Changes in diffusivity due to sintering in metallized iron oxide pellets
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pellets and a finer band of pores (reduction band, - 0 . 0 3 / x m radius) representing the intragranular pores produced during reduction. As the temperature was increased, the average pore size of the reduction band also increased. This was accompanied by a progressive decrease in the overall pellet pore volume as the finer pores in the structure were eliminated by sintering. A more complete explanation of the structural changes shown by the pore size distributions has been published elsewhere. 4'5'6 To estimate the effect of these structural changes on the transport of gaseous reactants in the metallized pellets, effective gas diffusivities were estimated using the random pore model of Wakao and Smith] This model requires that, in a bimodal structure, the overall diffusion flux is given by the sum of the individual fluxes through the macro and micro pores plus the flux by series diffusion through macro and micro pores. For an equimolar countercurrent reduction system ( e . g . , reaction of iron oxides with hydrogen), 10
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Fig. 2 - - P o r e size distribution of iron oxide pellets reduced at 500 ~ for 3 h then heated at 8.3 ~ min -~ to various temperatures.
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ot
Temp. (*C)
Fo (~m)
go
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500 600 700 800 900 1000 1100
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Fig. 3 - Pore size distribution of iron oxide pellets reduced isothermally at various temperatures. METALLURGICALTRANSACTIONS B
VOLUME 13B, SEPTEMBER 1982- - 519
In order to test the validity of the model, a series of pellets was isothermally reduced at temperatures in the range 500 to 1100 ~ Pore size distributions for these pellets are shown in Figure 3. Deff values, calculated from the pore structure data, were compared with the results of previous investigators. 9'1~ This comparison is shown by the upper curve in Figure 4. Considering the relative simplicity of the model, the isothermal Deff results are in remarkably good agreement with results derived from direct measurements, reducibility data, and other pore structure calculations. Also shown in Figure 4 (by the lower curve) are the results of Deff calculations for the pellets initially reduced at
From
Turkdogan et at.
O Derived f r o m reduction data 6 Derived f r o m pore s t r u c t u r e 12 Derived from d i r e c t measurements
Present
3 --
~E o v
work
9
Derived f r o m pore s t r u c t u r e of isothermaLLy reduced peLLets
x
Derived f r o m pore s t r u c t u r e s of peLLe
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