The dramatic increase in reaction rates as a result of oxidation treatment is due to a change in reaction control from mass transport resistance to chemical reaction resistance except in the latter stages of reduction,
6.6.3, Thermally treated sinter pellets
Sinter pellets were thermally treated in an inert atmosphere,
o o o o
The isothermal ti’eatment tempera-cures v/ere 500 C, 6.50 C, 750 C, 850 C
o o
and 1000 C. The treated samples were reduced by hydrogen at 800 C. o
The optimum treatment temperature was found to be 750 C. At this temperature the rate of reduction was at a maximum (Fig. 144).
Variations in heating and cooling rates at this temperature showed that this improvement in reaction rates over untreated pellets only occurred at slow heating and cooling rates.
-
112
-at 750°C the chemical reaction law controlled from (O/Fe)^ = 1.35
to (O/Fe). = 1.2 compared with 1,35 to 1.28 for untreated sinters. The chemical reaction rate constant increased almost three-fold and the exponential rate constant ten fold (Fig. 191).
The very large increase in the exponential law
constant
inni—*cates an improvement in diffusion rates as a result of the thermal treatment,
A general law plot as calculated by the computer was found to give a good fit to the data indicating the necessity of mixed control reaction lav/s to describe the course of reduction (Fig, 193).
The restriction of the improvement in reaction rates to slow heating and slow cooling regimes indicates that some physical change is occurring within the matrix. X-ray diffraction studies indicate that this is not due to stress relieving effects which would in any case tend to reduce reaction rates. It is possible that sub-micros copic cracks are forming during the heating and cooling cycle which are undetectable by normal metallographic techniques.
6.7. Discussion
Isothermal reduction of untreated, reconstituted sinter pellets indicated a progressive increase in reaction rate with increase in temperature. Slow reaction rates were experienced at 700°C and 800°C and fast reaction rates at 10G0°C and 1100°C, For the intermediate temperatures intermediate reaction rates were found. The change in rate of reaction over a particular temperature range
o o
0 0 0 G - 900 C indicated a change in reaction control mechanism,
Examination of the reduced structures, the apparent activation energy plots and the individual reaction rate laws supported the existence of a change in reduction mode as the isothermal reaction temperature was increased.
For the low reaction temperatures 700°C, 800°C wholly topochemical reduction occurred. Partly reduced ferruginous grains could be seen at the interface and some reduction had occurred within the grains along specific planes. The grains had reduced from the edge nearest to the sample surface. The iron product was finely porous and relict of the original emboids. Although topochemical reduction is consistent with both chemical and mass transport control examination of partly reduced grains lead to the conclusion that mass transport was the controlling
mechanism. The activation energy values also supported this belief. Examination of the individual reaction rate laws showed that, for the low temperatures, chemical reaction control was limited to the very early stages of reduction (O/Fe = 1,34 - 1.28), It is probable therefore that initially the rate is controlled by chemical reaction at the sample surface. Once the surface layer has reacted then mass transport control becomes predominant. From the values of apparent activation energies this may possibly be gaseous diffusion. Examination of the microstructures indicate that gaseous diffusion through the matrix is controlling.
Therefore it can be concluded that for the low temperature reduction of lean magnetite sinter pellets the rate of reduction is initially controlled by the rate of reaction at the sample surface. Once the surface has reacted then mass transport through the matrix becomes rate controlling.
As the temperature increases through the intermediate range
o o
840 C, 870 C reduction was seen to occur in advance of the main interface. This effect was more pronounced at 07O°C thus indicating an increase in the rate of mass transport and the growing influence of the chemical reaction rate.
- 114 -
A p p a re n t a c t iv a t io n e n e rg j' c a lc u la t io n s g iv e v a lu e s o f th e order required for the diffusion of ferric ions in magnetite.
Examination of the individual reaction rate laws showed that chemical reaction rate control predominated over the range
(O/Fe)^ = 1.34 - 1,28 and 1,34 - 0.61 respectively, Changes in the slope of the exponential law straight lines indicated the possibility of changing diffusion modes controlling the transport mechanism.
