TECHNICALFIELD The present invention relates generally to a process for the efficient extraction and recovery of zinc, lead and cadmium products from industrial waste streams comprising zinc, lead, cadmium and iron compounds, while also providing for the enhanced product yield in an iron production process. The present invention relates more specifically to a process for subjecting a waste materials stream comprising zinc compounds and iron compounds, such as electric arc furnace (EAF) dust, to a beneficiation step, for increasing the level of zinc, lead and cadmium, while decreasing the iron content, in a recycling operation which recycles process solutions for reuse, and produces a briquette product from undissolved iron and carbon compounds which can be used as a supplemental feedstock for steel mills. Once the zinc, lead and cadmium compounds have been recovered, they are further purified by a process which is preferably based on the solubility of zinc oxide in a concentrated sodium hydroxide solution. This final purification process can be controlled in such a manner that the particle size and surface area of the zinc oxide produced can be controlled. Additionally, zinc compounds can be quickly, easily, and economically, synthesized from the aqueous zinc oxide slurry resulting from this process. The cadmium and lead compounds also can be recovered as chemical values.

The present invention also provides a further zinc oxide purification process which involves precipitating zinc oxide in such a manner that the desired purity and particle characteristics can be obtained. One method to control particle size is through the control of the conditions of the washing step. Additionally, although the zinc oxide purification process preferably utilizes a sodium hydroxide solution as the intermediate, the purification process of the present invention also provides for preparation of zinc oxide having particular purity and particle characteristics by utilizing intermediates such as ammonium chloride liquor, ammonium sulfate, ammonium phosphate, potassium hydroxide, ammonia/ammonium oxalate and ammonia/ammonium carbonate solutions. Once the zinc oxide has been dissolved in

the solution, controlled dilution results in the precipitation of zinc oxide having predetermined purity and particle characteristics.

BACKGROUND ART Zinc oxide typically is a coarse white or grayish powder which has a variety of uses including as an accelerator activator, as a pigment, as a dietary supplement and in the semiconductor field. Zinc oxide is found in commercial by-products including waste material streams such as fly ash and flue dust. Many of the uses of zinc oxide require that the zinc oxide have certain particle size, shape and purity characteristics. Therefore, many grades of zinc oxide having different purity and particle characteristics have been developed to meet the diverse industry requirements. Methods for recovering zinc oxides are known in the art, including recovering zinc oxide from industrial waste materials. Such previous methods have included leaching with mineral acid, caustic soda, ammonium hydroxide, and ammonium carbonate solutions. However, these methods have low yields of zinc oxide and typically do not recover pure zinc oxide, the recovered zinc oxide being contaminated with other metal salts. Therefore, in order to obtain pure zinc oxide, subsequent roasting and evaporation processes were necessary. Today, most zinc oxide is made by the so called French Process which involves controlled burning of zinc metal vapor in air to obtain zinc oxide having exceptional chemical purity. Typical EAF flue dust usually contains less than 20% zinc-containing compounds less than half of which is oxidic. Typical flue dust also can contain significant amounts of iron compounds. Zinc recovered from extractive processes is much more commercially valuable if the levels of insoluble (such as iron compounds) is decreased and a higher proportion ofthe zinc is oxidic rather then ferric. Thus there exists a need for a method for the beneficiation of EAF dust in which levels of zinc, lead, and cadmium are increased and the iron content is decreased.

Thus, there exists a need for a method which will economically extract zinc and other products from industrial waste streams. The present method relates to the beneficiation of EAF dust to increase the levels of zinc and other products in the dust, allowing more economical recovery of the products, while decreasing the level of iron

compounds in the dust. The present method also demonstrates how to increase the formation of desired zinc products and decrease the formation of undesired zinc products. Furthermore, once the desired zinc products have been obtained, zinc oxide can be further purified by a process which is based on the solubility of zinc oxide in a concentrated sodium hydroxide solution. This purification process can be controlled to produce zinc oxide having a desired size and surface area.

There also exists a need for a method which will allow the recovery of iron oxide from industrial waste streams which can be used with little or no additional treatment as the feedstock for other processes. Producing an iron oxide with a minimum amount of impurities, such as zinc ferrite, is advantageous because the iron oxide can be used as the feedstock for steel production processes. A method which results in the recovery of iron oxide would have additional value in that the iron oxide could be sold for use in other processes.

Additionally, there exists a need for an economical method of purifying zinc oxide which allows a highly purified zinc oxide to be obtained which has predeterminable and controllable purity, and particle size and shape characteristics.

DISCLOSURE OF INVENTION The present invention provides for the recovery of zinc and other metal and chemical values from EAF dust. The basic steps for the process comprise collecting EAF dust from one or more EAF batches, preferably three batches, briquetting the EAF dust with a carbon product, preferably coal fines, and then reintroducing the briquetted EAF dust into the arc furnace, preferably with a fourth batch of charge. The iron oxide contained in the EAF dust from the fourth batch is reduced to metallic iron, and the EAF dust from this fourth batch is higher in content of zinc and other metals, such as cadmium and lead. This higher content dust from the fourth batch is very suitable for further hydrometallurgical processing, as also disclosed herein.

Alternatively, the present process can be operated in a more continuous fashion. The EAF dust from each arc furnace batch is split into two streams, with the first stream being briquetted with carbon and insolubles from the second stream and recycled back to the arc furnace in the next batch. The second stream is fed to a

hydrometallurgical process where the iron insolubles are separated from the dust and sent to the briquetting mix. The compounds remaining in the dust now in the hydrometallurgical process are beneficiated and other chemical and/or metal values are recovered. After beneficiation, essentially pure zinc oxide may be recovered along with zinc metal and values of other metallic elements contained in the waste material such as lead, silver, and cadmium. The solutions used in the recovery process are recycled such that the process does not have any liquid wastes. The solids recovered from the process, namely, the zinc oxide, zinc, metal values, and other residues all can be used in other processes. The iron oxide cake, produced from the iron insolubles as discussed above, is sent to the briquetting mix, which can be used directly as the feedstock for the typical steel production process. The zinc oxide recovered in this process is further purified based on the solubility of zinc oxide in a concentrated sodium hydroxide solution. This purification process can be controlled to produce zinc oxide having a desired size and surface area.

