Reduction of Chromium Plating Wastes
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PI gress R P rt to Metro Recovery Systems (MRS) 2430 Rose Place Roseville, MN 55113 Minnesota Office of Waste Management Industrial Waste Reduction Grant Program REDUCTION OF CHROMIUM PLATING WASTES Task 7 FINAL REPORT Michael Wismer, Technical Manager, MRS and Malcolm T. Hepworth, Consultant November 9, 1990 MINERAL RESOURCES RESEARCH CENTER DEPARTMENT OF CML AND MINERAL ENGINEERING UNIVERSITY OF MINNESOTA TABLE OF CONTENTS Page PREFACE TASK 7 . FINAL REPORT Executive Summary ............................................. 1 Chromium Recovery Process Description ............................. 3 TASKS 1.2. and 3 Introduction ................................................. 1-1 TASK 1 . Survey of Plating Wastes. MRS: Quantities. Compositions ..................................... 1-3 TASK 2 . Survey of Chromium Users and Purity Requirements ........ 1-5 TASK 3 . Review of Literature ................................ 1-8 References ................................................. 1-12 TASK 4 . SOLVENT EXTRACTION SHAKER TESTS Introduction ................................................. 4-1 Test Procedure .............................................. 4-3 Results and Conclusions ........................................ 4-4 References ................................................. 4-5 TASK 4A . OPTIMIZATION OF SOLVENT EXTRACTION CONDITIONS AND CONCENTRATION OF REFINED SOLUTIONS Introduction ................................................ 4A-1 Test Procedure ............................................. 4A-2 Results ................................................... 4A-4 Conclusions & Recommendations for Future Work .................. 4A-7 References ................................................ 4A-8 TASK 6 . ESTIMATION OF PROCESS ECONOMICS FOR RECOVERY OF SALEABLE CHROMIUM PRODUCT Introduction .......... ...................................... 6-1 Test Results for Extension of Work Begun in Task 4A ................. 6-2 Discussion of Results with Respect to Process Economics ............... 6-3 References ................................................. 6-4 Appendix A Appendix B Appendix C Appendix D .Proposed Change in Scope of Project: Task 5 change Appendix E .Estimate of Capital Costs a Progress Report to Metro Recovery Systems (MRS) 2430 Rose Place Roseville, MN 55113 Minnesota Office of Waste Management Industrial Waste Reduction Grant Program REDUCTION OF CHROMIUM PLATING WASTES Task 7 Final Report Michael Wismer Technical Manager, MRS and Malcolm T. Hepworth Consultant November 1990 PREFACE To the reader, this report is divided into several tasks which were set up in the original work contract. The first section of this report is Task 7, the final report with executive summary. It gives a brief description of the chromium recovery process which is illustrated by a block flow diagram. The report explores a recovery method using solvent extraction:- a process whereby an organic phase is contacted in multiple contacts with the aqueous (water) phase which contains waste chromium plus other metallic species which occur in the plating wastes. The organic phase is practically insoluble in the aqueous phase, and vice-versa, so that only metal ions are transferred between each phase. By controlling the pH (acidity) of the aqueous phase, it is possible to transfer (extract) all metals except chromium from the aqueous to the organic phase by a process termed "loading" the organic. The purified aqueous stream (raffinate) contained the purified chromium salt. Tasks 1,2, and 3 follow in the report and cover surveying of the plating wastes which were collected over a time interval at the beginning of the project. These wastes were collected from local plating companies and are processed by Metro Recovery Systems (MRS) to recover and recycle some metallic species, but not chromium. Presently, MRS must precipitate the chromium as a sludge and subject it to land-filling. In Task 2 a survey was conducted of chromium users to determine if there was a market for a purified chromium product and to determined the purity requirements. Task 3 is a review of the literature to determine whether or not the technology which was to be explored was unique and appropriate to the process. Task 4 reports on solvent extraction testing on a bench scale (shaker tests) to determine the conditions most favorable for producing a purified chromium product from the aqueous stream. In the original test plan, a high temperature (pyrometallurgical) process (Task 5) was also to be explored for recovering chromium; however, this alternative was abandoned for several reasons, primarily because it would not be possible to conduct such a process in the MRS facilities in Roseville. Instead Task 4A was agreed