Note: Descriptions are shown in the official language in which they were submitted.
FIELD OF THE INVENTION
The present invention relates to the recovery of
nickel and other metal values from production waste materials.
BACKGRO~ND OF THE INVENTION
.
Large amounts of waste materials are formed during
the production of stainless steel and other alloyed metals.
Waste materials such as mill scale, pickling liquor, dust,
oily grindings and swarf are generally considered to have
relatively little value and consequently are assigned to use
as land fill or are disposed of by other means. This is
economically unrewarding and, equally important, does
not conserve valuable metals such as nickel, molybdenum and
chromium.
There are a number of methods presently known for
recovering metal values from wastes as well as several pro-
cesses dealing with the production of metallic elements rom
ores. However, such processes differ considerably from the
present invention.
For example, in one of the most recent proposals
regarding the treatment of stainless steel making waste in
which a carbonaceous reductant is employed, it is deemed
necessary that the feedstock contain appreciable amounts of
cement to provide sufficient pellet strength following
pelletization.
Until the developemnt of the present invention a number
of processes fortherecovery of metal values from wastemateri-
al have been disclosed. Among these disclosures are the fol-
lowing, all of which fail to perceive the essential steps o~
the present invention, proper blending of ingredients and
3D elimination of the need for a binding or cementing agent.
U.S. Patent 3,264,091, which deals with the pro-
cessing of ironore discloses that it is necessary to use
"_.. ~.
3:~6~
a flu~ and a hi~h -temperature bonding in order to provi.de
strong pelle-ts. In such a process s-trong pellets could
not be formed unless the speciEic bo.nding step is carried
out.
U S. patent 3,870,507 discloses a process for treat-
ing steel making waste in which tarwas used as a binder and
a heat treating operation in an oxidizing atmosphere employed
in order to provide pellets hav.ing the desired strenyth.
In U.S. Patent 4, 004, 918 the process disc:losed is
concerned with the treatment of stainless waste as is the
.
process of the presen-t inventlon. ~n this process, organic
and inorganic binders are required to form briquettes, car-
bonaceous materials are lacking for supplying a reducing
atmosphere. Reduction in this process of this patent is
: accomplished by heating the briquettes in an electric-arc
melting furnace.. Again the present invention process is an
importan~ improvement over the disclosure in this patent.
German Auslegeschrift 1,039,546 teaches the use of
ferrous hydroxides sludges from spent pickle liquors for
briquetting of flue dust and fines from ores. Briquettes
are formed and require a binder of fine tar and napthalene
adsorbed on the:ferrous hydroxides precipitates present.
U. K. Patent 853, 532 deais with extracting metal
values from ores and also discusses the treatment of steel
making waste. The process relies on the use of~hydrocarbon
oil emulsion and a hydraulic binding agent (e~./ Portland
cement) to form pellets.
Until the development of the process of the present
invention no one envisioned being able to recover nlckel
and other metal values Erom most of the undesirable wastes
produced in various metal facilities. Furthermore, until
the development of the process of the present invention, it
3~66
was never envisioned that pelletized or briquetted metal
wastes could be formed without the use of blinding agents
and their ensuing undesirable side effects in further
processing.
OBJECT OF T~IE INVENTION
. . .
The present invention aims at providing a process
which converts waste from specialty steel mills, such as
flue dust, mill scale and swarf, into a useful remelt alloy
for the stainless, alloy and specialty steel industries.
A further object of the present invention is to
provide a method of pelletizing for the recovery o metal
values from waste without the necessity of using bonding
agents.
SUMMARY OF THE INVENTION
-
According to the present invention, the important
disadvantages that have stemmed from the process of the
prior art are overcome. It has been found in accordance
herewith, and, in fact, one of the most important factors
in the success of the present invention, that by using a
2~ proper balance of feedstock ingredients, binding or cement
additions are ~uite unnecessary. Specifically, it has now
been discovered that by proper selection of metallurgical
wastes and blending of these wastes with a carbonaceous
material supplying a substantially oxygen free reducing at-
mosphere, strong pellets can be produced without the use of
binding materials, such pellets can be converted to melting
stock highly suitable for tlle production of metal alloys by
a re~uction roasting and melting operation.
