Nigeria is the highest consumer of aluminium in Africa but lacks bauxite deposits. The replacement of bauxite alumina with other alumina bearing resources (clays in particular) has been proposed. The present study investigated the thermal treatment required to activate Edda clay from southeastern Nigeria for optimal leaching of alumina. The clay is composed mainly of kaolinite and quartz, assaying 24.65% Al 2O 3 and 52.81% SiO 2. Thermal activation of the clay prior to leaching transformed the crystalline kaolinite mineral to an amorphous phase (metakaolinite) in which the alumina became soluble. Clay samples passing 300 μm sieve were calcined at temperatures of 500 °C, 600 °C, 700 °C, 800 °C, and 900 °C at holding times of 30, 60, and 90 minutes in each case. The uncalcined clay and sample heated at 1000 °C (for 60 minutes) were used in the control experiments. Leaching of alumina from the resulting clay calcines was done in 1 M hydrochloric acid solution at room temperature using a solid/liquid ratio of 0.02 g/ml and shaking speed of 100 rpm. The solubility data based on the percentage of Al ions taken into leach solution showed that the sample calcined at 700 °C (for 60 minutes) responded to leaching better than other samples. Samples calcined for 60 minutes at all temperatures studied were found to respond more than those held for 30 or 90 minutes. Studies on the activation energy of leaching revealed that calcines produced at 700 °C (for 60 minutes) had both the highest leaching response (49.96% after 1 hour at leaching temperature of 100 °C) and the lowest activation energy of 24.47 kJ/mol. It is concluded therefore that Edda kaolinite clay should be thermally activated for alumina yield by heating up to 700 °C and holding for 60 minutes. The clay deposit is therefore a potential alternative resource for alumina production.
The feedstock required to produce the metal aluminium in the Hall-Héroult process is alumina (Al2O3), which is normally refined from bauxite [
Despite the scarcity of bauxite in Nigeria, the demand for and consumption of aluminium in the country is the highest in the sub-region [
Kaolinite is structurally composed of sheets of silica tetrahedral layer ( Si 2 O 5 ) 2 − and aluminium octahedral layer Al 2 ( OH ) 4 2 + [
However, the alumina content of kaolinite (and other clay minerals) is chemically locked up in the clay structure; and becomes extractable or leachable after proper thermal treatment or activation [
Al 2 O 3 ⋅ 2 SiO 2 ⋅ 2 H 2 O ︸ Kaolin → Dehydration above 600 ℃ Al 2 O 3 ⋅ 2 SiO 2 ︸ Metakaolinite + 2 H 2 O ︸ Constitutional WaterRemoved (1)
4 ( Al 2 O 3 ⋅ 2 SiO 2 ) ︸ Metakaolinite → 950 ℃ − 980 ℃ 3Al 2 O 3 ⋅ 2 SiO 2 ︸ Primary Mullite + γ − Al 2 O 3 ︸ Gamma − Alumina ( fcc ) + 6 SiO 2 ︸ Amorphous Silica (2)
γ − 3 Al 2 O 3 ︸ Gamma − Alumina ( fcc ) + 3 SiO 2 ︸ Amorphous Silica → 1000 ℃ − 1400 ℃ 3Al 2 O 3 ⋅ 2 SiO 2 ︸ SecondaryMullite + SiO 2 ︸ Tridymite (3)
3Al 2 O 3 ⋅ 2 SiO 2 ︸ SecondaryMullite + SiO 2 ︸ Tridymite → 1400 ℃ − 1580 ℃ 3Al 2 O 3 ⋅ 2 SiO 2 ︸ Mullite + SiO 2 ︸ Cristobalite (4)
The goal of heating a clay before leaching is to produce metakaolinite ( Al 2 O 3 ⋅ 2 SiO 2 ), an anhydrous amorphous phase in which the alumina is soluble. Since clays from different localities vary considerably in mineralogy, chemical composition, degree of crystallinity and purity [
The raw clay sample was obtained from Edda clay deposit (Lat. 5˚57'54"N, Long. 7˚51'56"E) in Afikpo-South LGA of Ebonyi state, Nigeria. The clay was sun-dried, crushed and ground. The ground clay sample was placed on a 300 μm ASTM sieve and shaken for 5 minutes. The oversize was further ground and sieved on
the same sieve. The procedure was repeated until the entire clay sample passed through the 300 μm sieve (50 standard Tyler mesh).
The mineralogical and chemical compositions of the sieved clay were obtained by x-ray diffraction (Pananalytical XRD, Empyrean model) and atomic absorption spectrophotometery (Buckscientific AAS, model 210 VGP). The optimal clay calcine (which responded most to leaching) was also analysed with XRD to determine the effect of the selected thermal treatment on the transformation of the clay. The loss on ignition (LOI) was determined by placing one gramme of dry clay sample (dried at 105˚C for 12 hours in an oven) into a platinum crucible of known weight ( W d ). This was subsequently fired at 1000˚C for 1 hour in a muffle furnace. At lapse of time, the crucible was brought out of the furnace and reweighed ( W f ) after cooling down to room temperature. The LOI was calculated in percentage as ( W d − W f / W d ) × 100 .
