Furnace dust recovery process

A process for selective vaporization of oxides from recovered waste dust collected from a bag house. Pellets are formed from the waste dust and dried to permit charging in an oxidizing chamber wherein an oxidizing atmospheric is maintained. The chamber is heated to a temperature sufficient to vaporize lead oxide and, if present, also oxides of cadmium, potassium and sodium. The vapors are cooled and separated from furnace gas. The residual oxidized mass is cooled after removal from the oxidizing chamber and fed into a reduction chamber wherein a reducing atmosphere is maintained. The reduction chamber is heated to a temperature of between 1800 and 2000 degrees Fahrenheit to reduce zinc oxide and form zinc vapors which are cooled and separated from furnace gas. Lead oxide particles recovered from the oxidizing process are reduced to lead.

BACKGROUND OF THE INVENTION 
This invention relates to a process for removing one or more toxic oxides 
forming undesirable contaminants in recovered waste dust and more 
particularly, to the removal of lead oxide and, when present, cadmium 
oxide and, when desired, zinc oxide by vaporization. The vaporization is 
essentially carried out in the oxidizing atmosphere at least at the 
vaporization temperature of lead oxide and, additional vaporization may be 
carried out in a reducing atmosphere at least at the vaporization 
temperature of zinc. 
Melting or refining processes generate substantial quantities of dust which 
must be collected to prevent environmental pollution. Steel-making 
furnaces for example, liberate airborne particles which are typically 
collected by directing gases containing the particles to a filter situated 
in a bag house. Such steel-making facilities include an electric arc 
furnace, basic oxygen furnace, and an open hearth. While not so limited, 
the present invention is particularly useful to treat waste dust recovered 
from an electric arc steel-making facility in which it was a conventional 
practice to collect, filter, and dispose of all airborne particles as, for 
example, in a land fill operation. Such disposal is environmentally 
unacceptable because the waste dust contains leachable metal oxides of 
lead, cadmium, and chromium. It is an unnecessary economic burden on the 
steel-making process to dispose of the waste dust as a hazardous waste. 
It is known in the art to pelletize steel-making dust and then heat the 
pellets in a reducing atmosphere to recover zinc by vaporization as 
disclosed in U.S. Pat. Nos. 3,756,804; 3,770,416; 4,266,966; 4,283,223; 
4,396,423; 4,404,027; 4,434,001; and 4,488,905. The reduction process, 
however, cannot be utilized to remove lead and cadmium oxides which, if 
allowed to remain in the recovered mass from the reduction process, render 
the mass a hazardous material and importantly, prohibit recycling of the 
mass as a part of a furnace charge. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a process for treating 
waste dust recovered from a melting or refining process to remove lead 
oxide and, when present, to remove cadmium oxide and to stabilize chromium 
oxide to permit handling of the of the residual mass as a non-toxic waste. 
It is a further object of the present invention to provide a process for 
treating waste dust recovered from a melting or refining process, to 
remove lead oxide and, when present, to remove cadmium and zinc oxides and 
to stabilize chromium oxide to permit handling of the residual mass as a 
non-toxic waste, but importantly, to permit use of the residual mass as 
part of a furnace charge. 
It is a further object of the present invention to provide a process for 
treating waste dust recovered from melting or refining process such as a 
steel-making furnace by the vaporization of a constituent oxide of lead 
and recovery of the vapors in a form which is easily handled, thus, 
economically beneficial to the process while the residual mass can be 
treated to remove zinc oxide. 
More particularly according to the present invention, there is provided a 
process for removing toxic lead oxide and, when present, cadmium oxide 
forming leachable contaminants in waste dust recovered from a melting or 
refining operation, the process including the steps of pelletizing the 
waste dust, heating the pelletized waste dust in an oxidizing atmosphere 
to a temperature sufficient to essentially form lead oxide vapors, 
separating the vapors from the oxidized residual mass, causing the vapors 
to solidify, and recovering the solidified vapors. 
