Functional and Performance Characteristics of Soluble Silicates in

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Functional and Performance Characteristics of Soluble Silicates in Deinking. Part I: Alkaline Deinking of NewsprinVMagazine

T. ALI, F. McLELLAN, J. ADIWINATA, M. MAY and T. EVANS

The function(s) of soluble silicates in repulpinghashing and flotation deinking of waste newsprint/magazine were investigated extensively, under optimized conditions. During repulping/washing, silicate ’s response was characteristic of a buffer and a detergent. It was responsible for a 3.0% increase in I S 0 brightness. In repulping/flotation, silicate accounted for a 6.1% increase in I S 0 brightness. Part of this enhancement was likely caused by silicate’s ability to create larger ink particles which were more easily removed. Silicate also dem- onstrated detergency functions. In the presence of hydrogen peroxide, approximately half of silicate 5 function was the deactivation of metal ions, resulting in stabilization of peroxide.

INTRODUCTION Deinked paper has traditionally been

used as a substitute for virgin fibres. In North America, the impetus for using deinked fibre is based on a number of influencing factors. These include decreasing landfill space, a

JP T. Ali, F. McLellan, J. Adiwinata, and T. Evans

P.O. Box 69, Station N Toronto, Ontario M8V 3S7

University of Toronto Dept. Chem. Eng’g. Toronto, Ont. L5L 1C6

ps National Silicates Ltd.

M. May

greater global demand for paper, diminish- ing wood supply coupled with social con- cern over deforestation and, impending legislation mandating the use of recycled fibre.

The process of deinking often in- volves the physical separation of oil-based printing inks from a mechanical fibre sub- strate. A wide variety of chemical additives is used in the deinking process, including soluble silicates.

Soluble silicates have been used extensively in peroxide brightening of virgin mechanical fibres. Recent work demon- strated conclusively that the most important function of soluble silicates was the stabilization of hydrogen peroxide, either by direct deactivation of transition metals and/or indi- rectly by pH buffering [l-31. While it is reasonable to expect these important functions of silicate to have significance in deinking of wastepaper, its other role(s) as a dispersant, emulsifier, anti-redeposition and wetting agent (i.e. detergency) may also be important. Furthermore, it is conceivable that silicate performs other important functions, for example, in the collection and flotation of ink.

The focus of this study was to systematically investigate and quantify the relative importance of the various functional characteristics of soluble silicates in deinking.

EXPERIMENTAL All experiments reported in this paper

were performed using 70% Globe & Mail

and 30% magazine. The magazine was a composite of Time, Newsweek, T.V. Guide, and Business Week. The Globe & Mail is printed almost exclusively by an offset process and was chosen because of its brightness repeatability from sample to sample.

The deinking procedures outlined below were developed at the National Silicates Limited Technical Centre.

REPULPING 1. Fifty grams 0.d. (70% Globe & Mail +

30% magazine) were weighed out. 2. Repulping chemicals were weighed out

individually. 3. The waste paper was placed in a Hobart

mixer modified with a constant temperature control unit and lid. Hot water was added to obtain the desired temperature and consistency.

4. The repulping chemicals were added in the following order: caustic, silicate, sur- factant and peroxide.

5. The sample was mixed into a slush for 1 min. A sample was then extracted for pH measurement.

6. The sample was then repulped at the desired speed for the required time (optimum 20 min).

7. After repulping, a sample was removed for residual pH and chemical determination. A portion of the sample was washed or floated to remove the ink. The remain- der of the original sample was sealed in a polyethylene bag and placed in a constant temperature bath (usually 60°C) for

JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 1 JANUARY 1994 J3

the desired bleaching time (optimum 20 min).

8. After bleaching, a sample was removed and the ink removal step outlined below was conducted.

WASHING 1. After repulping andor bleaching, each

sample was diluted to 0.5% consistency, "soured" with sulphurous acid if peroxide was present or sulphuric acid if not, to a pH of 5.5.