One method for purifying zinc oxide to obtain zinc oxide crystals having predetermined purity and particle characteristics comprises the steps of dissolving the zinc oxide containing product in an intermediate, filtering out any undissolved materials, precipitating zinc oxide crystals out of the intermediate in a controlled manner such that the zinc oxide crystals have predetermined purity and particle characteristics, filtering out the zinc oxide crystals, washing the zinc oxide crystals, and then drying the zinc oxide crystals.

The intermediate preferably is selected from the group consisting of sodium hydroxide, ammonium sulfate, ammonium chloride liquor, ammonium phosphate, potassium hydroxide, ammonia/ammonium oxalate, and ammonia/ammonium carbonate solutions. Most preferably, the intermediate is a concentrated 50% - 70% sodium hydroxide solution. The precipitation step is accomplished by diluting the solution at a predetermined rate by a factor ranging from 3 to 30 at a temperature ranging from about 25°C to 100°C at atmospheric pressure, and even over 100°C at pressures greater than atmospheric pressure, to precipitate the zinc oxide crystals.

The preferred method for purifying zinc oxide to obtain zinc oxide crystals having predetermined purity and particle characteristics comprises controlling the method by which the intermediate is added to the zinc oxide to control the particle size of the resulting purified zinc oxide. Preferably, the sodium hydroxide is dispersed into droplets averaging in size from 100 to 300 microns, with 150-250 microns being preferred, and the best results being obtained at approximately 180 microns. Generally, the smaller the droplet size the larger the surface area of the resulting zinc oxide particles.

Once the substantially pure zinc oxide is recovered from the process, the purification process takes place resulting in zinc oxide which is at least 99.8% pure. Most of the metal impurities contained in the zinc oxide will not dissolve, including manganese, iron and cadmium. Any lead, calcium or chloride contained in the zinc oxide will dissolve. The solution then is filtered to remove the undissolved solids, which then are recycled back to the metals recovery section of the plant and thereby returned to the recycling process ofthe present invention.

The diluted sodium hydroxide solution then is sent to an evaporative condenser where the solution is concentrated back to 50% - 70% sodium hydroxide so that it can be reused. When a steady state has been achieved, this step will result in the formation of sodium chloride crystals which will be filtered out ofthe solution and recovered. This is because sodium chloride formed by the chloride present in the zinc oxide is less soluble in a concentrated sodium hydroxide solution than in dilute sodium hydroxide. After the sodium chloride is filtered out, the concentrated solution can be reused in the purification process ofthe present invention.

Periodically, lead will be removed from the sodium hydroxide solution by cementation. This involves the addition of zinc dust which will displace the lead in solution. The lead will be filtered out and sent to the lead recovery portion of the plant.

By controlling the manner in which the zinc oxide precipitates out of the intermediate during the zinc oxide crystallization step, it is possible to control the particle size hence the surface area of the zinc oxide produced as well as the purity.

Furthermore, the purification process of the present invention can be used to purify zinc oxide obtained from other sources.

Therefore, it is an object of the present invention to provide a method for beneficiating and recovering zinc oxide from waste materials, such as fly ash or flue dust, which contain other metals, such as iron oxide, lead oxide, cadmium and other materials.

Yet another object of the present invention is to provide a method for recovering zinc oxide in which all leaching and washing solutions are recycled for further use, and no leaching or washing solutions are disposed of into the sewers or the environment.

Still another object of the present invention is to provide a method for recovering zinc oxide which also results in the precipitation in elemental form of any lead and cadmium metals contained in the starting materials.

It is another object ofthe present invention to provide a method for recovering zinc oxide in which all of the zinc can be recycled so that all of the zinc eventually will be converted to zinc oxide.

Still another object of the present invention is to provide a method for recovering iron oxide from waste materials, such as fly ash or flue dust, which contain other metals, such as zinc, lead oxide, and cadmium, and using the iron oxide as a precursor to a supplemental feed to an iron or steel making process.

Another object of the present invention is to provide a method for recovering zinc metal, zinc oxide and/or iron oxide which is economical, quick and efficient.

A further object of the present invention is to provide methods of purifying zinc oxide which allows a highly purified zinc oxide to be obtained which has predeterminable and controllable purity, and particle size and shape characteristics.

It is a further object of this invention to provide a method for generating reduced iron units without the need for a separate reduction furnace.

It is yet another object of this invention to provide a method of EAF dust beneficiation useful for on-site hydrometallurgical dust treatments.

These objects and other objects, features and advantages of the present invention will become apparent to one skilled in the art when the following Detailed Description of a Preferred Embodiment is read in conjunction with the attached figures. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic diagram illustrating the general process used in the present invention.

Fig. 2 is a schematic diagram illustrating an alternate process used in the present invention. Fig. 3 is a schematic diagram illustrating the general process used in the present invention in combination with the production of usable iron products.

Fig. 4 is a schematic diagram illustrating the use ofthe general process used in the present invention in combination with a complete chemical and metal values recovery process.

BEST MODE FOR CARRYING OUT THE INVENTION

A typical industrial waste stream used is a flue dust where the charge contains galvanized steel, having the following percent composition:

TABLE I

Λnalvsis of Flue Dust

Component Percent By Weight

Zinc Oxide 39-40

Iron Oxide 36-37

Lead Oxide 5-6

Inert Materials 9-10

Calcium Oxide 2-3

Potassium Oxide 2-3

Manganese Oxide 1-2

Tin Oxide 1-2

Aluminum Oxide 0-1

Magnesium Oxide 0-1

Chromium Oxide 0-1

Copper Oxide 0-1

Silver 0-1

Unidentified Materials 0-1

I. BENEFICIATION OF EAF DUST

Beneficiation of EAF dust is the process by which levels of certain compounds, such as zinc, lead, and cadmium, are increased and the iron content is decreased. Zinc recovered from extractive processes is much more commercially valuable if the levels of insoluables (such as iron compounds) is decreased and the zinc is of a higher value if a higher proportion of the zinc is oxidic rather then ferric. Disclosed herein are two related methods for the beneficiation of EAF dust.

