Effect of Thermal Treatment of Bauxite Ore on Carbon - American

Dec 22, 2012 - College of Biology, Chemistry and Environmental Sciences, Northern Caribbean University, Mandeville, Jamaica, W.I.. Department of Scien...
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Effect of Thermal Treatment of Bauxite Ore on Carbon (Organic and Inorganic) Content and Solubility in Bayer Process Liquor Max A. Wellington* College of Biology, Chemistry and Environmental Sciences, Northern Caribbean University, Mandeville, Jamaica, W.I. Department of Science and Engineering, Atlantic International University, Honolulu, Hawaii 96813, United States S Supporting Information *

ABSTRACT: Bauxite inputs into the Bayer process at a Jamaican refinery were found to contain approximately 0.4% inorganic carbon (or 3.2% CaCO3) and 0.2% organic carbon. Thermal treatment or heating of 10 g bauxite samples for 30 min at temperatures ranging from 300 to 1000 °C was shown to significantly reduce levels of both organic (up to 93%) and inorganic carbon (up to 100%); however, there was an accompanying noticeable decrease in solubility of bauxite ore calcined at temperatures above 300 °C based on predigestion alumina/caustic (A/C) ratios in spent liquor at 99 °C. X-ray diffraction and Xray database (XDB) spectral analysis of the calcined bauxite (>600 °C) demonstrated the appearance of corundum (α-alumina), a form of aluminum oxide much less soluble than boehmite or gibbsite.

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INTRODUCTION Carbon in bauxite ore occur mainly in the form of organic compounds or as inorganic carbonates. Both forms can have deleterious effects on Bayer process operations used to produce smelter grade alumina from bauxite ore. Jamaican bauxite which is comprised mainly of gibbsite (Al2O3·3H2O) and boehmite (Al2O3·H2O) and which accounts for approximately 9% of world bauxite production1 has been reported to contain calcite or limestone2,3 and varying concentrations of iron, titanium, and silica based mineral oxides4,5 as well as organic carbon.6−8 Organic carbon levels in bauxite ore can impose severe problems during its processing in the Bayer Process to produce alumina. Due to the high temperatures, pressures, and the cyclical and oxidizing nature of the process, most of the bauxite organic compounds decompose eventually to form sodium oxalate which can negatively impact the size and morphology of the alumina crystals produced in the precipitation circuit.6,9,10 Also the carbonation of the process streams by inorganic carbonates such as calcite can significantly reduce alumina extraction efficiency and productivity.3 Losses from the alumina refineries due to organic and inorganic carbon in bauxite ores can be tremendous and as such the exploration of technologies that can serve to reduce these are cogent. This paper examines organic and inorganic content as well as the thermal treatment of bauxite ore used in a Jamaican alumina refinery to reduce organic and inorganic carbon levels and the potential impact on its subsequent processability in the Bayer process to produce smelter grade alumina.

random days for over a period of about 1 month for 12 months from the process feed and size fractionated using the following wire sieves: 3, 2, 1, and 1/2 in. All stones that collected on the sieves were washed and dried in an oven at 110 °C for 2 h and thereafter weighed and their weight calculated as a percentage of the weight of the bauxite used. The −1/2 in. fraction (devoid of large stones) was collected and dried in oven at 110 °C for 2 h and pulverized, and a subsample of each monthly composite analyzed in duplicate for percentage inorganic and organic carbon using a Leco Sulfur−Carbon Dual Range Analyzer. The percentage inorganic carbon in the −1/2 in. fraction was expressed as percent CaCO3 by multiplying by a factor of 100/ 12 (assuming that sample inorganic carbon was all due to limestone). Determination of Percent Carbon in Bauxite Using a Leco Sulfur−Carbon 144 Dual Range Analyzer. Approximately 0.5 g of the finely ground bauxite sample was weighed accurately into two Leco ceramic crucibles one of which contained a Leco Nickel liner to prevent leakage. Approximately, 2.0 mL of phosphoric acid (10% v/v) was added to the one with the nickel liner, and the sample was allowed to sit for at least 10 min (to allow the decarboxylation and purging of inorganic carbon as CO2) and then placed in an oven at 110 °C for 90 min to dry. Both samples were then subsequently analyzed by combustion in the furnace (∼1300 °C) of the Leco analyzer and the percent carbon was measured by infrared detection of the CO2 released from the sample. The result obtained for the phosphoric acid treated sample represents percent organic carbon whereas the untreated sample gave the total carbon in the bauxite sample. The percent inorganic carbon was obtained by subtracting the percentage organic carbon from the