2. The sample was placed in a DDJ (dynamic drainage jar) and washed at 2000 rpm using a 450-500 mesh (19.3 pm) screen.

3. Three grams 0.d. brightness pads were made by filtering the accepts through filter paper in a Buchner funnel.

FLOTATION 1. Six hundred grams 0.d. of wastepaper

and deinking chemicals were weighed out as before.

2. Chemicals were added in the following order: hot water, caustic, silicate + fatty acid, DTPA (if desired), and peroxide.

3. After repulping andor bleaching, the sample was transferred to a flotation cell. Calcium chloride (200 ppm as CaCO3) was added to the water in the cell. The consistency for flotation was 0.8%.

4. The sample was adjusted to pH 8.5 in the flotation unit.

5. A sample (3.0 g 0.d.) was taken for brightness reference.

6. Foamer (usually Triton X-100) was added and the sample was floated for the desired time, usually 10 min.

7. Rejects were collected for metals analysis and yield loss determination. Accepts were removed from the cell and the pH was adjusted to 5.5.

8. Brightness pads were made as before.

BRIGHTNESS Brightness sheets were prepared in

accordance with Technical Section, CPPA Standard C.5, except that the sheets were formed on a #4 filter paper in a Buchner funnel. Brightness readings were recorded on a Data Colour 2000 Spectrophotometer

as described in Technical Section, CPPA Standard E. 1.

RESULTS AND DISCUSSION RepulpingMlashing

Many roles have been suggested for soluble silicate during the repulping of waste newsprint/magazine; these include: absorp- tion of metal ions [4,5], pH buffering [2,6,7], peroxide stabilization [ 1,3,6], dispersing agent [8,9], emulsification [9-111 and wetting agent [8,9]. There has not, however, been any previous attempt to systematically reconcile and quantify the results of empiri- cal deinking data with these reported functions.

Before investigating the role -of sodium silicate in repulping, conditions (consistency, time and ' temperature) were optimized with and without hydrogen peroxide, using a dynamic drainage jar (DDJ) as a precise ink removal technique. The DDJ did not result in fibre cutting as determined by fibre fractionation following treatment over a range of mixing speeds (500-4000 rpm). The total solids removed by the technique were typically 8.0 f 1.5%, thus only ink particles of diameter 19.0 pm or less and colloidal material were removed, while most of the fines and fibres were retained.

Consistency The optimum repulping consistency

was estimated to lie in the range of 10-14%. Repulping at lower consistencies gave incomplete defiberization and consequently poor ink removal. At higher consistencies (214%), the limited volume of liquid phase was insufficient to suspend the ink. Conse- quently, the ink was smeared and redepo- sited back on to the fibre and could not be removed by laboratory washing (DDJ).

Temperature Repulping experiments were per-

formed over a temperature range of 22- 100°C. Significant brightness improvement resulted when the temperature was increased from 22 to 60"C, however, there was only a slight advantage (0.5% ISO) to repulping at higher temperatures (60-100°C).

i

Time The optimum repulping time was de-

termined to be approximately 20 min. This was essentially the minimum time required for complete defiberization.

With consistency, time and temperature optimized, the next experiments were designed to examine the effects of silicate and caustic during repulping. Figure 1 de- picts the effect of silicate, at various alkalinities, with respect to pulp brightness following DDJ ink removal. In the absence of sodium silicate, a maximum brightness of 48.4% I S 0 could be achieved with minimum addition of alkali (0.25% on pulp); however, it was evident that moderate alkali charges (20.5%) were required to achieve complete defiberization of the wastepaper. At low alkali dosage the repulped waste paper contained large pieces of ONP/magazine with readable prints. Increasing the alkalinity resulted in significant alkali darkening of the samples.

In the presence of sodium silicate (see Fig. 1), significant brightness improvements were noted over the full range of alkalinities tested; for example at 0.5% total alkalinity, addition of 5.0% sodium silicate was responsible for a 3.0% I S 0 brightness improvement versus no silicate. Irrespective of the total alkalinity, increasing the dosage of sodium silicate increased the post DDJ brightness of the deinked pulps. This is important because the addition of silicate expands the practical range of alkali charges that may be used with minimal alkali darkening.

