METHOD FOR PREVENTING ACCUMULATION OF METALS IN A CLOSED PULP BLEACHING PROCESS

Abstract: A method at the bleaching of pulp, comprising bleaching with peracetic acid where the pH value during the bleaching reaction is held within 5-7. In the final phase of the bleaching with peracetic acid, metals released in the pulp are rebound to the fibers in the pulp by increasing the pH value to at least 9, whereafter the pulp is washed. The pH value is increased preferably 1-10 minutes before the washing. It is thereby possible to close the bleaching process in counterflow without risk of accumulation of the metals in the process.

Description: METHOD FOR PREVENTING ACCUMULATION OF METALS IN A CLOSED PULP BLEACHING PROCESS Pulp bleaching This invention relates to chlorine-free bleaching of pulp from ligno- cellulosic fiber material comprising bleaching with peracetic acid. Totally chlorine-free bleaching (TCF) of chemical papermaking pulp has become a very great success, and the making of chlorine-free pulp in the world will soon have reached five millions tons per year, which is an exceptional development as the bleaching method was introduced less than five years ago. The reasons for such a breakthrough of TCF-pulp are that it was consid¬ ered favourable for the environment, because emission of chlorinated impurities from pulp mills disappears completely, that the bleached pulp does not contain appreciable amounts of chlorinated matter, and that it is generally considered easier to close the process (recover bleach plant waste) at TCF bleaching than at conventional bleaching based on chlorine dioxide. The older method of using also chlorine gas at bleaching disappeared at the beginning of the 1990s in Sweden, be¬ cause such bleaching gave rise to large emissions of matter harmful to the environment. The latter bleaching method, besides, is difficult to use.ina closed bleach plant because of the great amounts of chlor¬ ide ions which then should be recycled to the chemical recovery. TCF bleaching mostly is carried out with oxygen gas, hydrogen peroxide ozone and/or some type of peracids such as peracetic acid (Paa) . Due to the fact that neither chlorine nor chlorine dioxide are used as bleaching chemical, chloride ions are prevented from accumulating at the chemical recovery when the bleach plant is closed, and thereby the risk of corrosion attacks a.o. in the soda recovery boiler is reduced substantially. It was now found, however, that it is more difficult than assumed to close TCF bleach plants, in spite of the solution of the chloride problems. One problem is that hydrogen. peroxide bleaching is very sensit¬ ive to the presence of certain metals, a.o. manganese, copper and iron. These metals must be separated from the pulp before the peroxide bleaching, and resulting metalliferous filtrates can at the present state of art only with difficulty be recycled to the process. The reason is that then these metals will accumulate in the filtrate flows of the pulp mill, and the contents in the pulp mill finally become so high that the hydrogen peroxide bleaching will be significantly more difficult. Another problem is that calcium ions in the pulp are re¬ leased, which takes place to an extent increasing with increasing acidity of the step. At increased concentration of calcium ions in the process liquors there will be a greater risk of precipitations of calcium compounds of the type calcium oxalate, calcium carbonate a.o., which, if it comes to the worst, can make continuation of the operation impossible. For removing metals from the pulp, a chelating agent treatment is carr¬ ied out in a separate step before the hydrogen peroxide step (so-called Q-step) . Traditionally, the Q-step is carried out in such a way that the pulp is treated with a chelating agent of the type EDTA or DTPA, whereafter the pulp is washed in order to eliminate released metals. The conditions normally applied in the Q-step are 70-90 C, 1-2 hours, 10-14% pulp concentration, pH 5-7 and charge of chelating agent mostly of the magnitude 1-3 kg/;;. Many mills are today planning to combine hydrogen peroxide bleaching with peracetic acid bleaching in order thereby to increase the brightness and strength of the pulp. The bleaching, therefore, is carried out in several steps, with subsequent washing, for example as follows: oxygen gas - chelating agent - peracetic acid - chelating agent -. hydrogen peroxide Such a sequence normally is designated 0Q(Paa)QP. As the Q-steps and the Paa-step normally are carried out at pH 5-6, there is a great risk that calcium and other disturbing metals are released in these positions, and the resulting washing liquor will contain these released ions. As appears from above, it is desired on one hand to separate disturb¬ ing'metals from the pulp and discharge them with the waste water, and on the other hand for environmental reasons not to discharge the waste water, but to re-use it in the pulp mill. By increasing pH in all steps to about 9-10, however, calcium and the other disturbing metals will not leave the pulp, but remain therein and follow along with the bleach¬ ed pulp out of the mill. In this way, accumulation can be prevented. Under certain conditions the Q-steps can be carried out at higher pH without thereby rendering the bleaching capacity of subsequent P-steps considerably more difficult, which makes the above process change poss¬ ible. A Paa-step, however, bleaches the pulp much inferior when the pH is increased above pH 7, which renders it difficult to carry out an alkaline TCF bleaching containing peracetic acid. The present invention has the object to solve the aforesaid problems. It implies that the peracetic acid bleaching can be carried out under optimum pH conditions for the bleaching while at the same time the metals are bound in the pulp and thereby can follow along out of the bleach plant, so that the bleaching process can be closed. The characterizing features of the invention are apparent from the attached claims. The invention is described in greater detail in the following, with reference to some embodiments thereof. In a TCF bleaching process with a bleaching sequence of the type 0Q(Paa)QP the Paa-step is carried out at a temperature of at least 70°C, preferably 80-120°C, for 1-2 hours at pH 5-7, preferably 5-6. The Paa-step can be pressurized. This means optimum pH conditions for the peracetic acid bleaching, but also that metals such as calcium and manganese are released. After the Paa-step the pulp is washed before it is transferred to the next step in the bleaching sequence. In order to prevent the released metals from being washed out, the pH value is increased to at least 9, preferably 9-12, in the final phase of the Paa-step in order thereby to effect a rebinding of the metals to the fibers in the pulp. The bleaching process can thereby be closed in counterflow without risk of accumulation of the metals. The pH value should be increased 1-10 preferably 2-5 minutes before the washing. This can be made with help of alkali, which preferably is added in the form of alkaline white water from other parts of the bleaching process. The invention, of course, is not restricted to the embodiments described above, but can be varied within the scope of the invention idea.

Claims: Claims 1. A method at the bleaching of pulp, comprising bleaching with peracetic acid where the pH value during the bleaching reaction is maintained within 5-7, c h a r a c t e r i z e d i n that metals released in the pulp are rebound to the fibers of the pulp in the final phase of the bleaching with peracetic acid by increasing the pH value to at least 9, whereafter the pulp is washed, whereby the bleaching process can be closed in counterflow without risk of accum¬ ulation of the metals in the process. 2. A method as defined in claim 1, c h a r a c t e r i z e d i n that the pH value is increased 1-10 minutes before the washing. 3. A method as defined in claim 1 or 2, c h a r a c t e r i z e d i n that the bleaching with peracetic acid is carried out at a temp- ertaure above 70°C, preferably within 80-120°C. 4. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the peracetic acid bleaching is carried out pressurized. 5. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the pH value is increased with help of alkali, preferably in the form of alkaline white water from other parts of the bleaching process.

Inventors: GERMGAARD ULF, LINDQUIST JONAS, GERMGAARD, ULF, LINDQUIST, JONAS
Applicants: GERMGAARD ULF, LINDQUIST JONAS, SUNDS DEFIBRATOR IND AB, GERMGAARD, ULF, LINDQUIST, JONAS, SUNDS DEFIBRATOR INDUSTRIES AB
Application year: 1996
Main classification: D21C9
IPC codes: D21C9