BACKGROUND OF THE INVENTION

1. Field of the Invention The present invention relates generally to a method for the recovery of essentially pure zinc oxide and specifically to a method utilizing a combination of leaching and roasting steps, for the recovery of essentially pure zinc oxide in a recycling operation from metal dust containing zinc compounds.

2. Prior Art

Zinc oxide typically is a coarse white or grayish powder which has a variety of uses including as an accelerator activator, as a pigment, as a dietary supplement and in the semiconductor field. Zinc oxide is found in commercial by-products including waste material streams such as fly ash and flue dust. Methods for recovering zinc oxides are known in the art, including recovering zinc oxide from industrial waste materials. Such previous methods have included leaching with mineral acid, caustic soda, ammonium hydroxide, and ammonium carbonate solutions. However, these methods have low yields of zinc oxide and typically do not recovery pure zinc oxide, the recovered zinc oxide being contaminated with other metal salts. Therefore, in order to obtain pure zinc oxide, subsequent roasting and evaporation processes were necessary.

U.S. Patent No. 3,849,121 to Burrows, assigned to a principal of the assignee of the present invention, discloses a method for the selective recovery of zinc oxide from industrial waste. The Burrows method comprises leaching the a waste material with an ammonium chloride solution at elevated temperatures, separating iron from solution, treating the solution with zinc metal and cooling the solution to precipitate zinc oxide. The

Burrows patent discloses a method to take metal dust which is mainly a mixture of iron and zinc oxides and, in a series of steps, to separate out the iron oxides and waste metals. However, the material obtained in the last step is a mixture of a small amount of zinc oxide, hydrated zinc phases which can include hydrates of zinc oxide and zinc hydroxide, as well as other phases and a large amount of diamino zinc dichloride Zn( H ₃ ) ₂ Cl2 or other similar compounds containing zinc and chlorine ions. Currently, the Burrows method is not economically viable because of Environmental Protection Agency guidelines established subsequent of the issuance of the Burrows patent. Additionally, the Burrows method is not a continuous method and, therefore, is not economical as a continuous process.

Thus, there exists a need for a method which will recover zinc oxide from industrial waste which results in a product the majority of which is zinc oxide, and not mixtures of zinc oxide and other zinc phases. The method disclosed below relates to the preparation of pure zinc oxide. In addition, since zinc oxide is the desired product and diamino zinc dichloride is undesired, the method disclosed herein demonstrates how to increase the formation of zinc oxide and decrease the formation of diamino zinc dichloride.

Waste metal process dust typically has varying amounts of lead, cadmium and other metals contained in the dust. For various reasons, it is desirable to remove such metals from the waste metal dust, for example to recycle the lead and cadmium and/or to prevent introduction of the lead and cadmium into the atmosphere. The Burrows patent includes a method for removing dissolved lead and cadmium from the ammonium chloride solutions which have been used to treat the waste metal dust. In the Burrows method, powdered zinc dust is added to the ammonium chloride

solutions and an electrochemical reaction results in which lead in elemental form deposits on the surface of the powdered zinc dust. For this reaction to proceed, a large surface area of zinc initially must be present because as the lead covers the zinc dust particle, the particle becomes no longer available for the electrochemical reaction. For this reason, very fine powder is used. However, in the Burrows method as disclosed, there is a major disadvantage in that the powdered zinc dust, when added to the solutions, immediately aggregates to form large clumps which sink to the bottom of the vessel. Rapid agitation does not prevent this from happening. Because of the aggregation of zinc, a large amount of zinc must be added to remove all of the lead, a poor practice for economic reasons. Further, if it is desired to separate the lead and some cadmium from the zinc so that all of these metals can be sold or reused, the higher the zinc concentration in the metals, the larger the mass to be processed per unit mass of zinc. U.S. Patent No. 4,071,357 to Peters discloses a method for recovering metal values which includes a steam distillation step and a calcining step to precipitate zinc carbonate and to convert the zinc carbonate to zinc oxide, respectively. Peters further discloses the use of a solution containing approximately equal amounts of ammonia and carbon to leach the flue dust at room temperature, resulting in the extraction of only about half of the zinc in the dust, almost 7% of the iron, less than 5% of the lead, and less than half of the cadmium. Steam distillation is directly contrary to the temperature lowering step of the present invention. Steam distillation precipitates zinc carbonate, whereas temperature lowering precipitates a number of crystalline zinc compounds. Calcining at temperatures between 200 ^β C and 1100°C converts the zinc carbonate to zinc oxide,

whereas, washing and drying at temperatures between 100°C and 200°C converts the zinc compounds to zinc oxide. The present process also employs a 23% NH ₄ C1 solution at temperatures ranging from 90-110 ^β C, and is distinct from the Peters process for a number of reasons:

