The size of refractory raw material particles has an impact on the performance of refractory ramming materials, especially the tensile force and impact resistance of the materials.
Granular Materials of Refractory Ramming Materials
The particles of refractory ramming materials are generally of different particle gradations of 0-5mm, and the particles are also 0-7mm, and will not exceed 8mm particles. Because the particles are small, the ramming will be more compact.
However, there are also 0-3 particles of refractory ramming materials. This is because some parts with too small gaps have special particle requirements. Otherwise, the 0-3 particles have low tensile force and insufficient impact resistance. In many cases, the 0-3 particle grading will not be used for process proportioning. For example, the particles of the baking-free ramming material used in the blast furnace slag skimmer will be larger. There are also refractory castable manufacturers that put the particles into 10mm, so that the impact resistance is strong during use, and it is also more wear-resistant.
Reasonable Particle Grading and Reasonable Process
Refractory materials themselves belong to applied disciplines, with applicability as the ultimate goal. If there is no special requirement under special circumstances, the particle grading between 0-8mm should be used as the proportion. Too large ramming is not dense, too small tension is not good and the impact resistance is not enough.
Reasonable particle grading and reasonable process ratio will produce products suitable for use. Therefore, the production of refractory ramming materials should be based on the use of different furnace linings, and reasonable particle grading is the most scientific.
According to the current market usage, 0-7mm particle grading is a more suitable ratio, and it is also a way to use the ratio combined with the particle grading of the ramming material. Although larger particles are OK, it is most suitable not to exceed 10mm, which is easy to construct.
In summary, the size of the particles does affect the performance of refractory ramming materials. Therefore, it is also a matter of attention in production and use. Although indicators are important, only those suitable for use are the best.
Why is Boric Acid Used in Refractory Ramming Materials?
Different refractory materials are suitable for smelting different metals or alloys. At present, medium frequency induction furnaces are widely used in metallurgy and foundry industries. During the use of induction furnaces, the impact, friction and electromagnetic stirring of the charge will aggravate the erosion of the furnace lining. The factors that cause the reduction of service life are:
- (1) Operating conditions.
- (2) Furnace building and baking process.
- (3) Rationality of refractory raw materials.
Quartz sand refractory ramming material is widely used in the production of molten cast iron and cast iron alloys because of its high cost performance and good thermal shock stability, mechanical strength and resistance to acidic slag corrosion.
However, there are many types of quartz ores, and their important performance indicators such as impurity content and crystallinity are different. As a result, the service life of quartz refractory materials in medium frequency furnaces varies greatly.
Theoretically, quartz with low impurity content and high crystallinity has high mechanical strength and excellent thermal shock stability, and is particularly suitable as a lining material for large-capacity medium-frequency furnaces. However, the sintering performance of this quartz is poor, which will affect the service life of the lining.
Quartz ramming material is made of high-quality large-grained quartz sand as the main raw material, supplemented by a binder and a sintering promoter. During the production process, an important binder needs to be added, and boric acid is used as a sintering binder. During the sintering process of the furnace lining, boric acid is dehydrated and converted into boric oxide. At high temperatures, boric oxide plays the role of a high-temperature binder and promotes the sintering of the workpiece.
In addition, during the high-temperature use of quartz ramming material, quartz will react with boric acid B2O3 to form tridymite and cristobalite. Boric oxide can reduce the synthesis and sintering temperature of quartz without causing obvious harm to the material properties. Therefore, boric acid is an important raw material for quartz ramming material.
We know from a set of experiments that boric acid plays an important role in quartz ramming material. The main raw material is quartz sand, whose critical particle size is 5mm. Different particle gradings are carried out according to the maximum stacking density, and boric acid is added as a sintering agent. Then the test block is made and the total weight loss of quartz is observed to be about 3.0 when the temperature rises from 25℃ to 1500℃ in the furnace. When the temperature rises from 25℃ to 1000℃, the weight loss of quartz is about 2.2, and this part of the loss mainly comes from the evaporation of free water and crystal water. Due to the evaporation of free water, an endothermic peak appears at 80℃. Due to the transformation of β-quartz to α-quartz, there is an endothermic peak at 580℃. Due to the transformation of α-tridymite to α-quartz, a large endothermic peak appears at 1250℃. There are also thermal effects of other crystal transformations, but they are not very obvious, indicating that its phase change is easier to occur.
The linear change rate of the ramming material after sintering increases with the increase of the content of the sintering agent boric acid. The change rate of the ramming material sintered at 1100℃ generally shows an increasing trend, and the linear change of the ramming material sintered at 1600℃ first increases and then decreases. It is worth noting that the linear change rate of the ramming material sintered at 1100℃ is negative. The reason is that the sintering agent boric acid begins to form a liquid phase at this temperature, causing the sample to shrink. The linear change rate of the ramming material sintered at 1600℃ is positive, and the sample expands. The reason is that quartz has a transition from a low-temperature phase to a high-temperature phase at high temperature, accompanied by volume expansion. This phase change is irreversible, and the volume expansion still exists after the temperature is reduced.
Effect of the content of sintering agent boric acid on the compressive strength of the ramming material. With the increase of the boric acid content, the compressive strength of the ramming material shows a significant increasing trend. It can be seen from the B2O3-SiO2 phase diagram that after adding boric acid, a liquid phase appears in the ramming material at about 440℃, thereby promoting sintering and improving the strength of the ramming material. As the temperature rises, the amount of liquid phase increases, and the strength of the ramming material is bound to increase. However, due to the phase change of quartz at high temperature, the volume expansion associated with the phase change will partially offset the sintering effect of boric acid, making the influence of boric acid content on the strength of ramming material sintered at 1600℃ smaller. Boric acid mainly improves the medium-temperature sintering strength of the ramming material, while the role of the sintering promoter during high-temperature sintering is relatively small, and at this time it still mainly depends on quartz.
