NOVEL METHOD OF TREATMENT OF EAF DUST FOR RECOVERY OF ENRICHED ZINC OXIDE POWDER
References Cited
U.S. PATENT DOCUMENTS
3,849,121 11/1974 Burrows
4,071,357 1/1978 Peters
4,525,208 6/1985 Yasukawa et al.
5,942,198 8/1999 Myerson et al.
6,102,982 8/2000 Isozaki et al.
6,464,753 B2 10/2002 Home et al.
Other Publications
"Direct Smelting of Stainless Steel Plant Dust", T.J. Goff and G.M. Denton, Pyrometallurgy Division, Mintek, Private Bag X3015, Randburg.
"EAF Stainless Steel Dust Processing", G.M. Denton, N.A. Barcza, P.D. Scott and T. Fulton.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present process relates to a method for recovering zinc oxide with high zinc oxide purity and reduced halide content from steel scrap used for remelting in EAF
Electric arc furnaces are known to generate hazardous flue dust that contains high proportion of valuable oxides of metals like zinc, iron, manganese etc due to large amount of scrap materials used in such a steel making process. In industries producing alloy and stainless steel, the dust may also contain oxides of chromium, nickel etc. This flue dust, collected in bag-houses of arc furnaces can be briquetted along with coke and quartz and later on, used in another type of arc furnace known as submerged arc furnace. In this manner, the metals like chromium, nickel, iron and manganese go into the liquid metal and flue dust from submerged arc furnace is enriched in zinc oxide content.
The present invention thus presents a commercially economic method of obtaining zinc oxide powder with suitable purity that can become a saleable product to various zinc smelters and also finds its use for manufacture of paints.
2. Background Art
Zinc Oxide typically is a coarse white or grayish powder which has a variety of uses including zinc smelting, as an accelerator activator, as a pigment, as a dietary supplement and in the semi-conductor field. Methods for recovering zinc oxides are known in the art, including recovering zinc oxide from industrial waste materials by leaching with mineral acid, caustic soda, ammonium hydroxide and ammonium carbonate solutions. Low yield and less purity have made these methods unpopular.
U.S. Pat No. 3,849,121 to Burrows discloses a method for selective recovery of zinc oxide from EAF dust. It comprises leaching of dust with an ammonium chloride solution at elevated temperatures, separating iron from solution, treating the solution with zinc metal and cooling the solution to precipitate zinc oxide. The material obtained in the last step is a mixture of a small amount of zinc oxide, hydrates of zinc oxide and a large amount of diamine zinc dichloride Zn(NH3)2Cl2 or other similar compounds containing zinc and chlorine ions.
U.S. Pat No. 4,071,357 to Peters discloses a method for recovering a substantially pure zinc product from steel making flue dust containing impurities like copper, lead, cadmium, iron, sulfur and chromium impurities. This objective is accomplished by leaching the flue dust with concentrated ammonia and carbon dioxide to solubilize a major portion of the zinc in the dust and form a zinc-ammonia-carbonate complex. The product of this leach is then filtered and the solids which contain zinc ferrite can either be discarded or processed further

with acetic acid to recover some of the zinc from the zinc ferrite in the residue, as is well known in the art. The leach filtrate is then cemented at room temperature with zinc to replace impurities in the complex, such as copper, lead and cadmium with zinc metal. The cementation filtrate having high soluble zinc content is distilled with steam to remove ammonia and carbon dioxide and precipitate basic zinc carbonate. The final steam distillation product is then filtered and filtrate discarded from the process, resulting in a residue containing primary basic zinc carbonate with sulfur and chromium impurities. The residue is dried at approximately 125 "C and washed to solubilize sulfur compounds in the basic zinc carbonate. The basic zinc carbonate is then calcined at a temperature high enough to break it down into zinc oxide and carbon dioxide. A water wash of the product of this calcining solubilizes chromium compounds, resulting in a zinc oxide product after a final diying at approximately 125 "C which is substantially free of impurities.
Enviroplas process was developed at MINTEK for treatment of solid wastes from EAF dust and alloy steel dust. The main objective of the process was to produce disposable slag and to recover chromium and nickel into a crude ferro-alloy. The equipment consisted of a 5.6MVA d.c. power supply, raw material feed system, a DC arc furnace, a gas cleaning system and instrumentation for data logging and control.. The average analyses of samples of fumes contained around 35.5% of zinc oxide along with containinated elements like chlorine, fluorine, arsenic, sodium and potassium.