1. The solubility of zinc (or zinc oxide) is relatively high in NH ₄ C1 solution which is important to the efficiency of the present process in terms of the rate of the leaching, the mass of dust that can be processed, and the ability to recycle the solution. The rate of the leaching (which is a dissolution process) is a function of the difference between the zinc concentration in solution and the saturation concentration; the higher the saturation concentration the more rapid the leaching. The present process leaches for only 1 hour, while the Peters process leaches for at least several hours. In addition, the ammonium chloride solution has the added property that the solubility of zinc (or zinc oxide) in the solution declines rapidly with temperature, which is the basis for the crystallization-based separation which is used later in the process.

2. Lead and lead oxide, as well as cadmium and cadmium oxide, are soluble in the ammonium chloride solution while iron oxide is virtually insoluble. During the leaching process of the present invention, 95-100% of the zinc present as zinc oxide is extracted, compared to about 55% in Peters; 50-70% of the lead present is removed, compared to less than 5% in Peters: as is 50-70% of the cadmium, compared to less than half in Peters. In effect, Peters does not remove a significant amount of the impurities so as to leave an acceptably clean effluent. Peters indicates that his residue, which is high in lead and is a hazardous waste, is discarded. By leaching out a significant portion of the lead and cadmium, the present process produces a material which can be used by the steel producer as they use scrap metal.

Peters adds powdered zinc to the solution, which has a tendency to clump reducing the surface area available for the dissolution of the zinc and the plating of the lead and cadmium. The present process teaches a method to minimize this effect through the use of an organic dispersant. Peters steam distills the filtrate, resulting in an increase in temperature which drives off ammonia and carbon dioxide, resulting in the precipitation of iron impurities and then zinc carbonate and other dissolved metals. The purity of the zinc carbonate obtained depends on the rate of steam distillation and the efficiency of solids separation as a function of time. Peters also precipitates iron impurities, zinc carbonate and other carbonates, while the present process only crystallizes zinc salts. Any metal impurities (less than 1%) are surface absorbed or lattice substituted.

In the present process the filtrate from the cementation step is already hot (90-110°C) and contains a large amount of dissolved zinc with small amounts of trace impurities. Upon controlled cooling of the solution crystals of zinc salts begin to appear. Control of the cooling rate and temperature versus time profile is important in controlling the size distribution of the crystals and in reducing or eliminating many of the impurities which might occur. This is especially true of included solution; control of the crystallization can reduce this to virtually zero. In addition, since crystallization is based on differential solubility, and none of the impurities is present in a concentration which can crystallize, the zinc salts are virtually free of any metal impurities.

The final purification step in Peters is a calcining of the zinc carbonate at 600°C to zinc oxide. In the present process, the mixture of zinc oxide hydrates and diamino zinc dichloride are suspended in hot

(90-100°C) water. The zinc oxide is not soluble; however, the zinc diamino dichloride is very soluble and completely dissolves. The remaining solid which is zinc oxide hydrates is then filtered and dried at 100-200°C to remove the water of hydration. The result is a very pure zinc oxide powder of controlled particle size.

Thus, there exists a need for a method which will allow the recovery of elemental lead, cadmium, and other metals from industrial waste streams which allows the powdered zinc dust to remain dispersed in the solution so as to minimize the amount of zinc dust needed to remove lead, cadmium and other metals. Minimizing the amount of zinc required increases the economy of the process first by reducing the quantity of zinc needed, second by reducing the mass of material to be processed, and third by allowing the removal of a proportionally greater quantity of lead and cadmium.

