Maximilian F Chami
email: maximilian.chami@b-tu.de

Gabriel Kaminyoge
email: 86kaminyoge@gmail.com

Closed House of Wonders museum: Implications to the tourism of Zanzibar Stone Town, UNESCO World Heritage Site

Keywords: House of Wonders, Tourism, Zanzibar Stone Town, Museum, Heritage JEL Classification: G14, M31, Z33

This paper examines the impact of the closed House of Wonders Museum in the tourism industry of Stone Town, Zanzibar. The paper aims to propose the best practices taken into account due to the impact raised by the closure of the Museum. There has been no clear information on the overall situation which faces the site since 2012 when the Museum closed. Data collected through mixed methods, including the sample size of 105 tourists who visited the House of Wonders Museum, 8 Government Official, 6 Tour Guides and 8 Tour Operators. The findings show that the closed museum has affected the level of tourists' satisfaction, tour operators, community and tour guides economically. The paper recommends quick rehabilitation and reconstruction of the Museum to save the integrity and authenticity of this World Heritage Site.

This paper reports a piece of research undertaken in Zanzibar Island on the impact of closed House of Wonders Museum since December 2012 to the tourism cycle of Zanzibar Stone Town, UNESCO World Heritage Site. The Museum was closed down by the Revolutionary Government of Zanzibar for maintenance and repair after a large corner of the Museum collapsed, taking with it several essential iron pillars and threatening the structural integrity of the building and its façade. Furthermore, the roof of buildings is in a precarious state, especially after heavy rainfall of November 2015 caused the partial collapse of the roof (WMF 2017). The House of Wonders (in Arabic: Beit-al-Ajaib) is a landmark building in Stone Town, Zanzibar Island. It is the largest and tallest building of Stone Town facing the Forodhani Gardens on the old town's seafront, in Mizingani Road. It's located between the Old Fort and the Palace Museum as one of six palaces built by Barghash bin Said, the second Sultan of Zanzibar (ibid.). The building was constructed in 1883 with the intention to act as a ceremonial or stately palace to the Sultan´s visitors. It was named "House of Wonders" because it was believed as the first building in Zanzibar Island and Africa to have electricity and elevators. The design of the palace attributed to a British marine engineer and after Sultan Barghash bin Said inspired by its visit to Paris and London in 1875, where he enthusiastically took the idea to modernise Zanzibar Island by installing a piped drinking water system, amongst other infrastructure projects [START_REF] Folkers | Early Modern African Architecture. The House of Wonders Revisited[END_REF][START_REF] Meffert | Where to, Fair Beauty? Zanzibar Guide. The Attempt of an Inventory, Zanzibar[END_REF]. The building has the wide external verandas supported by castiron columns, which allowed for uniquely high ceilings while construction materials consisted of an original combination of coral rag, concrete slabs, mangrove shoots and steel beams [START_REF] Folkers | Early Modern African Architecture. The House of Wonders Revisited[END_REF]. In August 1896, the building suffered damage due to Anglo-Zanzibar War and reconstructed in 1897. After the bombardment and reconstruction, in 1911 the building was transformed into government offices and as the central secretariat for the British governing authorities in Zanzibar Island and later in 1964 converted into a school and a museum for the ruling Afro-Shirazi Party. Since the 2000s, the House of Wonders became the Museum of History and Culture of Zanzibar and the Swahili Civilization along the Coast of Indian Ocean [START_REF] Sheriff | The Zanzibar House of Wonders Museum. Self-reliance and Partnership. A Case Study in Culture and Development[END_REF]. In this paper, we examine the implication of the closed House of Wonders Museum to the tourism cycles in Zanzibar Stone Town. While we try to discuss the impact of closed Museum on the tourism cycle in Stone Town; it should be well-known that many studies in Zanzibar Island (e.g. Stone Town) have concentrated more on the tourism satisfaction [START_REF] Salim | What Satisfies Tourists in Cultural Heritage Sites? Evidence of Zanzibar Stone Town[END_REF]Chami 2018a;Chami 2018b), heritage management and archaeological excavations [START_REF] Chami | The Excavation of Mapangani Cave, Pemba Island, Zanzibar[END_REF][START_REF] Chami | Excavation of Kiwengwa Cave, Zanzibar[END_REF]Chami 2017;[START_REF] Juma | Unguja Ukuu on Zanzibar, Archaeological Study of Early Urbanism[END_REF][START_REF] Juma | Management of Zanzibar Cultural Heritage[END_REF]. Unfortunately, many of the studies or researchers in Zanzibar Island have ignored the impact of closed of House of Museum on the tourism cycle in Stone Town of Zanzibar. As such, before this study, there was no precise information on whether or not the closed museum has affected the tourism cycle in Zanzibar stone Town UNESCO World Heritage Site. It is essential to find out and determine whether or not the closed House of Wonders Museum has affected the tourism cycle and activities in Stone Town of Zanzibar. This idea is vital to government bodies especially Zanzibar Commission for Tourism, Stone Town Conservation Development Authority, Department of Antiquities and Museum on how to solve the tourism challenges and opportunities which might raise in Zanzibar Stone Town due to the House of Wonders closed for reconstruction. Therefore, it is fundamentally essential to investigate whether or not the closed House of Wonders Museum has affected the tourism activities in Zanzibar Stone Town. We further argue that there is a significant need for Zanzibar Commission for Tourism (ZCT), Department of Museum, Antiquities and Stone Town Conservation Development Authority (STCDA) to work together in developing an alternative means to the tourists who want to visit and explore the closed Museum.

DATA COLLECTION METHODS

Data presented in this study drawn from various interviews with the Zanzibar Commission for Tourism, Department of Antiquities staffs. In this occasion, a non structured with open-ended question were prepared to acquire relevant information from the informed respondents. Also, closedended questionnaires were designed and distributed to 120 tourists, but only 105 returned. To complement the above two methods, non-participation observation employed in assessing the state of the building. All of these activities conducted between July-October 2018 in Zanzibar Stone Town. These questionnaires were used to catch up on the opinion and views of tourists on the level of satisfaction with House of Wonders as one of the attributes in Zanzibar Stone Town. Personal observations on tourism activities going on in and around the House of Wonders Museum also included in this data. One of us (MC) has frequently visited the site for his research activities since 2013 [START_REF] Chami | Assessment of Tourist Satisfaction in Cultural Heritage Tourism Destination in Tanzania: The Case of Zanzibar Stone Town Tourism[END_REF]Chami 2018a;Chami 2018b). During all these visits, both of us experienced that tourists were demoralised due to the failure to get inside the Museum. This situation might have a significant adverse impact on the tourism cycle of Zanzibar Stone Town in Zanzibar Island.

