Effective Solutions For Magnesia Brick Failure in Glass Furnace Regenerators Due To Petroleum Coke

Petroleum coke is the final residue obtained from the distillation and cracking of crude oil in the petrochemical industry, with a carbon content of more than 95% by mass. The chemical composition (mass fraction) of petroleum coke is as follows: moisture 1.44%, ash 0.16%, C 88.87%, H 3.69%, N 2.27%, S 0.87%, and O 2.7%. Compared with residual oil, petroleum coke has higher carbon, nitrogen, and oxygen contents, but lower hydrogen content and calorific value. Petroleum coke has a wide range of applications: about 40% is used as an alternative fuel in cement production, 22% as a raw material for carbon materials, 14% as fuel for thermal power generation, 7% as a carburizer in steelmaking, 1% as fuel for heating, and 16% for other purposes.

Fuel costs account for approximately half of the total production cost of glass. Replacing heavy oil with relatively low-cost petroleum coke can significantly reduce production costs. However, most of the petroleum coke used in the glass industry is imported, much of which is low-grade petroleum coke that cannot be utilized abroad and contains large amounts of sulfur, vanadium, and other elements. After petroleum coke replaces heavy oil, changes occur in the thermal system, especially in the composition and acidity/alkalinity of the furnace slag, which significantly affect the service life of refractory materials, particularly those used in regenerators. The service life of checker bricks drops sharply from 10 years to 2–5 years, or even to less than 1 year.

To extend the service life of the regenerator checkerwork in glass furnaces, the chemical composition and alkali–sulfur ratio of glass furnace slag after the use of petroleum coke, the corrosion mechanism of magnesia-based residual bricks in the regenerator checkerwork, and the slag resistance of direct-bonded magnesia–chrome bricks, fused-rebonded magnesia–chrome bricks, and fused-rebonded high-purity magnesia–alumina spinel bricks to glass furnace slag were first investigated. On this basis, a series of targeted countermeasures were adopted, which significantly improved the service life of the checker bricks.

01 Chemical Composition of Glass Furnace Slag After Using Petroleum Coke

After a large glass enterprise replaced heavy oil with petroleum coke, chemical analysis was carried out on a total of 14 slag samples from 9 production lines and 5 small furnaces. The results are shown in Table 1. The alkali–sulfur ratio refers to the molar ratio of R₂O to SO₃. When the alkali–sulfur ratio is 1, R₂O reacts with SO₃ to form sulfate. When the ratio is greater than 1, the excess free alkali strongly corrodes aluminosilicate refractories. When the ratio is less than 1, the excess SO₃ first reacts with CaO to form CaSO₄, and the remaining SO₃ strongly corrodes basic refractories. It can be seen from Table 1 that the V₂O₅ content in the furnace slag is very high, and the alkali–sulfur ratio fluctuates greatly.

02 Study on Corrosion of Basic Refractories by Slag

2.1 Test Procedure

Firstly, the microstructure of magnesia residual bricks taken from the regenerator checkerwork of a glass furnace using petroleum coke was analyzed by scanning electron microscopy (SEM) and compared with that of two unused magnesia bricks of the same grade to investigate the corrosion mechanism of magnesia bricks. The average chemical compositions (mass fraction) of the two unused magnesia bricks were: MgO 95.43%, SiO₂ 1.43%, CaO 1.14%, and Fe₂O₃ 0.76%. Their average physical properties were: bulk density 3.02 g·cm⁻³, apparent porosity 14.2%, and cold crushing strength 85 MPa.

Then, direct-bonded magnesia–chrome brick, fused-rebonded magnesia–chrome brick, and fused-rebonded high-purity magnesia–alumina spinel brick were selected for slag resistance tests using neutral synthetic slag with a chemical composition close to that of Slag No. 6 in Table 1. The physical and chemical properties of the three test bricks are shown in Table 2. The chemical composition (mass fraction) of the test slag was: Al₂O₃ 10%, SiO₂ 30%, CaSO₄ 5%, Na₂SO₄ 30%, K₂SO₄ 2%, CaCO₃ 12%, Fe₂O₃ 7%, NiO 1%, and V₂O₅ 3%.

The three test bricks were processed into crucible specimens. Slag was placed in each crucible, and the crucibles were put into a high-temperature furnace, heated to 1450 ℃ at a rate of 2.5–1.5 ℃·min⁻¹, and held at 1450 ℃ for 6 h. After the furnace was shut down, the crucibles were cooled naturally. The crucibles were then cut along the axis of the slag hole to observe the residual slag, as well as the penetration and corrosion of the crucibles by the slag.

