US3098738A - Method of heating and sintering - Google Patents

Description

July 23, 1963 3,098,738

J. D. MADARAS METHOD OF HEATING AND SINTERING Filed Aug. 50, 1954 IN VEN TOR. Jul/U5 0 MHOARHS ATTORNEYS 3,098,733 METHOD ()1? HEATING AND SINTERING Julius D. Madaras, Longview, Mich, assignor to Gas Incorporated, Detroit, Mich, a corporation of Michigan Filed Aug. 30, 1954, Ser. No. 452,913 4 Claims. (Cl. 75-34) My invention relates to the heating and sintering of mineral materials, principally ores, that are mined or produced in such a physical state that they must be fused into larger masses for further handling, heated for further chemical treatment or otherwise subjected to an oxidizing roast. This application is a continuation in part of my copending application Serial No. 125,932, filed November 7, 1949, now abandoned. By the application of my method to these well known steps great savings in time, fuel and handling costs can be achieved. Like- Wise certain physical properties of mineral materials can be developed leaving these materials in such a condition that further processing can be performed in Ways which, though well known, have not been economically practical.

My method is limited only in its application to those materials that can be heated in the presence of burning carbonaceous, hydrocarbon, sulphur and other fuels with air or in some cases oxygen or oxygen enriched air is described.

The material to be heated is crushed, if necessary, to a size that will permit relatively free passage to air and other gases through a bed of the material or if originally in a more finely divided state, the material is agglomerated to a similar gross particle size in the approximate range of inch to 2 inches or more. The material to be heated is mixed with a fuel material such as coke breeze, powdered coal, hydrocarbon fuel or powdered sulphur in such a manner that the fuel material is more or less equally distributed throughout the mixture. In the case of sulfide ores a large part or all of the necessary fuel for heating or sintering is naturally distributed in the ore. It is important that the fuel material and all other charged materials be fixed in position in the mixture in such a manner that they shall not be separated or disintegrated (in the case of glomerules) during the subsequent steps described below.

To achieve adequate fixation in place of the mixed materials as required, I use a liquid binder which may be water or water containing carbohydrate such as wood pulp sludge Waste or other pastes. Any binders which are used in pelletizing generally are adequate. I have also used liquid hydrocarbons as a binder. In such cases the fuel content of the binder must be accounted for as described below as replacing all or part of the fuel materials which must be added to achieve desired heating.

The mixing of crushed ore, such as raw limestone, with the calculated amount of fuel and binder may be carried out in any suitable mixing device, not shown. The amount of binder to be used is best described as the minimum amount needed to wet the charged material, limestone in this case, and the finely divided carbonaceous fuel to such an extent that substantially all the fuel is evenly distributed around and adheres to the surfaces of the larger limestone ore particles. This minimum amount of binder will be diiferent for difierent binders and different minerals but for limestone and most iron ores water varies between 4% and 6% of the weight of ore. For reasons explained below Water or water producing fuel binders should be held to the The mixture is then charged into a suitably designed pressure retort.

In the drawing is illustrated a type of apparatus suitable for carrying out my process. As shown, 10 is the outer casing of a pressure retort which is suitably lined with a refractory lining =11, which in many cases may be 3,698,738 Patented July 23, 1963 omitted. .A gas-tight cover 12 is secured to the retort by means of brackets :13 and bolts 14, and may be removed when charging or removing the mixture from the retort. An inlet conduit 15 provided with a control valve 16 is connected to a source of compressed air or gas, while another conduit 17 serves as an outlet and is controlled by a valve 18. The control valve 18 serves to connect the outlet conduit to a vacuum tank 19, and may be so operated as to by-pass the vacuum tank and connect the out-let directly to an exhaust conduit 20 when desired. This valve may be of any desired construction, and may be manually or mechanically operated. It may also be automatically operated and controlled by the difference in pressure between the retort and the vacuum tank or the exhaust conduit.

For purposes of illustration, the retort is shown as charged with a mixture of limestone 21 and coke 22, although other materials may be processed by my method as will hereinafter be set forth.

