There are around 2,000 tanneries spread all over the country with an annual processing capacity of over 500,000 tonnes of hides and skins. A majority of these tanneries use chrome in their tanning operations. About 60 per cent of the chrome applied in tanning operations is absorbed by the leather and the remaining is discharged in the waste water. Annually, 25,000 tonnes of chromium salt is used, out of which around 10,000 tonnes is discharged into the waste water, causing environmental pollution and making waste water treatment and sludge disposal complicated and costly. It is, therefore, worthwhile to recover the chromium being discharged down the drain and reuse it for tanning.
Chromium from the waste chrome tanning liquor in its trivalent form is generally insoluble at a pH of 8 to 12. So addition of sufficient alkali to spent chrome liquor will cause precipitation of the chromium in the form of high basic complexes. These can be separated from the liquor by settling and/or filtering under pressure. The liquor is almost free of chromium and contains most of the dissolved solids and other impurities. The chrome sludge (cake) can be dissolved in sulphuric acid to form a tanning liquor which can be reused.
Within the framework of the Indo-Dutch Environmental & Sanitary Engineering Project in Kanpur under Ganga Action Plan Phase-I, a simple chrome recovery pilot plant was developed and installed in one of the large tanneries (i.e. Pioneer Tannery) at Jajmau, Kanpur using the above principle.
The chromium containing waste water, including wash water, was collected via gutters mounted under the tanning drums and led to a collection cum treatment pit after which it was screened and collected in a treatment tank. Magnesium Oxide (MgO) was added to the liquor and stirred till the pH gradually rose to the required value of about 8. After stabilisation of the pH, stirring was stopped. The chromium precipitates and settles to a very compact sludge within an hour, which is only about 8 per cent by volume of the exhaust chrome liquor. The supernatant liquor is decanted and the sludge is dissolved in Sulphuric Acid so that again Basic Chromium Sulphate (BCS) is formed, which can be reused as a tanning agent.
The pilot plant was monitored for a period of one year and the recovered chromium was reused for chrome tanning within the tannery. Hides were tanned with a mixture of 70 per cent fresh chromium and 30 per cent recovered chromium and compared with the hides tanned with 100 per cent fresh chromium. The leather tanned with the mixture of recovered chromium and fresh chromium appeared remarkably equal to the leather tanned in the normal way.
For a medium scale tannery with a processing capacity of 5 tonnes per day, the capital investment for the chrome recovery system is about Rs 3.5 lakhs (January 1992 rates). Assuming that production takes place during 250 days/year and the wastage is 30 per cent of the used chromium, then about 30 tonnes per year of chromium can be recovered and reused which is equivalent to Rs 5.4 lakhs. The annual operating costs of the plant, including depreciation, are about Rs 2.5 lakhs. Thus, the net profit/year is Rs 2.9 lakhs.
The calculations only cover the investment costs and operating costs. The costs of floor space and the buildings in which the plants will be installed are not taken into account since the space required for chrome recovery systems is very small and is generally available in most of the tanneries adjacent to the tanning drums.
The cost of recovered chromium is about Rs 8,000/tonne, whereas fresh chromium salt costs more than Rs 18,000 per tonne. Profits will be relatively higher if the production capacity increases. In a tannery with a processing capacity of 10 tonnes of hides per day, the cost of recovered chromium will be about Rs 6,000 per tonne which is only 30 per cent of the cost of fresh chromium salt. The pay back period of the chrome recovery plant will be one or two years depending upon the processing capacity.
Based on the success of the pilot plant, five large tanneries have already installed chrome recovery plants in the Jajmau area of Kanpur and besides controlling chromium pollution are also making profits. However, there is a need to popularise this clean technology and make the other tanneries aware of its inherent advantages, so that the 100-odd chrome tanning tanneries in Jajmau and others all over India install chrome recovery systems.
For the first time in the country under the Indo-Dutch Environmental and Sanitary Engineering Project at Kanpur, under Ganga Action Plan Phase-I, a clean technology has been experimented by CLRI, a premium National Institute on Leather Research, along with Dutch consultants and demonstrated a chrome recovery plant in one of the chrome tannery in the Jajmau Tannery Cluster, 90% of recovery of chrome within their chrome tanning unit using this simple and cost-effective technology. With a cost recovery period of less than 2 years and subsequent saving of foreign exchange for tanners on account of imported chromium salt as a make up for its loss in the effluent, the economics of this technology motivated five large chrome tanneries in the Jajmau complex and on a sewage farm. C.L.R.I., Chennai has promoted this clean technology by extension programme through Dutch and UNDP Programmes to other technology complexes in the country.