Principles for Blast Furnace Refractory Lining Inspection and Monitoring
Blast furnaces are complex pieces of equipment that involve the transformation, interaction and flow of liquids, solids and gasses at high temperatures throughout the vessel. Blast furnaces are lined by carbon and graphite refractory bricks to protect the integrity of the vessel. Carbon and graphite refractories are primarily used for blast furnace linings because of their high temperature conductivity and their ability to reduce the heat load by moving the excessive heat to the shell where the heat can be transferred to the water by film cooling or through the cast iron or copper staves.
There are four main factors that affect the campaign life of a blast furnace: 1) blast furnace design, 2) refractory quality and reliability, 3) refractory lining quality and tightness, 4) amount of care and attention paid to maintaining the furnace lining. Over the past two decades, significant improvements have been made to the design of blast furnaces. These improvements include increased furnace volume and operating pressure; new furnace structures and cooling designs; improved burden distribution methods; advances in monitoring instrumentation and control; and the introduction of techniques to lower energy requirements and to save labor. Additionally, there have been improvements in the science and understanding of lining blast furnaces. Today the refractory lining is frequently maintained and improved by adding various elements such as titania. All of these developments have helped result in lower iron costs, increased blast furnace campaign life and improved working conditions for operational and maintenance personnel. The typical campaign life of a modern blast furnace is 15 to 20 years, depending on the cooling and refractory design as a whole and the hearth area in particular.
Hearth repairs and relinings are undoubtedly the most costly items for blast furnaces. Therefore, it is desirable to extend the existing hearth life as much as possible to reduce these expenses. Under normal operation and process conditions, hearth lining deterioration is generally slow and gradual throughout the life of the furnace. However, many furnaces experience irregular refractory wear due to a number of reasons. Some of the most common areas of excessive wear are on the furnace wall below the taphole, at the opposite side of the taphole and at the base of the hearth (more commonly known as the “elephant foot”). Recent studies show that furnace design, taphole position and refractory quality have profound effects on the furnace lining wear rate. In addition to the expected wearing of the lining, incidents such as gas leakage, water attacks, premature furnace shut downs, and thermal fluctuations can cause severe damage to local areas of the refractory lining. If local damage spreads to greater areas in the furnace, it may necessitate a refractory repair or relining. The degree of damage affects the repair costs, the reduction in productivity and the resulting safety concerns. Therefore, the reliable assessment of refractory lining quality and thickness is crucial for maintaining a healthy and productive blast furnace campaign life.