Tourist Satisfaction in Zanzibar Stone Town: House of Wonders Museum

In this regard, tourists were requested to give a score to the closed House of Wonders Museum in each of the questionnaires using a five-point Likert scale that is ranging from 1. Very Satisfied 2. Satisfied 3. Moderate 4. Dissatisfied 5. Very Dissatisfied. Here, the respondents provided a score to the closed Museum on the level of satisfaction to this attribute. Tourists (respondents) selected in a random way whereby 120 questionnaires distributed to tourists, but only 105 questionnaires returned. It revealed that 38 respondents equivalent to 36.2% and 18 (17.1%) were both dissatisfied and very dissatisfied with the House of Wonders museum. Also, the study found out that 2 (1.9%) respondents did not visit, the Museum. However, 15 (14.3%) and 6 (5.7%) were satisfied and very satisfied with the attribute respectively, but 26 (24.8%) felt that they were moderately satisfied with this closed Museum. These results summarised in Table 1 below These results perfectly accord with previous studies [START_REF] Chaudhary | Tourist Satisfaction and Management of Heritage Sites in Amritsar[END_REF][START_REF] Huh | Tourist Satisfaction with Cultural/Heritage[END_REF][START_REF] Hou | An Investigation into Visitors' Satisfactions with Port Elizabeth's Heritage Museum[END_REF][START_REF] Mensah | Residents' Satisfaction and Behavioural Intentions with Asogli Yam Festival in Ghana[END_REF]) which found out similar results on the variation in the level of attribute satisfaction in different tourism attributes. These results further suggest that the difference in the level of tourists' satisfaction with the House of Wonders Museum might have a significant adverse impact on the tourism cycle in Zanzibar Stone Town. The higher number of tourists (53.3%) who revealed not satisfied with House of Wonders which closed for reconstruction can have the significant negative influence to the tourist touching loyalty to the Zanzibar Stone Town UNESCO World (Chami 2018b;[START_REF] Alsaqre | Investigating the Effects of Tangible and Intangible Factors on Customers' Perceived Service Quality and Loyalty in Hotel Industry in AL-LADHIQIYAH[END_REF]. The closed House of Wonders Museum can influence the tourists to lack the sense of willingness to revisit the site or even not to recommend Zanzibar Stone Town to their relatives in the future [START_REF] Ijeomah | Tourism Management in Selected Ecotourism Destinations in the Niger Delta, Nigeria[END_REF]Nowack 2013). For example, Nowack (2013) argued that revisit intention can be subjective or somewhat influenced by a specific site or attribute in any heritage site. We can further discuss that intention to the tourist revisit, and loyalty with the Zanzibar Stone Town can be severely affected by the closed House of Wonders museum as one of the attributes in this site if government bodies will take no action.

Opinions of Government Staff Regarding the Closed House of Wonders Museum: Department of Antiquities and ZCT

Focusing on the importance of the House of Wonder museum holds for the Revolutionary Government of Zanzibar, the study interviewed eight (8) employees from the Department of Antiquities (5) and Zanzibar Commission for Tourism (3) respectively. The respondents provided their views and opinion on the importance of the closed Museum to the people of Zanzibar Island. Regarding the position that the House of Wonders depicts to the tourism of Zanzibar and Stone Town, in particular, all eight (8) respondents revealed that, the House of Wonders regarded as a symbol and one of the prominent structures in Zanzibar Island and the World of Architecture [START_REF] Garlake | The Early Islamic Architecture of the East African Coast[END_REF][START_REF] Sheriff | Zanzibar Stone Town: An Architectural Exploration[END_REF]. To show the value of this building, the Revolutionary Government of Zanzibar uses the building in their passport and currency. It is the most famous building in the area of Stone Town. This building used as a museum that showed the history and Swahili culture of Zanzibar and the Coast of East Africa. In other words, the House of Wonders complements another tourism attribute available in Zanzibar (Chami 2018a). It is even said the visit to Zanzibar Island without getting into the House of Wonder, such tour considered as incomplete. Therefore, the House of Wonder is considered as a benchmark of Zanzibar's tourism as it attracted many visitors than any other building within the Zanzibar Stone Town.

It further revealed by one respondent that the building by itself is a museum apart from what is housed inside as artefacts. The tourism of Zanzibar Stone Town embedded in this building. The building as a museum houses different artefact belonging to the Swahili culture [START_REF] Steyn | An Analysis of an Omani House in Stone Town, Zanzibar, Department of Architecture[END_REF]. They also added that the building attracts many visitors from different parts of the world just because of its name and uniqueness. One staff further added, to show the value and uniqueness of this building the Revolutionary Government of Zanzibar has even enacted a law that regulates the size of buildings within Stone Town not to transcend the House of Wonder. On this fact, the collapse and close of the building have brought a significant loss to the government regarding the economy and tourism activities at large. Furthermore, the closed museum led to the government to lose an authentic and sustainable source of income from visitor and thus helping with the funding of conservation activities and other projects. For the visitors have missed an opportunity to learn and experience the sense of the architectural, historical and the collection housed inside the house. During the interview one antiquities staff said:

"the closure of this museum have not only Denise the rights of publicity and visitors to access the collection, but also the artefacts impacted due to storage spacing, and others were affected by moisture and dust due to leakage".

Regarding tourism, the closure of the museum might have led to the decline in the number of visitors, particularly those interested in the museum staffs though no precise statistical data obtained from these government bodies. But, as it stands the number of visitors to Zanzibar is increasing year after year; however, with the closure of the House of Wonder visitor have shifted to other tourism activities. This tendency if allowed to prevail for an extended period may lead to mass tourism and the destruction of the facility and surrounding vicinity like in Greece [START_REF] Schouten | Professionals and Visitors: Closing the Gap[END_REF]. Also, we can argue that this situation may result in the reduction of the number of nights that visitors stay in Zanzibar destinations due to the decrease in tourism activities. The decline in the number of visitors' nights has serious implication for the expenditure and government revenues. Other respondents showed that the closure of the museum contributes to the loss of memories about the collection, which is the strongest tool for visitor's orientation to the museum. Secondly, the closure of this museum had affected the workers economically as they used to receiving tips when the museum was operating but now no longer receiving it. As a department of antiquities, this building uses to generate more than 75% of the total fund created from all buildings governed by the antiquities. Lastly, the study observed severe input to the budget and conservation activities of the department of antiquities. We can further discuss that; this downfall has perhaps contributed to weak conservation efforts. As a result, the government end up relies on the funding from donors which is not sufficient. Additionally, the closure of this building jeopardises the position of Zanzibar Stone Town as a UNESCO World Heritage Site which could end up being positioned on the list of World Heritage Site in Danger. It should well know that tourism is the backbone of the economy of Zanzibar Island; hence a huge effort is needed to retain this status and the authenticity of House of Wonders Museum. The closure of the House of Wonder has demoralised the attitude of workers as they are not exercising their areas of specialisation and do not benefit anymore ending up allocated to the different working stations. As a result, this perhaps suggests that House of Wonders has a significance value not only to the antiquities government officers but also to the Stone Town of Zanzibar in general. The building might cause the Stone Town World Heritage Site to lose its authenticity hence being in danger of removed from the UNESCO World Heritage Site if the conservation and rehabilitation activities won't be finished as soon as possible.

