Introduction

The incorporation of secondary fiber into the paper making process is a strategy that has been used for decades. The information relating to the reuse of secondary fiber dates back to the 17^th century in Denmark; however, it was not until the apparition of the paper machine, at the end of the 18^th century -perfected by the Fourdrinier brothers and with progressive implementation throughout the 19^th century-, that the secondary fiber began to be utilized on a large scale (^{Sánchez, 2000}).

In the last 20 years, there was a sharp worldwide increase in the reuse of secondary fibers, which rose from 5.9 to 45 million tons per year (^{Costa and Rubio, 2005}).

Recycled paper (secondary fiber) is used for the manufacture of toilet paper, tissue paper and paperboard, which is considered to be low quality. Indeed, the whiter the product, the better its quality; a less white hue is the result of the presence of ink residues in the recycled paper pulp (^{Alliot et al., 2004}).

The development of information technology and the sustained growth in the use of personal computers have increased laser and xerographic printing, and therefore the consumption of paper, resulting in a good availability of high quality fibers for reuse. However, it should be noted that the deinking process is closely related to the printing method, the type of ink and the method utilized to dry the ink (^{Alzate y Alzate., 2012}).

^{Manning and Thompson (2004)} point out that the particles of ink used in laser printing are made of carbon black, iron oxide, polystyrene, polyamide and polyester, among other products that are used to adhere the carbon black to the paper when it melts during the printing process. According to ^{Álvarez (2006)}, among the main problems related to the use of recycled printing paper by electronic methods, there is notably little information about the chemical characteristics and the physico-chemical properties of the residues of ink that are left after the deinking, and of the remnants after this process that can contribute to the formation of anionic impurities.

On the other hand, in countries like Japan and the United States of America, most of the plants normally use the washing process in addition to flotation; while, in regions of Europe and Canada only flotation is used, in an alkaline medium and at moderate temperatures(^{Theander y Pugh, 2004}). The elimination of the ink in the flotation phase is one of the key points for obtaining high-quality deinked fiber (^{Pélach, 2015}), through physico-chemical mechanisms. The success of the flotation process lies in three main characteristics: the detachment of the particles of ink on the fibers; the effective adhesion of the particles of ink to the air bubbles which are dragged to the surface of the cell; the elimination of the foam and inks. The use of surfactants in the flotation and washing phases can have a positive or negative effect on the deinking process. Its primary function is to release the ink particles from the fibers, as well as to stabilize them; this contributes to the elimination of inks and improves the properties of the surface of the fibers during flotation (^{Zhao et al., 2004}). ^{Theander and Pugh (2004)} indicate that the formation of foam and the hydrophilic-lipophilic balance (HLB) are important parameters, when using non-ionic surfactants, as these affect the whiteness, and also the efficiency of the washing and of the flotation.

According to research carried out in the mid-90s on the use of ultrasonic devices with frequencies of 22, 23 and 54 kHz as a new method (^{Norman et al., 1994}), the ink particles are not easily removed from the fibrous suspension by conventional methods, due to their large size. ^{Norman et al. (1994)} proved that the low frequencies (22 kHz) were more effective to break ink particles larger than 400 µm, while the high frequencies facilitated the removal of small particles by flotation. In a study of bond paper deinking at a frequency of 40 kHz with laser printing, ^{Ramírez (2004)} points out that the best conditions to separate particles of less than 100 µm were: 0.5 % of consistency, high modulation in ultrasound, 55 °C of temperature in the fibrous suspension, a pH of 5, and 20 minutes of ultrasound treatment. An increase of 91 % of the ink particles in the 1-50 µm interval, and of 27 % in the 50-100 µm interval was achieved; this facilitated efficient separation of the toner particles through flotation and washing.

The purpose of the present study was to evaluate the effect of deinking laser printing paper, through the application of intensive action ultrasound and in the later phases of flotation and washing; to understand the phenomena associated with the deinking process, and to determine the optimal values of the parameters involved in the process in order to achieve the highest efficiency per m² in the removal of ink particles.

