2.        Tanning involves stabilization of the hide material by treatment with chemicals to produce leather. The most common tanning agent is Chrome or Basic Chrome sulphate and the process consists of pickling, tanning and neutralization. While there is no need for extensive pickling in the case of vegetable tanning, the pickling and Chrome tanning processes are of major environmental consequence. Large amount of common salt and sulphuric acid during pickling gives solids (35000-70000 mg/l), COD (1000 to 3000 mg/l) very high chlorides (20000-30000 mg/l) and sulphates (12000 – 18000 mg/l). The volume of effluent is expected to be 1 m3 per tonne hide processed with a low pH of 2.0 to 3.0.
Chrome tanning operations generate about 1 to 2 m3 of effluents almost similar in characteristics to pickling operations except for total chromium which ranges from 1500 to 4000 mg/l. This is a major concern.
The neutralization process generates 2 to 3 m3 per tonne of hide/skins as effluent with a high BOD (800 to 1100 mg/l), COD (2000 to 4500 g/l) and a total chromium concentration of 15-30 mg/l. Rechroming, dyeing and fat liquoring are important steps in the post tanning operations which impart special properties to leather, increase penetration of tanning solutions, replenish oils in the hides impart colors to the leather and generate acidic effluents (pH 3.5 to 4.5). While the general characteristics resemble the neutralization effluents the total chromium concentration may range from 50 to 300 mg/l.
The composite effluents turbid, colored and foul smelling discharged from a tannery are expected to have the following characteristics:
1 | Volume per/tonne of hide/skins | 30 – 40 m3 |
2 | pH | 7.0 -9.0 |
3 | BOD 200c | 1200 – 3000 mg/l |
4 | COD | 2500 – 8000 mg/l |
5 | Sulfides (as S) | 30 – 150 mg/l |
6 | Total Solids | 12000 – 23000 mg/l |
7 | Total Dissolved Solids | 9000 – 18000 mg/l |
8 | Suspended Solids | 2000 – 5000 mg/l |
9 | Chlorides as (Cl) | 6000 – 9500 mg/l |
10 | Sulfate as SO4– – | 1600 – 2500 mg/l |
11 | Chromium as total Cr | 120 – 200 mg/l |
Air Emissions
Leather production processes can also discharge considerable emissions into the atmosphere which include the emission of ammonia during deliming and unhairing, sulphate during liming, chrome during chromate reduction and from buffering and also hydrogen sulphide due to the conversion of alkaline suphate to H2S at low pH (less than 8.0).
Solid Waste and Waste Utilization
Significant amount of solid waste are produced including trimmings, degraded hide and hair from the beam house process. The solid wastes can represent up to 70 % of the (wet) weight of the original hides. Large amounts of sludges are also generated. Solid wastes can be utilized to manufacture utilizable products like dog toys, gelatin, shoes etc. The salt obtained from mechanical desalting and solar evaporation could be used for curing and pickling or can be disposed off into the sea where feasible. Fleshings and solid sludge from primary and secondary treatment process can be utilized to produce biogas. The hair removed could be used in the manufacture of low priced rugs and carpets. Hair protein can be used as a supplement in animal feed. In England, waste hair has been used in the production of flower pots that are easily biodegradable, hair hydrolysate has found limited applications in the pharmaceutical and cosmetics industry, as a thermal and acoustic insulation material and as a manure (30%) supplement with household and garden refuse (70%). Sludge from lime pits can be used for land filling as well as construction of low price houses. Barks and nuts from vegetable tannery can be used as fuel for boiler and brick kilns. The shavings, trimmings and buffings of vegetables and chrome tanned leather could be used in the manufacture of leather boards. Sludge from treatment plants can also be used in brick manufacture, land filling for low lying areas and as a fertilizer (given that the chromium level is within permissible standards). A special “Oxidative” firing and ‘Reductive’ cooling technology has been developed to manufacturing bricks ensuring the safe deposition of chromium with out any leaching in industrial bricks.
we are interested to plan Leather Tanning business, we need to know more details of this project, machinery and also we want to know cost of production for this project to run, please email me the details.
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