Quicklime -- both high calcium and dolomitic -- enjoys its most extensive use as a flux in purifying steel in the electric arc furnace (EAF) and basic oxygen furnace (BOF). Lime is particularly effective in removing phosphorus, sulfur, and silica, and, to a lesser extent, manganese. Lime also has important uses in secondary refining of steel and in the manufacture of steel products.
Electric Arc Furnaces
In electric arc furnaces, scrap iron and steel, scrap substitutes such as DRI and HBI, pig iron, iron ore, and beneficiated iron ore are placed in a furnace and melted by the use of heat from an electric current. A lime flux consisting of quicklime or a blend of quicklime and dolomitic lime is added. The total flux amount varies from 50 to 120 pounds per ton of steel, and up to 50% may be dolomitic lime.
Benefits of Lime Flux
The lime flux removes impurities and forms a slag that can be separated from the steel and poured from the furnace as a liquid. It also reduces refractory wear and gunning, and can provide a foaming slag for long arc operation. Pebble quicklime is used, unless a finer product is required by specialty furnace injection applications.
Lime Flux vs. Imported Magnesite
Some steelmakers have experimented with using magnesia, usually imported from China, (magnesium oxide) (often referred to as magnesite) in the fluxing process, in place of dolomitic lime (which contains both magnesium oxide and calcium oxide). Although this imported material is substantially more expensive than dolomitic lime, some steel companies considered using it because of claims that it dissolved more quickly, and thus improved performance. However, studies have shown that this imported magnesite does not in fact go into solution more quickly than MgO from dolomitic quicklime, and that it has no performance advantage. Thus, the choice of dolomitic quicklime for this application remains the cost-effective option. (A paper detailing the studies referred to above was presented at ISS Tech in April 2003.) A number of steel manufacturers are using lime for fluxing after trying imported magnesite, finding that the magnesite provided inferior or at best equivalent performance at a much higher cost.
Basic Oxygen Furnaces (BOF)
In basic oxygen steelmaking, molten iron from a blast furnace is charged into a refractory-lined steelmaking furnace, and then oxygen is injected into molten iron at high speeds, resulting in oxidation of carbon and impurities. Lime is used in several steps in this process. Many steel plants desulfurize the hot metal externally in torpedo cars or ladles following the blast furnace, utilizing a flow-aided pulverized lime blend before charging into the BOF. Lime may be used for sulfur and phosphorus removal at this stage as well. Most importantly, quicklime is typically added to the mixture in the steelmaking furnace after the beginning of the oxygen “blow” where it reacts with impurities (primarily silica and phosphorus) to form a slag which is later removed. The lime factor per ton of steel ingot averages 150 lb./ton. Although steel plants flux with high calcium quicklime, most of the basic oxygen plants substitute or add 30 to 50% dolomitic (high magnesium) quicklime because experience shows that this extends the refractory lining life of the furnaces. While most basic oxygen steel plants use pebble quicklime, the injection systems used in certain processes (such as QBOP) require pulverized quicklime.
Whether produced in a basic oxygen or electric arc furnace, steel often requires secondary refining to transform it into a saleable product, especially where ultra pure steel is required. Many secondary refining processes use lime to perform key functions, such as the adjustment of steel temperature or chemistry, the removal of additional impurities, and the prevention of reabsorption of impurities from slags. In addition, quicklime may be used with other materials, such as fluorspar or alumina, to form a synthetic slag, which is used as a flux to remove additional sulfur and phosphorus after the initial steel refining process.
Hydrated lime (either dry or as a slurry) has a number of miscellaneous applications in the manufacture of steel products. It is commonly used in wire drawing, acting as a lubricant as the steel rods or wires are drawn through dies, and in pig and slag casting in which a lime whitewash coating on the molds prevents sticking. Lime is used to neutralize the acid-based waste pickle liquor in which iron salts are also precipitated. After pickling, steel products are often given a lime bath to neutralize the last traces of the pickling acid adhering to the metal. Hydrated lime is used to provide temporary corrosion protection in the form of a whitewash coating on steel products and to neutralize acid in coke by-product plants.