Pulp Phase Research Articles

An Empirical Mathematical Model for the Predictive Analysis of the Chemical Absorption of Hydroxide in Eucalyptus Wood

Chip steaming, heating, impregnation, and sugar acid neutralization of wood chips by an alkaline kraft liquor during the pretreatment and initial phase of pulping are critical physical−chemical phe...

Positron emission particle tracking as a method to map the movement of particles in the pulp and froth phases

Positron emission particle tracking (PEPT) is a method of detecting the position of particles in vessels over a period of time. It has been used effectively in a number of mixing applications, and this work introduces the concept to the froth flotation process. Using PEPT, it was possible to map the position of a pyrite particle as it mixed in the pulp phase of a Denver Cell batch flotation experiment. Once the air flow was switched on, it was then possible to observe the particle entering the froth after attaching to an air bubble, and observing the detachment from the froth, re-entry into the pulp phase and then re-attachment to another bubble leading to the particle entering the froth once again. From this work, PEPT has the potential to become a powerful tool in order to understand exactly what happens in flotation practice, and to enable more accurate mathematical models to be developed.

Impact of effective alkali and sulfide profiling on Eucalyptus globulus kraft pulping. Selectivity of the impregnation phase and its effect on final pulping results

AbstractBACKGROUND: Eucalyptus globulus is an important wood source for paper production and, in the last few years, great efforts have been made to assess its chemical specificities and improve the kraft pulping efficiency. Despite the existence of several works concerning mostly the kinetics of E. globulus kraft pulping there is a lack of systematic studies on the initial phase of pulping as well as on the impact of effective alkali (EA) charge profiling on the kraft pulping performance of this species. The aim of the present work is to assess the effect of initial effective alkali and sulfide charges on the lignin and carbohydrates removal on the impregnation phase and to investigate the effect of EA splitting charge on the whole E. globulus pulping process efficiency.RESULTS: When the EA charge at impregnation phase increases, the amount of dissolved wood increases until it reaches a constant value of about 20%. Maximum polysaccharide removal at impregnation phase was about 10% of total wood weight. Glucose removal during impregnation was attributed to the degradation and/or dissolution of E. globulus glucans. For EA charges at impregnation phase higher than ∼15%, xylan retention on wood was roughly constant. Despite the differences found at impregnation phase on the amount of dissolved wood and lignin removal, it was demonstrated that these differences are almost completely attenuated until the end of the kraft pulping process.CONCLUSION: In the case of E. globulus, for a constant effective alkali charge, alkali profiling does not affect the whole kraft pulping performance. Copyright © 2007 Society of Chemical Industry

Characterization of non-cellulosic glucans in Eucalyptus globulus Labill. wood and kraft pulp

Abstract The amount of non-cellulosic glucans in Eucalyptus globulus wood (approx. 4.5%) was assessed by extraction of sawdust with a 0.5 M NaOH solution at 90–120°C and the structural features were elucidated in a glucan-enriched polysaccharide fraction isolated by hot water extraction (120°C, 2 h). Isolated soluble glucans were characterized by wet chemistry methods and a set of 1D and 2D NMR techniques. The major proportion of glucans consisted of amylose (20%) and amylopectin (80%), although a small proportion of β-(1→3)-glucan (<0.2% on wood weight) was also detected. Although glucans are easily removed in the initial phase of kraft pulping, notable amounts were found in unbleached pulp and it was supposed that in the final stage of kraft pulping a fraction of α-glucan is precipitated onto the fiber surface and contributes to the pulp yield.

Convective–dispersive gangue transport in flotation froth

The transport of gangue through flotation froth has been described by solving the convection–diffusion equation. Gangue recovery is predicted to be proportional to liquid recovery, which is consistent with experimental observation. In addition, it is seen that the dependency of gangue recovery upon particle size is due to processes within the pulp phase rather than the froth, insofar as the transport of particles in a given froth is approximately independent of size. The importance of maintenance of positive bias in column flotation, previously stressed by other workers, is reinforced. This model utilises a simplified representation of the froth and, as a consequence, it does not necessarily give accurate gangue recovery estimates for practical flotation processes. However, the convective–diffusive model does illuminate the physical processes behind gangue recovery in the concentrate which will aid the development of automatic control strategies.

