The maintenance of the blast furnace, the repair of the hearth by bricklaying or monolithic casting, and the repair of the lining by replacement bricking or shotcreting, will all involve heating-up before the BF start-up. Heating-up is a very important process. It is not advisable to blindly shorten the heating-up time in pursuit of short maintenance cycle. In order to improve the competitive advantage of the maintenance period, some refractory manufacturers have reduced the overall casting and heating-up of the hearth to less than 3 days, or even no heating-up. In fact, they only focus on the evaporation of water from the casting body, and ignore the requirements of the furnace body for preheating.
The blast furnace heating-up is to drain the water inside the refractory, drain the free water in the casting body by evaporation, and then convert the combined water into free water by medium temperature decomposition to volatilize and discharge. If the thickness of the casting body is ≤ 200mm, this process may only take about 3 days; For the casting of used carbon bricks, the casting thickness of the furnace bottom is generally more than 800mm, the thickness of the elephant foot area is more than 600mm, and the thickness of the taphole area can even reach more than 2000mm. For the casting body with this thickness, the water can be completely discharged after heating-up for more than 1 week. If the heating-up time is insufficient, the water will continue to volatilize in the early stage of furnace start-up, then the iron splashing and gas channeling of the blast furnace may occur, thus prolonging the production cycle.
It is only one of the functions of heating-up that the refractory moisture is discharged, and the more important preheating and stress distribution of the blast furnace are mostly ignored. For the hearth repaired by masonry, wood wedges will be used to hold the carbon bricks with gaps during construction, and then tamping materials will be used to fill them. Therefore, local stress will be left. When the heating-up time is sufficient, the stress will gradually be evenly distributed to avoid local brittle fracture due to uneven stress during the operation of the blast furnace (except for the temperature factor of brittle fracture of carbon bricks); While for the blast furnace hearth with monolithic casting, the evaporation of water is accompanied by stress concentration and stress release during the heating-up process. If the heating-up speed is slowed down, it is beneficial to dissipate the concentrated stress of the casting body, reduce local defects, eliminate concentrated cracks, and ensure the smooth production of the furnace.
For hearth casting with old carbon bricks, shortening the heating-up time means sharply increasing the heating-up temperature. The expansion and contraction of the casting body and the old carbon bricks are not uniform under the condition of rapid temperature change, and the casting body and the carbon bricks will peel off. Even if the expansion and tightening of the high-temperature operation in the later period, it is impossible to ensure that the casting body and the old carbon bricks are closely connected, so there is a local hidden danger of gap insulation.
The above is based on the experience of a large number of blast furnace maintenance practices. For refractory materials, full heating-up can exert 100% characteristics, while the effect of no heating-up is less than 50%. The fast heating-up performance of materials only improves its heating-up efficiency, but the fundamental principle will not change. Sufficient heating-up time has a great effect. Blindly shortening the heating-up time or no heating-up may lead to the failure the blast furnace maintenance.