US3836353A

US3836353A - Pellet reclamation process

Info

Publication number: US3836353A
Application number: US00127748A
Authority: US
Inventors: C Holley
Original assignee: C Holley
Current assignee: Zia Technology Inc
Priority date: 1968-10-18
Filing date: 1971-03-24
Publication date: 1974-09-17
Anticipated expiration: 1991-09-17

Abstract

A method of recovering iron and oxide impurities from steel furnace dust in which the dust first is mixed with finely divided coke and then this mixture is pelletized. The green pellets thus formed are deposited over a layer of burnt pellets on a rotary hearth which successively conveys the pellets first through a drying zone, then through an initial heating zone in which the pellets are gradually raised to a temperature at which the coke starts to burn, then through a decontamination zone in which the pellet temperature is rapidly raised to a degree at which zinc, lead and sulfur impurities vaporize and in which these impurities are carried off and collected as oxides, and finally the pellets are carried through a reoxidation and hardening zone in which the temperature thereof is further increased to a sufficient degree and held for a long enough period of time to permit the growth of grains of an oxide of iron on the surface of the pellets, thus to form hard bonded pellets which are not fused together.

Description

Unite States Patent [191 Holley 51 Sept. 17, 1974 [54] PELLET RECLAMATION PROCESS [76] Inventor: Carl A. Holley, 116 Marian Ave.,

Glenshaw, Pa. 15116 22 Filed: Mar. 24, 1971 21 Appl. No.: 127,748

Related U.S. Application Data [63] Continuation of Ser. No. 768,727, Oct. 18, 1968,

abandoned.

[52] U.S. Cl. 75/3, 75/25 [51] Int. Cl C2lb l/30 [58] Field ofSearch 75/3, 4

[56] References Cited UNITED STATES PATENTS 1,930,010 10/1933 Haswell 75/25 X 2,040,825 5/1936 Betterton et a1 75/25 2,919,231 12/1959 Donath 75/4 3,050,298 8/1962 Hall 263/28 3,149,958 9/1964 Ward 75/5 3,180,723 4/1965 McCauley.. 75/3 X 3,264,092 8/1966 Ban 75/3 3,386,816 6/1968 English 75/3 3,460,818 8/1969 Greanes et al 75/3 X Primary Examiner-A. B. Curtis Attorney, Agent, or Firm-Shenier & OConnor [57] ABSTRACT A method of recovering iron and oxide impurities from steel furnace dust in which the dust first is mixed with finely divided coke and then this mixture is pelletized. The green pellets thus formed are deposited over a layer of burnt pellets on a rotary hearth which successively conveys the pellets first through a drying zone, then through an initial heating zone in which the pellets are gradually raised to a temperature at which the coke starts to burn, then through a decontamination zone in which the pellet temperature is rapidly raised to a degree at which zinc, lead and sulfur impurities vaporize and in which these impurities are carried off and collected as oxides, and finally the pellets are carried through a reoxidation and hardening zone in which the temperature thereof is further increased to a sufficient degree and held for a long enough period of time to permit the growth of grains of an oxide of iron on the surface of the pellets, thus to form hard bonded pellets which are not fused together.

7 Claims, 5 Drawing Figures PATENTEDSEP! 7 1914 SHEU 2 OF 5 W QUX H TTORNEYS PATENIEDSEPI 7:914

SHEET 5 0F 5 INVENTOR C r/ H. HO/ky BY QTTOP/VEYS PEILLIET RECLAMATION PROCESS This application is a continuation of my copending application Ser. No. 768,727 fied Oct. I8, 1968, now abandoned.

BACKGROUND OF THE INVENTION Both the basic oxygen furnace (BOF) and the open hearth (OH) processes of making steel employ large amounts of scrap, much of which is galvanized. In the course of each process, there is produced a dust containing oxides of iron as well as oxides of impurities such as zinc, lead and sulfur. In the prior art, this dust not only is waste, but also creates a problem, since it must be disposed of if contamination of the surrounding atmosphere is to be avoided. Particularly, the dust contains a relatively large percentage of zinc ferrite, zinc oxide and sulfur. Because of the presence of these impurities, attempts to reclaim it directly for re-use have not proved to be practical.

