Use of Bauxite Residue (Red Mud) As CO2 Absorbent

Use of Bauxite Residue (Red Mud) as CO2 Absorbent

Paulo Braga1, Flavio Lemos1, Ronaldo dos Santos1, Christine Nascimento2 and Sílvia França1 1. Centre for Mineral Technology, CETEM, Brazil 2. Institute of Macromolecules of Rio de Janeiro Federal University, Brazil

ABSTRACT

The large amount of bauxite residue (red mud) -a highly alkaline by-product of the Bayer process of refining bauxite into alumina- has a relevant environmental impact and poses a challenge to the aluminum industry. In particular, the high alkalinity (pH~13) of red mud can cause adverse environmental consequences if no treatment is applied before final disposal. Although industrial plants wash the red mud for maximum recovery of caustic soda and soluble aluminum, the alkalinity remains high, reaching pH values in the range of 10 to 13. On the one hand, the high alkalinity of the red mud complicates its final disposal due to the need for pH adjustment. On the other hand, this characteristic makes red mud a potential material for absorption and neutralization of acid wastes, which is the objective of this research project. The use of red mud as an acid gas absorbent can be an alternative to mitigate the effects of greenhouse gases produced by fossil fuel power plants, benefiting the environment. This practice is already adopted industrially for desulfurization (SO2 removal) of combustion gases, and consists of promoting the contact of the slurry with the gaseous effluent in such a way that the gas passes countercurrent to the mud flow through a column equipped with perforated devices.

The main purpose of this study is to evaluate the technical feasibility of using red mud for CO2 capture from gas effluents generated by fossil fuel power plants, by evaluating the influence of process and design variables on the efficiency of this removal. In addition, we evaluate the influence of the captured CO2 on the amelioration of chemical characteristics of the red mud and possible environmental benefits for disposal.

INTRODUCTION

The generation of solid wastes and emission of CO2 are two grave environmental problems faced by the aluminum industry. The production of calcined alumina and primary aluminum is responsible for 90% of the CO2 emissions from the aluminum production chain. According to data from the Brazilian Association of Aluminum Producers (ABAL), although Brazil obtains most of its electricity from hydro generation, considered clean and renewable regarding CO2 emissions, the production of

1.0 metric ton of primary aluminum causes the emission of 2,661 kg of CO2. A significant volume of solid waste (generally called red mud) is also generated during the digestion of bauxite with caustic soda (NaOH). It is estimated that for each ton of alumina produced, the same quantity of red mud is generated. Although factories wash the red mud to recover NaOH and soluble aluminum, the alkalinity of the red mud discarded is still high, with pH values in the interval of 10 – 13. While this high alkalinity requires special precautions in storage in industrial landfills, this characteristic means the red mud has high potential to absorb and neutralize acidic effluents. This is the focus of the present study.

Bonenfant et al. (2008) found that the neutralization of CO2 by applying red mud occurs according to reaction equations (1) to (5).

NaAl(OH)4 + CO2 ⇄ NaAlCO3(OH)2 + H2O (1)

NaOH + CO2 ⇄ NaHCO3 (2)

Na2CO3 + CO2 + H2O ⇄ 2NaHCO3 (3)

3 Ca(OH)2·2Al(OH)3 + 3CO2 ⇄ 3CaCO3 + Al2O3·3H2O + 3H2O (4)

Na6[AlSiO4]6·2NaOH + 2CO2 ⇄ Na6[AlSiO4]6 + 2NaHCO3 (5)

The concern for proper disposal of red mud has prompted the conduction of various studies involving the use of this material to abate levels of acid gases. Table 1 lists patents covering processes employing red mud to abate gases, developed by different companies that produce alumina or aluminum. These processes rely on a variety of equipment, operational procedures and analytical techniques, hampering comparison of the results obtained. Besides the patents listed in Table 1, the literature contains many studies investigating the use of red mud in various applications.

Table 1 Patents of companies covering processes for neutralization, carbonation or abatement of gases using red mud.

Patent Year Holder Title

JP52094862 1977 Kogyo Gijutsuin Treatment of exhaust gas with industrial waste materials.

Removing method for sulfur oxides and nitrogen oxides in JP53113761 1978 Sumitomo exhaust gas.

Process for the removal of sulfur oxides from exhaust US4222992 1980 Sumitomo gases using slurry of red mud containing calcium ion.

JP56130224 1981 Sumitomo Removal method for carbonyl sulfide.

