Evaluation of South African Chromite sand sintering behaviour
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Evaluation of South African Chromite sand sintering behaviour Jonathan K Kabasele Department of Metallurgy, University of Johannesburg, South Africa Didier K Nyembwe Department of Metallurgy, University of Johannesburg, South Africa Franklin Ochonogor Department of Metallurgy, University of Johannesburg, South Africa ABSTRACT This research was conducted on 6 different supplies of South African chromite sand samples: Lil sales, Insimbi, S11594, Rand York, Pentagon and Mineral Loy. The aim of the experiment was to determine the foundry properties of chromite sand that affect its sintering temperature. Three foundry properties of chromite sand were studied: turbidity or the amount of impurities, the percentage of fine particles and percentage of silica content. The approach selected to perform the sintering test was the VDG procedure 26 from the German foundry association. The test observes the sintering behaviour of chromite sand over a range of temperature and is able to identify its sintering temperature. Using simple linear regression analysis, it was concluded that the presence of impurities, high silica content and fine particles decreases significantly the sintering temperature of chromite sand which contributed to an increase in the occurrence of casting defects. Keywords: Sintering temperature, turbidity, silica content, fine particles, South African chromite sand INTRODUCTION Chromite sand has a chemical composition of FeCr 2 O 4 and a high specific gravity of 4.5 which is a very good heat conductor. This thermal conductive characteristic facilitates chilling effect. Chromite sand has a low thermal expansion, therefore defects associated to expansion are less likely to occur. Chromite has a coarser grading than Zircon however it is relatively more resistant to metal penetration when compared to Zircon. Chromite sand has a high acid demand which translate to greater addition of acid catalyst for furane binder to effectively bind the particles. Furane binder as an exception, chromite sand generally compatible with all the usual binder systems. Sand reclamation of chromite sand is a challenge, especially if it is contaminated with silica. Silica may reduce the refractoriness of chromite sand, it is prone to react with the molten iron and form low melting component called fayalite 1 . Chromite has unique characteristics ideal for difficult metal casting applications and this proven performance enable foundries to perform castings of tough jobs. Chromite is preferred to other moulding materials to produce castings of the highest quality. Chromite sand is used in foundry for preventing defects such as metal penetration, veining and sand burning on. Chromite sand is preferred to silica for large steel casting, high alloy castings and copper based alloys due to its high resistance to metal penetration, good thermal stability and most importantly its rapid chilling properties 2 . It should be noted that rapid cooling can be detrimental to thin section ductile iron castings as it promotes the
Evaluation of South African Chromite sand sintering behaviour
Jonathan K Kabasele
Didier K Nyembwe
Franklin Ochonogor
ABSTRACT
This research was conducted on 6 different supplies of South
African chromite sand samples: Lil sales, Insimbi, S11594, Rand
York, Pentagon and Mineral Loy. The aim of the experiment was to
determine the foundry properties of chromite sand that affect its
sintering temperature.
Three foundry properties of chromite sand were studied: turbidity
or the amount of impurities, the percentage of fine particles and
percentage of silica content. The approach selected to perform the
sintering test was the VDG procedure 26 from the German foundry
association. The test observes the sintering behaviour of chromite
sand over a range of temperature and is able to identify its
sintering temperature.
Using simple linear regression analysis, it was concluded that the
presence of impurities, high silica content and fine particles
decreases significantly the sintering temperature of chromite sand
which contributed to an increase in the occurrence of casting
defects.
Keywords: Sintering temperature, turbidity, silica content, fine
particles, South African chromite sand
INTRODUCTION
Chromite sand has a chemical composition of FeCr2O4 and a high
specific gravity of 4.5 which is a
very good heat conductor. This thermal conductive characteristic
facilitates chilling effect. Chromite sand has a low thermal
expansion, therefore defects associated to expansion are less
likely to occur. Chromite has a coarser grading than Zircon however
it is relatively more resistant to metal penetration when compared
to Zircon. Chromite sand has a high acid demand which translate to
greater addition of acid catalyst for furane binder to effectively
bind the particles. Furane binder as an exception, chromite sand
generally compatible with all the usual binder systems. Sand
reclamation of chromite sand is a challenge, especially if it is
contaminated with silica. Silica may reduce the refractoriness of
chromite sand, it is prone to react with the molten iron and form
low melting component called fayalite1.