As an alternative to direct recovery, it has heretofore been proposed that the dust first be pelletized and then subjected to a very high temperature for a short period of time to sinter the pellets and to volatilize the impurities. The very high temperature involved in this operation partially melts the iron oxide which, when it cools, rapidly forms a glass-like material which fuses the pellets together. Not only is the resultant mass difficult to reduce, but the recovery ofimpurities is relatively ineffective.

I have invented a process for recovering iron from steel mill dust which overcomes the problems involved in attempts of the prior art to recover the iron. My process produces hard pellets of partially reduced iron with an iron oxide surface, which pellets are not fused together. My process permits effective removal of impurities and recovery of the same as oxides. Particularly, my process permits removal of substantially all of the zinc from the dust and collection of the same as a zinc oxide. My process not only permits efficient recovery of what formerly was waste material, but itreduces contamination of the atmosphere in an economical manner.

BRIEF DESCRIPTION OF THE INVENTION One object of my invention is to provide a method for recovering both iron and impurities from steel mill dust which overcomes the difficulties involved in recovery attempts of the prior art.

Another object of my invention is to provide a method for recovering iron from steel mill dust in the form of hard, unfused pellets of iron.

Another object of my invention is to provide a method of removing and recovering impurities such as zinc in the form of an oxide thereof.

A still further object of my invention is to provide a method of reclaiming what formerly was waste material in the steel-making process, while at the same time reducing contamination of the atmosphere.

Other and further objects of my invention will appear from the following description.

In general, my invention contemplates the provision of a pellet reclamation process in which I form green pellets ofa mixture of steel furnace dust and an internal fuel such as coke or coal. I feed the green pellets over a layer of burnt pellets on a rotary hearth which successively conveys the pellets first through a drying zone in which the pellets are dried, then to the next zone in which the pellets are heated to a "temperature at which the coke begins to burn, then through a decontamination zone in which the pellet temperature is rapidly raised to a degree at which the impurities such as zinc are first volatilized and then carried off as oxides of the impurity. Finally, the pellets are carried into a reoxidation and hardening zone wherein the temperature thereof is further raised to a degree just below the melting point of oxides of iron wherein the iron is reoxidized by the growth of grains of an oxide of iron to form a hard dense shell on the pellets which are not fused together. Also the calcium oxide combines with ferric oxide to form calcium ferrites which also densify the surface layer.

BRIEF DESCRIPTION OF Til-IE DRAWINGS In the accompanying drawings which form part of the instant specification and which are to be read in conjunction therewith and in which like reference numerals are used to indicate like parts in the various views:

FIG. 1 is a block diagram illustrating the various steps which are performed in the practice of my pellet reclamation process.

FIG. 2 is a schematic view illustrating the relationship among the various pieces of apparatus which are employed in practicing my process.

FIG. 3 is a plan view with parts shown schematically of a typical plant which may be used to carry out my process.

FIG. 4 is a front elevation of the plant shown in FIG. 3.

FIG. 5 is a side elevation of the plant shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, the initial step or steps in carrying out my process are indicated by the block 10 in FIG. 1. These initial steps are the collection and drying of the dust from the mill furnace which may be either the BOF or the OH type. Clarifiers l2 deliver a slurry of dust to an underflow pump 14, which feeds the slurry containing about 40 percent solids to a spray drier I6 of any suitable type known to the art. I have indicated the flow ofthe slurry from the pump 14 to the drier 16 by the broken line 18 in FIG. 2. I supply hot drying air to the drier as indicated by the dot-dash line 20 in FIG. 2. As will be explained more fully hereinafter, I obtain the air supplied to the drier 16 from a rotary hearth to be described. Dust-laden air from the drier 16 passes through a bag collector 26 of any suitable type known to the art, from which it is carried by a fan 28 to a stack 30. The dust is delivered by the bag collector 26 to bin 34. Bin 34 feeds a measured amount of the collected dust to the conveyor 24.

In practice of my method, the next step, indicated by the block 40 in FIG. l, is the addition of finely divided coke dust to the dust 22. A dry coke breeze, indicated by the dot-dash line 42 in FIG. 2, supplies the coke dust to a collector 44 having an exhaust air filter 46. A scale 48 associated with conveyor 24 permits me to measure the amount of dried coke dust added by collector 44 to the dust 22. Preferably, I add coke dust in an amount of about from 10 to 12% by weight of the mixture.

