01 Hydrogen diffusion degree
Hydrogen diffusion degree is an indicator to measure the foaming performance of bottom bricks, and controls the diffusion of H2 within a suitable range. It is expressed as the difference between the pressure of the air chamber enclosed by the sample and the atmospheric pressure. During the test, open the vent valve first, then open the air intake valve. At this time, the U-shaped meter indicates zero. After the air intake continues for a few minutes, close both valves at the same time (or close the air intake valve slightly in advance, and then close the vent valve). At this time, it can be observed that the pressure in the air chamber indicated by the U-shaped meter drops rapidly, and then rises after reaching the lowest point. The reading at the lowest point is the hydrogen diffusion degree usually referred to. It is generally believed that as long as this value is less than 1471Pa (150mmH2O), it can be guaranteed that there is no bubbling.
Hydrogen diffusion degree is essentially the balance point of the two opposite processes of hydrogen diffusion outward and air diffusion inward. In the early stage, due to the small size of hydrogen molecules and strong diffusion ability, the hydrogen diffusion rate is much higher than the air diffusion rate, resulting in a decrease in the amount of gas in the air chamber and a decrease in pressure. As the diffusion progresses, the hydrogen in the air chamber becomes less and less, the diffusion driving force gradually fades, the diffusion speed gradually slows down, and gradually approaches the air diffusion speed. When the two are equal, the U-shaped pressure gauge indicates the lowest point (i.e., the maximum negative pressure). After that, the air diffusion speed is higher than the hydrogen diffusion speed, so the pressure rises.
It can be seen that the hydrogen diffusion index does not represent the ability of hydrogen to pass through a sample of a certain area and thickness, but represents the relative ability of hydrogen and air to pass through the sample. Its value is restricted by two factors: hydrogen diffusion speed and air diffusion speed. A small hydrogen diffusion does not necessarily mean that the ability of hydrogen to pass through the sample is weak. Two completely different refractory material samples were used to measure hydrogen diffusion, but the values obtained were basically the same. One sample is a fused brick sample with an extremely low porosity. Obviously, the ability of hydrogen to pass through this sample is very small. The other is a refractory concrete sample that has been dried at 300°C. After a large amount of water in the sample is dried, many fast channels for hydrogen diffusion are formed, which has a strong hydrogen permeability. The measured values of hydrogen diffusion of the two samples are very low, only a few millimeters of water column. Obviously, the porosity of the first sample is extremely low, and the permeability of hydrogen and air is very weak. The amount of gas in the air chamber changes very little, so the pressure changes very little, and the hydrogen diffusion index is very low. The diameter of the micropores left after drying in the second sample is larger, providing the same fast channel for air and hydrogen. As a result, hydrogen permeates out very quickly, air diffuses in very quickly, and the amount of gas in the air chamber also changes very little. The reading of the hydrogen diffusion index is of course also very small. If the hydrogen diffusion is understood mechanically from the literal and numerical meanings, it is bound to come to the wrong conclusion that "the ability of hydrogen to diffuse through the second sample is also weak."
02 Permeability
The permeability index is generally used to measure the ability of refractory materials to resist penetration and erosion. A high permeability indicates that there are more diversion-type pores in the refractory material, the pore size is relatively large, and the corrosive medium is easy to penetrate into the refractory material. Specifically for the bottom brick refractory material, sodium oxide is easier to penetrate into the brick body and cause nepheline petrification and peeling. If other factors are not considered, from the perspective of anti-nepheline petrochemicals, the smaller the air permeability of the bottom brick, the better.
Air permeability refers to the ability of gas with pressure difference to pass through refractory materials. The air permeability index of different types of sintered refractory materials varies greatly, ranging from a few centimeters of cm3/m3 to tens of cm3/m3.
03 Strain rate
The bottom bricks with a large strain rate have better elasticity, and the higher extrusion stress generated by thermal expansion can be partially released through deformation, avoiding stress accumulation and causing the brick body to break and float. In the 1970s, there were many accidents of "7-inch break and float" of bottom bricks abroad. At that time, the strain rate of the bottom bricks used was generally less than 0.5%, and the expansion joints between bricks were also small. The brick body was subjected to great stress, resulting in horizontal fracture along the cone of the fixed bolt hole, and the upper part of the brick body floated. After switching to high-strain-rate machine-pressed bricks and more advanced vacuum-cast bricks, although small expansion joints were still used, such accidents have never occurred again. The strain rate of the latest machine-pressed bricks abroad has exceeded 0.7%, and the strain rate of vacuum-cast bricks is as high as 1.5%. In China, the practice of large expansion joints has always been used in the design of the bottom structure of the trough, and there is no requirement for the strain rate index. Therefore, the bottom brick manufacturer has not deliberately controlled this index, and the strain rate of the bottom bricks produced is about 0.5%.
To measure the strain rate of the tin bath bottom brick, a 60mm×60mm×130mm specimen is generally used. It is placed vertically on a material testing machine and pressurized. The deformation of the specimen height under load is recorded. The percentage of the maximum deformation of the specimen before reaching the limit yield to its original height is the strain rate.
Generally speaking, for bottom bricks produced by a specific forming method, the strain rate is proportional to the porosity, that is, the larger the porosity, the higher the strain rate.
04 Relationship between hydrogen diffusivity and permeability
When the total porosity is a constant, there is a dependent relationship between hydrogen diffusivity and permeability, and this relationship depends on the distribution of pores. If the pore distribution is dominated by large-diameter pores through which air molecules can pass, the air permeability is large, while the hydrogen diffusion is small. When the pore distribution is dominated by fine pores, hydrogen molecules can pass through these fine pores while larger air molecules are difficult to pass through. At this time, the air permeability index value is small, while the hydrogen diffusion index is large.
It should be noted that when the air permeability is large, it means that the sample has a large ability to pass through air and hydrogen. For the pores that large molecules of air can penetrate, small molecules of hydrogen can also penetrate. The basis for understanding the inverse relationship between hydrogen diffusion and air permeability is to avoid confusing the two different concepts of hydrogen diffusion and hydrogen permeability.
05 Relationship between porosity and hydrogen diffusion, air permeability and strain rate
In general, if you do not deliberately pursue pore distribution and only artificially control the size of the porosity, the following situation generally occurs in practice: the higher the porosity, the greater the hydrogen diffusion, and the greater the air permeability and the strain rate of the brick. In the production process of bottom bricks, if the strain rate control parameter is not considered, bricks with low hydrogen diffusion and low air permeability can be obtained by reducing the porosity of the bricks. This is exactly the path that domestic bottom bricks take. In the early days, since there were no professional manufacturers to produce finished bricks, float glass plants could only directly purchase unprocessed clay bricks and process them on site. These bricks are generally used at the bottom of large kiln pools. For safety reasons, the expansion joints between the bottom bricks of the trough are designed to be relatively large to avoid the bricks squeezing each other and causing layer cracking and floating. Although many professional bottom brick manufacturers have emerged later, the design still uses large expansion joints, so there has been no strain rate requirement for bricks. Manufacturers do not have to bother to balance the strain rate and hydrogen diffusion indicators. Simply reducing the porosity can obtain bricks with ideal hydrogen diffusion and air permeability indicators.
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