Electric arc furnaces operate on a batch system wherein each charge quantity 400 of scrap is melted in a cycle, usually around 50 minutes. In the first beneficiation process, shown in Fig. 1, the EAF dust from several batches 101, preferably three, can be collected and briquetted with a carbon product, such as coal fines («60#) in a ratio of approximately 4:1 dust to coal in a briquetting process 60. The briquettes 64 are added to the fourth arc furnace batch along with the charge 400. The high temperature and reducing conditions in the arc furnace 12 reduce the iron oxide in the briquettes 64 to metallic iron, thus adding to the overall product yield from the arc furnace 12. The oxidic and ferritic zinc in the briquettes 64 is driven off as a zinc oxide fume 102 and collected. The EAF dust from the fourth batch 102 contains effectively all the zinc, lead and cadmium from the previous three batches 101. The zinc recovered from the EAF dust from the fourth batch 102 will be more than 80% oxidic rather than the normally less than 50% found in non-beneficiated dust 101. Further, the proportion of zinc in the EAF dust from the fourth batch 102 is typically more than 50% rather than below 20% for the previous batches 101 , with a corresponding reduction in iron content. Such a beneficiated dust 102 is also very suited for further hydrometallurgical processing 100, and any iron-containing insoluables 68 recovered from that process may be added to the briquetting mix 60 for further process cycles. The resulting product solution 74 is optionally processed further 300 to recover zinc and other chemical values. Further, any iron insoluables 86 recovered from the chemical value recovery subprocess 300 may be reintroduced to the briquetting process 60.

The second beneficiation process, alternative to the one above and shown in Fig. 2, is useful in instances when the steel company may not want to operate a batch procedure. The dust 10 from the EAF 12 is split into a first stream 104 and a second stream 103. Dust collected from the first stream 104 is briquetted 60 with coal dust and the briquettes 64 are returned to the furnace 12 in an on-going basis. The second stream 103 is fed to the hydrometallurgical process 100 and the iron insoluables 68 are returned to the briquetting mix 60. The solubilized compounds 74 can be processed further 300 to recover zinc and other chemical values. Iron insoluables 86

recovered from subprocess 300 may also be reintroduced to the briquetting process 60.

It can be seen by one skilled in the art that by choosing a particular ratio for the first stream 104 and the second stream 103, a steady state will be reached in which the dust 10 arising from the electric arc furnace 12 is beneficiated with respect to zinc, lead, and cadmium. This ratio should be chosen in consideration of the level of beneficiation compared to the level of iron insoluables 68 and the amount of coal added.

In choosing between the two beneficiation strategies described herein, one skilled in the art will consider the benefits of a continuous process (Fig. 2) over those of a batch process (Fig. 1). Additionally one skilled in the art will consider the efficiency ofthe batch process in terms of energy and coal utilization in comparison to a continuous process. The operating cycles and conditions may be changed to meet individual requirements of the actual operation. Process strategies embodying aspects of both beneficiation strategies, either in part or in their entirety, could be employed to achieve a cost-effective beneficiation of EAF dust 10. The operation of the dust baghouse may be modified to allow an increase in the beneficiation. Each of these decisions is well within the knowledge of one skilled in the art.

The beneficiated EAF dust 102, 103 is subjected to various processes to separate the iron insolubles 68 and other insolubles from the dust, and to purify and recover zinc, zinc compounds, cadmium, lead, and/or other chemical and/or metal values, depending on the make-up of the charge 400 to the arc furnace 12. Other processes may be added to these basic steps to recover whatever may be contained in the initial charge 400 to the arc furnace 12. Several of these other processes are described below and are the subject of other patents and patent applications owned by the owner of the present invention. The two primary purification processes described, the ammonium chloride leach 100 and the sodium hydroxide leach 300, can be used individually or in combination with each other to further purify the beneficiated EAF dust.

II. GENERAL DESCRIPTION OF A PROCESS FOR PRODUCING A ZINC OXIDE PRODUCT USING AN AMMONIUM CHLORIDE LEACH

The beneficiated EAF dust 102, 103 is leached in process 100 with an ammonium chloride solution resulting in a product solution 74 and undissolved materials 68. The product solution and the undissolved materials are separated, with both the product solution and the undissolved materials being further treated 300 to . recover valuable components. Zinc metal is added to the product solution to cement out any lead and cadmium contained in the product solution. The remaining product solution is rich in zinc compounds.

The remaining product solution then can be treated in one or more of at least two manners. First, the remaining product solution can be cooled thereby precipitating the zinc components from the product solution as a mixture of crystallized zinc compounds. These crystallized zinc compounds are separated from the product solution, washed and then dried at elevated temperatures, resulting in a zinc oxide product of 99% or greater purity. Second, the remaining product solution 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 is recycled back to treat incoming waste material. The zinc oxide of 99% or greater purity then can be further purified by a purification process which produces zinc oxide which is at least 99.8% pure. This purification process is preferably based on the solubility of zinc oxide in a concentrated sodium hydroxide solution. However, intermediates other than sodium hydroxide can also be used. The result is zinc oxide which is even of greater purity. Furthermore, the process can be controlled to obtain zinc oxide having a desired size and surface area.

The undissolved material separated from the product solution is rich in iron oxide, and typically has some impurities such as zinc ferrite. The undissolved materials can be used as a feedstock for steel mills so long as the quantity of impurities is not too great. It is preferable to remove the impurities from the iron oxide prior to using the iron oxide as a feedstock. Reducing the iron oxide to direct-

reduced iron (DRI) also is desired as DR1 can be used to replace part or all of the steel scrap charge. This is accomplished in the briquetting step.

Prior to being leached by the ammonium chloride solution, the waste material, typically including franklinite and magnetite, may be roasted at temperatures greater than 500°C for a predetermined period of time. The roasting causes a decomposition of the franklinite zinc oxide-iron oxide complex into zinc oxide, iron oxide and other components. The roasting process generally comprises the steps of adding heat to the waste material and/or passing heated reducing gases through the waste material. 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 material and roasting 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 roasting step. Additionally, iron and iron oxides are not soluble to any degree in the basic ammonium chloride solution. In process 100, a leach may be used in which 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 74 is separated from the undissolved materials 68, such as iron oxides and inert materials such as silicates, which will not dissolve in the ammonium chloride solution. In process 300, finely powdered zinc metal can be added to the resultant solution 74 at a temperature of about 90°C or above. A dispersant may be added at this point to prevent the finely powdered zinc metal from flocculating and becoming less effective. Through an electrochemical reaction, lead metal and some cadmium plates out on the surface of the zinc metal particles. The addition of sufficient powdered zinc metal results in the removal of virtually all of the lead from the resultant solution 74. The resultant solution 74 is filtered to remove the solid lead, zinc and cadmium. These initial steps, with the exception of adding the dispersant, have been generally disclosed in the prior art, yet have not resulted in the production of essentially pure zinc oxide.

