DETERMINATION OF FEASIBLE OPERATING CONDITIONS FOR METAL RECOVERY FROM ELECTROPLATING RINSEWATER EFFLUENT

Transcription

1 DETERMINATION OF FEASIBLE OPERATING CONDITIONS FOR METAL RECOVERY FROM ELECTROPLATING RINSEWATER EFFLUENT Prof. CZifotd W. Wdton, CEF, and Steven S. Bray1 University of Nebraska-Lincoln, Dept. of Chemical Engineering Lincoln, NE Thomas J. Thompson, CEF Lincoln Plating Company, 600 West E Street Lincoln, NE Abstract A process scheme using an electrodialytic ion exchange (EDIX) cell for the reduction and recovery of metal ion content from electroplating rinsewater is presented. The EDIX cell is comprised of bipolar and cation permeable membranes held between electrodes producing oxygen and hydrogen gases. Chemical engineering principles are used to provide details of the expected recovery rates and identify constraints on the operation, information which is necessary for the construction of prototype and actual operating recovery systems. Results presented are based on operating data from an actual nickel plating line. The described method of analysis can be used to evaluate the feasible operation of any metal recovery system. Background Any new waste management process that hopes to find use in the electroplating industry must meet certain standards of operation. The most common current primary waste management technique, chemical precipitation, results in the formation of a solid sludge product that requires subsequent disposal [1,2]. This method is expensive in both the cost of disposal and the loss of raw materials. Alternate methods which reduce the amount of lost raw materials is needed. Methods other than precipitation being used in the plating industry include ionexchange, evaporation, reverse osmosis, and electrolytic metal recovery [3]. All of these waste management processes are alternatives to precipitation and, although none of the methods have been able to eliminate completely the need for precipitation, all can reduce greatly the amount of sludge produced. Unfortunately, none 'Current address: Union Carbide Corporation, Process Design Group, Industrial Chemicals Division, P.Q. Box 8361, South Charleston, WV

2 of the current methods are able to produce a concentrated metal stream and a U cleaned water stream with one process. As a means of eliminating the problem of resource loss and water contamination, a new method for waste management is proposed. The intent of the electrodialytic ion exchange (EDIX) cell is to achieve both of these goals. The initial design criteria were to return a concentrated metal ion stream to the plating bath at the same concentration as the bath while the residual wastewater stream would meet EPA effluent guidelines and would not require any further treatment before exiting the plant. The EDIX cell is shown in Fig. 1. It contains chambers separated alternately by bipolar membranes and cation permeable membranes. A typical cation permeable membrane consists of a fluorosulphonate polymer with fixed negative groups (-SO;) which are part of the membrane s physical structure. A bipolar membrane consists of a cation permeable membrane and an anion permeable membrane (fixed positive groups) which are laminated together. The wastewater stream from the rinse tank flows into the regenerate chamber where metal and hydrogen ions migrate across the cation permeable membrane into the concentrate chamber. To balance the negative ions remaining in the regenerate chamber, water is split into hydrogen and hydroxide ions by electromotive force within the bipolar membranes. The hydrogen ions replace the migrating positive ions while the hydroxide ions migrate to the anode and react to form oxygen gas and water. In the concentrate chamber the formation of hydroxide ions balance the positive ions that have migrated from the regenerate chamber through the cation permeable membrane, while the hydrogen ions formed migrate to the cathode and react to form hydrogen gas. The water flowing from the regenerate chamber would be reused in the rinse tanks or released from the plant, as it would now meet Effluent Limitation Guidelines set by the EPA in 1983 for heavy metal content 141. Part of the stream flowing from the concentrate chamber would be sent back to the plating bath at the same concentration as the plating bath. This proposed method minimizes losses of valuable resources and eliminates the problems associated with sludge production. Objectives This study was undertaken to develop physical and mathematical models that would provide the capability to predict and evaluate the operation of the proposed electrodialytic ion exchange (EDIX) cell under a wide range of operating conditions, including variations in flow rates and compositions. This allows for the study of the feasibility of using the EDIX cell for many different plating bath operations. A mathematical model was developed to characterize the system in order to minimize the amount of future experimental work required to evaluate the applicability of the EDIX cell to specific applications within the electroplating industry. 2

