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Chrome~9 August 2000
UNITED NATIONS INDUSTRIAL DEVELOPMENT ORGANIZATION
Regional Programme for Pollution Control in the Tanning Industry
in South-East Asia
CHROME BALANCE IN LEATHER PROCESSING
UNIDO Consultant in Leather Pollution Control
J. Buljan, Agro-Industries and Sectoral Support Branch
This report has not been edited.
The views presented are those of the author and are not necessarily shared by UNIDO.
Reference herein to any specific commercial product, process, or manufacturer does not necessarily
constitute or imply its endorsement or recommendation by UNIDO.
This is a revised version of an earlier printed document.
1. INTRODUCTION. 1
2. BASIC ASSUMPTIONS. 1
3. CHROME TANNING.1
3. 3. Tanning with direct and indirect chrome recycling .3 3. 3. 1. Conventional tanning in conjunction with direct chrome recycling.3 3. 3. 2. High-exhaustion tanning in conjunction with chrome precipitation……………….3 4. POST-TANNING.4
4. 1. Post-tanning with chrome retanning .4 4. 2. Post-tanning with chrome retanning and chrome precipitation.5 4. 3. Post-tanning without chrome retanning.5 5. SUMMARY TABLES AND FIGURES………………………………………………….5
Chrome tanning is the most common type of tanning in the world. Chrome tanned leathers are characterised by top handling quality, high hydro-thermal stability and excellent user properties. Chrome waste from leather processing poses a significant disposal problem. It occurs in three forms: liquid waste, solid tanned waste and sludge. In most countries, regulations governing chrome discharge from tanneries are stringent. Today, all tanneries must thoroughly check their waste streams. Chrome discharge into those streams is one of the components that has to be strictly controlled. The environmental impact of chrome waste from tanneries has been a subject of extensive scientific and technical dispute. Statutory limits have since been set for chrome discharge and disposal, and relevant guidelines have been drawn up throughout the world. Given the close correlation between chrome tanning and the environmental impact of leather processing, auditing the efficiency of processing operations takes on prime importance. Tanneries have to bear in mind that only some 53 % of the corium collagen and 15 % of the chemicals offered are retained in the finished leather, while the residue (47 % corium collagen and 85 % chemicals) enter the waste streams (1). BASIC ASSUMPTIONS
Given the broad variations in raw materials, processing methods and equipment, as well as the different final product specification, certain basic assumptions have to be made. (See Annex 1 for the assumptions that have been made for the production of grain leather for shoe uppers). Although this chrome balance corresponds reasonably well to the situation in some factories, figures may differ considerably in many other factories. That notwithstanding, it is hoped that this paper will serve as a suitable working tool for tanners doing their own computations. CHROME TANNING
Balancing chrome in leather processing, i.e. maximising the chrome uptake in the tanning process and minimising the amount of liquid/solid chrome wastes, is a matter of great concern to all tanners. In the following section, the chrome balance is computed for: - Conventional tanning, - High-exhaustion tanning, - Tanning with chrome recovery/chrome precipitation.
3. 1. Conventional tanning
Basic chrome sulphate containing 25 % Cr2O3 in the amount of 8 %, corresponding to 2 % Cr2O3 on pelt weight. About 75 %, i.e. 1.5% Cr2O3, reacts with pelt collagen in the form of a binuclear basic chrome sulphate complex. Typical chrome distribution in leather, split, solid waste and effluent according to (1) is shown in Table 1. Chrome distribution after conventional tanning
Table 1 shows that some 75 % of the Cr2O3 offer is retained in the collagen structure of wet
blue (grain leather, usable split, solid waste) and 25 % remains in the effluent.
