The Formation of Freeze Linings: A Microstructural Perspective (De vorming van vriesbekledingen: een microstructurele benadering)

Een vriesbekleding wordt reeds veelvuldig gebruikt om de corrosie van de reactorwand in pyrometallurgische processen te beperken. Het gebruik va n een vriesbekleding is voordelig aangezien een vriesbekleding zelfregen erend is en omdat extreme procescondities bereikt kunnen worden. Wanneer een vriesbekleding wordt toegepast, is het echter van groot belang dat zijn vorming en stabiliteit gegarandeerd kunnen worden en dus moet het g edrag van een vriesbekleding voorspelbaar en controleerbaar zijn. Om het inzicht in het vriesbekledinggedrag te verbeteren is in deze thes is een methodologie ontwikkeld voor het bestuderen van vriesbekledingmic rostructuren. Deze methodologie is toegepast voor het bepalen van vormin gsmechanismen van een vriesbekleding, voor de studie van haar thermische geschiedenis en voor het bestuderen van de invloed van slakeigenschappe n op haar vorming. Uit de resultaten blijkt dat de initiële vorming van een vriesbekleding afhangt van de groei van een glas of hoogvisceuze vlo eistof laag welke wordt bepaald door warmtetransport. Op het moment dat kristallen vormen en deze zich met elkaar verbinden, hangt de vriesbekle dinggroei af van kristalgroei. Met dalende groeisnelheid van de vriesbek leding stijgt de impact van de uitwisseling van componenten tussen bad e n vriesbekleding. Hierdoor kunnen grote kristallen vormen bij lage groei snelheden waardoor een afsluitende kristallijne laag vormt. De studie va n de thermische geschiedenis toont dat warmtetransport zeer snel kan geb euren. Als gevolg vormen de "verbindende" kristallen in een onderkoelde vloeistof en varieert de bad-vriesbekleding grensvlaktemperatuur tussen de glastransitietemperatuur en de liquidustemperatuur van de slak. Verde r blijkt dat slakeigenschappen zoals het kristallisatiegedrag, de viscos iteit, het type kristallijne fasen in de slak ("verbindend" of niet) en het temperatuurgebied waarin ze stabiel zijn een grote impact hebben op de vorming van een vriesbekleding. Als het gevolg kan het aanpassen van de slaksamenstelling leiden tot optimaal vriesbekledinggedrag. De operat ionele eisen (snelle vorming and stabiliteit) kunnen behaald worden indi en de initiële groei van de vriesbekleding bepaald wordt door de vorming van "verbindende" kristallen en als uiteindelijk een hoogsmeltende kris tallijne laag vormt aan de badkant van de vriesbekleding die in evenwich t is met het bad en wiens samenstelling verschilt van het bad.status: publishe

Influence of Cr and Ni content in stainless steel on the degradation mechanisms in PbO-CaO-SiO2 slag

Degradation of stainless steel in contact with PbO-CaO-SiO2 slag at 1200 degrees C is experimentally investigated. The contact between steel and slag leads to the reduction of PbO from the slag to liquid Pb, and concurrent oxidation of the steel. The degradation mechanisms can be divided into liquid slag and liquid metal corrosion and oxidation. High Cr content is beneficial as it promotes the formation of a Cr2O3 scale which shields the steel from the slag and liquid Pb. The high solubility of Ni in liquid Pb is responsible for the increasing dissolution rate with increasing Ni content of the steel. (C) 2012 Elsevier Ltd. All rights reserved.status: publishe

The importance of slag engineering in freeze-lining applications

Freeze linings are frequently used to protect the reactor wall in pyrometallurgical processes. In order to minimize reactor wall corrosion, the stability of a freeze lining has to be guaranteed. To illustrate the importance of slag engineering in the optimization of freeze-lining behavior, the freeze-lining formation of six synthetic lead slags is studied. Lab-scale freeze linings are obtained using a cooled-probe technique and their microstructures are characterized using light optical microscopy (LOM) and microprobe analysis. The results show that slag engineering can have a major impact on the operative freeze-lining formation mechanisms. Some slag properties found to affect the freeze-lining formation are the viscosity, the temperature stability range of the relevant phases, the type of phases that form (interlocking or not), and the crystallization behavior of the slag. The operational demands of a protective freeze lining are defined by the authors as follows: (1) a rapid formation to limit the contact between the reactor wall and the corrosive bath material and (2) a sufficient stability during changes in heat input from the bath and in bath composition. From the comparison of the microstructural features of the freeze linings formed with the studied slags, it is concluded that these demands can be fulfilled with the growth of an initial layer dominated by the presence of interlocking crystals in combination with the subsequent formation of a high-melting crystalline layer at the bath-freeze-lining interface, which is in equilibrium with the slag bath and the composition of which differs sufficiently from the bath composition.status: publishe