3 w Q 0 'd E-r 4 u cu 0 3

4 $ This was accomplished by formulating governing equations and boundary conditions for the entire process (shown in Fig. 2). A component material balance model was constructed to evaluate different stream configurations, as well as possible output concentrations and flow rates based on input concentrations and known volumetric flow rates. The model requires estimates for the fraction of moles of metal ion that would flow across the membrane in one pass. These were estimated based on past knowledge of membrane performance. Initial EDIX Recovery System Design The initial design for the process as proposed by Thompson and Walton [5] is shown in Fig. 2. For ease of identification, streams referenced with descriptive names will also be followed with a capitalized italic S and a stream number (e.g., S4). The plating bath is kept at a constant composition by replacing exactly what is lost from evaporation (S7) and dragout (5'10). Evaporation is the result of the elevated temperatures at which electroplating is operated usually. It is assumed that only water is lost due to evaporation although some ions may be released as particulate matter. Dragout occurs due to the tendency of some electrolyte to adhere to the plated parts upon removal from the plating bath. It is assumed that the concentration of the dragout liquid is the same as that in the plating tank. Also, it is assumed that anode dissolution and metal plating rates are equal. This assumption is reasonable for all types of electroplating except for chromium deposition [6]. The plated parts from the plating bath are sent to rinse tanks where the parts are cleaned, resulting in the dilution of the ion concentrations. Flow from a down stream rinse tank (Sl) is then sent to the EDIX cell. Electrode reactions within the EDIX cell produce hydrogen (S9) and oxygen gas (S8). The outlet stream from the regenerate chamber (S2) is sent out of the plant to a water treatment system, while the outlet stream from the concentrate chamber (S3) goes to a concentrate recycle tank. The concentrate recycle tank, having a concentration equal to the that of the evaporation stream and the dragout stream combined, is used to replace electrolyte in the plating bath. The concentration of the tank is also the concentration of the recycle stream entering the EDIX cell (S5). New acid and salt solution enters the concentrate recycle tank from a feed tank (234) to replace what is lost from the system through the outlet stream of the regenerate chamber (S2). The goal of the model was to define all stream conditions based on the input of a minimum of variables. The model does not consider how the membranes work within the EDIX cell, but rather is used to identify regions of operating feasibility of the system. The input variables needed are the dragout flow rate, the evaporation rate, the rinse stream flow rate, the plating bath and the rinse tank composition 4

5 I I PLATING I TANK I LED 1 TANK 1 5 LEDIX CELL I 3 Figure 2: Original schematic design of EDM recovery system.

6 Table 1: Selected EPA electroplating effluent guidelines [4]. Metal ion Concentration Limit (mg/l) daily average over four days Cadmium 0.7 Chrome 4.O Copper 2.7 Lead 0.4 Nickel 2.6 Zinc 2.6 Total metals 6.8 and the concentration of metal ion in the treated water stream. Typical allowable values for some metal ions are shown in Table 1. The concentration limits are based on the average of daily values for four consecutive days. Estimates of the value of the molar fraction of species i that will cross the cation permeable membrane (given the variable name ri) are also required for all species except water. Other required data are the oxidation number and molecular weight of each species, and the liquid or solid density of all acids and salts introduced into the concentrate recycle tank. The existence of small quantities of organic additives are neglected in this model. The system is considered to be continuous even though the plating operation is not actually operated this way. Plating and dragout are processes which occur in steps. The flow rates for the step processes were measured during actual operation of an Watts nickel plating line and then averaged over the time of operation to obtain values in a continuous form [7]. Nickel, chloride, sulphate, and hydrogen ions, as well as water and boric acid as neutral compounds, were the chemical species included in the model analysis. The stream conditions for the entire process are found using component molar balances. The following is a step-by-step description of the methods used to accomplish the goals of the model. The known variables in this case are streams S1, S7, and S10, as well as the metal ion concentration in stream S2. A volumetric balance and molar component balances around the plating tank are used to calculate the volumetric flow rate and molar concentrations of the plating bath inlet stream (S6). This assumes that the density of S6 and S10 are constant. The concentrations of the inlet recycle stream to the EDIX cell (S5) are equal to the concentrations of stream S6. The volumetric flow rate of the treated water stream from the EDIX cell (S2) is calculated by using a molar flow rate balance on 6

