The role and change of bentonite in clay-bonded sand

Foundry bentonite mainly has two types of sodium bentonite and calcium sodium bentonite, sodium bentonite is made of natural sodium bentonite or artificial sodium bentonite processing, with its good reusability and high wet pressure strength and welcomed by the casting industry. Because of its good plasticity, it can prevent the casting from sand inclusion, scarring, block falling, sand collapse and other phenomena. In addition, it has strong formability and high cavity strength, which is convenient for metal industry to cast wet or dry molds. It is the preferred sand binder for precision castings. Calcium-sodium bentonite is processed from natural-based calcium-sodium bentonite and is a common sand binder for light castings. It is popular with users for its practical and convenient casting sand cleaning.

Compared with other binders, bentonite has an important feature, that is, it has a certain heat resistance. As long as the heating temperature is not too high, the bentonite removed from free water can still recover the bonding ability as long as water is added. The binding ability of bentonite can only be expressed after adding water. Bentonite loses its ability to bond and is also related to its dehydration. So far, it is believed that there are three forms of water in bentonite: one kind of water is free water, that is, water adsorbed by bentonite particles, which can be heated above 100 ℃ to remove free water, bentonite with free water removed, and the bonding ability is not affected; The second kind of water is firmly bound water. After heating at 110 ℃ for a long time, bentonite can completely remove free water, but will not remove firmly bound water. Bentonite that has been completely removed from free water will continue to lose weight when heated at higher temperatures (such as 200°C, 300°C), indicating that there is still water loss. After the bentonite is heated and dehydrated in this way, as long as water is added, the bonding ability can be completely restored; the three kinds of water are lattice water, which is also called structural water. The lattice water can only be partially or completely removed at relatively high temperatures. After the lattice water of bentonite is removed, it loses its binding ability and becomes dead clay.

Different bentonite, loss of bonding capacity of different temperatures. It is proved by the differential thermal analysis experiment that, in general, the failure temperature of natural sodium bentonite is 638 degrees C, calcium bentonite is 316 degrees C. Artificial activation of sodium bentonite, due to different activation conditions, failure temperature is different, but are lower than the natural sodium bentonite. It can be seen that there are two kinds of bentonite in clay-bonded sand, one is effective bentonite and the other is dead clay. The more dead clay, the worse the performance of clay-bonded sand.

When sodium bentonite is heated below 600°C, its binding ability is essentially unaffected. When the heating temperature exceeds 600 ° C., the adhesive ability is abruptly lost. When heated to above 700°C, the bonding ability is almost completely lost. Calcium bentonite begins to slowly lose its bonding ability above 100 ℃. The higher the heating temperature, the more pronounced the loss of bonding ability. Artificial activation of sodium bentonite is not good, in the initial use, the failure temperature is only slightly lower than the natural sodium bentonite, several times after repeated heating, and calcium bentonite is similar.

When the molten metal is injected into the mold, if the sand close to the surface of the casting is heated to above 800°C (some non-ferrous alloys cannot reach this temperature). No matter what kind of bentonite you use, the bentonite in this part of the sand will become dead clay. Part of this dead clay, which is attached to the surface of the casting along with the molding sand, is taken away by the casting and part remains in the recycled old sand. In addition to the manufacture of large castings, in the casting process, most of the sand can not reach this temperature. However, because different foundry manufacturers use bentonite produced by different manufacturers, the temperature of removing lattice water is different, and the rate of removing lattice water is also different. If the bentonite which is easy to remove the lattice water is used, even in the sand which is not directly close to the casting, more bentonite will fail and become dead clay. If the use of the lattice water is not easy to remove the clay, the resulting dead clay will be less. In short, the less the amount of bentonite added, the better, the higher the effective bentonite content in the clay-bonded sand, and the more bentonite that is heated and ineffective.

　　Control of old sand

After molding and casting with clay wet sand, most of the sand can be recycled except that the active bentonite in the sand close to the casting becomes dead clay when heated. When preparing clay wet sand, the amount of old sand is generally above 90%. If the old sand is not handled properly, no matter how to strengthen the sand mixing, no matter what auxiliary materials are added, good sand cannot be obtained. Therefore, the effective treatment of old sand is the premise to ensure the quality of molding sand.

In order to ensure the stable performance of the sand, the old sand temperature control should be kept below 50 ℃, more than 50 ℃ even hot sand. The hot sand problem has been recognized as the biggest problem that clay wet sand casting must face. Sand temperature is too high, the casting is prone to sand, rough surface, sand, porosity and other defects. At present, there are many varieties and specifications of sand cooling devices, mainly including cooling rollers, double-plate coolers and cooling fluidized beds, all of which use water evaporation to cool sand. Among them, the effect of cooling the fluidized bed is better.

