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Investigations of the Kinetics of Reduction and Reduction/Carburization of NiO-WO3 Precursors.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Process Science.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Kinetic studies of reduction of the mixtures of NiO and WO3 having different Ni/(Ni+W) molar ratios in flowing hydrogen gas were investigated by means of Thermo Gravimetric Analysis (TGA), Fluidized Bed (FB) technique as well as Thermal diffusivity measurements under isothermal conditions. In the case of TGA, the reaction progress was monitored by mass loss, while evolved gas analysis by a gas chromatograph was the indicator of the reaction progress in the case of FB. The results indicate that the reduction reaction proceeds through three consecutive steps, viz.

NiO-WO3 Ni-WO3 Ni-WO2 Ni-W

The present results show that the fluidized bed technique can be successfully utilized in bulk production of intermetallics containing W and a transition metal (or a composite material) wherein the process conditions would have a strong impact on the particle size of the end product.

During the investigations, it was found that there was a delay in the reaction during the hydrogen reduction of NiO-WO3 mixed oxides in a fluidized bed reactor. In order to understand the same, a theoretical model was developed to estimate the apparent reaction rate constant for the reduction reaction from the intrinsic chemical reaction rate constant. Appropriate differential mass balance equations based on intrinsic chemical reaction rate constants and thermodynamic equilibria were developed. The proposed model was successfully applied in predicting the overall reaction kinetics of a fluidized bed reactor. This model is also suitable for scale-up calculations.

SEM images showed that the particle size of the final product was dependent on the Ni/(Ni+W) molar ratio; smaller particles were formed at higher nickel contents. X-ray diffractions of the reduced precursors exhibited slight shift of Ni peaks from the standard one indicating the dissolution of W into Ni.

A new method for studying kinetics of the hydrogen reduction of NiO-WO3 precursors was developed in which the reaction progress was monitored by following the change of thermal diffusivity of the precursors. Activation energies of reduction as well as sintering were calculated. This method is considered unique as it provides information regarding the physical changes like sintering, change of porosity and agglomeration along with the chemical changes occurring during the gas/solid reaction.

As a continuation of the kinetic studies, Ni-W-C ternary carbides were synthesized by simultaneous reduction–carburization of Ni-W-O system using H2-CH4 gas mixtures by TGA. The results showed that the reduction of the oxide mixture was complete before the carburization took place. The nascent particles of the metals formed by reduction could react with the gas mixture with well-defined carbon potential to form a uniform product of Ni-W-C. The above-mentioned experiments were conducted in such a way to ensure that the reaction was controlled by the chemical reaction. The activation energies of the reduction as well as carburization processes at different stages were calculated accordingly.

The present dissertation demonstrates the potential of the investigations of gas/solid reactions towards tailoring the process towards materials with optimized properties as for example introduction of interstitials. The present process design is extremely environment-friendly with reduced number of unit processes and the product being H2O.

Place, publisher, year, edition, pages
Stockholm: KTH , 2010. , viii, 54 p.
Keyword [en]
Reduction, Reduction-Carburization, Gas-solid Reactions, Kinetics, Hydrogen, Methane, Thermogravimetric analysis, Fluidized bed, Simulation, Thermal diffusivity, Thermal conductivity, Laser flash
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-26015ISRN: KTH/MSE--10/51--SE+THMETU/AVHISBN: 978-91-7415-774-1 (print)OAI: oai:DiVA.org:kth-26015DiVA: diva2:366653
Public defence
2010-11-25, Salongen Lavoisier, Biblioteket, Osquars Backe 31, KTH, Stockholm, 09:00 (English)
Opponent
Supervisors
Note
QC 20101112Available from: 2010-11-12 Created: 2010-11-09 Last updated: 2012-03-23Bibliographically approved
List of papers
1. Kinetics of Reduction of NiO-WO3 Mixtures by Hydrogen
Open this publication in new window or tab >>Kinetics of Reduction of NiO-WO3 Mixtures by Hydrogen
2010 (English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 41, no 1, 161-172 p.Article in journal (Refereed) Published
Abstract [en]

The kinetics of reduction of the oxide mixtures of Ni-W with different Ni/(Ni-W) molar ratios within the range of 923 K to 1173 K in flowing hydrogen gas was investigated by means of thermogravimetric analysis under isothermal conditions. The products were examined by X-ray diffraction, scanning electron microscope (SEM), and electron dispersion spectroscopy (EDS) analyses. Five different oxide mixtures apart from the pure oxides were studied in the present work. The results indicate that the reduction reaction proceeds through three consecutive steps that are as follows: NiO-WO3 -> Ni-WO3 -> Ni-WO2 -> Ni-W From the experimental results, the Arrhenius activation energies of the three steps were evaluated for all of the studied compositions. The activation energy for the first step was calculated to be approximately 18 kJ/mol. For the second and third stages, the activation energy values varied from 62 to 38 kJ/mol for the second stage and 51 to 34 kJ/mol for the third stage depending on the Ni/(Ni + W) molar ratio in the precursors; the activation energy increased with increasing ratios. SEM images showed that the grain size of the final product was dependent on the Ni/(Ni + W) molar ratio; smaller grains were formed at higher nickel contents.