upon between the contractor and sponsor as a series of tests to optimize the solvent extraction process and to concentrate the refined solutions to produce a product. Task 6 is an estimation of the process economics for the recovery of a saleable chromium product. As this section details, the process is not economic at this time because of the dilute nature of the solutions which MRS is currently receiving from its clients. The primary unit of concentration used in this report is grams of metal species per liter of solution. For example 1gram per liter (gpl) represents a concentration of 1000 parts per million (ppm). Additionally the unit mg/liter is used, which is identical with parts per million. Some of the terms used in the description of the organic phase may be unfamiliar to most readers. The organic is primarily D2EHPA (di-2-ethylhexylphosphoric acid) which contains an additive DMOO (3,7-dimethyloctanol). In order for these two organic constituents to function properly, they are dissolved in kerosene (actually ESCAID, a Shell brand aliphatic diluent). For the purposes, therefore, of the average reader, the organic phase can be considered a kerosene-like material containing specific additives, which is essentially insoluble in water. .-. EXECUTIVE SUMMARY Metro Recovery Systems ,(AIRS) of Roseville, MN receives chromium plating wastes from electroplating and etching solutions as part of its processing operations. The chromium is reduced from the hexavalent state to a non-hazardous trivalent state via treatment with a chemical reductant, sulfur dioxide. The resulting solutions contain chromium in solution with several other plating residues, notably iron, nickel, copper, zinc, and cadmium.. Recovery methods for the latter four constituents of potential value are in place at MRS, and iron represents a valueless constituent. However, the separation of the above elements, especially iron from trivalent chromium solutions is not readily achieved by any commercial method. Because of the hazardous nature of hexavalent plating solutions, platers are beginning to attempt to redirect their operations to trivalent chromium, but at this time, the bright plating finishes characterized by plating from hexavalent solutions is not readily achievable from the trivalent solutions. Therefore there is a potential but not current market for trivalent chromium solutions in the plating industry; however, the solutions must be pure and free from contaminants. There is a lesser market for trivalent chromium solutions in the tanning industry; however, iron in particular, is an impurity which must be kept at very low levels. This study reports on a method of achieving the purification of trivalent chromium solutions and their concentration to a salt of chromium which could be saleable especially . to tanners. The method of purification is via solvent extraction in which the aqueous stream is contacted with an organic stream which removes all heavy metal cations except chromium. The aqueous stream is then treated with caustic additions to produce a precipitated salt which can be separated from the filtrate. This residual salt assayed 34.9% chromium, 10.6% sodium, 6.0% carbon, 6.7% sulfur, less than 100 parts per million (ppm) copper, 37ppm cadmium, less than 50 ppm iron, less than 200 ppm nickel and less than 50 ppm zinc. This salt contains residues of the organic phase and co-precipitated sodium sulfate. With further processing, such as removal of the organic via treatment with activated carbon and further washing to remove sodium sulfate, this salt would meet many of the requirements of tanners and may be saleable to potential customers. Further work is required to firm up this question. As will be documented subsequently, this study was divided into seven tasks. One of these was an analysis of the streams received by MRS from its clients. Early projections upon which this study was based indicated that the received solutions would contain approximately 21,000 pounds of chromium per month; however, assays on chromium solutions over the period September, 1989 through July, 1990 indicated a monthly average of a little over 800 pounds of chromium per month. This difference in quantity has a significant negative impact on the process economics. Several factors account for this shortfall in the amount of chromium received at the MRS facility. When this project was first proposed to the Office of Waste Management, MRS had been in operation only for three months with a limited client base. Data on the 1 potential amount of chromium available in the region was taken from a 1985-86 market survey study done for the MRS partnership. This study concluded that 250,000 pounds of chromium per year of chromium-containing process wastes were available in the MRS market area. Since this study,