Red mud (bauxite residue) is an alkaline suspension that is the by-product of alumina production via the Bayer process. Its elevated annual production and the global inventory of red mud determine its valorisation. Granite can be used as a source of fluxing oxides for the ceramic industry, as can the flake-shaped waste generated during the flaming of granite. In this work, a set of ceramic pieces made of red mud and granite waste are prepared and characterised via X-ray diffraction, a hardness test, electron scanning microscopy, a leaching test, and determining open porosity, water absorption, bulk density and flexural strength of the samples. The main crystalline phases in the high-temperature fired products are hematite, pseudobrookite and anorthite; the presence of magnetite reveals their ferrimagnetic character. All samples present high mechanical properties. Leaching results are below critical levels established by regulations.
Red mud is the residue generated during the Bayer process, which produces alumina from bauxite. Due to the way the alumina is extracted, the red mud generated constitutes an alkaline solution with a pH between 10 and 13. The annual residue production is estimated to be over 100 million tonnes (Mtpa) and the global inventory reached an estimated 3.5 billion tonnes (Bt) in 2014 (
Nowadays, discharging red mud into the sea is avoided (
The great amount of mud generated during the process and its accumulated value determine the importance of its valorisation. Different solutions for using red mud have been presented (
The flaming of granite generates a flake-shaped residue which has barely been altered compositionally or morphologically, and thus these flakes could be used as a source of fluxing oxides, like feldspars, for the ceramic industry. The flakes are landfilled in the surroundings of the granite processing plant due to the perception that the risk of contamination is low. The production of granite waste can be estimated to be 22 Mtpa, taking in consideration that the amount of waste generated during industrial cutting and processing is 65% (
Choosing this material provides an outlet for a residue that is rarely used. Most research on the utilisation of granite waste is limited to the use of the slurry from the cutting/polishing processes or crushed granite itself as aggregate in the production of concrete and ceramic tiles (
In this paper, the use of red mud and granite flakes from flaming (a treatment that confers a rustic finish to the granite surface) is presented as an option for changing an unused residue which is usually landfilled around transformation points into a raw material for producing ceramic tiles with high mechanical properties. These properties will provide a high added value to the final products.
A set of samples (10 mm × 80 mm × 10 mm) were prepared using different quantities of the wastes taken as raw materials in this work: red mud from the Alcoa plant in San Ciprián (Lugo, Spain) and residues from the flaming of granite. The red mud was air dried and ground to obtain a fine dust with homogeneous granulometry. The granite flakes were milled in a Fritsch Pulverisette 7 ball mill with tungsten carbide jars, and later they were sieved in a 75 micron mesh.
For this work, the selected red mud/granite waste proportions (% w/w) were 50/50, 60/40 and 70/30. The green ceramic bodies were formed by uniaxially pressing the dry dust with a hydraulic Perkin-Elmer press of 15 Tn and under a load of 10 tons for 90 seconds.
The calcination treatment consisted of: a heating rate of 10 ºC/min, from room temperature to 573 ºC; a soaking time of 10 minutes; a heating rate of 10 ºC/min to maximum firing temperature; a soaking time of 30 minutes; a cooling rate of 5 ºC/min to 573 ºC; a soaking time of 5 minutes; free cooling from 573 ºC to room temperature. Maximum firing temperatures were: 1100 ºC, 1150 ºC, 1160 ºC and 1170 ºC. For the hardness test and scanning electron microscopy (SEM), the ceramic pieces were polished to a specular finish using a Buehler Phoenix Beta automatic polisher at 500 rpm for 5 minutes with each cloth and diamond paste (9 micron to 1 micron).
The specimens were named according to the percentage of red mud in the paste: LG50, LG60 and LG70.
A flexural strength test was conducted following the method described by ISO standard 10545-4 (
Vickers hardness measurements were obtained in accordance with the UNE-EN 843-4:2005 standard (
The chemical composition of the raw materials was determined by x-ray fluorescence (XRF), using a 4 kW sequential wavelength-dispersive X-ray spectrometer (Philips Magix PRO PW-2440).
