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QUESTIONS ON ION EXCHANGE

Some questions about our ion exchange systems reoccur with such regularity that we have written this short question and answer brief to cover the most commonly asked questions.

1. What is the level of compliance I can expect?
2. What is the difference between deionization and metal recovery ion exchange.
3. How does Ion Exchange work?
4. I have a Total Dissolved Solids(TDS) limit, what does ion exchange do to my TDS?
5. How much floor space does an ion exchange system occupy?.
6. How much does it cost to run an ion exchange system?
7. How much regenerant is used and can I reuse my regenerant?
8. How much water is used during a regeneration cycle? .
9. Where does the water used for rinsing go?
10. How much floor space will it take up?
11. What do I do with the regenerant? .
12. What is the difference between elements?
13. What types of resins are there?
14. What are Chelated Resins?.
15. How long does a regeneration take?
16. Do I have to shut down for regeneration?.
17. Do I have to pH adjust before the ion exchange system?
18. How automatic is automatic?.
19. What is leakage?
20. What is breakthrough?.
21. Do I have to segregate effluent lines?
22. What types of problems will I have?.
23. What do I do with the regenerant?
24. What is electrowinning?.
25. Should I evaporate to concentrate regenerant?
26. Should I evaporate to go to Zero Discharge?.
27. Should I use dragouts to reduce metal concentrations in the rinses?.
28. What kind of filtration is necessary?
29. What does a sand/media filter do?.
30. What does a carbon filter do?
31. How long will my resin last?.
32. How often will I have to haul resin?
33. How big of a system will I need?


Q. What is the level of compliance I can expect?

A. With most divalent ions (Copper, Lead, Nickel, etc) the effluent will usually contain less than 0.5 ppm (usually between .05 and .2 ppm). Trivalent ions such as Chrome and Iron are lower. The key to reducing effluent metal levels is having the correct sizing of the resin bed for the flow rate. We use 1 gallon per minute per cubic foot of resin with a minimum of a 30 inch bed. With two columns in line, the leakage level is very low.

Q. What is the difference between deionization and metal recovery ion exchange.

A. Deionization removes all of the ions from the water, leaving only non-ionic materials in solution. Deionizing is best for incoming water of low TDS. In California where TDS is sometimes very high, an RO (reverse osmosis) system is usually used first to reduce the TDS to very low levels followed by the DI system as a polisher. Treating waste water is more complicated and DI systems leave most non-ionic organics in solution.

Metal recovery ion exchange targets only the ions that are required to be removed from the waste stream. The rest, including Sodium, sulfate, chloride, phosphate, etc, are not removed. Unless you wish to recycle water, it doesn't make much sense to remove all of the ions that are not regulated. Chelated resins have a "preference" for heavy metals and pass Calcium and Magnesium (the hardness ions) which a DI type system will take out. The main difference between the two systems is with a DI systems, you regenerate a lot, treat a lot of regenerant with low metal levels, use a lot of chemicals and reuse the water.
A metal recover IX system has a low regeneration frequency based only on the metal captured, not otherions, creates about 1/3 as much regenerant per cu. ft of resin regenerated and up to 1/10 as much regenerant per pound of metal, but you cannot reuse the water unless you use a following RO (still cheaper than DI overall).

Q. How does Ion Exchange work?

A. The best analogy I have found is comparing the resin bead to a ball of yarn. The fuzz on the yarn strands are the exchange sites. Like the yarn, the resin bead is a long string (molecule) wrapped around itself. The exchange sites are where ions such as H+ or Na+ are exchanged for the heavy metals. The resins prefer the metals over H+ or Sodium and readily take up the metal and release the H+ or Na+. When the resin is freshly regenerated, many surface sites are available and the rate of exchange is high. As the resin sites fill up, the availability of sites decrease and the rate of exchange slows. This is why, at a constant concentration and flow rate, the leakage increases during a run. When the only sites available are deep inside the beads, the rate of exchange is slow and more metal leaks through.

Q. I have a Total Dissolved Solids(TDS) limit, what does ion exchange do to my TDS?

A. In theory, chelated resin systems does not change the TDS in the effluent stream. The heavy metal is exchanged for Sodium so the TDS remains constant. Some increase in TDS is apparent due to pH and/or ORP adjustment in the conditioning stage. During regeneration, some excess caustic is added which will increase the total TDS slightly. This is still much less of an increase than is used in a precipitation system. The total TDS increase in a precipitation system can be 2 to 3 times the TDS in the actual effluent stream.

