US3005699A - Method for converting iron oxide to magnetic oxide - Google Patents

Description

Oct. 24, 1961 J. ERCK ETAL METHOD FOR CONVERTING IRON OXIDE TO MAGNETIC OXIDE Filed Oct. 9, 1957 ifiq- A WUBQMN min INVENTORS (OI/l6 1 ERCK A THOMAS E- BAN- N N no r s N g a w W Y wr "M BMW United States Patent 3,005,699 METHOD FOR CONVERTING IRON OXIDE T0 MAGNETIC OXIDE Louis J. Erck, Negaunee, Mich., and Thomas E. Ban,

Cleveland Heights, Ohio, assignors to The Cleveland- Clilfs Iron Co., Cleveland, Ohio, a corporation of Ohio Filed Oct. 9, 1957, Ser. No. 689,167 9 Claims. (Cl. 751) The present invention relates generally to the ore processing art and is more particularly concerned with a novel method for converting non-magnetic iron minerals into a magnetic state and to new apparatus implementing this method.

It has long been recognized that in the magnetite state iron minerals in lean ores can be readily concentrated from non-magnetic gangue constituents by mineral dressing operations employing both comminution of the lean ore and magnetic separation of the liberated magnetite. Magnetic separation is easy and economical to carry out in large-scale operations with excellent iron value recoveries. This advantage over non-magnetic separation procedures is such that the substantial investment in heat, equipment, and labor essential to chemically convert hematite minerals of marginal ores into magnetic oxide has been commercially attractive for as long as marginal iron ores have been important to the industry.

A comparatively new branch of the iron ore benefication art has grown up on the basis of efforts of others to provide a method or means by which magnetic oxide conversion could be carried out economically and on a large scale on various lean, non-magnetic ores which are abundantly avail-able. Inasmuch as the chemistry involved necessitates elevated temperatures, a furnace is essential to the conversion process and it is in respect to furnace design and operation that this field has further divided itself several ways. Thus shaft furnaces, multiple hearth vertical furnaces, multiple hearth duo-solids furnaces and horizontal rotary kilns have all been used to carry out magnetic oxide conversion reactions.

All these furnaces of the prior art, however, have shortcomings and drawbacks of one kind or another which have restricted the use of magnetic oxide conversion as a tool in the iron ore beneficiation art. Thus shaft furnaces can tolerate only a relatively small quantity of fine material because excess amounts of fines cause preferential gas flow and consequent poor reaction efliciency. Furthermore, shaft furnaces have a capacity limitation which prevents large units from operating efficiently due to continual temperature and :gas composition variation.

The multiple hearth or rabble hearth vertical furnace, on the contrary, requires finely-ground feed which necessitates a special feed preparation step adding materially to the over-all cost of the process. Furthermore, furnaces of this type inherently have poor thermal efiiciencies because of the abnormally large furnace surface per unit capacity.

The multiple hearth fluoro-solids furnace likewise necessitates special preparation of the ore feed since it requires finely-ground ore for satisfactory operation. In addition, this furnace has a relatively high initial cost and the power requirements for fluidizing and conversion make operational charges excessive.

The horizontal rotary kiln of the prior art characten istically has poor thermal efliciency and it presents a serious dust-loss problem when the ore feed contains substantial quantities of fine material. Another disadvantage of this furnace is that it provides no means for isolating and removing gases for reduction and heating with the result operational costs run high.

