Most plating process water is used to
cleanse the surface of the parts after each process bath. To
maintain quality standards, the level of dissolved solids in the
rinse water must be regulated. Fresh water or recycled water is
added to the rinse tank accomplishes this purpose, and the overflow
water is treated to remove pollutants and then is discharged or
reduced in volume and recycled using a vacuum distillation process.
As the metal salts, acids, and bases used in metal finishing are
primarily inorganic compounds, they are ionized in water and could
be removed by contact with ion exchange resins. In a water
deionization process, the resins exchange hydrogen ions (H+) for the
positively charged ions (such as nickel, copper, and sodium) and
hydroxyl ions (OH-) for negatively charged sulfates, chromates and
chlorides. Because the quantity of H+ and OH- ions is balanced, the
result of the ion exchange treatment is relatively pure, neutral
water.
Ion exchange reactions are
stoichiometric and reversible, and in that way they are similar to
other solution phase reactions. For example:
NiSO4
+ Ca(OH)2
= Ni(OH)2
+ CaSO4
In this reaction, the nickel ions of
the nickel sulfate (NiSO4)
are exchanged for the calcium ions of the calcium hydroxide Ca(OH)2
molecule. Similarly, a resin with hydrogen ions available for
exchange will exchange those ions for nickel ions from the
wastewater solution. The reaction can be written as follows:
(R–SO3H)2
+ NiSO4
= (R-SO3)2Ni
+ H2SO4
R indicates the organic portion of
the resin and SO3
is the immobile portion of the ion active group. Two resin sites are
needed for nickel ions with a plus 2 valence (Ni+2).
Trivalent ferric ions would require three resin sites.
As shown, the ion exchange reaction
is reversible. The degree the reaction proceeds to the right will
depend on the resins preference or selectivity for nickel ions
compared with its preference for hydrogen ions. The selectivity of a
resin for a given ion is measured by the selectivity coefficient K,
which in its simplest form for the reaction
R–A+ + B+ =
R–B+ + A+
is expressed as: K = ( concentration
of B+ in resin/concentration of A+ in resin) x
(concentration of A+ in solution/concentration of B+
in solution.)
The selectivity coefficient expresses
the relative distribution of the ions when a resin in the A+ form is
placed in a solution containing B+ ions. Table 1 gives the
selectivity’s of strong acid and strong base ion exchange resins for
various ionic compounds. It should be pointed out that the
selectivity coefficient is not constant but varies with changes in
solution conditions. It does provide a means of determining what to
expect when various ions are involved. As indicated in Table 1,
strong acid resins have a preference for nickel over hydrogen.
Despite this preference, the resin can be converted back to the
hydrogen form by contact with a concentrated solution of sulfuric
acid (H2SO4):
(R–SO3)2Ni
+ H2SO4
–> 2(R-SO3H)
+ NiSO4
This step is known as regeneration.
In general terms, the higher the preference a resin exhibits for a
particular ion, the greater the exchange efficiency in terms of
resin capacity for removal of that ion from the wastewater solution.
Greater preference for a particular ion, however, will result in
increased consumption of chemicals for regeneration.
Resins currently available exhibit a
range of selectivity’s and thus have broad application. As an
example, for a strong acid resin the relative preference for
divalent calcium ions (Ca+2) over divalent copper ions
(Cu+2) is approximately 1.5 to 1. For a heavy metal
selective resin, the preference is reversed and favors copper by a
ratio of 2,300 to 1.
