US2295811A - Treatment of finely divided material - Google Patents

Description

an ore property with which I am familiar.

Patented Sept. 1 5, 1942 TREATMENT OF FINELY DIVIDED MATERIAL Percy Lea Stefiensen, Cornwall, Pa., assig'nor to Bethlehem Steel Company, a corporation of Pennsylvania No Drawing. Application June 17, 1940, Serial No. 341,020

7 Claims.

My invention relates to the preparation of materials for treatment with a gas. More specifically, my invention relates to the preparation of ores for reduction, and particularly to the preparation of finely divided ores for sintering.

More specifically my invention relates to the preparation of finely divided ores for sintering so as to provide satisfactory and substantially uniform porosity throughout the charge on the sintering machine and thereby facilitate the passage of combustion supporting air into and through the charge during and after the ignition thereof.

It is well known that inorder to satisfactorily concentrate certain ores they must first be reduced to a finely divided condition by crushing and grinding in order to free the different mineral particles from each other and from the gangue material. When it is necessary to sinter such concentrated ores in order to prepare them for. subsequent reduction, the difliculty of sintering finely divided particles limits the degree of fineness to which the ore may be ground. This limitation restricts the efiiciency and the economy of the concentration operation.

By way of example, I will refer to iron ore from This iron ore in its natural state is too low in' iron and too high in sulphur and copper to be satisfactory or economical for reduction to pig iron. Much of this ore can be concentrated after grinding to 10 mesh size to produce iron concentrates averaging about 59% iron and 015% to 0.20% copper. By finer grinding to about 65 mesh size a better separation of the mineral particles is accomplished and more of the gangue material is released. Consequently the grade of the iron concentrates can be improved to an average close to 65% iron and 0.06% copper, which is a much more valuable raw material for an iron blast furnace after sintering than sinter from the 10 mesh concentrates. However, the difiiculties encountered in sintering the 65 mesh concentrates have made it uneconomical to produce the higher grade product.

As a second example: From another section of the same ore property a large part of the ore as mined is so easily ground that in the ordina y 10 mesh grinding 55% of the concentrates are finer than 200 mesh, making the concentrates extremely diflicult to sinter. Likewise, fiue dust, resulting from blast furnace operations, is rich iniron, but is extremely difficult to sinter since at least 50% of it is finer than 50 mesh.

To make the dimculties of sintering fine mathe returns.

terial clear, I might explain that in the sintering operation, as usually practiced, the ore, fiue dust or similar material to be sintered is thoroughly mixed with suitable fuel and a certain amount of returns, which are the fines removed from the product of the sintering machine by screening. To this dry charge, while it is being mixed, or before, a suitable amount of moisture is added to produce the desired consistency of the charge. The mixture thus prepared is fed on to the grate of a sintering machine. After igniting the exposed upper surface of the charge, combustion of the fuel in the charge is maintained by air sucked through the charge from the open ignited surface to the grate below thus propagating the sintering action downwardly until the entire charge has been'more or less completely sintered. The degree of completeness of the sintering is very largely dependent on the consistency or porosity of the charge.

That portion of the returns which is coarser than the fine ore feed is depended upon to open up the charge in order to obtain the necessary porosity. But it is difiicult if not impossible, particularly when sintering very finely divided ores, to control the quantity of returns and the proportion of the desirable coarser portion of If the charge is not uniform in porosity over the sintering area the denser sections will sinter very slowly or not at all, and from these sections much unsintered material will be discharged with the product and recirculated as returns. This unsintered material in the returns, however, will be of no use in opening up the charge in which it is included but merely replaces new feed and therefore reduces the producing capacity of the sintering machine.

Moreover, attempts to increase the amount of coarse returns, by coarser screening of the previously sintered product, are not always successful. In attempts to sinter the fine concentrates described in the second example above it was found that screening out 50% to of the sintered product as returns did not result in satisfactory sintering of the chargewith which such returns were mixed.