As is indicated by the block in FIG. 1, the next step in the practice of my process is mulling or mixing the dried coke dust with the furnace dust. In order to achieve that operation, conveyor 24 delivers the dust to a dry pan muller 52 of a type known in the art. Muller 52 delivers the dust to a second conveyor 54.

As shown in FIGS. 3 to 5, the elements of my apparatus which have been thus far described, as well as those to be described, are housed in a suitable building, indicated generally by the reference character 56, having a floor 58, a roof and a plurality of side walls 62. The means which carries the mixture of dust and coke from the muller 52 may comprise a bucket elevator 64 leading to the conveyor 54 which carries the material to a pelletizer 66 in which the step, indicated by block 68 in FIG. 1, of pelletizing the material is carried out. This pelletizer 66 may be of any suitable type known to the art.

The apparatus for carrying out my process further includes a rotary oven, indicated generally by the reference character 70, comprising a hearth 72 enclosed by a hood 74 and adapted to be driven in the direction of the arrow A in FIG. 3. The structural details of an oven such as I employ in my process are more fully shown and described in U.S. Pat. No. 3,050,298. The oven 70 includes an entry 76 through which the material to be treated is delivered for deposit on the hearth 72. In the particular use to which I put the oven 70, I divide the hood into respective zones, indicated generally by the

reference characters

78, 79, 80, 82 and 83, by means of

partitions

84, 86, 87 and 88. An inlet duct 90 is supplied with preheated air to provide combustion air for the burners (not shown) in the wall of the hood. It will readily be appreciated that the burners in the respective zones and the air supplied thereto may be so controlled as to provide the proper conditions to be described below to which the pellets are subjected in a manner to be described. A combustion fan 93 supplies air through a heat exchanger 94 to the duct 90 which leads to a distributing duct 96 which supplies the various zones. A heated air fan supplies hot air through exchanger 94.

I provide the

respective zones

78, 80 and 82 with

gas collecting adapters

98, 100, 102 and 103. A respective exhaust duct 104 is connected to each of the

headers

98, and 102.

In the next step of my process I form on the hearth 70 a base layer of burnt pellets which performs the dual functions of protecting the refractory material of the hearth 70 and of protecting the green pellets deposited thereover in a manner to be described from the heat of the direct contact with the hearth. For this purpose I provide a burnt pellet bin 108 at a location which is prior to the beginning of zone 78 so as to distribute the burnt pellet layer thereon. Preferably, l deposit only a single layer of such pellets which may have a diameter of about one-half inch on the surface of the hearth 72.

A reciprocating conveyor 110 leading from the pelletizer 66 to the entry 76 deposits green pellets on top of the layer of burnt pellets which has been deposited on the hearth. Preferably, the layer of green pellets is about one and one-half times the diameter of an individual pellet. Owing to the fact that the bed of green pellets is only one or at most two pellets deep, all are exposed to radiation from the walls and roof and thus are heated faster.

With the system in operation, hearth 72 rotates to carry the green pellet layer supported on the burnt pellets from the entry 76 around the hood 74 successively through the drying zone 78, through the heating zone 80, through the decontamination zone 82 and through the reoxidation and hardening zone 83 to a point just ahead of the entry 76 wherein the pellets are removed and deposited in the cooler 112.

I so control the supply of air and the gas burners in the various zones as to heat the pellets as follows. In drying zone 78 or the first zone, the pellets are heated for about 10 to about 15 minutes to a temperature of 500F. to 600F. in the presence of a slightly oxidizing atmosphere. All of the heat in this zone comes through the hearth layer pellets from the hearth. Since, in the course of the drying step, air is not passed through the bed of pellets, they can be dried at a higher temperature than would be possible otherwise.

In the second zone, the temperature is raised to 1500F. to l600F. in the presence of a slightly oxidizing atmosphere. The heat in this zone is supplied primarily by combustion of gas in the furnace hood. The pellets are in this zone for about 7 to 10 minutes. In this zone the temperature of the pellets is raised to a degree at which the internal fuel coke in the pellets begins to burn but which temperature is lower than that at I which the impurities begin to volatilize.

