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Ind. Eng. Chem. Res. 2010, 49, 2380–2387
Electroflotation Combined with Flotation Deinking of Flexographic Newsprint Akpojotor Shemi and Jeffery S. Hsieh* Pulp and Paper Engineering, School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst DriVe, N.W., Atlanta, Georgia 30332-0100
A semicontinuous flotation cell, which incorporates an electrolytic cell, was used to collect inks from newsprint pulp slurry. In general, longer pulping time has an adverse impact on deinking of 100% flexographic old newspaper or newsprint (flexo-ONP) while temperature has a positive impact on deinking. However, the use of electric field improved deinking efficiencysmeasured by the percent reduction in effective residual ink concentration (ERIC)sby as much as 5% at longer pulping time (25 min compared to 5 min). Higher temperature (40 °C compared to 25 °C) combined with the use of electric field further improved deinking efficiency by 5%. Furthermore, the impact of electric field assisted flotation is discussed in the context of the following variables: alkali charge (sodium hydroxide), float consistency (percent solids in the pulp suspension), surfactant and soap concentration, and the amount of flexo newsprint in a mixture of newspaper (or furnish)-newsprint. Unlike 100% flexo-ONP, the mixture of 48% flexo-ONP, 30% offset-ONP and 22% old magazine (OMG) contains both flexographic and offset inks, which have different ink sizes and chemistries. On average, considering all the variables, the use of electric field improved deinking efficiency by 3%. The best deinking efficiency was achieved at 1% consistency, when sodium hydroxide and surfactant and soap were added, combined with electric field. The deinking efficiency was corroborated with thermogravimetric analysis (TGA). Deinking efficiency is also discussed in terms of a deinking selectivity parameter called the Z-weighted factor. 1. Introduction The motivation behind the use of water-based flexographic ink is the reduction of the emission of volatile organic carbon compounds associated with many conventional solvent-based types of ink. The printing quality of water-based inks is crisp and the print has a lower ruboff and see-through.1,2 A wastepaper furnish which contains flexographic old newspaper or newsprint (ONP) is difficult to deink by the conventional flotation process. The problem with flexographic inks is related to their composition and properties. Flexographic inks are made of hydrophilic pigment particles, which are difficult to deink, by air bubble flotation. The flexographic ink particle diameter is 0.3-1 µm, and the offset ink particle diameter is 2-30 µm.3 McCool describes the removal efficiency of contaminants as a function of their particle size, and shows that flotation deinking is more effective in removing particles larger than 10 µm.4 Flexographic inks consist of carbon black pigments and acrylic resin binders that disperse the pigments in water. The poor floatability of water-based inks can be ascribed to the water-based acrylic resin, which in an alkaline medium prevents ink agglomeration by electrostatic and steric repulsion mechanisms.5,6 These small hydrophilic inks are ideal for deinking by washing. Washing can effectively remove ink particles of less than 10 µm, and it involves a series of diluting and dewatering (thickening) processes.7 Washing is especially effective for the removal of well-dispersed ink particles with the exception of some irreversibly redeposited inks in the fiber. However, washing consumes a large amount of water, and an elaborate water decontamination process is needed to clean the wastewater. Also, the yield loss associated with washing is economically unacceptable. A single wash deinking stage can have up to 35% yield loss.8 * To whom correspondence should be addressed. Tel.: (404) 8943556. E-mail:
[email protected].
Neutralization of the dispersion power of the acrylic resin by nonalkaline deinking conditions has been partially successful. Based on the fact that flexographic inks are not heavily dispersed in an acidic medium, Galland and Vernac9 used a two-stage flotation process for deinking of wastepaper that contains flexographic printed material and offset printed paper.10 The first stage or loop removes the flexographic ink particles in a neutral condition. The second loop uses an alkaline medium to remove the conventional ink particles. Using 100% flexo newsprint, with a starting 35% ISO brightness, the first stage acidic flotation gave a final brightness of 40%. After thickening and bleaching, a second stage of alkaline flotation improved the brightness from 46% to 55% ISO brightness. An alkaline flotation only process for 100% flexo newsprint improved the ISO brightness from 26% to 28%. At 26% ISO brightness, the effective residual ink concentration (ERIC) was 4760 ppm.10 Typical gains of pulp brightness around 10% ISO brightness11 and ERIC reduction of 65% through flotation are common in laboratory or mill operations for conventional offset-ONP. In a separate study with 100% flexo newsprint, only 10% reduction in ERIC was attained in conventional alkaline deinking.12 A new electric technology is proposed to improve deinking even in alkaline conditions. Besides ERIC and brightness, deinking efficiency was reported in terms of a thermographic analysis method based on ink and fiber decomposition temperature,13 and deinking selectivity (Z-weighted factor parameter, calculated as the ratio of ERIC or brightness and yield loss).14 The thermographic analysis (TGA) is used to corroborate ERIC measurements. While ERIC is based on ink optical properties, TGA is based on the estimated grams of ink. 1.1. New Electric Technology for Deinking Flexographic Ink. The electric field technology is essentially an electrolytic cell that consists of inert stainless steel electrodes and electrolyte solution. The electrolytic cell is incorporated into the flotation deinking process. The electrodes are connected
10.1021/ie9012985 2010 American Chemical Society Published on Web 01/27/2010
Ind. Eng. Chem. Res., Vol. 49, No. 5, 2010
to an external power source that drives nonspontaneous chemical reactions.15 Water constitutes the electrolyte. Electrolysis of water is the decomposition of water (H2O) into oxygen (O2) and hydrogen gas (H2) due to an electric current being passed through the water. In deinking operation, in addition to water, deinking chemicals, such as NaOH, Na2SiO3, and CaCl2, may be present and act as electrolytes. The premise for using an electric field in deinking waterbased inks is based on the fact that a process called electroflotation has been employed in the removal of suspended particles from effluents. These suspended particles are normally less than 20 µm and have close to neutral buoyancy. Empirically, very small bubbles, often less than 100 µm in diameter, can be used for the removal of fine particles. One of the methods to accomplish this aim is electroflotation, in which bubbles are generated by electrolysis.16,17 The removal of fine (