2.2 Results and Analysis

2.2.1 Microstructure Analysis of Used Magnesia Bricks and Unused Bricks

The backscattered electron (BSE) images of the cross-section of unused magnesia brick specimens of the same grade as those used in the regenerator checkerwork of glass furnaces fired with petroleum coke are shown in Figure 1. Since the brightness of a backscattered electron image is related to the atomic number, each phase in the figure can be rapidly identified in combination with electron probe microanalysis (EPMA).

It can be seen that the microstructure of the unused magnesia brick consists of aggregates bonded by a porous matrix. The grain size of the aggregates is relatively small (< 1 mm), with many pores, indicating that sintered magnesia was used. The main impurity phases in the brick are forsterite (M₂S) and monticellite (CMS), which almost completely surround the periclase grains, while direct bonding is still barely maintained between the periclase grains.

Figure 2 shows the backscattered electron image of the cracked magnesia brick taken from the regenerator under hot conditions. In the low-magnification image (Figure 2(a)), the dark gray phase is forsterite (M₂S), the light gray phase is monticellite (CMS), and the blackish-gray grains are periclase (M). In the high-magnification image (Figure 2(b)), CVP stands for calcium vanadate phosphate, NAS stands for sodium nepheline, and Glass refers to the glass phase.

It can be seen from Figure 2 that SiO₂ and CaO from fly materials and ash have penetrated into the brick body in large quantities. Under severe corrosion, M₂S (dark gray) and CMS (light gray) surround M (blackish gray) to form a three-dimensional continuous network, resulting in the complete disintegration of the bonding between periclase grains. Furthermore, the low-melting phase also contains vanadates, which further degrade the high-temperature properties of the magnesia brick. Softening of the bonding phase at high temperatures leads to the collapse of the checker bricks under compression.

2.2.2 Corrosion Resistance of Direct-Bonded Magnesia–Chrome Bricks

After the corrosion test, the macrostructure of the direct-bonded magnesia–chrome brick crucible specimen remained generally intact; however, little residual molten slag was left, as it had almost completely penetrated into the specimen. A large amount of yellowish-green substance was observed on the cut section of the crucible. This occurred because sulfates had penetrated into the specimen and decomposed into SO₃ and R₂O, and R₂O reacted with Cr₂O₃ to form hexavalent chromium compounds.

The backscattered electron image near the corroded layer of the specimen section is shown in Figure 3. The electron probe microanalysis results of the selected areas indicate the following:

Zone 1: sodium–calcium aluminosilicate glass phase and magnesia–chrome spinel;

Zone 2: sodium aluminosilicate glass phase and magnesia–chrome spinel;

Zone 3: periclase solid solution containing chromium and iron;

Zone 4: magnesia–alumina–chrome spinel containing a small amount of iron;

Zone 5: periclase and a small amount of forsterite;

Zone 6: periclase.

It can be concluded that the microstructure of the direct-bonded magnesia–chrome brick specimen near the inner hole surface had changed greatly, mainly due to the complete reaction of chromite to form a magnesia–chrome spinel solid solution and the penetration of the glass phase. The damage in the deeper region was mainly caused by the penetration of SiO₂.

2.2.3 Corrosion Resistance of Fused-Rebonded Magnesia–Chrome Bricks

After the corrosion test, the macrostructure of the fused-rebonded magnesia–chrome brick crucible specimen remained intact. A large amount of molten slag remained inside the crucible, while only a small amount penetrated into the specimen. No yellowish-green substance was observed on the cut section of the crucible, indicating that the fused-rebonded magnesia–chrome brick exhibits excellent slag corrosion resistance.

The backscattered electron image near the slag–refractory interface of the specimen section is shown in Figure 4. Electron probe microanalysis of the selected areas shows the following:

Zone 1: sodium–aluminosilicate glass phase with a small amount of calcium from the slag;

Zone 2: sodium–aluminosilicate glass phase with a small amount of calcium and iron;

Zone 3: chromite;

Zone 4: sodium–aluminosilicate glass phase with a small amount of calcium from the slag;

Zone 5: magnesia–alumina spinel with a small amount of iron and chromium;

Zone 6: silicate glass phase at the edge of chromite.

It can be seen that the fused-rebonded magnesia–chrome brick has excellent slag corrosion resistance. Changes occurred only within 500 μm of the interface after corrosion. This is attributed to the spinel barrier layer formed during corrosion, which maintained the stability of the internal microstructure.

2.2.4 Corrosion Resistance of High-Purity Magnesia–Alumina Spinel Bricks

After the corrosion test, most of the slag remained in the crucible specimen of the high-purity magnesia–alumina spinel brick, and the crucible suffered little penetration and corrosion. However, a relatively large crack appeared in the crucible. Observation of the cut section revealed a large amount of white salt substances, indicating that sulfates had penetrated into the specimen.

