Characterisation of scale products formed on the heat- exchanger surfaces in the Bayer process November 20 th, 2014 Lina Shi Dr. Nobuyuki Kawashima Dr.

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Presentation transcript:

Characterisation of scale products formed on the heat- exchanger surfaces in the Bayer process November 20 th, 2014 Lina Shi Dr. Nobuyuki Kawashima Dr. Ning Xu Dr. Jun Li Prof. Andrea Gerson

Bayer Process — background Australia has been the global leader of alumina production and the 5 th largest aluminium producer for the last decade –invented by Karl Josef Bayer in 1887 Gibbsite ( Al(OH)3) Alumina Bayer Process Bauxite ore

Bayer Process — Scale Incomplete desilication the presence of titanium

Bayer Process — background Scaling — a major technical problem Reduced volume capacity and heat-transfer efficiency Lost production De-scaling operations and OH&S issues Scaling in heat transfer & evaporation equipment Equipment downtime

Bayer Process — background Scaling — a major technical problem Scale Sodium Aluminosilicate & Titanate XRD EDS XANES Less thermodynamically stable phases more stable species –Amorphous ZeoliteSodalite Cancrinite Same At% of Al and Si in sodium aluminosilicate

Sample list Industrial Scale samples across a number of alumina refineries Quantity of depth resolved samples Scale A Heater, 200°C. Collected from the inner wall of heat exchanger tube after installing new tubes and running for 2 weeks. The thickness of the scale was approximately 2 mm on the tube wall. 12 Scale B 14 Collected on from the end caps of heater (194°C) Scale C 7 Slurry heater, 148°C.

XRD-Quantitative Phase Analysis Scale A Scale BScale C Cancrinite (Na 6 Ca 2 (Al 6 Si 6 O 24 )(CO 3 ) 2 ·2H 2 O 65  412  225  3 Sodalite (Na 8 (Al 6 Si 6 O 24 )(OH) 2 ) 8  2 Boehmite (AlO(OH)) 2  18  21  1 Nacrite Al 2 Si 2 O 5 (OH) 4 1  1 Perovskite (CaTiO 3 ) 6  213  2 Hematite (Fe 2 O 3 ) 2  16  21  1 Quartz (SiO 2 ) 5  2 Amorphous25  361  454  4

The aim of using XANES To identify the large amorphous components in scale samples obtained from XRD. To characterise and compare the scale sample at different depth from the wall

Experimental Methodology for XANES XANES spectra at Al K-edge have been collected on the Soft X-ray Spectroscopy beamline at the Australian Synchrotron. X-ray absorption spectra were recorded in partial electron yield (PEY) detection mode in the photon energy range eV (Al) with 0.1 eV steps and 0.5 s integration time, using a microchannel plate detector.

NameStoichiometries Source DiasporeAlOOHPurchased from e-bay and tested by XRD GibbsiteAl(OH) 3 KB-30, Alcoa World Alumina- Australia CorundumAl 2 O 3 Sigma-Aldrich, -100 mesh, 99 % pure Sodium aluminate NaAlO 2 Riedel-de Häen NordstranditeAl(OH) 3 Synthesised, (Peskleway et al. 2005) Peskleway et al KaoliniteAl 2 Si 2 O 5 (OH) 4 Unimin Limited Australia BoehmiteAlO(OH)Synthesised, (Dash et al. 2007)Dash et al SodaliteNa 8 Al 6 Si 6 O 24 (OH) 2 Synthesised, (Zheng 1997)Zheng 1997 CancriniteNa 8 Al 6 Si 6 O 24 (CO 3 ) 1.44 H 0.88 (H 2 O) 2 Synthesised, (Zheng 1997)Zheng 1997 Standard Samples for XANES

Standard samples - Al Tetrahedral Octahedral Distorted octahedral structure Three dimensional framework of corner linked AlO 4 tetrahedra

Scale A (Heater, 200°C)

Scale A-Quantitative Comparison XRD Results PhaseWeight % Cancrinite 65  4 Hematite 1  1 Boehmite 2  1 Perovskite 6  2 Amorphous 25  3 EDS Results (At%) ElementNaAlSiSCaFe bright contrast phase dark contrast phase Linear Fitting

Scale B (Heater,194°C) XRD Results PhaseWeight % Cancrinite 12  2 Hematite 6  2 Boehmite 8  2 Perovskite 13  2 Nacrite 1  1 Amorphous 61  4 EDS Results ElementNaAlSiPSCaTiFe bright contrast layer dark contrast layer Linear Fitting

Scale C- (Slurry heater, 148°C) XRD Results PhaseWeight % Quartz 5  2 Hematite 1  1 Cancrinite 25  3 Boehmite 1  1 Sodalite 8  2 Amorphous 54  2 Linear fitting EDS Results AreaNaAlSiSClKFe

Summary No significant trend in the concentration of any particular phase was found as the depth changes. The amorphous component in all these three scale samples has been demonstrated to contain a large proportion of poorly crystallised sodalite. Scale B may also contain some poorly crystallised cancrinite in amorphous component. The conclusion is not exclusive and there might be other phases in the amorphous that have not been confirmed.

Acknowledgement Beamline Team - Soft x-ray spectroscopy Dr Bruce Cowie Dr Lars Thomsen Dr Anton Tadich

Thank you