Generally speaking, the present invention contem-
plates a process for reclaiming metallic values from wastes
and comprises blending a waste material containing one or
more of mill scale, furnace scale, swarf and high-grade grit
with a carbonaceous reductant, water and, at times when a
high amount of CaO is present, pickle liquor or pickle
liquor and water and at least one material selected from
the group consisting of furnace flue dust, and process
sludge to form a blended feed material, pelletizing said
blended feed material to form strongly bound pellets,heating
said pellets in the- presence of an oxygen free carbonaceous
material not capable of reacting with said waste materials
for the purpose of binding but capable of providing a re-
ducing atmosphere (particularl~ in accordance with the two
stage heating-reducing operation described hexein) t
reacting the oxidized metal waste materials of said pellets
with said formed reducing atmosphere thereby forming a
metal-containing pellet, and thereafter melting said metal-
containing pellet to separate and recover a metal value.
For the purpose of convenience, the following de-
scription is largely directed to recovery of metal values,
e.g., nic~el, chromium, iron, molybdenum, etc., from stain~
less steel wastes; however, it is to be understood that the
invention is not restricted to such recovery but is appli-
cable to the recovery of metals generally fxom other metal
containing wastes, e.g., nickel-containing wastes, copper-
containing wastes, ferrous and ferrous allo~ wastes, etc.
It is contemplated that metal-containing pellets
formed by the reduction-roasting step could also be used
as a supplemental addition to a melt of stainless steel,
despite the large amount of gangue contained therein.
Within the melting furnace, it has been found advan-
tageous to add su~stances such as burnt lime, limestone, mag-
nesia, dplomite and crushed steel-making slag to the charge
during melt-down to ease tha separation of ~olten metal from
gangue. It is preferred ~o maintain a basic slag in which
the ratio of basic constituents, e.g., CaO, MgO, etc., to
acid constituents, e.g., sio2~ Al203,etc., be ~etween about
0.8 and 1.8 to insure the desired chromium recovery. For
example, using a ratio of 0.6, the chromium recovery was 28%,
whereas with a ratio of 1.3, the chromium recovery was 96%.
This ratio can be maintained by the addition of burnt lime
dolomite or limestone to the charge within the melting
furnace, The amount of burnt lime or other basic rock
added to the melting furnace is largely influenced by the
lD amount of CaO and MgO present within the pellets. By reason
of an appropriate selection and propor~ioning of waste
materials as discussed below, it is possible in addition to
providing strongly bound pellets, to actually minimize the
limestone or other basic additions to the melting furnace by
utilizing the CaO in the pellet for such purpose, even though
CaO and MgO would have been considered a contaminant. Also,
chromium-rich substances, e.g., ferrochrome, chromite oret or,
for that matter, nickel-rich substances, e.g., niçkel, nickel
oxide, nickel alloys, etc. can be added to the molten
charge with the pellets to provide flexibility in the com-
position of the final product; i.eO, to provide a composi-
tion better adapted to stainless steel production. It has
been found expedient in stainless steel production to ad-
just the blend of waste materials and additions so that
the pig produced in the melting furnace contains ~p to
about 20~ nickel, about 2~ to about 30% chromium, and the
- balance essentially iron.
The various distinct forms of metal-containing
wastes suitable for conversion to metal values, such as
stainless steel pig, include mill scale, furnace scale,
high-grade gxit grinding swarf, furnace flue dusts, pickle
--5--
3~6
li~uor and process sludge. Mill scale is the oxide scale
that forms on the suxface o~ hot metal and subsequently
breaks away during the deformation imposed b~ hot~working
operations. ~urnace scale is very similar to mill scale,
it forms during high temperature heating of ingots, billets
and slabs in preparation for hot working. Dry grinding oper-
ations which are used to surface condition billets, e.g~, by
removing seams and smoothing defects, produce a was~e material
known in the industry as high grade grit. ~urnace flue dusts
include the dust collected from simple dust-catchers located
immediately adjacent to steel melting furnaces as well as the
dust removed from primary and secondary wet and dry cleaners
including bag ~ilters and electrostatic precipitators~ To
fully remove scale formed during hot rolling operations and
to clean the surface of cold-rolled stainless steel, an
acid pickling operation may be employed. The pickling solu-
tion after use is termed spent pickle liquor and containsmetal ions as well as variable amounts of residual acid. The
used pickle liquor poses a difficult disposal proble~. Still
another form of waste is process sludge. This is the
precipitate collected from process water treatment.
Typical process sludges can contain about 3% Ni, 8% Cr,
30~ Fe, 7% C, 7~ CaO and 4~ SiO2. Grinding swarf is produced
in finishing operations employed for sheet and strip products
and also during centerless grinding. Usually belt grinding
equipment is used with oil or oil-water emulsions as cool-
ants. This resulting waste is an agglomeration of fine
elongated metal particles interwoven with grinding media
debris and soaked in coolant. Due to i~s low bulk density
this valuable material is difficult to remelt in steel making
~31~6
furnaces. In addition the containe2 oi~ content produces
large volumes of smoky fumes on heating that cannGt be
treated in normal steel making melt shop pollution control
equipment.
