CN1166862A - Method for recovering metal and chemical values - Google Patents

Abstract

A method for recovering metals from industrial waste streams (500) which entails treating the waste streams with an ammonium chloride solution (100), separating the undissolved precipitates comprising iron compounds from the leachant solution, further treating the undissolved precipitants by elevated temperature roasting resulting in an iron-based feedstock (200), displacing metal ions including lead and cadmium from the solution using zinc metal, further treating the solution to remove zinc compounds therefrom, further treating the zinc compounds and the undissolved components with a dissolving solution, and further treating the displaced metal ions to recover lead, cadmium and zinc therein using various methods including electrolysis.

Description

Method for recovering useful metals and chemicals

Background

1. Field of the invention

The present invention relates generally to a process for recovering useful metals and chemicals from an industrial waste stream comprising zinc compounds and iron compounds. The invention more particularly relates to a method for treating a waste stream comprising zinc compounds and iron compounds, such as Electric Arc Furnace (EAF) dust, in a combined leaching and reduction step that enables the separation and recovery of zinc oxide, zinc metal, iron and carbon compounds, lead and cadmium from the waste stream.

What is effective in practicing the invention is a process in which additional waste streams, such as iron-rich and iron-depleted waste, are mixed with a waste stream that typically contains zinc compounds and iron compounds. The mixed waste is treated in a combined step comprising leaching to produce a precipitate comprising iron oxides, which precipitate is then subjected to roasting to produce an enriched directly reduced iron compound which can be used as a raw material in a steel mill. During the recovery process, carbon compounds may be added to the waste stream, thereby producing a cake product of insoluble iron and carbon compounds, which may also be used as a feedstock for a steel mill.

2. Description of the Prior Art

Generally, dust from the processing of scrap metal is known in the art. In particular, the recovery of zinc oxide and other zinc products is known. However, it is not known to completely continuously recycle dust from scrap metal processing processes to recover useful chemicals and metals and produce feedstock for steel mills.

Zinc oxide is included in industrial by-products including waste streams such as fly ash and flue dust for a variety of purposes. Known methods for recovering zinc oxide, including zinc oxide from industrial waste, include leaching with solutions of mineral acids, caustic soda, ammonium hydroxide and ammonium carbonate. These processes have low zinc oxide yields and generally do not recover pure zinc oxide because the recovered zinc oxide is contaminated with other metal salts. In order to obtain pure zinc oxide, a subsequent calcination and evaporation treatment is necessary.

U.S. patent 3,849,121 to Burrows, which is now expiring, but assigned to the assignee of the present invention, discloses a batch process for selectively recovering zinc oxide from industrial waste. The Burrows process involves leaching the waste material with an ammonium chloride solution at elevated temperatures, separating iron from the resulting solution, treating the solution with zinc metal, and then cooling the solution to precipitate zinc oxide.The material obtained in the last step is a mixture containing small amounts of zinc oxide, hydrated zinc phases which may include hydrates of zinc oxide and zinc hydroxide, and other phases and large amounts of diaminozinc dichloride Zn (NH)₃)₂Cl₂Or other similar compounds containing zinc and chloride ions. Currently, the Burrows approach is not economically feasible due to environmental standards specified after the issuance of Burrows.

The first step in the Burrows patent is to treat the EAF dust with an ammonium chloride solution. Since 20-50% of the zinc contained in Burrows' dust is an iron-zinc complex (called spinel) that cannot be leached by ammonium chloride solution, the Burrows process cannot leach and recover most of the zinc contained in EAF dust. The second step in the Burrows process is displacement with zinc powder. The zinc powder causes an electrochemical reaction that results in the deposition of lead and cadmium on the zinc particles. Burrows does not teach the need to effectively remove lead and cadmium in this step without using large amounts of zinc. The third step in the Burrows patent is to cool the filtrate from the displacement process to obtain zinc oxide crystals. However, the zinc oxide produced by Burrows is of no purity; the X-ray diffraction pattern clearly shows that the crystals are still heterogeneous mixtures. Burrows does not teach cooling or methods for controlling purity or particle size, and thus the particles produced do not meet commercial requirements. Furthermore, most of the ammonium chloride is lost in the washing step of the crystals when the diamino zinc dichloride decomposes.

Typically, dust from scrap metal processing contains varying amounts of lead, cadmium and other metals. There is a need for extracting these metals from waste metal dust for various reasons, such as recovering lead and cadmium and/or preventing lead and cadmium from entering the atmosphere. The Burrows patent includes a process for extracting dissolved lead and cadmium from an ammonium chloride solution to which powdered zinc dust is added for treating waste metal dust. The resulting electrochemical reaction produces elemental lead and deposits on the surface of the powdered zinc powder. For this reaction to proceed, a large surface area of zinc must be present initially, because when the lead covers the particles of the zinc powder, the particles cannot be reused in the electrochemical reaction. For this reason, very fine powders are used, which unfortunately immediately agglomerate to form chunks and sink to the bottom of the container. Rapid stirring does not prevent this. This is a very uneconomical practice, since a large amount of zinc must be added to separate out all the lead due to the accumulation of zinc. Furthermore, if it is desired to separate lead and some cadmium from zinc so that all of these metals can be sold or recycled, the higher the zinc content in the metal, the greater the zinc throughput per unit mass.

Peters, U.S. patent 4,071,357, discloses a process for recovering valuable metals that includes a steam distillation step and a calcination step to separately precipitate and then convert zinc carbonate to zinc oxide. Peters also discloses that flue dust is leached at room temperature using a solution with approximately equal ammonia and carbon content, resulting in extraction of only about half of the zinc in the dust, nearly 7% iron, less than 5% lead and less than half of the cadmium. Steam distillation can precipitate zinc carbonate, other carbonate and iron impurities, however lower temperatures favor the precipitation of some crystalline zinc compounds. Steam distillation may also increase the temperature of the system, driving off ammonia and carbon dioxide, which may result in precipitation of iron impurities, zinc carbonate, and other dissolved metals.

Zinc and zinc oxide in NH₄The solubility in Cl solution is high and the solubility of zinc and zinc oxide in this solution decreases rapidly with temperature, which is the underlying principle of separation based on crystallization for the process of the present invention. The leaching rate is a function of the difference between the zinc concentration and the saturation concentration in the solution; the higher the saturation concentration, the faster the leaching. The process of the present invention requires only 1 hour for leaching, whereas the process of Peters requires at least several hours for leaching.

Lead and lead oxide and cadmium oxide are soluble in ammonium chloride solution, while iron oxide is practically insoluble. In contrast to the Peters process, 95-100% of the zinc present in the form of zinc oxide can be extracted during leaching according to the invention, whereas in Peters it is about 55%; the method of the invention can remove 50-70% of lead, while in Peters the dry rate is low, 5%; the process of the present invention removes 50-70% of the cadmium, while less than half of it is in Peters. Peters indicated that his residue was high in lead and was discarded as a hazardous waste. By leaching out most of the lead and cadmium, the method of the invention can produce a material for use as scrap metal in a steel plant. Also, the process of the present invention allows for the separation and recovery of substantially pure lead and cadmium, thereby reducing the total amount of scrap and recovering materials of potential economic value.

Another method, offered by Engitec imperianti SpA in milan, italy, suggests the use of electrolytic techniques to extract metals from soluble salts in electrolytic cells to recover zinc metal and deposit displaced lead. In the Engitec process, EAF flue dust is leached with spent electrolyte, such as ammonium chloride, which dissolves the zinc, lead, copper and cadmium in the EAF dust in solution, leaving the iron in solid form. The solution containing dissolved zinc is charged to an electrolytic cell, causing zinc to be attracted from the solution to the cathode plate, while other heavy metals are filtered out as a solid mass of gelled lumps. It is evident that the electrolysis of zinc ammine occurs in conventional open cells using titanium permanent blank cathodes and patented graphite anodes. In the cell, zinc is deposited on the titanium cathode. However, the deposition time of zinc varies from 24 to 48 hours depending on the current density. The cell consumes ammonia and evolves nitrogen and in order to maintain the pH of the electrolyte in the desired 6-6.5 range, an additional 180 kg of ammonium per ton of product zinc must be added. In practice, the Engitec process takes the finished solution from the rrows process and subjects it to electrolysis.

The use of an electrolytic cell adds to the cost of the process. The Engitec process also produces metallic zinc which is less valuable than zinc oxide. The residue removed from the Engitec process contains zinc ferrite, an added impurity for any future process. It would be advantageous if a residue could be obtained that contained primarily iron oxide and no or only small amounts of zinc ferrite or other impurities.

Fray, U.S. patent 4,292,147, discloses and claims a method for the electrolytic deposition of cadmium or zinc from a chloride solution obtained by chlorinating a leach material. Mixing an aqueous solution containing 15-30 wt% of zinc chloride or cadmium chloride at a pH of 2-3.5 and a temperature below 35 deg.C under gas stirring and at a current density above 100A/m²Electrolysis under conditions to form aggregated zinc or cadmium on the cathode. Typical zinc-bearing materials such as flue dust are leached with a saturated chlorine-containing solution, preferably in the presence of a chlorine-containing hydrate. The zinc chloride solution preferably contains 20 to 30% by weight of zinc chloride or cadmium chloride and up to 20% by weight of alkali metal or ammonium chloride. Preferably at 0 ℃ to 9 ℃ and with interrupted current reversal, higher than 2500A/m²Electrolysis is carried out under the conditions of (1). The chlorine-containing hydrates released at the anode can be recycled to the leaching to play a role.

Summary of The Invention

The present invention is a process for recovering iron-containing compounds, zinc oxide, and other useful chemicals and metals from waste materials. In addition to the recovery of zinc oxide, zinc metal, other useful metal elements contained in the waste material such as lead, silver and cadmium can be recovered. The solution used in the process of the invention can be recycled, and therefore the process is completely free of liquid waste. The solids recovered from the process may be used in its entirety in other processes. Some of these residues, including iron oxide cakes, lead metal residues and cadmium, are of a quality that can be used directly as feedstock for the production of various commercial products.

