BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates generally to a process for the recovery of usable economically valuable products, including a relatively pure iron or direct reduced iron product feedstock and, optionally, an iron oxide and an iron-carbon residual, from industrial waste streams typically comprising zinc compounds and iron compounds. During the process, both iron-rich waste materials and iron-poor waste materials can be added to the waste stream being treated.

A waste materials stream typically comprising zinc compounds and iron compounds, such as electric arc furnace (EAF) dust or the dust from metals production processes, is subjected to a combination of steps including leaching (digesting), resulting in a precipitate comprising iron oxides (an iron cake or IC), which then is subjected to roasting, resulting in an enriched iron compound (an enriched iron cake or EIC) which can be used as a feedstock for steel mills. The EIC typically is rich in direct reduced iron (DRI). During the recovery process, carbon compounds can be added to the waste stream, and a cake product is produced from the undissolved iron and carbon compounds, which also can be used as a feedstock for steel mills.

The present invention also relates to an enhanced recycling process which utilizes iron-rich materials produced by the invention as a feedstock ultimately to a steel mill. The iron-rich materials are fed to a reduction furnace in which the iron-rich cake is reduced to a higher purity iron product which may be fed a steel mill as the feed. Fumes exhausted from the reduction furnace can be processed by a baghouse or/and by a wet scrubber and the captured materials are then recycled to the present process where they are used in the recovery process of the invention. Fumes emanating from the reduction furnace contain paniculate matter, and may include potentially valuable zinc, cadmium, and lead constituents. The fumes are filtered in a baghouse, either at the steel mill's baghouse or at an independent baghouse. The filter

cake, which is an iron-poor mixture, may be combined with the initial waste feed (such as EAF dust) to the present process and/or other iron-rich materials, .and processed according to this invention.

An alternative method of removing the particulate matter from the reduction furnace fumes is the use of a wet scrubber. A primary embodiment of the alternative recycle of the present invention is to pass the reduction furnace fumes through a recirculated water wet scrubber, such as a venturi scrubber. The fume constituents soluble in water will be removed from the fumes by the recirculated water. The loaded recirculated water then may be introduced to the ammonium chloride leach step. Alternatively, the wet scrubber can use an ammonium chloride solution instead of water. The particulate matter soluble in ammonium chloride, such as for example zinc, cadmium, and lead constituents, will be removed in the ammonium chloride solution in the wet scrubber. The loaded ammonium chloride solution then can be combined with the leaching step discussed above, which preferably is an ammonium chloride leach, resulting in an exceptional increase in the recycle of waste streams from, for example, the steel making process. 2. Prior Art

Industrial waste streams typically contain components which have economic value if they can be recovered in an economic fashion. For example, U.S. Patent No. 3,849,121 to Burrows, now expired but which was assigned to a principal of the assignee of the present invention, discloses a method for the selective recovery of zinc oxide from industrial waste. The Burrows method comprises leaching a waste material 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 Burrows patent discloses a method to take EAF dust which is mainly a mixture of iron and zinc oxides and, in a series of steps, to separate out and discard the iron oxides and waste metals, so that the resulting zinc-compound- rich solution can be further treated to recover the zinc compounds.

Waste metal process dust typically has varying amounts of iron, lead, cadmium and other metals, in various forms, contained in the dust. The first step in

the Burrows patent is the treating of the EAF dust with an ammonium chloride solution to leach any zinc oxide, lead oxide and cadmium oxide present in the dust into solution, without any leaching of the iron oxides present. The second step in the Burrows process is cementation in which the solution obtained from the initial leach is filtered to remove any remaining solids and then zinc dust is added. The third step in the Burrows patent then takes the filtrate from the cementation process and cools the filtrate and obtains what are called zinc oxide crystals. The Burrows patent does not teach the treatment or recovery of any values from the discarded iron oxide containing precipitates. U.S. Patent No. 4,071,357 to Peters discloses a method for recovering metal values which includes a steam distillation step and a calcining step to precipitate zinc carbonate and to convert the zinc carbonate to zinc oxide, respectively. Peters further discloses the use of a solution containing approximately equal amounts of ammonia and carbon to leach the flue dust at room temperature, resulting in the extraction of only about half of the zinc in the dust, almost 7% of the iron, less than 5% of the lead, and less than half of the cadmium. However, Peters does not disclose a method for further treating the removed components not containing zinc compounds.

Thus, there exists a need for a method which will allow the recovery of an iron product from industrial waste streams which can be subjected to further treatments, resulting in a relatively pure iron product, such as direct reduced iron, which can be used as the feedstock for other processes. The industrial waste streams of most interest for this invention include a typical electric arc furnace waste stream and the particulate matter filtered or otherwise removed from various substeps of the invention or the steel mill, particularly from the fumes of a reduction furnace, such as for example a rotary hearth furnace. Producing an iron product with a minimum amount of impurities, such as zinc ferrite, is advantageous because the iron product can be used as the feedstock for steel production processes. A method which results in the recovery of an iron product has additional value in that the iron product can be sold for use in other processes. Furthermore, recovery and retreatment of exhaust and other waste products from the present invention and from other processes and

subprocesses has a beneficial effect on the environment, and a beneficial, economic effect on the cost of the steel making process.

Iron is smelted, or refined, in a furnace in which iron ore, coke and limestone are heated. Scrap iron also can be used as a feed to the iron smelting furnace. Prior to introducing scrap iron to the furnace, it is de-scaled of iron oxide, or rust. The mill scale, as it is called, is a waste product typically disposed of and not used in the iron production process. Steel is basically an iron alloyed with other chemical elements. Scrap steel also can be used as a feed in the making of steel. Mill scale also is not used in the steel production process. Finding an economical and/or beneficial use for this mill scale would provide iron and steel mills an opportunity to dispose of the mill scale. Likewise, used batteries provide a waste disposal problem. Used batteries also are not typically used in the steel making process. Rather than disposal in a landfill, it generally is preferable to recycle the used batteries, which -are rich in iron oxide. Finding an economical and/or beneficial use for used batteries would reduce the quantity of such material sent to landfills and provide a recycle for usable components. All of these iron oxide rich materials can be added to the waste stream feed which is fed into the present process.

