Options for cleaner chromium tanning
Chrome tanning has been a major focus for cleaner production and various systems have been developed:

High exhaust systems
Require good control. Suitable for some tanneries. Effluent needs treatment.

Direct chrome liquor recycling
Simple. Suitable for some small and larger tanneries. Reduces TDS.

Direct chrome liquor recycling with precipitation and re-use of chrome in excess liquors
Best for Australia, when effluent is irrigated. Thorstensen recommends this option for small tanneries in developing countries.

Chrome precipitation and re-use
Increases TDS.

Chrome precipitation and re-use with use of supernatant in pickle
Little lowering of TDS.

Total chrome liquor reuse
Total chrome liquor recycling is a system developed by CSIR07 for reuse of all the chromium and salts in tanning liquors. The liquors are concentrated before reuse. The capital cost of flash evaporation has been too high for commercial adoption; the challenge is to reduce the capital and operating costs. Solar evaporation may be possible in some climates. Each system has advantages and disadvantages and some of these are shown in Table 2. Mass balances are necessary to establish the real benefits of the options available.

HE High exhaust systems.
DR Direct Chrome Liquor Recycling.
DRP Direct Chrome Liquor Recycling with precipitation and re-use of chrome in excess liquors.
PR Chrome precipitation and re-use.
PRS Chrome precipitation and re-use with use of supernatant in pickle.
TCLR Total chrome liquor reuse.

Direct chrome liquor recycling

The Importance of process Flow Sheets and Material Balances
Figs. 1 and 2 give material balances for 10 tonnes of delimed hide being processed to sammed wet-blue. In Fig. 1, the hides and drum are washed with a total of 100% water and there is 150% excess chrome containing liquor. In Fig. 2, the hides are not washed before samming and the drum is washed with 5% water (500L) and only 50% float requires precipitation·

Pickling and tannage

• After drainage the required salt is added to the delimed hides and drummed before the pickle liquor is added
• Best practise requires less than 2% salt addition but it ivital that the SG is sufficient to control swelling to the same degree as in the normal tannery process.
• The recovered chrome liquor must be acidified before it is reused for the next pack of delimed hides. The pH is < 1 and this prevents chrome staining. At this low pH, the chromium species present are of low molecular weight and rapidly penetrate the hide.
• Good practice will allow indefinite re-use of the chrome liquor

Direct chrome recycling plus reuse of excess chrome liquors
If a tannery is already recovering chromium, it is very easy to combine this with direct recycling (see Fig. 3). The greatest savings in salt use are made if the spent chrome liquor from the drum is collected undiluted for recycle and diluted liquors are precipitated.

Direct recycling of chrome plus alutan
Ramasami et al. have developed a recycling chrome plus aluminium system.

Appropriate regulations
Appropriate regulations should vary between countries and between different regions within a country. For example, salinity is of concern in some inland areas but is not a problem for disposal to the ocean. Regulations will also vary for different types of sewage treatment plants with different loads and capacities. Mass loading regulations for pollutants rather than concentration limits encourage reduced water consumption. It is encouraging that a number of authorities around the world are reconsidering their regulations for chromium in the environment. The industry must provide authorities with scientific information to counteract misinformation and emotion.

In the USA, reduction of Cr(VI) to Cr(III) is now being used to remediate Cr(VI) contaminated sites and ground water. Recent publications by James, on soil remediation, state that Cr(III) is considered non toxic in most forms and reduction of Cr(VI) to Cr(III) in contaminated waters and soils diminish or eliminate the threat to aquatic life and to human health posed by Cr(VI) contaminations. The waste forms of Cr and redox related properties of Cr contaminated soils must be understood. Other workers state that Cr(III) is benign and that iron filings mixed with quartz sand completely reduce Cr (VI) and Cr(III) is incorporated into sparingly soluble species. It is important to note that the proposed US EPA analytical method for Cr(VI) in soils, SW-846 Method 3060A, has shown partial method induced oxidation of Cr(III) to Cr(VI) when soluble Cr(III) or freshly precipitated Cr(OH)3 is present. In such cases the method specifies the addition of Mg2+, which suppresses oxidation of Cr(III). This is relevant for soils treated with tannery effluent or sludges because false positive Cr(VI) analyses can occur. Conclusion Tanneries can be environmentally sustainable with clean technologies available. However, society demands continuous improvement. We need to exploit existing knowledge and to invent new technologies and that is a challenge.

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