Another process developed by Engitec Impianti SpA, Milan, Italy purports to recover zinc metal & lead cement directly from EAF flue dust using an Electro winning technology. Electrowinning is the method of extracting a metal from its soluble salt by an electrolytic cell. It is used in the recovery of zinc by subjecting the zinc sah in solution to electrolysis and electrodepositing the elemental metal on a zinc cathode in the electrolytic cell. In the Engitec process, the EAF flue dust is leached with a spent electrolyte, such as ammonium chloride, which dissolves the zinc, lead, copper and cadmium in the EAF dust into solution while leaving the iron in solid form.
A commonly & most widely used process for processing of flue dust is the Waelz process. This process mixes carbon in the form of fine coke or coal with the arc furnace flue dust to form a compact mixture. Water is added to this mixture to form 'self-reducing' pellets. These pellets are heated in a reducing atmosphere of coke or anthracite to volatilize the metallic zinc and later on, the metallic zinc is re-oxidized. This process is carried out in a counter flow type rotary kiln - the solid material advancing from feeding side by rotation of kiln (about 1 rpm) and the inclination (2-3%) while the air is sucked in at the slag outlet side. The iron is discharged in the form of reduced compacts and the zinc oxide is carried gas-borne along with the kiln exhaust streams. The heat required to volatilize the zinc is generated by burning of the carbon in the carbon/flue dust mixture inside the kiln. The carryover from the feed is separated in the dust settling chambers to achieve a high zinc concentration in the product. Crude Waelz oxide powder is separated in bag house filters. 2-stage counter-flow washing is required to achieve high zinc content Waelz oxide (-60-68%) with low halide and iron oxide (<4%) values.
Waelz process is an established technology that is used in 80% of the processing capacity worldwide. It requires simple installation and Waelz oxide is welcomed by all zinc smelters. The by-product waelz slag can be used for road construction. Single leached Waelz oxide contains about 55-58% zinc with a very high chloride and fluoride content and insoluble iron compounds. Double leaching is required to dissolve the halides and improve the purity of waelz oxide.
EAF flue dust in a stainless steel plant contains less than 30% zinc containing compounds and significant amount of valuable metal oxides. It is therefore desirable to recover both zinc oxide rich dust as well as valuable alloy of iron, chromium, nickel & manganese etc in two separate streams so that both could be gainfully utilized. Zinc oxide rich dust for sale and a master alloy of valuable metals which could be fed to EAF for manufacture of appropriate stainless steels.
The present invention achieves this objectives.
SUMMARY OF THE INVENTION
An object of the present invention is to provide saleable zinc oxide through an economic & productive process having low halogen and iron contents. The basic steps of the process includes collection of flue dust from bag-house of electric arc furnace, briquetting of the dust, use of the briquettes in a submerged arc furnace for precious metal recovery, collection of flue dust from submerged arc furnace bag-house filters, washing with distilled hot water, filtering and drying in an oven to get a high purity zinc oxide powder with very low halide content. The molten alloy from the submerged arc furnace recovers valuable iron, chromium, nickel, manganese etc as a master alloy.

BRIEF DESCRIPTION OF THE DRAWINGS
The single drawing (Figure.l) is a flow sheet illustrating the preferred embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Stainless Steel plants are known to produce dusts that are enriched in various kinds of elements and their oxides. A typical analysis of dusts collected from different sources in a stainless steel plant is shown in the following table (Table I):
TABLE 1
ANALYSIS OF STAINLESS STEEL PLANT DUSTS
_ . . Iron Chromium Manganese Silicon Magnesium Calcium Zinc
Oxide Oxide Oxide Oxide Oxide Oxide Oxide
Pollution Dust (EAF) 40.3% 11.1% 3.4% 7.4% 1.5% 2.4% 23.0%
AGO Dust 23.8% 9.2% 6.6% 6.0% 18.3% 33.7% 1.8%
SAF Pollution Dust 22.1% 2.5% 3.1% 12.5% 2.1% 11.3% 16.5%
Shot Blasting Dust 81.6% 14.0% 1.8% 1.5% 0.3% 0.2% 0.1%
Mill Scale 72.5% 11.4% 6.8% 3.0% 0.6% 0.9% 0.0%
Pickling Sludge 36.4% 5^0% 0.3% 3.0% 0.5% 27.0% 0.3%
The preferred embodiment of the invention is illustrated by the flow sheet (Fig. I). A typical analysis of a sample of electric arc furnace flue dust where the charge contains stainless steel scrap shows the following percent composition (Table 2):
TABLE 2
ANALYSIS OF ELECTRIC ARC FURNACE FLUE DUST
Iron Oxide 36.20%
Zinc Oxide 22.80%
Chromium Oxide 10.30%
Silicon oxide 9.22%
Manganese Oxide 3.64%
Chlorine 3.23%>
Sodium Oxide 2.94%
Sulphur tri-oxide 2.78%
Calcium Oxide 2.33%
Potassium Oxide 1.48%)
Aluminium Oxide 1.33%
Magnesium Oxide 1.25%
Lead Oxide 0.97%)
Nitrous Oxide 0.58%
Copper Oxide 0.51%
Cadmium Oxide 0.22%)
Phosphorus Penta-oxide 0.13%)
Titanium Di-oxide 0.09%
TOTAL 100.00%)
The electric arc furnace flue dust collected in bag house filters contains in between 10-30% Zinc oxide. This flue dust is then briquetted using molasses as binder. The briquettes are then charged in a submerged arc ftirnace along with coke, quartz and slags from electric arc furnace for recovery of precious metals like chromium, nickel, manganese etc which can be again reused in steel making process.