As can be seen, there exists a need for a method which will allow exhausts and fumes from reduction furnaces or the like to be filtered in a baghouse or/and a wet scrubber so that the filtrate can be recycled back to the leaching step of the recovery process. This need is addressed by the present invention.

BRIEF SUMMARY OF THE INVENTION The present invention satisfies these needs in a method which recovers a relatively pure iron product from waste materials from industrial processes, such as waste streams from electric arc furnaces, typically containing zinc or zinc oxide, exhaust fumes from reduction furnaces, which typically are iron-poor, waste streams from metals production processes, waste metals themselves, including scrap iron or steel, or combinations of the waste streams. The non-iron solids and feed and product solutions used and/or produced in the process can be recycled such that the process has minimal solid or liquid wastes. Other solids can be recovered by treating other

compounds in the waste materials, for example zinc oxide, zinc, metal values, chemical values and other residues, all of which can be used in other processes. Iron- rich waste products, such as for example mill scale and used batteries, also can be added to the waste stream feed of the present process. The preferred iron-poor waste feed stream is taken from fumes emanating from industrial processes. Fumes from reduction furnaces and from the iron and steel making processes typically are filtered in baghouses. The waste product removed from the fumes in the baghouses may be subjected to the present process for recovery of chemical values and production of an iron-rich product. Likewise, the fumes emanating from direct-reduced iron reduction furnaces may be filtered, with the filtrate recycled to the present process. Alternatively, the fumes may be cleaned using a recirculating water or ammonium chloride solution wet scrubber. The loaded recirculating water or ammonium chloride solution (the scrubbant) may be recycled to the ammonium chloride leach step of the present invention. The preferred iron-rich waste material is common mill scale which is primarily iron oxides and/or used batteries which contain an economically recoverable quantity of iron oxide. Mill scale is the rust which is removed from scrap iron or steel before the scrap iron or steel is used as a feedstock for the iron or steel making process, respectively. Mill scale is not used in the iron or steel making process, but, if properly pretreated, is a significant source of iron which can be used as a feedstock for the iron and steel making process. Used batteries are a source of iron oxide. Other iron oxide rich materials are suitable.

The waste material is leached with an ammonium chloride solution resulting in a product solution (leachate) and undissolved materials (precipitate). In the leaching step, the zinc and/or zinc oxide dissolves in the ammonium chloride solution along with other metal oxides contained in the waste material, such as lead oxide and cadmium oxide. The resultant solution is filtered to remove the undissolved materials, including iron oxides and inert materials such as silicates, which will not dissolve in the ammonium chloride solution. The product solution and the undissolved materials are separated, with both the product solution and the

undissolved materials being further treated to recover valuable components. The remaining product solution can be treated to produce a zinc oxide product of 99% or greater purity. The remaining product solution also can be subjected to electrolysis in which zinc metal plates onto the cathode of the electrolysis cell. Any remaining product solution after crystallization or electrolysis can be recycled back to treat incoming waste material.

The undissolved material separated from the product solution is rich in iron oxides, and typically has some impurities such as zinc ferrite. The undissolved materials can be used without further treatment as a feedstock for steel mills so long as the quantity of impurities is not too great. It is preferable, however, to remove the impurities from the iron oxide prior to using the iron oxide as a feedstock. Even more preferably, reducing the iron oxide to direct reduced iron (DRI) is desired as DRI can be used to replace part or all of the steel scrap charge. Many steelmakers purchase DRI produced through other processes off site to add to the steelmaking process. The present process can be carried out on site and can recover valuable constituents, including possibly some wasted iron constituents, normally exhausted from the steelmaking process.

The iron oxide in the undissolved materials can be reduced to DRI in several manners. First, the undissolved materials may be subjected to a high temperature roasting step, preferably in the 980°C to 1315°C range, to reduce the iron oxide present in the undissolved materials to direct reduced iron. Roasting at this elevated temperature oxidizes and/or drives off the vast majority of the remaining impurities. To assist in the formation of a more usable direct reduced iron, the undissolved materials can be pelletized with carbon or sodium silicate, or another suitable material, at the end of or after the roasting step. Second, carbon, in the form of activated carbon, carbon dust, carbon pellets or the like, can be introduced to the ammonium chloride and waste material mixture during the leaching process. Third, carbon can be introduced to the dried undissolved material cake. When the iron oxide and carbon are heated under a reducing atmosphere, such as CO or CO ₂ or other common reducing gases, the carbon will react with the iron oxide, assisting in

reducing the iron oxide to DRI. Combining any of these methods can result in an even purer direct reduced iron product.

Prior to being leached (digested) by the ammonium chloride solution, the waste material mixture, which typically includes franklinite and magnetite, may be preroasted at temperatures greater than 500°C for a predetermined period of time. The preroasting causes a decomposition of the franklinite zinc oxide-iron oxide complex into zinc oxide, iron oxide and other components. The preroasting process generally comprises the steps of adding heat to the waste material mixture and/or passing heated reducing gases through the waste material mixture. Although all reducing gases are suitable, hydrogen and carbon-containing gases such as carbon dioxide are preferred, as well as mixing carbon (activated) with the waste material mixture and preroasting in a gas containing oxygen. While some iron oxide is reduced from Fe ₂ O ₃ and Fe ₃ O ₄ to FeO, no elemental iron is produced during the prero-asting step. Additionally, iron and iron oxides are not soluble to any degree in the basic ammonium chloride solution.

