Note: Descriptions are shown in the official language in which they were submitted.
CA 02150793 2007-12-28
CONTROLLING DEPOSITS IN THE CALCINATION
OF FLUXED IRON ORE PELLETS
FIELD OF THE INVENTION
The present invention relates to compositions and methods for
inhibiting deposits during calcination of fluxed iron ore pellets.
BACKGROUND OF THE INVENTION
Crude iron ore cannot be used directly in the steel making process,
but must first be concentrated and refined. When the iron content of the
ore is increased, the process generally is referred to as concentration,
and this can sometimes be accomplished simply by crushing, screening,
and washing. Other times, the ore is ground to very small particles before
the iron oxides can be separated from the rest of the material, called
gangue, which is normally accomplished by magnetic drums.
However, even where there is satisfactory concentration, iron ore
consisting of fine particles must first be agglomerated into a coarser form,
and this process is referred to as agglomeration. The most desirable size
for blast-furnace feed is from 6-25 mm, and pelletizing is one of the meth-
ods frequently used to achieve this type of coarse iron ore feed.
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In the pelletizing process, which accounts for about two-thirds of
U.S. agglomerate production, the ore must be ground to a very fine size,
less than 75 m. The ground ore is mixed with the proper amount of
water, and sometimes with a small amount of bentonite, and this is rolled
into small balls 10-20 mm in diameter in a balling drum or disk. These
green pellets are dried, then are heated to 1200 -1370 C to bond the
small particles, and finally are cooled. The heating can be done on a
traveling grate, or in a shaft furnace, or by a combination of a traveling
grate and a rotary kiln.
Another of the chief raw materials in the steel making process in
addition to the iron ore, is the fluxing material, consisting of lime (CaCO3)
and/or dolomite (CaCO3-MgCO3). Typically, limestone is crushed and
screened to the desired particle size, and burnt lime for steel making is
then prepared from the limestone by calcination in a long rotary kiln. It is
common to combine the iron ore pelletizing operation described above
with the limestone and/or dolomite flux preparation and calcination by add-
ing the limestone and/or dolomite particles directly to the iron ore particles
which are to be formed into pellets. This mixture is then heated in the
same device, usually a long rotary kiln, often with a traveling grate, so that
the pelletizing and limestone and/or dolomite calcination are accomplished
in the same step and in the same heating furnace. This combined step is
usually referred to as calcination of the iron ore, although the chief result
is
the hardening of the green iron ore pellets.
During the heating of the mixture of particles of limestone and/or
dolomite flux and particles of iron ore formed into pellets, which will be
referred to as flux pellet kilning, a problem is frequently encountered in-
volving deposits which form on the walls of the rotary kiln or other furnace
or heating device being used. These deposits are formed as a result of
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the flux pellet kilning operation, perhaps as a result of a combination of
mechanical adhesion and condensation on the cooler skin of the kiln or
furnace surface. The predominant constituent of such deposits is ferric
oxide (hematite), with the majority of the remainder being magnetic iron
oxide (magnetite). However, there is frequently a significant amount,
about 2-10% by weight of the total deposit, of calcium phosphate,
Ca1 p(PO4)6(OH)2 (hydroxyapetite).
Such deposits create substantial problems in the kilning operation,
e.g., large portions of such deposits can break away and become admix-
ed with the pellets being calcined, thus resulting in an unacceptable final
product. Also, as a result of the formation of these deposits, significant
removal problems are created.
For example, there is a significant down time for the kilns, furnaces
or other heating devices being used, during which the deposits are me-
chanically removed by such off-line cleaning methods as compressed air
driven jack-hammers, small charges of blasting explosives, or more time-
consuming approaches utilizing hammers and chisels, etc. These proc-
esses of mechanical removal present serious problems in addition to the
down time which they entail. An on-line method of cleaning which is fre-
quently used involves mechanical removal of these deposits by "shoot-
ing", in which the deposits are blasted away by repeated discharging of
shotguns against the deposits. This procedure poses the obvious risks to
the personnel performing it, but also has been known to result in serious
damage to the walls of the kiln or other furnace heating device being
used.
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In order to significantly inhibit the formation of these flux pellet kiln
deposits, and thereby significantly increase the efficiency of the flux pellet
kilning operation, the present invention provides for the administration of
a water soluble magnesium compound that undergoes thermal decompo-
sition, preferably to form magnesium oxide at temperatures of about 100 -
1200 C.
BRIEF DESCRIPTION OF THE PRIOR ART
U.S. Pat. No. 4,503,019 discloses the use of blends of magnesium
oxide and copper oxychloride for inhibiting and dispersing calcium oxide
deposit formation in coal-fired kilns.
