Phase 2: Step by step baking process (the most essential preparation step for starting a new furnace)

Natural drying can only remove the shallow free water in refractory masonry. The deep adsorbed water and mineral crystal water inside the refractory material cannot be discharged by room temperature drying. At the same time, the newly built refractory bricks and magnesia sand knotting layer have a loose structure and have not formed a stable high-temperature sintering structure, which cannot withstand the high-temperature conditions of aluminum alloy melting. Therefore, an oven drying process must be carried out. Regardless of whether it is a new, major, medium, or minor repair of a furnace, or a smelting furnace that has been shut down for a long time or a short period of time before being put back into use, the complete drying process must be carried out without exception. Drying is the most critical part of the preparation work for starting a new furnace.

The two core functions of the oven drying process

• Firstly, gradually remove all moisture in layers to prevent cracking of the furnace lining. The oven adopts a low-temperature starting and stepwise slow heating mode, which evenly and synchronously heats up the inner and outer layers of the furnace lining, avoiding thermal stress cracking caused by excessive temperature difference between the inside and outside of the lining; At the same time, the deeply adsorbed water and crystal water in the refractory masonry are gradually discharged at a very slow heating rate. Water vapor slowly escapes from the micropores of the refractory material, without forming high-pressure internal stress, completely avoiding the problems of refractory brick explosion and magnesia sand furnace bottom bulging and falling off. If the heating rate is too fast, the surface of the furnace lining has already hardened and sealed at high temperature, and the internal water vapor cannot be discharged, which will inevitably cause the furnace lining to bulge and crack.
• Secondly, complete the high-temperature sintering molding of refractory masonry and construct a stable heat-resistant furnace lining. In the high-temperature insulation stage of the later stage of the oven drying, refractory bricks, magnesia sand knotting layers, and refractory castables will undergo solid-state sintering reactions at high temperatures. Loose refractory particles will bond with each other to form a dense, high-strength, high temperature resistant, and anti aluminum liquid penetration stable overall furnace lining. After sintering, the furnace lining can withstand aluminum alloy melting temperatures of 700-850 ℃ for a long time, resist aluminum liquid penetration erosion and alkali corrosion, and greatly improve the service life of the melting furnace.

In addition, after fully drying the furnace, the moisture inside the furnace lining is completely drained, and there will be no continuous release of water vapor during the subsequent formal aluminum melting process. This eliminates the problem of excessive hydrogen absorption and oxidation slag formation in the aluminum liquid from the source, ensuring the basic quality of the molten material for subsequent aluminum alloy melting and casting.

2. Core Control Points for Oven Operation

2.1 Strictly implement the step heating oven curve and strictly prohibit rapid heating

The oven must be divided into multiple stages of low-temperature insulation, slow heating, and high-temperature insulation according to the preset heating curve. It is absolutely forbidden to rapidly increase the temperature with a single large fire. The low-temperature range (room temperature~200 ℃) is the key stage of dehydration, and the heating rate must be controlled at 5-10 ℃/h. For every 50-80 ℃ increase, a long-term constant temperature insulation should be carried out to allow the internal moisture to fully escape; The temperature range of 200~600 ℃ is the stage of crystal water removal and preliminary sintering, with a heating rate not exceeding 15 ℃/h. The high temperature range above 600 ℃ is the dense sintering stage of the refractory layer, and the heating rate can be relaxed to 20~30 ℃/h. The final insulation temperature needs to reach 750~800 ℃ and maintain constant temperature for a long time to ensure that the magnesium sand layer at the bottom of the furnace is completely sintered and dense. The entire baking process usually lasts for 10-25 days, and the larger the furnace tonnage and the higher the repair level, the longer the total baking cycle.

2.2 Maintain reasonable furnace pressure throughout the entire baking process

Aluminum alloy melting furnaces are mostly gas/oil fired flame furnaces. During the drying stage, the flue damper needs to be synchronously adjusted to stabilize the furnace pressure within the micro positive pressure range of 2-4mmH ₂ O (1mmH ₂ O ≈ 9.8Pa); If the furnace pressure is too low, cold air will flow back into the furnace, causing local rapid cooling and cracking of the furnace lining; If the furnace pressure is too high, the water vapor in the furnace cannot be smoothly discharged into the flue, and the water vapor remains in the furnace and infiltrates the refractory layer, prolonging the drying cycle. During the heating process, adjust the opening of the flue damper flexibly according to the flame state inside the furnace. The optimal state is when there is a slight amount of smoke at the bottom of the furnace door and the flame is gentle without spraying. When the flame overflows violently, open the flue damper moderately to enhance the smoke exhaust capacity.

2.3 Inspection Record of the Whole Process of Oven Drying

During the baking period, assign a dedicated person to be on duty 24 hours a day, regularly record the furnace temperature, furnace pressure, and flame status, and focus on observing whether there are cracks, bulges, or abnormal smoke on the furnace top, wall, and bottom; Once the refractory layer cracks and emits water vapor, immediately stop heating and keep it at a constant temperature for a long time. Wait until the moisture is fully drained before continuing to heat up to avoid further expansion of defects.

3. Qualification criteria for oven drying

After the continuous insulation of the high-temperature constant temperature stage of the oven is completed, the inner wall of the furnace is dry and there is no moisture precipitation, the furnace lining has no visible cracking or bulging, and there is no more continuous water vapor coming out of the flue outlet. The overall color of the furnace lining shows uniform sintering, which means that the oven drying process is qualified before entering the furnace cleaning process. If there are large cracks or magnesium sand bulges on the furnace lining after baking, it is necessary to repair the area locally and re bake the furnace. It is strictly prohibited to enter the washing and aluminum melting stages with defects.