The present invention provides a method by which iron-rich by-products produced by the recovery process are reduced in a reduction furnace to DRI. Fumes exhausted by the reduction furnace are filtered through a baghouse or/and a wet scrubber. The materials captured by the baghouse or/and wet scrubber can be recycled back into the leaching step where they are used in the recovery process. The solid particles captured by the baghouse can be combined with the primary waste stream feed, or fed as a separate primary feed to the ammonium chloride leach. The loaded scrubbant liquid from the wet scrubber can be combined with the primary ammonium chloride leachant or, if an ammonium chloride solution is used as the scrubbing liquid, used as the primary ammonium chloride leachant.

The iron-rich by-products produced by the present process can be reduced into DRI by heating in an reduction furnace. The DRI then can be sent to a steel mill where it is used during the steel production process. The fumes exhausted by the steel mill, or any other reduction furnace, which typically are iron-poor, are filtered through a baghouse, which may be located at the steel mill, or/and through a wet scrubber,

which may also be located at the steel mill. The materials captured by the baghouse or/and wet scrubber can be recycled back to the present recovery process.

If the fumes are filtered through a baghouse, the captured materials will be solids which are placed into the primary waste material stream and added to the ammonium chloride solution of the leaching step. If the fumes are filtered through a wet scrubber, the captured materials will be discharged from the wet scrubber in a liquid stream directly into the ammonium chloride solution of the leaching step. If ammonium chloride is used as the scrubbing liquid, the ammonium chloride scrubbant may be used as the leaching (digesting) solution. If the scrubbant becomes too loaded, make-up ammonium chloride can be added. If the wet scrubber uses an ammonium chloride solution as the scrubbing liquid, it should be maintained at approximately 90°C and 23% by weight ammonium chloride in water.

It is an object of the present invention to provide a waste material recovery process which recovers chemical values from industrial waste streams, recycles exhaust fumes from furnaces such as electric arc furnaces and reduction furnaces, recycles exhaust fumes from industrial processes such as iron and steel making processes, and recycles other waste materials, including both iron-rich and iron-poor waste materials, to produce valuable products.

Another object of the present invention is to provide a process which uses the waste streams of various industrial processes, particularly the iron and steel making processes, so as to achieve an economical, environmentally friendly recycle process in the steel making industry.

Another object of the present invention is to provide a method for recovering iron and iron oxide from waste materials, such as exhaust fumes, furnace fumes, mill scale, used batteries, fly ash or flue dust, which contain other metals, such as zinc, lead oxide, and cadmium. Another object of the present invention is to provide a method for recovering iron oxide which can be used as is as a feedstock for steel production processes.

Another object of the present invention is to provide a method for producing direct reduced iron from iron oxide recovered as a residue from an ammonium

chloride leached waste material, such as exhaust fumes, furnace fumes, mill scale, used batteries, fly ash or flue dust. Another object of the present invention is to provide a method for recovering an iron product such as direct reduced iron and/or iron oxide which is economical, quick and efficient. These objects and other objects, features and advantages of the present invention will become apparent to one skilled in the art after reading the following Detailed Description of a Preferred Embodiment.

BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a schematic of a representative process of the present invention. Fig. 2 is a basic flow diagram of a representative process of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

By taking an iron cake comprising for the most part iron oxides, and roasting it at elevated temperatures under a reducing atmosphere, a product can be made which is equivalent to direct reduced iron. In general terms, heating the iron cake above 980°C up to about 1260°C, and typically no higher than 1315°C, a direct reduced iron product is formed. This direct reduced iron product then can be pelletized with carbon or with a sodium silicate, or other suitable compound, after it comes out of the furnace. The final product then can be used as a feedstock for steel mills without any additional treatment.

As discussed below, the additional step of roasting the iron cake, which is the undissolved precipitate, to reduce the iron oxide and to drive off any zinc, cadmium, and lead, and other impurities, is added to the end of a zinc oxide recovery process. The resulting iron product may have been reduced from several forms of the iron, such as FeO, Fe ₂ O ₃ , or Fe ₃ O ₄ , reduced to an iron extremely usable as the feedstock for steel mills.

The waste material is leached using ammonium chloride, and the remaining undissolved precipitate is, for the most part, an iron oxide cake. Iron-rich and iron- poor materials also may be added to be leached and further processed. During the roasting of the undissolved precipitate, the bond to the non-leachable zinc oxide-iron

oxide complex, franklinite, contained in the undissolved precipitate is broken, and the zinc oxide compounds are exhausted in the off gas and captured in a pollution control device, such as a baghouse, leaving the iron oxide cake as the residue. The iron oxide cake is roasted at an elevated temperature, causing the reduction of the iron oxide, leaving the iron metal values. The iron then can be mixed with a binder and formed into briquettes or cubes to be used as the feedstock. The exhausted impurities then can be recycled to recover, for example, zinc oxide, cadmium metal, and lead metal.

