Understanding Hydrogen in Bayer Process Emissions. 1. Hydrogen Production during the Degradation of Hydroxycarboxylic Acids in Sodium Hydroxide Solutions

Abstract

The formation of potentially explosive gas mixtures during Bayer process digestion and the wet oxidation of Bayer process liquors underscores the need for an improved understanding of the degradation reactions of organic compounds that produce flammable gases. This study is the first of a series investigating the production of hydrogen from different classes of organic compounds in sodium hydroxide solutions. The alkaline degradation of a range of aliphatic and aromatic carboxylates and hydroxycarboxylates was investigated under anaerobic conditions in an autoclave. It was found that aliphatic C 4 carboxylates possessing a single β-hydroxy substituent are particularly reactive under these conditions, generating hydrogen gas and a range of low molecular weight (LMW) carboxylates. The effect of temperature (175À275°C) and NaOH concentration (0À6 M) on the degradation of 3-hydroxybutanoate and hydroxybutanedioate (malate) was investigated in detail for reaction times up to 120 min. Under conditions that promote the total degradation of the organic compound, the β-hydroxycarboxylates have similar hydrogen production capacities, each generating about 1 mol of hydrogen gas per mole of organic compound consumed. The results provide direct evidence for an ionic degradation mechanism involving base-catalyzed oxidation by water, consistent with the stoichiometry of the formation of hydrogen gas and the other main reaction products (LMW carboxylates). These findings have important implications for the production of hydrogen in Bayer process digestion and the safe application of wet oxidation technologies for the treatment of organic compounds in alkaline liquors, such as those found in the Bayer and Kraft processes. The results also shed new light on the mechanism of hydrogen production during the base-catalyzed gasification of biomass at low temperature.

Experimental Study on the Molecular Hydrogen Release Mechanism during Low-Temperature Oxidation of Coal

“…Besides, these results may also be caused by the reactions between aldehyde groups and some specific organic components in coal. The carbonyl carbon is attacked by hydroxyl radicals resulting in an intermediate species [27] . These intermediate species further form into carboxylate, which is accompanied with the production of hydrogen gas.…”

Organic compounds in the processing of lateritic bauxites to alumina Part 2: Effects of organics in the Bayer process

“…Further insight into the mechanisms of hydrogen production has been provided by the recent work of Costine et al (2010Costine et al ( , 2011aCostine et al ( , 2011bCostine et al ( , 2012. The oxidation of complex organic compounds by water leads to hydrogen as the major reduction product, but can also lead to the generation of methane, propane and other volatile hydrocarbons as well as alcohols, aldehydes and ketones, as reduction products (Loh et al, 2010b).…”

Evaluation of the Current State of Knowledge for Thermolysis of Organics within SRS Waste Forming Volatile Organic Compounds (VOCs)

“…These estimated methane generation rates from methyl mercury in neutral water in the range of 2.2E-07 ft 3 /gal/h at 26 °C up to 4.5E--02 ft 3 /gal/h at 170 °C are higher than the methane generation rates for each specified temperature estimated from the dimethyl mercury decomposition data from 7 M NaOH simulated tank waste shown in Table 2-17. 9,10,11,12 This process is an analog of the aluminum leaching process at SRS although the temperatures are appreciably higher. Some of these references were noted in the recent study related to hydrogen production from radiolytic and thermolytic degradation of either formate or glycolate in SRS Tank Farm wastes.…”

“…The detection limit for hydrogen analysis was indicated as 0.01 mol%. The first two papers in this series 9,10 focused exclusively on hydrogen as the measured gaseous products and did not address the potential formation of hydrocarbon gases. The latter two papers in the series 11,12 did address possible formation of hydrocarbon gases.…”