South African Coal Fly Ash Research Articles

Synthesis and Evaluation of Mesoporous Silica Nanoparticle Catalyst Supports Prepared from South African Coal Fly Ash

The development of environmentally sustainable catalytic reaction systems for the oxidation of organic compounds has gained significant interest, with widespread application across various industries. This study investigates the suitability of mesoporous silica nanoparticles, synthesized from coal fly ash, as catalyst supports in the oxidation of veratryl alcohol, a compound that serves as a model to mimic the oxidation behavior of lignin-derived structures commonly found in biomass waste. A Cu(II) salicylaldimine complex was employed as the catalyst, with tert-Butyl hydrogen peroxide as the oxidant. Silica catalyst supports were synthesized from coal fly ash derived Na2SiO3 solutions. The effect of pre-dealumination of coal fly ash during Na2SiO3 preparation, variation of the surfactant in silica nanoparticle synthesis, and storage conditions of the catalyst supports on the performance of the coal fly ash derived silica catalyst supports were evaluated and compared to the performance of MCM-41 and SBA-15. The textural properties of the coal fly ash derived silica catalyst supports were notably influenced by the choice of surfactant used during its synthesis, while storage conditions affected the abundance of silanol groups necessary for successful catalyst immobilisation.Graphical

Synthesis and characterization of amorphous nano silica from South African coal fly ash

In this study, amorphous silica nanoparticle was extracted from South African coal fly ash (SACFA), using a facile sol–gel synthesis method. Prepared amorphous silica nanoparticles (ASNPs) were characterized employing X-ray fluorescence (XRF) spectroscopy, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), transition electron microscope (TEM), Brauer Emmett Teller (SBET) surface area analysis, and ultraviolet–visible diffuse reflection spectroscopy (UV–vis DRS). XRF analysis suggests SACFA as fly ash class F. the XRD pattern, reveals the structural compositing of SACFA and the amorphous nature of extracted SNP, while the FTIR assay shows the presence of silanol and siloxane groups. SEM and TEM analysis revealed primary silica nanoparticles were roughly spherical with sizes of about 98 nm. EDX spectral confirms the presence of SiO2. The findings of this study demonstrate a simple sol–gel method of synthesizing amorphous nano silica from SACFA, thus suggesting the utilization of SACFA in high-tech and complex material preparation and applications in a more inventive manner.

Microwave-assisted sol–gel template-free synthesis and characterization of silica nanoparticles obtained from South African coal fly ash

Abstract In this study, we prepared a silica nanoparticle from South African fly ash (SAFA), using a facile microwave (MW)-assisted sol–gel template free syntheses method. Prepared silica nanoparticles (SNPMW) were characterized using X-ray fluorescence (XRF) spectroscopy, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), transition electron microscope (TEM), Brunauer–Emmett–Teller (S BET) surface area analysis, and ultraviolet visible diffuse reflection spectroscopy. XRF analysis suggests SAFA as fly ash class F. The XRD pattern reveals the structural composition of SAFA and the amorphous nature of extracted SNP, while the FTIR assay shows the presence of silanol and siloxane groups. SEM and TEM analyses revealed primary silica nanoparticles were roughly spherical with sizes of about <200 nm. EDX spectra confirm the presence of SiO2. The optical bandgap of SNPMW was established to be 4.67 eV. The application of SNPMW demonstrated that it can be used to remove Pb2+ from an aqueous solution. Test results show the optimum treatment time as 60 minutes, while removal efficiency increases from 66.76 to 96.64% as the pH rises from 3 to 5, but as the pH rises above 5, the efficiency decreases. The use of an MW-assisted sol–gel preparation method gave rise to an elevated reaction rate with minimal contamination and thinner particle size SNPMW, which was utilized for the removal of Pb2+ in an aqueous solution.

