Transforming Sludge from Intaglio Printing into Green Alternative

The production of a million banknotes (or one shift of an intaglio printing line) produces 100 kg of intaglio ‘sludge’. Sludge is frequently sent to landfill because of its contamination in the process used to extract it from the printing process.

Biobanknote, the Colombian-based developed of solutions to turn banknotes and banknote waste into compost, has developed a new organic treatment that allows printing works to convert contaminated sludge safely and within national standards into an alternative capable of enhancing soil.

The intaglio printing process

Intaglio printing, which produces the relief print and unique feel of a banknote, presses the substrate against a metallic plate into which the banknote’s image has been engraved. The engraving is filled with ink. Before that ink can be transferred to the substrate, any excess must be removed.

The ink is wiped and dissolved off using a plastic cylinder immersed in a tank containing a solution of soft water, caustic soda (NaOH – sodium hydroxide) and sulfonated castor oil. It is constantly refreshed, removing the contaminated solution.

To reduce the consumption of the solution, a ‘Depuration System’ allows the solution to be recycled. This recovery process requires the following stages:

1. Flocculation – a flocculant agent is added, usually calcium chloride (CaCl2) or ferric chloride (FECL3), to the solution that has been removed from the wiping tank. Using either a decantation tank or a centrifuge, the solid material is separated from the contaminated solution.

2. Filtration – the contaminated solution is transferred to a treatment tank in which it is mixed with a filter medium, Perlite or Fossil Flour, and then passed through a filter that separates the remaining solids. This delivers a clean solution, although it may not necessarily be ready for be reused. The solid extracted during filtration is added to that obtained from the flocculation process.

3. Recycling – under those conditions the recovered solution can be reincorporated into the solution preparation tank, mixed with new solution in appropriate proportions.

The problem

But disposal of the solid material, referred to as ‘sludge’, is a problem. Currently it is usually chemically treated, incinerated or deposited in landfill, with the associated contamination risks.

The amount of sludge generated by banknote production is high because the proportion of ink that actually transfers on to the substrate is a low percentage of how much actually goes into the ink ducts on the presses.

The solution

Biobanknote has developed a successful organic treatment for intaglio printing sludge. Its solution can be incorporated into the waste management procedure and the resulting material used to enhance the quality of soil.

The organic treatment involves adding, mixing and dosing the sludge with natural waste materials, such as food and vegetable waste. This takes place under controlled oxygen, humidity and temperature conditions.

A recent trial treatment took 12 weeks. The result was the production of a soil enhancement material which met the standards set by Colombian regulations. This trial is now being confirmed by a second test to validate the effectiveness of the process and the quality of the resulting product.

Steps to use the solution

The composition of the sludge resulting from the printing process is different in each banknote printing plant because they have different depuration systems, specifications of intaglio inks, concentrations of the formulae of special inks used, such as those of optical, fluorescent, magnetic effects, IR etc. and different colours.

As a result, if a printworks wishes to adopt this new solution the first step is to find out whether their sludge produced is compatible with this organic treatment.

Biobanknote will need to know the characteristics of the ink used and nature and proportions of all chemicals used in the process. It will then carry out laboratory analysis and field tests where it is based, in Colombia, on representative samples of the sludge. It will also need a description of the depuration process used by the printworks.

Depending on the results, ideally there will be an industrial test with at least one ton of sludge carried out at the treatment plant, 35 km from Bogota. The result of these steps will be a decision about whether the organic treatment will produce the necessary results to meet the national standards of fertilizer products.

Biobanknote uses the results of its banknote composting solution as fertilizer to help trees grow as part of a reforestation programme. One of its products is BiobankMulch, a protective ring made of the compost that is put around newly-planted saplings and help their growth improving the soil and protecting them from weeds and fungi (see CN February 2023).

Together with Louisenthal, Biobanknote has produced a documentary on the conversion of cotton banknote paper waste to compost for trees, and will be releasing this on 5 September.