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EXPERIMENTAL SECTION Bayer Process Materials. All bauxite and process liquors used in this work were obtained from a local alumina refinery in Jamaica. Determination of Percent Limestone in Bauxite Fractions. One gallon bucket equivalents of monohydrate (boehmite) and trihydrate (gibbsite) bauxite were collected and weighed on © 2012 American Chemical Society

Received: Revised: Accepted: Published: 1434

September 5, 2012 November 27, 2012 December 21, 2012 December 22, 2012 dx.doi.org/10.1021/ie3024005 | Ind. Eng. Chem. Res. 2013, 52, 1434−1438

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Table 1. Effect of Roasting Bauxite on Percent Organic and Inorganic Carbon (CaCO3) Content as well as on Predigestion in Process Liquora digestion at 99 °C for 1 h in liquor temp °C

org carbon %

CaCO3 %

LOI %

alumina

caustic

soda

A/Cb

C/Sc

110 300 400 500 650 750 850 1000

0.14 0.11 0.08 0.07 0.06 0.06 0.05 0.01

3.25 2.66 2.75 2.83 2.63 0.67 0.25 0.00

9.06 21.88 23.44 25.35 26.37 26.88 27.59

124.08 125.17 116.15 113.49 110.81 107.45 106.80 105.17

243.45 244.76 247.24 248.02 248.48 248.13 247.43 256.01

329.67 328.13 331.08 332.85 333.56 333.19 332.21 339.22

0.5097 0.5114 0.4698 0.4576 0.4460 0.4330 0.4316 0.4108

0.738 0.746 0.747 0.745 0.745 0.745 0.745 0.755

a Initial spent liquor parameters: alumina (105.8 g/L); caustic (254.5 g/L); soda (337.6 g/L); sodium carbonate 83.1 g/L; A/C (0.4145); C/S (0.754). bA/C = alumina/caustic. cC/S = caustic/soda.

percentage total carbon of each bauxite sample. The inorganic carbon was expressed as percent CaCO3. X-ray Diffraction (XRD) and X-ray Fluorescence (XRF) Analysis of Bauxite. The dried, pulverized sample was tumbled in a 250 mL polyethylene bottle, and a 5 g subsample was placed in a spex cap and pressed into a pellet at 20 000 psi using a briquetting sample press. The pellet was then analyzed using XRD (Phillips) and XRF (Phillips). A NIST698 Bauxite standard was run before each sample to ensure the veracity of the results. The samples were analyzed for Trihydrate Alumina (Gibbsite), Monohydrate Alumina (Boehmite), Lattice Bound Alumina (LBA), Fe2O3, CaO, SiO2, P2O5, MnO, and TiO2. A composite of the bauxite analyzed was prepared and used in further experiments. The composite was also analyzed for percent organic and inorganic carbon using the Leco SC144DR Range Analyzer. XRD studies on the bauxite samples was performed at an accelerating voltage of 40 kV and a current of 40 mA. Diffraction patterns were collected at 5−70° 2θ using Cu−Kα radiation. The scans had a counting time of 10 s per step. XDB Phase Analysis of Bauxite. A 5 g subsample of the bauxite sample was placed in a spex cap and pressed into a pellet at 20 000 psi using a briquetting sample press. The pellet was then analyzed by X-ray diffraction (Phillips) and X-ray Fluorescence (Phillips), and the resulting spectra were then integrated using XDB (or X-ray database) software to yield the XDB phase spectra, using mass balances to refine the final phase composition.11 Bauxite Predigestion. The spent liquor used for this experiment was collected from the process stream and analyzed for alumina, caustic, and soda by Worsely/Metrohm titration. A 70 mL portion of the liquor was poured into a series of eight 250 mL leak-proof polyethylene bottles. These were then placed in an electrostatically controlled water bath at 99 °C and allowed to equilibrate for about 15 min. To each bottle (in duplicate) was added 10.0 g of trihydrate bauxite . The bauxite was slurried in hot spent liquor to a total volume 30 mL. The 30 mL hot slurry was added to the 70 mL spent liquor in the polyethylene bottles at 99 °C. The bottles were then shaken three times to allow mixing and then placed in the 99 °C water bath for 1 h, with agitation after which 12 mL of liquor was removed from each bottle and filtered using a 0.45 μm syringe filter. The filtered sample was then cooled and immediately analyzed for alumina, caustic, and soda using a Metrohm autotitrator. Alumina, Caustic, and Soda Analysis. Analyses was by the Worsely/Metrohm titration method.12