3. 2. High-exhaustion tanning
Basic chrome sulphate containing 25 % Cr2O3 in the amount of 4.6 % corresponding to 1.2 % Cr2O3 on pelt weight. Self-basifying chrome tannin with fixing effect containing 7 % Cr2O3 in the amount of 2.6 % corresponding to 0.2 % Cr2O3 on pelt weight. The total offer of chrome tannin is 7.2 % corresponding to 1.4 % Cr2O3 on pelt weight. About 98 %, i.e. 1.37 % Cr2O3, reacts with pelt collagen in the form of a binuclear basic chrome sulphate complex. Typical distribution of chrome in leather, split, solid waste and effluent is shown in Table 2. Chrome distribution after high-exhaustion tanning Table 2 shows that about 98 % of the Cr2O3 offer is retained in the collagen structure of wet blue (grain leather, usable split, solid waste) and only 2 % remains in the effluent. 3. 3. Tanning with direct and indirect chrome recycling
In practice, two ways of chrome recycling and re-use are practised: direct and indirect recycling. The direct form entails spent float being recycled direct to the chrome tanning processing for re-use. The indirect form entails precipitating and separating the chrome from the float containing residual chrome, and then re-dissolving it in acid for re-use. The efficiency of both methods can be very high (more than 90 %); it depends on the effectiveness of the float collection process and the recycling/reusing technique. Of the two approaches, chrome recovery is more widely used than chrome precipitation. Normally, chrome recovery is used in conjunction with convential tanning. In high-exhaustion tanning, however,the chrome can, as a rule, only be precipitated, separated and de-watered, but not re-used in processing. 3. 3. 1. Conventional tanning in conjunction with direct chrome recycling
A model for chrome balance in tanning using direct chrome recycling has been presented in other papers (2,3). On the basis of that model and the assumptions given above for a conventional tanning process, computation shows that about 24 % of the chrome offer can be recovered from the spent tanning float and sammying/draining water, and 1.4 % is discharged in effluent. In principle, the chrome so recovered can be recycled and re-used in the form of a basic chrome sulphate solution for the subsequent tanning batches. 3. 3. 2. High-exhaustion tanning in conjunction with chrome precipitation
A model for chrome balance in high-exhaustion tanning using chrome precipitation has also been presented in other papers (2,3). On the basis of that model and the assumptions given above for a high-exhaustion tanning process, computation shows that at the most some 1.9 % of the chrome offer can be precipitated from the spent tanning float and sammying/draining water, and about 0.1 % is discharged in effluent. POST TANNING
The range of post tanning wet-work formulations applied by tanners is very broad. For the purposes of this paper, two basic post tanning processes are considered: post tanning with or without chrome retanning. 4. 1. Post tanning with chrome retanning
Basic chrome sulphate containing 25 % Cr2O3 in the amount of 5 % corresponding to 1.25 % Cr2O3 on shaved weight. About 40 %, i.e. 0.5 % Cr2O3, interacts with wet blue grain leather in the form of a binuclear basic chrome sulphate complex. Typical distribution of chrome in crust leather, waste crust and effluent is shown in Table 3. Chrome distribution after post tanning with chrome retanning (related to shaved weight) Table 3 shows that about 40 % of the Cr2O3 offer is retained in the leather material (grain crust, waste crust) with 60 % remaining in the effluent. This proportion has to be adjusted slightly, given the leaching of unfixed chrome during neutralisation, retanning with organic tannins, dyeing and fatliquoring. In conventional tanning, we can reckon with leaching in the order of 0.7 kg Cr/t of wet salted weight (w/s) (or 0.3 kg Cr/t w/s in high- exhaustion tanning) (3, 4, 5).
4. 2. Post tanning with chrome retanning and chrome precipitation
In principle, the chrome remaining in the spent retanning float can be precipitated, separated and dewatered, but it cannot be re-used. A model for chrome balance in post tanning with chrome retanning and chrome precipitation was also presented in earlier papers (2, 3). On the basis of that model and the assumptions given above for chrome retanning , computations show that in chrome retanning some 50 % of the chrome offer can be precipitated and 10 % of the chrome offer is drawn off in effluent. Here again, however, the proportion of chrome offer discharged in effluent must be slightly adjusted to account for chrome leaching(0.3 - 0.7 kg Cr/t w/s). 4. 3. Post tanning without chrome retanning
The post tanning formulation is subject to various modifications. In order to balance the chrome in leather processing, account has to be taken of the amount of chrome that might leach. SUMMARY TABLES AND FIGURES
A general chrome balance in leather processing by various types of chrome tanning and post tanning is given in Table 4. The balance expressed in a proportional chrome distribution is illustrated in Figure 1. General chrome balance in leather processing Conventional tanning without chrome retanning. Conventional tanning with chrome recovery and without chrome retanning. Conventional tanning and chrome retanning. Conventional tanning with chrome recovery and chrome retanning. Conventional tanning/chrome retanning with chrome recovery/ precipitation. High-exhaustion tanning without chrome retanning. VII High-exhaustion tanning with chrome precipitation and without chrome retanning. VIII High-exhaustion tanning and chrome retanning. High-exhaustion tanning with chrome precipitation and chrome retanning. High-exhaustion tanning/chrome retanning with chrome precipitation. Proportional distribution of chrome in leather processing Solid waste