Ground and sieved clay samples passing 300 μm sieve were subjected to thermal treatments in a muffle furnace at a heating rate of 15˚C per minute up to the holding temperature (500˚C, 600˚C, 700˚C, 800˚C or 900˚C) and held for 30, 60 or 90 minutes before furnace-cooling to room temperature. A sample of the raw or uncalcined clay and another sample heated up to 1000˚C (and held for 60 minutes) were used for the control experiments.
Analytical grade HCl acid and pure deionized water were used to prepare the leaching reagent. 1 molar standard solution of hydrochloric acid was prepared using deionized water. This was used as the leaching reagent for the leaching studies to determine the leaching response of the various clay calcines and the controls. The leaching was done under moderate leaching conditions, i.e. hydrochloric acid concentration of 1 M, leaching temperature at ambient (~25˚C), shaking speed of 100 rpm, and solid/liquid ratio or clay weight to acid volume ratio of 0.02 g/ml, using clay particles passing 300 μm (−50 mesh).
The leach contact time or duration ranged from 0 to 120 minutes at intervals of 30 minutes. An orbital shaker (KOMA, model KED11) was used to shake the reaction bottles at a constant rate of 100 rpm during the contact time (duration of leaching) to facilitate the reaction. By the end of leaching, the resulting slurry was filtered to separate undissolved materials (residue) from the filtrate (leachate or pregnant solution). The leachate was analysed for aluminium ion concentration by means of the Buckscientific AAS.
This procedure was repeated for all the clay calcines and the control samples, which were prepared at different temperatures and duration of calcination. The optimum condition for calcining the clay was determined using solubility data (concentration or fraction of aluminium ion solubilized).
After examination of the extent of aluminium extraction or solubilization from the clay calcines, the calcine with the best leaching response for each calcination temperature was subjected to activation energy studies by leaching them at different temperatures. This was done to establish the thermodynamic basis for their better leaching kinetics in accordance with the Arrhenius rate law (Equation (5)), in which k is the leaching rate constant, A is the Arrhenius frequency factor, E a is the activation energy for the leaching reaction, R is the universal gas constant (=8.314 J×mol−1×K−1), and T is the leaching temperature on the absolute scale.
k = A e − ( E a R T ) or ln k = − ( E a R ) ⋅ 1 T + ln A (5)
Consequently, samples held for 60 minutes at different temperatures of calcination were leached in 1 M HCl for 60 minutes at 25˚C, 50˚C, 75˚C and 100˚C by means of a constant temperature water bath. All other variables such as clay particle size, shaking speed, and solid/liquid ratio were kept constant at less than 300 µm (−50 mesh), 100 rpm and 0.02 g/ml, respectively. The natural logarithm of rate constants ( ln k ) as deduced from the solubility data (based on second-order reaction kinetics which appeared to fit the data) and the reciprocal of absolute
temperature ( 1 T ) were used for Arrhenius plots to determine the activation energy ( E a ) in the case of each clay calcine [
The x-ray diffraction pattern of the raw clay is shown in
amorphous metakaolinite ( Al 2 O 3 ⋅ 2 SiO 2 ) that releases its alumina ( Al 2 O 3 ) for dissolution making the clay respond to acid leaching. This behaviour, known as thermal activation, is due to proper thermal treatment and transformation of the clay [
The chemical composition is shown in
The extent of dissolution of alumina from the clay is given by the concentration of Al ions detected in leach solutions after leaching the various clay calcines. This was expressed in percentage or as a fraction. The fraction of Al ions ( X Al ) leached out of the clay into the solution was given as:
X Al = concentration of Al ions in the solution concentration of Al ions in the original clay sample (6)
Multiplication of Equation (6) by 100 gives the percentage of Al ions (%Al) leached out of the clay. The presence of Al 3 + ions in the leach solutions shows that a dissolution reaction occurred between the alumina in the clay calcine and the leaching reagent. It is proposed that hydrochloric acid ionized in water to produce protons or hydrogen ions ( H + ) in the hydrated form known as hydronium ( H 3 O + ) in accordance with reaction (7) [
HCl + H 2 O ⇌ H 3 O aq + + Cl aq − or HCl aq ⇌ H aq + + Cl aq − (7)
The dissolution of alumina ( Al 2 O 3 ) from the clay is therefore the result of chemical interaction between hydrogen ions ( H + ) and aluminium atoms of the alumina according to reaction (8):
Al 2 O 3 ⋅ 2 SiO 2 + 6 ( H + Cl − ) → ( 2 Al 3 + 6 Cl − ) +3H 2 O+ 2 SiO 2 (8)
The results of leaching showed that thermal treatment (particularly, calcination
Chemical Composition (%) | ||||||||
---|---|---|---|---|---|---|---|---|
SiO2 | Al2O3 | Fe2O3 | TiO2 | CaO | MgO | Na2O | K2O | LOI |