BRIEF SUMMARY OF THE INVENTION The present invention satisfies these needs in a method which recovers essentially pure zinc oxide from waste material containing zinc or zinc oxide. The waste material, typically including Franklinite and Magnetite, is roasted at temperatures greater than 500°C for a predetermined period of time, before and/or after the material is added to an ammonium chloride solution at a temperature of about 90°C or above. The roasting causes a decomposition of Franklinite into zinc oxide and other components. The roasting process generally comprises the steps of adding heat to the waste material and/or passing heated reducing gases through the waste material. Although all reducing gases are suitable, hydrogen and carbon dioxide are preferred, as well as mixing carbon (activated) with the material and roasting in a gas containing oxygen.

The zinc and/or zinc oxide dissolves in the ammonium chloride solution along with other metal oxides contained in the waste material, such as lead oxide and cadmium oxide. The resultant solution is filtered to remove the undissolved materials, such as iron oxides and inert materials such as silicates, which will not dissolve in the ammonium chloride solution, and then dried at a temperature of at least 100°C for at least 60 minutes.

Alternatively, before drying the filtered solution, finely powdered zinc metal can be added to the resultant, solution at a temperature of about 90°C or above. A dispersant may be added at this point to prevent the finely powdered zinc metal from flocculating and becoming less effective. Through an electrochemical reaction, lead metal and some cadmium plates out on the surface of the zinc metal particles. The addition of sufficient powdered zinc metal results in the removal of virtually all of the lead from the resultant solution. The resultant solution is filtered to remove the solid lead, zinc and cadmium. These initial steps, with the exception of adding the dispersant, have been generally disclosed in the prior art, yet have not resulted in the production of essentially pure zinc oxide.

The filtrate then is cooled to a temperature of between about 20°C and 60°C resulting in the crystallization of a mixture of zinc compounds. The crystallization step helps to achieve a high purity zinc oxide of controlled particle size. During the crystallization step, the filtrate can be cooled to its final temperature by controlling the cooling profile. The use of a reverse natural cooling profile is preferred as its results in a more desirable nucleation to crystal growth ratio.

This mixture contains a significant amount of diamino zinc dichloride, or other complex compounds which

involve zinc amino complexes, as well as hydrated zinc oxide and hydroxide species. The solid precipitate is filtered from the solution, the solution recycled, and the solid precipitate washed with water at a temperature between about 25°C and 100°C. The diamino zinc dichloride dissolves in the wash water leaving the majority of the hydrated zinc oxide species as the precipitated solid. The precipitated solid then is filtered from the solution, the resulting solution being recycled, and the solid precipitate placed in a drying oven at a temperature of between about 100°C and 200°C, resulting in a dry white zinc oxide powder. These additional steps allow the production and recovery of substantially pure zinc oxide.

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

It is another object of the present invention to provide a method for recovering high grade purity zinc oxide.

Yet another object of the present invention is to provide a method for recovering zinc oxide in which the leaching and washing solutions are recycled for further use.

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

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

A further object of the present invention is to provide a method for recovering zinc oxide in which iron oxide contained in the starting materials is not put into solution. An additional object of the present invention is to provide a method for recovering zinc oxide in which lead, cadmium and other metals contained in the starting materials can be removed from the process using a minimal amount of powdered zinc dust. Yet another object of the present invention is to provide a method for recovering zinc oxide in which the powdered zinc dust added to the intermediate solutions is kept dispersed using water soluble polymers which act as antiflocculants or dispersants. Still another object of the present invention is to provide a method for recovering zinc oxide in which Franklinite is decomposed to zinc oxide and other materials through roasting.

A final object of the present invention is to provide a method for recovering zinc oxide which is economical, quick and efficient.

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

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. IA is an X-ray diffraction of the precipitate obtained in Example 1 (many phases).

Fig. IB is an X-ray diffraction of the precipitate after drying ZnO + Zn(NH ₃ ) ₂ Cl ₂ .

Fig. 1C is an X-ray diffraction of the precipitate after washing and drying ZnO.

Fig. 2A is an X-ray diffraction of the precipitate obtained in Example 4 (many phases).