After the retort is charged, hot compressed air is admitted through valve 16 while valve 18 remains closed. The hot air ignites the carbonaceous material in the charge, and after a suitable length of time valve 16 is closed to shut off the air and valve 18 is opened to permit the escape of the gaseous products of combustion through exhaust conduit 20. Valve 18 is then closed and more hot air is admitted into the retort, and the cycle is repeated. By alternately opening and closing the inlet and outlet valves, air is repeatedly injected into the charge and the products of combustion along with the other gases and vapors are exhausted. In this Way the entire charge in the retort is heated and its chemical composition is changed. In some cases only one of the valves may be operated while the other is kept open, still providing suflicient pulsation of the air through the mass to accomplish etficient heating and reaction of the charge. The nature and advantages of the invention will be made more clear by the following discussion.

Hot air burning with coke or other carbonaceous material gives a high temperature and the heat .is conducted rapidly toward the inside of the lumps of the charge. The heat gradient between the surface and the inside of the lumps is greater than can be provided if unheated air is used. The high temperature of air and coke at the place of combustion helps in igniting the coke faster and burning it more completely, however unheated air may also be used without departing from my invention.

As the lumps of limestone are heated, the carbon dioxide in gaseous form leaves the limestone leaving lumps of lime behind. As the combustion gradually proceeds downward, or upward if such a heating arrangement is desired, it will leave behind a hot layer in which the burning has been completed. Upon injecting hot air into the charge part of the air goes around the lumps or particles of the charge, but as the pressure builds up the hot air penetrates the pores of the lumps. When the pressure is released upon opening the exhaust valve, the air will burn as it passes through the hot unburned layers of coke while escaping from the retort.

Repeatedly injecting hot air into the charge eliminates the channeling of air through the mass, burns the combustible more economically, provides higher temperature and heats the charge more uniformly than do the methods commonly used,

The temperature of the hot air that is admitted through the inlet valve is further increased when it contacts the surface of the hot lumps of the charge. As the pressure builds up within the retort the air is forced into the pores of the lumps and carries heat into the lumps at a higher rate than if the heat were carried to the inside of the lumps by conduction alone. This is especially true since the outside heated part of the lumps is made more porous and less heat conducting after the outer surface has become calcined. Without the pulsating pressure provided by my method, the calcining of the lumps becomes more diflicult as the outer portion becomes progressively more porous and less heat conducting. This difiic-ulty is increased as the CO begins to form inside the lumps, since in escaping from the lumps the CO tends to conteract the effect of heat conduction and also tends to cool the outer surface of the lumps. In contrast to this, when the pressure is pulsated, hot air is forced into the lumps through the porous outer surface. The hot air that is forced into the lumps carries its heat directly to the interior of the lumps, and also serves to mechanically drive some of the heat from the outer surface directly into the lumps.

In addition, the mechanical action of forcing the air into the lumps creates pores much more rapidly than they would otherwise be created by the action of the CO trying to escape from the lumps. With my pulsating method the calcining of the limestone, that is to say, the removal of from the lumps, is accomplished partially by the heat and partially by the mechanical force of the pressure differential.

An added advantage will be derived from calcining limestone by my method Wherever the exhausted C0 is to be further used for chemical purposes. Thus the CO Will be more concentrated in the exhaust gas due to more efiicient use of air which results in less air being used and therefore less nitrogen being present in the gases. This effect may be further enhanced by enriching the air with oxygen or if desirable, by using all oxygen. In the latter instance there is no need to preheat the oxygen.

A further advantage of my method is that the temperature of calcining may be lowered to a point where there is little tendency for the lime to fuse or ilux with the earthy impurities such as alumina or silica or materials in the retort wall insulation. Furthermore, my method overcomes the objection to ordinary methods of calcining wherein a considerable part of the CaCO in the center of the lumps is not calcined and must be removed at considerable expense.