It is essential to the present invention that
the types and grades of waste material be selected so as to
produce a pellet with high strength and resistance to im~
- pact and to make advantageous use of the components of the
waste materials. As a preliminary step, the waste materials
are pxeferably screened to remove excessively large pieces
of waste, i.e., those larger than about 2.54 cm diameter.
Oversize pieces are crushed and rescreened. Solid pieces
of metal are removed and added directly to the melting
furnace. The reductant which may be any carbon supplying
material that is capable of reacting with said waste
material and capable of providing a reducing atmosphere,
e.g., a low-cost anthracite, is added in crushed form.
It is essential that the quantity of the material
selected from the group consisting of furnace flue dust
and process sludge ~e present in an amouni of at least
about 20~ by weight, to produce the required strength and
impact resistance in the pellets since it is one of the
most important features of the present invention to elim-
inate the use of any binder material in the pellets formed
thereby avoiding one of the most undersirable drawbacks of
the prior art waste recovery processes. Mo more than about
85%, by weight, of these constituents should be present to
provide an economically useful pellet. The waste materials
consisting of mill scale, furnace scale, and high-grade
grit provide the balance Gf the dry charge in an amount
ranging` from about 10~ to about 75~, by weight. ~or ex-
ample, a mixture of 10% anthracite, 60~ mill scale and 30%
i6
furnace flue dust can be used in a blend. It has been
found that grinding swarf can be added to the mixtures
up to a level of about 20% by weight of all other
constituents. At this level, thle oil contained in the
swarf has only a modest effect on lowering pellet
strength.
The presence of flue dust and/or process sludge
is essential in order to provide fine material for void
filling and to promote agglomeration during pelletization.
Use of flue dust is also important from a commercial
standpoint as it is a rich waste posing a difficult and
costly disposal problem. In order to satisfy these needs,
the invention has incorporated a novel use for another
di~ficult to dispose of waste, namely pickle liquor.
A feature of the invention is the lack of need to
add binder materials to the blended waste mixture in order
to produce pellets of sufficient strength for further pro-
cessing. It has been found that pellet strength can be
increased by the use of pickle liquor substituted for all
~o or part of the water added during the blending operation.
~his approach has a number of benefits in addition to im-
proving pellet strength; another difficult t~ dispose of
waste is employed and the metals contained in the pickle
liquor (e.g. Fe, Ni, Cr and Mo) are recovered.
It has been found that when the normal blend of
waste material (as described above) containing flue dust
of typical CaO content (about 8~) is pelletized with use
of pickle liquor as part or all of the moisture an im-
provement in pellet strength is obtained over those made
3~ with water as the only source of r.loisture. Flue dusts with
--8--
~1~3~
higher CaO contents (up to 50~) can on occasion b~ pro-
duced by the steelmaker par-ticu]arly if a high lime slag
practice is being employed for enhanceci steel desulfur:i-
zation. Such high CaO flue dus-ts can be employed as part
of the blended wastes and good quality pellets can be
produced. However, said pellets rapidly lose strength
after short storage times to the polnt where their strength
is effectively zero. The failure cause had been identified
- to be the taking-up of pellet moisture by reaction with
CaO to ~orm slaked lime (Ca(OH)2). Thus the moisture
essential to pellet strength is removed. The situation is
further aggravated by the almost double volume expansion
in the transformation of CaO to Ca(OH) 2 which causes a
build up of pressure within the already weakened pellet
resulting in fracture and decrepitation.
. .
The addition of pickle liquor during -the blending
operation overcomes this problem by reacting CaO with the
q
residual acid content of the pickle li~uor to produce
dense crystaline calcium salts. Salts of the valuable
metals contained in the pickle liquor also precipitate at
this time thus incorporating these metals in the pellet
~for recovery in the subsequent processing steps. The xesidual
acid level in the pickle liquor can be variable and the
amount of pickle liquor added to the blend can vary without
- appreciably affecting pellet strength. This is because it
- is not necessary to transform all the CaO to calcium salts;
A coating of calcium salt around a predominantely CaO particle
effectively seals and blocks the CaQ from reaction wi~h water
to form slaked lime. We have found that addition of pickle
liquor of up to about 20~ by 7eight of the blena is effective if
3~6~
high CaO flue dust (~35% CaO) is present in the blend.
The amount of spent pickle li~uor can be reduced to about 5%
if low CaO flue dust (<~5% CaO) is use~ although the higher
pickle liquor level can be maintained without deleterious
effects.