Briefly, the preferred waste material, typically fly ash or flue dust such as EAF, is leached with an ammonium chloride solution to produce a finished solution containing dissolved zinc and/or zinc oxide and other metal oxides and insoluble materials including iron oxides. Ammonium salts may be added, the anions of which form insoluble compounds with calcium, in order to remove impurities of the calcium compounds precipitated from the finished solution. Separating the product solution from the insoluble material, and further processing the product solution and insoluble material to recover the useful component. Zinc metal is preferably added to the finished solution at a temperature of 90 cor higher to displace all of the lead and cadmium contained in the finished solution. Dispersants may also be added to prevent flocculation of the zinc metal. The finished solution that remains is rich in zinc compounds.

The remaining finished solution may then be processed in some way. For example, the finished solution may be cooled to about 20 ℃ to 60 ℃ to precipitate the zinc component from the finished solution as a mixture of crystalline zinc compounds. These crystalline zinc compounds are separated from the finished solution, washed with wash water at 25-100 ℃, and then dried at elevated temperatures above 100 ℃ to produce zinc oxide products of 99% or greater purity. In another example, the finished solution may be subjected to electrolysis, wherein zinc metal is deposited on the cathode of the cell. After crystallization or electrolysis, all remaining finished solution is recycled back for treatment of incoming waste.

The insoluble material separated from the finished solution is rich in iron oxide and usually contains some impurities such as zinc ferrite. The insoluble material can be used as a raw material for a steel mill as long as the amount of impurities is not too large. However, before using iron oxide as a starting material, it is preferred to separate impurities from the iron oxide. It is more preferred to reduce the iron oxide to Direct Reduced Iron (DRI) because DRI can be used to replace part or all of the scrap.

The waste material, typically containing zinc ferrite and magnetite, may be calcined at a temperature above 500 ℃ for a predetermined period of time prior to leaching with ammonium chloride solution. The firing process generally includes the following steps: the waste material is heated and/or hot reducing gas is passed through the waste material to decompose the gahnite-type zinc oxide-iron oxide complex into zinc oxide, iron oxide and other constituents. A rotary hearth furnace has been found to be a suitable apparatus for this firing process. Although all reducing gases are suitable, hydrogen and carbon-containing gases such as carbon dioxide are preferred, and carbon (activated carbon) is mixed with the material and calcined in an oxygen-containing gas.

In the cementation step lead, cadmium and copper are precipitated and attached to zinc particles introduced into the solution, thereby forming a scrap metal cake, which is filtered and removed from the solution. The scrap metal cake can be further processed to separate and purify constituent elements such as lead and copper, which can then be sold as a product. The waste metal cake is washed with water and transferred to a vessel containing sulphuric acid. The sulfuric acid can dissolve zinc, cadmium and copper contained in the waste metal cake. However, the lead metal is not soluble in sulfuric acid, and all of the lead oxide in the cake will dissolve and then precipitate as lead sulfate. The resulting solid was filtered, washed with water, and then dried under nitrogen. The solids are primarily lead metal with trace amounts of lead oxide, lead sulfate, copper, zinc and cadmium impurities. This lead metal can be resold and is suitable for various uses. The remaining sulfuric acid solution contains cadmium and zinc as well as a small amount of copper. Cadmium can be removed electrochemically by inserting zinc metal flakes into the solution to produce sponge cadmium suitable for resale as a product. On the other hand, cadmium can be recovered by electrolysis. The remaining solution is mainly zinc and sulphuric acid, which can be recycled to the original leach solution for final recovery of zinc in the form of zinc oxide.

The filtrate from the cementation step is hot (90-110 c) and contains a large amount of dissolved zinc as well as a few trace impurities. On controlled cooling of the solution, crystallisation of the zinc salt begins to occur. Controlling the cooling rate and temperature versus time profile is important to control the particle size distribution of the crystals and to reduce or eliminate many of the impurities that may be produced. This is particularly useful for controlling entrained solution, and controlling crystallization can effectively reduce entrained solution to zero. In addition, the zinc salt does not actually contain any metal impurities since crystallization is based on different solubilities and no impurities are present at concentrations that can crystallize.

Iron-depleted and iron-enriched waste may be added to the waste stream. A preferred iron-depleted waste feed stream is taken from flue dust discharged from an industrial process. For example, soot from reduction furnaces and steel production processes is typically filtered through a bag house. Other industrial processes also produce soot that can be filtered through a cloth bag dust collection chamber. The waste material from the dust in the bag house can be treated by the process of the invention to recover useful chemicals and produce iron rich products. Also, the fumes discharged from the reduction furnace for direct reduction of iron may be filtered, and the filtrate recycled to the process of the present invention. Alternatively, the fumes can be purified using a wet scrubber with circulating water or ammonium chloride solution. The load-bearing recycled water or ammonium chloride solution (detergent) may be recycled to the ammonium chloride leaching step of the present invention, as described below.

Iron-rich waste may be added to the mixed waste stream to facilitate disposal of such iron-rich waste and to produce a higher percentage iron-based feedstock. The use of iron scale as an iron fortifier in steel production processes is contrary to conventional techniques, as iron scale is considered to be a waste material or impurity. As with the spent batteries. Iron-rich raw materials suitable for steel production processes can be produced by adding iron oxide-rich materials to EAF dust and then processing the mixed waste.

The iron oxide in the insoluble material may be reduced to DRI by several methods. Firstly, insoluble materials can be treated by a high-temperature roasting step at 980-1315 DEG CThe iron oxide contained in the insoluble material is reduced to DRI. Calcination at the above-mentioned elevated temperatures may oxidize and/or drive off most of the remaining impurities. To promote the formation of more suitable DRI, the insoluble material may be granulated with carbon or sodium silicate or another suitable material at the end of or after the firing step. Secondly, carbon in the form of activated carbon, carbon powder, carbon granules, etc. may be added to the mixture of ammonium chloride and waste during leaching. Third, carbon may be added to the dried insoluble cake. When iron oxide and carbon are present in, for example, CO or CO₂Or other common reducing gases, the carbon reacts with the iron oxide to promote the reduction of the iron oxide to DRI. Combining any of the above methods can produce a more pure DRI product.

The present invention also provides a process whereby the iron-rich by-product produced by the recovery process can be reduced in a reduction furnace that reduces iron oxide to DRI. The smoke dust discharged from the reduction furnace is filtered by a cloth bag dust collecting chamber or/and a wet scrubber. The material captured by the bag house or/and the wet scrubber may be recycled back to the leaching step of the recovery process of the invention, where it is used for the recovery process. The solid particles captured by the bag house can be mixed with the feed of the original waste stream, such as EAF dust, or supplied as a separate original feed to ammonium chloride leaching. The loaded detergent liquor flowing from the wet scrubber may be mixed with the original ammoniumchloride lixiviant or, if an ammonium chloride solution is used as the scrubbing liquor, used as the original ammonium chloride lixiviant.

The flue dust discharged from the reduction furnace used to reduce the iron-rich material to DRI is fed into a bag house or/and a wet scrubber containing hot ammonium chloride solution. These fumes, which are typically iron-depleted, contain primarily zinc, lead and cadmium. The material that is captured by the bag house or filtered by the wet scrubber can then be recycled back to the leaching step of the recovery process of the present invention. If the fumes are filtered through a bag house, the trapped material is a solid, which is sent to a waste stream from which it is added to the ammonium chloride solution in the leaching step. If the fumes are filtered through a wet scrubber, the captured material is discharged from the wet scrubber as a liquid stream directly into the ammonium chloride solution of the leaching step. On the other hand, if an ammonium chloride solution is used as the washing liquid, an ammonium chloride detergent may be used as the leaching (dissolving) liquid.

Due to the continuity of the process, calcium impurities can foul resulting in reduced efficiency. The use of a second generation ammonium salt other than ammonium chloride helps to mitigate calcium impurity scaling while maintaining efficiency. The calcium contained in the fumes can be leached out by the ammonium chloride solution. Calcium scaling during ammonium chloride leaching reduces the ability of ammonium chloride to leach zinc from the waste. Prior to charging the waste material, a dibasic ammonium salt, such as preferably ammonium sulfate or ammonium hydroxide, is added to the leaching tank, and calcium ions in the form of calcium sulfate are precipitated. The loaded recycle water or ammonium chloride solution (detergent) can then be recycled to the ammonium chloride leaching step of the present invention as described below without causing calcium scaling in the detergent.

Brief Description of Drawings

FIG. 1 is a schematic representation of a flow scheme of the present invention.

Detailed description of the preferred embodiments

The disclosed method for recovering valuable chemicals and metals achieves in its best mode the recovery of such materials from waste streams of industrial or other processes. A typical industrial waste stream used is flue gas such as Electric Arc Furnace (EAF) dust containing galvanized steel in the charge, and has the following percentage composition:

TABLE I

Analysis of flue dust

Weight percent of the ingredients

Zinc oxide 39.64

Iron oxide 36.74

Lead oxide 5.72

Inert substances 19.10

Calcium oxide 2.80

Potassium oxide 2.41

Manganese oxide 1.29

Tin oxide 1.13

Alumina 0.38

Magnesium oxide 0.33

Chromium oxide 0.16

Copper oxide 0.06

Silver 0.05

Unidentified substance 20.22

Total 100.00

1 siliceous material, such as slag with entrained carbon particles.

2 molybdenum, antimony, indium, cadmium, germanium, bismuth, titanium, nickel and boron.

General description of the flow

In summary, the present invention is a continuous process for the recovery of zinc oxide and iron-containing compounds from a waste stream comprising zinc compounds, the process comprising the initial steps of:

a. roasting the scrap at elevated temperature in a reducing atmosphere and/or in the presence of a portion of the carbon to reduce any iron oxides in the scrap to Direct Reduced Iron (DRI) for use as scrap for further recovery of useful chemicals and metals;

b. treating the waste material with an ammonium chloride solution at elevated temperature to produce a finished solution comprising dissolved zinc and dissolved zinc oxide such that any iron oxide in the waste material does not go into solution;

c. separating the finished solution from any undissolved material, including any iron oxide, contained in the finished solution;

d. adding zinc metal and a dispersant selected from dispersants that prevent aggregation of said zinc metal to the finished solution such that all lead and cadmium ions contained in the finished solution are replaced by zinc metal and precipitate out of the finished solution as lead and cadmium metals;

e. separating the finished solution from lead and cadmium metals that may be further processed so that they may be purified and recovered;

f. further processing the finished solution to recover zinc compounds and other useful chemicals and metals; and

g. the insoluble material is further processed to recover iron products suitable as a raw material for a steel mill.

