Commercial hardening techniques for ferrous metal components typically involve the steps of heating the components in a furnace or a bath of molten salt, such as a chloride, quenching the components in a bath of another molten salt such as a nitrate, nitrite, or mixture thereof, and thereafter causing the parts to be passed through one or more rinse baths to clean off salts which cling to the parts as a result of the heating and quenching steps. A certain amount of each of the molten salts is transferred by drag-out to the rinse bath, thus requiring occasional replenishment of the molten salts, as well as replacement of the rinse water.
It is desirable to recover the salts from the rinse water and return them to the heat treating baths. However, a number of problems have heretofore prevented such recycling on a straightforward and economical basis. First, it is difficult to justify economically the installation of recovery apparatus unless the quantities of reclaimed salts are very high, and the recovery process can be carried out without shut-down of the heat treating system. Secondly, it has been found that the impurities which typically find their way into heat treating baths tend to build up to unacceptable levels particularly fast when the salts are recovered from the rinse and recycled into the heat treat bath. For example, chloride salts from a heating bath are dragged into the quench bath and thence into the rinse where recycling tends to return them to the quench bath. Recycled chloride impurities quickly reach insolubility levels in the quench, and crystallize out as a sludge which must be removed from time to time. In addition, chloride tends to substantially reduce the quench efficiency of a nitrate/nitrite bath and, thus, removing the chlorides is highly desirable.
Another impurity which will build up quickly as a result of salt recycling is carbonate, a reaction product of decomposition of the salt itself. For example, a sodium nitrate/nitrite salt tends to form Na.sub.2 CO.sub.3, the carbon being taken out of the CO.sub.2 present in the air. The sodium carbonate is quite soluble in the rinse water but only sparingly soluble in the nitrate/nitrite quench bath and, thus forms a precipitate in the bath which impairs quenching efficiency and coats mechanical components in contact with the bath.
It should be understood that both the chloride and carbonate impurities, as well as many other impurities such as hard water ions, are typically present in the heat treating salt baths of prior art systems. However, the levels of concentration of such impurities are kept reasonably and acceptably low for longer periods of use by reason of the fact that impurities as well as desirable salts are continuously lost by drag-out. Lost salt, of course, is replaced by fresh salt which does not contain the impurities. Where salt is recycled, however, undesirable impurities dragged into the rinse and which are soluble in the rinse water are retained in the recycled materials and, after a short time, reach unacceptable concentrations.