U.S. patent number 3,836,354 [Application Number 05/360,640] was granted by the patent office on 1974-09-17 for production of pellets.
Invention is credited to Fritz O. Wienert.
United States Patent |
3,836,354 |
Wienert |
September 17, 1974 |
PRODUCTION OF PELLETS
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.
Inventors: |
Wienert; Fritz O. (Niagara
Falls, NY) |
Family
ID: |
26853641 |
Appl.
No.: |
05/360,640 |
Filed: |
May 16, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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156910 |
Jun 25, 1971 |
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785132 |
Dec 19, 1968 |
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Current U.S.
Class: |
75/766; 75/10.61;
75/771; 264/15; 264/70; 264/144 |
Current CPC
Class: |
C22B
1/2406 (20130101); Y02P 10/234 (20151101); Y02P
10/20 (20151101) |
Current International
Class: |
C22B
1/14 (20060101); C22B 1/24 (20060101); C21b
001/22 (); C22b 001/24 () |
Field of
Search: |
;264/15,70,144
;75/3,5 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Curtis; A. B.
Attorney, Agent or Firm: Harlan, Jr.; Ashlan F.
Parent Case Text
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.
Claims
I claim:
1. 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 carbon-reducible 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 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.
Description
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 applicant's U.S. 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
the resultant 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 carbon-reducible 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.
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
surface 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 a solution 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, it is 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 suitable for 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, 11 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 15 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 of the 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 may be 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:
- 20 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 11.5 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 3/4 in. .times. 3/4 in. squares, thus forming a
plurality of approximately cubic bodies 3/4 in. .times. 3/4 in.
.times. 3/4 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
1,300.degree.C.
The magnetite ore concentrate used above was not suitable for
conventional formation of pellets by the so-called balling action
as was demonstrated by charging some of the dry mixture of ore
concentrate and bentonite 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 300 mesh. Often grinding for the
sake of pelletization has been necessary. This is costly,
especially with hard minerals such as magnetite, chromite ores,
ilmenite, specular hematite, silica, and others. The present
process does not require the fine grinding and consequently avoids
this expense.
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 9.0% - 32 mesh, + 48 mesh 17.0%
- 48 mesh, + 100 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 cube-like 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
1,275.degree.C., in an oxidizing atmosphere. The indurated pellets
had an apparent density of 3.58 g/cm.sup.3.
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 900.degree.C.,
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 5 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
1200.degree.C. 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.sub.2 and had the following size distribution:
+ 20 mesh 1% - 20 mesh, + 65 mesh 30% - 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 (-100 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 3/4 in. .times.
3/4 in. .times. 3/4 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 1,275.degree.C. 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.sub.2 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 900.degree.C. is illustrated in the succeeding two
examples.
EXAMPLE 5
A dried Brazilian hematite containing 97 percent Fe.sub.2 O.sub.3
(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, + 150 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
1,250.degree.C. 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 so-called "in-plant
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, mill 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:
- 1/4 inch, + 32 mesh 8.0% - 32 mesh, + 42 mesh 3.0% - 42 mesh, +
80 mesh 21.0% - 80 mesh, + 150 mesh 21.0% - 150 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.sub.2, CO, CO.sub.2, H.sub.2, and
H.sub.2 O 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 200.degree.C. through the bed. Upon
being heated in an oil-fired rotary furnace to 1,250.degree.C. 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 150.degree.C. 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 140.degree. C. to about
550.degree.C. 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 1,000.degree.C. 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.
100 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 flat surface. 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 1.11 g/cm.sup.3. 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 10
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 pellets produced 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% SiO.sub.2 and having a size distribution as
follows:
- 20 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%
was mixed with 18.4 parts of a -100 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 (-100 mesh), 1 part of
fluorspar (-100 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 1,240.degree.C. 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 150 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.sub.2 + 3C .fwdarw. SiC + 2CO.
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
heat-stable 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 10- 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 140.degree.C. 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 20 percent aqueous sodium silicate
solution, removed, and again dried at 140.degree.C. 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 use of 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 less spherical shape is
obtained. The size of such pellets may vary, as desired, from an
average diameter of about 6 mm (one-fourth in.) to about 50 mm (2
in.). By contrast, "briquettes" 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 sulfonates, 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 U.S. 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.
* * * * *