Maintenance and opening points of carbon bricks lining of ferroalloy submerged arc furnace

01. Lining construction method and refractory materials used

Carbon lining construction is divided into insulation lining construction, cooling lining construction and hot and cold mixed lining construction according to cooling method.

Carbon lining is divided into seam masonry, seamless masonry and integral ramming lining according to integrity.

At present, various ferroalloy varieties using carbon lining basically adopt the above three types. Different alloy varieties adopt different cooling methods: ferrosilicon and industrial silicon basically adopt insulation lining seam and seamless masonry; manganese silicon alloy and carbon ferromanganese have both insulation seamless and seam masonry and cooling seamless or integral ramming masonry. At present, most of them adopt composite masonry, with a layer of carbon bricks at the bottom and cold ramming paste (1~1.2m) at the top.

Carbon materials used for furnace construction: ordinary carbon bricks, electrode paste, self-baking carbon bricks, cold ramming paste, microporous high thermal conductivity carbon bricks.

Other furnace materials: clay bricks, high-alumina bricks, silicon carbide bricks, clay powder, clay particles, asbestos boards.

02. Various masonry methods of furnace lining masonry drawings

Commonly used carbonaceous UC condensing furnace linings for high-carbon ferromanganese, high-carbon ferrochrome, etc., electric furnace cold ramming paste (carbon brick) insulation furnace lining structure diagrams, and manganese silicon alloy electric furnace cooling furnace lining masonry lining diagrams are shown in Figures 1 to 3.

1/6/ 18-high-strength phosphate refractory concrete; 2-microporous graphite-silicon carbide iron mouth brick; 3-carbon ramming material; 4/7/ 16-clay brick; 5-1520 ℃ condensation isotherm; 8-light clay brick; 9-spray water surrounding pipe; 10/ 15-high alumina brick; 11-graphite ramming material; 12-low porosity roasted carbon block; 13-high thermal conductivity graphite tile; 14-high thermal conductivity graphite block; 17 furnace bottom water collection tank

Figure 1 Commonly used carbonaceous UC condensing furnace lining

1-asbestos board; 2-lightweight insulation brick; 3-ordinary refractory brick; 4-high alumina brick; 5-insulation carbon paste; 6-micro-expansion cold ramming paste; 7-graphite, semi-graphite brick or silicon carbide brick; 8-silicon carbide corundum castable; 9-fine gap paste; 10-protective brick; 11-elastic layer

Figure 2 Electric furnace cold ramming paste (carbon brick) insulation lining structure diagram

1-Refractory brick; 2/15-Elastic layer; 3/5/18-Carbon brick; 4/12/19-Cold ramming paste; 6-Aluminum silicate plate; 7-Semi-graphite silicon carbide iron mouth brick; 8-Paste; 9-Cover brick; 10/11-Corundum castable; 13-Quick-setting carbon ramming material; 14-Asbestos board; 16-Clay brick; 17-High alumina brick

Figure 3 Lining diagram of cooling lining of manganese silicon alloy electric furnace

03. Key points of lining construction in various ways

1. Insulation lining

Dry lining of furnace bottom

In principle, clay bricks or high alumina bricks at the bottom of the furnace are not wet-laid, and water is avoided as much as possible to prevent the future hot water vapor from evaporating and causing the furnace bottom to expand upward, causing the carbon brick structure to be damaged and causing leakage.

Pre-drying of furnace bottom refractory materials

If wet masonry is used or the bricks have high water content, coal or wood must be used to dry the clay bricks or high-alumina bricks at the furnace bottom after they are laid. Generally, they should be dried for more than 24 hours. After the water vapor is basically discharged, carbon bricks can be laid or cold rammed paste can be made to ensure that the furnace bottom is dry; even if dry masonry is used, because the refractory materials also contain different amounts of water, the water contained in them will be discharged after heating. Instead of discharging it after the furnace is dried and powered on, it is better to discharge it before the carbonaceous materials at the furnace bottom are laid (easy to discharge), so that the furnace lining is safer after power is supplied. Otherwise, the water vapor inside and outside the furnace that harms the furnace lining after the furnace is opened basically comes from the refractory bricks at the furnace bottom, which is the root cause of the furnace leakage.

