In many cases the deinking process itself or one of the subsequent bleaching steps (for example peroxide) is alkaline. This means the material must be neutralised before being fed to the paper machine.
Mineral acids such as hydrochloric and sulphuric acid are still traditionally used as neutralising agents. Neutralisation with carbon dioxide (CO2) has many advantages over the use of mineral acids:
- In water, carbon dioxide reacts as a weak acid. Characteristic for carbon dioxide is its flat neutralisation curve which makes it easier to control pH. Furthermore, acidification, which is a risk when using mineral acids, can be eliminated with the use of carbon dioxide.
- When using carbon dioxide to carry out neutralisation on paper machines there is no risk of danger to health from dangerous fumes, or of corrosion to the hardware. In addition, the storage vessel for CO2 does not require either a water-tight drip tray or other safety hardware as is the case with mineral acids. Thus, handling carbon dioxide is simple, safe and economic.
- The process and waste water salt load is not increased by chloride or sulphate when neutralising with carbon dioxide. This is ecologically advantageous and can be of relevance for the discharge permit or for freight-based waste water discharge.
To add and dissolve the carbon dioxide in highly viscous liquid/fibre mixtures, nozzles of a special construction, which minimises their tendency to block, need to be used.