CA1098711A - Recycling of iron values - Google Patents

Abstract

ABSTRACT

A method for recycling iron values such as the iron which is present in the iron oxide, primarily Fe3O4 found in millscale and other fine or waste materials containing oxides of iron. The method is applicable to basic oxygen refining operations wherein iron is charged to the basic oxygen furnace along with a flux. The method can also be applied to other iron or steel-making furnaces. In accordance with the inven-tion, a portion of the flux charged to the furnace is prepared by forming a mixture of iron or iron containing particles with the balance comprising dolomite, the mixture including suffi-cient iron values such that the product after calcination will contain in excess of 20 percent and up to 50 percent by weight iron values, calculated as Fe2O3. The mixture is heated in a kiln to a temperature in excess of 2000°F. whereby reaction products such as calcium and magnesium ferrites are formed and make up a substantial portion of the mixture. The mixture is then introduced as a part of the flux utilized in the furnace, the mixture preferably comprising from 20 to 100 percent of the dolomitic lime which would normally be utilized in said flux charge.

Description

This invention is concerned with the recyling of iron values. The invention is particularly concerned with the efficiency of iron refining operations through the utiliza-tion of iron oxide materials or metallics which would normally not be suitable for use as a charge in a refining operation.
It is well known that large amounts of so-called waste materials containing iron values are produced in steel-making operations. Millscale comprises a large portion of such materials, and the discussion hereinafter will be direct-ed to the utilization of the iron values present in such mate-rials. It will be understood, however, that the invention is considered to be applicable to the utilization of iron values from material of a similar character including particulate iron ore and other waste materials produced in the course of steel-making operations or other operations which result in the collection of significant amounts of iron oxide or other iron particles.
The utilization of the iron values in materials of the type described has the obvious advantage of providing ad-ditional sources of iron and steel. Furthermore, waste mate-rials of the type referred to are known to produce severe pollution problems. The recycling of such materials signifi-cantly reduces disposal problems whereby an important addition-al advantage is achieved with the practice of the invention.
It is a general object of this invention to provide an improved technique for utilizing the iron values from iron materials such as particulate iron or iron oxide.
It is a more specific object of this invention to provide a method for improving the efficiency of refining oper-3 ations by utilizing the iron values present in millscale and ql~L

similar materials which might otherwise be considered waste materials.
These and other objects of this invention will ap-pear hereinafter, and it will be understood that the specific examples hereinafter set forth are provided for purposes of illustration and not for purposes of limitation.
The method of this invention generally relates to the utilization of iron values in material which has often been considered to be waste material. The method is particu-larly applicable for use in association with a basic oxygen refining process wherein iron is introduced into a basic oxy-gen furnace. In the normal operation of such a furnace, oxy-gen is blown into the iron bath for the oxidation of silicon, manganese, phosphorus and carbon, and for removal of the thus oxidized material. In such operations, a flux forms part of the furnace charge, the flux normally consisting of lime, in-cluding a portion of dolomitic lime, and an auxiliary flux material such as fluorspar. During the operation, the furnace contains the molten iron bath and an overlying slag layer which is formed by chemical combination of the flux materials and the oxidized metallic elements (silicon, manganese, phos-phorus and some iron).
In the preferred practice of this invention, a mix-ture is prepared by forming a mixture of iron or iron contain-ing particles with the balance comprising dolomite, the mix-ture including sufficient iron values such that the product after calcination will contain in excess of 20 percent and up to 50 percent by weight iron values, calculated as Fe203. The dolomite comprises a combination of calcium and magnesium car-3 bonate, and it will be understood that where reference is made herein to "dolomite", that term shall include materials suchas magnesian limestone which have the required calcium and magnesium carbonate contents. The iron particles are de-fined as any of the valence states of Fe and its oxides, hy-droxides, and carbonates, and combinations thereof. The thusprepared mixture is heated in a kiln to a temperature in ex-cess of 2000 F. and up to 3000 F. whereby reaction products are formed.
The mixture which is produced in accordance with this invention is preferably introduced into the BOF along with the flux additions. The dolomitic lime portion of a nor-mal flux typically comprises from 15 to 60 pounds per ton of steel produced.
This invention contemplates the substitution of the mixture obtained from the kiln for all or part of this dolo-mitic lime portion of the flux. Specifically, the mixture may be employed as a substitute for from 20 to 100 percent of the dolomitic lime portion of the flux charge.
The iron or iron oxide particles which are employed in the mixture in the kiln will usually comprise millscale which is a material readily available, and which, therefore, comprises a highly desirable source of useful iron. The re-cycling and/or the disposal of millscale as waste has created considerable pollution problems and other problems. Accord-ingly, this invention provides solutions to such pollutionproblems in addition to providing more efficient use of iron resources. It will be appeciated that particles of a charac-ter similar to millscale could also be utilized in the forma-tion of applicants' mixtures, including other iron bearing 3 materials such as iron and iron oxide fines obtained in steel ~n~

