Freeze Lining Research Articles

Modeling Freeze‐Lining Formation: A Case Study in the Slag Fuming Process

Slag fuming (SF) is a metallurgical process designed to recycle Zn‐containing slags derived from various industrial residues. To protect the reactor from corrosive molten slag, a deliberate as‐solidified slag layer, known as a freeze lining (FL), is formed on the reactor walls using intense water‐cooled jackets. In this article, a computational‐fluid‐dynamics‐based model capable of simulating FL formation in a SF furnace is presented. To capture the complex multiphase flow dynamics, heat transfer, and FL formation during SF, a volume‐of‐fluid model is coupled with a mixture continuum solidification model. Three phases are considered: gas, liquid bulk slag, and solid slag (FL). Moreover, two types of FL are distinguished: one that solidifies on the reactor wall in the bulk slag region and another that solidifies on the reactor wall in the freeboard region owing to slag splashing. Comparisons between calculated FL thickness and heat fluxes and corresponding industrial data demonstrate satisfactory agreement. In this outcome, the robustness of the model is underscored and confidence in its accuracy is instilled. In the simulation results, valuable insights are provided into the evolution of the fuming process, particularly regarding the slag bath temperature, slag splashing dynamics, FL formation, local heat fluxes through the reactor wall, and global net energy balance.

On the Impact of a Change in Slag Batch on the Freeze Lining Structure and Stability

On the Impact of a Change in Slag Batch on the Freeze Lining Structure and Stability

Elastic damage characterization of an ilmenite smelter freeze lining

A furnace freeze lining is necessary for safety and economic reasons in several smelting operations. The integrity of the freeze lining is put at risk by furnace power imbalances. Although numerous models have been derived to model the growth and depletion of the freeze lining due to power imbalances, no model exists for thermomechanical damage to the freeze lining. This study provides an initial pathway for modelling thermomechanical damage to the freeze lining in an ilmenite smelting furnace using information from the literature and experimental work. A methodology under the framework of continuum damage mechanics is proposed to model mechanical damage to the freeze lining due to phase changes, thermophysical changes, and constrained thermal expansion. Drill cores of solidified slag ingots were used to represent the freeze lining. The modified power law proved to be the best predictor of the softening response of the drill cores. Damage driving parameters were extracted from the raw data, and governing equations of the parameters with respect to temperature were derived for use in a finite element method (FEM) code.

Modeling Framework for the Simulation of an Electric Smelting Furnace Considering Freeze Lining Formation

The use of freeze linings to protect pyrometallurgical furnaces from chemically corrosive molten slags is a widespread technique in industrial processes. The main goal of the present study is to establish a modeling framework that considers fluid flow, heat transfer, and slag solidification to simulate freeze-lining formation and its dependency on operating conditions. A mixture continuum solidification model, which had been used for the solidification of metal alloys, was employed. Several parametric studies have been conducted to better understand the smelting process. The results demonstrate that the model can capture freeze-lining formation and predict the global energy balance and flow behavior of the smelting furnace. The freeze-lining thickness was shown to depend on heat removal intensity during the process and slag bath chemistry. A direct relationship between the average temperature in the refractory and freeze-lining thickness was also observed. This is an important indicator for furnace operators in controlling the furnace operation parameters. This improved knowledge offers the potential to further optimize furnace operations and reduce energy costs and environmental impacts. A discussion was presented on the different modeling assumptions considered and potential future model refinements.

Investigation of Bath/Freeze Lining Interface Temperature Based on the Rheology of the Slag

Freeze lining is a solidified layer of slag formed on the inner side of a water-cooled pyrometallurgical reactor, which protects the reactor walls from thermal, physical, and chemical attacks. Because of the freeze lining's high thermal resistance, the reactor heat losses strongly depend on the freeze lining thickness. In a batch process such as slag fuming, the conditions change with time, affecting the freeze lining thickness. Determining the freeze lining thickness is challenging as it cannot be measured directly. In this study, a conceptual framework based on the morphology and microstructure of freeze lining and the rheology of the slag is discussed and experimentally evaluated to determine the freeze lining thickness. It was found that the bath/freeze lining interface lies just below critical viscosity temperature. The growth of the freeze lining is primarily controlled by the mechanical and thermal degradation of the crystals forming at the interface. The bath/freeze lining interface temperature for the measured slag lies in the range of 1035–1070°C.