Although the data in Fig. 1 clearly show that modest alkali addition darkens wastepaper, there was some quantitative evidence that substantial deinking was taking place. Starlab (L*) is often used as a measure of relative whiteness (or blackness) and is based on reflectance measurements taken using the Y filter. The data presented in Fig. 2 suggest that, for printed wastepaper, as the total alkalinity is increased the increase in L* could be explained by an ac- companying decrease in highly absorbing species (ink?). This observation was sup- ported by the fact that L* for unprinted virgin newsprint decreased systematically under identical conditions, presumably in response

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Fig. 2. Characteristic L* response of virgin news and waste newslmagazine to repulpinglwashing and alkali. Fig. 1. Repulpinghvashing with sodium silicate and caustic soda.

J4 JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 1 JANUARY 1994

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Fig. 3. Characteristic brightness of virgin newsprint and printed newsprintlmagazine after repulping/washing.

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Fig. 6. Image analysis of ONP/magazine after repulping/washing deinking.

to the formation of chromophores (alkali darkening). It should also be noted that in both cases the brightness decreased.

The 3.0% IS0 brightness improvement observed in the presence of sodium silicate (Fig. 1) may be explained by two distinct functional characteristics of soluble silicates: pH buffering and detergency.

Buffering The buffering properties of sodium

silicate are well documented [2,6]. If this is of any importance in wastepaper repulping, it has not been adequately reported. Con- sider the brightness of virgin and unprinted newsprint, in response to alkali treatment with and without silicate, shown in Fig 3. At

low alkalinities, the presence of sodium silicate reduced the brightness loss by 1.0% ISO. At higher alkalinities (2.0%), the brightness loss was reduced by as much as 3.0% ISO. From these observations it is ap- parent that silicate's buffering capacity is more important at high alkalinities compared to low alkalinities. Similar observations were noted for printed wastepaper under identical alkali treatments. Further evidence of the buffering role of silcate in repulping wastepaper is illustrated in Fig. 4. The amount of yellowing indicated by b* is affected very little by silicate addition at low alkalinities (0.5%). The degree of yellowing was significantly less in the presence of silicate over the alkali range of 0.5 to 2.0%. These data correlated well with the pH measurements made in each case.

Detergency In the case of sodium silicate, the term

detergency has been used to describe its ability to wet, disperse, emulsify and suspend soils in aqueous systems [%lo]. We have shown that the presence of silicate in alkaline wastepaper repulping improves the brightness of washed pulps significantly and that pH buffering plays a key role in its effect. In the absence of a brightening agent and with constant scattering coefficient, the only other way to improve brightness is to remove light-absorbing material. Figure 5 presents the Starlab L number (L*) of wastepaper samples repulped at various silicate dosages over a range of total alkalinities. As noted previously (see Fig. 2), L* increased with increasing alkali charge in response to greater ink removal. At low alkalinities, L* was improved significantly in the presence of sodium silicate. The ability of silicate to improve L* over and above that of caustic alone suggests that sodium silicate was removing more ink.

Further evidence of silicates detergency role is shown in Fig 6. The image analysis data clearly show two interesting trends. First, the addition of 5% silicate decreased the number of ink particles remaining in the sample over the full range of ink particle sizes. In order to remove more ink,

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JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 1 JANUARY 1994 I

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Fig. 8. Repulpinghvashing deinking with 1.0% hydrogen perox-

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Fig. 10. Performance characteristics of silicate in repulpinglflotation deinking. Optimum brightness vs. 'YO silicate.

silicate must disperse the ink in the aqueous phase and prevent its redeposition onto the fibres. Since L* of post-washed samples is higher for silicate-treated samples and the image analysis shows that there is less ink over the entire distribution, silicate must be acting as a dispersant and anti-redeposition agent. The second interesting trend can be seen when comparing the % of particles vs. particle size (Fig. 6). At low particle size (11 pm or less), the 5% silicate sample had a significantly higher percent of particles. In contrast, at the larger particle sizes (16 pm or greater) the 5% silicate has a smaller percent of particles. This suggests that silicate must be emulsifying and dispersing the ink particles, preventing coalescence into larger ink particles. Thus silicate improves the initial separation of ink from the fibre and its subsequent removal by washing.

