US3836354A

US3836354A - Production of pellets

Info

Publication number: US3836354A
Application number: US00360640A
Authority: US
Inventors: F Wienert
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-06-25
Filing date: 1973-05-16
Publication date: 1974-09-17
Anticipated expiration: 1991-09-17

Abstract

Durable pellets of a uniform desired size and uniform dense particle packing are produced from a mixture of a liquid, and if desired, fusible and pyrolyzible hydrocarbons and a binder, with finely divided particles of carbon-reducible oxides or oxidic materials, carbon, and mixtures thereof, the particles being in sizes favorable to dense packing. Such a mixture is spread on a horizontal surface to form a layer of uniform desired thickness which is consolidated, by moving a vibrating face under a load in contact with and over the upper surface of said layer, to a densely packed plastic layer of uniform thickness, a slight amount of the liquid being exuded and wetting the vibrating face. The layer is then divided into cube-like bodies of equal size which are rounded by tumbling to pellets of uniform size and uniform dense particle packing. The pellets may be dried for subsequent heat induration or partially dried and impregnated with a binder solution prior to heat induration, or heated for bonding by hydrocarbons and their pyrolysis residues when hydrocarbons are employed in the mixture from which the pellets are formed.

Description

tlnit States atet [1 1 Wienert [451 Sept. 17, 1974 PRODUCTION OF PlElLlLlETS [76] Inventor: Fritz 0. Wienert, 394 Roosevelt Ave., Niagara Falls, NY. 14305 [22] Filed: May 16, 1973 [21] Appl. No.: 360,640

Related US. Application Data [63] Continuation-impart of Ser. No. 156,910, June 25, 1971, abandoned, which is a continuation-in-part of Ser. No. 785,132, Dec. 19, 1968, abandoned.

2,145,899 2/1939 Simpson... 264/15 2,164,950 7/1939 Schulze 2,543,898 3/1951 DeVaney.....

2,869,850 l/1959 Wienert 75/3 3,307,927 3/1967 Muschenborn et a1. 75/3 X 3,400,179 9/1968 Wienert 264/15 3,608,003 9/1971 Klaue et a1 264/70 X FOREIGN PATENTS OR APPLICATIONS 780,667 8/1957 Great Britain 264/70 Primary Examiner-A. B. Curtis Attorney, Agent, or Firm-Ashlan F. Harlan, Jr.

[57] ABSTRAT Durable pellets of a uniform desired size and uniform dense particle packing are produced from a mixture of a liquid, and if desired, fusible and pyrolyzible hydrocarbons and a binder, with finely divided particles of carbon-reducible oxides or oxidic materials, carbon, and mixtures thereof, the particles being in sizes favorable to dense packing. Such a mixture is spread on a horizontal surface to form a layer of uniform desired thickness which is consolidated, by moving a vibrating face under a load in contact with and over the upper surface of said layer, to a densely packed plastic layer of uniform thickness, a slight amount of the liquid being exuded and wetting the vibrating face. The layer is then divided into cube-like bodies of equal size which are rounded by tumbling to pellets of uniform size and uniform dense particle packing. The pellets may be dried for subsequent heat induration or partially dried and impregnated with a binder solution prior to heat induration, or heated for bonding by hydrocarbons and their pyrolysis residues when hydrocarbons are employed in the mixture from which the pellets are formed.

23 Claims, 3 Drawing Figures PATEN TH) 35? 1 71974 3, B86 354 PRODUCTION OF PELLETS This application is in part a continuation of my copending application Ser. No. 156,910, filed June 25, 1971, which was in part a continuation of my copending application Ser. No. 785,132, filed Dec. 19, 1968, both now abandoned.

BACKGROUND OF THE INVENTION This invention relates to pellets and particularly to durable metallurgical pellets which comprise particles of a metallurgical material of the group consisting of carbon-reducible oxides and/or carbon and is particularly concerned with the production of such pellets.

It has previously been proposed to produce metallurgical briquettes and pellets in which ore particles are bonded with carbon. Little, if any, commercial success has been achieved with pellets so bonded. It has been found that when the ore contains substantial amounts of finely divided oxides, such as those of trivalent iron and quadrivalent manganese which are easily reduced by carbon monoxide and/or hydrogen, the pellets when heated in a reducing atmosphere become weak and tend to disintegrate. Similar results have been obtained with briquettes. Moreover, experience has shown generally that the production of pellets requires very fine grinding of the constituent materials, in many cases grinding to such a fineness that the materials pass through a 325 mesh screen, although in applicants US. Pat. No. 3,400,179, there is disclosed a process for producing pellets from materials containing some coarse particles. However, pellets could not be formed in a uniform size by such process.

SUMMARY OF THE INVENTION The present invention provides not only an improved process for forming pellets, particularly metallurgical pellets, but also novel procedure for indurating them for metallurgical processes. In forming pellets by the process of the present invention a mixture of particles having a particle size distribution favorable to dense packing is blended with a small, predetermined amount of liquid and theresultant moist mixture is thereafter spread on a horizontal surface to form a relatively loose layer of a uniform thickness yielding, after consolidation to densely pack the particles, a dense layer of the desired thickness. Such a loose layer is then consolidated to a plastic layer of uniform thickness and uniform dense particle packing by moving one or more vibrating faces under load in contact with the upper surface of the layer over said surface thus causing the exudation of a slight amount of said liquid on said surface and, consequently, wetting of the vibrating faces. The resulting densely packed plastic layer is then cut longitudinally and transversely into cube-like bodies which are discharged from the supporting surface and rounded by tumbling to pellets of uniform size and uniform dense particle packing.

In regard to the predetermined amount of liquid in the mixture it was found that with an amount such that there was insufficient exudation sticking of the moist material to the vibrated face occurred. Furthermore, such short" layer was not plastic enough and could not be divided into cube-like bodies without causing cracks in and breakage of such bodies. When bodies with cracks were tumbled for rounding they broke to pieces and pellets of various sizes were formed what is contrary to the present process. On the other hand, if

too much liquid was used the bodies formed by cutting stuck to each other and formed large aggregates as soon as they were tumbled.