The filtrate then can be cooled to a temperature of between about 20°C and 60°C resulting in the crystallization of a mixture of zinc compounds. The crystallization step helps to achieve a high purity zinc oxide of controlled particle size. During the crystallization step, the filtrate can be cooled to its final temperature by controlling the cooling profile. The use of a reverse natural cooling profile is preferred as its results in a more desirable nucieation to crystal growth ratio. The filtrate contains a significant amount of diamino zinc dichloride, or other complex compounds which involve zinc amino complexes, as well as hydrated zinc oxide and hydroxide species. The solid precipitate is filtered from the solution, the solution recycled, and the solid precipitate washed with water at a temperature between about 25°C and 100°C. The diamino zinc dichloride dissolves in the wash water leaving the majority of the hydrated zinc oxide species as the precipitated solid. The precipitated solid then is filtered from the solution, the resulting solution being recycled, and the solid precipitate placed in a drying oven at a temperature above 100°C and preferably between about 100°C and 350°C, resulting in a dry white zinc oxide powder. These additional steps allow the production and recovery of substantially pure zinc oxide. Alternatively, the filtrate can be subjected to electrolysis to recover zinc metal. Generally, the zinc oxide production process comprises the steps of: a. treating the waste material fumes 102 generated by a furnace 12 with an ammonium chloride solution at an elevated temperature to form a product solution 74 which comprises dissolved zinc and dissolved zinc oxide whereby any iron oxide in the waste material will not go into solution and will be contained in the undissolved precipitate 68; b. separating the product solution 74 from any undissolved materials 68 present in the product solution 74 including any ofthe iron oxide; c. further treating the product solution 74 by:

1. adding zinc metal and a dispersant to the product solution 74 whereby any lead and cadmium ions contained within the product solution 74 are displaced by the zinc metal and precipitate out of the product solution 74 as

lead and cadmium metals and the dispersant is selected from the group consisting of dispersants which will prevent the aggregation of said zinc metal;

2. separating the product solution 74 from the lead and cadmium metals; 3. lowering the temperature of the product solution 74 thereby precipitating the zinc component as a mixture of crystallized zinc compounds;

4. separating the precipitated zinc compounds from the product solution 74;

5. washing the zinc compounds solids with a wash water thereby solubilizing certain ofthe zinc compounds;

6. separating the remaining zinc compounds solids from the solution; and then

7. drying the remaining zinc compounds solids at a temperature of at least 100°C whereby the resulting product is zinc oxide of 99% or greater purity.

The undissolved materials 68 and precipitated non-zinc compounds 86 are sent to the briquetting process 60 for further treatment.

The process also can comprise a two-stage leaching process for even greater yields of zinc oxide. The two-stage process comprises the steps of: a. treating the waste material 400 a first time with an ammonium chloride solution at an elevated temperature to form a first product solution which comprises dissolved zinc constituents whereby any iron oxide in the waste material will not go into solution; b. separating the first product solution from the undissolved waste material compounds present in the first product solution including any of the iron oxide; c. roasting the undissolved waste material compounds at an elevated temperature and in a reducing atmosphere in a furnace 12; d. treating the waste material fumes 102 generated by roasting in a processing step 300 with the ammonium chloride solution at an elevated temperature

to form a second product solution which comprises dissolved zinc constituents whereby any iron oxide in the waste material fumes will not go into solution; e. combining the first and second product solutions to form a combined product solution 74; and then f. treating the combined product solution 74 similarly to the treatment of the product solution 74 in step c ofthe general method disclosed above.

An ammonium chloride solution in water is prepared in known quantities and concentrations. If the two-stage leaching process is used, the feed material which contains the zinc species, such as the waste material flue dust described in Table I or any other feed material source which contains zinc or zinc oxide mixed with other metals, is added to the ammonium chloride solution at a temperature of about 90°C or above. Otherwise, the feed material is roasted. The zinc and/or zinc oxide dissolves in the ammonium chloride solution along with other metal oxides, such as lead oxide and cadmium oxide. The iron oxide does not dissolve in the ammonium chloride solution. The solubility of zinc oxide in ammonium chloride solutions is shown in Table II.

TABLE II Solubility of ZnO in 23% NH ₄ C1 solution Temperature °C g Dissolved/1 QQ g

90 14.6

80 13.3

70 8.4 60 5.0

50 3.7

40 2.3

A 18-23% by weight ammonium chloride solution in water at a temperature of at least 90°C provides the best solubility of zinc oxide. Concentrations of ammonium

chloride below about 18% do not dissolve the maximum amount of zinc oxide from the flue dust, 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. The zinc oxide and ammonium chloride solution then is filtered to remove any undissolved material, including the iron oxide.

To recover the zinc oxide, while the filtered zinc oxide and ammonium chloride solution is still hot, that is 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 ofthe zinc metal particles. The addition of sufficient powdered zinc metal results in the removal of virtually all of the lead ofthe 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. To keep the zinc powder suspended and from aggregating in the zinc oxide and ammonium chloride solution, any one of a number of water soluble polymers which act as antiflocculants or dispersants, surface active materials, and/or compounds used in scale control may be used. 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. A discussion of these various materials can be found in the literature, such as Drew, Principles of Industrial Waste Treatment, pages 79-84, which is incorporated herein by reference. 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 74 and precipitate 68 are separated, with the precipitate 68 being further treated, if desired, to capture chemical values, but preferably sent to briquetting step 60. The filtrate 74 may be treated in process 300 several manners, two of which are preferred. First, the filtrate 74 may be cooled resulting in the crystallization and recovery of zinc oxide. Second, the filtrate 74 may be subjected to electrolysis resulting in the generation and recovery of metallic zinc.

To recover zinc oxide, the filtrate 74 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 ofthe cooling period and faster at the end ofthe 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. The solubility of diamino zinc dichloride in water is shown in Table III.