U.S. Pat. No. 5,221,320 discloses a method of inhibiting the forma-
tion of iron oxide containing deposits on the surfaces of heating devices
during fluxed iron ore pellet calcination, wherein the flux employed con-
tains phosphate, which consists of a treatment of magnesium hydroxide,
copper oxychloride and an alkyl benzene sulfonate suspending agent.
The phosphate content, as P205, of the flux in said fluxed iron ore pellet
must be less than 1% by weight of the total weight of flux and iron ore in
the pellets.
None of the above applications in any way suggest the composi-
tions and methods of the present invention.
CA 02150793 2006-12-15
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a method of inhibiting the forma-
tion of iron oxide containing deposits on the surfaces of heating devices
5 during fluxed iron ore pellet calcination comprising treating the atmos-
phere of said heating device in which said calcination takes place with a
deposit-inhibiting amount of an aqueous solution comprising a magne-
sium compound that undergoes thermal decomposition, preferably to
form magnesium oxide, at temperatures of about 100 -1200 C, with tem-
peratures of from about 100 -500 C particularly preferred. In a preferred
embodiment, the present invention comprises treating the atmosphere of
the heating device where calcination takes place with a deposit-inhibiting
amount of an aqueous solution comprising (1) a magnesium salt, e.g.,
magnesium acetate, magnesium sulfate, magnesium chloride, or magne-
sium nitrate (the latter particularly preferred) with (2) a surfactant select-
ed from the group consisting of ethoxylated alkylphenols, (e.g., ethoxy-
lated nonylphenols), phosphate esters (e.g., Triton QS-44, Union Car-
bide) or nonionic glucosides, particularly preferred (e:g., Triton BG-10).
The present invention, being an aqueous solution, is easier to
store, handle and feed than a suspension of a water insoluble salt as
found in, e.g., U.S. Patent 5,221,320. Suspensions, which have been
previously used for the purposes of the present invention are viscous,
require stirring to keep the solids suspended, and prove difficult to pump
and feed. The present invention is also more effective than prior art
methods at equivalent magnesium treatment rates. This is believed to be
due to the increased surface area of the magnesium salt decomposition
products as compared to the relatively large particle size of magnesium
hydroxide particles.
* trade-mark
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It has been found that water soluble magnesium compounds that
undergo thermal decomposition, preferably to form magnesium oxide at
temperatures of about 1000-1200 C are effective for inhibiting deposits on
the interior of iron ore pellet kilns. The magnesium salt can be formulated
as a concentrated solution, and then diluted with water and applied
through spray nozzles into the atmosphere of the kiln. Additional product
components believed to improve performance are nonionic or anionic sur-
factants for improved spray atomization due to surface tension reduction
and calcium salt inhibitors to inhibit spray nozzle deposition, e.g., CaCO3.
In a preferred embodiment of the present invention, the magnesium com-
pounds undergo thermal decomposition to form magnesium oxide at a
temperature of from about 100 -500 C. An exemplary magnesium com-
pound is magnesium nitrate. Exemplary surfactants are ethoxylated
nonylphenols, phosphate esters and nonionic glucosides. Exemplary
deposit control agents are 2-phosphono-butane-1,2,4-tricarboxylic acid
and 1 -hydroxyethylene-1, 1 -diphosphonic acid.
The present invention further relates to a.composition for inhibiting
the formation of iron oxide containing deposits on the surfaces of heating
devices during fluxed iron ore pellet calcination comprising an aqueous
solution containing (1) a magnesium salt, e.g., magnesium acetate, mag-
nesium sulfate, magnesium chloride, or magnesium nitrate (particularly
preferred) with (2) a surfactant selected from the group consisting of
ethoxylated alkylphenols, phosphate esters or nonionic glucosides.
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Field studies have revealed that a particularly preferred embodi-
ment of the present invention, an aqueous solution of magnesium nitrate
and a nonionic glucoside surfactant, is especially effective in inhibiting
deposition in a taconite pellet kiln. Specifically, the treatment has virtu-
ally eliminated down-time for off-line cleaning, as well as substantially
reducing deposit formation and the need for shot-gunning.
The aqueous solution containing magnesium is injected into the
kiln in an amount of from about 0.001-0.1 pounds of Mg as MgO per ton
of pellets, with from about 0.005-0.05 pounds of Mg as MgO per ton of
pellets being preferred. While the particularly preferred embodiment
described above contains about 63% by weight magnesium nitrate hexa-
hydrate (or 10% Mg as MgO) and 1% by weight nonionic glucoside sur-
factant, with the balance being water, a more meaningful treatment range
is as follows: the water soluble product of the present invention contains
from about 1-25% Mg as MgO, with from 5-15% Mg as MgO preferred.
While this invention has been described with respect to particular
embodiments thereof, it is apparent that numerous other forms and modi-
fications of this invention will be obvious to those skilled in the art. The
appended claims and this invention generally should be construed to
cover all such obvious forms and modifications which are within the true
spirit and scope of the present invention.
