Process for melting cast iron borings

Process for melting cast iron borings comprising continuously charging dried borings into a coreless induction furnace operated within controlled metal levels to achieve adequate stirring of the molten metal in the furnace, drawing incoming borings beneath the meniscus of the molten metal surface, controlling the dissolved oxygen content of the molten metal to ensure formation of carbon oxide and thus achieve a "slag free" operation. The rate of feed of the borings is controlled according to the power input to maintain a predetermined constant temperature of the molten metal. Apparatus relating to the foregoing process is also disclosed.

This invention relates to the melting of finely divided cast iron particles 
such as borings, chips and turnings. More particularly, it relates to the 
continuous "slag free" melting of this finely divided charge material in a 
coreless induction furnace. 
For the purpose of definition, borings, chips and turnings will hereinafter 
be referred to as borings. Cast iron borings are generated daily in 
sizeable quantities through various machining operations performed on cast 
iron castings. When the borings are charged directly into a coreless 
induction furnace of the type set forth in U.S. Letters Patent No. 
3,463,864 to Mario Tama, any entrained cutting oil on the borings 
vaporizes and burns producing considerable air pollution. When any wet or 
oily borings become submerged in the molten metal, the liquids vaporize, 
expand many times in volume and displace molten metal from the furnace. 
When borings are dried to avoid the aforementioned problems, they will not 
readily melt due to the generation of slag and their entrappment therein. 
This slag generation represents the loss of valuable metallic constituents 
through an oxidation reaction, increased refractory consumption, and 
general reduction in utilization of the coreless induction furnace. 
Consequently, cast iron borings are not used extensively as charge 
material for the coreless induction melting facilities. In fact, there is 
no known continuous coreless induction melting operations melting 100% 
cast iron borings other than by use of the invention herein described and 
claimed which achieves a controlled oxygen content of the molten metal 
within predetermined metal levels at predetermined temperatures and which 
secures a "slag free" operation and controlled metal chemistry. 
It is also interesting to note that of the other types of melting 
equipment, namely, (1) the arc furnace, (2) the cupola, or (3) the air 
furnace, none provides efficient melting of cast iron borings. The great 
majority of borings are currently (1) "hot or cold" briquetted for use as 
charge material in the cupola, or (2) used as a substitute for iron ore in 
the blast furnace steel making operations. Cold briquetted borings used in 
the cupola charge break apart as they descend in the cupola with 
considerable loss of the ferrous fines as they become oxidized and/or 
blown from the stack by the combustion gases. Hot briquetted borings 
represent a considerable increase in processing costs as compared to cold 
briquetted borings and can be used in limited quantities in the cupola. 
It is an objective of this invention to provide a means whereby cast iron 
borings can be melted efficiently and continuously in a coreless induction 
furnace without the problems hereinbefore recited in prior melting 
practices and achieves the economically favorable charge material 
processed by the invention herein. 
It is therefore an object of the present invention to provide an improved 
means and process for melting cast iron borings, and the invention 
represents improvements over previouslyfiled U.S. applications covering 
inventions of the present invention, namely, Ser. No. 165,922 filed July 
26, 1971, and Ser. No. 421,244 (both abandoned). 
Another object is to provide a means and process for melting cast iron 
borings in a continuous manner. 
Another specific object is to provide a means and process for avoiding the 
generation of slag in the melt of a coreless induction furnace used for 
melting cast iron borings. 
A further object is to melt cast iron borings in an efficient, effective 
and practicable manner. 
Other objects will be apparent from the description to follow and from the 
appended claims. 
The present invention involves means and a process for melting cast iron 
borings comprising continuously feeding borings into a coreless induction 
furnace wherein the melt level is regulated to achieve adequate stirring 
to immediately draw the borings beneath the meniscus of the molten metal 
surface in the furnace, and the borings are fed into the furnace at a rate 
whereby the dissolved oxygen content of the molten metal contained in the 
furnace will not exceed the silicon/silicon-dioxide (Si/SiO.sub.2) 
equilibrium, and the melt is maintained within a specified temperature 
range above the silicon-dioxide/carbon monoxide (SiO.sub.2 /CO) inversion 
temperature.