Another example illustrating the invention is that of burning or calcining cement. A predetermined mixture of ingredients for making a particular type of cement is ground and mixed together and pelletized to form small pellets or glomerules in a manner as described below. The glomerules then mixed with a proper amount of carbonaceous material, the amount of which is determined by the ultimate temperature desired. The temperatures in burning lime or cement as well as the ingredients of any particular cement are known to anyone versed in the art, and need not be specified here. The mixture is charged into the retort and burned by injecting air, preferably hot air or overventilated combustion products substantially in the manner previously described. If unheated air is used, or if necessary when using heated air, carbonaceous material may be spread over the top of the charge and ignited before closing the retort. Part of the combustible may also be powdered and mixed with the cement ingredients before pelletizing, and the balance of the combustible mixed with the charge and burned in the above described manner. After the burning is completed the charge may be cooled by injecting cold air into the retort.

Still another example consists of pretreating, heating and reducing metallic oxides, iron ore for instance. Crushed iron ore or mill scale, which may be lump, fairly fine or pelletized or agglomerated from fines, is mixed with the proper amount of carbonaceous material and a small amount of water as described below and charged into the retort. The amount of carbonaceous material, coke breeze for instance, will depend a great deal upon the heating value of the combustible and the tem perature to be obtained. In general, in the case of iron ore, approximately 2.5% to 3% coke breeze is required for heating purposes. When fusion of the mass is required, the amount may be increased to 4% or even 5%. After the vessel is charged with the mixture, hot air is admitted into the retort where it ignites and burns the combustible. As previously described, repeatedly injecting the air and exhausting the products of combustion will eliminate channeling and will help in heating the inside of the lumps thoroughly, uniformly and efiiciently. Moreover, the hot air will treat the ore by burning out all of the sulphur, and will leave the ore porous for the subsequent reduction of the ore by reducing gas.

When the iron ore is intended to be reduced to iron, it is preferable to keep the average firing temperature of the mass below the sintering, clinkering or fusing point so that the reduction of the ore with reducing gases is not made more difficult. If the fusion point is reached or exceeded, iron silicate will form and will materially hinder the reduction of the ore. Similarly, other oxides will fuse at the sintering temperature of the ore, and calcium carbonate or similar material will act as flux to lower the fusion point and thereby make subsequent reduction of the ore more diflicult.

My method, however, may be used for sintering or even fusing the entire mass of iron ore. Such fused or over sintered ore is much more dense than ordinary ore sinter, and may advantageously be used as lump iron oxide for decarbonizing the bath of steel in the open hearth or the electric furnace since the heavy, dense lumps readily sink through the molten slag covering the molten steel bath.

It should be noted that mill scale may also be sintered by my process, and fused so that it forms heavy lumps. This is particularly valuable due to the fact that mill scale is relatively pure iron oxide, and contains no slag material or only a very small amount of it. After treatment in the retort, the material may be put through a roller or press to shape it to the desired size.

If it is desired to bring about more complete melting or fusion of the mass after it is heated, reducing gas such as hydrogen or carbon monoxide is repeatedly injected into the hot charge so that it will be partially reduced. Hot air is then repeatedly injected to reoxidize the partially reduced ore, thereby producing an additional amount of heat within the charge sufficient to fuse the ore. The desired temperature can be reached by regulating the amount of combustible and the temperature of the air, as well as the extent of reduction of the ore which in turn is determined largely by the amount of reducing gas used. A more comprehensive description of the above outlined process is contained in my copending applications bearing Serial Numbers 125,931 and 125,933.

The mass of mixture charged into the retort has considerable resistance to the flow of gas therethrough. This resistance is determined mostly by the size of lumps, the fine particles in the mass, the moisture and the depth of the charge. If desired, the density of the mass may be increased at the start of heating by building up a high pressure of air within the retort. The air pressure on top of the charge can be easily regulated to provide the desired force to press down on the mass. Similarly, when greater porosity of the charge is desired the air pressure at the beginning of heating can be low until the air going through the mass has worked a certain porosity to lower the resistance to the flow of air or gas. The pressure may then be increased gradually to control the porosity of the heated or sintered mass. Also, pulsation may be started only a few minutes after the combustion of the top layer of the charge has been completed. The gas or air pressure as well as the valve operation that provides the pulsation may be mechanically controlled and varied at will, depending upon the results to be obtained.