The above part of the process of the invention may also include a two stage leach process with higher zinc oxide yields. The two-stage leaching method comprises the following steps:

a. first treating the waste material with an ammonium chloride solution at elevated temperature to producea first finished solution containing dissolved zinc components such that any iron oxide in the waste material does not enter the solution;

b. separating the first finished solution from insoluble waste compounds including any iron oxide contained in the first finished solution;

c. calcining the insoluble waste compounds at elevated temperature in a reducing atmosphere;

d. second treating the calcined insoluble waste compound with an ammonium chloride solution at an elevated temperature to produce a second finished solution containing a dissolved zinc component such that any iron oxide remaining in the calcined insoluble waste compound does not go into solution;

e. mixing the first and second finished solutions to produce a combined finished solution; then the

f. The d-g step in the above general flow is performed.

An ammonium chloride solution was prepared with water in known amounts and concentrations. If a two-stage leaching process is used, a feed containing a zinc component, such as the waste flue dust described in Table I, or any other source of feed containing zinc or zinc oxide mixed with other metals, may be added to the ammonium chloride solution at a temperature of about 90℃ or greater. Alternatively, the feed material is calcined. Zinc and/or zinc oxide is dissolved in an ammonium chloride solution together with other metal oxides such as lead oxide and cadmium oxide. Iron oxide is insoluble in ammonium chloride solution. The solubility of zinc oxide in ammonium chloride solution is listed in table II.

TABLE II

ZnO at 23% NH₄Solubility in Cl solution

Grams dissolved at temperature/100 grams H₂O

90 14.6

80 13.3

70 8.4

60 5.0

50 3.7

40 2.3

An aqueous solution of 23% by weight ammonium chloride at a temperature of at least 90 c provides the best solubility of zinc oxide and has been selected as the preferred concentration of ammonium chloride solution. Ammonium chloride concentrations below about 23% do not dissolve the maximum amount of zinc oxide from the flue dust, while ammonium chloride concentrations above about 23% tend to precipitate ammonium chloride as well as zinc oxide as the solution cools. Iron oxide and inert materials such as silicates are not soluble in the preferred solution.

Zinc oxide, as well as lower levels of lead or cadmium oxide, can be leached from the original dust by dissolution in ammonium chloride solution. The residual solids after the leaching step described above contain zinc, iron, lead and cadmium and possibly some other impurities. The solid is then calcined in a reducing atmosphere, generally at temperatures above 420 ℃ and often between 700 ℃ and 900 ℃. The reducing atmosphere may be formed by using hydrogen, a single component carbon-containing gas such as carbon dioxide, or by heating the material in an oxygen-containing gas in the presence of elemental carbon. Carbon in powder or granular form is preferred. The calcination time is usually 30 minutes to 4 hours. As mentioned above, the waste dust may be first calcined and then leached out, eliminating the first leaching step.

The calcined dust is leached with a 23% ammonium chloride solution at a temperature of at least 90 ℃. All zinc or zinc oxide formed in the calcination step is dissolved in the ammonium chloride solution. The zinc oxide-containingammonium chloride solution was then filtered to remove all insoluble material including iron oxide. While the filtered solution of zinc oxide and ammonium chloride is still maintained at a temperature of 90 c or higher, zinc metal in a finely powdered form is added to the solution. All of the lead metal and cadmium in the solution is deposited on the surface of the zinc metal particles by the electrochemical reaction. The addition of a sufficient amount of powdered zinc metal removes substantially all of the lead in the solution. The solution was then filtered to remove solid lead, zinc and cadmium.

To facilitate the zinc powder remaining suspended in the solution of zinc oxide and ammonium chloride, a water soluble polymer that can be used as a deflocculant or dispersant may be added. The surface active material may also function to keep the zinc powder in suspension as many compounds are used to control scale. The content of these materials is only 10-1000 ppm. A variety of suitable materials include water-soluble polymeric dispersants, scale control agents and surfactants such as lignosulfonates, polyphosphates, polyacrylates, polymethacrylates, maleic anhydride copolymers, polymaleic anhydride, phosphates and phosphonates. A detailed description of the various materials mentioned above can be found in the literature, for example, Drew's "Principles of Industrial Waste Treatment" (Principles of Industrial Waste Treatment), pages 79-84, which is incorporated herein by reference. The Flocon series products (manufactured by FMC Corporation) of Flocon100 and other maleic acid-based acrylic oligomers of water-soluble polymers of different molecular weights are also effective. The addition of dispersants to solutions containing various ions and having high ionic strength is objectionable to standard practice for dispersants that are generally insoluble in such high ionic strength solutions.

The cementation step described above ispreferably carried out by adding about twice the stoichiometric amount of zinc and dispersant. After the initial co-precipitation recovery step described above, the lead, cadmium and copper concentrations remaining in the solution were monitored. Then, a second advanced treatment step can be carried out, if necessary, by adding a small amount of powdered zinc or a dispersant.

At this stage, a filtrate rich in zinc compounds and precipitates of lead, cadmium and other products can be obtained. To recover the zinc oxide, the filtrate may be cooled to about 20 ℃ to 60 ℃, resulting in crystallization of the mixture of zinc compounds. The mixture contains significant amounts of diamino zinc dichloride or other complexes including amino zinc complexes, hydrated zinc oxides and hydroxides. Crystallization is carried out by controlling a temperature-time cooling curve, which is beneficial to obtaining high-purity zinc oxide with controllable granularity. Reverse natural cooling, i.e. cooling the solution slowly at the beginning of the cooling and more rapidly at the end of the cooling period, is preferred in order to control the nucleation effects with respect to the crystal growth ratio and the final crystal size distribution. The precipitated crystalline solid is filtered from the solution and washed with water at a temperature of about 25 ℃ to 100 ℃. The filtered solution is recycled for further carrying of the raw material. The solubility of diamino zinc dichloride in water is shown in table III.

TABLE III

Zn(NH₃)₂Cl₂Solubility in water

Grams dissolved at temperature/100 grams H₂O

90 32

80 24

40 21

2512.8

Very little hydrated zinc oxide dissolves in water. Then, the solution was filtered to remove the hydrated zinc oxide component, and it was dried in an oven at 100 ℃ or higher. After sufficient drying time, the resulting dry white powder was essentially pure zinc oxide. The filtrate obtained from this solution is recycled as a mixture carrying further zinc compounds.

The zinc oxide may be dried at a temperature of about 100 c. However, to ensure that the material is chloride free, it is preferred to heat the zinc oxide to a higher temperature. Diaminozinc dichloride decomposes at 271 ℃ and ammonium chloride sublimes at 340 ℃. Therefore, it is preferred to heat the zinc oxide to 271 ℃ to about 350 ℃ to avoid sublimation of large amounts of ammonium chloride. Generally, the zinc oxide should be dried at the above temperature for about 2 to 60 minutes, preferably 5 to 20 minutes. A drying time of 10 minutes is considered a satisfactory average.

Since the zinc, lead and cadmium contained in the feed are amphoteric substances, these substances are brought into solution by using an ammonium chloride solution, while all the iron oxide contained in the feed does not go into solution. Other solutions, such as strongly alkaline solutions having a pH greater than about 10 or strongly acidic solutions having a pH less than about 3, may also be used to dissolve zinc, lead, and cadmium; however, if a strongly acidic solution is used, iron oxide dissolves into the solution, and if a strongly alkaline solution is used, iron oxide becomes gelatinous. Lead and cadmium can be removed from the ammonium chloride solution by an electrochemical reaction that results in the precipitation of lead and cadmium in elemental form. The difference in solubility of the zinc diamino dichloride and zinc oxide in water and in the ammonium chloride solution allows selective dissolution of the zinc diamino dichloride, so that pure zinc oxide can be recovered. This can also be used in the crystallization step to adjust the relative amounts of diamino zinc dichloride and zinc oxide types. It is important that all of the zinc be recycled so that it can ultimately be converted to zinc oxide.

Recovery of iron feed

The insoluble precipitate obtained, mostly iron oxide, is calcined at high temperature in a reducing atmosphere,a product equivalent to DRI can be produced. In general, heating the discus to above about 980 ℃ to as much as about 1260 ℃, usually not above 1315 ℃ will produce a DRI product. The DRI product may be pelletized after discharge from the furnace with carbon or with sodium silicate, or other suitable compounds. The resulting end product can be used as a raw material for a steel mill without any additional treatment. Calcining the insoluble precipitate reduces the iron oxide and drives off all zinc, cadmium and lead as well as other impurities. The resulting iron product has been obtained from some form of iron, e.g. FeO, Fe₂O₃Or Fe₃O₄Reduced to iron feedstock which is extremely suitable for steel mills.

Iron-rich materials such as iron scale or spent batteries may also be added to the waste to be leached and further processed. During the calcination of the insoluble precipitate, the combination of the non-leachable zinc oxide-iron oxide complex contained in the insoluble precipitate is broken, and the zinc oxide compound is discharged in the exhaust gas and trapped in a pollution control device such as a bag house, leaving the iron oxide cake as a residue. The iron oxide cake is calcined at high temperature to reduce the iron oxide leaving the useful iron metal. The iron may then be mixed with a binder to form briquettes or cubes that are used as feedstock. The discharged impurities can be recycled to recover, for example, zinc oxide, cadmium metal, and lead metal.

The method of the present invention achieves in its best mode the recovery of waste material from a waste stream of an industrial or other process and mixing it with waste material recovered from a vent gas stream of a furnace. Many processes, such as reduction furnaces and steel production processes, produce iron-depleted waste streams. Many other processes produce waste streams rich in iron oxides. Other processes separate the iron oxide rich material before treatment. The iron-depleted material is mixed with a typical processed industrial waste stream to produce an iron-rich material suitable as a steel mill feedstock.

