by In recent years, with the development of high-temperature industries such as steel smelting, nonferrous smelting and cement kiln, researchers in various countries have studied and developed new varieties of magnesium castables, especially in improving the performance of ultra-fine powder combined magnesium castables, which has made magnesium castables widely used.
The production process of magnesia castable is relatively simple, which is basically carried out according to the processes of mixing, forming, curing, demoulding and drying.
However, some problems are often encountered in the production process.
This paper mainly discusses the common problems and treatment methods in the production process of magnesium prefabricated parts (including slag retaining plate, wall protection plate, impact plate and other tundish prefabricated parts).
The castable rises during curing and hollow magnesia castable occasionally rises and hollow during curing.
The rise range is generally 0.5% ~ 1.5% of the height of the castable, while the hollow phenomenon is generally generated with the rise of the castable.
When the cavity is serious, the diameter can reach 200 ~ 300mm.
Through long-term exploration, we believe that the main reasons for the rise of castable during curing are as follows: (1) the physical and chemical properties of silicon powder fluctuate.
Generally, the silicon powder used in the refractory industry is a by-product of the production of metal silicon and ferrosilicon.
It has the advantages of good filling, no agglomeration and significantly reducing the amount of water added in the mixing of castable.
However, when the physical and chemical properties of silicon powder fluctuate, it will seriously affect the properties of castable.
(2) When mixing, the metal aluminum powder is not mixed evenly or the addition amount exceeds the standard.
Metal aluminum powder is commonly used as an additive for castables.
After mixing with water, it releases exothermic reaction, generating Aluminium Hydroxide Gel and escaping hydrogen.
The reaction formula is as follows: 2Al + 6H2O → 2Al (OH) 3 + 3h2 ↑ when the metal aluminum powder is not mixed evenly or the addition amount exceeds the standard, it is easy to produce too much gas, resulting in the rise and hollow of the preform, and even stratification in serious cases.
The common methods to solve the rising and hollow of magnesium prefabricated parts are as follows: (1) accurately proportioning and appropriately prolonging the dry mixing and wet mixing time, so that the metal aluminum powder can be fully mixed; (2) To ensure the stability of the performance of silicon powder, it is best to screen the silicon powder with agglomeration before use; (3) In the curing process, low-temperature curing shall be carried out first, and then high-temperature curing shall be carried out after the surface of the castable is hardened.
The castable will collapse after curing or baking.
The magnesium castable will sometimes appear white spots and collapse after curing or baking.
Taking the wall panel produced by our company for a steel plant as an example, after curing and demoulding, there will be 6 ~ 8 white spots with a diameter of 3 ~ 5mm on the surface of the wall panel.
Through chemical analysis, the main reason for this phenomenon is that the calcium content in the raw material magnesia (especially sintered magnesia) exceeds the standard.
As we all know, Cao will undergo hydration reaction after encountering water.
The white spots of magnesia castable after baking are caused by the hydration of Cao.
The hydration reaction of Cao will produce volume expansion, resulting in the collapse of castable after baking.
Slight chipping has no great impact on the use of tundish prefabricated parts, and it can be used normally only by repairing.
However, when the block collapse is serious, it is often accompanied by cracks, which will cause quality hidden dangers in the use of prefabricated parts, and even quality accidents during steel pouring.
The main way to solve the problem of chunking after baking magnesia castable is to ensure that the physical and chemical indexes of magnesia raw materials are qualified.
In order to prevent the CaO content from exceeding the standard, when magnesia must be used, it is recommended to put the magnesia in the open air for a period of time before use.
Cracks in castables after baking are the most common problems encountered in the production of magnesia preforms, which can be attributed to the hydration of magnesia.
The hydration process of magnesia is as follows: during the mixing, curing and baking of castable, MgO will hydrate after contacting with water and form the following hydration products: MgO + H2O → Mg (OH) 2 (2) MgO + H2O + CO2 → MgO + H2CO3 → MgCO3 + H2O (3).
If there is more free water in the castable, magnesium hydroxide and brucite crystals obtained from reaction formula (2) can be produced.
The formation of brucite can significantly improve the pH value.
At this time, CO2 is dissolved in the water and H2CO3 is produced.
H2CO3 reacts with MgO to form magnesium carbonate or magnesite, as shown in reaction formula (3).
The two kinds of hydration ways have similar effects, and the volume expansion is 2.5 times.
Serious volume expansion may lead to cracks in castable.
Therefore, for magnesia castable, it is very important to formulate a reasonable maintenance and baking system to reduce the hydration degree of magnesia.
In the curing stage of magnesium prefabricated parts, some studies show that when the curing temperature is lower than 50 ℃, the amount of brucite formed by magnesium castable is small and the speed is slow.
If the curing temperature is too high, a large amount of brucite will be formed in a short time, and the internal volume of the castable will expand, resulting in stress and cracks.
Therefore, the curing temperature of magnesia castable should be strictly controlled.
The experimental results show that the hydration degree of magnesia castable cured at 70 ℃ for 8h is higher than that cured at 50 ℃ for 20h.
In the baking stage of magnesia castable, since the hydration rate is significantly accelerated when MgO is above 90 ~ 100 ℃, a rapid baking scheme should be formulated according to the conditions of baking equipment to make the free water in the pouring body discharge as quickly as possible.
Of course, the baking rate should not be too fast, so as to avoid cracks caused by too fast evaporation of water in the castable and the generated steam pressure being greater than the tensile strength of the surface of the castor.
According to our field test, when the temperature rise rate is lower than 10 ℃ · H-1, the cracks of slag retaining wall prefabricated parts are serious after baking; When the heating rate reaches 20 ℃ · H-1, the product is basically free of cracks after baking..
