WO1988003912A1

WO1988003912A1 - Process for recovering metal values from ferrite wastes

Info

Publication number: WO1988003912A1
Application number: PCT/US1986/002477
Authority: WO
Inventors: Mark A. Peters; Wayne W. Hazen
Original assignee: Resource Technology Associates
Priority date: 1986-11-26
Filing date: 1986-11-26
Publication date: 1988-06-02

Abstract

Process for recovering metal values from a zinc ferrite-containing waste by mixing the ferrite (2) with a solution containing sulfuric acid and a source of ammonium ions to form a sulfuric acid leach mixture (4), heating the leach mixture at a temperature above 50°C and below the decomposition temperature of ammonium jarosite to form solid ammonium jarosite (8) and solubilized zinc sulfate, separating the phases, recovering zinc from the leachate (6) treating the solid with a solution of calcium chloride (10) at a temperature no greater than the boiling point of the solution to form a liquid leachate (14) and a solid residue (12) followed by separation of the leachate from the residue and recovery of metal values from the liquid phase.

Description

"PROCESS FOR RECOVERING METAL VALUES FROM FERRITE WASTES"

Field of the Invention

This invention relates to an integrated process for recovering metal values from ferrite containing wastes from electrolytic metal recovery plants and for providing recycle streams from the recovery process back to an electrolytic zinc plant.

Background of the Invention

An electrolytic zinc process treats complex ores that cannot readily be treated by pyrometallurgical recovery. The usual steps in such an electrolytic process include: (a) roasting the zinc concentrate to eliminate sulfur and produce zinc calcine; (b) leaching the zinc calcine to provide an impure zinc sulfate solution; (c) precipitating the iron present; (d) purifying the zinc sulfate solution; and (e) subjecting the zinc sulfate solution to electrolysis to recover the zinc metal. U.S. Patent 4,128,617 (1978) of DeGuire et al. Additional details of various process modifications can be found in "The Encyclopedia of Chemical Technology", Kirk-Othmer, Vol. 24, pp. 812-824, 3rd Ed, incorporated herein by reference.

Modern electrolytic zinc processes use a two step leaching process as depicted in Fig. 1. The second leaching step involves a hot acid leach which is intended to dissolve the zinc ferrite present. However, in the past, only a neutral leach was used which allowed much of the zinc ferrite and associated metal values to be discarded as wastes. Therefore, there are many existing waste lagoons which contain substantial quantities of zinc ferrite which contains metal values such as silver, copper, lead, etc. The waste can also contain toxic heavy metals such as cadmium as well as arsenic which can create environmental hazards if leached into the ground- water. The term "ferrite" is used herein to refer to a combined metal oxide-ferric oxide material, e.g. zinc ferrite (ZnO.Fe₂O₃).

A number of processes have been developed to recover this zinc. One such process is disclosed by Rastas et al. in U.S. Patent No. 3,959,437 (1976). In the Rastas et al. process, the ferrite of a non-ferrous metal, as well as the oxide of the non-ferrous metal, is subjected to a neutral leach which dissolves the oxide but leaves the ferrite unaffected. The non-ferrous values in the solution are recovered, and the undissolved ferrite material is further treated in a "conversion" stage with sulfuric acid-bearing solution at atmospheric pressure and at a temperature of about 80°C to about 105°C in the presence of alkali or ammonium ions. Under these conditions, the non-ferrous metals dissolve as sulfates while iron is simultaneously precipitated as an insoluble complex sulfate, i.e., jarosite. The term "jarosite" is used herein to refer to a complex iron substrate of general formula MFe₃(SO₄)₂(OH)₆ where M is a monovalent ion usually an alkali metal such as sodium, potassium or ammonium. U.S. Patent No. 4,355,005 of Rastas et al. (1982), U.S. Patent No. 4,366,127 of Rastas et al. (1982), as well as U.S. Patent No. 4,383,979 of Rastas et al. (1983) each disclose modifications to the process disclosed in the 437 patent. U.S. Patent No. 3,691,038 of Von Roepenack et al. (1972) discloses a method for recovering zinc from oxides containing zinc and iron. The oxide is leached with sulfuric acid at a temperature of 95° to 100°C with an excess of sulfuric acid to solubilize the zinc and iron. Alkali metal or ammonium ions are added to the liquid phase along with a zinc-containing oxidic material at a temperature of 95° to 100°C to precipitate jarosite.