In the third zone, the temperature is raised rapidly to about 2000F. to 2IO0F. and is held for about 7 to 15 minutes in a slightly reducing or oxygen poor atmosphere. At this temperature carbon has a greater affinity for oxygen than does either iron or zinc, so that it takes oxygen both from the oxides of zinc and lead and the oxides of iron. The zinc in the pellets is both in the form of zinc oxide and zinc ferrite. The former material reduces at approximately 1700F. while the latter requires a temperature of about 2000F. to reduce. At the same time, the carbon reduces the iron oxide toward metallic iron with the result that the iron oxides end up in this zone as FeO and Fe. The zinc, lead and sulfur all vaporize. The sulfur goes off with the flue gases. The lead fumes off as a part of the zinc vapor under the action of the exhaust applied to header 100. These elements reoxidize above the bed and are carried away through duct 104 to a bag collector 114 through which exhaust gases are drawn by a fan 116, the exhaust of which is connected to a stack 118. Any suitable means such, for example, as a pneumatic pump 120, carries the zinc oxide from the collector 114 to a bin 122 having an exhaust air filter 124. In this manner the zinc oxide is reclaimed. The zinc oxide may also be granulated and dried so it can be handled and transported easier.

After the impurities are removed in the manner just described, the pellets are carried on through the fourth or reoxidation and hardening zone 83. In this last zone the partially reduced iron is heated to a temperature of about 2300F. to 2450F. Air is introduced into this region to oxidize the surface of the pellets and to cause the pellets to shrink and thus harden. I so control the rate and temperature of the air added to maintain the pellet at the desired temperature. It is important to note that I so select this temperature as to be just below the melting point of the oxide. I maintain this temperature for a period of about 7 to l5 minutes to cause the pellets to gradually shrink and thus to form a hard bonded pellet. In this respect, my process differs from sintering processes of the prior art; that is, in the sintering processes the pellets are subjected to a very high temperature which melts the iron oxide so that when it cools rapidly it forms a glass-like material which is brittle and hard to reduce. In my process, no melting of the pellet takes place. The dense outer shell of my pellet is composed ofa thin layer of hematite at the surface followed by a dense mixture of magnetite and calcium ferrite with most of the pores filled with a silicate slag phase. This surface zone is very dense and inhibits oxidation of the core. The center of the pellet is made up of a porous FeO zone and a FeO, metallic Fe core.

I have indicated the respective steps of depositing the green pellets on the base layer and of passing the pellets through the respective zones by the

blocks

126, 127, 128, 130 and 132 in FIG. 1. When the pellets reach the end of the reoxidizing zone 83, they are removed to the cooler 112. A fan 134 supplies cooling air to the pellet cooler 112 to cool the pellets in the step indicated by block 1136 in FIG. 1. I take the heated air from cooler 112 and supply it with the air from

zones

78 and 83 to the drier 16.

A vibrating feeder 138 located below the cooler 112 receives pellets from the cooler and delivers them to a vertical bucket elevator 148 which delivers the cooled pellets to a screen 142. A reciprocating conveyor 144 is so operated as to deliver a portion of the pellets from screen 142 to bin 108. The remaining pellets are received by a conveyor 146 which delivers them to a pellet bin 148. It will be appreciated that the pellets received by conveyor 144 and delivered to bin 108 form the base layer of burnt pellets on the hearth 72.

In practice of my pellet reclamation method, the slurry fed by pump 14 to drier 16 contains about 40% solids, including oxides of lead and iron sulfates and oxides of iron. The heated air supplied to the drier 16 from the initial heating zone 78 and from the reoxidizing zone 83 and from the cooler 112 removes water from the slurry so that particles of dust 22 are fed directly by the drier to the bag collector 26 and from the dust bin 34. Finely divided coke from the coke collector 44 is mixed with the steel furnace dust in muller 52 and is pelletized in pelletizer 66.

Bin 108 delivers a base layer of burnt pellets to hearth 72 and conveyor 118 spreads a layer of green pellets over the base layer of burnt pellets. Hearth 72 carries the pellets first through the initial drying zone 78 and then through a heating zone 83 in which the pellets are heated to a temperature which is sufficient to begin oxidation of the internal coke but which is lower than the temperature at which the impurity oxides are reduced. After the initial heating, the pellets are carried through the decontamination zone 82 in which they are heated rapidly to a temperature sufficient to reduce the impurity oxides and to begin reducing the iron oxides. The impurities are taken away from the hearth in metallic form, reoxidized above the hearth and are carried away and collected in bag collector 114. Following removal of the impurities, the pellets are carried into the reoxidation and hardening zone 83 to which hot air is supplied so as to reoxidize the surface without at the same time raising the pellets to a temperature above the melting point of the oxides. The pellets are held in this last zone for a period of time which is sufficient to permit shrinkage and hardening so as to form the hard surface skin described hereinabove.

