Ind. Eng. Chem. Res. 2000, 39, 3631-3639
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APPLIED CHEMISTRY Structure of Molecular Weight Fractions of Bayer Humic Substances. 2. Pyrolysis Behavior of High-Temperature Products Damian E. Smeulders,† Michael A. Wilson,*,† Harish Patney,† and Lyndon Armstrong‡ Department of Chemistry, Materials and Forensic Science, University of Technology, Sydney, Broadway 2007, Australia, and Queensland Alumina Ltd., Parsons Point, Queensland 4680, Australia
Sodium hydroxide is used at elevated temperatures to separate aluminum hydroxide from ferric oxide (red mud) in bauxite in the Bayer process. Organic material in the bauxite can intefere with the precipitation of aluminum hydroxide. In this paper we study organic material from a plant operating at 250-255 °C. Under these conditions large amounts of the acidified organic matter are volatile (approximately 89% at 100 °C and 0.1 atm of pressure under rotary evaporation). In this paper nonvolatile organic material obtained by rotary evaporation has been fractionated by dialysis to produce different molecular weight fractions. These fractions have been analyzed by thermal and spectroscopic methods (differential thermal analysis and calorimetry, nuclear magnetic resonance, infrared spectroscopy, pyrolysis-gas chromatography/ mass spectrometry). The 300 kDa) behaves as a soluble char. An interesting new finding is that small volatile molecules can attach themselves to large molecules, possibly by hydrogen bonding, so that during dialysis they are an integral part of the larger molecular weight fractions and hence dialyze as if they were components of the larger molecules. Introduction Geo-organic matter is introduced with ore during processing of bauxite to form alumina by the Bayer process. However, it is poisonous to the process, and it is desirable that methods be developed for its removal.1-3 Bauxite is subjected to a high-temperature sodium hydroxide digestion, during which most of the organic matter associated with the bauxite ends up in the liquor. The soluble organic species accumulate in the process liquor because the caustic is recycled for the digestion of fresh bauxite after the precipitation of aluminum hydroxide. Repetitive caustic digestion degrades some of the organics to a multitude of low molecular weight aliphatic and aromatic carboxylic acids. This paper investigates the chemical composition of size-exclusion fractions of Bayer organic matter (described as humic substances by organic geochemists) separated by dialysis and analyzed by spectroscopic (nuclear magnetic resonance and infrared spectroscopy) and pyrolysis techniques. Elsewhere, we have published a similar study on organic matter in a plant operating at 145-150 °C.4 However, because the operating temperature could affect the organic matter composition, in this paper we investigate the organic matter in a plant operating at around 250-255 °C. In practice, † ‡
University of Technology. Queensland Alumina Ltd.
plants operate at different temperatures because the aluminum-containing components of different bauxites are variable, and therefore different temperatures are needed to dissolve the bauxite at economic rates. Spectroscopic properties and the pyrolysis behavior of each molecular weight fraction is investigated by infrared spectroscopy, 13C solid-state nuclear magnetic resonance spectroscopy, differental thermal analysis, modulated differential scanning calorimetry, and pyrolysis-gas chromatography/mass spectrometry. Pyrolysis is a potentially useful method of analyzing the structure of these organics, and it may also be useful for removing them and/or generating a source of useful byproducts,5-9 although if employed on an industrial scale, there would be a necessity for pollution control.10 There have been a number of previous studies of the pyrolysis of humic substances,11-19 but only our recent work at a refinery operating at lower temperatures has been concerned with Bayer process fractions separated on the basis of molecular weight.4 Experimental Section Extraction of Humic Substances from Bayer Liquors. Isolation of the organic substances from the Bayer process was carried out using a method similar to that described by Sihombing et al.15 The refinery from which these liquors were isolated operates at a higher temperature (250-255 °C and 3500 kPa steam pres-
10.1021/ie000290s CCC: $19.00 © 2000 American Chemical Society Published on Web 08/25/2000
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Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000
Table 1. Yields, Acidity, and Elemental Analysis of the Bayer Humic Substance Fractions (Dry Ash Free Basis) fraction no. PLb SLb 1 2 3 4 5 6 7 8 a
mol wt fraction (kDa) 300 300 300
%C
%H
%N
%S
%O difference
O/C
H/C
N/C
S/C
pH at 5 g/L in water
mass yield (%)a
56.32 53.13 50.79 48.25 54.32 53.87 55.58 48.85 49.13 54.52
3.67 3.58 4.22 3.78 4.23 3.67 5.41 4.05 4.85 3.78
0.36 0.11 0.56 1.87 3.22 1.39 3.69 4.92 5.92 2.05
0.23 0.11 0.17 0.32 0.60 0.44 0.42 0.68 0.99 0.42
39.42 43.07 44.26 45.78 37.62 40.62 34.90 41.50 39.11 39.24
0.53 0.61 0.65 0.71 0.52 0.57 0.47 0.64 0.60 0.54
0.78 0.80 0.99 0.93 0.93 0.81 1.16 0.99 1.18 0.83
0.0055 0.0018 0.0095 0.033 0.051 0.022 0.057 0.086 0.10 0.032
0.0015 0.00078 0.0013 0.0025 0.0041 0.0031 0.0028 0.0052 0.0075 0.0029
2.43 2.39 2.48 2.96 3.39 2.90 3.79 3.99 4.64 4.36
100.00 100.00 87.0 3.3 0.6 1.6 1.0 3.8 0.7 2.0
The weight of the fraction over the total weight of the material recovered. b PL ) pregnant liquors; SL ) spent liqours.