We have found that these and other shortcomings and derelictions of the prior art can be overcome to a large extent or eliminated altogether by carrying out the roasting operation in a novel manner which is quite different from any of the procedures involved in the operation of the various furnaces and kilns and described above. Thus we have discovered that by using an entirely diiferent type of furnace and operating it in a certain new manner, a uniform high temperature can be imparted to a bed of ore. Also, a wide variation in sizes of the feed material can be tolerated and in fact can be advantageous from the standpoint of obtaining maximum operational efficiency when sizes are graded in a certain way through a bed. Still further, we have found that it is possible easily to control the operation of this conversion process very closely and without in any way diminishing the efficiency of the conversion reaction. Additionally, according to our findings, size limitations on the iron ore feed may be governed by mechanical construction rather than by gas flow principles and operational data on relatively small units can on this basis be reliably extrapolated in the design of large units. Another important finding is that the very fine material can advantageously be pelletized prior to feeding because the compacted material will not undergo degradation under stock loads as it does in a typical stack furnace operation. We have also found that compacts made from line material retain suificient micro-porosity to permit gas penetration during firing to afford maximum heat transfer efiiciency during cooling.

These discoveries arose out of work carried out by us in the course of testing our fundamental and novel concept on which the present invention is predicated. According to this concept, magnetic oxide conversion of iron ore is accomplished by contacting the ore in a relatively shallow bed with a reducing gas at a temperature conducive to oxide-reduction reactions leading to the synthesis of magnetite. The fundamental departure which this procedure constitutes over the prior practice as outlined above is essentially in the physical form of the ore during its residence in the furnace, that is, the idea of treating separate, relatively small increments of ore in a shallow but static bed. This is in contrast to the massive bed treatment of the stack furnace and the active or flowing charge of furnaces more like the rotary kiln. It will be understood that production rates in accordance with this invention compare favorably with the best obtainable in accordance with prior art practice.

Another novel concept underlying this invention is in the manner of heating and cooling the ore charge at the beginning and at the end of the conversion process. This procedure holds the important advantages of heat con servation and over-all good heating efliciency.

The method aspect of this invention, broadly speaking, comprises the steps of providing a gas-permeable bed of lean iron ore, heating the ore to an elevated temperature conducive'to oxide-reduction reactions, flowing reducing gas through the ore bed at the elevated temperature and thereby reducing a substantial proportion of the iron oxides in the ore and forming magnetite therefrom, and flowing additional quantities of reducing gas at relatively low temperatures through the bed and thereby cooling the ore in the bed and heating said additional quantities of the gas for introduction into another bed in a continuation of the process.

In accordance With the preferred practice of our invention, the temperature of the ore bed during the oxidereduction phase is maintained between about 1100 F. and about 1600 F. with best results being consistently obtained when the bed temperature approximates 1200 'F. At temperatures below 1100 F. the reaction is too slow for good economy and While the rate of reaction increases with increases in temperature, the problem of control also becomes more difiicult. At high temperatures, such as 1600 F. and above, in order to keep the reduction reactions from going too rapidly and too far with the ultimate production of non-magnetic Wustite, it is necessary to maintain a low carbon monoxide-carbon dioxide ratio and a low hydrogen-water ratio, or in other words, a low reductant concentration. This can be both expensive and difficult to do, particularly when operating at a high input rate and at temperatures well above 1600 F.

The reducing gas used in this process can be hydrogen or carbon monoxide, or a mixture thereof such as commercial producer gas. In the interest of heat recovery and control of the reduction reactions, this reducing gas is preferably mixed with spent, scrubbed, recirculated gas which also serves as a diluent to insure that the gas mix ture will be below the critical explosion mixture range as it enters the furnace. The bulk of the reducing gas thus obtained serves as an effective heat carrier in abstracting sensible heat from the reduced ore and conducting it to the reduction zone of the furnace.

In carrying out this invention process, the step of heating the ore bed preliminary to the reduction reaction stage can be carried out through the use of any suitable high temperature source such as a gas which can be flowed through the bed to bring the temperature of the bed up to the desired level. However, we prefer to use both a gas and a flame and to apply the flame directly to the top of the ore bed to bring the ore temperature up rapidly to the level at which the'oxide-reduction step is to be carried out.

