Aluminum compounds are widely used in the paper industry: in rosin sizing, as drainage and retention aids, and as fixing agents for anionic trash (dissolved organic compounds e. g. extracts from wood fibers, degraded starch). Important applications are the treatment of circuit water and effluents or the thickening of sludge, where the flocculation and precipitation effects of aluminum ions are utilised.
The most frequently applied aluminum compounds are aluminum sulfate, polyaluminum chloride (PAC), polyaluminum hydroxidechloride sulfate, and recently also (poly)-aluminum nitrate and polyaluminum nitrate sulfate. Aluminum compounds have a favorable price-performance ratio.
An analysis of the aluminum concentration in the production of a wood-containing printing paper (at neutral pH, with 0.45 % aluminum sulfate added to the white water) showed an aluminum level in the water circuit (at a pH of approx. 7.5) of about 0.1 mg l–1, maximum 0.2 mg l–1 (determined by UV/VIS spectroscopy). This indicates that the paper retains nearly all of the aluminum added in the form of aluminum hydroxide. The anionic counterions sulfate and chloride, however, are nearly fully discharged into the effluent.
The paper retains the share of ions dissolved in its residual moisture at the end of the drying section. Sulfate and chloride ions are nonvolatile, i. e. they are not released into the atmosphere. The loss via papermaking residues is negligible. The two anions may be considered environmentally harmless. As sulfate promotes the corrosion of concrete, there may be statutory discharge limits for this substance (500 mg sulfate l–1).
There are usually no restrictions, however, on the discharge of chloride. The use of polyaluminum nitrate and polyaluminum nitrate sulfate is fairly new. The two compounds help to prevent or combat malodours caused by the conversion of sulfate into hydrogen sulfide under anaerobic conditions. The results of mill trials suggest that nitrate is converted into elemental nitrogen because no nitrate could be detected in the effluent – nitrate was found only in the white water. According to information from mills using large quantities of polyaluminum nitrate, a minor discharge of nitrate into the effluent can occur.