Przem Pruszynski and David Sirois, Nalco, an Ecolab Company, Naperville, IL
Dec. 10, 2012
Replacement of HWK pulp with high-yield pulps (HYP) in UCFS and CFS offers some furnish cost optimization, but is also driven by certain quality improvements and sustainability benefits. Recent renewed interest in this trend could be expected as HYP addition assists another very important trend in the industry - increasing sheet filler content. The increased bulk, stiffness and opacity offered by HYP have allowed papermakers to reach even higher filler levels.
There are technical and regulatory barriers that affect the introduction of HYP into fine paper furnish. Standards for paper permanence vary by country, but most agree to; the paper being produced under alkaline pH, inclusion of carbonate as filler (minimum 2% is often quoted), and defining maximum level of lignin content (typically <1%). The lignin content limitation was included in early standards and was based on the belief that lignin contributes to the increased rate of degradation of paper produced with mechanical fiber. Extensive research in Canada determined that the effect of lignin on permanence of paper produced at alkaline pH, filled with calcium carbonate and without alum is negligible. This led to the new Canadian permanence standard (CAN/CGSB-9.70-2000), the first to be based on physical performance criteria rather than on furnish composition. The limit on lignin content (below 1%) remains in place only for papers with a brightness stability requirement.
Furnish cost reduction is different for integrated mills versus mills using purchased HYP and depends strongly on normal pulp price fluctuations. With $100/ton price differential, a mill producing 200,000 tons/yr at 10% substitution rate could save $20,000,000/yr. With all other conditions constant, 20% HYP substitution increases bulk up to 10%, stiffness up to 5%, and opacity up to 2%. Although these numbers may differ between applications, they provide a general picture of the benefits that HYP may offer. Increased wood utilization, lower effluent flows and concentrations, lower air emissions and less solid waste sent to landfill are some of the sustainability benefits when comparing the production processes of HYP and HWK.
It may be expected that with further understanding of the permanence mechanisms and the benefits offered by HYP application, changes in regulations will gradually follow and lead to further extension of grades and regions where these pulps are utilized. In some countries, HYP levels in woodfree paper are commonly 10-15%, with some mills exceeding 30%.
As regulatory barriers on permanence diminish and HYP levels increase, both operational and quality issues will become more important. The purpose of this paper is to define the current solutions and future needs for papermakers seeking higher utilization of HYP
Different HYP production processes (BCTMP, APMP, PRC-APMP) involve mild chemical treatment, mechanical refining and bleaching resulting in pulps of similar quality regardless of process at a yield of 80-90%. According to Ni, up to 50% of HYP could be used in most printing and writing grades, coated and uncoated.
As mentioned previously, HYP offer many benefits in terms of economics, environmental footprint and paper properties. At the same time, the use of HYP can impact paper machine operation and final product quality therefore presenting serious challenges to papermakers. . The benefits and challenges presented by the use of HYP are listed in Table 1.
|Benefits ||Negative Impact |
|Cost ||More anionic surface |
|Bulk ||More soluble anionic ”trash” |
|Bending stiffness ||Reduced brightness |
|Opacity ||Reduced brightness stability |
|Printability ||OBA efficiency |
|Sustainability benefit ||Increased system conductivity |
Increased extractives content
Reduced strength (tensile, surface)
Surface roughening on re-wetting
Exposure of HYP to high pH in the bleaching and/or refining stages affects the surface chemistry of the fiber and the level of soluble and colloidal impurities impacting paper machine performance and final product quality. Higher the brightness and strength specifications for HYP pulps require higher caustic dosages in pre-treatment or bleaching stages, and cause higher levels of contamination. The limited washing capability of HYP manufacturing installations and a trend to reduced water consumption further magnifies levels of contaminations and related challenges.
De-acetylation of hemicelluloses and de-methylation of pectins generates low-molecular weight organic acids, contributing to COD and higher-molecular weight pectic acids, such as polygalacturonic acids (PGA) that contribute to increased cationic demand and affect performance of retention, drainage, pitch control and sizing applications. PGA may represent up to 50% of the cationic demand found in peroxide bleached mechanical pulps. Ni reported un-pressed aspen HYP having a total cationic demand of 108.4µeq/g as compared with 16.6µeq/g and 19.4µeq/g for bleached SWK and HWK pulps, respectively. The dissolved colloidal fraction of cationic demand was reported as 34.5, 2.7 and 3.9µEq/g for HYP, BSWK and BHWK, respectively. It was observed in our field studies that increasing the pH of furnish from 5 to 7 results in approximately a 30% increase in cationic demand. Our experience indicates that at 30% HYP replacement, FPAR can drop by as much as 20% if no modifications to chemical programs are made. This is in agreement with data published by Ni. Pruszynski discusses the pros and cons of various strategies used to control anionic trash. Ni et al., reported that once soluble anionic trash is neutralized with coagulants, HYP fibers, due to their higher surface charge, can contribute positively to filler retention.
- Closely monitor HYP cationic demand levels for variability.
- Pruszynski and Orlowski presented the development of cationic demand specifications for market BCTMP pulp.
- Coagulant application:
- Set reasonable goals for reducing cationic demand. Excessive charge reduction targets may not be cost effective and could lead to runnability problems.
- Different ratios of organic and inorganic coagulants used in a given system often provide best results.
- In selecting and applying coagulants for charge control in fine paper applications, avoid eroding surface charge and agglomeration of colloidal material, and minimize effect on optical brightening agents (OBA). Nalco HYBRIDTM technology of lower charge fixatives meets all these requirements.
- Since most fine paper systems operate at high pH, quaternized coagulants are recommended to eliminate pH charge sensitivity.
- Target HYP stream prior to mixing with other components of furnish.
- Use coagulants strategically prior to such applications as starch, synthetic strength products and sizing to shield from undesired interaction with anionic trash.
- Optimize bleaching processes (replacing NaOH with Mg(OH)2, optimizing the alkalinity ratio in peroxide bleaching, optimizing the usage of chelants and sodium silicate) and maximize washing through available thickening stages.
- Enzymatic treatment of PGA with pectinase displays high effectiveness without any negative impact on paper machine operations or paper quality[4,6-8] (Fig.1). Although many mills would be prevented from using this technology, mainly due to process pH (pH 5-5.5 optimum), the exceptional effectiveness of pectinase justifies reviewing this opportunity.
- Modification of the retention program with the goal of reducing sensitivity to cationic demand level and its variability, may include new chemistry selection, modification of addition points and filler pre-treatment strategy.
Filler pre-treatment was found to be a very effective solution in a case based on a machine producing SC grades with an operating system running at very high cationic demand conditions. Figure 2 demonstrates how the removal of 1kg/ton of coagulant added directly to the filler line resulted in approximately a 30% increase of the flocculant flow required to reach sheet ash target at a constant whitewater consistency.
To be continued ... Click here to read Part II
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