Keyword
nickel oxide, alloys, tungstate, gas
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-19365 (URN)10.1007/s11663-009-9302-7 (DOI)000276235900017 ()2-s2.0-77951297719 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
2. Kinetic Studies of the Hydrogen Reduction of NiO-WO3 precursors in a Fluidized-bed reactor
Open this publication in new window or tab >>Kinetic Studies of the Hydrogen Reduction of NiO-WO3 precursors in a Fluidized-bed reactor
(English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916Article in journal (Other academic) Submitted
Identifiers
urn:nbn:se:kth:diva-26080 (URN)
Note
QC 20101112Available from: 2010-11-12 Created: 2010-11-12 Last updated: 2017-12-12Bibliographically approved
3. On The Kinetics of Hydrogen Reduction of NiO-WO3 Precursors in Fluidized Bed- A Modeling Approach
Open this publication in new window or tab >>On The Kinetics of Hydrogen Reduction of NiO-WO3 Precursors in Fluidized Bed- A Modeling Approach
2011 (English)In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 51, no 9, 1383-1391 p.Article in journal (Refereed) Published
Abstract [en]

In the present work, Fluidized bed reduction of NiO-WO3 precursors was investigated isothermally at temperatures 973-1 273 K. The reaction progress was monitored by analysis of H2O evolved during the reaction process using a gas chromatograph instrument. A theoretical model based on intrinsic chemical reaction rate constants and thermodynamic equilibria was developed to estimate the apparent reaction rate constant for the reduction reaction. In developing the model, the particles are considered to be in a completely mixed condition and gas flow is described as plug flow. The proposed model is also suitable for scale-up calculations. The interfacial chemical reaction model was found to fit the experimental results. The apparent activation energy values of the reduction process at different stages were calculated accordingly. The present investigation proved that the fluidized bed technique can be successfully utilized in bulk production of intermetallics containing W and a transition metal (or a composite material) wherein the process conditions would have a strong impact on the particle size of the end product.

Keyword
Fluidized-bed, Gas-solid reactions, H2, Kinetics; Modeling, NiO-WO3
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-26081 (URN)
Note
Updated from submitted to published. QC 20120323Available from: 2010-11-12 Created: 2010-11-12 Last updated: 2017-12-12Bibliographically approved
4. Reduction-Carburization of NiO-WO3 Under Isothermal Conditions Using H2-CH4 Gas Mixture
Open this publication in new window or tab >>Reduction-Carburization of NiO-WO3 Under Isothermal Conditions Using H2-CH4 Gas Mixture
2010 (English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 41, no 1, 173-181 p.Article in journal (Refereed) Published
Abstract [en]

Ni-W-C ternary carbides were synthesized by simultaneous reduction–carburization of NiO-WO3 oxide precursors using H2-CH4 gas mixtures in the temperature range of 973 to 1273 K. The kinetics of the gas–solid reaction were followed closely by monitoring the mass changes using the thermogravimetric method (TGA). As a thin bed of the precursors were used, each particle was in direct contact with the gas mixture. The results showed that the hydrogen reduction of the oxide mixture was complete before the carburization took place. The nascent particles of the metals formed by reduction could react with the gas mixture with well-defined carbon potential to form a uniform product of Ni-W-C. Consequently, the reaction rate could be conceived as being controlled by the chemical reaction. From the reaction rate, Arrhenius activation energies for reduction and carburization were evaluated. Characterization of the carbides produced was carried out using X-ray diffraction and a scanning electron microscope (SEM) combined with electron dispersion spectroscopy (SEM-EDS) analyses. The grain sizes also were determined. The process parameters, such as the temperature of the reduction–carburization reaction and the composition of the gas mixture, had a strong impact on the carbide composition as well as on the grain size. The results are discussed in light of the reduction kinetics of the oxides and the thermodynamic constraints.

National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-26067 (URN)10.1007/s11663-009-9307-2 (DOI)000276235900018 ()2-s2.0-77951295646 (Scopus ID)
Note
QC 20101111Available from: 2010-11-11 Created: 2010-11-11 Last updated: 2017-12-12Bibliographically approved
5. Isothermal dynamic thermal diffusivity studies of the reduction of NiO and NiWO(4) precursors by hydrogen
Open this publication in new window or tab >>Isothermal dynamic thermal diffusivity studies of the reduction of NiO and NiWO(4) precursors by hydrogen
2011 (English)In: International Journal of Materials Research - Zeitschrift für Metallkunde, ISSN 1862-5282, E-ISSN 2195-8556, Vol. 102, no 11, 1336-1344 p.Article in journal (Refereed) Published
Abstract [en]

Thermal diffusivity measurements of uniaxially cold pressed NiO and NiWO(4) were carried out in a dynamic mode in order to monitor the kinetics of hydrogen reduction of the above-mentioned materials using a laser flash unit. The calculated activation energy was found to be higher than that for chemically-controlled reaction obtained earlier by thermogravimetry. The difference has been attributed to physical changes occurring along with the chemical reaction. The activation energy of sintering of the products was evaluated to be 33 and 36 kJ.mol(-1) for NiO and NiWO(4), respectively. Thermal conductivities were calculated taking into consideration the change in heat capacity considering the compositional and the structural changes with the progress of the reaction. The potentiality of the laser-flash method as a complementary technique to thermogravimetry in understanding the mechanism of gas solid reactions is discussed.

Keyword
Thermal diffusivity, Gas-solid reactions, Reduction by hydrogen, Ni-W-O system, Laser flash
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-58823 (URN)10.3139/146.110591 (DOI)000297955000004 ()
Note
QC 20120109Available from: 2012-01-09 Created: 2012-01-09 Last updated: 2017-12-08Bibliographically approved

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