STRESS DEVELOPMENT IN REFRACTORY DUE TO THE RATE OF TEMPERATURE CHANGE 57

LAUZON, P., KONING, A., and DONOHUE, I. Stress development in refractory due to the rate of temperaturechange: a pressure vessel refractory lining design consideration. Hydrometallurgy Conference 2009, TheSouthern African Institute of Mining and Metallurgy, 2009.

Stress development in refractory due to the rate oftemperature change: a pressure vessel refractory

lining design consideration

P. LAUZON*, A. KONING*, and I. DONOHUE*

*Hatch, Ontario, Canada

Vessels that are used for pressure hydrometallurgical operationsrequire an impermeable membrane that provides corrosion protectionand one or more courses of refractory lining or ceramic brick. Asignificant amount of work goes into the design of a refractory liningto ensure that it is mechanically stable at steady state processconditions. Steady state analysis techniques are covered in detail inthe paper ‘Design fundamentals for hydrometallurgy pressure vesselrefractory linings’ by A. Koning and P. Lauzon. Since newhydrometallurgical processes are pushing the pressure andtemperature envelope with each new generation of plants it becomesequally important to perform transient analysis.

The purpose of this paper is to build on the design fundamentals ofrefractory linings by demonstrating the importance of transientthermal and stress analysis. A transient analysis is necessary becauseheating or cooling a vessel too quickly can result in lining failure as aresult of exceeding the stress limits of the refractory. To demonstratethe effects that the rate of temperature change has on the stresses in arefractory lining, a transient thermal and stress analysis wasconducted on a typical refractory lining design for an autoclave.

The results obtained from the analysis shows a significant increasein peak stress when the rate of heating or cooling is increased by5°C/h. Peak stresses were increased by approximately 2 MPa. A stressincrease of this magnitude is significant because tensile failure ofrefractory brick occurs in a range of 6 to 10 MPa. The magnitude ofstress development is highly dependent on geometry, materials, andthe thermal boundary conditions. Since stress development is affectedby multiple factors it is important to analyse the transient effects ofheating or cooling a vessel.

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Introduction

Pressure hydrometallurgy operations require vessels to be lined with an impermeablemembrane for corrosion protection and one or more courses of refractory or ceramic brick.Examples of unit operations that utilize composite lining systems include pressure oxidationautoclaves, sulphide precipitation autoclaves, chloride leach reactors, flash vessels, cycloneseparators, and direct contact condensers (heater vessels and quench vessels). The refractorylining must satisfy multiple requirements: it must thermally insulate the membrane fromprocess fluid, be structurally stable, provide erosion resistance, be chemically compatible withprocess fluid, and provide an economic service life.

Refractory lining design begins with selecting lining materials based on chemical stability inthe process environment. The thickness of the different refractory layers is initiallydetermined using 1D heat transfer calculations. If the membrane or shell temperature isgreater than what is permitted then the refractory thickness is increased. Following thethermal analysis is a 1D mechanical stability analysis, which is used to examine variousloading conditions and the effects of material properties. Mechanical stability is determinedby ensuring that the hot face refractory layer is in compression through all operatingconditions and that the stresses do not exceed the material’s failure strength. An equallyimportant consideration is the calculated overlap between the brick lining and the membrane.This stability factor requires that the overlap should be positive, that is, that the steel shellshould be in contact with the refractory lining throughout all operating conditions.

The main drawback of 1D calculations, is the assumption of an infinitely long cylindricalvessel that does not take into account the effects of additional attachments: supports, nozzlesand hemi-heads. A designer must resort to the use of finite element analysis (FEA) becausethe various attachments require complex mathematical analysis for which exact formulae aredifficult or impossible to obtain. With the use of 2D axisymmetric and full 3D models thedesigner will perform thermal and stress analysis for various load conditions. FEA willhighlight hot spots in the vessel or stress concentrations, which would lead to failure. Thedesigner can then concentrate on the hot spots or stress concentrations and modify parameterssuch as gaps, geometry, and material selection in order to eliminate these hot spots or stressconcentrations.

