Non-Sulfite Neutral Deinking for Recyled Fibre
By Pulp & Paper Canada
By Pulp & Paper Canada
The increasing utilization of recycled fibre (RCF) in papermaking is due to multiple factors. This trend is a result of ongoing attempts to embrace environmental stewardship as well as to increase pro…
The increasing utilization of recycled fibre (RCF) in papermaking is due to multiple factors. This trend is a result of ongoing attempts to embrace environmental stewardship as well as to increase profitability. This increased usage of RCF also requires continuous improvement in the quality of pulp produced by such recycling processes.
More specifically, the recycling of wastepaper has become increasingly important globally. Recovering paper for recycled paper production is well advanced in parts of western Europe and Japan. In these areas about 60% of paper and board is recovered for recycling. The equivalent figure for North America is a little under 50% and Europe’s best recycling countries can achieve recovery figures of more than 70%.
These industry desires and societal trends to embrace environmental stewardship and improve profitability have resulted in an increased interest in new chemistry scenarios for RCF processing. Such an area of interest is the production of pulp from old newspapers (ONP) and old magazines (OMG) using a process that eliminates or reduces the alkalinity used for repulping.
Reduced alkaline processing with mixed office waste (MOW) is common and has been successfully employed for years. However, because of many differences between MOW and ONP/OMG inks, physical characteristics, printing processes, and chemistry, reduced alkaline processing of ONP/OMG has been more challenging.
Reduction or elimination of sodium hydroxide use in repulping is needed to make such processing possible. Associated with such a reduction is also reduced alkaline yellowing that is often accentuated by introduction of the caustic chemistry. If this yellowing is reduced, then it becomes possible to also reduce or eliminate other chemicals such as peroxide, chelants, sodium silicate and biocide for catalase control.
The scenarios of RCF processing utilizing reduced alkalinity have often been generally termed “neutral deinking.” Some of the common benefits often discussed in relation to this neutral processing of RCF include lower total chemical costs, lower COD release, the ability to remove alkaline sensitive printing inks and improved control of various pressure sensitive adhesive contaminants. Common concerns in moving towards neutral processing often include understanding the impact on ink release, ink fragmentation, yield, and ink removal efficiency.
A number of scenarios exist to accomplish reduced alkaline deinking of ONP/OMG. One of the better known approaches includes incorporation of sodium sulphite. This approach has been explored both in the laboratory and also in mill scale. In this approach, sodium sulphite often reduces or replaces sodium hydroxide, hydrogen peroxide and silicate in the pulper.
It has often been suggested that effective reduced alkaline processing will require post-bleaching to meet brightness targets. Previous results reflect a variety of outcomes, ranging from conclusions that suggest the bleaching response of the reduced alkaline pulp is the same as traditional alkaline pulp, to the other extreme of suggesting that the bleaching response of reduced alkaline/neutral pulp is less than that of pulp produced under traditional conditions. Certainly, whatever the case, the role of ink elimination is crucial in properly understanding and making such conclusions regarding the apparent brightness relationship to bleaching response.
This question is further complicated by realizing that the bleaching response of the pulp can also vary depending where in the process the chemistry is added. Many would suggest that bleaching after flotation is believed to be more effective than bleaching before flotation because of the reduced amount of disturbing substances in the pre-treated suspension. Results favouring the pulper bleaching compared to post-bleaching have also been published.
Regardless of the details of the process, it is clear several benefits can be observed by successful transition from alkaline deinking to reduced alkaline processing. In general, such a move in processing results in a pH of about 8 and sometimes even less in order to make possible these potential positives from both cost and process perspectives. A partial list often considered is as follows:
* Potential for significant chemical cost savings in most mills
* Decreased DAF clarifier and sludge dewatering polymer usage
* Reduced clarified effluent turbidity
* Reduced anionic trash to the paper machine
* Reduced COD to effluent treatment
* Increased removal of flexographic inks
* Reduced potential for stickies aggravation
* Reduced acid requirement for pH adjustment
This variety of results and benefits in tandem with a lack of total clarity is fundamentally associated with the realization that recycled fibre is a complex and variable raw material. In order to better understand these many facets of reduced alkaline RCF production, a high level of expertise and technical know-how is useful. It is reasonable to believe that development of novel technologies in this area will require highly regimented and technical exploration.
Several new concepts and innovations in RCF have recently been developed by Kemira. All of these developments have come from a common strategic framework and approach. The foundation of these developments is based on viewing recycle systems as a series of interrelated chemical and physical micro processes.
The strategic foundation utilized in exploring potential reduced alkaline solutions is based on considering this process as several interrelated micro processes. These deinking micro processes include facilitation of ink release, prevention of ink redeposition, control of ink agglomeration, control of ink hydrophobicity and surface energy, froth generation and froth stability, among others.
These micro processes are stated specifically in reference to the deinking process, but one can talk more generally about contaminants in RCF processing of which ink is only one species. Other contaminants of interest can include metals, stickies and microbiological species, as well as many other forms of contaminants from the non-paper constituents contained in wastepaper. From this more general perspective, the above philosophy can be restated to include various phenomena such as:
* enabling of contaminant release from the RCF;
* limiting the unwanted redeposition of the released contaminants, control and agglomeration of various contaminants as appropriate and, finally,
* systematic modification and control of the surface energy of the contaminants.
Expanding this concept, each component in a product formulation contributes in unique proportion to the overall activity in each of the micro processes within the process in its entirety. By understanding the micro process(es) with the greatest limitation(s), new technologies can be designed utilizing a variety of surfactant and/or polymer-based constituents in relative amounts that are tailored to meet the unique performance demands of a particular recycle system.