The basic iron feedstock production process is a continuous process comprising the steps of:

a. mixing a waste stream, e.g. from a typical industrial process of metal or metal product production, with iron-depleted waste, e.g. from a reduction furnace or steel production process;

b. treating the waste mixture with an ammonium chloride solution at elevated temperature to produce a finished solution and an insoluble precipitate comprising iron oxide;

c. separating the finished solution from the insoluble precipitate comprising iron oxide; and

d. the insoluble precipitate is further processed during roasting to recover a purer iron product.

All additional iron-depleted waste, if in solid form, e.g. from a bag house, is added to step a of the basic production process. On the other hand, all additional iron-depleted waste, if in solution form, e.g. from a wet scrubber, is added to step b of the basic production process.

Additional steps may be added to the basic production process depending on the production conditions and the desired iron characteristics. The additional steps include, alone or in some combination:

1. pre-roasting the solid waste at high temperature and/or partially producing DRI in a reducing atmosphere;

2. pretreating the solid waste material with an ammonium chloride solution at elevated temperature to produce a finished solution and an insoluble precipitate comprising iron oxide, calcining the insoluble precipitate at elevated temperature and optionally in a reducing atmosphere, and then treating the insoluble precipitate with an ammonium chloride solution at elevated temperature to produce a finished solution and an insoluble precipitate comprising iron oxide;

3. pre-roasting the solid waste at an elevated temperature and optionally in a reducing atmosphere, pre-treating the waste at an elevated temperature with an ammonium chloride solution to produce a finished solution and an insoluble precipitate comprising iron oxide, roasting the insoluble precipitate at an elevated temperature and optionally in a reducing atmosphere, and then treating the insoluble precipitate at an elevated temperature with an ammonium chloride solution to produce a finished solution and an insoluble precipitate comprising iron oxide; and/or

4. Off-gases, typically containing zinc, cadmium, lead and other metals and compounds, are collected from the pre-firing process and sent to an ammonium chloride leaching step for recovery of zinc, cadmium, lead and/or other useful metals and compounds.

A step of purifying the iron product may be additionally added to the basic production process. For example

1. Elemental carbon may be added in the leaching step or steps to initiate the reduction of iron oxide to DRI in the leaching step or steps. The elemental carbon may be added in some forms including, but not limited to, powders, granules, and pellets. The elemental carbon does not go into solution and remains in an insoluble precipitate.

2. After the insoluble precipitate is separated from the finished solution, elemental carbon may be added to the insoluble precipitate. Mixing the elemental carbon and iron oxide at elevated temperatures and in a reducing atmosphere in this manner also initiates the reduction of the iron oxide to DRI. The elemental carbon may be incorporated into the insoluble precipitate by some means including, but not limited to, a ribbon mixer or blender.

Pre-baking

The pre-calcination step may be carried out before the initial leaching step or between the first and second leaching steps, or both. The waste dust or the mixture of waste dust and iron oxide rich material is heated to above 500 ℃. This temperature causes a reaction that results in the decomposition of the stable zinc ferrite phase to zinc oxide and other components, but does not completely reduce the zinc oxide to zinc metal. The zinc oxide formed can be isolated by sublimation or extraction with an ammonium chloride solution. The material obtained after extraction has less than 1% by weight of zinc.

The solid waste material may be pre-calcined using a number of conventional calcination processes, for example using a rotary hearth furnace, direct or indirect heating and flowing hot gases through the dust. For example, a non-explosive mixture of reducing gases, such as hydrogen and nitrogen or carbon dioxide, can be flowed over the powder containing the zinc hercynite and magnetite. Hydrogen is not the only gas that can be used to reductively decompose zinc ferrite. Carbon or a purely carbonaceous material may be used, including carbonaceous reducing gases and elemental carbon. Heterogeneous gas phase reduction is faster than solid state reduction at low temperatures, and therefore the use of carbon monoxide is recommended. Carbon monoxide can be generated in situ by mixing zinc iron spinel powder with carbon and heating at high temperature in the presence of oxygen. The oxygen concentration is controlled to optimize the production of CO. Carbon monoxide may be introduced as a separate source to more clearly distinguish the yield of carbon monoxide production from the decomposition rate of zinc iron spinel. The zinc oxide produced can then be isolated by extraction with ammonium chloride or sublimation.

Addition of carbon

The process of the invention can also be applied to the production of high quality iron-carbon cakes in the form of by-products. The iron oxide contained in the waste stream does not pass into the ammonium chloride solution but is filtered out of the finished solution as undissolved material. The iron oxide cake can be used as it is as a raw material for steel works; however, as previously mentioned, its value is higher if it is reduced by reaction with elemental carbon to produce an iron-carbon or DRI product.

The steel industry typically uses a mixture of iron oxide and carbon as a feedstock for an electric arc furnace. The iron oxide cake discharged from the leaching step as undissolved material is mainly in the form of Fe₂O₃And Fe₃O₄Iron oxide in the form of a mixture. The iron oxide cake can be treated in three ways. In the first method, carbon may be added to the leaching step, so that the iron oxide cake contains carbon plus iron oxide. The iron oxide carbon cake can be sent directly to the steel plant, and if it is sent directly to the steel plant, the iron oxide is reduced in the steel furnace. In a second method, the iron oxide-carbon cake may be pelletized and then fired in a reduction furnace to produce DRI. Iron oxide precipitates, which typically contain about 80% solids, are milled with carbon to form pellets, briquettes, or cubes, which are then heated. These pellets, lumps or cubes can then be fed into a steelmaking furnace. The difference between the materials fed to the furnace produced by the first and second processes is that in the case of the second process the DRI is fed to the furnace and so onAccording to the first method, a mixture of iron oxide and carbon is fed to the steel plant. The iron oxide plus carbon may be supplied to a steel mill as it is. When this carbon-rich iron oxide is melted, it forms a foamy slag, which is required by steel works. In a third method, the carbon is added by means of a ribbon mixer and the iron oxide-carbon cake is then fed directly into the steelmaking furnace or preferablyThe DRI, which is preferred by steel works, is first baked in a reduction furnace.

Carbon and iron oxide are mixed in a reducing atmosphere at elevated temperatures, resulting in the reduction of the iron oxide to produce DRI. DRI can be used to replace part or all of the steel mill charged scrap. In certain operations, DRI fines are preferred because they have a known uniform composition and are generally free of residual elements such as chromium, copper, nickel, and tin. When the carbon-rich iron oxide is melted, the desired foamy slag is formed because it contains carbon and iron oxide. Because scrap is generally less expensive than DRI, the use of DRI often does not prove economically viable. The typical advance per ton of DRI is $120.00 and higher. However, since iron oxide is a by-product of an economical recovery process (e.g., zinc oxide from flue dust as outlined below, the primary value of which is derived from the zinc oxide product), iron oxide or DRI can be produced more economically. Thus, the iron oxide produced as a by-product of the process is of greater value.

Typically, the iron oxide and carbon products are pressed into cakes that are easy to handle and use. Such cakes typically contain about 82% solids, but may also contain 78% -86% solids for ease of handling and use. Although a cake with less than 78% solids canbe made, the remaining 22% or more of the material is the finished solution, and if this cake is used as a feed to a steel mill, the finished solution is sent back to the steel making process, which is uneconomical. Also, it is uneconomical to dry this cake to over 86% solids.

The firing process produces vapors from zinc, lead, cadmium and other impurities that have been condensed into the dust. These impurities can be sent to the bag house at the end of the steelmaking process, incorporated into the original spent dust and then sent to the initial leaching step in a recycled manner. Alternatively, the vapors and dust discharged from the calcination step may be sent to another bag house at a separate facility.

Regardless of the method, the fumes discharged from steel and reduction furnaces are iron-depleted, but contain other useful components. The waste flue dust of such furnaces is an excellent source of iron-poor waste that can be used for the recovery of the present invention. The waste flue dust may be filtered in a bag house and the resulting filtrate added to the waste feed stream of the present invention, or the resulting filtrate may be used as the main waste feed stream of the present invention. The waste flue gas can also be scrubbed in a wet scrubber and the resulting loaded scrubbing solution added to the ammonium chloride lixiviant of the present invention. If an ammonium chloride wash is used instead of water, this loaded ammonium chloride wash can be used as the primary lixiviant in the process of the invention.

Enhanced zinc recovery

The zinc dust obtained from various sources contains 20% to 25% by weight of zinc, as shown by chemical analysis. There are some crystalline phases in the dust, in particular zinc oxide. The definitive identification of iron phases is complicated by the fact that some possible structural types (i.e. spinel-type iron phases show almost the same diffraction patterns). Zinc oxide (and lower lead or cadmium oxides) is separated from the starting dust by dissolution in a concentrated ammonium chloride solution (23% ammonium chloride).

Insoluble material was filtered and washed leaving a residual powder. The powder shows a still higher zinc content (i.e. 10-13% by weight), but this is not zinc oxide. All crystalline phases can be identified by the spinel type phase. The powder is zinc-iron spinel (Fe, Mn, Zn) (FeMn)₂O₄And magnetite (iron oxide: Fe)₃O₄) A mixture of (a). The two phases have very similar spinel structures. The zinc in the zinc ferrite cannot be extracted by dissolution of ammonium chloride. Furthermore, there is no simple extraction method to extract zinc from this stable oxide phase. Although this compound is stable to oxidation (all elements are in the highest oxidation state), it can be relatively easily destroyed by reduction at high temperatures. Reducing zinc ferrite in an atmospheric environment does not make it possible to easily reduce zinc oxide or to rapidly oxidize zinc to zinc oxide after reduction, and then recover the zinc oxide by ammonium chloride extraction or sublimation (highly volatile zinc oxide will sublime from the mixture at lower temperatures and recondense on the cooling section of the furnace). Can be prepared by mixing zinc ferriteAll reduced to zinc metal and the molten zinc is then separated by distillation extraction or by sedimentation techniques.