The proper amount of exuded liquid was found to be that which just wets the vibrating face whereby sticking to the latter was prevented. Consequently the amount of liquid necessary to achieve the desired plasticity in the mass, since it varies with the materials being pelletized, the particle size distribution of the materials, and other factors, must be finally determined by experiment in each case. When such a layer was cut into cube-like bodies the exuded liquid was re-absorbed in the bodies. This indicates that the dense packing of the particles was disturbed by the cutting. However, when the bodies are tumbled for rounding dense packing of the particles was restored and liquid was exuded again on the surface of the bodies while-being rounded. With certain materials, for instance, when the bodies were made from finely ground coal particles there was a tendency for them to adhere to each other in the course of rounding. This was prevented by evaporating some liquid during tumbling. In other cases, some excessive liquid was deliberately exuded during tumbling and dry powder was added to form a coat around the pellets. Pellets comprising essentially only one or more carbonreducible oxides or oxidic materials can be produced as well as pellets comprising a mixture of oxides and carbon and pellets comprising essentially only carbon. The liquid used is preferably water. I

The use of vibratory means is essential for carrying out the invention. The solid material used in forming pellets is a collection of particles of various sizes, each with a film of liquid, preferably water, covering it. Plasticity is achieved by decreasing the number and size of voids and thus bringing the particles into closer contact. Then the film of liquid is thin in the areas of contact so that the particles are held together with sur face tensional forces. With dense packing of various sized particles, each particle is in close contact with others at many points and plasticity is achieved, the excess liquid being exuded from the mass. Vibration of the moist mass is essential, firstly, to achieve dense packing of the particles and thereby obtain uniform bulk density thereof, secondly, to minimize the amount of liquid held in the mass of particles, and thirdly, to produce plasticity of the mass.

As set forth above, small cube-like bodies of essentially uniform size are formed from the plastic mass of solid particles and liquid and these are rounded to form pellets of uniform size and uniform dense particle packing. The rounding may be carried out in any desired way, in some cases tumbling in a rotary drum being reasonably satisfactory. It is preferred, however, to utilize a shaking table which may be oscillated in a reciprocating'manner or with an oval or circular motion and which preferably is also vibrated. The bodies when freely moving thereon are rounded with great efficiency.

Induration of the rounded, green pellets thus formed may be accomplished in severaal ways. Initially, they are at least partially dried. Pellets comprising essentially only oxides may then be indurated by heating in an oxidizing atmosphere as is conventionally done with iron oxide pellets. Where the pellets contain more than a few percent of carbon, however, they are preferably indurated by impregnating them, after partial drying, with asolution of a binder and thereafter completing the drying. The latter procedure can also be used, if desired, with pellets which contain essentially only oxides or oxidic materials.

An advantage of the present novel process is that it does not require all of the materials in the pellets to be very finely divided. Conventional pelletizing processes, such as balling, must avoid large particles because they will not adhere to a growing pellet. By first producing a dense, plastic mass and then making pellets therefrom, this problem is completely avoided. Satisfactory pellets according to the invention can be made from materials in which over 60 percent of the particles are larger than 100 mesh. The expense involved in fine grinding for conventional pelletizing processes is thus avoided. Moreover, since heat stability of the pellets appears to be aided by the presence of larger particles, pellets according to the invention are not so readily broken down during heating in a rotary furnace. While employing materials having a relatively wide range of particle sizes is advantageous, his not essential, as pellets can also be produced when all of the particles are very fine, e.g. 200 mesh.

The present invention presents certain advantages over the process for forming pellets disclosed in my U.S. Pat. No. 3,400,179, mentioned above. In said patent, the moist mixture is filled into molds and then vibrated to form dense, plastic bodies. In the present invention, the moist mixture is vibrated so as to become a plastic and uniformly dense layer which is then divided into bodies which are of essentially uniform size and are subsequently rounded into pellets. It will be appreciated that the instant procedure is adapted for the use of very large scale equipment, since the plastic layer can be formed in a great width. The expense of molds is eliminated, and the problems of filling and emptying them are avoided. In addition, with the present invention it is easier and less expensive to change the volume of the small bodies which are rounded to form pellets.

DESCRIPTION OF THE DRAWINGS In the accompanying drawings apparatus suitablefor carrying out the process of the present invention in a continuous operation is shown schematically.

FIG. 1 illustrates apparatus for compacting a mixture of particles and a liquid into a dense, plastic layer, cutting cube-like bodies from said mass, and rounding said bodies to form pellets;

FIG. 2 is a fragmentary plan view of a portion of a shaking table for rounding pellets; and

FIG. 3 is profile view of a portion of the surface of the table shown in FIG. 2.

In FIG. 1, ll designates a moist mixture, of the type described, to be pelletized by the present process. The mix, conveniently, has been deposited by a swinging chute 5 leading from a adjustable rate feeder (not shown) on the endless belt 13 where it forms a loose layer with an irregular surface. The belt 13, which can be formed of any suitable material, e.g. rubber or steel, passes around and is carried by the longitudinally spaced rolls l5 and 17, either or both of which may be driven by suitable means, e.g. a motor (not shown). The belt 13 is also supported, in its upper flight, by a plurality of longitudinally arrayed idler rolls 19. The loose mixture 11 is spread more uniformly over the width of the belt 13 and consolidated to some extent into a layer, which is, however, not dense, by a rotating roll 21. The roll 21 is driven, by means not shown, at a circumferential speed faster than the surface speed of belt 13. If desired, the roll 21 may be loaded by a spring or weight (not shown). Suitable side-walls (not shown) are preferably provided to retain the mixture on the belt 13. To further level the layer and consolidate it into denser packing, whereby some of the liquid in the mixture is exuded on the surface of the layer, a flap 6 is provided. The flap 6 is carried on a hinge or pivot rod 7 and extends across the width of the layer of mixture in a generally horizontal direction with the lower face ofthe flap contacting the upper surface of the layer. On the upper face of the flap there is mounted an adjustable vibrator 8, preferably electrically operated and, if desired, additional force can be applied by a spring or weight (not shown) acting downwardly on the flap. The flap may be conveniently formed from metal, such as stainless steel. Use the flap is optional in forming pellets from ore-containing mixes but such use is usually helpful in forming coke pellets and the like where dense packing is less easily achieved.