This method is carried out in its best mode in recovering the waste material from the waste streams of industrial or other processes, and combining it with waste material recovered from furnace exhaust streams or with other scrap metal waste streams. Many processes produce an iron poor waste stream, such as reduction furnaces and iron and steel making processes. Many other processes produce an iron oxide rich waste stream. Other processes remove iron oxide rich materials prior to processing. The iron poor materials are combined with a typical industrial waste stream which, after treatment, results in an iron-rich material suitable for use as a feedstock to a steel mill. Iron oxide rich materials also can be combined with the typical industrial waste stream and the iron poor waste stream. A typical industrial waste stream used is a flue gas where the charge contains galvanized steel, having the following percent composition:

TABLE I

Analvsis of Flue Dust

Component Percent By Weight

Zinc Oxide 39.64

Iron Oxide 36.74

Lead Oxide 5.72

Inert Materials 9.10

Calcium Oxide 2.80

Potassium Oxide 2.41

Manganese Oxide 1.29

Tin Oxide 1.13

Aluminum Oxide 0.38

Magnesium Oxide 0.33

Chromium Oxide 0.16

Copper Oxide 0.06

Silver 0.05

Unidentified Materials 0.22

General Process Description

Generally, the present process is a continuous method for the recovery of an iron product feedstock from waste material streams. The basic process steps comprise:

Basic Process Steps a. treating the waste material with an ammonium chloride solution at an elevated temperature to form a product solution and an undissolved precipitate comprising iron oxide; b. separating the product solution from the undissolved precipitate comprising the iron oxide; and

c. further treating the undissolved precipitate in a roasting process resulting in the recovery of a relatively pure iron product.

The iron poor .and iron rich waste materials, if in solid form such as from a baghouse, is added to the waste stream prior to treatment in Basic Process Step a. Alternatively, the iron poor and iron rich waste materials, if in solution form such as from a wet scrubber, is added directly to Basic Process Step a.

To the basic process steps, a number of additional steps may be added depending on the process conditions and iron properties desired. The additional steps include, either individually or in some combination: 1. preroasting the solid waste material at an elevated temperature;

2. preroasting the solid waste material at an elevated temperature and in a reducing atmosphere;

3. pretreating the solid waste material with an ammonium chloride solution at an elevated temperature to form a product solution and an undissolved precipitate comprising iron oxide, separating the product solution from the undissolved precipitate, roasting the undissolved precipitate at an elevated temperature and optionally in a reducing atmosphere, and then treating the undissolved precipitate with an ammonium chloride solution at an elevated temperature to form a product solution and an undissolved precipitate comprising iron oxide, and separating the product solution from the undissolved precipitate; and/or

4. preroasting the solid waste material at an elevated temperature and optionally in a reducing atmosphere, pretreating the waste material with an ammonium chloride solution at an elevated temperature to form a product solution and an undissolved precipitate comprising iron oxide, separating the product solution from the undissolved precipitate, roasting the undissolved precipitate at an elevated temperature and optionally in a reducing atmosphere, and then treating the undissolved precipitate with an ammonium chloride solution at an elevated temperature to form a product solution and an undissolved precipitate comprising iron oxide, and separating the product solution from the undissolved precipitate.

To the basic process steps, additional iron product purification steps may be added. For example:

1. elemental carbon can be added during the leaching step or steps to initiate the reduction of the iron oxide into direct reduced iron during the leaching step or steps. The elemental carbon may be added in a number of forms including, but not limited to, dust, granules, and pellets. The elemental carbon does not go into solution and remains with the undissolved precipitate.

2. elemental carbon can be added to the undissolved precipitate after it has been separated from the product solution. Combining elemental carbon and iron oxide in this manner at an elevated temperature and under a reducing atmosphere also will initiate the reduction of the iron oxide into direct reduced iron. The elemental carbon can be mixed into the undissolved precipitate in a number of manners including, but not limited to ribbon blenders and mixers.

Preferred Embodiment Referring to Fig. 1, a preferred embodiment of the process is shown.

Subprocess 100 comprises the digestion and filtration steps. Subprocess 200 comprises the direct reduced iron production steps. Subprocess 300 comprises the chemical values recovery steps. Subprocess 400 comprises the enhanced direct reduced iron production steps. Subprocess 500 comprises the feed process to the present invention.

Feed streams such as iron poor waste fume streams from electric arc furnaces 12 and other furnaces such as reduction furnaces or smelters 14 are filtered in a baghouse 16. Other feed streams such as iron rich direct reduced iron and pig iron, as well as scrap iron and steel, are subjected to the iron or steel making process. Exhaust fumes from such processes, which typically include an electric arc furnace or other reduction furnace, also are filtered in a baghouse 16. The constituents filtered out in baghouse 16 comprise the waste stream feed to subprocess 100.

In subprocess 100, the waste stream feed is leached in digester 18 with ammonium chloride, preferably at approximately 90°C and approximately 23% by weight concentration. Constituents soluble in ammonium chloride go into solution,

while constituents insoluble in ammonium chloride, such as iron oxides, precipitate out. The precipitates are filtered from the solution in filter 20. The filtered solution is sent to cementer 22, and subjected to subprocess 200 to recover other chemical values. The precipitate, which is an iron cake (IC), is sent to subprocess 300. In subprocess 300, the precipitate is dried and crushed in dryer/crusher 24.

Exhaust gases from dryer/crusher 24 may be sent to a baghouse such as baghouse 16, but more typically are sent to an air scrubber such as air scrubber 26 for cleaning, as the exhaust gases from dryer/crusher 24 typically do not have a significant quantity of recoverable constituents. The dried and crushed precipitates are compacted in compactor 28 and sent to a reduction furnace or smelter 14. In reduction furnace 14, the dried and crushed iron cake is roasted at between 980°C and 1315°C, producing an enriched iron cake (EIC) which can comprise direct reduced iron (DRI) .and pig iron, which can be in liquid form. The EIC can be compacted in a second compactor 30, and then cooled by cooling water in a cooling conveyor 32, to produce the DRI. The DRI then can be used as the feed to a steel mill EAF, and the process cycle starts over.