The amount and temperature of the wash water can be selected to achieve a particular size/surface area of the zinc oxide crystals. At a fixed temperature, the more wash water used, the larger the surface area (smaller particles) and at a fixed amount of wash water, the higher the temperature, the larger the surface area (smaller particles). Any temperature above 25°C may be used in this process to control the particle characteristics. Temperatures above 100°C may be used if done under pressure. However, because of the added expense of heating and maintaining a temperature above 100°C, and because ZnOH forms at a greater rate as the temperature decreases below 60°C, it is preferred to use a temperature of 60 - 100°C.

Through variations in the two conditions, using the method disclosed herein, one skilled in the art can produce zinc crystals with a predetermined particle size, and thus, surface area.

TABLE III

Solubility of Zn(NH ₃ ) ₂ Cl ₂ in water

Temperature °C g Dissolved/100 _g H _; O

90 32

80 24 40 21

25 12.8

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

Examples of the process are contained in issued patents owned by Metals Recycling Technologies Corp. of Atlanta, Georgia US, the owner of this application, such as U.S. Patent No. 5,464,596, and are applicable here. 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) ₂ 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.

The roasting step in furnace 12 can be carried out prior to the initial leaching step in process 100, or between a first and second leaching step. The powder containing the franklinite and magnetite, such as the waste dust, is heated to temperatures greater than 420°C to 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 waste materials 400 can be roasted 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 for example hydrogen gas and nitrogen or carbon dioxide, can be passed through the powder containing franklinite and magnetite. Hydrogen gas is not the only species

7/49637 PO7US97/11404

21

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 roasting process also can be performed to complete reduction by using carbon, introduced during the briquetting step 60 for example, at high temperatures and collecting zinc metal that will melt at very low temperatures (420°C) and boil at 907°C. In this process, zinc metal is obtained that, if desired, can be converted readily to the oxide by air roasting. The roasting process generates fumes comprising the components to be recovered. The fumes are subjected to the leach step. The use of the term, "dust," in EAF dust is typically synonymous with "fumes."

The purpose of the crystallization/washing step in process 300 is to produce a high purity zinc oxide of controlled particle size. This is accomplished through control of the temperature-time profile during cooling in the crystallization. The crystallization step in the process takes the filtrate from the cementation step at 90- 100°C. This filtrate contains the dissolved zinc with small amounts of trace impurities such as lead and cadmium. In order to prepare a pure zinc oxide it is necessary to prevent the formation of solvent inclusions inside the grown crystals. Solvent inclusions are pockets of liquid trapped as a second phase inside the crystals. Control of crystallization conditions can be employed to reduce these impurities. An example is given below.

The use of a reverse natural cooling profile, which is the opposite shape as by natural cooling, is preferred. In reverse natural cooling, the cooling is slower at the

beginning and faster at the end; in natural cooling, the cooling is faster at the beginning and slower at the end. This type of cooling profile also is used to control the crystal size distribution (CSD) of the zinc oxide obtained. The cooling profile controls the ratio of nucleation (birth of a new crystal) to crystal growth (growth of existing crystals). The ratio of nucleation/growth determines the final CSD. Controlling the temperature with a reverse natural cooling curve results in a larger average size than by linear cooling (A) or natural cooling (B). This principle can be employed to design cooling profiles to produce zinc oxides of a desired average size and distribution. To produce pure zinc oxide from waste dust containing zinc efficiently and in a safe and cost effective way, the process recycles all zinc which is not removed from the leachate in the crystallization step. In addition, the diamino zinc dichloride which is redissolved in water in the washing step also is recycled. The recycle of zinc increases the overall zinc concentration in liquid solution in the process. This allows the crystallizer to operate at a higher temperature due to the rapid change in zinc oxide solubility with temperature in ammonium chloride solution.

The recycle has the advantage that the solution becomes saturated relative to certain materials present in the dust, such as CaO. When this occurs, CaO no longer is leached from the dust but remains with the iron in the iron cake. This increases the value of the cake since CaO is still present and will not have to be added when the iron cake is fed to a furnace in steel making. Another important advantage is that there is no liquid effluent in this process. The only products are solid (iron cake, zinc oxide, waste metals), which are then sold for use in various industrial processes. No waste is produced since all liquid is recycled. Iron-rich by-products produced during the recovery process are sent to the briquetting step 60 to obtain an end product which can be recycled back into the furnace 12. In a first embodiment, the iron-rich by-products are briquetted 60 and sent to a steel mill furnace 12 where they are used in the production of steel. The steel production process results in exhaust fumes 10, 101, 102 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 ofthe leaching step.

In a second embodiment, the fumes exhausted from the arc furnace 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 stream, 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 ofthe leaching step. 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 arc furnace are also a matter of design choice, plant efficiency and convenience.

Electrolysis Step For Zinc Recovery The process can be operated to recover zinc metal by replacing the crystallization steps with an electrolysis step in process 300. The combined product solution 74 from the leaching steps comprises zinc ions in solution as Zn . When the combined product solution is subjected to electrolysis in an electrolytic cell containing an anode and a cathode, the zinc metal is electrodeposited on the cathode. Although it is preferable to have the cathode made from zinc metal, cathodes of other material also will allow the electrodeposition of zinc metal from the combined product solution.

Any of the electrolysis cells discussed in the literature are suitable, as long as such cells are configured for the electrolysis of zinc ion containing solutions. The two electrodes of the electrolysis cells are connected externally to a power supply capable of impressing a suitable voltage across the electrodes. The zinc ions, being positive in nature, migrate toward the negative electrode, or cathode, where they combine with electrons supplied by the external circuit to form neutral zinc metal atoms. When this happens, the zinc metal, in effect, electroplates onto the cathode. By using a zinc cathode, the entire cathode can be removed and used as necessary as a source of zinc.

Alternatively, a cathode on which electroplated zinc metal can be easily removed can be used.

Periodic Precipitation Of Other Solubles From The Product Solution

The product solution 74 also may contain sodium, potassium, magnesium, calcium, and other solubles in solution. These solubles can be recovered by introducing an electrolyte either in the leaching step or in the ammonium chloride storage tanks receiving the recycled product solution. As ammonium chloride is used as the leachant, ammonium salts in solution is the preferred electrolyte. For example, if some ammonium sulfate is added, one could precipitate out calcium sulfate. Ammonium sulfate is a preferred electrolyte to add because the process already uses ammonium in the form of ammonium chloride. The preferred electrolytes include ammonium sulfate, ammonium hydroxide, or ammonium carbonate to precipitate out various solubles.

