Article continued (page 2): Modern Treatment Methods of Strong Chelates in Surface Technology

DTC (Dimethyl Dithiocarbamate)

Dimethyl Dithiocarbamate - also known as DTC has been very successfully in applications. The DTC is used in the presence of all chelating agents, including those with tartrate, quadrol, EDTA etc. DTC will react with a number of metals, forming an insoluble metal / DTC compound which will precipitate as sludge. The concentration of the chelated material has a significant impact on the dosage rate. The disadvantage of DTC treatment is that it cannot be safely adjusted to treat fluctuations in feed metal concentrations. An advantage is that DTC produces less sludge than the Iron process. This treatment route can be used where the metal concentrations are relatively low and where there is adequate equalization and full process segregation. For treatment of concentrated solutions such as electroless nickel and copper bail out solutions the cost of treatment may be prohibitive.

Conventional Treatment of Electroless Nickel

With the classic lime precipitation, a large amount of nickel as well as phosphite can be separated. Depending on the concentration of the chelates, a significant amount of nickel still remains in the solution. The concentration of the chelates can rarely be reduced through precipitation therefore the resulting wastewater filtrate still contains large amounts of nickel and chelates. Hypophosphite does not build a low solubility calcium salt like phosphate and it also cannot be separated with lime. There have been many attempts to change hypophosphite and phosphite to phosphate. Some applications use calcium hypochlorite in excess to achieve oxidation and precipitation. Permanganate has also been suggested and tried. The use of calcium hypochlorite, not only creates unwanted AOX, but also causes a strong increase in salt load. This causes subsequent problems with flocculation and filtration, sludge generation can also be significant. In practice, the disposal of rinse waters and used baths is often undertaken by special waste disposal companies.

Decomplex Treatment – Advanced Oxidation

EDTA and other organic chelates are widely employed in electroless plating processes used in the PCB and metal finishing industries. The main concern about these compounds is their poor biodegradability. Chelates can still be present as much as 15 years after their disposal, with EDTA being among the most persistent. Restrictive legislation on the discharge of chelating agents and general organics is becoming ever more stringent. The legislation is especially restrictive in Germany, where Annex 40 of the wastewater legislation only allows EDTA based processes to function where the EDTA is either recycled or completely destroyed.

Kurion has addressed the issues of treating chelated waste solutions with the application of advanced oxidation. The Kurion ACK process uses hydrogen peroxide in combination with UV to produce hydroxyl radicals that subsequently oxidise species such as EDTA to a level where after treatment the wastewater can be simply treated via hydroxide precipitation. In addition, the process also eliminates other sources of COD such as formaldehyde etc.

The wastewater is treated in a batch tank from where it is recirculated through a UV reactor system. The key of success is the use of a performance regulated UV-reactor and application of subsequent process steps. After completing approximately 16 treatment steps, the wastewater is free of EDTA, the COD is significantly decreased and the wastewater can now be treated with conventional treatment techniques. Advanced oxidation yields a waste suitable for discharge that meets future legislation and greatly reduces operating costs compared with off site haulage or DTC treatment.

Table 2 Treatment of CuEDTA. Comparison of sulfide – DTC Precipitation and UV-Oxidation/Precipitation

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This page was last reviewed 06.06.08