Damage to refractory materials caused by corrosive gases

The cement kiln exhaust gas fueled by waste oil contains a large amount of sulfur and chlorine, the glass kiln exhaust gas contains a large amount of SO3, K2O, Na2O, etc., and the waste incinerator and melting furnace exhaust gas will also produce acid gases such as sulfuric acid gas and hydrogen chloride gas. These acid gases are highly corrosive and corrosive to refractories.

1. Damage caused by Cl2 and HCl

In the absence of oxygen, the high-temperature reaction of refractory oxides with Cl2 and HCl can be described by the following general formula:

MO(s) +Cl2(g) = MCl2(g) +1/2O2(g) (6-3)

MO(s) +2HCl(g) = MCl2(g) +H2O(g) (6-4)

Through thermodynamic calculations, it is determined that the reaction between various refractory oxides and Cl2 and HCl at 1300K 6-3.The standard free energy △Ce value is positive in any case, so the positive reaction cannot be carried out. However, under the condition of gas flow, due to the very low partial pressure of the O2 (g), H2O (g) and chloride gas generated, the second term in Eq. 6-5 will become very negative, and △G will all become negative.

△G = △Gθ + RTlnKp (6-5)

In this case, the positive reaction described above is possible. If the activation energy of the chloride generation reaction is low, it is predicted that the quality of the refractory will be reduced due to chloride volatilization at a lower boiling point. The results of the actual observation are that in Cl2 (g), when the concentration exceeds 5000 PPm, special attention needs to be paid to the refractory materials used. To sum up:

(1) Non-oxide refractories have high corrosion resistance to HCl. Fe2O3 does not react with Cl2 (g) thermodynamically, but the generated FeCl3 volatilizes and loses equilibrium because it is a gas phase, so it can react.

(2) CaO reacts easily with Cl2 and forms a CaCl2 liquid phase, thereby reducing the heat resistance of the refractory.

When the gas contains HCl, the effect of the gas on its corrosion can be ignored because the refractory materials (refractory bricks and refractory castables) have good stability to HCl. However, corrosion caused by HCl should be considered when the temperature in the furnace drops below the dew point during kiln shutdown operation. In particular, CaO in refractory castables will dissolve into strong acids with pH = 1 ~2, causing tissue damage.

In the study of the possible reactions of Cl2 and HCl with non-oxide substances such as SiC, Si3N4 and AlN, it is concluded that the standard free energy △Gθ value of the reaction between Cl2 and SiC, Si3N4 and AlN at 1300K is negative, so it is considered that it can be carried out thermodynamically, which indicates that these non-oxide refractories will also accelerate the corrosion due to the formation of chloride volatilization. It can be seen that when using in gases with high Cl2 content, attention should be paid to the correct selection of refractory materials.

However, in the case of HCl, with the exception of SiC, △Gθ1300K is a positive value, indicating that they are thermodynamically stable and therefore have high corrosion resistance to HCl.

2. Corrosion caused by SO3 gas

When waste oil is used as fuel for cement rotary kiln, because waste oil generally contains a large amount of chlorine and sulfur, the combustion gas contains not only Cl2 (g) but also SO3 (g), which increases the erosion of kiln lining refractory materials.

SO3 (g) and Cl2 (g) react with MgO at about 700 ~1000 °C to form MgSO4 and chloride, which will react with the matrix of alkaline lining refractory materials in contact with the working surface, resulting in loose working surface and seriously reducing its service life.

In addition to chlorine, the gas in the regenerator of the glass kiln contains V2O5 and alkalis. The upper part of the middle lattice is the V2O5 erosion zone. V2O5 comes from heavy oil, which mainly erodes the magnesia lattice bricks in the upper lattice layer with 2CaO SiO2 as the binding phase. The reaction process is that under the condition of 1150 ~ 1250 °C in the oxidizing atmosphere, V2O5 reacts with CaO to form calcium vanadate with low melting point, and volatile calcium vanadate is generated under the reducing atmosphere, resulting in the loss of part of CaO in the bonded phase of magnesia bricks, so that the CaO/SiO2 ratio changes, that is, the silicate phase changes from 2CaO· SiO2→3CaO· MgO·2SiO2→CaO· MgO· SiO2. and the magnesia lattice bricks are severely eroded.