​Nickel-Iron Submerged Arc Furnaces: Design, Construction, and Material Selection Methods for the Lining

Nickel-Iron Submerged Arc Furnaces: Design, Construction, and Material Selection Methods for the Lining

Design of the Lining of Nickel-Iron Submerged Arc Furnaces, Selection of Refractory Materials, and Construction Techniques for the Lining

With the introduction of RKEF technology, domestic ferronickel technology has made rapid progress, with the lifespan of Nickel-Iron Submerged Arc Furnaces linings increasing from less than six months initially to seven years now. Based on the development experience of the domestic ferronickel industry over the past decade, the author has improved the lifespan of ferronickel furnace linings to ten years or even longer through technical improvements in design, masonry, refractory material selection, smelting slag type control, and operation and maintenance.

1.  Furnace Lining Design

Currently, the mainstream process for producing nickel iron from laterite ore is the RKEF process, where the submerged arc furnace is typically a large electric furnace with a capacity of over 33MVA. The core equipment of the RKEF process includes the submerged arc furnace and rotary kiln. The design of the submerged arc furnace first requires determining reasonable parameters such as the diameter and height of the furnace shell, the diameter and depth of the furnace chamber, and the diameter of the electrodes (for rectangular electric furnaces, the length, width, and height are determined). These parameters are closely related to the determination of furnace lining parameters. Based on empirical data from over 10 years of nickel iron submerged arc furnace operation in China, generally speaking, a furnace bottom thickness of 2.0~2.2m and a furnace wall thickness of 1.0~1.3m can fully meet production needs and ensure stable operation of the furnace lining. Some domestic manufacturers choose deeper furnace bottoms and thicker furnace walls than these data, but there is no significant advantage in actual operation and it cannot effectively improve the service life of the furnace lining. Specific design considerations from the following aspects can determine an appropriate furnace lining scheme.

1. Design of Taphole and Slag Tap

The schematic diagram of copper water jacket slag and iron tap is shown in Figure 1. Both the iron tap and slag tap utilize copper water jackets, which are beneficial for stable operation and maintenance during production. The depth of the iron tap is approximately 1.5m, and the depth of the slag tap is around 1.3m. The iron tap diameter is 60mm, and the slag tap diameter is 100mm, which effectively meets production requirements. There are 3 to 4 iron taps and 2 slag taps. The height difference between the slag tap and iron tap is 600 to 800mm, and it is recommended to be no less than 600mm. If the height difference is less than 600mm, there is a high risk of molten iron overflowing from the slag tap during production, which can lead to a decrease in nickel recovery rate and, in severe cases, may cause an explosion of the slag, posing a significant safety hazard. The common furnace lining parameters and furnace lining life overview are shown in Table 1.

2. Selection of Refractory Materials

Nickel-iron has a high specific gravity, high temperature, and strong permeability. In the early days, using carbon bricks or magnesia bricks to build the furnace bottom would result in burnthrough of the furnace bottom in a short period of time. Choosing magnesia ramming material to construct a seamless furnace bottom through integral bonding effectively prevents the occurrence of iron melt penetrating through the furnace bottom, greatly improving the lifespan of the furnace lining. The slag line part of the furnace wall uses 97 and 95 magnesia bricks, which can withstand high-temperature slag erosion during smelting. Through slag hanging operations, the overall structure of the furnace lining can be stabilized. For the parts above the slag line, 75 high-alumina bricks can be selected as the insulation layer.

3. Setting of Elastic Layers and Expansion Joints

Generally, large-scale nickel-iron electric furnaces nowadays have a diameter of over 17 meters, with refractory lining weighing more than 2,000 tons. During production, the weight of high-temperature molten iron, slag, and furnace materials is approximately 1,200 to 1,600 tons. The refractory lining will bear significant pressure and the expansion stress of the refractory materials will gradually increase with rising temperature. Therefore, a reasonably designed elastic layer is necessary to balance the pressure and expansion stress, preventing excessive stress from causing the furnace shell to crack, leading to instability in the refractory lining structure and reducing its lifespan. Through theoretical calculations and empirical data, it is reasonable for the elastic layer of a large-scale electric furnace to be approximately 300mm thick. During the start-up process of the brick-lined furnace, a certain amount of expansion will occur as the temperature rises from room temperature to around 1,600°C. According to the expansion coefficient provided by the manufacturer, 1mm of expansion paper should be placed between the bricks to eliminate the expansion stress and achieve overall stability of the refractory lining.

II. Furnace Lining Construction

Good design is the foundation for ensuring the lifespan of the furnace lining, while the quality of masonry construction is the key to ensuring the lifespan of the furnace lining. Construction is a complex and comprehensive project. Quality assurance in the production process of magnesium refractory materials is a prerequisite for ensuring the quality of the furnace lining. During transportation and construction, waterproofing must be ensured. Before construction, all refractory materials need to be sampled for physical and chemical index analysis and strength data analysis.

