Caustic-free repulping for newsprint production - pilot plant trials

Daniel J Dingman and Philip Hoekstra

Abstract
Caustic-free repulping of ONP/OMG furnishes has the potential for significant cost savings when compared to the conventional alkaline repulping process. Part I of this work detailed laboratory studies which showed that a high quality pulp could be produced under caustic-free conditions by pulping for the shortest possible time at the lowest reasonable consistency and temperature, along with addition of an optimized dosage of deinking surfactant. Caustic-free repulping eliminates the need for the conventional alkaline chemistry usually added to the pulper, and in turn can significantly reduce cost. To confirm these results a series of pilot plant trials were performed on ONP/OMG furnishes from North America and Europe. Pulps produced using caustic-free repulping on these furnishes were equal or higher in quality compared to those produced using conventional alkaline repulping. Bleaching studies showed that the caustic-free pulps could be post bleached to a 60+ brightness. Micro and macro sticky measurements showed that the pulps produced under caustic-free conditions contained significantly less stickies than did those produced using alkaline conditions. In addition, the process water from the North American caustic-free trial was easier to clarify than the process water from the alkaline pulping run.

Introduction
Part I of this study explored the possibility of producing high quality pulps for newsprint production through caustic-free repulping [1]. Results of that study showed that by minimizing pulping time, pulping temperature, and consistency, a pulp deinked pulp of high quality could be made, comparable in quality to those made using conventional alkaline chemistry. Conversely, it was found that excessive pulping time and high consistency contributed to a significant drop in final brightness and in increase in ERIC. This is consistent with work done by Ben et al. [2] where it was found that irreversible ink redeposition occurred during repulping with increasing pulp consistency and pulping times.

In response to these encouraging results, a series of pilot plant trials were performed at the Comer Pilot Plant in Scio, Italy the week of January 17-21, 2000. The goal of these trials was to compare pulps produced using caustic-free repulping under conditions predicted in our previous work with those using more conventional alkaline conditions.  The facility allowed us to effectively model a commercial deinking plant using the following process flow:

Pulper > Epurex (Detrasher) > H.D. Cleaners > Coarse Screens (0.4 mm holes) > Fine Screens (0.15 mm slots) > Flotation > Post Flotation > L.C. Cleaners > Thickener

Furnish for our work came from both North America and Europe.  North American furnish was supplied from the warehouse of a commercial ONP/OMG deinking facility. The ONP was approximately two months old and contained a noticeable amount of contaminants, such as OCC, food cartons, plastic and glass. The "OMG" was mostly coated groundwood sections, with very little true magazine present. The European furnish for this study  was supplied from the warehouse of a commercial deinking facility in Northern Italy. The ONP and OMG was also about 2 months old, and consisted entirely of bundles of over-runs from a printing facility. Pulping conditions for the caustic-free repulping trials were based on those identified as optimal in Part I of our work.  Pulping conditions for the alkaline repulping trials were consistent with those used in commercial deinking facilities in North America and Europe.  All pulper batches were 800 kg.  A proprietary deinking aid, BRD 2345, was added to the pulper in all cases to facilitate ink detachment and flotation. Fresh water was used for all dilutions.  The water was slightly alkaline, with a pH of about 8 and a hardness in excess of 200 ppm (as CaCO₃).

Process measures included temperature, pH, brightness, ERIC, consistency, ash, and yield.  In addition, micro sticky content was measured on filtrate from the pulper stock, and clarification demand was tested on filtrate from the thickener. Final pulps from all of the trials were shipped to Buckman Laboratories in Memphis, TN where post bleaching response and macro sticky content was investigated.

Repulping and Deinking Trials

North American Furnish
Repulping conditions for the trials on North American furnish are shown in Table 1.  Chemical dosages for the alkaline conditions are shown in Table 2. The furnish ratio for the two runs was  80% ONP, 20% CGS.