Requirements for material moisture

Before clay bricks or high-alumina bricks enter the factory, they must be coordinated with the supplier and used as furnace bottom masonry (separate from furnace wall masonry). The water content must be within the required range (<0.2%). Control the moisture from the source, and use is not allowed if it exceeds the standard, otherwise there will be huge hidden dangers.

Exhaust holes in the furnace shell

Before laying the furnace, two rows of square exhaust holes (100mm×100mm, with a net inside) are drilled in the furnace shell on the refractory brick layer (middle and upper parts) at the bottom of the furnace in the longitudinal direction and 16 square exhaust holes (100mm×100mm, with a net inside) are drilled in the transverse direction to prepare for the later heating and exhaust (applicable to all masonry methods, especially cooling masonry). After power is supplied, the gas is exhausted and the square hole is re-welded with the original furnace shell plate (new exhaust channel is added).

Exhaust holes in the furnace wall clay bricks

For traditional thermal insulation furnace linings, 14 to 16 exhaust holes should be evenly left on the furnace wall clay bricks (230mm) from the bottom of the furnace above the clay brick layer, and all the way to the top of the furnace wall as the main exhaust channel (secondary exhaust channel).

Leave a good elastic layer on the furnace wall

The thickness of the elastic layer should be reasonable in all parts where the elastic layer is left, and its filler should be clay particles with a certain particle size (8~20mm), which plays the role of elastic layer and exhaust. It should not be too thin (80~100mm should be left) or use small particle size fillers (affecting the air permeability of the elastic layer).

Furnace wall exhaust holes

Use cold ramming paste to ram or cool the furnace lining, and leave 14 to 16 exhaust holes evenly on the self-cooling ramming paste or high thermal conductivity carbon bricks.

Masonry of key parts of furnace wall

The furnace wall generally has a height of 1200mm for carbon bricks. The iron outlet and the upper part of the electrode face are prone to rise in the furnace bottom (especially high silicon), which seriously erodes clay bricks or high alumina bricks. For this reason, these parts can be rammed with 600 to 800mm electrode paste on the basis of the original carbon bricks (the carbon material of the furnace wall is masonry in the shape of the Great Wall), or the sandwich method is used to ram the bricks on both sides with electrode paste in the middle to prevent burning through; clay bricks are better than high alumina bricks in slag erosion resistance, but the refractoriness is not as good as high alumina bricks. In principle, clay bricks are used. When filling the gap with electrode paste, the electrode paste is required to be basically volatilized to 70% to 80%, and it is rammed manually. If the paste is made by a paste machine, it needs to be heated and volatilized in layers with a baking gun.

Selection of seam paste for seam masonry

When seam masonry is used, it is not advisable to use cold ramming paste (or fine seam paste) to fill the gaps between carbon bricks (low softening point, high volatile matter, not melted and volatilized in advance), which may cause slag iron to penetrate and cause the furnace bottom to flip; low volatile electrode paste (melted and volatilized to 70% before filling) should be used to fill and tamp (the lower part of the carbon brick seam can be filled with graphite powder and tamped to about 1/2 height).

Furnace bottom interlayer masonry

Tie 50~70mm of electrode paste (cold ramming paste or electrode paste melted and volatilized not less than 50%, poor volatilization is easy to bulge, large porosity and fluffy) between the three layers of carbon bricks 1~2 and 2~3, tamp and flatten, and then lay the second layer of carbon bricks on top. Use seam masonry between each horizontal layer of carbon bricks to form a whole. Existing problems: mainly include the control of the melting degree of the knotted electrode paste and the drying speed, how to ensure that the electrode paste is well sintered, with less volatile voids and no bulging, and no accidents.