It will be seen that I have accomplished the objects of my invention. I have provided a pellet reclamation process which overcomes the defects of reclamation processes of the prior art. My process forms hard iron oxide pellets which are not fused. It permits the formation of such pellets while at the same time effectively removing and recovering impurities. Thus my process not only permits recovery both of iron and of impurities in furnace dust, but it also greatly reduces the atmosphere pollution problems.

It will be understood that certain features and subcombinations are of utility and may be employed with out reference to other features and subcombinations. This is contemplated by and is within the scope of my claims. It is further obvious that various changes may be made in details within the scope of my claims without departing from the spirit of my invention. It is, therefore, to be understood that my invention is not to be limited to the specific details shown and described.

Having thus described my invention, what I claim is:

l. A method of recovering iron from steel furnace dust containing oxides of iron and zinc including the steps of passing wet dust through a drier, supplying said drier with hot air from the hood of a hearth to dry said dust, mixing said dust with about 5 to 7 percent by weight of finely divided coke, forming green pellets of said mixture, burning some of said pellets, depositing a layer of said burnt pellets on a hearth, dividing said hood into a drying zone, a heating zone, a decontamination zone and a reoxidation and hardening zone, said zones being separate and distinct "from each other, depositing said green pellets on said layer of burnt pellets, passing said pellets while on said! hearth successively through said drying zone and heating zone and said decontamination zone and said reoxidizing and hardening zone, drying said pellets in said drying zone at a temperature of about 500F. to about. 600F., heating said pellets in said heating zone to a temperature of about l500F. to 1600F., then rapidly heating said pellets in said decontamination zone to an elevated temperature of about 2000F. to 2100F. and maintaining said pellets at said elevated temperature in said decontamination zone for sufficient time to volatilize said zinc, removing said volatilized zinc from said decontamination zone as an oxide of zinc, collecting said zinc ox ide, heating said pellets in said reoxidation zone to a higher temperature of from about 2300F. to about 2450F., maintaining said pellets at said higher temperature in said reoxidation zone for sufficient time to cause grains of hematite to grow to form a hard bonded pellet and removing said pellets from said hearth after passage through said reoxidation zone.

2. A method as in claim 1 in which said heating step at l500F. to l600F. is for about 10 to l5 minutes.

3. A method as in claim 1 in which said drying step comprises heating said pellets for about 10 to 15 minutes at a temperature of about 500F. to 600F.

4. A method as in claim 1 in which said rapidly heating step comprises heating said pellets to a temperature of about 2000F. and holding said pellets at said temperature for about 7 to about 15 minutes.

5. A method as in claim 1 in which said further heating step comprises heating said pellets to a temperature of about 2300F. to about 2450F. and maintaining said pellets at said temperature for about 7 to about 15 min- 7. A method as in claim 1 including the steps of reutes to form said hematite grains. moving said pellets from said hearth after passage 6. A method as in claim 1 including the steps of rethrough said reoxidizing zone and cooling said removed moving said wet dust from a slurry and drying said dust pellets. before forming said mixture.

Claims

2. A method as in claim 1 in which said heating step at 1500*F. to 1600*F. is for about 10 to 15 minutes.
3. A method as in claim 1 in which said drying step comprises heating said pellets for about 10 to 15 minutes at a temperature of about 500*F. to 600*F.
4. A method as in claim 1 in which said rapidly heating step comprises heating said pellets to a temperature of about 2000*F. and holding said pellets at said temperature for about 7 to about 15 minutes.
5. A method as in claim 1 in which said further heating step comprises heating said pellets to a temperature of about 2300*F. to about 2450*F. and maintaining said pellets at said temperature for about 7 to about 15 minutes to form said hematite grains.
6. A method as in claim 1 including the steps of removing said wet dust from a slurry and drying said dust before forming said mixture.
7. A method as in claim 1 including the steps of removing said pellets from said hearth after passage through said reoxidizing zone and cooling said removed pellets.