Freeze Linings Research Articles

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.

Fundamental and Formation Aspects of Slag Freeze Linings: A Review

Fundamental and Formation Aspects of Slag Freeze Linings: A Review

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.

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.

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.

Understanding Slag Freeze Linings

Slag freeze linings, the formation of protective deposit layers on the inner walls of furnaces and reactors, are increasingly used in industrial pyrometallurgical processes to ensure that furnace integrity is maintained in these aggressive, high-temperature environments. Most previous studies of freeze-linings have analyzed the formation of slag deposits based solely on heat transfer considerations. These thermal models have assumed that the interface between the stationary frozen layer and the agitated molten bath at steady-state deposit thickness consists of the primary phase, which stays in contact with the bulk liquid at the liquidus temperature. Recent experimental studies, however, have clearly demonstrated that the temperature of the deposit/liquid bath interface can be lower than the liquidus temperature of the bulk liquid. A conceptual framework has been proposed to explain the observations and the factors influencing the microstructure and the temperature of the interface at steady-state conditions. The observations are consistent with a dynamic steady state that is a balance between (I) the rate of nucleation and growth of solids on detached crystals in a subliquidus layer as this fluid material moves toward the stagnant deposit interface and (II) the dissolution of these detached crystals as they are transported away from the interface by turbulent eddies. It is argued that the assumption that the interface temperature is the liquidus of the bulk material represents only a limiting condition, and that the interface temperature can be between T liquidus and T solidus depending on the process conditions and bath chemistry. These findings have implications for the modeling approach and boundary conditions required to accurately describe these systems. They also indicate the opportunity to integrate considerations of heat and mass flows with the selection of melt chemistries in the design of future high temperature industrial reactors.

Investigation of the Freeze-Lining Formed in an Industrial Copper Converting Calcium Ferrite Slag

Pyrometallurgical coppermaking processes are operated under intensive reaction conditions; high process temperatures and vigorous bath agitation is used to increase the kinetics of reactions and to achieve high smelter throughput. Slag freeze-lining reactor wall protection is a widely used technology in coppermaking processes, such as flash smelting and converting reactors. Freeze-linings mitigate and resist the effects of thermal and chemical attack by aggressive slags. In this laboratory-based study, a water-cooled probe “cold finger” technique has been used to investigate freeze-lining formation with calcium ferrite slags in equilibrium with metallic copper; the slag composition reflects that used in the industrial copper flash converting furnace of Rio Tinto—Kennecott Utah Copper. The effects of probe immersion times on the thickness and microstructures in the freeze-lining deposits have been investigated. A range of complex oxide solutions and copper-containing phases have been found in the deposits. The phase assemblages formed from the industrial calcium ferrite slag in the steady-state deposit are very complex and information on the phase equilibria of the multi-component systems with addition of minor elements may not be available. Subsolidus and subliquidus phase equilibria in the Cu-Ca-Fe-O system at metallic copper saturation along with interpolated temperature across the deposit, microstructural changes and compositional trends in the phases in the deposit have been used to understand the formation and characteristics of the phases in the steady-state freeze-lining. Also, it has been shown that under steady-state conditions a dense sealing layer consisting primarily of the spinel primary phase is formed at the deposit/liquid interface; however, the interface temperature is below the liquidus temperature. The findings of the study have potentially important implications for the operation of the converting furnace and the design of freeze linings in metallurgical systems.

Investigation of Freeze Lining in Copper-Containing Slag Systems: Part III. High-Temperature Experimental Investigation of the Effect of Bath Agitation

Freeze-lining technologies have been employed to protect the cooling walls of reactors from chemically aggressive molten reactants. To date, the designs of freeze linings for pyrometallurgical reactors have been based on the basic assumption that the interface between the deposit and the bath remains at the liquidus temperature of the bulk liquid. There is, however, increasing evidence that interface temperature between stagnant deposit and bath is less than the liquidus of the bulk liquid. A previous study also demonstrated that the effects of bath chemistry need to be taken into account in freeze-lining designs. To investigate the fundamental processes involved in the formation and stability of the deposit, experimental laboratory studies have been undertaken in the Cu-Fe-Si-Al-O slag system in equilibrium with metallic copper using an air-cooled probe technique. In the current study, the effects of bath agitation on the microstructure, morphologies of the phases, and formation of various layers across the freeze-lining deposit were studied at steady-state conditions. It appears that the changes in the fluid flow pattern through changes in shear intensities result in corresponding changes in the deposit microstructure, formation of the sealing primary phase layer, thus influencing the interface temperature between freeze-lining deposit and the liquid bath.

