Process Solutions to Handle Small Amounts of Flexo in Wood-containing Raw Material, 2002 Fall Technical Conference

Standard waterbased inks are not deinkable in conventional alkaline deinking process. A research programme has been coordinated, since 1998, in the framework of the taskforce " FOR THE DEVELOPMENT OF A COST EFFECTIVE SOLUTION TO THE DEINKING OF WATERBASED INK PRINTED PAPERS. Investigations were focused on the formulation and development of improved deinkability inks, by ink suppliers and necessary the adaptations of the deinking process to improve its efficiency.
Ink suppliers participating in the taskforce have achieved significant improvements of deinkability of waterbased ink, for news printing as well as envelope printing. An ink supplier has developed deinkable waterbased inks which were successfully used for industrial printing of envelopes while another is in the process of doing the same. Deinkability of flexo waterbased inks for news has been largely improved but newly developed inks cannot be already considered as correctly deinkable.

The deinking conditions, especially pulping conditions that could favour the deinkability of flexo print (low alkalinity or neutral conditions) cannot be applied since high alkalinity is required to deink conventional print. Reducing alkalinity whilst respecting deinkability of conventional print has a limited effect and could give a solution for only very low amount of flexo content of 2-3 %.
The improvement of deinking efficiency by introducing a specific chemical at the pulping stage in order to favour the agglomeration of the ink and its removal by flotation could be a solution for mixtures containing less than 10 % flexo. The drawback of such an approach is an increase of the flotation losses and a chemical overcost. One chemical tested has led to interesting results but it needs to be evaluated at industrial scale during a long trial run.

• Cleaning up the process water, in order to remove small size ink particles which build up in the circuit due to the washing effect induced by the thickeners, seems to have a limited effect on the final brightness of the deinked pulp, since thickening implemented after flotation is performed by high retention thickeners.
• Post-bleaching, by sodium hydrosulphite or hydrogen peroxide, could permit to process mixtures including up to 8 % flexo.
• Additional washing, implemented directly after flotation, could enable the handling of mixtures including up to 10 % with a maximum of 10 % additional losses. It could permit to process up to 13 % flexo mixtures but the amount of losses produced is not economically acceptable.
• Combination of washing and bleaching could allow to process mixtures including up to 15 % with moderate additional losses not exceeding 5 %.

These treatments lead to additional costs (investment and running costs). For mixtures including 5 to 10 % flexo, the over cost ranges from 8 to 20 €/t and reaches 23 €/t for 15 % flexo (based on a cost of raw material of 100 €/t).

With a first loop operated in neutral conditions (pulping and flotation) followed by a second loop with intermediate an alkaline bleaching and a second flotation loop in alkaline conditions, mixtures including 30 % flexo (and probably more) can be deinked at the pilot plant scale. Compared to an alkaline neutral two-flotation stage deinking process, the over running cost is around 8 €/t, including bleaching investment. A first feasibility trial has been conducted in a mill including two flotation stages (with intermediate peroxide bleaching) by gradually reducing alkalinity at the pulping stage, no detrimental consequences on DIP quality and plant runnability were observed.