7 the metal ion around the concentrate chamber. This is given by the equation where Fj is the volumetric flow rate of stream j and Cj,i is the concentration of species i in stream j. The membrane permeation factor, 7M+, is defined as Substituting for FCIacr088 mcmbronc,m+ from Eq. (2), rearrangement of Eq. (1) yields After the outlet flow rate (Fa) has been calculated, the concentrations of the other species can be calculated with a similar molar component balance. This is done for all of the species except water. The water concentration in the outlet stream is based on the assumption that the specific gravity of the stream is equal to 1.00, since the ion concentration should be very small resulting in little deviation from the density of pure water at 20" C. The values for the amount of oxygen and hydrogen gas produced in the EDIX cell are calculated from the following equations, respectively H2 (e) sec N = 0.50 F1C1,i37i i=l where q is the oxidation number for species i (e.g., +2 for Ni2+ and -1 for C1-). These values are based on the net sum of the moles of charge passing through the cation permeable membrane (the value given by the summation term in Eq. (4) and (5)) and the stoichiometric ratios of the reactions (5) at the cathode and 40H- * H20 + 4e- at the anode. These reactions result in a net consumption of water equal to 7

8 Table 2: Antoine equation constants for water [8]. Range, "C A (log mm of Hg) B (log mm of Hg-"C) C ("C) 0 to to because two moles of water are split for each mole of net charge that crosses the membrane, all hydroxide in the regenerate chamber will form water, and one-half mole of water will be produced for every mole of hydroxide that reacts at the anode. The oxygen and hydrogen streams are also assumed to be saturated with water vapor. The Antoine equation [8] where p* is the vapor pressure of water in mm of mercury and T is the temperature of the system in degrees Celsius, is used to calculate the vapor pressure of water at the cell temperature. Values for the constants A, B, and C used in the computer program are shown in Table 2. This equation gives the vapor pressure with an accuracy of within 1% and allows simple calculation of the vapor pressure at a given temperature. The total pressure is assumed to be one atmosphere and the partial pressure of the hydrogen or oxygen gas is the total pressure minus the vapor pressure of water. The ideal gas law is then used to estimate molar volumes for the three species. The molar flow rate of water in S8 and S9 is calculated based on the molar flow rate of the gas produced and a ratio of the molar volumes of water to oxygen and water to hydrogen, respectively. After these calculations have been completed, the value for 7water is calculated by forming a balance around the regenerate chamber similar to Eq. (2). This balance gives the molar flow rate of water that crosses the cation permeable membrane, which is divided by the molar flow rate entering the EDIX cell from the rinse tank to give the value for the fraction of water that crosses the membrane in one pass ("later). An overall balance around the entire system (Fig. 2) allows for calculations of the individual molar flow rates for the inlet stream to the concentrate recycle tank (S4). These flow rates are equal to the amount of each species that leaves the system either in the treated water stream from the EDIX cell (S2) or by evaporation from the plating tank (S7). The next step involves choosing a value for the molar flow of hydrogen ion for stream S3. This insures that the stream remains acidic, preventing any possibility of hydroxide precipitates forming in the EDIX cell. The value was selected close to the hydrogen ion molar flow rate of the inlet stream to the plating bath (S6). With this 8

