Mg-Al-Fe Spinel Refractory Brick 2-year Safe Operation Experience

With the improvement of environmental awareness and standards, magnesium-iron spinel refractory bricks have been adopted for cement kilns in place of directly bonded magnesium-chromium refractory bricks, in order to reduce the environmental impact of hexavalent chromium (Cr⁶⁺). However, compared with directly bonded magnesium-chromium bricks, magnesium-iron spinel bricks have disadvantages such as higher thermal conductivity and poor kiln skin adherence, resulting in shorter service life, higher shell temperatures, and the occurrence of red kilns.

After continuous adjustment and optimization, magnesium-iron-aluminum spinel bricks used in the burning zone have now achieved stable performance with a service life of up to two years. The following summarizes the adjustment experience for reference by industry peers.

A 4,000 t/d clinker production line with a Φ4.6m × 68m rotary kiln is configured with three types of refractory bricks: magnesium-aluminum spinel, magnesium-iron spinel, and silica-mullite bricks. The kiln mouth and kiln tail use monolithic (castable) refractory materials. The brick configuration follows the design institute's specifications (see Fig. 1).

Magnesium-aluminum spinel and magnesium-iron spinel bricks are used in the firing zone as replacements for directly bonded magnesium-chromium bricks. However, compared to magnesium-chromium bricks, both magnesium-aluminum and magnesium-iron spinel bricks have poorer kiln skin adherence and higher thermal conductivity.

These performance issues lead to frequent problems in the firing zone. High-temperature failure points typically appear after 6–8 months of operation, concentrated at both ends of the firing zone-from 0.8 to 3 meters and 18 to 23 meters from the kiln mouth. These sections often need to be excavated and repaired annually just to maintain operation until the next scheduled overhaul, thereby affecting the kiln's continuous and stable performance.

1. Damage at 0.8–3 m from the Kiln Mouth

Each time high temperatures and red kiln conditions occur, shutdowns are triggered in this area. The kiln skin here is thin and unstable, and the refractory bricks show severe overall spalling-typically around 5 cm deep, with some areas completely devoid of brick, exposing the kiln shell. However, no brick misalignment is observed.

This section lies at the transition between the firing zone and the cooling zone, where temperature fluctuations are frequent and the kiln skin is unstable, forming and peeling off repeatedly. Additionally, the abrasive action of the clinker as it exits the kiln contributes to mechanical wear and accelerates brick spalling. These conditions indicate a need to adjust the refractory brick configuration in this area.

2. Damage at 18–23 m from the Kiln Mouth

The high magnesium oxide content in limestone from the mine results in clinker with an average MgO content of 5.0% (see Table 1 for composition details). From Table 1, the clinker liquid phase content reaches 29.29%, and the crust factor is 40.82%, both exceeding the upper limits of the optimal control values (26% and 37%, respectively).

Additionally, thick kiln skin and ring formation are observed around 16–17 m, which is located toward the rear of the firing zone where the temperature is relatively lower and kiln skin stability is poor. Combined with the presence of a ring in front of this area, the abrasion from the clinker intensifies the damage to the refractory bricks. This suggests that the brick configuration and process conditions in this zone also require adjustment.

Table 1 Chemical Composition Analysis of Clinker

Considering that the high magnesium oxide content in limestone from the mine cannot be improved, the performance of magnesium-aluminum spinel and magnesium-iron spinel refractory bricks must be addressed from the beginning by optimizing brick selection indicators and other related work. The specific measures are as follows:

01. One Kiln, One Design: Optimizing Brick Allocation to Maximize Refractory Performance

Based on the actual brick usage and long-term application feedback from customers and refractory manufacturers, the brick configuration of the kiln has been adjusted. At the kiln mouth, magnesium-aluminum spinel bricks are used in combination with 1680-grade silica bricks and magnesium-iron spinel bricks to adapt to the higher magnesium oxide content in the clinker and the unstable kiln skin.

Since the kiln skin adherence performance of magnesium-iron spinel bricks is not as good as that of directly bonded magnesium-chromium bricks, the firing zone was shortened by 1 meter. Considering energy efficiency, low thermal conductivity refractory bricks with good spalling resistance were used after the transition zone. The detailed brick configuration is shown in Figure 2.

At the same time, some of the physical and chemical indexes of the magnesium-iron spinel bricks were partially adjusted. The use of magnesium-iron-aluminum spinel bricks is recommended, combining the advantages of both magnesium-iron spinel and magnesium-aluminum spinel. This ensures improved kiln skin adherence along with enhanced brick strength and wear resistance.

Kiln Mouth and Kiln Tail

Kiln Mouth Castable Material: 0.8–2.4 m

(Silica-mullite brick 1680)

Gear Brick Ring Dimensions: 40 × 60 mm, with 15 mm spacing; divided into 12 sections, composed of magnesium-iron-aluminum spinel bricks.

Total: 94 rings covering 18.8 m (from 3.2 to 22 m)

Brick ratio: F3 : MF4 = 87 : 10

Silica-Mullite Brick 1680 Section:

Total: 95 rings covering 19 m (from 22 to 41 m)

Brick ratio: MF1 : MF2 = 38 : 57

Low Thermal Conductivity and Anti-Scaling Bricks Section:

Total: 131 rings covering 26.2 m (from 41 to 67.2 m)

Brick ratio: AM1 : AM2 = 47 : 91

Kiln Tail Castables: 0.79 m

02 Optimize the Batching Program to Promote the Leveling of Kiln Skin and Enhance the Performance of Hanging Kiln Skin

The quality of the kiln skin directly determines the service life of refractory bricks in the firing zone. Under normal kiln skin thickness, the surface temperature of alkaline bricks can be reduced from 1400–1450 ℃ to 600–700 ℃, keeping the brick lining in a safe state. Even a kiln skin thickness as thin as 23 mm can reduce the brick surface temperature from 1450 ℃ to about 1230 ℃.

