Multi-salt coagulation of soft pitch colloids

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Abstract

The dynamic coagulation of colloidal pitch was quantified under orthokinetic conditions. The effects of single salt, multiple salt and cation valency on the stability of the colloidal pitch were investigated as a function of salt concentration. Critical coagulation concentrations (CCCs) were determined for a range of individual cations. The CCC in the presence of a number of divalent or trivalent cations was investigated to gain an understanding of the effect of multiple salts normally found in industrial systems.

Electrostatic destabilisation of wood resin colloids by a single salt is strongly influenced by salt valency (z) and mostly independent of the individual cation (at constant z). Addition of a second cation to solution resulted in a decrease in the CCC for both calcium and aluminium ions in the presence of sodium ions. The decrease in the CCC for the wood resin colloids was non-linear and showed restabilisation of the colloids above the CCC, unlike the effects observed for a single salt. Comparison of the experimental CCC results with the DVLO theory indicates that a higher Hamaker constant than reported for the interaction of abietic acid with talc in water (used here as a model interaction) is needed. This suggests that the aggregation process of the wood resin colloids involves a stronger interaction than abietic acid with talc. Experimental CCC values for divalent and trivalent ions were also lower than those predicted theoretically. To obtain agreement between theory and experimental results, reduced Stern potentials and higher Hamaker constants were required, suggesting specific ion adsorption of the multivalent cations is occurring to reduce surface charge of the colloid.

Highlights

► Effects of single and multiple salts on stability of wood resin colloids studied. ► Addition of second salt decreases first salt CCC and causes restabilsation of colloid. ► Zeta potential measurements show charge reversal with addition of aluminium ions. ► Experimental CCC values lower than predicted from theory. ► Differences suggest specific ion adsorption occurs with calcium and aluminium ions.

Introduction

The stability and aggregation of colloidal particles are important to many industrial processes such as paint, food, petrochemical, pharmaceutical, mineral processing, and papermaking [1], [2], [3]. Industrial colloidal systems can be quite complex and a sound understanding of the factors affecting colloidal stability is necessary in order for good process control to be achieved.

In papermaking the naturally occurring lipophilic wood extractives (also called wood resins) released during wood pulping, form colloidal pitch particles that can aggregate and form sticky deposits that cause significant process problems [4], [5], [6]. These problems are aggravated as most pulp and paper mills actively reduce water usage through intense process water recycling. Water recycling increases the concentration of the lipophilic extractives and other substances, such as dissolved organic compounds and inorganic salts that affect colloidal stability of the wood extractives in the process water [6], [7], [8].

The aggregation process can be understood in terms of the attractive and repulsive forces acting on the particles [9], [10]. The DLVO theory defines the coagulation forces in terms of the attractive van der Waals forces and the repulsive electrostatic forces under perikinetic (Brownian motion) controlled coagulation [9], [10]. However, in most industrial systems orthokinetic (fluid shear) controlled coagulation predominates. This will influence the rate of particle–particle interaction and inter/intra particle forces resulting in discrepancies on comparison to DLVO predictions [1], [11], [12].

Experiments with systems of rigid, weakly charged particles at relatively low ionic strength conditions (<5 × 10−2 M) with monovalent electrolytes have shown agreement with traditional DLVO theory [13], [14], [15]. The addition of multivalent salts has been found to result in deviations from predicted values in some cases [16]. This is attributed to the multivalent salts not behaving as indifferent electrolytes but adsorbing and interacting with the surface to neutralise the surface charge [16] and the side reactions that they can undergo in solution [17]. High ionic strength (>0.1 M) and highly charged particles [13], [14] also result in discrepancies. At high ionic strengths, the charge on the particle is screened and the barrier to coagulation lies at separations of less than 2 nm. At these short distances, it is believed that other short-range non-DLVO forces come into play [14], [15].

Wood resins in solution form a soft colloidal system capable of reconformation [16] and coalescence of particles under orthokinetic conditions [17]. This paper aims to quantify the effect of various cations of different valency on the colloidal stability of a soft pitch colloid under orthokinetic conditions. The effect of multiple cations on the coagulation of the wood resin colloidal dispersions is also studied to determine if the addition of a second destabilising salt to the colloidal dispersion results in a competitive or additive effect on the destablisation of the colloids.

Section snippets

Experimental

NaCl, KCl, CaCl2 and KNO3 (all 99.8%) and AR HNO3 were purchased from BDH. MgCl2, Al2(SO4)3 and FeCl3 (99.8%) were obtained from Merck. All electrolytes were dissolved in deionised water as stock solutions.

Thermomechanical pulp (TMP) made from Pinus radiata was collected from the primary refiners at Norske Skog, Boyer, Tasmania. The pulp was freeze-dried and soxhlet extracted for 8 h with hexane. The hexane was evaporated to obtain the lipophilic wood resins that were stored at −20 °C until

Results

Table 2 summarises the chemical composition, particle size and zeta potential of P. radiata wood resin.

Salt induced coagulation experiments were performed with monovalent (NaCl and KCl), divalent (CaCl2 and MgCl2) and trivalent (FeCl3 and Al2(SO4)3) salts. Fig. 1 shows the aggregation behaviour observed by PDA, on the addition of various concentrations of CaCl2 to the colloidal wood resin dispersion. Similar curves were obtained for all the salts. The slope of the aggregation region represents

Discussion

The Na+ and Ca2+ CCCs measured for a pitch colloidal suspension (Table 3) are higher than those previously reported [7], [21], [22]. The published Na+ CCC for pitch are 100 mM [22], 150 mM [7] and 200 mM [21]. These differences in CCC can be explained in two ways: differences in pitch composition and variations in analytical method and level of shear during the measurement. P. radiata wood resins are higher in resin acid content than spruce and so have a very different extractives composition (

Conclusion

The mechanisms by which multiple salt addition and salt valency can affect colloid stability under orthokinetic conditions were studied. A wood resin (pitch) colloidal suspension was selected as model system due to the high industrial impact of colloid destabilisation (pitch deposition). The addition of multiple cations significantly reduces the critical salt coagulation concentration (CCC) of a wood resin colloid suspension compared to that expected from single salt addition. Restabilisation

Acknowledgements

Financial support for this project was provided by Norske-Skog Paper and an Australian Research Council Linkage Grant LP882355.

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