Pollution and energy management in tanning industry

Tanning industry uses a number of chemicals such as Common Salt, Lime (Calcium Hydroxide), Sodium Sulfide and Basic Chromium Sulfate etc. During process, only a part of the chemical is consumed and the rest ends up in the effluent as pollutant. This paper deals with the techniques, locally developed or published in literature to recycle these chemicals and also discusses some energy saving techniques which can be used in tanning industry. Basic Chromium Sulfate (BCS) is one of the expensive chemicals used in 'Chrome Tanning'. By precipitating d filtering basic chromium sulfate, the recovery is nearly complete and the effluent obtained contains less than 1ppm Chromium. Dried raw hides contain up to 15% sodium chloride (w/w) and this can be removed in solid form by using mechanical brushes and can be re-used. The recovered salt contains foreign matter as impurities. After dissolution in water, the salt solution is filtered through cartridge filters and can be used in pickle bath. Liming slurry containing sodium sulfide is wasted as it contains fleshing and hair etc. A self cleaning 'J' type screen has no moving parts and removes fleshing and hair from the lime suspension. 'Counter Current Washing Technique,' reduces the wash water quantity by a factor of five to six. Air born pollution generated during buffing and dyeing can be captured by properly designed air filters. The solvents released in atmosphere during dyeing and finishing can be recovered by absorption. Fat, gelatin and protein can be recovered from waste fleshing. In tanning industry, drying of hides is the major consumer of thermal energy. Hot air can be produced by steam, hot water or solar energy. Advantages and disadvantages of these options are discussed. Wastage of thermal energy in dryers can be reduced by improving the existing designs. Hot water for tanning purposes can be generated by recovering waste heat present in the boiler flue gases. Boiler efficiency can also be improved by cycling heat in flue gases through a thermal wheel. Savings in the use of electrical and mechanical energy can carried out by using simple techniques. (author)