Crystallization of

The purpose of the crystallization/washing step is to produce high purity zinc oxide of controllable particle size. This can be achieved by controlling the temperature/time profile during the cooling of the crystallization. The crystallization step in the process of the present invention takes the filtrate from the displacement precipitation step at 90-100 ℃. The filtrate contains dissolved zinc as well as a few trace impurities such as lead and cadmium. In order to produce pure zinc oxide, the formation of solvent inclusions in the growing crystal must be prevented. Solvent entrainment is pockets of a second phase of entrained liquid within the crystals. These impurities can be reduced by controlling the crystallization conditions.

Recycle of

One purpose of the method is to produce pure zinc oxide from zinc-containing waste smoke. In order to carry out this efficient, safe and cost-effective process, the process recycles all the zinc not extracted from the leachate in the crystallization step. In addition, the zinc diaminodichloride redissolved in water in the washing step can also be recycled. The recycling of zinc can increase the total concentration of zinc in the solution of the process. Thus, the crystallizer can be operated at a higher temperature since the solubility of zinc oxide in the ammonium chloride solution varies rapidly with temperature.

Recovery of lead and cadmium

The process also produces substantially pure lead and cadmium from the waste metal cake filtered from the solution after the cementation step. After recovery, these metals can be sold as products suitable for various uses. A preferred method of recovering the metal byproducts comprises:

a. washing the waste metal cake filtered from the solution after the displacement precipitation step with water;

b. treating the waste metal cake with sulfuric acid capable of dissolving zinc, cadmium and copper contained in the waste metal cake;

c. dissolving and re-precipitating lead oxide as lead sulfate, while removing lead metal that is insoluble in sulfuric acid from the solution; and

d. cadmium is extracted from the solution electrochemically by placing zinc metal pieces in the solution to deposit cadmium thereon to produce sponge cadmium.

The lead solid is filtered and washed with water and then dried in an inert gas such as nitrogen to produce relatively pure lead metal. Some impurities are present in the form of lead oxide, lead sulfate, copper, zinc, and cadmium. Alternatively, electrolytic methods can be used to deposit cadmium in solution as a sponge on other cathode materials. The zinc remaining in solution, primarily in sulphuric acid, may be recycled back to the leach solution for recovery of zinc in the form of zinc oxide.

In the cementation step, when zinc powder is added to an ammonium chloride solution, an electrochemical reaction is caused to occur in the solution, causing lead, cadmium and copper atoms to be deposited on the surface of the zinc powder, thereby producing a scrap metal cake. The resulting solid is filtered from the solution to produce a scrap metal cake. The metal scrap cake was washed with water to remove any residual solution. The wash water may be purified and recycled. Then, the waste metal cake is treated with sulfuric acid which can dissolve zinc, cadmium and copper contained in the waste metal cake, and the zinc and cadmium contained in the cake are dissolved at a faster rate than the dissolved copper. The lead metal is insoluble in sulfuric acid and, therefore, the lead contained in the scrap metal cake is still in solid form.

The spent metal cake is preferably leached by adding a dilute sulfuric acid solution to form a slurry. The reaction of zinc oxide in the dust with sulfuric acid is exothermic. Cadmium oxide, copper oxide and lead chloride react relatively slowly with sulfuric acid, while copper oxide reacts the slowest with sulfuric acid. Insoluble lead sulphate is formed, which is removed by filtration. The cadmium solids resulting from the cementation step can be dissolved with sulfuric acid resultingin the formation of soluble zinc and cadmium sulfates. The zinc sulphate is recycled to the acid leach. The above treatment results in the following main reactions:

(1) (fast reaction, exothermic)

(2)

(3) (slow reaction)

(4)

Metal ions and sulfate ions are generated, and insoluble lead sulfate is generated. Sulfuric acid may be additionally added until the solution remains acidic (pH<2 and preferably pH 1). Thus, the consumption of acid can be saved to the utmost extent. The reaction (1) having a high speed can be completed without delay. Reaction (1) releases a large amount of heat, thus eliminating the need for external heating of the solution. Reactions (2) and (3) take a long time to complete in the leaching stage, leaving detectable Cd and Cu in solution. Leaching may be continued until all the extractable copper has dissolved. Leachable Cu and Cd are preferably separated before proceeding. It is also preferred to rinse with water or perform a second dilution leach to remove any entrained ions by rinsing the solids to prevent contamination in the next leach step.

The soluble lead oxide contained in these dusts is converted into highly insoluble lead sulfate. The large amount of sulfate ions contained in the solution greatly suppresses the solubility of lead ions. Due to reaction (4), chloride ions are introduced into the leachate. Zinc sulfate, which is the target of electrochemical applications, must be kept at a low chloride level, but even so, zinc chloride, which has a high solubility, is less likely to cause contamination and is easily removed by recrystallization. The lead solid was filtered from the solution, washed with water, and dried under nitrogen. The solids are primarily lead metal and some impurities including lead oxide, lead sulfate, copper, zinc, and cadmium.

After filtering off the neutral precipitate, the pH of the neutral filtrate can be adjusted to slightly acidic (pH4-5) by adding an acid, preferably sulfuric acid. The remaining solution contains cadmium and zinc as well as small amounts of copper and possibly lead. The pH of the solution can be adjusted to and maintained in the preferred pH range of 4-5 by the addition of zinc oxide. Cadmium contained in the solution can be separated electrochemically by placing zinc metal pieces in the solution, producing sponge cadmium as follows:

(5) on the other hand, the cadmium contained in the solution can be recovered by electrolysis. The sponge cadmium can be separated from zinc metal sheets, rinsed and redissolved in sulfuric acid to produce high purity cadmium sulfate by the following reaction:

(6) cadmium oxide air accelerates the rate of the chemical reaction of the sulfuric acid solution, greatly increasing the rate of dissolution, and forming water as the sole reaction byproduct:

(7) likewise, the sponge cadmium product can be removed from the zinc sheet and sold directly as cadmium metal.

After removal of the sponge cadmium and zinc flakes, the solution left was mainly zinc and sulfuric acid. The solution can be recycled by mixing with the original leach solution, so that the zinc can be finally recovered in the form of zinc oxide. The sulphate radicals will react with the calcium contained in the original leach solution and precipitate as calcium sulphate.

Electrolysis

The process can recover zinc metal by replacing the crystallization step with an electrolysis step. The mixed product solution from the leaching step contains Zn in solution²⁺The zinc ion of (1). When the mixed finished solution is electrolyzed in an electrolytic cell equipped with an anode and a cathode, zinc metal is electrodeposited on the cathode. While cathodes made of zinc metal are preferred, cathodes of other materials can also electrodeposit zinc metal from the mixed finished solution.

Any of the cells described in the literature are suitable, provided that they are configured to electrolyze a solution containing zinc ions. The two electrodes of the electrolytic cell are externally connected to a power supply which can apply proper voltage through the electrodes. The positive zinc ions migrate to the negative electrode, the cathode, where they combine with electrons supplied by an external current to form neutral zinc metal atoms. When this occurs, zinc metal is actually electrodeposited on the cathode. By using a zinc cathode, the entire cathode can be removed and used as a source of zinc as desired. On the other hand, a cathode from which electrodeposited zinc metal is easily taken out may be employed.

Periodically precipitating other soluble substances

The finished solution may also contain sodium, potassium, magnesium, calcium, manganese and other solubles in the solution. These solubles can be recovered by introducing electrolyte either in the leaching step or in the ammonium chloride storage tank that receives the recycled finished solution. When ammonium chloride is used as the leaching agent, ammonium salts in solution are the preferred electrolytes. For example, if some ammonium sulfate is added, calcium sulfate canprecipitate. Ammonium sulfate is the preferred electrolyte to add because the process already employs ammonium in the form of ammonium chloride. Preferred electrolytes include ammonium sulfate, ammonium hydroxide or ammonium carbonate to precipitate out the various solubles. Manganese can be removed by adding an oxidizing agent such as potassium permanganate or hydrogen peroxide. This oxidation process results in Mn which will be soluble²⁺Oxidation to insoluble Mn which precipitates as manganese solids⁴⁺。

Removal of calcium compounds

Due to the continuity of the process, calcium impurities may foul resulting in reduced efficiency. The use of a second generation ammonium salt other than ammonium chloride helps to mitigate calcium impurity scaling. The calcium contained in the fumes can be leached out by the ammonium chloride solution. Calcium scaling during ammonium chloride leaching reduces the ability of ammonium chloride to leach zinc from the waste. Prior to charging the waste material, a dibasic ammonium salt, such as preferably ammonium sulfate or ammonium hydroxide, is added to the leaching tank to precipitate calcium ions in the form of calcium sulfate. The loaded recycle water or ammonium chloride solution (detergent) can then be recycled to the ammonium chloride leaching step of the present invention as described below without causing calcium scaling in the detergent.

The addition of a soluble ammonium salt to which the anion can form an insoluble compound with calcium allows the calcium to be removed from the leachate while balancing the ammonium and chloride ions. The two salts are ammonium hydroxide (NH)₄OH)And ammonium sulfate ((NH)₄)₂SO₄). Addition of hydrogen hydroxideAmmonium can lead to the formation of insoluble calcium hydroxide. Ammonium hydroxide increases the pH of the system, resulting in a large loss of ammonia as the pH becomes more basic due to a change in the ammonium/ammonia balance. The addition of ammonium sulphate results in the formation of calcium sulphate which is also insoluble. Ammonium sulfate can maintain a near neutral pH while precipitating calcium sulfate. Although other ammonium salts are considered to be within the scope of the present invention, the preferred ammonium salt is ammonium sulfate. Iron-rich material may also be added for leaching and further processing.

The calcium in the waste is usually in the form of lime (CaO), some of which may also be leached by ammonium chloride. The solubility of calcium in freshly prepared ammonium chloride solutions is low (approximately 2-3%). As leachate is treated at various stages in the process, calcium ions increase, causing ammonium ions to be converted to ammonia and lost through the ventilation and scrubber system. Loss of ammonium ions imbalances the balance of ammonium and chloride ions, resulting in the formation of calcium chloride. As the solution is repeatedly recirculated, the concentration of calcium (and thus calcium chloride) is increased.