U.S. Patent No. 4,192,852 of Pammenter et al. (1980) discloses a process for treating zinc plant residues containing zinc ferrite and precipitating the iron as a jarosite. The sulfate solution containing ferric iron free acid and other non-ferrous metals is cooled, part- ially neutralized and then heated to a temperature not exceeding the boiling point at atmospheric pressure in the presence of sodium, potassium or ammonium ions. U.S. Patent No. 4,305,914 of Pammenter et al. (1981) discloses a process similar to that in the ' 852 patent.

U.S. Patent 4,128,617, (1978), describes a three- step process for the treatment of zinc calcine containing zinc oxide, zinc sulfates, and zinc ferrites. The first step involves the neutral leaching of the zinc calcine with an effective amount of aqueous sulfuric acid containing solution. The leach residue is subjected to hot acid leaching with sulfuric acid followed by jarosite precipitation by alkali with the subsequent recycling of the jarosite-containing pulp. The preferred temperature range for the hot acid leaching is from about 80°C to the boiling point of the leach solution and preferably the temperature is greater than 90°C.

These processes have the disadvantages of either (1 ) having low rates of extraction of metal values, (2) providing low levels of recovery of certain metal values, (3) having poor filterability of the iron containing residue, and/or (4) solubilizing large amounts of iron. Also, most of these processes form a jarosite waste which can contain metal values and toxic components. There is presently no process which can recover substantially all of the metal values, from zinc ferrite waste, provide a solid waste suitable for non-hazardous disposal, and provide valuable recycle streams which can be used in a zinc plant.

A number of methods have been disclosed as being useful in treating jarosite wastes. Steintveit et al. in Norwegian Patent 142,406 (1980) disclose a process which involves leaching with a chloride-containing, acidic solution at a temperature between 50°C and the boiling point of the solution. An alkali metal chloride or an alkaline earth metal chloride is used as the source of the chloride with calcium chloride being disclosed as the preferred material. The pH of the solution is adjusted during the leaching step so that the iron is not dissolved while the valuable metals are leached into the hot solution. The pH is adjusted to between about 2 and 4 preferably with calcium hydroxide. It is disclosed that these conditions allow extraction of up to 95 percent of the lead and silver content of the waste.

U.S. Patent No. 4,054,638 of Dreulle et al. (1977) is directed to a process for recovering metals from sulfated residues from electrolytic zinc plants. The residue is digested preferably at a temperature between 95° and 115°C with hydrochloric acid in the presence of calcium chloride. This leaching process dissolves the metals present, including the iron, by forming the corresponding metal chloride. Consequently, the process requires that the iron chloride be removed by extraction by an organic solvent. This process has a disadvantage of solubilizing the iron and requiring a separate separation step.

U.S. Patent 4,070,437 of Van Ceulen (1978), discloses a process for recovery of metals from jarosite sludges. The process involves leaching the jarosite with an acidic calcium chloride solution, preferably formed by mixing hydrochloric acid and calcium hydroxide or calcium carbonate. The leaching is preferably carried out close to the boiling point of the leaching medium. Insoluble calcium sulfate is formed and is separated by filtration. This process has the disadvantage of solubilizing essentially all of the iron in the jarosite.

Several processes have used elevated temperature and pressure leaching steps in treating zinc plant residues. U.S. Patent No. 3,143,486 of Pickering et al. (1964) discloses a process for the extraction of zinc from zinc ferrite containing residue. The process involves subjecting the residue to a first-stage leaching treatment under non-oxidizing conditions in a closed vessel in the presence of excess sulfuric acid at a temperature between 140°C and 260°C. The leachate is then subjected to a second-stage pressure leaching treatment under oxidizing conditions, also in the temperature range of 140° to 260°C.

U.S. Patent No. 3,493,365 of Pickering et al. (1970) discloses a method of treating zinc plant residue containing zinc ferrite. The residue is leached in excess sulfuric acid at a temperature near the boiling point of the solution at atmospheric pressure to dissolve substantially all of the zinc and iron. The resulting leach solution is then treated at super atmospheric pressure and a temperature in the range of 140° to 240°C with a source of a cation selected from the group consisting of sodium, potassium and ammonium is added in order to precipitate the iron from the liquor as a jarosite material.