Opinions of Private Sector Regarding the Closed House of Wonders Museum: Tour Operators and Tour Guides

In different parts of the World, researchers have been conducting studies on the impact of tour guides, tour operators and their role in the overall development of the tourism industry [START_REF] Sandaruwani | The Role of Tourist Guides and their Impacts on Sustainable Tourism Development: A Critique on Sri Lanka[END_REF][START_REF] Blyablina | The Contribution of Guides in Developing Tourist Experiences During Historical Theatrical Tours: The Case Of Stockholm Ghost Walk[END_REF][START_REF] Nkonoki | Challenges of Tour Operators: Case: Dar es Salaam, Tanzania[END_REF]. Some of the researchers have moved even further and tried to show how tour guides and tour operators can provide tourism satisfaction and experience within the heritage site [START_REF] Hsu | Tour Guide Performance and Tourist Satisfaction: a Study of the Package Tours in Shanghai[END_REF][START_REF] Weiler | Enhancing the Visitor Experience: Reconceptualising the Tour Guide's Communicative Role[END_REF]. Hence, following up from this review, the is no clear information on the impact of the closed museum to the tour guide and tour operators in Tanzania. This situation even made the need to include the opinions of these two stakeholders in this study. Therefore, the views from private sectors on the closed House of Wonders Museum of Zanzibar Stone Town is of paramount. The study also consulted at least Eight (8) tour operators and Six (6) tour guides around the site. These two categories of stakeholders considered because of their duties and position in the tourism activities of Zanzibar Stone Town World Heritage Site in particular. At this end, at least all respondents cohere that, the House of Wonder is important evidence of the unique architecture of Zanzibar and mirror / iconic building of Zanzibar Stone Town. Also, information on this building was the first to be published on the internet before any facility found in the Zanzibar Stone Town. The name wonder attracted many visitors both domestic and foreign who have been motivated to visit and see what does the building embrace. This status, therefore, was used as an attracting force to the visitor to come to Zanzibar. As for this case, every visitor who sets foot in the land of Zanzibar wished to visit the building and experience the sense of wonder of the building as the name suggests [START_REF] Weiler | Enhancing the Visitor Experience: Reconceptualising the Tour Guide's Communicative Role[END_REF]. Therefore, three tour operators and two tour guides shared that before the closure of the Museum, the building received nearly 90% of the total visit to Zanzibar Stone Town. The estimated figure from both operators and guides rounded to 200 to 300 hundred visitors per day. They further said the government received a lot of foreign currency as a direct entrance fee that was standing at 2$ per visitor. After the closure of the Museum, the respondent 's showed a sudden drop in visitors and activities around the building. Regarding the revenues, three tour guides suggested that nowadays the government has lost the very sustainable source of income from the tourists who were coming to this building. One tour guide added that the closure of the building has increased troubles resulting from frustrations and dissatisfaction from visitors who always insist there need to go to the building even if closed [START_REF] Sandaruwani | The Role of Tourist Guides and their Impacts on Sustainable Tourism Development: A Critique on Sri Lanka[END_REF]. For instance, in 2017 one royal family travelled from Saud Arabia to Zanzibar to see the building, when I told them that it's not open to the public due to leakages they were very disappointed, he added. In addition to this, the tour guide explained that we had lost some tips from visitors following the closure of the museum as this was helping to support themselves and our families.

As the issue of concern for tour operators, two operators revealed that the closure of the famous building of House of Wonders has led to the shutdown and the collapse of some tour operators/ organisers who were specialised in the Stone Town World Heritage Site walking tours especially those dealing with visitors from Italy. And some others have shifted to other forms of tourism activities and destinations, for example, organising tours to Prisoners Island, Beach Safaris, Dolphin tours and spice tours to mention a few [START_REF] Nkonoki | Challenges of Tour Operators: Case: Dar es Salaam, Tanzania[END_REF]). Additionally, one tour operator added that the time when House of Wonder was active, even our school children used to benefit from tourists who used to give them with books, pen and other school materials during recession hours.

Often time these were visitors from Italy who were having an undivided interest in the architecture and the content of the museum. Today our kids have not received this stuff anymore. At one point this was helping to inform our kids about the importance of tourism to them and thus valuing and respecting their culture and its immediate attributes which attract visitors. These results further suggest that not only the government of Zanzibar losing a lot of revenue but also tour guides and tour operators due to the closure of the building. On top of this, we can argue that there is a big possibility that the closure of the museum has increased mass flow of visitors to other attributes that might increase chances to mass tourism which have a devastating impact on the same. We further argue that the closure of the building might have discouraged some of tour guides and operators who many of them opted to work on other economic activities or the different working environment. Similarly, the closure of this building had altered some entrepreneurial activities which operated greenly when the House of Wonders was in operation. For instance, tax drivers complained about the decline of business as many visitors from peripheral areas (Shamba) are no more flowing to the museum and other small business owners like Machingas selling "batiks" "spices" and other products to visitors have either closed their business or shifted to less lucrative business. This situation has increased competition among taxi drivers and business people to areas that still invite visitors. As a result, whatever the reasons one could provide, it suffices to note that the closed House of Wonders Museum has severely affected the economic situation not only for the revolutionary government of Zanzibar but also to many tour operators and tour guides around Stone Town World Heritage site.

CONCLUSION

Based on the results and discussion above, we conclude that the closed House of Wonders Museum has clear negative implications on tourist satisfaction and the tourism cycle of Zanzibar Stone Town at present. The closed museum also might cause the Stone Town World Heritage Site to lose its authenticity hence being in danger of being removed from the UNESCO World Heritage Site if the conservation and rehabilitation activities won't be finished as soon as possible.

Lastly, we conclude that the closed House of Wonders Museum has badly affected the economic situation of the many tour operators and tour guides and the community around Stone Town World Heritage site which threatening the tourism business around the area.

RECOMMENDATIONS

It's acknowledged that tourism is the backbone of the economy of Zanzibar Island [START_REF] Rotarou | Tourism in Zanzibar: Challenges for Pro-Poor growth[END_REF]. It is from this fact that the Revolutionary Government of Zanzibar and the community at large should embrace this vital sector important in the economy, employment and linkage with other sectors. This study proposes five recommendations identified as follows; 1. The STDCA as the caretaker of the Stone Town buildings including House of Wonders Museum should continue to identify and rehabilitate buildings requiring to be repaired immediately once spotted. The authority should not wait until the buildings collapse like House of Wonders Museum to react. For this, prevention is better than cure. Also, tour guides, tour operators and other people in business who by one way or another depend on House of Wonders museum should be encouraged to work on other activities which will help to solve their economic problems at present. 2. The government should continue to raise awareness among the public on the value of repairing their building while obeying to their originality, authenticity and available guideline, especially from UNESCO and ICOMOS. They should not use modern materials that alter the originality and authenticity of their historic city that can lead to the lose its value and popularity among tourists and as a World Heritage Town. For instance, today there so many buildings with protruding air conditioners, grills, grass windows which are against the guideline of the STDCA, UNESCO and ICOMOS. 3. The government should allocate more funding to enable the STDCA to implement their duties of rehabilitating and maintaining the Stone Town World Heritage Site especially House of Wonders Museum. There is a continuing complains from this authority that the amount of money returned by the government from those generated by tourism does not tally with reality. A very small percentage is given back for the management of the buildings. Instead, the STDCA continues to rely on donors and the UNESCO to fund for rehabilitation and maintenance of the buildings. The lack of a reliable internal source of fund to run STDCA activities is the threat to the survival of the House of Wonders Museum and Stone Town World Heritage site as the external sources are neither reliable nor sustainable. 4. Immediate efforts are required from the STDCA and Antiquities department to find an alternative modality to continue showing cases of the artefacts and other materials that were displayed (exhibitions) in the House of Wonder to the visitors and the public elsewhere. The essence will help to make a tour to Zanzibar Island most fascinating and complete as tourists will learn more about the history, culture and leadership regime of Zanzibar through material remains hence tourist satisfaction.