The somewhat compacted layer emerging from under the flap 6, when one is used, is further densified, which is accompanied by exudation of liquid from the mixture, by a pair of rolls 27. The latter are mounted on a carriage 25 which is reciprocated longitudinally of the belt 3 (see arrows 29) by suitable means (not shown) in such manner that the rolls contact the layer of moist mixture. Densification of the layer is facilitated by an adjustable vibrator 31, preferably electrically driven, mounted on the carriage 25, the vibration being conducted through the rolls 27 to the mixture layer through the upper surface of the layer. If desired, additional pressure can be applied to the layer through the rolls 27 by a spring or weight (not shown) acting downwardly on the carriage 25. The reciprocating speed of the carriage 25 should be such as to minimize variations in thickness of the layer and the roll surfaces or faces and the lower face of the flap 6 should impart vibration to the layer.

The densely packed plastic layer leaving the zone of the reciprocating rolls is cut, without major disturbance because of its plasticity, longitudinally by two staggered rows of rotary cutting disks 33 into strips the width of which is preferably, approximately equal to the thickness of the layer. The strips are then cut transversely by a flying knife 35, of any conventional or desired design, into a plurality of cube-like bodies 37. It will be understood that a set of rotary cutting disks similar to those designated by 33 may be used for transverse cutting, although this requires travel of the disks with the belt.

The cube-like bodies 37 are discharged onto a shaking table 39 for rounding. The table 39 is oscillated in the directions shown by the arrows 41 and is also slightly inclined or sloped towards the discharge end. In moving down the slope the bodies collide with each other frequently thereby becoming rounded into pellets 53 which are discharged from the table. Preferably the table 39 is provided with suitable vibrating means 43. It is also preferred that the surface of the shaking table 39 be provided with a plurality of upstanding hilly protrusions or knobs 51. As shown in FIGS. 2 and 3 these knobs are distributed or arranged in a regular pattern and are preferably of less height than the diameter of a pellet, a typical pellet being illustrated at 53 in FIG. 3. Similar knobs have been found advantageous on the surface of other tumbling apparatus such as a rotary drum. Such knobs facilitate rounding by increasing the irregularity of the pellet paths.

Various modifications can be made in the illustrated apparatus without departing from the spirit of the present invention. For example, although a shaking table is preferred for rounding the cube-like bodies into pellets, other tumbling means may be employed such, for example, as a rotary drum.

In some cases, during tumbling of the cube-like bodies to form pellets an excessive amount of moisture may be exuded from the bodies. Such excess moisture may be evaporated by passing hot gas over the shaking table or through the tumbling drum or may be absorbed by dry, fine, particulate material. The latter may be spread over the tumbling bodies in any desired manner and forms a coating or shell on the pellets. If desired, successive coatings of dry material and moisture maybe employed thereby building up the size of the pellets. The shells may have the same composition as the inner part of the pellets or a different composition.

DESCRIPTION OF PREFERRED EMBODIMENTS In the first two of the following examples the present novel process is employed in producing and indurating green pellets formed from a mixture of coarse and fine iron oxide particles. Such pellets after induration are desirable for use as a charge for blast furnaces.

EXAMPLE 1 A relatively coarse magnetite concentrate containing 64 percent Fe as oxide and having the following size distribution was used:

mesh, 32 mesh 7.4% 32 mesh, 80 mesh 41.8% 80 mesh, 150 mesh 22.5% 150 mesh, 200 mesh 10.2% 200 mesh 18.1%

100 parts of this ore concentrate was mixed with 0.75 parts of bentonite and 1 1.5 parts of water. The mixture was spread on a horizontal surface to form a loose layer which was densified, by moving a vibrating face under slight pressure in contact with the upper surface of the layer, into a layer approximately three-fourth in. (19 mm) thick. As a result of such vibration and pressure the surface of the layer glistened because of the water exuded as the ore particles packed into a dense, plastic or malleable mass and the vibrating face was wetted by the exuded water. The layer was then cut into A in. X in. squares, thus forming a plurality of approximately cubic bodies in. X A in. X A in. These bodies were rounded by tumbling in a rotary drum until substantially spherical pellets were produced.

The resulting green pellets wee dried and indurated in a conventional manner by blowing hot oxidizing flame gases through a bed of the pellets, thereby heating them to about 1,300C.

The magnetite ore concentrate used above was not suitable for conventional formation of pellets by the socalled balling action as was demonstrated by charging some of the dry mixture of ore concentrate and benton' ite gradually onto an inclined rotary disk while spraying fine droplets of water on the charge. It was observed that the coarse particles did not cohere to form balls. It has been common experience that, for conventional processes of pelletizing, a major portion of the particles must be finer than 200 mesh and preferably finer than EXAMPLE 2 A relatively coarse iron ore of the so-called lateritic type composed essentially of hermatite, goethite, and limonite was used. This ore, containing 58.1 percent Fe as oxide, had the following size distribution:

16 mesh 1.0% 16 mesh, 32 mesh 32 mesh, 48 mesh 17.0% 48 mesh. mesh 17.0% 100 mesh. 200 mesh 12.0% 200 mesh, 325 mesh 3.0% 325 mesh, 41.0%

100 parts of this ore was mixed with 0.6 parts of bentonite. Of this mixture 98.6 parts was mixed with 12.8 parts of water. The moist mixture was spread on a horizontal surface where it was formed, under the influence of a vibrating face exerting pressure on the layer as described in Example 1, into a densely packed layer approximately three-fourths in. thick. When the surface of the layer was glistening due to the exudation of water therefrom, the layer was cut into a plurality of cubelike bodies which were rounded by tumbling on a shaking table until they closely approached spherical shape.