In a deionizing system where both cation and anions are removed, the TDS in the effluent stream is lowered during normal operation, but due to the high levels of regenerant required, the total TDS loading is higher than with a Chelate resin system for just metal recovery.

Q. How much floor space does an ion exchange system occupy?

A. Integrated chelated resin systems are compact, a 25 gpm system will occupy an area 8' x 12'. Only one storage tank is needed and that is mounted on the skid. DI type systems are more compact but require more overall floor space due to the waste regenerant storage. At least two large tanks are required for dilute acid and dilute caustic storage. In addition, the regenerant plateout cell, if used, will require more space due to the 3x volume of regenerant per pound of metal recovered.

Q. How much does it cost to run an ion exchange system?

A. The cost to operate is a total of three variables. These are, power, chemistry (regenerant and pH adjust) and labor.

Power Costs-

The power requirement is about 1.5 hp for 15 gpm up to 3 hp for 40 gpm. This is constant while the system is running. A 3 hp, 3 ph motor costs about 5 cents per hour to operate (or less). Power costs compare favorably with precipitation systems and are much lower than systems using membranes or evaporation.

Chemistry Costs-

Chelated resins for Copper and Nickel recovery.

The chemistry costs for a chelate resin is about 1 gallon of sulfuric per cubic foot of resin and one gallon of 50% caustic per cubic foot of resin per regeneration. Approximately 70% of the sulfuric can be recovered per regeneration. This is a net cost of .3 gallons of sulfuric and 1 gallon of 50 % caustic. This is per 1.9 pounds of Copper recovered. The cost of pH adjusting is low as the systems run at a pH between 3.5 and 4.5. pH is increased as the effluent passes through the column.

DI resin systems.

The chemistry cost of DI type systems is higher as it is difficult to reuse the regenerants due to the huge volume of regenerant used. It takes approximately 3 gallons of sulfuric per 1.9 pounds of Copper recovered plus an equivalent amount of Caustic to regenerate the anion resin.

Labor costs-

Labor costs for the system is usually very low due to the automated nature of the systems. If the proper segregation is used, little or no maintenance is required between regenerations except for chemical maintenance for the pH adjust system. Calibration of pH and ORP probes is a weekly project. Cleaning is a simple backwash procedure taking 5 minutes per column or filter and is usually done once per day. Batch treatment costs may also be lowered because some solutions with low metal levels may be bled directly into the system. Usually 2 gallons of sulfuric to regenerate costs less than an hour or two of labor to batch treat.

Q. How much regenerant is used and can I reuse my regenerant?

A. For a Copper or Nickel system, about 1 gallon of 93% sulfuric acid is used per cubic foot of resin. This is diluted to a 15% solution so a total of 11 gallons of regenerant is used per cubic foot (15 lb acid, 85 lb water). Our unique process also leaches the resin after the regeneration and we us about 2.5 gallons of leach water per cubic foot of resin also. This is added to the regenerant as it is high in acid and metal. We increase the volume of the regenerant by about 20% each cycle so with some acid adjustment, about 75-80% of the regenerant can be reused the next regeneration (if you plate the metal out in between cycles.)

We do not use an eductor to add regenerant to the system, we have a batch tank to hold the correct solution and recirculate it through the system. This method reduces the volume of regenerant generated.

Q. How much water is used during a regeneration cycle?

A. Standard DI systems can use up to 135 gallons per cubic foot of resin during the regeneration cycle. A 40 gpm system regenerates 20 cubic feet each cycle. This would generate 2700 gallons of rinse water each cycle. Remco Engineering systems use a much different rinsing scheme. By using a programmable controller for valve control, we can create a much more effective rinsing cycle. We use a fill-leach-fill-leach cycle to reduce water use. The resin beads are small porous spheres which transfer chemicals from the inside to the outside at a slow rate. Running water continuously over the beads doesn't really effect the transfer rate and uses a lot of water. We fill the column to cover the beads, hold till we are close to equilibrium, then blow the water out of the tank. We do this several times until the beads quit leaching chemicals. We generally use about 30-40 gallons per cubic foot of resin or 600-800 gallons for a 20 cubic foot regeneration.

