# Oxygen for pulp delignification and bleaching

The EU best available techniques (BAT) conclusions for pulp and paper (2014) describe oxygen delignification as reacting pulp with oxygen under alkaline conditions to remove part of the lignin left after cooking, which lowers the kappa number before bleaching and so reduces bleaching chemicals and the COD load of waste water.

## What the oxygen stage does

Commission Implementing Decision 2014/687/EU, which sets the BAT conclusions for the production of pulp, paper and board, describes oxygen delignification as an option to remove a substantial fraction of the lignin remaining after cooking, in case the cooking plant has to be operated with higher kappa numbers. It is one of two forms of extended delignification, with modified cooking. Lowering the kappa number by one unit before bleaching can reduce the COD released in the bleach plant by approximately 2 kg COD per air dry tonne of pulp (ADt), and the lignin removed can be recovered and sent to the chemicals and energy recovery system.

The same document describes modern elemental chlorine free (ECF) bleaching as minimising chlorine dioxide consumption with one or a combination of bleaching stages, oxygen among them, and brown stock washing, in which the pulp may be washed in high-efficiency washers before and after oxygen delignification.

## Where the BAT conclusions list it

For kraft mills, BAT 19 combines TCF or modern ECF bleaching with techniques that include oxygen delignification before bleaching, to reduce emissions to receiving waters from the whole mill. The sulphite section lists oxygen delignification before bleaching again, under BAT 33.

## Conditions and limits in one study

A 2007 peer-reviewed study in BioResources on kraft pulp from Muli bamboo found that oxygen delignification dropped the kappa number by 40 to 75% over 70 to 100 °C, oxygen pressures of 2 to 7 bar, alkali charges of 2 to 4% and reaction times of 5 to 60 minutes, and the pulp could then be bleached to 86% brightness with a CEHH sequence. Conditions that gave more kappa reduction also reduced pulp viscosity more, which called for an economic balance between environmental benefits and pulp degradation.

The authors, citing earlier literature, note that oxygen delignification is used to reduce the kappa number by only about 50%, because a greater reduction is expected to cause unacceptable carbohydrate degradation and loss of pulp strength, and that it decreases the BOD, COD and colour of the effluents whatever bleaching chemicals follow.

## On-site oxygen from ULTRAOX VSA

The ULTRAOX VSA datasheet lists bleaching of pulp among the applications of this Ultra Controlo industrial oxygen plant. It delivers 75% to 95% oxygen at 50 kPa; higher pressures can be achieved with an additional oxygen booster, supplied on demand. It comes on a skid or in a container and can replace cryogenic systems with liquid oxygen tanks or work as the main source of supply. Its seven models deliver 18 to 240 m³/h at 93% oxygen, and the datasheet says purity data can vary up to 2%.

### How does oxygen fit into ECF bleaching?

The EU BAT conclusions (2014) list oxygen among the stages that modern ECF bleaching uses, alone or combined, to minimise chlorine dioxide consumption.

### What conditions were tested in the 2007 bamboo study?

Temperatures of 70 to 100 °C, oxygen pressures of 2 to 7 bar, alkali charges of 2 to 4% and reaction times of 5 to 60 minutes, which dropped the kappa number by 40 to 75%.

## Sources

- [Commission Implementing Decision 2014/687/EU establishing the BAT conclusions for the production of pulp, paper and board (Official Journal L 284, 30.9.2014)](http://publications.europa.eu/resource/celex/32014D0687)
- [Thomas, Singh and Subrahmanyam, A study on oxygen delignification of Melocanna baccifera (Muli bamboo) kraft pulp, BioResources 2(3), 430 to 441 (2007)](https://bioresources.cnr.ncsu.edu/BioRes_02/BioRes_02_3_430_441_Thomas_SS_Ox_Delig_Bamboo.pdf)
- [ULTRAOX VSA datasheet, pp. 2 and 3](https://www.ultracontrolo.com/products/ultraox-vsa)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/industry/oxygen-for-pulp-bleaching/