Microwave-Assisted Synthesis and Characterization of Zeolite Prepared From South Africa Coal Fly Ash

In industry, synthetic zeolites are commonly used as ion-exchange materials, catalyst supports, and adsorbents. As a result, a more energy-efficient alternative for its synthesis from low-cost and accessible raw materials is needed. This study, presents the possibility of using class F South African coal fly ash (SACFA) from Lethaba thermal-power station as a precursor to produce zeolites via a microwave (MW) assisted synthesis route. The mineral content of synthetic materials was determined using X-ray florescence (XRF). Morphology was determined using a Scanning electron microscopy (SEM), elemental composition by energy dispersive spectrometer (EDS). X-ray diffractometry (XRD) was used to get structural characterization. Microwave (MW) irradiation time and intensity enhance the crystallization of the zeolite phase as a result of sufficient energy required to enable the solubility of alumina and silica from coal fly ash. The use of MW irradiation provides a green alternative to zeolite synthesis from fly ash (FA) than traditional thermal and fusion techniques, which uses a great deal of energy consumption and a longer reaction time.

The effect of titanium slag on the properties of glass-ceramic composites from South African coal fly ash

In this study, the impact of titanomagnetite slag on the structure of glass-ceramic composites was investigated. The slag was added as a source of titania which promotes nucleation within the parent glass matrix and magnesium oxide which facilitates crystallisation. A mixture of fly ash and varying quantities of waste beverage glass and titanomagnetite slag were melted. The melts were then annealed at lower temperatures to reduce the internal stress and, subsequently, cooled to room temperature to produce parent glasses. The mass of fly ash was held constant at 60%, while the mass of the waste beverage glass and titanomagnetite slag was varied between 10-30% and 0-30% respectively. Afterwards, the parent glasses were converted to glass-ceramic composites through a controlled double-staged thermal treatment. Characterisation of these composites showed that an increase in titanomagnetite slag resulted in many crystal phases forming within the glass-ceramic matrix and an increased porosity. The desired diopside phase was not detected in the composites same as when pure magnesium oxide was added instead of the titanomagnetite slag. The compressive strengths, the bulk densities and melting temperatures of the glass-ceramic composites were found to be lower as compared to glass-ceramic composites produced from a mixture containing pure magnesium oxide. However, they were highly resistant to attack by nitric acid and sodium hydroxide.

The Behaviour of Rare Earth Elements from South African Coal Fly Ash during Enrichment Processes: Wet, Magnetic Separation and Zeolitisation

Rare earth elements (REEs) are essential raw materials in a variety of industries including clean energy technologies such as electric vehicles and wind turbines. This places an ever-increasing demand on global rare earth element production. Coal fly ash (CFA) possesses appreciable levels of REEs. CFA, a waste by-product of coal combustion, is therefore a readily available source of REEs that does not require mining. CFA valorisation to zeolites has been achieved via various synthesis pathways. This study aimed to evaluate one such pathway by monitoring how REEs partition during CFA processing by the wet, magnetic separation process and zeolitisation. South African CFA was subjected to wet, magnetic separation and subsequent zeolitisation of the nonmagnetic fraction (NMF); solid products were characterised by XRD, SEM, XRF and LA-ICP-MS. The wet, magnetic separation process resulted in the partitioning of a specific set of transition metals (such as Fe, Mn, Cr, V, Ni, Zn, Cu, Co and Mo) into the magnetic fraction (MF) of CFA, while REEs partitioned into the NMF with a total REE content of 530.2 ppm; thus, the matrix elements of CFA were extracted with ease. Zeolitisation resulted in a solid zeolite product (hydroxysodalite) with a total REE content of 537.6 ppm. The process of zeolitisation also resulted in the selective enrichment of Ce (259.1 ppm) into the solid zeolite product (hydroxysodalite), while other REEs were largely partitioned into the liquid phase. CFA valorisation by wet, magnetic separation and zeolitisation therefore allowed for the partitioning of REEs into various extraction products while recovering the matrix elements of CFA such as Fe, Si and Al. The findings of this study highlight the geopolitical importance of REEs in terms of the development of alternative processes for REE recovery from waste and alternative sources, which may potentially give countries that employ and develop the technology a key advantage in the production of REEs for the global market.

Microwave Irradiation-Assisted Synthesis of Zeolites from Coal Fly Ash: An Optimization Study for a Sustainable and Efficient Production Process.