Q. Where does the water used for rinsing go?

A. In our chelated resin systems, regenerant rinse water is returned to the main holding tank for pH adjust and for processing through the good column. We oversize our metering pumps so we can rapidly adjust pH during the regeneration cycle. We also use a special program which both extends the regeneration time and places time delays during critical portions of the rinse cycle to allow for pH adjustment.

DI type systems either dump the water to drain or require large tanks to hold the water for pH adjust and possibly further processing. Even anion regenerant can have chelated metal in it and may require treatment.

Q. How much floor space will it take up?

A. Compact skid mounted integrated systems are designed for small areas. A 15 gpm system with a 30 minute buffer tank takes an area of about 5' x 8' plus 3' of access. A 25 gpm system is about 8' x 12' including a plate out cell. On a gpm per square foot of floor or pound of metal recovered per gpm/sq.ft, ion exchange is the most cost effective waste treatment method.

Q. What do I do with the regenerant?

A. Regenerants with heavy metals such as Copper, Nickel, Cadmium, or Lead are usually electrowinned (plated out) or precipitated and sludged. Anion regenerants from DI systems are usually chemically treated and sludged. Cyanide effluents can be both electrowinned and chemically treated to destroy the cyanide. Hexavalent Chrome is chemically reduced and precipitated. Other treatment technologies are possible including R.O. concentration and evaporation.

Q. How much metal is recovered in the regenerant?

A. Chelated resin systems in actual operation have a capacity of about 1.9 pounds of Copper or 1.8 pounds of Nickel per cubic foot of resin at operational pH. We have a system running on an alkaline etcher rinse which may reach 20 pounds of copper per cu. ft. Capacity of other metals vary with their atomic weight. In DI systems, strong cation resins can hold about 0.7 times as much metal as the equivalent volume of chelated resin. Anion resins have about half of the capacity of cation resins. The observed capacity may differ due to several factors including high levels of TDS, complexing agents, incomplete regeneration, or an incomplete conversion to the sodium form during the last stage of regeneration.

Q. What types of resins are there?

A. There are many types of resins used for ion exchange purposes including many specialty resins used in the lab for specific separations. There are three parameters which differentiate resins, they are:
1. Base polymer
2. Functional group (reactive site)
3. Size (standard vs. fine)

Base Polymer - There are generally two types of resin bases which are reacted to make ion exchange polymers. These are styrene-divinylbenzene and acrylic backbones. Generally the styrene-divinylbenzene polymers are the backbone for chelated resins and standard DI resins.

Functional group - The base polymer is reacted to form a functional group along the polymer. This functional group determines whether a resin exchanges cations or anions, is strong or a weak resin. Resins reacted to for a SO3= group are strong cation resins, a carbonate group results in a weak cation resin, an iminodiacetic acid, or aminophosphate reaction results in chelated cation resins. Anion resins are formed by adding an amine to the resin (notice the fishy amine smell on startup). Various amines form strong and weak versions of the resins.

Cation resins are resins that exchange Cations, such as Nickel, Copper, Lead, Cadmium, Chrome+3, and water hardness ions (Calcium and Magnesium). Anion resins exchange anions such as sulfate, formate, Chloride, hexavalent Chrome, cyanide, etc. Together a strong cation resin and a strong anion will deionize the water passing through them.

Strong resins hold the exchanged ion tightly and require a high concentration of regenerant, about 3 times more than the theoretical equivalent. This is true for both anion and cation resins. They are mainly used in deionizing systems which require very pure water. They show little selectivity and remove all ions that pass through them. If you are not interested in DI water as a result, these resins are costly to operate for just waste treatment and metal recovery.

Weak resins do not hold the exchange ions tightly and are easily regenerated with a slight excess of regenerant. They generally have a lower pH range with cation resins being active over a pH of 6 and anion resins being active at a ph under 8. Weak cation resins are used in the sodium form to soften water, exchanging the sodium for Calcium and Magnesium. A saturate brine solution regenerates the resin. The pH limits for these resins limit their applications (except for chelated resins) in waste water systems.