The following basic information can be derived from the general balance and chrome distribution: The type of chrome tanning and post tanning significantly affects the balance and distribution of When processing leather by conventional tanning and/or chrome retanning with a chrome offer of 15-17 kg Cr/t w/s, 40 % of the chrome offer remains in the leather, 26-30 % in the solid waste and 30-34 % enters the effluent. A certain amount of the chrome entering the effluent can be recovered from the tanning float and sammying/draining water (21-24 % of chrome offer) or precipitated from the retanning float without being re-used (6 % of chrome offer). When processing leather by high-exhaustion tanning and/or chrome retanning, the chrome offer can actually decrease from 15-17 kg Cr/t w/s to 10-13 kg Cr/t w/s and it is used more effectively: 54-57 % remains in the leather, 31-38 % in the solid waste and 5-15 % enters the effluent. A certain amount of the chrome entering the effluent can be precipitated from both the tanning and retanning floats without being re-used (2-11 % of chrome offer). An overview of the chrome content in tannery effluent by type of chrome tanning and post tanning is presented in Table 5 and Figure 2. Chrome content in tannery effluent, by stream The following main conclusions can be drawn with respect to the quantities presented in Table 5 and Figure 2: The lowest level of chrome in effluent, i.e. 0.31 kg Cr/t w/s, is achieved when the leather is processed by means of high- exhaustion tanning with chrome precipitation, but without chrome retanning in the post tanning process. If the leather has to be retanned using chrome tannins, it is advisable to precipitate the chrome from both the spent tanning float and the separated spent retanning float. The level of chrome in the effluent can thus drop from 1.67-1.87 kg Cr/t w/s to 0.41 kg Cr/t w/s. The actual chrome concentration might be slightly higher owing to such residues as shavings and buffing dust. In many instances, the main source of chrome in effluent (up to 97 %) is the unfixed chrome leaching from the leather. This could be considered a challenge to tanners to improve tanning and post tanning formulations in order to enhance chrome fixing in leather. Selected figures relating to the levels of chrome concentrate in mixed effluents are to be seen in Table 6. Chrome concentration in mixed effluents. Legislative requirements governing chrome content in effluents and its discharge into surface water vary greatly from 0.05 to 10 mg Cr/l. In the case of discharge into sewers the range extends from 1 to 50 mg Cr/l (7). However, the most frequent requirements range from 1 to 4 mg Cr/l. In order to meet the rigorous requirement of 1 mg Cr/l in mixed effluent, it is essential to start by collecting all waste streams emanating from the tanning and chrome retanning operations after precipitating the chrome,, as well as all waste streams, i.e. spent floats from other post tanning operations (neutralisation, dyeing, fatliquoring). The latter waste streams which contain residual chrome in soluble/insoluble form also have to undergo precipitation. Calcium hydroxide is the most frequently used precipitant. In order to achieve complete chrome precipitation, ferric and/or aluminium salt is added together with lime. According to recent information (7) it has been demonstrated that chrome can be almost totally precipitated from waste stream residues in tanning/post tanning operations. It has been claimed that concentrations of 1 mg Cr/l corresponding to 0.03 kg Cr/t w/s can be obtained in mixed effluent. In some countries, tannery sludge containing chrome cannot be re-used and must be dumped in special landfills. CONCLUSION
Building on our previous work on mass balance in leather processing (1), the attempt was made to design a specific model for computing chrome balance. Computations were made on the basis of an input of one ton wet salted cattle hide processed into grain leather for shoe uppers using the most common types of tanning/post tanning procedures. A survey of chrome balance shows that when processing leather by means of conventional tanning and/or chrome retanning with a chrome offer of 15-17 kg Cr/t w/s, some 40 % of the chrome offer remains in the leather, 26-30% in the solid waste and 30-34 % in effluent. A certain amount of the chrome entering the effluent can be recovered from the tanning float (21-24 % of chrome offer) and re-used or precipitated from the retanning float without being re- used (6% of chrome offer). When processing the leather by high-exhaustion tanning and/or chrome retanning with a chrome offer of 10-13 kg Cr/t w/s, 54-57 % of the chrome offer remains in the leather, 31-38 % in the solid waste and 5-15 % in the effluent. A certain prportion of the chrome entering the effluent can be precipitated from the tanning/retanning floats without being re-used (2-11 % of the chrome offer). The lowest practically attainable chrome content in effluent ranges somewhere between 0.3-0.6 kg Cr/t w/s. With a level of standard effluent production of 30 m3/t w/s, chrome concentration thus ranges between 10 and 18 mg Cr/l. However, statutory requirements most frequently stipulate levels in the range of 1 to 4 mg Cr/l. In that case, all waste streams from tanning/post-tanning operations have to undergo further precipitation after precipitating the chrome. Calcium hydroxide combined with ferric and/or aluminium salt is considered the most suitable precipitant. Only after further precipitation can a concentration of about 1 mg Cr/l corresponding to 0.03 kg Cr/t w/s be obtained. REFERENCES
(1) Buljan, J., Reich, G., Ludvik, J: Leder u. Häutemarkt No.4 (1998), p.30, No.5(1998), (2) Covington, A.D: Chrome Management. Proceedings of the Workshop on Pollution Abatement and Waste Management in the Tanning Industry. Ljubljana, 1995 (3) Ludvik, J: Chrome Management in the Tannery. UNIDO, Vienna, Rev. 1, 2000 (4) Ludvik, J: Study on the Scope for Decreasing the Pollution Load in Leather Processing. UNIDO, Vienna, Rev. 1, 2000 (5) Francke, H: Das Leder 44 (1993), p. 89 (6) Bosnic, M., Buljan, J., Daniels, R.P: Pollutants in Tannery Effluents. UNIDO, Vienna, (7) Rydin, S: Reduction of Chrome Discharge from the Leather Industry. Minutes of the European Workshop on Environmental Technologies in Leather Industry. Bologna, BASIC ASSUMPTIONS
1000 kg wet salted cattle hides. Weight class of 25-29.5 kg green weight Conventional chrome tanning with/without chrome recovery High exhaustion chrome tanning with/without chrome precipitation. Sammying, splitting, trimming, shaving. Post tanning Neutralisation Retanning with/without chrome precipitation, dyeing, fatliquoring Lightly corrected shoe upper grain leather
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