Exhaust fumes from the reduction furnace 14 are sent to scrubber 34, which preferably is a recirculating wet scrubber using water or an aqueous ammonium chloride solution. Exhaust fumes from EAFs such as EAF 12 also can be sent to scrubber 34. In scrubber 34, the exhaust fumes are scrubbed and the scrubbed off-gas released. The water or aqueous ammonium chloride solution containing the constituents scrubbed from the exhaust fumes is sent either to cementer 22 or digester 18, depending on purity; more pure solutions typically are sent to digester 18, while less pure solutions typically are sent to cementer 22. In the preferred embodiment, the furnace 12, 14 off-gases comprise ZnO and other particulate impurities. If the off-gases are scrubbed in scrubber 34, the water balance is maintained using a temperature control such as heat exchanger 36. Additionally, the concentration of ZnO and other solubles in the scrubbing liquid may be controlled by the addition of water W to the cementer 22, or ammonium chloride to the scrubber 34. As discussed above, if an ammonium chloride solution is used as the

scrubbing liquid, it is preferred to maintain the solution at approximately 90°C and approximately 23% NH ₄ C1.

Preroasting Process The preroasting step, as mentioned above, can be carried out prior to the initial leaching step, or between a first and second leaching step, or both. The waste material, which typically contains franklinite and magnetite is heated to temperatures greater than 500°C. This temperature causes a reaction which causes a decomposition of the stable franklinite phase into zinc oxide and other components, and yet does not allow for the complete reduction of zinc oxide to zinc metal. The resulting zinc oxide can be removed by sublimation or extraction with an ammonium chloride solution, such as by following the steps detailed above under the general process. The resulting material after extraction has less than 1% by weight zinc.

The solid waste material can be preroasted using many conventional roasting processes, such as, for example, direct or indirect heating and the passing of hot gases through the dust. For example, non-explosive mixtures of reducing gases, such as hydrogen gas and nitrogen or carbon dioxide, can be passed through the powder containing franklinite and magnetite. Hydrogen gas is not the only species that may be used for reductive decomposition of franklinite. It is possible to use carbon or simple carbon containing species, including carbon-containing reducing gases and elemental carbon. Heterogeneous gas phase reductions are faster than solid state reductions at lower temperatures and therefore suggest the use of carbon monoxide. The carbon monoxide can be generated in situ by mixing the franklinite powder with carbon and heating in the presence of oxygen at elevated temperatures. The oxygen concentration is controlled to optimize CO production. The carbon monoxide may be introduced as a separate source to more clearly separate the rate of carbon monoxide preparation from the rate of Franklinite decomposition. The prepared zinc oxide then can be removed by either ammonium chloride extraction or sublimation.

The iron rich and iron poor materials may be added to the process either before or after this preroasting step. As the preroasting step mainly is to assist in the

decomposition of franklinite, if the iron oxide rich material is devoid of franklinite, it need not be subjected to preroasting.

Leaching Treatment The waste material is subjected to an ammonium chloride leach. An ammonium chloride solution in water is prepared in known quantities and concentrations. If a two-stage leaching process is used, the feed material, such as the waste material flue dust described in Table I combined with any other feed material source, is added to the ammonium chloride solution. Otherwise, the feed material first is roasted. The majority of the waste material mixture, including any zinc and/or zinc oxide, lead oxide, cadmium oxide, and other metal oxides, dissolves in the ammonium chloride solution. The iron oxide does not dissolve in the ammonium chloride solution.

A 23% by weight ammonium chloride solution in water at a temperature of at least 90°C provides the best solubility for a waste stream comprising a significant quantity of zinc oxide. Concentrations of ammonium chloride below about 23% do not dissolve the maximum amount of zinc oxide from the waste material, and concentrations of ammonium chloride above about 23% tend to precipitate out ammonium chloride along with the zinc oxide when the solution is cooled. Iron oxide and inert materials such as silicates will not dissolve in the preferred solution. Zinc oxide, lead and cadmium oxide, are removed from the waste material by the dissolution in the ammonium chloride solution. The solid remaining after leaching contains iron oxides and some impurities including zinc, lead, cadmium, and possibly some other impurities. The remaining solid can be roasted in a reducing atmosphere, typically at a temperature greater than 420°C and often at 700°C to 900°C. The reducing atmosphere can be created by using hydrogen gas, simple carbon species gases such as carbon dioxide, or by heating the material in an oxygen containing gas in the presence of elemental carbon. The carbon preferably is in the form of dust or pellets. Typical preroasting times are from 30 minutes to 4 hours.

If the iron poor material is removed from the industrial waste stream using a wet scrubber, the preferred wet scrubber is an ammonium chloride solution wet

scrubber. By using an ammonium chloride wet scrubber, the loaded scrubbing solution, ammonium chloride, can be combined directly with the ammonium chloride leachant, or sent directly to the cementation step for removal of certain non-iron products. Alternatively, the loaded ammonium chloride scrubbing solution may act as the primary leachant in Basic Process Step a. Depending on the degree of loadedness of the ammonium chloride scrubbing solution, pure make-up ammonium chloride solution can be added to increase the effectiveness of the ammonium chloride leachant. Iron-poor waste streams in both solid and liquid form may be added for treatment. The solid stream may be added prior to Basic Process Step a, while the liquid stream may be added directly to Basic Process Step a.

Carbon Addition Step The present process can be operated to produce a high-quality iron-carbon cake as a residual product. The iron oxide contained in the waste stream does not go into solution in the ammonium chloride solution, but is filtered from the product solution as undissolved material. This iron oxide cake can be used as is as the feedstock to a steel mill; however, it is more valuable if reduced by reaction with elemental carbon to produce an iron-carbon or direct-reduced iron product. One preferred method for producing such an iron-carbon or direct-reduced iron product from the waste material comprises the steps of: a. treating the waste material combination with an ammonium chloride solution at an elevated temperature to form a product solution which comprises dissolved zinc and dissolved zinc oxide whereby any iron oxide in the waste material will not go into solution; b. adding carbon to the product solution whereby the carbon will not go into solution; and then c. separating the product solution from the undissolved materials present in the product solution including any of the iron oxide and the carbon.