Investigation of Freeze Linings in Copper-Containing Slag Systems: Part II. Mechanism of the Deposit Stabilization

A major industrial problem in high-temperature liquid reaction systems is the attack of furnace components by chemically aggressive molten reactants. Freeze-lining technologies involving the deliberate formation of controlled frozen deposits are increasingly being applied to extend the range of liquid bath compositions and process temperatures that can be used; this has resulted in significant increases in process performance and productivity. It has been widely assumed that the interface between the stationary frozen layer and the agitated molten bath at steady state consists of the primary phase, which stays in contact with the bulk liquid at the liquidus temperature, Tliquidus. It has been shown in the current laboratory-based studies through the use of a cold finger technique that, at steady state and in selected ranges of process conditions and bath compositions, the phase assemblage present at the deposit/liquid interface is not that of the primary phase alone. The microstructural observations clearly demonstrate that the temperature of the deposit/liquid bath interface, Tf, can be lower than the liquidus temperature of the bulk liquid, Tliquidus. These observations point to a significant change in the mechanism and behavior of the systems. To explain this phenomenon, it is proposed that the steady-state thickness of freeze linings is not the result of equilibrium freezing but rather represents a state of dynamic equilibrium that is critically dependent on the relative rates of crystallization, mass, and heat transfer processes, occurring close to and at the deposit interface. The mechanisms taking place in the boundary liquid layer involve both partial crystallization/remelting and continuous removal of solids. This finding has important implications for the design of the high-temperature industrial reactors and selection of ranges of melt chemistries and conditions that can be used. This finding means that temperatures below the liquidus can be selected for some processes, resulting potentially in significant savings of energy and increases in throughput of pyrometallurgical reactors. The findings are generic and are not limited to the specific chemical systems reported in the article.

Freeze-lining Formation and Microstructure in a Direct-to-blister Flash Smelting Slag

Copper smelting in one processing step directly from concentrates to blister copper has been realized on an industrial scale in a few smelters, using concentrates with a high Cu/Fe-ratio. The fluxing of any direct-to-blister slag is demanding task as it must be fluid and maintain suitable properties in the oxidising conditions of copper making, and the reducing conditions of slag cleaning. The smelting slags in the direct-to-blister furnaces contain much more chemically dissolved copper than typical matte making slags. In this investigation, an industrial direct-to-blister slag was used in a freeze lining growth kinetics study. The freeze lining was formed on a water cooled metal finger at typical smelting temperatures using different dipping times from 5 to 120 min. The growth kinetics of the lining was very fast in the initial stage of the slag contact with the cooled metal surface. The quenched samples showed characteristic solidification zones from the cold end towards the hot side of the freeze lining and the molten slag shown already in other freeze linings and different slag types. The slag chemistry modifies the solidification pattern very much and thus the crystalline phases in the lining included also phases created by the high copper oxide concentration as well as the specific gangue assay of the smelters feed mixture. The thermal stability of the freeze lining in high-in-copper DB slags is discussed as well as the mechanism of delafossite precipitation.

Investigating the effect of slag bath conditions on the existence of multiphase emulsion zones in PGM smelting furnaces using computation fluid dynamics

The presence of chrome in electric arc furnaces smelting platinum group metals (PGM’s) has a number of potentially negative consequences. In cases where the slag chrome content is above the saturation limit the existence of near-stagnant conditions near the slag/matte interface increases the risk of chromite spinels settling and consolidating into a “mushy layer”, a three-phase suspension of slag, chromite and matte. The hold-up of matte above the elevation of the slag/matte interface can lead to the attack of freeze linings and copper cooling elements potentially causing failures of the furnace lining and significant downtimes as well as major safety risks. This paper investigates the relationship between typical furnace operating parameters and the behavior of the slag bath with respect to the formation of the “mushy” layer at the slag/matte interface using computational fluid dynamics (CFD). The extent of the potential “mushy” layer is seen to increase with decreasing electrode immersion and furnace power. Electrode immersion is, however, a considerably stronger driver is this regard. The CFD modelling results have aided in selecting appropriate furnace electrode immersion/power combinations intended to minimize “mushy” layer formation.

