ELECTROLYTIC RECOVERY FROM RINSE WATERS. C.A.Swank. ERC/LANCY Division of Dart & Kraft, Inc. 525 West New Castle Street Zelienople, PA 16063

Transcription

1 ELECTROLYTIC RECOVERY FROM RINSE WATERS By: C.A.Swank ERC/LANCY Division of Dart & Kraft, Inc. 525 West New Castle Street Zelienople, PA 16063

2 I, Introduction The recovery and reuse of various industrial waste products is looking more and more attractive as raw material costs increase and tighter restrictions are put on the disposal of wastes. The increase in the number of recovery technologies is keeping pace with the interest in recovery. Electrolytic metal recovery is one of the options available, and I would Like to put it into perspective by'briefly examining a number of the other major options. The current recovery technologies can be broken into two major groups: concentrative and selective techniques. The concentrative techniques return the major portion of the dragout to the plating bath, and the selective recover only a portion of the dragout, usually the metal. The choice of which route to follow is basically an economic one. In some cases the metal is the most valuable constituent of the dragout, and the cost of replacing the other components is not high enough to warrant the cost of recovering them. In other instances the metal is relatively inexpensive and recovery of the entire dragout makes more econdhnic sense. The recovery technologies can also be used in conjunction with each other; for example, ion exchange can be used to concentrate a metal ion from a very dilute rinse stream and the metal can then be electrolytically recovered from the more concentrated regenerant. Problems Associated with Dragout Concentration and Return A large number of the commonly used recovery systems are based on concentrative techniques. Concentrative methods of recovery are susceptible to some problems, which must be considered along with the economic factors. It is almost always the case that when soluble anodes are used the anode efficiency is higher than the cathode efficiency. This difference can range from 3-4% for some nickel baths to extremely high values for 1

3 .. I some barrel plating lines. The resultant build up in metal concentration is usually not noticed because it is compensated for by dragout, but when dragout return is practiced, the build up can become a problem. Accumulation of impurities is another problem that can devolop when dragout return is practiced. Plating baths are normally purged by dragout and the impurity level is kept to a minimum. When dragout is artificially reduced, the trace impurities can build up to significant levels. The build up of carbonate in a cyanide plating bath, for example, can become so extreme when dragout return is practiced that the loss of the bath when the carbonate is crystallized out more than compensates' for any gain due to dragout return. Because of this build up of impurities, any concentrative technology will need to have some form of purification associated with it and any water entering the system must be of high purity to minimize contamination. Common Recovery Technologies The most commonly used recovery technologies for electro-. plating baths are: simple dragout recovery, evaporative concentration, reverse osmosis, electrodialysis, ion exchange and electrolysis. Simple dragout recovery is an often overlooked method that can recover up to 60% of the losses due to dragout. When the first rinse after the plating bath is segregated and made stagnant, it can be used to make up evaporation and dragout losses and to mix up chemical additions, and a significant savings can be achieved without the use of any complicated "black box" technology. Evaporative recovery is the oldest, most widely used and most flexible of the recovery technologies. It is also the most energy intensive. There are two basic types of evaporative units: atmospheric and vacuum. In the atmospheric evaporator rinse water is heated and passed through a packed 3

4 column, countercurrent to dry air, which is exhausted from the system at atmospheric pressure. In the vacuum evaporator the rinse water is concentrated by boiling at reduced pressure. The moisture that is driven off is condensed and made available for use as rinse water. In both cases a concentrate that contains essentially everything that was in the plating bath is made available for return to the bath. The vacuum units are generally much more expensive than the atmospheric units and the operating costs are roughly the same for both systems. The most useful application for the atmospheric evaporator is the recovery of chrome plating baths, because it can serve three functions: recovery, scrubbing and cooling. The air above the plating bath, which must be scrubbed, can be used as the drying air in the evaporator. Chrome baths tend to increase in temperature and can be cooled by passing through the heat exchanger for the evaporator. Reverse osmosis is one of the newer technologies to be applied to rinse recovery. It is much less energy intensive than evaporative recovery, but it has a number of drawbacks. In reverse osmosis pressure is used to concentrate the recovery rinse by forcing water molecules through a semipermeable membrane against a concentration gradient. The problems with these systems are associated with the limitations of the membranes, which are fragile, easily fouled and difficult to replace. The membrane problems can be minimized by using soft water to eliminate calcium and magnesium, adjusting the ph to ensure that there is no precipitation of metals on the membrane, and filtering the rinse solution to remove suspended solids that might clog the membrane. The highest pressure that can be used is on the order of only psi, so the output from reverse osmosis is not very concentrated, and it is best applied where there are high evaporation losses. The major egonomic factor to be considered with reverse osmosis is the effective life of the membrane, which is only on the order of two to three years, even when the system is operating ideally. The necessary high 3