A mixture of iron oxide and carbon is used by the steel industry as a feedstock for electric arc furnaces. The iron oxide cake which is removed as undissolved material from the leaching step is primarily iron oxide, being a mixture of Fe ₂ O ₃ and

Fe ₃ O ₄ . The iron oxide cake can be made into the mixture of iron oxide and carbon by adding elemental carbon to the iron oxide cake in several manners. First, carbon can be added to the leaching tank at the end of the leaching step but before the undissolved materials are separated from the product solution. Since the carbon is not soluble in the ammonium chloride solution and will not react in an aqueous solution, the iron oxide cake and the carbon can be separated from the product solution and made into a hard cake. Different size carbon, such as dust, granules, or pellets, may be used depending on the desires of the steel makers. Second, the carbon can be added to the iron oxide after the iron oxide has been separated from the product solution. The dried iron oxide and the carbon can be ribbon blended in a separate process.

Combining carbon and iron oxide in a reducing atmosphere and at an elevated temperature results in the reduction of the iron oxide, producing direct-reduced iron (DRI). DRI can be used to replace part or all of the steel scrap charged to a steel mill. In some operations, DRI is preferred to scrap because it has a known uniform composition -and generally contains no residual elements such as chromium, copper, nickel, and tin. When carbon-enriched iron oxide is melted, it forms a desired foamy slag because it contains both carbon and iron oxide. Because the price of steel scrap usually is lower than DRI, the use of DRI usually cannot be economically justified. DRI typically runs in the $120.00 and higher per ton range. However, since the iron oxide is a residual product of an economical recovery process, such as the recovery of zinc oxide from flue dust described generally below, with the main value of the process being from the zinc oxide product, the iron oxide or direct-reduced iron can be produced more economically. The roasting process produces vapors, from the zinc, lead and cadmium and other impurities, that have to be condensed into dust. These impurities can be sent to the baghouse at the end of the steel making process, mixed into the original waste dust, and then sent to the first leaching step, in a recycle fashion. Alternatively, the exhaust vapors and dust from the roasting step may be sent to a separate baghouse at a stand alone facility.

There are two preferred ways to add carbon to the iron oxide cake. First, it may be beneficial when the iron oxide cake comes out of the reclamation process to grind up the iron oxide cake, pelletize it with carbon and put it in a roasting furnace. Second, carbon can be added to the furnace with the iron oxide. The iron oxide cake can be treated in three manners. First, carbon can be added to the leaching step and the iron oxide cake will have carbon plus iron oxide. The iron oxide-carbon cake can go directly to the steel mill and, if it goes directly to the steel mill, then the reduction of the iron oxide would take place in the steel mill furnace. Second, the iron oxide-carbon cake can be pelletized and roasted in a reduction furnace to form direct reduced iron. The iron oxide precipitate, which typically contains around 80% solids, is ground up with carbon and formed into pellets, briquettes or cubes and then heated. These pellets, briquettes or cubes then can be introduced to a steel making furnace. The difference in the material that would be introduced to the furnace from the first manner and the second manner is that in the second manner, direct reduced iron is introduced to the steel making furnace, while in the first manner, a combination of iron oxide and carbon is introduced to the steel making furnace. The iron oxide plus carbon can be supplied to the steel mill as is. When this carbon enriched iron oxide is melted, it forms a foamy slag, and a foamy slag is desirable in steel making. Third, the carbon can be added through a ribbon blender, and then the iron oxide-carbon cake can be introduced either directly into the furnace or, preferably roasted in a reduction furnace first to form direct reduced iron, which would be preferred for steel making.

In order of preference, the first manner is the least preferable, that is adding the material itself as a mixture of carbon and iron oxide without any reducing agents mixed in with it. The second most preferable is the third manner, adding the material with carbon added to it either through the leaching step or through a ribbon blender and put directly into the furnace. The most preferable is the second manner, where carbon is added either though the leaching step or a ribbon blender, pelletizing or briquetting it, roasting it, and introducing it to the steel furnace.

In any manner, the fumes exhausting from the steel mill furnace and the reduction furnace typically are iron poor, but comprise other valuable components. The furnace exhaust fumes are an excellent source of iron poor waste materials useful for recovery in the present process. The exhaust fumes may be filtered in a baghouse, with the resulting filtrate being added to the waste stream feed of the present process, or with the resulting filtrate being the primary waste stream feed of the present process. The exhaust fumes also may be scrubbed in a wet scrubber, with the resulting loaded scrubbing solution being added to the ammonium chloride leachant of the present process. If an ammonium chloride scrubbing solution is used instead of water, the loaded ammonium chloride scrubbing solution may be used as the primary leachant of the present process.

Examples The following Examples demonstrate ways to increase the formation of zinc oxide from the product solution produced according to the present invention. Examples 1 and 2 do not include roasting and Examples 3 and 4 include roasting. The Examples are intended to illustrate the initial treatment of the waste material stream, including the leaching treatment and the initial waste material stream roasting, if employed.

Example 1 Prior Art

A metal dust of composition listed in Table I of the Burrows patent is added to 23% by weight NH ₄ C1 solution (30g NH ₄ C1 per lOOg H ₂ O), as discussed in the Burrows patent, in the amount of 1 gram of dust per 10 grams of solution. The solution is heated to a temperature of 90°C and stirred for a period of 1 hour, during which the zinc oxide in the dust dissolves. The remaining solid, which has a composition of approximately 60% iron oxide, 5% calcium oxide, 5% manganese, 30% other materials, is filtered out of the solution. This remaining solid is further treated according to the invention to recover feedstock grade direct reduced iron or iron oxide.

