1. 1. 1. Leather industrial waste:
Prominent effectiveness of leather industry is amplified by high input and expenditure but on other side it causes huge waste of resource, incredible environmental pollution and biological chain destruction. Streams of gaseous, liquid and solid waste are resulted by environmental blow of tanneries. Global leather industry generates 4 million tones of solid waste per year. People use products of the leather-processing industry on a daily basis. These include especially shoes, leather and textile goods; we normally encounter leather products even in both public and private transport.
The primary raw material for final products is hide from animals from slaughter houses and hide from game—i. e. waste from the meat industry, which is processed in tanneries and turned into leather. Therefore, the tanning industry can be considered one of the first industries to use and recycle secondary raw materials. Although the tanning industry is environmentally important as a principal user of meat industry waste, the industry is perceived as a consumer of resources and a producer of pollutants.
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Processing one metric ton of raw hide generates 200 kg of final leather product (containing 3 kg of chromium), 250 kg of non-tanned solid waste, 200 kg of tanned waste (containing 3 kg of chromium), and 50,000 kg of wastewater (containing 5 kg of chromium). Thus, only 20% of the raw material is converted into leather, and more than 60% of the chromium is in the solid and liquid waste. During the production of leather goods, especially shoes, manipulation waste is produced, whichmakes about 15–20% of the entry material—leather. The last kinds ofwaste are used leather products which have lost their utility value.
1. 1. The possibility of oxidation of CrIII to CrVI
The basic question is the possible oxidation reaction from chromium III to chromium VI. In basic solutions, the oxidation of CrIII to CrVI by oxidants such as peroxides and hypohalide occurs with ease. Such strong oxidation conditions are realized in the process of the sterilization of drinking water. This is the first threat to human health and life.
Rain (especially acid rain) can leach chromium III from waste dumps, and soluble salts can then reach sources of drinking water. During the sterilization process by ozone or hypochloride, chromium III is converted into chromium VI and reacts with magnesium and calcium ions occurring in drinking water to produce carcinogenic magnesium and calcium chromate or dichromate salts. Another problem concerns the possibility of oxidation of CrIII into CrVI in gentle conditions by air in the wide range of pH. Principally, oxidation can be realized after the following equations:
According to the European Commission (EC) the quantities of solid waste produced by tanneries depend on the type of leather processed, the source of hides and skins, and the techniques applied. On an average, at the end of the process, about 20% of the weight of the raw hides is (grain side) leather. On the other hand, in Rio Grande do Sul, approximately 40% of the initial raw material is transformed into solid and liquid wastes . In the tanning industry, raw skin is transformed into leather by means of a series of chemical and mechanical operations.
Chromium salts (in particular, chromium sulfate) are the most widely used tanning substances today. Hides that have been tanned with chromium salts have a good mechanical resistance, an extraordinary dyeing suitability and a better hydrothermal resistance in comparison with hides treated with plant substances. Chromium salts also have a high rate of penetration into the inter fibrillar spaces of the skin, what represents a saving in terms of production time and a better control of the process.
In Brazil, approximately 90% of the leather industry uses chromium in hide processing, resulting in hazardous The conventional tannery methods lead to discharge of solutions with chromium concentrations in the range of 1500–4000 mg/l. The specification for the discharge of chromium containing liquid wastes stipulates a range of 0. 3–2 mg/l. The tanning treatments to produce the wet blue leather yield sludge containing approximately 3% (w/w) of chromium. The method commonly used for this waste disposal presents high operational costs.
The production of chromium containing leather wastes (including chrome shavings and tanned splits) in leather industry has been recognized as a real problem for many years [ref]. The chromium leather wastes are generated principally during mechanical treatments carried out after tanning process. In this latter, chromium is bound with the collagen matrix, by cross linking with collagen carboxylic groups through coordinate covalent linkage. The final chemical structure of the waste illustrated in Eq. (1), is obtained through two chemical phenomena “olation and oxolation”.
As reported by numerous authors, the olation phenomenon is observed gradually with the increase of the alkalinity of the tanning medium. The olified complex continues its evolution through time and an acid discharge takes place while the oxygen-chrome coordinate links are transformed into covalent links (oxolation bridges) Eq. The great stability of the collagen–chromium complex produced makes the waste a non-biodegradable and toxic material, due to the chromium and nitrogen content about 4. 3% and 14%, respectively.
A large amount of waste still goes into land disposal. Incineration in air atmosphere generates other forms of residual pollutant (gaseous emission and ashes) more noxious. The solid wastes generated _presented in Table 1. from leather industry can be broadly classified as untanned collagenous, tanned collagenous and non-proteinous wastes. Among the tanned collagenous waste, the one resulting from the finishing operation called buffing dust draws the most attention from the public and pollution control authorities.
Buffing dust appears in a considerable proportion with processing of raw hides skins _i. e. 2–6 kg per ton of raw hides skins.. Buffing dust is a micro fined solid particulate impregnated with chromium, synthetic fat, oil, tanning agents and dye chemicals. Buffing dust carries about 2. 7% chromium on dry weight basis. This is carcinogenic in nature and it causes clinical problems like respiratory tract ailments w1x, allergic dermatitis, ulcers, perforated nasal septum, kidney malfunctions w2x and lung cancer w3x in humans exposed to the environment containing buffing dust particulates.
