OPERATING BLAST FURNACES. 53
and is tapped off as slag through the slag hole T, Fig. 10, page 49, while the iron is delivered at the tap hole X. The amount of fuel and limestone necessary, depends upon the nature of the ore charged and the grade of iron desired. All material charged into a furnace passes off either as a liquid or as a gas. The gas which comes off at the top is made to pass through the down comer into the ovens and burned there. There the blast is heated while passing to the furnace. The liquid products which pass off are iron and slag, both formed at a point ranging from a level with the tuyeres to a height of about four feet above them, a portion generally called the " melting zone," or bosh, the hottest part of a furnace.
If ore is not properly reduced a percentage of its iron may pass off with the slag, the reason for this being that it is not thoroughly extracted from the ore and non-metallic matter. This is generally due to an insufficient amount of fuel, or decrease in temperature from other causes. Moreover, too small an amount of silicon is reduced at the same time from the fuel and ore, and consequently the iron obtained is smaller in amount and silicon contents and richer in sulphur. "The furnace is working cold, or "off," and a greater per cent of fuel may make it work better.
Sulphur in iron is generally largely obtained from the fuel in a furnace. Iron from the ore, as well as the lirne in the flux absorbs sulphur. Which of these two elements, in the process of reducing the ore, will absorb the greater percentage of sulphur from the fuel depends upon the degree of heat obtained. Lime has a great affinity for sulphur, and if the slag is made thin and hot it can counteract the absorbing power ofee reactiony they are often composed of about equal parts of silica and alumina. Bricks should contain silica or alumina in proportion to the amount of heat or friction they are if j