Furnace shell sealing

Keep the furnace shell and the iron outlet well sealed to prevent oxidation of the carbonaceous materials in the furnace wall and the iron outlet.

2. Condensation furnace lining (high thermal conductivity bricks)

The carbonaceous materials of the cooling furnace lining, especially those close to the furnace shell, must have excellent thermal conductivity to ensure that the hot surface temperature of the refractory material is lower than its critical temperature. The refractory material working close to the hot surface should also have excellent corrosion resistance and good slag hanging properties, which can not only reduce the erosion temperature of the hot surface refractory material, but also facilitate the formation of slag skin (i.e. false furnace lining) to protect the refractory material.

Furnace bottom structure

The bottom layer is three layers of high-alumina bricks, which mainly play a supporting and heat-insulating role to protect the furnace bottom steel plate. On top are high-thermal-conductivity graphite blocks, and on top are ultra-microporous carbon blocks. The top layer of ultra-microporous carbon blocks is built in an anti-floating oblique shape, and the top layer is a ceramic refractory material protective layer.

Furnace wall structure

The furnace wall structure adopts thin furnace lining plus furnace spraying technology. Close to the furnace shell is a high-thermal-conductivity graphite tile, and the inner side of the graphite tile is an ultra-microporous carbon brick (this material is both heat-conductive and easy to hang on the slag skin). The thermal conductivity is used to reduce the temperature of the slag iron, so that the slag iron becomes thicker and forms a slag shell. The innermost layer is a protective layer, which plays a protective role during furnace drying and feeding.

Tap hole structure

Tap hole bricks are an important part of the electric furnace lining. Only when the life of the tap hole bricks is synchronized with the life of the lining can a truly long-lived lining be achieved.

3. Ramming of cold ramming paste at room temperature

Cold ramming paste at room temperature has good plasticity. It does not need to be preheated during the furnace construction process. No gas is generated, which can improve the furnace construction environment, reduce the labor intensity of furnace construction, and reduce the cost of furnace construction. It can be constructed intermittently. The ramming quality is significantly better than that of heated cold ramming paste. It also avoids the quality difference between parts caused by raw frying or over-frying of heated cold ramming paste during the baking process. The ramming quality of the entire furnace lining is uniform. It has the following advantages:

(1) Lowering the softening point of heated cold ramming paste reduces the construction temperature from 40℃ to room temperature (construction can also be carried out at -10℃), and furnace construction does not require preheating.

(2) Increase the compressive strength and true density of cold ramming paste after sintering.

(3) The furnace bottom has no cracks, strong anti-penetration performance, no stratification phenomenon, small thermal expansion coefficient, and various technical parameters are better than carbon bricks.

(4) The furnace construction operation process is simple, the construction is easy, the material cost is low, the furnace construction period is short, and the quality is easy to guarantee.

(5) Furnace lining masonry. Two layers of asbestos boards are laid on the steel plate at the bottom of the furnace. 80~100mm clay particles are laid on the asbestos boards for leveling. High-alumina bricks are laid on the top of the clay particles (leaving an 80~100mm elastic layer with the furnace wall). A layer (400mm) of prepared carbon bricks is laid on the top of the high-alumina bricks. 1000mm of normal temperature cold ramming paste is rammed on the prepared carbon bricks. From the first layer of prepared carbon bricks, all the contact parts of the furnace shell are rammed with normal temperature cold ramming paste (no elastic layer is left). The thickness of the furnace wall is 500~600mm, and the height depends on the size of the electric furnace. Several layers of high-alumina bricks are laid on the annular furnace wall, and clay bricks are laid on the top of the high-alumina bricks (leaving an elastic layer).