U.S. Patent 4,225,342 of Freeman et al. (1980) discloses a process for treating zinc plant leach residues to recover metal values. The residue is subjected to a sulfation step in which sulfuric acid in a concentration of about 90 to 100 weight percent acid is contacted with the residue at a temperature of from 100°C to 700°C under pressure to convert the metal values to their sulfate forms. The sulfation products are then subjected to a step-wise recovery process in which silver, tin and lead are recovered.

None of these patents discloses an intregrated process suitable for recovering metal values from zinc ferrite waste and for providing a waste product which can be directed to non-hazardous waste disposal. There is no suggestion that an improved process for recovering metal values might be obtained with an integrated process which is capable of leaching ferrite and treating the residue from the ferrite leach. Also, there is no suggestion or disclosure of a process in which economic benefit and other advantages are obtained by the recycle of particular process streams to other steps of the process and/or to a zinc plant. Summary of the Invention

It has now been found that the above described disadvantages of known processes can be eliminated or minimized and the indicated benefits can be provided by the practice of the instant invention. According to the present invention, a unique two-stage process is provided for recovering metal values from ferrite wastes said process comprising a first leach of the ferrite waste with a sulfuric acid solution in the presence of ammonium ions to form ammonium jarosite and a second-stage leach of the ammonium jarosite with an acidic solution of calcium chloride to solubilize additional metal values for subsequent recovery. Specific streams are generated which can provide added value to the process by recycle to a zinc plant or to other steps in the process. Another embodiment of the instant invention comprises contacting a ferrite material which contains metal values with a leach solution of sulfuric acid and ammonium ions. The leach mixture is heated at a temperature between about 50°C and the decomposition temperature of ammonium jarosite to form a solid ammonium jarosite containing metal values and a liquid leachate containing most of the zinc present. The leachate is transferred to the zinc plant for recovery of the zinc. The solid jarosite is mixed with an acidic leaching solution containing calcium chloride and heated at a temperature below the boiling point to solubilize metal values. The solid tails are discarded and the liquid phase is contacted with a reducing metal to cement out silver and copper. Remaining zinc is removed by precipitation or liquid/liquid extraction. The liquid phase from which the zinc is extracted is subjected to a lime boil to form ammonia vapor and provide a calcium chloride brine suitable for recycle to the jarosite leaching step. The ammonia can be recycled to the zinc plant and/or the ferrite leaching step.

In a further embodiment, the instant invention comprises separating the liquid leachate from the solid residue in the ammonium jarosite leach. The liquid leachate is contacted with a reducing metal to reduce silver ions contained in the leachate to metallic silver which is separated from the liquid solution. The liquid solution is sulfided by mixing the solution with a sulfide compound to precipitate the lead contained in the solution as lead sulfide. The solid lead sulfide is separated from the liquid phase. Substantially all of the zinc in the remaining liquid phase is recovered using a zinc recovery process to provide a liquid solution substantially free of zinc. The resulting liquid solution is heated with calcium oxide to provide a vapor containing ammonia and bottoms which contain calcium chloride. The ammonia is dissolved in water or conveyed directly back to the zinc plant or to the ferrite leach. The calcium chloride brine solution is recycled to the jarosite leach.

Brief Description of the Drawing

Fig. 1 shows a typical process flowsheet for an electrolytic zinc plant;

Fig. 2 shows a process flowsheet for a ferrite leach in conjunction with a jarosite leach; and

Fig. 3 shows a process flowsheet for a preferred embodiment for metals recovery from ferrite and jarosite in association with a zinc plant.

Description of the Preferred Embodiments

The instant invention comprises an integrated process for recovering metal values from ferritecontaining wastes from electrolytic metal recovery plants. The ferrite waste is leached with sulfuric acid in the presence of ammonium ions to solubilize zinc and form an ammonium jarosite precipitate. The solid and liquid phases are separated with the zinc-containing liquid phase being recycled to a zinc plant for recovery of the zinc. The solid ammonium jarosite material containing metal values is subjected to an acidic calcium chloride leach to solubilize a substantial amount of these metal values. The liquid leachate is subjected to additional treatment to recover the metal values contained therein.