9 assumption, a hydrogen molar balance around the EDM cell allows the calculation of the volumetric flow rate of the EDM recycle inlet stream (S5). Rearranging the balance by solving for Fa produces After this value is known, the molar flow rates for all components in stream S3 are calculated using the molar flow rate balance around the EDM cell (MF)S,i = FlC1,i - F2c2,i + F6c6,i (11) For streams where only the molar flow rate can be calculated, a stream density approximation is used to calculate a volumetric flow rate and molar component concentrations. This approximation involves the use of densities for species that would be in the system if the acids and salts did not disassociate. Mass fractions of each are estimated and the equation for the stream density is This process allows the calculation of volumetric flow rates from the sum of the component mass flow rates, calculated previously for use in the average density equation, and the average density, yielding. mj Fj = - 7 Component concentrations are calculated from the component molar flow rates and the volumetric flow rate. To evaluate a given set of conditions, the input variables are selected and the value for the fraction of each ion that will cross the membrane in one pass are selected. Values for the membrane permeation factor of the cations were estimated to be between 0.5 and 0.8. These cation values were chosen from past experience and the assumption that any value below 0.5 would not be worthwhile and that any value above 0.8 would require too much energy to be considered a reasonable operating condition. The values of gamma for the neutral'compounds (except water) and anions were initially chosen as zero because of the properties of an ideal cation exchange membrane and an assumption that convection of these species was negligible. Results of Initial System Design The input data was based on actual conditions for a nickel plating tank [7]. The inlet nickel ion concentration in the rinse tank varied from 10 mg/l to 150 mg/l 9

10 and the outlet metal concentration from the EDIX cell varied from mg/l. The concentrations of all species are adjusted to the changes in the metal ion concentration in accordance with electroneutrality. Problems in this system design were discovered after running the computer program based on the system model described as appearing in Fig. 2. Figure 3 shows the results of the first runs for the volumetric flow rate of stream S2 versus the metal ion separation factor (7M+). The mathematical equation used is given by Eq. (3). The figure shows that the volumetric flow rate for S2 was much too large to operate the cell under the range of 7 ~ that + were assumed to be reasonable by the previous discussion. In this case, the amount of metal ion remaining was high enough that the outlet stream (S2) effectively had to be diluted with water to reduce the concentration to the chosen value. This caused a large flow of water across the cation exchange membrane from the regenerate chamber to the concentrate chamber (right to left in Fig. 2). This cannot be allowed since a large bulk convection flow in the direction opposite to the migration flow would disrupt the operation of the EDIX system. These runs showed that the only way to meet the EPA effluent guidelines with a realistic volumetric flow rate would be if the cell achieved separation factors greater than 0.95 for the cations. This is not a realistic operating condition, so the value of the outlet metal concentration was changed to a value that would allow for realistic separation factors for the cations while utilizing a flow rate for stream S2 that was close to the flow rate of the inlet stream to the EDIX cell (Sl). The new requirement that the flow rates of streams S1 and S2 be of the same order of magnitude identified another problem. All streams must be kept acidic to avoid metal hydroxide precipitation in the EDIX cell. This necessity caused the flow rate of both recycle streams (S3 and S5) to be extremely high. This is due to the fact that the EDIX concentrate inlet stream (S5) has a relatively low hydrogen ion concentration. A large flow rate is needed to neutralize the hydroxide ions that are produced from the bipolar membrane to counter the metal ions which migrate into the concentrate chamber. Examples of different runs are shown in Table 3. Flow rates for S5 vary from 5 l/s for the smallest of cation separations to /s for the largest fractions. It should be noted that these flow rates can be reduced if some of the anions are assumed to cross the membrane also (i.e., the membrane is not 100% effective). This would happen if the convection forces were large enough to drag the anions across with the solution counter to migration effects. The results for this sort of assumption are shown in Table 4. The table shows that even in the case where 20% of the anions cross the membrane, the flow rates are not reduced enough to be helpful. Since this scenario is counter to the initial goals, this type of design change was not given any further consideration. 10

11 c Metal ion membrane permeation fraction 0 Figure 3: Volumetric flow rate of outlet stream of regenerate chamber (S2) as a function of 7 ~ (CI,Ni~+=150 + mg/l, CZ,Ni~+=2.6 mg/l). 11

12 Table 3: Flow rate the recycle stream entering the EDIX cell (S5) for varying cation separation factors O Table 4: Flow rate the recycle stream entering the EDIX cell (S5) for varying anion separation factors