During the tumbling, water was exuded from the surface of the bodies and 2 parts of the dry ore bentonite mixture described above was distributed over the tumbling bodies and adhered to their surface because of the film of moisture thereon. The finished pellets were dried and indurated by heating to about 1,275C., in an oxidizing atmosphere. The indurated pellets had an apparent density of 3.58 g/cm.

The following example illustrates the making of pellets from a magnetite ore with carbon particles uniformly distributed throughout the pellets in such proportion that upon heating the pellets to temperatures higher than about 900C, substantially all of the magnetite is reduced to metallic iron.

EXAMPLE 3 A magnetite ore concentrate with 59.2% Fe as oxide and the following size distribution was used:

65 mesh 52% 65 mesh, 100 mesh 16% 100 mesh 32% Sixty parts of the concentrate was mixed with 11.9 parts of a bituminous coal having 67.9 percent fixed carbon and 29.7 percent volatile matter and the mixture was ground so that 97 percent passed a 100 mesh sieve. The ground mixture was moistened with 16 parts of a 10 percent aqueous solution of sodium silicate. The moist mass was converted to a dense, plastic layer three-fourths in. thick as described in Example 1. This layer was cut into a plurality of substantially cubic bodies of uniform size one portion of which was tumbled in a rotary drum to round them to substantially spherical shape. It took approximately minutes in a specific rotary drum having a smooth inner wall to obtain spherical pellets. When the interior surface of the same drum was provided with hilly protrusions or knobs like those shown at 51, in FIGS. 2 and 3, rounding was accelerated and substantially spherical pellets were produced from the other portion of cube-like bodies in 2 minutes.

Upon heating the dried pellets to a maximum temperature of 1200C. in a rotary furnace heated internally by combustion and thereafter cooling them in an inert atmosphere, it was found that the magnetite was reduced, the pellets then containing 73.65 percent total Fe, 70.1 percent metallic Fe, and 1.4 percent carbon. They had shrunk and were about 10 percent smaller in diameter than before reduction.

The following example illustrates the use of a relatively coarse, titaniferous magnetite ore in the production of pellets which contain carbon particles uniformly distributed therethrough in such proportion that upon heating the pellets in a rotary kiln fired with combustion gases, the major part of the iron oxide is reduced to metallic iron and sufficient carbon is left for reducing and combining with the titanium content when the kiln-reduced pellets are smelted in an electric arc furnace.

EXAMPLE 4 The titaniferous magnetite used contained 48.6% Fe as oxide and 19.7% TiO and had the following size distribution:

mesh 1% 20 mesh. 65 mesh 65 mesh, 100 mesh 15% 100 mesh 54% The required amount of carbon was provided by a bituminous coal containing 78.04 percent fixed carbon, 15.9 percent volatile matter, and 5.9 percent ash, which was crushed to pass a 12 mesh screen. 33 parts of the coal was mixed with 100 parts of the ground ore and 2.7 parts of fine silica (l00 mesh), the latter to serve as a flux during electric smelting. The mixture was ground in a ball mill until about 95 percent of the coal passed a 100 mesh screen, the hard particles of the ore not being noticeably comminuted. However, the ore and silica particles had prevented the coal particles from caking together as they did when ground alone, without the ore and silica. The ore-coal mixture was moistened with 26 parts of an 8 percent aqueous solution of sodium silicate. The moist mass was spread on a surface and was then transformed into a densely packed plastic layer by vibration and cut into small, approximately cubic bodies of substantially equal size as described in Example 1. Some of these cubes, approximately /1 in. X in. X in. were tumbled on a shaking table to round them to pellets while 5 parts of said dry ore-coal mixture was spread over the tumbling bodies during approximately 5 minutes and was taken up by the moisture exuded from the bodies to form a coating or shell on the pellets produced. When, however, another portion of the cubes was tumbled on the shaking table while vibration was applied to the table there was such an increased exudation of water that 7 parts of the dry mixture spread over the bodies during approximately 4 minutes was taken up as a coating when the table was vibrated as well as shaken.

The green pellets rounded in both ways were dried before heating. The dried pellets after heating in a rotary furnace with combustion gases to a maximum temperature of 1,275C. and cooling in an inert atmosphere were found to contain 47.36 percent total Fe, 43.26 percent metallic Fe, and 10.82 percent fixed C and to be very satisfactory for smelting in an electric arc furnace.

The novel process of the present invention is also useful in forming pellets from partially pre-reduced metal oxides. For example, hematite and manganese dioxide are so easily reduced to lower oxides by such gases as CO and H or by fine carbonaceous particles that pellets made from them by conventional processes turn soft and disintegrate while they are being heated to the temperatures required to reduce the oxides to metal. This can be avoided by partially pre-reducing the easily reducible oxides before forming pellets therefrom. The production of pellets suitable for reduction to metal by heating to temperatures above about 900C. is illustrated in the succeeding two examples.

EXAMPLE 5 A dried Brazilian hematite containing 97 percent Fe- O (67.8 percent Fe) was used. The ore had the following size distribution:

12 mesh, 20 mesh 3.8% 20 mesh, 32 mesh 13.0% 32 mesh, 60 mesh 13.0% 60 mesh, 100 mesh 6.5% 100 mesh, mesh 3.4% 150 mesh, 200 mesh 5.0%

200 mesh 55.3%

100 parts of the ore was pre-reduced with the reducing combustion products from bituminous coal until there was a weight loss of 8 percent. The resulting partially pre-reduced oxide was mixed with 18 parts of a 32 mesh char resulting from the incomplete combustion of bituminous coal, and 13 parts of a 10 percent aqueous solution of sodium silicate. The moist mixture was spread on a surface and was then converted to a densely packed plastic layer about three-fourths in. thick by vibration and cut into cube-like bodies of equal volume which were rounded by tumbling in a rotary drum, as in Example 1.

