On the formation of oxalate in bleach plant filtrates on hot storage

One of the main problems seen today in a bleached kraft pulp mill with a hi gh degree of system closure is the deposition of sparingly soluble compound s based on the alkaline earth metal ions, i.e. Ca2+ and Ba2+. Calcium oxala te may precipitate in acidic or near neutral bleaching stages in a bleach p lant for ECF (elemental chlorine free)- or TCF (total chlorine free)-bleach ing in a kraft pulp mill. It was found that oxalate ions are formed not only during the cooking and b leaching processes but also when acidic bleach plant filtrates, i.e. from a chlorine dioxide (D)- or an ozone (Z)-stage, are stored at a high temperat ure. The amount of oxalate ions formed increased when the amount of organic subs tance (as GOD, chemical oxygen demand) in the filtrate was increased, when the temperature was raised or when the storage time was increased. The rate of formation in a D-filtrate from the first D-stage in an ECF sequence was about 0.02 and 0.04 mmol/(g COD. h) at 70 degrees C and at 90 degrees C re spectively, during the first hours. The formation of oxalate ions in alkali ne filtrates from an extraction (E)- or a peroxide (P)-stage was much lower than that in a D-filtrate. When the pH of D-filtrates was increased to abo ve 8 the formation of oxalate ions an hot storage increased slightly in one filtrate, but was unchanged in another filtrate. When the pH of an E-filtr ate was lowered to below 3 the formation of oxalate ions las mmol oxalate/( g GOD. h) was about doubled. However, the amount of oxalate ions formed in the E-filtrate was still much lower than that in the D-filtrates.