Class F South African coal fly ash was used as a precursor for the synthesis of zeolite A via complete microwave irradiation. To attain optimal conditions for the synthesis of zeolite A with minimum impurities, the microwave synthesis time, irradiation power, and Si/Al ratio were varied. Sodalite with fly ash phases were obtained when the Si/Al ratio in the coal fly ash was not adjusted and when the microwave irradiated coal fly ash slurry was used instead of the extract solution. Increased microwave irradiation time power and time favored the crystallization of zeolite A phase due to sufficient energy needed to ensure the dissolution of Al and Si from coal fly ash. A Brunauer–Emmett–Teller surface area of 29.54 m2/g and a cation exchange capacity of 3.10 mequiv/g were achieved for zeolite A, suggesting its potential application as an adsorbent and cation exchange material for environmental remediation. Complete microwave irradiation offers a greener approach toward zeolite synthesis from coal fly ash compared to conventional hydrothermal and fusion methods that consume a lot of energy and require longer reaction times.

Synthesis, characterisation and CO2 adsorption potential of NaA and NaX zeolites and hydrotalcite obtained from the same coal fly ash

Hydrotalcites and zeolites are materials characterised by a high capacity for CO2 adsorption and can therefore be useful adsorbents for the capture of CO2 from flue gases. In this study, two types of zeolites (NaXFA and NaAFA) were successfully synthesised from South African coal combustion fly ash. Their CO2 adsorption properties, as well as those of a hydrotalcite (HTFA) synthesized from the same fly ash in our previous work, were investigated and compared. In addition, this paper reports the CO2 adsorption capacity of a novel hierarchical morphology NaX-type zeolite (NaXFA) made from fly ash. This material was characterized by plate-like structures arranged in ball-like clusters with crystal widths of between 63 nm and 99 nm. NaXFA was characterised by very high CO2 physisorption properties and was therefore the most suitable candidate for low temperature and pressure swing adsorption generally required in short-term CO2 adsorption applications. In contrast, both physisorption and chemisorption properties were observed for NaAFA and HTFA. However, HTFA was the most suitable material for chemisorption of CO2. The findings demonstrated the suitability of utilising HTFA for both short- and long-term capture of CO2.

Preparation and characterization of glass-ceramic composites from South African coal fly ash

Though fly ash in South Africa has mainly been utilized in the construction industry, a large portion of it is still discarded. Production of glass-ceramic composites through fly ash devitrification has gradually become one of the applications being examined internationally. This study reports the investigation of producing glass-ceramic composites from fly ash and varying quantities of beverage waste glass and magnesium oxide as main raw materials. Glasses were obtained by melting fly ash and its additives using an arc furnace. Based on their behaviour when subjected to differential thermal analysis, the glasses were then crystallised through a double-staged thermal treatment to form glass-ceramic composites. The composition and behaviour of the glass-ceramic composites were investigated using X-ray diffraction, Scanning Electron Microscopy, mechanical and chemical assessments. The crystal content in the glass-ceramic composites increased as the magnesium oxide content was increased. The desired diopside phase decreased with an increased magnesium oxide content and ultimately resulted in the formation of forsterite and anorthite as the major and second major phases respectively. The glass-ceramic composites with lesser amounts of magnesium oxide were chemically and thermally resistant and they also demonstrated high cold compressive strength.

Preparation of Sodium Silicate Solutions and Silica Nanoparticles from South African Coal Fly Ash

The production of amorphous mesoporous silica nanoparticles can be achieved using sodium silicate (Na2SiO3) solutions prepared from South African coal fly ash waste. The first part of this study compared two processes for the preparation of Na2SiO3 solutions. The first process, hereafter called sequential acid-alkaline leaching (SAAL), is a two-stage process, which involves (i) a H2SO4 leaching step for the preferential extraction of reactive aluminium over silicon, followed by (ii) the preferential extraction of silicon over aluminium from the resulting residues using NaOH. The second process is a direct alkaline leaching (DAL) process, which consists of a single-stage elemental extraction from ash using NaOH, i.e. without the preceding acid leaching step used in SAAL. The two processes generated Na2SiO3 solutions with identical pH (11.8), similar silicon (10.2–10.3 g/L), iron (ca. 200 mg/L) and potassium (ca. 800 mg/L) content, and low calcium concentrations (≤ 29 mg/L). However, the inclusion of the acid leaching step in the SAAL process yielded a Na2SiO3 solution with significantly lower aluminium content (166 mg/L vs. 1158 mg/L). The Na2SiO3 solutions obtained from the SAAL and DAL processes were then used as silica precursors to synthesise silica nanoparticles via a sol–gel method using polyethylene glycol (PEG) as surfactant and sulphuric acid as catalyst. All samples of synthesised silica nanoparticles were characterised by a high level of purity (up to 99.3 wt % SiO2). The insight gained is now being used to improve existing processes for the production of high-grade ultra-pure silica nanoparticles (i.e. ≥ 99.9 wt % SiO2) for catalyst support applications.