Any zinc ferrite (ZnFe₂O₄) containing waste material can be used as a feed in the instant process. The usual source of the zinc ferrite containing waste is waste lagoons from zinc plants since these wastes commonly contain economically significant levels of other metal values such as silver, lead, and copper. These wastes also contain arsenic and toxic heavy metals such as lead and cadmium which require that the wastes be classified as hazardous wastes.

An alternative source of zinc ferrite material is the dust formed during steelmaking in an electric arc furnace. The dust primarily comprises zinc ferrite, zinc oxide, and various forms of ferric oxide. The dust can also contain lead, cadmium and chromium and is therefore usually classified as a hazardous waste. The dust can additionally contain metal values such as silver. Thus , the dust can be advantageously used in the process of the instant invention to allow recovery of metal values and/or removal of the toxic metals.

In the present process, the ferrite containing waste is subjected to a leach operation in which the waste is contacted with sulfuric acid and an ammonium ion source. This results in a simultaneous leach of the ferrite and a precipitation of the iron in the form of jarosite. A method for such a simultaneous leach and precipitation is described by Rastas et al. in U.S. Patent 3,959,437 (1976). Rastas, however, is only concerned with solubilizing the zinc in the ferrite and does not recover metal values from the jarosite which is formed.

To accomplish the leach, the ferrite material is combined with sulfuric acid and a source of ammonium ions at a temperature above about 50°C and below the decomposition temperature of ammonium jarosite. Most preferably the temperature is between about 80°C and about 170°C. The leach is preferably conducted under autogeneous pressure when leaching temperatures above the boiling point of the solution are used. The sulfuric acid used is preferably spent electrolyte from a zinc plant and is present in the leach solution to the extent of about 20 to about 60 grams H₂SO₄ per liter of solution. Excess sulfuric acid increases the solubility of the iron and retards the formation of jarosite. Sufficient ammonium ion is added to accomplish precipitation of the iron as ammonium jarosite. The ammonium ion is preferably recovered from the lime boil process step described hereinbelow although any ammonium source can be used. Suitable ammonium sources include without limitation ammonia, ammonia water, ammonium hydroxide and ammonium salts such as ammonium sulfate. Ammonium ions are present at the end of the leach to the extent of about 10 g/1 to about 20 g/1 of solution.

The ferrite leach is preferably conducted for a period of about 12 to about 24 hours with the length of time depending on the amount of zinc which must be leached while minimizing the amount of iron which remains in the solution (i.e. maximizing formation of insoluble jarosite). Formation of solid jarosite can be aided by the addition of a small amount of "seed" jarosite. The solid jarosite and any remaining untreated ferrite is separated from the liquid phase by standard solid-liquid separation techniques such as filtration or centrifugation. Preferably a flocculating agent is added and the resulting solids are allowed to settle and the liquid layer is decanted. Suitable flocculating agents are those known to those skilled in the art as effective.

The liquor from the leach step which is rich in zinc and also contains dissolved iron and free sulfuric acid is preferably conveyed to a zinc plant for recovery of the zinc. The solid jarosite containing other metal values is subjected to a calcium chloride leach process. Ordinarily, the leach liquid from the ferrite leach contains greater than about 70 percent and preferably greater than about 80 percent of the zinc present in the ferrite feed. The leach liquid contains essentially no silver or lead and commonly less than about 5 percent of the iron present in the ferrite feed.

In the calcium chloride leaching process, the jarosite is added to a leaching vessel containing calcium chloride as represented in Fig. 2. During the leaching process the pH of the solution tends to decrease due to formation of acid. The pH is adjusted to the preferred leaching range by adding a base. The solution is heated to the leaching temperature, preferably by steam, with agitation of the mixture. After the appropriate leaching time, the resulting slurry is subjected to a liquid-solid separation to provide solid tails and a liquid leachate. The solid tails consist of iron oxides and calcium sulfate while the liquid leachate contains solubilized zinc and other metal values. The liquid leachate ordinarily contains in excess of 70 percent of the silver and lead present in the jarosite feed. The tails are discarded while the leachate is subjected to further downstream treatment as discussed in more detail hereinbelow. Optionally, a portion of the leachate is recycled to the calcium chloride leach step to increase the concentration of the dissolved metal values.