The green pellets were dried and after being heated to 1,250C. in a rotary furnace heated by internal combustion and being cooled in an inert atmosphere, the pellets were found to contain 98 percent of the iron content in the metallic state and little disintegration of pellets had occurred.

EXAMPLE 6 The iron oxide material used was a blend of the socalled implant fines of an integrated steel plant. This material included dust from a blast furnace, very fine dust from the oxygen blow of a basic oxygen steel furnace, mil] scale, and other waste ferrous material such as scarfings, grindings, and the like. To this was added coke breeze. The dry mixture was ball milled, mainly for mixing, about 15 minutes and thereafter found to have a size distribution as follows:

Va inch, 32 mesh 8.0% 32 mesh, 42 mesh 3.0% 42 mesh, 80 mesh 21.0%

-Contmued 80 mesh, 150 mesh 21.0% l50 mesh. 200 mesh 12.2% 200 mesh 34.8%

100 parts of the dry mixture, which analyzed 53.0 percent total Fe, 37 percent trivalent Fe, 3 percent metallic Fe, and 11.4 percent C. was heated in a mixture of N C0, C H and H 0 to partially pre-reduce the ferric oxide.

After cooling, it was found that the mixture had lost weight and amounted to only 94.1 parts. The cooled mixture was blended with 19.5 parts of an 8 percent aqueous solution of sodium silicate and the resulting moist mixture was formed into a dense plastic layer about three-fourths in. thick by vibration on a surface as in Example 1. The layer was cut into small blocks of equal volume and approximately cubic in shape, which were tumbled in a rotary drum to form substantially spherical pellets. These were dried in a bed by blowing hot gas at about 200C. through the bed. Upon being heated in an oil-fired rotary furnace to 1,250C. and cooled in an inert atmosphere the pellets were found to contain 75.1 percent total Fe, 73.4 percent metallic iron, and 1.23 percent C. and were not substantially broken.

The present novel process for making pellets is especially useful if pellets are to be made from carbonaceous particles which are difficult to wet so that the conventional balling action would result in slow and irregular growth of pellets. For instance, the new process may be applied to the production of so-called metallized coke pellets of a pre-determined, uniform size for the blast furnace. Such pellets, made from ore fines and fine ore concentrates and particles of carbonaceous matter of sizes not suitable directly for the blast furnace, will improve the production rate of the furnace. The following example illustrates the formation of such pellets.

EXAMPLE 7 A mixture was made of 5 parts of coke breeze containing 86 percent fixed carbon, 5 parts of blast furnace dust containing 45.6 percent Fe as oxide and 10.3 percent fixed carbon, and 7 parts of a bituminous coal containing 61.45 percent fixed carbon and 33 percent volatile matter. The mixture was ground until approximately 95 percent passed a 150 mesh sieve. 17 parts of the powdered mixture was moistened with 6.2 parts of a 0.2 percent aqueous solution of a surfactant of the anionic wetting agent type and the moist mass obtained was spread on a surface and,as described in Example 1, subjected to vibration and pressure to produce a densely packed, plastic layer which was divided by knives into small cube-like bodies of equal size. Substantially spherical pellets were formed by tumbling the bodies in a rotary drum while gas at about 150C. was blown over the tumbling bodies to evaporate exuded water.

When the'surfaces of the pellets became dull, i.e., glistening was no longer observed, the still damp pellets were laid on a heat-resistant, gas-permeable support to form a bed several pellets thick and combustion gases having less than 2 percent free oxygen and a temperature increasing from about 140C. to about 550C. were blown through the bed, alternately downwardly and upwardly, whereby the water was first evaporated and, as the heating was continued, the bituminous coal particles within the pellets were transiently fused and then carbonized thereby bonding the pellets. The carbonized pellets were discharged from the support and heated further to about 1,000C. in a rotary kiln heated by combustion gases. The hot pellets were cooled in an inert atmosphere. The cold pellets had a high crush strength and contained 64 percent C. and 17 percent metallic Fe. A metallic screen, grate, or the like may be used as a support for the pellets during drying and carbonizing.

It has also been found that coke pellets of metallurgical grade can be made by the present novel process from carbonaceous materials which are not suitable for the conventional methods of producing metallurgical coke. The following two examples provide illustrations of such pellet production.

EXAMPLE 8 The coal used in this example was not suitable for conventional coking procedures because it did not fuse and swell when heated. The dry coal contained 74.7 percent fixed carbon, 16.56 percent volatile matter, and 8.2 percent ash.

parts of the coal was ground to pass a 32 mesh screen, mixed with 37 parts of water containing 0.1 percent of a surfactant of the anionic wetting agent type, and spread on a flatsurface. The resulting moist, loose layer was vibrated as in Example 1 to form a three-fourths in. thick dense plastic layer and cut into approximately cubic bodies of equal size. The bodies were tumbled to form pellets on a shaking table provided with vibration and the pellets were dried and carbonized as described in Example 7. The carbonized pellets had shrunk in size during carbonization and had an apparent density of 1.1 1 g/cm They contained 86.9 percent fixed carbon and 9.8 percent ash. Their average crush load or compressive strength as determined on single pellets with the help of a hydraulic piston was 315 lbs.

Although, as shown above, very good pellets were formed from a rather coarse coal, finer grinding is in some cases desirable because it results in increased pellet strength. Thus, following the same procedures, coal was ground to minus 100 mesh and converted to pellets which had a compressive strength of roughly 500 lbs. They were quite suitable for use as part of a charge for a blast furnace or a cupola.