making operations, and iron oxide fines obtained in mining operations.
It will be appreciated that the mixture which appli-cants obtain from the kiln contains a high percentage of iron values whereby a significant amount of additional iron is realized from each BOF heat when compared with heats obtained using regular procedures. In a typical mixture obtained from the kiln, the amount of iron oxide, calculated as Fe2O3, will be in excess of 20 percent by weight.
In an example of the practice of the invention, the kiln is charged with dolomitic stone at a rate of approximate-ly 13 tons per hourO A continuous operation is involved with millscale being added to the stone at a rate between about 2 and 3.5 tons per hour for admixture with the stone, the rate f addition of the millscale being varied with the coarseness of the material and being controlled to achieve the minimum requirement for iron values in the product of the kiln.
The kiln is utilized for heating the dolomite to a temperature in excess of 2100 F. thereby driving off carbon dioxide. The millscale is preferably mixed with the dolomitic material and located on a belt conveyor or the like for intro-duction into the kiln. In the kiln, the millscale reacts with the dolomitic material to form reaction products which may in-clude oxides, ferrites, and complex combinations thereof. The following comprises an analysis of a typical kiln product, it being understood that the reference to Fe2O3 is made as a con-venient means of reflecting the iron content and is not in-tended to express the actual iron compounds involved which, as indicated, are typically of a complex nature:
3o 1 ~9 ~

SiO2 1.07%
Fe2O3 22.24%
A123 0.42%
CaO 42.26%
MgO 33.13%
S 0.014%
L.O.I. 0.83%
The mixture thus obtained was introduced into a BOF
along with the flux in a refining operation. In this in-stance,the total charge of mixture and flux was 150 pounds per ton with the mixture comprising 50 pounds per ton of thatamount. The slag obtained after pouring of the heat did not reveal any significant additional percentage of iron values when compared with analyses of slag from BOF operations run without the use of the mixture. This is believed to confirm that the iron values present in the mixture are recovered in the metal bath, thus increasing the over-all yield. More spe-cifically, it is recognized that in a typical BOF operation, the amount of iron oxide in the slag reaches an equilibrium ~ for any given set of operating conditions. This equilibrium level will typically be from 12 to 25 percent by weight of the slag calculated as FeO. The presence of the iron values in the mixture in accordance with the concepts of this invention provides a source of iron oxide to satisfy the equilibrium of oxide in the slag thereby reducing or eliminating the normal loss of iron values from the bath to the slag during a refin-ing operation. The heat balance in the refining operation us-ing the mixture of this invention was also otherwise of stand-ard character, that is, no steel temperature loss was ob-3 served by reason of the use of the mixture in any test heats.

7~1 In the foregoing example, reference is made to theformation of a mixture of millscale or other iron-rich parti-cles with the dolomite followed by the introduction of the mix-ture into the kiln. It will be appreciated that the invention contemplates the addition of the millscale to the kiln at var-ious locations therealong, for example subsequent to heating of the dolomite above 2000 F. with the formation of reaction products occurring toward the end of the kiln cycle.
The procedures of this invention represent signifi-cant improvements when considering prior efforts to recycle or dispose of millscale and similar iron values. Such at-tempts usually have involved sintering of the oxides, charging to the blast furnace, and then refining in the BOF. This in-vention, of course, by-passes the sintering and blast furnace operations.
It will be understood that various changes and modi-fications may be made in the invention described without de-parting from the spirit of the invention, particularly as de-fined in the following claims.

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Claims (5)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. In the operation of a basic oxygen furnace where-in iron is introduced along with a flux for refining the iron, said flux normally including dolomitic lime in amounts be-tween 15 and 60 pounds per ton, the improvement comprising a method for recycling iron values from compounds comprising iron and iron oxide, said method comprising the steps of pre-paring a mixture by mixing such compounds in particulate form together with dolomite, the mixture including sufficient iron values such that the products after calcination will contain in excess of 20 percent and up to 50 percent by weight iron values, calculated as Fe2O3, heating the mixture in a kiln to a temperature in excess of 2000° F. whereby the reaction pro-ducts are formed, and introducing the mixture from the kiln into said furnace along with said iron and said flux, said mixture being substituted for from 20 to 100 percent of said dolomitic lime portion of the flux.

2. A method in accordance with Claim 1 including the step of introducing the dolomite into the kiln, heating the dolomite to a temperature of at least about 2000° F., and thereafter adding said particles to the kiln.

3. A method in accordance with Claim 1 wherein said mixture is heated to a temperature between 2000° F. and 3000°
F.

4. A method in accordance with Claim 1 wherein said particles comprise at least one member selected from the group consisting of iron and the carbonates, hydroxides and oxides of iron.

5. A method in accordance with Claim 4 wherein said particles comprise millscale.