The temperature of the calcium chloride leaching mixture is maintained at or below the boiling point of the solution. Preferably, the temperature is maintained below about 105°C.

The time required for the extraction depends upon the concentration of calcium chloride in the leach solution, the temperature at which the leaching is conducted, the pH of the solution and the particular waste feed being treated. Ordinarily, the leaching requires less than about 5 hours with leach times in the range of about 1 to 3 hours being preferred.

The concentration of calcium chloride should be maintained above about 0.5 molar in the leaching solution to achieve the best extraction of metal values. The concentration can range from about 0.5 molar up to the saturation point for calcium chloride for the particular temperature and solution. It is preferred that the concentration of calcium chloride be between about 1 molar and about 4 molar in the leaching solution.

Calcium chloride can be added to the leaching solution directly or can be formed in situ. When the calcium chloride is formed in situ, it is preferred that the chloride be added to the leach solution in the form of hydrogen chloride and the calcium be added in the form of lime (CaO), calcium carbonate, calcium hydroxide or mixtures thereof; however, it is contemplated that other sources of chloride such as sodium chloride, etc., can be used. The use of the calcium oxide, calcium hydroxide, calcium carbonate or mixtures thereof also allows the pH of the solution to be adjusted to the desired range.

For effective leaching with calcium chloride, it is important that the pH of the solution be less than about 5. It is preferred that the pH be less than about 4 and most preferred that the pH be less than about 3. For best results, the extraction should be carried in a pH range of about 2.0 to 3.0 During the calcium chloride leach of the jarosite, the pH of the solution decreases to a pH of less than 1.0 unless adjusted by the addition of a base. Although any base, such as sodium hydroxide or sodium carbonate can be used, it is advantageous to use a calcium containing base such as calcium oxide, calcium hydroxide, calcium carbonate, or mixtures of these materials.

The leaching operation can be carried out in either a batch or a continuous mode. The particular choice of operation will depend upon the leaching time necessary to extract the desired amount of the metal values. In a batch operation, the necessary quantity of calcium oxide and/or calcium hydroxide needed for pH control can be introduced initially instead of being added incrementally during the leaching process. The final pH of the slurry can be calculated by methods known to those skilled in the art based upon the ingredients added to the leaching mixture. The initial quantitative addition of the calcium oxide is operationally advantageous since it eliminates the equipment required to add the material incrementally throughout the leaching process as well as the need to monitor the pH of the system. We have found that there is no difference in the final level of the metal values extracted when the calcium oxide is added initially as opposed to incrementally during the leaching operation as long as the final pH of the leaching solution is within the desired range.

At the conclusion of the leaching process, the liquid-solid slurry is separated into solid tails and a liquid leachate. This can be accomplished by any known method for solid-liquid separations although with this type of slurry it is preferred that the separation be accomplished by filtration. The solid tails which contain gypsum, i.e. CaSO₄.2H₂O and iron oxides are discarded. Optionally, a slip stream from the leachate is recycled to the calcium chloride leaching process to allow further build up of metal values in solution. The remaining leachate liquid is subjected to further processing for recovery of metal values.

The recovery of the metal values can be accomplished by any known method useful for separating such materials. A preferred embodiment is set forth in Fig. 3. The leachate from the calcium chloride leach is contacted with a metal capable of reducing silver. Preferably zinc metal is used although lead is also suitable and the choice is ordinarily a matter of economics. The metal, preferably in the form of fine particulates, is contacted with the leachate preferably at a temperature between about 40°C and 80°C. The silver cement is separated from the liquid by filtration although centrifugation could also be used. The silver is recovered from the silver cement by conventional methods.

Lead can be separated from the liquor from the cementation process by sulfiding with hydrogen sulfide. The hydrogen sulfide gas is sparged through the liquor for a time sufficient to precipitate substantially all of the lead as lead sulfide. Ordinarily, hydrogen sulfide is added until the measured EMF decreases to about 0 mv

(vs. Standard Calomel Electrode). Commonly, the liquor will contain less than about 0.02 g/l lead. The lead sulfide is separated from the liquid by filtration although centrifuging can also be used.