In summary, at low alkalinities the buffering effect of silicate is negligible and the improvement in brightness achieved must be due to silicate's ability to emulsify and disperse ink, preventing its redeposition onto the fibre surface. At high alkalinities, the detergency advantage of silicate ap- peared to be only marginally greater than that of caustic alone. Consequently, the brightness improvements were due to silicate's buffering action.

Silicate, Caustic and Hydrogen Peroxide

Under conditions of optimum total alkalinity, addition of 0.5% hydrogen peroxide (Fig. 7) to the repulping experiments produced a 1.6% IS0 improvement in brightness with no silicate present versus caustic alone (Fig 1). Under optimized conditions (temperature, time, consistency, and total alkalinity) the addition of silicate produced an additional 4.5% I S 0 brightness increase (0% silicate compared to 5.0% silicate).

Inclusion of peroxide to the repulping experiments resulted in a shift of the optimum alkalinities. The optimum alkalinity was also dependent on silicate dosage. (Figs. 8, 9). In both cases (1 and 2% peroxide) there was a strong dependence of brightness gain on the alkalinity dosage, as expected. The benefits derived from silicate with respect to brightness gain were 5.0,7.2 and 7.9% IS0 for 0.5, 1.0 and 2.0% peroxide, respectively. Therefore, the stabilizing function of silicate played a more dominant role as the peroxide level was increased. It was shown in our discussion under "detergency" that silicate alone was responsible for 3.0% ISO. In the presence of 0.5% peroxide this increased to 5.0% ISO. Thus, approximately half the role of silicate in repulping/washing was peroxide stabiliza-

tion, the other half could be attributed to its function as a detergent and buffer.

REPULPING/FLOTATION In flotation deinking, the objective of

the "deinking" chemical is to remove ink particles from the fibre surface and agglom- erate them into a particle size that is large enough for removal by flotation (air bub- bles). As before, in the washing experiments, the objective was to identify the performance characteristics of sodium silicate and to quantify its benefits. In order to do this, optimum repulping conditions were estab- lished for two peroxide levels (0.0 and 1 .O%) over a range of silicate dosages. The optimum T.A.s for 0.0 and 1 .O% peroxide were determined to be 0.5 and 1.0% respectively. Unlike the washing data, the optimum alkalinity was not dependent on silicate dosage, but was dependent on peroxide charge and fatty acid addition rates.

Silicate's effect on repulping/flotation deinking under optimum repulping conditions is shown in Fig. 10, for both peroxide charges. In the absence of peroxide, silicate was responsible for a 6.1 % IS0 increase in the post-floated brightness. The benefit of silicate use in repulping/flotation ink removal was substantially greater than the benefit derived from its use in repulpingl washing (6.1 compared to 3.0% ISO). Using


1.0% peroxide, the benefit of silicate was 9.6% IS0 and the total brightness gain was 12.4% ISO. The delta brightness line (plot- ted on Fig. 10) is indicative of the peroxide stabilization effect of sodium silicate. The brightness increase due to peroxide stabilization was linearly dependent on silicate dosage. For example, at 0.0% silicate the benefit due to peroxide was 2.7% ISO, compared to 6.3% I S 0 at 5.0% silicate. Since the total brightness gain was 12.4% IS0 under optimized conditions, and silicate's peroxide stabilization role was responsible for 6.3% ISO, the remaining 6.1% IS0 can be explained by silicate's other roles. In repulpinglwashing these functions were shown to be anti-redeposition, dispersion and emulsification. Whether or not these same mechanisms take place in a flotation process remains unknown.