New Insights Into Refractory Lining Wear Mechanisms on a 22MVA FeMn Furnace

During February 2020 Assmang Manganese Cato Ridge Works #2 furnace was demolished with the intention to rebuild the furnace after 9 years of operation. Assmang Mn #2 was installed with a GrafTech freeze lining in 2011. This lining technology has exceptionally long lifetimes on ferromanganese and silicomanganese producing furnaces and no reported dig-out or excavation findings have been published to date utilizing this technology. Prior studies mainly focused on lining wear on carbon “big-block “lining technology or “hybrid” carbon rammed/alumina brick linings. Assmang Mn currently utilizes this lining technology on both their #1 and #5 furnaces, with #5 furnace now in its 13th year of operation. The aim of this study was to identify the main areas of wear on the tap-hole areas and sidewalls but most importantly on the hearth. Over the lifetime of #2 furnace bottom plate lifting has been observed from the grillage beams. The exact reason for this phenomenon was not certain and possible reasons could only be established by demolishing the existing lining and noting findings. The results of this dig-out will outline the exact lining condition over its campaign life. It was found that there was no significant wear on the tap holes or sidewalls, but metal ingress was found into hearth refractories. The findings of this report will greatly assist Assmang Mn on all future lining design considerations as well as give insight to possible hearth wear mechanisms.

New Refractory Lining Direction at Maringá Ferro-Liga Silico-Manganese Furnace #5

The Maringa Group operates 5 Submerged Arc Furnaces (SAF) in Itapeva, Brazil. The company is among the largest producers of manganese alloys in South America. Maringa #5 furnace is a 17 MVA furnace designed by DELP S.A. The original hearth design was made by Magnesita S.A. The furnace start-up was in 1993, and the current daily hot metal production is approximately 80 tons. The furnace has four tap holes, a rotating shell and a shell diameter of 8,5m. The grade produced is currently FeSiMn. After adding water shell cooling and changing the refractory philosophy, the latest furnace campaign started in May of 2019. In its first year, it has showed exceptional operational and lining performance. The wall and bottom lining temperatures are monitored by 72 single thermocouples. In addition, the cooling water temperature increase is monitored. This furnace was rebuilt incorporating the refractory freeze lining technology. Maringa expected that by making this critical change to their furnace they would accomplish a long and trouble-free campaign, with increased production by increased power and inner furnace volume. This well proven refractory concept is used today by several ferroalloy producers and the experiences from this together with the latest technical improvements made to the design will be discussed in this paper. Notable is that this furnace was originally installed using conventional rammed and insulated lining technology, resulting in a furnace campaign life of about 8 years. These experiences will also be described in this paper. It also elaborates on the details of the #5 furnace installation and describes the measures employed to ensure optimum furnace availability.

An investigation into the permeability of a PGM slag freeze lining to sulphur

Paper written on project work carried out in partial fulfilment of B.Eng (Metallurgy) degree

Thermal conductivity of titanium slags

In ilmenite smelting furnaces, a freeze lining of solidified slag is used to protect the furnace refractories against the aggressive titanium slag. Freeze lining thickness cannot be measured directly due to harshness of conditions inside the process, thus process modelling is required. Several parameters influence the thickness of the freeze-lining, one of them being thermal conductivity of the frozen slag. However, there is a lack of thermal conductivity values for high titanium slags −especially as a function of temperature. In this study, thermal conductivity of three titanium slag samples and an additional sample of freeze-lining was measured from room temperature to 1100/1400 °C with the laser flash analysis method. In addition, thermal expansion and microstructures of the samples were studied to provide an extensive understanding of how microstructure will affect thermal conductivity. The thermal conductivity of the slag samples was found to increase from 1.2 to a maximum of 2.4 W/(m K) when increasing temperature from room temperature to 1100 °C. An additional experiment at 1400 °C showed that the thermal conductivity increased further as the temperature increased. The freeze-lining sample behaves differently, with conductivity being the highest at room temperature, 2.2 W/(m K).

