Bentonites used for foundry can be classified basically in two types, both coming from volcanic ashes and transformed over millions of years, the natural sodium bentonite and the calcium bentonite.
The natural sodium bentonite, from which DEL LAGO Bentonite belongs, has a high swelling capacity in water and excellent thixotropic properties. It is good to remember that there are few reserves of natural sodium bentonite.
Also, natural sodium bentonite has a high resistance to heat and dryness. These properties are necessary to eliminate faults of expansion, washing away and erosion of the mould. This type of bentonite also shows high durability, thus requiring less aggregates in the sand systems.
Moulding sands that are bound with natural sodium bentonite, seem to have a high plasticity when mixed with an adequate amount of water. This makes moulding easier, especially in order to avoid breaking the edges of the mould.
A second type is calcium bentonite, which is more abundant in nature. In its natural state, it has exchangeable calcium cations and a low swelling capacity.
Calcium bentonite has a lower resistance to heat and dryness than sodium bentonite, it collapses more easily and it prevents breaking faults when exposed to heat. It has a higher resistance in green than natural sodium bentonite and better flow properties. It also presents better properties for unmoulding, which makes it particularly interesting when wanting to obtain pieces that have a lower adherence than moulding sand.
Calcium bentonite can be turned into sodium bentonite if we add sodium carbonate (Na2 CO3) in an amount that is adequate to its cation exchange capacity. Sodium bentonite obtained this way is called acid activated bentonite, and its quality is similar to that of natural sodium bentonite.
Many foundries use mixtures of natural sodium and calcium bentonite in their sand systems, in order to profit from the good properties that each of them can provide to green moulding sand. A common proportion is 2:1 sodium bentonite: calcium bentonite.
Specific features
Unit |
Max 10,0% |
• Granulometry |
99,5% en tamiz de #200 (0,053mm) |
• Real Specific Weight |
2,21 g/cm3 |
• Specific weight |
0,95 g/cm3 |
• Ph (water suspension 6%) |
8,5 m |
• Hardness |
1 a 1,5 mohs |
• Color |
Cream |
Table N°2 – DEL LAGO bentonite chemical analysis
Swelling capacity
DEL LAGO bentonite adsorbs up to nearly 16 times its weight in water and, after complete saturation, occupies a volume that is 12 to 15 times its original volume. Bentonite adsorbs and swells more quickly in hot water and it does not swell in alcohol or gas. It only swells moderately in concentrated chemical solutions such as: acids, alkalines or salts. bentonite adsorbs up to nearly 16 times its weight in water and, after complete saturation, occupies a volume that is 12 to 15 times its original volume. Bentonite adsorbs and swells more quickly in hot water and it does not swell in alcohol or gas. It only swells moderately in concentrated chemical solutions such as: acids, alkalines or salts.
Swelling in distilled water: after 24 hours is 16 cm 3/g
Adsorption of methylene blue
Methylene blue has been used to evaluate the capacity of exchanging cations in bentonites, as well as to evaluate their purity.
DEL LAGObentonite is 55 -60 ml.
Impurities
DEL LAGO bentonite presents a 1% maximum of impurities. The low grade of impurities of CaO, FeO, Fe203 y K20 explains its exceptional quality during moulding and solidification of the pieces. This peculiarity is reflected in the final cost of the process, since a high purity bentonite presents a high thermal durability.
Technological characteristics of DEL LAGO bentonite
The evaluation tests of technological characteristics are generally started with a standard mixture that is made up of sand, 5% of bentonite and water, 45% of compactability plus a mixing time of 17 minutes in a laboratory blender.
|
Del Lago ® |
Wyoming |
Rcv (ni cm2) |
14,82 |
13,35 |
Rtu (ni cm2) |
0,33 |
0,29 |
Permeability (cm4/(gxmin) |
170 |
170 |
Table N°3 - Comparative results of Resistence to compression in green (RCV) and Resistence to traction in wet (RTU) between both DEL LAGO and Wyoming Bentonites.
Binding power
In order to develop all the binding power a natural sodium bentonite has, it is necessary for the moulding sand to be prepared in a correct way, that is to say, that the mixing time and the degree of the humidity, together with the set bentonite /sand, should be enough to disperse the bentonite completely, covering the grains of sand uniformly.
The following figures show comparatively results of green moulding sands prepared with both DEL LAGO and Wyoming Bentonites, with different compactability and mixing times of 20 minutes and 8 hours.
DEL LAGO Bentonite presents a similar performance to that of Wyomings bentonite on regards of binding properties.
Durability of natural sodium bentonite
Durability is one of the most important properties of bentonite for foundry, since it is responsible for the economy of the process of green sand moulding. The use of a high durability bentonite reduces the need of adittions in each cycle of the foundry process.
DEL LAGO, Wyoming and activated bentonite.
Differential thermal analysis
The differential thermal analysis (DTA) is used to determine the predominant type of mineral clay that exists in a particular sample of clay, but it can also be used to evaluate the resistance of a bentonite, when it is under a thermal request.
The figure shows a differential thermal analysis (DTA) of the natural sodium bentonites: DEL LAGO and Wyoming, and activated sodium bentonite.
