Electrolytic Wastewater Treatment. Dr. Clarence H. Roy, CEF Aqualogic, Inc. Bethany, CT

Transcription

1 Electrolytic Wastewater Treatment Dr. Clarence H. Roy, CEF Aqualogic, Inc. Bethany, CT

2 ELECTROLYTIC WASTEWATER TREATMENT ABSTRACT The electrolytic methods for wastewater treatment are described herein. Applications of this technology include metal recovery, bath regeneration and cyanide oxidation. LIST OF SLIDES ~ AES Slide Title Slide Electrolytic Methods Removing Cathode from Plating Tank Scraping Gold Deposit from Cathode Peeli-nq Copper Deposit from Stainless Steel Factors Influencing Metal Deposit High Cathode Area Removal of Metallic Mud from Cell Rapid Agitation Commercial Installation Solution Chemistry Effect of Temperature Electrodialysis Electrolysis--Packed Beds Summary All rights reserved. Printed in the United Statesof America. This publication may not be reproduced. stored in a retrieval system. or transmitted in whole or in part. in any form or by any means, electronic. mechanical, photocopying. recording. or otherwise. without the prior written permission of AES, 1201 Louisiana Ave.. Winter Park. FL Copyright 1980, American Electroplaters' Society, Inc.

3 ELECTROLYTIC WASTEWATER TREATMENT 7. Slide 1 This is one in a series of Illustrated Lectures on Environmental Compliance and Control sponsored by the American Electroplaters' Society, Inc. Slide 2

4 Slide 3 Electrowinning has been practiced in one form or another in the electroplating industry since its early days, but the name comes from the mining industry where it achieved more significant commercial importance. Unfortunately the majority of electroplaters have not seized upon this very familiar and simple technology to recover metal and reduce pollution. Perhaps dummying can be regarded as a form of electrowinning, but the potential of the technology extends far beyond bath purification. Electrowinning is widely employed in the mining industry to recover metals from dilute solutions, but has not been extensively used by electroplaters to do the same thing with their wastewaters and spent solutions. Presently noble metal recovery, particularly silver and gold, is perhaps the most popular application of electrowinning. While the name is not often applied to the apparatus, a number of suppliers produce a variety of equipment

5 Slide 4 Essentially these devices electroplate metal on inert, usually stainless steel, cathodes. In simple form the equipment is just a plating tank with alternating, closely-spaced rows of anodes and cathodes. Slide 5 The meta.1 depo sits are mechanically stripped from the cathodes with I a kni fe or a chisel. The gold deposit is scraped f :rom the pa ssive stainless steel surface

6 While the noble metals have attracted the most attention it should be noted that copper, tin and cadmium are also being recovered electrolytically. Slide 6 Copper recovery is particularly attractive from pickle baths in copper and brass mills, since the recovered metal can be renelted in their own furnaces. Even if the metal cannot be reused in-house, it should be noted that metal values as metal are easier to handle, store, transport, and recover than sludges and salts sent out for reclaim. This is an important consideration when one contemplates the complexities of RCRA

7 ,qlide 7 In the next series of slides we will discuss factors influencing the nature and rate of metal deposition in electrowinning. Slide 8-5 -

8 The process is similar to many other plating systems. The ratio of cathode to anode area varies in the different commercial systems. In general, a high cathode area favors deposition of a smooth deposit. Slide 9 Another supplier uses high current densities to encourage a dendritic crystal growth (burning) and actually recovers the metal as a slurry or mud through dump valves in the sloped bottom of the cell. Slide 10 The factor of rapid agitation favors high rates of deposition and production of smooth deposits

9 slide 11 This commercial installation features high recirculation rates to achieve a smooth deposit at high cathode current densities. Slide 12 As with any electrolytic process, such as plating and electrocleaning, conductive salts may be added to improve the electrical conductivity of the solutions. Without salt &&itions to the rinse waters, the conductivity is relatively poor

10 ~ - S ide 13 The optimum temperature range varies with the type of metal being recovered. Gold recovery is normally carried out at ambient temperature. In most cyanide systems, the processes are operated at elevated temperatures for two reasons: Higher temper- atures favor production of smooth deposits, and the other important factor of high temperatures ( F) is the destruction of cyanide ions. Naturally as the ionic content of the solution diminishes during electrolysis so does the efficiency. There is in this case a practical limit of about 500 ppm, below which alkaline chlorination is more efficient. Addition of sodium chloride to the process to extend the efficiency range by generating chlorine at the anode has also been used. Slide

11 Electrodialysis in the connotation used here might better be termed electrophoresis or electromigration. The chemical composition of the diaphragmmaterial is not necessarily of the ion exchange type as in the true electrodialysis. In these applications the nature of the membrane or diaphragm is more of a porous partition through which ions can migrate freely in accordance with their specific charge, and the applied potential. Thus cations migrate toward the cathode and anions toward the anode. The partition then simply serves to prevent the loss of the enriched material from the catholyte or anolyte compartment (s) or cells by back diffusion or mixing. There are two commercial applications of this type of electrolytic treatment. Shown on the slide is an application of the process to regenerate chromic acid solutions used to strip copper or activate plastic surfaces prior to plating. The trivalent chromium is oxidized at the lead alloy and the copper ions migrate to the platinized titanium and are deposited as metallic copper. Another application of this technology is the con- centration and recovery of chromate ions from rinsewaters from chrome plating. The cell used here employs a '/2 inch thick porous plastic diaphragm. Concentrations approaching 10% of both strengthsare achieved in the anode compartment. Further concentration by evaporation may be required prior to returning this solution to the plating bath

12 Slide Electrolysis in packed beds has been used in a variety of applications over the years. An early patent by Gage, dated 1903, describes the use of a packed carbon bed to capture gold from dilute cyanide solutions. Zadra patented the use of charred wood shavings (excelsior) in a similar application. More recent patents and papers by Roy, Strier and many others, describe the use of carbon pellets, carbon fiber, tin shot, steel shot and almost anything that provides reasonable conductivity and expanded surface area. Metal removal in these applications is by either electrowinning or electroprecipitation of the metal hydroxide. The difficulty with this type of electrowinning resides in removal of the metal from the substrate. Gold and perhaps silver can be recovered by burning away the carbon, but with other metals this practice is not feasible. Acid stripping of the recovered metal from the packing material is being studied but practicality of this approach has not yet been demonstrated. Another application is destruction of cyanides (1) in -~ dilute solutions. (1) D. T. Chin and B. Eckert, Electroplating & Metal Finishing, 63, October, 41 (October 1976)

13 Slide 16 The features as listed on the summary slide are self evident. As with any technology, the intelligent application of electrolytic methods to wastewater treatment will provide ideal solutions to specific problems