Once a refractory lining design passes the mechanical stability and thermal requirements atsteady state process conditions it becomes necessary to examine the effects of start-up andshutdown procedures. Steady state analysis techniques are covered in detail in the paper‘Design fundamentals for hydrometallurgy pressure vessel refractory linings’ by A. Koningand P. Lauzon. Analysis of start-up and shutdown procedures for a vessel is required becausea refractory lining is dramatically affected by the rate of temperature change. The purpose ofthis paper is to demonstrate the effect that the rate of temperature change has on refractorystresses during start-up and shutdown.

If the vessel is heated or cooled too quickly stresses can be generated that exceed the stresslimits of the refractory, leading to failure of the lining. It is the hot face of the process brickthat will experience the greatest effects of thermal shock and will crack or spall if the rate oftemperature change is too high. To assist in prolonging the life of the refractory lining it isimportant that appropriate heating and cooling rates be determined and followed. Thedamaging effects of a large rate of temperature change will be discussed and examined usingFEA.

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Transient thermal stresses

Refractory material is selected for its excellent insulating material properties: lowconductivity, high density, and high specific heat capacity. The insulating ability of therefractory is required for the thermal protection of the vessel membrane. The downside is thelength of time required to heat or cool the refractory lined vessel.

When the refractory in a pressure vessel is being heated it resists the transfer of heat fromthe hot face to the cold face, generating steep thermal gradients through the material. Stressesare generated within the refractory as a result of uneven thermal expansion due to the non-uniform temperature distribution. If the refractory is heated quickly, large enough compressivestresses can be generated, which cause the lining to fail due to thermal shock. The same typeof failure can also occur when the vessel is cooled too quickly. In this case the hot facebecomes cooler than the remainder of the refractory and develops a tensile stress. The resultsof a transient thermal and stress analysis, using FEA, effectively shows the magnitude ofstresses generated as a result of rapid temperature change.

Transient analysis using FEA

The effects of rapid temperature change on refractory will be shown using the results from atwo-dimensional FEA model. Figure 1(a) shows the general arrangement of the model usedfor the analysis, which approximates a cross-section of a quarter of the pressure vessel’scylindrical body. It is assumed that this geometry represents an infinitely long cylinder intothe page. In the model, plane strain applies. This model geometry will behave identically to acomplete annulus and reduces the computational time because the model is smaller.

The model is made up of 8 different layers as shown in Figure 1(b): steel shell, leadmembrane, 3 mortar layers, and 3 brick layers. The brick layers are modelled as monolithiclayers that do not account for the mortar that is between the bricks circumferentially oraxially. These mortar joints could be modelled but are not necessary to show the effects of therate of temperature change.

Figure 1. (a) General arrangement of the model used for the transient analysis and (b) the different material layers

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The model geometry is a representation of the cylindrical portion of an autoclave.Thicknesses used for each layer in the model are from the design of an existing autoclave. Theautoclave is being used for the processing of an orebody that contains gold. Transient thermaland stress analysis was conducted using the 210°C process design temperature for thisparticular autoclave. Autoclaves used to process an orebody containing gold can have processtemperatures that range from about 200°C to 250°C. Process conditions will vary dependingon the orebody being processed, the chemical reaction taking place, and the type of vesselbeing used.

The effect of heating rate on refractory

The refractory used to line a vessel is selected for its excellent insulating properties to keepthe membrane below a particular temperature at process conditions. At steady state operatingconditions the thermal gradient through the refractory generates compressive hot face stressand tensile cold face stress. Larger stresses will be developed in the refractory as the vessel isbeing heated, as shown in Figure 2. The peak stress occurs when the vessel reaches the 210°Cprocess temperature and then stabilizes to a steady state position. For the given analysis a5°C/h increase in the heating rate increased the peak stress by approximately 3 MPa. A peakstress is formed due to the thermal gradient through the refractory.

