Bosh & Shaft Gunning Lining Spalling

Background

The bosh, belly and lower shaft of a blast furnace are where ascending gas meets descending burden. The lining here endures continuous scouring by 1100–1400℃ dust-laden gas, plus chemical attack by alkali vapors (K₂O, Na₂O). This zone is normally lined by gunning-applied refractory working together with cooling staves. As intensification and campaign-life demands rise, the gunned lining's service life has become the key constraint on the mid-campaign maintenance interval.

Failure Mechanism

Gas mechanical scouring: high-velocity dust-laden gas continuously erodes the lining surface, thinning the gunned layer step by step. Alkali attack: alkali vapor penetrates pores and reacts with the matrix to form low-melting minerals such as kalsilite and leucite; the accompanying volume expansion causes "alkali cracking." Thermal-shock spalling: blow-in, blow-out and burden-slip operations cause rapid temperature changes; thermal-expansion mismatch between the gunned layer and the cooling stave triggers laminar spalling. Hydration damage: excessive water in gunning or inadequate bake-out leaves residual moisture that vaporizes at high temperature, worsening looseness and block fall-out. These factors combine so that the bosh/shaft gunned lining spalls and sheds blocks within the mid-campaign interval.

Consequences

After the gunned lining spalls, cooling staves are exposed directly to hot gas flow: cooling load rises, water temperature differentials increase, and in severe cases staves burn out and the shell turns red, forcing an unscheduled mid-campaign repair that shortens stable operation of the campaign.