The solubility of zinc (from zinc oxide) in freshly prepared ammonium chloride solution (20% at 96 ℃) was about 13%. As shown in table IV, an increase in the calcium chloride concentration in the leachate decreased the solubility of zinc. The reduced solubility of zinc results in successively lower efficiencies per leach, since only a smaller amount of material can be leached per cycle.

TABLE IV

The solubility of ZnO in the aqueous solution at 96 ℃ contains 20% NH₄Cl

40％NaCl

34％KCl

1.6％MgCl₂

And varying amounts of CaCl₂

CaCl₂Concentration of

5% 10% 15% saturated solution containing:

calcium 1.9% 3.7% 6.5%

Magnesium 0.4%, 0.46%

Sodium 1.2% 1.2% 1.4%

Zinc 10.8%, 8.4%, 4.95%

As mentioned above, it is preferred to add ammonium sulphate to the leach solution. Ammonium sulfate may be added to the leaching tank prior to charging the dust. The calcium sulphate produced is filtered out with the discus and returned to the steelmaking furnace. Calcium can be calcined to calcium oxide when heated during the steel making process.

Recovery of ammonium chloride and purification of wash water

The wash water used to wash the zinc compound precipitated from the finished solution contains some ammonium chloride as well as other compounds. Rather than disposing of such contaminated wash water, it is treated to produce pure water and a more concentrated solution containing ammonium chloride and other compounds. Pure water may be returned to wash further zinc compounds precipitated from the finished solution, while the concentrated solution may be recycled back to the leaching step. Purification can be achieved using evaporator condenser or reverse osmosis membrane technology.

In view of economic competition, energy costs are saved by using 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. It is sometimes necessary to backwash the membranes of salt and recover it for further use at the next time. In general, reverse osmosis membrane technology uses a pump to pump wash water through the membrane, which is significantlyless expensive than burning natural gas to evaporate and then recondensing distilled water when using an evaporator condenser.

Recycling of iron by-product

The iron-rich by-product produced in the recovery process can be further processed to obtain a final product that can be recycled back to the leaching step of the recovery process of the present invention. The iron-rich by-product is preferably reduced to DRI in a reduction furnace. During the reduction process, waste flue gases are generated in the reduction furnace, which mainly consist of zinc, lead and cadmium.

In a first embodiment, the DRI is sent to a steel mill for use in the production of steel. The steel production process results in the production of waste fumes that can be treated by a cloth bag house or/and a wet scrubber or both that can be located at a steel mill. The fumes treated by the bag house are filtered and the captured solid residue is recycled back into the waste stream, with a supplementary amount of EAF dust, and thence to the leaching step of the recovery process. The fumes treated by the wet scrubber are scrubbed in a liquid stream, and the residual impurities resulting from the scrubbing process are discharged from the wet scrubber directly into the ammonium chloride solution of the leaching step.

In a second embodiment, the flue dust discharged from the reduction furnace used to produce DRI is treated by a cloth bag house or/and a wet scrubber. The fumes treated by the bag house are filtered and the captured solid residue is recycled back into the waste stream and thus into the ammonium chloride solution of the leaching step. In this embodiment, no EAF dust needs to be added to the solid residue. The fumes treated by the wet scrubber are scrubbed in a liquid stream and the residual impurities resulting from the filtration process are then discharged from the wet scrubber directly into the ammonium chloride solution of the leaching step.

The iron-rich product produced in the recovery process of the present invention may be further processed to produce fumes consisting primarily of zinc, lead and cadmium, which are captured in a bag house or/and wet scrubber and then recycled back to the ammonium chloride solution of the leaching step for use in the recovery process. The configuration of the bag house and wet scrubber is a matter of design choice, plant efficiency, and convenience. For example, steel mills are equipped with bag houses and wet scrubbers, which can be used in the recycling process of the present invention. Also, the configuration of the bag house or wet scrubber for treating the flue dust from the DRI reduction furnace is a matter of design choice, plant efficiency and convenience.

Preferred embodiments

Referring to FIG. 1, a preferred embodiment of the process of the present invention is shown. The sub-process 500 includes the feed process of the present invention. Feed streams such as iron-depleted waste flue gas streams from an electric arc furnace 12 and other furnaces such as a reduction furnace or a smelting furnace 14 are filtered in a bag house 16. Other feed streams such as iron-rich DRI and pig iron as well as scrap iron and steel must be subjected to an iron or steel making process. The fumes from these processes, which typically include electric arc furnaces or other reduction furnaces, are also filtered in a bag house 16. The filtered components in the bag house 16 comprise the reject stream that is supplied to the sub-process 100.

In sub-process 100, the waste stream feedstock is preferably leached in digester 18 at a temperature of about 90 deg.C using about 23% by weight ammonium chloride. Ammonium chloride-soluble constituents such as zinc oxide go into solution, while ammonium chloride-insoluble constituents such as iron oxides precipitate out. The precipitate in solution is filtered off in a filter 20. The filtered solution is sent to the displacement precipitator 22, through sub-process 200, to recover other useful chemicals. The precipitate in the form of a discus (IC) is sent to a sub-process 300.

The calcium contained in the fumes can be leached out by the ammonium chloride solution. Due to the continuity of the process, calcium impurities can foul resulting in reduced efficiency. Calcium scaling during ammonium chloride leaching reduces the ability of ammonium chloride to leach zinc from the waste. Prior to charging the waste material, a dibasic ammonium salt, such as preferably ammonium sulfate or ammonium hydroxide, is added to the digester 18, precipitating calcium ions in the form of calcium sulfate. The loaded recycle water or ammonium chloride solution (detergent) can then be recycled to the leaching step of the invention as described below without causing calcium scaling in the detergent.

In sub-process 300, the precipitate is dried and broken up in dryer/breaker 24. The exhaust air from the dryer/shredder 24 may be routed to a bag house, such as the bag house 16, but is typically routed to an air scrubber, such as the air scrubber 26, for scrubbing because the amount of recoverable components in the exhaust air from the dryer/shredder 24 is not significant. The dried and crushed precipitate is compressed in a compressor 28 and then fed to a reduction or smelting furnace 14. In the reduction furnace 14, the dried and crushed iron cake is roasted at 980 ℃ to 1315 ℃ to produce iron-rich cake (EIC) which may include DRI and pig iron may be in liquid form. The EIC may be compressed in a second compressor 30 and then cooled with cooling water in a cooling conveyor 32 to produce DRI. The DRI can be used as a feedstock for the EAF of a steel plant, and the cycle of the process then begins.

The flue dustdischarged from the reduction furnace 14 is sent to a scrubber 34, which is preferably a wet scrubber with water or an aqueous ammonium chloride solution recycled. The fumes exiting the EAF12 may also be sent to the scrubber 34. In the scrubber 34, the discharged soot is scrubbed, and the scrubbed exhaust gas is discharged. The water or aqueous ammonium chloride solution containing the components washed from the waste flue dust is sent to the cementation precipitator 22 or the digester 18 according to its purity; more pure solution is typically sent to digester 18, while less pure solution is typically sent to displacement settler 22.

In a preferred embodiment, the off-gas of the furnaces 12, 14 comprises ZnO and other particulate impurities. If the exhaust gas is scrubbed in the scrubber 34, the water balance is maintained using temperature control, such as a heat exchanger 36. Alternatively, the concentration of ZnO and other solubles in the wash liquor can be controlled by adding water W to the displacement precipitator 22 or ammonium chloride to the scrubber 34. As mentioned above, if an ammonium chloride solution is used as the scrubbing solution, it is preferred to maintain the solution at about 90℃ and about 23% NH₄Cl。

Example 1

Prior Art

As described in the Burrows patent, metal dust of the composition listed in Table 1 of the Burrows patent was added to 23 wt.% NH at 1 gram of dust per 10 grams of solution₄Cl solution (30 g NH per 100g water)₄Cl). The solution was heated to 90 ℃ and stirred for 1 hour, during which time the zinc oxide in the dust dissolved. The residual solids, which had a composition of about 60% iron oxide, 5% calcium oxide, 5% manganese, 30% other materials, were removed from the solutionAnd (6) filtering. Powdered zinc was then added to the filtrate at 90 c to precipitate the scrap metal, which precipitate contained about 60% lead, 40% zinc, 2% calcium and 8% other metals. The scrap metal was then filtered off and the filtrate was cooled to room temperature (about 18 c to 30 c) over about 2 hours. The solution now contains a white precipitate which is not essentially pure zinc oxide but a mixture of a hydrated zinc phase and zinc diamino dichloride.

Example 2

Metal dusts of the compositions listed in Table 1 were added to 23% by weight of NH₄Cl solution (30 g NH per 100g water)₄Cl). 1 gram of dust was used per 10 grams of solution. Adding the solution toHeated to 90 ℃ and stirred for 1 hour. During which the zinc oxide in the dust dissolves. The remaining solids, which had a composition of about 60% iron oxide, 5% calcium oxide, 5% manganese, 30% other materials, were filtered from the solution. Then, powdered zinc was added to the filtrate at 90 ℃. This precipitates scrap metal, which contains about 60% lead, 40% zinc, 2% calcium and 8% other metals. The scrap metal was then filtered off and the filtrate was cooled to room temperature (about 18 c to 30 c) over about 2 hours. The solution now contained a white precipitate.

The precipitate is a mixture of a hydrated zinc phase and diamino zinc dichloride. The hydrated zinc phase is practically insoluble in water; however, the results of the measurements in Table III show that diaminozinc dichloride is very soluble in water. A portion of the white precipitate was dried, i.e. zinc oxide and zinc diamino dichloride as well as some other ingredients appeared. The white precipitate was then filtered from the solution and resuspended in water at 90 ℃ and stirred for 1 hour. The suspension was then filtered off and the product was dried in an oven at 140 ℃. The white solid obtained was zinc oxide of 99% or more. The amount of zinc oxide obtained was 47.8% of the original precipitate mass.