Fig. 2B is an X-ray diffraction of the precipitate after drying ZnO + Zn(NH ₃ ) Cl ₂ . Fig. 2C is an X-ray diffraction of the precipitate after washing and drying ZnO.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The method for recovering zinc oxide disclosed herein is carried out in its best mode in recovering zinc oxide from the waste streams of industrial or other processes. A typical industrial waste stream used is a flue gas where the charge contains galvanized steel, having the following percent composition:

TABLE I Analysis of Flue Gas

siliceous material, such as slag, with carbon granules occluded molybdinum, antimony, indium, cadmium, germanium, bismuth, titanium, nickel and boron.

General Process Description

An ammonium chloride solution in water is prepared in known quantities and concentrations. The feed material which contains the zinc species, such as the dust described immediately above or the waste material flue dust described in Table I or any other feed material source containing zinc or zinc oxide mixed with other metals, is added to the ammonium chloride solution

at a temperature of about 90 ^β C or above. The zinc and/or zinc oxide dissolves in the ammonium chloride solution along with other metal oxides, such as lead oxide and cadmium oxide. The solubility of zinc oxide in ammonium chloride solutions is shown in Table II.

TABLE II Solubility of ZnO in 23% NH ₄ C1 solution

ure °C g Dissolved/100 g H ₂ Ω

14.6

13.3 8.4

5.0 3.7 2.3

It has been found that a 23% by weight ammonium chloride solution in water at a temperature of at least 90°C provides the best solubility of zinc oxide. Concentrations of ammonium chloride below about 23% do not dissolve the maximum amount of zinc oxide from the flue dust, and concentrations of ammonium chloride above about 23% tend to precipitate out ammonium chloride along with the zinc oxide when the solution is cooled. Therefore, 23% has been chosen as the preferred ammonium chloride solution concentration. Iron oxide and inert materials such as silicates will not dissolve in the preferred solution.

The zinc oxide, as well as smaller concentrations of lead or cadmium oxide, are removed from the initial dust by the dissolution in the ammonium chloride

solution. The solid remaining after this leaching step contains zinc, iron, lead and cadmium, and possibly some other impurities. The remaining solid is then roasted in a reducing atmosphere, typically at a temperature greater than 420°C and often at 700°C to 900°C. The reducing atmosphere can be created by using hydrogen gas, simple carbon species gases such as carbon dioxide, or by heating the material in an oxygen containing gas in the presence of elemental carbon. Typical roasting times are from 30 minutes to 4 hours. Alternatively, the waste dust first may be roasted and second may be leached, omitting the first leaching step.

After the dust has been roasted, it is subjected to another leaching step in 23% by weight ammonium chloride solution in water at a temperature of at least 90°C. Any zinc or zinc oxide formed during the roasting step dissolves in the ammonium chloride solution. The zinc oxide and ammonium chloride solution then is filtered to remove any undissolved material. After filtering, for analysis, the solid may be separated out and dried at a temperature of over 100°C, typically between 100°C and 200°C, for about 30 minutes to 2 hours, typically 1 hour.

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

Powdered zinc metal alone may be added to the zinc oxide and ammonium chloride solution in order to remove the solid lead and cadmium. However, the zinc

powder typically aggregates to form large clumps in the solution which sink to the bottom of the vessel. Rapid agitation typically will not prevent this aggregation from occurring. To keep the zinc powder suspended in the zinc oxide and ammonium chloride solution, any one of a number of water soluble polymers which act as antiflocculants or dispersants may be used. In addition, a number of surface active materials also will act to keep the zinc powder suspended, as will many compounds used in scale control. These materials only need be present in concentrations of 10 - 1000 ppm. Various suitable materials include water soluble polymer dispersants, scale controllers, and surfactants, such as lignosulfonates, polyphosphates, polyacrylates, polymethacrylates, maleic anhydride copolymers, polymaleic anhydride, phosphate esters and phosponates. A discussion of these various materials can be found in the literature, such as Drew, Principles of Industrial Waste Treatment, pages 79-84, which is incorporated herein by reference. Flocon 100 and other members of the Flocon series of maleic-based acrylic oligmers of various molecular weights of water soluble polymers, produced by FMC Corporation, also are effective. Adding the dispersants to a very high ionic strength solution containing a wide variety of ionic species is anathema to standard practice as dispersants often are not soluble in such high ionic strength solutions.