The formation of LC and other aromatic end-group structures in O-alkyl-substituted cellulose during kraft pulping conditions

Ultraviolet resonance Raman spectroscopy (UVRRS) was used to study the formation of aromatic and/or lignin-carbohydrate (LC) structures at the reducing end groups of O-alkyl-substituted cellulose under conditions simulating the initial phase of kraft pulping. The derivatives studied were methyl cellulose (MC) with degree of substitution (DS) of 1.64–1.95, carboxymethyl cellulose (CMC) with DS ∼0.6, and a lignin model compound, creosol. The total alkali concentrations in the treatments were 0.1 M and 0.5 M and the sulfidities were 1%, 10%, and 30%. HS− ions and creosol are both strong nucleophiles and they compete for the hot-alkali-generated unsaturated electrophilic reaction sites in the reducing end groups of the polysaccharides. The results indicated that conditions similar to those in the initial phase of conventional kraft cooking (high OH− and low HS− ion concentrations) increased the aromatic nature of the end groups and conditions similar to those in the initial phase of super batch cooking (low OH− and high HS− ion concentrations) partly inhibited the formation of aromatic and LC end groups.

Assessment of potential approaches to improve Eucalyptus globulus kraft pulping yield

AbstractThe main goal of this work is to study the potential approaches to improve polysaccharides retention during Eucalyptus globulus kraft pulping. The addition of anthraquinone to kraft pulping leads to the highest pulp yield while the addition of urea promotes lower depolymerization of polysaccharides (higher pulp viscosity), but does not have a significant effect on yield. The early interruption of kraft cooking followed by oxygen delignification is a reliable approach to increase pulp yield, particularly when pulping is interrupted at the end of the faster and more selective kinetic regime (bulk phase). Yield loss during oxygen delignification is considerably lower than that incurred in the last phase of kraft pulping. Pulping with OH−/HS− charge profiling, carried out with liquor injection in three different phases leads to a yield increase. However, this increase results from a lower total alkali charge applied when profiling pulping is compared to standard pulping conditions, rather than to alkali profiling. Standard kraft pulping with different active alkali (AA) charges demonstrated that this operational variable is determinant for pulp yield and viscosity. Pulping experiences with lower AA (14%) resulted in a higher and almost constant pulp viscosity and in a higher pulp yield, assigned to improved retention of both cellulose and xylan. During the last stage of pulping, cellulose content decreases, this being mainly responsible for the decrease of pulp yield, while xylan content is almost constant, a feature attributed to the peculiar structure of this E. globulus's hemicellulose. Copyright © 2007 Society of Chemical Industry

Characterization of thin water layers in pulp by tritium exchange. Part 1: Methods development

Abstract A new technique for measuring the monolayer water content of fiber is presented. Tritiated water is added to a pulp/water suspension, whereupon the tritium partitions between the bulk water and the pulp. In the pulp phase the tritium can exchange with free water, bound water, and with hydroxyl and other protons present in the pulp matrix. The free water in the pulp is then removed by displacement with acetone. The tritium remaining in the pulp is mostly associated with tightly bound water, with a small fraction being tied up with the exchangeable hydrogen in pulp. The procedure provides a value of 10% for the tightly bound water content of hardwood or softwood fiber, either bleached or unbleached. If this water is assumed to cover the fiber surface as a monolayer, then an estimate of the wet surface area of the fiber can be obtained. This estimate compares well with independent measurements of surface area.

