US3425823A - Method of improving shock temperature of metallic pellets - Google Patents

Description

United states Patent Ofice 3,425,823 Patented Feb. 4, 1969 3,425,823 METHOD OF IMPROVING SHOCK TEMPERATURE OF METALLIC PELLETS William J. Ward, Naperville, Ill., assignor to Nalco Chemical Company, Chicago, Ill., a corporation of Delaware No Drawing. Filed Dec. 5, 1966, Ser. No. 598,950 The portion of the term of the patent subsequent to Sept. 22, 1981, has been disclaimed US. Cl. 75-5 2 Claims Int. Cl. C22b 1/16 ABSTRACT OF THE DISCLOSURE The shock temperature of self-agglomerating green hematite ore pellets is improved :by treating the ore prior to pelletizing with an ammonium or alkali metal humate salt.

This invention relates to a method of improving the shock temperature of metallic green mineral and ore pellets. More particularly, this invention is concerned with the use of certain humate salts which when added to particulate masses of metals prior to pellet formation tend to increase the shock temperature of such pellets when they are fired.

In recent years, the metal refining industry has undergone striking changes in processing ores. In particular, emphasis has been placed upon production of pellets making a part or all of the ore charge since their use has resulted in immense increases in efiiciency in operation of blast furnaces. In particular, the melting of such pellets in a blast furnacecan be achieved at increased rates, with concurrent decrease in emission of small particles of flue dust which for most efficient operation must be collected and reused. Also, a permeable bed of pellets allows rapid transfer of the hot reducing gases through its entire dimension. In many instances, up to 20% increases in efficiency have been noted when ore pellets are used in the blast furnace. The reduction of iron ore and manufacture of steel are particularly improved through use of such pellets.

While in some instances relatively pure ores containing a high percentage of the valuable mineral constituent have been pelletized after minor prior processing such as crushing, of more importance is the production of pellets from relatively impure ores, such as ores having a low content of iron oxide. Since these ores of necessity must be beneficiated, that is, increased in relative proportion of desired metal content, and since after such process of beneficiation the purified mineral is already in the wetted particulate state, a pelletization process is particularly suit-able. The ore mass then can be easily and conveniently made into pellets for subsequent blast furnace use. Also, not only are the lower grade ore deposits such as finely divided hematite iron ore deposits more useful in such pelletizing process, but also t-ailings from prior ore processing, which have heretofore been discarded and stockpiled, can also be made into convenient pellet size. Thus, in the case of these relatively impure minerals or by-product fines, not only is it desirable to pelletize these materials, but in many instances it is also essential. For example, in the case of working low grade iron ores the products cannot be fed directly into a blast furnace due to the fact that the high content of impure materials would cause considerable slag with result of inefliciency of operation and repeated breakdown. Likewise, many of these relatively impure ore deposits are in such a fine state of aggregation that they would be lost through the flue of the blast furnace long before any liquification into a fluid state could take place. The same result occurs through use of flue dust, which, while it could be collected and reused, nevertheless in subsequent reuse without further processing would again be lost through the flue. Therefore, these fine particles must be processed into larger dense masses such as small integral units or pellets. Thus, it can be seen that the pelletizing process is admirably suited to use of the above type materials.

As is mentioned above, in the case of impure mineral deposits the valued or desired metal constituent in the mineral must be concentrated prior to pe'lletization. For example, many impure iron ore deposits have an iron content as low as 30% or less. These deposits must be increased in iron content to above about 60% in order for them to be economically and efliciently employed in the blast furnace as pellets.

In general, these impure ore deposits are processed to the desired wetted particulate state prior to pelletization as follows: The first step normally involves crushing or comm-inution of the raw ore as mined or in the presence of a liquid media, most generally water, by means of successive reduction steps to the final particulate stage. in the crushing operation in which the final particles are usually ground to a fine or intermediate state, two gener-al types of mechanisms are employed to apply gradual pressure to the particles. The first mechanism involves reciprocating breakers, that is, alternate approach and withdrawal of the crushing surfaces in the crushing zone to a substantially fixed predetermined minimum spacing. Reciprocating pressure brakers include such mechanisms as jaw, gyratory, cone and gyrasphere crushers. The continuous pressure breakers include rolls, single roll crushers and roller mills. Of lesser importance are impact crushers such as stamps, hammer mills and tumbling mills.

