Freeze lining formation in continuous converting calcium ferrite slags. II J. Jansson, P. Taskinen* and M. Kaskiala 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 m21 K21. 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 e´tudie´ les proprie´te´s de transfert de chaleur de reveˆtements de gel engendre´s en laboratoire par des scories industrielles de ferrite de calcium, a` partir d’un four e´clair en continu de ce´mentation de matte de cuivre, dans la scorie fondue en utilisant la technique du capteur refroidi a` l’eau. La conductibilite´ thermique mesure´e du reveˆtement de gel forme´, estime´e par des mesures directes en conditions de re´gime permanent, e´tait de 8?0¡1?5 W m21 K21. La conductibilite´ thermique obtenue du reveˆtement de gel est de 50 a` 100% plus e´leve´e que celle des reveˆtements de gel de scorie de silicate de fer. La scorie de ferrite de calcium forme un reveˆtement de gel entie`rement cristallin. Les diffe´rentes ferrites et le cuivre me´tallique de´veloppent la conductibilite´ thermique e´leve´e observe´e lorsque le cuivre pre´cipite´ a` partir de la scorie pendant la solidification remplit herme´tiquement les cavite´s intergranulaires des cristaux de ferrite dans la couche de reveˆtement de gel en formation. Keywords: Copper converting, Calcium ferrite, Slag, Freeze lining, Cooling element, Blister copper, Heat conductivity
Introduction In high-intensity smelting technologies, energy and resource usage can be minimised and throughput as well as the reactor mass flow rates maximised. The number as well as physical sizes of the processing equipments are smaller in those smelters compared to the plant throughput and production rate, and thus is also the capex or investment cost. In the high-intensity smelting process engineering designs, vessel integrity can be maintained and secured with a large number of externally cooled, well conducting cooling elements embedded in various critical locations of the refractory linings and inside the vessel, Aalto University, School of Chemical Technology, Department of Materials Science and Engineering, Metallurgical Thermodynamics and Modelling Research Group *Corresponding author, e-mail
[email protected] ß 2014 Canadian Institute of Mining, Metallurgy and Petroleum Published by Maney on behalf of the Institute Received 1 March 2013; accepted 27 May 2013 DOI 10.1179/1879139513Y.0000000093
typically on side walls. They generate protective freeze linings or ‘autogenous’ linings on the furnace walls and their hot faces from the smelter products, typically the slag.1 Pure silica-free calcium ferrite slags, i.e. the system CaO–Fe–O for fluxing solid iron oxides, were taken into industrial use in copper smelting in late 1970s and 1980s when the modern continuous copper matte converting technologies were adopted into large scale industrial operations.2,3 This decision was followed by an intensive academic research focussed on the thermodynamic and phase equilibrium properties of such slag systems. Only limited information has so far been available about their physical or transport properties4–7 other than viscosity and electrical conductivity.8–13 The aim of this work was to evaluate the freeze lining properties at the copper converting temperatures when the molten calcium ferrite slag maintains direct contact
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1 Design of the water cooled probe: water temperatures inside the probe were measured with six A Class PRTs and temperatures inside the freeze lining with two S-type thermocouples, located in alumina sheaths with a diameter of 4 mm; a schematic illustration of the experimental procedure and growth of a freeze lining on the probe is shown on the right
with a water cooled metal surface and the solid slag layer is formed on the cooling element. The heat transfer properties will be recorded in situ continuously from the first moments of transient heat transfer and freeze lining growth to the steady state condition, when the freezelayer growth rate is zero and the dissipated thermal energy to the cooling element is in balance with the energy supply to the molten slag bath.
Experimental Apparatus The experiments were carried out with a 25 kVA vertical 155 mm i.d. tube furnace, delivered by Entech ¨ ngelholm, Sweden). The furnace temperature was (A regulated by a Eurotherm 2408 PID control unit. The studied slag was melted in cylindrical, dense 99?4% MgO crucibles supplied by Ozark Technical Ceramics Inc. (MO, USA) with outer diameters from 80 to 100 mm. More detailed information on the experimental furnace can be found in Refs.14–16 The freeze-lining sample was solidified on the surface of a water cooled probe built from two concentric AISI 316L pipes with outer diameters of 8 and 14 mm and a wall thickness of 1 mm. The temperature of the water flow inside the probe was monitored with six sensitive and stable A Class PT100 temperature sensors (platinum resistance thermometers, PRT), manufactured by SKS Automation (Finland), located on two levels of the cooling water circuit. The temperature of the freeze lining was monitored with two calibrated S-type thermocouples at fixed distances, on one side of the probe. The assembly of the thermal sensors in the probe together with an illustration of the experimental procedure is shown schematically in Fig. 1.