Q. What are Chelated Resins?

A. Chelated resins are a special catagory of weak cation resins that are very useful in certain heavy metal recovery application. Their pH range is lower for many heavy metals and they are selective. The advantage of low pH range is that the metals do not precipitate as oxides or hydroxides while you try to grab them. Precipitated metals are not removed by ion exchange systems, the metals must be dissolved. Selectivity is a very important feature of chelated resins because it is possible to use them with ordinary city water effluents and almost exclusively strip out heavy metals. While strong cation resins prefer Calcium and Magnesium over Copper, Lead, Cadmium, etc, the chelated resins reverse the order and pass the Calcium and Magnesium at low pH and pick up the heavy metals.

Q. How long does a regeneration take?

A. Depending on the size of the system, from 1.5 to 5 hours.

Q. Do I have to shut down for regeneration?

A. No. Regeneration is automatic and the system isolates the column to be regenerated and runs the effluent through the other column. If you let both columns breakthrough, you will have to shut down but somebody was asleep to let both columns breakthrough. I'll bet you won't let it happen again.

Q. Do I have to pH adjust before the ion exchange system?

A. No, unless your stream is very low in pH and the hydraulic capacity of the system is very close to your flow rate. We use in-line pH sensors and large metering pumps to insure rapid on-line pH adjustments but very low pH effluent (less than 2.0) require some pH adjust time and if there is no excess hydraulic capacity, you must pre-adjust pH.

Q. How automatic is automatic?

A. Full automatic controls are available including sensors to determine the level in the dosing tanks, no flow alarms, regenerant sensing, and breakthrough detection. Full data monitoring, storage and analysis is available as are network interfaces.

Q. What is leakage?

A. Resins are not 100% efficient and as the concentration of ions gets low, some can slip past the resin and "leak". Leakage is constant until breakthrough. Usually the leakage is much lower than 0.5 ppm until the column is almost spent.

Q. What is breakthrough?

A. When the resin column becomes saturated with ions and the capacity is almost all used up, leakage increases until the concentration of ions in equals the concentration out. When the leakage starts to increase rapidly, indicating the end of the useful life of the column, breakthrough is said to occur.

Q. Do I have to segregate effluent lines?

A. Maybe. Resins systems vary with their tolerances but as a general rule, some segregation will make your life easier. The key thing to watch for is streams with high particulate levels that may cause rapid plugging of the filter, organic solvents that may attack the elastomer or piping parts of the system, or in the case of DI type systems, any organics that may attack the anion resin. For clear metal streams that do not settle out when you let it stand overnight, no segregation is probably needed. We will evaluate each stream in you facility and advise you how it should be treated.

Q. What types of problems will I have?

A. The main problem we see is plugging. Usually some one dumps something in a rinse tank to get rid of it and it sludges out and plugs up the sand filter. Sometimes the pumps will clog instead. The kinds of thing that have caused problems for our customers are:
Cement Sand (from an unprotected sump)
Photoresist waste (organic goo)
Screen ink waste (more organic goo)
Mop head strings (dumping floor waste through the system)
Pencil ends, nuts and bolts in pump
Precipitated Tin Oxide (white sludge)

Q. What do I do with the regenerant?

A. There are several choices, evaporation, precipitation, electrowinning (metals), hauling and/or combining with other recycle materials. The proper choice depends on the frequence of regeneration and the size of the batch. Recovery may not be econominical for very small facilites, electrowinning or evaporation may not be practical or the best choice in some applications. In certain applications, the regenerant may be in a useable form which allows close to 100% recycling.

Q. What is electrowinning?

A. Electrowinning is plating the metal onto a cathode (plate) using a direct current rectifier. Metals such as Copper, Nickel and Cadmium can be recovered by introducing a direct current between and anode (positive charge) and a cathode (negative charge). Anodes and cathodes are usually flat plates. Anode materials vary with the solution chemistry. Cathode materials are usually stainless steel or Copper. More on electrowinning

Q. Should I evaporate to concentrate regenerant?

A. Evaporation is an expensive way to concentrate solutions unless there is a lot of waste heat available. Regenerants can usually be plated out much faster than they can be evaporated. Evaporation is best for special applications where the recovered salt has a very high value or can be directly reused.

