w- LlkJ-/ rpdf Pollution Prevention - Source Reduction with Electrodialytic Processes by Daniel J. Vaughan This paper is focused on how not to make waste or how to prevent pollution at the source. I know of no profit incentive to make waste. Why do we make lots of waste? Could it be that there is no technology that provides an economical alternative? Waste is obviously not a useful or desirable substance. Waste is an expense, it creates liabilities and it pollutes the environment. Why do we continue to make waste? Compliance has been our way of regulating pollution, defining waste and prescribing what can and cannot be done with waste. If we define waste as an unwanted or undesirable substance in its environment and pollution prevention as not making waste, we can determine if pollution is increased or decreased in any operation or action; we just make a mass or material balance of the desirable and undesirable substances in their environment. Each year about 74 million tons of sulfuric, hydrochloric, nitric, phosphoric and hydrofluoric acids are made, used, neutralized and discharged, mostly to fresh water sources. Obviously, these salts are not desirable in our fresh water sources. Yes, they are waste. Many chemical solutions are "waste treated" by chemical oxidation-reduction, precipitation and neutralization. None of these operations reduces waste. Indeed, waste is increased when we use chemicals to change the form of some material so that it can become landfill. Some solutions are evaporated, distilled or separated by reverse osmosis into a concentrate of waste and unused chemicals, but the waste is not changed to a usable substance; the amount remains the same. Ion exchange is often used to remove chemicals from a dilute solution. The ion-exchange resins must be regenerated with chemicals. The regenerant chemicals are waste unless the regenerants are reformed without using chemicals. None of these technologies reduce waste or convert waste to a useful material. Some of these technologies actually increase chemical waste. To carry out chemical reactions without forming a waste, we must use a basic particle of all matter, an electron. Electrons, when properly engineered, provide a way to oxidize-reduce substances and to separate ions. Oxidation is adding electrons to a substance and reduction is removing electrons from a substance. Separation of ions is often referred to as electrodialysis, membrane electrolysis and electrodialytic processes. The electrodialytic processes involve oxidation and reduction of water and the separation of ions. These processes, when engineered properly, provide for source reduction and pollution prevention of many solutions of chemicals without forming a new waste. In the early 198Os, the Ionsep Corporation invented, patented and developed IONSEW Electrodialytic Processes for converting salts of multivalent metal cations into the acid of the salt anion and an insoluble hydroxide of the multivalent metal cation. It is broadly applicable for purifying, reforming and maintaining the chemistry of process solutions as the solutions are being 8 Trademark of Ionsep Corporation Inc.
-2- used. Shown schematically in Figure 1, the process is carried out in an electrochemical cell having an anode, an anolyte, a membrane separator, a cathode and a catholyte. A rectifier (direct current electrical power source) is connected to the electrodes, positive to anode and negative to cathode. When an electrolyzing current is passed through the cell, electrons are removed from the anolyte at the anode to oxidize water and form oxygen and hydrogen cations, while electrons are added to the catholyte at the cathode to reduce water and form hydrogen and hydroxide anions. The membrane separator is a traffic controller for ions. When the membrane separator is permeable to cations, cations go from the anolyte to the catholyte; the anions remain in the anolyte and form acids with the hydrogen ions which are formed at the anode. The cations that go through the membrane, for example, copper ions, react with the hydroxide ions to form insoluble copper hydroxide. The copper hydroxide is then removed from the catholyte. The catholyte chemicals are not consumed; only electricity and water are consumed. The copper ions were an undesirable substance in the anolyte or process solution and, therefore, no new waste was produced. A large number of process variations can be created by dividing an electrochemical cell into compartments separated by cation and anion permeable membranes and bipolar membranes. Figure 2 shows a cell having three compartments separated by two cation permeable membranes. This cell configuration is often used to remove metal and alkali cations from an acidic anolyte