I have found that as a result of my invention these difiiculties in sintering fine materials can be overcome and the sintering operation can be carried on as satisfactorily with fine ores as when the normal 10 mesh size limit is maintained. To accomplish this result I add to the normal charge a small percentage by weight of a suitable bulky but light combustible material for the purpose of opening up the charge uniformly to permit the preferably be less than about one inch in length and flufly or springy. The chunky shavings produced when machine planing the edge of a board, and also ordinary sawdust, are particularly undesirable.

The chunky chips are unsatisfactory becaus they have too little bulk for their weight and also because they do not burn completely during the short sintering period. Ordinary sawdust is .iifiicult to mix uniformly with a fine moist charge and it tends to close up the voids rather than increase them. However, I have found that the product of very coarse shredding saws is quite satisfactory for the purpose. The long, curly shavings similar to those produced when hand planing the fiat surface of the board are undesirable also as these shavings obstruct the passage of air instead of facilitating it.

While the size and springiness of the shavings are among the best criterions of their suitability for the present purpose, I have found that the weight of the shavings per unit of volume is an accurate indication of their suitability for this purpose. Specifically I have found that the most satisfactory pine or equivalent wood shavings have a weight of approximately between 7 and 8 pounds per cubic foot in a loose condition. Similar shavings from'heavier woods would be satisfactory but would have a greater weight per cubic foot. A greater unit weight than this for pine or equivalent shavings indicates that the shavings contain an undesirable amount of chips or sawdust while a lighter unit weight indicates that the shavings are too large.

For convenient reference, I list below the specifications of the type of shavings which I have found most suitable for my purposes:

Size.-'Ihe shavings should pass thru a 1" square mesh screen and be retained on a or A" mesh screen.

Thickness-From about $6 up to about 3%".

Weight.About '1 to 8 pounds per cubic foot in a loose condition.

W00d.-White pine, cedar and other soft woods have given the best results although some of the oaks and other hard woods may give good results if properly prepared.

Physical properties.-The shavings should preferably be planed from well seasoned lumber. The wood should not be so brittle that shavings will break up easily upon handling. Shavings should curl into small cylinders and other shapes giving maximum porosity. Good shavings should be as free as possible from dust, sawdust, chips and splinters.

As a specific example of the value of my invention, I will recite my experience in its development and in applying it to regular operations in the sintering of iron ore concentrates on a large scale.

In my first attempts to sinter, on a commercial scale under normal sintering conditions, the fine concentrates referred to in the second example above, in which approximately 55% of the concentrates were finer than 200 mesh, fair results were obtained as long as returns from previous coarse concentrate sintering were available to mix with the charge, but when returns from the sintering of these fine concentrates began to come through, the charge became tighter until finally not enough air was sucked through to maintain combustion, and sintering ceased. Coarser screening of the sintered product was resorted to, as hereinabove mentioned, in order to increase the proportion of coarse returns in the charge but it was still impossible to produce a satisfactory sinter. However, I found that with no other change from normal sintering practice than the addition to the charge of approximately between 1% and 2% by weight of wood shavings of the characteristics hereinabove described, it was possible to sinter satisfactorily such fine concentrates, and I have since been following such practice regularly. The rate of production, the time of ignition and all operating conditions, as well as the quality of the sinter have been substantially the same as when coarser concentrates are sintered.

The foregoing results are obtained with shavings of suitable characteristics as hereinbefore described. If the shavings are too heavy, on account of an excess of chips or sawdust, a greater percentage of shavings by weight must be added,-

as the chips and sawdust do not contribute to improving the porosity of the charge. But the extent to which unsuitable shavings can be utilized by increasing the proportion of shavings is limited, as too great an addition interferes with sintering. I therefore limit the amount of shavings to about 3% by weight of the-sintering charge, although this limit might have to be increased with an increase in the fineness of the material to be sintered or in its unit weight beyond those of the ores I have been sintering.