The backscattered electron image near the slag–refractory interface of the specimen section is shown in Figure 5. Figure 5(b) shows the slag penetration into the brick:

Zone 3: sodium–aluminosilicate glass;

Zone 4: magnesia–alumina–iron spinel and sodium–aluminosilicate glass;

Zone 5: sodium–aluminosilicate glass with a small amount of calcium;

Zone 6: magnesia–alumina spinel with a small amount of iron.

It can be seen from Figure 5 that the structure of the magnesia–alumina spinel brick was relatively intact. Area composition analysis was conducted twice on an approximately 1 mm × 1 mm uncorroded region of the magnesia–alumina spinel brick. The results show that the atomic percentage of Mg is 14.35%–14.58% and that of Al is 33.52%–33.65%, indicating that the material is alumina-rich magnesia–alumina spinel.

The reaction between sodium sulfate in the slag and alumina released from the spinel may be one of the causes of material cracking.

Magnesia–alumina spinel bricks exhibit excellent resistance to the test slag. However, since the dissolved Al₂O₃ in alumina-rich spinel may react with alkalis, and the dissolved MgO in magnesia-rich spinel may react with acidic components, stoichiometric spinel should be used. High-purity magnesia–alumina spinel bricks are expensive and have poor thermal shock resistance, so they cannot yet be widely used as regenerator checker bricks in the glass industry. Nevertheless, as a chromium-free solution for glass furnace refractories, further research on magnesia–alumina spinel refractories is still needed.

03 Countermeasures and Effects

Based on the changes in slag composition, as well as the corrosion behavior and mechanisms of magnesia bricks, direct-bonded magnesia–chrome bricks, fused-rebonded magnesia–chrome bricks, and fused-rebonded high-purity magnesia–alumina spinel bricks, corresponding countermeasures were adopted in three stages:

Stage 1: Control the glass production process and petroleum coke quality, and replace 95-grade magnesia checker bricks with 97-grade magnesia bricks and direct-bonded magnesia–chrome bricks.

Stage 2: Replace direct-bonded magnesia–chrome bricks with fused-rebonded magnesia–chrome bricks, and increase the proportion of fused magnesia in 97-grade magnesia bricks.

Stage 3: Develop customized solutions according to actual working conditions and provide optimal matching schemes with balanced cost-performance.

Through the gradual implementation of these countermeasures, the service life of regenerator checker bricks has been steadily improved.

Since the glass plant initially used Al₂O₃ and materials containing B₂O₃ and P₂O₅ as fluxes, which caused corrosion and blockage of basic checker bricks, the use of these materials was stopped or greatly reduced. As the creep of 95-grade magnesia bricks after corrosion was the main cause of checkerwork damage, 95-grade magnesia bricks in the upper layer of the checkerwork were eliminated. The top layer was expanded with 97-grade magnesia bricks, and the middle layer was lined with direct-bonded magnesia–chrome bricks. These adjustments alleviated, to a certain extent, the problem of excessively short service life of checker bricks in glass furnace regenerators.

Due to the insufficient corrosion resistance of 97-grade magnesia bricks and direct-bonded magnesia–chrome bricks, more fused magnesia was used, and a high-temperature firing process was adopted in the production of 97-grade magnesia bricks. Furthermore, direct-bonded magnesia–chrome bricks with insufficient corrosion resistance were replaced with fused-rebonded magnesia–chrome bricks. Because fused-rebonded materials have excellent microstructural stability, their corrosion resistance and service life are significantly better than similar products made with sintered magnesia. By replacing direct-bonded magnesia–chrome bricks with fused-rebonded magnesia–chrome bricks, the service life of checker bricks can be further increased by 150%.

04 Conclusion

The corrosion mechanism of refractories is determined by the characteristics of glass ash and slag. Basic refractories should be used when the slag is basic (containing free Na₂O), and neutral refractories such as alumina–chrome bricks can be used when the slag is acidic (containing free SO₃).

When using petroleum coke, indicators such as its calorific value, volatile matter, ash content, moisture, sulfur content, vanadium content, as well as sulfur and alkali contents in regenerator ash, should be tested to select suitable refractories and control the glass production process.

When conditions permit:

Produce magnesia checker bricks using fused magnesia and the three-high process (high-purity raw materials, high-pressure forming, high-temperature firing).

Expand the application of magnesia–chrome bricks, and use fused-rebonded magnesia–chrome bricks instead of direct-bonded magnesia–chrome bricks for regenerator checker bricks.

According to different service conditions, the top layer and part of the upper layer can be replaced with high-temperature fused magnesia–zirconia bricks, chrome–corundum bricks, chrome–zirconia–corundum bricks, and similar materials.