Alberta Chem. Fab. Process for eliminating acidic components from waste US4341745 1982 GMBH gases.

Aquatech Method for the multistage, waste-free processing of red US5053144 1991 Kernyezeteedelmi mud to recover basic materials of chemical industry.

Catalytic absorbent production from mixed inorganic and Inst. Energetik DE4102557 1992 organic waste - contg. catalytic heavy metal and carbon by GMBH pyrolysis and activation, used in waste gas purification.

WO93/16003 1993 Alcoa Process for the treatment of red mud.

Taiheiyo Cement JP2002011346 2002 Exhaust gas treatment agent. Corp.

WO 2005/ 2005 Alcoa Treatment of alkaline Bayer process residues. 077830 A1

Process for absorption of sulfur dioxide waste gas by CN1883766 2006 Guizhou University Bayer red mud.

This work investigates the neutralization of red mud with CO2 gas in a stirred reactor, to ascertain the influence of the variables percent of suspended solids in red mud, stirring speed, flow rate and temperature of the gas current. The gas utilized contained 10% CO2, similar to that generated in coal combustion processes.

METHODOLOGY The red mud used in the tests came from an alumina refinery located in northern Brazil and had the following contents: SiO2 19.1%; Al2O3 21.5%; Fe2O3 33.0%; CaO 1.2%; Na2O 9.4%; TiO2 4.7% and; LOI 10.8%. The pH value varied from 11.0 to 12.0. The gas current used was a synthetic mixture, prepared from compressed air and 99.9% carbon dioxide - grade 4.0, supplied by White Martins.

Figure 1 shows the experimental apparatus used in the bench tests of CO2 absorption, composed of: a Testo model 350-S flue gas analyzer (A); Digimed model DM-22 pH meter with TS-52P-AM probe and electrode (C), connected to a HP computer (B) for acquisition and treatment of data; gas heating system composed of an Inova model INV-1713 oven (G) and temperature controller (D); IKA Eurostar mechanical stirrer (E), with power of 130 W and stirring speed control of 50 – 2000 rpm; and a steel reactor (F) with capacity of 3 L and internal diameter of 15.7 cm.

Figure 1 Experimental apparatus used in the absorption tests.

The synthetic CO2 flow gas was prepared in a gas mixer equipped with two flowmeters fed by flows of compressed air and CO2. The flows of the gases were adjusted to attain the desired CO2 concentrations in the gas current. Two experimental designs were used, the first in two levels involving five independent variables and the second with two levels with central and axial points involving four independent variables. Table 2 gives the values of the independent variables used in the bench tests.

Table 2 Experimental designs used in the tests of CO2 abatement and neutralization of red mud.

Independent Variables

Design Solids CO2 concentration Stirring Gas flow Gas temperature content (%) (%) speed (rpm) (L/min) (°C) 30 20 1000 7.0 40 1 10 10 500 5.0 70 10 7 350 3.5 - 15 10 500 5.0 - 2 25 15 750 6.0 - 35 20 1000 7.0 - 50 30 1250 9.5 -

The response variables studied were absorption percentage and specific absorption capacity, determined according to the concentrations of CO2 at the input and output of the reactor, and the weight of red mud employed in the batch experiments, as described in equations (6) and (7).

CO  absorption (%)  2 waste x 100 (6) CO2  feed

weight of CO absorbed(kg) specific absorption capacity  2 (7) weight of red mud (t)

RESULTS AND DISCUSSION

The CO2 concentrations were measured continuously during the experiment at the outlet of the reactor, using a gas analyzer. This is a differential in relation to other experiments described in the literature, in which the mass of CO2 absorbed is determined indirectly.

Since the tests were performed with a fixed red mud weight and continuous addition of gas (CO2), it was necessary to set a standard time (2 min) for performing the statistical tests. In this interval, the mean CO2 concentration in the treated effluent was 4%, as can be observed in the in Figure 2. This value corresponded to abatement of about 50% of the CO2 contained in the initial gas flow.

Figure 2 Variation of CO2 concentration at the reactor output with absorption time.

We studied the influence of the variables on the CO2 abatement (absorption percentage) and red mud neutralization (absorption capacity). The Pareto diagrams in Figure 3 demonstrate that the content of

5 suspended solids is the most significant variable in these processes. A higher content of suspended solids favors CO2 abatement, while a reduction in suspended solids favors red mud neutralization.