Chromite has unique characteristics ideal for difficult metal
casting applications and this proven performance enable foundries
to perform castings of tough jobs. Chromite is preferred to other
moulding materials to produce castings of the highest quality.
Chromite sand is used in foundry for preventing defects such as
metal penetration, veining and sand burning on. Chromite sand is
preferred to silica for large steel casting, high alloy castings
and copper based alloys due to its high resistance to metal
penetration, good thermal stability and most importantly its rapid
chilling properties2. It should be noted that rapid cooling can be
detrimental to thin section ductile iron castings as it promotes
the
formation of carbide. The carbide tendency can be evaluated using
thermal analysis and removed by using the correct combination of
treatment alloys and inoculation3. Chromite can be bonded with
other aggregates to refine mold density and have similar effect on
the mold as Zircon. Chromite moulds produce castings with superior
surface finish resulting to its first rate peel characteristics.
Mold and cores made of chromite sand benefit from high rate of heat
diffusion and matchless chilling effects, this reduces additional
cost from adding metal chills. Chromite sand requires a minimal
amount of binder and has high permeability, this decrease the
likelihood of scrap as a result of gas problems. Chromite sand
experiences only slight changes in terms of thermal expansion under
the effect of thermal shock or cooling. The excellent thermal
stability prevents defects such as veins and scrabs, rattails,
buckles and dimensional inaccuracy2.
SINTERING BEHAVIOR The term “refractoriness’’ is broad in its
elucidation compared to “fusibility”. Fusibilty is explained by the
softening point or melting point. Refractoriness refers to the
temperature at which the material starts to lose its shape.
Refractoriness is observed in several stages over a range of
temperature. The sintering of the material is characterized by
bending and shrinkage, but the phenomenon does not take place at
the melting point. Refractoriness is therefore explained better by
the sintering behavior of the material rather than by its melting
point. The fusion of the lower melting-point constituents may also
produce sintering, the formation of eutectics and solutions, and
the chemical reaction of the constituents. In the case of chromite
sand, silica is the lower melting element4
Several researchers have investigated the sintering behaviour of
chromite sand. Some of the researches were conducted to study the
sintering behaviour of chromite sand without interaction with
molten steel. Others studied the behaviour of chromite sand during
its interaction with molten steel. A comparison was made between
the sintering responses of chromite sand interacting with steel and
chromite sand without any interaction at 1873 K. It was concluded
that liquid steel could intensify the sintering process of the sand
and dissolved elements such as Manganese further enhance the
sintering behavior of chromite5.
The sintering behavior is also influenced by the foundry properties
of chromite. There are disparities
across several chromite sand supplies in terms of foundry
properties. This is due to inconsistency during the mineral
processing in one hand and mishandling by the supplier on the other
hand. There are limitations with operating spiral plants to produce
South African foundry chromite sand which cause inefficiency. These
limitations are caused by the variation in the feedstock, pump
pressure, slurry density, agglomerates fines, etc. All these
limitations make control very difficult. The process also requires
large quantities of clean water, in what is a very arid area of
South Africa (Bushveld Complex) this result in the need to recycle
water as much as possible. All variables considered the final
foundry chromite sand product to vary in terms of quality. Most of
foundry chromite sand are used with resin binder systems. These
systems rely on highly efficient mixers and low binder
additions(resin 0.8% + acid catalyst 0.125) therefore even the
slightest variation in the quality of chromite will have a
significant effect on these small additions and in turn on the
mould6.
FOUNDRY PROPERTIES AFFECTING SINTERINGT Turbidity testing performed
on chromite sand consist in the measurement of light scattering
caused by suspended particles in a liquid. The test procedure shows
that a sample of chromite sand, in a flask or other recipient, is
agitated in water for a fixed amount of time. Thereafter allowing
the heavy grain particles to settle at the bottom of the recipient
for a fixed period of time. Finally measuring the agitated water in
the turbidity equipment. The material in suspension are believed to
be low melting point silicate impurities which can cause casting
defects, such as chromite double skin defects. As pointed out by JD
Howden in his research on the subject, higher level of impurities
leading to higher formation of fayalite. The test is prone to
variation due to the differences in shaking velocity between users.