The various waste forms are generally blended
with a small amount of water and/or pickle li~uor to
facilitate the pelletizing operation. Up to about 20
water, e.g., 8%, by weight of charge is added
during blending in a conventional blending device such
as a pug mill.
The pellets are formed by any suitable means,
e.g., on a disc pelletizer. In addition to the water
and at times the pickle liquor added during blending,
additional water is added during pelletization to pro-
vide~a total moisture content of from about 5% to about
20% by weight of dry charge wqight, and preferably from
about 8~ or 10~ to about 13~ or 15%. After the pelle~s
have been formed on the pelletizer, they have sufficient
green s rength for further handling. Pellets from about
one to about four centimeters diameter are preferred for
roasting and melting operations.
The reductant blended with the waste materials can
be any carbonaceous substance as long as it is capable
of reaction with the waste materials a~d is capable of pro-
viding a reducing atmosphere for said waste materials.
Anthracite coal and petroleum coke are examples of rela-
tively inexpensive substances that are particularly effec-
tive for performing the reduction step. The quantity of
reductant added should be from about 5~ to about 30~, by
--10--
3~66
weight of the dry charge, and preferably from about 8%
to about 20~. Reduction is incomplete and not as satis-
factory in pellets containing less than about 5~ reductant.
More than about 30~ reductant provides little added benefit.
The reductant should be in a fine crushed form, e.g., not
greater than 6mm. It is preferred that the particles have
no dimension larger than about 4mm.
It can be seen from the reaction below, that the
carbonaceous reduction material, in fact, does not serve
as a binding material for the formation of the pellets but
does serve as a medium for supplying the necessary reducing
a~mosphere for the recovery of the metals present in the
waste materials. In addition to direct reaction with the
wastes the carbon present in the reduction material reacts
with the ambient carbon dioxide and the water to form
carbon monoxide which in turn reacts with the metal oxide
present in the waste material. The reaction is as follows
C02+C~H 2 C0
H20+C~ C0+ H2
CO*M0 ~ C~ 2 + M
In the above reactions M is a metal.
The carbon dioxide andthe water shown in the above
reactions are evolved from the combustions taking place as
a result of the heat required for maintaining roasting
temperatures.
166
Reduction roasting prior to smelting serves to
substantially lower the levels of volatile elements such
as sulfur, lead, zinc, etc., at an early stage in the pro-
cess so that these elements can be more easily removed and
recovered by pollution control equipment. The early re-
moval of such elements provides for advantageous, cleaner
conditions during the melting operation with resultant im-
proved cleanliness of the final product. By providing
efficient atmosphere control, the reduction roast increases
the effectiveness of the carbon contained within the pellets
and consequently the overall process efficiency.
During roasting, the volume of the pellets decreases
substantially, by as much as about 40%, which lessens melt-
ing time considerably. Decreased bulk and increased thermal
conductivity leads to rapid and less expensive melting which
is combined with advantageous use of the electric-arc
furnace for melting.
In additlon to the presence of the reducing atmo-
sphere supplied to the pellets by the carbonaceous reducing
? material, the pellets should be subjected to a neutral,
slightly reducing or preferably a moderately reducing atmo-
sphere, e.g., an atmosphere containing 30% CO and 70% Co2,
in the reduction unit.
In one ~referred embodiment of this invention,
pellets are gradually heated to a temperature between about
1000C to 1300~C in a reducing atmosphere. This may be
accomplished as described hereinafter by passing stainless
steel wire mesh boats containing green pellets through a
mu~fle furnace.
-12-
~3~
A most advantageous variation of this process
which is particularly adaptable ~o production conditions
is contemplated in which a shaft preheater is used in
combination with a rotary hearth furnace. The shaft pre-
heater is a larger diameter cylindrical vessel which can
be constructed from refractory-brick lined steel. The
unit has a slot at its base which can be opened and closed
to control the flow of pellets into the rotary hearth
furnace. The use of a shaft preheater reduces the overall
hearth area requirements for heating and reduction as well
as providing a method for waste heat recuperation. The
shaft preheater is not an essential feature of the process.
Preheating of the pellets can be done on the hearth and
economic objectives met by heat recuperation from the
furnace off-gases.
The shaft preheater is positioned atop the
rotary hearth furnace and a portion, roughly half, of the
off-gases from the rotary hearth furnace pass through the
shaft preheater. Green pellets are charged into the
shaft preheater where they encounter the reducing off-gas
from the rotary hearth ~urnace which drys, heats and
partially reduces the metallic values of the pellets.