Q. Should I evaporate to go to Zero Discharge?

A. The only way to truly get to zero discharge is to evaporate the DI regenerant and/or RO reject from a recycling system. The operating cost of this is usually quite high and the benefit questionable unless you have very special circumstances such as no POTW and a ground discharge. The only easy call for evaporation is if your discharge requirement is so low and your water allocation so small that you cannot operate and stay in compliance.

Q. Should I use dragouts to reduce metal concentrations in the rinses?

A. Dragouts reduce the metal levels in the following rinse, but unless the solution you are concentrating is valuable or can be added back into the process tank directly, a dragout is not necessary for an ion exchange system. A dragout usually does not get concentrated enough to electrowinn directly so you would probably bleed it into the ion exchange system with the rinsewater. The only other reason for a dragout when using ion exchange to capture metals is if you have a strict water conservation program and wish the following rinse to remain very clean with low water flow.

Q.What kind of filtration is necessary?

A. Ion exchange systems remove ions in solution. Anything not in solution and anything in solution that is not ionic (alcohols such a isopropyl and glycols, surfactants, etc.) is not removed. Particulate materials such as flux residues, solder balls, photoresist chips, pencils, etc need to be removed before the ion exchange system. Many types of filtration have been tried and many have failed. Cartridge and other fixed media filters that you change out and dispose of tend to have very short lives in waste water treatment application. These generally require much maintenance and few still survive on existing systems. A media or sand filter is easily cleaned and there are no consumables to change.

Q.What does a sand/media filter do?

A. We use a sand type filter to remove particulates that may clog the resin bed. The sand filter is easier to clean up than the resin bed. Resin beads have very small holes which are difficult to clean if fine particulates enter the bed. Cleaning a sand filter is much easier than cleaning a resin bed.

Q.What does a carbon filter do?

A. Carbon is derived from either wood or coal. The starting material is heated in an inert atmosphere to convert it to pure carbon. The way it is processed results in a very high surface area and an ability to adsorb various material. The capacity for both organic and inorganic materials is high but carbon is usually used for adsorbing organics. Each organic and each type of carbon have a different adsorption capacity.
Carbon is used to adsorb organics, chromates, sulfides and chlorine. It is used to prevent strong oxidizing materials, such as peroxides, nitric acid, and chlorine, from attacking ion exchange resins and it is used as a general adsorbent in recycling systems.

Q.How long will my resin last?

A. DI resins usually many years. Some of the resin beads break during the swell cycle when regenerating. The life is partially dependent on the number of regenerations and partially on the quantity of oxidizers passed through the column.

Q.How often will I have to haul resin?

A. Only if you totally destroy the resin should you change it out and haul it away. Usually a little bit is lost each regeneration (due to breakage) and each year you will top up the columns with 10% or so. The resin should last a long time if the system is run correctly. Following the manufacturers segregation suggestions will help increase resin life.

Q. How big of a system will I need?

A. Systems are sized on flow and recovery requirements. When you provide flow data for each type of rinse stream with a general analysis of the level of contamination, it is relatively easy to calculate the size of the system required. If you are presently running a 50 gpm sludging system, your probably can get by with a 20-25 gpm metal recovery system. Some examples are shown below.

Example 1: a system requiring a recovery of 20 pounds per day of copper and having a segregated rinse flow rate of 22 gpm peak for 4 hours maximum.
Using a chelated resin at 1 gpm per cubic foot of resin would be 22 cu. ft. of resin nominal which would be rounded up to a 25 gpm system (with only 4 hours peak flow, a 20 gpm system may be enough).

The resin would be in 2 columns of 12.5 cu. feet each. Each column would hold 1.9 pounds of copper per cu.ft or 23.75 pounds per regeneration. You would have to regenerate once per day. A 20 cubic foot system would be to small. In this case, you would look for the source of copper and reduce it if possible.

Example 2: A small facility has a nickel sulfamate plating bath. Flow peaks at 4 gpm for no more than 5 minutes, twice an hour. Average flow is 120 gallons per hour. Nickel levels range up to 20 ppm maximum. What size system would be required?
A 2 gpm system would be enough. Using the maximum Nickel concentration times the average flow, we get 2.4 grams of nickel per hour. On a 8 hour shift, about 19.2 grams. One shift per day, is 134 grams per week. About three weeks to get a pound and for a one cubic foot chelate resin column, it would take about 5 and one half weeks for breakthrough.