In the case of the red mud and the ceramic products, powder x-ray diffraction (XRD) was performed on a Bruker D8 diffractometer, using MoKa1 radiation (0.907300 Å) and a LYNXEYE XE detector. The recorded diffraction angle was from 1º to 35º (2q). For the granite residue, a PANalytical X’Pert PRO MPD diffractometer was used, with CuKα1 radiation (1.5406 Å) and an X’Celerator RTMS detector. The recording was done with a diffraction angle of 10° to 80° (2θ).
Water absorption, open porosity and bulk density were evaluated according to the UNE-EN ISO 10545-3 standard (
The morphology of the raw materials and the final products was analysed via SEM using a JEOL JSM-6490 LV, with a Si(Li) Oxford Inca X-Sight detector for energy dispersive x-ray spectroscopy (EDX). Prior to analysis, the samples were gold-sputtered in order to avoid surface charging.
The leaching test was performed according to the UNE-EN 12457-4 standard (
The raw material compositions as determined by XRF were presented in
X-ray fluorescence of the starting materials (%)
Fe2O3 | Al2O3 | TiO2 | Na2O | SiO2 | CaO | P2O5 | MgO | K2O | Pb2O5 | MnO | |
---|---|---|---|---|---|---|---|---|---|---|---|
47.35 | 19.99 | 9.81 | 8.31 | 7.42 | 6.16 | 0.45 | 0.33 | 0.11 | – | 0.07 | |
3.69 | 14.19 | 0.66 | 3.16 | 67.73 | 4.18 | – | 0.77 | 5.32 | 0.30 | – |
The XRD analysis in
Powder X ray diffraction for red mud (A) and granite waste (B). Ab: albite, Ano: anorthoclase, Bt: biotite, Bhm: boehmite, Cal: calcite, CAFS: calcium aluminium iron silicate hydroxide (Ca3AlFe(SiO4)(OH)8), Gth: goethite/aluminous goethite, Hem: hematite, Mag: magnetite, Ms: muscovite, NAS4: sodium aluminium silicate (Na4Al2Si2O9), NAS6: sodium aluminium silicate (Na6Al6Si10O32), Ntd: natrodavyne, Or: orthoclase, Qz: quartz, Rt: rutile.
The generation of open porosity depended on both composition and temperature, as
Water absorption (solid line) and linear shrinkage (dashed line) according to the final firing temperature. Inset: Open porosity according to the final firing temperature. Black line: LG50; Dark grey line: LG60; Light grey line: LG70.
Water absorption was directly related to the porosity developed during sintering. It can, therefore, be considered a simple way to predict the technological properties of the final products. These results match those for bulk density, except for the case of LG50, as shown in
Bulk density according to the final firing temperature, g/cm3. Black line: LG50; Dark grey line: LG60; Light grey line: LG70.
Soaking times were introduced in the firing process at 573 ºC to 1) facilitate the polymorphic transformation of quartz from phase α to the more reactive phase β during the heating process and 2) minimise the stress originating during the cooling process from the reduction of volume due to the transformation of quartz from phase β to phase α. Thus, the presence of a more reactive phase of quartz led to the best densification of the specimens by generating a greater amount of glassy phase, while mechanical behaviour was improved by the reduction of stress during the cooling process.
One of the most representative properties of the mechanical behaviour of a ceramic material is flexural strength. The densification of samples led to better mechanical behaviour, with higher flexural strength (
Flexural strength according to the final firing temperature, N/mm2. Black line: LG50; Dark grey line: LG60; Light grey line: LG70.
