INCREASING THE REACTIVITY OF BAUXITE RESIDUE FOR ITS USE

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Increasing the reactivity of bauxite residue FOR its USE as building material: an alternative thermal ACTIVATION treatment

Lukas ARNOUT1, Tobias HERTEL1, Lucas Beirao DO VALLE2, Adrien NELIS2, Marcel DORMANN2, Theodoros karachalios1, Yiannis PONTIKES1

1 Department of Materials Engineering, KU Leuven, 3001 Heverlee, Belgium

2 CRM Group (Centre for Research in Metallurgy), 4000 Liege, Belgium

[email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected]

Abstract

An alternative process is suggested herein, aiming to transform bauxite residue into a reactive material that can be used as an inorganic polymer binder. For this, bauxite residue with minor additives was granulated and sintered in a sinter pot, conventionally used for sintering of iron ores at pilot scale. The resulting material had a semi-vitreous microstructure and the first results on mortar samples are promising. This proof-of-concept study demonstrates that it is conceivable to have a reactive precursor based on bauxite residue, simply by incorporating a robust, industrially-proven process, of relatively low complexity and cost.


Introduction

Bauxite residue (BR) is mainly unreactive as a binder material. The residue is sparingly soluble in water and in an alkaline solution1, and the possibility of using it as an active binder, for instance as a cement or as an inorganic polymer in construction, is thus limited.


To address the above, several transformations of BR are suggested, for instance, thermal treatments2 or a combined thermal and chemical treatment3 in order to increase its solubility and thus reactivity. It was demonstrated that a mix consisting of BR, carbon and SiO2 (and/or CaO; predominantly in the FeO – Al2O3 – SiO2 – CaO system) melts at lower temperatures compared to BR only (predominantly Fe2O3 – Al2O3 – SiO2 – CaO system). Following melting, quenching is performed and a semi-vitrified material is produced. The latter demonstrated chemical reactivity in an alkaline environment: the final products were alternative binder products with satisfactory mechanical properties.3,4


The bottleneck of such processes is the heat treatment step which is accompanied by high energy requirement, cost and environmental footprint. Alternative heat treatment technologies, especially for waste and residue valorisation, are therefore much in demand in order to lower the footprint of such a processing step.


One candidate is the so-called (iron ore) sintering process,5 which is a technique conventionally used to agglomerate iron ore fines and concentrates in the presence of fluxes into strong and reducible particles. These can be fed into blast furnaces (BF) guaranteeing enough permeability inside the furnace. Iron ore sintering is applied worldwide in most of the integrated steel plants and its principle dates back to the early 1900s with the first Dwight-Lloyd travelling grate continuous machine. Iron ore sintering can be illustrated as a burning cigarette: the raw material is ignited on the top by an external energy source (ignition hood) and the air is sucked downwards while the flame progresses until the bottom of the layer. The goal is to heat up the raw material to temperature up to 1250 to 1400 °C, inducing partial melting, followed by rapid quenching (Figure 1).


INCREASING THE REACTIVITY OF BAUXITE RESIDUE FOR ITS USE

Figure 1: Illustrative picture of the sintering process

An industrial operation is illustrated in Figure 2. The most important points of the industrial process are highlighted below:


INCREASING THE REACTIVITY OF BAUXITE RESIDUE FOR ITS USE

Figure 2: Illustrative image of industrial Dwight-Lloyd travelling gate type of iron ore sintering

In the conventional sintering process of iron ores, a carbon source (also called solid fuel, such as coke breeze and anthracite) is added mainly as energy source in order to promote the melting of oxides and to create the aimed product. The carbon is burnt in the excess of O2, decreasing the local pO2 in the flame front allowing the partial melting of raw materials, solid state reactions and the reduction of part of the Fe2O3. Just after the flame front, the material is quenched by air. Normal operations use about 40-80 kg of solid fuel per kilo of sinter.


In a proof-of-concept study, batch experiments (similar to that displayed in Figure 1) were performed at pilot scale in the CRM Group Liège sintering pilot station. A mix consisting of BR and additives, such as coke breeze (used as carbon source), was treated in the sinter pot. The resulting material was ground and alkali-activated using a potassium silicate solution to form a so-called inorganic polymer, an alternative building material. The mechanical properties of the resulting products were assessed.


Material and Methods

Bauxite Residue from RUSAL Auginish was microwave-oven dried to a residual moisture of 5 wt% in order to ease handling. The chemical and mineralogical composition of BR was determined using X-ray fluorescence and X-ray diffraction, respectively.


Based on thermodynamic calculations (not presented herein) a mix of 88 wt% of BR, 2 wt% of carbon (added as coke breeze 0 – 5 mm), 5.2 wt% of sand (99.4 wt% SiO2) and 4.8 wt% of lime (93.7 wt% of CaO) was chosen in order to maximise the formation of melt phase during a heat treatment. In order to provide enough energy to guarantee water evaporation, partial melting and reduction of Fe2O3 during furnace trial, 10 wt% of additional cokebreeze was added.


The mixing and granulation of the mix was carried out in order to assure a certain homogeneity and permeability in the bed. A vertical intensive mixer was used for the pilot sinter pot tests. The raw materials were humidified and homogenised, and then granulated with a total of about 200 g water per kg mix (corresponding to about 17-19 % final moisture), in order to form granules.


Based on conventional iron ore working conditions, some of the parameters were adapted in order to promote an increased melt formation. The process parameters of the sinter pot pilot trial are listed in Table 1.