The formation of aromatic end group structures in O-alkyl substituted celluloses under alkaline pulping conditions

Abstract UV resonance Raman (UVRR) spectroscopy was used to study changes in the structure of reducing end groups of O-alkyl substituted celluloses under conditions simulating the initial phase of soda pulping. The derivatives studied were carboxymethyl cellulose (CMC) with a degree of substitution (DS) of 0.58 and methyl cellulose (MC) with DS of 1.64–1.95. Hot alkaline treatments were carried out in solutions of sodium hydroxide (0.1–1 M) at 95°C in the presence of creosol as a lignin model compound. Alkali-stable aromatic end groups were generated in both cellulose ethers. Moreover, the end groups of methyl cellulose were extensively condensed with creosol. The results demonstrate that aromatisation and condensation depend on the pH and reaction time.

Radical coupling – A major obstacle to delignification in kraft pulping

Lignin dissolution is very slow in the final delignification phase of kraft pulping whereas carbohydrate degradation and dissolution proceeds at a rate that is about the same as that in the bulk ph ...

Development of methodologies to improve the assessment of reagent behaviour in flotation with particular reference to collectors and depressants

Reagents are added to flotation pulps to perform specific roles, however due to the complexity of the sub-processes and the interactions between reagents and these sub processes, it is not always possible to know whether the intentions have been achieved—either directly or indirectly. It is also not possible to reliably predict whether the behaviour or trends observed in the laboratory can be scaled up to plant conditions. This paper seeks to unravel the behaviour of reagents and improve the understanding of the mechanisms on the sub processes which contribute to the resulting overall flotation performance. In particular, to decouple the material reporting to the concentrate by true flotation from that recovered by entrainment and to take into account the froth phase behaviour as well as that of the pulp phase. This makes it possible to get a better indication of how trends will be translated to a larger scale.

Transport phenomena at the pulp–froth interface in a flotation column: I. Recovery profiles

A calculation procedure is proposed whereby the bubble load and interstitial concentration of different species can be estimated at different levels in the pulp phase of a pilot-scale flotation column. The model requires experimentally measured concentrations of species in the pulp (overall concentrations), the flow rates of species in the concentrate and tailings, air hold-up, the rate of aeration, the wash water rate, as well as mineralogical liberation data as input parameters. Various transport mechanisms, such as the preferential rejection of particles from bubble wakes, settling of particles, entrainment of particles, the flow of the bulk slurry and bubble transport, are incorporated in the calculation procedure. Calculated results can be used to investigate transport phenomena at the pulp–froth interface, to estimate pulp recoveries and froth recoveries. The flotation of chromite in a 15-cm diameter column serves as a case study to demonstrate the application of the calculation procedure. The ore originates from the Western Bushveld Complex in South Africa, and contains chromite, enstatite and feldspar as major minerals. Conclusions regarding the froth zone recovery are made in conjunction with a proper quantification of the bubble load and interstitial concentration of species directly below the pulp–froth interface. If the upward transport by entrainment directly below the pulp–froth interface is negligible relative to the corresponding bubble transport, then the froth recovery is largely determined by particle–bubble detachment at and above the pulp–froth interface. Conversely, if entrainment contributes significantly to the upward transport below the pulp–froth interface, then the rejection of these entrained species from the froth could strongly reduce froth recovery. The froth recovery in the chromite case study is insensitive to froth depth, and decreases with a decrease in particle size. Furthermore, the pulp–froth interface plays a dominant role in the upgrading process during flotation.

BEHAVIOR OF EUCALYPTUS GLOBULUS LIGNIN DURING KRAFT PULPING. I. ANALYSIS BY CHEMICAL DEGRADATION METHODS

ABSTRACT Eucalyptus globulus wood was subjected to kraft pulping, reaching different extents of delignification. The residual and dissolved lignins were isolated by soft acidolysis and acidic precipitation, respectively, and submitted to analysis of residual sugars and methoxyl groups, as well as to analysis by nitrobenzene and permanganate oxidation and thioacidolysis. Results from both residual and dissolved lignins analyses indicated that in the initial phase of pulping there is a preferential removal of lignin enriched in guaiacylpropane (G) and p-hydroxyphenylpropane (H) units, which is highly condensed and bonded with polysaccharides, predominantly with xylan. During the bulk and residual phases of pulping, lignin enriched in syringylpropane (S) units is progressively removed, leading to the increase of S/G ratio of black liquor lignin. After a purification step of dissolved lignins, two fractions with distinct levels of carbohydrates and with different structural features were obtained. The overall results were interpreted in terms of the topochemistry of the kraft pulping process.

