Article pubs.acs.org/IECR
Understanding Hydrogen in Bayer Process Emissions. 4. Hydrogen Production during the Wet Oxidation of Industrial Bayer Liquor Allan Costine* and Joanne S.C. Loh CSIRO Mineral Resources, P.O. Box 7229, Karawara, Western Australia 6152, Australia ABSTRACT: To provide a basis for strategies to prevent the formation of potentially explosive gas mixtures during Bayer digestion and wet oxidation processes, a quantitative description of the hydrogen production capacities of industrial liquor is required, along with an improved understanding of the reactions that produce hydrogen gas. This study is the fourth in a series aimed at addressing these knowledge gaps. For a low-temperature refinery liquor, it was found that doubling the TOC conversion from 20 to 40% resulted in a 5-fold increase in the amount of hydrogen evolved (∼0.06 mol H2/L of liquor), indicating that the degradation of refractory organic compounds in the liquor contributes to a significant amount of the hydrogen produced. Critically, more hydrogen was produced under wet oxidation conditions than under the largely anoxic conditions encountered in digestion, with the magnitude of this effect diminishing with an increase in temperature (180−270 °C). The wet oxidation of several model organic compounds representing various structural classes known to be present in the liquor was also investigated. With oxygen addition, aliphatic compounds (3-hydroxybutanoic acid, maleic acid, and erythritol) produced less hydrogen compared with the same reactions conducted under anoxic conditions; conversely, aromatic compounds (benzoic acid, m-salicylic acid, gallic acid, and catechol) produced more hydrogen in the presence of oxygen, which was attributed to benzene ring-opening reactions generating unsaturated intermediates which are known hydrogen producers. The effect of oxygen on hydrogen production rates is therefore expected to be largely governed by the relative and total amounts of aromatic and aliphatic carbon in the liquor. Because the formation of hydrogen from complex mixtures of low molecular weight (LMW) organic compounds often occurs through short-lived aldehyde intermediates, the reactions of acetaldehyde, butyraldehyde, benzaldehyde, and glyoxylate were studied under similar conditions. A high-temperature pathway for the quantitative conversion of aldehydes to hydrogen and their corresponding carboxylic acids was identified in the case where the aldehydes were injected at low concentration into a preheated sodium hydroxide solution (≤1 mM aldehyde, 3 M NaOH, 250 °C, N2).
1. INTRODUCTION
Oxalate is a common degradation product that has a negative impact on gibbsite precipitation,11,12 such that many refineries practice oxalate removal, which represents a significant exit for organic carbon from the liquor. Wet oxidation13 using oxygen or air can be effective and economical in reducing the concentration of organic carbon in the liquor, and has been applied commercially;14 however, safety remains a serious concern with this process, as explosive mixtures of hydrogen gas can be produced. The hightemperature (220−320 °C) wet oxidation of spent Bayer liquors in continuous-flow tube reactors was previously studied,14−16 with wide variability in the hydrogen concentrations of the waste gases reported. Defining the hydrogen production capacity of the liquor and understanding the key reactions that produce hydrogen under wet oxidation conditions are important aspects in the improvement of this technology. In the present work, the amount of hydrogen produced per liter of spent liquor and the evolution of LMW carboxylate products was measured under wet oxidation conditions as a
In the Bayer process for refining alumina, bauxite ore is digested in hot, concentrated, sodium hydroxide solution to produce a pregnant sodium aluminate solution, which is then cooled and seeded to allow precipitation of gibbsite. The gibbsite is separated from the spent liquor and calcined to produce alumina suitable for smelting to metal. As the process is cyclic, the spent liquor is recycled and used to digest a fresh batch of bauxite. The recirculating liquor also contains many hundreds of organic compounds that form through the oxidative and nonoxidative (alkaline) degradation of high molecular weight (HMW) substances present in the bauxite (e.g., humic and fulvic acids, humin, lignin, and cellulose).1−4 Organic compounds in the liquor, particularly those with adjacent hydroxyl groups,5−9 have detrimental effects on productivity and product quality. Lever10 classified the organic compounds in Bayer liquor into three groups, as follows: (1) HMW humictype substances, which give Bayer liquors their characteristic brown/black color; (2) “humic building block” materials, which are mainly aromatic carboxylates and phenolic carboxylates; (3) low molecular weight (LMW) degradation products such as formate and oxalate. Published XXXX by the American Chemical Society
Received: March 2, 2016 Accepted: April 1, 2016
A
DOI: 10.1021/acs.iecr.6b00853 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Figure 1. Liquor and gas analysis as a function of TOC conversion. (A) Humate and fulvate destruction, color reduction (691 nm), oxygen consumption, and number of moles of H2 produced per liter of liquor, (B) evolution of LMW organic and inorganic compounds, where ox = oxalate; car = carbonate; ace = acetate; suc = succinate; lac = lactate; for = formate; mal = malonate; and OH¯ = free hydroxide. Hydrogen measurements are annotated with the reaction times (h) for each experiment. Conditions: low-temperature refinery liquor; temperature = 270 °C; time at temperature = 0−3 h; O2/TOC mole ratio = 0.5.