13 Final EDIX Recovery System Design The volumetric flow rates predicted by the initial model were unacceptable, so alternate designs were considered to alleviate this problem. The final process design is shown in Fig. 4. In this schematic, the outlet recycle stream from the EDIX cell (S3) is set to the concentration of the plating bath inlet stream (S6) and flows directly to the plating bath. A control valve would be used to vary the amount of solution flowing into the plating bath. The rest of the recycle stream flows into the concentrate recycle tank. This allows the inlet stream of the concentrate recycle tank (S4) to be of any concentration necessary. In this manner, the volumetric flow rate of the inlet stream can be run in a feasible operating range while still guaranteeing that the streams remain acidic. In order to keep the flow of water from flowing backwards across the membrane for some cases, the outlet metal ion concentration in stream S2 was changed to a higher concentration. This change would require further treatment of the outlet stream from the EDIX cell before disposal. The new design caused only minor changes in the computational procedure. The flow rate and concentrations of the treated water stream (S2) and the inlet stream to the concentrate recycle tank (S4) are determined the same way. The plating bath inlet stream (S6) is also the same, although now the concentrations are equal to the outlet recycle stream from the EDIX cell (S3). The values for oxygen and hydrogen production, along with the water in these streams (S8 and S9) are also calculated in the same manner. The difference arises from the selection of the volumetric flow rate of outlet stream S3 instead of making an initial assumption of the hydrogen ion molar flow rate in this stream. The flow rate can take on any value with its lower limit being what is needed for the return flow rate to the plating tank (S6). In this case S4 and S5 would be identical. From this value and the concentration of the outlet concentrate stream, a material balance around the EDIX cell allows for calculation of the molar flow rate of the inlet recycle stream (S5). The equation used is (MF)F# = F2C2,i + w s, i - Wl,i (14) The density approximation (Eq. (12)) is again utilized to obtain concentrations and a volumetric flow rate for the inlet recycle stream. Results of Final Design The same input data was used as for the first design. It was found that the design shown in Fig. 4 worked much better than the initial configuration. The results showed that there were feasible operating regions where the volumetric flow rates could be set while still insuring that all streams were at least slightly acidic. Problems with water having to flow the reverse direction across the cation-exchange 13

14 I W 14

15 membrane were still present for most runs, but cases were found where the water flow is in the desired direction or only slightly in the reverse direction. Figure 5 shows the molar flow rate of water across the membrane versus the value of rm+ at different values of the outlet stream metal ion concentration while holding the inlet stream concentration constant. The graph shows that for most cases the molar flow rate of water is in the direction opposite of the desired direction, but that there are values for 'ym+ that will give positive molar flow rates for each system. Each system reaches values in the positive range before the results of the system model yield negative flow rates for stream S4, an impossible operating condition. This graph is described by the equation The slope of the line is C2,HaO Fl Cl,M+ c2,m+ As the inlet concentration becomes smaller (and therefore the molar flow rate) the slope of the line will also become smaller. Since the amount of metal ion that must be removed is less, the amount of water added to this stream is less. Each line on the graph is linear due to the fact that the flow rate of stream S2 varies linearly with respect to the permeation factor. This is shown in Eq. (3). Figure 6 shows the same results for various outlet concentrations of S2 while holding the inlet stream metal concentration constant. It can be seen that the line with the greatest slope on both graphs is for the same set of conditions. Figure 6 shows that its the amount of metal ion removed is reduced, the amount of water needed to achieve the ion concentrations in the regenerate stream (S2) is much less. The slopes of the lines vary due to the changes of the inlet concentration. Conclusions The system model presented here details the feasible operating conditions for the EDIX cell itself. Using a minimum of input variables and easily obtained species data, all of the streams in the EDIX cell recovery system can be determined. Only a minimum of constraints are necessary to provide the system a basis for normal operating conditions. Feasible operating conditions are expected to be limited to those where the outlet metal ion concentration does not require the membrane permeation fraction to be above 0.8 and the volumetric flow rate of the outlet regenerate stream can be kept near the flow rate of the inlet regenerate stream to the EDIX cell. Operating constraints prove that the proposed one pass process is not capable of providing a "cleaned" water stream that is suitable for disposal, as was proposed originally. The outlet water stream (S2) would have to be sent on 15

16 Figure 5: Molar flow rate of water across the cation permeable membrane for various regenerate outlet stream (S2) concentrations (C1,Ni2+=150 mg/l). 16