Materials and Methods

The study was carried out with Maxbrite^TM letter-size bond paper sheets of 75 g m^-². In order to standardize the amount of ink deposited on each sheet, one of their faces was printed, using a laser printer (HP LaserJet 1022), with the following legend: Deinking of laser printing, using sequences with intensive action ultrasound, oriented toward the production of high purity cellulose, which is repeated several times until 45 rows were completed; a size 12 bold Times New Roman font was used at double space. An approximate amount of 0.1872 g of toner was thus deposited on each paper sheet.

Results and Discussion

Tables 2 and 3 show the optical characteristics and the chemical analysis of the bond paper sheets, respectively. The information in Table 2 indicates that the paper contains non-degraded cellulose (α-cellulose) and relatively high hemicelluloses (β and γ-celluloses). There is also a high Kappa number, which implies an unusually high lignin content for a bleached chemical pulp. Both values suggest that the raw material may include mechanical pulp mixed with chemical pulp in its composition.

The quantity of ashes in the sample is not very high, which suggests that the amount of mineral loads in the paper corresponds with the limits considered as normal in a bond paper formulation.

Table 3 shows the whiteness and the parameters of color, according to the space of the 1976 CIE L*, a*, b* hues, measured on the bond paper. An average whiteness of 94.41 % ISO is observed, with a brightness or luminosity (L) of 91.54 %, and a coloring within the red/blue color plane, leaning more toward the blue color (b=-10.73). This trend reflects the fact that the paper is bleached using an oxidative process combined with the use of optical brighteners, which would account for the high level of whiteness observed.

One of the main objectives of this research was to apply an ultrasound treatment in order to promote effective fragmentation of toner particles, in such a way that it will generate the greatest possible amount of ink particles, with sizes between the intervals of 25-50 and 50-100 µm, to facilitate its removal by the flotation and washing stages.

Figure 1 shows a response surface graph based on the statistical analysis of the particle size distribution data, obtained through the application of experimental design at the treatment phase with high-gain ultrasound. The response variable considered was the count of residual ink particles per m² in 25-50 µm. It is evident that in order to generate a larger quantity of ink particles per m², it is necessary to work using 20 min time periods, consistencies of 0.5 % at a temperature of 25 °C and with a continuous application of ultrasound.

No. partículas tinta/m² = Number of ink particles/m²; Tiempo = Tiempo; Consistencia = Consistency; Superficie de respuesta estimada = Estimated response surface; Tratamiento con ultrasonido = Treatment with ultrasound; Part entre 25-50 μm = Particle between 25-50 μm; Temp. = Temperature; Forma de aplic. = Application form; Continua = Continuous.

Figure 1 Response surface for the ultrasound treatment phase with ultrasound for the detachment of ink particles per m² in the range of 25-50 µm, time vs consistency. 

Figure 2 shows the response surface graph in the case of the statistical analysis, in which the response variable was the content of residual ink particles in the paper with size of 50-100 µm. The trend observed in Figure 2 indicates that in order to generate a greater amount of ink particles m² of 50-100 µm, a 20 min ultrasound treatment with a consistency of 0.5 % is required.

No. partículas tinta/m² = Number of ink particles/m²; Tiempo = Tiempo; Consistencia= Consistency; Superficie de respuesta estimada = Estimated response surface; Tratamiento con ultrasonido = Treatment with ultrasound; Part entre 50-100 μm = Particle between 25-50 μm; Temp. = Temperature; Forma de aplic. = Application form; Continua = Continuous.

Figure 2 Response surface for the ultrasound treatment phase for the detachment of ink particles per m², 50-100 µm, time vs consistency. 

Because the laser printing inks are usually made of a pigment and a thermoplastic resin, the polymers of the resin melt and become resistant to the action of chemicals, which renders the ink on the printed paper difficult to remove with conventional processes (^{Borchardt et al., 1998}).

The statistical analysis of the data of the treatment phase with high-gain ultrasound revealed that the optimum conditions to maximize the number of ink particles, sized within the ranges of interest, were the following: treatment time of 20 minutes, consistency of 0.5 %, temperature of 25 °C and continuous application of ultrasound. These values were used to deink paper, under optimized conditions of treatment with ultrasound, and to analyze the general behavior of the proposed deinking process.