refineries processing Australian bauxites. The liquors are typical of the liquors found in many alumina refineries, although the exact composition of individual liquors will be a function of the bauxite source, the digestion temperature, and other factors. The liquors were pressure filtered through a 0.45 μm Supor membrane prior to use. 2.3. Organic Compounds. Nine aliphatic and aromatic compounds were studied as starting substrates to help identify the key pathways that produce hydrogen gas under anoxic and oxic conditions. The compounds represented various structural classes known to be present in Bayer liquors.2 The aliphatic compounds were DL-3-hydroxybutanoate, DL-malate, maleate, trans-2-hexenoate, and meso-erythritol; the aromatic compounds were benzoate, m-salicylate, gallate, and catechol. All compounds were analytical grade reagents purchased from Sigma-Aldrich (Sydney, Australia). Solutions of each compound (20 mM) were prepared in 3 M sodium hydroxide solution (>97%, 1% Na2CO3, Rowe Scientific, Perth, Australia). Four aldehydesacetaldehyde, butyraldehyde, benzaldehyde, and glyoxylatewere also tested because the production of hydrogen during the alkaline degradation of LMW compounds often occurs through reactive aldehyde intermediates. 2.4. System Preparation. A liquor/solution volume of 1 L was used for each test. For heat treatment (anoxic) tests, the liquor/solution was degassed by sparging with nitrogen until
function of total organic carbon (TOC) removal. Several aliphatic and aromatic compounds were studied as starting substrates to help identify the key reactions that produce hydrogen under digestion and wet oxidation conditions. The previous studies in this series of papers showed that the production of hydrogen during the alkaline degradation of βhydroxycarboxylates,17 unsaturated carboxylates,18 and polyols19 occurs through aldehyde intermediates. The reactions of several aldehydes were therefore studied under high-temperature alkaline conditions.