Iron ore may also be mixed withvlime or limestone and carbonaceous combustible before being charged into the retort. This charge will be treated and heated as above described. However separating the CO from one pound of lime requires 1380 B.t.u. of heat for which fuel has to be added in addition to the fuel needed for raising the temperature of the mass. Since only about 650 B.t.u. is needed to preheat a pound of iron, the iron ore would be melted or fused too much for subsequent best reduction in solid form by reducing gas. Therefore, it is desirable in some instances to change the retort with alternate layers of iron ore mixed with its proper share of carbonaceous fuel and layers of limestone mixed with its proper share of fuel. The amount of fuel mixed with the limestone amounts to approximately twice to three times the amount required to preheat the iron ore.

The above cited applications of my method all recite the use of crushed mineral or ore materials. In each case a binder must be used for the following reasons. As described the charged mixture is subjected to passage of gases through the charge, pulsated or not as described. This gas flow, if admitted through the charge at a practical economical rate, will tend to segregate the fuel material from the ore material resulting in poor heating in pants of the charge and over sintering in other parts. The wetting of the mixture material in the mixing step sufiicient to prevent this segregation in later steps is described above.

=In addition to the use of air or oxygen in the manner described above the inlet air may be burned with a desired amount of fuel in the top of or ahead of the vessel to be passedthrough the charge at a predetermined temperature and containing sufficient excess oxygen to burn with the fuel in the charge.

In the described cases the best heating is done when the combustion air is pulsated into the mixture charge. By pulsating the pressure fresh air is brought into direct contact with the fuel in a narrow burning zone extending across the vessel as it progresses through the charge in the direction of air flow. Where the burning is restricted to a narrow zone at any particular time there is a desirable concentration of the combustion reaction. This permits or causes more intense local heat at the burning face with great savings of fuel to accomplish a given degree of heating or sintering as compared with achieving similar minimum heating throughout the charge by burning with air admitted with a steady flow.

A very uniform heating or sintering may be accomplished very quickly and efficiently by my method. With a uniform distribution of fuel and pulsated air all parts of the charge are heated equally and as quickly as the air is admitted in any cross section of the vessel. This is particularly true of sintering where a minimum fusion temperature is required in all parts of the charge. By using my method no part of the charge need be oversintered to insure sintering of all parts of the charge.

By taking advantage of this narrow zone of concentrated combustion loose mineral materials may be very loosely sintered or fused only at the contact points between the ore particles or where particles of fuel were present. This type of sintering is very easily controlled by measurement of the fuel and inlet air temperature for any type of ore. With this sintering the internal structure of the ore particles is changed very little while the entire vessel charge is fused into a semi solid mass that can withstand severe handling in later treatment. This treatment is particularly adapted to the sintering of agglomerated finely ground ores such as taconite and other flotation concentrates. In this case the glomerules need to be fixed in a permanent condition so they withstand severe handling. It is desirable to fuse the interior of the glomerules or pellet-s as little as possible to permit easier chemical treatment subsequently. 'By my method it is very easy to give these pellets a tough sintered surface of desired thickness leaving the interior unchanged.

Using taconite concentrates as an example of the use of my process on very fine minerals, I will describe the treatment to sinter them into a usable form for further smelting and the treatment to produce metallic sponge iron in .a single vessel directly. Taconite is a flotation concentrate of finely divided iron ore sized approximately 200 to 400 mesh. My method of treating is as follows: Starting with dry taconite I place it in a standard drum pelletizer and adding water I work the ore into pellets or glomerules sized in a range from 4 inch to 1", ideally /2 inch; larger pellets do not sinter as easily being too much larger than average and smaller pellets obstruct gas flow in a later step. The amount of water is critical at this step as too much water destroys the pellets and will interfere later as described below. By using an organic binder such as beer, pulp waste liquor or equivalent the necessary water may be decreased. After separating oifsize pellets I add 2% to 2 /2% by weight of coke breeze to the ore pellets. This carbon should be crushed small with respect to the size of pellets, less than 20 mesh, although coarser fuel has been used successfully. The fuel and ore are further mixed in the drum mixer until all the carbon is impressed into the surface of the damp pellets or adheres to the pellets in a substantially uniform distribution. These pellets or glomerules are now charged into a vessel described in the drawing. The pellets made in the above described manner have suflicient strength to Withstand the handling involved in transferring them from the mixer to the pressure vessel.