Once the paper was deinked under optimum conditions through a process that included disintegration, ultrasound, flotation and washing, the sheets were formed for their respective evaluation. Below are the results of tests for the determination of: α, β and γ-cellulose, and percentage of ashes (Table 4). The determination of α, β and γ-pulp was carried out under the norm T 203 om-88 (^{TAPPI, 1998}).

The ash content was determined according to the norm T 211 om-02 (^{TAPPI, 2002}).

Table 5 shows the measures of whiteness and the L*, a*, b* of the formed paper sheets, made after the phases listed above. The degree of whiteness was measured with the Elrepho 3000 spectrophotometer.

Low values were registered for whiteness at the disintegration phase because only the paper was disintegrated (Table 5).

In the ultrasound phase, the results for the whiteness and L*, a*, b*, appear to be low. However, the generation and division of the ink particles per m² at this stage corresponded to the intervals of 25 to 50 µm and 50 to 100 µm; these ink particles were easily removed by flotation and washing. The level of whiteness was low, and so were the L*, a*, b*, values that represent the tone of the fibrous suspension treated with ultrasound. L* was positive, with a blue-gray hue; a* was also positive, with a reddish-blue hue; conversely, b* was negative, with a blue-gray hue in a grayish fibrous suspension.

As a result of flotation, it was possible to obtain an optimal whiteness of 83.74 % without the use of any chemical reagent. The results for L*, a* and b* are good in the color scheme of the fibrous suspension; L* was positive, with a grayish-blue hue; a* was positive with a reddish-blue hue; and b*, was negative, blue-gray. These shades are reflected on the tonality of the fibrous suspension as positive in terms of the coloring and the whiteness obtained.

With regard to the washing phase, there has been an increase in the whiteness; with an optimal of 86.98 %, which confirms the removal of particles of ink through the use and deinking of bond paper with intensive ultrasound. According to ^{Fricker et al. (2007)}, the washing phase is highly effective for removing particles sized under 10 µm. The results in the tonality of the L*, a*, b* fibrous suspension in the washing phase were positive, which increased the final coloring of the fibrous suspension even more.

Figure 3a shows the surface of a sheet with disintegrated paste before treatment with ultrasound. Figure 3b exhibits the image of he sheet prepared with an intensive ultrasound treatment. As can be seen, a large number of ink particles were generated, or many were fragmented, which led to a blue-gray coloration on the surface of the formed sheet.

A) Disintegration; b) Treatment with ultrasound; c) Floating; d) Washing.

Figure 3 Images of the surface of the sheets obtained using the deinking process with intensive ultrasound. 

Figure 3c shows the surface of a sheet obtained with treated paste at the later stages of ultrasound and flotation, in which the elimination of a large amount of toner particles is evident. The surface of the sheet showed a reduction of the gray-blue shade originally observed. Finally, Figure 3d shows the properties of the sheet formed after the ultrasound, flotation and washing phases. This surface represents the highest reduction of ink particles, which gave a greater whiteness to the paper.

Conclusions

Deinking of laser printed bond paper is possible when previously treated with ultrasound of intensive action, since it made it feasible to generate a greater amount of ink particles per m² at intervals of 25-50 µm and 50-100 µm, which are easy to remove through flotation and washing.

There is an important relationship between the temperature and the detachment of the toner particles from the fibers; i.e. higher temperatures, of or above 80 °C, during the application of the ultrasound are not conducive to a good ink particle detachment from the fibers; this is reflected in a reduction of whiteness. However, a temperature of 25 °C results in a better removal of the toner particles present in the fibers; this is one of the main factors in the deinking process with ultrasound.

The treatment time, temperature, consistency and form of application of the ultrasound to the fibrous suspension are very important in the process. It was observed that 20 minute periods at a temperature of 25 °C, a consistency of 0.5 % and continuous application of ultrasound result in a better detachment of ink particles per m²; and in an improved whiteness at the later stages.

Flotation and washing are very important in the removal of particles of ink after the implementation of the process with ultrasound to the fibrous suspension; however, it is necessary to add a surfactant froth, as this allows the removal of the ink particles of 25-50 µm and 50-100 µm, respectively.

The properties of the pulp obtained under optimized conditions indicate that it can be used to generate high purity cellulose through subsequent bleaching.