Foam drainage on a sloping weir.

Foam drainage is considered in a froth flotation tank with a sloping weir. The drainage is shown to be gravity dominated in most of the foam, except for thin boundary layers at the base of the froth, and along the sloping weir. The mathematical reason for the boundary layers is that capillary suction is a much weaker effect than gravity, but cannot be ignored altogether, because it represents a singular perturbation. The relative weakness of capillary suction with respect to gravity is represented by a key dimensionless parameter, denoted K, which satisfies K<<1. The volumetric flow at any point along the weir boundary layer is the accumulation of all liquid that has rained onto the weir above the point in question: typically, this flow is linear in distance measured downward from the weir lip. All liquid raining onto the weir is ultimately returned to the pulp phase as a high-speed jet. The jet velocity scales with the (2/3) power of distance from the weir lip, and is O(K(-2/3)) times larger than the typical velocity in the gravity-dominated flow in the bulk of the flotation tank. The liquid volume fraction in the jet is likewise O(K(-2/3)) larger than that in the bulk. Across the jet, the foam exhibits a known profile of liquid fraction vs. distance from the weir: this is known as the equilibrium profile. The foam requires a distance equivalent to O(K(4/3)) weir lengths to dry out significantly from the wetness value on the weir, but a larger O(K) distance to fall back to a wetness comparable with that in the bulk of the froth.

The use of bubble loads to interpret transport phenomena at the pulp–froth interface in a flotation column

A calculation procedure is proposed whereby the bubble load and interstitial concentration of different species can be estimated at different levels in the pulp phase of a pilot-scale flotation column. The model requires experimentally measured concentrations of species in the pulp (overall concentrations), the flow rates of species in the concentrate and tailings, air hold-up, the rate of aeration, the wash water rate, as well as mineralogical liberation data as input parameters. Various transport mechanisms, such as the preferential rejection of particles from bubble wakes, settling of particles, entrainment of particles, the flow of the bulk slurry and bubble transport, are incorporated in the calculation procedure. Calculated results can be used to investigate transport phenomena at the pulp–froth interface (PFI), to estimate pulp recoveries and froth recoveries, as well as to identify regions of high and low particle–bubble attachment in flotation columns. The flotation of chromite in a 15 cm diameter column serves as a case study to demonstrate the application of the calculation procedure.

The effect of mothers on bubble size distributions in flotation pulp phases and surface froths

Froth dynamics, i.e. the stability and mobility of the froth, are crucial indicators of various important features of flotation systems. For example, it is desirable that the froth collapses as soon as possible after it is skimmed off the cell in order to curtail losses in throughput. On the other hand, if the froth is too prone to collapse, it will not be sufficiently stable to support its load prior to skimming. Likewise, the mobility of the froth gives similar information on the performance of the flotation cell. For example, a sharp contrast can be observed between dry viscous (immobile) froths and watery runny froths with high mobility. Several authors have recently shown that analysis of the structure of the froth in a flotation cell can be used to assess the performance of the cell. This implies that there is a close relationship between the bubble size distribution in the pulp and froth phases. Until very recently, it was not possible to verify this hypothesis direct, since reliable measurements of bubble size distributions in especially the froth phase could not be obtained. With recent improvements in the machine vision technology originally developed at the University of Stellenbosch, it is now possible to measure bubble size distributions and stability in froth structure with a high degree of accuracy. Unlike previous methods, these improved algorithms can provide a detailed map of flow patterns in the froths, which can give a significantly better idea of operating conditions in the flotation cell. Consequently, in this experimental study the bubble size distribution in the pulp phase of a laboratory flotation cell was measurement with a capillary tube system (UCT bubble size analyser), while the bubble size distribution in the froth phase was measured by use of digital image analysis. The relationship between these bubble size distributions in the pulp and froth phases is discussed.

