II WORKSHOP ON CHEMICAL SENSORS AND BIOSENSORS TANNERY PROCESS REGULATION WITH ON LINE CHROMIUM (III) MONITORING Massimo Pizzichini 1, Mauro Spadoni 1, Rosalba Montani 2. 1 ENEA, C.R. Casaccia, Via Anguillarese 301, 00060 Roma 2 INTEAM s.r.l., Via Chiana 48, 00198 Roma Abstract: A special sensor for chromium (III) on line monitoring into the tanning reactor was studied and developed for a practical use in tannery industry. The sensor was carried out combining different apparatus as ultrafiltration unit for cleaning the tanning solution, flow injection system to increase the monitoring range, and a photometer detector with wavelength set between 440 and 460 nm. The chromium sensor is hydraulically connected with a tanning big drum, through a recycling loop in which are inserted some filtration apparatus. The sensor flow cell is fed with water solution at a flow rate of 5 ml/min twich assures a suitable sample carrying out and a continuous cleaning of the system. A sample loop of about 30 µl provide a suitable photometric response, with characteristic peaks having an area proportional to chromium concentration. The sensor shows a satisfactory linear response in the 2-20 g/l chromium concentrations range, with a mean accuracy of 5 %. A single determination takes place in 30-50 seconds and the measure is not disturbed by organic chemicals present in the tanning medium. The peaks area are integrated in order to have directly the chromium concentration in g/l, as requested by tannery operators. Keywords: Chromium(III), automation and process control, tannery. INTRODUCTION The chrome tannage process is based on the complex bounds formation between chromium (III) sulphate and the carboxylic groups of the proteins (collagen), that represent the 33 % of the raw hide [1 ]. This process takes place in aqueous medium with ph variation in the range 2,5-3,8, for a reaction time of several hours (8-16) depending on the hide thickness and its origin (bovine, ovine, etc.). During the chromium tanning process the raw hides are transformed into leather to avoid putrefaction, cracking of the fibber and hardness after drying. In addition, the skin shows the mechanical and commercial characteristic of the leather goods. In Italy, second world producer of leather goods (more than 2,500 manufactures, 25,000 employees and 600,000 ton/y of treated hides), 257
M.PIZZICHINI ET AL. "TANNERY PROCESS REGULATION WITH ON LINE CHROMIUM (III) " tannery wastewater amount reaches the relevant level of 40 million of ton/y [2]. Wastewater treatment plants produce more than 700,000 ton/y of sludges and solid residues, containing about 2 % of chromium. Treatment and disposal costs amount to about 840,000 millions Italian lire/y (560 millions of US $). The impressive dimension of this environmental problem pushes to transfer in tanneries any available technological improvement to reduce the environmental impact and recover and recycle water and the main chemicals. More than one hundred different chemicals (350.000 ton/y of inorganic and heavy metal salts, soaps, oils waxes, solvents, dyes. etc.) used in tanning processes are found in process wastes and wastewater. Chromium (III) sulphate employed in tanning process and sulfur compounds used for unhairing-liming process are the main pollutants released in the tannery wastewater and in the atmosphere (sludges incineration) [3]. As far as the chromium salts is concerned the tanning process consumes only 70 % of the chromium of the tanning bath and the possibility of recover the residual metal represents a main goal in process improvement. An improvement of the chromium recovery cycle by means of membranes processes as ultra and microfiltration was reported in a recent papers [4-7]. The membrane process allows to purify the chromium solution in order to increase its recovery and at same time reduces the environmental impact. In fact, the species chromium (III) and specially chromium (VI) are toxic for plants and animals [8]. In order to reduce the chromium release in the environmental and to control the tanning process, a system for chromium(iii) on line monitoring was studied and developed for practical use in the tannery farms. Actually the chromium determination into tannery bath solutions is performed only sometimes and at the end of the process, using a