In the preferred form, the process of the present invention provides for 
treating the oxidized residual mass which is laden with zinc oxide by 
heating the oxidized residual mass in a reducing atmosphere to a 
temperature sufficient to reduce the zinc oxide and form zinc vapors, 
separating the zinc vapors from the reduced residual mass, recovering 
solidified zinc vapors and recovering the reduced residual mass which 
mainly comprises iron oxide. 
Heating of the pelletized waste dust is carried out preferably at a 
temperature of about 2700 degrees Fahrenheit and preferably not greater 
than 2800 degrees Fahrenheit. The heating of the oxidized residual mass in 
a reducing atmosphere is preferably carried out at a temperature of about 
1800 degrees Fahrenheit, but usually not greater than 2000 degrees 
Fahrenheit. Lead oxide and cadmium, when present, may be treated in a 
reducing atmosphere to form a liquid mixture of lead and cadmium which is 
nontoxic. 
These features and advantages of the present invention as well as others 
will be more fully understood when the following description is read in 
light of the accompanying drawing.

The process of the present invention is useful for treating dust recovered 
from any of known melting or refining process in which the recovered dust 
contains a toxic contaminant comprising oxides of lead, cadmium and 
chromium and typically also includes zinc oxide. The process of the 
present invention is particularly useful for treating dust recovered from 
an electric arc furnace, because, typically, such furnaces are charged 
with substantial quantities of scrap metal. The composition of the 
electric arc furnace dust will vary from furnace to furnace dependent on 
the type and quantity of the scrap charge and the final product analysis. 
A stainless steel producer using an electric furnace would experience 
relatively larger amounts of chromium and nickel oxides in the waste dust 
whereas the amount of zinc oxide will vary with the amount of galvanized 
and automotive scrap. The increased use of galvanized material in 
automotive fabrication will increase the amount of recoverable zinc when 
the derived scrap is recycled. A typical composition of particulate 
generated by an electric arc furnace producing carbion steel is given in 
table 1. 
TABLE 1 
______________________________________ 
COMPONENT WEIGHT % 
______________________________________ 
Iron Oxide (Fe.sub.2 O.sub.3) 
61.96% 
Cadmium Oxide (CdO) 
0.01% 
Lead Oxide (PbO) 0.89% 
Zinc Oxide (ZnO) 3.01% 
Chromium Oxide (Cr.sub.2 O.sub.3) 
1.62% 
Calcium Oxide (CaO) 
13.36% 
Silicon Oxide (SiO.sub.2) 
4.55% 
Magnesium Oxide (MgO) 
5.61% 
Manganese Oxide (MnO) 
5.42% 
Molybdenous Oxide (MoO.sub.3) 
0.71% 
Copper Oxide (CuO) 0.18% 
Nickel Oxide (NiO) 0.051% 
Sodium Oxide (Na.sub.2 O) 
1.81% 
Potassium Oxide (K.sub.2 O) 
0.53% 
Trace elements remainder 
______________________________________ 
The present invention is concerned with the treatment of waste dust for the 
removal of oxides which may be considered toxic oxides and essentially 
include lead and, if present, also cadmium and, if desired, zinc oxide 
although not considered toxic. Eventually, a vaporization process is 
carried out in an oxidizing atmosphere at a temperature sufficient to 
vaporize lead oxide. The melting and vaporization temperatures of lead and 
lead oxide as well as typical other constituent in waste furnace dust are 
given in table 2 below: 
TABLE 2 
______________________________________ 
MELTING AND VAPORIZATION POINT OF VARIOUS 
COMPONENTS OF ELECTRIC ARC FURNACE DUST 
MELTING VAPORIZATION 
TEMPERATURE TEMPERATURE 
COMPONENT (.degree.F.) (.degree.F.) 
