Second life

The last phase of recycling: the pulper waste


A study of technologies for separating and initiating the recycling of mixed pulp and plastics from pulper waste has allowed to identify solutions to increase separation efficiency and material stream quality, improving paper recycling efficiency. 

Involved in numerous studies with its three operational areas, i.e. Research and Innovation, Paper Quality Centre (CQC) Laboratory, ICT Products and Services, Lucense deals at national and international level with industrial research, experimental development, technology transfer and dissemination, with specific reference to innovation and sustainability issues for the paper industry. For about three years now, it has been working on a topic that is particularly interesting, as it is in line with the current requirements of an increasingly circular economy: technologies for separating pulper waste materials from recycling.

The weight of pulper waste

Figures clearly show that the circular economy is an integral part of the paper industry and is becoming fundamental: a packaging recycling rate of over 80% and around 60% of the fibrous raw material being recycled paper.

«Within the paper recycling process, however, each step also corresponds to a loss of material. And despite the records achieved, there is still a residual quota of waste, equal to 5-6% of materials that are not recyclable in the paper production process» says Simone Giangrandi, head of Lucense’s Research and Innovation Area, who spoke about it at the Aticelca 2020 Congress.

«This small percentage of residual pulper waste, given the very high volumes, still has an important impact. We are talking about 250,000 tonnes per year, with all the problems, which are well known to the industry, related to waste management». That is why further improving even these last recycling steps can make an important contribution to paper mills.

Separation technologies

Lucense has initiated a project to evaluate the efficiency of material separation plants. The objective is to identify the quality of the various fractions and then estimate the benefits in terms of waste volumes and recoverable fibre and, ultimately, in overall economic terms. «We started with a survey of suppliers and material separation technologies, analysing 7 manufacturers with 9 solutions in total». Giangrandi explains how there are «plants that only separate the fibrous material to be fed into the paper mill process, in which all other materials form a second, single stream, and more complex plants in which, in addition to the fibre, the lighter component is also separated, in particular the polyolefin plastic, and three material streams are generated».

Specifically, in plants with the basic configuration, the separation and recovery of the fibrous material takes place with friction washers or similar machines. These work at very high rotational speeds in order to separate, by centrifugal force, the fibrous component which is then expelled. While the fibre is fed back into the production plant directly during stock preparation, the other stream of material, consisting of the non-fibrous fraction, undergoes passage through a compactor or press to reduce its moisture content.

In more complex plants, this basic configuration can be extended with an initial shredding stage to reduce the size of the materials and any metal components, and a homogenising reactor to separate the remaining materials. In the following part of the plant, a further step can be added by inserting washing or settling tanks, or air flows, with the task of separating heavy materials from the polyolefin plastic fraction.

Analyses and experiments begin

Lucense’s analysis work was relevant. «We started with sample analyses and tests», explains Giangrandi, «and then we carried out more significant tests at some suppliers and industrial trials, conducted over longer periods, at some paper mills. This activity was accompanied by laboratory analyses to assess the quality and efficiency of separation». In total, pulper waste from 10 paper mills was analyzed.

The initial part of the characterisation of the material streams separated from the pulper waste concerned the mixed plastics component and the fibrous component. «As far as the non-fibrous part is concerned, a distinction must be made between the two cases: the one in which light plastics are actually separated and the one in which the waste fraction of pulper cleaned of the fibrous component is actually taken into account» (Figure 1). In the first case, explains Lucense’s manager, «it is possible to obtain a very high composition of mixed plastics with a large polyolefin majority, with reduced contamination of fibre, wood and other materials»; the more or less relevant presence of these materials depends on the quality and type of the initial waste. «The level of fibrous contamination can vary significantly, but by optimising and tailoring the implant, results can be at least partially uniform. This result is linked to the initial composition of the waste: if it is richer in fibre, it will require a stronger action to remove it».

In the case of plants that only separate the fibrous component, «you can still achieve important values with significant efficiencies. In this case, the waste used at the start has fibre concentrations of between 30 and 50% and it is easy to drop below 10% at the output» (Figure 2).

The analyses carried out for the characterisation of the fibrous fraction were performed at the Lucense CQC laboratory. «They focused mainly on the assessment of non-cellulosic impurities, macro stickies, flakes and ash» (Figure 3). The most significant results were obtained on the post-screening waste from the Somerville screening, which «gives a direct indication of the percentage of the fraction that can be recovered at the mill. The other parameters – such as ash and fibre analysis – are within the expected values, which are typical for recycled paper, and therefore do not pose any problems or criticalities» and were therefore considered less relevant for the study carried out. Finally, mechanical tests or evaluation of optical impurities were also carried out in the laboratory.

Results: the efficiency of waste recovery

The combination of tests carried out with different scraps, equipment and configurations allowed Lucense to create a useful matrix to evaluate the work of different machines under different operating conditions. These findings were then further investigated in medium-term experiments carried out in the paper mill. «These allowed us, on the one hand, to have periodic samplings that also allowed us to evaluate the inevitable fluctuations due to differences in the starting material, to the variation of the pulper waste according to the production and the pulping recipe used; on the other hand, they allowed us to see improvements in the efficiency of the work of the machines as measures were introduced, process parameters modified and the necessary changes made».

From this and the quality of the fractions, the researchers were able to estimate the performance, in terms of efficiency, of the separation activities. «Considering a fibre input waste of between 30 and 50% on the dry stock» explains Giangrandi, «the estimated recovery of the fibrous component is between 70 and 90%, which may be closer to 90% in the case of more complete and complex plants; the separation of polyolefin plastics is around 80%, which is very significant».

From the point of view of the volumes involved, evaluated on the dry stock, «it is estimated for a fibre separation plant alone to be able to achieve a separation and reuse percentage of fibre in the paper mill of around 60-70%. Considering then that the residual fraction of waste, due to the reduction of the fibrous component, can be brought to a higher dryness, the overall reduction of waste can be estimated to be around 30-35%». This is a range of values in which most of the cases studied fall, which can be refined according to specific conditions, Giangrandi points out. «Again, a large part of the reduction in waste is due to a reduction in the moisture content, thus making it easier to remove water by pressing or squeezing».

Finally, analyses related to water properties were carried out. «This could prove to be an important parameter, but in terms of stickies and flakes we have not seen any significant impact». 

Costs, benefits and guidelines

Finally, Lucense’s experts considered all the results obtained and made an overall assessment of the costs and benefits for paper mills. «From a quality point of view, fibre-only plants are simpler, take up less space and are easier to operate. More complex plants, on the other hand, have higher investment costs and management and space problems. On the other hand, however, they can achieve better results and therefore a greater reuse of cellulose, the possibility of also enhancing the polyolefin plastic component and a higher average reduction in waste volumes».

Economic impact and feasibility assessments have also been carried out, «considering a plant with a size of 2 or 2.5 tonnes per hour, which corresponds to a production of pulper waste of between 16,000 and 20,000 tonnes per year, the various simulations tell us that, regardless of the type of plant and its complexity, and regardless of the mill’s conditions, a payback of between 1.8 and 2.5 years can be achieved» (Figure 4). So, a very quick return on investment.

The project was then completed by the guidelines. It is a simple tool which, by means of an evaluation questionnaire and on the basis of the characteristics of the individual company, provides guidance on the course to be followed. If necessary, it is then possible to go into the specifics of the individual company and suggest customised solutions.

«The implementation of the solutions presented» concludes Giangrandi, «will further improve paper recycling efficiency and the paper industry’s leading role in the circular economy».