In order to concentrate the valuable or desired metal in the mineral, now in a particulate state, it is necessary to separate it from the impurities or gangue which nor mally contains a substantial amount Qf silica which is later discarded. One means by which the mineral maybe separated from the gangue is by gravity concentration since the desired metal portion of the mineral and gangue difier appreciably in specific gravity. The theory of gravity separation depends upon a difference in movement in response to joint simultaneous actions upon the mineral and impurity or gangue by gravity and one or more other forces. This type concentration may be carried out by means of pulsated, shaken, or stirred beds, water impulse separators, pneumatic concentration, etc. Normally, water is employed as the impregnating fluid. Another method of concentration of the valuable mineral constituent is by means of flotation processes. Normally these consist either of froth flotation or agglomerate or table flotation. Another excellent method of concentrating the desired constituent is by magnetic means. This is eifective only with magnetic iron oxide ores. In particular, magnetic iron ores such as taconite are admirably suited to beneficiating by treatment of the ores with electromagnets or magnets.

After the desired mineral content has been increased to a relatively high content via any of the above means, the material must normally be partially dewatered either by draining or thickening. For best pellet formation the mass of the particulate mineral substance to be processed must be in a damp or wetted condition. Normally from 5-10% by weight of Water content is required for efiicient pellet formation. In conjunction with the dewatering process, normally the beneficiated ore must be filtered to give a wet ore mass having water in the above range. This filtration may be carried out by use of continuous vacuum, sand, pressure, vacuum-leaf, centrifugal, etc., filters. After such processing, the mineral is now in the desired state for pellet formation.

The concentrated dewatered ore may be pelletized alone or flue dust mey be added thereto and the resultant mixture pelletized. Likewise moist flue dust may be pelletized singly. Generally, prior to the pellet forming step,

coke is added as a fuel media for use in the subsequent step of firing or setting the formed pellets to a hard mass suitable for mechanical handling and shipping. Normally since the bulk of the siliceous material has been removed in the separation of the gangue from valuable mineral constituent, no flux is necessary. However, in some instances a small amount of calcium carbonate-containing material, preferably limestone, may be used as a fiux. This flux intermingled in the pellet causes ore such as iron ore to melt more readily by dissolving the outside or surface impurities, thereby increasing the fluidity of the impure ore in bringing all of its components to a more intimate contact during the liquification step at the blast furnace.

In addition to sources of ore such as the low grade ores which have been processed as generally outlined above, ore flue dust or sludges may also be used. In the case of iron ore a convenient source is found in the use of sludge which is obtained in aqueous suspension or slurry, from flue gases which have been collected from blast furnace stacks, wetted in gas washers and then concentrated. Other iron ore sludges may be found, for example, in holds of iron ore barges or around iron ore shiploading areas. Also, in addition to use of flue dust as a by-product from the blast furnace melting process, cold and hot fines falling from a sinter process, and tailings from other iron ore processing may be used. For convenience sake these finely divided dry iron materials, generally in the form of impure iron oxides, may be referred to as iron dust. Again, these may be used alone, in combination with each other or with the beneficiated ore concentrate and/or the ore sludge.

In order to form an ore pellet of sufficient strength to hold its shape prior to the firing step, it is necessary, as mentioned above, that it contain sufficient water to form a damp mass suitable for formation of a cohesive pellet. Normally in the case of impure ore deposits which have been processed by the above stated steps, sufficient water remains after the filtration step so that no additional water need be added. In some cases though, it may be necessary that water be added to the material to be pelletized in order to give it more compactness in pelletforming tendency. The water may be added directly to the material at any point prior to the pelletizing, and/or it may be introduced into the system by use of ore sludge. A burden from material to be pelletized is then available in a form for ready pelletization by means of known machinery.