In the preparation of the bed of ore, coarse material is deposited last as a top layer of the bed and intermediate sizes and fine sizes are deposited in sequence to provide the bed central and bottom portions or layers, respectively. In other words, the ore is generally sized and deposited in layers in making up the bed so that the coarser particles will receive the most intense heat for the longest period of time and will be subjected to the richest gas concentrations. In a refinement of this procedure contemplated in commercial operations, sized returns comprising converted material are placed at a convenient depth immediately on top of the furnace grates. In this way, fine material will be prevented from shifting through the grates and the grates will be insulated to a substantial degree from the high temperature heat of the bed.

In its apparatus aspect this invention generally comprises a traveling grate-type compartmented furnace having heating, reducing and cooling chambers, a traveling grate to run continuously through a course including the furnace chambers in the order named and through a charging station and a discharging station outside the furnace, and a closed gas-flow circuit whereby the ore bed burden on the grate is heated, reduced and cooled in the respective furnace chambers and the make-up reductan't is mixed and diluted and is heated preparatory to entering the reducing chamber.

An embodiment of this apparatus invention isdiagrammatically illustrated in the drawing accompanying and forming a part of this specification.

This illustrated apparatus comprises a traveling gratetype furnace F which includes a continuous chain of pallets arranged as a horizontal belt around a driven sprocket 12 and an idler sprocket 13-. Furnace F includes also a closed shell 15 which is compartmented to provide a preheated chamber 16, heating chamber 17, a reducing chamber 18, and a cooling chamber 19. Slide seal mechanisms indicated at 20, 21, and 22 serve to support the pallet chain horizontally within the furnace while the slide seals shown at 23 and 24 serve to support the pallet chain at the discharge end of the furnace.

The individual pallets preferably, as indicated in the drawing, consist of interlocking, two-sided steel boxes with grate bottom portions. The link mechanisms be- 4 r tween two adjacent pallets are connected to adjacent grates in accordance with heretofore common practice in continuous sintering machines. Thus the pallets in traveling in their horizontal upper course with sides disposed upright form a continuous grate enclosed by two continuous side Walls fitting flush in a vertical manner to provide adequate support for a bed of ore to be fired and reduced in the furnace.

In the diagrammatic showing of the drawing, the usual hood elements and wind boxes associated with a continuous sintering machine of this general type are indicated only in a general way. However, those skilled in the art will understand that the furnace may be constructed in accordance with conventional practice in respect to these elements and providing the various gas line connections indicated in the drawing are made and the novel closed circuit is thus established, consistently good results in the operation of this furnace in carrying out the aforesaid method can be obtained.

In the operation of the illustrated furnace for magnetic oxide conversion according'to this invention, know is continuously charged on the traveling grate at a point just in advance of chamber '16 by means of a plurality of hoppers 25. As mentioned above, the charge material will be deposited in layers and sized returns will constitute the charge from the first hopper 25 in the series to make up the basis for the ore bed. Fine particles of ore will be deposited on the traveling grate from the second hopper and the coarser grades will be deposited from the third and fourth hoppers. It will be understood, however, that this ore charge may be carried out in one stage following preliminary preparation such as crushing, rougher concentration, agglomeration, or the like. In any event,

though, the rate of discharge of the material from the hoppers will, according to this invention, be regulated in accordance with the rate of travel of the grate in order to produce a bed depth and a layer depth within the bed of predetermined desired dimensions.

As the freshly-formed bed is carried into furnace shell 15 and under the preheat hood 16 and over the first wind box, hotgases from the hood pass through the bed to expand sensible heat to the ore therein. Burner 28 is operated continuously during the conversion process and is disposed in the heating hood or chamber 17 to assure an adequate temperature through the depth of the ore bed prior to introducing the ore bed into the reducing zone. Thus pallets entering chamber 17 bear their bed charge under the burner flame and then producer gas at elevated temperature is flowed through the bed with the result that the top portion of the ore is heated to incandescence rapidly. The pallets are then conducted into the reduction chamber 18 under a hood through which preheated producergases are introduced into the furnace and the preheated gases are flowed through the incandescent ore bed, exchanging heat with the higher temperature top layers of the heated ore and transferring the heat to the lower layers thereof. This heat-exchanging operation is carried out concurrently with the reducing reaction and magnetite formation which proceeds in accordance with one or both the following basic equations:

On leaving the reducing chamber 18 the ore bed will possess sensible heat from the ignition operation, the heat exchanger operation and from the above exothermic chemical reactions. Accordingly, it is the function of this stage of the furnace operation to extract as much of this heat as possible and this is accomplished by mixing cool, raw, producer gas with scrubbed spent gas and flowing the resulting non-explosive gas mixture through the bed, either updraft or downdraft, and collecting the gas thus heated for use in the reducing zone of the furnace, where it will be efiective to reduce ore in the bed to magnetite.

The traveling pallets emerging from the furnace through slide seal mechanisms indicated at 23 and 24 are relatively cool and the ore bed is ready for discharge, which is accomplished by gravity as the pallets travel around sprocket 13.

The reducing gas used in this process is suitably producer gas and it is introduced into the furnace at two different locations. A substantial amount, preferably a principal portion, of the reducing gas is introduced into the furnace through a line 30 by means of a pump so that the gas as it flows through chamber 19 is forced through the traveling grate and the ore bed thereon to be cooled. The remainder of the reducing gas is introduced into heating chamber 17 in admixture with air and some proportion of spent hot gases from reducing chamber 18. Line 34 with suitable connections to an air source and a line 35 communicating with chamber 18 serve to perform this function. Preheated gas is withdrawn through line 36 and pumped into chamber 18 through its hood by means of a blower 38 on a line 39. Spent gas from chamber 18 is drawn through wind boxes serving that chamber through line 40 and this hot gas is pumped by blower 41 through line 42 to line 35 and line 43 which opens into the hood of chamber 16.

Gas vents from the system through the wind box serving chamber 16 and 17 and a line 45 having a vent branch 46 with suitable valve means (not shown) for controlling the volume of gas discharged from the system. The quantity of gas thus vented will in the normal operation be equivalent to the sum of the products of combustion in the furnace, the moisture as steam from the hood and the raw producer gas input. The residual quantity of spent gas thus is recycled to the raw producer gas stream through line 45, a scrubber 47 and line 30 and the reducing gas mixture to be introduced into the furnace is made up according to predetermined desired fuel values and ratios. Blower 48 serves to maintain flow through lines 45 and 46 and through scrubber 47.

-It is contemplated that in some operations in accordance with this invention the product will not be sutficiently cooled by raw reducing gas and that additional cooling means will be required. For this purpose, a vertical cylindrical vessel 50 encloses a portion of sprocket 13 and the pallets disposed in discharging position around this sprocket to receive the reduced ore product. As a heatrecovery feature, steam is generated by quenching this product with water in vessel 50 and this steam is introduced into the gas circuit through a line 51 connected to line 40, a blower 52 being provided in line 51. The water to quench the furnace discharge is normally maintained as a body or pool having its surface below the inlet end of line 51. Periodically water and solids are removed cordance with the reqiurements of the quenching operaated manually or automatically as desired. Make-up water is introduced into vessel 50 through pipe 57 in accordance with the requirements of the quenching operation.

Having thus described this invention in such full, clear, concise and exact terms as to enable any person skilled in the art to which it pertains to make and use the same, and having set forth the best mode contemplated of carrying out this invention, we state that the subject matter which we regard as being our invention is particularly pointed out and distinctly claimed in what is claimed, it being understood that equivalents or modifications of, or substitution for, parts of the above specifically described embodiments of the invention may be made without departing from the scope of the invention as set forth in what is claimed.