From the results, it can be deduced that the specimens with better mechanical properties (high flexural strength and low water absorption) were those that were sintered at 1150 °C and above. Moreover,
Vickers hardness values for all specimens were 440-550 kgf/mm2 (4.3-5.4 GPa), as shown in
Vickers micro-hardness values of the specimens at 1150ºC, 1160 ºC and 1170ºC (kgf/mm2)
Temperature | 1150 ºC | 1160 ºC | 1170 ºC |
---|---|---|---|
504.1 | 513.5 | 519.0 | |
545.6 | 507.5 | 449.9 | |
446.9 | 475.3 | 354.8 |
The mineralogical composition was determined by X-ray diffraction. The main phases for the three studied compositions were: hematite, pseudobrookite, magnetite, sodium anorthite, rutile, nepheline and quartz. A semiquantitative analysis of the fired samples is presented in
Albite + Calcite + Quartz = Anorthite
Na-feldspar + K-feldspar = Nepheline
Biotite + Calcite + Quartz = Augite + Alkali-feldspar
Biotite + Calcium iron aluminium silicate + Quartz = Augite + Alkali-feldspar
Semiquantitative analysis of fired samples (%)
Temperature (ºC) | LG50 |
LG60 |
LG70 |
|||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
1100 | 1150 | 1160 | 1170 | 1100 | 1150 | 1160 | 1170 | 1100 | 1150 | 1160 | 1170 | |
43.3 | 43.2 | 42.0 | 41.5 | 38.8 | 39.4 | 37.1 | 35.4 | 28.2 | 26.9 | 23.8 | 23.2 | |
18.0 | 20.2 | 21.0 | 22.3 | 20.9 | 23.6 | 24.1 | 26.0 | 28.0 | 29.5 | 29.1 | 27.7 | |
6.2 | 5.1 | 5.5 | 5.4 | 6.7 | 5.4 | 6.1 | 6.2 | 10.4 | 10.9 | 11.1 | 12.3 | |
8.2 | 9.2 | 9.4 | 10.5 | 9.4 | 10.6 | 12.2 | 13.7 | 9.2 | 9.2 | 10.1 | 12.0 | |
12 | 11.3 | 10.8 | 10.0 | 10.8 | 9.6 | 8.2 | 7.3 | 8.2 | 6.4 | 5.9 | 5.2 | |
3.3 | 2.9 | 2.8 | 3.1 | 4.3 | 2.9 | 3.1 | 3.2 | 4.1 | 3.3 | 3.6 | 3.9 | |
7.9 | 8.2 | 8.5 | 7.2 | 9.1 | 8.6 | 9.1 | 8.4 | 11.9 | 13.8 | 16.4 | 15.6 |
The presence of magnetite in the final products was evidence of their ferrimagnetic character, as magnetite is a ferrimagnetic mineral itself. Even though magnetic properties were not an object of study in this work, the samples were subject to an empirical test, proving that they were attracted by a magnet. The content of magnetite in the fired products varied from 5.1% to 12.3%, as presented in
Although it could not be identified in the diffractions, ilmenite might have been formed during the firing. The development of ilmenite in the process is essential to explaining the presence of pseudobrookite in the fired samples, as other researchers have found previously (
The formation of ilmenite from hematite and rutile happened during the heating stage, where a local reducing environment around the samples was created by the diffusion of air to the upper part of the furnace and helped by the release of CO2 from the decomposition of carbonated phases (calcite and natrodavyne). When the temperature increased, ilmenite decomposed. During the cooling stage, the environment around the samples became oxidising and magnetite and hematite were generated from wustite.
Hematite + Rutile = Ilmenite + O2 (during the heating stage)
Ilmenite = Wustite + Rutile (during the heating stage)
Wustite + O2 = Magnetite + Hematite (during the cooling stage)
Scanning electron microscopy shows the morphological and compositional heterogeneity of the samples.
6FeO(OH) → 2Fe3O4 + 3H2O + O2↑
3Fe2O3 → 2Fe3O4 + 1/2O2↑
A-D: SEM image (SEI) of specimens LG50 and LG70 at 1150 ºC and 1170 ºC (X100). Scale bar: 100 µm. Inset: BEC image of detail (X2000). Scale bar: 10 µm. Open pores marked by arrows. E: BEC image of fracture view (X3000) of specimen LG60-1150ºC. Points show where the EDX analysis were performed.
