How to systematically protect refractory materials in high temperature use (1)

For thermal equipment in metallurgical and other industrial sectors, only continuous automatic control of the status of refractory lining and system protection with refractory materials can make the equipment operate reliably.

The first step in the system protection of refractory materials is to measure the thickness of the lining frequently during use.

We already know that there are several ways to find out the rate of failure of refractory materials, using the naked eye, tracing atoms, measuring the temperature of the lining and so on. Laser interferometric analysis has been developed to determine the residual thickness of refractory lining with an accuracy of less than 1mm.

The thickness of masonry billets can be the same by constantly measuring the failure rate of refractory materials in different areas of the lining.

There are several aspects to the protection of refractory materials:

(1) Cool the refractory material of the masonry until the water-based guard completely replaces the lining;

(2) Spray, smear, adhesion and other methods to restore the damaged layer of refractory materials;

(3) Reduce the aggressiveness of the erosion;

(4) Refractory lining, the standard amount of temperature and gas system;

(5) Improvement of masonry members and masonry structures, the purpose of which is to reduce thermal mechanical stress.

First, lining cooling

According to the cooling intensity, it will affect the life of the refractory according to various mechanisms. The cooling system is divided into slagging layer and gradient: slagging layer, when the working surface temperature is significantly reduced, and at a certain level, the water cooling system is equivalent to the interaction balance between the solid state product and the liquid state of the refractory and the erosion, that is, the conditions for the formation of the slagging layer; Gradient, when the temperature of the refractory working surface is still unchanged, and is approximately equal to the furnace space temperature, the temperature of the cold surface is reduced, so that the lining increases according to the temperature gradient of the thickness, and at the specified level, the furnace is used for a certain period of time to support the water cooling system.

Because the slag and metal melt not only form a slagging layer when the lining surface becomes cold, but also because the fusible component of the refractory itself migrates to the working surface. When the viscosity of the metal melt of the oxide is 0.5~1Pa·s, it loses its fluidity. Depending on the type of chemical reaction, the composition of the melt and the nature of the gaseous medium, there is such a viscosity value in the range of 1250~1550 ° C. Therefore, when cooling the slagging layer, the working surface temperature should be reduced beyond the indicated range.

Another starting point for determining the working surface temperature is to have a temperature condition equivalent to the temperature at which the chemical reaction begins on the hot surface of the lining.

Use direct water cooling like vaporization cooling system. The slagging layer cooling system structure consists of a plate cooling unit, box or tube type furnace wall panel, whose lattice is initially filled with a thin layer of refractory material.

Blast furnace cooling achieved maximum success, with only water cooling under the furnace body.

When the electric furnace is cooled, the cooling wall is also the furnace wall. Such a structure, from the point of view of cooling, is most effective, but it puts high demands on the quality of the water, and when using it, strict measures must be maintained in accordance with safety techniques (in the metal melt pool, no water breaks are allowed). High power arc steelmaking furnace (500~600kW/t), when cooling the slag layer, the life of the furnace wall reaches 400 furnace, while the furnace productivity is increased by 3%~5%, due to the extension of the service life and the maintenance downtime is shortened, the duration of the same furnace is still the same as that of ordinary lining. For the additional power consumption of cooling, lt steel is considered to be 5% to 10%. The main effect of the slagging layer is to reduce the consumption of refractory materials by 50% to 90%.

As the lining is damaged, the temperature gradient increases, as shown in Figure 11-19, and the average temperature of the lining increases. Assume that the temperature gradient (dt/dx) 1 of the initial thickness of the lining is equal to the gradient (dt/dx) 2 of the subsequent (minimum) thickness. At that time, the heat flow should be equal to λ1 (dt/dx) 1=d1t1 and λ2 (dt/dx) 2=d2t2, resulting in λ2λ1=d2t2/d1t1 and λ2 =λ1d2t2. For example, when the temperature of the cold wall is increased from 40 ° C to 800 ° C, α2≈α₁, the thermal conductivity λ₂ should be increased by 20 times is unrealistic. This means that the gradients are assumed to be equal with respect to the abnormal, when in fact (dt/dx) 2 is greater than (dt/dx) 1

This situation is of great significance in the process of refractory failure: the temperature of a product with an increased temperature gradient at a certain depth (depending on the temperature of the whole process of the chemical attack refractory) is lower than that of the temperature gradient from the hot surface to the same depth (Figure 11-19). The temperature gradient in the refractory increases, and the penetration depth of the melt decreases, resulting in damage.

When cooling the gradient manually, the water-cooled lining on the outer surface causes the gradient value to increase.

The relationship between refractory life and temperature gradient, which is determined by tests on various furnaces without artificial cooling operations, is expressed as the tangent of a hyperbola. The characteristic of this relationship is that the range of values is small, and the final value of the function is obtained by changing the argument. This means that there is no continuous relationship between temperature gradient and refractory damage. The significance of a gradient is only expressed when its value is determined.

Solutions to gradient cooling structures may differ. The efficiency of this cooling form is lower than that of the slag layer from the point of view of refractory consumption, and the energy consumption of cooling may be higher from the overall economic calculation.

For refractories, gradient cooling makes more sense, as the temperature increases, its thermal conductivity decreases. With the increase of masonry thickness, the temperature curve has the form of concave line, convex downward. This shows that the direct hot surface temperature falls more significantly.

With the increase of temperature, the thermal conductivity of refractory increases, and the curve of temperature change according to the thickness of masonry is convex upward.

At the same time, the hot surface temperature drops more slowly (resulting in a small temperature gradient value), and gradient cooling may not be effective.

Two layers of lining are often implemented: dense, placed close to the heat direction and insulated. In this case, different temperature gradients are created in the layers: the gradient of the dense layer is small, while the gradient of the insulation layer is larger. The gradient of the dense layer decreases, accompanied by the drastic destruction of the layer. So it may not be reasonable to implement a two-layer lining. A lining that continuously increases the porosity from the hot side to the cold side seems more reasonable. The lining of the furnace can be easily achieved by ramming or pouring.