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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.5 ACID SORPTION
4.5.3 Applications and Restrictions
Exhibit 4-14 shows the basic configuration for acid purification using acid sorption. The process is typically performed on a continuous basis, although the same configuration could be used for batch treatment. The process is usually not performed on a batch basis because it is relatively time consuming to perform and would require that the process bath be inactive for days. For example, to purify a 2,600 gallon sulfuric acid anodizing bath from 15 g/l to 8 g/l Al would take approximately 200 hours using a resin bed with a volume of 0.42 ft3 (12 liters) (ref. Eco-Tec file).
The acid sorption technology is applicable to a range of acids and applications, including the following (ref. Eco-Tec file):
- Aluminum anodizing, using sulfuric acid
- Steel and galvanized steel pickling, using sulfuric or hydrochloric acid
- Copper and brass pickling, etching and brightening, using sulfuric or nitric acid
- Hydrometallurgical ore leach circuits
- Stainless steel and titanium pickling using nitric and hydrofluoric acids
- Electroplating rack or rework stripping, using nitric acid
- Aluminum bright dipping or electropolishing, using phosphoric and nitric acids or phosphoric and sulfuric acids
- Stainless steel electropolishing, using phosphoric and/or sulfuric acids
- Spent regenerants from acid cycle cation exchangers
- Continuously removing acid from electrowinning solutions to prevent acid interference.
Five respondents to the Users Survey indicated that they use or have used acid sorption. Their responses included the following applications: two applications for the purification of sulfuric acid anodizing solutions; one application to a hard anodize solution and two applications to hydrochloric acid pickle baths.
The five shops that use acid sorption represent 1.6% of the total number of shops responding to the Users Survey. It is interesting to note that approximately 200 acid sorption units have been sold by the only known North American manufacturer of this technology. Assuming that each of their units were sold to a different plating shop in North America, then approximately 1.5% of all North American shops have purchased this technology (based on combined EPA and Environment Canada estimates of 13,640 shops).
There are several important restrictions for acid sorption. First, the process cannot be applied to highly concentrated acids. For example, the concentration limit for nitric acid is 35% (by weight). Second, acids containing chromates should not be purified with this process. Chromates, as anions, will consume resin sites and prevent acid recovery. Third, hydrochloric acid solutions containing zinc and lead should not be purified using acid sorption. The zinc and lead form a strong chloride complex that preferably attach to the resin and prevent the sorption of acid. In these cases, it is possible to remove the zinc and lead from the acid using a process where the contaminated acid solution is passed through the resin bed two times. During the first pass, the zinc and lead chloride complex attach to the resin and are subsequently eluted with water. Then the acid would be passed through a second time to remove the acid and again be eluted with water. The key problem with this procedure is that the quantity of water needed to elude the zinc and lead chloride complex is approximately ten times greater than the quantity needed for a normal acid removal step, and therefore a large volume of metal bearing wastewater is generated.