5 pressures crush the membrane, making it increasingly less permeable with time, and the cost of the membrane is a major portion of the cost of the unit. A lot of research is currently being done on reverse osmosis and it has the potential for being very useful on those systems whose dragout is not easily recovered by the other methods. Electrodialysis is another of the newer technologies. It is more energy efficient than reverse osmosis and produces a more concentrated output, but it is also a membrane technology and so has similar limitations. There is no applied pressure associated with electrodialysis, so the membranes can last for several years when the system is operated properly. An electrodialysis stack consists of alternating cationic selective and anionic selective membranes. The rinse water is passed through the stack; under the influence of an electric field positive and negative ions move in opposite directions, forming alternating cells of concentrate and relatively clean solution. Electrodialysis is a relatively complex technology, but has been applied to a number of different recovery applications. It is currently most effective when used for the recovery of gold and nickel, but research is being done on other applications. Both reverse osmosis and electrodialysis require a high level of skill on the part of the operator. The systems must be closely monitored to ensure that the softeners and filters are functioning properly and that the ph is maintained within a narrow range. Ion exchange is the most energy efficient of the recovery methods. The dragout rinse is passed through a column in which a cation exchange resin removes the metal ions, holding them until the resin is fully charged. The metal ions, and other positive ions, are then removed when the column is regenerated with acid. It is necessary to use three to four times the stoichiometric amount of acid to regenerate the resin, so the concentrate is rather dilute. There are systems available that

6 , I.,. minimize this problem, but they are rather complex and require a lot of operator attention. The most useful application of ion exchange is in the recovery of gold. In this instance the resin is not regenerated, but sent to a refiner, where the gold is usually recovered by burning off the resin. It is generally a recommended practice for the customer to make his own careful assay of the gold before sending it off to the refiner, but the gold value is determined by the refiner. Ion exchange is the only one of the commonly used recovery technologies that can be used for treating a very dilute stream on a once through basis. Electrolytic Metal Recovery Electrolytic metal recovery is radically different from the other methods of recovery; it is selective and removes only the metal, and thus decouples the production and recovery processes. Electrolytic metal recovery doesn't concentrate and return to the bath everything that was dragged out, so if a plating bath is performing satisfactorily before an electrolytic metal recovery cell is added to the system, it will continue to operate satisfactorily after, and no additional purification of water will have to be added to the system. What electrolytic metal recovery does do is recover the most valuable constituent of the dragout - the metal. The metal is also the constituent of the dragout that is responsible for the formation of sludge. One pound of copper, for example, will form 8.3 gallons of sludge. ' Electrolytic metal rec0very.i.s an age old technology, used for many years in the mining industry for electrowinning and electrorefining of ores. It has also been used for a number of years to recover copper from pickle liquors. For about the last twenty five years electrolytic metal recovery has been studied as a means of recovery of the dragout from plating tanks, with most of the active research being done in the last ten years. In recent years there has been a lot of interest 5