Evaluation of the Bioavailability and Translocation of Selected Heavy Metals by Brassica juncea and Spinacea oleracea L for a South African Power Utility Coal Fly Ash.

This study evaluated the physicochemical and mineralogical properties, mobile chemical species bioavailability and translocation in Brassica juncea and Spinacea oleracea L. plants of a South African coal-fired power utility. Coal-fly-ash (CFA) disposal is associated with various environmental and health risks, including air, soil, surface, and groundwater pollution due to the leaching of toxic heavy metals; these ends up in food webs affecting human health, while repeated inhalation causes bronchitis, silicosis, hair loss, and lung cancer. The morphology and chemical and mineralogical composition of CFA were determined using Scanning Electron Microscopy (SEM), X-ray fluorescence (XRF), and X-ray diffraction, respectively. In pot-culture experiments, S. oleracea L. and B. juncea plants were grown in three sets of pots containing CFA (Set 1), soil (Set 2), and a mixture of CFA plus soil at a ratio of 1:1 (50% CFA: 50% soil, Set 3), while no plants were grown in Set 4 as a control for the leachate samples. SEM showed that the surface morphology of CFA has a lower degree of sphericity with the irregular agglomerations of many particles. XRF results revealed that CFA contains 43.65%, 22.68%, and 10.89% of SiO2, Al2O3, and Fe2O3, respectively, which indicates that CFA is an aluminosilicate material. X-ray diffraction (XRD) showed that CFA contains mullite as a major phase, followed by quartz mineral phases. Chemical species such as B, Ba, Mo, and Cr were occurring at higher concentrations in the leachates for most weeks in the pot-culture experiments, especially for CFA and soil + CFA growth media. However, there was a common trend for all growth media of chemical-species concentrations declining with time, which might have been caused by plant uptake or wash-off with water during irrigation; even for the growth media as well, where no plants were grown. Chemical species, such as Fe, Mn, B, Ba, and Zn, accumulated highly in most parts of the plant species. However, B. juncea showed higher potential to accumulate chemical species as compared to S. oleracea L. Bioconcentration and translocation factors (BF and TF) showed that B. juncea was the most effective in terms of bioconcentration and translocation of most of the chemical species. This indicates that B. juncea has potential in application for the phytoremediation of CFA dumps, and could contribute to the remediation of CFA dumps and the reduction of potential health and environmental impact associated with CFA.

Conversion of South African Coal Fly Ash into High-Purity ZSM-5 Zeolite without Additional Source of Silica or Alumina and Its Application as a Methanol-to-Olefins Catalyst

Characteristics of ZSM-5 synthesized from H2SO4-treated coal fly ash and fused coal fly ash extracts are compared in this study. In the synthesis process, fused coal fly ash extract (without an additional silica source) was used in the synthesis of ZSM-5. The effect of the structure-directing agent (tetrapropylammonium bromide, 1,6-hexanediamine or 1-propylamine) on the properties and methanol-to-olefins (MTO) effectiveness of the fly ash-based ZSM-5 was also investigated. A pure ZSM-5 synthesized from the fused coal fly ash extract led to a methanol conversion higher than 90% after 5 h on stream. The template 1,6-hexanediamine led to the synthesis of the most stable fly ash-based catalyst keeping a 44% methanol conversion after 24 h on stream.