traditional analytical method (atomic spectroscopy) that provide concentration data after some days from the process end. For this reason the chromium concentration during the tanning process is evalueted in empirical manner based on the practical experience of the workers. In this conditions any opportunity to tannery process control and regulation are impossible with high risk to damage the leather quality, owing to the presence of stain, and consequently discharge at the process end the chromium as reactive chemicals. From an analytical point of view, it is well known that chromium shows an unstable oxidation status, especially in an high complexed medium as tannery bath, consequently all the potentiometric analytical methods are unreliable [9]. 258
II WORKSHOP ON CHEMICAL SENSORS AND BIOSENSORS Also other powerful and sensible method like stripping voltammetry [10] are not able to operate at the high chromium levels present in the tanning bath solution(2-25 g/l). The objective of this work is to develop a special device suitable to monitoring the chromium (III) in tanning bath solution, in order to optimise and standardise the process conditions for a definite process manufacturing. In addition this device should permit a maximum chromium utilization into the tannery reactor, in order to minimize the reactive damping into the environment, with the consequent costs reduction for disposal. Materials and Methods In this section the following experimental activities performed for build up the chromium sensor are discussed. 1. analyse the physical and chemical composition of the tanning bath, 2. tanning bath purification with membrane technology, 3. build up and assembling the chrome sensor, 4. sensor calibration and optimisation with real tanning solution. Physical-chemical composition of the tanning bath solution The chemical composition of the chromium tanning solution especially during the manufacturing process when the organic matter of the hide are dispersed in the medium, depends upon the process conditions and can vary in an broad range of values, related with the specific process manufacturing, and also with the reaction time. In fact, the tanning procedure requires at the process beginning (1-5 hours) a low ph (2-0) for increase the chromium diffusion through the hide. On the contrary, at the process end (12-18 hours) the ph must be increased (ph 3.5) for facilitate the chromium up take on the hide s surface. All the tanning solutions studied in this paper are real industrial solutions, supplied by an Italian farm ( Conceria Tre effe). An average composition for the tanning solutions tested during 2 years of industrial tanning process, is reported in table 1. Table 1 shows a complex chemical compositions of the real tanning bath in which the chromium as (Cr 2 SO 4 )2H 2 O is present in the 20-25 g/l (fresh solution) and in 2-5 g/l (exhausted solution) in which the bath contamination by organic compounds is maximum. 259
M.PIZZICHINI ET AL. "TANNERY PROCESS REGULATION WITH ON LINE CHROMIUM (III) " Table 1. Chemical composition of chromium tanning solutions Cr 3+ Fe 3+ Al 3+ Ca 2+ Mg 2+ Na + Mn 2+ Parameter Concentration 3,0-25 g/l 20-45 mg/l 100-400 (60-120) mg/l 400 800 (400-500) mg/l 400-800 (90-150) mg/l 300-700 (300) mg/l 0,5-3 mg/l Proteic Nitrogen (Kjeldahl) 1,108 g/l Nitrogen (NH + 4 ) 633 mg/l Chloride (11-16 g/l range) 11,34 g/l Sulphate(22-33 g/l range) 22,75 g/l Sosp. Solids (0,67-2,9 g/l range) 1,09 g/l Fats and oils 305 mg/l ph 3,9 COD 8.568 ppm The bath tanning solution contains also traces of organic chemical as formiate, phthalate, oxalate, etc. that forms stable chemical bond with chromium (III). It is interesting to note that in the solutions chromium is present only for a little percentage in the ionic form, while 35-45% of the total chromium is present as binder in very stable coordination complexes with organic compounds and about 15-25% is present in the colloidal form. Consequently, the chromium is mainly present as complexes form, not as free ions. Purification of the bath solution with membrane technology The tanning bath is really a dirty suspension with fragment of hide, hears, fats, and proteins in solution or in suspension. This solution is treated with ultra and microfiltration membrane, in a tangential flow conditions, in order to obtain a purified permeate usable for the sensor. This techniques provide good performance in terms of productivity and chromium purification. In fact, a tubular ceramic membrane (alumina) with a cut of in the range 0,2-0,9?m, with a surface area of about 100 cm 2 and a pressure difference of 1,3 bar, assures a permeate flux of about 1-2 ml/min that remains stable for 20-30 hours of continuous run. The permeate clean solution, without suspended solids, shows a chromium concentration in equilibrium with the retentate. 260