7 in recovering the metals from dilute rinse solutions; thousands of hours have been spent working on the challenge and hundreds of patents have been issued. Most of the work has been done on a once through basis; very dilute rinse waters pass through the recovery unit on the way down the drain. Dilute rinse waters pose a special electroplating problem. The cathode polarization that all platers must be concerned with is a much more acute problem to someone who is trying to plate out of a very dilute solution. As plating proceeds, the area of solution next to the cathode'bec-omes- d-epletedltn metal ions. The ions must diffuse into and across this layer before they can plate out. In dilute solutions there are fewer ions present, so the rate of diffusion into and across the depleted layer is much slower, and the layer becomes thicker and more depleted. Cathode polarization can cause poor quality deposits, with dark, powdery, burned areas and treeing can occur, with trees growing across to the anodes and shorting out the cell. The efficiency at the cathode can be greatly reduced, with electricity being used to decompose water, forming hydrogen gas, instead of to plate out the metal. There are a number of ways that the problems associated with cathode polarization can be reduced: by running at a lower current density, adjusting the solution chemistry, agitating the solution, and adjusting the temperature of the solution. When plating is carried out at a low current density, the depleted layer will be narrower and metal ions can diffuse into and across it more easily. A given amount of metal can be plated out with a low current density when a large cathode surface area is used. One way of increasing the cathode surface area is to use a large tank with many rows of cathodes and anodes. This approach is cumbersome and to recover from low concentration rinse waters, hundreds of pairs would be needed, and this approach would be ridiculous. A more practical way to achieve high surface area, in a low volume, is to use stainless steel wool or a porous carbon for the cathode. There are companies working on both of these approaches. It is easy to remove the metal from dilute solutions this way,

8 .. L *. but the metal can't be recovered until it is dissolved out of the cathode into a concentrated solution and plated out by conventional methods.? There are a couple of ways that the characteristics of the plating solution can be chemically altered. Chemical additions can be made; for example, electrolytes can be added to improve the conductivity of the solution and grain refiners can be added to improve the quality of the deposit. Increasing the concentration of the metal ion in the solution is another chemical adjustment that can be made. It would be very expensive, however, to add chemicals to a dilute waste stream that only passes once through the recovery area on the way to waste disposal. Every plater knows that if he agitates his plating solution or his cathodes he can either plate at a higher current density or lower the concentration of metal in his plating bath. Metal has been electrolytically recovered from waste water containing as little as 100 ppm of metal with the aid of rapidly rotating cathodes. The metal is recovered as a powder, however, and this poses a collection problem, as well as exposing the metal to oxidation. When the temperature of a plating bath is elevated, the metal ions in the solution become much more mobile and can diffuse much more rapidly through the depleted layer. It is impractical to heat a large volume of water as it is going down the drain, though. Many of the potentially favorable adjustments that can be made to an electrolytic recovery system are impractical for a once through system, but can be taken advantage of if a closed loop is utilized. The first rinse after the plating tank can be isolated and continually recirculated through an electrolytic metal recovery cell. Since the basic recovery solution is being reused, it can be heated, the concentration of metal Tons 7

9 can be allowed to build up to a reasonable level, and other adjustments can be made to the chemistry of the solution. The electrolytic metal recovery cells pictured above have impellors on both ends of the cathode compartment which recir- culate the recovery solution through the cell. The impellor design has been optimized to provide a uniform flow of three to four feet per second past the surface of the cathodes. cathodes are stainless steel and are provided with edge guards that aid in the removal of the metal deopsit. The stainless steel cathodes are reusuable; when the deposit is k - %'I thick, the metal can be easily peeled off the cathode. produced with this electrolytic metal recovery cell is of high quality, and can either be reused as the soluble anode in the plating tank or sold for top dollar. The metal The Currently, the most popular application for the electro- lytic metal recovery cell is for the recovery of copper from copper sulfate plating solutions in printed wire board shops. The cell can also be used for the recovery of gold, silver and copper from cyanide plating solutions an.d the recovery of tin from alkaline, acid sulfate and fluoborate solutions, and research

10 ._ 9. -, '. is currently being done on recovery from solutiom containing chlorides and nitrates, and from electroless plating solutions. Besides being used for recovery from dilute rinse solutions, the cells are also used for the recovery of metals from sludges and crystals, and for the regeneration of process solutions, such as sulfuric/peroxide etch so1,utions. The tin shown below was recovered in one of these cells. Summary None of the recovery technologies discussed is a universal panacea. There are applications for which one of the technologies is best suited, and there are applications for which two or more of the technologies can be successfully specified. In these instances, the decision must be based on economic considerations. It should be decided whether the metal or the other constituents of the bath are most valuable, and the capital cost of the equipment and the operating costs must also be considered. The complexity of the equipment and the skill required of the operator, as well as the amount of operator time necessary are also importantant factors in the decision. 9