After the vessel 10 is charged with the pelletized ore mixture 21, 22, the cover is closed and the vessel made gas tight, preheated air or over ventilated combustion products are admitted through valve 16, if necessary incandescent charcoal on top of the charge may be used to start the sintering. Combustion products and steam are exhausted through valve 18. In the same manner as described above the sintering operation is continued to completion.

As the burning zone progresses downward through the charge the water used in making the pellets is vaporized when the combustion face approaches any particular part of the ore. This water vapor joins the stream of combustion products and moves toward the exhaust. By this method gases pulsated through solid mass are made to give up their heat to the solid most effectively. There fore, after leaving the burning zone, these combustion products cools very rapidly by heat exchange with the ore at lower levels and the water vapor in the stream condenses. As can readily be seen, this water accumulates in front of the burning zone and moves downward with it. If excessive water is used in the original mixture of ore and fuel this accumulated water will wash much of the fuel down to the bottom of the charge and the heat distribution will be poor in the lower part of the charge with over sintering at the very bottom. This problem is even more important with taconite and other fine materials that in pellet form have only wet strength and disintegrate readily in an excess of water. -I have noted that these accumulations of water can disintegrate the taconite pellets and completely cut off gas flow as described. Using Mesabi taconite pelletized with the minimum amount of water as a binder a charge depth of 4 to 5 feet is the maximum depth of charge that will permit no excessive accumulation of water when fired by this method. Certain pyritic ores require so much water in pelletizing that less than one foot of charge depth can be tolerated without disrupting the pellets and interfering with the gas flow.

It can be seen from the above paragraph that the exhaust temperature is near 212 F. as shown by the presence of accumulated water. The outlet temperature remains steady in this range until a matter of moments before completion of the heating. In a 5 foot depth of ore charge heated in 20 minutes, the finish temperature swings from a steady 200 F. to finish temperature of 1700 F. in less than one minute indicates the concentration of heat exchange and combustion zones that can be accomplished by my method of heating using a pulsating gas flow.

A modification of the above described method of treating taconites can be used to prevent accummulations of water that affect treatment of the charge. By reversing the inlet as shown in the drawing from top to bottom and the outlet from bottom to top, or in some cases not using the vessel cover and exhausting to the atmosphere directly, I can apply the same method and avoid accumulation of water. In the vessel I place the pelletized taconite fuel mixture to a depth of not more than one foot and ignite the fuel with preheated air. Immediately the temperature at the top of the charge will rise considerably above 212 F. and prevent accumulation of water at the top with excess water exhausting as vapor. By feeding in new charge material at the rate burning progresses up the vessel as measured by a constant temperature at the charge surface in the vessel, the vessel can be completely filled. By the time the vessel is filled the sintering is almost complete and the vessel may be closed as described and the hot charge subjected to further treatment. A short period of pulsating air through the charge will remove last traces of carbon or sulphur present in the ore.

I have used liquid fuels such as kerosene as a binder in making pellets suitable for sintering by my method. The procedure is the same except allowance must be made for the heat value of the fuel and an equivalent reduction made in the amount of solid fuel added. In the case of pyrites all of the carbon or other fuel may be replaced and the combustion may have to be moderated by using cooler air or by diluting the air with additional steam to keep from over sintering. In cases where the sulfur dioxide concentrations warrant it the exhaust products may be passed through sulfur recovery apparatus.

In all the above cited cases an intermediate result can be a charge of oxide ore heated to any desired temperature in a pressure vessel. An additional step in this invention would be to reduce this ore in place without further handling in the same vessel by the cyclic injection and exhausting of reducing gas through the same valves, reducing to any degree, desired or possible, under the chemical conditions available for gaseous reduction at less than fusion temperature.

I claim:

1. A method of heating sintering and reducing a mass of divided iron oxide solids comprising the steps of homogeneously mixing carbonaceous fuel with the iron oxide solid using an amount of liquid binder to provide substantial fixation in place of all elements of the mixture during the subsequent steps, charging the mixture into a pressure vessel, closing said vessel to make it gas tight, injecting air to burn with the fuel thereby raising the pressure above atmospheric, exhausting the products from the charge to lower the pressure and adding the step of cyclicly injecting reducing gas and exhausting products from the charge to reach the desired state of reduction in the charge and thereby produce sponge iron.

