P. Hart, A. Rudie
Nov 1, 2014
Citations
3
Influential Citations
40
Citations
Quality indicators
Journal
October
Abstract
The application of anthraquinone (AQ) as a pulping catalyst has been well documented in scientific studies and mill applications. AQ is known to increase the rate of delignification, enabling a reduction in pulping time, temperature, or chemical charge and an increase in pulp yield. This review does not focus extensively on specific details of AQ use but rather on critical milestones in the AQ process lifecycle, including its initial introduction, investigation of the reaction mechanism, and evaluation of best use by the pulping industry. The importance and difficulty of an economic justification for use of AQ are discussed, including their complication by modest improvement in yield obtained using AQ and low cost of the displaced chemicals. In many mills, documenting increased net mill revenue resulting from the use of AQ has been impossible. Recent health and safety studies and regulatory decisions have put the continuing use of AQ by industry in jeopardy. Given the unknown health risks, international regulatory environment, modest improvements available using AQ, and difficulty in economically accounting for the benefits, this likely represents the final chapter in the AQ life cycle. Application: The industry can use this work to better understand the context of recent decisions by many pulp and paper companies to stop using AQ in the production of direct food contact products. O ne of the first applications of anthraquinone (AQ) or AQ-derivatives in pulping was reported in 1972 by Bach and Fiehn [1]. In their work, 25 different compounds were refluxed with hydrocellulose and 2N sodium hydroxide for 2 h. Weight loss was recorded for each reaction. Anthraquinonesulfonates and hydroxylamine were the only two compounds found to stabilize cellulose toward caustic under these reaction conditions. Sodium 2-anthraquinonesulfonate was then used as an additive for kraft and soda laboratory pulping of pine. Increased yield, reduced rejects, and lower kappa numbers were obtained in kraft pulping with the addition of 1.0% anthraquinonesulfonate. Pulp strength properties were only slightly affected. A moderate amount of research work was conducted over the next several years, but the use of anthraquinonesulfonate in kraft pulping was not sufficiently effective from a cost and performance perspective [2,3]. The initial work of Bach and Fiehn suggested that AQ itself was not an effective yield-preserving pulp catalyst under the conditions they tried [1]. The use of AQ itself as an effective pulping catalyst was first reported by Holton in 1976 [4-6]. Holton reported that AQ could be used to reduce the kappa number under otherwise identical cooking conditions. In the early 1980s, a significant amount of laboratory pulping work was performed to identify and quantify the potential of low kappa pulping with AQ [7-11]. When the AQ charge was increased to 0.2%-0.3% on ovendry (o.d.) wood, softwood pulps of reasonable strength with kappa numbers as low as 15-17 could be obtained. Other researchers obtained similar results using AQ charges of 0.1%-.8% on o.d. wood [12]. With the usual yield increase from using AQ, low kappa kraft—AQ-cooked pulps have similar yields to pulp produced without AQ and to conventional brownstock kappa numbers. When AQ is added to a conventional kappa target kraft process, the resulting pulps retain a larger percentage of the carbohydrate material than the kraft pulp; that is, the pulp yield is higher at a given kappa number [13]. Thus, AQ not only accelerated the kraft pulping reaction, it also stabilized or preserved pulp yield. Over the next 15 years, researchers worked diligently to evaluate potential applications for AQ in industrial practice. It was determined that AQ could be used for incremental pulp production increases [14-19], environmental improvement [7,11,20,21], cost reduction [17,22-24], and nonconven-tional alkaline pulping processes [14,24-27]. AQ was also found to be beneficial in pulping multiple nonwood species [28-33]• During the late 1980s and into the 1990s, the desire to pulp to extremely low kappa numbers for environmental reasons led to a resurgence in mill interest in AQ [34]. AQ was well documented to provide an almost zero capital solution to the demand for lower impact bleach plant operations with minimal effect on recovery. Additionally, the development of dispersed AQ made mill application easier, [35] resulting in several successful mill trials followed by rapid adoption of AQ in mill kraft pulping processes [36,37]• Mills rapidly shifted from using AQ to pulp to lower kappa OCTOBER 2014 1 VOL. 13 NO. 10 1 TAPPI JOURNAL 23