Clearly as the reduction temperature increases diffusion through the matrix will increase and the rate of chemical reaction will become more significant in the early stages. In the later stages ferric ion diffusion may become controlling.
For the high reaction temperatures 900°C, 1000°C, 1100°C
topochemical reduction disappears. This indicates that mass transport
is no longer the controlling mechanism, f
For the temperature range 900°C - 1100°C the values of the apparent activation energies vary and it becomes difficult to use such values to determine the possible controlling mechanism.
Analysis of the individual rate laws show that the influence of chemical control extends from (O/Fe) = 1.34 ~ 1.16 at 900°C, 1,34 - 0.28 at 1000°C and 1.34 - 0.11 at 1100°C with mass transport
predominating only in the final stages. The probability is that at higher reaction temperatures matrix diffusion rates increase
and the chemical reaction becomes rate controlling. Except in the.
\
final stages where diffusion paths are long and mass transport contrrol predominates.
Clearly the reduction of sinter pellets even in the present idealised conditions is characterised by complex reduction
mechanisms. Chemical reaction and mass transfer both ploy a part to a greater or lesser degree.
Because of the low reaction at 800°C it was considered that oxidation of sinter pellets prior to reduction may enhance the • reaction rate. Oxidation at varying temperatures gave typically
higher oxide structures except at 500^0 when maghemite ( ^ “^e2°3^
was formed.
Metallographically the reduced microstructures changed from the topochemical reaction of the untreated pellets to general reduction in the oxidised pellets, indicating a change in the reduction
mechanism from predominantly mass trsnsport to predominantly chemical control.
Study of the reaction rate laws showed that the extent of chemical control was, typically, from (0/Fe).j. = 1.55 - 0,31 with diffusion
control becoming predominant in the final stages. Hie improvement in diffusion is doubtless due to the change in structure of the
V
ferruginous grains and the change in structure of the matrix. Matrix oxidation has clearly occurred since the gain in weight on oxidation is greater than the stochiometric weight gain of Fe^O^'-^Fe^Og,
possibly by oxidation of MnO-5* IvIno0„.
The change from diffusion to chemical control is even more marked when the known increased reaction rates as a result of oxidation are taken into account.
It was considered that thermal treatment of sinters may have an effect separately from that encountered on oxidation. It lias been shown that isothermal heat treatment in an inert atmosphere can result in increases in reaction rates. The optimum treatment temperature
o
was found to be 750 C,* A study of various heating and cooling rates for this temperature revealed that only slow heating and -cooling cycles could successfully induce the required increase in reaction rate. The necessary combination of slow heating and slow heating and slow cooling may mean that- time above a critical temperature
- 116 -
is important. Such a critical temperature may be the * triple
point* where 4 F e 0 ^ F e30^+Fe.
Metallographic examination of the reduced samples showed some reduction in advance of the interface indicating an increase in the extent of chemical control.
This was borne out by a study of the reaction rates laws the chemical control extending from (O/Fe)^ - 1.35-1.28 to (O/Fe)^ = 1.35-1.20. The chemical reaction rate constant was found to have increased threefold and the diffusion rate constant tenfold.
No metallographic, crystallographic or chemical change could be detected in the thermally treated samples. There was undoubtedly an increase in reaction rates. The restriction to the slow heating and cooling cycles indicating the probability that some physical change had taken place.
In addition to the improvement of reaction rates of untreated sinters as the reaction temperature increases there was found to be
o o
a critical temperature range 800 C - 900 C above which the rate increased markedly.
Further the low temperature oxidation of sinters to either maghemite or hematite can greatly enhance reaction rates.
Significantly the low temperature (i.e.< devitrification temperature) thermal treatment of sinters can be used to enhance reaction rates provided the correct heat treatment of sinters can be used to enhance reaction rates provided the correct heat treatment cycle can be selected.
CHAPTER 7