The steepest thermal gradient occurs when the vessel reaches the desired processtemperature. This is illustrated by the thermal gradient at the time of 19 h for Case 2 in Figure 3. This occurs because the hot face has achieved its steady state temperature but theremainder of the refractory is still being heated. The faster a vessel is heated the steeper thethermal gradient through the refractory when the vessel reaches the process temperature, asshown in Figure 4 for Case 1 and Case 2. This accounts for the largest peak stress for Case 2.The slope of the thermal gradient through the refractory and stresses are reduced over time asthe refractory reaches steady state.

The 12 h hold shown for Case 3 and 4 in Figure 2 is an example of a temperature holdtypically included in a vessel heating schedule during commissioning or after prolonged shut-down. The hold period allows the lining to settle into place and helps prevent damage to the

Figure 2. Hot face stress of the process brick

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lining. It provides time for the acid to completely soak through the refractory to preventflashing. A hold period allows the vessel to reach an equilibrium state so the operator has anopportunity to check that nothing is out of the ordinary (i.e. hot spots) before continuing theheating process.

Figure 3. Process brick thermal gradient for Case 2

Figure 4. Process brick thermal gradient

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The effect of cooling rate on refractory

The tensile strength of refractory typically falls within a 6–10 MPa range. When a vessel iscooled there is greater potential for lining failure due to the possibility of exceeding the lowtensile strength of the refractory. Looking at Cases 2, 3, and 4 in Figure 5 it can be seen thatthe peak stress is approaching the tensile strength of refractory as the cooling rate is increased.The stress on the hot face has become tensile because it is now cooler than the cold face. Forthe given analysis a 5°C/h increase in the cooling rate increased the peak stress byapproximately 2 MPa. A peak stress is formed due to the thermal gradient through therefractory.

For Cases 2, 3, and 4 the steepest thermal gradient occurs when the vessel temperaturereaches 60°C and cooling by means of natural convection is initiated. This is illustrated by thethermal gradient at a time of 6 h for Case 4 in Figure 6. It can be seen that at the 7 h mark thehot face increased in temperature and the remainder of the refractory continued cooling. Thisoccurred because the rate of heat removal due to convection is slower than the rate of heatingby the latent heat remaining in the hotter portion of the refractory.

The faster a vessel is cooled the steeper the thermal gradient through the refractory when thevessel temperature reached 60°C, as shown in Figure 7 for Cases 2, 3, and 4. This accountsfor the largest peak stress for Case 4. The slope of the thermal gradient through the refractoryand stresses are reduced over time as the refractory reaches steady state.

The effect of cooling the vessel interior from 60°C to 20°C using convection can be seenlooking at Case 1 and 5 in Figure 5. These two cases do not have a stress peak because thevessel is cooled to 60°C using a slow enough rate that the convective air passing though thevessel has a larger influence on the tensile stress development. The tensile stress is caused bythe 40°C temperature difference between the convective air and the hot face of the processbrick. Case 5 has a larger tensile stress because forced convection has a larger effect on thethermal gradient through the brick because it removes more heat than natural convection. Theforced convection rapidly decreased the hot face temperature of the process brick causing thegeneration of the larger tensile stress on the hot face.

Figure 5. Hot face stress of the process brick

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Conclusions

The rate of temperature change has a noteworthy effect on the stresses that are developed in arefractory lining. The stress observed, during heating or cooling, is significantly higher thansteady state stress, which demonstrates the need for transient thermal and stress analysis whendesigning a lining system. The stresses are highly dependent on geometry considerations,material properties, and thermal boundary conditions. So there are many variables that can bechanged to reduce the stresses within the refractory. Performing a transient thermal and stressanalysis during the design phase will result in a better engineered refractory lining with animproved life expectancy.

Figure 6. Process brick thermal gradient for Case 4

Figure 7. Process brick thermal gradient

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Al KoningMechanical Engineer, Hatch, Ontario, Canada

Mechanical engineer for the design and construction of chemical andmetallurgical processing plants, encompassing detailed mechanicaldesign and project engineering for the extraction of non-ferrous metalssuch as gold, nickel, cobalt and copper. Extensive experience usingfinite element analysis (FEA) and other analytical methods in thedesign, diagnoses and retrofitting of pressure vessels and pipingcomponents.

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