into a reactor electrolyte where the metal cations are precipitated and the alkali ions, Na', go through the second membrane into the catholyte and form a pure sodium hydroxide. This provides a separation of metal and alkali cations. An electrochemical cell could also be separated by an anion permeable membrane as shown in Figure 3. In this example, the process solution is fed to the catholyte where metal cations form insoluble hydroxides by hydroxide ions formed at the cathode and the acid anions pass from the catholyte to the anolyte and form a pure acid with hydrogen ions formed at the anode. Electrodialytic processes require the proper selection of materials, system, configuration and components to operate successfully. Each application demands different combinations of equipment, maintenance requirements, and operational procedures. Improper design and selection of any component can result in an inoperable process or one which requires constant attention and high operating and maintenance costs in order to function. A properly designed system is very easy to operate, requires little operator assistance and operates for at least one year with essentially no maintenance. The process must be properly engineered. The capacity of an electrodialytic system is measured in terms of the amperage at which the cells are operated. As more current is applied across an electrochemical cell, the more oxidationreduction and ion separation. The system capacity can range from as small as 50 amps for an experimental unit to over 150,000 amps for large commercial units. Electrical cost for operating electrodialytic processes is not as high as commonly perceived. The systems operate at a low voltage and, for example, a 500 amp system operating at 5 volts would use 2.5 kilowatts per hour. At 5 cents per KWH, the cost would be 12.5 cents per hour.
-3- Electrodialytic processes provide a versatile technology for preventing pollution by not making waste while purifying, reforming and maintaining the chemistry of a process solution as the solution is being used. In addition, these processes using electrons offer savings in energy, natural resources and more efficient manufacturing. The electrodialytic processes that have been illustrated herein are in operation in the United States, Mexico and Europe and in the Orient. Some processes have operated for a decade. The following examples show how source reduction and pollution prevention are carried out using electrodialytic processes. A comparison of waste treatment and electrodialytic processing is also shown. Reforming Chromic Acid Solutions with Electrons A cell divided by a cation permeable membrane can be used in many ways. For example, when chromium is electroplated from a solution of chromic acid, some of the chromium6 (chromate anion) is reduced to ~hro +~ and metals are dissolved in the plating solution; both form chromate salts. The electrical conductivity of the plating bath decreases as salts are formed and to keep the same deposition rate, voltage must be increased. This increases electrical cost and decreases quality of the deposit. Electrons can be used as shown in Figure 4 to remove the metals at the rate salts are formed and to oxidize the to chromic acid. Controlling the chemistry of the plating bath produces optimum conditions, making manufacturing more reliable, improving product quality and reducing electrical cost, labor and chemical usage. These benefits are usually realized when chromic acid plating solutions are maintained continuously at 4 to 6 g/l of dissolved metals and chromi~m ~. The by-product from the process is the metals which were dissolved in the plating solution. Similar electrodialytic processes are in use to reform anodizing, etching and chromating solutions, chromate seals and other solutions of acids and salts. Reforming Pickling Solutions with Electrons When stainless steel is pickled with nitric and hydrofluoric acids, metal salts and metal complexes are formed. The pickling rate decreases with build-up of salts and complexes and the capacity of the pickling plant is decreased. In addition, the quality of the pickled stainless steel becomes unacceptable at high levels of salts. To reduce chemical usage and waste disposal costs, the pickling operation is cut back and the production of steel decreases below design capacity of the plant. The high usage of acids in these pickling processes requires frequent production down-time to change solutions and regain quality production. Since productivity of the pickling process varies with the ability to pickle the steel, productivity can be doubled by reducing and maintaining the concentration of metal salts and complexes in the pickling baths. Manufacturing is optimized, cost is reduced and pollution is prevented by use of electrons as shown in Fi,oure 5. The pickling solution is circulated through a third compartment in the cell to remove metals at the Same rate as the metals are dissolved and to control the concentration of salts and complexes continuously at a level for maximum production of quality product. The anolyte