Figure 1 :

 1 Figure 1: The front façade of the House of Wonders, now a Museum of Swahili culture in Zanzibar Island (Source: WFM 2017)

Figure 2 :

 2 Figure 2: A corner section of the House of Wonders that collapsed in November 2012 (Source: WMF 2017)

Table 1 :

 1 . Generally, the results indicate that 56 (53.3%) respondents were not satisfied with the closed House of Wonders Museum in Zanzibar Stone Town. Respondents Satisfaction with Zanzibar Stone Town Attributes (N=105, P=100%) Source: Field Data (see also Chami 2018a)

ACKNOWLEDGEMENTS

This work was supported and funded by Deutscher Akademischer Austauschdienst -DAAD through Graduate School Scholarship Programme 2017/2018.

) is a PhD Candidate (Heritage Studies) with the Brandenburg University of Technology-Cottbus, German. He currently works at UNESCO National Commission of the United Republic of Tanzania as the Culture and Heritage officer. He obtained his Bachelor of Arts in Tourism and Culture Heritage from University of Dodoma (2012) and Master of Arts in Heritage Management from the University of Dar es Salaam in 2015. Gabriel I.


François Julien

Saint Amand

François JULIEN SAINT AMAND Grzegorz Wojciechowski

Dr Dariusz Asendrych

Michel Favre-Marinet
email: michel.favre-marinet@hmg.inpg.fr

Dr Rachid Rahouadj
email: rachid.rahouadj@ensem.inpl-nancy.fr

Salaheddine Skali Lami

Grzegorz

Salaheddine Skali- Lami

Dr Salaheddine Skali-Lami
email: salaheddine.skali-lami@ensem.inpl-nancy.fr

Screening: Fundamental studies on the extrusion of stickies through slots

de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

conditions with low pressure during the positive pressure pulse as well as reduced temperature and pH should thus minimise stickies extrusion as expected. Both experimental and numerical simulation results led to the same conclusions about the effects of key screening parameters. Linear relations were observed between the normalised particle diameter and the extrusion pressure and time. Particles 2 or 3 times larger than the slot width were effectively extruded at typical extrusion pressures observed in screens, but the time required to pass the slot was up to two orders of magnitude higher than the duration of the positive pressure pulse, which suggests that particle extrusion should take place step by step over several revolutions of the rotor. The tests performed to compare milled slots with sharp inlet edges to wedge wire slots with higher inlet radius, showed less particle extrusion with the milled slots, as expected. Further investigations are however required to

Introduction

Adhesives and glues entering the papermaking cycle with the recovered papers lead to numerous stickies problems including deposits on the paper machine, optical defects in the paper and printing problems caused by adhesive spots. Despite progress in recycling technology and the efforts engaged to develop environmentally benign adhesive products, stickies problems are far from being solved. PSA stickies from pressure sensitive adhesives (labels, tapes, envelopes, etc.) are the main concern in the field of deinking while hot-melt glues (container sealing, book bindings, etc.) contribute strongly to the paper machine deposit problems in the field of packaging.The research presented in this paper was performed in the frame of a European project "ScreenClean" [START_REF] Skali-Lami | Optimisation of screening and cleaning technology to control deinking pulp cleanliness[END_REF], which aims at improving the removal of primary PSA stickies at the different deinking process steps, with special focus on pressure screening.

Screening is clearly the most effective technology to remove primary stickies (solid particles introduced with the raw material and showing an adhesive effect under standard test conditions) especially the "macro-stickies" fraction. Indeed, macro-stickies do not refer to particles in the visible size range, but to the particles retained by laboratory screening with typically 0.10 mm slots for deinking pulps, according to widely accepted standard test methods [START_REF]Analysis of Macro Stickies in Deinked Pulp (DIP)[END_REF][START_REF] Delagoutte | modified method for quantification of primary stickies in recycled pulp[END_REF][START_REF] Saint Amand | ScreenClean Progress Report 1: Raw Materials and Methods[END_REF]. By contrast, "micro-stickies", which are defined as the fraction of primary stickies passing the lab screen, are much more difficult to remove by screening. Large microstickies particles produce visible dots in the paper sheet as they are generally covered with inks, while micro-stickies are mainly responsible for paper machine deposits [START_REF] Delagoutte | Drying section deposits: identification of their origin[END_REF][START_REF] Hamann | Stickies: definitions, causes and control options[END_REF]. Secondary stickies (tacky particles produced in mill circuits by physico-chemical processes involving dissolved and colloidal stickies) are normally not removed by screening. Screening is currently performed in deinking mills with slots down to 0.10 or 0.12 mm for respectively wood-free (MOW) and wood-containing (ONP/OMG) deinking pulp [START_REF] Schabel | Screening: fundamental aspects of stickies removal[END_REF][START_REF] Serres | ID2/ID3 -Un nouveau bond dans les procédés d'épuration[END_REF][START_REF] Griffin | Optimizing pressure screen parameters in a deinking plant using mixed office waste[END_REF].Typically, fine screening systems in European DIP lines are operated with 0.15 mm wedge wire slots in the low consistency range [10].The specific behaviour of stickies in screens is due to their ability to be extruded through the slots by the pressure generated by the rotor and the pulp flow, as a consequence of the visco-elastic properties of the adhesive material. Soft deformable stickies particles which can be extruded though slots should be considered as "probability" contaminants (normally particles with at least one dimension smaller than the slot width) rather than "barrier" contaminants (normally particles with all dimensions larger than the slot width). The behaviour of such particles in pressure screens can be characterised, on average, by a particle passage ratio, which is defined by the ratio of the downstream to the upstream particle concentration in a screen cylinder section, according to the probability screening theory [11][12][13][14][15][16]. Particle passage ratios should be determined for particles in different size and shape classes to predict correctly the performance of industrial screening systems.

Basic studies were carried out over the last decade at several research institutes, as reviewed in [16], to develop the understanding of the screening process at the scales of both pulp "macro-flow" conditions around the rotor and unsteady "micro-flow" conditions and particle separation phenomena at the surface of the screen plate and through the slots. Investigations at CTP in this field were first performed with relevant probability contaminants, i.e. flat-shaped particles (0.5 mm 2 films) and long-shaped particles (shives) with lower thickness than the slot width. A major conclusion about these studies [16][17] was that all the parameters which improved the passage of the fibres through the slots, i.e. increasing slot width and the effective normalised slot velocity, also increased the passage of the tested probability contaminants. Increasing the passing velocity as calculated from the accept flow rate, reducing the slot friction factor with wedge wire design and increasing the intensity and duration of the positive pressure pulse created by the rotor were assumed to increase the effective slot velocity during the screening phase and thus to increase the probability for particles to be captured in the fluid exit layer and then to pass the slot. The effective normalised slot velocity was an extended definition of the normalised slot velocity [18], i.e. the ratio of the effective slot velocity to the tangential velocity at surface of the screen cylinder. Increasing the thickness of the exit fluid layer taken from the flow at the top of the profiles and turning into the slot (see figure 13) was assumed to enhance its ability to drive particles towards the slot inlet. CFD simulation showed that this fluid layer thickness was roughly proportional to the normalised slot velocity and to the slot width [19]. The effects observed when changing rotor velocity were attributed to changes induced by the tangential fluid velocity in the exit layer thickness [16][17]. Practically, probability screening efficiency, i.e. the selectivity of the separation between fibres and thin contaminants showed to be very difficult to improve by changing slot velocity and design or rotor velocity and design, at given profile design. Clear improvement of the screening efficiency was only achieved by reducing the height of the profiles, from 1.2 to 0.6 mm, which was attributed to particle contacts with the inclined profile walls assumed to reject more efficiently large and stiff contaminants compared to flexible fibres [17].