With kiln skin protection, the temperature gradient in the alkaline bricks is significantly flattened, the infiltration layer of the bricks is notably thinner, and deterioration and damage are reduced.

The amount of clinker liquid phase is a fundamental factor in the formation of kiln skin. The melting of C2S promotes the increase in kiln skin strength, while Fe2O3 significantly lowers the sintering temperature and liquid phase viscosity; all three factors facilitate kiln skin formation.

When the crust value is less than 30%, kiln skin formation is often difficult; when the crust value ranges from 33% to 37%, kiln skin forms more easily; when the crust value exceeds 40%, large pieces and nodules tend to form.

The crust value can be calculated using the following formula:

Crust value = liquid phase volume + 0.2 × C2S + 2 × Fe2O3

It can be seen that Fe2O3 in clinker has a dual effect: reducing the Fe2O3 content simultaneously lowers both the liquid phase amount and the crust value, making dosage adjustment easier and more practical.

Therefore, a low iron dosing scheme is adopted, controlling the crust value near the middle of the optimal range (35.43%). The optimized composition is shown in Table 1.

03 Development of Cooling Curves

Heating curves are well developed in the cement industry, but less attention is paid to cooling curves. According to the crust value formula, C2S is a key factor in promoting the strength of the kiln skin. However, C2S undergoes a crystalline transformation between 400 and 600 ℃ during cooling. If the cooling is slow in this temperature range, C2S easily transforms into the γ-type, which damages the kiln skin and causes refractory brick spalling.

Therefore, special attention should be paid to the cooling rate during temporary shutdowns, especially in this temperature interval. Developing appropriate cooling curves (see Figure 3) can help protect the kiln skin and extend the service life of the bricks.

The purpose of the cooling curve is to control the rate of temperature reduction to prevent damage such as bursting and spalling of silicomullite bricks in the transition zone, as well as peeling of kiln skin and spalling of magnesium-iron-aluminum spinel bricks in the firing zone, thereby promoting longer brick service life.

04 Kiln Mouth Castables Maintenance

Kiln mouth castables are in contact with the first row of silica-mullite bricks. It has been observed that when the kiln mouth castables are damaged, the adjacent silica-mullite bricks quickly develop spalling, cracks, and broken corners, resulting in red kiln downtime.

Damage to kiln mouth castables often occurs as overall shedding along the diameter direction. Inspection of the damaged areas reveals no residual rake nails. Analysis shows that some rake nail welds were not secure, or the welding rods absorbed moisture after drying over time in the field, causing local castable detachment, which then led to surrounding castable falling off.

Therefore, a new rake nail structure and an open construction process for the castables have been developed (see Figure 4).

The rake nail consists of a square base and a U-shaped steel bar, both made of heat-resistant steel. The square base is fully welded, increasing the welding surface compared to the original rake nail, resulting in greater strength. The U-shaped steel bar passes through the center hole of the square base and is connected by spot welding to eliminate stress.

The kiln's radial circumference is divided into 24 equal parts with a spacing of 600 mm. Steel plates are fixed on both sides to support the kiln mouth pouring material. After the initial setting of the poured material, exhaust holes are arranged on the working surface, followed by insulation and maintenance.

Construction proceeds in sections; after one section is completed, the next is started, and so on. Although this open construction process is slower and the surface is less flat, practical experience shows that after one year of use, there has been no occurrence of entire ring detachment, indicating improved quality.

05 Coal Injection Pipe Maintenance

The coal injection pipe has a significant impact on the kiln mouth bricks. There have been two incidents: one involved a deviation in the size of the inner cyclone head spare parts from the manufacturer, resulting in excessive inner wind speed. This caused the flame to scatter, leading to deep grooves appearing in the ring of silica-mullite bricks at the kiln mouth.

The other incident involved the appearance of three large pits in the silica-mullite bricks at the kiln mouth. The analysis suggests that the ratio of internal to external wind was inappropriate, causing the flame to be unstable and weak. Improper positioning of the pipe may also have contributed to the problem.

It is recommended to ensure the correct positioning of the coal pipe and the proper flame shape as a prerequisite for the safe operation of refractory bricks. Fine adjustments should be conservative and detailed, with careful maintenance to extend the service life of the bricks.

Before starting the kiln, the end face of the coal pipe must be kept clean, and its dimensions should be measured and approved. The flatness of the casting material on the end face should be checked, and the positioning data must be accurate. The positions of the kiln and the external coal pipe (X and Y axes) should correspond precisely, with proper recording and marking.

During operation, the coal nozzle end face should be cleaned of coking residue every shift to stabilize the flame shape. Material accumulated on the upper part of the coal nozzle and debris beneath should also be cleaned regularly to prevent deformation of the nozzle head over time, which could alter the flame direction.

The use of magnesium-iron aluminum spinel refractory bricks, which combine the advantages of magnesium-iron spinel and magnesium-aluminum spinel, increases the strength and wear resistance of the bricks while maintaining the hanging kiln skin properties. Optimizing the physical quality of the bricks and their allocation in the kiln is fundamental, while kiln skin maintenance is key.

Daily production should pay attention to the crust value to ensure the kiln skin remains even, thereby avoiding the formation of rings, nodules, and other process accidents. At the same time, attention should be given to the heating and cooling processes, as well as the operating conditions of castables and coal pipes. Careful and detailed maintenance can improve the service life of refractory bricks in the kiln firing belt.