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The lower PRTs in the probe were located completely below the surface of the molten slag in the crucible when the water cooled steel probe was inserted in the slag during the measurement period, thus providing a calorimetric arrangement for an accurately known length and well defined section of the probe, for measuring the heat flux through the forming freeze lining. The cooling water flow in the probe was arranged from the top into the tip through the inner tube and out along the annulus. The chemistry of the industrial Flash Converting Furnace (FCF) slag used in the experiments and the microstructures obtained in the solidified freeze linings have been described in detail in part I of this paper.16
Results Methods for the calculation The freeze linings grow as a result of the thermal imbalance among the superheated slag, latent heat released during the solidification, and the cooled wall of the furnace or smelting vessel. The thermal balance among the three factors at each moment can be expressed as qbath zqfusion ~qwall
(1)
where qbath is thermal flux by superheat of the solidifying slag, qfusion is the amount of latent heat released during the slag solidification and qwall is the amount of heat removed and dissipated by the cooling element of the furnace wall. In a very simplified 1D case, the components of the heat flow in equation (1) can be written17 as qbath ~hbath A(Tbath {Tsurface ),
(2)
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qfusion ~rHf
dbfl , dt
Freeze lining formation in calcium ferrite slags. II
(3)
dT , (4) j dx hotface where hbath is the convection coefficient from the bath to the freeze lining, A is the area of interface between the bath and the cooling element, Tbath is the average bath temperature, Tsurface is the surface temperature of the freeze lining hot face, often approximated as the liquidus temperature of the slag bath,17 Hf the amount of heat released during the solidification, bfl is the thickness of the freeze lining, kfl is thermal conductivity of the freeze lining and dT=dxjhotface is the thermal gradient in the freeze lining evaluated at the hot face of the freeze lining. In the steady state conditions, thickness of the freeze lining is constant as a function of time. This also means that qfusion50 and the temperature distribution in the wall is constant. In this case it can be written as qwall ~kfl A
qwall ~
Tsurface {Twater , bcooling bfl brefr 1 z z z zSRair{gaps kfl A krefr A kcooling A hwater A (5)
where Twater is the (average) temperature of cooling water, brefr is the thickness of the refractory layer and krefr is its heat conductivity, bcooling is the thickness of the cooling element and kcooling is its heat conductivity, hwater is the convection coefficient of cooling water, and SRair2gaps is the sum of thermal resistances, because of air gaps between the furnace shell and refractory, and between the refractory and freeze lining. If all the constants and factors in equation (5) are known, the maximum thickness of the freeze lining in the steady state conditions can be calculated.17 Such a calculation gives the maximum thickness of the freeze lining, due to the strong simplification made often in the literature.17 In those cases liquidus temperature of the slag is assumed to be the prevailing temperature on the hot face of the freeze lining even if this temperature, where the fraction of solid phase by definition is zero, does not provide any mechanical strength to the lining. Solving equation (5) for bfl we get AðTsurface {Twater Þ brefr bcooling 1 bfl ~kfl { { { qwall krefr kcooling hwater (6) In the experimental setup used, there were no refractory materials on the water cooled probe, thus equation (6) is simplified by the fact brefr/krefr50. It should be taken into account that the model (6) is for 1D-planar walls, so it can be employed for most industrial vessels after the simplifications mentioned. The thermal conductivity of the freeze lining formed was calculated based on the experimental temperature data, measured by the two S-type thermocouples fixed at known distances next to the probe, as shown in Fig. 1. The assumptions made for this calculus were:
2 Illustration of the experimental setup with the probe in the centre and two S-type thermocouples at distances r1 and r2.; in this arrangement the temperature profile was assumed to be symmetric so that temperature at a certain radius r is always constant, for example at r5r2 the temperature is T5Ts,2
(i) temperature profile inside the freeze lining is symmetric (ii) the heat transfer rate in the radial direction is constant. The assumed conditions and the heat transfer geometry are illustrated in Fig. 2. By assuming that the heat transfer rate is a constant in the radial direction, the heat transfer rate in a cylinder can be expressed as (Ref. 18, p. 137) qr ~
2pLkðTs,1 {Ts,2 Þ , lnðr2 =r1 Þ
(7)
where qr is the heat transfer rate, L is the length of the cylinder, k is the thermal conductivity of the material, Ts,x is the temperature at the surface x, and rx is the radius of the surface x. Solving equation (7) for heat conductivity k we get k~
qr lnðr2 =r1 Þ , 2pLðTs,1 {Ts,2 Þ
(8)
Equation (8) can be used for calculating thermal conductivity if the amount of heat transferred, length of the cylinder, temperatures at the two surfaces, and their radii are known. For the experimental setup used in this work we have r1525 mm, r2515 mm, and L531 mm. The heat transfer rate to the probe qr was calculated from the amount of heat transferred to the flowing cooling water in the probe, by assuming constant water flow and latent energy, steady state conditions, and no other energy generation (thermal, mechanical, etc.) : qr ~ mcp ðTout {Tin Þ, (9) : where m5mass flow rate of water, cp5specific heat of water at constant 1 atm pressure and Tin and Tout the water temperatures before and after submerging a part of the probe into the slag respectively (Ref. 18, p. 17). Tin was measured in the experiments with one PRT
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3 Calculated thermal conductivities of the freeze linings formed in the experiments with the industrial calcium ferrite FCF slag; the conductivities were calculated from equations (8) and (9) using measured temperature readings in the cooling water and the growing freeze lining
whereas for Tout the average of three simultaneously measured readings from similar PRTs was used, as shown in Fig. 1. As no other model for the freeze lining growth for a cylindrical wall is available in the literature, the provided model (8) with no simplifications or assumed conditions, except the geometry, was used also in this experimental context.
Heat transfer properties of the lining The heat transfer properties of the freeze lining layer were calculated from equation (8) in the experiments carried out with the FCF slag and the geometric arrangement of heat transfer described above. Based on the measurements and the calculation method provided above, thermal conductivity of the studied FCF slag was in the range of k57–10 W m21 K21 (8?0¡1?5 W m21 K21), obtained from the steady state conditions of each run. The measured thermal conductivity of the FCF slag is illustrated in Fig. 3 as a function of the immersion time in the molten slag. The thermal conductivity evaluated in Fig. 3 suggests that the steady state conditions were reached after approximately 50 min (t
