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 v. imported magnesite--In recent years, some steelmakers have experimented with using magnesia (magnesium oxide) (often referred to as magnesite), usually imported from China, 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, recent 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 ISSTech in April 2003.)
A number of steel manufacturers have returned to 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. 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 and 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 Ablow,@ 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 has shown 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.
Secondary Refining. 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.
Steel Products. 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 employed in the neutralization of 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. In addition, 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.
Lime is a key component in a number of nonferrous metal applications.
Ore Concentration. Both quicklime and hydrated lime are widely used in the flotation or recovery of many non-ferrous
ores--in particular copper ore flotation in which lime acts as a depressant (settling aid) and maintains proper alkalinity in the flotation circuit. In the recovery of mercury from cinnabar, lime is used to remove sulfur. Lime is also used in the flotation of zinc, nickel, and lead bearing ores. It is often used as a conserving agent to assist in the recovery of xanthates, another flotation chemical.
Lime is also extensively used in the recovery of gold and silver in the cyanide leaching process to curtail the loss of cyanide, a costly dissolution reagent, and for pH control. In both Amill@ and Aheap@ leaching, gold and silver ores are crushed and mixed with lime, and then combined for leaching with a cyanide solution. Lime maintains proper pH in the cyanide solution, thereby keeping it in the liquid phase, preventing the formation of hydrogen cyanide gas and its loss into the atmosphere.
Alumina & Bauxite. Quicklime is used in varying amounts to remove silica from bauxite ore and for causticization in the manufacture of' alumina. The extent of its use depends largely on the quality of the bauxite used. Purer ores require less lime and more limestone (Sinter process) while ores high in impurities require more lime (Bayer process). In both cases, lime is required for desilification.
Magnesium. Lime is used to produce metallic magnesium. In thermal reduction techniques, magnesium oxide is reduced with ferrosilicon at high temperatures, with dolomitic quicklime providing the magnesium oxide. This process produces a gaseous magnesium which is ultimately condensed. Lime can also be used in electrolytic processes of magnesium production.
Other Metallurgy. In the smelting and refining of copper, zinc, lead and other non-ferrous ores, noxious gas fumes of SO2 can be neutralized by passing these gases through "milk-of-lime" (dilute hydrated lime in an aqueous suspension) in a scrubber to avert the formation of sulfuric acid in the atmosphere, as well as the corrosion of plant equipment.
After the smelting of nickel, the nickel is precipitated in a boiling solution with "milk-of-lime." In the electrolytic refining of copper the cathode sheets are dipped in lime water to protect them from sulfur in the "melting down" process. Some plants have used lime to reduce zinc chloride from galvanizing skimmings, reclaiming zinc hydroxide in the process. Substantial quantities of quicklime are used as a flux in the manufacture of low carbon and ferro chromes.
Lime is employed in uranium beneficiation mills operating acid leach systems. Lime neutralizes the acidic waste effluent before discharge.
In the concentration of rock phosphate, there is frequently a build-up of waste fluorine. Lime is used to precipitate these fluorides.
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