Example 2

A metal dust of composition listed in Table I is added to 23% weight NH ₄ C1 solution (30g NH ₄ C1 per lOOg H ₂ O). 1 gram of dust is used per 10 grams of solution. The solution is heated to a temperature of 90°C and stirred for a period of 1 hour. During this period the zinc oxide in the dust dissolves. The remaining solid, having a composition of approximately 60% iron oxide, 5% calcium oxide, 5% manganese, 30% other materials, is filtered out of the solution. This remaining solid is further treated according to the invention to recover feedstock grade direct reduced iron or iron oxide. Example 3

A dust containing 19.63% Zn, 27.75% Fe, 1.31% Pb, 9.99% Ca, and 0.024% Cd (analysis based on elements not oxides) was leached at 100°C in a 23% ammonium chloride solution. The solid remaining after the leaching process was dried and analyzed to contain 12.67% Zn, 4.6% Ca, 35.23% Fe, 0.7% Pb, and 0.01% Cd. This material was placed in a quartz boat in the presence of activated carbon and heated at 900°C for two hours in an atmosphere of 95% N ₂ and 5% O ₂ . After two hours, the material was removed and added to a 23% ammonium chloride solution at 100°C. The material was filtered and dried at 140°C for one hour to determine its composition. Analysis of this remaining solid was 42.84% Fe, 0.28% Zn, < 0.1% Pb, and < 0.01% Cd. This remaining solid is further treated according to the invention to recover feedstock grade direct reduced iron or iron oxide.

Example 4 A dust with the composition given in Table I is leached in 23% ammonium chloride solution for 1 hour at 100°C. The solid remaining (which contained 14% Zn) was placed in a quartz boat and heated to 700°C in an atmosphere of 8% H ₂ and 92% Ar. The material was cooled and reheated at 100°C in 23% ammonium chloride solution at 100°C. The solid was separated, dried and analyzed for zinc. The zinc was found to be less than 1%. The leached-roasted-leached material then can be subjected to the remainder of the general process.

Recovery of Zinc Oxide From Product Solution

To recover the zinc oxide from the product solution in subprocess 300, while the filtered zinc oxide and ammonium chloride solution is still at a temperature of 90°C or above, finely powdered zinc metal is added to the solution. Through an electrochemical reaction, any lead metal and cadmium in solution plates out onto the surfaces of the zinc metal particles. The addition of sufficient powdered zinc metal results in the removal of virtually all of the lead of the solution. The solution then is filtered to remove the solid lead, zinc and cadmium.

Powdered zinc metal alone may be added to the zinc oxide and ammonium chloride solution in order to remove the solid lead and cadmium. However, the zinc powder typically aggregates to form large clumps in the solution which sink to the bottom of the vessel. Rapid agitation typically will not prevent this aggregation from occurring. To keep the zinc powder suspended in the zinc oxide and ammonium chloride solution, any one of a number of water soluble polymers which act as antiflocculants or dispersants may be used. In addition, a number of surface active materials also will act to keep the zinc powder suspended, as will many compounds used in scale control. These materials only need be present in concentrations of 10 - 1000 ppm. Various suitable materials include water soluble polymer dispersants, scale controllers, and surfactants, such as lignosulfonates, polyphosphates, polyacrylates, polymethacrylates, maleic anhydride copolymers, polymaleic anhydride, phosphate esters and phosponates. Flocon 100 and other members of the Flocon series of maleic-based acrylic oligomers of various molecular weights of water soluble polymers, produced by FMC Corporation, also are effective. Adding the dispersants to a very high ionic strength solution containing a wide variety of ionic species is anathema to standard practice as dispersants often are not soluble in such high ionic strength solutions.

At this stage there is a filtrate rich in zinc compounds and a precipitate of lead, cadmium and other products. The filtrate and precipitate are separated, with the precipitate being further treated, if desired, to capture chemical values. The filtrate may be treated in several manners, two of which are preferred. First, the filtrate may

be cooled resulting in the crystallization and recovery of zinc oxide. Second, the filtrate may be subjected to electrolysis resulting in the generation and recovery of metallic zinc.

To recover zinc oxide, the filtrate then is cooled to a temperature of between about 20°C and 60°C resulting in the crystallization of a mixture of zinc compounds. The mixture contains a significant amount of diamino zinc dichloride, or other complex compounds which involves zinc amino complexes, hydrated zinc oxides and hydroxide species. Crystallization helps to achieve a high purity zinc oxide of controlled particle size, typically through control of the temperature-time cooling profile. Reverse natural cooling, that is cooling the solution slower at the beginning of the cooling period and faster at the end of the cooling period, is preferred to control the nucleation to crystal growth ratio and, ultimately, the crystal size distribution. The precipitated crystallized solid is filtered from the solution and washed with water at a temperature of between about 25°C and 100°C. The filtered solution is recycled for further charging with feed material. The diamino zinc dichloride dissolves in water.

Very little of the hydrated zinc oxide dissolves in the water. This resultant solution then is filtered to remove the hydrated zinc oxide species. The solid hydrated zinc oxide species filtered from the solution is placed in a drying oven at a temperature of over 100°C. After a sufficient drying period, the resultant dry white powder is essentially pure zinc oxide. The filtrate from the solution is recycled for charging with additional zinc compound mixture.