52.81 | 24.65 | 3.05 | 0.76 | 1.23 | 0.92 | 1.82 | 1.04 | 11.8 |
temperature) has significant effect on the amount of Al 3 + ions taken into solution. The effect of calcination temperatures at different durations of heating (30, 60 or 90 minutes) is illustrated in Figures 4-6. As shown, leaching response is lowest for the uncalcined clay sample (control 1, ~25˚C) followed by that calcined at 1000˚C (control 2) at all three durations of calcination. This means that the ability of the acid (1M HCl) to extract alumina from the clay (and enrich the leach solution with aluminium ions) varied as calcination temperature increased from 500˚C to 900˚C. However, at 1000˚C (control 2), the ability of the acid to attack the clay and remove alumina dropped drastically. Under 30, 60 and 90 minutes holding times (Figures 4-6); the peak leaching response occurred in the case of samples calcined at 700˚C. The response of Edda clay to leaching by acid attack increased and dropped between 500˚C and 900˚C; reaching its peak at 700˚C. This behaviour is probably due to the transformation of the clay from its room temperature crystalline form (kaolinitic) to various degrees of amorphous state and back to high temperature crystalline form at 1000˚C and beyond, as the clay steadily lost physically combined water (through drying and dehydration) and chemically combined water (de-hydroxylation) as temperature increased. In other words, the clay transformed completely to an amorphous phase (metakaolinite) when it was heated to 700˚C, while crystallinity or various degrees of crystallinity existed in the clay below and above 700˚C.
Another observation is that samples held for 60 minutes in the furnace responded better on leaching than those held for 30 or 90 minutes at all calcination temperatures investigated (
These observations are in conformity with XRD patterns shown in
As observed previously, clay calcines produced by holding for 60 minutes in the furnace (at all calcination temperatures studied) responded to leaching better than those held for 30 or 90 minutes. The effect of leaching temperature on the leaching response of calcines produced under 60 minutes duration of calcination is shown in
followed by a steady decline in Al extraction to 24.22% (for calcine produced at 1000˚C). Similar trends were also observed at leaching temperatures of 25˚C, 50˚C and 75˚C. This can be explained based on solid phase changes or transformations in the clay brought about by thermal treatment [
The activation energy of leaching was found to depend on the thermal treatment given to the clay. Thermal treatment itself determined whether the clay is in its hydrated crystalline form, anhydrous amorphous form, or anhydrous crystalline form, and the degree of crystallinity or otherwise.
The preceding discussion has shown that activation energy of leaching depended on the thermal treatment given to the clay.
As shown in
The effect of thermal treatment on the response of Edda kaolinite clay to acid leaching was investigated. Different thermal treatments produced different degrees of leaching response from the clay. X-ray diffraction studies confirmed that the optimal leaching response corresponded with the thermal treatment that transformed the clay to an amorphous phase or metakaolinite. The clay was transformed from its low-temperature crystalline form (below 600˚C) to an amorphous form (at 700˚C) and then back to high-temperature crystalline phases above 900˚C. Prolonged heating (above 90 minutes) under isothermal conditions also caused the appearance of crystalline phases and loss of solubility. Best results were achieved under 60 minutes heating duration at 700˚C. Thermal treatment also affected the activation energy of leaching such that high solubility (high leaching response) corresponded with low activation energy. Thus, the thermal activation of Edda clay for alumina leaching is possible and the clay can be used as an alternative resource for alumina in Nigeria where bauxite is scarce or non-existent.
Whereas the present work has established the optimal calcination conditions for Edda clay; further work is recommended to investigate the effects of the leaching variables, viz. clay particle size, acid concentration, leaching temperature, solid/liquid ratio of clay weight to acid volume, and shaking rate. This will establish the optimal leaching conditions for the clay calcine. Recovery of alumina from the leach solution is also recommended.
This work was funded through a research fellowship (REG/EST/SPMU/SP.2318) of the Federal University of Technology, Owerri. The authors would like to thank the unknown reviewers for their helpful comments and suggestions.
The authors declare no conflicts of interest regarding the publication of this paper.
Mark, U., Anyakwo, C.N., Onyemaobi, O.O. and Nwobodo, C.S. (2019) Determination of the Calcination Procedure Required to Activate Edda Clay for Optimal Leaching of Alumina. Journal of Minerals and Materials Characterization and Engineering, 7, 49-63. https://doi.org/10.4236/jmmce.2019.72004