Recovery of Ammonium Chloride and Wash Water Purification

The wash water used to wash the zinc compounds precipitated from the product solution 74 contains some ammonium chloride, as well as other compounds.

Rather than dispose of this polluted wash water, it can be treated to produce pure water and a more concentrated solution containing ammonium chloride and other compounds. The pure water can be recycled to wash additional zinc compounds precipitated from the product solution, and the concentrated solution can be recycled

back to the leaching step. The purification can be accomplished using evaporator condensers or reverse osmosis membrane technology.

From an economically competitive situation, the use of reverse osmosis membrane technology to filter the wash water containing ammonium chloride solution to obtain pure water on one side of the membrane and a concentrated ammonium chloride solution on the other side of the membrane, will save feed costs.

Every so often it will be necessary to back flush the salts off of the membrane to recover them for makeup use in the future. In essence, reverse osmosis membrane technology is using a pump to pump the wash water through a membrane, which is significantly lower in cost than burning natural gas in an evaporator condenser to evaporate and recondense distilled water. This technology is used to filter out sodium chloride and the minerals out of sea water to make distilled water.

III. GENERAL DESCRIPTION OF A PROCESS FOR PRODUCING A ZINC OXIDE PRODUCT USING A SODIUM HYDROXIDE LEACH

Once the essentially pure zinc oxide has been recovered, as discussed above, a further zinc oxide purification process can be utilized which, in a preferred embodiment, is based on the solubility of zinc oxide in a concentrated sodium hydroxide solution. The solubility of zinc oxide in sodium hydroxide increases significantly with increasing sodium hydroxide concentration. For example, a 16 molar sodium hydroxide solution (640g per liter) will dissolve 4 mole (320g) of zinc oxide. If this solution is then diluted by a factor of 4, the solubility will decline so that approximately 180g of zinc oxide/zinc hydroxide will precipitate. In accordance with the preferred embodiment, the zinc oxide purification process utilizes this phenomenon to produce zinc oxide which is at least 99.8% pure.

In the first step of the preferred process, zinc oxide is dissolved in a 50% - 70% sodium hydroxide solution. Since most metals are not soluble in concentrated sodium hydroxide, most of the metal impurities in the zinc oxide will not dissolve, including manganese, iron and cadmium. Lead and calcium are soluble in concentrated sodium hydroxide and therefore will dissolve, as will chloride. The

solution is then filtered to remove the undissolved materials which are then sent to the metals recovery section ofthe plant.

The solution is then diluted with water by a factor ranging from 3 to 30, but preferably 3 to 8, and optimally around 4, which appears to be optimum from the point of view of product recovery and energy costs. The resulting zinc oxide crystals which form are then filtered out, sent to a wash tank where they are washed with water, and then sent to a dryer where they are dried.

By controlling the rate of dilution of the sodium hydroxide solution or its method of addition during the zinc oxide crystallization step, it is possible to control the particle size hence the surface area of the zinc oxide produced. Furthermore, it should be observed that the zinc oxide purification process is not limited to the purification of zinc oxide recovered by the zinc oxide recovery process of the present invention and can be used to purify zinc oxide provided from any source.

Additionally, by selecting the method of addition of the ¹ intermediate solution, preferably sodium hydroxide, during the zinc oxide crystallization step, it is possible to control the particle size hence the surface area of the zinc oxide produced. It has been found that the smaller the droplet size in which the solution is added, the smaller the particle size (larger surface area). By dispersing the sodium hydroxide into droplets by a hydraulic atomizer, the particle size can be controlled. Additionally, at a constant droplet size, vigorous mixing will result in a larger surface area. The principle can be employed through selection of the appropriate droplet size and amount of mixing, to obtain highly purified zinc oxide with a predetermined surface area. This general relationship is shown in Table IV.

TABLE IV Approximate Droplet Size Approximate Surface Area

250 microns 2.0 m /g

180 microns 3.0 m ² /g

150 microns 4.0 m ² /g

100 microns 10.0 m ² /g

As the concentration of the sodium hydroxide increases, the number of moles of zinc oxide which can be dissolved in the sodium hydroxide solution increases. As the sodium hydroxide solution is diluted, the number of moles which can be dissolved in the solution decreases, i.e., the zinc oxide in the solution begins to precipitate. This solubility characteristic of zinc oxide in sodium hydroxide is used by the present invention to purify zinc oxide by first dissolving the zinc oxide in a highly concentrated solution of sodium hydroxide and filtering out the impurities which do not dissolve, and then by diluting the sodium hydroxide solution to cause the zinc oxide to precipitate. By controlling the rate of dilution, the particle size and surface area ofthe zinc oxide produced can be controlled.

An apparatus for performing this purification process comprises a hold tank maintaining a 50% - 70% NaOH solution at 120°C to 150°C. The zinc oxide- containing product to be purified is dissolved in the concentrated NaOH solution in a digestion tank, which solution is delivered to the digestion tank from the hold tank. The undissolved impurities are filtered out at a tramp press and the concentrated solution containing the zinc oxide is delivered to a precipitation tank. The solution contained in the precipitation tank then is diluted at a predetermined rate with distilled water. As discussed above, preferably the dilution takes place at a temperature ranging from 70°C to 100°C, and preferably from 80°C to 100°C, so that the formation of zinc oxide as opposed to zinc hydroxide is favored. Temperatures outside of this range can be used, but are not as economically advantageous. The zinc oxide crystal precipitates due to the decreasing solubility of zinc oxide as the NaOH solution is diluted. The zinc oxide crystal is then filtered and washed with water. The zinc oxide crystal is then dried, preferably at approximately 160°C. The diluted solution, after the zinc oxide crystal has been filtered out, is collected in a feed tank from which it is delivered to an evaporator condenser which concentrates the solution back to 50% - 70%. When steady state is achieved, sodium chloride crystals will form which are filtered out as the re-concentrated NaOH solution is delivered back to the hold tank for reuse in the purification process.

