Structure of molecular weight fractions of Bayer humic substances. 2. Pyrolysis behavior of high-temperature products

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 degrees C.,Under these; conditions large amounts of the acidified organic matter are volatile (approximately 89% at 100 degrees C and 0.1 atm of pressure under rotary evaporation). In this paper nonvolatile organic material obtained ny 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 <1.2 kDa fraction appears to yield mainly (hydroxybenzene)carboxylic acids on pyrolysis and structurally resembles the highly oxidized humic acids that are found in the group of soils known as podsols. The 12-25 kDa fraction appears to resemble material more akin to oxidized kerogen. The highest molecular weight material (> 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.