burden

The burden is the solid charge loaded into a blast furnace, consisting of iron ore, coke, and limestone or other flux materials. It sits on top of the hot gases rising from below, gradually descending as combustion consumes the coke and reduces the ore. The burden determines how much iron can be produced per shift and how efficiently the furnace operates.

A typical burden composition by weight runs roughly 50 to 65 percent iron ore, 25 to 35 percent coke, and 10 to 15 percent limestone flux. These proportions are not fixed; they vary with ore grade, coke quality, the iron oxide minerals present, and the desired hot-metal chemistry. A furnace operator adjusts the burden mix to compensate for changes in raw material quality and to hit target silicon, manganese, and phosphorus levels in the cast iron or hot metal.

The term burden also refers to the mass of material charged per unit of hearth area, measured in tonnes per square metre per day. A high burden rate means pushing material through the furnace quickly; a low burden rate means longer residence time and more thorough reduction but lower throughput. Pushing too hard risks burden collapse, where the charge bridges and hangs up in the shaft, starving the furnace of gas flow and forcing a shutdown.

Why the name matters

The word "burden" comes from its literal sense: the weight that presses down and must be supported. In older texts, it is sometimes called the "charge," especially when discussing what goes in; "burden" specifically names the ongoing column of descending solids. The concept is ancient, traceable to charcoal-fired bloomeries, but the term crystallized once blast furnaces grew large enough to run continuously and require precise proportioning.

Burden quality depends on sizing. Ore and coke must be screened to consistent lumps, typically 10 to 50 mm, so that gas can flow evenly through the descending column. Fines, or material under 10 mm, create resistance, cause hanging, and waste fuel. This is why burden preparation, including crushing and screening, is a major cost and quality driver before material ever reaches the furnace mouth.