pseudo-total and extractable non-process elements in

PSEUDO-TOTAL AND EXTRACTABLE NON-PROCESS ELEMENTS IN GREEN LIQUOR DREGS M. MÄKELÄ*, R. PÖYKIÖ**, K. MANSKINEN***, H. NURMESNIEMI°, O. DAHL*, G: WATKINS* and R. HUSGAFVEL* *Department of Forest Products Technology, Aalto University, School of Chemical Technology, FI-00076 Aalto, Finland, **City of Kemi, Valtakatu 26, FI-94100 Kemi, Finland ***Stora Enso Oyj, Heinola Fluting Mill, FI-18101 Heinola, Finland, °Stora Enso Oyj, Veitsiluoto Mill, FI-94800 Kemi, Finland

SUMMARY: Extraction is a procedure that places a solid and a liquid in contact under defined conditions and is widely used as a tool to estimate the potential release of constituents, for example from waste materials, over a range of possible waste management activities, such as recycling, reuse or landfill disposal. For the partitioning of non-process elements (NPEs) in the green liquor dregs between the exchangeable (CH3 COOH), easily reduced (NH2OH HCl in nitric acid medium) and oxidizable (H2O2 + CH3COONH4) fractions, we used the three-stage sequential extraction procedure developed by the European Community Bureau of Reference (BCR) in an attempt to harmonize the different extraction schemes. Based on the results, the highest extractable concentrations of NPEs were recovered in the oxidisable BCR3 fraction, although certain NPEs were also extractable and quantitatively detectable in the BCR1 (Al 6.6 mg/kg and Ba 3.3 mg/kg, d.w.) and BCR2 (Ba 445 mg/kg, Co 7.5 mg/kg, Ni 11.1 mg/kg and Zn 366 mg/kg, d.w.) fractions. From the utilization point of view, the NPEs bound to the oxidisable fraction can be released under oxidising conditions due to the decomposition of, e.g., organic matter or sulphides. 1. INTRODUCTION Chemical pulping is currently dominated by the sulphate (kraft) process, following by the sulphite process. The sulphate (kraft) pulping process is an alkaline process based on the use of sodium hydroxide (NaOH) and sodium sulphide (Na2S), whereas in the sulphite process, the active cooking chemical is hydrogen sulphite (HSO3-). Semi-chemical pulping is generally defined as a two-stage pulping process, involving chemical treatment to remove part of the lignin or fibre-bonding substances followed by mechanical refining to complete the separation of the fibres. Semi-chemical pulp can be produced by modifying nearly any presently used pulping procedure: sulphate, sulphite, soda or cold caustic. As pulp mills move towards minimum impact manufacturing, one of the most difficult challenges in the recycling and reuse of both water and the various aqueous streams is the development of strategies for effectively dealing with the build-up, carryover and recovery of cationic and anionic non-process elements (NPEs). NPEs or more correctly inorganic process

contaminants are elements such as potassium, manganese, barium, iron, aluminium, copper, nickel, chromium and zinc that have no active part in the process and are detrimental to pulping, bleaching or recovery. According to Grace and Tran (2009), the term “non-process elements” usually refers to all of the chemicals in the system other than sodium, sulphur, carbon, hydrogen, and oxygen. NPEs are introduced into the process in the fibre raw materials (wood chips, sawdust), the make-up chemicals and process waters, and from the corrosion of process equipment. It is important to control the input of NPEs, since they tend to accumulate in the different cycles, leading to operating problems as well as dead load (Grace & Tran, 2009). However, since different elements are active in different parts of the pulp process, an element may be considered an NPE in one location but not in another. Cooking chemicals used in pulping are recovered in the chemical recovery unit, which is used to recover valuable inorganic chemicals used in the cooking liquor. The accumulation of NPEs in pulping processes may result in, e.g., scaling problems and filtration failures. Therefore, NPEs have to be removed from the process, for instance by precipitation in the form of green liquor dregs. Green liquor dregs are usually dumped in landfills. In Finland, and elsewhere in the European Union (EU), the properties of solid wastes and industrial residues, especially when they are utilized or taken to a landfill, have to be investigated. The general principles applied in landfill approval are that the composition and extractability of the waste have to be known. The total element concentrations represent a source term only for the unrealistic environmental scenario in which the entire mineral structure of the solid material is dissolved. Measurement of the total concentration of metals provides relatively misleading information for assessing the possible bioavailability and mobility of metals. In order to estimate the bioavailability of metals and their potential toxicity, it is necessary not only to determine the total concentrations but also the different forms or processes binding the heavy metals to the solid phase of the sample (Filgueiras et al., 2002). Extraction is a procedure that places a solid and a liquid in contact under defined conditions. Extractions tests are widely used as tools to estimate the potential release of constituents, for example from waste materials, over a range of possible waste management activities, including recycling, reuse or landfill disposal. One of the most frequently used extraction tests for assessing the release of heavy metals from ash (Bruder-Hubscher et al., 2002), sludge, and from other industrial waste materials is sequential extraction (Filgueiras et al., 2002), in which elements in the material are commonly fractionated between acid soluble, reducible and oxidisable fractions. This approach provides information on the chemical conditions needed to obtain different extraction efficiencies. The goal of this method is to divide the total extractable concentration of metals into separate fractions in order to assess the form in which the metals occur in the waste material. These extraction tests are carried out in the assessment of worst-case environmental scenarios, in which the components of the sample become soluble and mobile (Filgueiras et al., 2002; Rao et al., 2008).