Table 1: Process Parameters for Sinter Pot Pilot Trial


Moisture after granulation (%)

17 -19

Bed height (cm)

30

Ignition Time (min)

2

Underpressure (hPa)

78.5

Solid fuel dry – coke breeze (%)

10

Sintering time (min)

16.5

Charged Density (kg/dm3)

~ 1.14


The granuled mix was manually charged into the sinter pot, followed by ignition using a mix of natural gas (37 MJ/m3). After the trial, the product was drilled from the centre of the cake for sampling. After milling, the sample was used as solid starting material for the synthesis of inorganic polymers mortars. The blend consisted of 550 g of milled slag, 1400 g of an industrial Cu slag (90 wt% glass content) as aggregate (ground to < 1 mm) and 234 g of activation solution (SiO2/K2O of 1.5 and a water content of 57.1 wt%). The liquid to solid ratio was 0.425 and the aggregate to slag ratio was 2.55; both ratios were kept constant. The mortar samples were prepared by mixing the ingredients for 5 min in a Hobart mixer, followed by casting in 4 x 4 x 16 cm3 prismatic moulds, and then compaction in a jolting table. Curing took place under ambient conditions and compressive strength testing was performed at 2, 7 and 56 d. Another set of samples was cured for 2 d at 50 °C and tested at 7 d.


Results

Characterisation of BR

BR is a residue rich in iron oxides (44 wt% as Fe2O3) with moderate levels of alumina (19 wt%) and silica (12 wt%) as well as titania (9 wt%), soda (9 wt%) and minor calcia (6 wt%). Its mineralogy is mainly dominated by Fe3+ containing phases, such as hematite and goethite, next to desilication minerals (cancrinite and katoite) and unreacted or precipitated alumina hydrates (such as gibbsite).

Observations of the Sinter Pot Experiment

The temperature profile measured during the sinter pot trials is presented in Error: Reference source not found. The temperature was measured experimentally inside the materials at three vertical positions of the sinter pot. The maximum temperature reached was about 1260 °C. It is noted that the surface temperature decreases very slowly after reaching 200 °C, which is an indication of a heterogeneous flame front propagation.


INCREASING THE REACTIVITY OF BAUXITE RESIDUE FOR ITS USE

Figure 3: Temperature profile of Sinter Pot Trial at different positions

Characterisation of the slag and production of alkali-activated materials

The content of glass (amorphous) of the produced sinterpot slag can be considered rather low (Table 6), which might be due to the heterogeneous burning behavior and thus to an insufficient reduction and cooling speed. This is further corroborated by the presence of various Fe+++ bearing minerals, such as maghemite and hematite, also indicators of an insufficient reduction reaction.


Table 6: Mineralogical composition of sinterpot slag.

Phase

wt%

Amorphous

14

Crystalline

86

Spinel solid solutions

27

Iron

1

Perovskite

11

Wüstite

3

Gehlenite

2

Nepheline

19

Maghemite

18

Hematite

4

Quartz

1


Despite the obvious room for improvement, satisfactory compressive strength of about 40 MPa was reached after 56 days (Figure 4), which is comparable to certain conventional cements mortars. Part of the strength gain may be attributed to the reactive nature of the aggregates, which are expected to participate in the reactions.


INCREASING THE REACTIVITY OF BAUXITE RESIDUE FOR ITS USE

Figure 4: Mechanical properties of produced alkali-activated mortars cured under ambient conditions

For the sintering process, several factors can be optimised related to carbon burning efficiency (as coarse solid fuel particles were found encapsulated inside the products). These are namely:

By improving the before quoted factors, higher temperatures might be possible during the heat treatment that will boost melt formation and slag vitrification.

Extrapolation to a possible industrial implementation

Based on the results obtained above in the pilot scale, the following approximate figures can be extrapolated for such a process on at industrial level (considering room for the 20% optimization):

Conclusions

A novel approach to treat bauxite residue (BR) by the well-established sintering process of iron ore, was used in this study as a way to enhance the reactivity of BR and enable its use as a binder material. The trials at the CRM sintering pilot station demonstrated the first proof-of-concept. After sintering, a semi-vitreous material was formed, and the first results on inorganic polymer mortars were promising. Several optimisation parameters are conceivable to further increase the reactivity of this secondary slag. Considering that this sintering process is accompanied with significantly lower opex and capex costs than other thermal processes, the research herein may pave the way for a real-life implementation. That being said, the work is on-going and more data are needed before any conclusions can be made.

Acknowledgements

This work has received funding from EIT RawMaterials, a European Institute of Innovation and Technology (EIT) Knowledge and Innovation Community (KIC) under the Project Agreement No. 16365 (RECOVER). Project website: https://recover.technology.


References

1. D.D. Dimas, I.P. Giannopoulou, D. Panias, "Utilization of alumina red mud for synthesis of inorganic polymeric materials", Mineral Processing and Extractive Metallurgy Review, 30 (3) 211–239 (2009).

2. N. Ye, J. Zhu, J. Liu, Y. Li, X. Ke, J. Yang, "Influence of Thermal Treatment on Phase Transformation and Dissolubility of Aluminosilicate Phase in Red Mud", in MRS Proceedings, 1488 (2012).

3. T. Hertel, B. Blanpain, Y. Pontikes, "A Proposal for a 100 % Use of Bauxite Residue Towards Inorganic Polymer Mortar", J. Sustain. Metall., 2 (4) 394–404 (2016).

4. T. Hertel, R.I. Iacobescu, B. Blanpain, Y. Pontikes," Large-scale Valorization of Bauxite Residue for Inorganic Polymers", in Proceedings of 34th International ICSOBA Conference, Québec City, Canada, 2016.

5. F. van Loo, J. F. Douce, E. Pirard, R. Pietruck, M. Martinez-Pacheco, "Improved Sinter Mix preparation while using challenging materials ", in Proceedings Metec & 2nd Estad, Düsseldorf, Germany, 2015.





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