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Table of Contents: Process of Metallic Replacement Method
Commercial Use of Metafix MRCs Guidelines for Selecting Metafix MRCs
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Silver-rich solutions were collected from several photofinishing customers. The silver concentration in the solutions ranged from 1,500 ppm to 3,000 ppm, with an average of 2,700 ppm. The average pH of the solutions was approximately 6.7. While the lack of standardized silver concentration and pH introduced some variability into the study, it more accurately reflected a real-world situation. Atomic absorption spectrophotometry (AA) was used to determine the silver concentrations in both the MRC influent and effluent solutions. To save time and to minimize the amount of silver-rich solution required, mini-columns instead of true-size columns were used first to find the best candidate for the medium. Once the candidate was identified, it was used for true-size tests in Metafix MRCs. There were three true-size columns: the MIX2000P, the MIX5000 and the Oliver. The mini-columns were made from transparent PVC tubing, while the Metafix MRCs were made from polyethylene. TABLE 1 shows the physical dimensions and volumes of the mini-columns and the true-size columns.
Iron was mixed with different amounts of inert fiber and then packed into mini-columns or true-size columns using a special high-pressure procedure. Silver-rich solution was passed through these columns at controlled flow rates: approximately 20 ml/min for the mini-columns, 110 to 150 ml/min for the smaller MRCs and 1 to 12 L/min for the larger Metafix MRCs. Samples from the effluents of these columns were taken routinely and analyzed by AA. The tests were stopped when the silver concentration in the effluent reached or exceeded the breakthrough point -- 5 ppm of silver. The mixture recovering the largest amount of silver was considered to have the optimum iron to fiber ratio. This mixture was used for further tests and is in commercial use. There was some concern regarding sample preservation and sample preparation procedures of MRC effluents7. This study did not use sample preservation, sample digestion, or matrix match. Tests showed that eliminating these procedures caused no more than a 10 percent error in silver concentration which was considered tolerable in this study.
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