The results obtained also are coherent in relation to the CO2 concentration in the feed effluent of the reactor: the lower the CO2 concentration, the higher the absorption percentage of alkaline carbon dioxide. On the other hand, the neutralization of red mud is favored by higher concentrations of CO2.

Although the increase in the stirring speed of the suspension favors both the absorption of CO2 and the neutralization of red mud, this parameter is only significant for abatement of CO2 within the limits set for the study. The statistical analysis also indicated that in the temperature range chosen (40 to 70 °C), this parameter was not significant either for absorption of CO2 or neutralization of red mud.

Figure 3 Pareto diagrams for absorption percentage and absorption capacity.

The statistical analysis of the second experimental design indicated that when maintaining the red mud at 25.4% solids content and processing it at a stirring speed of 754 rpm, with the CO2 concentration at 15% (v/v), the forecast CO2 absorption percentage would be 63.8%, as indicated in the profile graphs in Figure 4.

Figure 4 Profile graphs of values predicted for CO2 absorption percentage.

We plotted a second set of profile graphs for predicted values of red mud absorption capacity (Figure 5). With the same operational conditions described previously, the estimated absorption capacity was

16.8 kg of CO2 for each ton of red mud used.

Several studies have been published investigating the use of red mud to capture CO2, using different experimental apparatuses, operational parameters and analytical methods, making comparisons of the results hard to interpret. Table 3 succinctly describes the operational data and values for absorption capacity found in the literature.

Another important point is that the optimization of CO2 absorption requires different operational conditions than those for enhancing the neutralization of red mud, hence making it difficult to obtain maximized values of these parameters simultaneously.

Figure 5 Profile graphs of values predicted for CO2 absorption capacity of red mud.

Table 3 Comparison of results reported in scientific articles with the results obtained in this study

Absorption Researchers Operational Description Capacities Utilization of liquid CO2. Reaction time between 5 and 15 minutes. Shi (2000) 23.0 kg/t Suspensions with 45% solids. Did not optimize the contact between phases. Employed suspensions with Khaitan (2009) 21.0 kg/t 40% solids. Determination of CO2 concentration by the difference of partial pressure. Stirred reactor operating at atmospheric pressure and room temperature. Dilmore (2009) 36.9 kg/t Suspensions with 40% solids prepared from brine samples. Gas current containing 40% CO2. Direct measurement of CO2. Tests conducted at 20 oC and atmospheric pressure, with gas flow of 5 Bonenfant (2008) 41.5 kg/t L/min, stirring of 300 rpm. Determination of CO2 by analysis of total carbon. Result obtained after 16 hours of reaction. Utilized stirred reactor, with rotations of 500 to 1000 rpm. Concentrations CETEM 16.8 kg/t of CO2 from 10 to 30%. Tests conducted at 40 and 70 oC. Reaction time of 2 minutes. Direct measurements of CO2. The absorption capacity value obtained in this study is lower than those presented in Table 3. This can be attributed to the short reaction time, 2 minutes, which was probably insufficient to saturate the red mud. This supposition is supported by the result presented by Bonenfant et al. (2008), where the absorption capacity of 41.5 kg/t was obtained after reaction for 16 hours.

CONCLUSION

The use of red mud to abate CO2 is interesting because it attenuates the effects of both of these wastes generated by the aluminum industry. The main challenge to the industrial-scale neutralization of red mud is to develop simple apparatuses with large capacity, high mass transfer and low energy cost. The statistical analysis of the data allowed calculating an average absorption percentage of 63.8%, corresponding to a specific absorption capacity of 16.8 kg CO2/t.

The operational parameters that led the greatest absorption capacity were: CO2 concentration in the gas current of 15% (v/v); effluent flowrate of 6.1 L/min; reactor stirring of 1000 rpm; solids content of 25.4%; and effluent temperature of 25 oC. The model also led to prediction that a reduction of the gas flow to 3.0 L/min would enable an increase in the absorption percentage to 84%, with exponential decay of this parameter during the reaction interval.

REFERENCES

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Cooling, D. J.; Hay, P. S. & Guilfoyle, L. M. (2005) Treatment of Alkaline Bayer Process Residues. WO 2005/077830 A1.

Dilmore, R.; Lu, P.; Allen, D.; Soong, Y.; Hedges, S.; Fu, J. K.; Dobbs, C. L.; Degalbo, A. & Zhu, C. (2008) Sequestration of CO2 in Mixtures of Bauxite Residue and Saline Wastewater. Energy & Fuels, v.22, p. 343 – 353.

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