The pelletization step itself may be carried out by such machinery as a disc or drum pelletizer. This machinery comprises a rotating inclined surface which agglomerates the burden material composite into pellets when the burden is flowed upon the revolving inclined surface. Multiple-cone drum pelletizers are particularly desirable for the pelletizing operation. Another type of machinery available for this use is a pug mill which in its simplest form is a long trough containing two parallel counterrotating paddle-bearing shafts, horizontally mounted in side by side relationship so that the paddles on respective shafts are moving upwardly and away from each other in the center of the trough. This paddle action tends to fluff the burden mix, and cause the dampened mass to be joined into cohesive particles.

One of the most serious problems with respect to pelletizing minerals and ores of varying types is tendency of many of these materials to have a low shock temperature when fired. The shock temperature of an ore pellet may be defined as the highest temperature to which a green pellet can be exposed without disintegration. When ore materials such as hematite ores are processed into pellets useful in blast furnace operations, the green pellets tend to decrepitate during the heating step. That is, the pellets when exposed to high temperatures tend to spall, explode or fracture. Thus, such green pellets must be slowly and carefully heated just prior to induration in order to prevent such decrepitation. As can be readily seen, the entire operation of heating such pellets susceptible to decrepitation takes an excessively uneconomic amount of time due to requirement of slow and gentle heating. Green pellets which have low shock temperatures, Whether indurated in a shaft furnace, traveling grate, kiln grate, etc., thus must be processed over an excessive amount of time which, of course, materially lessens production per unit time.

In many instances, materials like hematites generally require no pelletizing aids because they are self-agglomerating. However, on the other hand, as discussed above, green pellets of hematite ores have an excessively low shock temperature. The problem of low shock temperature is of less severity with materials like taconite rock. However, these materials generally do require treatment aids to increase cohesiveness and strength of the resultant green pellets. Thus, the trend is that materials requiring pelletizing aids do not require shock temperatures improvers. The converse is also true.

A number of materials have been proposed as aids in increasing shock temperature. One of these is bentonite, a naturally occurring clay. However, this material is only effective at relatively high dosages and has the important disadvantage of adding silica to the burden. To combat this problem, additional amounts of limestone are required to remove the silica contained in the bentonite during the blast furnace operation. This silica creates large amounts of unusable and deleterious slag in the blast furnace operation. Again, chemical starch has also been proposed for this purpose. Such materials are generally quite expensive, and materially increase the cost of the over-all pelletizing process.

It would therefore be an advantage to the art if an additive for green pellets could be introduced into a burden which is to be pelletized, which additive would increase the shock temperature of the green pellet being subjected to induration. Another advantage would be realized if this additive was relatively inexpensive and would not adversely affect any desired pellet properties.

It therefore becomes an object of the invention to provide a method of increasing the shock temperature of green pellets by prior treatment of the burden before pelletization.

A specific object of the invention is to provide a method of increasing the shock temperature of hematite ores which commonly have low shock temperature regardless of source.

Other objects will appear hereinafter.

In accordance with the invention it has been found that certain humate salts are extremely useful in improving the shock temperature of green metallic mineral pellets. Generally speaking, the process of the invention comprises treating a wetted particulate mass of mineral ore with an alkali metal or ammonium humate, pelletizing said treated ore to form integral green pellet units having an increased shock temperature and decreased tendency to decrepitate under heat, and firing said formed pellet units to a hardened state while maintaining their integral character. The invention is particularly adaptable to treatment of hematite ores, normally of excessively low shock temperature.

The processing of impure mineral ores into their final hardened pellet stage generally includes the steps of comminution of a mineral ore to particulate size, and concentrating from this impure ore the valued or desired mineral constituent thereof to any desired increased purity by treating the mineral such as by gravity concentration, flotation, or magnetic separation, etc., in order to obtain a wetted particulate mass. This wetted mass is then pelletized into integral or individual pellets and the formed pellet units fired into a hardened fused state for use in blast furnace operations. The particular improvement in this method, comprising the invention, is addition of an ammonium or alkali metal humate salt during or before the pelletization step whereby green pellets of increased shock temperature are formed.