Periodically, lead will be removed from the NaOH solution by cementation by adding zinc dust which displaces the lead in solution. The lead will be filtered out and sent to the lead recovery portion of the plant. The purified zinc oxide can be ground, sized and bagged. The following examples illustrate how the purification process in accordance with the preferred embodiment results in a zinc oxide product which is at least 99.8% pure.

Example 3-1

48 grams of NaOH are dissolved in 52 grams of distilled water making a 12 molar solution. 21 grams of zinc oxide is added making a saturated solution. The excess zinc oxide along with any insoluble impurities is filtered out. The zinc oxide used was obtained from the recovery process of the present invention and was approximately 1% chloride, 700 ppm manganese, 150 ppm iron, and 300 ppm lead.

The solution was then added to a volume of boiling water resulting in a dilution of 30 times. After boiling for a few minutes zinc oxide crystals appeared. These crystals were filtered, dried and washed. They were then heated for one hour at 160°C. The resulting zinc oxide was a fine white powder with a surface area of 7.3 m /g as measured by the BET method. Analysis of the zinc oxide showed no detectable iron or manganese using DCP analysis. Lead was present at 160 ppm and chloride was under 50 ppm. The material contained 99.8% or greater zinc oxide.

Example 3-2 The same solution as used in Example 3-1 was prepared. The solution was added to a volume of boiling water resulting in a dilution by a factor of six. Zinc oxide crystals appeared rapidly. These crystals were filtered, washed, dried and then heated for one hour at 160°C. The resulting zinc oxide was a fine white powder with no detectable iron or manganese and a chloride content of less then 50 ppm. The material contained 99.8% or greater zinc oxide.

Example 3-3 The solution used in Example 3-1 was prepared and placed in a one liter vessel and kept at 90°C. Water at 90°C was added slowly over a period of one hour until the

solution was diluted by a factor of five. The resulting zinc oxide was filtered out and dried for one hour at 160°C. It was a fine white powder with no detectable iron or manganese and a chloride content of less then 50 ppm. The material contained 99.8% or more zinc oxide. The type of intermediate used in the zinc oxide purification process will depend on the desired purity and particle characteristics to be obtained. For example, it has been determined that if ammonium sulfate is used as the intermediate instead of sodium hydroxide, the desired particle size of the purified zinc oxide can be obtained by controlling the cooling of the ammonium sulfate solution to precipitate zinc hydroxide, because the solubility of zinc oxide in ammonium sulfate is temperature dependent. The following example illustrates how this embodiment can be used to purify zinc oxide while obtaining desirable size and shape characteristics.

Example 3-4 A saturated, boiling solution of 100 gm of ammonium sulfate in lOOg of water was prepared. Zinc oxide prepared from EAF dust by the ammonium chloride recovery process discussed above which contains 4% chloride ion was added to the solution. The saturated solution was filtered, maintaining the temperature above 95°C. The level of chloride in this zinc oxide affects the quantity dissolved in the ammonium sulfate. On cooling to 60°C over 20 minutes zinc hydroxide was precipitated, filtered and washed. After heating to 150°C, 6 gm of zinc oxide having a surface area of 8 square meters per gram was obtained. The product has a considerably reduced level of chloride ion (below 0.01%) and also a lower lead content.

Intermediates other than sodium hydroxide and ammonium sulfate can also be used to precipitate zinc oxide having the desired purity and particle characteristics. The following intermediates are chosen in accordance with the desired purity and particle characteristics to be obtained: ammonium chloride liquor, ammonium phosphate, potassium hydroxide, ammonia/ammonium oxalate and ammonia/ammonium carbonate solutions.

Example 3-5

21g zinc oxide containing 2% chloride, 200 ppm lead, 100 ppm manganese, 50 ppm iron are added to 100 ml of sodium hydroxide solution (50% by weight) at 100°C. The solution was filtered to remove any insoluble materials and then added to 300 ml of water at 100°C. The solution is stirred and the resulting zinc oxide is filtered washed and dried. The zinc oxide obtained has no measurable lead, manganese or iron present and has under 50 ppm chloride. The surface area is 1.8 m ² /g.

This experiment was then repeated with the sodium hydroxide solution being dispersed into droplets by a hydraulic atomizer. The average droplet size of the solution was 180 microns. The resulting zinc oxide had a surface area of 3.0 m ² /g. The composition was the same.

IV. GENERALDESCRIPTIONOFTHEPROCESSASPARTOFAN OVERALLCHEMICALANDMETALRECOVERYOPERATION

The present invention also can be coupled to the iron product production process shown in Fig. 3 and in the complete process for the recovery of usable economically valuable products from the beneficiated EAF dust 102, 103 typically comprising zinc compounds and iron compounds shown in Fig. 4. The recoverable products include a relatively pure iron or direct reduced iron product feedstock, an iron oxide and an iron-carbon residual, and various chemical values besides zinc and zinc compounds, such as cadmium and lead products. Primary recovery and recycle processes are disclosed and claimed in patents and patent applications filed by the present inventors and assigned to the assignee of this invention and specification. The beneficiated EAF dust 102, 103, is subjected to a combination of steps including leaching, resulting in a precipitate comprising iron oxides (an iron cake), 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 DRI. Preferably, the precipitate containing iron oxides 68a, 68b is removed from a process for the recovery of zinc oxide and zinc metal from industrial waste streams. 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.

In general, as disclosed above, the recovery and recycling process utilizes the iron-rich materials produced by the process as a feedstock ultimately to a steel mill. The iron-rich materials either are processed either to a higher purity iron product which may be fed directly to a steel mill as the feed, or are fed directly to a small scale blast furnace or a cupola furnace. Fumes exhausted from the furnace are beneficiated as described above and can be processed by a baghouse or/and by a wet scrubber and the captured materials are then recycled. Fumes emanating from the furnace contain particulate 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 may be combined with the initial waste feed to the present process and/or other iron-rich materials, and processed further. An alternative method of removing the particulate matter from the reduction furnace fumes is the use of a 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 an ammonium chloride or sodium hydroxide leach step. Alternatively, the wet scrubber can use an ammonium chloride or sodium hydroxide solution instead of water. The particulate matter soluble in ammonium chloride, such as for example zinc, cadmium, and lead constituents, or in sodium hydroxide, will be removed in the ammonium chloride solution or sodium hydroxide solution, respectively, in the wet scrubber. The loaded solution then can be combined with the leaching step discussed above, resulting in an exceptional increase in the recycle of waste streams from, for example, the steel making process. Recovery and retreatment of exhaust and other waste products from the present invention has a beneficial effect on the environment, and a beneficial, economic effect on the cost ofthe steel making process.

