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Equilibrium and Kinetic Considerations in Refining of Silicon
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Micro-Modelling.ORCID iD: 0000-0001-9257-5407
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The purity requirements for photovoltaic-grade (PV) silicon are very stringent. In PV applications, purified silicon is either doped with boron or phosphorous, so the levels of these particular elements have to be tightly controlled. An alternative process route to photovoltaic-grade silicon that has been successfully implemented on an industrial scale is a multi-step process comprising carbothermic reduction, slag refining, hydrometallurgical extraction, and finally directional solidification. There are many fundamental aspects of the process that are not fully understood and need to be defined.

 

In the refining of silicon using slag treatment, boron dissolved in the silicon is oxidized and rejected to a liquid oxide (slag) phase. The partitioning of boron between liquid silicon and SiO2–CaO–MgO slags was examined at 1873. It was found that the distribution of boron is strongly dependent on the oxygen partial pressure and nitrogen partial pressure, as well as the slag composition. The concentration of MgO seems to have little impact on the partition ratio. The greatest partition ratios were achieved at 0.6 atm CO / 0.4 atm N2 with low silica content in the slag.

 

Experiments were carried out to study mass transfer rates between liquid silicon and CaO–SiO2 slag using mechanical stirring at 1823 K. The evidence suggests that the reduction of calcium oxide at the interface leads to a rapid, temporary drop in the apparent interfacial tension. At low apparent interfacial tension, mechanical agitation facilitates the dispersion of metal into the slag phase, which dramatically increases the interfacial area; here it has been estimated to increase by at least one order of magnitude. As the reaction rate slows down, the apparent interfacial tension increases and the metal re-coalesces. The rates of mass transfer of both Ca and B were found to increase by agitating the melt, which shows that without forced convection, the overall kinetic rates are mass-transfer controlled. From a reactor design perspective, this is ideal since it should be simpler to achieve optimal mixing conditions with less kinetic energy input into the melt.

 

The infiltration of silicon into graphite was found to be highly dependent on the internal structure of the graphite substrate. It was confirmed that the heating history of silicon in contact with a graphite substrate strongly influences the melting behavior, which is likely attributed to a gas-solid reaction that forms SiC below the liquidus temperature of silicon and alters the surface properties of the graphite. It was also observed that a partial pressure of CO greater than 0.05 atm in the inlet gas leads to SiC formation on the surface of the silicon and severely hinders proper melting. 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , iv, 84 p.
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-133701ISBN: 978-91-7501-844-7 (print)OAI: oai:DiVA.org:kth-133701DiVA: diva2:662840
Public defence
2013-11-29, Sal F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20131111

Available from: 2013-11-11 Created: 2013-11-08 Last updated: 2013-11-11Bibliographically approved
List of papers
1. The thermodynamics of boron extraction from liquid silicon using SiO2-CaO-MgO stag treatment
Open this publication in new window or tab >>The thermodynamics of boron extraction from liquid silicon using SiO2-CaO-MgO stag treatment
2013 (English)In: International Journal of Materials Research - Zeitschrift für Metallkunde, ISSN 1862-5282, E-ISSN 2195-8556, Vol. 104, no 3, 229-234 p.Article in journal (Refereed) Published
Abstract [en]

In the refining of solar-grade silicon using slag treatment, boron dissolved in the silicon is oxidized and rejected to a liquid oxide (slag) phase. In this study the partitioning of boron between liquid silicon and SiO2-CaO-MgO slags was examined at 1 873 K over a range of slag compositions and oxygen partial pressures. It was found that the distribution of boron is indeed dependent on both the slag composition and the oxygen partial pressure. The concentration of MgO seems to have little impact on the partition ratio.

Keyword
Silicon, Boron, Slag, Partition ratio, Oxygen potential
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-121483 (URN)10.3139/146.110867 (DOI)000316519200001 ()2-s2.0-84875870269 (Scopus ID)
Note

QC 20130507

Available from: 2013-05-07 Created: 2013-04-29 Last updated: 2017-12-06Bibliographically approved
2. Boron partitioning between SiO2-CaO-MgO slags and liquid silicon at controlled nitrogen potential
Open this publication in new window or tab >>Boron partitioning between SiO2-CaO-MgO slags and liquid silicon at controlled nitrogen potential
2013 (English)In: International Journal of Materials Research - Zeitschrift für Metallkunde, ISSN 1862-5282, E-ISSN 2195-8556, Vol. 104, no 7, 650-656 p.Article in journal (Refereed) Published
Abstract [en]

During refining of silicon using slag treatment, boron dissolved in the silicon is extracted to a liquid oxide (slag) phase. In this investigation the partitioning of boron between SiO2-CaO-MgO slags and liquid silicon was examined at 1873 K over a range of slag compositions and CO-N-2 gas mixtures. It was found that the distribution of boron is strongly dependent on the nitrogen partial pressure as well as the slag composition. The greatest partition ratios were achieved at 0.6 atm CO/0.4 atm N-2 with low silica content in the slag. The concentration of MgO in the slag seems to have little or no impact on the boron partition ratio.

