FAIR-CT96-1492 Enzymatic deinking as an environmentally friendly solution for recovered paper recycling (BIO-DEINKING)

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Biological Conversion : Economics : Enviromental Aspects : FAIR Area 1.3 - Forestry-Wood Chain : Paper/Pulp : Process Engineering

	Contract No:	FAIR-CT96-1492
	Date Prepared:	July 2000
	Source:	Final Report Executive Summary

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Final Report Executive Summary

Introduction

The use of recycled fibres in newsprint, tissue and currently in higher quality paper grades has significantly increased over the last 20 years, particularly as a result of consumers' attitude and the successful development of deinking processes. The most common deinking techniques are wash and flotation deinking. Deinking is a sophisticated process for the benefit of recycling. Using this technique for processing post-consumer or pre-consumer recovered paper can produce high-quality papers.

The removal of ink called deinking is based on two steps: ink detachment from the surface of the disintegrated fibres which is performed during pulping (slushing), followed by the removal of the detached ink particles from the pulp slurry by washing or flotation.

Ink detachment is performed by the combination of mechanical forces (agitation during slushing) and chemical action due to chemicals added in the pulper, which contributes to the disintegration of recovered paper. The chemical environment can be controlled by alkaline agents or can be neutral.

Alkaline deinking is widely used and generally considered as more efficient in respect of ink detachment than neutral. Its drawback is the formation of a significant COD concentration of the white water caused by the dissolution of carbohydrates and organic additives present in the fibrous material.

For some ink types, deinking is insufficiently performed because of the following difficulties: poor ink detatchment (because of ageing of offset newspaper) or insufficient removal of ink (paper printed with water-based flexographic inks and deinked in an alkaline environment) induce a smeared pulp or a low brightness. Toner printed papers generally lead to speckled pulp

Dispersing is a beneficial technique that contributes to the reduction of the size of large particles. The main drawback is a brightness drop which could be recovered by the use of bleaching chemicals in the disperser and by post-deinking. At the same time the use of a bleaching agent will further increase the COD concentration in the white water. Furthermore, the dispersion treatment results in a significant increase in energy consumption.

The use of enzymes could be an attractive alternative to chemicals in deinking: enzymes could improve the de-inkability of inks difficult to detach with the conventional deinking process and since enzymes are used in neutral conditions, they could be supposed to produce a lower COD concentration than conventional alkaline deinking.

In recent years, the application of enzymes in deinking has been studied on laboratory and pilot plant scale. This work has resulted in numerous patents. However, so far enzymatic deinking is rarely applied in commercial use.

Generally there are three different approaches available for the use of enzymes in deinking:

• the fibre surface can be attacked by enzymes
• the starch-based coating can be solubilised by enzymes
• vegetable oil-based binders of the printing inks can be hydrolysed.

Lipases can be used to hydrolyse soy-based ink carriers and carbohydrate hydrolysing enzymes, such as cellulases, xylanases or pectinases can be used to release ink from fibre surfaces. Most applications proposed relate to the use of cellulases and hemicellulases. In this case the detachment of ink results from a partial enzymatic hydrolysis of carbohydrate molecules on the fibre surface. However, many reported studies use non-optimised enzyme mixtures.

One of the advantages of enzymatic deinking is the avoidance of alkaline deinking chemicals. Deinking at a neutral pH prevents alkaline yellowing of the recycled fibre pulp and simplifies deinking chemistry. In an industrial operation, the use of enzymes as deinking agents could thus contribute to lower chemical costs and might decreased environmental impacts. Enzymatic deinking also changes the ink particle size distribution, apparently reducing the average particle size. In addition to ink removal, enzymatic deinking may contribute to improved strength properties of the paper sheets and freeness and reduced fines content of the recycled fibre pulp.

The use of enzymes in the deinking of non-contact (toner) printed papers such as laser and xero- graphic office waste is expected to be especially beneficial. These inks contain thermoplastic resin binders (copolymers of styrene and acrylate) which crosslink at high temperature during the copying process. The polymerised toner inks are very difficult to disperse and deink with conventional techniques. Due to the lack of an economically feasible mean to reduce the residual ink content of pulp based on these office papers to an acceptable level, the use of this high- quality fibre source has been limited.

Activities

The work conducted during this project was as follows:

• the selection of paper samples which are difficult to deink and particularly of paper printed with inks difficult to detach; this selection includes both wood-containing and wood-free paper
• the detailed description of the reasons for various difficulties encountered
• the development of enzymes mixtures that resulted in improved detachment of these inks
• the evaluation of deinking improvements performed on laboratory, pilot and industrial scale.

During the development of enzymes for deinking, three approaches have been evaluated:

• enzymes attacking the fibre surface
• enzymes acting on coatings
• enzymes attacking the vegetable oil-based binders of printing inks difficult to detach from the fibre surface.