Temperatures of about 400C to about 900C, e.g., 800C,
can be achieved in the shaft preheater. Subsequently,
the pellets are further reduced during heating to tem-
peratures of abou~ 600C to 1300C, e.g., 1100C, in
the rotary hearth furnace. Gas flow within the rotary
hearth furnace is opposite to the direction of pellet
travel. Substantially, all of the nickel and iron,and up
to about half of the chromium can be reduced to the metallic
state at temperatures within the aforementioned range. The
non-reduced chromium is completely reduced in the arc
furnace; however, with ideal conditions and elevated temper-
13-
3~G
atures all of the chromium may be reduced in the rotary
hearth.
In another preferred embodiment of the invention,
after passing through the shaft preheater, pellets are
heated to temperatures of about 600C to about 800~C in the
xotary hearth furnace for purposes of economy. At these
temperatures, only the iron and nickel values of the
wastes are reduced, whereas the chromium remains in the
oxide state. In this embodiment, chromium oxide is reduced
to metallic form within the melting furnace.
When a rotary hearth furnace is used for the re-
duction step, it is possible to introduce still another type
of stainless steel waste. A waste material known as oily
grindings can be added near the product discharge section
of the rotary furnace. The temperature in this section of
the rotary furnace should be in excess of 600~C to avoid
smoke formation.
,
Oily grindings are produced during operations such
as the wet-surface grinding of rolled shapes. This type of
waste is not favored fQrpelletizing due to its excessive
oiliness; however, its residual hydrocarbon content is used
advantageously as a fuel source.
The process of this invention is preferably per-
formed on a continuous or semi-continuous basis in order to
retain the sensible heat value of the pellets. Where pellets
are charged continuously to the melting furnace, e.g., via a
launder, molten metal and gangue are generally tapped from the
melting furnace with regular frequency. In another and pre-
ferred embodiment, the reduced pellets can be ~ransferred from
3~
the reduction unit to an insulated product bin and then into
a sealed refractory-lined container. The container advanta-
yeously provides a means for maintaining reduction and melting
units on the same level. The container retains the sensible
heat and prevents reoxidation of the pellets by contact with
the atmosphere. In this embodiment, the reduced pellets are
collected in a quantity suitable for charging to the meltin~
furnace which can be operated on a continuous or a batch basis
as desired.
The reduced pellets should be melted in an electric
arc melting furnace utilizing graphite electro~es. ~hile
other types of melting units might possibly be employed,
the conventional electric-arc ~urnace is greatly preferred
and considerably more advantageous commercially. For example,
in connection with furnaces of the induction type, the
reduced pellets do not have sufficient electrical conductiv;ty
as required in induction melting. Moxeover, this unit ~oes
not appear particularly useful unless a graphite lining
(which acts as a susceptor) is used. And even with a ~raphite
lining, induction melting is not as efficient as electric-
arc melting because of slagging difficulties, electrical
losscs, etc.
An electric-arc melting operation is started with
a metal charge, or heel, which forms the initial molten pool
beneath the electric arc. This heel can be formed by meltin~
a small charge of scrap metal or by leaving behind a molten
pool from a prior melt~
The carbon content of the initial molten pool and
the carbon content of the reduced pellets should be sufficien~
to provide a carbon content of at least about 2~ in the molten
-15-
~ 6 ~
metal. This amount of ca~bon is ~enerally required to provide
ad~itional chromium ~eduction during melt-down of the pellets
and to prevent chromium oxidation within the molten metal.
The rate at which the charye is melted in the
electric-arc furnace is largely a function of the size of tlle
furnace and to some degree the reduced metallic content of
the pelleLs. Generally, conditions are established so that
a molten slag or gan~ue layer floats upon the surface of the
molten ~etal pool. Pellets charged into the electric-arc furnace
sin~ beneath the surface of the molten slag while un~ergoiny
completion of the reduction xeaction and melting o the
l~clletsA With proper adjustment of the slag basicity, as
described previously, the pellets are pro~ected from oxida-
tion since they sink beneath the surface of the slag and the
slag is nonreactive with respect to the pellets. In a batch
operation, sufficient time must be allowed to provide for
settling or separation of the metal from the gangue.
The temperature of the furnace should be between
about 1550C and 1700C. Temperatures below 1650~C can lead
to solidification of the slag which is undesirable and
excessive temperatures can cause breakdown of the refractory
lining of the furnace as well as lead to excessive enerqy cost.
Following the settling period, the temperature of
the slag and metal is generally lowered to about 1500~C to
1600C. This somewhat lower temperature aids in avoidînc3
unwanted oxidation o the molten metal durinc3 the pourin~
of pigs.
