Article pubs.acs.org/IECR
Formation of Soluble Calcium in Alkaline Pulping of Acacia Hailong Li,† Nikolai DeMartini,*,‡,§ and Xin-Sheng Chai† †
State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China ‡ Laboratory of Inorganic Chemistry, Åbo Akademi University, Piispankatu 8, Turku 20500, Finland ABSTRACT: Calcium is the primary inorganic element in wood. During chemical pulping, in which wood is delignified to free cellulose fibers for pulp, a significant portion of the calcium in wood is released. The released calcium is mostly soluble at first, going through a metastable limit before reacting with carbonate to form calcium carbonate. The precipitation of calcium forms scale in the pulping digesters and in downstream processes such as bleaching and black liquor evaporation. This work provides new data for the tropical hardwood Acacia and gives equations for the release of calcium as a function of H factor in Kraft and soda pulping. Higher levels of soluble calcium were seen in the pulping of Acacia than has been seen in the batch pulping of pine in earlier studies, and this soluble calcium forms a more stable intermediate than is formed during the pulping of pine. The implication is that scaling in mills pulping tropical hardwoods such as Acacia can be more severe and more of this soluble calcium may leave the digester to result in fouling of black liquor evaporators.
1. INTRODUCTION In Kraft pulping, an aqueous solution containing the active pulping ions hydroxide and sulfide is used to delignify the wood and free the fibers for the production of pulp. Soda pulping liquor differs from Kraft pulping liquor in that it contains hydroxide but not sulfide. Calcium is a nonprocess element in chemical pulping and bleaching (for bleached grades of paper). It is the cause for hard-scale deposits on the heating surfaces and screens of pulping digesters, in bleaching tanks, and in black liquor (the spent pulping liquor) evaporators.1 The scaling reduces heat transfer and leads to unscheduled downtime due to scaling and plugging during the operation.2 Calcium in black liquor comes from three main sources: it comes from wood,3,4 water,5 and it can also come from the pulping liquor (white liquor) from the recausticizing process if there is overliming.1 Stemwood contains approximately 500− 1000 mg Ca/kg d.s.6,7 or more and accounts for the majority of the soluble calcium in the pulping and bleaching process.1,5,8 It has been reported that the concentration of soluble calcium in mill black liquors differs significantly between black liquors from different Kraft mills, from about 100 to 1000 mg Ca/kg dry solids or even higher in some cases.9 Therefore, a better understanding of the formation of soluble calcium during the pulping process is important to select liquor handling strategies aimed at controlling calcium scale problems in mill operations.10 Experimental results have been reported for soluble calcium during the pulping of pine.11−13 During pulping, the concentration of soluble calcium increases steadily until a maximum is reached before nucleation occurs and calcium precipitates. Even for experiments carried out in which no sodium carbonate is added to the liquor, calcium carbonate can form late in the cook because carbonate is formed during the pulping process,11,14 and this carbonate can eventually react with the soluble calcium to form calcium carbonate. If sodium carbonate is added to the pulping liquor, the maximum concentration of soluble calcium reached during the pulping of © 2014 American Chemical Society
pine is less and occurs at a lower temperature than if sodium carbonate is not added to the pulping liquor.11−13 Oxalate can form in both pulping15 and bleaching;16 thus, calcium oxalate can also form and precipitate2 during bleaching. It is important to understand the release of the calcium ions during the pulping process, so that a calcium scale control strategy around pulping, bleaching, and black liquor evaporation can be established. Thus, an improvement in energy efficiency can be achieved in the mill operation. In this study, acacia, a tropical hardwood, was used in the chemical pulping. This is new data and of particular importance to the pulping of tropical hardwood species. A relationship between H-factor and soluble calcium was obtained for both kraft and soda pulping in a laboratory digester. H-factor is a term that combines the time and temperature effects on delignification into one term.17 Carbonate formation was monitored during pulping. Additionally, an experiment with sodium carbonate added to the black liquor was carried out. In addition to measuring the calcium concentration, we followed the concentration of dissolved lignin during the cook. Lignin has been shown to affect the crystal structure of calcium carbonate,18 and lignin degradation products have active sites which may bond Ca to form soluble calcium. Thus, the dissolution of lignin was thought to be potentially relevant to the formation of soluble calcium during pulping.
2. MATERIALS AND METHODS 2.1. Pulping. All pulping experiments were carried out in a laboratory digester system with circulating cooking liquor. The system employs heaters that are controlled through Windows based temperature controllers. The total volume of the cooking system was 10 L. A 1000 g portion (OD wood) of acacia wood Received: Revised: Accepted: Published: 17282
July 18, 2014 October 2, 2014 October 5, 2014 October 6, 2014 dx.doi.org/10.1021/ie502866p | Ind. Eng. Chem. Res. 2014, 53, 17282−17285
Industrial & Engineering Chemistry Research
Article
pine,11−13 the metastable limit was not exceeded in these cooks when no sodium carbonate was added to the synthetic white liquor. More calcium was released to the black liquor in the Kraft cook than in the soda cook. The concentration of soluble calcium versus H-factor can be described by the equation
chips was charged to the digester. The active alkali charge AA (as Na2O) was 18% on wood for both the Kraft and soda cooks. Sulfidity was 0% or 30%. Liquor- to-wood ratio was 4:1. The white liquor was prepared in the lab using sodium hydroxide and sodium sulfide. The cooking temperature was increased at rate of 1.2 °C/min from 25 °C for 120 min and then maintained at 170 °C for 60 min. Black liquor samples (