Influence of As₂O₃ added to PbO-CaO-SiO₂ slag on the degradation mechanisms of stainless steel

The degradation of three stainless steel grades (23.8–25.0 wt.% Cr and 3.8–19.0 wt.% Ni) has been investigated after immersion in PbO–CaO–SiO₂–As₂O₃ slag for 3 h at 1200 °C. The degradation mechanisms are related to the steel and slag composition. The effect of As₂O₃ is most pronounced in the slag with the highest PbO/SiO₂ ratio, where dissolution of the steel in liquid Pb, coming from the reduction of PbO, is most severely present. The reduction of As₂O₃ to As causes alloying of the steel with As and subsequent liquid phase formation. The degradation degree increases with increasing Ni content of the steel.status: publishe

Collaboration: the key towards a resource resilient society

The global demand for technology materials is continuously increasing as the world's population grows and high standards of living are sought in developing and transition countries. To secure a reliable and sustainable supply of these metals, innovative solutions need to be developed along the entire value chain. This requires a system-wide, collaborative approach focusing on sustainable mining methods, substitution of critical metals and recovery of metals from secondary sources

Influence of simultaneous infiltration and reaction on the chemical degradation of magnesia-chrome bricks in contact with a synthetic lead slag

Chemical degradation of a refractory lining is most commonly considered as a dissolution process whereby the refractory components dissolve into the liquid slag. Hence, the lining thickness is steadily reduced. The dissolution process is not merely limited to the slag-refractory contact surface, but due to capillary forces, also occurs inside the porous refractory brick. This paper describes how the interaction between slag and brick during infiltration, changes the slag composition as a function of infiltration depth. Knowledge about the selective filtering of certain slag components by a refractory brick is indispensible in predicting the chemical degradation, especially when significant changes in slag composition are present. In this case, often seen with non-ferrous slags, the situation in the interior of the brick cannot be adequately predicted based on the global slag and brick composition. To study this “filter effect”, a magnesia-chrome finger in contact with a PbO-SiO2-ZnO-Al2O3-CaO liquid slag at constant temperature is tested. Using SEM-EDS analyses the slag composition as a function of infiltration depth is measured. Based on the microstructure at different positions, the interactions leading to modified slag compositions are determined. They can be classified into two categories: (1) slag components that diffuse into the brick phases and (2) reaction between slag components and dissolved brick components forming new solid phases. Slow diffusion prevents the slag from reaching its equilibrium concentration before it infiltrates further in the brick. The slag composition, therefore, not merely depends on the occurring reactions but also on the relative diffusion rate in the refractory phases (compared to the slag infiltration rate).status: publishe

Degradation behavior of stainless steel in PbO-CaO-SiO2 slag in the presence of sulphur

Five stainless steel grades are subjected to PbO-CaO-SiO2-S slag at 1200 degrees C. The degradation phenomena are identified as liquid slag and liquid metal corrosion, oxidation and sulphidation. The relation between sulphidation and steel and slag composition is discussed. For the slag with the lowest PbO/SiO2 ratio, sulphidation is mainly recognized through (Fe, Cr)(x)S1-x at the surface and in the subsurface of the steel, especially for the steel grades with the lowest Cr content. For the slag with the highest PbO/SiO2 ratio, sulphidation is mostly pronounced in the steel grades with the highest Ni content through the formation of a liquid (Ni, Pb, S) phase. (C) 2011 Elsevier Ltd. All rights reserved.status: publishe

The influence of slag compositional changes on the chemical degradation of magnesia-chromite refractories exposed to PbO-based non-ferrous slag saturated in spinel

In non-ferrous metallurgy the refractory life is application dependent, typically going from several months up to 2 years or more. Slag engineering is widely used to reduce the dissolution rate of a refractory lining by forming a solid protection layer at the slag-lining interface, thereby increasing the lining's lifetime. The non-ferrous slag in this paper is engineered to form a protective spinel layer. This phase, however, only forms near and at the slag-lining interface, while deeper inside the sample forsterite grains are detected, resulting in direct contact between the still unsaturated slag and the magnesia-chromite refractory phases. At this position the MgO dissolution increases with a factor 3–4 compared with the slag-refractory interface, attacking the bonding between grains and decreasing the brick's mechanical strength. As this happens deeper inside the sample, the spalling risk increases. Strategies to simultaneously reduce refractory dissolution and spalling are discussed.publisher: Elsevier articletitle: The influence of slag compositional changes on the chemical degradation of magnesia-chromite refractories exposed to PbO-based non-ferrous slag saturated in spinel journaltitle: Journal of the European Ceramic Society articlelink: http://dx.doi.org/10.1016/j.jeurceramsoc.2014.08.017 content_type: article copyright: Copyright © 2014 Elsevier Ltd. All rights reserved.status: publishe

Characterization methodology for copper-droplet losses in slags

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