Hence, it is cautioned by pollution control authorities to collect the buffing dust for safety disposal. The current practice of disposing of buffing dust consists of: _i. incineration in incinerators, _ii. land co-disposal w4–12x. Incineration causes serious air pollution problems because of release of toxic So and No gases w13x, and it has been observed x x that at 8008C, about 40% of Cr_III. is converted into Cr_VI. during the incineration of Cr laden solid waste w14x. The tanning industry is familiar with its being a potentially pollution-intensive industry. The nvironmental impacts from tanneries result from liquid, solid and gaseous waste streams. It must be emphasized that 4million tones of solid waste per year is generated by the global tannery industry. According to the estimation of Sreeram et al. , about 0. 8 million tons of chromium tanned shavings are generated per year globally. The solid wastes from tannery industries may have significant Cr (III) content. Even though Cr (III) is viewed as not toxic, possible oxidation of Cr(III) to Cr(VI), due to the acid rains or incineration, threats the environment since Cr(VI) is a more toxic species.
Therefore, the conventional disposal methods, land-filling and incineration, cannot be considered a solution to the disposal problem of tanned leather wastes in eco-friendly manner. In literature, there are many studies on the treatment of tanned leather wastes mainly including the extraction of chromium from wastes to re-use in the tanning process and isolation of protein fractions. The tanning industry generates a huge quantum of liquid and solid wastes while producing finished leather.
Tanning is the main process followed in leather manufacturing that protects the leather against some environmental effects such as microbial degradation, heat, sweat or moisture, etc. In tanning industry raw skins/hides are transformed into leather by means of a series of chemical and mechanical operations. The tanning process is usually accomplished in three distinct phases, i. e. , preparation of the raw live stock to tan with tanning agents, tanning with mineral/vegetable tanning agents and post tanning to impart colour to finished leather.
Basic chromium sulfate is the most widely used tanning agent for converting putrescible collagen fibres into non-putrescible leather matrix. Chrome tanned leathers have improved mechanical resistance, extraordinary dyeing suitability and better hydrothermal resistance in comparison with vegetable tanned leather. The solid wastes generated from leather industry can be broadly classified into untanned collagenous, tanned collagenous and non-proteinaceous wastes. Among the tanned collagenous waste, the one resulting from the finishing operation is called chrome buffing dust (CBD).
CBD is a micro fined solid particulate impregnated with chromium, synthetic fat, oil, tanning agents and dye chemicals. About 2–6 kg of CBD is generated as a solid waste per ton of skin/hide processed. CBD contains chromium, it is carcinogenic in nature and it causes clinical problems like respiratory tract ailments, ulcers, perforated nasal septum, kidney malfunction and lung cancer in humans exposed to the environment containing buffing dust particulates. Hence, it is advised by pollution control authorities to collect the CBD for safe disposal.
The current methods for disposing buffing dust are land codisposal and thermal incineration. Land co-disposal method is not preferred for the reasons such as overall high pollution emissions and low energy recovery. The leather industry generates a large amount of a Cr-containing solid waste (wet blue leather), with approximately 3% (w/w) of chromium. However, the leather industry has commonly been associated with high pollution due to the bad smell, organic wastes and high water consumption caused during traditional manufacturing processes.
Different forms of waste in quality and quantity, which emerge during the transformation of hides and skins into leathers in thousands of leather factories, from primitive to modern all around the world, have negative impacts on the environment. According to the data received from the studies of several researchers, approximately 200 kg of leather is manufactured from 1 tone of wet-salted hide. This amount constitutes about 20% of rawhide weight. More than 600 kg of solid waste is generated during the transformation of Raw hide into leather.
That is to say, solid wastes containing protein and fat that constitute more than 60% of rawhide weight are disposed to the environment by leather factories without turning them to good use In other words, besides the 30-35m3 waste water disposed to environment during the processing of every 1 ton of rawhide in world leather industry, the data from FAO reveals that approximately 8. 5 million tons of solid waste is generated during the production of 11 million tons of raw hide processed in the world. Solid wastes generated by the leather industry in these stages of processes may be classified as follows: i. astes from untanned hides/skins (trimmings, fleshing wastes) ii. wastes from tanned leather (shaving wastes, buffing dust) iii. wastes from dyed and finished leather (trimmings from leather) Data obtained from research reveals that 80% of solid wastes are generated during pre-tanning processes, while 20% of the wastes are caused by post-tanning processes Due to the bad smell they produce during their putrefaction and their harmful chemical content, untanned hide/skin wastes have negative effects on the soil and/or water resources of the environment where they are discharged, in other words n the local plant flora and animal fauna. Therefore, uncontrolled discharge of such wastes should be prevented without taking adequate precautions. Legal arrangements gradually gaining speed all over the world enforce the leather industry to apply innovations in terms of reusing solid wastes generated during leather production processes such as fleshing, shaving, trimming and splits. Solid wastes create a major problem for leather industry in terms of both their variety and quantity.
A high amount of reusable waste is generated in the leather industry. It is possible to recycle these products and even use them as raw materials for different industries. The variety and quantity of solid wastes depends on animal species, breeding conditions, slaughterhouse practices, conservation conditions, leather process stages, mechanical operations, qualification of the personnel, and chemicals used in processes. Yet this fact causes uncertainties in reusing the generated wastes.
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