______________________________________ 
Lead Oxide 1625 2687 
Metal 618 3137 
Zinc Oxide 3587 +4000 
Metal 788 1670 
Cadmium Oxide 1652 Decomposes 
Metal 610 1409 
Iron Oxide 2849 +4000 
Metal 2802 5432 
Calcium Oxide 4676 5162 
Metal 1540 2625 
Potassium 
Oxide 662 Decomposes 
Metal 147 1400 
Sodium Oxide sublimes 2327 
Metal 208 1590 
Magnesium 
Oxide 5072 6512 
Metal 1202 2012 
Manganese 
Oxide 2273 3900 
Metal 3227 +4000 
Silicon Oxide 3115 4046 
Metal 2570 4496 
Chromium 
Oxide 3380 4824 
Metal 4415 7232 
______________________________________ 
The embodiment of the process illustrated in the drawing is especially 
useful for treating electric arc furnace dust which is collected in the 
bag house. The particles of waste dust present difficult handling problems 
because the particles are relatively small, usually about 2.5 microns, and 
have a settling velocity less than 10.sup.-5 feet per second. The dust is 
loaded into a pelletizer 10 together with water sufficient to form a pug 
from which pellets are formed having a size preferably between 1/4 to 3/4 
inch diameter. The pellets emerging from the pelletizer typically have a 
moisture content of about 5 to 10 percent which is reduced by feeding the 
pellets into a dryer 12 to increase the porosity and strength of the 
pellets. The dust pellets can now be loaded into a feed bin for delivery 
into an oxidizing chamber 14 in a batch-like, but preferably, continuous 
fashion. Preferably, the oxidation chamber takes the form of a furnace 
having an elongated oxidizing chamber in which the waste dust pellets are 
introduced at one end and proceed along the chamber floor to a discharge 
outlet. An oxidizing atmosphere is maintained in the oxidizing chamber, 
preferably by feeding atmospheric air and/or oxygen into the chamber. The 
chamber is heated as by the use of fossil fuel or electric power to a 
temperature sufficient to vaporize the lead oxide constituent from the 
pelletized waste dust. This temperature must be at least 2687 degrees 
Fahrenheit, preferably, however, the oxidation chamber is operated at a 
temperature of about 2700 degrees but usually not greater than 2800 
degrees. In this temperature range and under oxidizing conditions in the 
chamber, any cadmium oxide present in the pelletized waste dust decomposes 
and vaporizes. The vapors of lead oxide and cadmium metal are drawn from 
the oxidation chamber with flue gas and delivered by a suitable conduit to 
a bag house 16 where the flue gases are separated from solidified 
particles of the vapors of lead oxide and cadmium present therein. 
Solidification of the lead oxide and cadmium occurs as the flue gas stream 
undergoes cooling while conducted by a conduit to the bag house. Also, 
under the operating conditions in the oxidizing chamber, sodium oxide and 
potassium will be vaporized when the oxides are present in the pelletized 
waste dust. These oxides also solidify as the flue gas stream cools and 
the particles are collected in the bag house. Devices other than a bag 
house, such as a cooling condenser 17, can be used to separate lead oxide 
and cadmium from the flue gases. 
The residual material remaining in the furnace is withdrawn through an 
opening as a fluid or sinter mass which is partially cooled or quenched. 
Rapid quenching of the mass produces a granular feed which can be conveyed 
while at an elevated temperature to a reduction chamber 18. The reduction 
chamber is preferably formed by an elongated furnace in which feed 
materials are introduced at one end and transported to a discharge port at 
the opposite end of the furnace. A reducing atmosphere is maintained in 
the furnace by the introduction of nitrogen and carbon monoxide and/or 
hydrogen gas from an atmospheric generator 20. The reducing chamber is 
maintained at a temperature of about 1800 degrees and preferably not 
greater than 2000 degrees. Carbon monoxide and hydrogen in the reducing 
atmosphere form reducing gases which react with zinc oxide present in the 
charge of granular material to form zinc which vaporizes. The zinc vapors 
are drawn off with flue gases from the reduction chamber for delivery by a 
conduit to which there is preferably introduced an inert coolant such as 
nitrogen or argon gas to solidify the zinc vapor for separation in a bag 
house 22. Also, delivered with the flue gas from the reduction chamber are 
nitrogen gas and carbon dioxide/water vapor. As shown in the drawing, zinc 
is recovered in the bag house as well as the nitrogen gas and carbon 
dioxide/water vapor. Again, devices other than a bag house such as a 
cooling condenser 21 can be used to separate zinc from the furnace gases. 
