kg/m³for bamboo liquor from mill F).

11000 12000 13000 14000 15000 16000 17000

0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 O/I (-)

HV (kJ/kg)

eucalyptus liquor - mill E pine liquor [21]

Fig. 6.Liquor heating value (HV) as function of the organic/inorganic ratio (O/I) for the eucalyptus black liquor from mill E and the pine black liquor reported by Zaman and Fricke[21].

Table 7

Mean heating value (HV) of the eucalyptus and bamboo black liquors.

Mill/Liquor HV (kJ/kg)

Mill A/eucalyptus 14593 (±162)

Mill E/eucalyptus 14615 (±268)

Mill F/bamboo 14673 (±226)

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mill E, for varying O/I (Fig. 6) and for varyingC_lig(Fig. 7). Contrari- wise, the HV of the eucalyptus black liquor depends explicitly on O/I, but slightly onClig. Both the organic components and the re- duced sulfur compounds in black liquor contribute to its heating values[4]. This non-expected result may indicate another organic compound presented at a significant concentration in the eucalyptus black liquor, affecting its heating value. Investigations about the presence of other organic compounds in the liquor of eucalyptus and their influence on the heating value, and also on the apparent viscosity, are currently being conducted. Additionally, It is important to emphasize that theslashpine liquor, reported here, has been obtained in the laboratory and its level of lignin differs from the industrial liquor.

The apparent viscosity (

g

) of eucalyptus and bamboo liquors as function of the shear strain (D) is shown inFig. 8, for different solid concentrations (Css) and liquor temperature (T). The apparent black liquor viscosity is influenced by its chemical composition, mainly by the concentration of organic compounds, such as lignin and polysaccharides[4].

As expected for both liquors, the lignin macromolecules entan- gle more easily at low shear rates increasing the black liquor viscosity. At high shear rates, these macromolecules tend to align together reducing their resistance to flow and so the liquor viscosity.

By comparing the rheological behavior of these two liquors, it is confirmed that the apparent viscosity of the bamboo liquor is higher than one of the eucalyptus liquor, since the bamboo liquor has the highest value of MMlig (Table 5). Furthermore, especially in non-wood soda bamboo liquor, a high silicon level exists, as shown inTable 4, in the form of water-soluble silicate ions. These ions agglomerate with organic matter to form colloidal structures, as pH is lowered. Such conditions can result in a high liquor apparent viscosity even at low solids contents as shown inFig. 8b. These higher

g

values prevent an efficient evaporation of the bamboo liquor in multiple-effect evaporators and, consequently, a stable combustion of this liquor in the furnace. Therefore, the recovery of the bamboo liquor in the soda process requires the development of new techniques to overcome problems related to its high viscosity. Passos et al.[22]have demonstrated that drying this liquor in a low-cost spouted bed dryer to produce powdery fuel is a feasible technique for recovering this liquor.

Based on Fig. 8a, one can see that, for C_ss640.5% at 30°C6T640°C, the reological behavior of eucalyptus black liquor approaches to a Newtonian fluid, with its viscosity indepen-

dent of shear rates (a constant of proportionality between the shear stress (

s

) and the shear rate (D)). ForC_ss> 40.5%, its reological behavior is typical of a pseudoplastic fluid, with

g

decreasing expo- nentially as the shear rate rises. As reported by Zaman and Fricke [16], at the same range of temperature, the reological behavior of the pine black liquor approaches to a Newtonian fluid forC_ss650%, corroborating the effect of lignin concentration and mass molar on 11000

12000 13000 14000 15000 16000 17000

31 33 35 37 39 41 43 45

C_lig(%)

HV (kJ/kg)

eucalyptus liquor - mill E pine liquor [21]

Fig. 7.Liquor heating value (HV) as function of lignin concentration (Clig) for the eucalyptus black liquor from mill E and the pine black liquor reported by Zaman and Fricke[21].

0 150 300 450 600 750 900 1050

0 5 10 15 20

(cP)

Css = 49.3% T= 35.3˚C Css = 51.5% T= 37.2˚C Css = 47,6% T= 37.3ºC Css = 40.5% T= 31.5˚C

0 10000 20000 30000 40000 50000

0 5 10 15 20

shear rate (s^-1) shear rate (s^-1)

(cP)

Css = 35.9 % T = 29.5 ˚C Css = 37.8 % T = 29.5 ˚C Css = 40.2 % T = 30.1 ˚C Css = 37.8 % T = 69.5 ˚C

ηη

a

b

Fig. 8.Liquor apparent viscosity,g, vs. shear rate,D, as function ofCssand T: (a) eucalyptus liquor from mill A and (b) bamboo liquor from mill F.

80 100 120 140 160 180 200 220

0 50 100 150 200 250

shear rate (s^-1)

(cP)

increasing D decreasing D

η

Fig. 9.Hysteresis occurred in eucalyptus black liquors for increasing and decreasing the shear rate (sample from mill A,Css= 52.0%,T= 30°C).

(8)

the liquor viscosity. Although the bamboo liquor presents a similar behavior (seeFig. 8b), its pseudoplastic behavior occurs at lower solid concentrations (Css< 35% at 29°C6T630°C).

In addition, the hysteresis, observed inFig. 9for the eucalyptus liquor, is not seen in bamboo and pine liquor rheological curves.

This hysteresis phenomenon occurs in thixotropic pseudoplastic fluids. Therefore, the black liquor of eucalyptus has the most complex rheological behavior.

4. Conclusion

Results show that the eucalyptus black liquor from Brazilian mills presents a higher content of non-processing elements (NPEs), a higher lignin concentration and a lower lignin molar mass than the pine liquor from northern Hemisphere mills does. On the other hand, the bamboo black liquor presents the lowest sulfur concentration (as expected in the soda pulping process), the highest silicon and lignin concentrations and the highest lignin molar mass in relation to the pine and eucalyptus liquors. This confers to this liquor the highest apparent viscosity. From these results, it is shown in this work how the chemical composition influences physical and rheological properties of the eucalyptus and bamboo liquors, imparting to them an own behavior in the industrial recovery evaporator and boiler — different from the pine black liquor behavior.

Acknowledgements

Authors are grateful to Brazilian governmental research founda- tions (FAPEMIG and CNPq) for the financial support; to Brazilian industries for supplying black liquor samples and to students (S.C. Kupidlowski, C.R.S. Gonçalves, A.O.S. Costa, R.M.S. Carmo, A.L.G. Trindade, N.S. Oliveira, T.M.G. Ribeiro, D.C. Rena) for collect- ing data and performing tests.

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