-16-
~3~
The pouring operation ~an ~e accomplished by tilt-
inq thc electric-arc furnace, or tapping through tap holes,
to first remov~ the slag layer which is subsequently dis-
carded ald then pouring the rnolten rnetal fraction. The
molten metal is preferably poured into molds on a continuous
pig casting machine.
The reclaimed stainless steel can also be poured
into an insulated container and transported to a nearby Ar~on~
Oxygen-Deoxidation unit for further processing. In such A~
; 10 AOD unit, the levels of carbon and sulEur as well as leve~s
of various other harmful or tramp elements can be substantially
lowered.
During the reduction operation as well as t:he melt--
ing operation, a number of volatile elements, e.g., Zn an~l S,
as well as dust must be prevented from entering the atmosphel-e.
This is accomplishea by the use of conventional dust collec-
tion, wet scrubbing and electrostatic precipitator devices.
Substantial quantities of elements such as zinc may be recovere~
from these dusts and gases by well-known processing sequences.
For the purpose of giving those skilled in the
art a better understanding of the invention and/or a better
appreciation of the advantages of the invention, the follow-
ing illustrative examples are given:
EX~IPLE I
A rotary mixer was used to blend a 900 kilogram
charge consisting of 30~ of flue dust A (Table I), 90% mill
scale B, 20~ mill scale C and 10% crushed anthracite. The
fine dust and mill scale were screened through a 2.5 cm mesh
and large metallics removed prior to introduction to the mixer.
The anthracite was screened to have particles no larger than
3 mm. The blendin~ ope~ation was carried out for a~out 20
-17-
~3~6
minutes and the aim and fina:L compositions of three
representative samples of the mixture are shown in Table II~
-18-
. I . ~3
.
O u~ o In
P' C~ .~ o
P~ C.~ ,. ..
o ~,
.~ .~ C~
E~ o c~
.. ... ..
~-~ ~D el o
U~ ~ C~ ~i
.. ..... -
cn ~ o
.r~
Z N e~ N
.. .~
~0 N ~ N
~ O O C~
8 ~ C3 r~
~ r cn r;
~ ~ c~
v
~ .
~ o o
o ~ cn
.r~ ~ co r~ co
~ c~
.r~ o o o
~q
o ~
~ t) N O
V U~ O C~
..
~ o ~o
O, O O
r~ r~
el~ O C~
O O O
~ .. , - ... -
V . r-l
~ r~ O
..
C~ ~ O
. ..
o N ~ ~
.. - ~ :
:: : m o
: ~
a) a u~ u~
~- ~ ~, .,
~n : ~
--19--
: ~ .
.
3~
Pelletixation wa~ accomplished on a three~foot
diameter pelletizing disc. The premixed powders were sprayed
with about 10% water (by weigh-t). Pellets were readily formed
having a mean diameter of about 1.2 centimeters, a size range
of about 0.6 to about 2.5 centimeters ~eing acceptable and
preferred for later processing. The green strength of the
pellets was adequate for transfer to the reducing furnace,
e.g., a 1.2 centimeter diameter pellet could support a load
of about 5 kilograms without crushing.
EXAMPLE II
Green pellets, allowed to air dr~v for one hour,
were placed in stainless steel mesh boats and passed ~hrough
a horizontal electrical-resistance heated muffle furnace
having a reducing atmosphere to simulate trea-tment i31 a com-
bination vertical-shaft/rotary-hearth furnace. The furnace
had a 18 cm by lg cm muffle cross section. The fllxnace atmo-
~here was 30~ carbon monoxide plus 70% carbon dioxid~ ~lowing
~t the rate of 1.3 cuhic meters per hour (3.5 cm per millute
vcloci~y) through the 0~16 cubic meter furnacc. Ilame-cllrtain
doors ~erc used at both ~nds of the muffle furnacc to prevent
oxidation of the pellets by the atmosphere. ~he ovexall
lensth of the furnace was 5.2 meters with the last 1.~ meter
water jacketed and used as a cooling zone so that pellets
could be removed from the protective reducing atmospherc with-
out reoxi~izing.
I~uring heating in the mu~le furnacc, a 2.5 cm dec~
1.~yer of ~cllets required about 25 minutes to achicve lO90~C.
'~hc ~ellcts remained in the hot zone of the furnace at a
temperature in excess of 1090C ~or about 20 minutes. A
maximum temperature of 1180~C was attained during passage
through the furnace.
-20-
.
~33L~6
o o o
V V
oo o
N N
O O O
.
¢1 O o
.