Almost any type of metallic mineral desired to be pelletized may advantageously be acted upon by the humate salts. For example, the predominant desired metal constituent of the mineral may be chosen from among lead, copper, nickel, zinc, uranium, iron, etc. Mixtures of the above or any other metal occurring in the free or molecularly combined natural state as a mineral, or any combination of the above, or other metals which are capable of pelletization may be acted upon by the humate salts.

As mentioned above, the burden or material to be pelletized may contain iron ore deposits coming directly from the mining site, from ore tailings, flue dust, cold and hot fines from a sinter process or iron ore which may be found in a sludge condition as aqueous iron ore concentrates from natural sources or recovered from various processes. For example, flue gases containing fine flue dust particles may be caught, wetted in gas washers and then concentrated by coagulation-type clarifiers into a relatively concentrated wet sludge which may be employed in the pelletizing operation. Any one of these sources of iron or any possible combination thereof may be employed according to their availability and particular process setup of the pelletizing unit, normally existing at the mine site itself. Iron ore or any of a wide variety of the following minerals may form a part of the burden: magnetite, hematite, limonite, goethite, siderite, franklinite, ilmenite, chromite, pyrrhotite, ch-olcopyrite, pyrite, etc.

Since the problem of low shock temperature is particularly prevalent in hematite ores it is greatly preferred that these ores be benefitted by the invention treatment.

To the burden may be added a flux material chosen from a number of substances. In most cases due to the fact that silica and related impurities have been removed in the processing prior to pelletization, a flux material is not needed. However, if such is necessary, a calcium carbonate-containing substance is generally employed because of availability and relatively low cost. Among these, limestone or an impure source of limestone, such as calcite are suitable. Calcite is also known as calcspar which is a hexagonal, normally colorless, rock-forming mineral composed of both crystalline species, such as iceland spar, corn spar and satin spar, and amorphous varieties including chalk, marble, limestone, stalactite and baryte. Also spongy and flake-like calcium-containing mineral forms, such as mountain milk and schifer spar may be employed.

Another element in the burden, normally essential in order to fire and fuse the formed burden pellets after pelletization, is a source of fuel. Generally coke is employed, but any other inexpensive source of fuel may be included in the operation. A particular advantage in the use of humate salts is that concurrent with its action in increasing pellet shock temperature is its ability to burn and thereby enhance the fuel value of coke or any other fuel used in the firing step. A typical sample of humate salt has a fuel value of 6,000-10,000 B.t.u./ lb. Therefore the humate binder not only helps to form pellets of requisite size and strength, but also volatilizes in the firing step thereby acting as a minor source of fuel without forming any deleterious slag deposits during this particular step of the process.

As outlined above, in order to promote compactness and adhesiveness of the pellets so that they may withstand handling subsequent to their formation and prior to the firing step, it is necessary that they be moist and in condition for ready and efficient pellet formation. It is necessary, then, that water in some form be added to the burden prior to the pelletization. In the wet processing of impure ores, whereby the desired metal constituent is increased to a usable amount, sufficient water for good compaction generally remains after the completion of this beneficiation. However, in some cases Water may be conveniently added by use of aqueous ore sludge, or may itself be added at any point in the overall pelletizing process.

It is preferred that the aqueous content of the burden prior to pelletization comprises 2-20% by weight of water based on the weight of the burden. More preferably the water content of the burden forms 5 to 10% by weight of the entire mixture. In its most favorable aspect a burden composition comprises 6-9% by weight of an aqueous liquid primarily composed of water.

The amount of hum-ate salt added to the burden may be widely varied. It has been determined that excellent results are obtained when from 0.-1-l00 pounds of ammonium or alkali metal humate salt per ton of burden are employed. More preferably, 0.5 20 pounds of humate salt are added per ton of burden with the most preferable results, from a standpoint of efficiency, and cost being obtained in the range of 0.5100.0 pounds per ton.