-4- serves to protect the anode from fluoride and to provide hydrogen ions which replace the metal ions removed from the pickling solution, continuously reforming the acids in the pickling solution. The dissolved metals are removed as insoluble hydroxides that have a potential use. The utilization of acids is over 95%. Reforming Caustic Solutions with Electrons Caustic soda (sodium hydroxide) is used in chemical milling etchants, flash etchants, rust removers, alkaline cleaners and electrocleaners. In the chemical milling of aluminum and aluminum alloys, aluminum dissolves in the caustic as sodium aluminate. The milling rate decreases as the level of dissolved aluminum increases and the quality of the milled part becomes unacceptable. Using an electrodialytic process as shown in Figure 6, the aluminum is removed from the solution at the rate it is dissolved which maintains the solution at the preferred composition for both production and quality. Electrons form hydrogen ions in the anolyte as sodium ions are transported to the catholyte. The sodium ions combine with the hydroxyl ions formed at the cathode making a solution of essentially pure sodium hydroxide that is returned for milling. The anolyte is controlled by caustic concentration or ph to convert sodium aluminate to sodium hydroxide and insoluble aluminum hydroxide. The electrons are good workers; they produce no waste. The by-product is the dissolved aluminum and alloy metals as insoluble hydroxides. Closing the Loop with Electrons Electrons can be used to provide a closed-loop operation alone or in combination with other technologies. A closed loop for a chromic acid plating operation is illustrated in Figure 7. Chromium is electrostripped from parts as sodium chromate. The sodium chromate is converted with electrons to sodium hydroxide and chromic acid. The chromic acid is used to plate chromium and the plating solution is continuously maintained as previously described in Figure 4. The plating chemicals in the rinse water can be removed in a membrane cell or removed by ion exchange as cations and anions. The rinse water is reused. The cations are removed from the cation resin with sulfuric acid as metal sulfates and the anions from the anion resin with sodium hydroxide as alkali salts. Electrons can be used to reform the spent regenerants and recover the ions as acids and hydroxides of metals. Reforming the spent regenerants can be done in several ways with electrons in membrane divided cells. One way to reform the spent anion regenerant and recover the chromic acid for use is illustrated in Figure 8. The cell is divided into four compartments using an anion permeable membrane to separate the anolyte from a feed compartment and cation permeable membranes to separate the reactor compartment from the feed compartment and catholyte. When an electric current is passed through the cell, anions go to the anolyte and form acids; metal cations and alkali cations go to the reactor compartment where the multivalent metal cations form insoluble hydroxides; the alkali cations go to the catholyte to form a solution of alkali hydroxides. The chromic acid is used in plating and the sodium hydroxide used as the regenerant. The recovery of sulfuric acid is a similar operation.
The advantages of using electrons for pollution prevention are several: + no additional chemicals are required the acid or alkaline bath is reformed to specifications unwanted metals are removed from the bath only electricity and water are used + production quality is improved It is reasonable to predict that electrons will increasingly be used to manufacture, purify, reform and reuse chemicals, to improve efficiency of manufacturing and to increase conservation of resources. Surface finishers who make full use of electrons will not only prevent pollution but will also realize these advantages. The electmdblytic processes illustrated are patented IONSEPa Electrodialytic Processes.
~~?CURE 1: 2 COMPARTMENT CELL ANODE (+) CAT1 0 N CATHOOE (-) MEMBRANE m SALTS CLTIONS METAL HYDROXIDES 1 ACIDS RGURE 2: 3 COMPARTMENT CELL ANODE (+) ' CATION CATION,I MEMBRANE MEMBRANE SALTS ANIONS ACIDS - I CATIONS 8 METAL HYDROXIDES CATHODE (-) SOLUTION BASES FIGURE 3: 2 COMPARTMENT CELL WITH ANION MEMBRANE ANODE (+) ANION CAkOOE (-) m MEMBRANE... WNSEP.ELECTROO(ALYTIC PRCCESSES
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