New and patented deinking technologies are now available for non-sulfite reduced alkali deinking for recovered paper compositions containing appreciable old newspaper. These technologies are currently being regularly used in deinking mills.
The interdependence of deinking chemistry and bleaching response are important. This review describes a portion of Kemira’s work in reduced alkaline processing and includes a summary of results from multiple pilot plant trials, as well as those results from a mill. The work substantiates the view of reduced brightness immediately following flotation of recovered paper systems when produced under reduced alkaline conditions. Furthermore, the results also conclusively show that this brightness loss can effect
ively be reclaimed by judicious selection and utilization of post-bleaching chemistry.
The mill evaluation shared here has previously been reported in extensive detail. In summary, the evaluation originally started as a four week trial and has been running continuously since that time.
Peroxide, sodium silicate, caustic, chelant, and sulfuric acid were completely eliminated, resulting in a pH of 7.0-7.2 in the pulper. The dramatic drop in pH at the start of the trial (indicated by red arrow) is illustrated in Figure 1. Also, interestingly, the process operation became more stable as demonstrated by dramatically reduced variations under these reduced alkaline processing conditions. The time frame for the x-axis is six months.
The pulp brightness following flotation, prior to dithionite post-bleaching, was consistent with pre-trial lab results. That is to say the brightness varies from the same to one point lower than the alkaline chemistry. These differences have been eliminated with the post-bleaching. The brightness consistently is the same or better compared to the brightness with alkaline chemistry, as shown in Figure 2.
Initial ERIC values were also consistent with lab results and were slightly higher, 20-40 ppm, than the alkaline chemistry baseline as shown in Figure 3. With optimization the ERIC numbers were improved further and in line with the alkaline baseline. Hyperwash data was the same before and after the change to reduced alkaline conditions, which suggests the ink removal efficiency is similar for both chemistry scenarios. The time frame for the x-axis is six months for both Figures 2 and 3.
Process yield was also comparable to the alkaline chemistry as shown in Figure 4. The time frame for the x-axis is six months.
In addition to these results, no deposits on paper machine felts or wires were observed and the paper machine quality, cleanliness, or runnability were the same or better during this period. The mill has also significantly reduced DAF clarification and sludge dewatering polymers. Cationic demand has been shown to have diminished greatly, thus leading to less anionic trash on the paper machine.
In addition to these positive mill results, extensive work has also been performed at two different locations at the pilot scale to further examine reduced alkaline RCF processing. One of these systems was at an independent contract pilot facility and the other is located at the University of Oulu in Oulu, Finland.
A total of three scenarios were examined. In two of the scenarios the impacts of post-bleaching using peroxide at post flotation disperging is evaluated. In the final case, also performed at the University of Oulu, the impact of post-flotation bleaching in the absence of disperging was evaluated.
The focus of these recent studies was to evaluate more carefully the performance of our sulfite-free reduced alkaline deinking technology for systems containing higher magazine content, up to approximately 50%, as well as to understand in a more regimented manner the bleaching response that could be obtained by various scenarios of post-bleaching.
Shown in Figure 5 are the results for a pulp produced in a pilot system using a reduced alkaline deinking at an independent contract facility. The brightness for the low-alkaline, also referred to as reduced alkaline deinking sequence, shows the brightness achieved after flotation is less than the brightness achieved in alkaline deinking.
This is common for reduced alkaline processing of recovered papers and is not particularly surprising. It is essential to remember that the alkaline fatty acid chemistry had peroxide present in the pulper, which provides some brightening, while the reduced alkaline chemistry had no peroxide present in the pulper and thus there was no possibility for such an increase in brightness.
Importantly, this gap in brightness after flotation obtained by reduced alkaline deinking compared to alkaline deinking can be essentially eliminated by post-bleaching with peroxide at the disperger in this model system.
Shown in figure 6 are results for reduced alkaline processing at the University of Oulu. The flotation cell in this pilot line has four cells in series. The results for each cell are reported. As before, the brightness achieved after flotation is less than the brightness achieved in alkaline deinking. Also as before, this brightness differential can be essentially eliminated by post-bleaching with peroxide at the disperser in this model system.
Shown in Figure 7 are results for a second study performed in Oulu, being careful to obtain the same ink contents as measured by ERIC values thus allowing interpretations regarding fibre responsiveness to post-bleaching.
In this case the pulp was processed through a wire-press immediately following flotation. As before, the brightness achieved after flotation is less for reduced alkaline conditions. Following post-bleaching with peroxide and dithionite, the brightness was even better for pulp produced using reduced alkaline conditions.
Furthermore, the bleachability of the reduced alkaline pulp was higher than alkaline upon peroxide post-bleaching. In other words, the final brightness was at the same level even though the alkaline deinking pulp had an additional 6 kg/t of hydrogen peroxide to the pulper for a total of 26 kg/t for the alkaline chemistry while the reduced alkaline system only experienced a dosage of 20 kg/t in post-bleaching.
In all cases presented, the brightness obtained by a patented sulfite-free reduced alkaline neutral RCF processing scenario are comparable or better than those obtained using traditional alkaline processing. It was also observed that the reduced alkaline pulp was significantly more responsive to the post bleaching chemicals than was the pulp processed using a traditional alkaline processing sequence and resulted in the same or better brightness and ERIC values.
Scott Rosencrance, Phd, is the Global R & D Manager RCF at Kemira.