What is claimed is:

1. In the beneficiation of low grade, iron bearing rock, the method of converting non-magnetic iron minerals into a magnetic state which comprises continuously carrying out each of the following steps: depositing a shallow gas permeable layer of non-magnetic iron ore on a traveling grate, moving said grate through successively arranged seal closed, communicating heating, reducing and cooling chambers, flowing gas through a closed circuit extending successively through the heating, cooling and reducing chambers, bringing a combustible gas into the circuit in advance of the heating chamber and burning said gas therein, passing the burning gas through the ore on the grate in the heating chamber, discarding part of the burned spent gas from the circuit after leaving the heat-ing chamber, mixing a cool gas substantially free from uncombined oxygen with the remainder of the burned spent gas and passing said mixture through the cooling and reducing chambers and through the ore on the grate in both chambers and bringing gas from the reducing chamber into the heating chamber.

2. The method set forth in claim 1 in which the grate moves through a preheating chamber in communication with said heating chamber and in which the gas from the reducing chamber is admitted into the preheating chamber.

3. The method set forth in claim 1 in which the combustible gas entering the heating chamber comprises a mixture of producer gas and air.

4. The method set forth in claim 1 in which the gas entering the heating chamber comprises producer gas, air and gas from the reducing chamber.

5. The method set forth in claim 1 in which producer gas is the cool gas which is added to the spent gas entering the cooling chamber.

6. The method set forth in claim 1 in which the gases burning in the heating chamber heat the top portions of the ore on the grate substantially to incandescence.

7. The method set forth in claim 1 in which the ore on the grate in the reducing chamber is maintained between about 1100 F. and about 1600" F.

8. The method set forth in claim 1 in which the ore on the grate in the reducing chamber is maintained at about 1200 F.

9. The method set forth in claim 1 in which the ore on the grate is cooled to below about 500 F. in the cooling chamber.

References Cited in the file of this patent UNITED STATES PATENTS 916,397 Dwight Mar. 23, 1909 1,370,915 Riveroll Mar. 8, 1921 1,832,731 Pardee Nov. 17, 1931 2,190,493 Stehli Feb. 13, 1940 2,477,454 Heath July 26, 1949 2,689,168 Dovey et a1 Sept. 14, 1954 2,693,409 Stephens Nov. 2, 1954 2,717,205 Edwards Sept. 6, 1955 2,750,272 Lellep June 12, 1956 FOREIGN PATENTS 573,539 Great Britain =No 26, 1945 707,583 Great Britain Apr. 21, 1954

Claims (1)

1. IN THE BENEFICIATION OF LOW GRADE, IRON BEARING ROCK, THE METHOD OF CONVERTING NON-MAGNETIC IRON MINERALS INTO A MAGNETIC STATE WHICH COMPRISES CONTINUOUSLY CARRYING OUT EACH OF THE FOLLOWING STEPS: DEPOSITING A SHALLOW GAS PERMEABLE LAYER OF NON-MAGNETIC IRON ORE ON A TRAVELING GRATE, MOVING SAID GRATE THROUGH SUCCESSIVELY ARRANGED SEAL CLOSED, COMMUNICATING HEATING, REDUCING AND COOLING CHAMBERS, FLOWING GAS THROUGH A CLOSED CIRCUIT EXTENDING SUCCESSIVELY THROUGH THE HEATING, COOLING AND REDUCING CHAMBERS, BRINGING A COMBUSTIBLE GAS INTO THE CIRCUIT IN ADVANCE OF THE HEATING CHAMBER AND BURNING SAID GAS THEREIN, PASSING THE BURNING GAS THROUGH THE ORE ON THE GRATE IN THE HEATING CHAMBER, DISCARDING PART OF THE BURNED SPENT GAS FROM THE CIRCUIT AFTER LEAVING THE HEATING CHAMBER, MIXING A COOL GAS SUBSTANTIALLY FREE FROM UNCOMBINED OXYGEN WITH THE REMAINDER OF THE BURNED SPENT GAS AND PASSING SAID MIXTURE THROUGH THE COOLING AND REDUCING CHAMBERS AND THROUGH THE ORE ON THE GRATE IN BOTH CHAMBERS AND BRINGING GAS FROM THE REDUCING CHAMBER INTO THE HEATING CHAMBER.

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