,CI ¦ NO ~
U)O O O
ZU7t~CO
N ~N
11~Ln d
U)' -
~ N`D
.~~0N NH
~ . . .
C I 00 ~t ~
. . .
_~ O
' ¦ ~~, N
C~ ~ N ~ Il~
E~ . . .
~rl ¦ 1~ N C~ ~
u.l E-~ I o ~ o o
ZH 1 N ~ 1
U.l ¢ ~ U~ N
~ . . .
C~
H ~--~1 ~ 1~ ' t~)
H ~~ I H 0 N ~)
C¢ C~l cdl ~
E-- ~ N ~ ~1 ~
O I ~
H ~ I N N
CJ~
~ ~ ~ ~ U~
¢ I ~ ~0 ~ ~
~ H H H N
o l 1~ r~ o u~
:~:1 o o ~, ~"
0~ ~ cn a u~
1~ N ~ ~
~D ~ ~ oO oO oO o\
Z ¦ N ~ ~ N
N ~D
a
~ .
N
- 21 -
~!
3~66
Followin~ the reductioh step, th2 pellets contained
a~out ~. iron, 11.3~ chromium and 2.6~ nic~e1 in elcmc~lltal
Eorm. ~rhcsc values reprcsent reductions of about 84% f~r
iron, 12~ for chromium and 9S~ for nickel. T~ble III sho~
thc cc)ml)osition of the pellets followinq the rcduction step
and in addition, sho~J~ that the content of undcsirable elements
such as lead and tin is substantially lowered durin~ this
portion of the process.
~ TABL~ III
Compositio_ of Pellets ~fter Reduction at 11~0C
Composition in Weight Percent
: ~e : Cr : Ni :Al : Si :Mq ~iln :Ca : Pb : Sn
Reduced ~c~lets:48.8:ll.3:2.86:l.3:2.7n:2.3:3.7:3.9:0.0~4:~0.()05
_ _ _ _ _ _ _ _ . _
EXAMPLE III
About 270 ky of the aforedescribed reauced pellets
were melted in an electric-arc furnace for the purpose of:
~i) completing the reduction reaction, tii) melting the pellets,
(iii) separating metal and gangue and ~iv3 providiny solidiied
pigs o~ reclaimed stainless steel.
~0 An initial charqe of 180 k~ of pi~ iron containi.nc~
about 3.6% carbon was melted in an electric arc furnace.
~ollo~ing meltin~, about 2.3 k~ of f~rro-silicon (containin~
75Y. silicon) was addcd to maintain the c~rbon content of thc
charge.
About 50 minutes were required to melt the cast iXOll
and achieve a temperature of lS00C at which time pellets and
limestone were introduced. Pellets were added at the rate of
2.7 kg every 2 minutes. To insure a favorable ratio of
-22-
3~
~ 'a~ ;J())/SiO2 of ~rom 0.8 to 1.8, a~o~lt 0.45 k~ of limt~st:one
w~s ~d~le~l with each pellet charge. The operatiny,temperature
of the electric arc furnace was maintained between about 165C)
alld l7onoc~ This temperature range ~as achieved about one
hour after start-up. After 3 hours of operation in this
manner, 272 kg of pellet and 25 kg of limestone were consu~ed.
The furnace ~as mainta'ined at 1650-1700C for an additional
1/2 hnur to complete the reductio;n, melting, and separat~on
reactions. Irhc tcmperature of the furnace was then lowcred
lD to 1580~C and the slag tapped. The reclaimed metal was
}~oured into cast iron ingot molds to produce ~i~s ~eighin~
about 20 pounds each.
The composition o~ the reclaimed metal ~nd slag
shown in Table IV during the meltin~ operation and aftex
separation of the slag and pouring. A materials balance ior
this electric-arc furnace melt showed that about 99~. of the
nick~, 9r~.8~ of the chromi~m and 95.8~ of the iron were
r~coverecl during the meltinc~ process. Only 0.04~j nickel,
1.59~. chromium and 6.8~ lxon were retained in the discarded
2 D s ~ a~.
Reclaimed pigs such as those produced in this
example and containing 9.1% chromium, 2.44~ nickel, balance
iron are well suited for prcparation of conventional stain-
less steels. Although the 2.95~ carbon content at first glance
appears high, this level of carbon is suitable in pig used
for supplemental additions ~o an electric-arc furnace and is
particularly useful in the Argon-Oxygen-Deoxidation process
for refin~ng of stainless steel, e.g., where high-carbon ferro-
chromium (6-9% carbon) is routinely used.