Keyword
Silicon, Boron, Partition, Nitrogen potential
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-127505 (URN)10.3139/146.110913 (DOI)000322499300005 ()2-s2.0-84880881726 (Scopus ID)
Note

QC 20130902

Available from: 2013-09-02 Created: 2013-08-30 Last updated: 2017-12-06Bibliographically approved
3. Mass Transfer in Slag Refining of Silicon with Mechanical Stirring: Transient Interfacial Phenomena
Open this publication in new window or tab >>Mass Transfer in Slag Refining of Silicon with Mechanical Stirring: Transient Interfacial Phenomena
2014 (English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 45, no 1, 96-105 p.Article in journal (Refereed) Published
Abstract [en]

Experiments have been carried out to study the rates of mass transfer between liquid silicon and CaO-SiO2 slag with impeller stirring at 1823 K (1550 A degrees C). The occurrence of transient interfacial phenomena related to the mass transfer of calcium has been observed; the evidence suggests that the reduction of calcium oxide at the interface leads to a rapid, temporary drop in the apparent interfacial tension. At low apparent interfacial tension, mechanical agitation facilitates the dispersion of metal into the slag phase, which dramatically increases the interfacial area; here, it has been estimated to increase by at least one order of magnitude. As the reaction rate slows down, the apparent interfacial tension increases and the metal recoalesces. The incidental transfer of calcium very likely promotes the transfer of boron by increasing the interfacial area. Mechanical mixing appears to be an extremely effective means to increase the reaction rate of boron extraction and could feasibly be implemented in the industrial slag refining of silicon to improve reaction rates.

Keyword
Molten Silicon, Boron, Iron, Kinetics, Thermodynamics, Behavior, Removal, Tension
National Category
Materials Engineering Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-133689 (URN)10.1007/s11663-013-0010-y (DOI)000330735200013 ()2-s2.0-84895059910 (Scopus ID)
Note

QC 20140306. Updated from accepted to published.

Available from: 2013-11-08 Created: 2013-11-08 Last updated: 2017-12-06Bibliographically approved
4. Mass Transfer in Slag Refining of Silicon with Mechanical Stirring: Rates of Ca and B Transfe
Open this publication in new window or tab >>Mass Transfer in Slag Refining of Silicon with Mechanical Stirring: Rates of Ca and B Transfe
(English)Manuscript (preprint) (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-133711 (URN)
Note

QS 2013

Available from: 2013-11-08 Created: 2013-11-08 Last updated: 2013-11-11Bibliographically approved
5. Reactions Between Silicon and Graphite Substrates at High Temperature: In Situ Observations
Open this publication in new window or tab >>Reactions Between Silicon and Graphite Substrates at High Temperature: In Situ Observations
2014 (English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 45, no 1, 150-160 p.Article in journal (Refereed) Published
Abstract [en]

Graphite as a refractory material has found wide application in many process steps to produce photovoltaic silicon. In the current study, the melting behavior of silicon in contact with different grades of graphite was investigated. The infiltration of silicon into graphite was found to be highly dependent on the internal structure of the graphite substrate. It was confirmed that the heating history of silicon in contact with a graphite substrate strongly influences the melting behavior, which is likely attributed to a gas-solid reaction that forms SiC at less than the liquidus temperature of silicon and alters the surface properties of the graphite. It was also observed that a concentration of CO greater than 5 pct in the inlet gas leads to SiC formation on the surface of the silicon and severely hinders melting.

Keyword
Gas-solid reaction, Graphite substrate, Heating history, High temperature, In-situ observations, Internal structure, Liquidus temperature, Melting behavior
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-133692 (URN)10.1007/s11663-013-9947-0 (DOI)000330735200019 ()2-s2.0-84895060880 (Scopus ID)
Note

QC 20140311

Available from: 2013-11-08 Created: 2013-11-08 Last updated: 2017-12-06Bibliographically approved

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