The liquid from the sulfiding step is treated to recover zinc. Solvent extraction and precipitation are preferred methods of separation although any method suitable for recovering zinc from such a stream can be used. The zinc solvent extraction process involves contacting the aqueous solution containing the zinc with an extractant which will extract the zinc into another phase. A preferred extractant is di-2-ethylhexyl phosphoric acid. In operation, the organic extractant dissolved in a suitable organic solvent is contacted with the zinc containing aqueous solution. The pH of the aqueous phase is maintained in the desired range with a base. Although bases such as sodium hydroxide, potassium hydroxide, etc. can be used, it is preferred that calcium oxide, calcium hydroxide, calcium carbonate or mixtures thereof be used. At a pH of between about 2 and 3, the extractant removes the zinc into a phase separate from the aqueous solution. The organic phase containing the zinc is separated from the aqueous phase and then stripped of the zinc by adding sulfuric acid. The zinc-containing solution can then be transported to a zinc plant for recovery of the zinc. A preferred process sequence is shown in Fig. 3. The liquid from the cementation step is subjected to solvent extraction to remove the zinc as described hereinabove. The liquor from the zinc solvent extraction is partially neutralized, preferably with calcium oxide, to a pH of between about 8 and 9 to precipitate lead hydroxide. The lead hydroxide solid is removed, for example by filtration or centrifugation. The lead can be recovered by known procedures.

The liquid recovered from this separation is combined with a base in an amount sufficient to provide a pH above about 9. Although bases such as NaOH and KOH can be used, it is preferred to use a basic calcium compound such as calcium oxide, calcium hydroxide, calcium carbonate or mixtures thereof. Most preferably, calcium oxide is added and the mixture is heated to about the boiling point in a so-called "lime boil" procedure. The ammonia formed is vaporized and recovered by condensing or passing the vapor through a scrubber to provide an ammonium hydroxide stream which can be recycled to the ferrite leach or to the zinc plant. The liquid condensate can optionally be neutralized with sulfuric acid to produce ammonium sulfate suitable for fertilizer or use in the zinc plant.

Alternatively (not shown), the zinc can be precipitated as zinc hydroxide by adding a base, preferably calcium hydroxide and/or calcium oxide to the liquor from the sulfiding process. Sufficient base is added to provide a solution pH in excess of about 8. The precipitated zinc hydroxide is separated by liquid-solid separation means preferably filtration. The recovered zinc hydroxide can be conveyed to the zinc plant for recovery of the zinc. The liquid from this separation is subjected to the "lime boil" described above to generate ammonia.

The residue from the "lime boil" procedure contains a concentrated calcium chloride brine. This brine stream is preferably recycled to the calcium chloride leach step to allow reuse of the chloride values. Additional calcium chloride can be added to the leach step as needed for make-up.

In an alternative procedure also not shown, the liquor from the cementation step can be combined with base, preferably calcium oxide, calcium hydroxide, calcium carbonate or mixtures thereof to cause the precipitation of a mixture of zinc hydroxide and lead hydroxide. Sufficient base is added to provide a pH in excess of about 8. The metal hydroxides are separated by usual solid-liquid separation techniques preferably by filtration. The liquor from the precipitation process is then subjected to a lime boil as discussed hereinabove.

The resulting brine solution is preferably recycled to the calcium chloride leach with the dilute ammonium hydroxide formed preferably being recycled to the zinc plant or to the ferrite leach.

Figs. 2 and 3 show process schemes in which the product from the ferrite leach is optionally combined with additional jarosite in a repulp tank and the resulting pulp is fed to the calcium chloride leach. The subsequent metal recovery steps have been described in detail hereinabove.

These Figs, also show the movement of various streams to and from a zinc plant. The zinc rich liquid from the ferrite leach is transferred to the zinc plant for recovery of the zinc. Also, the strip solution from the zinc extraction in the jarosite section of the process is transferred to the zinc plant. Ammonia from the lime boil is used in the zinc plant and/or the ferrite leach step to form jarosite. Spent sulfuric acid electrolyte from the zinc plant is used in the ferrite leach and in the zinc strip section to remove zinc from the loaded organic phase. As discussed hereinabove, an alternative method of zinc separation which is not shown is the precipitation of zinc hydroxide. This zinc hydroxide can also be conveyed to the zinc plant for recovery of the zinc metal.