Table 1. Pulping Conditions for Trials on North American Furnish

Pulping Conditions Caustic-Free Conventional Alkaline Pulping Time 10 minutes 10,5 minutes Temperature (oC) 42 46 Consistency 10,0% 10,2% Deinking Surfactant Dosage 0,3% 0,3%

Table 2. Chemical Dosages for North American Alkaline Run

Chemical Dosage Sodium Hydroxide 1,05% Sodium Silicate 1,0% Hydrogen Peroxide (as 100%) 0,88% Chelant 0,2%

The results for the two runs were very interesting.  Through pulping, detrashing, and screening, the alkaline conditions gave about 2 points higher brightness and 200 points lower ERIC.  However, the caustic-free conditions provided much better ink removal through primary flotation, resulting in the caustic-free pulp having about 1 point higher brightness and 100 points lower ERIC. One possible reason for this is that the ink was more efficiently detached in the pulper, but slightly more fragmented than in the alkaline pulp.  Also, peroxide added to the alkaline pulper likely contributed to some bleaching of the fibers. Post flotation resulted in the pulps having equivalent brightness, with the caustic-free pulp having about 50 points lower ERIC. 

Final thickening resulted in the alkaline pulp having a 56.6 brightness and a 137 ERIC, while the caustic-free pulp had a 55.4 brightness and a 164 ERIC.  Adjusting the pH of both pulps to 5.0 resulted in a 58.1 brightness for the alkaline pulp and a 56.2 brightness for the caustic-free pulp.  Considering that there was only a 30 point difference in ERIC between them, this difference in brightness is likely due to peroxide bleaching in the pulper. Hyperwashing the pulps to remove all of the remaining detached ink resulted in an ERIC of 86 for the alkaline pulp and 102 for the caustic-free pulp. 

Pulp yield was measured through post flotation and was 83% in both cases.  Ash removal was also similar in both cases, resulting in less than 3% ash in both pulps after thickening. However, in the case of the caustic-free pulp most of the ash was removed through flotation, while in the alkaline case most of the ash was removed through thickening.

Tables 3 and 4 summarize the results of these runs, and Figures 1 and 2 show the brightness and ERIC values throughout the process flow.

Table 3. Summary Summary of Results for Caustic-Free Deinking Trial of North American Furnish

Process Step pH Temp (oC) % Consistency %Ash Brightness ERIC Net Yield Pulper 7,5 42 10,0 8,7 44,8 886 - Epurex 7,5 41 5,0 - 42,7 1089 99,7% HD Screen 7,4 41 4,5 8,2 43,1 1005 93,7% Flotation 8,0 40 1,03 5,4 54,3 254 86,6% Post Flotation 8,0 40 0,94 3,7 55,4 188 83,2% Thickening - - 6,78 2,4 55,4 164 - Adjust to pH5 5,0 - - - 56,2 164 - Hyperwash - - - - 57,8 102 -

Table 4. Summary of Results for Alkaline Deinking Trial of North American Furnish

Process Step pH Temp (oC) % Consistency %Ash Brightness ERIC Net Yield Pulper 10,4 46 10,2 8,5 45,8 789 - Epurex 10,0 43 4,5 - 44,9 846 99,8% HD Screen 10,0 43 4,5 7,9 45,5 818 93,0% Flotation 8,8 43 0,98 7,2 53,6 350 87,4% Post Flotation 8,8 43 0,95 7,0 55,6 233 83,0% Thickening - - 6,12 2,6 56,6 137 - Adjust to pH5 5,0 - - - 58,1 137 - Hyperwash - - - - 59,6 86 -

Figure 1. Brightness comparison through deinking process steps - North American Furnis

Figure 2. ERIC comparison through process steps - North American Furnish

European Furnish
Repulping conditions for the deinking trials on European furnish are shown in Table 5. Chemical dosages for the alkaline conditions are shown in Table 6. The furnish ratio for all three runs was 80% ONP, 20% OMG.