Freeze linings in the Al2O3–CaO–SiO2 system

Degradation of refractory lining in pyrometallurgical furnaces can be prevented by cooling down the furnace shell and solidifying a protective layer of slag, called freeze lining, on top of the refractory. Freeze lining behaviour in the high and low SiO region of the AlO–CaO–SiO system was investigated using the previously established cooled probe technique. Temperatures in the freeze lining were determined through a combination of direct measurements and linking measured compositions with equilibrium temperatures. Bath–freeze lining interface temperatures ranging from the solidus to the liquidus temperature were observed depending on composition. As opposed to freeze linings in other systems, considerable porosity was found to be present. A freeze lining containing dicalcium silicate was found to shatter on quenching, suggesting this type of freeze lining may not be advisable to use in industrial applications.

Investigation of the effect of bath temperature on the bath–freeze lining interface temperature in the CuOx–FeOy–MgO–SiO2 system at copper metal saturation

AbstractIn pyrometallurgy, freeze lining technology is used to ensure the integrity of the furnace wall. This involves cooling down the wall and solidifying part of the liquid bath material onto the wall, creating a protective layer. In the current study, the cooled probe technique was used to perform a series of laboratory experiments, producing freeze linings under controlled conditions in the CuOx–FeOy–MgO–SiO2 system. Earlier research has shown the bath–freeze lining interface temperature at thermal steady state can be at subliquidus temperatures under certain conditions. The current study focuses on the effect of bath temperature on this bath–freeze lining interface temperature and has indicated that increasing the bath temperature increases the interface temperature. Furthermore, the crystals forming the interface did not include the primary phase and formed a non-planar interface at thermal steady state.

Responses of Lithium-Modified Bath to a Shift in Heat Input/Output Balance and Observation of Freeze-Lining Formation During the Heat Balance Shift

In the aluminum electrolysis process, new industrial aluminum/electricity power markets demand a new cell technology to extend the cell heat balance and amperage operating window of smelters by shifting the steady states. The current work investigates the responses of lithium-modified bath system when the input/output balance is shifted in a laboratory analogue to the industrial heat balance shift. Li2CO3 is added to the cryolite-AlF3-CaF2-Al2O3 system as a bath modifier. A freeze deposit is formed on a ‘cold finger’ dipped into the bath and investigated by X-ray diffraction analysis and electron probe X-ray microanalysis. The macro- and micro-structure of the freeze lining varies with the bath superheat (bath temperature minus bath liquidus temperature) and an open crystalline layer with entrapped liquid dominates the freeze thickness. Compared with the cryolite-AlF3-CaF2-Al2O3 bath system, the lithium-modified bath freeze is more sensitive to the heat balance shift. This freeze investigation provides primary information to understand the variation of the side ledge in an industrial cell when the lithium-modified bath system is used.

Investigation of the Influence of Heat Balance Shifts on the Freeze Microstructure and Composition in Aluminum Smelting Bath System: Cryolite-CaF2-AlF3-Al2O3

In an aluminum electrolysis cell, the side ledge forms on side walls to protect it from the corrosive cryolitic bath. In this study, a series of laboratory analogue experiments have been carried out to investigate the microstructure and composition of side ledge (freeze linings) at different heat balance steady states. Three distinct layers are found in the freeze linings formed in the designed Cryolite-CaF2-AlF3-Al2O3 electrolyte system: a closed (columnar) crystalline layer, an open crystalline layer, and a sealing layer. This layered structure changes when the heat balance is shifted between different steady states, by melting or freezing the open crystalline layer. Phase chemistry of the freeze lining is studied in this paper to understand the side ledge formation process upon heat balance shifts. Electron probe X-ray microanalysis (EPMA) is used to characterize the microstructure and compositions of distinct phases existing in the freeze linings, which are identified as cryolite, chiolite, Ca-cryolite, and alumina. A freeze formation mechanism is further developed based on these microstructural/compositional investigations and also thermodynamic calculations through the software-FactSage. It is found that entrapped liquid channels exist in the open crystalline layer, assisting with the mass transfer between solidified crystals and bulk molten bath.

