EthonlineS Reflections + Reviews + Reports

From Wastage to Usage

An Account of Chrome Recovery in Ethiopian Tanning Industry

Wondirad Seifu

Quality Control Service Head, Awash Tannery, Addis Ababa, Ethiopia

May-December, 1997, Vol.2, No. 2, ESALIA (Eastern and S. Africa Leather Ind. Asso.)

Introduction

Like many important discoveries, tanning was probably stumbled upon by historical accident. With time, leather-making has become an important form of trade, with knowledge of leather making having spread to many parts of the world.

The world’s leather industry of the mid-nineteenth century was very similar to that of the 300 or 400 years earlier. Despite the introduction of various alternative vegetable tanning materials, it still took 3-10 days to produce a piece of leather in a conventional tanning process. In addition to this, an immense amount of capital was required to run a small tannery, making it difficult to meet the needs of manufacturers.

It was against this background that chemists attempted to introduce a process called chrome tanning, in order to mitigate the problem faced by the tanning industry in particular, and the leather sector in general.

In Ethiopia, the leather industry is the second biggest export earner. However tanning processes are characterised by lack of waste treatment or recycling procedures. Of the various forms of wastes, pollutants include lime yard fleshing and trimmings, spent vegetable tans, sulphide, chromium and salts.

Out of these wastes, chrome recovery has been proven as viable, both economically and environmentally. In general, the exhaustion rate of chrome tanning process varies between 60-70 percent, and as such leaves at a minimum about 25 percent of chrome applied in the main tanning bath. Chrome recovery process is aimed at addressing this problem.

Chrome and the Tanning Industry

The history of industrial chrome tanning goes back to 1884, when Augustus carried out the first two-bath chrome tanning in the USA. The introduction of this new tanning material can be regarded as a “breakthrough” as it took the leather industry one step nearer to a 24 hour work cycle, as opposed to the vegetable tanning process. However, the innovation was resisted by the conservative leather industry, which still saw itself as a craft than an industry.

Chrome ranks sixth in the quantity of deposits on earth’s crust, fifteen in sea water, and fifteen in the human body. Roughly estimated, the global consumption of chromium by the leather industry is about 65,000 tonnes per year, which is about 0.5 percent of the total world consumption. In principle, however, all chrome tanning materials could be produced from chromium wastes of several galvanic and chemical industries.

The Ethiopian tanning industry at present time consumes around 223 tonnes of chrome tanning material (as Cr₂O₃) per year (see table) at the current level of net supply of raw hide and skins for tanneries.

Chrome Tanning

Before chrome tanning process starts, animal skins are subjected to preliminary cleaning and brought into acid media by pickling. After this, the resulting pure collagen matrix, the main component of the leather making, is hydrothermally stabilized by tanning with basic sulphates of trivalent chromium. In the process, the active groups of the tanning agent bind with the functional groups of the collagen, which gives the leather adequate strength properties and resistance to various biological and physical agents.

Unfortunately, despite more than a century experience with chrome tanning and extensive research work, complete fixation of chrome to the collagen fibre has not been achieved. This is due to the affinity of the tanning material to collagen which is in turn governed by the law of chemical thermo-dynamics. In addition to this, full replacement with other tanning minerals has been found unsuccessful due to the inherent characteristics of chrome tanned leather.

As a result of this, chrome tanning process is generally rather inefficient, sometimes leaving more than 40% of the chrome in the tanning bath. Even with the most sophisticated chrome-leather manufacturing process, complete prevention of chromium discharge to the environment by the leather industry is difficult. However, improvements are gained by application of highly exhaustive chrome tanning materials.

The above holds true in Ethiopian tanning industry. According to a study made in Awash Tannery, the fixation rate of chrome tanning process varies between 60-75%. As such it leaves at a minimum, 25% of the chromium applied in the tanning bath, ultimately goes to the environment. Roughly, Ethiopian tanning industry discharges to the environment every year, at least 56 tonnes of chromium in solution form, and mixed with other organic and inorganic compounds.

Chrome and the Environment

Increasingly, the leather industry is being confronted with the challenge of improving the tanning process, in order to reduce the amount of effluent discharge to the environment. Many countries have now imposed limits on the chrome content of the effluent due to the assumed toxicity nature of chromium.

The toxicity of chrome has been a widely debated issue, although no general consensus has emerged among tanners, researchers and the environmental authorities; however, the latter considered any metal, including chrome as a potential pollutant to the environment. The toxicity argument asserts that chrome at +3 oxidation state is neutral and stable, and therefore largely harmless to the environment. However, its conversion to +6 oxidation state is said to render it biologically active, and therefore may affect biological organisms. However, a great deal of additional data is needed in order to adequately establish the valancy, stability and mobility of Cr III in industry waste, as well as to establish an alternative analytical procedure for the determination of Cr VI in environmental samples.

In pursuit of a solution to minimize chrome discharge to the environment, many alternatives have been tried; among these are the “cost-conscious and responsible tanners’ deed” whose aim was to promote complete utilization of chromium. This is regarded as both an economic proposition as well as environmentally responsible measure.

Chrome Recovery

The technology of chrome recovery is very simple process. It starts with the collection of spent chromium liquor, and is followed by separation of the chromium by adding appropriate alkali, usually magnesium oxide, in order to precipitate the chromium as chromium hydroxide. The rest of the process is explained by the words of Dr. Lieselotte Ferikes as follows: “What is going to happen to the precipitated chromium hydroxide? ...My answer is to let it settle, drain and dissolve in a boiling sulphuric acid, creating a regenerated solution... I acquired this procedure and have used it for decades without problems. This procedure must, however, be accompanied by careful analytical control.”

Conclusion

Leather being the second major export commodity in Ethiopia, its importance to the economy is unquestionable. However, the waste it generates deserves critical attention in both economic and environmental terms. Latest developments in the chrome tanning provide prospects for improving the exhaustion rate of chromium in the tanning bath. Practically, the leather produced from 100% recovered chrome has showed no difference as compared with normal chrome tanned leather.

Of the two methods currently available for chrome waste treatment such as disposal after drying in drying beds and chrome recovery after precipitation, the later has great advantage. Chrome disposal after drying demands large areas for drying, is liable to produce strong odour and is contagious on weather conditions. In a country like Ethiopia where industrial land and foreign exchange are scarce, chrome recovery could contribute to land savings and increase exchange earnings.

References

1.      Beeby K.J. The Wonderful Story of Leather, The Leather Institute: London

2.      Thomson R.C., Journal of the Society of Leather Technologists and Chemists, (1985)

3.      Thorstensen T C. Practical Leather Technology, Robert E. Krieger Pub. Comp.; USA(1984)

4.      Luck Wolfhard, Journal of the Society of Leather Technologists and Chemists, (1986)

5.      Langlains J. S, Journal of the Society of Leather Technologists and Chemists, (1985)

6.      Wilson J.S, Journal of the American Leather Chemists Association(1990)

7.      Langerwerf J.A., Journal of the Society of Leather Technologists and Chemists

8.      Ruiland F.H., Journal of the American Leather Chemists Association(1987)

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