Design and Development of an Innovative Industrial Scale Process to Economically Treat Waste Zinc Residues

Principal Investigators
T.J. O'Keefe, University of Missouri

Abstract

Goal: The primary goal of this project is to design and develop a hydrometallurgical flow sheet to treat waste zinc residues containing iron and other heavy metal impurities such as lead and cadmium. The resulting flow sheet will be used at Big River Zinc Co., or any other industry desiring to treat similar wastes.

Rationale: A major problem faces the minerals industry in the form of huge t onnages of environmentally unacceptable zinc residues. Previously these oxidized dusts, which contain high iron and zinc contents with lead, cadmium, and other heavy metals, were precipitated in chemical forms acceptable for standard landfills. Under curr ent laws, this practice will not be allowed and costs of compliance are expected to increase dramatically. In fact, it may even be necessary to re-process all the wastes that have been stored and accumulated over the years. The technical challenge is to d evelop metallurgical and chemical processes to treat these hazardous wastes in an economically viable manner. The most serious technical impediment preventing treatment of these wastes is the inability to separate the iron from the zinc. The investigator on this project has developed a process, galvanic stripping, to separate the iron from the zinc. As the next step, it is important to develop unique in-line processes specifically for handling diversity in feed stock, particularly when certain categories of impurities are present in low concentrations. Many existing processes are basically sound, but supplementary unit processes must be developed to make them more amenable to treat impure metal wastes and residues in an economic fashion.

Approach: This project will be conducted in conjunction with Big River Zinc Co., where the commercial plant to treat 50 tons per day residue will be located. In addition, two suppliers of the organic extractant and another zinc producer, Noranda Ltd., will also provide support and assistance. Ultimately, this technology will be transferred to others in the industrial sector for use in treating a variety of similar wastes generated in the mining and mineral community. Research will include the following thre e areas: evaluation of the process parameters to optimize the reduction of Fe+3 to Fe+2 in the D2EHPA organic phase; determination of the type of aqueous stripping solution and design procedure alternative to be used to separate and recover the Fe+2 and produce the best, salable iron product; and identification of the influence of the various heavy metal impurities in the solutions, their distribution (aqueous vs. organic) and effect on subsequent iron and zinc recovery.

Status: Research conducted to date has demonstrated that the galvanic stripping procedure is technically feasible. The process has been successfully demonstrated at a pilot and small plant scale level by Cominco, Ltd., in Canada. However, as with many complex reactions which are essentially under kinetic control, the operating parameters used have a major influence on the extent or efficiency of the process. In this case it is imperative to identify those factors which affect the reactions a nd attempt to quantify their sensitivity in changing the degree of iron removal if galvanic stripping is to be used on a large industrial scale. A number of parameters have now been identified which are capable of producing such changes and these include temperature, agitation, oxygen content, time, aqueous/organic (A/O) ratio, and pH of the aqueous strip solution. Each is important as an individual variable, but there are complex interactions among them as well, and these require clarification. Studies a re now being made on the effect of these variables and the impact they may have on the economics of the industrial scale process. Based on the results obtained to date, the process continues to show promise as an alternative for treating waste zinc residu es. One particularly encouraging feature has been the initial success obtained using high iron concentration stripping solutions. This would provide a technically and economically attractive outlet for the iron impurity. The current thought is to use the solution in water treatment applications where high iron and low zinc and other impurity concentrations are desirable. This project is in its first year.

Clients/Users: This research will interest those in the mining and metals industry, U. S. Department of Defense, and regulatory community.

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