-23-
~3L43~6
oo ~
t~) N ~ ~ ~C) U~
ol g o g g g g ~ o o u~
. . . . . . u~
Ul ¦ O O O O I N N ~ O
¢. . . . G I O O O O
¦O O N ~1 ~ ~
~1 o Lt~ Lt') ~1 o~ v 00
V V V V
U I O O .~ N ~,~ C~ 00
c~
~ I 1~ 00 0 0 ~ ~ N ..
cdC~ f l
¢ ~~ ¦ ~L o~ ~ ~ o ,~
a v ¦ ~ I _, O N_~ O r~ ¦ ~ ~ 00 0 0
c~ . a~ Nu~ o ~ ~D `D
3 Il) N O O O ~ ~1 t~ CO 00 0 0
C U t~) N N ~ /7 N ~ Z
;~ l '`~ ~ ~ o~ ol 0 O 0 ~0
~ ~ ~ O ~I N N N ~~ 1 0 0 0 0
o ~ ~1 ei In 0, 11'1 ~ O
o ,~ O OV a.~l ~~) r` 00
o~ u~ v ~
~ h ¦ ~ oo10 0~i h
H ~LO I` 5~~c_~¦ 5- ~3 Ll~
E-' Z I ~I NN N N Z ¦ ~ ~ O
o
o
~ o
.,1 ~ ~ ~
oo~ ~ O O td
h ~ ,, h h
O ~ O ~
O ~ ~ h td
~H t~
¢ ~ ~ ~ ~ ¢
- 24 -
`
,
3~t~6
F.XA-r~P-~E-Iv
~ blend was made up containiny ~2.8~ flue dust
(containing 32P~ CaO) 28.8% mill scale 1~.0~ swarf and
1~.3% crushed coal as the reduction atmosphere suppl~iny
material. To this was added 10~; by weight of pickle liquor
containiny residual nitric and hydrofluoric acids. Pellets
were made on a pelletizing disc with water added to briny the
total water plus pickle liquor content to :L~. Two hours
after production, the peLlets had a crushing strength of
about 20 lbs.
Similar pellets were produced from the same mixture
but with water being the only source of moisture. Pellet
crushing strength measured was about 10 Ibs. immediately after
pellet formation but was so low after 2 hours stoxage as to
be unmeasurable. The pellets wer~ jwollen and d:isintegrated
upon touch.
,
EXAMPLE V
- A blend with a composition similar -to that shown
in Example IV was made up but with flue dust containin~ 15~
CaO instead of 32%. Two lots of pellets were prepared from
- this mixture in the manner prescribed in Example IV. One
lot had 10% pickle liquor added and the other lo-t was made
.
up with water as the only source of moisture. Three hours
after pelletization the pickle liquor containing pellets
had crushing strength of~about 18 lbs. and the pickle liquox
free pellets had crushing strengths of about 10 lbs. After
reduction under rotary hearth furnace condi-tions at 2200F
the analysis of both lots of pellets were essentially identi-
cal. This indicates that the calcium compounds formed by
reaction with the pickle liquor decomposed during the thermal
treatment and are not carried forward to the melting stage
of the process,
-25-
3~
EXAMPLE VI
.
A mixture consisting of 40% flue dust, 21 ~ mill
scale, 17% oily swarf, 6~ high grade (metal billet)grit and
1~% crushed coal was blended at a rate of 10,000 lbs. per
hour in a pug mill. Moisture was added in the form of
pickle liquor at the rate of 1 gal. per minute. The mixture
was pelletized on a 12 ft. diameter pelletizing disc at which
point water was added to raise the total moisture to 12%.
The pellets were then transferred to a rotary hearth furnace
and heated to a temperature of 1100C for a total residence
time of 25 minutes.
Pellets were discharged from the rotary hearth
furnace with 95~ of the nickel and iron content reduced to
the metallic state. Reduced pellets were added to an arc
furnace via a refractory lined transfer car. Following melt-
ing and chromium reduction in the arc furnace the metal tapped
from the furnace contained
10.22% Ni 20.4% Cr 0.8~`Mo 3O72 C
<0.005~ Pb <0.005% Zn
2D The temperature was 1600C at tapping.
The slag phase tapped from this furnace contained
the following
D.25~ Ni 1.3~ Cr, etc.
~he basicity as measured by the ratio of CaO+M~ O~SiO2+Al 2
was 1 6. The slag temperature was 1525C.
Although the present invention has been described
in conjunction with preferred embodiments, it is to be under-
stood that modific:ations and variations may be resorted to
without departing from the spirit and scope of the invention,
as those skilled in the art will readily understand. Such
modifications and variations are considered to be within the
purview and scope of the invention and appended claims.
-26-
. . _ . . _