The zinc oxide may be dried at approximately 100°C. To ensure that the material is free of chloride, however, it is preferable to heat the zinc oxide to a higher temperature. Diamino zinc dichloride decomposes at 271 °C and ammonium chloride sublimes at 340°C. Therefore, heating the zinc oxide to a temperature above 271 °C is useful. The drying temperature should be kept below approximately 350°C to prevent the sublimation of significant amount of ammonium chloride. Therefore, it is preferable to dry the zinc oxide at a temperature in the range of 271°C to 350°C. Typically, the zinc oxide should be dried in this temperature range for approximately

2 to 60 minutes, and preferably from 5 to 20 minutes. A 10 minute drying time has been found to be a satisfactory average.

As the zinc, lead and cadmium contained in the feed materials are amphoteric species, by using ammonium chloride solution these species will go into solution, while any iron oxide present in the feed material will not go into solution. Other solutions, such as strong basic solutions having a pH greater than about 10 or strong acidic solutions having a pH less than about 3, also can be used to dissolve the zinc, lead and cadmium species; however, if strong acidic solutions are used, iron oxide will dissolve into the solution, and if strong basic solutions are used, iron oxide will become gelatinous. The lead and cadmium can be removed from the ammonium chloride solution through an electrochemical reaction which results in the precipitation of lead and cadmium in elemental form. The difference in solubility between diamino zinc dichloride and zinc oxide in water and in ammonium chloride solutions allows the selective dissolution of the diamino zinc dichloride such that pure zinc oxide can be recovered. This also can be used in the crystallization step to improve the relative amounts of diamino zinc dichloride and zinc oxide species form. Significantly, all of the zinc can be recycled so that all of the zinc eventually will be converted into zinc oxide.

The crystallization step of the present process can be done continuously in order to increase the throughput and maximize the zinc oxide yield after the washing and drying step.

Preroasting Step for Enhanced Zinc Recovery The zinc dust obtained from various sources have shown by chemical analysis to contain from 20% - 25% zinc by weight. X-ray diffraction indicates clearly the existence of certain crystalline phases in this dust, specifically zinc oxide. The positive identification of the iron phase is complicated by the possible structural types (i.e. spinel type iron phases showing almost identical diffraction patterns). The zinc oxide (as well as smaller concentrations of lead or cadmium oxide) are removed from the initial dust by dissolution in a concentrated ammonium chloride solution (23% ammonium chloride).

Filtration and washing of the undissolved species leaves a residual powder. This powder shows a zinc concentration that is still elevated (i.e., 10 - 13% by weight), but that is not zinc oxide. X-ray diffraction indicates that all crystalline phases can be identified by spinel type phases. The combination of chemical analysis and x-ray diffraction indicates that this powder is a combination of magnetite (iron oxide: Fe ₃ O ₄ ). Both of these phases have very similar spinel type structures. The zinc within the franklinite, (Fe, Mn, Zn)(FeMn) O ₄ , cannot be removed by dissolution with ammonium chloride. In addition, no simple extraction process will remove zinc from this stable oxide phase. Although this compound is very stable to oxidation (all elements in the highest oxidation state), it is relatively easy to destroy this compound by reduction at elevated temperatures. The reduction of the franklinite in an atmosphere that cannot readily reduce zinc oxide or allow for the rapid oxidation of zinc to zinc oxide following reduction and subsequently recover the zinc oxide by ammonium chloride extraction or sublimation (the highly volatile zinc oxide will sublime from the mixture at relatively low temperatures and recondense at the cold locations of the roaster). The alternative will be complete reduction of the franklinite to zinc metal and removal by distillation or separation of the molten zinc by settling techniques. This can be accomplished by including the preroasting step disclosed above. Iron By-Product Recycle

Iron-rich by-products produced during the recovery process can be processed further to obtain an end product which can be recycled back into the leaching step of the recovery process of the present invention. The iron-rich by-products preferably are reduced to DRI in a reduction furnace. During the reduction process, exhausts fumes which consists primarily of zinc, lead and cadmium are produced in the reduction furnace.

In accordance with a first embodiment, the DRI is sent to a steel mill where it is used in the production of steel. The steel production process results in exhaust fumes which are processed through the baghouse or/and a wet scrubber, either or both of which can be located at the steel mill. Fumes processed through the baghouse are

filtered, and the captured solid residuum, along with an added amount of EAF dust, is recycled back into the waste materials stream whereby it is returned to the leaching step of the recovery process. Fumes processed through the wet scrubber are scrubbed in a liquid stream and the residual impurities obtained from the scrubbing process are discharged from the wet scrubber directly into the .ammonium chloride solution of the leaching step.

In accordance with a second embodiment, the fumes exhausted from the reduction furnace used to produce the DRI are processed through the baghouse or/and the wet scrubber. Fumes processed through the baghouse are filtered, and the captured solid residuum is recycled back into the waste material str&am, whereby it is returned to the ammonium chloride solution of the leaching step. In this embodiment, no EAF dust need be added in with the solid residuum. Fumes processed through the wet scrubber are scrubbed in a liquid stream and the residual impurities obtained from the filtering process are discharged from the wet scrubber directly into the ammonium chloride solution of the leaching step.

Therefore, iron-rich products which are produced during the recovery process of the present invention can be further processed to produce fumes consisting primarily of zinc, lead and cadmium which are captured in a baghouse or/and a wet scrubber and recycled back into the ammonium chloride solution of the leaching step to be used in the recovery process.

It should be noted that the locations of the baghouse and wet scrubber are a matter of design choice, plant efficiency and convenience. The present invention is not limited in this aspect. For example, steel mills are equipped with baghouses and wet scrubbers which can be used in the recycling process of the present invention. Similarly, the locations of the baghouse or wet scrubber used to process fumes from the DRI reduction furnace are also a matter of design choice, plant efficiency and convenience.

The above detailed description of a preferred embodiment is for illustrative purposes only and is not intended to limit the spirit or scope of the invention, or its equivalents, as defined in the appended claims.