Ammonium sulphate and/or ammonium bisulphate as extracting agents for the recovery of aluminium from ultrafine coal fly ash

We recently showed that the selective extraction of aluminium from the amorphous phase of a South African ultrafine coal fly ash can be achieved via thermochemical treatment with ammonium sulphate for 1h followed by aqueous dissolution, as an alternative to conventional hydrometallurgical processes. In this study, insight gained from the previous work was applied to investigate and compare total vs selective aluminium extraction efficiencies using ammonium sulphate or ammonium bisulphate either on its own, or as a mixture of the two salts as extracting agents during a 2h thermal treatment process. The effects of (i) ash-to-extractant mass ratio and (ii) temperature during thermal treatment on extraction efficiency was investigated. While a maximum, but non-selective, recovery of 46.6% total aluminium was achieved using ammonium bisulphate at 400°C, the most technically appropriate results for selective recovery yielded 37.3% aluminium, with only 0.3% silicon, 0.1% titanium and 3.9% iron having been co-extracted when using ammonium sulphate at a processing temperature of 600°C. Extraction of most of the calcium and magnesium could not be prevented. Using mixtures of ammonium salts as extracting agents during thermochemical treatment may however introduce technical difficulties on large scale. Our results indicate that any of the two ammonium salts could be used on their own during thermochemical treatment. Thermochemical treatment of coal fly ash using ammonium salts may therefore represent a promising technology for extracting aluminium from South African coal fly ash.

Synthesis and Characterization of Faujasite Zeolite and Geopolymer from South African Coal Fly Ash

AbstractSouth African coal fly ash (CFA) has been used as a source of silicon and aluminum in the synthesis of faujasite zeolite and geopolymer. However, the synthesis of faujasite zeolite from CFA...

Synthesis and characterization of hydrotalcite from South African Coal fly ash

The synthesis of adsorbents such as hydrotalcites (HT) from mineral waste (e.g. blast furnace slag) is receiving increasing attention. This study reports a novel procedure to prepare HT from a South African coal-combustion fly ash, an ubiquitous waste generated by coal-fired power stations. The second point of novelty is the optimization process, which highlights the boundary conditions for crystallization of this mineral phase. The parameters investigated for the optimization of HT synthesis from fly ash included HCl concentration, aging time and temperature, pH during the aging step, and crystallization time and temperature. The most suitable synthesis conditions were: HCl concentration of 3M, aging time of 30min, aging temperature of 65°C, pH of 11.5 during aging, crystallization time of 12h, and crystallization temperature of 70°C. HT had a large external surface area and very low microporosity. Morphologically, synthesized HT was mostly made up of sub-micron plate-like particles. The structural characteristics of HT synthesized from fly ash were similar to those of HT produced from analytical grade chemicals, except for the presence of calcite. A novel use of the waste South African Class F fly ash was a suitable feedstock for the synthesis of high quality HT under the optimized process conditions that minimized the formation of secondary undesired mineral phases such as calcite or hydrogarnet.

Synthesis of Phase-Pure Zeolite Sodalite from Clear Solution Extracted from Coal Fly Ash

The formation of zeolite sodalite (SOD) as a by-product in the synthesis of zeolite X from clear solution extracted from fused South African coal fly ash has recently been studied. However, the present study shows that zeolite X as well as zeolite A should be seen as intermediate structures and that phase-pure sodalite can be obtained by increasing crystallization time and temperature. To investigate the transition from zeolites A and X into SOD, the crystallization was conducted at 70, 80, and 90°C for different times. In addition to structural and chemical analyses by XRD and ICP, respectively, a detailed morphological characterization of the synthesis products by scanning electron microscopy (SEM) was undertaken in order to investigate the effect of the crystallization conditions on different crystal morphologies of zeolite SOD.

Transformation of South African coal fly ash into ZSM-5 zeolite and its application as an MTO catalyst