II WORKSHOP ON CHEMICAL SENSORS AND BIOSENSORS Assembling of chrome (III) sensor The experimentation of the chromium(iii) sensor was performed in the ENEA laboratories (Rome) and to the Tre effe tannery farm (Vicenza). In the same farm was studied the hydraulic connections between the industrial drum of 110 m 3 and the sensor system for chromium monitoring during the process time (figure 1). The reactive tannery solution is continuously recycled through a service tank having a volume of 1 m 3, used to introduce chemicals into the rotating reactor. The solution is pumped through mechanic filters with a porosity of 5?m, and the permeate was ultrafiltered with ceramic membrane. The UF retentate was recycled into the service tank and the permeate was injected into the chromium sensor device. Tannery drum UF retentate stream filters membrane cartridge service tank pump chromium sensor 261
M.PIZZICHINI ET AL. "TANNERY PROCESS REGULATION WITH ON LINE CHROMIUM (III) " peristaltic pump Figure 2. : Chromium(III) detection system optical fiber probe carrier solution Reodyne valve sample solution flux cell discharge photometer detector peaks integrator Figure 1 Tannery bath purification with ceramic membrane. The experimental apparatus for the determination of chromium(iii) is constituted by the apparatus reported in figure 2. 262
II WORKSHOP ON CHEMICAL SENSORS AND BIOSENSORS The (Cr 2 SO 4 )2H 2 O, with organic acids as complexion agents, for a more graduated leaching of chrome, used as tannery reactive shows a characteristic absorbance as reported in the figure 3. Assorbanza 1. peristaltic pump supplied by Gilson Minipuls 3, with four channels 2. Reodyne valve type Omnifit Gilson, electrically controlled 3. UV-Visible photometer with fibre-optic light guide, Metrohom662. 4. monitoring cell (made in ENEA) 5. integrator Shimadzu type C-R3A. The carrier solution (distilled water) is pumped by the first channel of Lunghrzza d onda(nm) the peristaltic pump at a flow rate on 4-5 ml/min. The second pump channel sends the chromium purified solution (UF permeate) with a flow rate of 1 ml/ min. The monitoring cell is realized in Plexiglass block having a cylindrical hole with a volume of 3 ml, in which is inserted the spectrophotometric probe. An o-ring placed in the top of the cell assures the hydraulic seal with the probe. All the reactive chemicals, especially the chromium salt used for sensor calibration are of analytical grade supplied by BDH. RESULTS AND DISCUSSION Chromium absorbance The chromium(iii) sulphate shows a characteristic absorbance spectrum with two peaks respectively at 406 and 574 nm. When the chromium is complexed with organic substance the maximum is shifted towards wavelength more high. The absorbance of the chromium(iii) depends by the degree of complex formation with organic substances, in fact the chromium free in solution (aqueous complexes) shows, for the first peak, a maximum in absorbance near 406 nm, while the organic complexes shift the maximum at 420 nm. 263
M.PIZZICHINI ET AL. "TANNERY PROCESS REGULATION WITH ON LINE CHROMIUM (III) " Figure 3 Wave length of chromium absorbance On the bases of our experimental evidences was choosen to operate at a wave length of 440 nm., but this value can be changed until 460 nm. Sensor calibration with standards solutions. Standards chromium solutions, obtained by solving chromium sulphate dibasic of analytical grade are prepared and successively controlled with atomic absorbance. Figure 4 shows the correlation between the chrome concentration and the area of the peaks obtained with flow injection analysis. Each point of the figure 4 is obtained with an average of 10 determination. The calibration straight line is obtained employing the commercial chromium salt used in tannery industry (cromo Stoppani). 264