Metals buildup in TCF bleach plant closure: laboratory simulation.Application: Stringent effluent regulations often compel kraft mills to reduce their effluent volumes through closing the bleach plant liquid circulations. This study adds to the knowledge of closure chemistry to help mills address related problems. In response to increasingly stringent effluent regulations, kraft pulp mills have focused much effort on reducing the volume of effluents. A significant goal of totally chlorine free (TCF See Trenton Computer Festival. ) bleaching is to "close" the bleach plant and recycle the filtrates. In a closed bleaching system, however, dissolved material accumulates in the system and intensifies the formation of harmful scales. Moreover, the contents of certain transition metals such as copper, iron, and manganese manganese (măng`gənēs, măn`–) [Lat.,=magnet], metallic chemical element; symbol Mn; at. no. 25; at. wt. 54.938; m.p. about 1,244°C;; b.p. about 1,962°C;; sp. gr. 7.2 to 7. must be low in the pulp and liquids when hydrogen peroxide hydrogen peroxide, chemical compound, H2O2, a colorless, syrupy liquid that is a strong oxidizing agent and, in water solution, a weak acid. It is miscible with cold water and is soluble in alcohol and ether. is used, because they can have a detrimental effect on the bleaching results. This investigation studied the consequences of partial closure of ozone and hydrogen peroxide stages on the buildup build·up also build-up n. 1. The act or process of amassing or increasing: a military buildup; a buildup of tension during the strike. 2. of metals. The experiments involved bleaching oxygen-delignified hardwood kraft pulp with ozone and hydrogen peroxide and recycling the filtrates. Simulating real bleach plant conditions enhanced the applicability of the results. The equilibrium concentration levels of different metals obviously depended on pH in each bleaching stage. Certain anions (e.g. sulfate sulfate, chemical compound containing the sulfate (SO4) radical. Sulfates are salts or esters of sulfuric acid, H2SO4, formed by replacing one or both of the hydrogens with a metal (e.g., sodium) or a radical (e.g., ammonium or ethyl). ions) also affected the equilibrium concentrations. After filtrate filtrate /fil·trate/ (fil´trat) a liquid or gas that has passed through a filter. fil·trate v. To put or go through a filter. n. recycling was started, most metal concentrations in the bleaching filtrates increased. Transition metals, however, did not build up in the system. The manganese concentration in the first ozone/chelation stage filtrates was relatively high (over 10 mg/L), while the concentrations of copper and iron were on a very low level. Calcium concentration was also relatively high (over 100 mg/t, in the Z/Q stage filtrates), but it increased little as a result of the closure, which was at least partially due to the calcium oxalate Calcium oxalate is a chemical compound that forms needle-shaped crystals. Large quantities are found in the poisonous plant dumb cane. It is also found in rhubarb leaves, various species of Oxalis, and agaves, and (in lower amounts) in spinach. scale formation. Barium barium (bâr`ēəm) [Gr.,=heavy], metallic chemical element; symbol Ba; at. no. 56; at. wt. 137.33; m.p. 725°C;; b.p. 1,640°C;; sp. gr. 3.5 at 20°C;; valence +2. concentration of the filtrates remained on a very low level (less than 1 mg/L). Magnesium concentration was high in all filtrates. The magnesium concentration, however, did not increase after a few cycles of bleaching. Therefore, all studied metal concentrations (and COD) were in equilibrium after two or three partially closed cycles of bleaching. The influx of all studied metals was then equivalent to the output rate. Fiskari is with Abo Akademi University, Porthansgatan 3, 20500 Turku, Finland. Contact him by e-mail at jfiskari@abo.fi. |
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