The image analysis data shown in Fig. 11 for flotation experiments under optimum conditions shed some light on the function of silicate. The data clearly show that, compared to ink removal by washing, there were substantially fewer ink particles, approximately 50% over the entire ink particle size range. This was likely a result of the oleic acid acting as an ink collector and the use of a more efficient method of ink removal (brightness for flotation is higher than that of washing). The image analysis data also show that 5% silicate compared to 0% silicate resulted in fewer small particles (300 particles between 0.00 and 5.66 pm vs. 110 particles for 5% silicate over the same particle size range). As the particle size increased the difference between the number of particles for a given range decreased until one reached the 22.63-32.00 pm particle size range. Here the number of particles for 0 and 5% silicate were essentially the same. Once above 32 pm in size, 5% silicate had a greater number of particles. The number of particles, for 5% silicate, over the entire distribution was relatively flat. These data along with the frequency data (Fig. 11) both suggest that silicate somehow enhanced ink removal by forming larger ink particles which were more effectively removed in dispersed air flotation.

Larsson et al. investigated the floatability of model ink particles in the presence of sodium stearate, sodium silicate and calcium chloride using a multifactorial experimental design [ 121. Their results showed that the presence of silicate had no significant effect on the mean ink particle diameter in the repulper and therefore no influence on the floatability of the ink. These data were in direct contradiction with our results which showed an increase in mean particle size from 11.0 pm with no silicate to 18.8 pm with 5.0% sodium silicate. One possible explanation for the difference in results could be proposed on the basis that different fatty acid collectors were used for the two series of experiments.

The exact mechanism for this enhancement of ink removal by repulping/ flotation remains unknown; but one explanation may be that silicate prevents the ink 1

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MODE OF ADDITION OF SILICATE

Fig. 12. Effect of silicate during flotation deinking.

suspended in the aqueous phase from rede- positing on the fibre surfaces. An alternative explanation for the enhancement of flotation ink removal by silicate could be based on its known action as a flocculation aid. Experi- ments investigating the mode of addition of silicate (Fig. 12) support both mechanisms for silicate improvement in flotation efficiency. Since peroxide was present in these experiments, the samples were soured with sulphurous acid after repulping. Silicate was added to the flotation cell and pH was adjusted to 8.5 as before. Pulp samples were also split in two after repulping so that the initial brightnesses were identical. Addition of silicate to the repulper resulted in the highest brightness achieved, yet when silicate was added to the flotation cell, a brightness improvement was still observed. Therefore, the data in Figs. 11 and 12 suggest that silicate has important roles in flotation deinking other than peroxide stabilization in the initial repulping phase.

\ JOURNAL OF PULP AND PAPER SCIENCE: VOL. 20 NO. 1 JANUARY 1994

SUMMARY Although some questions about so-

dium silicate's function in deinking still remain unanswered, this study has provided insight into the effects that caustic, silicate and peroxide have in repulping/washing and repulping/flotation. The following conclusions can be drawn keeping in mind that all were obtained using optimized repulping conditions: 1. For newsprinumagazine there is an op-

timum consistency under which repulping is carried out. Low consistency (14%) leads to incomplete defiberization while high consistencies (214%) lead to a smearing effect. Our optimum consistency was 12%.

2. There is a practical temperature for repulping (60°C) above which no real increase in repulping efficiency or brightness is achieved.

3. The optimum time for repulping was found to be 20 min; this has to be deter-

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5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