This study presents a way of using South African coal fly ash by extracting metals such as Al and Fe with concentrated sulphuric acid, and then using the solid residue as a feedstock for the synthesis of ZSM-5 zeolite. The percentage of aluminium and iron oxides decreased from 28.0 ± 0.2% and 5.0 ± 0.1% in coal fly ash to 24.6 ± 0.1% and 1.6 ± 0.01% in the acid treated coal fly ash respectively. The fly ash-based zeolite ZSM-5 sample synthesised from the solid residue after extraction of Al and Fe, contained 62% of ZSM-5 zeolite pure phase with a number of Brønsted acid site density of 0.61 mmol per gzeolite. By properly treating the as-prepared coal fly ash-based ZSM-5 zeolite, an active and selective methanol-to-olefins acid catalyst could be designed, leading to full methanol conversion during 15 h on stream. The optimised catalyst exhibited a cumulative methanol conversion capacity of 71 g(MeOHconverted)/g(catalyst) and a light olefin productivity of 21 g(C2=–C4=)/g(catalyst). Cette étude présente une stratégie de valorisation de cendres du charbon (origine : Afrique du Sud) après extraction de métaux tels que le fer ou l'aluminium. Le résidu solide est ensuite utilisé comme matière première pour synthétiser une zéolithe ZSM-5. Cette dernière est obtenue sous la forme d'un matériau composite zéolithe (62%)/cendre, présentant une acidité de Brønsted de 0,61 mmol par gzeolite. Par une conception sur mesure ou rational design, un composite ZSM-5/cendre s'est avéré être un catalyseur actif et sélectif dans la réaction de conversion du méthanol en oléfines légères (MTO). Une capacité totale de conversion du méthanol de 71g/gcatalyseur accompagnée d'une productivité de 21 g (C2=–C4=)/gcatalyseur a ainsi été obtenue.

Thermochemical processing of a South African ultrafine coal fly ash using ammonium sulphate as extracting agent for aluminium extraction

South African coal fly ash represents an untapped secondary resource of aluminium. Continuous research is conducted to develop suitable chemical and/or geometallurgical processes for aluminium extraction, preferably accompanied by minimal silicon extraction. The thermochemical treatment of a South African ultrafine coal fly ash was investigated to test the feasibility of recovering aluminium using ammonium sulphate, a widely available, low-cost, recyclable chemical agent. The optimum processing conditions were determined to be a temperature of 500°C and a fly ash to ammonium sulphate weight ratio of 2:6 when a reaction time of 1h was used. Water leaching of the reaction product obtained under these conditions resulted in the selective recovery of 95.0% aluminium from the amorphous phase, with <0.6% Si extracted. Mullite was unlikely to have reacted with the extracting agent. Except for Si, the process was not element-selective, but the extraction of iron could be minimized by increasing the treatment temperature to 600°C without compromising Al extraction. Thermochemical treatment using ammonium sulphate may therefore represent a promising technology for extracting aluminium from coal fly ash, which could be subsequently converted to value-added products such as alumina.

Synthesis of sodium silicate from South African coal fly ash and its use as an extender in oil well cement applications

chemical properties of the CFA-Na2SiO3 were compared to those of a commercial sodium silicate (com-Na 2SiO3) using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. OWC slurries with varying proportions of cement, distilled water, and 2% CaCl 2 by weight of water (BWOW) were prepared and extended using the CFA-Na 2SiO3 and the com-Na2SiO3 at compositions ranging from 0.25-2.5% by weight of cement (BWOC). Rheological properties of the slurries were evaluated using American Petroleum Institute procedures and compared. The physico-chemical properties of the CFA-Na 2SiO3 are consistent with those of com-Na2SiO3, indicating the purity of the CFA-Na2SiO3. A comparative study of the OWC slurries indicated that the slurries extended with CFA-Na 2SiO3 have slightly lower densities, lower viscosities, and higher compressive strength than those extended with com-Na 2SiO3. This indicates that CFA-Na2SiO3 slurries would be easier to pump and preferable where early strength development is critical. This report could be instrumental in providing a way for the beneficiation of South African CFA in the petroleum, oil, and gas industry.

Dissolution kinetics of South African coal fly ash and the development of a semi-empirical model to predict dissolution

Wet flue gas desulphurization (FGD) is a crucial technology which can be used to abate the emission of sulphur dioxide in coal power plants. The dissolution of coal fly ash in adipic acid is investigated by varying acid concentration (0.05-0.15M), particle size (45- 150?m), pH (5.5-7.0), temperature (318-363K) and solid to liquid ratio (5-15 wt %.) over a period of 60 minutes which is a crucial step in wet (FGD). Characterization of the sorbent was done using X-ray fluorescence (XRF), X-ray diffraction (XRD), Furrier transform infrared (FTIR), scanning electron microscope (SEM) and Branauer-Emmett-Teller (BET) surface area. BET surface area results showed an increase in the specific surface area and SEM observation indicated a porous structure was formed after dissolution. The experimental data was analyzed using the shrinking core model and the diffusion through the product layer was found to be the rate limiting step. The activation energy for the process was calculated to be 10.64kJ/mol.