The ZnO recovered by this example also contains the following components:

lead: 866ppm

Potassium: 45ppm of

Calcium: less than 25ppm

Manganese: less than 25ppm

Chromium: less than 25ppm

Example 3

The procedure of example 1 was followed until the zinc-containing filtrate cooling step. Since diamino zinc dichloride is more soluble than most other possible precipitates in ammonium chloride solution (except that zinc chloride is very soluble and does not appear), as the temperature is lowered, a larger portion of the diamino zinc dichloride appears as a solid. The filtrate was divided into two portions and each portion was cooled to a different temperature. The resulting solid was then filtered, resuspended in water at 90 ℃ for 1 hour, filtered and dried. The results obtained in all cases were more than 99% zinc oxide; however, the yield varied with the temperature to which the fraction was cooled, and the results were as follows:

crystallization temperature (. degree. C.) percent of ZnO obtained

75 65

70 60

60 60

5050 the yield of crystallized ZnO increases at temperatures above 60 deg.c.

Example 4

ZnO can also be recovered from the wash water used in the process of the invention. 50 g of the dry zinc phase precipitate obtained in the manner of example 1 (solid obtained after cooling to room temperature) are added at 90 ℃ to 100g of H₂And (4) in O. The diamino zinc dichloride dissolves, while the other zinc phases only dissolve to a small extent(due to the fact that ammonium chloride is part of the diamino zinc dichloride). Filtering to remove residual solid, and drying to obtain the final product with purity of over 99%And (3) zinc oxide. The filtrate was cooled to room temperature and the solid was filtered off. The solid is also a hydrated zinc phase and Zn (NH)₃)₂Cl₂A mixture of (a). The solid was washed with 90 ℃ water, filtered and dried to give 99% ZnO. The yield was 40% ZnO.

The yield can also be increased by crystallization at higher temperatures. Furthermore, the same wash water can be used instead of fresh water, since this part of the process depends on Zn (NH)₃)₂The solubility of (c) varies with temperature.

Example 5

The source of zinc need not necessarily be dust. If pure ZnO is added to 23% NH₄In Cl solution, the results were the same. For example, a 23% solution of ZnO saturated in ammonium chloride solution was prepared at a temperature of 40 ℃ to 90 ℃ using the solubility data of table II. The solutions were then cooled to room temperature over 1-2 hours. The resulting solid was filtered off, washed with water at 90 ℃ and then dried. As previously mentioned, the original solid was a mixture of a hydrated zinc phase and diamino zinc dichloride, and the final product was 99% ZnO. The yields obtained, expressed as a percentage of the original solid precipitate, are listed in the following table:

temperature (. degree. C.) added ZnO (g) ZnO (%)

90 14.6 64

80 13.2 62

70 8.4 60

60 5.0 60

50 3.7 45

40 2.3 40

The above results show that as the amount of dissolved ZnO increases (also meaning higher temperatures), the yield of ZnO increases.

Example 6

This example shows that the process of the invention can be operated using a continuous crystallization process to increase productivity and maximize zinc oxide yield. The procedure of example 1 was followed until the scrap metal was precipitated from the zinc oxide containing solution. 50 gallons of solution was used as a feedstock for a continuous crystallization process. The solution, initially at about 90 c, was pumped at a rate of 1 gallon per hour into a 1 gallon jacketed crystallizer equipped with baffles and draft tubes. The temperature of the crystallizer jacket was maintained at about 55 ℃ using a constant temperature circulating bath. The solution and the finished crystals are continuously withdrawn so as to keep the volume of material in the crystallizer constant. At steady state, the temperature in the crystallizer was maintained at about 60 ℃. The finished solution is passed through a filter that collects the solids. The solid product was then subjected to the washing and drying steps as described in example 2. The yield of zinc oxide from the continuous crystallization process was about 60% of the total solids crystallized.

The crystalliser can be operated at low temperature, however, as shown in example 3, low temperature reduces the final yield of the zinc oxide obtained. The flow rates employed may also be varied with the temperature of the crystallizer jacket to minimize crystallization on the crystallizer vessel wall. Furthermore, these changes with the crystallizer jacket temperature can be used to change the particle size distribution of the crystals.

Example 7

The metal dust of thecomposition listed in table 1 was dissolved in a 23% ammonium chloride solution at about 90 ℃.1 gram of zinc metal dust was used per 10 grams of ammonium chloride solution. After 1 hour, the residual solid was filtered from the solution. 500cc of the solution was poured into two containers, respectively, with stirring, and the temperature of the solution was maintained at 90 ℃. 500ppm of Flocon100 was added to one vessel and no further material was added to the other vessel. Then, four tenths of gram (0.4g) of 200 mesh zinc powder was added to each of the two solutions. In the solution containing Flocon100, the zinc powder remains in suspension, while in another solution without additives, the zinc powder agglomerates (flocculates) with each other. After a duration of 1 hour at about 90 ℃, the solids were filtered from the respective solutions, weighed and analyzed. The resulting solids amount from the solution containing the dispersant was 1.9 grams and contained about 21% zinc, 75% lead, 2% cadmium, and the balance other metals. The resulting solids from the solution without dispersant was 1.2 grams and contained about 33% zinc, 63% lead, 2% cadmium, and the balance other metals. As can be seen from this example, the additional step of adding a dispersant increases the amount of lead and other metals separated from the waste stream in the solution.

Example 8

Dust containing 19.63% Zn, 27.75% Fe, 1.31% Pb, 9.99% Ca and 0.024% Cd (analyzed on an elemental basis rather than an oxide basis) was leached at 100 ℃ with 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. Placing the material in a quartz boat filled with activated carbon at 95% N₂And 5% of O₂At 900 ℃ for 2 hours. After 2 hours, the mass was removed and added to a 23% ammonium chloride solution at 100 ℃. Theresulting material was filtered and dried at 140 ℃ for 1 hour, and the composition was determined. The analysis of the above residual solids gave 42.84% Fe, 0.28% Zn,<0.1% Pb and<0.01% Cd. The leached-roasted-leached material may be subjected to the remaining steps of the total flow to recover zinc oxide.

Example 9

The dusts of the compositions listed in table 1 were leached in a 23% ammonium chloride solution at 100 ℃ for 1 hour. The residual solids (containing 14% zinc) were placed in a quartz boat at 8% H₂And 92% Ar to 700 ℃. The mass was cooled and heated again in a 23% ammonium chloride solution at 100 ℃. The solid was isolated, dried and analyzed for zinc. Less than 1% zinc was measured. The leached-roasted-leached material may be subjected to the remaining steps of the total flow to recover zinc oxide.

Example 10

It was passed through leaching and cementation steps using dust of the composition listed in table 1. After the displacement precipitation, the filtrate was kept at 100 ℃. 500 ml of the filtrate was placed in a stirred tank equipped with a jacket at a temperature of 100 ℃. The temperature in the crystallizer decreases as follows:

time (minute) temperature (. degree. C.)

0 100

60 90

120 75

180 55

210 25

The resulting solid was washed and dried using the method described above. The analysis of the material obtained was as follows:

over 99 percent of zinc oxide

Lead less than 50ppm

Cadmium is less than 25ppm

Iron is less than 25ppm

The cooling curve in example 10 is referred to as the inverse natural cooling curve. This curve is the inverse of the shape observed with natural cooling. In the reverse natural cooling curve, cooling is slower at the beginning and faster at the end; in the natural cooling curve, cooling is faster at the beginning and slower at the end. Such cooling curves can also be used to control the Crystal Size Distribution (CSD) of the resulting zinc oxide. The cooling profile allows control of the ratio of nucleation (the creation of new crystals) to crystal growth (the growth of existing crystals). The nucleation/growth ratio may determine the final CSD.

Example 11

A23% ammonium chloride solution containing 11% by weight of dissolved ZnO at 100 ℃ was divided into 4 parts. Each portion was placed in a jacketed stirred tank. The cooling curve in each tank is given as follows:

groove A, groove B

Time (minute) temperature (. degree.C.)

0 100 0 100

60 75 60 50

120 50 120 37.5

180 25 180 25

Groove C groove D

Time (minute) temperature (. degree.C.)

0 100 0 100

60 87.5 60 87.5

120 75 120 75

180 25 180 62.5

270 25

The solid was washed using the general method described previously. The average particle size and particle size distribution of these materials were determined using a laser scattering particle size analyzer. The results are shown below:

cell average particle size

A 22

B 19

C 27

D 37

The above results show that controlling the temperature with an inverse free cooling curve results in a larger average particle size than either linear cooling (a) or free cooling (B). The principles described above can be used to design a cooling curve to produce zinc oxide of a desired average particle size and particle size distribution.

Example 12

By controlling the recycle, the steady state zinc concentration can be increased to 7 grams per 100 grams of solution. If the outlet of the crystallizer is maintained at 60 ℃, there will be 3 grams of solid (which is a mixture of zinc oxide and zinc diamino dichloride) crystallized per 100 grams of solution. Further cooling of the system is not necessary as this is an energy efficient method of operation (cooling is not necessary and then heating of the solution is performed). Furthermore, operating at higher Zn concentrations can increase the ZnO/diamino zinc dichloride ratio produced in the crystallizer.

The advantage of recycling is that the solution becomes saturated for some of the material contained in the dust, such as CaO. When this happens, CaO will no longer leach from the dust and remain with the iron in the discus. This increases the value of the discus, since CaO is still present and does not have to be added when the discus is fed to the steelmaking furnace. Another important advantage is that there is no liquid effluent in the process. All products are only solids (discus, zinc oxide, scrap metal) and can be sold for use in various industrial processes. No waste is produced as all the liquid is recycled.

Example 13

A sample of mixed scrap metal cake was freshly taken from a mixed metal press (mixed metals press) and placed in a gas tight plastic bag. The cakes were mixed in a bag and excess liquid was removed by draining through the bag. And tries to remove the air from the bag. The main element composition of this cake material was determined to be:

weight of element (%)

Cd 1.5

Cu 1.0

Pb 19.2

Other materials where Zn 2.1 is present include water and ammonium chloride and mixed metal oxides.

This mixed metal cake sample was removed from the bag, added to a beaker, and washed with an equal weight of warm water on a hot plate with mechanical agitation. Some white precipitate of mainly zinc oxide is formed by decomposition of the complexed diamino zinc salt. A washing step is carried out to remove ammonium chloride and other soluble components that may be present.