As the shaking time is extended foreign element or gangue are
liberated from the chromite grain by continuous agitation which
contributes to increasing the surface area of the suspended
material and light scattering effect7.
Foundries have the tendencies to focus on the AFS number while
neglecting to specify the sieve type, sizes and most importantly
what is considered fines. The AFS number is often meaningless, the
sand can meet the specification while at the same time not being
suitable for molding due to the amount of fine particles. Foundries
should provide specifications
which defines grain size distribution. In the case of this
evaluation fine particles are material with a size of 75 microns or
below7.
During chemical analysis, the element we are most interested in is
the content in silica. It is important to note that what is
reported as Si02 is not actually Si02 but a range of low melting
point silicates including enstatite, anorthite, and
phlogopite7.
CASTING DEFECTS AS A RESULT OF POOR SINTERING PROPERTIES Casting
defects are more prevalent in sand with low refractoriness or
sintering temperature. The presence of silica further enhanced the
occurrence of defects. Sand burn-on is a typical surface casting
defects in sand casting in general and chromite sand in particular.
The defect is caused by low sintering point of the molding material
and is enhanced by the presence of trump silicate. Cast iron react
with silica to form low viscosity iron silicate (Fayalite) this low
melting compound promotes further metal penetration or sand burn
on.8
Double skin is another surface defects affecting the casting of
heavy section steel in chromite sand mould. The defects depend on a
phenomenon called metallization. The metallization of chromite sand
consists of the reduction of iron from the chromite sand. Under
reducing conditions, Iron droplets are reduced from the chromite
sand especially when the temperature is raised to 1250 0C. The sand
mass expands as the iron droplets migrate to the surface of the
sand grains; the droplets will form an amalgam. During cooling, the
resin binder in the sand bums out and air is therefore able to
penetrate the sand interfaces which cause oxidation. Due to
oxidation, the mass of the double skin pieces increases and reduced
iron from the chromite sand will oxidizes. During the reaction,
Iron combines with the trump silicates within the sand and form
fayalite9.
Changes in the foundry properties of chromite as well as the
presence of silica affect the sintering behavior of chromite, which
in turn affects the refractoriness of the mould and lastly increase
the occurrence of casting defects. This research is conducted to
evaluate the extent of changes in the properties of chromite sand
and how it affects the sintering behavior of chromite sand. The
experiment is a study of the sintering behavior of chromite without
any interaction with molten steel. The method used to study the
sintering behavior of the sand is the VDG
procedure P26 from the German foundry association which is
effective at lower temperature compared to other conventional
tests.
EXPERIMENTAL PROCEDURE
This research follows a step by step procedure shown in figure
6.
STEP 1: The experiment begins with sample preparation. The
experiment is conducted on 6 different supplies of chromite
samples. These samples will be identified as samples A, B, C, D, E
and F.
STEP 2: During characterization the chemical composition of each
individual sample is determined by means of X- ray fluorescence. A
controlled X-ray tube in the equipment emit high energy X-rays
which will irradiate the solid sample. The X-ray struck with
sufficient energy an atom of the solid sample which dislodge an
electron from the atom’s inner orbital shells. The unstable atom
will regain its stability when an electron from one of the atom’s
higher orbital energy shells fills the gap or vacancy left in the
inner orbital shell. When the electron drops to the lower energy
state it releases a fluorescent X-ray. The energy of the
fluorescent X-ray is determined by measuring the difference in
energy between two quantum states of the electron. The measurement
of this energy is the foundation of X-ray fluorescence
analysis10.
STEP 3: The next stage consists of the determination of foundry
properties. Two properties of chromite are studied and these
include: turbidity and grain size.