EXAMPLE 9 The same coke breeze and bituminous coal employed in Example 7 were used. 10 parts of the coke breeze (-35 mesh) and 7 parts of the coal (-28 mesh) were mixed with 0.9 parts of coal tar, the: latter being roughly distributed in the powdered mixture by agitation. The 6.3 parts of water containing 0.2 percent of a sulfonated alcohol type surfactant was mixed with the carbon-tar blend. A moist mass with the tar well distributed therein as a film on the solid particles was thus obtained. It was spread on a flat surface to form a loosely packed moist layer.

The layer was vibrated and formed into pellets, and the pellets were dried and carbonized, all in the same manner as described in Example 8. The cooled, carbonized pellets had a crush strength of about 220 lbs.

When coke breeze or char is bonded by a fusible bituminous coal without the use of tar, wetting agent or surfactant, the fine particles desirable for a high compressive strength are easier to achieve if the coke breeze or char is mixed with the bituminous coal before grinding. It is known that fine particles of certain bituminous coals tend to compact together during grinding, and that coke and char are difficult to grind finely, due to the hardness of such particles. The following example illustrates this.

EXAMPLE 1O 24 parts of the same coke breeze as used in Example 7 were crushed to pass a 12 mesh sieve and then ground in a mill containing steel balls for 34 minutes. The screen analysis of the product was:

18.4% 100 mesh Tyler screen 40.1% 170 mesh Tyler screen 41.5% 170 mesh Tyler screen Another 24 parts of the same coke breeze passing a 12 mesh screen were mixed with 16 parts of the same bituminous coal used in Example 7 and passing a 12 mesh screen. The mixture was also ground in the same mill and under the same conditions and the screen analysis of the resulting product was:

4.1% 100 mesh Tyler screen 32.2% 170 mesh Tyler screen 63.7% 170 mesh Tyler screen The particles remaining on the 170 screen were coke particles. This demonstrates the relative hardness of coke compared with bituminous coal and that coke is ground more easily in admixture with coal. The ground mixture was free flowing. It was moistened with 17.5 parts of water, shaped into green pellets and converted into coke pellets in the same manner as described in Example 8. The compression strength of the obtained coke pellets was an average of 400 pounds.

As a further demonstration, the same mixture of coke breeze and bituminous coal was ground in the same ball mill under the same conditions as given above but for 45 minutes, instead of 34 minutes as above. The screen analysis was:

0.18% 100 mesh Tyler screen 11.8% 170 mesh Tyler screen 88.02% 170 mesh Tyler screen and the coke pelletsproduced in the same manner as described in Example 8 had an average compressive strength of 550 pounds, instead of the 400 pounds obtained with the above coarser mixture.

Most carbonaceous materials and certain iron oxide bearing materials contain small amounts of sulfur which is ordinarily harmful if present in iron or steel and must, therefore, be removed during the iron or steel manufacture. This is commonly done by strongly agitating the metal bath for relatively long periods in the presence of lime. When metallized pellets are made according to this invention, as in Example 3 for instance, and when such pellets are melted to iron and steel, preferably in an electric furnace, the rate of desulfurization is surprisingly accelerated if particles of limestone or other lime source compatible with the liquid employed in forming the pellets are incorporated in the iron oxide-carbon pellets. The following example illustrates the production of such pellets.

EXAMPLE 11 100 parts of a magnetite concentrate containing 64.9% Fe as oxide, 0.21% S, and 1.5% S10 and having a size distribution as follows:

- 20 mesh. 32 mesh 7.4% 32 mesh, mesh 41.8% 80 mesh. 150 mesh 22.5% 150 mesh. 200 mesh 10.2% 200 mesh 18.1%

was mixed with 18.4 parts of a l00 mesh bituminous coal containing 78.0 percent fixed carbon, 15.9 percent volatile matter, and 0.48 percent S and with 13 parts of limestone (lOO mesh), 1 part of fluorspar mesh), and 21 parts of a 10 percent aqueous solution of sodium silicate. Green pellets were formed from the resultant moist mass in substantially the manner described in Example 1. The pellets were dried and then heated in a rotary furnace to a maximum temperature of 1,240C. and cooled in an inert atmosphere as described in Example 4. No abrasion or breakage of the pellets occurred. They had shrunk considerably in size, had an average crush strength of lbs., and contained 82 percent total Fe of which 96 percent was in the metallic form, 9.6% CaO and 0.25% S.

Under certain conditions a higher lime content in the pellets than that necessary for binding the sulfur content of the pellet constituents may be useful. Pellets containing excess lime have been found to accelerate the desulfurization of an iron or steel bath, if such pellets are submerged therein and melted.

It may be mentioned here that, as shown in various examples, the present invention comprehends the inclusion of minor amounts of many different materials in pellets produced according to the invention. For example, in Example 11 in addition to limestone, a little fluorspar is also present and in Example 6 a small percentage of iron particles was present in the in plant fines employed. If desired, suitable metal particles such as sponge iron fines may be added to the mix for making pellets.

Oxides of metals other than iron and other oxidic materials which oxides and materials are reduced by solid carbon at elevated temperatures, may also be formed into pellets, with or without carbon particles, in accordance with the present invention. The following example illustrates the making of pellets from particles of silica and carbon by bonding these particles with carbon formed by the pyrolysis of hydrocarbon material such as fusible bituminous coal, pitch, and tar. The ratio of the total amount of carbon after pyrolysis, i.e., of the fixed carbon, was such that subsequent metallurgical heat treatment would yield silicon carbide according to the equation SiO 3C SiC 2C0.

EXAMPLE 12 A mixture was formed of 100 parts of silica ground to pass a 200 mesh sieve, 23.1 parts of calcined petroleum coke ground to the same fineness, 15 parts of hard pitch particles smaller than 0.04 in. (1 mm) in average diameter, 26 parts of coal tar, and 25 parts of water containing 0.12 percent of an anionic wetting agent type surfactant. Green pellets were formed from the resultant moist mass and were dried and carbonized in the same manner as described in Example 7. The pellets did not change their shape when heated gradually to produce pyrolysis of the pitch and tar.