Referring to the drawings, in all of which, like parts are designated by 
like reference characters, FIG. 1 shows the general concept of the melting 
process generally indicated at 10. Borings 11a are continuously fed into 
the coreless induction furnace 14 by means of a controlled rate feeder 15 
which may be of the vibratory, oscillating, screw or belt type conveyor 
type. In the event the borings contain noticeable moisture and/or 
hydrocarbons, a drier (not shown) would be used to vaporize and remove 
both said oil and water volatiles, and if desired, a storage hopper (not 
shown) could be provided to provide a surge storage bin for the dried 
borings 11a during non-charging periods to the furnace 14 and/or to supply 
dried borings 11a to the furnace 14 during down-time periods of the drier 
12. The coreless induction furnace 14 is equipped with a cover 16 in which 
there is a centrally located opening 17 through which the dried borings 
11a are fed from the feed at a controlled rate. It is important that the 
dried borings 11a be fed into the center of the molten metal heel 18 in 
the coreless induction furnace generally indicated as 14 so that the 
particulate matter is charged into the meniscus of the molten metal. 
The coreless induction furnace, generally indicated as 14, consists of a 
molten metal heel 18 contained within a refractory lining 20 which is 
surrounded by the power coil 21. Some coreless furnace are equipped with 
additional cooling coils at either and/or each end of the power coil. 
These are shown as the top cooling coil 22 and bottom cooling coil 23. 
When electrical energy is applied to the power coil 21, the molten metal 
18 is acted upon by the mutual repulsion of the magnetic field generated 
about the coil 21 and the magnetic field generated in the molten metal 
heel 18. The force of this magnetic repulsion upon the molten metal heel 
18 is depicted by arrows 24 and forces the metal away from the refractory 
sidewalls 20 at the center of the power coil 21. As the radially inward 
moving metal collides in the center of the furnace, quadrantial stirring 
is effected and a portion of the metal moves upward and a portion of the 
metal moves downward as depicted by the arrows 25 and 26 respectively. The 
metal moving upward in the center of the furnace depicted by the arrow 25 
forms a curved surface of the metal known as the meniscus 28. The 
resulting metal flow is depicted by the stirring pattern 27. The height of 
the meniscus 28 for a furnace varies directly with the level of metal 
within the furnace and the applied power input. 
FIG. 3 shows how the meniscus changes in form for a given power input as a 
function of the metal level in the furnace. The meniscus that results when 
power is applied for metal level 30 is shown at 31; in the same manner, 
the meniscus for metal level 32 is shown at 33, and the meniscus for metal 
level 34 is shown at 35. As the metal level moves above the power coil 21, 
the meniscus decreases in height approaching a flat surface. 
When melting borings, I require the borings to be drawn beneath the molten 
metal surface. This can only be accomplished when the stirring action is 
adequate. I have found that the metal level should never exceed 105% of 
the power coil 21 height for the present application. 
Also because the boring must be charged into a molten metal heel, the 
amount of metal tapped from the furnace should never reduce the metal 
level below 50% of the power coil 21 height. 
The borings 11a fed into the furnace melt, are always oxidized to some 
degree and are coated with iron oxide, Fe.sub.2 O.sub.3. If oxidized 
borings were charged to the molten metal and did not submerge beneath the 
surface, the iron oxide coating of said borings would form a slag. This 
slag would remain atop the molten metal surface and entrap other borings 
which in turn would further oxidize. The cycle then would worsen and 
little to no melting occurs. 
However, when the borings are submerged as described herein, the following 
reactions take place. First, the submerged borings do not form a surface 
slag. Rather, the iron oxide (Fe.sub.2 O.sub.3) coating is heated by the 
molten metal within which it is submerged and is changed to the more 
stable high temperature iron oxide (FeO) phase as defined by equation 1. 
EQU Fe.sub.2 O.sub.3 +Fe (1).fwdarw.3FeO (1) 
since oxygen will dissolve in molten iron, the FeO dissociates according to 
and defined by equation 2. 
EQU FeO (1).fwdarw.Fe (1)+(%0) (2) 
The amount of oxygen that can be dissolved in molten iron can be calculated 
with the aid of published thermodynamic data found in the technical 
literature and expressed in equation 3. 
EQU log (%0)=-6316/T-2.73 (3) 
where T is in degrees Kelvin. 
Curve 40 in FIG. 4 shows the equilibrium oxygen concentration of the Fe-O 
system as a function of temperature. 
Because cast iron contains carbon and silicon, I have also determined what 
effect these alloying elements have. The amount of oxygen that can 
dissolve in molten iron with carbon and silicon present is calculated and 
defined by equations 4 and 5. 
EQU log (%0)(%C)=-1169/T-2.07 (4) 
EQU log (%0).sup.2 (Si)=-31,031/T+12.02 (5) 
where T is in degrees Kelvin. 