1. Preparation Before Construction

For the acceptance of the furnace body's steel structure, according to the design drawings, measure the diameter and height of the furnace shell, calibrate the furnace core and centerline, determine the center and position of slag and iron eyes, and calibrate height and other data. The thickness of the steel plate for the furnace shell is approximately 30mm, with a large amount of welding. Each weld seam should be UT tested to meet the design requirements. After the temperature measurement points on the furnace bottom and walls are drilled and reserved, refractory masonry construction can be carried out.

2. Construction of Ramming Material for Furnace Bottom

Put the ramming material into the furnace, and the magnesium ramming material at the bottom of the furnace should be rammed in layers, with each layer being compacted to a thickness of about 160~200mm. According to the design requirements, put the ramming material into the furnace. After feeding, use a shovel or a rake to level it, then use an aluminum alloy ruler and a level meter to find the level. After that, lay a color strip cloth (canvas) on the leveled ramming material, which will cover all the ramming material at the bottom of the furnace. Place the plate vibrator on the color strip cloth (canvas), and each vibrator should be kept at a certain distance, balanced, and symmetrical.

(1) Vibration. 

Start the vibrator. Once the vibrator is activated, start vibrating from the outer ring, moving inward (towards the center of the furnace) in a circular pattern. Each subsequent ring should overlap the previous one by one-third to prevent any missed areas. Continue vibrating from the inside of the furnace shell until the center is considered one complete pass. After completing one pass, repeat the process from the outside inward several times. Each layer of material should be vibrated multiple times simultaneously using two or more vibrators.

(2) "Roughening" with a rake. 

After each layer of ramming is completed, remove the dustproof color strip cloth (canvas), and use a homemade rake to roughen the surface of the ramming material layer in the same direction, creating grooves to increase the contact area between the adjacent layers above and below, thus forming a seamless and integrated rammed furnace bottom.

(3) Knotting quality. 

The density of the rammed material after compaction should be above 2.8g/cm3. At this point, the vibrator bounces up and down with a larger amplitude, and the vibration sound is much louder than the previous passes. These phenomena indicate that the material layer is basically compacted. The method for inspecting knot quality is as follows: insert a 4mm diameter iron rod, such as a 4mm welding rod, with a depth of no more than 20mm. The vertical force applied to the iron rod is 5kg.

(4) Precautions. 

When adding ramming material to the furnace, the material in ton bags should be evenly added from several directions to facilitate manual leveling and uniform particle feeding. Plastic, silk bags, and wire ropes should be picked out at any time, and it is strictly prohibited to mix slag, iron pieces, and other impurities into the material. During vibration operation, the vibrator should move slowly and uniformly, with balanced and stable vibration, to ensure the bonding strength.

3. Furnace Lining Brick Masonry

(1) Identify the center of the furnace bottom, mark out the inner lining circle boundary according to the inner diameter size of the furnace lining, and lay the inner lining magnesia bricks according to the inner lining boundary. After laying the inner lining ring bricks to a certain height, fill the elastic layer between the outer circle of magnesia bricks and the furnace shell with ramming material (no need to compact it).

(2) Bricks for masonry walls should be straight and wedge-shaped, with circular joints filled with refractory powder and chiseled smooth. When laying bricks, a wooden or rubber hammer should be used for alignment, and an iron hammer should not be used. The door bricks of the furnace wall should be evenly distributed on both sides at an angle of 90° to the slag port.

(3) If the last door-fitting brick of each layer needs to be cut and processed, the size of this brick must be at least half of the original brick width, and the brick above this processed brick must be a whole brick.

(4) Depending on the diameter, each ring of bricks requires wedge-shaped bricks of different sizes. During masonry, the centerline of each brick should be aligned towards the center of the circle.

(5) The parts above the slag line of the furnace lining shall be filled with high-alumina bricks and high-alumina fine powder. The maximum thickness of the joint shall be 2mm, and the elastic layer between the brick and the furnace shell can be filled with ramming material.

4. Iron-eye and Slag-eye Masonry

(1) Before masonry, check whether the bricks at the iron outlet and slag outlet are intact.

(2) Pre-lay iron and slag port bricks, and polish and adjust unqualified parts.

(3) The refractory mortar applied on both sides of the iron taphole and slag tap hole should not use expansion joint paper. The front and rear bricks should have staggered joints, without any through joints, and the brick joints should be ≤1mm.

(4) The bricks for the iron outlet and slag outlet should be wet-laid using refractory mortar mixed with magnesium powder and water glass. The laying should proceed from the inside out, and the later-laid bricks should be tightly pressed against the earlier-laid ones. The brick joints should be fully filled with mortar, leaving no gaps, to prevent the infiltration of molten iron.