2. Process which comprises mixing finely divided iron ore about 200 to 400 mesh with a limited amount of water and an organic binder to form pellets of a size about A" to 1", further mixing with added coke breeze pellets into a pressure vessel having a gas inlet and a gas outlet, closing the vessel to make it gas-tight, introducing air into the gas inlet to burn with the carbonaceous ma terial thereby producing the heat to raise the temperature of said ore pellets and also raising the pressure above atmospheric, exhausting the combustion products through said gas outlet to lower the pressure, and repeating the cycle until the ore is heated to a final temperature about 1700 F.

3. Process which comprises mixing finely divided iron ore about 200 to 400 mesh with a limited amount of water and an organic binder to form pellets of a size about A" to 1", further mixing with added coke breeze to coat the surface of the pellets, charging said coated pellets into a pressure vessel having a gas inlet and a gas outlet, closing the vessel to make it gas-tight, introducing air into the gas inlet to burn with the carbonaceous material thereby producing the heat to raise the temperature of said ore pellets and also raising the pressure above atmospheric, exhausting the combustion products through said gas outlet to lower the pressure, repeating the cycle until the ore is heated to a final temperature about 1700 F., and further subjecting to cyclic injection of reducing gas followed by exhaustion of the reaction products until the ore is reduced to sponge iron.

4. Process which comprises mixing finely divided iron ore with a limited amount of Water and a binder to form pellets, further mixing with addedcarbonaceous material to coat the surface 'of the pellets, charging said coated pellets into a pressure-vessel having a gas inlet and a gas outlet, introducing air into the gas inlet to burn with the carbonaceous material thereby, producing the heat to raise the temperature of said ore pellets and also raising the pressure above atmospheric,- exhausting the combustion products through said gas outlet to lower the pressure, repeating the cycle untilthe pellets reach a desired final temperature at least about, 1700 PI, and further subjecting tocyclic injection ofreducing gas followed by exhaustion of the reaction products until the ore is reduced to sponge iron.

References Cited in the file of this patent UNITED STATES PATENTS 263,310 Browne Aug. 29, 1882 420,371 Willcox Jan. 28, 1890 434,830 Joy Aug. 19, 1890 685,064 Schubert Oct. 22, 1901 2,090,868 Hyde Aug. 24, 1937 2,243,110 Madaras May 27, 1941 2,450,343 Howard Sept. 28, 1948 2,468,738 Durfee et al. May 3, 1949 2,548,876 De Jahn Apr. 17, 1951 2,666,632 Culver et al Jan. 19, 1954 FOREIGN PATENTS 3,328 Great Britain 1885 24,803 Great Britain 1898

Claims (1)

1. 2. PROCESS WHICH COMPRISES MIXING FINELY DIVIDED IRON ORE ABOUT 200 TO 400 MESH WITH A LIMITED AMOUNT OF WATER AND AN ORGANIC BINDER TO FORM PELLETS OF A SIZE ABOUT 1/4" TO 1", FURTHER MIXING WITH ADDED COKE BREEZE TO COAT THE SURFACE OF THE PELLETS, CHARGING SAID COATED PELLETS INTO A PRESSURE VESSEL HAVING A GAS INLET AND A GAS OUTLET, CLOSING THE VESSEL TO MAKE IT GAS-TIGHT, INTRODUCING AIR INTO THE GAS INLET TO BURN WITH THE CARBONACEOUS MATERIAL THEREBY PRODUCING THE HEAT TO RAISE THE TEMPERATURE OF SAID ORE PELLETS AND ALSO RAISING THE PRESSURE ABOVE ATMOSPHEREIC, EXHAUSTING THE COMBINATION PRODUCTS THROUGH SAID GAS OUTLET TO LOWER THE PRESSURE, AND REPEATING THE CYCLE UNTIL ORE IS HEATED TO A FINAL TEMPERATURE ABOUT 1700* F.

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