Table 5. Pulping Conditions for Trials on European Furnish

Pulping Conditions Caustic-Free Conventional Alkaline I Conventional Alkaline II Pulping Time 10 minutes 10 minutes 10 minutes Temperature (oC) 42 40 43 Consistency 10,2% 9,9% 10,0% Deinking Surfactant dosage 0,3% 0,3% 0,5%

Table 6. Chemical Dosages for European Alkaline Run

Chemical Dosage Sodium Hydroxide 1,05% Sodium Silicate 1,0% Hydrogen Peroxide (as 100%) 0,88% Chelant 0,2%

Two alkaline runs were made on the European furnish. This was because insufficient foam development in the float cells during the initial alkaline run made a meaningful comparison impossible. Therefore, a second alkaline run was performed at a higher deinking surfactant dosage.

The results of these trials were also very interesting.  At equal deinking surfactant dosages the caustic-free and alkaline trials gave similar results through primary flotation, but due to lack of foam generation, very little ink was removed in post flotation for the alkaline trial.  This resulted in the first alkaline trial producing a much lower quality pulp compared with the caustic-free trial. The post thickening brightness and ERIC of the caustic-free trial were 1.4 points higher and 162 points lower respectively than those for the first alkaline run. However, when the deinking surfactant dosage was raised to 0.5% (an unusually high dosage), the second alkaline trial outperformed the caustic -free trial with regard to thickener brightness (58.1 vs. 55.5), but still produced a significantly higher ERIC than the caustic-free trial (271 vs. 190) through thickening. While more foam was produced using the higher dosage of deinking surfactant, it was still not as much as that produced in the caustic-free trial using a much lower amount of product.

Adjusting the pH of the thickened pulps to 5.0 increased the brightness of all three, but did not change their relative rankings in the study. Hyperwashing also improved the brightness and ERIC of all three pulps, but still had no effect on their relative rankings.  The caustic-free run produced a slightly lower yield through thickening than did the alkaline runs (3% to 6% lower), but removed about 2% more ash and significantly reduced the residual ink as evidenced by the ERIC values.

Tables 7, 8, and 9 summarize the three trials performed on the European furnish. Figures 3 and 4 show the brightness and ERIC values through the process flow.

Table 7. Summary of Results for Caustic-Free Deinking Trial of European Furnish

Process Step pH Temp (oC) % Consistency %Ash Brightness ERIC Net Yield Pulper 7,5 42 10,2 11,6 45,3 926 - Epurex 7,5 41 4,2 - 45,4 903 100,0% HD Screen 7,5 41 3,6 11,4 45,0 955 96,0% Flotation 7,8 40 1,2 8,2 51,0 470 89,8% Post Flotation 7,8 40 1,1 5,6 54,7 240 86,2% Thickening 7,8 39 5,5 3,3 55,5 190 - Adjust to pH5 5,0 - - - 56,6 190   Hyperwash - - - - 57,7 146 -

Table 8. Summary of Results for First Alkaline Deinking Trial of European Furnish

Process Step pH Temp (oC) % Consistency %Ash Brightness ERIC Net Yield Pulper 10,5 40 9,9 12,1 46,5 888 - Epurex 10,0 40 4,5 - 47,1 879 100,0% HD Screen 10,0 40 3,8 10,1 46,5 926 97,0% Flotation 9,0 40 1,0 10.0 50,5 614 93,9% Post Flotation 9,0 39 1,0 9,2 52,1 502 92,0% Thickening 9,0 39 5,5 5,2 54,1 352 - Adjust to pH5 5,0 - - - 56,0 352   Hyperwash - - - - 58,3 205 -

Table 9. Summary of Results for Second Alkaline Deinking Trial of European Furnish

Process Step pH Temp (oC) % Consistency %Ash Brightness ERIC Net Yield Pulper 10,5 43 10,0 12,1 49,1 832 - Epurex 10,0 43 5,1 - 46,3 1089 100,0% HD Screen 10,0 43 4,0 11,4 48,1 893 96,5% Flotation 8,5 40 1,1 9,9 52,6 581 91,7% Post Flotation 8,5 40 1,0 9,7 55,5 425 89,4% Thickening 8,5 40 5,5 5,1 58,1 271 - Adjust to pH5 5,0 - - - 59,4 271   Hyperwash - - - - 61,4 190 -