Zinc oxide in the dust is promptly reduced by adjacent carbonaceous material to generate zinc vapor, then the vapor is immediately re-oxidized and the oxidized product is collected in the bag house filters of submerged arc furnace. A typical analysis of flue dust from a submerged arc furnace shows the following percent composition (Table 3):

TABLE 3
ANALYSIS OF SUBMERGED ARC Fl'RNACE FLUE Dl'ST
Zinc Oxide 55.10%
Ciiiorine 10.60%
Potassium Oxide 10.00%
Sodium Oxide 5.74%
Calcium Oxide 4.91%
Iron Oxide 3.43%
Magnesium Oxide 2.90%
Lead Oxide 2.01%
Manganese Oxide 1.59%
Aluminium Oxide 1.24%
Sulphur tri-oxide 1.05%
Chromium Oxide 0.79%
Cadmium Oxide 0.12%
Fluoride 0.12%
TOTAL 99.61%
The flue dust collected from submerged arc furnace bag house contains enriched zinc oxide along with impurities like chloride, potassium oxide etc. To reduce the chloride content in the dust, the flue dust was washed with water by continuous stirring. The washing was carried out at 40 °C. The dust was then filtered and dried in oven. The final analysis shows the reduction in chloride content and increase in zinc oxide content up to 76.6%. A typical analysis of flue dust (Table 4) after washing shows the following percent coinposition:
TABLE 4
ANALYSIS OF SUBMERGED ARC FlIRNACE FLUE DUST AFTER WARM DISTILLATION
Zinc Oxide 76.60%
Chlorine 0.33%
Potassium Oxide 0.60%
Silicon Oxide 5.91%
Calcium Oxide 4.42%
Iron Oxide 2.78%
Magnesium Oxide 2.69%
Lead Oxide 2.94%
Manganese Oxide 1.49%
Aluminium Oxide 1.06%
Sulphur tri-oxide 0.11%
Chromium Oxide 0.64%
Cadmium Oxide 0.22%
Fluoride
TOTAL 99.79%

We claim:
1. A method for selective recovery of valuable elements from Submerged Arc furnace dust containing
high commercial value oxides of Zinc, Iron, chromium, nickel & manganese, comprising the steps of:
a. Collection of Zinc Oxide Dust from EAF bag house.
b. Briquetting the dust using Binder.
c. Using the briquettes in Submerged Arc Furnace for metal recovery.
d. Collection of SAF dust in bag house filters.
e. Washing with hot water to get enriched Zinc Oxide content having an average purity of 75%.
f Recover a master alloy of iron, chromium, nickel & manganese etc from the melt.
2. A method as described in claim 1 wherein said flue dust collected from EAF bag house contains 10-35 % Zinc Oxide Dust.
3. A method as described in claim 1 wherein said binder used in briquetting the EAF flue dust contains 5-15% molasses ideally around 7-8%
4. A method as described in claim I wherein said briquettes are again charged in submerged arc furnace along with coke and quartz for recovery of precious metals like chromium, manganese & nickel along with which can be again used in electric arc furnaces for steel making process.
5. A method as described in claim I wherein flue dust is collected in SAF bag houses having enriched zinc oxide content.
6. A method as described in claim I wherein said water used for warm washing should have a pH of 5-9 preferably in range of 6-8.
7. A method as described in claim 1 wherein the product obtained after washing SAF flue dust is rich in zinc oxide content and having a high commercial value.
8. A method as described in claim 1 wherein the product obtained after washing SAF flue dust is low in halide and iron content.
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