The filtrate then is cooled to a temperature of between about 20°C and 60°C resulting in the crystallization of a mixture of zinc compounds. The mixture contains a significant amount of diamino zinc dichloride, or other complex compounds which involves zinc amino complexes, hydrated zinc oxides and hydroxide species. Crystallization helps to achieve a high purity zinc oxide of controlled particle size, typically through

control of the temperature-time cooling profile. Reverse natural cooling, that is cooling the solution slower at the beginning of the cooling period and faster at the end of the cooling period, is preferred to control the nucleation to crystal growth ratio and, ultimately, the crystal size distribution. The precipitated crystallized solid is filtered from the solution and washed with water at a temperature of between about 25°C and 100°C. The filtered solution is recycled for further charging with feed material. The diamino zinc dichloride dissolves in the water. The solubility of diamino zinc dichloride in water is shown in Table III.

TABLE III

Solubility of Zn(NH3) ₂ Cl ₂ in water

Tempera ure °C σ Dissolved/100 α H^O

90 32 80 24

40 21

25 12.8

Very little of the hydrated zinc oxide dissolves in the water. This resultant solution then is filtered to remove the hydrated zinc oxide species. The solid hydrated zinc oxide species filtered from the solution is placed in a drying oven at a temperature of between about 100°C and 200°C. After a sufficient drying period, the resultant dry white powder is essentially pure zinc oxide. The filtrate from the solution is recycled for charging with additional zinc compound mixture.

As the zinc, lead and cadmium contained in the feed materials are amphoteric species, by using ammonium

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

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

The following Examples demonstrate ways to increase the formation of zinc oxide according to the present invention. Examples 1-7 do not include roasting and Examples 8-13 include roasting. Examples 10-12 also show variations on the crystallization step, and Example 13 also illustrates the recycle results. X-ray diffraction analyses of the zinc oxide prepared according to these examples indicate the recovery of high purity zinc oxide.

Example 1

Prior Art A metal dust of composition listed in Table I of the Burrows patent is added to 23% by weight NH ₄ C1 solution (30g NH ₄ C1 per lOOg H ₂ 0) , as discussed in the Burrows patent, in the amount of 1 gram of dust per 10 grams of solution. The solution is heated to a temperature of 90°C and stirred for a period of 1 hour, during which the zinc oxide in the dust dissolves. The remaining solid, which has a composition of approximately 60% iron oxide, 5% calcium oxide, 5% manganese, 30% other materials, is filtered out of the solution. Powdered zinc is then added to the filtrate at 90°C, causing the precipitation of waste metals, the precipitate containing about 60% lead, 40% zinc, 2% cadmium and 8% other metals. The waste metals are then filtered out and the filtrate is cooled to room temperature (between about 18°C and 30°C) over a period of about two hours. The solution then contains a white precipitate which is not essentially pure zinc oxide but is a mixture of hydrated zinc phases and diamino zinc dichloride.

Example 2 A metal dust of composition listed in Table I is added to 23% by weight NH ₄ C1 solution (30g NH C1 per lOOg H ₂ 0) . 1 gram of dust is used per 10 grams of solution. The solution is heated to a temperature of 90°C and stirred for a period of 1 hour. During this period the zinc oxide in the dust dissolves. The remaining solid, having a composition of approximately 60% iron oxide, 5% calcium oxide, 5% manganese, 30% other materials, is filtered out of the solution. Powdered zinc is then added to the filtrate at 90°C. This causes the precipitation of waste metals, the waste metal precipitate containing about 60% lead, 40% zinc, 2% cadmium and 8% other metals. The waste metals are then filtered out and the filtrate is

cooled to room temperature (between about 18°C and 30°C) over a period of about two hours. The solution then contains a white precipitate.

As shown in Fig. IA, X-ray diffraction of the precipitate indicates that it is a mixture of hydrated zinc phases and diamino zinc dichloride. The hydrated zinc phases are virtually insoluble in water, however the measurements in Table III show that diamino zinc dichloride is quite soluble in water. A portion of the white precipitate was dried and, as shown in Fig. IB, zinc oxide and diamino zinc dichloride, as well as some other components, are present. The white precipitate is then filtered from the solution and resuspended in water at

90°C and stirred for a period of one hour. This suspension is then filtered and product dried in an oven at 140°C. As shown in Fig. 1C, the resulting white solid is 99%+ zinc oxide. The amount of zinc oxide obtained was

47.8% of the mass of the original precipitate.