sure) and is expected to produce organic materials different from those reported from refineries operating at lower temperatures4 and with a different feed bauxite. Recovered organics from pregnant liquor (i.e., prior to plant precipitation of gibbsite aluminum hydroxide produced by digestive extraction of bauxite) and spent liquor (i.e., after plant precipitation of gibbsite aluminum hydroxide) were examined, but only organic matter from spent liquor was further separated into molecular weight fractions. Aliquots of the Bayer process liquor were diluted at a ratio of 1:5 (v/v) with singly distilled water. Dilute hydrochloric acid [1:1 (v/v) with water] was added to the mixture dropwise with stirring to give a pH of approximately 1.5. The mixture was then left for 24 h to allow insolubles to settle to the bottom of the flask. The acidified solution was decanted, filtered through a glass microfiber filter (Whatman GF/D 15 cm), and then filtered again through a sintered glass filter of porosity 4. The decanted solution was chromatographed on a water-washed and acidified Amberlite XAD-7 column (2 cm × 60 cm). The filtrate was passed through the column with a flow rate of approximately 1 mL/min. More filtrate was added until the eluant from the column developed a yellow tinge, signifying that the column was overloaded. The column was then rinsed with 2 L of 0.1 M hydrochloric acid, followed by 2 L of distilled water to remove any salts. The solutions used to rinse the column were retained and later passed through a regenerated XAD-7 column again to recover the dissolved organics that were eluted off the column. The Bayer organics were eluted by passing 500 mL of 0.1 M potassium hydroxide through the column. The eluant was captured along with an additional 300 mL of distilled water, which was added to rinse the column. The organic compounds contained in the eluant from the XAD-7 column were protonated by passing the eluate through a column containing an IR-120(H+) resin (BDH; 2 cm × 60 cm). The organic compounds remaining on the IR-120(H+) resin were removed by passing 300 mL of distilled water through the column. This rinse water was also collected and combined with the initial eluate. The combined solution was concentrated by rotary evaporation. The resin was regenerated as described previously.4 The protonated humic substances obtained from the above procedure were concentrated by rotary evaporation and then freeze-dried. This procedure was performed on 10 L each of the spent and pregnant Bayer liquors. The pregnant liquor gave 5.26 g/L, and the spent liquor yielded 5.10 g/L. This represents only 11% of the plant organic matter (on a dissolved organic carbon basis) because volatiles are lost on evaporation.
The volatiles range from methanol to simple carboxylic acids and phenols. The protonated organics from the spent Bayer liquor were separated into their molecular weight fractions using Spectra/Por molecular porous dialysis membranes constructed from cellulose and cellulose-ester with molecular weight cutoffs (MWCO) of 1.2, 6, 12, 25, 50, 100, and 300 kDa. The membranes were cut into 45 cm lengths and conditioned prior to use. Each membrane was rinsed under running tap water for 2 h to remove any free organics. The membranes were then left to soak overnight in distilled water before being given a final rinse with redistilled, low-conductivity water. The 50 kDa MWCO membranes were the first to be used. One end of the membrane was tied in a knot, and the membrane was filled with the freeze-dried humic substances dissolved in doubly distilled water at a concentration of 5 mg/mL. The other end of the membrane was tied, and the membrane was placed in a beaker to dialyze against doubly distilled water. Each membrane was secured in a beaker using an elastic band, ensuring that both ends of the membrane were on the outside of the beaker. This was to ensure that any solution which leaked from the interior of the membrane did not go into the water. Each beaker was covered with Parafilm to prevent contamination. The water surrounding each membrane was changed on a regular basis for up to 21 days. The collected water was stored in a clean glass bottle and then concentrated by rotary evaporation. The solution which remains inside the membrane represents all of the compounds with a molecular weight above that of the MWCO of the membrane (>50 kDa). The compounds which passed through the membrane into the surrounding water had molecular weights less than that of the membrane (