mined individually for each type of repulper. The addition of caustic alone to a wastepaper sample comprising 70% newsprint and 30% magazine resulted in immediate alkali darkening. Modest alkali (>OS%), however, was required for complete defiberization. The addition of alkali alone results in increased ink removal as measured by L”. In repulping/washing, sodium silicate was responsible for a 3.0% IS0 brightness increase. Silicate increased the brightness at every alkalinity tested allowing for the use of higher alkalinities without suffering the consequences of alkali darkening. This was achieved by silicate’s role as a buffer and was re- flected in b* and pH measurements. The optimum total alkalinity was dependent on silicate dosage. As shown by image analysis for repulping/washing, silicate must act as a detergent (anti-redeposition agent, dispersant, wetting agent and emulsifier) by removing more ink particles and creating a greater number of small ink particles which are easily removed by washing deinking. In the absence of peroxide, the detergency effect of silicate was the dominant function at low alkalinities. As the alkalinity was increased the buffering role became more important. Under optimized conditions of washing deinking, silicate was responsible for a 5.0,7.2 and 7.9% IS0 increase in brightness for 0.5, 1.0 and 2.0% peroxide, respectively. Approximately half of silicate’s function (in the presence of peroxide) was the deactivation of metal ions preventing peroxide decomposition. This is prob- ably the mechanism by which silicate acts as a stabilizer. In repulping/flotation, the optimum total alkalinity was dependent on the dosages of peroxide and oleic acid, and inde- pendent of silicate dosage. In the case for repulping/washing the opposite was true. The peroxide stabilization effect of silicate was approximately the same for repulping/washing and repulping/flotation (7.0% ISO). Silicate enhances flotation by creating larger ink particles which are more easily removed by dispersed air flotation and by acting as an anti-redeposition agent.

Some of what is reported is not new but has never been adequately reported specifically for deinking applications. The data were also internally consistent, allowing comparisons of various parameters and chemical addition rates under optimized conditions. It is hoped that these data will lead to a better understanding of soluble silicate’s chemistry in deinking and help to evolve new and more efficient methods of ink removal.

This is the first in a series of papers designed to systematically quantify the role of soluble silicates in wastepaper (newdmagazine) deinking. Future papers will deal with such factors as surfactants, the compatibility of silicate with other deinking chemicals, calcium carbonate concentration, other wastepaper grades (ledger, CPO etc.) as well as address the various other types of inks commercially available for printing (Le. flexo). The eventual conclusions will lead to a full understanding of ink removal and the role of silicates in deinking.

ACKNOWLEDGEMENTS The authors wish to thank Dr. Jorge

Miranda and Hart Chemical Limited

QUNO Corp. for assistance with the design of the laboratory flotation cell.

REFERENCES 1. ALI,T., McARTHUR, D., STO’IT, D.,FAIR-

BANK, M. and WHITING, P., J . Pulp Paper Sci. 12(6):5166 (1986).

2. FAIRBANK, M.G., COLODETTE, J.L., ALI, T., McLELLAN, F. and WHITING, P., J . Pulp Paper Sci. 15(4):J132 (1989).

3. ALI. T., FAIRBANK, M., McARTHUR, D., EVANS, T. and WHITING, P., J . Pulp Paper Sci. 14(2):523 (1988).

4. STRUNK, W.G., Pulp & Paper. 54(6):156 (1980).

5. ANDREWS, D.H., Pulp Paper Can. 60(11):T273 (1968).

6 . REICHERT, R.S. and PETE, R.H., Tappi 32(3):97 (1949).

7. KANTER, M.Y., RASKINA. I.K. and BOG- DANOV, G.A., Zhurnel Prihlandnoi Khemii 39(1):35 (1966).

8. SHRINATH, A,, SZEWCZAK, J.T. and BOWEN, J.I., Tappi 74(7):85 (1991).

9. VAIL, J.G., “Soluble Silicates Their Proper- ties and Uses’’, Vol. 1, Chemistry, Reinhold Publishing Corp., New York (1952).

10. FORESTER, W.K., Proc. TAPPI Pulping Conf., Toronto, Ont., p. 419 (1986).

(Guel~h, Ontario) for Providing assistance and expertise in the completion of the image analysis data. The authors also wish to thank Mr. Tom Mah and Dr. Anthony Yau of

11. BAST, I., Proc. TAPPI Pulping Conf.

12. LARSSON, A,, STENIUS, S . , ODEBERG, CONCLUDING REMARKS

A review of the research efforts at National Silicates Technical Centre is given.

Toronto, Ont., p. 90 (1991).

L., Svensk Papperstid. 87(18):R158 (1984).


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Functional and Performance Characteristics of Soluble Silicates in