Palladium Reclamation

Q: We’re looking into a wastewater reclamation system for rinse waters from our palladium plating process. We are using the palladosamine chloride bath with a palladium concentration of about 7 g/L and operating the bath at 40°C (104°F). After the plating bath, we have two counterflow rinses flowing at 6 gpm, and the first counterflow rinse right after plating has a concentration of 10 mg/L. What is the best way to treat this rinse water to reclaim the palladium? I’ve heard reclaiming palladium can be costly. Is it even worthwhile to treat this rinse water? N.Y.

A: I’m not familiar with the palladium plating process, so I cannot tell you up front whether or not reclaiming it is “worthwhile.” Palladium is very expensive at over $300/oz, so evaluating its recovery is surely worthwhile.

Before investing in expensive technology, using a series of counterflow recovery tanks after the plating tank can recover a very high percentage of your dragout. Based upon the information you provided, we can estimate your dragout rate by the equation D = (Cr × Qr)/Cp, where D is dragout rate (gph), Cr is rinse concentration (mg/L), Qr is rinse flow rate (gph) and Cp is plating solution concentration (mg/L).

Plugging in your numbers, we have D = (10 mg/L × 360 gph)/7,000 mg/L = 0.51 gph, say 0.5 gph. With this dragout rate, a bath palladium concentration of 7,000 mg/L (0.85 oz/gal), and assuming palladium at $300/ounce, this dragout represents a potential loss of about $130/hour.

Assuming that this 6-gpm rinse water flow rate provides the desirable rinse water quality in the second and final rinse tank, we can estimate the palladium concentration in this tank as follows: C2 = (C1 × D)/Qr, where C1 and C2 are the concentrations of the first and second rinses. So C2 = (10 mg/L × 0.5 gph)/360 gph = 0.014 mg/L. As expected, this final rinse is very high--quality.

Due to your relatively low dragout rate and assuming you have room, you can create a closed loop reclamation system by using the power of “N”—that is, by adding additional counterflow rinse tanks. To estimate the amount of counterflow rinse water needed after a process tank we use the following equation: Cn = Cp/(1 + R + R2 + R3... +Rn) where Cn is the concentration in the last rinse tank and R is the rinse ratio—that is, counterflow rinse water flow rate Qr/D, the dragout rate.

Since we know that Cn = 0.014 mg/L and Cp = 7,000 mg/L, through trial and error, we can solve for R knowing the number of counterflow rinses. By increasing the number of counterflow rinses from two to four, our needed R value decreases from 720 (360 gph/0.5 gph) to 28 and our needed flow rate decreases from 360 gph to 14 gph! Hence the power of “N.”

Due to your bath’s relatively low temperature, you surely do not have nearly enough evaporation to make room for 14 gph of rinse water back into the process tank, and the alkaline nature of your bath gives me pause in recommending an atmospheric evaporator on your process tank due to possible scale buildup as the bath’s alkalinity reacts with carbon dioxide in the air. However, reverse osmosis (RO) has proved successful in palladium recovery.

RO separates water from larger-molecular-weight compounds by forcing the waste stream, under high pressure, against a semi-permeable membrane. RO splits the flow into two streams, and “clean” permeate, and has the added advantage of recovering all your bath constituents, not just the precious metal.

I propose the following reclamation scheme: To increase recovery and overcome the low evaporation rate of the process tank, install a dragin/dragout tank in front of the palladium plating tank and install four counterflow rinse recovery tanks after palladium plating. The RO unit will be fed from the dragin/dragout tank, its permeate will flow to the last counterflow rinse recovery tank while the concentrate goes back to the dragin/dragout tank. The permeate will then counterflow through the recovery rinses to the dragin/dragout tank.

In this way, the palladium and its bath constituents are recovered and concentrated in the dragin/dragout tank. As parts enter and leave this tank, they drag in the palladium and bath constituents back into the bath at a rate about equal to dragout rate, thus helping to overcome low evaporation rate. It’s likely you may need to heat the dragin/dragout tank to increase evaporation or decant recovered solution into a heated off-line tank to evaporate water and concentrate solution before adding back to process tank.

Another option is to add a fifth counterflow rinse and reduce rinse water flow to 7 gph. The bottom line is that the evaporation rate of your plating tank needs to be equal to the RO concentrate rate.

Whatever reclamation system you decide to implement, make sure you use high-quality makeup water in the system. Numerous recovery/recycling projects have failed due to lack of attention paid to incoming water supply. Since many recovery technologies, such as RO, recover all chemicals in the rinses, contaminants found in “tap water” can build up to levels that cause failure of bath chemistry and result in expensive bath restoration or disposal and makeup.

The two most common technologies used for treatment of “tap” water are ion exchange and reverse osmosis. In the ion exchange process, columns are packed with resin beads which provide a large surface for the cation and anion sites. Cationic sites exchange hydrogen ions (H+) for positive charged ions such as calcium, magnesium, iron, zinc, sodium, and other metals; anionic resins exchange hydroxyl ions (OH-) for negatively charged sulfates, phosphates, and chlorides. If you decide to go the ion exchange route and don’t need additional high-quality water elsewhere in your facility, it likely makes more economic and safety sense to contract with a supplier of exchangeable ion exchange columns that are regenerated off-site rather than regenerating ion exchange columns on-site.