Metallurgical

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, silica, and, to a lesser extent, manganese. Lime also has important uses in the secondary refining of steel and 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 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.) Many 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 the 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.

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 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.

Lime is a key component in several 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. To process gold and silver, lime and cyanide are added to gold ore in a cyanide mill’s grinding circuit. The crushed lime, cyanide, and gold are then combined with cyanide and lime solutions. When lime and cyanide solutions are introduced, the gold particles are polished by the grinding action, and the solutions are heated by friction and liberated from the gold ore. Lime has several functions in this process, most notably it 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 amount of lime used depends largely on the quality of the bauxite. 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 from dolomitic quicklime is reduced with ferrosilicon at high temperatures. 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 them through a scrubber with “milk-of-lime” (diluted hydrated lime in an aqueous suspension) to avert the formation of sulfuric acid in the atmosphere as well as the corrosion of plant equipment. After smelting nickel, the nickel is precipitated in a boiling solution with “milk-of-lime.” In the electrolytic refining of copper, cathode sheets are dipped in lime water to protect them from sulfur in the “melting down” process. Some plants use 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 ferrochromes.

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, lime is used to precipitate the build-up of waste fluorine.