Figure 1 shows the equipment used to perform the turbidity test. In
order to get reproducible and accurate results in the turbidity
test, there is a need to somehow standardize the test. The approach
used in this experiment to ensure reproducibility is as follows:
The chromite sand is dried in oven at 1100C. 50g of chromite is
measured in a 500ml beaker. 150ml of water is added to the beaker.
The beaker is agitated for a minute. The agitated water is then
poured in the cuvette and inserted in the equipment for turbidity
testing. The test is repeated 5 times for the same samples. This
operation is the same for all chromite sand samples.
Figure 1 turbidity test performed on chromite sand sample
To determine the grain size, the test is performed according to the
standards AFS 1106-00-S and BCIRA 16-7. 100g of dry chromite sand
is weighed on a mass balance. The sieves are arranged in order, in
accordance with the standard (AFS 11106-00-S) as shown in figure
2.
Figure 2 set of sieves, arranged by the following size of aperture:
1700, 850, 600, 450, 300, 212, 150, 106, 75, and 53 microns
The arrangement of sieves is placed on the Mechanical shaker. The
sand is poured onto the top sieve and covered with the sieve lid.
The sieves are tightly gripped onto the mechanical shaker using the
pan and screws. On the arms of the mechanical shaker. The equipment
is configured to run for 15 minutes. After the shaking the
arrangement of sieves is carefully removed. The sand retained in
each sieve is weighed and recorded. A particular attention is given
to the amount of fine particles retained in smaller sieves (from 75
Microns and below)
STEP 4: The sintering test follows the German VDG procedure 26. In
figure 3 and figure 4, half of the crucible’s length is filled with
dry sand. The sample is then put into a heated furnace for five
minutes (when set temperature is reached again after opening the
furnace, time is started).
Figure 3 chromite sand filled in crucibles
Figure 4 chromite sand placed in the sinter furnace
Afterwards the filled porcelain crucible is taken out and cooled
down to room temperature. By turning the sample over a scale, the
amount of sand falling out is determined. When 2/3 of the sand
remains in the porcelain crucible, sinter beginning is determined.
If not, the temperature of the furnace must be increased by
50-degree K and testing must be repeated until 2/3 of the sand
remains in the crucible.in figure 5, the stereomicroscope gives a
caption of the sintering effect experienced by the chromite sand
samples. At a temperature T, the percentage of sand sintered is
given by:
% = − −
A is the mass of the crucible
B is the mass of the sand and the crucible (the sand has not been
placed in the oven yet)
C is the mass of the sintered sand and the crucible11
STEP 5: In this stage the sintering temperature obtain from each
individual sample is plotted against the chromite sand foundry
properties and chemical composition. This step is crucial in
determining the extent foundry properties of chromite sand affect
the refractoriness of the sand
Figure 5 caption of sintered sand at different temperatures
Figure 6 flow chart procedure
RESULTS AND DISCUSSION
Chemical composition of chromite samples The results obtained by
X-ray fluorescence chemical analysis are shown in figure 1. From
figure 7 it is observed that sample D, F and C are above the
tolerated amount of silica content of 1 %.
Figure 7 SiO2 content
FOUNDRY PROPERTIES The turbidity tests result and the percentage of
fines per samples are shown in figures 8 and 9. Sample C has the
highest value of turbidity compared to other samples as shown in
figure 8. Figure 9 reveals that all
samples have percentage of fine particles below 2% except for
sample C with a value of 3.72%.
Figure 8 Turbidity value in NTU
1,455 1,34
Turbidity in NTU
Figure 9 percentage of fine particles
SINTERING TEST Figure 10 gives an overview of the sintering
behavior of each sample. The critical temperature of sintering is
reached when 60 % of the chromite sample’s total mass undergoes
sintering. The samples with relatively good foundry properties will
reach the 60% limit at higher temperature compared to poor quality
chromite sand samples. From figure 10, it can be concluded that
samples F and D have better properties compared to the other
sample. The two samples have relatively high values of sintering
temperature compared to the other samples. Sample C cross the 60%
at relatively lower temperature therefore it has a relatively lower
thermal properties compared to the other samples. Table 1 gives the
sintering test results for each sample
0,14 0,22 0,52
4
%fines
Figure 10 sintering behavior of chromite sample using vdg procedure
P26
Table 1 Sintering test Results
Percentage sintered sand
Oven Temperature Sample A Sample B Sample C Sample D Sample E
Sample F
800 20.4 27.4 0 15 0 37.5
850 41.2 53.9 93.4 0 59.9 45.4
900 93.3 91.2 88.3 58.1 78.4 50.1
950 91.2 87.6 94.9 89.6 92.6 91.5
Figure 11 gives an example of the critical temperature of sintering
determination by calculation. Table 2 contains the critical
temperature for each chromite sand sample.