Indeed, the hydrodynamic screening mechanisms revealed through the investigations with thin flat and long-shaped particles should also apply to stickies at least until the particle has reached the slot inlet.Then the extrusion process should become decisive. Recent research was reported about experimental studies on the passage of acrylic PSA particles through a fine slot in a single slot laboratory device using vacuum to extrude the particle through the slot [20-21].The particle passage probability was shown to increase as temperature and pressure drop increased and as the particle width, thickness or area decreased. Particle size and shape were controlled by changing the pulping conditions which produced mainly string-like particles.

Slight changes in particle size and shape were observed after the extrusion of the particles through the slot.

The research work reported in this paper is devoted to basic studies about the extrusion of PSA particles through slots and the influence of the most relevant parameters. The first part deals with experimental results obtained at CTP with a single slot laboratory device equipped with a high-speed camera to visualise the extrusion process. The numerical simulation work performed at LEGI [22] is briefly reported in the second part. The third part about stickies rheological properties describes shortly the new method developed at LEMTA to determine the dynamic behaviour of the adhesive material at high strain rate.

Experimental study of stickies extrusion

The objective in this study was essentially to develop a better understanding of the stickies extrusion process through visualisation, in connexion with the numerical simulation work already engaged at LEGI in the frame of the ScreenClean project.

Experimental Set Up

The single slot channel screen (figure 1) was designed to simulate the flow conditions observed at the surface of screen plates and through the slots during the positive pressure pulse and to change and control easily slot width and design. The height of the transparent channel was 10 mm and the width 15 mm. The syringe shown in figure 1 was not used in the test procedure described later. It was intended to introduce stickies particles at the right level to drive them toward the slot inlet.

A high-speed CCD camera (Sony XC-HR300) was used at 100 or 200 i/s to visualise the particles in transmitted light, using continuous light source behind the transparent channel.The resolution was 2 or 8 µm depending on the magnification of the optics (x1 or x4).Visualisation was limited to the extrusion of stickies through the slot, as a conse- quence of the shape of the PSA particles and procedure used for the tests.

Adhesive Particles and Test Procedure

PSA stickies particles observed in mills have various size and shape, depending on the adhesive type and pulping conditions. Figure 2 shows handsheets with acrylic and SBR adhesives, which were obtained by re-pulping self adhesive labels previously covered with toner inks, in a low-consistency lab pulper with standard deinking chemistry. The acrylic labels gave more flat-shaped particles than the SBR labels, which was attributed to the higher trend of the less hydrophilic rubber based adhesive film to stick onto itself and form thick rolled up particles in the pulper. Laboratory screening tests with 0.08 mm slots showed low screening efficiency of about 50% with the acrylic adhesive (the total area of micro-stickies passing the slots and measured on handsheets as specks > 0.01 mm 2 was about half of the stickies area in the feed pulp) compared to 90% efficiency obtained with the thicker and larger SBR adhesive particles [START_REF] Saint Amand | ScreenClean Progress Report 1: Raw Materials and Methods[END_REF].

Long-shaped particles were considered as the most relevant to study the extrusion of mill PSA stickies. Indeed, flatshaped stickies with lower thickness than the slot width (down to 20 or 30 µm, the thickness of wet adhesive films) are not relevant while spherical particles are typically found after dispersing, especially kneading, i.e. in the second deinking loop after fine screening. Consequently it was decided to use the cylinder as the model particle shape in the extrusion studies. Special rolls of adhesive films between two silicone release papers were used to manufacture cylindrical adhesive particles of different diameters (figure 3). Two commercial adhesives from Avery Dennison Jackstädt were tested: -a water-based acrylic adhesive (ref E115) -a hot-melt based rubber adhesive (ref D170) These products were taken as reference adhesives in the ScreenClean project [START_REF] Saint Amand | ScreenClean Progress Report 1: Raw Materials and Methods[END_REF] as they represent respectively the most common type of adhesive used for label application in Europe and a growing market share among the other types of adhesives. The adhesive cylinders were measured and tested after soaking about one hour, which is particu-larly important with the acrylic adhesive which showed to absorb about 50% water.

The cylindrical shape of the adhesives did not allow to introduce them in the channel with the syringe (figures 1). The following procedure shown in figure 4 was adopted: -the adhesive cylinder was introduced in the device through the valve V2 while a very low flow rate was maintained in the channel until the cylinder reached the slot inlet and was properly aligned with the slot, -the valves V2 and V4 were closed, the flow rate in the channel was increased with the pump after opening V3 and the pressure was set to the desired value with a sufficient flow rate through the channel, -the valve V4 was then opened rapidly (< 0.5 s) to set the pressure on the adhesive particle.

This procedure was established in order to control, as far as possible, the pressure during the extrusion process, since the flow rate through V3 was much larger than through the open slot area, about half of the slot being covered by the adhesive cylinder.

Most of the tests were performed only to determine the pressure required to extrude the particle as a function of adhesive and slot characteristics. Images were recorded under certain conditions to analyse deformation of the particles and to determine the extrusion time. Some tests were also performed at extrusion pressures far above the usual range of positive pressure pulses observed in screens, i.e. typically 20 to 100 kPa.

Effect of Adhesive Material Parameters

A first test series was performed with 0.5 mm slots with "wedge wire like design" to study the adhesive material parameters. New slots with more accurate design were used later to study the effects of slot design parameters.

Figure 5 shows the pressure required to extrude the adhesive particle versus the normalised particle size defined as the ratio (d/w) of the cylinder diameter to the slot width, for the two reference adhesives at room temperature (22°C) and at typical temperature in deinking circuits (50°C).The extrusion pressure was significantly lower with the acrylic adhesive (E115) than with the hot-melt rubber adhesive (D170) at both temperatures, which suggests that the acrylic adhesive material is softer and that acrylic stickies should be more difficult to screen as larger particles are extruded through the slots for a given pressure.

Increasing the temperature reduced the extrusion pressure, especially with the acrylic adhesive, which suggests higher softening of the adhesive material.

It is interesting to note that the extrusion pressure is roughly proportional to the particle "oversize" (d-w) though the extrusion phenomena is probably strongly nonlinear. The results in figure 5 indicate that acrylic adhesive particles with 2 to 3 times larger diameter than the slot width were extruded through the slots in the temperature and extrusion pressure range of mill screening systems.

