Saving energy making CTMP
As well as saving energy, silicate-free bleaching’s other benefits include improved bulk, less off-grade production and fewer deposits
HIGH BRIGHTNESS mechanical pulps are today most commonly bleached with hydrogen peroxide. Since most of the peroxide is consumed by transition metal induced decompositionW.the control of peroxide decomposition is of crucial importance due to the fact that manganese, iron and copper always come in to the process with the wood raw material, with the process waters or as impurities with bleaching chemicals.  The type of metals and their complexes formed, their activity state, interactions and the local environment (wood type, process temperature and set-up, water loops, stabilizers etc.) will affect the chemistry involved in peroxide decomposition. Thus peroxide needs a special stabilizer to prevent this decomposition. The optimal behavior of a stabilizer in bleaching is a combination of complexing and deactivating detrimental metal ions.
SODIUM SILICATE AS A PEROXIDE STABILIZER
Sodium silicate (water glass) is a commodity product used to enhance peroxide bleaching of mechanical pulps. It is relatively cost-effective, rather easy to apply and universally available. Several theories on the role of silicate in peroxide bleaching have been suggested[2,3,4]. However, the exact stabilization mechanisms are still partially unknown. Silicate can act as a peroxide stabilizer, a metal ion sequestrant, a buffering agent and as a metal surface passivator.
With regard to peroxide stabilization, surface passivation and metal ion sequestration are the most important functions. Although silicate is beneficial in peroxide bleaching, it may generate serious detrimental effects in the pulp and paper making processes. Today, when most pulp and papermaking processes take place in closed loops using a minimum amount of water, silicate builds up in the process.
Silicate may also cause deposits on the paper machine’5! and it makes the paper making system more anionic, which leads to a higher consumption of wet-end chemicals. Typically, the bleaching plant may not have any problems with silicate as long as the operating pH is alkaline. However, paper machines operating at lower pH will encounter problems related to silicate chemistry. Hence, there is a strong demand for silicate-free peroxide stabilizers in order to avoid the problems described.
SILICATE-FREE PEROXIDE BLEACHING
The use of organic polymers as stabilizers in peroxide bleaching has been studied since the 1950s. Peroxide bleaching is a challenging environment for organic polymers due to the very high operating pH. It is difficult to estimate the stabilizing ability of polymers, since the process environment is rather complex. The ability of polymers to bind metal ions is not a well-known area.
Around 2000, Kemira started to develop polymer-based, totally silicate-free peroxide stabilizers to replace silicate in the bleaching of mechanical pulps. Since, a series of new products have been commercialized under the tradename Fennobrite®. Kemira has accumulated extensive knowledge about the functionality of these products, based on experiences from laboratory tests, full-scale mill trials and cooperation with regular customers.
One major finding of these products is their beneficial effect on dewatering. By using Fennobrite, dewatering can be improved significantly, compared with the use of sodium silicate. More efficient dewatering will lead to significant energy and cost savings meaning that less energy is needed in wash presses to reach same dry matter of pulp, and less drying energy is needed in the drying section.
What follows are some results from a commercial-scale mill trial where silicate was replaced with a silicate-free bleaching concept, leading to significant energy savings.
SILICATE-FREE BLEACHING IN A CTMP PLANT
This mill uses spruce chips as a raw material in its production of chemi-thermomechanical pulp (CTMP). The mill produces 110,000 tonnes/yr of market CTMP with a main grade having a brightness of 70% ISO. However, grades of higher brightness (77-78% ISO) have been produced. The pulp is used mainly in board production. A simplified process flow sheet of the plant is presented in Figure 1.
A two-stage peroxide bleaching is applied. The first bleaching is carried out in a medium consistency stage (7.5-10%, 600 -70°C, 30 min) using the filtrate from the second bleaching stage. The second bleaching stage is a normal high consistency stage (25-30%, 600 -70°C, 60 min) where bleaching chemicals are dosed into the chemical mixer. When silicate was used as a peroxide stabilizer, it was dosed like the other bleaching chemicals, i.e., via its own pipeline into the chemical mixer. Fennobrite was dosed into the conveyor screw before the chemical mixer together with dilution water to ensure a proper mixing of the chemical. Bleaching chemical dosages used in the low and high brightness production are presented in Table 1.
EFFECTS OF SILICATEFREE BLEACHING
During the mill trial with Kemira’s silicate-free bleaching concept Fennobrite, both lower and higher brightness grades were produced. Figure 2 shows that the final product reached a higher brightness 2-3 hours faster with Fennobrite than with silicate.
When silicate was used as a peroxide stabilizer, the drying section normally had to run at its higher limits and formed the bottleneck of the process. Improved dewatering was obtained with Fennobrite; therefore, less drying energy (natural gas, electricity for suction, etc.) was needed. Consequently, the drying energy demands for low and high brightness grades were on the same level.
Another important finding was that the wash presses before and after the high consistency bleaching showed a much lower energy demand when using Fennobrite. Compared with the situation with silicate, energy demand in the wash presses both before and after bleaching was decreased by approximately 50%. Thus, dewatering of pulp with Fennobrite was clearly more efficient than by using silicate.
By applying the silicate-free bleaching concept, the reject line could be shut down and bypassed, which led to major energy savings. Earlier, silicate-containing waters were normally used in the refiners. Fennobrite led to cleaner filtrates (no silicate deposits, etc.). Consequently, refining and screening worked more efficiently and no reject was produced.
The CTMP mill obtained major energy savings after transferring to Kemira’s silicate-free bleaching concept. This mill has used a silicatefree peroxide stabilizer as a standard product for more than a year. Table 2 has a summary of its energy saving effects (silicate vs. Fennobrite).
The following advantages were noted when the mill changed to a silicate-free bleaching concept:
* Major energy savings: less energy needed in the wash presses and in the drying section due to better dewatering, the reject line could be shut down
* Final product brightness was reached 2-3 hours faster when switching from a lower brightness grade (70 % ISO) to a higher brightness grade (77-78% ISO); time savings lead to money savings and less low-grade pulp
* The pulp had better bulk, according to the end user (a board producer)
* No changes in the waste water treatment plant
* silicate-free product Fennobrite is easy to handle
* Fewer deposits in the system: this means cleaner pipelines with no scaling and no agglomeration of silicate deposits on the process equipment.
1. XU, E. C., Journal of Pulp and Paper Science, vol. 28, no. 11, 2002.
2. COLODETTE, J.L., ROTHENBERG, S. and DENCE, C.W, Journal of Pulp Paper Science, vol. 15, no.l, 1989.
3. FAIRBANK, M. G., COLODETTE, J.L. ALI, T., McLELLAN, E and WHITING, P., Journal of Pulp and Paper Science, vol. 15, no. 4,1989.
4. BURTON, J. T., Journal of Pulp and Paper Science, vol. 12, no. 4, 1986.
5. ALI, T, EVANS T.D., FAIRBANK, M.G., 74th Annual Meeting, Tech. sect, CPPA, 1989.
6. WUORIMAA, A, JOKELA, R. and AKSELA, R., Nordic Pulp & Paper Research Journal, vol. 21, no. 4, 2006.
Copyright Paperloop, Inc. Apr 2008
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