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 or steel production process. Scrap steel also can be used as a feed in the making of steel. 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. It 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. Additionally, iron-poor materials can be added to the waste stream feed for the present invention. Iron-poor materials comprising chemical values recoverable in the present process can be added, resulting in the recycle of waste streams and the recovery of chemical and metal values.

The preferred feed stream is beneficiated dust 102, 103 produced as disclosed herein. For example, fumes from reduction furnaces and from the iron and steel making processes can be beneficiated by the batch method represented in Fig. 1 or the continuous method represented in Fig. 2. The beneficiated dust 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 or sodium hydroxide solution wet scrubber. The loaded recirculating water or solution (the scrubbant) may be recycled to the ammonium chloride leach step of the present invention. Referring to Fig. 4, a preferred embodiment of the complete iron products and chemical values recycle and recovery process is shown. Subprocess 100, the digestion and filtration steps, generally comprises the process disclosed and claimed in U.S. Patent No. 5,464,596. Subprocess 200, the direct reduced iron production steps, generally comprises the process disclosed and claimed in parent application Serial No. 08/348,446. The present process discloses an alternative to subprocess

200. Subprocess 300, the chemical values recovery steps, when combined with subprocess 100, generally comprises the process disclosed and claimed in U.S. Patent No. 5,453,1 1 1. Subprocess 400, the enhanced direct reduced iron production steps, when combined with subprocess 200, generally comprises the process disclosed and claimed in U.S. Patent No. 5,571,306.

Subprocess 500 comprises a feed process for 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 are filtered in a capture means 16, such as a baghouse or scrubber. 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 capture means 16, The constituents filtered out in capture means 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 300, which is the preferred embodiment of this invention disclosed above, to recover other chemical values. The precipitate, which is an iron cake (IC), is sent to subprocess 200.

In subprocess 200, the precipitate is dried and crushed in dryer/crusher 24. Exhaust gases from dryer/crusher 24 may be sent to a capture means 16 such as baghouse, 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 rushed precipitates are compacted in a compactor and sent to a reduction furnace or smelter 14. In reduction furnace 14B (see Fig. 3), the dried and crushed iron cake is roasted at between 980°C and 1315°C, producing an EIC which can comprise DRI. Pig iron, which can be in

liquid form, can be produced in furnace 14A (see Fig. 3). 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 furnace 14 can be 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.

Generally, 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<C1.

The preferred embodiment of the present invention also can be combined with an method for producing usable iron products, as shown in Fig. 3. A waste material stream 10 such as EAF dust typically comprises iron oxide, cadmium, zinc, lead, lime and ash. Waste material stream 10 may be the stream from subprocesses 400 and 500 as described above and in other related patent specifications. In accordance with the process described above, the waste materials stream 10 from the furnace 12, but which also can be from furnace 14, is provided to the leaching process 100. During the leaching process, any iron oxide contained in the waste materials stream 10 does not go into solution whereas other materials such as zinc, lead and cadmium compounds dissolve. Still other materials, such as lime and ash and other lights, do not go into

solution, but float on the top of the solution. The undissolved iron oxide 68a is then separated from the solution and the lights and sent to the briquetting process 60 where it is briquetted with carbon at approximately 50°C to 250°C and then sent to a reduction furnace 14B where it is converted to DRI or/and to the pig iron furnace 14A for the production of pig iron.

The briquettes 64 are sent to the pig iron production furnace 14A or the reduction furnace 14B. In the furnace 14A, 14B, fumes 70a, 70b are exhausted which typically comprise lead, zinc and cadmium. The fumes 70a, 70b are captured in a capture means 16, such as a baghouse or scrubber. The captured product 72a, 72b, 82a, 82b also can be considered the same as the original charge 10 and subjected to the beneficiation process disclosed above. For example, the captured product 82a, 82b may be sent in several consecutive batches to the briquetting process 60 for reintroduction to the furnace 14A, 14B. As disclosed above, a final batch of captured product 72a, 72b, which will be higher in the non-iron compounds, then is sent to directly to the leaching process 100. The undissolved iron oxide 68a, 68b separated from the leaching process 100 then is recycled to the briquetting process 60 and the briquettes 64 then are sent to the furnace 14A, 14B. The materials which go into solution during the leaching process 100 constitute other values 74a, 74b which are recovered and may be further treated or used in other processes, such as subprocess 300, which may be the sodium hydroxide process disclosed above. Any iron insolubles 86a, 86b recovered from subprocess 300 may be reintroduced to the briquetting process 60. The direct reduced iron 76 produced in the reduction furnace 14B can also be liquefied in pig iron furnace 14A as indicated on Fig. 3.

In accordance with a second embodiment, the unseparated waste materials 10 are sent directly to briquetting process 60 where they are briquetted with carbon C at temperatures ranging from approximately 50°F to 250°F. The briquettes 64 then are fed to the reduction furnace 14B or/and to the pig iron furnace 14A. If the briquettes 64 are provided to the pig iron furnace 14 A, the impurities such as zinc, lead, cadmium, silica and lime become slag which can be skimmed off. The iron-based products form pig iron which can be used in the steel making process. If the

briquettes 64 are provided to a reduction furnace 14B, DRI is produced and any fumes 70b exhausted are captured by the capture means 16 and are provided either to the leaching process 100 or the briquetting process 60 as disclosed above. The resulting undissolved iron oxide 68b then is recycled back to the briquetting process 60, as discussed above. When the briquettes 64 are subsequently subjected to the reduction 14B or/and liquification processes 14A, the iron and iron oxide is separated from the other materials as either slag (in the liquification process) or fumes (in the reduction process).

The above description sets forth the best mode of the invention as known to the inventor at this time, and the above Examples are for illustrative purposes only, as it is obvious to one skilled in the art to make modifications to this process without departing from the spirit and scope of the invention and its equivalents as set forth in the appended claims.