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Selective ion exchange

Metal in treated
waste water
Concentration
[mg Me/L]
Chromium (Cr) 0,023
Copper (Cu) 0,033
Lead (Pb <0,001
Nickel (Ni) 0,011
Zinc (Zn) 0,288
Silver (Ag) <0,001


pH


6,9 - 7,6
Table 1: Average surface treatment plant effluent concentrations after seletive ion excangers. [Period of six months]

Nowadays, production effluents are effectively controlled by the authorities, who are advised of the decisions made by Paris and Helsinki Commissions. Even more stringent limits can be set locally for each case. Conventional precipitation does not produce an effluent that is pure enough. For this reason modern metal treating facilities are equipped with microfilters or sand filters followed by selective ion exchangers.

Strong ion exchange resins tend to remove all the strong and weak ions, whether this is preferred or not. Selective ion exchange can be used to selectively pick up certain ions by their affinities, typically heavy metals, precious metals or nitrate.

Heavy metals can be separated from high concentrations of univalent cations and also from common divalent cations. Removal can be achieved both from weakly acidic and weakly basic solutions; weak complex forming agents such as phosphates or ammonia do not interfere. Certain strong chelates prefer to form a liquid complex which may be more stable than that which the corresponding resin counterpart could offer.

The WatMan tailor-made Selix-series IX offers an excellent choice to follow the letter of the environmental law. All the materials in contact with the chemicals are made of plastic, so no corrosion will take place in future years. The units are logic-controlled, semi-automatic or volume-based fully automated. The separate valves can be manually overdriven. The construction is very simple in order to avoid time-consuming services, and the only ongoing service is to add chemicals. To guarantee the best possible spare parts service, components with an excellent reputation have been chosen.

The best macroreticular chelating resins with standard bead size distribution, advanced kinetics of ion exchange and outstanding mechanical stability are used. The units are designed to purify the effluent flawlessly. This also applies to maximum flows. The highest possible purification result can be obtained when installing two units in series. In each case one, two or three units can be installed in one system.

The selective extraction of heavy metals typically follows the affinity series: copper > lead > nickel > zinc > cadmium > iron > calcium > sodium

Depending on the nature of the matrix, the most favourable copper at the left might have a K value of several thousands, meaning nearly absolute selectivity over calcium and sodium. Solutions with a matrix of strong complexing agents can be treated with special resins. Low regenerant costs as well as low investment costs are achieved due to the high operating capacity of the resins. Low effluent heavy metal leakage level can still be measured after 1000-2000 bed volumes of treated water.

 
 
 
 
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