The ZnO recovered by this Example also had the following components: lead: 866 ppm potassium: 45 ppm calcium: less than 25 ppm manganese: less than 25 ppm chromium: less than 25 ppm

Example 3

The procedure of Example 1 is followed until the step in which the zinc containing filtrate is cooled.

Since the diamino zinc dichloride is more soluble then the majority of the other possible precipitates in the ammonium chloride solution (except for zinc chloride which is so soluble that it will not appear), the diamino zinc dichloride appears as a larger fraction of the solid as the temperature declines. The filtrate was divided into fractions and each fraction cooled to a different

temperature. The resulting solids were then filtered, resuspended in water at 90°C for one hour, filtered and dried. The result was 99%+ zinc oxide in all cases, however the yield changed with temperature to which the fraction was cooled as follows:

Crystallization Percent ZnO Temp (°o Ob ained

75 65

70 60 60 60

50 50

Crystallization at temperatures from 60°C up improve the yield of ZnO.

Example 4 ZnO also can be recovered from the wash water used in the process. Fifty grams of dried zinc phase precipitate (the solid obtained after cooling to room temperature) obtained using the procedure of Example 1 is added to lOOg of H ₂ 0 at 90°C. The diamino zinc dichloride dissolves while only a small amount of the other zinc phases dissolve (due to the ammonium chloride which is part of the diamino zinc dichloride) . The remaining solid is filtered out and is dried resulting in 99%+ zinc oxide. The filtrate is cooled to room temperature and the solid filtered out. The solid is again a mixture of hydrated zinc phases and Zn(NH ₃ ) ₂ Cl ₂ . The solid is washed in 90°C water, filtered and dried resulting in 99% ZnO. The yield is 40% ZnO.

The yield can also be improved by crystallizing at higher temperatures. In addition, the same wash water can be used again instead of fresh water since this part of the process relies on the change in Zn(NH ₃ ) solubility with temperature (see data section).

Example 5

The source of the zinc does not have to be dust. If pure ZnO is added to a 23% NH ₄ C1 solution, the result is the same. As an example, saturated solutions of ZnO in 23% ammonium chloride solutions were prepared at temperatures ranging from 40°C - 90°C, using the solubility data of Table II. These solutions were then cooled to room temperature over a period of 1 - 2 hours. The resulting solid was filtered, washed in 90°C water, and dried. As before, and as shown in Fig. 2A, the original solid was a mixture of hydrated zinc phases and diamino zinc dichloride. As shown in Fig. 2C, the final product was 99% ZnO. Fig. 2B shows the analysis of the intermediate zinc oxide and diamino zinc dichloride precipitate. The yields obtained as a fraction of the original solid precipitate are listed below:

These results indicate that the yield of ZnO improves as the amount of dissolved ZnO increases (which also means higher temperatures). Example 6

This example shows the present procedure run in a continuous crystallization process to increase the through put and to maximize the zinc oxide yield. The procedure of Example 1 is followed until the step in which the waste metals are precipitated out of the zinc oxide containing

solution. Fifty gallons of the solution are used as the feedstock for a continuous crystallization process. The solution, initially at about 90°C, is pumped into a 1-gallon jacketed crystallizer equipped with baffles and a draft tube at a rate of 1 gallon per hour. The crystallizer jacket temperature is maintained at about 55 ^β C by use of a constant temperature circulating bath. The solution and the product crystals are removed continuously so as to keep the volume of material present in the crystallizer constant. At steady state, the temperature in the crystallizer is maintained at about 60°C. The product solution flows through a filter which collects the solid. The solid product then undergoes the washing and drying steps as discussed in Example 1. The yield of zinc oxide from this continuous crystallization process is about 60% of the total mass of the solid crystallized.