2. EXPERIMENTAL 2.1 Sampling The green liquor dregs investigated in this study were sampled from a semi-chemical pulp mill located in Finland. The pulp mill produces unbleached semi-chemical pulp, which is used as the corrugated medium between the liners of corrugated board. The main functional operations in the semi-chemical pulp mill investigated in this study are: cooking, washing, evaporation, the soda recovery boiler and the recovery process. In this study, the wood-based fibre raw materials

(i.e., wood chips, sawdust) were cooked with a mixture of sodium sulphite (Na2SO3) and sodium carbonate (Na2CO3). Green liquor dregs were sampled over a period of one week, with individual daily samples being combined to give one composite sample with a weight of 10 kg (wet weight). The sampling period represented the normal process operating conditions for the pulp mill. Approximately 93% of the wood material was birch (Betula verrucosa and Betula pubescens) and 7.0% was aspen (Populus tremula). During the sampling period, approximately 67% of the wood material used for pulping was roundwood and 33% consisted of wood chips from the mechanical wood industry. After sampling, the samples were stored in polyethylene bottles in a refrigerator (+ 4 °C). A coning and quartering method was repeatedly applied to reduce the sample of grits to a size suitable for conducting laboratory analyses (Gerlach et al., 2002). 2.2 Determination of the mineralogical composition of the dregs For the determination of the mineralogical composition of the dregs, X-ray diffractogram of powdered sample was obtained with a Siemens D 5000 diffractometer (Siemens AG, Karlsruhe, Germany) using CuK radiation. The scan was run from 2 to 80° (2-theta-scale), with increments of 0.02° and a counting time of 1.5 seconds per step. Operating conditions were 40 kV and 40 mA. Peak identification was done with the DIFFRACplus BASIC Evaluation Package PDFMaint 12 (Bruker axs, Germany) and ICDD PDF-2 Release 2006 software package (Pennsylvania, USA). 2.3 Determination of the pseudo-total heavy metal concentrations in the dregs For the determination of pseudo-total element concentrations in the dregs, the dried sample was digested with a mixture of HCl (3 mL) and HNO3 (9 mL) in a CEM Mars 5 microprocessor controlled microwave oven with CEM HP 500 Teflon vessels (CEM corp., Matthews, USA) using USEPA method 3051A (Yafa and Farmer, 2006). The cooled solution was transferred to 100 mL volumetric flask and the solution was diluted to volume with ultrapure water generated by an Elgastat Prima reverse osmosis and Elgastat Maxima ion exchange water purification system (Elga, Ltd; Bucks, England). All reagents and acids were suprapure or pro analysis quality. The pseudo-total element concentrations in the dregs were determined with a Thermo Fisher Scientific iCAP6500 Duo (United Kingdom) inductively coupled plasma-optical emission spectrometer (ICP-OES). 2.4 Sequential extraction partitioning of NPEs in the dregs For the partitioning of NPEs in the green liquor dregs between the exchangeable (CH3COOH), easily reduced (NH2OH-HCl in nitric acid medium) and oxidizable (H2O2 + CH3COONH4) fractions, we used the three-stage sequential extraction procedure, which is described in more detail in our previous publication (Manskinen et al., 2011). This extraction procedure was developed by the European Community Bureau of Reference (BCR) as an attempt to harmonize different extraction schemes. This extraction procedure is a widely applied for heavy metal fractionation in various environmental matrices, e.g. ash, soil, sediment and sludge (Bacon & Davidson, 2008; Filgueiras et al., 2002). Extraction was carried out by shaking 5 g of the green liquor dregs in a polypropylene bottle. In order to minimize possible chemical and/or microbiological changes in the material, the extraction was carried out using an undried sample instead of a dried sample since, according to Kosson et al. (2002), it is preferable to avoid sample drying before extraction. After each extraction step the extracts were separated from the solid residue by filtration through a 0.45 µm

membrane filter (47 mm diameter; Schleicher & Schuell, Dassel, Germany). In order to avoid losses between the extraction stages, the filters and adhering dregs particles from the previous extraction stage were also included in the next stage. After the addition of 200 µL of 65% HNO3 to the supernatant phase, the extracts were stored in a refrigerator (+4 °C) until element determinations. The element concentrations in the extracts (i.e., extraction stages 1-3) were determined with a Thermo Fisher Scientific iCAP6500 Duo (United Kingdom) ICP-OES.