An investigation of factors influencing freeze lining behaviour

ABSTRACTRecent studies have indicated that the steady state thicknesses and interface temperatures of freeze linings can be influenced by factors other than the thermal parameters of the systems. To explore these possibilities further cold modelling of freeze linings was undertaken in the CaCl2–H2O system using an experimental apparatus that enabled the variation of both the bath temperature and the fluid flow rate. Through in situ experimental observations, it was shown that the phases formed, the deposit/liquid interface temperature and the freeze lining thicknesses depend strongly on chemical parameters and elementary reaction steps, which are not considered by the conventional thermal treatment of freeze linings. These results indicate the need for further systematic investigation of various process parameters that influence the elementary reaction steps that may be active in these systems under dynamic steady state conditions.

Freeze lining formation on water cooled refractory wall

The experimental study was focused into effects of a ceramic refractory lining, covering a metallic cooling element, as well as to properties and microstructures of the freeze lining formed in such cases in copper smelting and converting slags. The modified heat transfer pattern, due to the ceramic refractory lining and its air gap(s), increases effectively the average freeze lining temperature, heating its cold face from 30 to 50°C of the direct contact close to 800–900°C, even with refractory thicknesses of a few millimetres. This general feature has direct consequences to the microstructure and thickness of the freeze lining and probably also to its growth rate. The observations about effects of high copper iron silicate slag on the chemical corrosion of direct bonded magnesia–chromia refractories suggest that copper oxides are not the most aggressive components of direct to blister smelting slags. Impregnation of the fluid direct to blister slag into the open porosity of the brick, even on the cooled furnace wall, extends quickly in the initial contact several hundred micrometres into the refractory.

Microstructure characterisation of freeze linings formed in a copper slag cleaning slag

The initial growth rate of freeze linings on water-cooled elements submerged in molten iron silicate slag is fast. The freeze lining microstructure forming on water cooled steel surface in a high-silica, slag cleaning furnace slag of a direct-to-blister copper smelter is mostly glassy or amorphous. It contains 5-30 ?m magnetite crystals, very small and larger copper droplets as well as small magnetite and silicate nuclei embedded in the glassy silica-rich matrix. Chemically the formed freeze linings are more silica-rich than the slag from which they were generated. Magnetite (spinel) is the primary phase of the solidifying SCF slag but it does not form a continuous network through the freeze lining. Its strength is given by the intergranular silica-rich phase which initially is glassy or microcrystalline. Due to only partial slag reduction in the SCF process, large magnetite crystals are present in the freeze lining and seem to interact physically with copper droplets.

Crystallization of iron-containing Si–Al–Mg–O glasses under laser floating zone conditions

The glass system MgO–Al2O3–SiO2–FeOy has been mentioned as possible electrolyte for pyroelectrolysis of iron. This work was focused on the study of crystallization behaviour of the iron-containing Al–Mg–Si–O glass system under laser floating zone (LFZ) to identify expected changes occurring under freeze lining or other high temperature gradients at large industrial scale. Lower iron content and faster fibre growth were found favourable for the formation of isolated iron cations in the glass after cooling. The crystallization process, accompanied with separation of mullite and cordierite-type phases, is strongly affected by the formation of nanosized iron-containing clusters, confirmed by Mössbauer and EPR spectroscopies. LFZ method shows good prospects for studying crystallization/vitrification mechanisms in silicate-based glasses with additions of redox-active cations, by providing flexibility in tuning their oxidation state and attaining frozen-in conditions.

Freeze lining formation in continuous converting calcium ferrite slags. II

The heat transfer properties of freeze linings generated in laboratory conditions, by industrial calcium ferrite slags from a continuous copper matte flash converting furnace, have been studied in situ in the molten slag using a water cooled probe technique. The measured heat conductivity of the freeze lining formed, estimated from direct measurements in steady state conditions, was 8·0±1·5 W m−1 K−1. The obtained heat conductivity of the freeze lining is 50–100% higher than that of the iron silicate slag freeze linings. The calcium ferrite slag forms a fully crystalline freeze lining. Various ferrites and metallic copper develop the observed high heat conductivity when copper precipitated from the slag during solidification fills the intergranular cavities of the ferrite crystals tightly in forming the freeze lining layer.On a étudié les propriétés de transfert de chaleur de revêtements de gel engendrés en laboratoire par des scories industrielles de ferrite de calcium, à partir d’un four éclair en continu de cémentation de matte de cuivre, dans la scorie fondue en utilisant la technique du capteur refroidi à l’eau. La conductibilité thermique mesurée du revêtement de gel formé, estimée par des mesures directes en conditions de régime permanent, était de 8·0±1·5 W m−1 K−1. La conductibilité thermique obtenue du revêtement de gel est de 50 à 100% plus élevée que celle des revêtements de gel de scorie de silicate de fer. La scorie de ferrite de calcium forme un revêtement de gel entièrement cristallin. Les différentes ferrites et le cuivre métallique développent la conductibilité thermique élevée observée lorsque le cuivre précipité à partir de la scorie pendant la solidification remplit hermétiquement les cavités intergranulaires des cristaux de ferrite dans la couche de revêtement de gel en formation.