As far as the deinking process is concerned, the evaluation includes mainly the application of enzymes before deinking, in the pulping stage in addition to or substitution of conventional deinking chemicals. The use of enzymes after the first deinking stage in substitution of a hot dispersion treatment in order to improve post-deinking (substitution of conventional deinking chemicals) was only studied in preliminary laboratory trials.

The enzymatic deinking process was carried out under neutral conditions of pH and (both regarding production of selected enzymes and de-inking operations) was carried out in the first at laboratory scale. The most promising results were then evaluated in a continuous process on pilot plant scale. Finally, industrial trials have been conducted in order to confirm the proposed enzymatic process for wood-free paper grades.

Results

The work carried out during this project has contributed to an increase in knowledge concerning the enzyme application for deinking. Information has been obtained concerning the best types of enzymes and the best operating conditions to be used according to the type of. The laboratory work was a prerequisite for the selection of enzyme mixtures most suitable for either a typical wood-containing paper or a wood-free paper. On the basis of these results, suitable enzymes mixtures have been chosen for use in enzymatic deinking at pilot scale, using similar typical paper wood and wood-free compositions. The paper mixtures were chosen as representative of various types of papers that were known to be difficult to deink. The most promising results obtained at pilot scale were validated in full-scale trials.

The comparison at pilot scale of standard alkaline flotation deinking and neutral enzymatic flotation deinking on a typical wood-free papers composition (30 % coated sheet fed offset printing, 35 % BP laser printing, 35 % OCE copy) gave promising results. In particularly, a very positive effect of enzymes was the reduction of the number and area of the specks in the final deinked pulp (a decrease of 88 % in number and area). This is of interest in the case of mixed office waste that contain a lot of laser printing papers, that results in specks which are very detrimental to the visual aspect of the re-cycled pulp.

From these promising results, full scale enzymatic deinking was carried out in the Moulin Vieux mill that produces wood-free deinked pulp from 100 % printed coated woodfree papers. Enzymatic deinking efficiency was compared to efficiency of standard mill deinking using chemicals.. The trial was successful. It was found that:

• global efficiency of the deinking was improved in terms of brightness (4 to 6 additional brightness points)
• residual ink content was lower in the enzyme treated deinked pulp
• the mechanical properties could be kept to a correct level, but with a reduced dosage of enzymes.
• on the paper machine no modification of the runnability was observed, but a low proportion of the deinked pulp was sent to the machine for a specific type of production.

The main drawback observed was a higher COD (+ 20 to 40 % COD) that would have to be treated. Depending on the capacity of the effluent plant associated with a specific re-cycling mill, this higher COD could represent more or less of a problem. It is possible that the COD level could be reduced by optimisation of the enzyme dose.

On the basis of the results obtained it was difficult to make a cost evaluation. If only the cost of products (chemicals or enzymes) are taken into account, the calculation will be in favour of the chemical route, since enzymes are more expensive at present. However, in general the relative cost of commercial plant wall degrading enzymes is falling. Depending on the mill capacity, The cost of enzymatic treatment would also vary with the capacity of the plant. Further, the specific cost of the enzymes is of less significance than the effect of their use on the total process operating costs. This includes in particular the impact of handling effluents with a higher COD (where this occurs) as well as the benefits associated with the price obtained for improved pulp quality (optical properties) and yield as well as any impact on the runnability. This balance could only be obtained by longer industrial trials over several weeks, that lay outside the scope of this work) .

For wood-containing wastepaper, the results obtained on pilot scale show that good ink removal and lower specks contamination can be obtained by enzymatic treatment in neutral conditions but brightness is lower because of the lack of bleaching agent. Hence, the use of enzyme in wood-containing papers can only be value in combination with a subsequent bleaching step, preferably using hydrosulfite to reduce COD, to recover the brightness required. To validate these results, an industrial trial is suggested, using the enzyme mixture in neutral conditions, followed by a bleaching step. These conclusions resulted in Stora Enso, one of the industrial partners in the project, deciding not to run the industrial trial, due to the need for this bleaching step that would result in a significant increase in the cost of production of the re-cycled pulps.

Conclusions

The results obtained from this work show that for wood-containing wastepaper, the use of enzymes can be interest only in combination with a bleaching step. In the case of a line producing newspaper, such a bleaching step could not be considered due to cost considerations. . In the case of a line producing super-percalendared paper, some deinking lines do include a post-pulping bleaching step. The possibility of using enzymes for deinking of wood-containing waste paper without implementing a bleaching step has not been solved. More fundamental research work is required in this area.

The pilot trials also showed that residual ink content and excess COD generated are strongly dependent on the level of enzymes used and the level of alkali. To obtain more rigorous conclusions a more detailed comparison between alkaline deinking and enzymatic deinking at various dosages of both alkali and enzymes would have to be carried out.