Effluents from the woodroom, the brownstock area and the evaporation plant have the highest toxic emission factor per COD unit, compared to the effluent from ECF or TCF bleaching [1]. Steps taken to reuse these effluents lead to high levels of non-process elements (NPEs) in the liquor cycle. The levels of NPEs for five Swedish mills were monitored for 7 years, and the data were used to evaluate the effect of increased mill closure, and to identify suitable methods to purge NPEs.
Si
Si (in the form of silica) enters the mill mainly with the contaminated wood and with the make-up lime. The input of silica in the Swedish study varied between 7 to 45 kg/tp at the five mills; the silica content of the black liquors ranged from about 250 to 2100 mg SiO2/kg d.s. No lime mud was purged, except for the pre-coat of the green liquor (GL) filters. The mill with highest intake of make-up lime had problems with ring formation in the lime kiln.
Mg and Al
A mill with double oxygen delignification and recovery of alkaline filtrates had an excess of Al and Mg. Excess Mg appears in the form of hydroxide, and has poor settling characteristics in GL. The mill added Al to the dissolving tank to obtain a desirable ratio between Mg and Al, in order to improve GL filterability. Mg and Al form a double salt (hydrotalcite) which is easier to remove with the GL dregs. Hydrotalcite formation can also be used to decrease the Al/Mg ratio in the liquor cycle, as sodium aluminosilicates cause scaling in the digester and black liquor evaporators.
Cl and K
Cl and K accumulate in the recovery cycle when bleaching filtrates are recovered. They are enriched in the ESP dust and can be removed from the system by purging of the dust. For more information on Cl and K see the posting for February 02, 2008 (Managing Cl and K in a kraft mill).
Heavy Metals
Heavy metals such as Cd, As, Hg, Zn and Pb are also enriched in the ESP dust. The enrichment factor is high if the boiler is operating with a high bed temperature. These metals will pose a problem when the ESP dust is purged, in areas where heavy metals content is regulated. The main input of heavy metals is with the wood, and the purge points are the GL dregs, the unbleached pulp, or the bleaching effluents. To remove Cd in one mill, the ESP dust was dissolved to make a saturated solution. Heavy metal compounds were not dissolved and remained in the sludge, which was separated by sedimentation. At a sludge pH of >10.5, 90-95% of the Cd was removed. The sludge was mixed with GL before clarification; the heavy metals were then removed with the green liquor dregs, which were land filled.
To study the effect of acid rain on the stability of the land-filled sludge, laboratory leaching trials were conducted. Cd(OH)2 precipitates at high pH but, at pH levels below 9, Cd will be re-dissolved. At a sludge pH of 10, less than 1% of the Cd was dissolved. However, if the sludge pH was lowered to 5.7, 35% of the Cd content was released. Since CdS has a lower solubility than Cd(OH)2, addition of sulphides improved the stability of Cd , even at low pH levels.
1. Bjork, M. and Engstrom, I., “Handling of non-process elements for improved runnability and reduced environmental impact”, Pulp & Paper Canada, T102-T105, 103:4 (2002).
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