Curve 41 in FIG. 4 represents the amount of oxygen that would be in 
equilibrium in an iron-carbon alloy containing 3.4% carbon. Curve 42 in 
FIG. 4 represents the amount of oxygen that would be in equilibrium in an 
iron-silicon alloy containing 2.4% silicon. 
FIG. 4 illustrates that if the oxygen concentration in molten iron gets 
above curve 40, oxides of iron (FeO), silicon (SiO.sub.2), and carbon (CO) 
will simultaneously form. The FeO and the SiO.sub.2 will combine and form 
slag constituents which will rise to the surface of the melt or be 
deposited as a build-up on the refractory lining (20), while CO will 
evolve from the melt as a gas. 
If the oxygen concentration is maintained below curve 40, but above curve 
42, only oxides of silicon (SiO.sub.2) and carbon (CO) will form. Again, 
the SiO.sub.2 will either form a surface slag or be deposited on the 
refractory lining 20 while CO evolves from the melt as a gas. If the 
oxygen concentration is held below curve 42 and above curve 41, only oxide 
of carbon (CO) will form. Since it is a gas, no surface slag will result. 
Hence, to provide "slag free" melting of cast iron borings containing 3.4% 
carbon and 2.4% silicon in a molten heel of metal contained in the 
induction furnace of the same composition of contained carbon and silicon, 
the borings are preferably introduced at a temperature above point 43 and 
at a rate that will not allow the oxygen concentration to exceed the 
limits defined by curve 42. I prefer to charge the borings at a 
temperature of not less than 100.degree. F. or above the temperature at 
point 43 of FIG. 4. 
The point 43 where curves 41 and 42 intersect, is called the inversion 
temperature for the reaction defined by equation 6. 
EQU SiO.sub.2 +2C.fwdarw.Si+2CO (6) 
this temperature can be calculated for any carbon/silicon analysis in 
molten iron with the aid of equation 7. For the 3.4% carbon and 2.4% 
silicon alloy of iron as mentioned above, the inversion temperature 
calculates to be 2607.degree. F. 
##EQU1## 
where T is in degrees Kelvin. 
The melting of cast iron borings in the coreless induction furnace must be 
a tap and charge operation. That is, when the melt level has reached the 
upper limit, as previously defined, the charging and power application to 
the furnace is discontinued. The controlled rate feeder 15 of FIG. 1 is 
then moved to allow the coreless furnace 14 to be tapped of a given weight 
of molten metal. 
I have experimentally melted borings in the manner described above for 
varying analysis cast iron borings to produce varying predetermined 
analysis molten metal by the addition of alloying elements to be used for 
the production of both cast iron castings and for the production of 
secondary metal in the form of cast pigs. Cast iron borings which I melted 
experimentally analyzed a nominal 3.45% carbon, 2.00% silicon, 0.53% 
manganese, 0.30% chromium, 0.35% molybdenum, 0.50% nickel, 0.40% copper, 
0.035% phosphorus, and 0.11% sulfur, balance iron containing iron oxide 
levels in excess of 1.7% Fe.sub.2 O.sub.3. The molten heel in the coreless 
furnace was maintained at the same nominal chemistry as the borings. The 
borings were dried to a temperature of 650.degree. F. before they were 
charged into a 16-ton 60 hertz coreless furnace operating at a nominal 
power level of 3000 KW. When the metal level was varied between 95% and 
105% of the power coil height, the borings melted readily "slag free" on a 
continuous basic with the molten metal heel being maintained at about 
2730.degree. F. which was 140.degree. F. above the SiO.sub.2 /CO inversion 
temperature as calculated by equation 7 listed herein. When the metal 
level was raised above the 105% power coil height limit and chips were fed 
in the manner described maintaining temperatures as high as 2820.degree. 
F., considerable surface slag resulted and the melting operation was 
drastically impaired. During a particular test melt where 300 pounds of 
borings were fed above the 105% power coil height at a temperature of 
2820.degree. F. and at a power input of 2900 KW over 50 pounds of slag was 
generated which analyzed 46.05% SiO.sub.2, 29.18% Fe-O, and 9.91% MnO. 
Manganese losses are inevitable when melting cast iron borings in a slag 
generating operation because in such case the manganese/oxygen equilibrium 
curve 44 of FIG. 4 falls between curves 40 and 42 of FIG. 4 and will not 
intersect curve 41 for the required manganese levels contained in the 
alloy of cast irons. 
The invention has been described in detail with particular reference to the 
preferred embodiment thereof, but it will be understood that variations 
and modifications within the spirit and scope of the invention may occur 
to those skilled in the art to which the invention pertains.