If substantially more than about 30-35 percent (based on the total of oxide and coke) of the hydrocarbons is added in forming the densely packed plastic layer, the resulting pellets tend to flatten out and fuse together on heating. If substantially less than about 30 percent of the hydrocarbons is used, the carbon bond formed by their pyrolysis is insufficient, i.e., the carbonized pellets are too soft and break easily. It has also been found that substantially more tar and less pitch than in the preceding example also makes the pellets lose their shapes during carbonization. Generally, the amount and type of hydrocarbons required for heatstable and strong pellets must be pre-determined by experiment, but will be in the range from about 25 to 40 percent.

It has been further discovered that pellets made according to this invention can be made heat stable in a manner which is more convenient and economical since less binding agent is required. The improvement comprises evaporating at least a part of the liquid remaining in the pellets after rounding, impregnating the at least partially dried pellets with a solution of a bonding material in a liquid and thereafter completely evaporating the liquid from the pellets. Although an organic liquid may be used for making pellets and a binder soluble in such liquid may be used for impregnation, water is preferably used as a solvent and water soluble binders such as alkali silicates, waste sulfite liquor, molasses, sugars, and soluble starch are particularly useful. Generally, a solution containing -30 percent of binder is satisfactory. The actual amount of binder picked up by the pellets will vary with many factors, including the concentration of the binder solution and the moisture content of the pellets. Satisfactory results are obtained in many instances with as little as about 0.5 percent binder (based on the weight of the partially dried pellets) and no more than about 2 percent binder is required. Obviously, an excess of binder can be used, but it is not desirable. The following example illustrates this improvement using a sodium silicate solution for the impregnation.

EXAMPLE 13 The oxide-carbon mixture was made from in-plant fines and coke breeze as in Example 6. 100 parts of the dry mixture was mixed with 11.5 parts of water and formed into dense green pellets of equal size by the procedure described in Example 1. The green pellets were dried at 140C. in a current of combustion gases until the free water content of the pellets was about 1 percent. After cooling to room temperature, the partially dried pellets were immersed for 3 minutes in a percent aqueous sodium silicate solution, removed, and again dried at 140C. to about 1 percent moisture. It was determined that 100 parts of the pellets had absorbed about 4.3 parts of the silicate solution and that the outer portions of the pellets had been converted to a hard shell approximately 2 mm thick.

When these pellets were heated in a rotary furnace as described in Example 6, swelling and disintegration of the pellets did not occur and abrasion of the pellets was minimized. The useof sodium silicate is particularly desirable in this service, in that, after drying, the pellet has a silicious shell or outer portion. It has been found that such shells retard reactions of the pellets with the atmosphere surrounding the pellets and thus contribute to thermal stability of the pellets.

Pellets with hard, heat-stable shells may be formed by other methods than immersion. For instance, the binder solution may be sprayed on rolling pellets. Also, the dried and still hot pellets may be exposed to a flow of air at room temperature by which the outer portions of the pellets are cooled preferentially whereupon the pellets are treated with a solution of the binder for a period determined by the desired thickness of the hard shell.

By vibration, as used herein with reference to the conversion of a damp or moist mixture of liquid and particles to a plastic, densely packed mass, is meant such a mechanical, electric, or sonic vibrator contacting or connected to a face which contacts the mixture, as will cause minute but noticeable oscillation of the particles and their movement to achieve dense packing. In this connection, it will be understood that the horizontal surface on which the moist layer is vibrated may be a moving belt, thereby permitting continuous production of bodies to be rounded into green pellets.

As used herein, the term pellets is intended to refer to shaped bodies comprising densely packed particles held together at least partially by cohesion of said particles, said shaped bodies having no specific form but being characterized by rounding of the corners and edges thereof whereby a more or lessspherical shape is obtained. The size of such pellets may vary, as desired, from an average diameter of about 6 mm (onefourth in.) to about 50 mm (2 in.). By contrast, bri quettes are compacted larger bodies, usually bonded and consolidated by high pressure.

By carbon as the term is used herein is meant a carbonaceous material having more than 55 percent fixed cabon.

Where surfactants are used herein in the production of green pellets any wetting type anionic or non-ionic surfactant may be employed. Specifically, sulfated and sulfonated alcohols, alkyl-aryl sulf onates, esters of sulfonated dibasic acids, sulfonated amides, and ethylene oxide-fatty acid alcohol condensates are usable.

Percentages and parts as specified herein refer to percentages and parts by weight unless otherwise indicated, and mesh sizes, unless otherwise specified, to the US. Standard Sieve Series.

Various changes in the details, steps, materials and arrangements of parts, which have been herein described and illustrated in order to explain the invention, may be made by those skilled in the art within the principles and scope of the invention as defined in the appended claims and their equivalents.

I claim:

l. A process for producing rounded pellets of uniform size and uniform dense particle packing, the size being as desired in the range of an average diameter between about 6 mm and 50 mm, which comprises:

forming a mixture of a liquid and solid particles in a particle size distribution favorable to dense packing, said particles being a material such as carbonreducible oxides and oxidic materials and/or carbon, said liquid being present in a pre-determined quantity such as to slightly exceed the amount held by said particles when densely packed;

spreading said mixture on a horizontal surface to form a relatively loose layer of a uniform thickness yielding, after consolidation to a plastic layer of uniform thickness and uniform dense particle packing, the desired uniform thickness;

consolidating said loose layer to form a plastic layer of uniform thickness and uniform dense particle packing by moving, preferably repeatedly, a vibrating face under a load over and in contact with the upper surface of said layer, thereby causing wetting of said vibrating face by exudation of a slight amount of liquid from said layer; cutting the resulting densely packed plastic layer into cube-like bodies the side length of which is approximately the same as the thickness of said layer;

rounding said cube-like bodies by tumbling to form pellets of uniform size and uniform dense particle packing;

at least partially drying said pellets; and

thereafter heating said pellets in a furnace.

