Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Patent
1989-05-26
1992-01-07
Rosenberg, Peter D.
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
75726, 75733, 75736, 75961, C22B 300
Patent
active
050787861
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a process for recovering metal values from the jarosite containing wastes from electrolytic zinc recovery plants.
BACKGROUND OF THE INVENTION
Electrolytic zinc processes are used to treat complex zinc-containing ores that cannot readily be treated by pyrometallurgical recovery. The usual steps in such an electrolytic process include: (a) concentrating the zinc ores; (b) roasting the zinc concentrate to eliminate sulfur and produce zinc calcine; (c) leaching the zinc calcine to provide an impure zinc sulfate solution; (d) separating the iron present usually by forming a jarosite precipitate; (e) purifying the zinc sulfate solution; and (f) subjecting the zinc sulfate solution to electrolysis to recover the zinc metal. Such a process is described in U.S. Pat. No. 4,128,617 (1978) of DeGuire et al. which is incorporated herein by reference. A simplified process flow sheet for an electrolytic zinc plant is shown in FIG. 1.
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.
As indicated above, a common method of removing the iron present in the leachate is through the formation of a "jarosite" precipitate. Jarosite, MFe.sub.3 (SO.sub.4).sub.2 (OH).sub.6 where M is a monovalent ion, usually an alkali metal (general sodium or potassium) or ammonium, is commonly formed by adding a source of ammonium or sodium ions to the leach solution and maintaining the solution at an appropriate pH by the addition of base. This process is also shown in FIG. 1. Various modifications to the so-called "jarosite process" are discussed in the article entitled "The Jarosite Process--Past, Present and Future", V. Arregui et al., Lead-Zinc-Tin, TMS-AIME World Symposium on Metallurgy and Environmental Control, 1980, J. M. Cigon, T. S. Mackey and T. J. O'Keefe, Ed., pp. 97-123, incorporated herein by reference.
There are a number of problems associated with the formation of jarosite waste material. The jarosite can contain valuable metals such as silver, zinc, copper, lead, indium, etc. which require numerous expensive process steps to recover. The jarosite can also contain toxic species which can be leached into the environment by rain and groundwater. Therefore to avoid environmental contamination it is usually necessary to store the jarosite wastes in sealed lagoons which are expensive to build.
A number of methods for treating such wastes have been disclosed. Steintveit et al. in Norwegian Patent 142,406 (1980) disclose a process for leaching iron-containing waste with chloride-containing, acidic solution at a temperature between 50.degree. 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 remains as a precipitate 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. However, Applicants have found that unpredictably with some jarosite wastes this process provides recoveries of less than 20 percent of the silver present.
U.S. Pat. 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.degree. and 115.degree. 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 chlorides. 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 se
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"The Encyclopedia of Chemical Technology", Kirk-Othmer, vol. 24, pp. 812-824, 3rd Edition.
"The Jarosite Process-Past, Present & Future", V. Arregui et al. Lead-Zinc-Tin, TMS-AIME World Symposium on Metallurgy and Environmental Control, 1980, J. M. Cigon, T. S. Mackey and T. J. O'Keefe, Eds. pp. 97-123.
Hazen Wayne W.
Peters Mark A.
Reynolds James E.
Resource Technology Associates
Rosenberg Peter D.
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