Extrusion tests were performed after soaking the adhesive particles in cold and hot water at pH 10, in order to investigate the effect of caustic soda added to the pulper in deinking mills.The results in figure 6 show a large decrease of the extrusion pressure when the acrylic adhesive particles were soaked at room temperature with caustic soda. At high temperature the decrease of the extrusion pressure was lower as soaking pH was increased.These results suggest that the penetration of water and/or the dissolution of adhesive components, assumed to be responsible for softening the adhesive particle, required higher pH at low temperature.

Soaking caustic soda produced the opposite effect with the hot-melt rubber adhesive, as a significant increase of the extrusion pressure, by about 50%, was observed as soaking pH was increased (pH 10) at both temperatures (22 and 50°C).

Effect of Slot Design Parameters

Milled and wedge wire screen plates were compared to investigate the effect of slot inlet design which was supposed to influence significantly the extrusion process. Metal blocs shown in figure 7 were manufactured at scales 1 and 5 to improve the accuracy of the slot inlet radius. The dimensions in table 1 were defined on the basis of 0.10 mm slots and chosen to cover the variations observed with commercial screen plates. Slot print measurements were performed on various screen plates of CTP' experimental screen [17] including Microvortex (Kadant Lamort) and Macroflow (AFT) designs.

The stickies extrusion tests were first performed with the slots at scale 5 in order to improve the accuracy of the size and shape of the adhesive cylinders.The results in figure 8, which were obtained at 47°C after soaking the adhesives in tap water, confirmed the higher extrusion ability of the acrylic adhesive. The extrusion of both types of adhesive particles is clearly increased with the wedge wire slot compared to the milled slot, as expected when looking at the shape of the slot inlet section. In short, the particle is submitted to lower stresses when extruded through one round edge with slightly converging profile walls (165°a ngle with the wedge wire profile in figure 7) than through two sharp edges (4 times lower radius with the milled compared to the wedge wire slot) with slot inlet location on a flat profile bottom.

The time needed for the adhesive particles to pass completely the slot has been recorded under the same test conditions as in figure 8, except for the pressure which was 50 kPa for all the tests. The results in figure 9 showed a much higher extrusion time with the milled slot compared to the wedge wire slot, but only with the acrylic adhesive. The higher trend of the hot-melt rubber adhesive to stick at the slot walls was responsible for the very high extrusion time measured with wedge wire design. In fact, the extrusion time is over-estimated as it was recorded when the whole adhesive cylinder had passed the slot, after detachment of all the cylinder parts stuck on the slot outlet wall, as illustrated in figure 12.The sharp inlet edges and the higher length of the milled slot are responsible for the longer extrusion time observed with the acrylic adhesive. Indeed, the most important conclusion about the results in figure 9 is that the extrusion time is about two order of magnitude higher than the duration of the screening phase, i.e. a few seconds compared to typically 20 ms, which gives new insight in the stickies extrusion mechanisms in pressure screens.This means that adhesive particles with significantly larger thickness than the slot width cannot pass the slot during one positive pressure pulse and that the passage of such particles through the slots takes most probably place over several rotor revolutions in a multi-step extrusion process. One should however keep in mind that the experimental procedure does not take the inertia of the particle approaching the slot into account, which means that extrusion times in figure 9 are over-estimated compared to the real situation, especially at low extrusion time.

Scale effects should also be kept in mind when analysing the results obtained with slots at scale 5, especially the results in figure 9, since equal extrusion time with slots at scale 1 would correspond to a decrease of the average strain rate by a factor 5, at given normalised particle size.

The extrusion time should then in fact be lower with slots at scale 1 since the elasticity modulus of adhesive materials decreases as the strain rate is decreased. The extrusion times should then however still be about one order of magnitude higher than the duration of the positive pressure pulse, and suggest multi-step extrusion mechanisms of large PSA stickies in pressure screens.

Some tests were performed with the slots at scale 1 to evaluate extrusion pressure at mill relevant conditions, i.e. at 47°C and with acrylic adhesives after neutral and alkaline soaking.The results shown in figure 10 were less accurate, as expected, but gave similar pressure values and led to the same conclusions regarding the influence of slot design and alkaline soaking of the adhesive, though the effects were lower.

The pictures in figures 11 and 12 illustrate the extrusion of acrylic adhesive cylinders through the milled and wedge wire slots at scale 5. The ends of the cylinders passed the slot first, especially with the small cylinders. One can observe with the wedge wire slots that parts of the adhesive particle still stick at the wall, generally at the right side, when the particle has passed the slot.

Numerical simulation of particle extrusion

Screen Plate Flow Simulation

The numerical simulation of the extrusion of stickies through slots first requires the simulation of the flow conditions at the screen plate in order to establish the pressure exerted on a particle at the slot inlet.The velocity at which the particle approaches the slot is also an important parameter when taking particle inertia into account.

Figure 13 shows a 2D CFD simulation of typical flow pattern at the screen plate during the screening phase, where the exit layer was coloured in blue [19,23].

Further 2D CFD simulations were performed with the commercial code FLUENT (version 5.4) to characterise the micro-flow at the slot inlet in a large range of tangential velocities (Vt = 5 to 20 m/s) and slot velocities (Vs = 1 to 10 m/s). Figure 14 shows the thickness of the exit layer over the detachment point at the top of the profile (L od ) and under the vortex (L uv ) as a function of the normalised slot velocity (Vs/Vt) for 0.10 mm slots. Particles were then introduced at the slot inlet in the simulations, which were performed in 3D with both spherical and cylindrical particles. Figure 15 shows the flow pattern in the central section of a cylinder placed at the inlet of a milled slot.

Figure 14. CFD simulation of the exit layer thickness in the case of milled slots.

The pressure distribution at the surface of cylinder revealed two zones with roughly constant pressure, a high pressure area (in red) at the feed side and a low pressure area (in blue) at the accept side of the slot (figure 15 right).The "extrusion pressure" can then be approximated, in such a case, as the pressure difference between these two areas.

Simulation of Particle Extrusion

The results in figure 15 led to strong simplification of the simulation of the extrusion of cylindrical particles, as the simulation could be treated in 2 dimensions, though it is believed that three-dimensional effects could occur during the deformation of soft particle.The rheological properties of the adhesive material as well as the friction factor between the particle and the slot wall are required as input data for the simulation. The simulations of the particle deformation were done with the commercial code ANSYS (version 5.7) and with the use of a viscoelastic constitutive model where the material properties are represented by the elementary Maxwell model :

where σ is the stress, ε the strain (dot over symbol denotes time differentiation) G the shear (elastic) modulus, η viscosity and λ = η/G the relaxation time (the ability of the material to relax stresses).

The deformation of a cylindrical viscoelastic particle placed at the inlet of milled and wedge wire slots is shown in figure 16 at the initial time (deformation caused by elasticity only) and in figure 17 after 20s under a pressure of 10 kPa. The extrusion is slower with the milled slot because of the concentration of deformation and stress in the particle at the contact edges compared to the wedge wire slot where particle deformation is more uniform.

