Final Report: Metals Recycling From Waste Sludges by Ammoniacal Leaching Followed by Solvent ExtractionEPA Contract Number: 68D01033
Title: Metals Recycling From Waste Sludges by Ammoniacal Leaching Followed by Solvent Extraction
Investigators: Park, Brian
Small Business: MSE Technology Applications Inc.
EPA Contact: Manager, SBIR Program
Project Period: April 1, 2001 through September 1, 2001
Project Amount: $70,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2001) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , SBIR - Waste , Small Business Innovation Research (SBIR)
Description:The purpose of this project was to evaluate the technical and economic feasibility of applying ammoniacal leaching and solvent extraction, followed by standard metallurgical recovery steps, to recycle nickel, copper, cobalt, zinc, and cadmium from hydroxide sludges. The target sludges would be those produced by electroplating shops, metal finishers, treatment of acid mine drainage, and industrial wastewater in general. A specific area of interest was to evaluate the effects of leach slurry oxidation/reduction potential (EH) on leach recovery; evidence suggested that leaching under reducing conditions would produce significantly increased recoveries over those seen in similar previous research efforts.
MSE Technology Applications, Inc. (MSE) prepared a representative sludge for leaching tests, including the target metals of nickel, copper, cobalt, zinc, and cadmium as well as other typical sludge constituents such as aluminum, chromium, iron, and tin. All metals present were at an approximate concentration of 4 percent by weight (dry basis). The sludge was produced by dissolving metal salts into sulfuric acid, neutralizing with caustic, and filtering. The filtered sludge was approximately 27 percent solids. Leach tests were performed at 10 percent solids at varying total ammonia strengths (2M, 6M, 10M, and 14M), over a pH range of approximately 8?11 achieved by varying the proportions of ammonium hydroxide and ammonium sulfate, and over an EH range achieved by dosing ferrous sulfate solution to produce reducing conditions and hydrogen peroxide solution to produce oxidizing conditions. Further tests were performed evaluating the possibility of increasing recovery by multistage leaching. Copper was seen to precipitate from the slurry as metallic copper at low EH conditions. This also was investigated further as a possible copper recovery technique.
Solvent extraction tests were performed using LIX 84-I, LIX 84-IT, LIX 54-100, and LIX 860N-I obtained from Cognis Corporation, along with di(2-ethylhexyl) phosphoric acid (DEHPA), based on discussions with reagent manufacturers. Extraction tests were performed at constant reagent strength on synthetic leach solutions consisting of 2 g/L of each target metal alone in solution, at varying total ammonia strength (2M, 5M, and 8M). Further extraction tests were performed on synthetic leach solutions containing all five target metals at 1 g/L, at constant total ammonia strength (5M), and at varying solvent extraction (SX) reagent concentrations. These tests provided information on what separations were achievable via solvent extraction. The test data generated were used to develop a conceptual flowsheet of a process to recover, in order, cobalt, copper, nickel, cadmium, and zinc.
The conceptual flowsheet included: (1) sludge recycle to increase overall
process recoveries (at a penalty of increased capital and operating costs); (2)
copper and cobalt recovery by co-extraction with LIX 84-I, with cobalt stripped
at pH 4.0 with sulfuric acid and crystallized as cobalt sulfate, and copper
stripped at pH 1.0 with sulfuric acid for feed to an electrowinning recovery
(3) nickel recovery by extraction with LIX 84-I, then stripping at pH 4.0 with sulfuric acid and crystallizing as nickel sulfate; (4) cadmium recovery by cementation with zinc dust; and (5) zinc recovery by extraction with LIX 860N-I, stripping at pH 3.0 with sulfuric acid, and crystallizing as zinc sulfate. The metal-depleted, ammonia-rich leach solution would be recycled to the head of the process. It should be noted that there are a number of ways to recover nickel, cobalt, and zinc from the relatively high pH strip solutions. Recovery as their sulfates via crystallization was only one technique; it is possible that other approaches, such as precipitation as the carbonate or precipitation as the hydroxide followed by calcining, may be more economical.
A mass balance of the process was prepared at a design basis of 50 wet tons per day. The mass balance was prepared in a spreadsheet, which included calculations for generating capital and operating costs and projected revenues for the treatment process tied to the mass balance. The cost and revenue information were used to perform simple economic analyses evaluating the process as compared to alternatives of shipping sludge to a hazardous waste handler or to a recycler.
Summary/Accomplishments (Outputs/Outcomes):The major findings were:
- Ammoniacal leaching will very selectively leach the target metals into solution. The "gangue" metals of aluminum, chromium, iron, and tin leached into solution at values less than 10 mg/L as compared to values in the g/L range for the target metals. The exception to this was at low EH values, at which some ferrous iron went into solution.
- Solvent extraction is a very effective way to separate most of the target metals from the leach solution. Separation of cadmium from zinc is always difficult, and a solvent extraction separation could not be justified in the conceptual flowsheet prepared based on the data generated by the project. Therefore, cadmium cementation using zinc dust was used for cadmium recovery in the conceptual flowsheet.
- Metals recoveries could be improved by reducing the system EH, but the improvement was only marginal and recoveries were disappointing for cobalt (~20 percent), nickel (~40 percent), and zinc (~50 percent), as compared with 90?100 percent for copper and 80?90 percent for cadmium. With these relatively low leach recoveries for cobalt, nickel, and zinc, sludge recycle would be required in an actual process to increase overall recoveries.
- Ammonia could be recycled for leaching purposes without the requirement of steam-stripping for its recycle, a potentially difficult and expensive step.
- Process economics show that the technology is only potentially attractive if considered as an alternative to an expensive sludge disposal option, such as a hazardous waste handler.
Conclusions:This project showed that ammoniacal leaching followed by solvent extraction and subsequent metallurgical processing steps is a viable technique for recycling certain metals from hydroxide sludges. The approach is technically feasible, with all equipment and materials being off-the-shelf. Economic feasibility is more complicated. Although leach recoveries were marginally increased by leaching under very reducing conditions, the recoveries overall were relatively disappointing, particularly for cobalt and nickel. This requires sludge recycle as part of the treatment process, which in turn drives up capital and operating costs. The economics are heavily driven by the content of the feed sludge, both from a standpoint of the specific contained metals as well as the number of metals targeted for recovery. The economics also are heavily driven by the cost of alternatives (e.g., cost of sludge disposal if not handled by this process). Relatively attractive economics were seen based on the content of the feed sludge prepared for testing purposes, but primarily because this sludge contained about 4 percent cobalt. Cobalt is far more valuable than the other target metals, and is unlikely to appear in an electroplating sludge. If the revenues from cobalt are removed, the economics are not overly attractive. There are sludges, such as electromachining sludges, that contain significant cobalt, and this process should be quite effective on those types of feedstocks. As far as electroplating-type sludges, the process would be most effective on sludges rich in nickel and copper, or on more typical sludges but with only one or two, or at most three metals targeted for recovery.
Further work in this area should focus on:
- Improving recoveries of the target metals, particularly nickel and zinc because they would be most commonly found of the poorer leaching metals, to approach thermodynamic expectations.
- Application of ammoniacal leaching to other oxidized waste streams, particularly those containing cobalt, nickel, and copper.
- Evaluation of the optimum recovery method for the metals in solution not easily obtainable by electrowinning (e.g., by crystallization, precipitation with caustic and calcining to the oxide, and precipitation as a carbonate).
- Additional bench testing of SX reagents to produce pH isotherms, extraction isotherms, and stripping isotherms.