While the maximum quantity of shavings which may be used satisfactorily will not ordinarily exceed 3% by weight ofthe sintering charge, the minimum quantity may on occasion fall below 1% by weight of the charge. I have found that with shavings having optimum characteristics, the quantity used per ton of charge may not exceed 6 or 7 pounds, or approximately of 1% by weight of the charge. A certain amount of experimentation will be required in any case to determine the proper amount of shavings to be added to a given charge. However, I have indicated hereinabove the quantity which I normally use, as well as the minimum and maximum quantities which I have found it possible to employ, and the limits so specified will make it possible for anyone skilled in the art to employ, and obtain the benefits of, my invention.

Although my experimental work and subsequent operations, as hereinbefore described, have had for their object the development of means for the satisfactory sintering of finely divided iron ores by increasing the porosity of the charge, the methods which I have developed can also be applied to the preparation of fine materials through which it is desired to pass a suitable gas in order to obtain intimate contact between all portions of the bed of material and the gas for the purpose of accomplishing desired chemical reactions in the bed more completely than has been possible heretofore.

For example, certain low grade hematite or limonite iron ores may be concentrated to make them economically useful for reduction to pig iron by subjecting such hematite or limonite ores to a reduction roast followed by magnetic con- :entration. Reduction roasting is accomplished by exposing the ore at a suitable temperature between 250 C. and 500 C. to solid or gaseous reducing agents such as fine coal or carbon monoxide gas. The FezOa of the ore is thus reduced to Fe3O4 or magnetite. The efficiency of the concentration depends largely on the completeness of the con-version of the F8203 to F6304. When treating finely divided ores with a reducing gas with or without the aid of solid reducing agents mixed with the ore it is very important that the bed of ore being treated should have a porosity which will permit the reducing gas to come in contact very completely and very intimately with the iron oxide to be partially reduced. This is ordinarily quite difficult if the ore is in a finely divided state, but preparation of the ore by the methods hereinbefore disclosed will facilitate the operation very materially.

Similarly, processes have been developed for the complete reduction'of iron from its oxide ores to sponge iron by passing reducing gases through the ore, which may or may not have a solid reducing agent mixed with it, while the temperaturev of the charge is raised .to between 850 C. and 1000 C. For this operation the methods which I have developed, as described, for increasing the porosity of the charge will be of great benefit in providing for more complete exposure of the iron oxide to the reducing gas, which will result in more complete reduction of the iron oxide as desired.

The same method of preparation can be applied to advantage when drying or heating finely divided material by the passage of hot air or other gases through a bed of such fine material. For this purpose the lightweight flufiy addition would be mixed with the fine material and suitably moistened if necessary to provide the proper initial consistency before it is deposited in a bed.

The hot air or other gas can then be passed claim as new and desire to secure by Letters Patent is:

1. In the preparation of finely divided metalliferous materials for sintering, the process comprising mixing with the charge a sufficient amount of short, curly wood shavings to open up the charge and thusprovide suflicient porosity therein to permit the satisfactory sintering thereof.

2. In the preparation of finely divided metalliferous material for sintering, the process comprising mixing with the charge a quantity of short, fluffy wood shavings, said quantity comprising less than 3% of the charge by weight.

3. In the preparation of finely divided metalliferous materials for sintering, the process com-= prising mixing with the charge a quantity equr to approximately between 1% and 2% of thc charge by weight of short, fiuffy wood shavings having a weight of between 7 and 8 pounds per cubic foot.

4. In the preparation of finely divided metalliferous materials for sintering, the process comprising mixing with the charge a quantity equal to approximately between 1% and 2% of the charge by weight of a bulky, light weight,- flufly material weighing approximately 7 to 8 pounds per cubic foot.

5. A mixture for sintering comprising approximately between 98% and 99% by weight of metalliferous material, fuel and moisture, and approximately between 2% and 1% by weight of short, flufiy wood shavings.

6. A mixture for sintering comprising approximately between 98% and 99% by weight of metalliferous material, fuel and moisture, and approximately between 2% and 1% by weight of a bulky, light weight and flufiy cellulosic material.

7. A mixture for sintering comprising between 4% and 3% by weight of a bulky, light weight and fluffy cellulosic material and the remainder metalliferous material, fuel and moisture.

PERCY LEA STEFFENSEN.