The humate salt may be added at any place during or prior to the pelletization operation. In the normal operating procedure the finely divided iron ore in any of its various forms and the coke, and, if necessary, flux malteriall, which components when combined with the above required amounts of water comprises a mixture known as a burden, are mixed together first. The humate may be added in at any spot of the operation, before, during or after addition of any of the components of the burden. It is preferred, however, that the burden be prepared first and that the humate salt subsequently be added in required amounts. Partial mixing may 'be effected. by transfer of the burden 'and binder, but more nearly complete mixing is effected during the balling step itself, in order to give a fairly homogeneous iron ore composite.

Any of the many well-known types of pelletizing apparatus may be used in this operation, but the preferred embodiment involves the use of what is known as a revolving disc or revolving drum-type pelletizing machine. In this type of operation the composite comprising the burden and humate salt flow over revolving surfaces, and are retained thereon for a sufficient amount of time, generally only a few seconds or so, to impart a centrifugal force to the composite and form or ball it into numerous agglomerates or pellets. These pellets spinning off the surface of the revolving drum or disc are then caught on a conveyor belt for transfer to the firing furnace. In the case of revolving discs the surfaces are normally set at an angle of inclination ranging from 40 to 60. Additional water may be added to the rotating pelletizing disc in order to promote better pellet fonmation.

As mentioned above, the ore-containing materials may also be composed of fines, hot or cold, or flue dust. In another embodiment of the invention these fines are added to the already formed pellets, and are *held in contact with the moist adhesive-type pellets prior to entrance into the ignition furnace.

The humate salts, of course, are the resultant products from reaction of a source of humic acid which is a generic term for acids derived from humus or the top layer of the soil containing organic decomposition products of vegetation, etc., with alkali metal hydroxide or ammonia. Sources of the humic acid may be from peat, brown coal,

lignite, and the like. Of course, the invention contemplates ,salts prepared from the above raw materials containing varying amounts of humic acid. In fact, it is preferred that the impure or just mined material be used as a starting reagent due to low cost, availability, and lack of need for costly processing prior to salt formation.

One of the preferred sources of humic acid as used in preparing the pelletizing aids of the invention is leonardite, often found in association with lignite. This is a specific organic substance named after A. G. Leonard who was associated with its discovery. It is considered to be more in the nature of a chemical useful in various additive processes rather than as a fuel, due to its relatively poor combustibility and low B.t.u. content per unit weight. Leonardite is primarily mined from the Harmon bed in Bowman County, ND, and Divide County, ND, and in and around Alpine, Tex. Although physically similar to lignite, leonardite has a much richer oxygen content than does lignite, ranging in oxygen content from 2733% by weight, whereas lignite contains about 1920% oxygen by weight. The high oxygen content of leonardite is ascribed to the presence of carboxylic acid and phenolic groups in the leonardite molecule. Spectral analysis has indicated that leonardite is generically speaking a mixture of humic acids and salts thereof which upon excitation for such analysis, causes certain distinctive spectral patterns to appear. Although not proved conclusively, leonardite is probably a large condensed ring polymeric molecule containing carboxyl groups.

A typical leonardite sample normally said to be con prised of calcium, sodium, magnesium, potassium, etc., salts of complex organic acid and free organic acid is partially analyzed as follows:

Ash 14.01

CH 1.26 011 0.44 CH CO 0.38

The equivalent weight of the above sample of leonardite was determined to be 256.

In order to synthesize the ammonium alkali metal humate salts of the invention it is only necessary to add an alkali metal hydroxide or ammonia to the above humates. The salt-forming reaction is preferably carried out in the presence of water. A more preferred humate salt is sodium humate and most preferably is sodium leonardite.

If desired, the above salt forming reaction may be carried out either at room temperature or at elevated temperatures. The amount of time necessary to effect the reaction is quite minimal and usually reaction is considered complete in times varying from 2-60 minutes.