These results can be related to the water absorption and bulk density results. The shape of the pores varied with composition and temperature, and thus the more granite there was in the paste or the higher the firing temperature was, the more rounded the pores were. The presence of different phases was demonstrated by the different tones of the backscattered electron compositional images (BEC) in
Point EDX analysis of selected specimens at 1150 °C and 1170 ºC (Atomic%). ND: not detected
O | Na | Mg | Al | Si | K | Ca | Ti | Fe | |||
---|---|---|---|---|---|---|---|---|---|---|---|
48.74 | 2.32 | ND | 10.88 | 3.32 | 0.40 | 0.35 | 1.11 | 32.87 | |||
57.01 | 4.54 | 0.02 | 10.27 | 21.14 | 0.59 | 3.50 | 0.43 | 2.50 | |||
45.58 | 2.26 | ND | 14.08 | 3.76 | 0.57 | 3.77 | 4.47 | 25.51 | |||
65.61 | 2.14 | 0.37 | 4.98 | 1.59 | ND | 0.44 | 1.70 | 23.18 | |||
44.92 | 2.94 | 0.58 | 11.23 | 8.80 | 0.34 | 6.86 | 3.15 | 21.17 | |||
34.52 | 1.38 | 0.33 | 3.15 | 2.09 | ND | 0.87 | 19.62 | 38.05 | |||
60.03 | 1.16 | 0.10 | 7.44 | 4.72 | 0.07 | 1.56 | 9.10 | 15.84 | |||
58.33 | ND | ND | 0.06 | 41.25 | 0.01 | 0.04 | ND | 0.30 | |||
53.45 | 7.32 | 0.43 | 10.36 | 24.25 | 1.35 | 2.49 | ND | 0.35 | |||
45.54 | 3.17 | 0.19 | 4.43 | 15.03 | 0.77 | 11.41 | 7.17 | 12.27 | |||
53.37 | 3.36 | 0.79 | 4.39 | 6.54 | 0.38 | 0.45 | 1.72 | 29.00 | |||
53.61 | 0.40 | 3.07 | 4.54 | 7.27 | 0.40 | 1.47 | 11.01 | 18.23 |
According to the EDX and XRD data, in specimen LG50 fired at 1150 ºC, area 1 was compatible with an aluminium-hematite or hematite supported in aluminium silicate matrix. However, area 2 was characterised by a high content of silicon, much higher than aluminium, so it was composed of a vitreous matrix and some quartz. At 1160 ºC, areas 1 and 2 corresponded to hematite particles with different structures, area 3 seemed to be an aluminium silicate (anorthite) with the presence of hematite, and area 4 presented a composition similar to that of pseudobrookite. When this composition was fired at 1170 ºC, area 1 was rich in iron and the amount of titanium could guide the presence of pseudobrookite and rutile. Area 2 was constituted by silicon and oxygen, which corresponds to quartz. In LG70 at 1150 ºC, area 1 was constituted by aluminium silicates (anorthite and nepheline) and was enriched in silicon, which may indicate the presence of quartz. The Ti/Fe relation in area 2 indicated the presence of pseudobrookite in an aluminium silicate matrix. At 1170 ºC, area 1 for LG70 showed hematite supported in a nepheline/anorthite matrix; while area 2 corresponded to pseudobrookite with a presence of anorthite.
The leaching test determines material stability under ordinary environmental conditions and the release of hazardous elements. The most restrictive standard applicable for this case, the “Guideline for drinking-water quality” published by WHO (
This work has demonstrated that the valorisation of red mud is possible through ceramisation to obtain pieces with high physical and mechanical properties. Specimens with more granite content that have been fired at high temperature (>1150 ºC) show the best behaviour due to greater densification, since these are above their vitrification temperature. The main crystalline phases in the fired products are hematite, pseudobrookite, anorthite, rutile, nepheline and quartz; magnetite has been found in all cases, revealing their ferrimagnetic character. SEM-EDX analyses reveal that the specimens present a high degree of sintering and that they are composed of an aluminium silicate matrix that contains small particles and some aggregates of iron and titanium rich phases. ICP-MS of the leachates generated by the materials shows that their release does not involve health or environmental risk, and the stability of the products working under ordinary environmental conditions is guaranteed.