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Optimization of the use of polyethylene oxide and calcium carbonate in papermaking Pang, Peter

Abstract

Traditionally, high molecular weight polyethylene oxide (PEO) has been used as a flocculant (retention aid) in conjunction with another chemical called cofactor for fiber fines and mineral filler retention when using mechanical pulps. The role of the cofactor is to enhance the retention of PEO. An alternative approach is to use PEO alone above the cloud point temperature (CPT) of the PEO-water solution. It has been shown to enhance retention (flocculation). Clay is a typical mineral filler for paper. Calcium carbonate is another one with a number of desirable characteristics. However, application of calcium carbonate in acidic papermaking is limited by its extensive dissolution under acidic conditions. In addition, under its natural pH (8-9), calcium carbonate is not suitable for use because at these pH conditions alkaline darkening of mechanical pulp occurs. Hence, there is an optimum pH window typically between 6.9-7.2 and a requirement to minimize the dissolution of calcium carbonate. One method to minimize dissolution of calcium carbonate is to use phosphoric acid. The main objectives of this work were (i) to illustrate how the flocculating capability of the PEO can be significantly enhanced in the absence of a cofactor by inducing PEO-water phase separation, (ii) to understand how the inhibition mechanism operates and to develop an improved treatment process for inhibiting the dissolution of ground calcium carbonate (GCC) and (iii) to propose methods for implementation in papermaking. Retention of fines, clay and GCC under papermaking conditions was significantly enhanced at temperatures above the cloud point temperature (CPT) of the PEO-water system. The CPT is defined as the temperature at which a homogenous aqueous PEO solution turns into two phases. The CPT was found to decrease with increasing electrolyte content and shear. PEO was found to flocculate clay and phosphate-pretreated GCC effectively at temperatures above the CPT. Particle size measurements showed that the kinetics of flocculation was affected by the temperature deviation from the CPT, shear, PEO concentration and molecular weight. Surface erosion caused by the shearing action of the water surrounding the floes is believed to be the main mechanism for the breakup of the floes. In the absence of colloidal particles, the phase-separated PEO polymers were also found to interact with themselves and form macroscopic aggregates. It is proposed that the mechanism for the flocculation of colloidal particles involves a cooperative action of enhanced adsorption of phase-separated PEO onto colloidal particles (clay and GCC) and entrapment, of colloidal particles into the phase-separated PEO aggregates. A pretreatment method, which involves mixing a thick stock GCC suspension with phosphoric acid for 24 hours, was found effective in inhibiting the GCC dissolution in acidic mechanical pulp suspension. Surface analysis confirmed that the inhibition was brought about by the precipitation of insoluble calcium-phosphate containing overgrowths on the GCC surface, which act as a protective barrier.

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