Figure 3.  Brightness Comparison Through Process Steps - European Furnish

Figure 4.  ERIC Comparison Through Process Steps - European Furnish

Post Bleaching Studies

North American Furnish
Furnish samples were taken after the thickener from both of the trials run on North American furnish. These samples were bleached using sodium hydrosulfite, the predominant post bleaching chemical used in North America.  Bag bleaches were performed using 25 g O.D. samples. Bleaching was done at 3.5% consistency at a temperature of 70° C.  The samples were placed into the bags, and 70° C water was added to adjust to the desired consistency. Then, 0.5% Sodium Silicate and 0.2% DTPA was added and mixed well to tie up any metals present. The pH was adjusted to 6.0 with 5% H2SO4. Nitrogen was added and mixed into the pulp three times to purge any air from the bag, and the proper dosage of fresh 1% Sodium Hydrosulfite solution was added. Three dosages were used on each pulp; 0.25%, 0.50% and 0.75%. After sealing and thorough mixing, the bag was adjusted back to 70° C in a microwave and placed in a 70° C water bath for 90 minutes. Results are listed in Table 10 and shown in Figure 5.

Table 10.  Sodium Hydrosulfite Bleaching Results from North American Furnish Trials

Sodium Hydrosulfite Dosage Control 0,25% 0,50% 0,75% Alkaline Trial Pulp 56,6 62,1 63,9 64,7 Caustic-Free Trial Pulp 55,4 60,2 60,5 63,0

Figure 5.  Sodium Hydrosulfite Bleaching Results from North American Furnish Trials

The data show that pulps from the alkaline and caustic-free trials gave about the same bleach response at each dosage .  However, it should be noted that the caustic-free pulp exceeded a brightness of 60 at only 0.25% hydrosulfite.  Depending on current chemical costs, caustic-free deinking followed by hydrosulfite post bleaching could provide significant cost savings over conventional alkaline deinking while producing an equal or better quality pulp.

European Furnish
Furnish samples were taken after the thickener from all three of the trials run on European furnish. These samples were bleached using hydrogen peroxide, the predominant post bleaching chemical used in Europe.  Bag bleaches were performed using 25 g O.D. samples. Bleaching was done at 5.0% consistency at a temperature of 70° C.  The samples were placed into the bags, and 70° C water was added to adjust to the desired consistency. Then, 0.5% Sodium Silicate and 0.2% DTPA was added and mixed well to tie up any metals present.  The pH was adjusted to 10.0 with 5% NaOH, and the proper dosage of 1% hydrogen peroxide solution was added.  Three dosages were used on each pulp; 0.25%, 0.50% and 1.0%. After sealing and thorough mixing, the bag was adjusted back to 70° C in a microwave and placed in a 70° C water bath for 90 minutes. Results are listed in Table 11 and shown in Figure 6.

Table 11.  Hydrogen Peroxide Bleaching Results from Trials on European Furnish

Hydrogen Peroxide Dosage Control 0,25% 0,50% 1,0% Alkaline I Pulp 54,1 57,5 58,4 59,9 Alkaline II Pulp 58,1 60,3 61,4 62,3 Caustic-Free Pulp 55,5 58,0 58,8 61,0

As was the case with the North American trials, the results show that all pulps responded about the same at each dosage.  It should be noted that the caustic-free pulp produced a 58+ brightness at 0.50% peroxide dosage, and a 61+ brightness at a 1.0% dosage. Depending on current chemical costs in Europe, caustic-free deinking followed by hydrogen peroxide post bleaching may provide significant chemical cost savings without sacrificing pulp quality.

