Lime Usage
Lime Usage In The Iron and Steel
Industry
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. We are manufacturers and suppliers of calcium hydroxide,
calcium oxide and other lime chemicals.
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 Usage In The
Nonferrous Metals Industries
Lime is a key component in a number of nonferrous metal applications.
Other Uses of Lime
Chunna Udyog
Lime Udyog
Rajasthan Lime Udyog
Gotan Lime Udyog
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.