The ammonium or alkali metal salts of the humic acid materials, and preferably sodium leonardite, achieve best results when the salt has been prepared by reaction of humic acid material with sodium hydroxide or potassium hydroxide in order to give a product which has a pH greater than 7.0, measured as a dispersion in Water. Preferably, samples of humic acid such as leonardite give better results as a pellet shock improving agent when they have pHs as a 10% aqueous solution, of between 8 and 12 and most preferably between 8 and 9.

The method of preparation of ammonium or alkali humate salt by reaction of the respective salt-forming ingredients may be considerably varied. A representative method comprises mixing thoroughly the humate, such as leonardite, with an alkali metal hydroxide or ammonia in water, and then allowing the salts to air-dry from the liquid media. The resultant salt is then broken up and is immediately ready for use. The mode of addition of reactants to water or to each other, is immaterial. For example, the humus material may first be dispersed in water, and the alkali metal hydroxide added thereto. Likewise, an aqueous ammonia or alkali metal hydroxide solution may be prepared, to which is added the humic acid material. During the reaction the base reacts with the carboxyl groups existing on the humic acid, forming salts having requisite water solubility. It is generally preferred that 0.1 to 1.0 equivalents of ammonia or alkali metal hydroxide is used for each equivalent of humic acid. If desired the salt-forming reaction may be carried out either at room temperature of at elevated temperatures.

To determine the efiicacy of the invention, the humate salt shock temperature aids were tested for their utility in forming green pellets having the essential character of being able to be fired or fused at relatively high temperatures without decrepitation.

A standard procedure for evaluating shock temperature of pellets was devised as follows. A hematite ore was mulled dry for 5 minutes during which time the additive was mixed in. The ore was pelletized by placing a scoop of on the rolling mass until the appropriate size seed green pellets are formed. The remaining ore is added to the seed pellets with spraying of water as needed. This process required 5 minutes.

To determine the shock temperature of the formed pellets, 10 pellets, about /2 inches in size, were placed in a single layer in a basket and inserted into a preheated furnace. Hot air at 300 standard cubic feet per minute was passed through the pellet bed for 10 minutes. The basket was removed and percent survival noted for that temperature. Table I below summarizes the data from this test.

As can be readily seen from the above data shock temperatures of the hematite green pellets were substantially increased by addition of the humate salts of the invention. In this particular case, the humate source was leonardite.

It was interesting to note results when a number of quite similar materials were also tested for ability to increase pellet shock temperature. Specifically, organic amine humates (leonardite source) were prepared and also tested for effectiveness. The additives yielded results the same as the blank, that is, no increase in shock temperature, or merely increased the shock temperature to a minimal degree. Again, in order to reach equal effectiveness reached by addition of 6 pounds per ton of sodium humate, 20 pounds per ton of bentonite had to be added to the burden. Thus, it can be readily seen that the additives of the invention have surprising activity in materially increasing shock temperature of green pellets, and particularly pellets of the hematite ore type.

It was also noted that addition of the additives of the invention did not affect other desirable properties of the green pellets such as strength. In fact, a number of cases, green pellet strength was increased somewhat through addition of humate salt.

It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

The invention is hereby claimed as follows:

1. The method of improving the shock temperature of green hematite ore pellets which comprises the steps of treating a wetted particulate mass of hematite ore with from 0.5 to 10 pounds per ton of ore of a compound 3,425,823 9 i 10 selected from ammonium and alkali metal humate, pellet- References Cited izing said treated ore to form integral green pellet units UNITED STATES PATENTS having an increased shock temperature and decreased tendency to decrepitate under heat and firing said formed 3,149,958 9/1964 Ward 755 pellet units to a hardened state while maintaining their 5 3,154,403 10/ 1964 Stickley t a1. 75 3 integral character, said hematite ore being characterized 3,266,887 8/1966 Kr e et 1, 753

as being self-agglomerating. 2. The method of claim 1 wherein said alkali humate D EWAYNE RUTLEDGE Pnmm'y Examine,

is the sodium salt of leonardite. ERNEST L. WEISE, Assistant Examiner.