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Ind. Eng. Chem. Res. 1998, 37, 2410-2415
Aluminophilicity of the Humic Degradation Product 5-Hydroxybenzene-1,3-dicarboxylic Acid Michael A. Wilson,*,† Graeme J. Farquharson,‡ James M. Tippett,‡ Robinson A. Quezada,§ and Lyndon Armstrong| Department of Chemistry, Materials and Forensic Science, University of Technology Sydney, Broadway, New South Wales 2007, Australia, Nalco (Australia) Ltd., 2 Anderson Street, Botany, New South Wales 2019, Australia, CSIRO Petroleum, P.O. Box 136, North Ryde, New South Wales 2113, Australia, Queensland Alumina Ltd., Parsons Point, Queensland 4680, Australia
The humic substances in Bayer liquor (the sodium hydroxide soluble part of bauxite in alumina refinery plants) precipitated with the aluminum hydroxide cake at pH 7 have been studied. Five types of organic carbon were found by solid-state 13C nuclear magnetic resonance, corresponding to alkyl (24.9 ppm), alcoholic (81.1 ppm), aromatic (137.3 ppm), oxalate (164.8 ppm), and carboxylate (183.4 ppm) functionalities. Gas chromatography/mass spectrometry was also used to study the adsorbed organics. An interesting finding was the detection and prominence of 5-hydroxybenzene-1,3-dicarboxylic acid, yet it had no effect on sizing or alumina trihydrate yield. Thus, there appears to be no correlation of adsorbivity on aluminum hydroxide with inhibition of crystallization. The results assist in the understanding of the mechanism of activity of hydroxy organic compounds as aluminum hydroxide poisons in the Bayer precipitation process. Introduction Humic substances are important during the industrial processing of bauxite. During the Bayer process for the production of alumina, bauxite is subjected to a hightemperature caustic digestion. Most of the organic matter associated with the bauxite ends up in the liquor (Grocott, 1988; Atkins and Grocott, 1988), although an insoluble portion is removed with the red mud. Since the process liquor is recycled after precipitation of the aluminum hydroxide for digestion of fresh bauxite, the soluble organic species can accumulate. Repetitive caustic digestion degrades some of the organics to a multitude of low-molecular-weight aliphatic and aromatic carboxylic acids (Atkins and Grocott, 1988). Bayer liquor organic concentrations range up to 40 g of carbon/L (Grocott and Rosenberg, 1988). They range in molecular weight from less than 100 to over 50 000 Da. Certain low-molecular-weight organic impurities such as formate, acetate, and oxalate (in solution) have little impact on the precipitation of aluminum hydroxide in the Bayer process (Lever, 1978). However, some compounds clearly affect the process by, for example, suppressing precipitation yields, adversely altering desired product size distributions (reduce agglomeration), or increasing undesirable impurities such as sodium ions (Grocott and Rosenberg, 1988; Armstrong, 1993). The nature of these compounds may vary with each plant, but little is in the open literature because of industrial confidentiality. Species possessing relatively acidic hydroxycarboxylic acid functionalities and * To whom correspondence should be addressed. Professor of Chemistry, Department of Chemistry, Materials and Forensic Science, P.O. Box 123, Broadway, NSW 2007 Australia. † University of Technology Sydney. ‡ Nalco (Australia) Ltd. § CSIRO Petroleum. | Queensland Alumina Ltd.