Particle-bubble attachment in flotation froths

During flotation, hydrophobic particles detach from the bubbles in the froth phase due to bubble coalescence, bubble bursting and froth overloading. While some detached particles drain back to the pulp phase, a fraction may selectively re-attach to the bubbles rising in the froth. This should result in grade profiles through the froth. Although this process has not been studied in much detail, qualitative evidence suggests that it occurs in deep, lightly loaded froths, such as in flotation columns. A mathematical model was developed to describe the process. The model takes account of the extent to which bubble surfaces are covered by floating particles and the changing characteristics of the froth with increasing height above the pulp-froth interface. These include the changes in the gas holdup and the bubble size. The model has been verified qualitatively by testwork in a laboratory-scale column flotation cell. Results indicated that the probability of attachment increased with an increase in the hydrophobicity of the draining particles, an increased bubble frequency and an increased gas holdup. It decreased when the coating of bubble surfaces was increased by the addition of fine floatable material. It further appears that a critical ratio of water to hydrophobic solids in the froth is required if selective attachment is to occur.

Reduction of magnesia in nickel concentrates by modification of the froth zone in column flotation

The reduction of the magnesia (MgO) content in a nickel concentrate was investigated for a nickel sulphide ore from Western Australia. Tests were conducted using both a conventional flotation cell and a column flotation cell. The conventional flotation tests show that the use of magnesia depressants were effective in controlling the magnesia levels in the concentrate. Of the depressants tested carboxy-methyl cellulose (CMC) and guargum were found to be the most effective. All of the column flotation tests were performed using a 7.0 cm diameter column. Depression of magnesia was studied by modifying the froth phase chemistry of the column via the wash water system. This allowed large addition rates of flotation modifiers to be used economically compared to high dosages normally added to the pulp phase. The depressant dosages tested in the wash water were equivalent to 0.66, 2.6, 4.6 and 6.6 kg/t in the froth respectively. It was found that CMC is a more effective magnesia depressant than guargum when added to both the pulp and wash water. Excessive addition of CMC show no obvious effects on the nickel grade and recovery compared with guargum where further addition above the optimum dosage had a detrimental effect on nickel grade and recovery. It was also found that the wash water pH can have a positive effect on concentrate grade and recovery.

Bubble characteristics in three-phase systems used for pulp and paper processing

Experiments are conducted to study the hydrodynamic behavior of three-phase pulp slurry systems at consistencies ranging from 0.1 to 1.0% (weight percent pulp). The pulp phase is a low-density fibrous material which swells to several times its original volume in water. The overall hydrodynamic behavior, including the regime transitions, of a multi-bubble, three-phase pulp slurry is studied by measuring the overall gas holdup and by using a light transmittance probe to determine the bubble characteristics. The results demonstrate that the hydrodynamic behavior of a three-phase pulp slurry varies from the behavior of a gas-liquid system. The deviation in the behavior becomes more pronounced as the pulp consistency increases. The bubble characteristics of bubbles injected through a single bubble injector are studied in a small rectangular column through the use of flow visualization and a particle image velocimetry technique. The measured change in the behavior of single bubbles injected into the pulp slurry through a single injector, is the basis for a proposed mechanism for the increased bubble coalescence in a multi-bubble system.

Probability of collision between particles and bubbles in flotation: the theoretical inertialess model involving a swarm of bubbles in pulp phase

The collision interaction between particles and air bubbles in the flotation pulp phase has been analyzed. The inertial effects on the particle-bubble collision probability are ignored (inertialess model). Special attention has been paid to the influence of a swarm of bubbles on the water velocity near the bubble surface. The approximation for water radial velocity near the bubble surface permits derivation of an expression for both collision probability and collision angle as a function, besides others, of gas holdup. This provides useful information for further understanding the particle collection (collision, sliding and adhesion) process in flotation. The model is for gas holdup in the range 0≤ ϵ≤0.30, for bubble Reynolds number up to 500 and for a ratio of particle size to bubble size D p/ D b⪡1.