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 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:
% 𝑠𝑖𝑛𝑡𝑒𝑟𝑒𝑑 𝑎𝑡 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑇=𝐵 − 𝐶 𝐴 − 𝐵 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
0,852 0,676 0,839 1,183 0
0,5 1 1,5 2
SiO2 content %
379,2 231,296
428,1 468,168 381,538
510,488
0 100 200 300 400 500 600
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
0,08 0 3,72
0,501 1,52 2,53 3,54
%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
93,3 91,2
y = 0,529x - 401,35 0
20 40 60 80 100 120
750 800 850 900 950 1000
average percentage sintered sand
Sample A
Table 2 Critical temperatures of sintering Sand
samples Function, y=f(x) Percentage sintered, y=60%
Critical
Temperature at y=60% (in degree Celsius)
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
y = -67,628x + 1654,9 R² = 0,7905
1570 1580 1590 1600 1610 1620
0 0,5 1 1,5 2
TEMPERATURE OF SINTERING
SIO2 CONTENT
TEMPERATURE VS SIO2
1586,246 1668,704
1627,916
y = -195,59x + 1685 R² = 0,9032
1560 1580 1600 1620 1640 1660 1680
0 1 2 3 4
CRITICAL TEMPERATURE
PERCENTAGE FINE PARTICLES
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 1668,704
1613,912 y = -0,5853x + 1873
R² = 0,631
1560 1580 1600 1620 1640 1660 1680
0 200 400 600
TEMPERATURE OF SINTERING
TURBIDITY VALUE IN NTU
TEMPERATURE VS
TURBIDITY
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