The wash water was decanted from the product. Fresh water was added enough to cover the solids to prevent the solids from becoming reoxidized by air, and excess sulfuric acid was added in stages over several days during which time foaming was observed with care. No heat was applied. When the pH of the mixture reached 5, sulfuric acid was further added. The solutions were mixed and then allowed to react without further mixing. The elemental composition was analyzed with the results:

weight of element (%)

Cd 0.19

Cu 4.1

Pb 43.2

Zn 0.04

The solid was observed under a microscope using crossed polarizers and many crystalline- "non-metallic" materials were observed. Most of these materials may be lead sulfate formed by the reaction of lead metal with acid. The remainder may be copper and lead metals. The solution produced by this process was analyzed and found to contain a significant amount of zinc and cadmium, with very low concentrations of copper and lead. If it is desired to produce high purity cadmium metal, the solution can be further processed using a displacement precipitation process.

Example 14

The second batch of washed mixed metal was taken from the scrap metal cake described in example 13 and heat treated with a limited amount of sulfuric acid (10 grams of acid per 100 grams of mixed metal). The reaction was allowed to continue for 4 hours. At the end of this time, the pH rose to 6, meaning that the acid had completely reacted with the product.

Samples of residual solids were collected and air dried on hot glass slides. After analysis, it contained:

weight of element (%)

Cd 1.9

Cu 4.0

Pb 78

Zn 2.9

The product showed more metal solids than previous tests, as observed by microscopy. However, some of the crystallized material was similar to that observed previously.

The foregoing description sets forth the best mode of the invention known to the inventors at the time, the foregoing examples being provided for illustration only, it being apparent that modifications may be made thereto by those skilled in the art without departing from the spirit and scope of the invention and the equivalents thereof set forth in the claims appended hereto.

Claims (30)

1. A continuous process for recovering valuable metals and chemicals from a waste stream comprising iron and zinc compounds, the process comprising the steps of:

a. pre-roasting the waste stream at elevated temperature resulting in reduction of at least a portion of the iron oxides to direct reduced iron and production of waste steam comprising zinc, lead and cadmium compounds;

b. treating the waste stream with an ammonium chloride solution at elevated temperature to produce a finished solution comprising dissolved constituents and insoluble precipitates such that all iron oxide in the waste material is contained in the insoluble precipitates and not passed into solution;

c. separating the finished solution from the insoluble precipitate; and

d. the insoluble precipitate is calcined at a high temperature to reduce the iron oxide to direct reduced iron.

2. The method of claim 1, further comprising the step of mixing an iron oxide rich material with the waste stream prior to pre-roasting the waste stream.

3. The method of claim 1, further comprising the step of mixing the iron-depleted material with the waste stream prior to pre-roasting the waste stream.

4. The method of claim 1, further comprising the steps of:

e. adding a zinc compound to the finished solution so that lead, copper and cadmium contained in the finished solution are separated by precipitation with the zinc compound, thereby producing a waste metal cake including lead and cadmium;

f. treating the scrap metal cake with a dissolving liquor in which zinc and cadmium are soluble and lead is insoluble;

g. recovering lead by separating lead from the dissolved solution; and

h. and recovering cadmium from the dissolving solution by an electrochemical method.

5. The method of claim 1, further comprising the steps of:

e. cooling said finished solution by a controlled process, and

f. the zinc oxide crystals are precipitated from the finished solution in a controlled manner such that the zinc oxide crystals have a predetermined purity and particle characteristics.

6. The method of claim 1, further comprising the steps of:

e. adding zinc metal to the finished solution such that all zinc-replaceable metal ions contained in the finished solution are replaced by the zinc metal and precipitate as metal from the finished solution;

f. separating the metals from the finished solution and reducing the temperature of the finished solution to precipitate at least a portion of the zinc component in the finished solution as a mixture of crystalline zinc compounds;

g. separating the crystalline zinc compound from the finished solution and washing the crystalline zinc compound with wash water, whereby some of the zinc compound is dissolved; and

h. any remaining crystalline zinccompound is separated from the finished solution and then dried at a temperature of about 100 c to 200 c, resulting in the recovery of 99% or more pure zinc oxide product.

7. The method of claim 1, further comprising the steps of:

a. initiating mixing of an iron oxide rich material with the waste stream to produce a waste mixture;

b. first pretreating said waste mixture with a 23% by weight ammonium chloride solution at elevated temperature to produce a first finished solution comprising dissolved constituents and insoluble precipitates such that any iron oxide in the waste is contained in the insoluble precipitates and does not go into solution;

c. separating the first finished product solution from the insoluble precipitate;

d. pre-calcining the insoluble precipitate in a reducing atmosphere at a temperature of at least 500 ℃ resulting in reduction of at least a portion of the iron oxide to direct reduced iron and generation of waste steam; and

e. performing the b-d step of claim 1, wherein the calcining is performed at a temperature of 980 ℃ to 1315 ℃.

8. The method of claim 7, further comprising the step of adding carbon to the mixture of waste vapor and ammonium chloride without bringing the carbon into solution.

9. The method of claim 1, further comprising the steps of:

d. treating the finished solution with a lixiviant solution consisting of sulphuric acid until at least most of the zinc is dissolved and all lead in the finished solution does not enter the solution and separating the lead from the solution;

e. adding a compound selected from the group consisting of zinc oxide and zinc hydroxide to the finished solution to raise the pH of the finished solution to a sufficient level to cause precipitation of sulfate, and then separating the salt from the finished solution;

f. adding zinc metal powder to the finished solution to effect a first cementation step resulting in the formation of copper solids and separating at least a portion of the copper solids from the finished solution; and

g. adding zinc metal pieces to the finished solution of the electrolysis step results in sponge cadmium being produced on the zinc metal pieces and separating at least a portion of the sponge cadmium from the finished solution.

10. The method of claim 1, further comprising the steps of: the finished solution is treated with a stoichiometric amount of a soluble ammonium salt wherein the anion forms an insoluble compound with calcium to produce a finished solution substantially free of calcium product and insoluble calcium compound precipitate.

11. The method of claim 10, further comprising the steps of: the waste material is pre-calcined in a reducing atmosphere at an elevated temperature of at least 500 ℃ prior to treatment of the waste vapor with an ammonium chloride solution.

12. The process of claim 11 wherein the insoluble precipitate is calcined at elevated temperature to reduce all of the iron oxide in the insoluble precipitate to direct reduced iron.

13. The method of claim 12, wherein the pre-firing step is performed in a rotary hearth furnace.

14. The method of claim 13, wherein the waste stream is selected from the group consisting of a waste stream from a roasting step, a waste stream from an ore smelting process, a waste stream from a metal working process, a waste stream from a metal product working process, a waste stream from an iron making process, and a waste stream from a steel making process.

15. The process according to claim 14, wherein the insoluble precipitate is calcined at a temperature of 980 ℃ to 1315 ℃.

16. The method of claim 1, further comprising the steps of: the waste vapor is treated in an ammonium chloride solution with a soluble ammonium salt in which the anions can form insoluble compounds with the calcium component.

17. The process according to claim 16, wherein the insoluble precipitate is calcined at a temperature of 980 ℃ to 1315 ℃ and the concentration of the ammonium chloride solution is about 23% by weight.

18. The process according to claims 1-6, wherein the insoluble precipitate is calcined at a temperature of 980 ℃ to 1315 ℃.

19. The method of claim 18 wherein said ammonium chloride solution is a 23% by weight aqueous ammonium chloride solution.

20. The method according to claims 1-19, further comprising the steps of:

e. treating said pre-roasted waste material with an ammonium chloride solution at elevated temperature to produce a second finished solution comprising dissolved constituents and a second insoluble precipitate such that any iron oxide in the waste material is contained in the second insoluble precipitate and does not go into solution;

f. separating the second finished solution from the second insoluble precipitate; and

g. calcining the second insoluble precipitate at high temperature results in the reduction of iron oxide to direct reduced iron.

21. The method of claim 20, further comprising the step of pre-mixing the waste material with carbon prior to pre-firing the waste material.

22. The method of claim 20, further comprising the steps of:

h. adding zinc metal powder to the second finished solution resulting in cementation of lead and cadmium; and

i. cooling the second finished solution results in crystallization of the zinc compound.

23. The process according to claim 22, wherein the insoluble precipitate is calcined at a temperature of 980 ℃ to 1315 ℃.

24. The method of claim 21, wherein the scrap material is pre-fired at an elevated temperature of at least 500 ℃ and in a reducing atmosphere comprising carbon.

25. The method of claim 24, further comprising the steps of:

h. adding zinc metal to the second finished solution such that all zinc-replaceable metal ions contained in the second finished solution are replaced by the zinc metal and precipitate as metal from the second finished solution;

i. separating the metals from the second finished solution and reducing the temperature of the second finished solution to precipitate at least a portion of the zinc component in the second finished solution as a mixture of crystalline zinc compounds;

j. separating the crystalline zinc compound from the second finished solution and washing the crystalline zinc compound with wash water whereby some of the zinc compound is dissolved; and

k. any remaining crystalline zinc compound is separated from the second finished solution and then dried at a temperature of about 100 c to 200 c, resulting in the recovery of a 99% or more pure zinc oxide product.

26. A process according to claim 5, 6, 22 or 25, further comprising the step of washing and drying the zinc oxide crystals.

27. The method according to claim 26, wherein zinc oxide crystals are precipitated by diluting said finished solution at a predetermined ratio of 3-30 times at 70-100 ℃.

28. The method according to claim 27, wherein the finished solution is diluted 3-5 times at about 90-100 ℃.

29. A method according to claim 28, wherein the waste stream is first treated with a medium selected from the group consisting of sodium hydroxide, ammonium sulfate, ammonium chloride solution, ammonium phosphate, potassium hydroxide, ammonia/ammonium oxalate and ammonia/ammonium carbonate solution to produce the finished solution.

30. The method according to claim 29, wherein said medium is a sodium hydroxide solution having a concentration of 50% to 70%.

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