3. RESULTS AND DISCUSSION 3.1 Mineralogical composition of the dregs The XRD spectra provided in Figure 1 illustrates that the green liquor dregs obtained from the chemical recovery circuit with sodium sulphite (Na2SO3) and sodium carbonate (Na2CO3) as the cooking chemicals only contained carbonate minerals such as pirssonite (Na2Ca(CO3)2 2H20), calcite (CaCO3) and barentsite (Na7AlH2(CO3)4F4). However, the present XRD data are consistent with the findings of Castro et al. (2009), as well as with Garcia and Sousa-Coutinho (2010), who reported the existence of calcite in green liquor dregs, as well as Martins et al. (2007), who detected both calcite and pirssonite. Moreover, Garcia and Sousa-Coutinho (2010) also detected of a sodium-calcium carbonate mineral in their dregs, as we did. However, Garcia and Sousa-Coutinho (2010) reported it to be of the chemical formula Na2Ca2(CO3)3 (i.e., shortite), whereas our dregs contained the hydrated formula (Na2Ca(CO3)2 2H20) (i.e., pirssonite). As the cooking chemicals were not reported by Castro et al. (2009), Garcia and Sousa-Coutinho (2010), or Martins et al. (2007), the reasons for differences in mineral composition between dregs cannot be further discussed.

Pir

Lin (Counts)

6000

5000

4000

Green liquor dregs

Pir 3000

2000

Bar Cal

1000

0 5

10

20

30

40

50

60

70

80

2-Theta - Scale

Figure 1. X-ray diffraction pattern of green liquor dregs. Mineral abbreviations and their abundances (%) are: Pir = Pirssonite (90.2%); Bar = Barentsite (7.7%); Cal = Calcite (2.1%).

3.2 Pseudo-total and extractable NPE concentrations in the dregs The distribution of NPEs in the green liquor dregs after a three-stage BCR extraction (leaching) procedure between acid soluble (CH3COOH), reducible (NH2OH-HCl in nitric acid medium) and oxidizable (H2O2 + CH3COONH4) fractions as well as the pseudo-total NPE concentrations in the dregs are presented in Table 1. The acid soluble fraction (i.e., BCR 1 fraction), extractable with acetic acid (CH3COOH), gives an indication of the amount of metals bound on the surface of the particles, as well as those that are released as acid-soluble salts such as carbonates (Filgueiraes et al., 2002). This fraction is potentially bioavailable and corresponds to the form of metals that is most available for plant uptake, and can be released by merely changing the ionic strength of the medium. If we disregard the NPEs with concentrations lower than the respective detection limits, according to the results in Table 1, acetic acid (CH3COOH) was only capable of releasing aluminium (6.6 mg/kg; d.w.) and barium (3.3 mg/kg; d.w.) from the green liquor dregs, whereas the release of other NPEs was low. However, compared to the pseudo-total concentrations, the partitioning of aluminium and barium in this fraction was low: Al (0.6%) and Ba (0.2%). Table 1 – Pseudo-total and extractable concentrations of NPEs in green liquor dregs determined using USEPA method 3051A, and the extractable concentrations of non-process elements (NPEs) in extraction stages BCR1 (CH3COOH), BCR2 (NH2OH-HCL in nitric acid medium) and BCR3 (H2O2 + CH3COONH4), as well as the pH of the extract prior to (i.e., only extract) and after (i.e., extract + dregs) extraction. Extractable concentration Pseudo-total pH of extract / concentration Fraction Fraction Fraction USEPA 3051A BCR1 BCR2 BCR3 NPE (mg/kg; d.w.) (mg/kg; d.w.) (mg/kg; d.w.) (mg/kg; d.w.) pH (prio to extraction) pH (post-extraction) Al As Ba Be Cd Co Cr Cu Ni Pb Se V Zn

2.9 9.1 6.6 < 0.6 3.3 < 0.2 < 0.08 < 0.1 < 0.4 < 0.4 < 0.2 < 0.6 < 0.8 < 0.4 < 0.4

2.0 5.9