Application of Magnesia in Foundry Binder(2)
Source: internal company
1. Influence of the ratio of magnesium oxide to magnesium chloride
1.1 The activity of light burning magnesium oxide
As magnesium oxide is a magnesite ore that is calcine and then ground into powder, When the calcination temperature is 750-850 ℃, the content of magnesium oxide can become 75%-85%, of which only 60%-65% is active magnesium oxide. The activity of magnesium oxide gradually decreases with the extension of storage time. As the activity of magnesium oxide is an important basis for batching, it is necessary to know the activity of magnesium oxide when leaving the factory before batching. After leaving the factory and using for several months, the activity needs to be tested again. If it is left for too long, magnesium oxide will become magnesium hydroxide due to moisture, and the caked magnesium hydroxide has almost no activity, so it cannot be used, but it can be regenerated.
1.2 Purity of magnesium chloride
The specifications of magnesium chloride purchased on the market are generally 44%-45% and more than 97% of magnesium chloride, and the ingredients are subject to the content of magnesium chloride. As magnesium chloride is extremely easy to absorb moisture, magnesium chloride particles are generally placed in a closed container and kept dry to facilitate operation.
1.3 Ratio of magnesium oxide to magnesium chloride
The molar ratio of magnesium oxide to magnesium chloride is between 6 and 9. From the test, it is concluded that after magnesium oxide is fixed, when the molar ratio is 6, magnesium chloride should be added more, magnesium cement has high strength, but the risk of moisture absorption will be higher. If the molar ratio of magnesium oxide to magnesium chloride is 9, the strength is low and the moisture absorption is small.
1.4 Configuration of core sand
First of all, the addition amount of magnesium oxide is determined according to the quality of the original sand. For sand with good quality, the addition amount of magnesium oxide is generally 6%-8%. For sand with poor quality, such as high mud content, fineness above 200 and poor angular coefficient, the addition amount of magnesium oxide should be 8%-10%, and should be appropriately increased or decreased according to special requirements. After we have determined the addition amount of magnesium oxide, the addition amount of magnesium chloride can be determined according to the molar ratio of magnesium oxide to magnesium chloride. According to the molar ratio of water and magnesium chloride, the addition amount of water can be determined. It should be noted that we should adjust the addition of water according to the addition, specific operation and hand feeling. When we know the amount of water to be added, dissolve magnesium chloride in water and add wet mixed sand until it is transparent.
2. Use of modifiers
2.1 Adjust the strength of core sand
Since magnesium cement has a needle-like structure and is arranged in a disordered and staggered manner, it is necessary to add elements that can increase the number of crystal nuclei, thus making magnesium cement crystals round or short needle-like compact structures, such as the use of Al (OH) 3 and Al2 (SO4) 3. The addition of polymers such as starch and water-resistant powder can not only improve the wet strength of the core sand, but also plug the micropores between the core sand and magnesium cement, thus reducing the problems of halogen return and frost return of magnesium cement.
2.2 Controlling the solidification time of magnesium cement
The usable time of magnesium cement core sand is generally 1.0-1.5 hours. Adding phosphoric acid accounting for 0.5%-1% of magnesium oxide can appropriately delay the setting time of magnesium cement and improve the water resistance of magnesium cement, thus leaving sufficient operation time for molding core making. The addition of sulfuric acid can react with magnesium oxide to become magnesium sulfate, which has certain coagulation promoting effect and reduces the halogen and frost return effect of magnesium cement core sand.
3. Operating process
3.1 Sand mixing process
Sand + magnesium oxide + additives are dry mixed for 3 minutes, then magnesium chloride aqueous solution is added for wet mixing for 3 minutes, and sand is produced after the wet strength is adjusted.
• Magnesium chloride is extremely easy to absorb moisture. It must be placed in a closed container, taken out the dosage, dissolved in water, and stirred until transparent before use.
• After sand mixing, it should be molded and cored as quickly as possible. Generally, the usable time of wet sand is about 1.5-2 hours.
• After the core is made, leave it for 48 hours, brush the spraying material after self-hardening, and then dry the coating.
• Self-hardening conditions, room temperature below 50 ℃, air humidity greater than about 70%, self-hardening, not in the furnace drying. Medium and large pieces can be self-hardened in the furnace under the conditions that the furnace temperature is less than 60 ℃ and the humidity is more than 70%. The storage time should be controlled according to the specific conditions. The self-hardened molding sand can be thoroughly dried and painted above 100 ℃.
4.1 Active magnesium oxide and magnesium chloride belong to inorganic materials. Like sodium silicate, they are also hydrate gel binders, so they are environmentally friendly and non-toxic. Meet the requirements of green casting.
4.2 The price is low. The cost of preparing molded sand is about 300 yuan per ton, and "self-hardening" does not need to consume any energy, and there is no special requirement for raw sand. Any dry and wet sand for casting can be prepared.
4.3 The wet strength is easy to control, does not creep, does not stick to the mold, does not stick to the sand, and has good collapsibility.
4.4 The production arrangement should pay attention to the curing time, because it takes longer to harden itself.
4.5 The residual moisture of core sand after self-hardening should be controlled.