The crystallizer can be operated at lower temperatures; however, lower temperatures decrease the final yield of zinc oxide obtained as shown in Example 2. The flow rate employed also can be altered along with the crystallizer jacket temperature to minimize crystallization on the crystallizer vessel walls. In addition, these variables, along with the crystallizer jacket temperature, can be used to alter the crystal size distribution.

Example 7 Metal dust of the composition shown in Table I is digested in 23% ammonium chloride solution at about 90°C One gram of zinc metal dust is used per 10 grams of ammonium chloride solution. After one hour the remaining solid is filtered out of the solution. 500 cc of the solution is put into each of two vessels with stirrers and the temperature of the solutions is maintained at 90°C. 500 ppm of Flocon 100 is added to one of the vessels,

while nothing is added to the other vessel. Four-tenths of a gram (0.4g) of 200 mesh zinc dust then is added to each of the two solutions. In the solution containing the Flocon 100, the zinc dust remains suspended, while in the solution containing any additive the zinc dust clumps together (flocculates). After one hour at about 90°C the solids are filtered out of each of the solutions, weighed and analyzed. The mass of solid from the solution which contained the dispersant was 1.9 grams and comprised approximately 21% zinc, 75% lead, 2% cadmium and the remaining, amount other metals. The mass of solid obtained from the solution with no dispersant was 1.2 grams and comprised approximately 33% zinc, 63% lead, 2% cadmium and the remaining amount other metals. From this example, it can be seen that the additional step of adding a dispersant increases the amount of lead and other metals removed from the waste stream in solution.

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

Filtration and washing of the undissolved species leaves a residual powder. This powder shows a zinc concentration that is still elevated (i.e., 10 - 13% by weight), but that is not zinc oxide. X-ray diffraction indicates that all crystalline phases can be identified by

Spinel type phases. The combination of chemical analysis and x-ray diffraction indicates that this powder is a combination of Magnetite (iron oxide: Fe ₃ 0 ₄ ). Both of these phases have very similar Spinel type structures. The zinc within the Franklinite [(Fe,Mn,Zn) (FeMn) ₂ 0 ₄ ] cannot be removed by dissolution with ammonium chloride. In addition, no simple extraction process will remove zinc from this stable oxide phase. Although this compound is very stable to oxidation (all elements in the highest oxidation state) , it is relatively easy to destroy this compound by reduction at elevated temperatures. The reduction of the Franklinite in an atmosphere that cannot readily reduce zinc oxide or allow for the rapid oxidation of zinc to zinc oxide following reduction and subsequently recover the zinc oxide by ammonium chloride extraction or sublimation (the highly volatile zinc oxide will sublime from the mixture at relatively low temperatures and recondense at the cold locations of the roaster) . The alternative will be complete reduction of the Franklinite to zinc metal and removal by distillation or separation of the molten zinc by settling techniques. 1. Roasting Process:

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

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

The roasting process also can be performed to complete reduction by using carbon at high temperatures and collecting zinc metal that will melt at very low temperatures (420 ^β C) and boil at 907°C. In this process, zinc metal is obtained that can, if desired, be readily converted to the oxide by air roasting.

Example 8 A dust containing 19.63% Zn, 27.75% Fe, 1.31%

Pb, 9.99% Ca, 0.024% Cd (analysis based on elements not oxides) was leached at 100 ^β C in a 23% ammonium chloride solution. The solid remaining after the leaching process was dried and analyzed to contain 12.67% Zn, 4.6% Ca, 35.23% Fe, 0.7% Pb, 0.01% Cd. This material was placed in

a quartz boat in the presence of activated carbon and heated at 900°C for two hours with a mixture of 95% N ₂ , 5% 0 ₂ • After two hours the material was removed and added to a 23% amnonium chloride solution at 100°C. The material was filtered and dried at 140°C for one hour to determine its composition. Analysis of this remaining solid was 42.84% Fe, 0.28% Zn, Pb less than 0.1% and Cd less than 0.01%. The leached-roasted-leached material then can be subjected to the remainder of the general process to recover zinc oxide.

