Good practice will allow indefinite re-use of the chrome liquor.

It is not the purpose of this paper to give detailed practical recommendations for chrome liquor recycling but rather to present the important principles involved.

Freeze concentration was found to be impractical at present as a method of concentrating chrome liquors.

The action of neutral salts upon chrome liquors.

Time factor in the adsorption of the constituents of chrome liquor by hide substance.

A number of other observations (37) have been made during introduction of the chrome liquor recycling process into industry:

Titration of chrome liquors by the conductance method.

Four different methods of direct recycling chrome liquors have been wet-blue tannery processing 10,000 hides per week.

Rapid estimation of chromium in chrome liquor by use of the immersion refractometer.

The acid titration of chrome liquors.

Estimated weekly operating costs of four different methods for recycling chrome liquors in Melbourne and Geelong.

Equilibria between tetrachrome collagen and chrome liquors.

The adsorption of chrome from chrome liquors by hide substance and negative adsorption.

Further work is continuing on improving chromium fixation but where, despite the above precautions, an excess of chrome liquor is accumulated it can be either:

Direct chrome liquor recycling3 reduces the use of both sodium chloride and chrome powder, which contains up to 30% sodium sulphate.

After such an assessment by Australian tanneries, direct chrome liquor recycling with precipitation of excess chromium containing liquors is usually preferred to high-exhaust systems.

Chrome precipitation from the total volume of spent chrome liquors results in far higher TDS levels and should not be used when salinity is a concern.

After conventional pickling, the spent liquor was discarded and the pickled stock was tanned with spent chrome liquor from the previous run after reconstitution.

Figures in brackets ( ) are achieved with efficient chrome liquor recycling Large numbers of Australian sheepskins are air dried in open sheds using solar and wind energy.

First, the feasibility of chrome liquor recycling was tested using a variety of techniques for the separation and characterization of the chromium complexes present in tanning liquors (1,12).

Before discussing some of the practical aspects of the process in the tannery, it is worth considering the characteristics of a recycled chrome liquor as they illustrate clearly the scientific rationale involved.

It is an important principle of this method that, by addition of the pickle acid before presentation of the recycled chrome liquor to the delimed/bated hide, the low pH (ca.

Concentration using membrane technology was not viable for concentration of chrome liquors as the liquors are very dirty, containing large amounts of fats and oils, protein, dirt, hair and flesh.

Before the start of this work, the possibility of chrome liquor recycling had been mentioned in the literature (12,40) and recycling had apparently been practiced commercially to a limited extent in the past.

In Australia, there was some initial resistance by the industry to full-scale implementation of chrome liquor recycling but this was gradually overcome by showing that leather quality is unimpaired when the system is used correctly.