Approximately 10 mL of the sample to be analyzed was placed in a sample tube and placed on the autosampler of the Metrohm Autotitrator. A 2 mL portion of the sample (pH 12.2) was automatically pipetted by the instrument and analyzed by titration with a 0.5 N HCl. A 40 mL portion of a 400 g/L sodium gluconate (Jungbunzlaeur brand) at pH 8.30 was automatically added after the free caustic titration (∼pH 10.5). Half of the soda is then titrated (∼pH 8.1), and alumina, caustic, and soda are determined by gram equivalence plot. All solutions were made up using Millipore water (conductivity 0.054 μ Siemens). Calcination of Bauxite. Samples of 10 g of trihydrate bauxite were weighed into platinum crucibles, in duplicate, and placed in a Lindberg Blue Furnace oven at 1000 °C. The samples were allowed to calcine for 5, 10, 15, 30, 45, and 60 min after which they were removed, cooled in a desiccator, and then weighed to determine the optimum calcination time.. The test was repeated for 30 min with duplicate bauxite samples at 850, 750, 650, 500, 400, and 300 °C. The percent loss due to calcinations or ignition (LOI) was calculated and the percent organic and inorganic carbon determined using the Leco SC144DR Analyzer. Analysis was also done using XRD and XDB to examine the phase alumina composition in some of the samples.

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RESULTS AND DISCUSSION The calcite or calcium carbonate levels in bauxite feed randomly sampled from the Bayer process at a Jamaican alumina refinery was found to range from 0.9 to 7.1% with an overall average of 3.2% (Supporting Information). The calcite in the +1/2 in. fractions was determined by weight of the stones relative to the weight of the bauxite analyzed whereas the calcite in the −1/2 in. fraction was determined by thermal analysis. Calcite or limestone levels in bauxite can vary but can be controlled to some extent by mining practices and by screening of the bauxite feed.2 Calcite has the effect of reacting with caustic in Bayer liquors to form sodium carbonate. Sodium carbonate does not react with alumina ores and as such will reduce the effectiveness of Bayer liquor in extracting alumina during digestion. Decrease in the caustic to soda (caustic to sodium carbonate or C/S) ratio does not only affect alumina extraction from bauxite but also has a strong influence on precipitation performance. A direct impact of about 2.1 g/L yield loss for a 0.06 drop in C/S has been reported which translates to a significant amount on a daily basis.13 Further analysis of the −1/2 in. fraction of gibbsitic and boehmitic bauxite used as inputs to the alumina refinery 1435

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Table 2. Comparison of X-ray Spectral Analysis of Dried Gibbsite (XRD/XRF) and Calcined Gibbsite (XDB)

a

sample

% gibbsite

% boehmite

% LBAa

gibbsite (100 °C) gibbsiteb (1000 °C)

44.1 43.96

1.89 1.77

0.47 NRc

% corundum

% Fe2O3

% CaO

% SiO2

% MnO

% TiO2

0.18

18.15 18.62

1.92 2.07

2.17 2.15

0.38 0.37

2.49 2.48

Lattice bound alumina. bCorrected for loss in ignition (LOI). cNot reported.

Figure 1. X-ray diffractogram of trihydrate bauxite.

generation of excessive foam in the liquor process and increased scale formation.15 Predigestion of the roasted bauxite samples in Bayer process spent liquor showed differing alumina solubilities with the highest solubility being observed in the 300 °C treated sample as reflected by the highest alumina/caustic ratio of 0.5114. It is also observed that the caustic/soda (C/S) ratios increased in predigestion samples prepared using bauxites roasted at 300 °C and above. The increase in C/S was due to a relatively higher caustic levels of about 2−13 g/L in each sample and a reduced sodium carbonate level (soda minus caustic) of 1.2−3 g/L. One possible suggestion for this observation could be increased causticization of sodium carbonate due to the lime produced during roasting of the bauxite. Lime has also been shown to reduce chemical caustic soda losses by the formation of Ca−Alhydrosilicates from Na−Al-hydrosilicates and Ca-TiO3 from Na-Titanates.15 The significant increase in caustic levels (∼13 g/L) in the predigestion of the 1000 °C roasted bauxite could be due to the unreactive nature of the ore with respect to both alumina and silica16 as well as the disappearance of the rutile− TiO2 phase.17 The calcium oxide produced during the heating could readily react with water and any sodium salts present in the bauxite to form additional caustic soda, hence the increase when compared to the initial spent liquor. The characteristics of kaolinite or reactive silica [Al2Si2O5(OH)4] undergo changes at temperatures of 400 °C and above and as such could have