(6) Integrally rammed iron outlet. If silicon carbide bricks are used to build the iron outlet and the cold ramming paste, it is difficult to integrate them, which may cause cracks at the junction and cause furnace leakage. For this reason, oxidation-resistant cold ramming paste (mixed with some silicon carbide, also used for the furnace wall) should be used to truly make the furnace lining an integrally rammed furnace lining. The ramming materials at both sides of the iron outlet are roasted separately (heating inside the furnace eye).

04. Furnace lining maintenance

1. Furnace bottom ventilation

The furnace bottom ventilation matches the electric furnace to ensure that the heat conducted from the upper part of the electric furnace is discharged from the furnace bottom in time, so that the furnace lining is relatively in a thermal dynamic balance state at different times, and the service life of the furnace bottom is improved; the furnace bottom ventilation should consider both ventilation cooling to protect the furnace lining and excessive heat loss affecting the index, control different air volumes at different times, and control the temperature within a reasonable setting range. The fan uses a variable frequency fan.

Furnace wall water (air) cooling

The high temperature area of the furnace wall adopts water cooling (spraying or closed water cooling or air cooling) to promote the formation of a false furnace wall on the inside of the furnace wall to prevent oxidation of the furnace wall and slag iron erosion, and create a permanent furnace lining atmosphere (insulation furnace lining can be omitted) Furnace bottom: generally two layers are arranged, four on each layer, and arranged at the center and the bottom of the three electrodes respectively. The upper layer is arranged at the junction of carbon bricks and clay bricks (or high-alumina bricks), and the lower layer is arranged on the first layer of clay bricks. The upper layer monitors the temperature of the lower part of the furnace lining carbon bricks, and the lower layer monitors the middle and late temperature of the furnace lining clay bricks (or high-alumina bricks).

Furnace wall: Two layers of thermocouples are placed within the height of the vertical carbon bricks in the condensing furnace lining, with two rows in each layer, and 6 to 8 in each row (arranged at 200mm and 400mm in each layer of the furnace wall).

About 3 months after the furnace is opened normally, the temperature of each layer of thermocouples reaches stability (controlled not to exceed 900℃).

The furnace bottom and furnace wall are built with ordinary clay bricks. Clay bricks are better than high-alumina bricks in resistance to rapid cooling and heating, slag erosion and hydrolysis, and are cheaper. Large electric furnaces can consider building 2 to 3 layers of high-alumina bricks under the first layer of carbon bricks.

2. Reasonable control of process operation

(1) Control the reasonable electrode length to prevent the electrode from being too long and the arc eroding the furnace bottom and the electrode from being too short (the electrode is inserted shallowly) and the furnace bottom rising and eroding the refractory wall.

(2) Ensure that the furnace shell as a whole, especially around the tap hole, is leak-proof to prevent oxidation of carbonaceous materials.

(3) Ensure stable operation of the electric furnace, prevent long hot stop time due to accidents, and stabilize the furnace lining temperature (avoid damage to the furnace lining caused by rapid cooling and heating).

(4) Strengthen daily tap hole maintenance, use cement-free balls for deep plugging (full plugging), reduce the use of oxygen to open the eye to avoid oxygen oxidation damage to carbonaceous materials at the tap hole, regularly repair the eye and chute; timely reverse the eye (every furnace or every shift).

(5) Keep good records of furnace bottom and furnace wall temperature measurement, and handle abnormalities in a timely manner.

05. Furnace opening

1. Furnace drying

When ramming the furnace lining and opening the furnace, the heating speed of the furnace bottom and furnace wall should be controlled. During the period of 300~450℃, the volatile matter escapes the most (about two days). If the temperature rises too fast, the volatile matter evaporates too fast, which will cause bulging, holes or cracks, and loose sintering paste; ordinary insulation furnace lining can reduce the furnace drying time, and the normal sintering of the seam paste is the standard. The furnace bottom and furnace wall can evaporate according to the normal channel.