Freeze-Lining Formation of a Synthetic Lead Slag: Part II. Thermal History

Recently, freeze linings have been selected more frequently to protect pyrometallurgical reactor walls, due to a number of advantages over conventional refractory lining such as a self-regenerating capability and the possibility of operating under high-intensity process conditions. A freeze lining is formed on a cooled reactor wall in a time-dependent temperature gradient. To model freeze-lining behavior, input data on several assumptions, such as the phase formation and the temperature at the bath–freeze-lining interface during freeze-lining formation, are needed. In order to provide experimental data, the freeze-lining formation of a synthetic lead slag system (PbO-FeO-Fe2O3-ZnO-CaO-SiO2) is investigated. A lab-scale freeze lining was produced by submerging an air-cooled probe into a liquid slag bath for 120 minutes. The temperature evolution during freeze-lining formation was estimated using the experimentally determined position and composition of the phases, the phase-temperature relations predicted with the thermodynamic computer package FactSage, and the results of reference experiments. For the studied slag system, it is concluded that heat transfer is much faster than mass transfer and crystallization. As a result, the liquid in front of the freeze lining undercools. The degree of undercooling depends on the solidification rate. It is concluded that the temperature at the bath–freeze-lining interface varies between the glass transition and liquidus temperatures of the slag bath during freeze-lining formation.

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.

Freeze-Lining Formation of a Synthetic Lead Slag: Part I. Microstructure Formation

Recently, freeze linings have been selected more frequently to protect pyrometallurgical reactor walls, due to a number of advantages over conventional refractory linings, such as a self-regenerating capability and the possibility of operating under high-intensity process conditions. A freeze lining is formed on a cooled reactor wall in a time-dependent temperature gradient. A full description of freeze-lining development, including phase formation as a function of temperature, time, and position, is important in understanding freeze-lining formation mechanisms and may be instrumental for the design of a sustainable freeze-lining concept. Freeze-lining formation is therefore investigated in a synthetic lead slag system: PbO-FeO-Fe2O3-ZnO-CaO-SiO2. Lab-scale freeze linings were produced by submerging an air-cooled probe into liquid slag for different times ranging from 1 to 120 minutes. The freeze-lining microstructures were characterized with optical microscopy, scanning electron microscopy (SEM), and electron probe X-ray microanalysis. The results were compared with the results of reference experiments. The freeze-lining formation of the studied slag system is initially dominated by the formation of glass and a highly viscous liquid. After 1 minute, extensive crystallization occurs and further growth of the freeze lining is determined by the growth of the melilite phase, which forms networking crystals. Because the heat transfer occurs very quickly, these melilite crystals form in undercooled liquid. Because the initial solidification rate is high, mass exchange between the freeze lining and bath affects the freeze-lining growth only when the freeze lining almost reaches its steady-state thickness.

Continuous Fuming of Zinc-Bearing Residues: Part I. Model Development

The disposal of zinc-containing residues is an important sustainability issue currently facing the metallurgical industry. These residues can be treated by using zinc slag-fuming processes in which heavy metals are chemically reduced and evaporated from a molten slag bath. Continuous operation of these processes requires high zinc-fuming rates while retaining vessel integrity. To meet these challenges, a new generation of technologies is being developed that depends on the formation of a freeze lining on the internal reactor walls. A general zinc-fuming process model that simultaneously describes the chemical reactions, phase equilibria, and thermal- and heat-transfer outcomes of these processes has been developed using the FactSage thermodynamic databank system and the ChemApp programmer’s library for thermochemical applications. This simultaneous description of these processes allows the model to be used as part of a new approach to the design of freeze linings, namely, the use of slag engineering and composition adjustment to obtain optimum process efficiency and freeze lining behavior.

Water-cooled probe technique for the study of freeze lining formation

Furnace protection by water-cooled freeze linings becomes increasingly important as the metal producing industry attempts to achieve higher process intensities. Systematic investigations of the growth and the resulting microstructure and compositional profile of freeze linings are necessary to understand the behavior of freeze linings, their relation with the industrial process, and their interaction with the wall cooling system. We have developed a technique based on the submergence of a water-cooled probe into a liquid slag bath. Freeze linings of two industrial nonferrous slags have been produced using this technique and their growth, microstructural, and compositional profiles as a function of submergence time were determined. Thermodynamic equilibrium for the investigated slag systems was calculated and compared with the observed microstructures. The freeze linings form in approximately 15 minutes. Close to the water cooling, the freeze linings are predominantly amorphous in structure. With increasing distance from the water cooling, the proportion of crystalline phases increases and bath material is entrapped in the microstructure. Cellular crystals are observed close to the bath. The freeze linings exhibit an approximate homogeneous composition. The results demonstrate that the technique is a successful tool in obtaining information on the growth, microstructure, and composition of freeze linings in industrial water-cooled furnaces.