Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1998-11-09
2002-05-07
Reddick, Judy M. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S445000, C524S447000, C524S492000
Reexamination Certificate
active
06384126
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to binder formulations for use in agglomerating mineral concentrates or fines comprised of colloidal silica and a polymeric binder. More particularly, it is directed to a mineral pellet including the binder formulation and related low temperature hardening process for making the same.
BACKGROUND OF THE INVENTION
Pelletizing is the most desirable agglomerating process for iron ore. The concentrates produced are of an extremely fine size (85%—44 micrometers) and are readily formed into green pellets.
The process encompasses two basic steps: (1) the formation of sized (−½+⅝-inch) green (wet) pellets from a moist filter cake concentrate via the balling process, and (2) the oxidation and induration of the green pellet by high temperature heat treatment in an oxidizing atmosphere to produce a fired pellet with sufficient strength and abrasion resistance to withstand the rigors of handling, transportation, storage and blast furnace reduction/smelting.
Different types of additives, which can be classified as binders, fluxes, and fluxing binders are sometimes used to aid in pellet forming, induration and blast furnace reduction. Bentonite, composed mostly of the clay mineral, montomorillonite, is the binder most commonly used to minimize degradation of the green and dry pellet during the induration process. Recently, several water soluble organic binders have been used, in lieu of bentonite, to reduce silica contamination and improve the reducibility of the fired pellet. These organic binders, which include Carboxy Methyl Cellulose (CMC), Alcotac-Acrylate/Acrylamide copolymer and modified starch, are used to eliminate the additional 0.5% silica that bentonite adds to the pellet and also to improve the ‘reducibility’ of the fired pellet. A lower ‘silica’ pellet reduces the slag volume in the blast furnace and a higher ‘reducibility’ pellet increases blast furnace productivity and lowers the coke rate requirement.
Most of the iron ore concentrates produced in North America contain over 95% magnetite. During the heat treating process the magnetite (Fe
3
O
4
) is oxidized to hematite (Fe
2
O
3
) according to the reaction:
4Fe
3
O
4
+O
2
→6Fe
2
O
3
This reaction begins at about 1600° F. and is completed around 2000° F. if sufficient oxygen is available. The reaction is exothermic and releases about 210 Btus per pound of magnetite. The heat generated provides over half the total heat required for the process. The oxide bonding produced by the conversion of the magnetite to hematite, however, is not sufficient to produce a competent pellet and the pellet must be further heated to a temperature of 1288 to 1343° C. (2350-2450° F.) to complete the sintering of the hematite grains and slag bonding of the gangue constituents. The final pellet has a compressive strength of over 500 pounds.
The fuel (natural gas) and electric power requirements for producing pellets from magnetic taconite concentrates is currently about 350,000 Btus and 35 KWH per long ton pellets, respectively. The pelletizing process for hematite concentrate is similar to that for magnetite but because there is no exothermic heat release the total fuel requirements are considerably higher.
The mechanical specifications for good shipping ‘fired’ pellets consist of the following:
(1) Size structure—Pellets should be closely sized, preferably in the −½+⅜-inch size range with less than 2% finer than ¼-inch.
(2) Weatherability—Pellets must have excellent resistance to long term stock piling and outdoor winter storage. Maximum moisture content should not exceed 3 percent and freezing must not be a problem.
(3) Resistance to Breakage during Handling and Shipping—Pellets must be strong enough to withstand (without substantial breakage) normal handling between the pellet plant (mine site) and the blast furnace. Two standard ASTM tests are available to predict the pellet strength performance. They are the tumble test and the pellet compressive test.
In the tumble test, a 25 pound sample of plus ¼-inch pellets are tumbled in a 3-foot diameter by 18-inch wide steel drum (fitted with lifters) for 200 revolutions at a speed of 24 RPM. After tumbling, the pellets are screened at ¼-inch and 28 mesh. The weight percent plus ¼-inch is referred to as the ‘tumble index’ and the percentage of minus 28-mesh fines produced as the ‘abrasion (dust) index’. Fired pellets normally have a tumble index greater than 95 percent and an abrasion index of less than 3.5 percent.
In the compressive test, the compressive strength of 60 individual pellets is determined at room temperature with an automatic compressive tester using a constant speed load. The average compressive strength of the −½+⅜-inch pellets should exceed 450 pounds. The percentage of pellets that have a compressive strength of less than 200 pounds is also important since most of the weaker pellets tend to break up during handling, transportation and blast furnace reduction.
Iron ore pellets containing 4-5% silica are used in North America, primarily as a feed stock for blast furnace reduction and smelting. The blast furnace is a counter current furnace which has the ability to reduce and melt burdens and use coke as the source of heat and reducing gases.
In the upper part of the shaft, sometimes referred to as the Massive Zone, the hematite pellets are slowly heated and reduced while descending. Beginning at a temperature of about 450-500° C. the hematite in the pellet is reduced to magnetite according to the reaction:
3Fe
2
O
3
(hematite)+CO→2Fe
3
O
4
(magnetite)+CO
2
The reduction of hematite-to-magnetite results in a change in crystal structure that sets up stresses in the pellet that are strong enough to cause significant pellet degradation. The fine particles produced can be either carried out of the furnace in the gas stream or fill the interstices of the burden and reduce its permeability. The performance of pellets in this section of the furnace can be predicted with the standard low temperature breakage (LTB) test.
At intermediate levels in the Massive Zone, the pellets begin to increase in temperature, i.e. 500-1000° C. and the magnetite begins to reduce to wustite and wustite to metallic iron according to the reactions:
Fe
3
O
4
(magnetite)+CO→3Fe
x
O (wustite)+CO
2
Fe
x
O (Wustite)+CO→Fe (metallic)+CO
2
The relative rate of reduction is the critical test parameter in this section of the Massive Zone. It is measured by a test procedure known as the ISO-reducibility, —i.e. the rate of oxygen removal measured as percent per minute up to the 40% reduction level. The swelling property of the pellet is also very important in this section of the furnace.
At temperatures above 1000° C., the pellet burden begins to soften slightly and at 1100° C. molten slag begins to be produced and flows out with the rise in temperature. A standard reduction test under load (RTuL) is used to evaluate the pellet contraction at this temperature. At temperatures approaching 1200° C., a cohesive layer is formed by the combination of metallic iron which grows in the shell portion, normally referred to as the Cohesive Zone. When the temperature exceeds 1200° C. metallic iron and slag separate and in the vicinity of 1400-1500° C. these begin to melt down (Melting Zone).
Higher grade iron ore pellets containing less than 2% silica can also be used as a feed stock for a coal or gas based direct reduced iron (DRI) process. The DRI processes normally operate at maximum temperatures of from 800 to 1100° C. and the iron oxide pellets are reduced to metallic iron in the solid state below the softening/melting temperature of the iron oxide and gangue constituents in the pellet. The highly metallized pellets, i.e. 92+% metallization, are normally used as melting stock in combination with ferrous scrap for electric arc furnace steelmaking.
Iron ore pellets produced commercially owe their hardness to being fired
Nigro John C.
Pirtle James
Onofrio Law
Onofrio, Esq. Dara L.
Pirtle James
Reddick Judy M.
LandOfFree
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