2. A process as set forth in claim 1 in which substantially all of said particles are of a size to pass a screen of approximately mesh and said liquid comprises water. I

3. A process as set forth in claim 1 wherein said rounding is carried out by tumbling said bodies freely on an oscillating surface.

4. A process as set forth in claim 1 wherein said rounding is carried out by tumbling said bodies on a surface provided with a plurality of smoothly rounded protrusions or knobs.

5. A process as set forth in claim 1 wherein said consolidation is at least in part produced by rolling said layer with a vibrating roller having a reciprocating movement longitudinally of said layer.

6. A process as set forth in claim 1 wherein said consolidation is in part produced by the compressive action of a vibrating flap in contact with said upper surface of said layer.

7. A process as set forth in claim 2, in which said particles include a substantial percentage of iron oxide particles.

8. A process as set forth in claim 2, in which said particles include iron oxide particles and carbon particles.

9. A process as set forth in claim 2, in which said liquid comprises an aqueous solution of a surfactant.

10. A process as set forth in claim 2, in which a binder is included in said mixture.

11. A process as set forth in claim 2, in which said liquid comprises an aqueous solution of a binder.

12. A process as set forth in claim 2, in which said mixture contains organic material yielding carbon by pyrolysis, said organic material being liquid at a temperature lower than that at which pyrolysis occurs and being present in such amounts that said pellets may be heated to pyrolyze said organic material for bonding without change of shape.

13. A process as set forth in claim 2, in which said particles are carbon.

14. A process as set forth in claim 7, in which the heat-stability of said pellets is improved by partially reducing said iron oxide particles before forming said mixture.

15. A process as set forth in claim 5 wherein said consolidation is in part produced by the compressive ac tion of a vibrating flap in contact with said upper surface of said layer.

16. A process as claimed in claim 3 wherein said oscillating surface is vibrated.

17. A process as set forth in claim 8, in which said carbon particles are present in such amount as to cause reduction of a substantial portion of said iron oxide when heated to reduction temperatures.

18. A process as set forth in claim 8 in which at least some of said particles contain sulfur and said mixture also includes particles of lime-bearing material.

19. A process as set forth in claim 8 in which said pellets are at least partially dried and the outer portions thereof are impregnated with a solution of bonding material prior to heating in said furnace.

20. A process as set forth in claim 12, in which said organic material comprises a hydrocarbon material selected from the group consisting of tar and hard pitch and said mixture also comprises a surfactant effective to cause coating of said particles by said hydrocarbon material.

21. A process as set forth in claim 12, wherein said organic material is a fusible bituminous coal, said particles are carbon, and said coal and carbon are ground together prior to forming said mixture.

22. A process as set forth in claim 19 in which said bonding material is a water-soluble binder selected from the group consisting of alkali silicates, waste sulfite liquor, molasses, sugars, and soluble starch.

23. A process as set forth in claim 19 in which the bonding material is provided in the outer portions of said pellets by partially drying said pellets, applying a solution of said bonding material to said partially dried pellts, and drying said pellets.

Claims

2. A process as set forth in claim 1 in which substantially all of said particles are of a size to pass a screen of approximately 20 mesh and said liquid comprises water.
3. A process as set forth in claim 1 wherein said rounding is carried out by tumbling said bodies freely on an oscillating surface.
4. A process as set forth in claim 1 wherein said rounding is carried out by tumbling said bodies on a surface provided with a plurality of smoothly rounded protrusions or knobs.
5. A process as set forth in claim 1 wherein said consolidation is at least in part produced by rolling said layer with a vibrating roller having a reciprocating movement longitudinally of said layer.
6. A process as set forth in claim 1 wherein said consolidation is in part produced by the compressive action of a vibrating flap in contact with said upper surface of said layer.
7. A process as set forth in claim 2, in which said particles include a substantial percentage of iron oxide particles.
8. A process as set forth in claim 2, in which said particles include iron oxide particles and carbon particles.
9. A process as set forth in claim 2, in which said liquid comprises an aqueous solution of a surfactant.
10. A process as set forth in claim 2, in which a binder is included in said mixture.
11. A process as set forth in claim 2, in which said liquid comprises an aqueous solution of a binder.
12. A process as set forth in claim 2, in which said mixture contains organic material yielding carbon by pyrolysis, said organic material being liquid at a temperature lower than that at which pyrolysis occurs and being present in such amounts that said pellets may be heated to pyrolyze said organic material for bonding without change of shape.
13. A process as set forth in claim 2, in which said particles are carbon.
14. A process as set forth in claim 7, in which the heat-stability of said pellets is improved by partially reducing said iron oxide particles before forming said mixture.
15. A process as set forth in claim 5 wherein said consolidation is in part produced by the compressive action of a vibrating flap in contact with said upper surface of said layer.
16. A process as claimed in claim 3 wherein said oscillating surface is vibrated.
17. A process as set forth in claim 8, in which said carbon particles are present in such amount as to cause reduction of a substantial portion of said iron oxide when heated to reduction temperatures.
18. A process as set forth in claim 8 in which at least some of said particles contain sulfur and said mixture also includes particles of lime-bearing material.
19. A process as set forth in claim 8 in which said pellets are at least partially dried and the outer portions thereof are impregnated with a solution of bonding material prior to heating in said furnace.
20. A process as set forth in claim 12, in which said organic material comprises a hydrocarbon material selected from the group consisting of tar and hard pitch and said mixture also comprises a surfactant effective to cause coating of said particles by said hydrocarbon material.
21. A process as set forth in claim 12, wherein said organic material is a fusible bituminous coal, said particles are carbon, and said coal and carbon are ground together prior to forming said mixture.
22. A process as set forth in claim 19 in which said bonding material is a water-soluble binder selected from the group consisting of alkali silicates, waste sulfite liquor, molasses, sugars, and soluble starch.
23. A process as set forth in claim 19 in which the bonding material is provided in the outer portions of said pellets by partially drying said pellets, applying a solution of said bonding material to said partially dried pellts, and drying said pellets.