Stickies Measurements

Stickies measurements were performed on all the pulps from the North American and European furnish trials. Macro stickies were measured by screening 100 g O.D. of  fiber through a Pulmac Master Screen fitted with a 4-cut (0.004 inch ) screen.  Stickies retained by the screen were collected on a filter paper and pressed against a second filter paper in a Carver Press at 300° C and 10,000 PSI. Stickies were transferred to the second filter paper, which was then dyed with India ink and allowed to dry.  The ink was absorbed onto the filter paper, but not onto the stickies, allowing for "white on black" image analysis to be performed in order to determine the stickies content of the original pulp.

Micro stickies were semi-qualitatively measured in filtrate taken from the pulper stock for all the trial runs.  Testing was done using a hemacytometer and a microscope set at 500X. Colloidal particulates were counted and the average number of particulates per square was estimated.

Results from the measurements on pulps from the North American furnish trials showed that caustic-free repulping produced only about 50 micro stickies per square.  This is contrasted against more than 100 per square for alkaline repulping. Macro stickies were effected even more significantly.  The final deinked pulp from the caustic-free trial contained only 34 mm2/kg of macro stickies versus 203 mm2/kg in the pulp from the alkaline trial.  These results support the theory that stickies remain large in the absence of caustic during repulping, allowing them to be more easily removed by screening and cleaning processes. 

Results from the measurements on pulps from the European furnish trials were consistent with those observed for the North American furnish trials. In the case of the caustic-free trials, only about 20 micro stickies per square were observed versus about 200 per square for both of the alkaline trials.  Unfortunately, it was not possible to measure macro stickies in pulps from the European furnish trials.  As was previously mentioned, the furnish consisted entirely of bundles of overrun newspapers and magazines.  Because of this, the furnish was extremely clean and free of materials that could produce macro stickies. While measurements were attempted, the number of stickies detected was so low it did not allow for any meaningful interpretation.

Water Clarification

Filtrate from the thickener stage was sampled during the two trials performed on North American furnish. Initial filtrate turbidity was 828 FTU for the alkaline run and 348 FTU for the caustic-free run. Polyaluminum Chloride (PAC) was used to test ease of clarification on these filtrates. In the case of the alkaline trial, 12.5 ppm of PAC reduced the turbidity to 26 ppm. In the case of the caustic-free trial, 9.4ppm of PAC reduced the turbidity to 7 FTU. This difference in ease of clarification is likely due to the same effect discussed above with regard to micro stickies. Less colloidal material is produced when pulping in the absence of caustic, so less colloidal material is present at the end of the process.

Conclusions

From the data generated in this series of pilot plant trials it can be concluded that a high quality pulp can be produced using caustic-free repulping in a commercial scale setting.  Pulps produced by this process are of equal or better quality compared to those produced by conventional alkaline repulping. Bleaching studies showed that pulps from the caustic-free trials had the same bleaching response as those from the alkaline trials. This result suggests that caustic-free deinking followed by post bleaching may be a lower cost alternative than conventional alkaline deinking, depending on the mill's process equipment and chemical costs.  Repulping in the absence of caustic produces less micro and macro stickies in the final pulp, which would likely result in better runnability on the paper machine.  Process water clarification is also easier and less costly using caustic-free repulping due to the lower amount of colloidal materials generated during the pulping process.

Acknowledgements

The authors would like to thank the employees and management of Comer S.p.A. for their hard work and gracious hospitality during our visit to their facility.  We would especially like to single out Mr. Flavio Magaraggia and Mr. Matteo Scapin , without whose help this study would not have been possible.

References

[1] Dingman, D.; Perry, C., "Caustic-Free Repulping for Newsprint Production - Part 1" Proceedings, 1999 TAPPI Pulping Conference, November 1-3, 1999

[2] Ben, Y., Dagenais, M. and Dorris, G.M., "Irreversible Ink Redeposition During Repulping; Part I, Model Deinking Systems" Journal of Pulp and Paper Science, 23(3): pp.83-89, March 2000