Example 9 A dust with composition given in Table I is leached in 23% ammonium chloride solution for 1 hour at 100°C. The solid remaining (which contained 14% Zn) was placed in a quartz boat and heated to 700°C in an atmosphere of 8% hydrogen, 92% argon in mixture. The material was cooled and reheated at 100°C in 23% ammonium chloride solution at 100°C. The solid was separated, dried and analyzed for zinc. The zinc was found to be less than 1%. The leached-roasted-leached material then can be subjected to the remainder of the general process to recover zinc oxide. 2. Crystallization Step Variations:

The purpose of the crystallization/washing step is to produce a high purity zinc oxide of controlled particle size. This is accomplished through control of the temperature-time profile during cooling in the crystallization.

The crystallization step in the process takes the filtrate from the cementation step at 90-110°C. This filtrate contains the dissolved zinc with small amounts of trace impurities such as lead and cadmium. In order to prepare a pure zinc oxide it is necessary to prevent the formation of solvent inclusions inside the grown crystals. Solvent inclusions are pockets of liquid

trapped as a second phase inside the crystals. Control of crystallization conditions can be employed to reduce these impurities. An example is given below.

Example 10 A dust of composition given in Table I is taken through the leaching and cementation steps. After cementation the filtrate is at 100 ^β C. 500 ml of this filtrate is placed in a jacketed stirred vessel with the jacket temperature at 100°C. The temperature is lowered in the crystallizer as follows:

The resulting solid was washed and dried employing the procedure described above. The resulting material was analyzed as follows:

ZnO 99 + %

Lead less than 50 ppm Cd less than 25 ppm

Fe less than 25 ppm

The cooling profile in Example 10 is known as a reverse natural cooling profile. Such a profile is the opposite shape as that which is observed by natural cooling. In a reverse natural cooling profile, the cooling is slower at the beginning and faster at the end; in a natural cooling profile, the cooling is faster at the beginning and slower at the end. This type of cooling profile also is used to control the crystal size

distribution (CSD) of the zinc oxide obtained. The cooling profile controls the ratio of nucleation (birth of a new crystal) to crystal growth (growth of existing crystals). The ratio of nucleation/growth determines the final CSD.

Example 12

A 23% ammonium chloride solution at 100°C containing 11 wt% dissolved ZnO is divided into 4 portions. Each portion is placed in a jacketed agitated vessel. The cooling profiles in each vessel are given below:

The solid is washed using the usual procedures described previously. The average size and size distribution of these materials were measured using a laser light scattering particle size analyzer. The results were as shown below:

Mean Size

A 22

B 19

C 27 D 37

The results show that controlling the temperature with a reverse cooling curve (slower cooling at the beginning, faster at the end) results in a larger average size than by linear cooling (A) or natural cooling (B) (fast at the beginning, slower at the end). This principle can be employed to design cooling profiles to produce zinc oxides of a desired average size and distribution.

3. Recycle:

The purpose of this process is to produce pure zinc oxide from waste dust containing zinc. To do so this efficiently and in a safe and cost effective way, the process recycles all zinc which in not removed from the leachate in the crystallization step. In addition, the diamino zinc dichloride which is redissolved in water in the washing step also is recycled. The recycle of zinc increases the overall zinc concentration in liquid solution in the process. This allows the crystallizer to operate at a higher temperature. This is due to the rapid change in zinc oxide solubility with temperature in ammonium chloride solution. An example of the process with recycle is given below: Example 13

By controlling the recycle, the steady state zinc concentration can be raised to 7g/100g of solution. If the outlet of the crystallizer is kept at 60°C, 3g/100g solution of solid will crystallize (the solid is a mixture of zinc oxide and diamino zinc dichloride). The system

does not have to be cooled further since this is an efficient way to operate to conserve energy (one does not have to cool then reheat the solution) . In addition, operating at the higher Zn concentration improves the ratio of ZnO/diamino zinc dichloride produced in the crystallizer.

The recycle has the advantage that the solution becomes saturated relative to certain materials present in the dust, such as CaO. When this occurs, CaO no longer is leached from the dust but remains with the iron in the iron cake. This increases the value of the cake since CaO is still present and will not have to be added when the iron cake is fed to a furnace in steel making. Another important advantage in that there is no liquid effluent in this process. The only products are solid (iron cake, zinc oxide, waste metals), which are then sold for use in various industrial processes. No waste is produced since all liquid is recycled.

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