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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.8 DIFFUSION DIALYSIS
4.8.4 Technology/Equipment Description
4.8.4.1 General
4.8.4.2 Commercially Available Equipment
Diffusion dialysis equipment resembles that of other membrane technologies such as electrodialysis. These units are designed with a stacked membrane configuration. A "membrane stack" consists of a number of thin membrane compartments created by alternating layers of ion-specific membranes and spacers. The spacers are present to permit solutions (i.e., contaminated acid and DI water) to flow between the membrane layers. The membranes and flow spacers are held together by a cell frame and clamping unit, giving the membrane stack the appearance of a plate and frame filter press (ref. 252, Kinetic Recovery Corporation file).
The commercial diffusion dialysis units usually employ co-polymer membranes of polystyrene and divinylben-zene. These units permit the diffusion of highly disassociated acids through the polymer structure while cations (metals) remain behind because of their positive electrical charge. The mass transfer through the membrane was described by Reinhard (ref. 252) as:
jn = K x A x Cn
where, jn is the overall transport rate of solute n,
Cn is the average solute concentration difference between the fluids,
A is the membrane area, and
K is the overall rate constant (dialysis coefficient). The proper average solute (acid) concentration difference is the logarithmic mean of the inlet and outlet differences. The dialysis coefficient is a function of: (1) chemical characteristics of the acid; (2) concentration of the acid; (3) chemical characteristics of the salt; and (4) temperature of liquid (ref. 252).
The temperature of the feed appears to be a significant variable. One vendor indicates that a temperature drop of 3.6 °F (2.0° C) will reduce the recycling rate by approximately 1.5%. The membranes can typically withstand temperatures up to 122°F (50°C). They also indicate that heating of the feed stream may be required for winter operation (ref. Kinetic Recovery Corporation file).
The diffusion dialysis technology is commercially available in a wide range of capacities. Existing systems were identified with inlet acid flows ranging from 100 gpd to 2,000 gpd (ref. 192, 336).
This subsection contains a description of commercially available diffusion dialysis equipment that is manufactured and/or sold by Vendor Survey respondents. This is intended to provide the reader with information and data on a cross section of available equipment. Mention of trade names or commercial products is not intended to constitute endorsement for use.
Kinetic Recovery Corporation markets a series of diffusion dialysis units that have been used in Germany and Sweden for more than two years on the following applications: HNO3/HF (stainless steel); H2SO4/HCl (zinc or aluminum); H2SO4/HNO3 (zinc or aluminum); H2SO4 (iron or zinc); and HCl (iron or zinc). They have also tested a unit at United Technologies on a HNO3/HF (titanium). Each of their systems are custom made and include: tanks for acid and DI water; level controls; membrane stack, internal plumbing and connections to tanks; automatic valves; feed pump; cartridge filter; and control panel. The units are skid mounted and factory assembled. The normal range of capacities is from 30 to 415 gpd of acid feed.