DEL LAGO and Wyoming bentonites have a similar and better performance than the activated sodium bentonite.
Figure 3 – Differential thermal analysis (DTA) of different bentonites
Reduction of the adsorption of methylene blue
When analyzing the reduction of methylene blue (RMB) in bentonites burned at 550°C in relationship with those just dried at 100ºC, it is possible to estimate the thermal durability of bentonites. The higher the thermal durability, the lower the reduction of methylene blue.
The figure shows that there is practically no difference in values between the two natural bentonites (Wyoming and DEL LAGO). However, a significant difference can be seen between these and the activated bentonite.
This verifies the high thermal durability of DEL LAGO Bentonite.
Figure 4 - Reduction of absorption of methylene blue
Constant of burning
The constant of burning is a characteristic that represents the degree of a thermally desactivated bentonite per unit of active clay, and per unit of sand/ iron compound. The lower the constant of burning, the better the performance of the bentonite during pouring into the mould at high temperature. As a consequence, there is less consumption.
It can be observed, once again, that the results obtained with natural sodium bentonites are similar and have advantages over activated bentonite.
Figure 5 - Constant of burning of Del Lago, Wyoming and activated bentonites..
Effect of the degree of smectite
The purity of the bentonite, measured by the degree of smectite that is present, is a decisive factor in obtaining good properties in green moulding sand.
Figure 6 - Effect of the degree of smectite versus resistance in green and permeability.
It can be observed that there is a strong evolution in the resistance and permeability as bentonite turns more pure. In this way, the choice of a high purity bentonite is extremely important for a good performance in the casting process. DEL LAGO Bentonite belongs to the high purity class and, for this reason it has a high performance when used for green moulding sand.
Expansion flakes
The faults of expansion in melted pieces, obtained from green sand moulds, are related to compressive stress, due to the expansion of silica, the resistance to traction in wet during the maximum moulding time. The more common expansion fault is called «flake».
Figure 7 – Graphic representation of a flake's fault mechanism
The figure shows the mechanism by which a flake is formed in melted pieces of green sand and the fault itself, as presented in melted pieces.
The factors that affect the formation of the flake are the following:
Two factors that can cause this fault are the type and quality of the bentonite.
Variation de RTU/AA
It was made a comparative study, in which DEL LAGO natural sodium bentonite and activated sodium bentonite were added to moulding sands in different cycles of the casting process, and it showed that DEL LAGO Bentonite presents a better performance concerning the fault, as it keeps a high resistance to traction when wet per unit of activated clay - which does not occur with an activated bentonite. As shown in figure 8 keeping a high resistance as the moulding sand is recircled, garantees the production of faultless pieces that are free from flakes.
Figure 8 shows that the relation «RTU/ AA» (resistance to traction in wet/ activated clay) for green sand bound with DEL LAGO Bentonite, has remained practically the same, after the different cycles of casting. However, when activated sodium bentonite is used, the RTU/ AA drops dramatically to 35% reduction, after the fourth cycle in the casting process. This drop has come along with an increase of flakes in the pieces.
Analyzing the specific and technological characteristics of DEL LAGO Bentonite, we can see that it possesses exceptional levels of purity, high durability and low tendency to expansion faults. These features enable the use of DEL LAGO Bentonite in the manufacturing of high quality melted pieces, with low cost. This is the reason why it has an outstanding performance in Latin America.
Castiglioni Pes & Cía., provides natural sodium bentonite to the most important foundry companies in MERCOSUR. This is due to the superior quality of our product which meets the strict requirement of the most specialized foundries in Brazil together with its purity qualities, comparable only to Wyoming Bentonite (USA).
Figure 8 -Variation of resistance to traction in wet/ activated clay (RTU/AA) with cycles casting
Sodium bentonite vs. Calcium bentonite
There is a summary of properties of each type of bentonite mention above, and also their tendency to certain faults produced during foundry .
Properties |
Sodium bentonite |
Calcium bentonite |
Resistance to compression in green |
lower |
higher |
Resistance to compression in dry |
higher |
lower |
Permeability |
higher |
lower |
Need for water |
lower |
higher |
Durability |
higher |
lower |
Flow properties |
lower |
lower |
Collapsibility |
higher |
lower |
Plasticity |
lower |
higher |
Resistance to heat |
lower |
higher |
Contraction |
higher |
lower |
Number 01 dots during unmoulding |
lower |
higher |
Effort for mixing |
higher |
higher |
Table N° 1- Comparison of properties of calcium and sodium bentonites
As regards some important faults related to moulding sand, the bentonite with less tendency to the fault would be:
Fault in foundry process |
Bentonite with the lower tendency |
Expansion flake |
natural sodium |
Gases, pinholes |
natural sodium |
Erosion of the mould |
natural sodium |
Breaking in heat |
calcium |
Broken mould |
natural sodium |
Because of its high durability, DEL LAGO Bentonite is desactivated in a lower scale when it is under a thermal request. In this way its consumption gets reduced in 30 or 40%, compared to activated bentonites. Therefore, the use of DEL LAGO Bentonite enables the production of melted pieces in a more economical way.
Industrial uses
Special uses
Brig. Juan Manuel de Rosas S/N
Cinco Saltos - Rio Negro
Argentina