In Fig. 3 is shown a preferred process scheme for the treatment of ferrite and jarosite wastes. A ferrite containing feed is subjected to a leach, by contacting it with sulfuric acid and an ammonium source as described hereinabove. The source of the sulfuric acid is spent electrolyte from a zinc plant. The ammonium source is ammonia or ammonium hydroxide recycled from the lime boil. The liquid, containing zinc, is conveyed to a zinc plant for zinc recovery. The solids, containing ammonium jarosite and metal values, are transferred to a repulp tank where these solids can optionally be mixed with additional jarosite feed and slurried with a calcium chloride source. This calcium chloride leach mixture is heated as described hereinabove. The pH of the leach is adjusted to the desired level by the addition of calcium oxide and/or calcium hydroxide. After leaching, the slurry is filtered with washing to separate the solid tails and the leachate.

The leachate is contacted with metallic zinc to form a silver/copper cement. The cement is removed, preferably by filtration, and the silver and cooper are purified by standard metallurgical methods. The liquid is extracted using an extractant with the pH of the solution adjusted by adding a base preferably sodium hydroxide, calcium oxide, calcium hydroxide, calcium carbonate or mixtures thereof. The extractant combines with the zinc and effects the transfer of the zinc into an organic phase. The phases are separated and the zinc-containing organic phase is contacted with spent electrolyte from a zinc plant to strip the zinc from the organic phase and into an aqueous phase. The aqueous phase containing the zinc is transferred to a zinc plant for recovery of the zinc. The free extractant in the organic phase is recycled to contact fresh zinc-containing solution.

The aqueous phase from which substantially all of the zinc has been removed is contacted with a base such as calcium oxide, calcium hydroxide, calcium carbonate or mixtures thereof to increase the pH to between about 8 and 9 to precipitate lead hydroxide. The solid lead hydroxide is separated from the liquid by filtration. The liquid phase is subjected to a lime boil as described hereinabove. The vapor containing ammonia is condensed with cold water and the resulting ammonium hydroxide solution is recycled to the zinc plant and/or to the ferrite leach. The liquid residue from the lime boil is a calcium chloride-brine solution and is recycled to the calcium chloride leaching step.

The following examples are given for illustrative purposes only and are not to be a limitation on the subject invention.

Examples A sample of ferrite was assayed and found to contain the following components.

Weight %

Zn 13.0

Ag oz/Ton 5.06

Fe 33.4

Pb 0.325

NH₄ 1.12

K 0.095

Na 0.104

This ferrite was subjected to a sulfuric acid leach using conditions and reagents given in Table 1 and using the following procedure:

The ferrite feed was mixed with the indicated sulfuric solution and ammonium sulfate solution. The initial leach mixture had 20 wt/vol percent solids. The mixture was heated and agitated for 24 hours with samples taken and analyzed at the times and with the results given in Table 2. Additional sulfuric acid was added as needed to maintain the concentration at the target level. The resulting slurry was filtered to remove leachate. The residue containing ammonium jarosite was washed with water. ₄

The liquid leachate was assayed with the following results obtained:

The residue from the sulfuric acid leach in Runs 3 and 4 was subjected to a calcium chloride leach. The following procedure was used with the conditions given in Table 3:

Each sample of residue was mixed with the aqueous calcium chloride solution to provide the indicated initial solids content. The mixture was heated as indicated for 5 hours with agitation. Calcium oxide was added as needed to maintain the pH in the desired range. Samples were taken at 1, 3, and 5 hours. Calcium hypochlorite was added as indicated to assure that all iron was in the ferric form. The mixture was filtered to remove the liquid leachate and the residue was washed with two to three displacements of 80°- 85°C, 250 g/l calcium chloride solution.

*Reflux

The leachate from the CaCl₂ leach was assayed with the results given in Table 4 and Table 6. The "percent extracted" for each metal is based on the amount of the metal in the residue from the sulfuric acid leach, i.e. the feed to the CaCl₂ leach.