Freeze lining formation in continuous converting calcium ferrite slags. I

The initial freeze-lining growth rate in calcium ferrite slags at copper saturation is high and comparable to iron silicate slags. The entire freeze-lining layer is crystalline, from the cold end to its hot-face in contact with the molten slag. Industrial copper converting slags from a continuous flash converting furnace, processing solid high-grade matte to blister copper, generate a thin calcium sulphate bonding layer against the water-cooled metal surface during the very first minutes of the slag-to-cooling element contact. The rare solidification behaviour was observed using the water-cooled probe technique in rotating MgO crucibles, at slag temperatures of 1325°C when liquidus temperature of the flash converting slag was estimated to locate at about 1245°C. The body of the freeze lining is mostly composed of magnetite, various mixed calcium–copper ferrites and delafossite embedded in an intergranular phase of metallic copper and some copper oxide. The arsenic oxides dissolved in the slag are precipitated as solid calcium arsenates in the freeze lining.La vitesse initiale de croissance du revêtement de gel des scories de ferrite de calcium saturées en cuivre est élevée et comparable à celle des scories de silicate de fer. Toute la couche du revêtement de gel est cristalline, de l’extrémité froide jusqu’à sa face chaude en contact avec la scorie fondue. Les scories industrielles de cémentation de cuivre d’un four éclair de cémentation en continu, transformant une matte solide de haute qualité en un cuivre ampoulé, engendrent une mince couche de liaison de sulphate de calcium contre la surface de métal refroidie à l’eau lors des toutes premières minutes du contact de la scorie avec l’élément de refroidissement. On a observé ce comportement rare de solidification en utilisant la technique de la sonde refroidie à l’eau dans des creusets de MgO en rotation. La température de la scorie était de 1325°C alors que la température de liquidus de la scorie de cémentation éclair était estimée à environ 1245°C. Le corps du revêtement de gel est composé principalement de magnétite, de diverses ferrites mélangées de calcium et cuivre et de délafossite, qui sont incluses dans une phase intergranulaire de cuivre métallique et d’un peu d’oxyde de cuivre. Les oxydes d’arsenic dissous dans la scorie sont précipités dans le revêtement de gel sous forme d’arséniates solides de calcium.

Balance of Raw Input and Energy of Titanium Slag Smelting in Closed High Power Direct Current Electric Arc Furnace

There were two controlling factors, which should be kept balance of raw input and energy during titanium slag smelting in closed high power direct current electric arc furnace, DC furnace for short. The first factor contains compositions and ratio of input raw materials. Compositions of the raw could influence the progress and technical indicators of DC furnace smelting, and on the meanwhile, it could determine the compositions of impurity and contents of TiO2 in titanium slag. On the other hand, the ratio of input infected the compositions of the slag, while too much anthracite would disadvantage to produce high quality slag because of impurity being reduced continually. Meanwhile much more low-state titanium would be generated. The second factor was an important one that energy equilibrium, which influence the production safety of DC furnace. It could keep balance of input and output energy, while the heatloss was 4.971 MVA. While the ratio of anthracite remained the same, the energy would effect slag viscosity and equilibrium of freeze lining directly. The energy input higher than smelting need would result in burnthrough of furnace wall and hearth, on the contrary, lower than smelting need would bring about slagging difficulties. Consequently, raw input and energy must be kept dynamic balance in order to achieve high quality titanium slag and protect freeze lining.