The residue from the reducing operation in the reduction chamber is 
discharged through the outlet opening. The residual material is mainly 
comprised of iron oxide which can be formed into briquettes, cooled and 
used to recharge the steel-making furnace. Carbonious material, such as 
coke, can be added to the reduction chamber or mixed with the reduced 
product therefrom to enhance the use of the briquettes as a charge 
material in a steel-making furnace. The removal of lead oxide from the 
waste dust is important not only because lead oxide is leachable and a 
hazard constituent in waste material but also lead is detrimental to the 
steel-making process because lead does not alloy with iron. As a result, 
if lead is present in the charge material for an electric arc furnace, it 
will build up in the furnace causing operational problems. As can be seen 
from table 2, metallic lead vaporizes at a temperature of 3137 degrees 
Fahrenheit whereas lead oxide vaporizes at a much lower temperature of 
2689 degrees Fahrenheit. It is more feasible in terms of energy and 
refractory requirements to vaporize lead oxide rather than reduce the 
oxide to metal. The oxidizing atmosphere in the chamber vaporizes lead 
oxide without reduction of even trace amounts to lead. Operation of the 
oxidation chamber at a temperature of 2700 degrees is preferable because 
it is above the vaporization temperature of lead oxide but below the 
temperature range where iron oxide looses stability. 
The present invention thus insures complete removal of lead which might 
otherwise constitute a hazardous material in the waste dust. Some lead may 
be alloyed with zinc which can be removed during the reduction process 
with zinc at a lower temperature. Elemental lead, however, cannot be 
removed in this manner. A surprising discovery arising out of the process 
of the present invention is an enhancement of any chromium oxide present 
in the waste dust. Although not completely understood, it has been 
discovered that when chromium oxide is present in the waste dust, the 
chromium oxide is not leachable from the iron oxide residue. It is 
believed that the chromium oxide in the waste dust undergoes some type of 
transformation into a more stable form, such as chromic oxide Cr.sub.2 
O.sub.3 which is non-leachable. However, chromium and its oxides are 
economically beneficial to electric arc steel making and enhance the 
economics arising out of the use of the residual material from the process 
of the present invention as a charge material for an electric arc furnace. 
As can be seen from table 2, the vaporization temperatures of the metal 
and oxide are higher than the melting point of iron oxide matrix whereby 
it is difficult to carry out a separation process in a solid state. The 
particulate which is collected in the bag house 16 from the oxidation 
chamber will contan large quantities of lead oxide. 
Referring again to the drawing, the solid residue which is collected in bag 
house 16 or oxide and metal condensate from condenser 17 is preferably 
treated to reduce the lead oxide constituent to lead thereby avoiding a 
need to dispose of lead oxide as a toxide waste in the residue. For this 
purose, the solid residue is fed into a reducing chamber 24 which is 
separate and apart from the reducing chamber 18 which receives the 
residual mass from the oxidizing chamber. The reducing chambers 18 and 24 
are preferably situated within the same furnace to carry out reducing 
operations at the same temperature which is maintained in the furnace. 
This temperature, as previously described, is preferably between 1800 
degrees and 2000 degrees Fahrenheit. Into chamber 24, there is also 
introduced reducing gases of nitrogen and carbon monoxide/hydrogen. Under 
these conditions in reducing chamber 24, lead oxide present in the charge 
material is reduced to lead which can be drawn off from the chamber as a 
mixture with cadmium, sodium oxide, and potassium, when present, together 
with carbon dioxide and water vapor. The output from chamber 24 is fed 
through a cooling chamber which can be cooled with coolant circulating in 
an outer shell. The product in the cooling chamber is cooled to at least 
about 1200 degrees Fahrenheit to produce a liquid mixture of lead and 
cadmium. This mixture can be fed to pig molds and the pigs when solidifed 
can be safely handled as a non-toxic substance. A gaseous residue is 
recovered from the cooling chamber and will essentially include nitrogen, 
water vapor, carbon dioxide, carbon monoxide and hydrogen which can be 
safely removed from the system. 
Although the invention has been shown in connection with a certain specific 
embodiment, it will be readily apparent to those skilled in the art that 
various changes in form and arrangement may be made to suit requirements 
without departing from the spirit and scope of the invention.