The overall percent extractions for the two leaches are given in Table 5 and Table 7. The percent is based on the total metal extracted compared to the total amount of the metal contained in the ferrite feed.

*Overall percent extractions for ferrite leaching followed by CaCl₂ leaching

*Overall percent extractions for ferrite leaching followed by CaCl₂ leaching

It will be understood that the above description of the present invention is susceptible to various modifications , changes and adaptations and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

Claims

What Is Claimed Is:

1. A process for recovering metal values from a zinc ferrite containing waste said process comprising: a) leaching said waste at a first temperature above about 50°C and below the decomposition temperature of ammonium jarosite with a solution containing sulfuric acid and ammonium ions to form a liquid leachate containing zinc and an ammonium jarosite solid containing metal values; and b) contacting said ammonium jarosite with a leaching solution containing calcium chloride at a second temperature no greater than the boiling point of the solution and under atmospheric pressure to solubilize a substantial portion of said metal values and provide a solid residue.

2. The process of Claim 1 wherein said first temperature is between about 80°C and 170°C.

3. The process of Claim 1 wherein the liquid leachate containing zinc is transferred to a zinc plant.

4. The process of Claim 1 wherein the amount of said sulfuric acid is maintained below about 60 grams per liter of leach solution.

5. The process of Claim 1 wherein the calcium chloride leach solution containing solubilized metal values is separated from said solid residue and said metal values are recovered.

6. The process of Claim 5 wherein said metal values are recovered by: a) contacting said leach solution with a reducing metal to cement out silver and copper; b) separating said cement from said liquid phase; c) removing zinc in said liquid phase by extraction to provide a zinc-containing first stream and a substantially zinc-free second stream; d) returning said zinc-containing first stream to a zinc plant for recovery of said zinc; and e) subjecting said substantially zinc-free second stream to a lime boil to form ammonia vapor and a liquid residue.

7. The process of Claim 6 wherein said ammonia is recycled to the ferrite leaching step.

8. The process of Claim 6 wherein said ammonia is recycled to a zinc plant.

9. The process of Claim 6 wherein the liquid residue from step (e) comprises calcium chloride and is recycled to said ammonium jarosite leaching step.

10. A process for recovering metal values from a zinc ferrite containing waste said process comprising: a) leaching said waste at a temperature of about 80°C to about 170°C with a solution containing sulfuric acid and ammonium ions to form a first liquid leachate containing zinc and an ammonium jarosite solid containing metal values; b) separating the first liquid leachate from the solid ammonium jarosite; c) transferring the first liquid leachate to a zinc plant for recovery of contained zinc; d) contacting the solid ammonium jarosite with a second leaching solution containing calcium chloride at a temperature no greater than the boiling point of the leaching solution for a time sufficient to dissolve the metal values in a second liquid leachate and provide a solid residue; e) separating the second liquid leachate from the solid residue; f) contacting said second liquid leachate with a reducing metal selected from the group consisting of lead and zinc to reduce silver ions contained in said leachate to metallic silver and provide a substantially silverfree liquid phase; g) separating said metallic silver cement from said liquid phase; h) recovering zinc contained in said liquid phase by contacting said liquid phase with an extractant which selectively removes said zinc from said liquid phase to a second phase; i) contacting the substantially zinc-free liquid phase from step (h) with sufficient calcium oxide, calcium hydroxide, calcium carbonate or mixtures thereof to provide a solution pH of between about 8 and 9 to precipitate lead hydroxide to form a solid/liquid slurry; j) removing the solid lead hydroxide from the slurry of step (i) and provide a substantially lead-free liquid phase; k) adding sufficient calcium oxide or calcium hydroxide to the liquid phase from step (j) to increase the pH to above about 10; l) increasing the temperature of the resulting solution in step (k) to provide a vapor containing ammonia and a liquid residue containing calcium chloride; m) condensing the vapor from said solution by contacting with water to provide a solution containing ammonium hydroxide; n) transferring the solution containing ammonium hydroxide to the zinc plant or to the zinc ferrite leaching step; and o) recycling the liquid residue from step (1) to the calcium chloride leaching step.