revealed no significant difference in the carbon content between the two bauxite types with both having an average organic carbon content of approximately 0.2% and inorganic carbon of approximately 0.3% or 2.5% calcite (Supporting Information). The organic levels compare well to the reported range of organic carbon in bauxites of 0.1−0.3%.6−8,14 No report was found that highlighted the levels of calcite in Jamaican bauxites. Table 1 shows the effect of temperature on the organic and inorganic carbon levels in gibbsitic bauxite obtained from a local alumina refinery. From the table, it is seen that the levels of inorganic and organic carbon in the bauxite decreased with increased temperature trending to zero as the temperature approached 1000 °C. The LOI or loss on ignition shows the respective percent weight loss in the sample associated with each temperature (300−1000 °C). Roasting of the bauxite ore invariably converts organics to carbon dioxide and calcite to calcium oxide or lime and as such should be more beneficial to the process. Organic impurities can cause a number of difficulties in the Bayer process such as lower alumina yield; generation of excessive fine alumina hydroxide particles; increased density, viscosity, specific heat, and boiling point of the sodium aluminate liquor; increased impurity content in the alumina; lower red mud settling rate; loss of caustic due to formation of sodium organic salts; coprecipitation of sodium oxalate with the product hydrate; 1436

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Figure 2. X-ray diffractogram of gibbsite calcined at 650 °C.

results of the thermal treatment further confirming the results obtained by XDB spectra. From this study, it was seen that roasting of bauxite to temperatures of 300 °C prior to predigestion slightly improved the alumina extraction and caustic losses. The roasting at 300 °C also reduced the organic carbon levels by 21% and calcium carbonate levels by 18%. However, given the current high cost of energy and the reduced demand for alumina on the world market the thermal treatment of bauxite prior to digestion may not be an economically viable option in Bayer process operations at this time.

become less reactive with the caustic thereby resulting in higher overall caustic values in the samples 400 °C and above.18 Kaolinite dissolves in the Bayer liquor and reprecipitates by combining with caustic and other impurities as desilication product (sodium aluminum silicate) or DSP which results in a loss of caustic and alumina from the Bayer process.19,20 Increasing the roasting temperature could concomitantly reduce the available reactive silica which would result in the increased caustic levels observed. Quartz, the other source of silica in bauxites, has been reported to not dissolve in Bayer liquor at temperatures below 180 °C.20 In order to keep the silica concentration in the liquor at an acceptable level the digestion stage is normally preceded by a predesilication step performed at a lower temperature (60−100 °C) for up to an hour to facilitate dissolving of the silica and its subsequent precipitation as DSP.21 Table 2 highlights some elemental differences between the dried bauxite and the bauxite roasted at 1000 °C. The major quantitative differences observed is the presence of corundum or α-alumina, and this could be the underlying factor responsible for the reduced alumina/caustic ratios observed in samples heated in excess of 400 °C. Alumina has been shown to undergo phase changes with increased temperature. From the figures presented it appears that corundum produced emanated from boehmite. This agrees with a previous report which concluded that α-alumina during heating was derived mainly from boehmite, while kaolinite is transformed to an amorphous phase during the ashing of bauxite bearing coal at 815 °C.22 Boehmite transformation was shown to occur via dehydration to form metastable γ-alumina, θ-alumina, crystal nucleation, and finally α-alumina or corundum crystal growth during heating to temperatures of 1000 °C and above.22 Figures 1 and 2 illustrate the XRD spectra of dried bauxite (oven heated to 100 °C for 30 min to remove moisture) and bauxite roasted at 650 °C. These spectra highlight the appearance of corundum or α-alumina in the sample as a

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ASSOCIATED CONTENT

S Supporting Information *

Table 1 and Figures 1 and 2. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or mwellington2000@ yahoo.com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Alumina Partners of Jamaica for their support and Mr. Robert Finlayson for assistance with the X-ray analyses

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REFERENCES

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