2. Furnace drying curve

The furnace using cold ramming paste needs to be laid flat on the furnace bottom according to the manufacturer's regulations. The furnace bottom is paved with 300~400mm of coke, and the furnace is powered and dried according to the given furnace drying curve and furnace drying time to ensure that the furnace lining is heated evenly. Generally, the final furnace drying temperature reaches about 400℃ (cold ramming paste surface temperature).

3. Electrode baking

Using seamed or seamless carbon bricks for masonry, no wood, coal, or coke oven is required. Electrode baking can be carried out simultaneously with the oven. After the electrode is basically baked, it is switched to an electric oven to continue baking until the baking is completed.

4. Electric oven

During the electric oven, as the current increases, the oven can be baked with materials, the furnace temperature can be controlled, and the furnace bottom temperature can be increased. The maximum current of the electric oven does not exceed 1/3 of the normal current, and the time is generally controlled at about 24 hours. During the electric oven, the newly opened electric furnace is paused every 2 to 4 hours (10 to 15 minutes to release water vapor). The baking time of the newly built electric furnace is appropriately extended to ensure that the moisture of the furnace bottom and lining is fully volatilized.

5. Charge

After the electric furnace is dried, start charging normally and control the charging speed to suppress the arc and current and control the furnace temperature. Generally, when the first furnace of alloy is discharged, the material surface is controlled at about 1/2 of the furnace depth. The power consumption of the first furnace is 2.5~3 times the normal discharge amount (based on the front of the furnace). Bring slag iron to plug the eyes in advance to ensure the opening of the second furnace.

6. Sealing the material surface

The third or fourth furnace seals the material surface and switches to the normal (except for ferrosilicon, which generally takes one to two weeks) material surface.

7. Load

After sealing the material surface, the current is basically full but not overloaded. As the number of discharges increases, the furnace temperature gradually increases, the electric furnace tends to be normal, and the voltage gradually increases. Generally, the voltage is slightly lower than the normal voltage and compensation operation is put into operation two weeks after discharge, and normal operation is switched after one month.

8. Use of star-angle conversion

Electric furnaces use star connection for baking electrodes. Low voltage, high current and low power are conducive to electrode baking and controlling the temperature in the furnace. After adding materials, they should be switched to angle connection. Gradually increasing the voltage is conducive to increasing the power entering the furnace and raising the temperature at the bottom of the furnace.

9. Load increase speed after opening the furnace

At present, after most electric furnaces are opened and iron is produced, long-term low voltage and low load operation can easily lead to the rise of the furnace bottom. The electrode is inserted too deep, resulting in local overheating of the lower part of the three-phase electrode and erosion of the carbon bricks at the bottom of the furnace. The speed of load depends on the masonry of the furnace lining. If the masonry is proper, there will be less gas and it can be discharged in time, which will not cause harm to the furnace bottom. Accelerating the heat storage of the furnace lining is conducive to protecting the furnace lining. If the masonry is improper, there will be more gas and it cannot be discharged in time. Even if the load is slowly increased, it is easy to have problems. Whether the furnace leaks or not depends on the masonry.

10. Normal temperature cold ramming paste integral furnace lining

Unlike carbon brick lining, normal temperature cold ramming paste integral furnace lining is still in the process of self-baking of heat in the furnace during the initial stage of furnace baking and commissioning and even within half a year of use. Cold ramming layer coking and carbonization are a systematic project. Strictly control the length of the working end of the electrode, especially in the initial stage of furnace opening, the three-phase electrode should be inserted into the furnace evenly, and overload production is not allowed. Try to operate without carbon shortage during the production process, reduce the amount of raw materials with high moisture and high powder entering the furnace, ensure the stability of the furnace condition, and reduce the erosion of the furnace bottom, furnace wall and iron outlet by the fire collapse.

11. Cold ramming paste drying furnace

Dry the furnace according to the designed drying curve to ensure that the power supply requirements are met.