The simulations were done with a set of rheological properties (G = 70 kPa, G∞ = G/10 and λ = 1s) taken from average preliminary test results, since more relevant data were not available. Friction was included later. The simulation results were however consistent with experimental results and allowed to determine the effects of the screening parameters. Both visualisation (fig. 12) and numerical simulation (fig. 17) indicated elastic expansion in the divergent outlet section of the wedge wire slot. The following parameters turned out to increase particle extrusion: -high screening pressure, -particle "softness" (small elastic/shear moduli), -short shear relaxation (more viscous behaviour), -wedge wire compared to milled slots, -slot inlet edge roundness (higher radius), -reduced friction, smooth profile surfaces. It should also be mentioned that the simulations gave about linear relations between the particle oversize (d-w) and the extrusion pressure or time [22] as observed experimentally (fig. 6, 8, 9, 10).

Dynamic behaviour of adhesive material

The analysis of possible single step extrusion of adhesives through slots shows that both strain and strain rate would be very high with particles much larger than the slots, if the particle effectively passes the slot during the screening phase.With integral calculation of the average strain at high deformation (ε = ln d 0 /d = ln d 0 /w) and for a particle squeezed by a factor 3 in 10 ms, the strain rate (ε/∂t) would be of the order of 10 2 s -1 . Results about the dynamic moduli, G' and G", of adhesive products have been reported but the measurements were performed in dry state at much lower strain rate [26].

The analysis of the dynamic behaviour of adhesives, was subcontracted, in the frame of the ScreenClean project to provide input for the numerical simulation, to LEMTA where expertise was available in this field as well as for the analysis of friction factors with soft materials [27].The test equipment at LEMTA had however to be adapted to the

σ / η + σ / G = ε + σ = G ε . . .. σ λ
very soft adhesive materials to be tested. In the mean time preliminary low-speed elongation tests were done at CTP to provide some input for the simulation.

Low-speed elongation tests

Preliminary tests were performed with paper testing equipment at elongation velocities up to 1 m/min, which gave a maximum strain rate of 0.4 s-1 in the narrow part of the adhesive test samples (40 mm length, 15 mm width and 2 mm thickness). The dimensions of the test samples were not exactly those defined for standard rubber testing procedure because of the difficulties encountered in preparing samples with very tacky material (figure 18).

Adhesive sheets were produced from the two reference adhesive raw materials, cut in water and tested after soaking. Both adhesives recovered almost their initial length in a few minutes after elongation by a factor 10. Figure 18 shows pictures of an adhesive sample back to its initial position after a first elongation and submitted to a second elongation.

The typical stress strain relations are shown in figure 19. Both adhesives showed a first elastic part followed by a drop of the modulus with lower strength at the second elongation, especially with the hot-melt rubber product. The first pass elastic modulus was about one order of magnitude higher with the hot-melt rubber than with the acrylic adhesive, which gave G ≈ 20 kPa at 1m/min after soaking in hot water.The elastic modulus showed to increase with increasing elongation velocity and to decrease with increasing temperature.

High-speed compression tests

The experimental equipment and method illustrated in figure 20 were developed to record stress strain relations applied on adhesive materials at high strain rates: -a cylindrical adhesive sample (18 mm diameter and 28 mm height) is placed on a bottom plate equipped with a load sensor, -a weight is dropped from different heights on the top plate above the adhesive sample, -the compression of the adhesive sample is measured with a laser recording the height of the bottom plate.

The initial strain rate are controlled by the drop height (0.15 to 0.9 m, e.g. 0.8 m gives a drop velocity of 4 m/s and an initial strain rate of 150 s -1 ) and the stress level by the weight. The samples are covered with talcum to allow adhesive material to slip between the plates and avoid barrelling of the cylinder during the compression. The recorded data (figure 21) are converted into stresses of Cauchy and true axial strains

The dynamic compression test is distinguished from the usual quasi-static tensile test by the fact that the strain rate drastically decreases during compression, towards zero at the end of the tests.The residual stress due to the weight at the end of the test is considerably lower than the one due to the high deceleration of the weight at the impact.

The strain level reached with adhesive material can be rela- tively significant, up to 1. Figure 22 shows a typical example of stress strain curve obtained with the dynamic compression test, at non-constant strain rate.

In principle, a large number of tests should be performed to determine, as a function of the strain rate, the stress strain relations, which are required for the simulation of particle extrusion.The development of a model describing the dynamic behaviour of viscoelastic material will enable to determine these relations from the experimental data.

The theoretical curve plotted in figure 22

Conclusion

The experimental and numerical simulation studies of the extrusion of PSA particles through slots led to clear and consistent results about the effects of key screening parameters. Some aspects which have not yet been taken into account should however be discussed before to conclude about the optimisation of stickies screening.

Discussion

Both the numerical simulation and the lab screening tests were limited to the screening phase with the adhesive particle initially at rest at the slot inlet.The reverse flow phase and particle inertia should also be taken into account to further investigate stickies extrusion in pressure screens. The average slot or passing velocity Vp in pressure screens is controlled by the accept flow rate, while the effective slot velocity during the screening phase depends on the pressure pulse. CFD simulations revealed high slot velocity fluctuations (for example +5 to -20 m/s with a foil rotor and wedge wire slots) since the effect of fluid inertia on time lag and slot velocity fluctuation was relatively low [23,24].The average effective slot velocity during the screening phase Vs (+) can also be evaluated by simple flow rate balance (t (+) Vs (+) -t (-) Vs (-) = t Vp) assuming constant slot velocity over the duration of both positive and negative pressure pulses t (+) and t (-) . Increasing for example the duration of the negative pressure pulse from 10 to 20 % of the whole pulse duration t would then give Vs (+) = 2.8 to 5 m/s for Vs (-) = 15 m/s and a typical passing velocity Vp = 1 m/s. The flow velocity in the exit layer under the vortex would then be roughly equal to Vs (+) according to figure 13 for a tangential flow velocity Vt = 10 to 15 m/s.

Under these conditions the inertia of the particle approaching the slot inlet at the same velocity as the fluid in the exit layer cannot be neglected. Particle inertia should reduce or delay particle extrusion in the case of milled slots located in the middle of flat bottom profiles (with large b value in table 1) as the particle might pass along the slot inlet. By contrast the inclined wall of the convergent inlet of wedge wire slots should increase particle extrusion as a result of the inertia of the particle turning smoothly into the slot, as illustrated in figure 23.Assuming a constant deceleration of the particle over a distance equal to particle radius, which gives γ = V 2 / d, and inertial forces applied on a section equal to the particle diameter, the "equivalent pressure" is given by P = (π/4) ρ V 2 , for a cylindrical particle. Such additional "inertial extrusion pressure" (e.g. 20 kPa for Vs (+) = 5 m/s) should have similar effect than the hydraulic extrusion pressure, which is generally higher (typically 50 kPa) but with resulting pressure limited to the particle section between the contact lines with the slot inlet (see figures 16 and 17). The behaviour of partly extruded adhesive particles during the reverse pulse should also play an important role in a multi-step stickies extrusion process, since the duration of the reverse pulse is extremely short, down to 2 ms with foil rotors.This means that the detachment of the adhesive particle from the slot wall could strongly retain the particle as long as water has not penetrated in the surface between the adhesive and the slot wall, assuming that time and pressure were sufficient to expel the water from the contact surface during the positive pressure pulse.The viscous component of the adhesive material might help to detach the particle during the reverse pulse if relaxation time is much longer time than the reverse pulse. Figure 23 illustrates possible "reverse extrusion" mechanisms of cylindrical adhesive particles.With the milled slot, the particle should be extracted very quickly from the slot inlet assuming low penetration, but difficult to detach from the corner along the profile wall. By contrast, the extraction of the adhesive particle from the wedge wire slot should take more time, especially if the particle having passed the narrowest slot section is submitted to "reverse extrusion". Low friction factor and high relaxation time should promote reverse extrusion. The particle should then, after detachment, be ejected very quickly back to the main flow since particle inertia can be neglected and the velocity of the reverse jet out of wedge wire slots is very high [23,24]. Unfortunately the possible extrusion mechanisms described in figure 23 do not allow to conclude easily about the most effective slot design regarding the efficiency of the reverse pulse in rejecting stickies retained at the screen plate.