Figure 11 determination of sintering behavior linear function
Sample A sintering temperature, if the average temperature is
60%
y=60 in equation y= 0.529x – 401.35,
60=0.529x-401.35, x= 872.11C or 1601.8F
20,4
41,2
average percentage sintered sand
Sand samples Function, y=f(x)
Percentage sintered, y=60%
critical temperature in (Fahrenheit)
A y=0,529x-401,35 60=f(x) 872,11 1601,798
B y=0,4358x-316,3 60=f(x) 863,47 1586,246
C y=0,5592x-420,15 60=f(x) 858,64 1577,552
D y=0,5638x-452,65 60=f(x) 909,28 1668,704
E y=0,5926x-460,8 60=f(x) 878,84 1613,912
F y=0,334x-235,6 60=f(x) 886,62 1627,916
EFFECT OF CHROMITE SAND PROPERTIES ON THE CRITICAL SINTERING
TEMPERATURE Chemical composition and sintering temperature Simple
linear regression analysis was used to assess the effect of silica
content on the sintering temperature. Figure 12 shows that the
relation between SiO2 content and the sintering temperature is
negative. Any increase of silica in the chromite sample reduces the
sintering temperature. A slight increase of silica content from
0,676 to 1,1831 % (0.5% increase) decrease the sintering
temperature by about 36 degrees from 1613.912F (878.46 0C) to
1577.552F (858.33 0C).
Figure 12 Effect of silica content on the sintering
temperature
Fine percentage and sintering temperature Figure 13 shows that the
presence of fine particles reduces the critical temperature of
chromite sand. An increase of 0.4% (from 0.14% to 0.52%) in fines
reduces the critical temperature by more than 80 degrees from
1668.70F (909, 23C) to 1586.25F (863 0C). The relation between % of
fine particles and sintering temperature is very strong (R2=0.903)
which indicates that fine particles should be kept under allowed
limit (not more than 2%).
Figure 13 Effect of fine particles on the sintering
temperature
1601,798
1586,246
1577,552
1613,912
1570
1580
1590
1600
1610
1620
PE RA
TU RE
O F
SI N
TE RI
1560
1580
1600
1620
1640
1660
1680
CR IT
IC AL
T EM
PE RA
TU RE
TEMPERATURE VS FINE PARTICLES
Turbidity and sintering temperature Turbidity or the amount of
impurities in chromite sand has an impact on the sintering
temperature of chromite sand. These impurities are low melting
silicates and other trump material liberated from the chromite
grain. Figure 14 shows that the increase in turbidity value which
or the increase in the amount of foreign element in chromite is
detrimental to the critical sintering temperature.
Figure 14 Effect of turbidity on the sintering temperature
CONCLUSION
The evaluation suggest that, the sintering temperature define the
refractoriness of chromite sand and its overall quality. It is
therefore important to also note, in terms of recommendation that
foundries test their chromite for the properties described in this
paper( SiO2 content, turbidity and percentage of fine particles) if
they are concerned about sintering temperatures.
1586,246 1577,552
1560
1580
1600
1620
1640
1660
1680
PE RA
TU RE
O F
SI N
TE RI
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ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURE
STEP 1:
STEP 2:
STEP 3:
STEP 4:
STEP 5:
FOUNDRY PROPERTIES
SINTERING TEST
EFFECT OF CHROMITE SAND PROPERTIES ON THE CRITICAL SINTERING
TEMPERATURE
Chemical composition and sintering temperature
Fine percentage and sintering temperature
Turbidity and sintering temperature