2. EXPERIMENTAL SECTION 2.1. Autoclave Facility. Heat treatment (N2) and wet oxidation (O2) tests were carried out at 180−270 °C in a 2 L Inconel 600 autoclave (Parr Instruments Co., IL, U.S.A.) fitted with a serpentine cooling coil, a thermocouple well, a pressure transducer, and a magnetically driven stirrer with twin sixpitched-blade impellers. A Parr 4848 BM reactor controller provided continuous logging of the autoclave temperature and heater output. The autoclave was controlled to within ±2 °C of the specified temperature through the reactor’s heating element and internal cooling coil circuit. A detailed description of the system can be found elsewhere.20 2.2. Bayer Liquors. The spent liquors used in the present work originated from low- and high-temperature alumina B
DOI: 10.1021/acs.iecr.6b00853 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Liquor decolorization as a result of wet oxidation was measured using a Cary 1C UV−vis spectrophotometer at 691 nm. The manufacture of specialty grades of gibbsite often requires a high level of product brightness, which is adversely affected by the coprecipitation or adsorption of colored compounds with or on the gibbsite product. TOC in the liquor samples was measured indirectly, reported here as the difference between the total carbon (TC) and total inorganic carbon (TIC) content, as determined by a Shimadzu TOC-V total organic carbon analyzer. TC and TIC were calculated from the infrared measurement of the CO2 gas produced by catalytic combustion (680 °C) of the sample and by the acid−base reaction of H 3PO 4 with carbonate, respectively. Capillary electrophoresis (CE) and ion chromatography (IC) were used to measure the concentrations of formate, acetate, oxalate, malonate, succinate and lactate after wet oxidation. CE was performed on a Beckman-Coulter MDQ instrument using a diethanolamine−molybdate buffer and a fused-silica capillary. IC was performed on a Dionex dual ICS 3000 instrument, equipped with eluent generators, electrolytic suppressors, and matrix elimination by ion-trapping.23,24 The carbonate and free hydroxide concentrations were measured using an automated potentiometric titration system,25 and the results in this work are the average of duplicate analysis. 2.7. Determination of Flammability Limits. The Federal Institute for Materials Research and Testing (BAM, Germany) was commissioned to determine the flammability regions of the H2−O2−H2O/N2 systems under high temperature (140 and 280 °C) and high pressure (3.6 and 54.8 atm) conditions. These measurements followed the procedures specified in the European standard EN 1839(B) “Determination of explosion limits of gases and vapours”. The ignition source, the criterion for an explosive mixture, and the step size in hydrogen concentration were in accordance with the requirements of the standard. Gas mixtures were prepared in a separate mixing vessel and then transferred to a stainless steel autoclave for the determination of the explosion limits. The gas compositions representing the explosion limits were validated by five repeat tests.
the concentration of residual oxygen in the headspace was below 0.01 mol %. This procedure ensured that reproducible oxygen-deficient conditions were created prior to heat-up, to minimize the possibility of oxygen-induced degradation reactions. For wet oxidation tests, the autoclave headspace was flushed with oxygen (>99.5%, BOC, Perth, Australia) for 30 s, then oxygen was added at ambient temperature to the required pressure (O2/TOC mole ratio was 0.5 for Bayer liquors and 1.0 for solutions containing model organic compounds). The liquor/solution was heated at a rate of approximately 5 °C/min, without stirring, to the designated temperature. The time at which the target temperature was reached was assigned to be time zero, at which point the stirring was activated at 700 rpm. 2.5. Gas Analysis. The hydrogen and oxygen concentrations in the headspace were measured using Orbisphere thermal conductivity and electrochemical-based sensors, respectively, which form part of a purpose-built wet oxidation facility. Each online sensor features a gas permeable membrane that regulates the gas flow into the sensor. The detection limit of the sensors is 0.01 mol % hydrogen or oxygen and their accuracy is ±1% of the reading. For each analysis, it was necessary to stop the reaction by cooling the autoclave to enable the hydrogen and oxygen content of the headspace gas to be measured. Each gas measurement therefore represents an independent autoclave run carried out for the nominated reaction time in the range 0− 5 h (i.e., the data entries in Figure 1A and B at a given TOC conversion represent individual experiments at a specified reaction time). After cool-down, the autoclave was pressurized with a known amount of nitrogen to establish a controlled flow of the headspace gas to the sensors. The internal pressure and temperature of the cooled autoclave were noted just before gas sampling. The measurements from the gas analyzers were obtained in mole percent and were converted to moles of hydrogen or oxygen using the general gas equation. Hydrogen molar yields were repeatable within ±0.01 mol of hydrogen formed per liter of liquor or per mole of organic compound used. The possibility of leakage of gases from the autoclave during experiments was checked in preliminary tests. These tests involved the measurement of a range of calibration gas mixtures and the direct measurement of hydrogen production from the reaction of aluminum wire in sodium hydroxide solution over similar timeframes.21 The results indicated that the loss of hydrogen by leakage was negligible (
