A Review of Ultrasound and its Applications in Papermaking

Progress in Paper Recycling is an international journal and the most important peer reviewed outlet for research in the area of paper recycling.

The subject of ultrasound and its applications in paper technology has not been reviewed for twenty five years. During this time, much international research has been conducted on various applications of high intensity ultrasound in pulp preparation. This paper reviews the research presented in seventy eight journal publications over the last twenty five years.

In this paper the authors review and update the research findings for the applications of ultrasound to paper manufacture, in particular pulp preparation and the de-inking of printed papers. Work on the de-inking of difficult to de-ink papers as well as conventional waste such as newsprint is reviewed and its potential to reduce the use of harmful chemicals examined. The potential for ultrasound to supplement conventional mechanical refining of fibres to yield improved pulps through enhanced fibrillation of fibres is explored. Exposure to ultrasound is shown to be of particular benefit in the recycling of printed papers, specifically those printed with ‘difficult to de-ink’ systems such as heat fused toners and UV-cured inks.

The physics of cavitation bubbles is discussed and their action on slurry media is explored. Cavitation processes in liquids result in the generation pressures up to 200 atmospheres and temperatures of 5,550 kelvin producing high velocity jets of liquid which impact on the paper pulp, dislodging ink from its surface.
Developments in ultrasound generators are examined alongside techniques being developed to measure the intensity of cavitation in situ. Large scale ultrasound generators are still in their relative infancy and a reliable and industrially useful means of measuring high intensity vibrations is still being sought.