alcohols have been implicated (Grocott and Rosenberg, 1988; Armstrong, 1993; The, 1980; Alamdari et al., 1993; Coyne et al., 1994; Tran et al., 1996). The poisoning effect appears to increase as the number of hydroxyl groups increases. Nevertheless, the poisoning effect of the C-5 polyols, ribitol, arabinitol, and xylitol differ and mesotartaric acid is unusually active (Wattling et al., 1996; Smith et al., 1996). The poisoning effect of the compound discussed here, 5-hydroxybenzene-1,3-dicarboxylic acid, has not been established. Published procedures for the separation of Bayer liquor organics (Guthrie et al. 1984) typically require acidification to a low pH followed by the addition of a derivatizing agent. Hundreds of low-molecular-weight compounds have been identified, but a wide range of large high-molecular-weight compounds are still present. Elucidating which compounds are particularly detrimental to the Bayer process is thus difficult, and it may well be that a compound in only very trace amounts is more important than a less active but more common material. A place to start, however, is to look at those compounds selectivity adsorbed onto the aluminum hydroxide cake. Recent studies have demonstrated adsorption onto aluminum hydroxide (Coyne et al., 1994) or onto calcined alumina (Mitchell et al., 1991) by some organics which significantly retard the precipitation of aluminum hydroxide, although no identification of specific organics were reported. A priori we might expect the most readily adsorbed organics to affect crystal growth. Hence, in this work we study the chemical composition of an adsorbed material. Bayer liquors were acidified to pH 7.0 rather than to pH 1-2 conventionally used to precipitate all aluminum hydroxide and leave humic substances in solution. The choice of a neutral pH allows aluminophilic material to adsorb.
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Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2411
Experimental Section Separation Procedure. Pregnant Bayer liquor was neutralized at pH 7 with 50% hydrochloric acid at 0 °C and the resulting slurry was filtered on a glass fiber filter under vacuum to remove as much of the filtrate as possible from the fine solids:
Bayer liquor + HCl f
aluminate cake A + filtrate A
The organics adsorbed on the aluminate cake were further characterized after Soxhlet extractions with methanol or water: (a) The aluminate cake (A) was Soxhlet-extracted for 8 h with methanol to give fraction M: methanol
cake A 98 cake M + fraction M (b) The aluminate cake (A) was water-washed to give fraction B: water
cake A 98 cake B + fraction B (c) Cake B was then Soxhlet-extracted for 8 h with methanol to give cake C: methanol
cake B 98 cake C + fraction C A carbon mass balance was maintained in order to account for possible losses during these separation procedures. Whereas carbon loss can be restricted to less than 7% by neutralization at 0 °C, evaporation at acidic pH causes serious loss of carbon. The loss was traced to evaporation of volatile low-molecular-weight aliphatics. Monitoring of fractions left to stand at room temperature for up to 3 weeks by 13C nuclear magnetic resonance (NMR) (Wilson et al., 1988) showed that evaporation of methanol, formic acid, and acetic acid were the major source of carbon loss. Derivatization. Samples for gas chromatography/ mass spectrometry (GC/MS) were first methylated to ensure that the products of pyrolysis are volatile and can elute from the GC column. The methylation procedure used was as follows: (1) An aliquot of extract (∼20 gm) was eluted through a short column of XAD-8 resin to eliminate chloride. The column was washed with water and the eluate was tested for chloride with silver nitrate. The organic material was then eluted from the column with NaOH (0.1 M). (2) The eluate was protonated on a IR-120 (H+) column. (3) Tetrabutylammonium hydroxide solution was slowly added until pH 8.5. (4) The solution was stirred for 2 h and then evaporated and freeze-dried. (5) Iodomethane or iodoethane was added and the freeze-dried product was observed to dissolve. After the sample was stirred for 1 h, it was evaporated and freezedried. The derivatized samples in chloroform or deuteriochloroform were analyzed by a number of different spectroscopic methods as outlined below. Characterization. Gas-liquid chromatography was carried out with a Varian 3400 instrument with a 30-m DB-1 0.25 µm × 0.25 mm column. Gas chromatograph/ mass spectrometry was performed on a Hewlett-Pack-
Table 1. Elemental Compositions of Fractions and Cake from Extraction of Bayer Aluminum Hydroxide element
filtrate A (ppm)
fraction B (ppm)
cake B (%) as oxides, organic free
sodium calcium potassium molybdenum vanadium aluminum carbonate as CO2 boron phosphorus silica sulfur chloride organic carbon
81 000 n.d. 450 29 36 n.d. nil 3