Synthesis

The extrusion of viscoelastic pressure sensitive adhesive particles through slots during the screening phase in pressure screens was studied numerically and experimentally by visualisation with a single slot channel screen. Special research effort was placed on the analysis of the rheological properties of the adhesive material. The effects of screen operating, slot design and adhesive material parameters were investigated with cylindrical particles to cover typical stickies screening conditions in deinking mills.

Concerning the effects of PSA material and process parameters it was shown that :

-Water-based acrylic adhesives, which represent the major part of the PSA stickies in deinking pulps, were softer and were extruded through the slots at lower pressure than hot-melt rubber adhesives, especially after soaking as they absorbed about 50 % water.

-Increasing the temperature increased significantly particle extrusion as the adhesive became softer.

-Alkaline soaking further increased the extrusion of acrylic adhesives as the adhesive became softer, while the opposite effect was observed with the hot-melt rubber adhesives.

Consequently screening should not be performed at high temperature and pH, as acrylic adhesives are the most problematic stickies.

Concerning the effects of technological screening parameand of geometrical particle and slot design parameters it was shown that : -Pressure and time required to extrude cylindrical particles through slots increased linearly with particle oversize with respect to slot width (d-w).

-Particles 2 to 3 times larger than slot width were extruded at pressures in the range of typical positive pressure pulses observed in screens.

-Extrusion pressure and time were larger with milled slots compared to wedge wire slots.

-The extrusion time (a few seconds) showed to be about 2 orders of magnitude higher than the duration of the screening phase (about 20 ms).

Consequently, screens should not be operated with rotors producing high positive pressure pulses and at too high slot velocities in order to minimise stickies extrusion. However, the most important conclusion was that the extrusion of adhesive particles with significantly higher thickness than the slot width cannot take place during a single positive pressure pulse, which suggests that the extrusion process in pressure screens should take place in subsequent steps over several revolutions of the rotor.

The experimental and numerical simulation results as well as further analyses about the effects of particle inertia, showed that the extrusion of a particle reaching the slot inlet (first step or single extrusion step with small particles) should be reduced with milled compared to wedge wire screen plates, which should then be less efficient in removing stickies. However, since wedge wire screen plates might be more effective in rejecting stickies during the reverse pulse if reverse extrusion hindrance can be neglected, the advantages of milled screen plates have still to be demonstrated with respect to multi-step extrusion of large adhesive particles.

Finally it should also be kept in mind that particle extrusion is not the only micro-process step to consider in the optimisation of stickies screening since all particles do not reach the slots as they can be rejected through contacts with the profile walls.

Perspectives

The fundamental studies engaged on the extrusion of stickies through slots have clearly improved the knowledge about the physical mechanisms involved in the extrusion of soft deformable particles, but also raised a number of questions about possible multi-step particle extrusion processes.

Further analyses of adhesive rheological properties are in progress to determine more completely the dynamic behaviour of the adhesive material at high strain rates. Experimental and numerical simulation work about the extrusion of viscoelastic particles through slots should be continued with special focus on the effects of particle inertia and on the analysis of the reverse extrusion and rejection of partly extruded adhesive particles during the negative pressure pulse.

Pilot stickies screening tests are under progress to verify the conclusions of this paper at industrial scale and to investigate the effects of other key screening parameters, especially the influence of profile height and design on the removal efficiency of PSA stickies and on the performance of screening systems in terms of production capacity and fibre losses.

Figure 1 .

 1 Figure 1. Single slot channel screen with camera.

Figure 3 .

 3 Figure 3. Adhesive particles manufactured for the tests.

Figure 4 .

 4 Figure 4. Adhesive extrusion test procedure.

Figure 2 .

 2 Figure 2. Handsheets of re-pulped adhesive labels.

Figure 6 .

 6 Figure 6. Effect of alkaline soaking of acrylic adhesive.

Figure 7 .

 7 Figure 7. Design and dimensions of the tested slots.

Figure 5 .

 5 Figure 5. Effects of adhesive material and temperature.

Figure 8 .

 8 Figure 8. Effect of slot design on extrusion pressure.

Figure 9 .Figure 10 .

 910 Figure 9. Effect of slot design on extrusion time.

  Application of CFD simulation to screening started at LEGI in the frame of a thesis prepared in cooperation with CTP and Kadant Lamort [19,23-25] and was continued in the frame of the ScreenClean project [22].

Figure 13 .

 13 Figure 13. CFD simulation of screen plate flow [23].

Figure 15 .

 15 Figure 15. CFD simulation of the flow pattern and of the pressure exerted on a cylinder at the slot inlet.

Figure 12 .

 12 Figure 12. Stickies extrusion through WW slots.

Figure 11 .

 11 Figure 11. Stickies extrusion through milled slots.

Figure 16 .

 16 Figure 16. Particle extrusion at p = 10 kPa, t =0.

Figure 17 .

 17 Figure 17. Particle extrusion at p = 10 kPa, t = 20 s.

Figure 20 .

 20 Figure 20. High-speed adhesive compression tests.

Figure 21 .

 21 Figure 21. Dynamic test: recorded load and compression.

Figure 18 .

 18 Figure 18. Low-speed adhesive elongation tests.

Figure 19 .

 19 Figure 19.Typical elongation stress strain curves.

  represents the estimated result obtained from a viscoelastic model with two relaxation times. Different relaxations times generally refer to the components of the generalised Maxwell model where infinite viscosity of one component gives the solid behaviour. The new theoretical model under development is based on the thermodynamics of irreversible processes produced by the stresses and strains applied to the polymers [28-30]. Indeed, polymer chains exhibit the highest disorder, i.e. maximum entropy, at steady relaxed state, while stresses tend to stabilise. The complex behaviour of adhesive material depends on the relaxation times spectrum which is related to the micro-structural reorganisations observed at different scales.

Figure 22 .

 22 Figure 22.Typical compression test results and model.

Table 1 -Slot dimensions according to figure 7.

 1

Acknowledgements

The authors would like to thank gratefully • the European Commission, the French Government for their financial support to this research, • Avery Dennison Jackstädt, Kadant Lamort Advanced Fiber Technologies (AFT), for their technical cooperation in this study • B. Perrin, D. Guerin and J.Veiga (CTP)

for their contributions to this work.