It is very important to strictly control the moisture of the old sand. If the moisture of the old sand entering the mixed sand is too low, the impact on the quality of the mixed sand may be no less than that of the high sand temperature. After casting, due to the influence of hot metal, many sand on the surface of the soil-water bond film are dehydrated and dried, adding water to make it absorb water and restore plasticity is not easy. The lower the moisture content of the old sand, the longer it takes to add water to the sand mixer to achieve the required performance. Because the time of mixing sand in production is limited, the lower the moisture of the old sand, the worse the overall quality of the mixed sand. The moisture of the old sand entering the sand mixer can only be slightly lower than that of the mixed sand. In this way, there is a considerable period of time from the cooling of the sand to the sand mixer, and the water can fully wet the bentonite on the surface of the old sand grains.

For cast iron parts made of clay wet sand, the particle size of the sand is better to be fine. Because the amount of old sand is generally more than 90% when mixing sand, the main factor that determines the size of the sand is the old sand. The amount of new sand added is very small, and it is impossible to change the particle size of the molding sand by adding new sand. Therefore, the particle size of the old sand should be detected frequently: the sand particles on the 140 mesh screen should be between 10% and 15%, the sum of the fine sand on the 200 mesh screen, the 270 mesh screen and the chassis should be as little as possible, and the sum of the fine sand should generally be less than 4%. The content of water-absorbing fine powder is best controlled between 2% and 5%. If the content of water-absorbing fine powder is too high, the water content of molding sand will be higher, which will easily lead to the defects of pinholes, rough surface and sand holes on the casting. If the content of water-absorbing fine powder is too low, the performance of molding sand (especially compactability) will not be stable. The water-absorbing fine powder is mainly dead clay, but also includes coked coal fines and other fine powders.

　　Amount of additional bentonite

When mixing sand, a certain amount of bentonite must be added to keep the necessary amount of bentonite in the sand. The bentonite content is usually measured by the methylene blue method, and the amount of bentonite measured by the methylene blue method is called the amount of effective bentonite.

　　New sand supplement

The additional new sand should not be too much, in order to maintain the stability of the total amount of sand in the system. Too much new sand will have a negative impact on the quality of the sand. China's foundry generally scattered sand are more, many factories new sand to add 5% to 8%. Of course, the amount of new sand to be added to consider many factors, such as equipment conditions, the amount of core sand.

　　Coal powder supplementary quantity

The additional quantity of pulverized coal shall be determined according to the measured value of effective pulverized coal in used sand. The content of pulverized coal in clay wet sand should be controlled between 3.5 and 5.5. Of course, the amount of pulverized coal supplement should be determined according to the characteristics of the casting and the quality of the pulverized coal.

　　sand mixing

The main function of the mixed sand is to mix the components of the clay bonded sand evenly, so that the water can fully wet the bentonite, and the effective bentonite paste is coated on the surface of the sand. The goal is to make the clay-bonded sand with properties suitable for molding equipment. Since bentonite paste is a semi-solid viscous material, the energy required to achieve this goal is very large. If the power of the sand mixing equipment is not high, or the sand mixing time is not enough, the bentonite cannot give full play to its bonding effect, the strength of the molding sand is not high, and other properties are not good. If the moisture in the sand is increased, the moisture of the bentonite paste is increased and the viscosity is decreased, the energy required for coating the bentonite paste can be reduced, that is, the mixing time can be shortened. However, due to the decrease in the viscosity of the bentonite paste, the wet compressive strength of the molding sand also drops sharply, which is actually undesirable.

In order to reduce the energy required for mixing sand, it is important to adopt a reasonable feeding sequence. When mixing old sand, do not add dry materials (sand and bentonite) according to the habit, dry mix for a period of time, and then add water to mix. Sand and water should be added first, then bentonite, because the water has been dispersed, there is no larger water droplets, after adding bentonite can only form a large number of smaller clay balls. It is relatively easy to press open these small clay balls and requires less energy. That is to say, with the same sand mixing equipment, the same quality of the sand, the mixing time required is shorter. Some foreign foundries, under the condition of using intermittent sand mixer, add water to the sand mixer before mixing sand and run for a few seconds (of course, the equipment ensures that the water does not leak). In this way, not only the above-mentioned benefits of adding water first, but also the grinding wheel and scraper can be washed before each sand mixing, so as to improve the efficiency of sand mixing.

　　wet compressive strength

The control target of wet pressure strength must be selected according to the modeling method, not high strength. The higher the strength of the sand, the greater the energy required for pounding when modeling. Now, many foundry sand strength is too high. Some hand molding or seismic molding machine with sand, wet compressive strength of up to 130 ~ 170kPa. It is difficult to use such a sand to make the mold very tight. As a result, the surface quality of the casting is not good, and it is easy to produce shrinkage defects.

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