17 20 c I I 1 System operation impossible - I Metal ion membrane permeation fraction Figure 6: Molar flow rates of water across the cation permeable membrane for various input stream concentrations (CZ,Nii+=2.6 mg/l). 17

18 to further waste treatment, but the amount of ions to be removed will have been reduced greatly. The fact that the concentrations of streams S1 and 52 will have a great effect on the range of conditions possible for use of the EDIX recovery system was also demonstrated. Recommendat ions The design of a dependable operating metal recovery system will require thorough knowledge of the system and component performance on a bench and pilot plant scale. Once feasible operating conditions have been established by application of chemical engineering principles as demonstrated in this study, the following steps are needed: 1. Evaluate both mathematically and experimentally the performance of the major system components, Le., the EDM cell in this case. (a) Develop a mathematical model of the EDIX cell. This is done in order to determine estimates of the values of the membrane permeation fraction of each species (7i) through the cation permeable membrane. This includes the use of approximate cell variables (e.g., free-stream and membrane diffusivities of each species, and cell dimensions) to find the actual direction of bulk flow of the solution across the membrane, as well as 7i. (b) Perform experiments to determine actual ion-permeable membrane performance for typical electroplating bath solutions. Prior to experimentation, the previously mentioned mathematical models need to be developed to both determine particular cell operating conditions to be tested and provide a systematic means for evaluating actual experimental data collected. This will help to avoid expensive, trial and error-type of experiments. 2. Construct working bench scale models of the major system components (i.e., the EDIX cell) and test these using actual rinse baths, determining actual performance. Results can be used to refine the previously developed component system models. 3. Construct a working bench scale model of the complete recovery system and evaluate its performance at operating conditions suggested by the previous mat hematical and experiment a1 results. (a) Identify alternate system configurations. For the EDIX recovery system, the results of this study would suggest a series arrangement of EDIX cells 18

19 may be capable of reaching the stated goals of a clean water stream and a reusable metal ion stream. (b) Determine scale-up factors needed for the design of the pilot scale plant. Since most electroplating operations are done on a small industrial scale, the pilot plant size operation could be the size required for actual operat ions. If each of the above steps identify economically feasible operations, then a dependable metal recovery system will have been developed. On the other hand, a tremendous savings in plant investment may be achieved by identifying overly optimistic recovery systems, which are either technically or economically unfeasible, prior to making a major economic investment. Nomenclature Greek letters Antoine Constant, log (mm of Hg) Antoine Constant, log (mm of Hg)- C Antoine Constant, C Concentration of species i, mol/l Concentration of species i in stream j, mol/l Volumetric flow rate of stream j, l/s Molar flow rate of species i in stream j, mol/s Total number of species Universal gas constant, J/mol-K Temperature, C or K Mass fraction of species k that would be present if it did not disassociate Oxidation number of species i, zero for neutral species 7i 7i+ 7i- p ij Mebrane permeation fraction of species i Mebrane permeation fraction of cation species i Mebrane permeation fraction of anion species i Density of liquid or solid compound, g/ml Average solution density, g/ml 19

20 , Subscripts j i chemical species stream number k refers to chemical species that would be present if compounds did not disassociate into the ionic species References [l] R. Peters, Y. Ku, and B. Bhattacharyya, AIChE Symposium Series, 81 (243), 165 (1985). [2] G. Ryder, Metal Finishing, 85 (7), 23 (1987). [3] K. Cherry, Plating Waste Treatment, Ann Arbor Science Publishers, Ann Arbor, Michigan (1982). [4] M. Marino and M. Banger, Metal Finishing, 84 (la), 802 (1986). [5] T.J. Thompson and C.W. Walton, A Proposed Technique for Recovering Metal from Plating Rinsewater, SBIR Proposal to the EPA, RFP No. D800008M1, by The Lincoln Plating Company, Lincoln, Nebraska (January 1988). [6] D. Pletcher, Industrial Electrochemistry, Chapman and Hall, London, (1984). [7] T.J. Thompson, process data, The Lincoln Plating Company, Lincoln, Nebraska (March 1988). [8] R. Felder and R. Rousseau, Elementary Principles of Chemical Processes, John Wiley and Sons, New York (1978). 20